Preamble signature selection for contention based random access in mobile communication

Techniques and examples pertaining to reducing preamble collision in contention-based random access (CBRA) from a user equipment (UE) to a base station of a mobile communication network are described. A method for selecting a preamble signature may involve identifying a quantity of preamble signatures reserved for CBRA in the mobile communication network. The method may also involve performing a modulo computation based on a unique identifier of the UE and the quantity of preamble signatures. The method may also involve designating one of the preamble signatures as a chosen preamble signature based on a result of the modulo computation. The method may further involve initiating a random access procedure using the chosen preamble signature to establish a communication link between the UE and the base station.

BACKGROUND

In mobile or cellular communication, a user equipment (UE), such as a cellular phone, connects to a communication network by establishing a communication link with one of the base stations of the communication network over an air interface. An initial step of establishing such a communication link, or a “connection”, from a UE to a base station may involve performing a “random access procedure”. The random access procedure involves exchanging messages between the UE and the base station using a channel of the air interface called “random access channel”, or RACH. Thus, the random access procedure is often referred to as a “RACH process”. Often, when a UE needs to establish a connection to a base station, the UE would initiate or otherwise trigger a RACH process to attempt to access (i.e., connect to) the base station.

Specifically, RACH may include a sequence of time-frequency resources called “random access slots”. Random access requests from the UE may be transmitted to the base station in random access slots. Depending on system configurations, a random access slot may be on the order of 1 millisecond in time. A RACH process starts by transmitting a message, called a “preamble”, in a random access slot (RAS). The preamble is transmitted during a RAS using a complex sequence or pattern called a “preamble signature”. There are multiple preamble signatures available to use for the RACH process, allowing multiple UEs to originate respective RACH processes at the same time (i.e., in the same RAS) over a shared RACH. As long as the multiple UEs use different preamble signatures for the RACH processes, the RACH processes would not interfere with each other, even if they are happening during a same period of time. However, if two UEs are using a same preamble signature to connect to a same base station, a so-called “preamble collision” happens. The two RACH processes using the same preamble signature at the same time would interfere with one another, and consequently at least one of the two RACH processes may lead to an unsuccessful establishment of a communication link with the base station. Given that the total number of preamble signatures available for RACH is fixed, and that according to existing technologies each of the UEs chooses a preamble signature randomly and independently, preamble collision is common when the number of UEs initiating RACH processes at the same time becomes significant as compared to the number of preamble signatures available. The problem is especially eminent when the number of UEs served by a base station increases. That is, the more UEs a base station serves, the more UEs may initiate RACH processes at the same time, and thus the higher the chance of preamble collision. A higher probability of preamble collision leads to a lower radio resource control (RRC) success ratio, a lower RACH success ratio, as well as a longer call setup time, both negatively impacting the quality of service (QoS) of mobile communication and quality of experience (QoE) of the users thereof.

DETAILED DESCRIPTION

As mentioned above, there is a problem of preamble collision due to a possibly large number of user equipments (UEs) each randomly choosing one of a fixed number of preamble signatures for random access channel (RACH) process. Aspects of the present disclosure address this problem. Further details are described below.

RACH process is one of the most important steps in mobile communication protocols, including long term evolution (LTE) technology. It is the first message from a UE to a base station (hereinafter interchangeably referred to as “eNodeB”, or “eNB”) during a mobile-originating (MO) call initialization process. RACH in LTE is used to obtain uplink synchronization between UE and eNB, and also to obtain the resource for the following radio resource control (RRC)connection request.

In LTE, synchronization in downlink channels is obtained by special synchronization channels in downlink (i.e., communication link from eNB to UE). The special synchronization channels include primary synchronization signal (PSS) channels and secondary synchronization signal (SSS) channels. A downlink synchronization signal is broadcast to every UE constantly with a certain interval. In contrast, synchronization processes in uplink (i.e., communication link from UE to eNB) need to meet a different set of criteria: (1) The synchronization process should happen when there is immediate necessity; and (2) The synchronization should be dedicated to a specific UE. Such synchronization process in uplink is called “random access”. There are two types of random access processes: contention-based random access (CBRA) and contention-free random access (CFRA). CBRA and CFRA differ in a way of using a preamble signature in the RACH process, as detailed below.

When a UE transmits a RACH preamble for either CBRA or CFRA, the preamble is transmitted with a specific pattern, and this specific pattern is called a “signature”, or “preamble signature”. A mobile communication network has a plurality of base stations each serving UEs within a respective area called a “cell”. A total number of preamble signatures available for RACH processes within each cell is a fixed number, and the fixed number has a same value for each of the cells in the mobile communication network. For example, each LTE cell has a total of 64 preambles signatures available for the RACH process. Some of the 64 preambles signatures are reserved for use by CFRA, whereas the rest of the 64 preambles signatures are reserved for use by CBRA. The preamble signatures are mathematically mutually orthogonal to each other. The orthogonality of the preamble signatures ensures that two RACH processes happening at the same time within a cell would not interfere with one another as long as the two RACH processes use different preamble signatures.

Each CFRA is initiated by the network and uses a dedicated random preamble signature allocated to a specific UE. CFRA can be used as part of a handover process to gain time synchronization with a new cell. CFRA can also be used prior to downlink data transfer in RRC-connected state when the UE is not time synchronized with the network. Since CFRA is initiated by the network, the network can decide and assign which preamble signature is to be used by an eNB for a specific UE, and thus there is no risk of preamble collision.

In contrast, CBRA implies an inherent risk of preamble collision. CBRA is initiated by a Medium Access Control (MAC) layer of a UE to gain access to network (i.e., to eNB). CBRA involves the UE selecting a random-access preamble signature from a list of contention-based preamble signatures available for selection by all UEs in the cell. The list of contention-based preamble signatures is broadcast to every UE constantly with a certain interval. According to existing LTE mobile communication protocols, each UE selects a random one of the preamble signatures in the list for the RACH process. Namely, each UE may pick any of the signatures in the list with equal probability. Thus, there is a chance of two or more UEs within the cell trying to access the eNB using a same preamble signature and in a same RAS, resulting in a preamble collision. Therefore, CBRA requires additional signaling steps to resolve contention when multiple UEs attempt to access the cell using the same preamble signature at the same time.

Obviously, the additional signaling steps required for contention resolution will add to call setup time and network access time. This can be explained by an example preamble collision as described in data flow chart100ofFIG. 1A. As shown inFIG. 1A, two UEs in a cell, UE-A and UE-B, may attempt to access a base station, eNB, using a same random access slot (RAS) during timeframe101. Moreover, UE-A and UE-B may happen to select a same preamble signature, such that each of UE-A and UE-B initiates a RACH process by sending a respective preamble using the same preamble signature. The eNB would receive both the preamble111sent from UE-A and the preamble112sent from UE-B, but only respond in a subsequent timeframe102with a RACH response123based on one of the two preambles, say, preamble111sent from UE-A. Therefore, RACH response123would include information intended for UE-A but not for UE-B. For example, RACH response123may include an uplink timing adjustment information intended only for UE-A. That is, the uplink timing adjustment information would work for UE-A but not for UE-B. Since UE-A and UE-B use the same preamble signature, RACH response123would be detected by both UE-A and UE-B. Each of UE-A and UE-B would subsequently adjust its transmission timing according to the uplink timing adjustment information included in RACH response123and send out a respective RRC connection request to eNB using the adjusted timing in a subsequent timeframe103. The RRC connection request includes a terminal identity, i.e., an identifier that uniquely identifies the UE that transmits the RRC connection. That is, the RRC connection request131sent out by UE-A includes an identity of UE-A (i.e., ID-A), whereas the RRC connection request132sent out by UE-B includes an identity of UE-B (i.e., ID-B). Since the uplink timing adjustment information is intended for UE-A but not for UE-B, the RRC connection request131sent by UE-A would be successfully detected by eNB, whereas the RRC connection request132sent by UE-B would not. In a subsequent timeframe104, the eNB would then respond by confirming the identity of UE-A in an RRC setup message143sent in the downlink, which would be received by both UE-A and UE-B. RRC setup message143represents a contention resolution between UE-A and UE-B. Since the RRC setup message143contains the identity of UE-A (i.e., ID-A), UE-A would know the RACH process it initiated is successful upon receiving RRC setup message143. On the other hand, upon receiving RRC setup message143, UE-B would know the RACH process it initiated has failed, as the identity included in RRC setup message143is the identity of UE-A, not the identity of UE-B. In order to gain access to eNB, UE-B would need to initiate another RACH process in a later timeframe105by transmitting the preamble152in another RAS, which may or may not use the same preamble signature. Namely, the UE-B has initiated a RACH process that has failed due to a preamble collision, and thus has to start another RACH process to gain access to the eNB. As explained above, preamble collision does add to call setup time and network access time for UE-B, and negatively impacts the quality of service (QoS) and quality of experience (QoE) for UE-B. Clearly, the higher the probability of preamble collision within a cell, the longer the call setup time and network access time for the UEs within the cell will be, and consequently the worse the QoS and QoE of the mobile communication.

For conventional implementations of CBRA, the probability of preamble collision within a cell increases as the load of the cell becomes heavier. That is, the greater number of UEs that are served by an eNB, the more debilitating the QoS and QoE would become. Specifically, the probability of two RACH processes having a preamble collision can be expressed as

P⁡(C,⁢n)=1-C!Cn·(C-n)!(Eq.⁢1)
whereas C is the total number of preamble signatures available for CBRA in a cell, n is the number of UEs in the cell initiating RACH processes in a same RAS, and ! is the factorial operator.

Take LTE as an example. Each LTE cell has a total of 64 preambles signatures available for RACH process. Out of the 64 preamble signatures, 24 preamble signatures are reserved for CFRA processes, whereas the rest of 40 preamble signatures are available for CBRA processes. When there are five UEs in a cell initiating CBRA processes in a same RAS, the probability of two of the five UEs having preamble collision can be calculated using Eq. 1 as:

P⁡(4⁢0,5)=1-4⁢0!4⁢05·(40-5)!=2⁢2.9⁢%(Eq.⁢2)
Similarly, when there are twelve UEs in a cell initiating CBRA processes in a same RAS, the probability of two of the twelve UEs having preamble collision is:

P⁡(4⁢0,1⁢2)=1-4⁢0!4⁢01⁢2·(40-12)!=8⁢4.0⁢%(Eq.⁢3)
The increased probability (i.e., from 22.9% to 84.0%) illustrates that when the traffic becomes heavier in the cell, the probability of preamble collision increases rapidly. Moreover, when there are 23 UEs in the cell attempting to access eNB via CBRA processes in a same RAS, preamble collision is almost guaranteed to happen, as the probability of two of the 21 UEs having preamble collision increases to:

For further illustrating the LTE example whereas 40 preambles are available for CBRA processes,FIG. 1Bshows a probability curve185of a UE having a preamble collision with one or more other UEs in a cell. That is, curve185ofFIG. 1Brepresents a chance of multiple UEs in the cell initiating CBRA processes during a same RAS using a same preamble signature. As shown inFIG. 1B, the probability of preamble collision increases rapidly as the number of the UEs increases.

Accordingly, aspects of the present disclosure are directed to techniques for a UE to choose a preamble signature for CBRA process in a way that the chance of preamble collision is reduced. Rather than randomly choosing one preamble signature from the list of the preamble signatures reserved for CBRA, a UE using techniques in the present disclosure would pick a preamble signature according to a result of a modulo computation that involves a unique identifier of the UE and a size of the list (i.e., a quantity of the preamble signatures reserved for CBRA). This may result in a more uniform usage of the preamble signatures when multiple UEs attempt to access eNB at the same time, thereby reducing the chance of preamble collision.

As explained above, the ability for a UE to establish a successful communication link to a base station without having a preamble collision is crucial to the QoS and QoE of mobile communication. The techniques described in the present disclosure enable a UE to choose a preamble signature for RACH process based on a unique identifier of the UE, instead of randomly picking one preamble. When some, most, or all of the UEs served by a sector of the base station choose a preamble using the techniques described herein, the usage of the preamble signatures becomes more evenly distributed, and the chance of preamble collision is reduced. As a consequence, overall RRC success ratio for the UEs is enhanced, and the average setup time for placing a call is improved. The higher RRC success ratio and the shortened call setup time directly lead to an improved QoS and QoE of mobile communication service provided by the base station to the UEs.

Example User Equipment Components

FIG. 2illustrates a block diagram showing various components of UE200, which may be UE-A or UE-B ofFIG. 1A. As shown inFIG. 2, UE200may include one or more processors210, wireless transceiver220, UE hardware230, and memory240.

Wireless transceiver220may include a transmitter222and a receiver226that enable UE200to perform wireless communication with a base station of a mobile communications network, such as eNB ofFIG. 1A. On the transmitting end, transmitter222may enable UE200to transmit a preamble (e.g., preamble111or112ofFIG. 1A) and a RRC connection request (e.g., RRC connection request131or132ofFIG. 1A) as part of RACH process. On the receiving end, receiver226may enable UE200to receive a RACH response (e.g., RACH response123ofFIG. 1A) or a RRC setup message (e.g., RRC setup message143ofFIG. 1A) from the base station. Receiver226may also enable UE200to receive the list of preamble signatures broadcast by the base station that are reserved for CBRA. Transmitter222may include a power amplifier (PA) whereas receiver226may include a low-noise amplifier (LNA).

UE hardware230may include other hardware that is typically located in a mobile communication terminal. For example, UE hardware230may include signal converters, transceivers, antennas, hardware decoders and encoders, graphic processors, a subscriber identification module (SIM) card slot, and/or the like that enable UE200to execute applications and collaborate with wireless transceiver220to provide wireless communication capabilities.

Memory240may include programs or software procedures that, when executed by processor(s)210, enable UE200to perform various functions as described herein. As shown inFIG. 2, memory240may include an operating system241, a preamble signature cache242, UE identifier245, modulo computation module246, message size margin calculator247, pathloss margin calculator248, as well as timing adjustment module249. Operating system241may include components that manage or otherwise coordinate processor(s)210and UE hardware230with software resources to perform various functions generally associated with a terminal device of a mobile communication network.

Preamble signature cache242is configured to store the list of preamble signatures reserved for CBRA that is broadcast by the base station (e.g., the eNB ofFIG. 1A). The list of preamble signatures may be stored in a certain order or sequence that is maintained in preamble signature cache242. In some embodiments, the list of preamble signatures may include group A preamble signatures243and group B preamble signatures244. The preamble signatures in group A preamble signatures243maintain a certain order or sequence that is unchanged, whereas the preamble signatures in group B preamble signatures244also maintain a certain order or sequence that is unchanged. Based on a message size margin calculated by message size margin calculator247and a pathloss margin calculated by pathloss margin calculator248, processor(s)210may select either one of group A preamble signatures243or one of group B preamble signatures244as a chosen preamble signature. Processor(s)210may subsequently direct transmitter222to transmit a preamble using the chose preamble signature in a RAS for initiating a RACH process.

UE identifier245is an identifier that uniquely identifies UE200to other parties (e.g., base stations, core network, other networks, as well as other terminal devices or UEs) of the mobile communication network. In some embodiments, UE identifier245may be a Mobile Station International Subscriber Directory Number (MSISDN) defined by E.164 numbering plan. In some embodiments, UE identifier245may be an Integrated Circuit Card Identifier (ICCID), namely, a SIM number. MSISDNs and SIM numbers are positive integers. Typically, mobile communication operators that provides mobile communication services to users of UEs assign MSISDNs and ICCIDs in sequential orders.

Modulo computation module246is capable of executing a modulo computation between two positive integers, a dividend and a divisor, and report a remainder of the modulo computation. That is, modulo computation module246operates a division operation by dividing the dividend by the divisor, and then finds the remainder of the division operation. With the dividend represented by a and the divisor by n, expression “a modulo n” (abbreviated as “a mod n”) gives the remainder of the Euclidean division of a by n. For example, 10 mod 3 would evaluate to 1. Modulo computation module246may operate a modulo computation that involves a MSISDN or a ICCID.

Message size margin calculator247is capable of calculating a message size margin related to data to be transmitted in an uplink from UE to eNB after a communication link between UE and eNB is established as a result of a CBRA process. Specifically, the message size margin is defined by how much less a sum of the size of the data to be transmitted, a size of a MAC header, and a size of MAC control elements is as compared to a message size threshold. The message size threshold is a cell parameter and is applicable to all CBRA attempts initiated within the cell. If the sum is less than the message size threshold, the message size margin is determined as positive. Namely,message size margin is positive ifsize of {data+MAC header+MAC control elements}<message size threshold

Pathloss margin calculator248is capable of calculating a pathloss margin related to a pathloss of the uplink from UE to eNB. The pathloss margin is related to the following parameters: (1) UplinkPathLoss, which represents reduction in power density from UE to eNB caused by propagation loss; (2) preambleInitialReceivedTargetPower, which represents an initial transmit power of the preamble; (3) deltaPreambleMsg3, which represents a nominal power used for uplink transmission; (4) messagePowerOffsetGroupB, which represents a pathloss threshold offset; and (5) PCMAX, which is a preamble power threshold that represents a maximum output power by each antenna of the cell, referenced at an antenna connector thereof. Specifically, the pathloss margin is defined by how much less a sum of UplinkPathLoss, preambleInitialReceivedTargetPower, deltaPreambleMsg3, and messagePowerOffsetGroupB is as compared to PCMAX. The preamble power threshold, PCMAX, is a cell parameter and is applicable to all CBRA attempts initiated within the cell. If the sum is less than the preamble power threshold, the pathloss margin is determined as positive. Namely,pathloss margin is positive if{UplinkPathLoss+preambleInitialReceivedTargetPower+deltaPreambleMsg3+messagePowerOffsetGroupB}<PCMAX

Timing adjustment module249is configured to adjust a timing of messages transmitted by transmitter222from UE to eNB. For example, timing adjustment module249may adjust the timing of transmitting the signal according to the uplink timing adjustment information included in RACH response123ofFIG. 1A. A main purpose of the timing adjustment is to compensate for propagation delay in the uplink so that the timing of the transmitted signal as received at eNB will better align with the timing of the mobile communication system.

Example Processes

FIG. 3Apresents an illustrative process300for a UE to select a preamble signature and initiate a random access procedure in an attempt to establish a communication link with an eNB.FIG. 3Bpresents an illustrative process333for a UE to actually establish the communication link with the eNB through CBRA. Specifically, process333may be utilized in conjunction with process300, where process300includes selecting a preamble signature and initiating a random access procedure, whereas process333includes establishing the communication link utilizing the selected preamble signature.FIG. 4presents an example process400for a UE to select or otherwise designate a preamble signature for CBRA process300. Each of processes300,333, and400are illustrated as a collection of blocks in a logical flow chart, which represents a sequence of operations that can be implemented in hardware, software, or a combination thereof. In the context of software, the blocks represent computer-executable instructions that, when executed by one or more processors, perform the recited operations. Generally, computer-executable instructions may include routines, programs, objects, components, data structures, and the like that perform particular functions or implement particular abstract data types. The order in which the operations are described is not intended to be construed as a limitation, and any number of the described blocks can be combined in any order and/or in mirror to implement the process. For discussion purposes, the processes300,333and400are described with reference to data flow chart100ofFIG. 1Aand UE200ofFIG. 2.

FIG. 3Ais a flow diagram of an example process300for a UE to initiate a random access procedure in an attempt to establish a communication link with a base station through CBRA. Process300may begin at block310.

At block310, UE200may identify a quantity of a plurality of preamble signatures reserved for CBRA to the base station. Specifically, the base station may broadcast a list of the preamble signatures to all UEs within the cell, including UE200. UE200may receive the list via receiver226of wireless transceiver220. UE200may subsequently store the list of the preamble signatures in preamble signature cache242. The size of the list indicates the quantity of the preamble signatures. UE200may identify the quantity of the preamble signatures reserved for CBRA from the size of list received by receiver226and subsequently stored in preamble signature cache242. As described above, for a LTE network, there may be sixty-four (64) preamble signatures, forty (40) of which may be reserved for CBRA. A list of those 40 preamble signatures would be broadcast by eNB, and UE200would receive the list and save it in preamble signature cache242. UE200would then identify that 40 is the quantity of the preamble signatures reserved for CBRA by examining preamble signature cache242that saves the list of the 40 preamble signatures. Process300may proceed from block310to block320.

At block320, modulo computation module246of UE200may perform a modulo computation based on a unique identifier of UE200(i.e., UE identifier245). Specifically, modulo computation module246may perform a modulo computation of “(UE identifier245) mod (quantity of preamble signatures stored in preamble signature cache242)” to find a remainder thereof. For example, if UE identifier245of UE200is a MSISDN, “5022409139”, and the quantity of the preamble signatures identified in block310is forty (40), then UE200may perform the modulo computation “5022409139 mod 40”, which results in a remainder of nineteen (19). Process300may proceed from block320to block330.

At block330, UE200may designate one of the preamble signatures stored in preamble signature cache242as a chosen preamble signature based on the remainder obtained by modulo computation module246in block320. As described above, the preamble signatures are stored in preamble signature cache242and maintain a certain order or sequence. Therefore, UE200may pick a preamble signature in the sequence based on the remainder. For example, if 40 preamble signatures are stored in preamble signature cache242and maintained in a sequence, given that the remainder is 19, UE200will designate the 19thpreamble signature in the sequence as the chosen signature. Process300may proceed from block330to block340.

At block340, UE200may initiate a CBRA process using the chosen preamble designated in block330to establish a communication link with the base station. For example, UE200, as either UE-A or UE-B ofFIG. 1A, may initiate a CBRA process to transmit preamble111or112to eNB in a RAS during timeframe101via transmitter222. Process300may proceed from block340to block350.

FIG. 3Bis a flow diagram of an example process333for a UE to establish a communication link with an eNB through CBRA. Process333may begin at block350in response to the initiating process block340ofFIG. 3A.

At block350, UE200may receive a random access response from the base station via receiver226. For example, UE200, as UE-A or UE-B ofFIG. 1A, may receive RACH response123from eNB. Process333may proceed from block350to block360.

At block360, UE200may transmit a RRC connection request to the base station via transmitter222. For example, UE200, as UE-A or UE-B ofFIG. 1A, may transmit RRC connection request131or132to eNB. Process333may proceed from block360to block370.

At block370, UE200may receive a contention resolution from the base station indicating whether the communication link is successfully established. For example, UE200, as UE-A or UE-B ofFIG. 1A, may receive RRC setup message143as a contention resolution. Process333may proceed from block370to block380.

At block380, UE200may determine whether the communication link is successfully established based on the contention resolution received in block370. As described above, since RRC setup message143confirms the identifier of UE-A rather than the identifier of EU-B, the contention resolution indicates that the CBRA process initiated by UE-A during timeframe101is successful, whereas the CBRA process initiated by UE-B during timeframe101has failed. That is, UE200, as UE-A ofFIG. 1A, may determine that a communication link between UE-A and eNB is successfully established based on RRC setup message143. However, UE200, as UE-B ofFIG. 1A, may determine that a communication link between UE-B and eNB has failed to establish, based on the same RRC setup message143. As described above, UE-B may initiate another RACH process in a following RAS, sending preamble152in a RAS during timeframe105ofFIG. 1A. In some embodiments, UE200may transmit preamble152using the same chosen preamble signature designated in block330ofFIG. 3A. In some embodiments, UE200may transmit preamble152using a different preamble signature in preamble signature cache242as the chosen signature. In summary, process333may proceed from block380to block385in an event that UE200determines that a communication link to the base station is successfully established. Process333may proceed from block380to either block390(i.e., as shown inFIG. 3B) or block340(i.e., an alternative embodiment not shown inFIG. 3B) in an event that UE200determines that a communication link to the base station has failed to establish.

At block385, UE200may communicate with the base station using the communication link established via wireless transceiver220.

At block390, UE200may designate a preamble signature in preamble signature cache242that is different from the previous chosen preamble signature designated in block330and used in block340. Process333may proceed from block390to block340ofFIG. 3A.

FIG. 4is a flow diagram of an example process400for performing blocks310,320and330of process300and process333under a scenario where there are two mutually exclusive groups of preamble signatures reserved for CBRA, such as group A preamble signatures243and group B preamble signatures244stored in preamble signature cache242ofFIG. 2. Process400illustrates how UE200may identify the quantity of the preamble signatures (i.e., block310), perform the modulo computation (i.e., block320), and select the chosen preamble signature (i.e., block330) under the scenario of two mutually exclusive groups of preamble signatures. UE200may select the chosen preamble from one of the two groups depending on a size of data to be transmitted in the uplink (e.g., the data transmitted in process block385), a pathloss of the uplink, or both. This involves message size margin calculator247to calculate a message size margin and pathloss margin calculator248to calculate a pathloss margin. Process400may begin at block410.

At block410, UE200may identify a quantity of the preamble signatures in each of the two groups. UE200may identify a first quantity which is the total number of preamble signatures in group A preamble signatures243, as well as a second quantity which is the total number of preamble signatures in group B preamble signatures244. As mentioned above, each LTE cell may have 40 preamble signatures reserved for CBRA. For example, if the 40 preamble signatures include 30 preamble signatures in Group A and 10 preamble signatures in Group B, then UE200will identify the first quantity as 30 and the second quantity as 10. Process400may proceed from block410to block420.

At block430, UE200may determine whether the message size margin is positive or not. As described above, message size margin is determined to be positive if the first sum is less than the message size threshold. In an event that the message size margin is positive, process400may proceed from430to450. In an event that the message size margin is not positive, process400may proceed from block430to block440.

At block440, UE200may determine whether the pathloss margin is positive or not. As described above, pathloss margin is determined to be positive if the second sum is less than the preamble power threshold. In an event that the pathloss margin is positive, process400may proceed from440to460. In an event that the pathloss margin is not positive, process400may proceed from block440to block450.

At block450, modulo computation module246of UE200may perform a modulo computation based on a unique identifier of UE200, i.e., UE identifier245, as well as the first quantity. Specifically, modulo computation module246may perform a modulo computation of “(UE identifier245) mod (first quantity)” to find a remainder thereof. For example, if UE identifier245of UE200is a MSISDN, “5022409139”, and the first quantity of the preamble signatures identified in block310is thirty (30), then UE200may perform the modulo computation “5022409139 mod 30”, which results in a remainder of twenty-nine (29). Process400may proceed from block450to block470.

At block460, modulo computation module246of UE200may perform a modulo computation based on a unique identifier of UE200, i.e., UE identifier245, as well as the second quantity. Specifically, modulo computation module246may perform a modulo computation of “(UE identifier245) mod (second quantity)” to find a remainder thereof. For example, if UE identifier245of UE200is a MSISDN, “5022409139”, and the second quantity of the preamble signatures identified in block310is ten (10), then UE200may perform the modulo computation “5022409139 mod 10”, which results in a remainder of nine (9). Process400may proceed from block460to block480.

At block470, UE200may designate one of group A preamble signatures243as a chosen preamble signature based on the remainder obtained by modulo computation module246in block450. As described above, group A preamble signatures243are stored in preamble signature cache242and maintain a certain order or sequence. Therefore, UE200may pick a preamble signature in the sequence based on the remainder. For example, if group A preamble signatures243has 30 preamble signatures that are stored in a sequence, given that the remainder is 29, UE200will designate the 29thpreamble signature in the sequence as the chosen signature.

At block480, UE200may designate one of group B preamble signatures244as a chosen preamble signature based on the remainder obtained by modulo computation module246in block460. As described above, group B preamble signatures244are stored in preamble signature cache242and maintain a certain order or sequence. Therefore, UE200may pick a preamble signature in the sequence based on the remainder. For example, if group B preamble signatures243has 10 preamble signatures that are stored in a sequence, given that the remainder is 9, UE200will designate the 9thpreamble signature in the sequence as the chosen signature.

In the scenario where there are two mutually exclusive groups of preamble signatures reserved for CBRA, process300and process333may replace blocks310,320and330with process400to designate the chosen preamble signature.

Conclusion

The ability for a UE to establish a successful communication link to a base station without having a preamble collision is crucial to the QoS and QoE of mobile communication. The techniques described in the present disclosure enable a UE to choose a preamble signature for RACH process according to a modulo computation that involves a unique identifier of the UE. The selection of the preamble signature is based on a remainder of the modulo computation. When all or most of the UEs served by a sector of the base station choose a preamble using the techniques described herein, the usage of the preamble signatures becomes more evenly distributed, and the chance of preamble collision is reduced. As a consequence, overall RRC success ratio for the UEs is enhanced, and the average setup time for placing a call is improved. The higher RRC success ratio and the shortened call setup time directly lead to an improved QoS and QoE of mobile communication service.