Final Report: ProSe 

Submitted to: Department of Commerce/ National Institute of Standards and 

Technology 

Federal Grant: 60NANB17D168 

Project Title: ProSe

 
Table of Contents 

Table of Contents 

1. Overview ............................................................................................................................................... 4 

2. Acronyms and Definitions ..................................................................................................................... 6 

2.1 Definitions ..................................................................................................................................... 6 

2.2 Acronyms ...................................................................................................................................... 6 

2.3 References .................................................................................................................................... 6 

3. Public Safety Direct Communications Range ........................................................................................ 7 

3.1 Use Cases: Direct Communication Range (Noise Limited) ............................................................ 8 

3.1.1 Mission Critical Voice Range (Noise Limited) ........................................................................ 8 

3.2 Use Cases: Direct Communication Range (Interference Limited) ............................................... 13 

3.2.1 Description: Voice Communications (legacy) ..................................................................... 13 

3.2.2 Issues ................................................................................................................................... 14 

3.3 Conclusions ................................................................................................................................. 21 

4. LTE-direct Description ......................................................................................................................... 22 

4.1 Standards .................................................................................................................................... 22 

4.2 LTE-direct Overview .................................................................................................................... 23 

4.3 Standards Based ProSe Evolution ............................................................................................... 23 

4.4 Extensions Beyond the Standards ............................................................................................... 23 

4.5 Public Safety Profiles ................................................................................................................... 23 

5. Results ................................................................................................................................................. 23 

5.1 Performance in Noise Limited Conditions .................................................................................. 23 

5.1.1 Audio Quality ...................................................................................................................... 25 

5.1.2 RF Channel Modeling .......................................................................................................... 26 

5.1.3 P25 Performance................................................................................................................. 26 

5.1.4 ProSe Performance ............................................................................................................. 28 

5.1.5 SLSS Processing ................................................................................................................... 60 

5.1.6 Enhancements to ProSe ......................................................... Error! Bookmark not defined.

5.1.7 Improved ProSe Performance ................................................ Error! Bookmark not defined.

5.1.8 Comparison of ProSe and P25 Coverage ............................... Error! Bookmark not defined.

6. Addendum – Description of Sidelink Operations ................................................................................ 64 

6.1 PHY Layer .................................................................................................................................... 64 
6.1.1 Synchronization ................................................................................................................... 65 

6.1.2 Control ................................................................................... Error! Bookmark not defined.

6.1.3 Shared .................................................................................... Error! Bookmark not defined.

6.1.4 Channel Efficiency .................................................................. Error! Bookmark not defined.

6.2 MAC Layer ...................................................................................... Error! Bookmark not defined.

6.3 RRC ................................................................................................. Error! Bookmark not defined.

7. Addendum – Direct Communications Requirements ......................................................................... 68 

8. Innovation Concepts ........................................................................................................................... 71 

8.1 Channel Aware CODEC ................................................................................................................ 71 

8.2 Macro-diversity Transmissions ................................................................................................... 71 

8.2.1 Description .......................................................................................................................... 71 

9. References to relevant 3GPP documents ........................................................................................... 76 

9.1 Interference Cancellation ........................................................................................................... 76 

9.2 ProSe ........................................................................................................................................... 76 

9.3 V2X .............................................................................................................................................. 76 

9.4 Voice Quality and Coding ............................................................................................................ 76 

9.5 NB-IoT ......................................................................................................................................... 78 

9.6 In-device Co-existence ................................................................................................................ 78 

9.7 Relays .......................................................................................................................................... 78 

9.8 High Power UE ............................................................................................................................ 78 

9.9 Misc ............................................................................................................................................. 79 

10. List of improvements and Issues ........................................................ Error! Bookmark not defined.

 
1. Overview 

This document provides high level use cases and analysis for public safety direct communication 

performance. NPSTC and other public safety focused organizations have developed requirements for 

public safety communications. These requirements capture the capabilities of legacy public safety 

communication systems while adding new requirements that anticipate the adoption of new services 

that enhance safety for both the first responder and the public at large. 

Globally most public safety organization have either adopted LTE as their next generation public safety 

wireless system or have begun implementing it. In support of this widespread adoption, 3GPP has 

responded by developing standards that support the needs of first responders. These include: higher 

power device specifications, group communication capabilities, priority & pre-emption, mission critical 

applications, and direct communications. 

Synergy between commercial interests and public safety interests have accelerated the adoption of 

some first responder capabilities while other capabilities with less commercial interest have seen slower 

adoption into devices. By example, LTE-direct communication standardized by 3GPP as Proximity 

Services (ProSe) has seen limited deployment into devices. In part, this slow adoption results from 

ProSe off network mode that is only supported for public safety use and is therefore applicable to a 

smaller market. 

3GPP standards bodies has taken input from public safety organizations, public safety officials, public 

safety manufacturers, and the cellular industry to develop a direct communications capability, called 

Proximity Services (ProSe), that is designed to address the direct communication needs of first 

responders. However, these efforts have not captured all expectations of public safety users and their 

organizations for direct communications. These shortcomings persist, because to date, ProSe has not 

been realistically tested or fielded in part due to a lack of UE chip sets that enable form factor trial 

devices. 

Beginning with 3GPP Release 14, 3GPP shifted its focus away from ProSe toward Vehicle to Everything 

(V2X). V2X communications is designed to provide ubiquitous communication for vehicles by providing 

communications with networks, with road side digital kiosks providing for example road conditions, and 

direct communications between vehicles and to pedestrians. This capability is an important part of 

Smart Car capabilities and has captured the interest of 3GPP standards committees with its huge 

potential for implementation in all vehicles, infrastructure, and cellphones. As such PC5 the interface 

between directly communicating nodes in ProSe and V2X has continued to evolve based on V2X 

requirements. Significant work in V2X has focused on lowering communication latency and improving 

performance in congested environments. These improvements are directly applicable to some public 

safety uses cases but V2X vehicle use cases miss some important public safety requirements. 

A cardinal requirement of public safety communications is to provide a direct communication capability 

that does not require any infrastructure and allows first responders to communicate effectively. Direct 

communications are essential when infrastructure-based communication is not available or when policy 

dictates that direct communication should be used. Given the slow commercialization of ProSe and the 

rapid development of V2X both as a standard and in field trials, the future of public safety direct 
communications could come either directly from ProSe standards or be based on V2X standards. Basing 

direct communications on V2X leverages a mainstream market where public safety use of V2X is a 

vertical application. Therefore, the important question then becomes initially for standards 

development and beyond, what additional requirements that may not exist for the mainstream V2X 

market are needed to support first responders. 

The use cases and recommended requirements developed in this document attempt to capture 

perceived shortcomings of both ProSe and V2X standards by creating quantifiable requirements that 

may be addressed as extensions to ProSe, V2X, or by specific configurations that are fully standards 

compliant. This document is designed as an extension to the requirements already addressed by 3GPP 

TS22.280 “Mission Critical Services Common Requirements” and TS22.179 “Mission Critical Push to Talk 

over LTE.” These requirements may also apply to TS22.181 “Mission Critical Video services over LTE” 

and TS22.182 “Mission Critical Data services over LTE,” however the current scope focuses on voice 

communications. 

 
2. Acronyms and Definitions 

2.1 Definitions 

 Mode – Operation of equipment in a defined high-level configuration that is constituted by a 

group of functional capabilities. Functional capabilities may appear in more than one mode, but 

modes are mutually exclusive. Thus, only one may be selected at a time and they cannot be 

combined. 

 Hot Spot: A device providing a Wi-FiTM AP that typically uses cellular transport for wide area 

networking access. 

 Flexible Hot Spot: A Hot Spot that expands the local network access to include other 

communication interfaces both wired and wireless. E.g: Bluetooth, Ethernet, USB, 7 etc. 

 Hybrid Communication Talkgroup – A talkgroup whose domain spans network and direct 

communication links. 

2.2 Acronyms 

 Ethernet 

 FHS Flexible Hot Spot 

 HS Hot Spot 

 HCT Hybrid Communication talkgroup 

 IC Incident Command 

 MODEM 

 NPSBN National Public Safety Broadband Network 

 OAP Opportunistic Access Point 

 USB Universal Serial Bus 

 Wi-FiTM Industry alliance for 802.11 products 

2.3 References 

The following documents contain provisions which, through reference in this text, constitute provisions 

of the present document. 

- References are either specific (identified by date of publication, edition number, version number, 

etc.) or non-specific. 

- For a specific reference, subsequent revisions do not apply. 

- For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP 

document (including a GSM document), a non-specific reference implicitly refers to the latest 

version of that document in the same Release as the present document. 

 [1] R1-133186: "Typical Public Safety Use Cases, Performance Values, and E-UTRAN 

Characteristics for D2D ProSe Group Communication", U.S. Department of Commerce 

[2] NPSTG Communications Report "Public Safety Broadband Push-to-Talk over Long 

Term Evolution Requirements", 7/18/2013. 

 
3. Public Safety Direct Communications Range 

The ability to directly communicate between user devices without intervening infrastructure has been 

recognized and accepted as a core requirement for public safety communications. However, the 

requirements to date have not quantified requirements for communication range. The use cases that 

follow provide a basis for range requirements. These requirements are based on legacy public safety 

communications and on new services that are enabled by ProSe. 

While the NPSBN is the primary, reliable transport of public safety voice and data, there are many 

situations where voice and data communications will be required in areas where the NPSBN service is not 

available either for initial response or for continuing operations. 

 NPSBN Users (NPSBN-U) may be outside of the range of the fixed network, such as first 

responders in a rural area assisting in a response to a plane crash or police officers inside a 

residence responding to a domestic issue. Off-network voice communications must be 

immediately accessible to users in the absence of the NPSBN. This need includes areas and 

locations where the ability to access non-terrestrial communications can be impaired such as 

within buildings and other enclosed areas where non-terrestrial communications may not be 

available. 

 Additionally, there may be times when users may wish to communicate off-network. Today, 

firefighters often join a local communications network, which does not leverage the fixed 

network, but rather, relies on either direct communication between the user devices or 

communications via a local repeater on-scene. Firefighters can voluntarily leave the fixed 

network either due to the unpredictable coverage of the fixed network, or if the coverage of 

direct communications or the local repeater is well known, based on experience. 

 A user within network coverage needs to communicate with users who are on the network and 

those that are off-network. By example an Incident Commander (IC) may be located within 

network coverage while some personnel responding to the fire may have more limited coverage. 

These users must be able to communicate to users on the fixed network, such as dispatch, as well 

as the local users who are off-network or when it is desirable to provide voice, data and video 

connections between users without connection to the network even if within network coverage. 

Note: – This description addresses current use where first responders must decide whether 

operations will be on or off network and then command their radios to operate either on 

network or through direct communications. However, future broadband communications should 

include that ability to automate this process allowing first responders to communicate with both 

on network and off network devices as possible by coverage. 

 A relay function is critical for off-network communications when NPSBN coverage is not enough 

to support the public safety mission. In the case of firefighters who are responding to a wildfire 

while outside of the coverage of the fixed network, if one user becomes encircled by the wildfire 

and is beyond the range of the IC, but within the range of another device that can act as a relay, 

the endangered firefighter can still update his status to the IC. A UE device operating on the 

NPSBN should be capable of relaying PTT traffic between a UE device operating off the network 

and a UE Device operating on the network.[2] 
ProSe provides both communication and discovery capabilities. Communication provides service 

information conveyance between devices, while discovery is designed to enable devices to find other 

direct devices operating in proximity. The focus of these use cases is on communication; discovery is 

treated as secondary. In many cases devices may be pre-configured to communicate for logistical, 

security, and latency reasons thereby minimizing the need for discovery. 

The following use cases are designed to reveal perceived weaknesses in legacy direct (LMR) and ProSe 

communications with a goal of addressing first responder expectations through these requirements. An 

attempt has been made to group use cases that have similar technical requirements. 

3.1 Use Cases: Direct Communication Range (Noise Limited) 

Direct communication noise limited range use cases are characterized by the communication range 

being limited by the noise floor of the receiver rather than interference in the operating environment. 

Typical environments providing low noise floors include: 

 Areas out of network coverage that may be rural or have geographic RF blocking whether 

natural or manmade 

 In-building coverage where RF losses are high even over potentially short distances and 

interference is typically low for PS bands 

 For direct communications, a low loading of competing ProSe communication links in range 

A noise limited use case is specifically provided to capture performance requirement for 

uncompromised coverage. The noise limited case that follows in the next section captures expectations 

for performance where interference from network operations, interference from other direct 

communication active links, or external interference may affect operations. 

3.1.1 Mission Critical Voice Range (Noise Limited) 

This use case captures the legacy public safety requirement for direct communications. 

3.1.1.1 Description: Voice Communications (legacy) 

A first responder outside of infrastructure coverage needs to communicate with other first responders 

in the area using voice PTT to coordinate activities and provide aid as needed. No interstitial 

communication nodes are available to provide communication relaying. The inability to support voice 

communication is a serious safety issue. 

 Wildfire fighting is performed over large, frequently rural areas with rugged terrain. Workers 

need reliable communications to coordinate efforts and to provide emergency response when 

needed. Communication traffic has a low duty cycle and consequently voice communications 

rarely collide. Legacy LMR technology such as P25 can fail to provide successful communications 

for remotely located workers, particularly due to natural terrain blockages or large propagation 

distances. Remote workers are aware that communications may be enabled or improved by 

moving to local high ground and may do so to enable critical communications as depicted in 

Figure 1. Fire fighters1 expect a technology that communicates on par with their existing 

technology experience with P25. Two requirements follow from this example: communication 

1

 As well as other first responders. 
range should be at least comparable with P25 and the communication system should allow 

users to communicate over longer distances by operating from physically advantaged locations. 

 SWAT teams operate in high intensity situations where real-time coordination is crucial to the 

mission. Network communication may be compromised by RF losses in penetrating buildings or 

other barriers. Generally, the reliability of network coverage in the operational zone is 

unknown. Direct communication can provide more reliable communications for the SWAT team 

member by greatly reducing the range and barriers that must be traversed by RF signals. The 

team would anticipate communication with a next generation technology would match or 

exceed the legacy capability. 

 Search and rescue operations benefit from reliable communications that may require direct 

communications to provide coverage to remote searchers. 

 Firefighting inside buildings is similar to SWAT team expectations for communications. 

 3GPP standards limited an explicit range requirement to the following: [22.280-C20 PR.73] It is 

desirable that an authorized public safety UE in or out of E-UTRAN coverage supports the 

capability to exchange data via ProSe from within a building to public safety UEs outside the 

building using a power class 3 E-UTRA UE [9].2 From a public safety perspective the requirement 

would simply exist. The need exists without consideration for the power class. 



{! REQUIREMENT – Comparable or better communications range than P25 

{! REQUIREMENT – Communication range should be extensible by the radio operator to large distances 

based on an advantaged position (line-of sight communications). 

2

 This is the only explicit range requirement in the ProSe study document. Clearly this requirement only captures a 

single narrow scenario. In addition, it does not define a testable criterion because a building could produce RF 

losses to great for practical communications, while a small residential building could be nearly loss free. 
3.1.1.2 Description: Voice Communications (new) 

In legacy communication systems (P25), talkgroups are generally either network based or in the case of 

P25 direct mode essentially mapped to a physical RF channel. Some proprietary solutions exist for 

vehicle-based repeaters that may extend the reach of talkgroups. Operation of these systems usually 

requires intervention by users to enable relay operations, however users are generally not privy to the 

information required to make an informed decision about which nodes at a scene should become relays 

or whether relay operation is even required. Therefore, activation of these “repeaters” tends to be 

based on policy rather than a substantiated physical need to achieve reliable communications. A next 

generation system should automatically evaluate whether relay operation is appropriate and either 

autonomously invoke proper relay operations or prompt an appropriate user for approval. This 

behavior is particularly important in broadband communication where the channel is re-used by each 

base station and several co-located active base stations/ relays would reduce capacity and reliability 

rather than enhance it. 

{! REQUIREMENT – Coverage extension capabilities including relays should be capable of automatic 

activation and deactivation based on an assessment of the RF environment and agency policy. 

3.1.1.3 Description: Voice Messaging (new) 

A first responder outside of infrastructure coverage needs to communicate with other first responders. 

Direct voice communications are breaking up. The user sends a voice message that can be delivered 

under these conditions. (Essentially rather than requiring immediate access and continuity of a link 

Figure 1, Improved Coverage with Advantaged Position 
between sender and receiver; messaging buffers, reassembles, and delivers the voice message once it 

can be presented. The first responder may also receive responses using the same mechanism. 

Voice messaging may also be applied at the application level. In this instance, multiple communications 

arrive concurrently at a device. Priority and latency rules for all communications cannot be achieved 

due to concurrent presentation limitations.3 The higher priority message is presented, while the lower 

priority messages if configured for voice messaging are presented later. 

{! REQUIREMENT – A public safety direct communication system should include the capability to support 

voice messaging for poor signal conditions where real-time voice is not possible. 

{! REQUIREMENT – A public safety direct communication system whose devices can receive more than 

one message coincidently should support voice messaging that allows a lower priority messages to be 

presented once a higher priority message thread is complete. 

3.1.1.4 Description: Emergency Messaging (new) 

A first responder outside of infrastructure coverage presses the emergency button on a device, because 

this message is especially critical for delivery the application may take special action when the transport 

service is not able to establish a voice connection. Typically, in response to an emergency button 

selection, three different types of information are transmitted: 

 A potentially single bit message declaring the emergency: 1 bit 

 A presence message providing various vital information about the initiator: hundreds of bits 

o Location 

o Identifier 

o Status 

o Bio-metrics 

 An open voice channel: a 15kbit/sec. transport with continuity, priority, and preemption. 

Each of these sessions varies widely in its transport throughput and service quality requirement. The 

application may request configurations of the transport that maximize delivery of the emergency 

declaration and presence when more demanding services cannot be supported. 

{! REQUIREMENT -The communication service should provide mechanisms to prioritize service delivery 

of emergency declaration and presence messages when an open voice channel may not be supportable. 

3.1.1.5 Applicability: Noise Limited Range 

Applicability: search and rescue, fugitive search, wild fire fighting, remote patrol, in-building 

communications, SWAT operations, firefighting, et al 

3.1.1.6 Trigger 

Pressing the PTT or emergency button. 

3

 The most obvious example would be a limitation of one voice presentation at a time by a device. 
3.1.1.7 Primary Actor 

A first responder is using PTT or the emergency button to communicate and expects to hear a voice 

acknowledgement upon successful communications. 

3.1.1.8 Supporting Actors 

The receiving first responder upon receiving an intelligible message will respond to the initiator. 

3.1.1.9 Stakeholders 

First responders 

Manager of first responders 

System managers 

3.1.1.10 Issues 

There is no obvious limit to the range over which the radio operater might desire communications. 

However, a minimum standard can be set by legacy technology. First responders are aware of the 

limitations of their current technology and could anticipate that a new technology would at least meet 

current capabilities. This requirement is also consistent with NPSBN recommendations provided in 

Table 9, row 3 and 4. 

3.1.1.11 Derived Requirements 

{!REQUIREMENT – The next generation direct communication solution should match or extend the 

maximum RF link losses (range) that support voice with a DAQ of 3.4.4

{!REQUIREMENT – The next generation direct communication solutions should provide a network 

bridging relay that extends communication services to off network UE by relaying traffic from its 

network link to its direct link. 

{!REQUIREMENT – The next generation direct communication solutions should provide a direct coverage 

extension relay that extends the effective coverage range of direct communication links via an 

intermediate relaying node. 

{!REQUIREMENT - [22.803-c20 PR.65] An authorized public safety UE may be capable of acting as a relay 

in or out of E-UTRAN coverage for other public safety UEs. 

{!REQUIREMENT - [22.803-c20 PR.66] An authorized public safety UE shall be capable of being 

enabled/disabled by a user or system in or out of E-UTRAN coverage to act as a relay for other public 

safety UEs. 

{!REQUIREMENT - [22.803-c20 PR.67] The user of a ProSe-enabled public safety UE acting as a relay 

should not perceive service degradation due to its use as a relay in or out of E-UTRAN coverage. 

4

 3GPP has extensively evaluated voice quality for public safety using MOS (mean opinion score) in 3x.yyy. For 

these evaluations, it uses a MOS score for its recommended mandatory CODEC AMBR-W at xxyy bps. In certain 

conditions talkgroups may use the P25 voice codec. 
{!REQUIREMENT – The next generation direct communication solution should support a mechanism to 

increase link margin for delivery as a non-real-time service like voice messaging, emergency 

declarations, presence, and texting.5

3.2 Use Cases: Direct Communication Range (Interference Limited) 

Direct communication interference limited range use cases are characterized by the communication 

range being limited by interference that exceeds the receiver noise floor. Typical environments with 

these interference conditions include: 

 Areas within network coverage, where network traffic generates interference for direct 

communications. 

 ProSe generated self-interference. Direct communication links in range may compete for the 

same RF resources (resource blocks) generating significant mutual interference. Similarly, direct 

communication links using disjoint RF resources but in the same band generate harmful 

interference due to limited inter-resource block isolation6. A typical ProSe transmitter while 

using a small number of available resource blocks for its transmission will also elevate the noise 

floor across the entire allocated bandwidth for a nearby receiver. Thereby limiting its ability to 

receive a concurrent message from a transmitter further away using different resource blocks. 

A second limiting mechanism is the receiver’s ability to reject in-band energy from a nearby 

transmitter using separate resource blocks. In this case the transmission power from the nearby 

transmitter’s selected resource blocks captures the AGC of the receiver. A weaker signal in a 

different part of the band is suppressed and may not be decodable. 

 External interference generated by signals outside the operational band or if within the band 

then external to the intended licensee of the band. 

3.2.1 Description: Voice Communications (legacy) 

A first responder outside of infrastructure coverage needs to communicate with other first responders 

in the area using voice PTT to coordinate activities and provide aid as needed. In this case the first 

responder is part of a multi-jurisdictional and multi-agency response to an emergency. Voice 

communication is organized into groups that may include: locally assigned personnel, mutual aid 

personnel, and command personnel. Additional voice groups are needed for the incident response. The 

large number of communicators and the segregation of these users into groups places a burden on the 

available RF resources. Communications may encounter interference associated with the inability to 

provide completely independent radio resources for each active talk group. Next generation 

communication systems should provide the same or better capacity, range, and call blocking 

performance. 

5

 This requirement is meant to ensure that crucial messages can be transferred successfully even with poor link 

conditions. Solutions could include application level persistent transmissions until acknowledgement or lower 

level protocol enhancements. 

6

 The limited isolation is a function of RF performance and protocol design. 
Each agency and jurisdiction can be expected to have its own communication groups as well as other 

groups for coordination. As part of incident management some groups might be combined, while others 

may be formed for coordination. Relays are excluded from this use case. 

{!REQUIREMENT – The next generation direct communication solution should match or extend the 

capacity, range, and call blocking performance in congested environments of previous technology for 

example P25.} 

3.2.1.1 Description 

A first responder that uses legacy (LMR) direct communications is provided with a ProSe capable device 

to evaluate during field exercises. Naturally comparisons are made between the observed capabilities. 

3.2.1.2 Level 

This use case is a high-level summary. 

3.2.1.3 Trigger 

3.2.1.4 Primary Actor 

A first responder is evaluating a device with direct communication capabilities that use ProSe. 

3.2.1.5 Supporting Actors 

Additional first responders are supporting the evaluation by providing a group communication scenario 

and providing responses to the primary actor communications. 

3.2.1.6 Stakeholders 

First responders 

Manager of first responders 

System managers 

3.2.2 Issues 

3.2.2.1 Scenario: Incident Command Center 

A command center is established to address a multi-department, multi-jurisdictional public safety 

incident. The distribution of UE includes multiple clusters of devices as well as distributed devices. For 

purposes of study two types of UE clusters are defined: command centers and teams. 

Teams share common group communication configurations. By example members of a team share a 

common talkgroup that is used for primary team communications. Team members may also have 

additional talkgroups that are secondary and primarily used for voice monitoring. Further, a first 

responder may switch the primary talkgroup by selection on the device. The monitored talkgroups are 

considered secondary in the sense that the principal talkgroup is preemptive for both inbound and 

outbound communications. While ProSe transport can deliver multiple simultaneously active talkgroups 

to a single device, the best way to present multiple overlapping conversations is an open area of study. 

For purposes of this work, principal talkgroups are delivered with priority and preemption, secondary 

talkgroups are delivered on a best effort basis. 
A command center (CC) UE cluster is characterized by a group of public safety command leaders that are 

coordinating between incident teams. Coordination is typically facilitated by physical proximity to other 

incident leaders but may also include remote communication particularly up a chain of command. 

These remote communications may share a common resource or use other communication resources. 

For purposes of this discussion communications with parties separated from the incident are considered 

to have a minimal effect on incident communications. 

Therefore, a principal concern is the significant harmful interference that may result from a command 

center cluster of UE operating in direct mode where members of the CC cluster would typically 

communicate on dissimilar talk groups to their teams. In this scenario, UE transmissions from the CC 

cluster are all nearby and create interference for any members of the cluster that need to receive while 

any other device is transmitting the vicinity. 

A secondary concern would be similar interference with Team clusters. However, Teams are generally 

expected to have less interference due to their use of a common talkgroup. Furthermore, with audio 

communications the group would naturally coordinate their voice. Local transmissions within the Team 

cluster would be easily processed by other team member devices. 

Finally, the use case where a single UE using a different principal talkgroup enters the vicinity of a team 

cluster which share the same principal talkgroup. However, this case is just a limiting case of the 

command center scenario. In the command center case devices are clustered and some number of 

these devices may transmit on different talk groups. The team cluster with one device on a different 

talkgroup is just the case of one potential transmitter of a dissimilar talkgroup. 

The command center and team cluster locus may include a mix of in network coverage and out of 

network coverage with the following environments possible. 

1. Both the command center and incident are completely outside network coverage. Direct 

communications are used for all communications. 

2. The command center is in coverage, but incident teams are primarily outside coverage. 

3. The command center is out of coverage, but the incident teams are in coverage. Since network 

communications are more efficient and more capable, best practices would suggest that this 

configuration should be avoided. Generally, the command center should be located such that it 

has network connectivity. 

4. Both the command center and incident are completely inside network coverage and no LTEdirect communications are used. 

3.2.2.1.1 Interference Mechanisms 

A transmitting LTE-direct device may interfere with other receiving devices in the following 

circumstances: 

1. The LTE-direct device transmits during a network connected device transmission and blocks the 

serving eNB from receiving the packet. 

a. This case can be avoided by using hybrid communication groups. Specifically, any device 

close enough to a base station to cause interference should establish a network 

connection. The network connection control channel will then manage this 

interference. LTE-direct only talkgroups should not be used in network coverage. 
b. It is also presumed that operation on LTE-direct and network in the same band with 

proximity involve the same spectrum owner. 

c. Adjacent band interference is also possible – In this case the interfering LTE-direct 

device is strong enough to affect the eNB from an adjacent band. 

2. The LTE-direct device transmits while another LTE-direct is attempting to receive a different LTEdirect. Reception is blocked. 

{! CONFIGURATION – LTE-direct only talkgroups should only be used outside of network coverage. Note 

while LTE-direct devices might use network resource scheduling while operating exclusively on an off 

network talkgroup and thereby avoid some interference, a significant amount of interference may also 

be generated by the shared use of the channel even with disjoint resource block allocations. LTE-direct 

even when using network scheduling is not able to manage its transmission power as effectively as a 

unicast uplink message that is power backed off by the serving eNB. Group based LTE-direct signals 

small generally operate at full power to effectively reach all members of a group. 

{! REQUIREMENT – A device within network coverage should establish synchronization with the local 

eNB even if its communications are exclusively direct. 

{! REQUIREMENT – A device with network connectivity should use the network connection for its 

communications when the required service is available. 

{! REQUIREMENT – When a service is not available, a device should consider based on policy, 

configuration, and RF metrics whether to become a network bridging relay. 

3.2.2.2 Environment 1a: Initial Incident Response: Network Coverage 

Responder perspective - Following notification of an incident, public safety officials begin responding. 

Typically, a police officer may be first to arrive. Following a quick initial assessment, the officer reports 

back to dispatch initial requirements for further response. While waiting for back-up the officer may 

step away from his vehicle to begin initial incident response. Having further appraised the incident the 

officer communicates with dispatch to request additional support requirements. Incidentally, the officer 

may notice a difference in channel grant tone or display notifications because his connection is using an 

LTE-direct relay to provide service. 

Behind the Scenes: The dispatch vehicle includes LMR and LTE equipment that provide greater reach 

(coverage) than portable devices. Proximity to the responding vehicle maximizes the officer’s ability to 

connect with dispatch directly. The vehicle equipment in conjunction with the officer’s portable device 

will automatically configure communications to maximize communication efficiency while maintaining 

coverage. Optionally the officer may choose to override automatic selection of a communication path, 

but generally the officer would not have the information to make a better decision than the system. 

Either due to natural or manmade geographic morphology or building penetration losses the officer’s 

portable radio loses coverage to serving networks as he steps away from his vehicle and into the 

incident. Prior to coverage loss and without user intervention, the officer’s radio initiates a request for 

relayed service using LTE-direct. The service is initiated and provides seamless service for the officer. 

[Relay – single client; low loading] 
{! REQUIREMENT – ProSe enabled devices should be configurable to automatically initiate methods to 

maintain connectivity as network communications become less reliable including direct communication 

links. 

{! REQUIREMENT – ProSe enable devices should be configurable to automatically initiate LTE-direct 

connectivity based on policy, events, and RF metrics. For example: ProSe may be enabled whenever an 

officer leaves a vehicle. 

{! REQUIREMENT – LTE-direct capable devices should be configurable to automatically return to network 

communications according to policy, events, and RF metrics. 

{! REQUIREMENT – LTE-direct enabled devices may be configured to automatically request via direct 

communications a connection to the macro network whenever it loses reliable network connectivity. 

Another LTE-direct device may respond by providing a network bridging relay. 

{! REQUIREMENT – When it has network coverage, a LTE-direct device configured to provide network 

bridging relay service shall respond to a requesting ProSe device with relay service after exchanging 

appropriate credentials that may include: security, priority and jurisdiction. 

3.2.2.3 Environment 2a: Back-up Arrives 

Responder perspective - Additional responders arrive at the incident. First responders are aware that 

communication groups (talkgroups) may be configured for network only, LTE-direct only, or to bridge 

both networks. In addition, first responders may recognize that their communication device display 

indicates, and audio annunciations provide status about the current scope of a communication group. 

Users may occasionally, be notified by the system or become aware through direct observation of 

communication responses that they may not be able to reach all members of a talkgroup. 

Behind the scenes – The communication system is tasked with maintaining reliable communications for 

first responders. Devices that may use LTE-direct present information or accept inputs for transmission 

in accordance with a hierarchy. From a reception perspective: 

 A signal must be present at the device antenna with enough quality that it can be processed by 

the device. For purpose of this discussion this is defined as RF coverage. The quality metric 

includes SINR and other signal attributes that may affect receiver processing. For instance, 

multipath with sufficiently long delay paths could preclude reception. 

 An LTE-direct device may fail to capture transport blocks even in RF coverage because its 

receiver does not capture signals on the target channel. For devices using network services, lost 

data due to radio availability is unusual. Devices transmit and receive simultaneously. Nondelay critical data is cached by the network or device and sent when the device is active on the 

required band. Delay critical data may fail because it cannot be sent in compliance with its delay 

constraint. Some data may be delivered late as appropriate for the application. Devices 

operating on network may be designed to minimize these types of outages using a combination 

of proper radio management and by using more than one receiver in the device. Proper design 

ensures that missed transport blocks are unusual for network operation. However, devices 

operating in LTE-direct mode are more subject to missed messages because the radio operates 

in time division duplex (TDD.) Neither the local network nor other communication devices are 

directly aware of the transmit or receive status of a device that is operating in LTE-direct due to 
distributed scheduling. By example, a transmitting ProSe device is blind to other ProSe 

transmissions in the same band and in general will not receive them and does not know that 

transmissions have occurred. To ensure proper prioritization of calls LTE-direct should 

implement a mechanism for a transmitting device to detect that another device is active and 

take appropriate preemption and prioritization actions. This action might include skipping 

transmission in some PSCCH periods to listen for higher priority transmitters. 

 The ProSe control message includes prioritization within its control message format. Therefore, 

a device may compare its transmission priorities with received message priorities to determine 

proper handling of messages that require arbitration. Likewise, V2X includes message priority. 

 Finally, a device application makes a final determination about service priorities and releasing TX 

data for transmission. By voice example, a device might receive simultaneously a telephone call, 

more than one network talkgroup, more than one talkgroup from a ProSe source, while the 

device operator also wishes to originate voice traffic. The device must make clear to the 

operator what services are being prioritized, while providing mechanisms to the operator for 

override. The emergency button would be an example of user override. 

{REQUIREMENT! – LTE-direct group communications should support self-preemption of an active 

transmission if another device begins transmitting using a higher priority on the same talkgroup. 

{REQUIREMENT! – LTE-direct multicast communications should support self-preemption of an active 

transmitter based on a combination of priority and congestion metrics. 

{REQUIREMENT! – LTE-direct unicast communications should support self-preemption of an active 

transmitter based on a combination of priority and congestion metrics. 

{REQUIREMENT! – LTE-direct reception should support configurable arbitration between its reception 

and transmission functions based on priority and policy. 

{REQUIREMENT! – LTE-direct devices should also support configurable arbitration between multiple 

services that may include multiple transport domains. 

3.2.2.4 Environment 3a: Incident Command 

Responder Perspective - Following an initial adhoc response, an incident command is set-up to 

coordinate between response groups, to enable senior command to manage the incident response, and 

to provide communications to political officials. From a communications perspective, two phases may 

exist for incident command. Initially team leaders may begin coordinating on preplanned talkgroups. As 

the incident response develops at least some team leads congregate to a physical command center that 

is in proximity to the incident. 

Behind the scenes –During the initial phases of response to an incident, communication is primarily a 

function of the capabilities of existing network coverage with an overlay of direct communication 

capable devices brought with the responders. For smaller incidents, local enhancement to this intrinsic 

communications capability may not be warranted. For larger incidents requiring an extended response 

additional equipment can be brought to the scene to enhance communication capabilities. The role of 
direct communication services may evolve over the course of an incident response7

, but the 

communication system should be designed to provide uniform and predictable service capability for first 

responders. 

For an incident where network coverage is widespread, but not ubiquitous direct communications 

provides a vital role in extending services into coverage gaps. As responders advance deeper into an 

incident their communications may seamlessly transfer to direct communications as network coverage 

limitations occur. This transfer to LTE-direct is enabled by devices either detecting LTE-direct activity or 

switching to it to maintain connectivity. Switching conditions are a function of configuration, public 

safety policy8, and RF metrics. 

Similarly, the network connection behavior is a function of connection logic, carrier policy, public safety 

policy, and RF metrics. Subject to device limitations, multiple simultaneous connections are presumed. 

To keep first responders’ focus on the incident and not on their communication devices, the 

communication application and airlink transport automatically provides pruning (arbitration) as 

necessary between multiple data sources. In simplest form, the highest priority is presented, however 

depending upon the data type multiple sources may be presented concurrently. For example: audio 

communications and a presence map update may be coincidently updated. 

For both network and LTE-direct the communication systems need to manage congestion, priority, and 

preemption. 

3.2.2.5 Environment 1b: No Network Coverage 

This section focuses on differences between partial network coverage and no network coverage at the 

incident. Following notification of an incident public safety officials begin responding to an incident. 

Typically, a police officer may be first to arrive. Following a quick initial assessment, the officer finds 

that communication with dispatch is not possible. While waiting for additional back-up the officer may 

step away from his vehicle to begin initial incident response. The officer’s portable radio and vehicle 

radios establish a ProSe connection. 

3.2.2.6 Environment 2a: Back-up Arrives 

As back-up arrives the officer is automatically alerted to arriving support as arriving devices 

communicate presence information and arriving personnel communicate verbally. Communication as a 

result is a combination of direct communication and relays that are automatically initiated based on 

conditions. Back-up personnel establish communication back to dispatch and provide assessment and 

support requirements. 

7

 An implicit presumption is that network connections will provide more services and better capacity than LTEdirect. Therefore, incident response is expected to enhance local network performance and coverage whenever 

the scale of the incident is sufficient to warrant such actions. Therefore, LTE-direct is expected to play an 

important role in: initial response, smaller incidents, and in isolated environments like buildings. 

8

 Public safety policy in this context may include some integration of local, jurisdictional, organizational, and 

national components. 
Behind the scenes – Communication support at the scene has more limited capacity than what is 

generally available from network coverage. The LTE-direct service must provide congestion 

management functions to ensure that communications are properly prioritized. 

3.2.2.7 Network Relays 

As back-up arrives to an incident that may benefit from relaying functions that extend network 

coverage, the system should automatically utilize the aggregate relaying capabilities effectively. Legacy 

LMR vehicle repeaters (relays) must be carefully managed to prevent harmful self-interference. Next 

generation systems should self-manage interference issues. 

Legacy LMR vehicle repeaters may operate by either generating a cross band bridge between frequency 

separated LMR bands or by time division multiplexing a single band. In both cases, vehicle repeaters are 

generally enabled by the vehicle operator when their application is expected to be helpful. Two 

problems that may arise include: a first responder leaving a vehicle without anticipating and enabling 

the repeater. The first responder subsequently loses coverage while away from the vehicle. Second, 

multiple repeaters may become active at a scene and generate harmful interference to relaying 

operations. Depending upon the affiliation of various potential vehicle repeaters at an incident, 

configurations may include a common operating channel or frequency spaced close enough to generate 

significant interference. Consequently, only a small number of repeaters that are carefully planned can 

operate in proximity. At larger incidents, distances required for interference free operation can be 

complicated and certainly difficult for personnel to manage in the field. 

Next Generation Behaviors – A next generation relay capability should not only match the current 

capabilities of existing vehicle repeaters but should also address shortcomings: 

 Automatic relay initiation based on need 

 Initialization by local (in-vehicle) or remote personnel via a portable device 

 Management and mitigation of harmful interference 

o Between network and direct networks 

o Between relays 

 Enhancement of relay coverage based on activation of multiple relays 

{REQUIREMENT! – An LTE-direct device that is configured to support relay operations should support 

automatic initiation. The device may base relay need on its own evaluation of the local RF environment. 

{REQUIREMENT! – An LTE-direct device that receives an explicit command either locally or remotely 

should initiate relay operation. 

{REQUIREMENT! – LTE-direct devices acting as relays should automatically assess their operating 

environment and take actions to minimize and mitigate harmful interference. 

{REQUIREMENT! – LTE-direct devices acting as relays should coordinate with other proximate relays to 

ensure that each relay is accretive to overall capacity and coverage while considering priority, 

preemption, and jurisdiction. 
3.3 Conclusions 

A next generation communications capability should provide as a minimum standard the same level of 

range, reliability, and predictability as its predecessor. Wherever possible it should provide improved 

performance, convenience, services, and coverage. 

 
4. LTE-direct Description 

LTE-direct is used throughout this document to refer to LTE communication capabilities that include 

direct communication or direct discovery of proximate devices via communication paths directly 

between devices. The term may refer to any of the following: 

 ProSe (Proximity Services) or D2D (Device to Device) communications – These terms are largely 

used synonymously within the 3GPP standards with RAN (Radio Access Network) documents 

primarily using D2D while other documents use ProSe. 

 V2X (Vehicle to everything) – Communications here may refer to V2V (Vehicle to Vehicle), V2I 

Vehicle to Infrastructure9, and V2P (Vehicle to Pedestrian) 

 The 3GPP based evolution of direct communication included in 5G standards 

 Communications capabilities that may be extensions to 3GPP standards 

Infrastructure may provide significant support to LTE-direct operations depending upon the operating 

scenario including: 

 Setting of operational parameters prior to LTE-direct operations 

 Provisioning of RF resource allocations for LTE-direct operations 

 Real time scheduling of LTE-direct resources for each transmitting UE 

 Real time modification of RF allocations or operational parameters 

LTE-direct may also operate independently and without any support of infrastructure as either a 

temporary or permanent operational mode. D2D public safety modes and some V2X modes support 

fully autonomous LTE-direct operations. 

4.1 Standards 

Primary standards for LTE-direct are developed by 3GPP. Beginning in Release 12 with a study and 

subsequent normative work ProSe basic capabilities were defined. Release 13 continued this work and 

completed additional features. Following Release 13 the focus of 3GPP has been V2X which uses the 

same underlying physical airlink interface (known as PC5), but includes additional requirements 

designed for low latency and congestion control. Table 1 illustrates a summary of primary features of 

ProSe. 

Table 1, ProSe Highlighted Features by 3GPP Release 

Features R12 R13 R14 R15

Discovery (EPC based) B

Discovery (direct) B

Communications (E-UTRAN) Multicast Unicast added

Communications (WLAN) B

Communications off network 

(PS only) 

Multicast

Relay UE to Network (PS only) E

Relay UE to UE (PS only) Not addressed

9

 Infrastructure that might use LTE-direct could include road side kiosks dispensing construction warnings, road 

conditions, or advertisements. 
4.2 LTE-direct Overview 

LTE-direct is intended as an inclusive term to include: 

 3GPP standards 

o Proximity Services that were developed to handle commercial applications for direct 

communications and public safety specific requirements. In particular the ability to 

communicate without a network. 

o Vehicle to Everything (V2X) that was developed to support Smart Car communications 

and rapid communication between vehicles and infrastructure to provide critical 

awareness of the surrounding environment for road hazards and nearby vehicles. This 

standard also supports off network communications and has main stream support 

because of its general applicability to all vehicles. 

o 5G versions of Proximity Services and V2X. 

 Profiles 

o Configurations of 3GPP standards that support public safety critical communication use 

cases. 

 Standard extensions 

o Capabilities in direct communications that are not directly covered by 3GPP standards. 

4.3 Standards Based ProSe Evolution 

ProSe particularly its use in public safety modes represent a very small part of the cellular market. 

Consequently, ProSe standards have not been quickly implemented. V2X by contrast has a very large 

potential market and has garnered significant focus within the 3GPP standards. An increasingly likely 

way forward would be to treat public safety direct communication requirements as a vertical market 

within a V2X umbrella. In this approach, only those requirements that were unique to public safety 

would need to be addressed within V2X standards. 

4.4 Extensions Beyond the Standards 

Potential additional capabilities and performance features that are not included in the 3GPP standards 

4.5 Public Safety Profiles 

Configuration of ProSe and its extensions to that provide improved performance that are designed to 

ensure public safety grade reliability. 

5. Results 

5.1 Performance in Noise Limited Conditions 

In interference free areas, communication range is limited by the maximum RF propagation losses that 

result in a signal at the receiver with enough quality to support a grade of service. Legacy LMR direct 

communications is primarily used for voice services. A natural comparison may be made between the 

relative range of LMR direct communications and LTE-direct while each provides voice services meeting 

a public safety grade. 
Legacy direct communications typically use either conventional FM or P25 CAI (Common Air Interface) 

for direct communications. Table 2 is an excerpt from TSB88.1-D Table A 1 illustrating voice quality 

measured as DAQ (Delivered Audio Quality) as a function of SINR (signal to interference plus noise ratio.) 

DAQ-3.4 is generally accepted as the minimum standard for mission critical voice and is defined as: 

“speech understandable without repetition. Some noise or distortion present.” As shown conventional 

FM SINR requirements vary significantly depending upon the channel width allocation and FCC rules 

associated with its emission mask. P25 is designed to fit in the narrowest channel supported by the FM 

configurations illustrated. Across all configurations P25 requires a lower SINR to deliver public safety 

quality voice. The table also provides pre-migration and post migration performance for P25. Post 

migration relies on a narrow receiver filter to provide better rejection of off channel interference and 

noise. Post migration is more representative of newer P25 receivers. To simplify comparisons with 

legacy LMR technologies, P25 post migration with a DAQ-3.4 will be used as a basis for comparison with 

broadband technologies unless otherwise noted. 

Table 2, Voice Quality versus Channel Quality 

 

DAQ-3.0 

(BER%/SINR)

DAQ-3.4 

(BER%/SINR)

DAQ-4.0 

(BER%/SINR)

Analog FM Radios 

FM +/- 2.5 kHz 23 26 33

FM +/- 4 kHz 19 22 29

FM +/- 5 kHz 17 20 27

P25 Radios 

C4FM (IMBE) (12.5 kHz) pre-migration 2.6/15.2 2.0/16.2 1.0/20.0

C4FM (IMBE) (12.5 kHz) post migration 2.6/16.5 2.0/17.7 1.0/21.2

To form a basis of comparison between LMR and LTE technologies a single approach to testing and 

modeling is required that equitably addresses fundamental differences in their technologies. LTE was 

developed to maximize capacity while minimizing cost for commercial service. In contrast, LMR was 

specifically designed for mission critical services to provide reliable communication, particularly voice. 

To provide reliable communications, LMR depends upon high performance equipment. It uses high 

power base stations, high RF power devices, and very selective receivers to maximize reliable 

communications and avoid interference from other LMR devices. In contrast LTE base stations have 

much lower power per MHz (spectral density), devices generally radiate about 0.1 W of RF power 

(includes some antenna losses), and receiver performance is heavily traded against cost models. This 

study identified areas of measurements differences and attempted to unify a single method to evaluate 

performance. Specifics areas that differed included: 

 Audio quality – P25 audio quality is measured by DAQ, while 3GPP standards use MOS (Mean 

Opinion Score). Unfortunately, a direct conversion does not exist between DAQ and MOS. 

 RF channel modeling – Most performance modeling in LMR is done with a flat fading (Rayleigh) 

model because the RF channel is narrow and expected to have relatively consistent 

characteristics across its bandwidth. LMR direct communications distances are generally 

shorter than network coverage and therefore multipath delay is not anticipated to be 

significant issue for LMR-D. By contrast, LTE-direct signals may be several MHz wide and 
therefore significant frequency selective fading can be anticipated across the LTE-direct 

channel. Consequently, we have adopted LTE channels models as a single common fading 

channel model for evaluations. 

 Protocol – LMR communications uses a fixed transmission signal, while LTE is adaptive across 

multiple dimensions. LMR direct communications uses a fixed transmission power, fixed 

modulation method, and fixed forward error correction code. In contrast, LTE-direct may vary 

its transmission signal in accordance with network provisioning, device configuration, channel 

conditions, and estimated RF loading. Transmission power, MCS (Modulation and Coding 

Schemes), and its use of frequency & time elements may all be varied. To simplify the 

comparison LTE will be configured to maximize its range. However, it’s important to 

understand that specific instances of LTE-direct may have shorter ranges due system tradeoffs 

with network performance or a requirement to meet throughput for the application. 

5.1.1 Audio Quality 

Audio quality in LMR systems and cellular communications are commonly measured with differing 

methodologies. While LMR frequently use DAQ (Delivered Audio Quality), the cellular industry 

commonly uses MOS (Mean Opinion Score). No commonly accepted conversions are provided between 

these two measures. However, DVSI the developer of the P25 voice coder has MOS data for the P25 

coder and TSB88 provides DAQ performance as a function of BER and equivalent SINR. Table 2 is an 

excerpt from TSB88 that provides the expected DAQ as a function of SINR and equivalent SINR. A DAQ 

3.4 is generally accepted as mission critical grade audio and requires a BER of 2% or equivalently 17.7 dB 

SINR. 

DVSI the developer of the P25 voice coder has test results for MOS as illustrated in Figure 2. Although 

this data does not allow direct correlation a comparison can be made with some interpolation of the 

MOS data. The red bars illustrate the original P25 vocoder performance that was selected for use in P25 

communication systems. The green bars show the “half-rate” vocoder that is used in trunking P25 

systems. The blue bars illustrate improved performance that DVSI introduced after the standardization 

of the “full-rate” coder. It is fully compatible with the original P25 vocoder and reflects current P25 

system performance. 
An estimate of MOS at 2% BER can be made by averaging the second, third, and fourth blue bars which 

results in a MOS of 3.2 for 2% BER. 

5.1.2 RF Channel Modeling 

For performance evaluations, 3GPP propagation fading models from 36.101 were used. The 3GPP 

models include multipath profiles for pedestrians and vehicles as well as geographic morphologies for 

urban and suburban areas. This model allows direct evaluation of air link protocols under a variety of 

multipath profiles. Figure 3 illustrates a summary of model profiles.10 

Figure 3, 3GPP RF Channel Models 

5.1.3 P25 Performance 

As a basis of comparison with LTE-direct the P25 CAI (Common Air Interface) used for direct mode 

operation was simulated for performance with 3GPP fading models. The models were applied with 5 Hz 

and 70 Hz doppler using EPA, EVA, and ETU models. The results are illustrated in Figure 4, and are 

consistent with performance expectations based on TSB-88 and previously illustrated in Table 2. To 

achieve DAQ-3.4 public safety grade voice a SINR of 17.7 dB providing a 2% BER was required in faded 

10 Detailed model parameters can be found in 36.101 Annex B.2.1. 

Figure 2, AMBE Vocoder Performance 
conditions with a doppler of 5 Hz. The simulation results show that a 2% BER is achieved at 17.7 dB; 

matching the TSB88 table. 

Figure 4, P25 BER Curves- PS Voice for BER < 0.02 

A total of six simulations were performed with performance curves clustering into two groups. The 

upper cluster illustrates the BER performance with a doppler shift of 70 Hz and three multipath profile 

models. Differences in performance between the multipath profiles are minimal. Similarly, the 5Hz 

doppler case are also clustered. From these graphs we can conclude that performance differences 

between the delay profiles in minimal, while the 70 Hz doppler cases lose about 0.5 dB of performance. 

Maximum RF losses for acceptable performance is an effective way to measure the performance of a 

communication transport. For P25 CAI operating in direct mode in the 700/800 MHz band, Table 3 

illustrates primary parameters associated with maximum link losses. Public safety grade LMR portable 

radios from major manufacturers generally provide 3 Watts of RF transmitter power in the 700/800 MHz 

band. MIMO processing including diversity is not supported. The IF bandwidth and noise figure are 

typical of public safety grade devices. Faded voice performance is taken from Table 2. From these 

parameters, the maximum RF coupling loss can be calculated. 

P25 direct communication out performs conventional FM by 7.5 dB for a DAQ-3.4.11 The relative 

coverage is based on an average propagation exponent of 4.0 that corresponds to higher average losses 

11 This difference results from the P25 digital protocol outperforming conventional FM for a given signal quality 

and because FM requires a wider IF filter that accepts more noise. Although conventional FM performs poorer at 

DAQ-3.4 some public safety users continue to use conventional FM for direct mode because in very poor RF 

conditions it can be possible to communicate with conventional FM, using repeated attempts, because it degrades 
for direct communications because path obstructions and losses are higher when antenna heights are 

low for both sides of the RF link. 

Table 3, LMR RF Link Performance 

 

P25 

Portable

Conv FM 

Portable

Nominal Tx Power 35 35

MIMO 1x1 1x1

Rx Effective IF Bandwidth (kHz) 6 12.6

Rx Noise Figure (dB) 6 6

Faded Performance Threshold DAQ3.4 

(dB) 17.7 22.0

Faded Performance 

Maximum RF Coupling Loss (dB) 146.5 139.0

Antenna Efficiency. (dB) 0 0

Maximum Link Loss 146.5 139.0

Radial Coverage Relative 1.00 0.65

Area Coverage Relative 1.00 0.42

 

5.1.4 ProSe Performance 

Evaluation of ProSe performance is much more complicated than LMR technologies because LTE-direct 

is an adaptive waveform whose performance changes as a function of its configuration, adaptation to 

channel conditions, channel loading, and transmitter power. 

 Configuration – ProSe provides significant configurability of the Sidelink signal. A significant 

aspect of this research is to understand configurations suitable for public safety use. 

 Adaptive Channel Coding & Allocation – The data transport channel of ProSe is managed by the 

upper layers of the protocol, that can affect tradeoffs between throughput and receiver 

sensitivity. Resources are allocated as PRB’s (Physical Resource Block) that consist of 0.5 msec. 

time slices and 180 kHz bandwidth. Generally, PRB’s are allocated in pairs in the time domain 

resulting in a time scheduling granularity of 1 msec. In the frequency domain, up to 50 PRB’s 

may be allocated in a 10 MHz carrier. Within each PRB ~20 different MCS (Modulation and 

Coding Schemes) may be selected that tradeoff larger transport blocks with higher SINR 

requirements. 

 Spectrum/ Channel Loading – ProSe performance is affected by channel activity in its allocated 

band and adjacent bands. When operating in off network public safety modes, scheduling of 

PRB resources is performed independently amongst active transmitters. Each transmitting 

device employs a Resource Allocation Algorithm (RAA) that attempts to minimize overall system 

interference. Receivers capture a composite signal of all transmitters, noise, and interference. 

Like any RF link, signals that are too weak relative to the receiver noise floor cannot be decoded. 

slowly, while P25 tends to have a strong threshold below DAQ-3.0. It is also believed to exhibit fewer artifacts in 

noisy audio situations.11 
In addition, ProSe has several mechanisms that introduce interference into the received signal 

and effect overall performance. 

o Two or more transmitters select and transmit on the same RF resources; commonly 

called a collision. In this case, successful communications occur only to receivers that 

capture a signal with significant signal quality to be decoded. Typically, a receiver may 

not decode any of the transmitted signals or if one signal dominates at the receiver then 

it may be decoded. 

o A receiver captures multiple signals that all use different RF resources. However, due to 

a limited receiver dynamic range12, the receiver is not able to decode weaker signals 

that are in effect suppressed by stronger ones that may capture the receiver automatic 

gain control. 

o A receiver captures signals in its allocated carrier bandwidth but is subject to a much 

stronger adjacent carrier that reduces receiver performance. 

 Transmitter Power – As part of minimizing interference between transmissions, ProSe is 

required to back off its transmission power under certain conditions. These rules make it harder 

to predict when communications will be successful. 

For performance evaluation LTE-direct may be partitioned into four separate signals that can be 

considered separately and together: 

 SLSS/ PSBCH – contains synchronization information and basic parameters of LTE-direct. It is 

used to share synchronization between ProSe devices. The synchronization signal is the primary 

mechanism for shared synchronization for off-network communications. ProSe devices in 

network coverage may also use this synchronization signal to enable communication between 

ProSe devices that are connected to separate eNB that are not synchronized. 

 PSDCH – is used for discovery and is not a primary part of the current investigation 

 PSCCH – provides the control channel that describes the format of the shared (data) channel 

and defines where in the time and frequency resource matrix that data will be sent. Decoding 

of the control channel is required to decode the PSSCH. 

 PSSCH – provides data transport 

5.1.4.1 Interference Free ProSe Coverage 

In order to compare coverage for a best-case scenario, ProSe can be configured for best sensitivity that 

can deliver the coded audio stream as recommended by the 3GPP standards. In setting this optimum 

operating point there two primary parameters to jointly optimize, while holding data throughput at ~13 

kbps as required by the recommended AMR-WB audio codec. First, the modulation and coding scheme 

directly tradeoffs data throughput and against required SINR with lower throughputs requiring a lower 

SINR to process the receiver signal. Second, the number of resource blocks used for transmission is also 

directly controllable. By example a transmission with 6 resource blocks has a 3 dB higher spectral 

density than a signal using 12 resource blocks. Figure 5 illustrates selection of an optimum selection of 

resource blocks and modulation coding scheme for transporting voice. As shown an SINR of -2 dB is 

12 In this context, limited dynamic range includes filter selectivity limitations in rejecting adjacent energy, receiver 

phase noise limitations commonly called reciprocal mixing, and non-linear effects like blocking or intermodulation. 
enough to provide acceptable audio performance. 

Figure 5, ProSe BLER in EPA5 Fading 

The required SINR may then be used as a basis for comparing maximum RF losses between LMR direct 

communications and ProSe configured for best coverage of coded voice. Figure 6 illustrates how P25 

compares with ProSe. Typical LMR radios transmit with 3 watts (35 dBm) of RF power. This signal is 

radiated by an external antenna on the top of the radio with relative efficiency. By contrast, ProSe 

typically has a transmitter power of 0.2 watts (23 dBm) and is generally configured with internal 

integrated antennas that typically have an efficiency of about -4 dBi. ProSe can partially make up this 

deficit in radiated power by using diversity reception, a wider bandwidth, and linear waveforms. 

However, the net effect is that ProSe is expected to have a coverage radius that is only 1/3 of LMR P25. 

A high-power UE (HPUE) has been defined by the standards, that increases transmitter power to 31 

dBm. Expected coverage improves to about 1⁄2 the coverage radius of P25. 

About half the coverage deficit in dB’s may be attributed to differences in the expected antenna. While 

LMR devices typically use external whip antennas, LTE device universally use integrated antennas with 

much poorer efficiencies. For in network coverage, that is usually capacity limited the esthetic of no 

protruding antenna has won out over better antenna efficiency that would only provide benefit in fringe 

RF coverage. However, a primary use case for ProSe is out of coverage communications with a group of 

devices. Direct communication RF loss is much higher because both the transmitter and receiver are 

presumed to be within local clutter at low elevation. Furthermore, the loss in antenna efficiency for 

direct communications effects both the transmitter effective radiated power and the receiver effective 

sensitivity. Additionally, choosing a low rate voice codec such as P25 AMBE reduces the coverage gap. 

Consequently, direct communication developers should consider, portable HPUE development, 

tradeoffs in deploying more efficient antennas, changes to the waveform design to improve coverage in 

low noise environments, and voice codec selection. Furthermore, data services presumed to require 

data rates faster that coded voice will have more reduced coverage. In calculating the coverage 
capabilities for ProSe it was configured for best sensitivity in transporting coded voice. Applications like 

still images or video require much faster data rates and consequently will require higher transmissions 

bandwidth and higher modulation coding schemes. Both of these methods of improving throughput 

have a direct effect in reducing coverage.

Figure 6, Relative Coverage of P25 and ProSe 

5.1.4.2 SLSS/PSBCH 

The ProSe waveform may be partitioned into different signals. The following sections described these 

signals and make recommendations. 

To establish communications or discovery the ProSe receiver must first acquire time and frequency 

synchronization with potential transmitters and obtain essential parameters. For ProSe devices 

exclusively using the network as a synchronization source, all required information is provided by the 

LTE downlink including: the primary synchronization signal (PSS), secondary synchronization signal (SSS), 

the PBCH, and MIB/ SIB information. Network synchronization is well established, should be used as the 

highest priority synchronization source, and is not part of this investigation. 

LTE-direct devices may also receive synchronization information from other LTE-direct devices via the 

Sidelink Synchronization Signal (SLSS.) Like the network synchronization signals the SLSS consists of a 

primary synchronization signal PSSS (Primary Sidelink Synchronization Signal) and a SSSS (Secondary 

Synchronization Signal). The PSBCH (Physical Sidelink Broadcast Channel) is also co-located within the 

same sub-frame and contains parameters describing the configuration and the state of the Sidelink. 

ProSe devices that broadcast the SLSS are known as synchronization sources. A ProSe device broadcasts 

the SLSS either as a synchronization source that periodically transmits SLSS/ PSBCH to provide a 

synchronization service to the surrounding area known as a synchronization beacon (SB) or it may also 

transmit SLSS/ PSBCH when it has a message to transmit during the same PSCCH period. All 

transmissions of SLSS/ PSBCH may be used by other ProSe devices for synchronization maintenance 

purposes. 

A typical environment for ProSe synchronization, illustrated in Figure 7 demonstrate several different 

synchronization scenarios. 
 Device (A) is in network coverage but receives a ProSe message from device (B). Device (B) 

transmits its message to device (A) with synchronization associated with its serving eNB. Device 

(A) served by a separate, potentially asynchronous eNB, accepts ProSe synchronization from 

device (B). 

 Device (C) receives network synchronization from its serving eNB and operates as a ProSe SB 

with timing based on network synchronization. Additional ProSe devices may use device (C) for 

ProSe synchronization including devices that may act as SB’s. 

 Device (D) operates as an autonomous SB and uses its internal reference as a basis. Its internal 

reference may be locked to GNSS if available. 

 Device (E) is in reception range of several synchronization sources. Therefore, signals based on 

these sources or even on a source not previously detected should be decoded by the device. 

Devices may have implementation limits on the number of separate synchronizations they are 

able to decode. As a collective, devices may therefore want to take actions that minimize the 

number of synchronization sources with different timing. 
Figure 7, Typical ProSe Synchronization Environment 

 The final bullet demonstrates a requirement for a receiving ProSe device to handle reception associated 

with multiple synchronization sources. Several issues result from this conclusion: 

1) The ProSe UE should be able to decode multiple synchronization signals including those that may 

occur with similar timing. For purposes of this statement multiple transmitters sending the same 

synchronization information with enough time alignment to be unresolvable would be considered a 

single synchronization signal but might later bifurcate due to reference precession or physical 

movement. Similar timing, but different ID’s or BCH content would be separate synchronization 

sources. 

2) For multiple reasons ProSe UE may not be able to decode all available synchronization signals each 

PSCCH period. 

a) Half Duplex – ProSe transceivers are half duplex. A device that is transmitting will miss all 

signals that are sent. 
b) The dynamic range of concurrent signals may exceed the processing dynamic range of the 

receiver. For example: typically, the strongest signal captures the receiver AGC. Weaker signals 

are suppressed by the ratio of the dominate signal to the weaker signal. Smaller signals will be 

expected to have a lower SINR associated with their smaller amplitude and higher relative noise 

processes due to linear dynamic range limitations of the receiver. 

c) Each signal including synchronization signals are subject to signal fading or shadowing by 

obstructions in the RF propagation path. 

d) Multiple synchronization signals with similar or the same configuration may arrive with time 

skews that are small enough to prevent separate discrimination. 

e) A practical implementation will likely limit the number separate synchronization sources 

tracked. 

3) A ProSe UE benefits from tracking synchronization sources across multiple PSCCH periods. Tracking 

synchronization sources provides a mechanism to manage finite processing resources and battery 

life. For instance, a device might limit the number of synchronization sources based on an 

operational requirement that a device operate without a battery swap out for a complete work 

shift. 

a) New synchronization should be accepted quickly but be removed only after a protracted period 

without detection, because the synchronization has been combined with another, or based on 

priority mechanisms when synchronizations must be pruned to device limitations. 

b) Synchronization sources can be categorized as adaptable or fixed sources. A fixed 

synchronization source has a basis for its timing that cannot be changed by the ProSe modem. 

Examples of a fixed basis would be networking timing, GNSS based timing that uses a fixed 

mapping between PSCCH periods and UTC, and any other basis that is supplied to the ProSe 

modem as fixed. Adaptable synchronization sources do not have a direct connection to a fixed 

synchronization source. Consequently, aligning adaptable sources with fixed sources and with 

clusters of adaptable sources may provide performance benefits. Some adaptable sources may 

choose synchronization based: 

i) on their internal reference because no acceptable external references are detected 

ii) on GNSS but not use a fixed mapping between PSCCH frames and UTC 

iii) on detecting synchronization references, but the detected references are not known to be 

fixed. 

c) ProSe devices should work in a distributed fashion to reduce the number of adaptable 

synchronization sources by providing mechanisms to shift timing toward preferred fixed 

synchronization sources or toward shared timing for clusters of devices with no fixed sources. 

4) The ProSe UE should perform reception processing for each synchronization source that it is 

tracking. 
5.1.4.2.1 Synchronization Beacons 

Synchronization Beacons (SB) are periodic transmissions of SLSS/ PSBCH from a subset of ProSe devices 

even if no data needs to be sent. Devices 

may be configured to act as SB’s, may 

become a SB based on not detecting a 

suitable SB already operating in its 

proximity, or may simply not become a SB. 

ProSe devices operating as SB’s may become 

the basis for synchronization of other 

devices that decode the SB. Sharing a 

common synchronization may benefit 

devices by reducing intra-system 

interference, reducing blind signal searches, 

and establishing priorities for devices with 

limited processing resources. 

As a synchronization distribution system, 

SB’s would ideally be positioned to provide 

continuous coverage of devices operating in 

ProSe modes. However, generally portable 

devices are simply where they are carried, and mobile devices are where they are installed. 

3GPP deals with this uncertainty by allowing 

for configured and autonomous SB’s. A device may, by network signaling or via configuration become a 

SB. Other ProSe devices may become SB’s based on their RF environment. A device in network 

coverage compares the signal strength (RSRP) of its serving eNB to syncTxThreshIC. If the RSRP is below 

threshold then it becomes a SB, otherwise it does not. Likewise, a device out of network coverage 

compares the signal strength of any decoded SB’s to syncTxThreshOC. If it finds no SB or only SB’s with 

signal strength less than the threshold then it becomes a SB, otherwise it does not. The proportion of 

active ProSe devices that become SB’s depends on the signal strength of decoded SB’s and the 

thresholds that are configured. For a given threshold, dense clusters of devices will have a small 

percentage of devices that become SB’s based on the RF environment. While in sparse device 

environments a much higher percentage of devices may become SB’s. 

The standard provides the flexibility to assign syncTxThreshIC and syncTxThreshOC levels but does not 

provide any guidance. Thresholds set for very high signal levels, will result in a larger proportion of 

devices in a given environment becoming SB’s because few detected SB’s will have the necessary signal 

strength to exceed the threshold. At the opposite limit where the threshold is set very low, nearly all 

detected SB would exceed the threshold and therefore the detecting devices would not become SB’s. 

The optimum settings of syncTxThreshIC and syncTxThreshOC are dependent upon several factors. 

The 3GPP standards implicitly suggest a single decoded SB is selected and is then used as the basis for SB 

timing. However, the ProSe system should benefit from SB that work collectively to minimize the 

number of separate synchronization sources. 

Network SLSS

Synchronization

A

B

C

Figure 8, Multiple Synchronization Sources 
5.1.4.2.2 Separate Synchronization Sources 

Synchronization sources are transmitting devices or eNB that provide a synchronization signal. ProSe 

devices sending PSCCH, PSSCH, or PSDCH precede these virtual channels with SLSS/ PSBCH and are 

therefore synchronization sources. Networks send periodic synchronization and are therefore 

synchronization sources. SB’s are ProSe enabled devices that send out periodic SLSS/ PSBCH without 

necessarily sending PSCCH, PSSCH, or PSDCH. 

The 3GPP standards define prioritizations of synchronization sources, but do not clearly define how 

priority should affect device operation. For some use cases the role of priority is clear, while for others 

implementation presumptions may be required. The standards appear to implicitly presume that a 

single synchronization source is maintained by a device and that prioritization is simply selecting the 

highest priority source. However, Figure 8 illustrates a simple example that shows that multiple 

synchronization sources may exist. Device (A) receives network synchronization and ProSe 

synchronizations from devices (B) and (C.) As depicted devices (B) and (C) are each autonomous 

synchronization sources. For device (A) to decode network communications or ProSe communications 

from devices (A) or (B), it must establish synchronization for each source. For network communications, 

synchronization is established before communications can occur. For ProSe, SLSS/ PSBCH is included 

with message carrying channels during a communications transmission. The ProSe transmission may be 

synchronized to a SB or may be asynchronous. Furthermore, even if the transmission is based on a SB, it 

may be asynchronous to a ProSe receiver that has never decoded that SB. For ProSe to achieve mission 

critical capability, it must be able to handle synchronization from multiple sources. 

5.1.4.2.3 Synchronization Source Priority 

Synchronization source prioritization in ProSe 3GPP standards recognizes 3 levels with diminishing 

priority. Synchronization signal content identifies each type: 

1) Synchronization directly obtained from network synchronization has highest priority for ProSe 

operations. 

2) Synchronization based on SLSS/ PSBCH received from a ProSe device that is network synchronized 

has secondary priority. 

3) All other synchronization sources have tertiary priority in the 3GPP standards. 

While the standards define synchronization source priority, they do not define how it should be used. 

Additional prioritization makes sense. Some of these sources could be based on absolute sources and 

may have a range of differing root synchronization sources. 

a) A ProSe device discovers an acceptable SB, with signal power that exceeds the signal threshold 

for acting as an SB. 

b) A ProSe device does not discover any acceptable SB and establishes its own arbitrary 

synchronization based on its internal reference. The internal reference is subject to frequency 

error that may affect some processing. The frequency error also produces time precession of 

the synchronization source relative to other sources. The device transmits as a SB. 

c) A ProSe device decodes a SB that that has a power level that is below the syncTxThreshOC. It 

may use the SB time and time/ frequency as a synchronization basis for its own SB transmissions.
d) A ProSe device does not discover any acceptable SB but is synchronized to GNSS. It establishes 

time arbitrary synchronization. However, GNSS eliminates time precession due to frequency 

tracking of the device internal reference. The device transmits as a SB. 

e) A ProSe device uses GNSS to establish synchronization based on mapping between UTC time 

and ProSe timing. 3GPP V2X standards establish such a mapping and could be used when GNSS 

is available. 

Synchronization priority may be used as one factor effecting the selection of the synchronization source 

for communicating. 

5.1.4.2.4 GNSS 

GNSS capabilities in commercial communication devices is nearly universal. When available, it can 

provide highly accurate synchronization between communicating network nodes and devices. GNSS is 

commonly used to synchronize eNB, however 3GPP standards do not dictate a relationship between 

UTC time and LTE ProSe framing. Different networks may have different mappings between UTC time 

and LTE framing. The standards for ProSe likewise do not provide a mapping between UTC time and LTE 

framing, but such a mapping is provided within 3GPP V2X standards. Therefore, while staying consistent 

with ProSe standards GNSS may be used in the following ways: 

 GNSS may be used to accurately match ProSe device frequency references to GNSS 

synchronization. This method ensures that ProSe radios are centered on their band allocation 

and that timing references do not drift over time. 

 A ProSe device with GNSS service and no network or SB detected may autonomously transmit 

synchronously to other devices based on a UTC time to LTE frame mapping that may either be 

configured or previously detected. This mapping can use the mapping from V2X standards to 

ensure that autonomous SB’s with GNS coverage will be synchronous within RF propagation 

tolerances. 

 A device temporarily out of GNSS coverage benefits for a limited amount of time from its prior 

GNSS synchronization and may benefit from other nearby devices that still have GNSS. 

5.1.4.2.5 Message priority 

As a mission critical protocol, message priority is essential in handling communication systems that may 

be congested. Like network operations communication priority may be addressed at the application 

level, at the protocol level, and at the network access level. 

Prioritization can now be defined for various use cases: 

 A device decides whether to track a decoded synchronization source 

 A device selects a synchronization source as the basis for a transmission 

 A device updates its synchronization tracking to reduce the number of separate 

synchronization sources. 

 A device lowers its energy consumption by limiting synchronization searches to certain 

windows in time. 

 Unlikely scenario perhaps – SLSS/ PSBCH not detected, but message bearing virtual channels 

are active. 
Synchronization is provided by the transmitting device, but may also have already been detected via SB, 

but synchronization may be first established during the PSCCH period that contains a message. Devices 

(B) and (C) may be acting as synchronization beacons. Then device (A) will detect the SLSS/ PSBCH 

signals and track these synchronization sources. At some later time, devices (B) and (C) may transmit 

messages. Device (A), in performing receiver processing for each of its tracked synchronization signals, 

detects and decodes messages from devices (B) and (C.) In a similar fashion device (A) might transmit a 

message. Device (C) should detect and decode this message from device (A.) 

Devices (B) and (C) may not be operating as synchronization beacons and may be receiving 

synchronization from another device not illustrated or may have their own autonomous 

synchronization. In this case device (B) or (C) may transmit a message that is preceded by SLSS/ PSBCH. 

Detection of this first transmission of SLSS/ PSBCH is usually required for subsequent decoding of the 

message. Message decoding may be successful without SLSS decoding because an existing tracked 

synchronization source in device A happens to have similar time and frequency synchronization and 

fortuitously acts as a proxy. Device A may also miss a first SLSS/ PSBCH because of reception limitations 

outlined including: half duplex operation, interference, RF fading, AGC capture, and to many 

synchronization sources. 

An important component of reliable communication with ProSe is the distribution of synchronization 

among ProSe devices. Network devices use PSS and SSS to establish synchronization. The eNB downlink 

signal sends these signals twice per frame allowing devices to establish and maintain network 

synchronization. Synchronization with other eNB is only required before handover or in certain dual 

connectivity scenarios. In ProSe, distribution of synchronization is more complicated. 

 ProSe devices may act as synchronization beacons that periodically send SLSS/ PSBCH 

o In network devices may act as SB’s for out-of-network ProSe devices. 

o Out-of-network devices may act as SB’s 

 Less commonly, ProSe devices may also be in an area served by separate eNB that are not 

synchronized. For normal network operations the UE will be served by one of the eNB and 

might handover to the other thereby switching its timing. For ProSe operations multiple cases 

exist: 

o A device may process and detect synchronization signals from both eNB’s and pass this 

information to ProSe for tracking. 

o In addition, the ProSe device may receive SLSS/ PSBCH from in range ProSe devices that 

are attached to either of the local eNB. 

 The ProSe device may receive SLSS/ PSBCH that has no direct affiliation with network timing. 

A ProSe device may track several synchronization signals, and then must establish how each is used by 

the ProSe device. 3GPP standards provide some guidance by establishing that network synchronization 

signals have the highest priority followed by SLSS that are directly tied to network synchronization 

followed by other SLSS. Other mechanisms to further prioritize synchronization sources are left to 

implementation. Furthermore, the standards do not make clear what is meant by prioritization of a 

synchronization signal. The following are all potential uses for prioritization of synchronization signals: 

1. Limiting the number of separate synchronization signals that are tracked by a device. 
2. Limiting the number of separate synchronization signals used in receiver processing to search 

for messages. 

a. Managing processor limitations 

b. Extending battery operation 

3. Identifying which synchronization signals may be transmitted as synchronization beacons. 

4. As one factor in determining which synchronization signal will form the timing basis for 

transmission of each message. 

5. In congested environments preferentially scheduling higher priority messages. 

5.1.4.2.6 Synchronization Sources 

The number of synchronization sources that need to be tracked? 

Network synchronization tracking is well established and is not included in this analysis. ProSe 

operation is not anticipated to place additional burden on network synchronization processing. 

However, ProSe operations that are using network synchronization are subject to handover. 

Predominately, the serving and destination eNB’s are part of a synchronized network. Devices 

synchronized by the network can anticipate small shifts in frequency and time associated with 

handovers. Handovers are presumed to be asynchronous events relative to the ProSe PSCCH period. 

Therefore, network timing can change but the ProSe device may choose to select synchronization 

sources at the beginning of a PSCCH period and maintain them for this period. This behavior ensures 

that SLSS, PSBCH, PSCCH, and PSSCH are all based on the same synchronization reference. Frequency 

changes are limited to the relative doppler shift between the eNB’s and the UE. A worse case geometry 

would be the UE following a path directly between the two sites that would result in a total doppler13 of 

about 140 Hz in the 700 MHz band. This change in frequency is expected to have little effect on ProSe 

performance. Especially as the shifts occur between PSCCH periods. Timing shift due to time 

differences in RF propagation from synchronized eNB’s could be large enough to effect decoding 

performance. Decoding performance is materially affected when timing is shifted by a significant 

fraction of the CP (cyclic prefix). This type of problem can be minimized by detecting a change in timing 

(when present) and maintaining both synchronizations for an overlapping period. In the more unusual 

case were eNB are not synchronized, timing offset may be fixed or precess between serving sites. A 

worst-case condition would be that handover occurred near but after the PSCCH period boundary. 

Transmissions would still be received based on the SLSS timing sent before the message. Acquisition of 

the new serving eNB timing could be delayed by a PSCCH period but is not expected to materially affect 

ProSe performance. Network synchronization and passing of serving site information is expected to be 

enough to enable robust ProSe communication using network synchronization. A UE near handoff is 

most likely able to track both the serving and destination eNB. The ProSe device may simply perform 

reception for both synchronization sources. 

Using SLSS for synchronization requires more maintenance of multiple synchronization sources. In 

principal there is no limit to the number of synchronization sources possible, but practical limits need to 

be applied as reception processing requirements increase in direct proportion to the number of 

synchronization sources. From a standards perspective a ProSe device in network coverage may 

become a SLSS beacon by command or configuration. Out-of-network ProSe device may also act as 

beacons based on the signal strength of SLSS decoded. The 3GPP standard specifies that if a device 

13Doppler of: f02v/c; V = 20 m/sec. 
receives SLSS above a configurable signal power level14 then it may use those sources as time and 

frequency synchronization sources and does not need to become a beacon. If, however, the SLSS are 

below threshold then the device should become a beacon. The standard conveys this requirement in 

the singular sense and does not address behavior when multiple SLSS may be decoded and tracked. The 

intent of the standard is clear, a device that decodes a strong synchronization signal may use it for 

synchronization and does not need to automatically become a beacon for its proximity. A device 

receiving a weak SLSS may also use it for synchronization but benefits its proximal area by providing a 

beacon signal. 

The proportion of active ProSe devices that become beacons depends on the strength of RF links 

between devices. In relatively dense clusters of devices a small percentage of devices presumably 

become beacons, while in sparse environments a much higher percentage of devices may become 

synchronization source beacons. The standard provides the flexibility to assign the syncTxThreshIC level. 

Set for very high signal levels, statistically nearly all detected SLSS will fail the threshold test. 

Accordingly, these devices would become synchronization beacons. At the opposite limit where the 

threshold is set very low, nearly all tracked synchronization sources would exceed the threshold and 

would not become synchronization beacons. The optimum setting of syncTxThreshIC is dependent on 

the density of devices and is related to the number of separate synchronization sources. 

Reducing the number of separate synchronization sources for tracking and reception has multiple 

benefits. Power consumption of a UE is directly affected by the amount of processing that is required 

for reception. Each synchronization source must be detected and tracked during each PSCCH period. In 

addition, receiver processing of at least the control channel must be performed for each synchronization 

source. Performance particularly in congested environments is also improved with fewer 

synchronization sources. ProSe devices operating with the same synchronization source can more 

effectively monitor activity on the channel and share the available resources. Separate synchronizations 

may precess through each other’s allocations resulting in additional interference to ProSe and to 

network operations. Finally, additional processing required for multiple synchronization sources 

requires more processing energy that quickly depletes portable device batteries. 

Asynchronous synchronization sources reduce the overall performance of ProSe; an overall 

synchronization strategy to lower the number of asynchronous sources is highly desirable. This goal 

may be achieved by controlling the birth rate of new asynchronous sources and by coalescing existing 

sources. 

The timing basis for synchronization sources has two primary forms adaptable and fixed. For these 

purposes network timing is always considered fixed. Likewise, a ProSe device receiving network 

synchronization and transmitting a beacon based on network timing is fixed. GNSS timing could be fixed 

or adaptable according to whether it uses an absolute mapping between UTC and ProSe frame timing15. 

Other synchronization sources are considered adaptable. That is their timing is arbitrary and therefore 

can be brought into alignment with other references to reduce the number of synchronization sources. 

Recommendation for tracking of synchronization sources: 

14 syncTxThreshIC – signal power threshold to initiate acting as a beacon synchronization source 

15 Such a mapping is providing in V2X standards and could be adopted for ProSe. (36.331 5.10.4) 
 In coverage: 3 network sources – Handover regions commonly have 3 similar strength eNB’s. 

The worst possible case is that all eNB’s are using separate synchronization timing which would 

be highly unusual in a fixed network. However, a tactical or deployable network could have 

conditions in which eNB synchronization was not achieved. Each eNB could be a 

synchronization source for ProSe capable devices. Therefore, a ProSe device should be able to 

track up to 3 network-based sources. 

 GNSS share a common absolute time reference. However, ProSe standards do not include a 

mapping between UTC and ProSe framing. Therefore, ProSe devices using GNSS may not be 

synchronized. Devices may be configured to be adaptable or have fixed timing according to 

whether they user a standardized relationship between GNSS time and ProSe framing. 

 Other sources may be treated as adjustable. Figure xxx illustrates an example geometry that 

may support multiple separate synchronization sources. RF signal barriers may exist in outside 

conditions and within structures. 

{!REQUIREMENT – ProSe devices should track up to 3 network synchronizations. 

{!REQUIREMENT – ProSe networks should adopt the relationship between GNSS based UTC and the 

frame timing specified for V2X. 

{!REQUIREMENT – ProSe device that are not synchronized to an absolute reference should support 

adjusting their timing based on received synchronization signals with the goal of reducing the number of 

separate synchronization sources. 

5.1.4.2.7 Synchronization Source Mangement 

Mechanisms to reduce the number of separate synchronization sources are not addressed in 3GPP 

standards. From all the possible ProSe transmitters in range of a ProSe receiver a separate 

synchronization source needs to be defined for each SLSS/ PSBCH signal that has been decoded and 

cannot be combined with another synchronization source. Differences that prevent combining include: 

timing skew greater than 1⁄4 to 1⁄2 of the active CP, significant frequency errors, different SL-ID, and 

different PSBCH content. Synchronization sources may be in a constant state of flux particularly with 

moving devices: 

 A receiving device may decode multiple synchronization sources 

o Existing synchronization sources 

o A new synchronization source for the first time 

 A device transmits for the first time in range of the receiver 

 New source due to device movement 

 New device as first transmission after ProSe enabled 

 Differences in device frequency references cause time precession resulting in an 

apparent single source splitting into two 

 Movement causes a single source to split into two due to time skew 

 Life cycle events – synchronization sources are tracked over long periods relative to the PSCCH 

period and likely are not detected during every PSCCH period 

o Authenticating synchronization sources include PSSS, SSSS, and PSBCH signals. 

Detection of PSBCH may be used to authenticate a synchronization source. PSSS and 

SSSS may generate false positives in low quality signal conditions. 
o Aging synchronization sources that are not detected for a succession of PSCCH periods 

may be deleted. 

o Combining synchronization sources that are adaptable reduces the number of separate 

sources within an area. 

5.1.4.2.8 Synchronization Source Pruning 

Minimizing the number of separate synchronization sources is highly desired for performance, reliability 

and battery operation. To minimize differences with and extensions of 3GPP standards, we recommend 

a distributed approach with minimum required signaling. An approach is described below. 

 A ProSe device can track multiple synchronization sources. All tracked synchronization sources 

should be presumed time adaptable unless a source can be identified as having a basis with an 

absolute reference. Network synchronization, SLSS with a SL-ID tied to network 

synchronization, and GNSS synchronization with a fixed mapping between UTC and Sidelink 

framing are examples of absolute references. 

 A ProSe device should adapt its internal frequency reference to high accuracy references 

including: GNSS and network synchronization. When it does not have a reliable high accuracy 

reference, it may not adapt it reference, but may track offsets from its reference to decode 

SLSS. 

 Synchronization beacons send out time and frequency synchronization information based on a 

current estimate for a collective of devices that may include devices nearby. 

o Beacon transmissions occur according to a transmission pattern set by the following 

considerations. 

 Devices transmitting non-adaptable synchronization sources are likely to receive 

synchronization signals matching their sources. Non-adaptable sources will not 

be adjusted. However, these same devices may receive additional 

synchronization signals that are adaptable, and these will be adjusted. 

 A beacon may transmit more than one synchronization source. For example, a 

ProSe device based on an absolute reference may attempt to slowly shift 

additional devices to that reference timing. It’s also possible that an absolute 

reference device could become a SB because it is receiving messages that are 

asynchronous. The principal idea is that the greater good is served by inducing 

the asynchronous transmitter to receive the SB and assume its timing for 

subsequent transmissions. 

 Multiple beacons can be sent out simultaneously and within the same 

PSCCH period. Gradual reference shifts may not always be necessary. A 

ProSe device that begins receiving an absolute source may begin using 

that source and perhaps become a beacon to bring other devices in its 

proximity to a single reference. 

 The average duty cycle of beacons is a function of their adaptability, their 3GPP 

priority, and other signal metrics. By example absolute reference may transmit 

each PSCCH period while asynchronous SB may adopt a lower duty cycle. 

 Transmission of a beacon synchronization signal prevents reception of 

synchronization signals with similar timing. 
 Receiving devices may capture timing, frequency estimates, and decode multiple 

synchronization sources with each applicable PSCCH period. 

o Network based synchronization signals are tracked and may be used as a basis for 

adapting other received synchronization sources. These sources are presumed to have 

a fixed time and frequency reference. SLSS with an SL-ID may also be treated as nonadaptive. 

o GNSS also provides absolute time and frequency references. However, ProSe does not 

require a specific mapping between ProSe framing and UTC. 

 Based on current standards a ProSe synchronization source does not send any 

information allowing a decoding device to know that an SLSS is based on GNSS. 

 Therefore, a device receiving a GNSS based SLSS does not know its basis 

and must treat it like other SLSS. 

 A device that has a reliable GNSS may use it as it as a synchronization 

source. 

o The device may use GNSS synchronization for calibrating its 

frequency reference. 

o A device implementation may choose to use the V2X mapping 

between UTC and ProSe framing; if it does then the device can 

treat the SLSS as non-adaptive. 

o A device that doesn’t implement a fixed mapping should be 

treated as an adaptive SLSS. 

o Synchronization sources not directly tied to network timing may also be used for 

adapting timing. 

o A ProSe device may implement an algorithm that attempts to coalesce differing SLSS 

into fewer separate sources. 

 A ProSe device may use SLSS it tracks as input. 

 The algorithm may weigh SLSS according to signal quality metrics and other 

configuration parameters. 

o Use an adaptation algorithm that calculates a new time reference that is closer to the 

collective timing for receiver sources. 

 Detected synchronization sources can be weighted as a function of their priority 

defined by 3GPP standards and by other signal metrics available to each UE. 

 Adaptation rates should be consistent with tracking rates for synchronization 

sources and not causing missed messages. 

o Receivers have limited instantaneous dynamic range. As a result, total received power 

will control AGC action. A dominate received signal will therefore suppress other 

coincident signals. Suppressed signals may include synchronization signals, but also may 

include PSCCH, PSDCH, and PSSCH. 

o Adaptable synchronizations may be resolved into separate sources 

 Different SL-ID or separated timing 

 Common SL-ID with similar timing 
 A ProSe UE may maintain multiple synchronization instances that may be categorized according 

to their adaptability as illustrated in Table 4. 

 A ProSE UE may receive multiple synchronization updates. These updates are used to update 

existing synchronization instances or create new ones. 

 Synchronization updates need to be mapped to tracked synchronization instances. 

o Absolute references like: (1) network timing, (2) a direct tie to network timing, (3) GNSS 

with a fixed mapping between UTC and frame timing (PSCCH period) 

o Non-drifting references like: (1) GNSS where there is no fixed mapping between UTC 

and PSCCH period, but where GNSS can be signaled. (4) Other synchronization signals 

that do not 

Table 4, Synchronization Instance Adaptability 

Non-adaptable Timing Adaptable Timing

Primary / Network

Frequency Reference 

(1) Network synchronization

(2) SLSS based on network 

synchronization 

(3) GNSS with fixed mapping 

to PSCCH period 

(1) GNSS with arbitrary 

time mapping 

(2) SLSS based on a 

synchronization chain 

leading back to a 

standard reference 

Adjustable Frequency 

Reference 

(1) UE based reference that 

adapts based on 

reference information it 

receives from other 

synchronization sources 

or GNSS information 

Device Frequency Reference (1) A synchronization beacon 

without a standard 

frequency reference, but 

that does not adapt its 

timing 

Time to associate a source with a fixed source 

Table 5, Decoded Synchronization Types 

Update Type Indication Standards Based

Network Synchronization Sourced from network 

synchronization of UE 

Yes

SLSS based on network 

synchronization 

Based on successful decoding of 

PSBCH with direct network 

synchronization SL-ID 

Yes

GNSS Sourced from regular update 

from GNSS 

No, however, V2X specifies a 

mapping between UTC and 

PSCCH timing 

Asynchronous SLSS Based on successful decoding of 

PSBCH without direct network 

Yes
synchronization indication from 

SL-ID 

A UE may use GNSS for frequency reference and timing based on decoded PSBCH. 

Prioritization of synchronization references 

An overall scheme for maintaining synchronization sources includes behaviors defined by 3GPP 

standards and by additional behavior necessary to provide a highly reliable communication system. 

At initialization a ProSe UE begins to search for synchronization sources. The following cases serve to 

motivate appropriate behaviors. Since, a UE may maintain multiple references simultaneously the cases 

describe a synchronization instance of a UE. A UE is generally expected to have multiple instances that 

may be born, track, merge, split, or die. A goal of the collective of devices is to reduce the number of 

synchronization sources that must be separately tracked by a UE. 

Case 1: Following initialization a UE receives network synchronization 

For each network synchronization that is successfully detected and subsequent ProSe related SIB 

decoded, the ProSe function shall update applicable SIB information from the decoded eNB. PSCCH 

timing and frequency tracking are shared from UE network processing functions. 

A UE may receive more than one network synchronization within a frame. Most commonly, the UE will 

decode additional synchronized eNB signals in coverage range. Although the eNB are synchronized 

differing propagation delays can result in significant skew between their relative timing. The UE will 

know these are separate eNB due to separate identifiers in PSS, SSS, MIB, and SIB. Depending upon the 

A

eNB 1 B eNB 2
relative delay between received eNB, an UE might track separately, treat synchronized eNB as a single 

synchronization source for ProSe purposes, or may only be able to decode some network 

synchronization. In any of these events, an applicable transmitting ProSe UE will transmit SLSS/ PSBCH 

based on its serving eNB during a PSCCH period in which it is performing either discovery and sending 

data. A receiving device that decodes this synchronization channel will use its timing for the applicable 

discovery or data that follow. Since the synchronization channel and the information channels will have 

followed the same RF path no differential delay is expected between these channels. Figure xxx 

illustrates this use case. UE A receives synchronization from eNB 1 and uses this timing as the basis for 

its transmission to UE B that includes: SLSS/ PSBCH and an information channel. 

A collision may occur in the synchronization channel that may adversely affect the transmission. As 

illustrated in Figure xxx, UE B is transmitting a message intended for UE A. At the same time UE C also 

transmits. It transmission may carry just SLSS/ PSBCH or may also include an information channel. The 

information channels for the transmissions from UE A and B are not likely to collide because each 

transmitter selects resources from a resource pool use an algorithm that attempts to minimize 

collisions. However, the SLSS/ PSBCH occur in a fixed location. Consequently, the synchronization 

transmissions of UE B and UE C overlap, skewed by the differences in their reference time and the 

propagation time to UE A. As illustrated UE C is closer and therefore has less transmission loss. UE A in 

processing the synchronization period captures the timing of UE C. UE B synchronization as a weaker 

signal is effectively blocked. If the differential delay between UE B and UE C transmissions is large 

enough then UE A is unable to process UE B information channels using synchronization from UE C. UE 

A may still successfully decode UE B messages provided it has a prior synchronization instance that it is 

tracking. Appropriate synchronization instances could include eNB2 timing or some other SLSS/ PSBCH 

signal including prior UE B or synchronization from another UE with similar delay. 

The amount of differential delay permitted is covered in section xxxx. Typically, public safety 

communications are group based and many devices are attempting to receive a transmission from UE B. 

A

B

eNB 1 eNB 2

C
The synchronization collision as depicted can significantly degrade reliable communication to the group, 

depending on the tolerance for delay skew between the synchronization channel and the information 

bearing channels. 

Case 1b: Network synchronization: local eNB not synchronized 

3GPP standards allow operation where physically adjacent eNB are not synchronized. Network 

operations and ProSe both benefit from synchronized eNB and therefore non-synchronized eNB is less 

common in fixed networks. However, operational boundaries in fixed networks and deployable/ mobile 

networks may not always be able to provide eNB synchronization. 

eNB generally use high quality frequency references therefore unsynchronized eNB timing may slowly 

precess due to the difference between these unsynchronized references. eNB may also be locked in 

frequency and therefore not drift, but not have the same frame timing. Each condition will typically 

manifest degraded performance that depend upon the current time offset. Time drift can be treated as 

fixed for PSCCH periods and therefore both cases have similar behavior for a current time offset. 

Typically, the principal repeating structure of ProSe is a PSCCH period of 40 msec. Within each PSCCH 

period some sub-frames are allocated for ProSe and the remaining may be allocated for network 

operations. Time offsets comparable or greater than a sub-frame will generate significant interference 

between network and ProSe operations as ProSe devices will transmit during network sub-frames. 

Smaller time offsets will require multiple synchronization instances to be tracked by the ProSe receiver. 

Offsets significantly smaller than a CP can be processed by a single synchronization instance. Given a 

random uniform distribution of time offsets, harmful interference between network and ProSe 

operation can be expected. 

Case 1: Network synchronization behaviors for reliable communications 

Given the potential for interference between network operations and ProSe communications, network 

operation is preferred under acceptable coverage conditions. Devices may operate in ProSe and 

network mode coincidently. Therefore, devices and their applications should preferentially use network 

operations when possible. 

{!REQUIREMENT - ProSe enabled devices should preferentially use network operations over ProSe 

operations. 

Synchronization channels are more robust than data bearing channels and consequently may be used 

when data transfer is not possible. Consequently, UE may directly synchronize to the network, but not 

be able to communicate with it. These devices would be limited to direct communications. 

Since adjacent eNB may not be synchronized, ProSe UE should be prepared to receive communications 

that are synchronous to any local eNB. This requirement includes eNB that may not be in range of a 

receiving UE. This condition is illustrated in Figure xxx. Therefore, a message may be sent by a UE that is 

synchronous to an eNB that is new (untracked) by that UE. Therefore, to process such a message, the 

UE must be continuously searching for potential new synchronization sources. 

Requirement: A ProSe device must continuously search for new synchronization sources in networks 

that include unsynchronized eNB. 
Case 2: Following initialization a UE receives SLSS/ PSBCH with a direct tie to network synchronization 

Behavior of SLSS/ PSBCH that is sourced by ProSe UE that have network synchronization has similar 

timing characteristic to network synchronization. These ProSe synchronization sources are synchronized 

to the network and therefore do not drift relative to their serving eNB. 

Like direct synchronization to eNB, the associated SLSS/ PSBCH signals may not be synchronized and 

therefore continuous searching is required to prevent missed messages. 

Case 3: Following initialization a UE receives SLS/ PSBCH without a direct tie to network synchronization 

The standards 

 

At point does a UE update its reference based on SLSS? 

The 3GPP standard provides some guidance on the prioritization of 

 if it To facillitatehe ould easily be large enough to cause probleto Worst case timing would be f is or d 

is expected to generally be caseypically this means that processing 

The number of synchronization sources that should be searched in reception processing? 

Tradeoffs in prioritizing synchronization sources for beacons? 

Selection of synchronization instance for a transmission? 

Effects of congestion on message scheduling. 

 and then used to search for potential messages can be left to implementation, but some lower bounds 

are provided here. Adjacent can be placed is it track timingcommunications between unsynchronized 

dep working presumption is that a ProSe UE will select at most one tracked/ decoded SLSS/ PSBCH as a 

periodic synchronization beacon. Further, that it will use a combination of 3GPP rules and other criteria 

to select at most a single decoded synchronization signal for periodic transmission. However, a ProSe 

device with data to send may associate and use a synchronization signal that correspond with a targeted 

device or set of devices that may not be the selected beacon signal. 

Reception Aspects 
Figure 9 illustrates a potential SS environment as seen from the perspective of an UE receiving 

synchronization signals. As illustrated UEA is receiving 3 SS, two asynchronous to the network and one 

to the serving eNB. To receive data traffic from any UE transmitting synchronous to an illustrated SS, 

UEA must track all the synchronization sources. UEA may receive scheduling information directly from 

the eNB for D2D traffic originating from UE in coverage, but also may receive traffic from UE that are 

synchronous to the eNB, but are using autonomous scheduling from the D2D resource pools. UE 

transmitting based on UEB or UEC must use the resource pools. Consequently, UEA must search 

applicable resource pools for 

each SS time reference to 

find applicable control 

messages. To ensure reliable 

public safety operation a UE 

should have the processing 

resources to process these 

asynchronous SS, control 

channels, and the shared 

channel that may follow. 

5.1.4.2.9 Synchronization Sources 

3GPP standards allow for multiple SS, but do not define several aspects of SS: 

 The life cycle process 

o Birth decisions 

o Death decisions 

 The density of SS 

 When a SS should be offset from a receiving source 

The management of SS is a critical component of reliable LTE-direct communications. While it is 

possible to configure D2D devices to operate off network without receiving a SS, these devices will 

operate at a disadvantage in radio performance and battery life. Devices operating without a SS must 

decode the PSCCH without prior timing and frequency synchronization. This approach is discussed 

further in the PSCCH section??? 

Synchronization with SS allows a device to save considerable battery power when it does not have active 

data sessions. Following synchronization, a device searches its Rx receiver pools for valid control 

messages. The location of control messages is limited to a relatively small number of subframes within 

the PSCCH period. Receivers make take advantage of this structure to idle their receivers and processing 

resources thereby saving substantially on battery life. 

Figure 9, Multiple Synchronization Sources 
As defined by 3GPP standards UE upon initiating D2D operation search for SS. Only three different 

signals are currently recognized for D2D primary synchronization: direct network using PSS (primary 

synchronization signal), D2D SS that are broadcast on Sidelink from a UE attached to a network, or other 

SS that do not have a direct synchronization to a network. Network synchronization using PSS/SSS is 

well established and is not consider here. 

3GPP standards considered and rejected “cluster head” architectures particularly for public safety 

communications using D2D. In a cluster head architecture, an UE within a cluster of D2D devices acts as 

the master while other devices operate as slaves. This approach improves resource efficiencies by 

having a single scheduler that reduces communication collisions. However, the cluster head is a single 

point of failure that is unlikely to provide reliable communications to a mobile group. The SS as defined 

can be interpreted to work as a loose cluster head capability and thereby has the same potential 

weakness. A cluster of devices may share a single SS, but if the serving SS suddenly shuts down or 

moves out of radio range then the collective must reach a new equilibrium of serving SS, meanwhile 

reliable communication may be interrupted or degraded. The standards define the initiation of a SS 

either by command or when a suitable source is not found, but do not define when a SS should be 

removed. The standard suggests that synchronization should be supplied by a low density of SS, but its 

structure does not prevent a high density of SS. It further prescribes the priority of SS but does not 

define how these priorities should be applied. A further consideration is that SS do not have a 

mechanism to evaluate how other devices may be affected by its absence as a SS. 

Battery life is an essential consideration for a practical public safety communications protocol. 

Significant factors for power consumption include: 

 Receiver power consumption during idle periods (no active data transfer) 

 Receiver power consumption during active periods 

 Transmitter power consumption during active periods 

The power consumption of D2D devices can be a strong function of its environment. Power 

consumption may be based on whether it acts as a SS, what transmitter power it is using, the median 

quality of received signals (additional error correction is required for poor quality signals). Therefore, 

battery life requirements should be provided under worst case scenarios for each of these parameters. 

Traditional models for voice communications in public safety environments use the 90-5-5 (idle-RX-Tx) 

rule. 

A couple use cases should be considered in considering the best operation of SS. 

 D2D devices using network synchronization – D2D devices within network coverage should 

simply use the available network signal for synchronization. D2D communication within 

network usually does not require D2D synchronization. An unusual situation supported by the 

D2D is when devices are within network coverage, but some of the devices are served by 

separate eNB that are not synchronized. For this case, the device wishing to transmit D2D 

should initiate D2D SS to facilitate synchronization with D2D receivers. For the asynchronous 

case, D2D SS may need to be initiated to provide references for receiving devices. 

 D2D devices operating as SS that have network synchronization – These devices in effect relay 

local network timing out to devices that may be operating outside of network coverage. With 
propagation delays significantly smaller than the CP, the timing provided by the D2D SS provides 

an accurate reference for D2D operations. 

 D2D SS without a direct tie to network time may operate synchronous (but delayed) to the 

network or may be completely asynchronous. Synchronization could be the result of a 

synchronization chain that traces back to a network or could be provided by an absolute 

reference supplied by GNSS. Asynchronous operation will occur whenever a device does not 

have an absolute time reference to a network. 

Case 1: Operation of an independent D2D network – A group of D2D UE wish to communicate, there is 

no network synchronization support. As the group assembles initially the first device will not find any 

other SB and will become a SB. As additional devices arrive each must decide whether to become a SS. 

The 3GPP standards calls for evaluation of the signal strength of any decoded SS. SS that exceed the 

configurable threshold do not automatically become SB but could be commanded or configured to do 

so. In the limit, a very low threshold, results in nearly every D2D devices becoming a SB. For a very high 

threshold very few devices become SB. 

1a: A sparse network of SB provide distributed synchronization for the network of users. This approach 

is the implicit working presumption of the 3GPP standards. Device are sufficiently clustered relative to 

the signal strength threshold that only a small percentage of devices become SB. SB devices are likely to 

be the first on scene. Most new devices arriving in a coverage area already served use existing SS. New 

devices not finding a satisfactory synchronization source become SB. 

This approach can be expected to minimize the total power consumption for a cluster of devices 

maintaining a synchronization field across the coverage area. However, islands of synchronicity are 

more likely. In this situation, a device initiates D2D operation and finding no suitable SB becomes a SB. 

The standard does not define how this device should obtain its absolute synchronization. D2D operation 

and more generally network operation performance is degraded by the presence of asynchronous D2D 

communications. Synchronized operations maintain a level of orthogonality between unrelated 

communications, however unsynchronized transmissions will precess through the timing of other 

communications. Consequently, it is highly desirable to avoid islands of synchronicity. However, it can 

be shown that islands become more likely as the relative density of SS decreases. Eventually devices 

only receive very weak SB and consequently become SB. Fewer islands will exist if devices receiving only 

weak SB use this weak signal as a basis rather than simply starting a SB that is asynchronous. 

Another approach is to allow a very high density of SS. 

1b: All devices act as SS for the distributed network 

The maximum density of SBS is reached when all D2D devices become SB. This behavior maintains 3GPP 

standards compliance and simply requires a lowering of the SB threshold or alternatively commanding 

every device to be a SB. Every initiating device finds proximate SS below threshold and becomes a 

source. SS may either take on the local synchronization or may be configured to offset reference timing 

by sub-frames. Offset timing allows a device to receive a SS and then transmit its own synchronization 

signal in a later frame within the PSCCH period. However, devices operating with different offsets 

increase power consumption for receivers that must search for additional signal timings. Offset timing 

also increases the likelihood of collisions between network and other D2D communications because 

subsequent sub-frames may be mapped to other virtual channels (control and shared.) If new SS 
devices offset then the potential exists for long chains of offsetting SS. This places additional demands 

on receivers for searching each SS it decodes for control messages. 

SS may also be deployed without offset. This approach requires that devices puncture transmissions to 

allow enough receptions of in range SS to provide synchronization. Puncturing rates may be adaptive to 

the local environment. Two isolated devices that need to communicate both send SS. With a random 

distribution each source punctures its transmission and instead listens for SS. In this isolated case, only 

the other devices can be heard, but only if it does not puncture during the same PSCCH period. 

Listening devices should accept the first peak during the correlation process as an update to its time 

reference. The device may use a filter to create a new time reference based on the update. UE can be 

expected to have a frequency reference accurate to +/- 1.5 PPM. Under this worst-case condition, time 

reference drift by 30% of the CP can occur in about 1 sec. To guarantee that time drift remains a small 

fraction of the CP, a UE should receive a SS at least twice per second. For a 40 msec. PSCCH period SS is 

scheduled 25 times per second. This sets an upper limit on the puncturing rate at about 90%. At high 

puncture rates, the UE can more quickly adapt its own timing, but not convey its own. These properties 

suggest that an improved strategy may be to have UE’s adapt their own puncturing rate based on what 

is known about the quality of their reference in time and frequency. Further devices may modify their 

reference averaging filters based on an estimation of the quality of their reference. Attributes effecting 

this behavior include: 

 Time errors can become significant within a second for worst case reference frequency errors. 

 UE may not be able to estimate their own frequency errors effectively without GNSS. 

 A UE with a reliable GNSS signal can lock its frequency very accurately. The frequency accuracy 

benefits both frequency tracking and slows time reference drift. 

 The D2D standard does not explicitly utilize GNSS, however V2X does. In V2X a time relationship 

is defined between UTC time and frame timing. This allows a UE to establish time and frequency 

lock independent of the SS. This explicit relationship can be adopted in D2D. 

 A UE with GNSS lock may modify its averaging filter to limit modification of its time reference. 

 A UE without an absolute time reference like GNSS or network synchronization may allow its 

averaging filter to allow the time reference to be modified more quickly. 

Transmission Aspects 

An important aspect of reliable D2D communications is the life cycle process of SS within a 

communication environment. 

SLSS is like the DL synchronization channels of LTE network operations. A primary synchronization signal 

(PSSS) allows the receiver to acquire time and frequency initialization and in the case to LTE-direct 

determine whether synchronization is based on a network or that it may be asynchronous to any 
networks. The secondary synchronization signal carries 1 of 168 

M-sequence codes. The PSSS and SSSS together determine the 

SLSS ID in the range of [ 0: 335]. The SLSS ID defines the 

scrambling and reference signals for the remaining physical 

channels. By varying the scrambling of the channels according to 

SLSS ID, other transmitters that may interfere but that are part of 

another synchronization group appear as noise rather than 

coherent energy. 

Figure 10, illustrate the allocation of resource elements (RE) to the 

D2D SLSS/ PSBCH. The PSSS signal shown in red provides initial 

synchronization. From a signal acquisition perspective, the SSSS 

shown in blue follows. Acquisition of these two signals provide 

sufficient information to decode the PSBCH that contains some 

basic parameters associated with the D2D signals that follow. 

The SLSS/ PSBCH is transmitted as the first subframe in a PSCCH 

period that typically lasts 40 subframes, but may be configured for 

longer frames. As shown with standard cyclic prefix the subframe 

consists of 14 symbols and 72 carriers in the frequency domain. 

The SLSS/ PSBCH is designed to fit into the smallest LTE allocation 

1.4 MHz and maintains the same allocations for all defined LTE 

bandwidths: 1.4, 3, 5, 10, 15, and 20 MHz. PSSS and SSSS only use 

62 carriers with the remaining carriers not allocated. The last or 

14th symbol of the subframe is blanked to provide an on/ off 

transition interval for the LTE-direct transmitter and to eliminate 

interference between network based UE transmissions and LTEdirect transmissions. 

SLSS/PSBH is not sent by all LTE-direct devices, but instead by a 

subset of devices or eNB referred to as synchronization sources. 

For LTE-direct devices within network coverage synchronization is 

simply based on acquiring the DL channel and using it as the 

reference for LTE-direct transmissions. LTE-direct devices out of 

network coverage may acquire synchronization from other LTEdirect devices that are transmitting the SLSS. Devices that receive 

the SLSS above an adjustable threshold simply use this signal as 

their synchronization source and do not become synchronization 

sources unless configured or commanded to. Those not receiving 

SLSS above threshold may begin transmitting their own SLSS. 

Typically, this SLSS is based on any synchronization information it 

may have available, which may include SLSS signals and GNSS. 

Synchronization sources basing their synchronization on SLSS 

typically offset their timing to minimize interference between 

synchronization sources that have overlapping coverage. A few 

test cases exist for SLSS performance: 

Figure 10, SLSS Allocations 
 Network derived synchronization – This case is simply LTE DL synchronization performance and 

is not evaluated in this study. This case also includes devices using network synchronization, but 

that may be served by separate, but synchronized eNB. 

 Asynchronous networks – Like the former case, devices use network synchronization, but their 

serving eNB are not synchronized. Consequently, the LTE-direct receiving devices must obtain 

synchronization from a synchronization source associated with the transmitting device. The 

network may either configure the transmitting device to.... 

 Off network device; no SLSS detected – A device that is not able to find a synchronization signal 

will become a synchronization source. 

 Off network device; SLSS found - Other devices that move within proximity of a synchronization 

source may use this device as their synchronization source. Devices receiving SLSS below 

threshold also become synchronization sources. 

Except for the first case not covered by this study SLSS is required. However, an important observation 

is that SLSS and PSBCH decoding do not need to be successful for every PSCCH period. Instead the 

receiver can maintain SLSS/ PSBCH information across multiple PSCCH periods. 

5.1.4.3 PSSS 

PSSS is designed to enable an LTE-direct receiver to acquire initial time and frequency synchronization. 

The signal consists of one of two possible sequences that convey whether the source of synchronization 

is directly tied to a network or may be asynchronous. A UE uses this information in part to determine 

priority in choosing synchronization sources as a basis for its transmissions. 

PSSS provides initial timing and frequency estimation along with whether its reference is directly tied to 

a network. The PSSS consists of one of two potential 62 bit Zadoff-Chu sequences more fully defined in 

by [36.211-9.7.1]. A root index of 26 conveys that the SLSS can be directly referenced to network 

synchronization. Specifically, the PSSS is transmitted from a UE that is synchronized to a network or is a 

UE listening to a directly network synchronized SLSS but transmitting its own PSSS. All other PSSS use a 

root sequence of 37 and may be connected via a SLSS chain to a network source or be completely 

asynchronous to any network. 

Simulation of PSSS performance under 3GPP fading profiles with 5 Hz doppler, illustrates the 

performance of PSSS time synchronization as a function of SNR. A small variance in performance is seen 

between the EPA, EVA, and ETU profiles which define progressively stronger and longer multipath. 

Single PSCCH period 95% detection is maintained to about -4 to -5 dB SNR, while the 50% detection 

occurs between -11 and -12 dB. PSSS is a very heavily coded signal with an effective efficiency of about 

0.01 bits/ Hz. This heavy coding provides a certain level of immunity to channel impairments. 

For valid time synchronization, the accuracy was required to be within a tolerance of its known value 

and have a normalized peak above a threshold. This tolerance is consistent with the combined errors 

associated with estimating timing for LTE-direct. 

In the case of LTE-direct three factors were identified as consuming CP margin. First performance 

degrades for timing errors approaching the CP and dramatically degrades for larger errors. Figure 11 

illustrates performance of the D2D shared (data) channel with timing offsets. Each curve on the graph 

shows the block error rate as a function of SNR for given delay offset. Best performance is near zero 

offset while offsets above 20 result in degraded performance. The delay offset is in sample delay which 
consists of about 40 samples per mile of propagation. Therefore, offsets representing more than 1⁄2 mile 

of airlink propagation degrade due to timing offset. D2D operates primarily as a broadcast channel 

without a tightly coupled HARQ (Hybrid Acknowledgement) process to correct transport errors by 

directed resends. Consequently, transport error rates must be tolerable to the application. MCPTT 

voice can tolerate a block error rate of 2% [TR 26.989 Table 5.1.1.6.3.4-1, Media, codecs and Multimedia 

Broadcast/Multicast Service (MBMS) enhancements for MCPTT over LTE] while maintaining acceptable 

voice quality.

Figure 11, Sidelink BLER as a function time offset 
Performance gets worse with more efficient 

modulations. 

Second, unlike LTE network uplink operations 

that adjust UE uplink transmitter timing to 

match the round trip airlink delay at the serving 

eNB, LTE-direct does not. Figure 12 provides an 

example where propagation delay consumes 

CP. As shown, unit B acts as the 

synchronization source for units’ A and B. Unit 

A receives its time reference delayed by d/c. 

Where d is the distance from unit A to its time 

reference (unit B). It then uses its time 

reference to transmit a message that is received 

by unit C with a relative delay of 2d/c.16 If d is 1⁄2 

mile, then the time skew is 5.4 μsec, which 

exceeds the Normal CP of 4.7 μsec. Without 

mitigation, a communication range limit can 

easily be reached due to timing margin. 

Range limitations from timing margin are problematic for multiple reasons: 

 LTE-direct in general is unconstrained from a user perspective on attempted communications. 

During a transition period attempts with legacy equipment and LTE-direct equipment would 

demonstrate an inferior capability particularly in line of sight conditions where distances could 

significantly exceed the Normal CP. 

 LTE-direct performance falls off very rapidly when timing margin is exceeded. 

 The user may have a signal strength indication suggesting that signal energy is being received 

but nothing is presented. 

Prediction of communication received signal strength for reliable communications with direct 

communication are associated with high variances that depend upon local “clutter” and land use at the 

transmitting UE and at the receiving device. However, when considering timing margin, maximum 

distance under more favorable conditions should be considered. First responders accustomed to 

communication challenges know that relocating to a more prominent location can lessen clutter effects 

and achieve or approach line of sight conditions to other devices17. Under this scenario, LTE-direct 

communication can propagate long distances and easily exceed delays given by the Normal CP. Under 

these conditions communication will fail. 

Third, the multipath propagation channel may consist of a range of multipath components whose extent 

may consume CP margin. Hess [Handbook of Land-Mobile Radio System Coverage, Garry C. Hess, Artech 

House, Inc., p 131] provides an empirical formula for estimating the effective multipath delay of a 

16 Units A and B both have the same d/c delay from the synchronization source. Therefore, the time skew relative 

to unit C’s time reference is: tskew = d/c + 2d/c -d/c = 2d/c. 

17 Failing to communicate in apparent line-of-sight conditions is particularly difficult for users to understand and 

accept particularly when legacy equipment works. 

Figure 12, Timing Offset 
propagation model with multiple delay components. It simplifies multipath profiles to a single number 

and is designed to be applied to a simplified model including two equal power sources. The first 

component with 0 relative delay and the second delayed by the Hess metric. Table 6 provides an 

illustration of the Hess metric for 3GPP models, Winner II and an LMR model. The propagation channels 

impact on timing uncertainty varies from insignificant to nearly as large as the Normal CP depending on 

the channel model. EPA, EVA, and ETU are 3GPP delay profile models with progressively larger 

multipath components. The Winner II model used for 3GPP simulations of D2D is a statistical model that 

randomly selects multipath delays for each time epoch. An equivalent delay is therefore more difficult 

to compute, but its extent can be bounded. Therefore, the equivalent delay for the Winner II B1 model 

has an upper bound of 240 nsec. [IST-4-027756 WINNER II D1.1.2 V1.2 Winner II Channel Models]. The 

LMR model is the TETRA HT200 model, which consists of a single secondary component delayed by 15 

μsec with a relative power of -8.6 dB. This model was provided as a compromise between P25 CAI 

performance and the other cellular based models. P25 maintains a 2% BER with a 33 μsec multipath 

component. [TIA TSB88.1-D, Wireless Communications Systems Performance in Noise and Interferencelimited Situations, Part 1: Table 20]. 

A potential area of further research could attempt to quantify the frequency that long multipath is 

encountered in realistic D2D situations, particularly where a user might choose to operate from a 

prominent position. 

Table 6, Channel Model Equivalent Delay 

Propagation Models

EPA EVA ETU WinnerII LMR

Tm (nsec.) 88 507 1122 <240 3639

Timing errors in receiver acquisition of the SLSS, airlink propagation delays, and multipath delay spread 

combine to degrade D2D performance particularly when users establish physically prominent locations 

for greater propagation distances. Three potential solution to this problem include: using the Extended 

CP, blindly searching for LTE-direct signals by searching multiple time skews beyond the receiving UE 

time reference and using the received PSCCH as the time reference. 

Based on the data above we can make a few statements about the limitations of using the Normal CP in 

D2D operations. 

If we allocate one third of the CP to propagation delay, delay tolerance, and timing accuracy then D2D 

operations using the Normal CP are fundamentally limited in range to about 1500 feet. 

Public safety scenarios cannot in general be constrained for range that is substantially lower than the RF 

loss-based range. In fact, based on this analysis public safety LTE-direct operation will need to use the 

extended CP for many scenarios to ensure that operations are not limited by airlink delays. A 

communication limit as short as 1500 feet could occur in situations where the apparent distance is much 
shorter. Figure 13 illustrates a simple example where two devices that are physically separated by a 

short distance may have a much longer effective RF path. 

Figure 13, Dilation of RF propagation distance

{! RECOMMENDED CAPABILITY – The next generation direct communication solution should support the 

extended CP.} 

{! RECOMMENDED CAPABILITY – The next generation direct communication solution may need to be 

configured for extended CP to prevent unexpected communication failure due to propagation 

distances.} 

{! RECOMMENDED CAPABILITY – The next generation direct communication solution should deploy 

receivers that search continuously for messages. 

A UE may receive synchronization signals from multiple synchronization sources concurrently which 

complicates signal handling. A few examples are provided here to motivate proper processing. 

Case 1: Single Active Synchronization Source 

In the simplest case, a UE receives a single synchronization signal. This signal may be: 

 A PSS/SSS signal decoded by a UE that is receiving a network signal. 
 A PSSS/SSSS signal that is received by a UE that is receiving a ProSe SLSS that is synchronized to a 

network. 

 A PSSS/SSSS signal that is not directly synchronized to a network. 

The decoded primary synchronization signal defines timing and frequency synchronization that simplify 

secondary synchronization signal decoding. Information from primary and secondary decoding may 

then be used to obtain ProSe configuration information from either applicable SIB with on network 

reception or from PSBCH for off network reception. 

Case 2: Multiple Synchronization Sources of Differing Type 

In a slightly more complex case, a UE may concurrently decode each of the three synchronization signals 

types bulleted in the paragraph above18. The UE should infer that each signal is an indication of a 

potential set of UE that expect to communicate. Therefore, it must decode PSBCH or SIB information as 

applicable for each decoded signal. Furthermore, the UE must also perform a separate search for 

control channel and discovery information for each active synchronization source. In general, each 

synchronization source will have separate timing, center frequency, and ProSe configuration. 

Case 3: Multiple Synchronization Sources of Same and Differing Types 

Finally, in the most complex case a UE may receive more than one synchronization source for each of 

the three types identified: 

 A UE in network coverage establishes a serving eNB. By standard a UE may encounter 

locations where it may be able to receive synchronization signals from eNB that are not 

synchronized. However, these events are anticipated to be the exception. Current LTE 

systems use synchronized eNB and usually to a basis standard. For example, GPS is directly 

traceable to a single standard. Small cells or temporary cells in indoor environments could be 

examples of potentially unsynchronized cells. An UE operating in such an environment simply 

roams or performs handover between cells. A ProSe UE is expected to use the timing of the 

current serving site. Therefore, ProSe essentially slaves its timing to its serving site and serving 

eNB changes may reset this timing when eNB are not synchronized. ProSe communication 

between devices that may be served by separate and unsynchronized eNB depend upon 

transmission of SLSS to allow the receiving ProSe UE to process the PC5 signal. From a user 

perspective serving cell changes with unsynchronized eNB are infrequent and expected to have 

little effect on user experience. 

 A UE receiving SLSS based on network synchronization – SLSS based on network 

synchronization occurs when a UE receives SLSS that is transmitted by a ProSe UE that is in 

network coverage. Like the prior synchronization type the transmitted SLSS is slaved to the 

serving eNB and is therefore subject to timing changes associated with serving eNB changes. In 

contrast to the prior case, the ProSe UE must track each SLSS signal that it can differentiate. As 

each identifiable SLSS may represent a set of devices using the applicable SLSS as the basis for 

18 In this context concurrent decoding includes all primary synchronization signals that are currently active within a 

UE. Due to signal shadowing and fading all primary synchronization signals may not be decoded each time they 

are available. The UE is expected to maintain a list of active SLSS or network sources that persist across missed 

detections, but that are removed after some timeout period with no detections. 
their communications. A UE not tracking a SLSS cannot communicate with the applicable UE 

set. Like the prior synchronization type, transitions between SLSS based on network timing 

that are unsynchronized are expected to be unusual events. Potential effects on user 

experience may be possible for this case due to propagation of delays. In this instance, the UE 

transmitting SLSS has an eNB change and accepts new timing. The UE must then update its 

SLSS timing and this new time basis must be recognized and accepted by the set of UE using 

this basis. The timing change may also have a cascading effect to other sets of UE that are 

using the third type of synchronization. 

 A UE receiving SLSS based on an SLSS beacon that does not declare synchronization to a 

network. For this third type of synchronization the basis of synchronization is not declared and 

may be indirectly based on network timing through a chain of synchronization sources, may be 

based on an absolute time reference like GPS either directly or through a synchronization 

chain, or may be asynchronous. Since each synchronization source can be anticipated to serve 

a set of UE, receiving UE need to maintain multiple synchronization sources to ensure 

communication with devices in communication range. 

5.1.5 SLSS Processing 

Because the PSSS does not carry a device unique identifier its reception does not provide any identity 

information. In fact, a UE may receive multiple PSSS that are a combination of both root sequences, 

different center frequencies, and separate timing. Under certain conditions it may not be possible to 

separately track PSSS that are aligned closely in time or that precess through each other’s time 

references. 

A summary of common PSSS reception cases is shown in Figure 14. Use 0 is ProSe operation using the 

network as the synchronization source. For this case synchronization is simply based on the local 

network timing. ProSe devices relying on PSS will have common timing and use SIB information for 

operations. 

Use 1 is operation with PSSS from a network based SLSS. Like the prior case the ProSe device tracks a 

single synchronization source that has a network basis. All timing is based on network time. 

Case 2 is for single PSSS reception from a SLSS without direct network synchronization. Therefore, 

timing may be based on a synchronization chain from a network source, may be based on a separate 

absolute reference like GPS, or may be completely asynchronous. For reception of a single 

synchronization source there is little expected difference in performance or processing. For 

asynchronous sources, new timing may need to be acquired, but this would likely involve multiple 

received sources involving one or more of the cases below. 

When ProSe operation includes reception of more than one synchronization source multiple timings and 

synchronization configurations may need to be tracked. 

Case 3 receives network-based PSS and PSSS. In this case the PSSS may be directly synchronized with 

the network or may not be. Consequently, the UE in general needs to maintain two synchronization 

sources and provide communication services for both. The UE should include logic to recognize and 

combine related sources to reduce processing requirements. 
Case 4 supports instances where a UE receives two SLSS one of each type: direct network-based and 

non-direct network based. The UE can track both SLSS independently provided their relative signal 

strengths both reside within the dynamic range of the receiver. 

Use # Use Case Active

SLSS 

Root 

Sequence 

SL-ID Dynamic 

Range 

Comment

0 Network sync 0 NA NA

1 Network based SS 1 25 {0:167} *

2 SS: Not directly 

network based 

1 37 {168:355} *

3 Network sync & 

SLSS 

1 25 or 37 {0:355}

4 Both SLSS 2 25 and 37 Two IDs

5 Multiple network 

based SS 

2 25 Two from: 

{0:167} 

6 Multiple Not 

network based SS 

2 37 Two from: 

{168:335} 

7 Mix of SS 3 25 and 37 Two from: 

{0:167} 

8 Mix of SS 3 25 and 37 Two from: 

{168:335} 

Figure 14, Synchronization use cases 

Cases 5 & 6 are for two concurrent PSSS which use the same base index. For PSSS with enough spacing 

in time the UE can resolve and recognize both synchronization signals. For closely spaced PSSS signals it 

may not be possible to separately resolve. However, the UE would search for SSSS and decode PSBCH to 

determine if separate synchronization signals/ BCH exist. Specific processing is implementation 

dependent, but separate SSSS or BCH configuration decodes would in general be tracked as separate 

synchronization sources, while the same configuration would be combined as a single source. If the 

same configuration exists with large time dispersal it may be necessary to handle it as two 

synchronization sources. 

Cases 7& 8 recognize that a mix of synchronization sources may exist and that tracking of multiple 

sources is necessary to ensure that communication is missed simply because it used an untracked 

synchronization source. 

While the PSSS only has two root sequences the SSSS has 336. Half are dedicated to direct network 

synchronized sources and the other half to non-direct synchronized sources. This allows for a significant 

discrimination of synchronization sources. The standards do not prescribe how SL-ID should be 

allocated. However, a few rules can be suggested: 

 Unique SL-IDs should be assigned for different BCH configurations within a potential 

communication zone. 
 SL-IDs may be assigned to provide additional information about the basis of synchronization. By 

example a synchronization source might use GPS. Prioritizing this source above an 

asynchronous source would be desirable. 

 More assigned SL-IDs increases SLSS processing burden and are likely to affect battery life. It 

may be desirable to limit the number of SL-ID that may be incident on a UE. 

 Using SL-ID to segregate ProSe devices can be counterproductive to reliable communications. 

Instead segregation should occur at higher layers based on unicast and group identifiers. 

 A UE operating off network without any available synchronization sources may pick an SL-ID 

randomly from a pool of available SL-IDs19 that is mutually exclusive to any SL-IDs it may already 

know are assigned. 

 A UE should support a mechanism to minimize the number of separate synchronization sources 

by bringing adaptable sources into alignment. Synchronization sources with a clear basis would 

not be considered adaptable and would include network based and GPS based sources. Other 

synchronization sources could be adapted overtime into synchronization. Methods to adapt 

synchronization sources are discussed in [20]. 

Tracking of SLSS is limited by the signal quality of each SLSS available to a ProSe receiver. Independent 

fading conditions can be presumed; by design synchronization sources are geographically dispersed. 

Total signal loss between a transmitter and receiver pair is frequently modeled as a slowly-time varying 

component associated with the distance and fixed obstructions along the communication path and a 

time varying component associated with multipath and associated fading processes. Due to the time 

varying component different SLSS may be received during each PSCCH period. SLSS /BCH signal are 

heavily coded signals and therefore can be decoded at SINR’s below 0 dB. Consequently, some SLSS 

with enough SINR may be decoded in a PSCCH period may be decoded, while weaker SLSS may only be 

decoded some of the time. Tracking of sources is dependent on the received signal quality. 

{! RECOMMENDED CAPABILITY – The next generation direct communication solution should maintain a 

long-term estimate of its frequency calibration that prioritizes sources.} 

{! RECOMMENDED CAPABILITY – The next generation direct communication solution should maintain 

separate short-term estimates for each synchronization source it decodes.} 

{! RECOMMENDED CAPABILITY – The next generation direct communication solution should provide a 

mechanism to update the long-term estimate from short term estimates that is based on a community 

19 The available pool size can also be one or equivalent to already assigned. 

20 Konstantinos Manolakis, Wen Xu, “Time Synchronization for Multi-Link D2D/V2X Communication,” in IEEE 84th

Vehicular Technology Conference, 2016. 
estimate that weights synchronization sources according to their priority and estimated quality.} 

0 10 20 30 40 50 60 70 80 90 100

Trials: PSCCH Periods (40 msec)

-1500

-1000

-500

0

500

1000

1500 Frequency Error (Hz)

Center Frequency Estimate
Asdjfkl; 

6. Addendum – Description of Sidelink Operations 

Cell phone radio transport is either network based between an eNB and device or direct transport 

between devices. In addition, devices with relay capability may act as bridges between network and 

direct communications or direct to direct communication providing coverage extension. 

Direct communication in LTE uses the Sidelink for communication between devices and is defined by the 

PC5 interface. The radio interface for Sidelink consists of 3 layers that include the physical layer (PHY), 

the medium access control (MAC), and Radio Resource Control (RRC). Detailed descriptions of these 

layers are provided in 3GPP 36.201 and its reference documents. This description is meant to highlight 

certain aspects of the Sidelink services applicable to this research program. 

 

6.1 PHY Layer 

The Sidelink PHY consists of several physical channels that support the radio interface and transfer 

information across the radio interface including: 

PSBCH (Physical Sidelink Broadcast Channel) – This channel is transmitted with the SLSS (Sidelink 

Synchronization Signals) and includes basic information about the transmission format. 

PSCCH (Physical Sidelink Control Channel) – This channel precedes the shared channel and defines 

transmission allocations that are used by the shared channel. 

0 10 20 30 40 50 60 70 80 90 100

Trials: PSCCH Periods (40 msec)

-700

-600

-500

-400

-300

-200

-100

0

100

200 Frequency Error (Hz)

Convergence of the UE Reference Oscillator
PSDCH (Physical Sidelink Discovery Channel) – This channel allows ProSe devices in proximity to 

discovery each other. 

PSSCH (Physical Sidelink Shared Channel) – This channel provides the actual transport of data over the 

air interface. 

The Sidelink PHY also includes physical signals that support synchronization and demodulation. These 

include: 

PSSS (Primary Sidelink Synchronization Signal) – This signal is for initial synchronization of a SLSS 

receiving device 

SSSS (Secondary Sidelink Synchronization Signal) – This signal may also be used for synchronization, but 

also includes certain critical information required for processing other channels. 

PDRS (Demodulation Reference Signal) – This signal consists of known symbols spread throughout the 

time and frequency space of the physical signal to enable efficient channel estimation and equalization. 

6.1.1 Synchronization 

For Sidelink operation, time and frequency synchronization must be established between UE. Several 

different scenarios may exist depending upon RF conditions and the availability of Sidelink 

synchronization signals. ProSe supports the following synchronization sources. 

In network – For UE within network coverage Sidelink communication is synchronized to the network. 

UE sharing a common serving eNB or served by synchronized eNB acquire synchronization to Sidelink 

directly from network timing. 

Async. Networks – UE within network coverage can establish Sidelink communications between UE that 

may be served by separate eNB that may not be synchronized. In this case, Sidelink communications 

must support offset timing. UE frequency synchronization may still benefit from eNB high stability 

references. 

Network Directed Synchronization Sources – The network can direct ProSe capable UE to act as Sidelink 

synchronization sources. 

Fringe Network Coverage UE – Devices supporting Sidelink that do not have a network RSRP above a 

provisioned threshold and that also do not have a Sidelink RSRP above a separately provisioned 

threshold may act as Sidelink synchronization sources. This synchronization source uses network timing 

it obtains from its fringe coverage to establish its synchronization. It then uses this synchronization for 

subsequent direct transmissions. It will also employ this synchronization if it is provisioned or directed 

to become a synchronization beacon. 

Off Network UE – Public safety devices supporting Sidelink that have no synchronization with a network, 

first attempt to find other synchronization sources. These may include other direct mode devices that 

either transmit a message which always include synchronization information or that transmit a 

synchronization beacon. The device may use any synchronization signals that it receives as a basis for its 

own transmission timing. However, reliable reception cannot rely on a single synchronization for 

reception. Devices may be RF isolated and may not receive a local synchronization beacon even when 
many nearby devices do. This device may then transmit asynchronously to the local synchronization 

beacon and is only heard by devices if they search for direct signals that may have arbitrary timing. 

Sidelink receivers prioritize synchronization sources according to Table 7. 

Sidelink receivers can maintain multiple synchronizations to support communication with Sidelink 

transmitters. The number of asynchronous sources varies with the scenario. 

Table 7, Synchronization Sources 

Scenario Number of Synchronization Sources

Asynchronous eNB; In 

network device 

In a regular tessellation of eNB, the handover region is typically 

characterized by 3 eNB’s with similar strength signals. In principal 

each of these eNB’s may have their own timing. While its true that 

fixed infrastructure is generally synchronized, synchronization may 

not be the case during public safety response when: 1) mobile eNB 

are brought in to increase capacity for an incident 2) when a disaster 

has knocked out high capacity backhaul links or 3) when mobile eNB 

are located within structures or geographic canyons. Therefore, a 

LTE-direct device may need to deal operate in an environment with at 

least 3 asynchronous eNB’s. 

In network Sidelink beacon An out of network (PS) device receives Sidelink beacons that are

synchronized to an eNB. 

Out of network beacons (PS) An out of network (PS) device receives several synchronization

beacons that are not directly based on an eNB. In a worst-case 

scenario, UE transmitting Sidelink beacons are radially dispersed from 

the target UE. Each beacon is far enough from the others to prevent 

synchronization via 3GPP standard behavior. E.g. Without topological 

obstructions, each could be equally distant from the target and 

spaced radially by slightly more than 1 radian. which would ensure 

the distance between beacons was greater than the distance to the 

target UE. In principal, up to 6 synchronization sources would then 

be possible. Obstructions may also prevent synchronization sources 

from hearing each other. 

The 3GPP standard defines a prioritization of synchronization sources and a device acting as a beacon 

may dynamically update whether the criteria to be a beacon is still valid. However, a beacon may not be 

aware of devices it is serving. Therefore, removing synchronization sources must be performed 

carefully. 

One method to reduce the number of asynchronous synchronization sources in overlapping areas is to 

shift synchronization sources toward a weighted average of the sources that can be detected. The idea 

is to bring disparate time sources to a smaller number of references. 

Unlike network unicast communications that adjust their UE transmission timing to compensate for 

airlink propagation, ProSe does not adjust its transmission timing. Therefore, the CP must be long 

enough to compensate for potential time skew between devices times. Scenarios 

1. Common SLSS 
2. Separate SLSS 

3. Async networks 

4. Sync networks 

Recommended Requirement: GNSS should be added to the priority list of synchronization sources for 

ProSe. 

Recommended Requirement: The PSSS signal should identify synchronization sources that are GNSS 

based. 

Recommended Requirement: The 3GPP standard should define the time relationship between UTC time 

and the beginning of a PSCCH frame. 

Recommended Requirement: The CP duration used in ProSe should exceed the worst-case propagation 

delay that may be expected in target PS scenarios. 

 
7. Addendum – Direct Communications Requirements 

This is an excerpt from the NPSTC Public Safety Communications Report, Public Safety Broadband Pushto-Talk over Long Term Evolution Requirements 7/18/2013. It provides requirements for direct 

communications. 

Public safety communications systems are designed and built to enable first responders to communicate 

in the direst of circumstances. This includes instances where the public safety radio system they are 

operating on has had its infrastructure damaged or has completely lost the ability to function. In such 

circumstances, it is paramount that first responders retain voice communications capability directly, 

from radio to radio, without depending upon any infrastructure. This capability is possible due to a 

commonality that is planned for and included (common agency-based channels, common 

interoperability channels, etc.) in each public safety radio. While the NPSBN will be a primary, reliable 

transport of public safety voice and data, there are many situations where voice and data 

communications will be required in areas where the NPSBN is not available. NPSBN Users (NPSBN-U) 

may be outside of the range of the fixed network, such as first responders in a rural area assisting in a 

response to a plane crash or police officers inside a residence responding to a domestic issue. Offnetwork voice communications must be immediately accessible to users in the absence of the NPSBN. 

This includes areas and locations where the ability to access nonterrestrial communications can be 

impaired such as within building and other enclosed areas where non-terrestrial communications may 

not be available. Additionally, there may be times when users may wish to communicate off-network. 

Today, firefighters often join a local communications network, which does not leverage the fixed 

network, but rather, relies on either direct communications between the user devices or 

communications via a local repeater on-scene. Firefighters can voluntarily leave the fixed network either 

due to the unpredictable coverage of the fixed network, or if the coverage of direct communications or 

the local repeater is well known, based on experience. There will be occasions where a user may be 

within network coverage and will need to communicate with users who are on the network and offnetwork, such as an Incident Commander (IC) supporting fire response activities. These users must be 

able to communicate to users on the fixed network, such as dispatch, as well as the local users who are 

off-network or when it is desirable to provide voice, data and video connections between users without 

connection to the network even if within network coverage. A relay function is critical for off-network 

communications when NPSBN coverage is not sufficient to support the public safety mission. In the case 

of firefighters who are responding to a wildfire while outside of the coverage of the fixed network, if one 

user becomes encircled by the wildfire and is beyond the range of the IC, but within the range of 

another device that can act as a relay, the endangered firefighter can still update his status to the IC. A 

UE device operating on the NPSBN SHALL be capable of relaying PTT traffic between a UE device 

operating off the network and a UE Device operating on the network. 7.1 Off-Network Operational 

Communications In today’s public safety environment, direct voice capability utilizes either mobile or 

portable radios with varying transmit values that operate in multiple frequency bands, providing a 

variety of range capabilities for users operating in the direct mode. Different frequency bands, by their 

nature, have varying range capabilities and that applies to operations in the direct mode. The transmit 

power, use of mobile or portable radios and the frequency band in use are all major factors in 

determining the distance over which two users can communicate in the direct mode. NPSTC 

Recommendations for PTT over LTE Requirements 31 PTT Off-Network Communications Requirements 

Public Safety Broadband The ability for NPSBN subscribers to operate in a peer-to-peer mode will 
enhance range possibilities between units and offer bandwidth extension between devices within urban 

areas. Direct mode communications could also offer the ability to enhance in-building coverage in the 

nation’s cities. Non-terrestrial communications should not be considered an alternative to off-network 

(direct mode) voice capabilities. Non-terrestrial communications may not offer public safety users the 

same capability or accessibility as off-network, direct mode capabilities. The following are requirements 

for off-network operations. 

 

Table 8, PTT Off-Network Communications Operational Requirements, 

# Requirement

1 Off-network PTT Communications SHALL not cause interference to on-network operations and 

on-network operations SHALL not cause interference to off-network operations. 

2 On-network operations SHOULD not cause interference to off-network PTT Communications

3 Off-network PTT Communications SHALL minimize interference to other off-network devices.

4 Public Safety Users SHALL have off-network PTT Communications, as necessary and authorized, 

in the complete absence of any fixed infrastructure. 

5 Off-network PTT Communications SHALL allow a minimum number of (N) simultaneous 

fffnetwork PTT Communication transmissions. 

6 Off-network PTT Communications SHALL only be available for authorized users.

7 The PTT Service SHALL provide a notification to a user when approaching the edge of the 

network.21

 7.2 Off-Network PTT Communications Status Following are requirements for the off-network 

communications status of users. 

Table 9,PTT Status Requirements of NPSBN User Off-Network Communications 

# Requirement

1 A UE SHALL be capable of switching to an off-network PTT Communications mode when 

detecting an off-network condition. 

2 The PTT Service SHALL allow an authorized user to move PTT Groups off network for use with 

off-network PTT Communications. 

3 An authorized user SHALL be capable of switching to an off-network PTT Communications 

mode. 

4 Off-network PTT Communications SHALL provide a range similar to what is offered by current 

LMR solutions at an outdoor incident scene.22

5 Off-network PTT Communications SHALL provide a range similar to what is offered by current 

LMR solutions between users within a building and users outside of the building.23

21 Could include audible, visual, or vibration notification. 

22 There are many ways to provide this capability, including but not limited to higher power UEs or 

portable infrastructure. 

23 The users within the building may be on different levels/floor and at varying distances within the 

building. 
6 Off-network PTT Communications SHALL support a number of (N) PTT Groups as authorized by 

the agencies System Administrator.24

7.3 Off-Network UE Functionality On-network communications is usually the preferred communications 

path. However, because fixed network coverage cannot be provided everywhere, it is beneficial for 

devices to be able to relay communications from off-network devices to the fixed network, when 

possible. An example of the benefit of this case, is when a user who enters an emergency condition is 

out of the fixed network coverage, but is within the coverage of another device that is in network 

coverage. By relaying the emergency state of the off-network user, critical information is passed to the 

network to protect the life of the user. The capability of NPSBN users to be able to communicate 

simultaneously with the NPSBN on-network and users who are off-network may also be required during 

mission critical incidents. Typically, users operating off-network will want to discover other users, and be 

discovered by other users on the scene. Sometimes, users will not want to be discovered, such as 

undercover agents in the area. Other times, users may not want to participate in off-network 

communications at all. Because of this, the ability to configure a device to discover other users, to be 

discoverable by other users, and to permit or restrict off-network communications, must be supported. 

Following are UE requirements for off-network communications. 

Table 10, Off-Network Communications UE Functionality Requirements 

# Requirement

1 A UE SHALL be capable of off-network PTT Communications and on-network PTT at the same 

time. 

2 Off-network PTT Communications SHALL provide a mechanism to dynamically create PTT 

Groups. 

3 Off-network PTT Communications SHALL provide a mechanism for a UE to monitor what PTT 

Groups are active. 

4 Off-network PTT Communications SHALL provide a mechanism for a UE to relay off-network PTT 

Group transmissions from an on-network UE to an off-network UE. 

5 Off-network PTT Communications SHALL provide a mechanism for a UE to relay off-network PTT 

Group transmissions between off-network UEs. 

6 A UE SHALL be capable of transmitting its location, if known, to other UEs when operating 

offnetwork. 

7 A UE SHALL be capable of utilizing off-network PTT communications while still connected to the 

NPSBN and access required services. 

8 A UE SHALL be capable of being connected to the NPSBN and utilizing required network services 

while operating off-network PTT communications. 

 

24 Breaking with the qualitative nature of this document, the minimum number of PTT Groups that must 

be supported is a minimum of 20. 
8. Innovation Concepts 

8.1 Channel Aware CODEC 

The EVS channel aware CODEC can maintain acceptable MOS results with a missing frame rate of 8%. 

This error rate is most obviously allocated to the receiver performance. However, it may also be 

proportioned to the transmitter not generating a frame. Can un-transmitted frames be used to greater 

advantage in system performance than allocating all frame loss to the receiver? 

8.2 Macro-diversity Transmissions 

Harris has identified a potential extension to ProSe that provides clear advantages to coverage when 

multiple ProSe devices are available and mutually within coverage. The following sections describe the 

techniques and provides simulation results demonstrating its capabilities. 

8.2.1 Description 

8.2.1.1 Background 

ProSe supports both broadcast and unicast communication capabilities, but broadcast operation is 

particularly important to first responders because they commonly communicate in groups. Broadcast 

allows communications to be delivered simultaneously to all members of the group and thereby by 

provides rapid communications that facilitate quick support and response. It also provides more 

efficient use of RF resources because a single message is decoded by all interested users, rather than 

individual messages for each user. Simulation performed during development of the ProSe standard 

identified that communications may encounter system self-interference that significantly limits range. 

This interference results from other devices transmitting in the same coverage proximity as a receiving 

device. These devices may be attached to the network or be operating off- network in public safety 

ProSe mode. While network devices may interfere with ProSe operations, off network device 

transmissions cause more interference. 

Network devices transmit according to scheduling from a serving eNB which dictates RF resources that 

are configured by design to provide isolation from resources used by ProSe devices. Furthermore, 

network device transmission power is tightly controlled by the eNB. It adjusts device power to the 

minimum required for efficient data transmission. Tight power control is possible because this network 

uplink traffic is unicast with a low latency HARQ process. The eNB calculates signal quality metrics that 

measure the performance of the device uplink. It then completes a feedback loop to the device to 

ensure that it operates at minimum transmission power for reliable data transport. Accordingly, devices 

can transmit at 23 dBm, but most commonly operate at much lower power. 

In contrast to network devices, PS mode ProSe devices that are providing broadcast communication 

services cannot take advantage of the tight feedback system used in network UL transmissions. First, 

public safety group communication reliability should be maximized to the most remote device in the 

group. However, RF losses to the most remote user are generally unavailable and rapidly change. 

Accordingly, the transmission must be performed at full power. In general, talkgroup members are each 

subject to separate and independent communication limitations that are a function of the desired signal 

quality as well as system self-interference. Figure xxx illustrates some simplified examples of a single 

talk group that spans: 

 Network coverage 
 Network accessible devices via network extension relay 

 Adhoc ProSe – segregated talkgroup subset of the same talk group 

 Adhoc coverage segregated talkgroup subset that includes a network extension-relay 

 Isolated devices – devices out of range of other networks and devices 

The 3GPP standards activities did not generate a solution to this problem and therefore the most 

effective way to provide reliable transmission to a group is full power transmission. 

{REQUIREMENT! – Broadcast ProSe communications should operate at maximum power. 

In addition to maximum transmission power, 3GPP decided to implement repeated transmissions to 

improve the reliability of receiving a message. ProSe self-interference is a complex function of 

interfering transmission activity in range of the victim receiver. The imposed interference depends on 

the incident power of the transmission intercepted by the receiver as well as the timing and frequency 

relationship between the transmission energy and the desired receiver resource elements. By simple 

example a receiver may encounter significant interference in the transport block, but very little 

interference 1 msec. later in the next transport block. Thus, multiple transmissions can substantially 

improve the likelihood for receiving a message. The standard decided that PSCCH messages that are 

relatively short and fixed in length would be sent twice. PSSCH messages which depend upon the MCS 

selected and could be much larger would be sent 4 times. During periods of low interference (low 

loading of RF resources available to ProSe communications) this redundancy properly processed at the 

receiver improves link margin, thereby extending coverage. During high interference some 

transmissions may be blocked, and redundancy makes it far more likely that the receiver will be able to 

decode the message by receiving some fraction of the redundant messages. The interference in effect 

reduces redundancy and therefore coverage is affected by loading. At sufficiently high loading the 

redundancy is counter-productive, and the capacity of the channel falls off. Judicious use of priority and 

congestion control measures implemented by the transmitters can prevent an inefficient channel. 

8.2.1.2 Macro-diversity Transmissions 

Macro-diversity ProSe transmission is a mechanism to improve the range and reliability of public safety 

ProSe communications. The principal concept is that retransmissions, already part of the ProSe 

protocol, can be reinforced by other ProSe devices. The likelihood that a ProSe receiver will be able to 

decode an initial ProSe transmission decreases with increasing equivalent RF distance to the transmitter. 

Consequently, receiving devices near the transmitter are likely to decode a first transmission while 

devices further away are increasingly less likely to be initially successful. Macro-diversity ProSe 

transmission employs these initially successful receivers to transmit during the redundant portions of 

the original message. Specifically, a receiver that successfully decodes an initial transmission, captures 

the transport block and synchronously transmits with the initial transmitter in subsequent redundant 

transmissions. 

Figure 15 illustrates how this form of macro-diversity works. In this example device A wants to 

communicate with the talkgroup that consists of all devices shown. However, its ability to do so is 

limited by obstacles and distance. Per ProSe standards it sends its transmission message 4 times. 

Devices B, C, and D that are within proximity to device A receive and decode the first transmission. They 

then synchronously repeat the message precisely echoing device A transmissions. This effectively 
removes the obstacle prevent communication with device E, while also extending range to devices F and 

G. 

Figure 15, Macro-diversity 

As illustrated in Figure 16, an originating device transmits and implicitly initiates support devices to 

mimic the first transmission on subsequent transmission periods. Potential support devices base 

whether they will support a transmission on several factors including: 

 Their configuration that may define whether devices are enabled to support transmissions and 

under what conditions support is provided. 

 Devices may also selectively support transmissions depending upon the priority. 

 Finally, devices may use radio metrics like signal strength to decide whether a transmission 

should be supported. 
Figure 16, Macro-diversity approach 

Macro-diversity offers multiple benefits from a coverage and reliability perspective. 

 More efficient than a relay – coverage extension-relays and network extension-relays operate 

by receiving complete transport blocks including all redundancy. Following complete 

reception, the relay selects additional resources and completes another transmission including 

redundancy. Consequently, twice as many resource blocks are required for each message 

passing through a relay. Macro-diversity transmissions do not increase the number of resource 

blocks required for message delivery. 

 Inherently adaptive to current RF conditions – Macro-diversity dynamically enlists other 

devices that can decode the initial transmission and are otherwise qualified to reinforce 

transmissions. As devices move around they move in and out of acting as reinforcing 

transmitters according to their ability to decode the initial transmission and other qualifying 

metrics. 

 Reduces near/ far interference issues – Most intra-system interference in ProSe results from 

receivers operating close to a transmitting device that is a member of a different 

communications group or is independently transmitting unicast traffic. Blocked reception is a 

strong function of the ratio of the incident interfering transmitter power to the desired 

transmitter power. Macro-diversity lowers this ratio on average. 

 Extends range – Macro-diversity extends range via higher transmission power, a richer diversity 

RF field, and by obviating RF obstacles. Each transmitter that operates during redundant 

periods adds to the average power of the transmission. Each transmitter also adds to the 

richness of the diversity field. Reinforcing devices will be far enough apart that their respective 

paths to the receiver are independent fading processes. It is therefore increasingly unlikely 

that multiple paths will simultaneously fade and prevent message transport. Finally, macrodiversity improves signal propagation around RF obstacles. RF signals have limited ability to 
transmit around obstacles, however reinforcing transmitters may eliminate obstacles simply by 

their separate location. 

 Improve communication reliability – The net effect of this richer diversity field, higher 

transmitter power, and location diversity is that group communications are more reliable. 

Multiple transmitters improve the likelihood that all members of a communication group 

receive a message. 

 Extends priority and preemption paradigm – Finally, diversity transmission may be applied 

selectively to messages. Decoding success is required for reinforcing transmissions, but 

transmissions may also be qualified by: received power, signal quality, and loading metrics. In 

addition, diversity transmission may be qualified by priority fields contained within the control 

channel for each message. In a heavily loaded channel this ensures that messages with the 

highest priority are given privilege. 

 
9. References to relevant 3GPP documents 

9.1 Interference Cancellation 

TR36.766 Study on interference cancellation receiver for LTE BS 

TR36.859 Study on Multi-user Superposition Transmissions 

TR36.884 MMSE-IRC receiver 

TR36.891 Network Assisted Interference Cancellation 

9.2 ProSe 

TR36.843 Study on LTE ProSe 

TR36.877 Study on ProSe User Equipment 

9.3 V2X 

TR36.785 V2X Radio Transmission 

TR36.786 V2X Services 

TR38.787 V2X New Bands 

TR38.788 V2X Phase 2 

TR36.885 Study on V2X 

9.4 Voice Quality and Coding 

TR26.952 

TR26.989 MCPTT Media, codecs, and Multimedia Broadcast/ Multicast Service (MBMS) enhancements 

for MCPTT over LTE 

TR26.989 provides a comprehensive comparison of voice codecs and their relative perceptual 

performance. In summary EVS was equivalent or outperformed AMR and AMR-WB vocoder 

under a battery of tests that included RF channel impairments as well as acoustic impairments 

defined by an NTIA report.25 Evaluations were performed using models for LTE unicast 

operations, LTE broadcast operations, and LTE-direct (ProSe) with frame erasures that were 

randomly distributed and per the Delay and Error profiles from TS 26.114 using the EVS JBM. 

Despite the EVS performance advantage, AMR-WB was selected as the recommended 

mandatory due to its wide current adoption in existing networks, as well as uncertainty over the 

25 NTIA Report 15-520: "Speech Codec Intelligibility Testing in Support of Mission-Critical Voice Applications for 

LTE", S.D. Voran & A.A. Catellier September 2015. 
licensing costs of EVS. EVS-SWB was also recommended as an optional CODEC for MCPTT 

operations. 

Table 11 illustrates the audio quality performance of the recommended CODECs in various 

acoustic environment scenarios. For comparison purposes the bit-rate for AMR-WB is 12.65 

kbps and EVS-SWB is 13.2 kbps. Under clean audio conditions the AMR-WB achieves a MOS 

score of 3.5 while EVS-SWB achieves 4.7. Likewise, under the car noise condition AMR-WB has a 

MOS of 3.0 while EVS-SWB achieves 3.8. Finally, with music and mixed content AMR-WB 

achieves MOS of 2.7, while EVS-SWB maintains 3.7. The 3GPP document notes that acoustic 

noise suppression capabilities may reduce the audio quality differences in noisy environments. 

Table 11, MOS Scores for Selected CODECs 

 

(a) Clean (b) Car Noise

(c) Music & Mixed Content - Chinese (d) Music & Mixed Content - US English

Table 12 illustrate the vocoder and transmitter mode combinations that were evaluated. The 

highlighted row shows the mandatory AMR-WB CODEC and forms the baseline of simulations to 
be used on this program. Additional simulations may be performed at the higher BLER allowed 

by the EVS CODEC. 

Table 12, Coded voice cases for LTE-direct 

Codec and mode On time Packet size BLER target

AMR 12.2 kbps/AMR-WB 12.65 kbps 75% 44 Bytes 2% 

EVS 13.2 kbps channel aware mode (Option 1) 72.5% 44 Bytes 8% 

EVS 13.2 kbps channel aware mode (Option 2) 66.5% 44 Bytes 2% 

EVS 13.2 kbps non channel aware mode (Option 1) 72.5% 44 Bytes 4% 

EVS 13.2 kbps non channel aware mode (Option 2) 70.5% 44 Bytes 2% 

EVS WB VBR 5.9 or 7.2 or 8 kbps 72.5% 31 Bytes 2% 

NOTE 1: Option 1 is RX side BLER relaxation. Option 2 is TX side relaxation where 6% of the packets are dropped at 

the transmitter while keeping the same RX BLER target of 2% (net FER is 8%) for the channel aware mode. 

For non-channel aware mode, 2% of packets are dropped at the transmitter and RX BLER target is kept at 2% 

(net FER is 4%). 

NOTE 2: The EVS-VBR (variable bit-rate) mode combines bit-rates of 2.8 kbps, 7.2 kbps and 8 kbps to achieve an 

average bit rate of 5.9 kbps over active speech. For purpose of this simulation, 2.8 kbps and 7.2 kbps packets 

were zero padded and sent at the same payload size as 8 kbps, i.e. 31 Bytes. 

TR26.114 IP Multimedia Subsystem (IMS); Multimedia Telephony; Media Handling and Interaction 

Includes the EVS JBM profiles. 

9.5 NB-IoT 

TR36.802 NB-IoT 

TR36.888 Study on IoT 

9.6 In-device Co-existence 

TR36.818 In-device coexistence 

9.7 Relays 

TR36.836 LTE Mobile Relay 

9.8 High Power UE 

TR36.837 HP-UE 

TR36.886 Study on B41 HPUE 
9.9 Misc 

TR36.824 LTE Coverage Enhancements 

TR36.868 Study on Group Communications 

TR36.898 Network Assistance for Network Synchronization