Special Feature

Wireless QoS Control Technologies
Takeshi Yamashita†, Hidetoshi Kayama, Lan Chen,
Daisuke Kitazawa, Masayuki Motegi, and Narumi Umeda
Abstract
In the fourth-generation (4G) mobile communication system, all communications including realtime
communications is expected to be carried by packets multiplexed on a shared radio channel to improve
the radio resource efficiency. Because the delay characteristics and the throughput of a packet radio
channel vary greatly with changes in both traffic volume and radio channel quality, NTT DoCoMo proposes novel wireless QoS (quality of service) control technologies to guarantee QoS requirements andachieve highly efficient realtime communication with a wide range of transmission rates.

1. Introduction
The fourth-generation (4G) mobile communication
system is expected to provide a wide variety of applications and services through high-data-rate radio
channels. All communications including realtime
communications such as VoIP, videophones, video
streaming, and digital TV broadcasting, will be packet multiplexed on a shared radio channel to improve
the radio resource efficiency. These communication
services have individual requirements for bandwidthand acceptable transmission delay, i.e., quality of service (QoS). However, packet radio systems sufferfrom drastic changes in the transmission delay andchannel throughput when packet traffic and radio
channel conditions fluctuate. To handle these dynamic conditions, a key technology for implementing thesystem is wireless QoS control for efficient wirelesspacket multiplexing that takes into account variations
in radio propagation and interference while satisfying
the QoS requirements of various applications [1], [2].
This article describes four novel wireless QoS control
technologies suitable for high-speed packet communication systems: radio-condition-aware admission
control, multistage hybrid scheduling, reservation† NTT DoCoMo, Inc.
Yokosuka-shi, 239-8536 Japan
E-mail: t_yamasita@nttdocomo.co.jp

32

based reverse channel access protocol, and adaptive
battery conservation management. We are implementing these technologies in a prototype 4G system.
2. Features of wireless QoS control
Wireless QoS control is designed to provide the following features.
(1) Guaranteed and best-effort services
Realtime communications has strict requirements regarding the bandwidth and acceptabletransmission delay. To cope with these applications, wireless QoS control provides guaranteedservices that can provide a flexible guaranteedbandwidth and delay suitable for realtime communications. Best-effort services will also be provided for non-realtime communications and these
services will take into account relative packet priorities with/without radio channel conditions.
(2) Smooth working with IP-QoS
By assigning guaranteed-class packets to the
expedited forwarding (EF) class for the differentiated services code point (DSCP) and best-effortclass packets to the best effort (BE) class for the
DSCP, we ensure that wireless QoS control canwork smoothly with Internet protocol (IP) QoS.
(3) Area-free guaranteed services
To improve radio channel efficiency, the adaptive modulation and coding (AMC) scheme is
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Special Feature

Table 1. Examples of services provided according to wireless QoS control.
Service/applied technology

Guaranteed-class services

Services• Guaranteed-rate service is provided
regardless of variation in interference anduser location.
• Guaranteed rate is set flexibly
considering a variable rate and the user
maximum and minimum requirements.

Best-effort service

Guaranteed QoSTransmission rate and delay

Target application

Realtime application

Non-realtime application

Applied (QoS request and radio conditions
are considered.)

Not applied

Admission control
Technologies

Best-effort-class services

Multistage
hybridscheduling

Scheduling between
guaranteed andbest-effort classes

Priority control is applied in the order: packets within the guaranteed rate, best-effort
packets, and packets that exceed the guaranteed rate.

Scheduling between
same-class users

Guaranteed rate, admitted delay, fairness,
etc. are considered.

assumed to be used in 4G. This scheme changes
the transmission rate according to the signal tointerference-plus-noise ratio (SINR). However,from the user’s viewpoint, the rate should be constant regardless of his/her location. Our radiocondition-aware admission control enables stableand guaranteed services with a flexible guaranteed rate to be provided to realtime users regardless of the variation in interference and the location of the mobile station. Examples of services
that can be provided depending on the type of
wireless QoS control used are shown in Table 1.

Fairness and radio conditions are considered.

3. Architecture of packet transmission using
wireless QoS control

ing their relative priorities. In multistage hybrid
scheduling, guaranteed scheduling performs traffic
shaping to guarantee both the admitted rate and delay
requirements of guaranteed-class users, while besteffort scheduling considers radio resource efficiency
and fairness among users and classes. In addition,
multistage scheduling gives priority to guaranteedclass users. Transmission parameters are determinedaccording to the QoS class and channel conditions
based on information sent from the receiver. In thedownlink, radio resources can be allocated in astraightforward scheme. In the uplink (reverse link),a reservation-based access protocol is employed toenable priority-based resource allocation required forthe wireless QoS control technology (See section 4.).

Wireless QoS control aims to provide a range ofguaranteed-rate and area-free realtime communication services regardless of the packet traffic and radio
condition fluctuations. Smooth working with IP-QoS
and channel efficiency are also taken into account.
Based on this concept, we propose a novel wireless
QoS control [3]. The architecture for packet transmission is shown in Fig. 1. It comprises several control factors for the medium access control (MAC) andradio resource control (RRC) layers and a radio
resource control factor over these two layers. These
factors can work smoothly with IP-QoS. IP packets
are first mapped to the best-effort or guaranteed classes. For guaranteed-class users, radio-condition-aware
admission control is carried out to avoid an overload
among these users. For best-effort-class users, classification is carried out according to the DSCP imply-

3.1 Radio-condition-aware admission control (RAC)
These days, many communication applications
have functions for responding flexibly to changes intransmission rates by using variable rate coding.
Therefore, the guaranteed rate can be set flexibly
within a variable rate range, that is the maximum andminimum requirements. This flexible admission control allows more users to be accommodated. Making
allowances for user movement and variations in interference, the lowest modulation coding set is used as
the channel capacity criterion for admission control
decision, as indicated in Fig. 2(a), which shows thetransmission rate for three different modulation coding sets (MCSs). To utilize the statistical multiplexing
characteristics of packet communications for radio
resource efficiency, the resource utilization rate of theguaranteed-class users is also considered. However,Vol. 2 No. 9 Sep. 2004

33

Special Feature

IP

Diffserv

Mapping
Guaranteed-class packets

Best-effort-class packets

Radio resource management

Radio-condition-aware
admission control

Classification

RRC

Multistage hybrid scheduling
Guaranteed scheduling

Best-effort schedulingMultistage scheduling
MAC
Transmission parameter selection

Resource allocation

Packet transmission

Fig. 1. Architecture of packet transmission.

Transmission rate
High-rate MCS

Reference capacity(y Mbit/s)

Low-rate MCS

SINR(a) Variation in transmission rate in adaptive modulation

Realtime
measured traffic (x Mbit/s) Acceptable rate

Margin (α)
Reference capacity (y Mbit/s)(b) Acceptable rate

Fig. 2. Principle of radio-condition-aware admission control.

not all the guaranteed-class users encounter poor conditions. To improve the resource utilization efficiency, the measured resource utilization rate for guaranteed-class users is considered, as shown in Fig. 2(b).
Here, the total of the bandwidth requests that can be
accepted (i.e., the acceptable rate) is given by the reference capacity (y) minus the measured resource utilization rate (i.e., the realtime measured traffic (x))
and a margin (α). That is, the acceptable rate = y - (x+ α).
3.2 Multistage hybrid scheduling (MHS)The scheduling that determines the transmission
34

order when all user packets are multiplexed onto one
common radio channel is important for maintaining
service quality and supporting various applications
and service classes. Furthermore, in wireless communication systems, an important purpose of schedulingis to increase the efficiency of radio resource utilization. There have been many proposals for schedulingthat takes into account QoS requests [4] or that aims
to increase the radio resource utilization efficiency[5]. However, in the 4G system, there are demands toprovide a range of guaranteed rates and maintain theservice quality for realtime communications without
deteriorating the radio resource utilization efficiency.
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Special Feature

EF

BE

Guaranteed class1

2

Best-effort classnn+1 n+2nBuffer per user

Buffer per user or class
Guaranteed scheduling

Best-effort schedulingMultistage schedulingFig. 3. Principle of multistage hybrid scheduling.

To satisfy these demands, multistage hybrid scheduling (MHS) has been proposed [6], [7]. Its principle isshown in Fig. 3. Here, we explain the downlink transmission case. As described above, each packet isclassified as either a guaranteed- or best-effort-classpacket; that is, IP packets with a header of EF and BE
are mapped into the guaranteed and best-effort classes, respectively. Either each user has his/her own
individual buffer or best-effort-class packets with thesame DSCP can share a buffer because, from theviewpoint of the QoS requirement, flow control basedon the guaranteed rate and acceptable delay are necessary for individual guaranteed-class users, whileclass-based flow control is sufficient for best-effortclass users. However, for scheduling that takes into
account the radio resource utilization efficiency, it isnecessary to consider the radio propagation conditions of each user, and individual buffers are required
not only for the guaranteed-class users, but also forbest-effort-class users.
In MHS, a different scheduling scheme is used
between the guaranteed- and best-effort-class users.
Guaranteed scheduling considers guaranteeing boththe admitted rate and delay requirements of guaranteed-class users, while best-effort scheduling considers the radio resource efficiency and fairness among
all best-effort-class users. Examples of schedulingapplicable to both types of users are described below.
(1) Guaranteed class• Weighted round robin (WRR): Each user inturn sends a weighted number of packets.
• Header early first (HEF) [8]: A time stamp is
attached to each packet. The time stamps of thepackets at the tops of the buffers are comparedVol. 2 No. 9 Sep. 2004

and the packet with the earliest time stamp isscheduled first.
In both of these methods, the number of packets transmitted within one turn depends on theguaranteed rate declared by the users.
(2) Best-effort class• Proportional fairness (PF) [9]: The ratio of
instantaneous SINR to averaged SINR is measured for each user. The packets are scheduled
based on this ratio in order from highest to lowest. This method takes into account the radio
resource efficiency and fairness among users.
• Class-based queuing (CBQ): When users of thesame DSCP class share a buffer and DSCP
class-based QoS control is used, weights are
assigned to each class according to its priorityand packets are taken from the buffers according to their weights.
(3) Scheduling between guaranteed and best-effort
classes (multistage scheduling)
In multistage scheduling, guaranteed-classpackets within the guaranteed rate are selected
first. Then, packets of best-effort-class users are
selected for transmission. Finally, packets thatexceed the guaranteed rate are selected.
Resources are assigned on a block basis, and in
the case of a guaranteed-class user, the guaranteed rate is also considered during resource allocation. Here, the transmission block size of a user
is determined according to his/her radio channel
conditions using a channel quality indicator during transmission.

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3.3 Performance of RAC and MHS
Computer simulation results for the satisfaction
rate for guaranteed-class users and the transmission
rate for best-effort-class users are shown in Fig. 4. In
these figures, round robin (RR) indicates the performance of a method without RAC and MHS, while
priority scheduling (PS) indicates the performance ofa method that gives unconditional priority to the guaranteed-class users. We can see that wireless QoS canincrease the satisfaction rate of guaranteed-classusers without greatly degrading the transmission rate
of best-effort-class users by setting the guaranteed
rate dynamically within the range between the maximum and minimum rates requested by users and by
performing scheduling that takes into consideration
the guaranteed rates. RAC and MHS make it possible
to aim for efficient utilization of the radio resources
while guaranteeing the delay and transmission rate
for guaranteed-class users. These methods can simultaneously maintain some degree of fairness among
best-effort-class users by taking into account theradio resource efficiency and fairness [3], [6]-[8].

ence. As a result, each realtime packet may suffer anunacceptable transmission delay caused by a random
access retry under relatively heavy traffic conditions.
According to the wireless QoS architecture (See section 3.), all packets are categorized into two classes:
guaranteed and best-effort. Realtime packets are categorized into a guaranteed class, in which the transmission rate and delay are guaranteed by RAC and
MHS. Incidentally, in MHS, the guaranteed-classpackets that are below the guaranteed rate are always
transmitted before best-effort-class packets. In thedownlink channel, the transmission order of the packets can be easily arranged because all packets areassembled at a base station and arranged by centralized control. On the other hand, packets in the reverse
channel are not; they are accumulated in distributed
queues from individual mobile stations. To satisfy the
QoS requirement for realtime communications and toincrease the radio resource utilization efficiency insuch a distributed queue environment, two schemes
have been proposed.
(1) Prioritized resource allocation (PRA) [11]This is a reservation-based scheme. Two different resource allocation areas in the time domain
are established for the guaranteed and best-effort
classes. The guaranteed-class packets have awider allocation area so that they can obtain radio
resources before the best-effort-class packets can.
With this scheme, almost all the bandwidth can
be occupied by guaranteed-class packets. Figure
5 shows an example of the time slot allocation in
the PRA scheme.
(2) Reservation-based random access protocol with atemporarily assigned dedicated control channel

4. Reservation-based reverse channel accessprotocol [10]For downlink packet transmission, resource allocation on a packet-by-packet basis is relatively easy
because the base station can manage all the transmissions on the downlink packet channel. On the other
hand, packets in the reverse channel are generallytransmitted in a random access manner. In this manner, a massive transmission with the same timing
causes packet loss due to multiple access interfer-

120

Satisfaction rate (%)

90
80
70
60

Wireless
QoS

50
40
30
0

PS
RR

Rv = 0.5
102030
Number of users per cell40(a) Satisfaction rate for guaranteed-class users

Transmission rate (kbit/s)

100

100
80
60
RR
Wireless
QoS
PS

40
20
Rv = 0.5
0
0

10

20
30
Number of users per cell40(b) Transmission rate for best-effort-class users
Rv = 0.5 means 50% of all users are guaranteed-class users.

Fig. 4. Performance of RAC and MHS.

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Special Feature

Guaranteed class
Best-effort class(Allocation not possible)(Allocation possible)

Allocated
Time
Time slots allocatable tobest-effort-class packets
Time slots allocatable to guaranteed-class packets
Initial allocatable pointerFig. 5. Example of time slot allocation of PRA.

(RAP-TDC) [12], [13]In this protocol, only a low-rate temporarily
dedicated control channel (TDCCH) is “temporarily” assigned between a base station and amobile station during a packet burst. In this channel, reservation signals for access and transmission control parameters are transmitted simultaneously. After a reservation has been accepted,
user packets are transmitted on assigned common
packet channel (CPCH) slots. This scheme prevents the packet-by-packet setup delay by maintaining transmission parameters using the
TDCCH. In addition, the mobile station cantransmit its reservation signal without any conflict with other reservation signals, so this scheme
enables the short delay packet transmission that isrequired for realtime communications. The interleave reservation mechanism that enables us toremove the channel inefficiency caused by theround trip delay of the reservation protocols is
also employed. In addition, because the reservation signal is transmitted without collisions, when
the reservation is denied because of a temporarylack of resources, the terminal can continuously
retry the request. Transmission control signals,
such as the transmission power control (TPC) command, channel quality indicator, and acknowledgment signals, are continuously exchanged in the
TDCCH to enable immediate packet (re)transmissions in both directions. If no packets aretransmitted in the CPCH during a predetermined
time, which is referred to as the release delay time
below, then the TDCCH is released. It takes a little time to reestablish the TDCCH, so the releaseVol. 2 No. 9 Sep. 2004

delay time should be adaptively set according to
the QoS requirement of the transmitted packets.
In addition, it is effective to coordinate the release
delay time with a dormant control timer such as
the timer described in section 5.
5. Adaptive battery conservation management
In existing mobile communication systems, amobile station intermittently receives paging signalsthat are periodically transmitted by a base station during its stand-by time (which is defined as the idle
mode in this paper). This scheme is generally used forbattery conservation. Moreover, since the terminationof a session can be clearly defined in a connection oriented manner, the mobile station can immediatelytransit to the idle mode after a session ends. On theother hand, in connectionless communications, session initiation and termination are generally indistinct, so it is effective to employ a timer that is initiated after each packet has been received or transmitted. When this timer expires, it initiates the transition
to idle mode. When a packet addressed to a mobile
station that is in idle mode reaches a base station, thebase station informs the mobile station by sending
packet arrival notifications (i.e., by paging it) using aperiodically transmitted signal. Therefore, thisscheme involves a wake-up delay. In the conventional battery conservation method, because there is only
one timer for entering idle mode and one paging
interval, the requirement for the acceptable transmission delay of realtime communications cannot be satisfied when applying this method to the 4G system.
To overcome these problems, adaptive battery con37

Special Feature

Activemode

Battery saving
mode
BSM-EF

BSM-BE

Idle mode

State in which the control signal from the base station is received intermittently

Fig. 6. State transitions in adaptive battery conservation management.

servation management (ABCM), which takes into
consideration the QoS requirements, has been proposed for connectionless communication systems
such as the 4G system [14].
In packet communications, the communication
state of the mobile station can be classified into threestates where i) packets are transmitted and received
continuously, ii) no packets are transmitted or
received for a long time, and iii) no packets are transmitted or received for a short time. ABCM pays attention to this feature, and a new state called battery saving mode is defined to improve battery conservation.
The ABCM state transition diagram for the mobile
station is shown in Fig. 6. Three mobile station states
are defined: the active, idle, and battery saving
modes. For the battery saving mode, two submodes(BSM-EF and BSM-BE) are defined that correspondto the two QoS classes. The guaranteed-class mobile
station transits to BSM-EF and the best-effort-classmobile station transits to BSM-BE. The transition to
the corresponding state is triggered by the transmission and reception of packets or by the expiration ofa timer. The battery saving mode is an intermediate
state between the active and idle modes, and the control signal from the base station is received intermittently, as in the idle mode. The difference between thebattery saving and idle modes is that the intermittent
receiving cycle is shorter in the former, and BSM-EF
is assigned a shorter period than BSM-BE because it
has stricter delay requirements. Furthermore, thetimer for the transition from active mode to batterysaving mode is also set corresponding to the guaranteed and best-effort classes, so the timer for the guaranteed class is longer than that for the best-effort
class. This method makes it possible to guarantee the38

packet transmission delay for the guaranteed class,although battery conservation is equivalent to that of
the conventional methods without an intermediate
state like the battery saving mode. For the best-effort
class, on the other hand, effective battery conservation is possible (although an increase in delay equivalent to at most the intermittent receiving cycle isunavoidable). Simulation results indicate that ABCM
achieves better battery conservation than conventional methods.
6. Conclusion
This article described some wireless QoS control
technologies that have been proposed for the 4G system and are currently under evaluation. In the future,
we plan to evaluate them in more detail by simulationand test their compatibility and effectiveness for various applications in a radio environment using a prototype system.
References
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[2]

[3]

[4]
[5]

[6]

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S. Ohmori, Y. Yamao, and N. Nakajima, “The future generations ofmobile communications based on broadband access technologies,”
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L. Chen, H. Kayama, Y. Kawabe, and N. Umeda, “Performance of a
Novel Wireless QoS Proposed for Broadband CDMA Packet Communication Systems,” IEEE Proc. VTC-2004 Spring, 2004.
P. Ferguson and C. Huston, “Quality of Service,” John Wiley & Sons,
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Y. Cao and V. O. K. Li, “Scheduling Algorithms in Broad-Band
Wireless Networks,” Proceedings of IEEE, Vol. 89, No. 1, pp. 76-87,
Jan. 2001.
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Conference, B-5-138, 2003.
L. Chen, H. Kayama, and N. Umeda, “Radio Condition Aware
Admission Control and Hybrid Scheduling for High Speed CDMA
Packet Cellular System,” IEICE 2003 Annual Conference, B-5-139,
2003.
[8] D. Kitazawa, L. Chen, H. Kayama, and N. Umeda, “A Study on Packet Scheduling Considering Buffering Delay for High Speed CDMA
Cellular System,” IEICE 2003 Annual Conference, B-5-140, 2003.
[9] A. Jalali, R. Padovani, and R. Pankaj, “Data Throughput of CDMAHDR a High Efficiency, High Data Rate Personal Communication
Wireless System,” IEEE Proc. VTC-2000 Spring, 2000.
[10] H. Kayama, H. Qiu, and N. Umeda, “Prioritized Reservation Type
MAC Protocol for Broadband CDMA Packet Communication Systems,” IEEE Proc. VTC-2004 Spring, 2004.
[7][11] H. Kayama, H. Qiu, and N. Umeda, “Prioritized Resource Allocation(PRA) for high-speed uplink packet channel,” IEICE 2003 Annual
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[12] M. Nagatsuka, Y. Ishikawa, J. Hagiwara, T. Nakamura, and S. Onoe,“Capacity evaluation of a Media Access Control method in WCDMA Packet Mobile Communication,” IEICE 1998 Annual Conference, B-5-164, 1998.
[13] H. Qiu, H. Kayama, and N. Umeda, “A Study on Reservation AccessProtocol with Dedicated Control Channel (RAP-DC) in High-speed
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IEICE Technical Reports, NS2001-242 IN2001-198, Mar. 2002.

Takeshi Yamashita

Daisuke Kitazawa

Research Engineer, Communication Systems
Laboratory, Wireless Laboratories, NTT DoCoMo, Inc.
He received the B.E. and M.E. degrees from
Nagaoka University of Technology, Nagaoka,
Niigata in 1996 and 1998, respectively. During
1998-2002, he was with NTT DoCoMo, Inc.,
Radio Network Development Department, where
he was involved in the development and study ofmobile satellite communication systems. Since
2002, he has been with the Communication Systems Laboratory of the Wireless Laboratories.
His research interests include handoff, wireless
QoS, and moving networks. He is a member of
the Institute of Electronics, Information andCommunication Engineers (IEICE).

Corporate Strategy & Planning Department,
NTT DoCoMo, Inc.
He received the B.S. and M.S. degrees in communication engineering from Osaka University,
Suita, Osaka in 1998 and 2000, respectively. At
university, he studied mobile communication
systems, multi-layer (hierarchical) cellular systems, and software-based multimedia communication systems. He was especially interested in
Software Radio and he devoted two years toward
his masters degree to improve the system capacity of a software-based mobile communication
system using two types of access schemes,
TDMA and CDMA. He made two presentations
about his research to IEICE. In 2000, he joined
the Wireless Laboratories, NTT DoCoMo, Inc.
and researched beyond-IMT-2000 (4G) mobile
communication systems, especially packet
scheduling, admission control, and radio
resource management. In 2003, he moved to thecorporate strategy & planning department.

Hidetoshi Kayama
Executive senior researcher, Director of Innovative Radio Transmission Laboratories, DoCoMo Beijing Communications Laboratories.
He received the B.S. and M.S. degrees in electrical engineering and the Ph.D. degree in informatics from Kyoto University, Kyoto in 1987,
1989, and 2004, respectively. In 1989, he joined
NTT Radio Communication System Laboratories, where he engaged in R&D of PHS packet
data systems, 19-GHz WLANs, and 802.11a.
From 1998 to 2004, he was with NTT DoCoMo,
Inc. where he engaged in R&D of PHS and 4G
mobile communication systems. His researchinterests include access protocols, QoS control,
system architecture, and transmission technologies for packet radio systems. He is a member of
IEICE and IEEE. He received the scholarshipencouragement award from IEICE in 1995 andthe best paper award of the International Conference on Telecommunications at ICT2002.

Masayuki Motegi
Radio Network System Group, IP Radio Network Development Department, NTT DoCoMo,Inc.
He received the M.E. degree in electrical andcomputer engineering from Yokohama National
University, Yokohama, Kanagawa in 2001 andthe same year he joined the Wireless Laboratories, NTT DoCoMo, Inc., Yokosuka, Kanagawa.
In 2004, he moved to the IP Radio Network
Development Department. He is a member of
IEICE.

Lan Chen

Narumi Umeda

Director, Advanced Radio System Lab, Executive senior researcher, DoCoMo Beijing Communications Laboratories.
She received the M.S. and Ph.D. degrees incommunications and computer engineering of
Kyoto University, Kyoto. She joined the Wireless Laboratories of NTT DoCoMo in 1999,
where she researched wireless QoS control andradio resource management for 4G and beyond.
Her research interests include wireless QoS,
radio resource management, packet scheduling,
and admission control. She received the best
paper award at ICT2002. She is a member ofIEEE and IEICE.

Director, Communication Systems Laboratory,
Wireless Laboratories, NTT DoCoMo, Inc.
He received the B.E. and M.E. degrees in electronic engineering from Hokkaido University,
Sapporo, Hokkaido in 1985 and 1987, respectively. He joined NTT Laboratories in 1987. In
1992, he transferred to NTT Mobile Communications Network, Inc. (now NTT DoCoMo, Inc.).
He has been engaged in research on the radio linkcontrol for the personal digital cellular (PDC)
system and IMT-2000. His current researchinterests are in the radio control for 4G mobile
communications systems (beyond 3G). He was aco-recipient of the Japan Institute of Invention
and Innovation (JIII) Imperial Invention Prize in1998 and the best paper award at ICT2002. He isa member of IEICE and IEEE.

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