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Cross-Layer Design for Resource Allocation in 3G Wireless ...
Cross-Layer Design for Resource Allocation in 3G Wireless ...
Cross-Layer Design for Resource Allocation in 3G Wireless ...
Cross-Layer Design for Resource Allocation in 3G Wireless ...
Cross-Layer Design for Resource Allocation in 3G Wireless ...
Cross-Layer Design for Resource Allocation in 3G Wireless ...
Cross-Layer Design for Resource Allocation in 3G Wireless ...
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Cross-Layer Design for Resource Allocation in 3G Wireless ...

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  • 1. CROSS-LAYER DESIGN Cross-Layer Design for Resource Allocation in 3G Wireless Networks and Beyond Hai Jiang, Weihua Zhuang, and Xuemin (Sherman) Shen, University of Waterloo ABSTRACT In order to meet the “anywhere and anytime” concept, the future wireless network architecture Cross-layer design approaches are critical is expected to converge into a heterogeneous, for efficient utilization of the scarce radio all-IP architecture that includes different wire- resources with QoS provisioning in the third- less access networks such as cellular networks, generation wireless networks and beyond. Bet- wireless local area networks (WLANs), and per- ter system performance can be obtained from sonal area networks (PANs, e.g., Bluetooth and information exchanges across protocol layers, ultra-wideband networks). It is well known that which may not be available in the traditional the success of today’s Internet has been based layering architecture. This article provides an on independent and transparent protocol design overview of cross-layer design approaches for in different layers, a traditional network design resource allocation in 3G CDMA networks, approach that defines a stack of protocol layers summarizes state-of-the-art research results, (e.g., the Open System Interconnection [OSI] and suggests further research issues. In addi- protocol stack). Using the services provided by tion, a cross-layer design approach for real-time the lower layer, each protocol layer deals with a video over time-varying CDMA channels is pro- specific task and provides transparent service to posed, where link layer resource allocation ben- the layer above it. Such an architecture allows efits from information in both the application the flexibility to modify or change the techniques and physical layers. Simulations results are in a protocol layer without significant impact on given to demonstrate the effectiveness of the overall system design. However, this strict layer- proposed approach. ing architecture may not be efficient for wireless networks when heterogeneous traffic is served INTRODUCTION over a wireless channel with limited and time- varying capacity and high bit error rate (BER). Third-generation (3G) wireless networks, also Efficiently utilizing the scarce radio resources known as International Mobile Telecommuni- with QoS provisioning requires a cross-layer cations 2000 (IMT-2000), aim at providing mul- joint design and optimization approach. Better timedia mobile services and achieving a performance can be obtained from information maximum bit rate of 2 Mb/s. The deployment exchanges across protocol layers. This article of 3G networks has already begun in different provides an overview of cross-layer design regions, and researchers have been proposing approaches over 3G and beyond wireless net- how 3G networks will evolve to beyond 3G or works, summarizes state-of-the-art research fourth-generation (4G) networks. To achieve a results, and identifies further research issues. successful and profitable commercial market The article focuses on code-division multiple for 3G and beyond, network service designers access (CDMA) networks, because: and providers need to pay much attention to • CDMA is a major candidate for 3G and efficient utilization of radio resources. Although beyond systems. the available bandwidth is much larger in 3G • CDMA offers more flexibility than time- and beyond networks (compared to 2G net- division multiple access (TDMA) to cross- works), it is still critical to efficiently utilize layer design for resource allocation due to radio resources due to fast growth of the wire- its capability to support simultaneous trans- less subscriber population, increasing demand missions. for new mobile multimedia services over wire- In the following we first summarize recent less networks, and more stringent quality of ser- research on cross-layer design for CDMA vice (QoS) requirements in terms of resource allocation, then present our research transmission accuracy, delay, jitter, throughput, work on video service over CDMA time-varying and so on. channels. 120 0163-6804/05/$20.00 © 2005 IEEE IEEE Communications Magazine • December 2005
  • 2. CROSS-LAYER DESIGN FOR CDMA RESOURCE ALLOCATION Request and update Source Scheduler One major challenge in multimedia services over CDMA cellular networks is QoS provisioning with efficient resource utilization. Compared to circuit-switched voice service in the 2G CDMA Buffer Decision systems (i.e., IS-95), heterogeneous multimedia (rate, power, and time) applications in future IP-based CDMA networks require a more complex QoS model and more sophisticated management of scarce radio Transmitter Uplink transmission Receiver resources. QoS can be classified according to its (spreading) (despreading) implementation in the networks, based on a hierarchy of four different levels: bit, packet, call, and application. Transmission accuracy, transmission rate (i.e., throughput), timeliness (i.e., delay and jitter), fairness, and user per- ceived quality are the main considerations in this classification. This classification also reflects the MSs BS principle of QoS categories from the customer point of view: • Bit-level QoS — To ensure some degree of n Figure 1. The centralized scheduler for the uplink transmission. transmission accuracy, a maximum BER for each user is required. • Packet-level QoS — As real-time applica- tions, such as voice over IP (VoIP) and Application videoconferencing, are delay-sensitive, each packet should be transmitted within a delay bound. On the other hand, data applica- Transport tions can tolerate delay to a certain degree, and throughput is a better QoS criterion. Each traffic type can also have a packet loss Network rate (PLR) requirement. • Call-level QoS — In a cellular system, a Link new (or handoff) call will be blocked (or dropped) if there is insufficient capacity. From the user’s point of view, handoff call Physical dropping is more disturbing than new call blocking. Effective call admission control (CAC) is necessary to guarantee a blocking n Figure 2. The cross-layer information for IP-based probability bound and a smaller dropping CDMA resource allocation. probability bound. • Application-level QoS — Bit- and packet- level QoS may not directly reflect service mission request upon new packet arrivals and quality perceived by the end user. On the update its link status to the BS, as shown in Fig. other hand, application layer perceived 1. The request and update information can be QoS parameters are more suitable to repre- transmitted in a request channel or piggybacked sent the service seen by the end user, for in the transmitted uplink packets to avoid possi- example, the peak signal to noise ratio ble contention in the request channel, and can (PSNR) for video application, and the end- be stored in the MS’s profile at theBS. The BS to-end throughput for data application pro- responds by broadcasting transmission decisions vided by the responsive Transmission to MSs. Control Protocol (TCP). To efficiently utilize scarce radio resources To guarantee the bit- and packet-level QoS and achieve overall QoS satisfaction, cross-layer requirements of mobile stations (MSs), an effec- information is necessary. In traditional layering tive link layer packet scheduling scheme with architecture, the link layer has statistical knowl- appropriate power allocation is necessary. Specif- edge of the lower physical layer, such as the ically, the power levels of all the MSs should be average channel capacity. However, to exploit managed in such a way that each MS achieves the CDMA time-varying channel, it is better for the required bit energy to interference-plus- the link layer to have knowledge of instanta- noise density ratio (denoted E b /I 0 ), and the neous channel status. Also, to guarantee the transmissions from/to all the MSs should be con- application-level QoS such as an acceptable visu- trolled to meet the delay, jitter, throughput, and al quality of video services or a guaranteed TCP PLR requirements. A centralized scheduler at throughput of data services, the application or the base station (BS) benefits from more pro- transport layer should be jointly designed with cessing power and more available information the link layer. In a five-layer reference model, than a distributed one. For the downlink, the BS Fig. 2 shows three possible cross-layer informa- has information on the traffic status of each MS. tion directions, from physical to link layer, from For the uplink, each MS needs to send a trans- link to transport layer and vice versa, and from IEEE Communications Magazine • December 2005 121
  • 3. link to application layer and vice versa. They or use fewer code channels in multicode CDMA. With the capability lead to three cross-layer design approaches: In a good channel state a previously sacrificed channel-aware scheduling, TCP over CDMA MS will be compensated (i.e., get a larger weight to support wireless links, and joint video source/channel or more code channels). simultaneous coding and power allocation, as discussed in the It should be mentioned that for real-time transmissions in a following. traffic (e.g., voice or video) with a delay con- straint, if an MS is in a bad channel state for a CDMA system, CHANNEL-AWARE SCHEDULING relatively long period, its packets will be discard- multiuser diversity In a multiple access wireless network, the radio ed when multiuser diversity is employed, as it channel is normally characterized by time-vary- has to wait until a good channel state. Hence, it can be employed ing fading. To exploit the time-varying character- is challenging to apply multiuser diversity to more effectively and istic, a kind of diversity (multiuser diversity) can real-time traffic. An effective way is to incorpo- flexibly than the be explored to improve system performance. rate the packet delay in the scheduling decision, The principle of multiuser diversity is that for a as in our proposed scheme discussed later. traditional channel- cellular system with multiple MSs having inde- aware scheduling pendent time-varying fading channels, it is very TCP OVER CDMA WIRELESS LINKS likely that there exists an MS with instantaneous For data services, TCP guarantees error-free schemes for a received signal power close to its peak value. delivery. TCP was originally designed for wire- TDMA system. Overall resource utilization can be maximized by line networks with a reliable physical layer, providing service at any time only to the MS where packet loss mainly results from network with the highest instantaneous channel quality. congestion. In such networks TCP adjusts its Multiuser diversity can be applied to CDMA sending rate based on the estimated network networks successfully. For the downlink (or congestion status so as to achieve congestion uplink), assume at each instant t only one MS i control or avoidance. In a wireless environment in a cell is receiving (or transmitting) with the TCP performance can be degraded severely as it target Eb/I0 value Γi (at the receiver side) while interprets losses due to unreliable wireless trans- other MSs are idle. Then the achieved data rate missions as signs of network congestion and of MS i is given by invokes unnecessary congestion control. To improve TCP performance over the wireless W P max ⋅ hi (t ) links, several solutions have been proposed to Ri (t ) = ⋅ (1) Γ i I io (t ) + vi (t ) alleviate the effects of non-congestion-related packet losses [2], among which snoop TCP and where W is the total downlink (or uplink) band- explicit loss notification (ELN) are based on width used by all the MSs, Pmax is the maximum cross-layer design. In snoop TCP, TCP layer power limit of the transmitter, hi is the channel knowledge is used by link layer schemes, while in gain of MS i, and Iio and vi are intercell interfer- ELN, the network layer takes advantage of cross- ence and background noise power at the receiver layer information from the physical layer. side of MS i’s connection, respectively. On the When a TCP connection is transmitted over right side of the equation, the second term is the CDMA cellular networks, further considerations received signal to interference-plus-noise ratio are needed. First, CDMA capacity is interfer- (SINR), represented by SINRi(t). In the down- ence limited. TCP transmission from an MS gen- link with the same required E b /I 0 value for all erates interference to other MSs. It is desired to MSs, obviously the maximum system throughput achieve acceptable TCP performance (e.g., a tar- can be achieved if at any time the BS only trans- get throughput) and at the same time introduce mits to the MS with the highest instantaneous minimum interference to other MSs (i.e., to channel quality (i.e., with the highest SINRi(t)). require minimum low-layer resources). Second, For the uplink, if there is sufficient power at MS power allocation and control in CDMA can lead transmitters and there is a limit on received to a controllable BER, which affects TCP per- power at the BS, when only the MS with the best formance. channel quality transmits, the minimum transmit For a TCP flow i over CDMA cellular net- power is needed. This leads to minimum inter- works, if there are packets to be transmitted, up ference to neighbor cells, thus increasing overall to a fixed number (denoted M i ) of link layer system capacity in a multicell environment. packets can be scheduled to be transmitted in With the capability to support simultaneous each link layer frame with E b/I 0 value Γ i. M i is transmissions in a CDMA system, multiuser called the target number of scheduled packets for diversity can be employed more effectively and flow i, and (Mi, Γi) is called the link layer design flexibly than traditional channel-aware schedul- parameter vector. It can be seen that TCP inter- ing schemes for a TDMA system. An MS does acts with link layer resource allocation. Specifi- not need to wait until it has the best channel cally, TCP dynamically adjusts the sending rate quality among all MSs, but rather can transmit of TCP segments (which will be fed into the link as long as its channel is good enough. All the layer transmission queue) according to network MSs are divided into two sets: bad channel state congestion status (e.g., packet loss and round- MSs if the channel gain is F dB less than the trip delay); on the other hand, the link layer average value, and good channel state MSs oth- design parameter vector (M i , Γ i ) ultimately erwise. The value F is called the good/bad thresh- determines the packet loss rate and transmission old. The scheduler keeps track of the obtained delay over the wireless link, and therefore affects services and channel states of all the MSs. MSs the TCP performance. From cross-layer parame- in a bad channel state postpone their transmis- ter design based on this interaction, a feasible sions until they have a good channel state, use a set of link layer design parameter vectors can be relatively small weight in resource allocation [1], obtained, which achieves the target TCP 122 IEEE Communications Magazine • December 2005
  • 4. throughput. If the design parameter vector is jointly designed such that the resulting PSNR is chosen (among the feasible parameter vectors) maximized, subject to a joint constraint on them Applying multiuser to minimize the amount of required resources, [7]. Transmission power consumption minimiza- diversity to real-time the optimal resource allocation can be achieved tion can also be achieved in joint source/channel for the TCP flow [3]. coding andpower allocation schemes subject to traffic is very acceptable video distortion [8]. Apparently, challenging due to JOINT SOURCE/CHANNEL CODING AND when transmission power consumption is reduced, CDMA system capacity can be the delay POWER ALLOCATION FOR VIDEO SERVICES enlarged. However, the above optimization is requirement of Video transmission is an important component complicated to achieve. The case is worse when real-time traffic. To of multimedia services. Typical video applica- time scheduling for multiplexed video traffic is tions include mobile videoconferencing, video implemented. Further investigation is necessary. address this problem, streaming, and distance learning. Due to real- we propose a time nature, video services typically require QoS guarantees such as a relatively large bandwidth PROPOSED CROSS-LAYER APPROACH cross-layer approach and a stringent delay bound. termed dynamic- For video services over a CDMA channel FOR VIDEO OVER weight generalized with limited capacity, an effective way is to pass TIME-VARYING CDMA CHANNELS source significance information (SSI) from the processor sharing source coder in the application layer to the Applying multiuser diversity to real-time traffic (GPS) for wireless channel coder in the physical layer. More power- is very challenging due to the delay requirement ful forward error correction (FEC) code (and of such traffic. To address this problem, we pro- video resource therefore more overhead) can be used to protect pose a cross-layer approach termed dynamic allocation with more important information, while no or weaker weight generalized processor sharing (DWGPS) service differentiation FEC may be applied to less important informa- for wireless video resource allocation with ser- tion. Such joint source/channel coding is a cross- vice differentiation over CDMA networks. To over CDMA layer approach, called unequal error protection implement UEP and multiuser diversity, the pro- networks. (UEP). UEP can easily be performed with Bose- posed cross-layer approach can benefit from Chaudhuri-Hocquenghem (BCH) codes, Reed- information in both the application and physical Solomon (RS) codes, and rate-compatible layers. Our focus is on video transmission in the punctured convolutional (RCPC) codes with dif- uplink, as resource allocation in the multiple ferent coding rates for packets with different pri- access uplink is much more complex than that in orities. UEP can also be implemented by means the broadcasting downlink. However, the pro- of power allocation in CDMA systems; for exam- posed solution should also be applicable to ple, transmission power can be managed so that downlink video transmission. a more important packet experiences a smaller error probability [4]. In case of capacity short- DYNAMIC-WEIGHT GENERALIZED age, UEP schemes can result in more graceful PROCESSOR SHARING quality degradation (and thus smaller distortion, or higher PSNR) than equal error protection For resource allocation, the well-known GPS [9, (EEP). Based on channel capacity, the optimal 10] is an ideal fair scheduling discipline original- transmission rate and power allocation for pack- ly proposed for wireline networks. The basic ets of each priority can be found to minimize the principle of GPS is to assign a fixed weight to average distortion of the received video by each session, and allocate bandwidth to all ses- means of an optimization formulation over sions according to their weights and traffic loads. CDMA channels [5]. It outperforms uniform GPS can provide each session with a minimum power allocation, as it exploits the degree of service rate. Also, a tight delay bound can be freedom added by CDMA power allocation. guaranteed by the GPS server for each session if A video codec has the ability to adjust its its traffic is shaped by a leaky bucket regulator. source coding rates. This flexibility can also be The minimum service rate and tight delay bound exploited to improve system performance. Con- guaranteed in GPS may seem attractive to real- sequently, it is desirable to employ a joint source/ time video transmission. However, as com- channel coding scheme that allocates bits for pressed video traffic is usually bursty, its peak source and channel coders so as to minimize the rate is likely to be much greater than its average end-to-end distortion under a given bandwidth rate. Therefore, a large weight should be constraint [6]. assigned to a video session in order to guarantee With interference-limited capacity, it is the peak rate. This means a video session will important to take into account the power man- get a large portion of the total capacity whenev- agement in CDMA systems when designing the er it has traffic to transmit, thus leading to ser- source and channel coding. More flexibility can vice degradation of other sessions. On the other be obtained when power allocation is considered hand, if the peak rate cannot be guaranteed, the jointly with source and/or channel coding. A delay bound of video traffic cannot be guaran- large source coding rate can lead to low quanti- teed either because of the latency in the leaky zation distortion, and a high transmission accu- bucket regulator. In order to apply GPS disci- racy level (i.e., high Eb/I0 from power allocation) pline to video transmission and extend it to wire- can achieve low channel-error-induced distor- less networks, dynamic weights in GPS are tion. However, for CDMA a large source coding introduced in our research, and the approach is rate (and therefore a large transmission rate) DWGPS. and a high Eb/I0 value are conflicting objectives. In DWGPS, each raw video frame is com- Hence, the source coding rate and Eb/I0 can be pressed to several batches of link layer (LL) IEEE Communications Magazine • December 2005 123
  • 5. LL packet loss rates of the L traffic classes in P b(k) order to avoid starvation of low-priority classes. i Aggressive In Eq. 2 {S i(k)/T i(k)} is the average service backoff capacity amount required by batch i in each sub- 1 sequent LL frame before the batch times out. With the weight proportional to its average required capacity portion, each batch is expected to be served smoothly rather than in burst within the delay bound. If a batch is expected to trans- mit all backlogged LL packets before it times out, all other batches in the same class are Moderate expected to do the same. Similarly, if a batch is backoff expected to lose a portion of LL packets, all other batches in the same class are expected to have the same share of packet loss. Therefore, fairness can be achieved among different traffic 0 Ti(k) flows. In this work, fairness means that all batch- 0 1 tm D es in the same class deliver successfully a similar portion of their arrival traffic (thus leading to a n Figure 3. The backoff probability of moderate and aggressive backoff similar packet loss rate). schemes. To implement DWGPS, upon each new batch arrival, the MS reports to the BS the batch class and batch size. For each LL frame, the BS deter- packets according to the priority of the coded mines how many LL packets can be transmitted information. When generated, each batch is clas- from each active batch of each MS, and broad- sified into one of total L classes (numbered from casts the decision to the MSs. Therefore, the BS 1 to L) according to its priority. Upon the arrival only needs to store the class number, remaining of each video frame, the MS creates a transmis- size, and timer value of each batch. And the sion queue for each batch of the video frame, information exchange overhead is not significant. assigns a timer with a timeout value to each batch, and reports to the BS the batch class and MULTIUSER DIVERSITY ADAPTATION batch arrival size in LL packets. The batch class To incorporate multiuser diversity and consider and batch arrival size are determined based on the delay bound of real-time traffic, in DWGPS, information passed from the application layer. each batch i with a bad channel at LL frame k is The timeout value in a unit of LL frame1 reflects kept idle with a probability Pib(k) (called backoff the maximum tolerable delay over the wireless probability) at this LL frame. If an MS is in a link. The timer will decrease by one after every bad channel state, we say all of its batches are in LL frame. If the timer expires, any LL packets a bad channel state. Intuitively, the smaller a remaining in the associated batch transmission batch’s timer value, the more urgent the batch’s queue are considered useless and discarded, and transmission, and the smaller the backoff proba- the batch transmission queue is deleted. bility for this batch should be. Figure 3 shows A session is defined as an active batch in the moderate and aggressive backoff schemes when transmission queue. Therefore, a video sequence batch i is in a bad channel state at LL frame k, may have multiple sessions simultaneously. At where D is the wireless delay bound and t m is LL frame k with a total number N(k) of active the point of timer value T i (k) from which the batches at all MSs, an active batch i is assigned a backoff probability of the aggressive backoff DWGPS weight scheme is linearly decreased. It is worth noting S (k ) that the relation of backoff probability vs. timer φi ( k ) = gi ( k ) i , 1 ≤ i ≤ N ( k ) (2) should depend on the channel fading rate: if the Ti ( k ) channel fades fast, relatively large backoff prob- where gi(k) is the importance weight of batch i’s abilities should be used in a bad channel state class at LL frame k, and Si(k) and Ti(k) are the with the expectation that the channel quality will remaining size and remaining timer value of get better and the affected batches will be com- batch i at the beginning of LL frame k, respec- pensated soon. Furthermore, the good/bad tively. Equation 2 is reasonable because the larg- threshold F should be determined carefully. This er a batch’s remaining size, the more capacity it threshold affects the probability of a batch being requires; and the smaller the batch’s timer value, considered in a bad channel state, and indirectly the more urgent the batch’s delivery. The selec- affects the performance of the backoff scheme. tion of the importance weight is based on the In our research, F could be determined based on criterion that a batch from a higher-priority class experimental results. will be assigned a relatively larger weight in Although originally designed for video trans- DWGPS, corresponding to a higher transmission missions, DWGPS can also be applied to real-time rate of this batch than those of lower-priority voice applications such as VoIP over CDMA with classes to better protect higher-priority classes a required delay bound. Furthermore, data traffic 1 In this article LL frame during capacity shortage. In DWGPS the impor- is usually deemed delay-insensitive, but this may means link layer time- tance weight selection is quite flexible. An opti- not always be true in practice. For example, for frame, while video frame, mization approach can be used for importance Web browsing, packets will be discarded if they I-frame, and P-frame all weight selection to protect higher-priority classes cannot be delivered successfully within a deadline. mean a frame of picture as much as possible. The importance weight can It is desired that data traffic should have a delay in a video sequence. also be configured to achieve a target ratio of bound, which can be much larger than that of 124 IEEE Communications Magazine • December 2005
  • 6. 5dB-M 5dB-M 10 10dB-M 10dB-M 5dB-A 10 5dB-A 10dB-A 10dB-A 8 8 Multiuser diversity gain (dB) Multiuser diversity gain (dB) 6 6 4 4 2 2 0 0 -2 -2 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Video MS ID Video MS ID (a) (b) n Figure 4. Multiuser diversity gains for MSs in the diversity DWGPS schemes in the slow and fast fading environments with F = 5 or 10 dB, and moderate (represented by M) or aggressive (represented by A) backoff: a) slow fading environment; b) fast fading environment. voice or video. In this context DWGPS is also rate 0.28) environments. In comparison with F = effective in supporting data traffic. 10 dB, F = 5 dB brings about more service degra- dation in terms of LL packet loss rates to high-pri- ority batch classes (not shown here due to space PERFORMANCE EVALUATION limitation). This is because F = 5 dB leads to a Computer simulations are carried out to evalu- larger probability of an MS being considered in a ate the performance of DWGPS. Consider 30 bad channel state, resulting in a larger probability video test sequences (with IDs ranging from 1 to of a batch being kept idle, which imposes more 30) are transmitted from 30 MSs to their corre- capacity requirements on later LL frames. Note spondence nodes. Each raw video test sequence that the probability of a channel 10 dB less than is in Quarter Common Intermediate Format average quality is 10 percent, while the probability (QCIF) with a duration of 3000 LL frames at a of a channel 5 dB less than average quality is 27 rate of 10 video frames/s, and is compressed by percent. For F = 10 dB, the aggressive backoff an MPEG-4 coder with a base layer and an scheme outperforms the moderate backoff scheme enhancement layer. In the base layer, only I- in terms of multiuser diversity gain as shown in frame and P-frame are used. B-frame is not used Fig. 4, at the cost of negligible (or non-negligible) due to the additional delay involved in its video service degradation in terms of LL packet loss compression and decompression process. Hence, rates of high-priority classes in the fast (or slow) there are three classes of batches from each fading environment. Hence, F = 10 dB, and the video sequence: I-frame batch in the base layer aggressive (or moderate) backoff scheme is a good (called IB batch), P-frame batch in the base choice for fast (or slow) fading. layer (called PB batch), and batch in the enhancement layer (called E batch). IB batch has the highest priority, E batch the lowest. CONCLUSION It is observed that in DWGPS, IB class traffic In this article the fundamentals of recent receives the best service (in terms of packet loss research efforts in cross-layer design for resource rate) and E class traffic receives the least. Hence, allocation in future CDMA-based 3G and DWGPS can implement UEP effectively. beyond networks have been presented, and a For multiuser diversity adaptation, we define a novel cross-layer approach for video service over multiuser diversity gain for an MS in a diversity time-varying CDMA channels is proposed. In DWGPS scheme as the ratio of average transmis- cross-layer design the overall system perfor- sion power for an LL packet from the MS in the mance can be improved by taking advantage of nondiversity DWGPS scheme to that in the diver- the available information across different layers. sity DWGPS scheme. Figure 4 shows the multius- To achieve this, an appropriate signaling method er diversity gain of the 30 MSs for F = 5 dB (or is necessary. Cross-layer signaling can be imple- 10 dB) and moderate (or aggressive) backoff mented by the following means [11]: scheme in the slow fading (with normalized fading • Cross-layer information is stored in packet rate 0.01) and fast fading (with normalized fading headers. IEEE Communications Magazine • December 2005 125
  • 7. • A third-party network service takes care of [6] L.P. Kondi, F. Ishtiaq, and A. K. Katsaggelos, “Joint Source-Channel Coding for Motion-Compensated DCT- For a CDMA the management of the cross-layer informa- Based SNR Scalable Video,” IEEE Trans. Image Proc., tion. vol. 11, no. 9, Sept. 2002, pp. 1043–52. network supporting • System profiles are used to collect and store [7] Y. S. Chan and J. W. Modestino, “A Joint Source Cod- heterogeneous cross-layer information, and access to the ing-power Control Approach for Video Transmission over CDMA Networks,” IEEE JSAC, vol. 21, no. 10, Dec. voice/video/data system profiles is provided to the related 2003, pp. 1516–25. layers. [8] Q. Zhang et al., “Power-Minimized Bit Allocation for traffic with different Recent research has provided preliminary Video Communication over Wireless Channels,” IEEE QoS requirements, it results for cross-layer design over all-IP CDMA Trans. Circuits Sys. Video Tech., vol. 12, no. 6, June 2002, pp. 398–410. networks. Further research efforts should is critical to consider [9] A. K. Parekh and R. G. Gallager, “A Generalized Proces- include: sor Sharing Approach to Flow Control in Integrated the tradeoff among •Joint source/channel coding and power allo- Services Networks: The Single-Node Case,” IEEE/ACM cation for multiplexed video services with time Trans. Net., vol. 1, no. 3, June 1993, pp. 344–57. the cross-layer [10] A. K. Parekh and R. G. Gallager, “A Generalized Pro- scheduling: when several video flows are multi- cessor Sharing Approach to Flow Control in Integrated approaches for plexed, it is desired to use time scheduling for Services Networks: The Multiple Node Case,” IEEE/ACM different traffic types, efficient bandwidth utilization. It is challenging Trans. Net., vol. 2, no. 2, Apr. 1994, pp. 137–50. to jointly allocate source coding rate, channel [11] Q. Wang and M. A. Abu-Rgheff, “Cross-layer Sig- and to achieve nalling for Next-generation Wireless Systems,” Proc. coding rate, and power level to video flows in a IEEE WCNC 2003, pp. 1084–89. desired overall multiplexed environment. [12] P. Giacomazzi, L. Musumeci, and G. Verticale, “Trans- system performance •Cross-layer design for differentiated services port of TCP/IP Traffic over Assured Forwarding IP-Differ- (DiffServ)-based QoS: DiffServ has emerged as entiated Services,” IEEE Network, vol. 17, no. 5, with efficient a scalable solution to ensure QoS in IP net- Sept.–Oct. 2003, pp. 18–28. resource utilization. works. When a data application transported by TCP is provided with DiffServ QoS, there is an BIOGRAPHIES interaction between TCP congestion manage- HAI JIANG [S’04] (hjiang@bbcr.uwaterloo.ca) received a B.S. ment and the IP-layer traffic conditioning/for- degree in 1995 and an M.S. degree in 1998, both in elec- tronics engineering, from Peking University, Beijing, China, warding mechanism. Poor performance can in 1995 and 1998, respectively. He is currently working result from mismatch between TCP and IP layer toward a Ph.D. degree at the Department of Electrical and mechanisms [12]. When DiffServ is applied to Computer Engineering, University of Waterloo, Canada. His CDMA wireless networks, this interaction should current research interests include QoS provisioning and resource management for multimedia communications in be extended to the lower link/ all-IP wireless networks. physical layers. To address this, effective cross- layer design from the TCP to the IP and finally WEIHUA ZHUANG [M93, SM’01] (wzhuang@bbcr.uwaterloo. to the link/physical layers is necessary to achieve ca) received B.Sc. and M.Sc. degrees from Dalian Maritime University, China, and a Ph.D. degree from the University of the desired DiffServ QoS performance. New Brunswick, Canada, all in electrical engineering. Since •Cross-layer design for heterogeneous voice/ October 1993 she has been with the Department of Electri- video/data traffic: a cross-layer design approach cal and Computer Engineering, University of Waterloo, usually focuses on a specific traffic type. For a Canada, where she is a full professor. She is a co-author of the textbook Wireless Communications and Networking CDMA network supporting heterogeneous (Prentice Hall, 2003). Her current research interests include voice/video/data traffic with different QoS multimedia wireless communications, wireless networks, requirements, it is critical to consider the trade- and radio positioning. She is an Associate Editor of IEEE off among cross-layer approaches for different Transactions on Wireless Communications, IEEE Transactions on Vehicular Technology, and EURASIP Journal on Wireless traffic types, and to achieve desired overall system Communications and Networking. performance with efficient resource utilization. XUEMIN (SHERMAN) SHEN [M’97, SM’02] (xshen@bbcr.uwater- ACKNOWLEDGMENTS loo.ca) received a B.Sc. (1982) degree from Dalian Maritime University, China, and M.Sc. (1987) and Ph.D. (1990) This work was supported by the Premier’s degrees from Rutgers University, New Jersey, all in electri- Research Excellence Award (PREA) from the cal engineering. Currently, he is with the Department of Ontario Government and by the Strategic Pro- Electrical and Computer Engineering, University of Water- ject (STPGP 257682 - 02) of the Natural Science loo, Canada, where he is a professor and associate chair for graduate studies. His research focuses on mobility and and Engineering Research Council (NSERC) of resource management in interconnected wireless/wireline Canada. networks, UWB wireless communications systems, wireless security, and ad hoc and sensor networks. He is a co- REFERENCES author of two books, and has published more than 200 papers and book chapters in wireless communications and [1] L. Xu, X. Shen, and J. W. Mark, “Fair Resource Allocation networks, control, and filtering. He was Technical Co-Chair with Guaranteed Statistical QoS for Multimedia Traffic in for IEEE GLOBECOM ’03, ISPAN ’04, QShine ’05, IEEE Wideband CDMA Cellular Network,” IEEE Trans. Mobile Broadnets ’05, and WirelessCom ’05, and is Special Track Comp., vol. 4, no. 2, Mar.-Apr. 2005, pp. 166–77. Chair of the 2005 IFIP Networking Conference. He serves as [2] G. Xylomenos et al., “TCP Performance Issues over Associate Editor for IEEE Transactions on Wireless Commu- Wireless Links,” IEEE Commun. Mag., vol. 39, no. 4, nications; IEEE Transactions on Vehicular Technology; Com- Apr. 2001, pp. 52–58. puter Networks; ACM/Wireless Networks; Wireless [3] H. Jiang and W. Zhuang, “Cross-Layer Resource Alloca- Communications and Mobile Computing (Wiley); and Inter- tion for Integrated Voice/Data Traffic in Wireless Cellu- national Journal Computer and Applications. He has also lar Networks,” to appear, IEEE Trans. Wireless Commun. served as Guest Editor for IEEE JSAC, IEEE Wireless Commu- [4] I.-M. Kim and H.-M. Kim, “Efficient Power Management nications, and IEEE Communications Magazine. He received Schemes for Video Service in CDMA Systems,” Elect. the Premier’s Research Excellence Award (PREA) in 2003 Lett., vol. 36, no. 13, June 2000, pp. 1149–50. from the Province of Ontario, Canada, for demonstrated [5] S. Zhao, Z. Xiong, and X. Wang, “Joint Error Control and excellence of scientific and academic contributions, and the Power Allocation for Video Transmission over CDMA Net- Distinguished Performance Award in 2002 from the Faculty works with Multiuser Detection,” IEEE Trans. Circuits Sys. of Engineering, University of Waterloo, for outstanding Video Tech., vol. 12, no. 6, June 2002, pp. 425–37. contributions in teaching, scholarship, and service. 126 IEEE Communications Magazine • December 2005

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