32
Cross-Layer Design in Wireless Communications
Sayantan Choudhury and Jerry D. Gibson
32.2 Fundamentals of Cross-Layer Design/Architecture
32.3 Applications of Cross-Layer Design to Wireless Networks
Multimedia Transmission over WLANs
Cross-Layer Design in OFDMA Networks
Cross-Layer Design for Future Wireless Devices
32.4 Challenges with Cross-Layer Design
32.1 Introduction
Wireless communication has become ubiquitous in the last two decades and has challenged modern communication system designers to innovate rapidly to meet the challenges of transmitting signal efficiently in a wireless medium. There has been outstanding progress in technology in all layers of the communication protocol stack from robust coding and physical layer transmission, more efficient medium access control (MAC) protocol design both from a throughput and from an energy standpoint, rethinking of the transport layer and routing in mobile ad hoc networks and also development of new application layer concepts that exploit the knowledge of the lower layers. This has caused wireless system engineers to rethink the traditional protocol stack designed for wire line communications and instead investigate cross-layer design approaches that can exploit the information across various layers. Cross-layer design relies on advances in signal processing, information theory and wireless networking. In this chapter, we try to highlight the use of cross-layer design for a few selected scenarios including wireless local area networks (WLAN) and cellular transmissions. We also provide some insight into the challenges faced by a cross-layer design approach.
32.2 Fundamentals of Cross-Layer Design/Architecture
The increased demand for real-time traffic such as voice, video, and data-traffic has challenged wireless engineers to design more efficient wireless networks. The goal from a system engineering point of view has been to build a wireless network where the user has a perceived quality close to a wireline network [1]. There has been tremendous development in standardization efforts including WLANs and cellular standards including the completion of Long-Term Evolution (LTE) Release 8–10. Apart from the signification developments in the individual layers of the protocol stack including physical and MAC layer transmission, a significant amount of research has been done investigating new wireless transmission architectures using cross-layer design.
Traditional communication networks were based on the layered Open Systems Interconnection (OSI) reference model (reference) shown in Figure 32.1a. A simplification of the modern internet architecture employing the TCP/IP model is shown in Figure 32.1b. The importance of well-designed network architecture cannot be overemphasized [2]. Layered architecture simplifies the network design and it is easier to develop and manage each of the individual elements of the protocol stack independently. In the layered architecture, there is typically communication only across the layers adjacent to a given layer. The physical layer provides an interface for transmission of information over a transmission medium (wired or wireless), the data link layer provides reliability to the transmission by handling acknowledgments and retransmissions, the network layer implements the routing of the packets including the most optimum paths a packet should take from source to destination and also handles congestion, the transport layer provides a higher-layer reliability mechanism ensuring the delivery of error-free data that are in sequence to the upper layers. The application, presentation, and session layers have various functions including data formatting, compression, encryption, initiating logical channels and various management functions required by different applications. An overview of the OSI reference model can be found in [3]. As mentioned earlier, having a clear distinction between the layers enables a system designer to concentrate on individual layers and optimize them individually.
FIGURE 32.1 (a) Layered OSI architecture and (b) TCP/IP model.
Wireless networks bring some unique challenges typically not encountered in a wireline network [4]. For instance, wireless channels vary over time and space due to multipath effects resulting in fading signals. Furthermore, propagation losses (including shadow fading caused by obstacles) and mobility cause the wireless signals to vary depending on the location of the users. Moreover, wireless channels being a broadcast medium are also susceptible to interference caused by neighboring transmissions. Finally, wireless networks have a scarcity of spectrum resource making the problems even more challenging. These challenges caused by wireless transmissions have caused researchers and system designers to rethink the traditional OSI architecture with the aim of developing newer architectures capable of optimizing across the protocol stacks.
An excellent survey on the different architectural designs is provided in Reference 5. Here, we summarize some of the essential concepts.
Figure 32.2 shows some of the different kinds of cross-layer design proposals. Traditional network architectures usually allow for interaction across adjacent layers only. In order to overcome this limitation, there have been proposals to create new interfaces that allow communication across nonadjacent layers as shown in Figures 32.2a–c. There have also been proposals to merge adjacent layers (Figure 32.2d), introducing coupling across layers (Figure 32.2e) and also for vertical calibration across layers (Figure 32.2f). Based on the various design proposals, some new architectural proposals that can be implemented in future wireless networks are shown in Figure 32.3, including direct communication between different layers, a shared database concept that allows different layers to share information and finally, developing completely new abstractions that allow rich interactions across the layers. In the next section, we show possible benefits of some of the proposed architecture with examples from WLAN, cellular and also future wireless networks including cognitive radios. In Section 32.4, we also highlight some of the challenges faced by these new cross-layer approaches.
FIGURE 32.2 Different kinds of cross-layer design proposals: (a) Upward information flow, (b) downward information flow, (c) back-and-forth information flow, (d) merging of adjacent layers, (e) design coupling without new interfaces. The designed layer's design is done keeping in mind the processing at the fixed layer, but no new interface is created, and (f) vertical calibration. (Adapted from V. Srivastava, and M. Motani, IEEE Communications Magazine, 43(12), 112–119, 2005.)
FIGURE 32.3 Some possible wireless communication architectures: (a) Direct communication between the different layers, (b) a shared database, and (c) completely new abstractions (no more protocol layers). (Adapted from V. Srivastava, and M. Motani, IEEE Communications Magazine, 43(12), 112–119, 2005.)
32.3 Applications of Cross-Layer Design to Wireless Networks
In this section, we review some case studies showing the benefits of cross-layer design principles in wireless networks including WLAN, cellular, and cognitive networks. We also try to show how the different cross-layer techniques can be implemented based on the architectural design principles mentioned in the previous section.
32.3.1 Multimedia Transmission over WLANs
Multimedia transmission over WLANs is quite challenging due to the different quality-of-service (QoS) requirements of different applications. For instance, voice and video traffic have tighter delay constraints while being somewhat more error tolerant compared to data traffic. Hence, depending on the application and usage scenario, there are different optimization metrics that can be used, for example, minimizing delay, maximizing throughput, minimizing energy, and so on. This compounded by the fact that wireless channel undergoes large- and small-scale fading and observed interference at the receiver makes multimedia transmission over WLAN quite challenging. Here, we sample some of the cross-layer approaches mentioned in the research literature that relies on the interactions between the physical, medium access control (MAC) and application layers.
Various design parameters can be optimized depending on the application and usage scenario. For instance, for voice and video traffic, the modulation and coding schemes could be adapted in the physical layer, the number of retransmissions and payload length in the MAC layer, the codec parameters and the application frame sizes in the application layer. We demonstrate the parameter interplay with an example of multimedia transmission over 802.11a network [6–9].
Table 32.1 shows the different modulation and coding rates that can be used for 802.11a transmissions. Depending on the channel condition, the transmitter can dynamically vary the transmission rates by using a different modulation and coding scheme. For instance, at lower signal-to-noise (SNR) ratio, the transmitter uses more robust modulation and coding scheme for example, use a low transmission rate 6Mbps (BPSK with rate 1/2code) while at higher SNRs, it can use a more aggressive transmission rate, for example, 54 Mbps (64 QAM with rate 3/4). Figures 32.4 and 32.5 show the payload headers and protocol overhead encountered during a packet transmission that result in additional overhead during data transmission.
TABLE 32.1 Different Modulation and Coding Rates in IEEE 802.11a Transmission
In Reference 6, a cross-layer design approach was proposed that dynamically adapts the PHY transmission rate and payload length depending on the channel conditions in order to maximize the effective throughput. The challenge involving rate and payload selection is that lower rates and smaller payloads are more robust while being inefficient since the MAC and protocol headers become quite large while larger payloads and more aggressive PHY rates reduce the header overhead but can lead to retransmissions in case they cannot be decoded correctly in the first transmission attempt. Hence, in Reference 6, a theoretical model was developed in order to maximize throughput by appropriate payload and rate selection. In Reference 7, the model was extended to incorporate a packet error rate constraint since employing larger payloads might lead to higher error rate in poor channel conditions. In Reference 8, the effect of retransmissions was considered and it was observed that by changing the maximum retransmission threshold value, different mix of data and multimedia traffic could be adapted.
FIGURE 32.4 Payload headers in OFDM based 802.11a/g transmission.
FIGURE 32.5 Protocol overhead in 802.11.
There has also been a lot of work on cross-layer design approaches that exploit the application characteristics. In Reference 9, the authors investigated the video user capacity of wireless networks subject to a multiuser perceptual quality constraint. Different parameters were studied including application layer parameters such as quantization parameter, group of picture size along with the PHY/MAC parameters, for example, payload size and data rate in order to optimize the delivered video quality based on a perceptual quality metric. Similar studies were also conducted for voice traffic in Reference 10. For the voice traffic studies, different codecs were also evaluated namely, G.729 and G.722 voice codec that have varying payload sizes and different concealment properties in order to evaluate the voice quality constrained capacity under different fading scenarios. Additional references on multimedia transmission over wireless networks can be obtained in Reference 11.
In the previous discussions, we have provided a few examples of the possible benefits of a cross-layer design approach for multimedia transmission over wireless LANs. Furthermore, the architectural framework that needs to be developed to allow such cross-layer interactions is consistent with the ones shown in Figure 32.3. In particular, either the shared database approach or the direct communication across the physical, MAC, and application layers could be employed.
32.3.2 Cross-Layer Design in OFDMA Networks
The Third Generation Partnership Project (3GPP) Long-Term Evolution (LTE) standardization efforts aim at high-speed broadband cellular data access transmission schemes. In this section, we show how some of the cross-layer interactions are an integral part of LTE design philosophy mainly focusing on the MAC and PHY layers.
Figure 32.6 shows the physical and MAC layer functional blocks and its interactions with the MAC scheduler [12]. Multiple Input Multiple Output (MIMO) transmission is an integral part of LTE design and can be used for transmit diversity, spatial multiplexing, beamforming, multiuser MIMO depending on the usage scenario. Additionally, there are 16 different modulation and coding schemes that are adapted based on the channel conditions. Furthermore, there is a Hybrid Automatic Repeat Request (HARQ) transmission mechanism that can provide fast retransmissions. In Reference 13, the authors investigated the optimal MIMO mode selection (spatial multiplexing or diversity) in order to maximize proportional fairness among users. The work was extended for multiuser MIMO transmission in Reference 14.
Cross-layer optimization for OFDM wireless networks has been extensively studied in References 15 and 16. In Reference 17, the authors present a cross-layer framework for efficient resource allocation in 802.16e OFDMA system. The proposed MAC architecture separates users into two groups: diversity subchannels group for high-mobility users and band adaptive modulation and coding (AMC) group for fairly static and/or high SNR users. Furthermore, the payload length and the number of HARQ retransmissions are also adapted based on the channel conditions. It is observed that by employing a cross-layer framework, the average cell throughput can be increased by 25–65%. The cross-layer framework and MAC simulator architecture is shown in Figure 32.7.
32.3.3 Cross-Layer Design for Future Wireless Devices
In the previous sections, we have shown the benefits of cross-layer design approach in increasing the throughput and multimedia quality. In this section, we provide a brief survey on the use of cross-layer design for future wireless devices focusing on two applications: (a) energy-efficient communication and (b) cognitive devices.
The improvement of battery technology has been slow in keeping up with the increased power consumption required by newer processors [18–20]. This compounded with the fact that newer wireless devices, especially cellular devices, are increasingly supporting multiple radios causing higher energy consumption, makes cross-layer optimization for energy-efficient communication quite important. The cross-layer framework for optimizing energy is similar to what was mentioned previously for throughput optimization, that is, at the PHY-MAC layers, the modulation and coding scheme, transmit power can be adapted to minimize the energy. However, the optimal solution for energy efficient communication is very different from that obtained for throughput maximization [21]. While throughput maximization aims at selecting the most spectrally efficient modulation schemes, traditional energy-efficient transmission schemes aim at transmitting packets over a long period of time [22]. In Reference 23, it was shown that when the transmitter circuitry energy dissipation is taken into account, it might be better to use higher-order modulation especially for short-range communication. In Reference 24, it was shown that it is more energy efficient to transmit a packet over multiple shorter routes compared to one long route. This suggests that the use of relays might lead to more energy-efficient communication. However, in Reference 24, it was shown that in some situations, it might be better to use longer hops. A detailed survey on cross-layer optimization for energy-efficient communication is found in [20].
FIGURE 32.6 LTE protocol structure. (Adapted from D. Astely et al. IEEE Communications Magazine, 47(4), 44–51, 2009.)
The energy efficiency based on modulation order and distance from the base station is shown in Figure 32.8 [19]. It is observed that for users near to the base station, it is better to use higher-order modulation rate while for users further from the base station, lower-order modulation is preferred. While the conclusion is quite similar in the case of traditional link adaptation algorithms, the actual data rates used for the energy-efficient algorithm are quite different as can be observed from the energy consumption for the different algorithms in Figure 32.8. The adaptive algorithms in Figure 32.8 refer to traditional adaptive modulation and coding algorithms with different fixed transmit power levels of 15, 20, 25, or 30 dB. It is quite evident from the figure that link adaptation algorithms based on minimizing energy consumption results in much lower energy consumption than throughput maximization-based algorithms. Hence, there is a trade-off between minimizing energy efficiency and maximizing throughput and an optimal scheduler needs to take into account the battery status and the QoS requirements.
FIGURE 32.7 Cross-layer scheme and MAC architecture for OFDMA networks: (a) Cross-layer adaptation scheme and (b) MAC simulator architecture. (Adapted from T. Kwon et al. IEEE Communications Magazine, 43(12), 136–146, 2005.)
As mentioned earlier, MIMO techniques add another dimension to the cross-layer framework. It is well known that MIMO techniques can be used to increase the data rate by using spatial multiplexing of the streams or to increase robustness by employing space-time coding. However, MIMO techniques require multiple transmit and receive antennas and power amplifiers that can increase the energy consumption. A comparison of energy consumption of MIMO techniques vs. single-antenna transmission was done in [25] where it was shown that using fixed modulation, single-antenna techniques are more energy efficient compared to MIMO, especially over small distances. However, by employing adaptive modulation, MIMO transmission can be made more energy efficient over all distances. The authors also showed that by using cooperative MIMO transmission, the energy transmission and delay could be reduced over certain distances.
FIGURE 32.8 Link-level energy efficient transmissions: (a) relationship of energy efficiency and (b) normalized energy consumption distance, modulation, and transmission on transmitting one million bits rate. (Adapted from G. Miao et al., IEEE International Conference on Communications, May 2008.)
We have illustrated the importance of cross-layer design for improving throughput, robustness and energy efficiency. Another area of increasing importance for cross-layer design is in the area of cognitive radios. In cognitive networks, secondary users use the channel only when it is not occupied by a primary user and if a primary user is detected, the secondary users need to vacate the channel. The primary user can be detected using spectrum sensing or by checking with a database where the primary users are required to register before accessing the channel. Hence, there are quite significant challenges for cross-layer design in cognitive networks. Firstly, channel selection whereby secondary users decide to use an available channel is an important research area. The channel to be used is based on the channel availability, QoS requirement, transport, and routing mechanisms and thus needs an integrated cross-layer framework. Secondly, spectrum handoff is another important problem since the handoff mechanisms need to be aware of the application requirements especially latency [26].
32.4 Challenges with Cross-Layer Design
In the previous sections, we have highlighted some of the benefits of cross-layer design in wireless communications. Cross-layer design can provide throughput enhancements, improved robustness, better multimedia transmission quality and also reduce energy consumption. However, in spite of all these benefits, there are quite a few challenges when it comes to implementing cross-layer protocols. In this section, we highlight a few challenges faced by a cross-layer approach.
As discussed in References 2 and 5, the OSI architecture has made the wireless system design problem scalable since each layer could be independently designed and optimized. However, using a cross-layer approach, there are no more clear boundaries across the protocol layers. Furthermore, there is a need to define a new architecture that allows interaction across the layers. Some of the open issues raised are whether such a cross-layer approach allows coexistence across devices with possible different architecture, whether the different interactions across layers consider thoroughly the interactions across the different layers and possible negative consequences that might not have been accounted for, whether it is possible to standardize the mechanisms/interfaces to share information and also more fundamentally, whether cross-layer design stifles innovation since there might not be a unifying framework for protocol development. It has also been argued that the cross-layer approach might lead to a spaghetti-like design and stifle innovation and proliferation due to the increasing number of new interactions across layers that might lead to every update having a complete redesign and replacement of the existing designs. It was shown in Reference 2 that the interaction of a rate-adaptive MAC protocol with a minimum hop routing protocol might perform worse than a plain 802.11 MAC due to the interaction of the MAC and routing protocols. Hence, from an engineering design perspective, one should be careful to thoroughly analyze the interactions across the layers ensuring that there are no unintended side effects.
32.5 Conclusion
In this chapter, we provide a brief tutorial of the applications of cross-layer design for wireless applications. After explaining the basic concept of cross-layer framework we showed using some applications of the benefits of cross-layer approaches in current and future wireless networks. First, we discussed the benefits of the cross-layer approach for multimedia transmission over WLANs. We then explained some of the integrated cross-layer frameworks in the 3GPP LTE standard and also applications of cross-layer design in future wireless networks focusing on energy enhancements and cognitive radio. Finally, we discussed some of the challenges faced by a cross-layer approach focusing on the some of the system design aspects.
Acknowledgments
Many thanks to Dr. Klaus Doppler, Dr. Chittabrata Ghosh, and Dr. Suchandrima Banerjee for their careful review and comments.
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