Cooperative Communication Xu Jiaming Thanks for yiming’s introduction. I am pleased to have this opportunity to present some of mine own ideas on wireless networks research. If you have any questions during my talk, feel free to interrupt me.

Outline Introduction Cooperative Routing Power and Delay trade-off in Cooperative Networks Conclusion In the introduction section, I try to answer the basic question: where the idea of cooperative communication comes from and what is it? Then I will introduce the basic concept of cooperative routing. Details will be presented by my partners. And present my own thoughts toward the power and delay trade-off problem Lastly, I will give my conclusion and point out some research trends in cooperative communication.

Introduction Two fundamental aspects of Wireless Networks multipath fading Interference As well known, there are two fundamental aspects of Wireless Networks that make the problem challenging and interesting. First is multipath fading, the variation of channel strengths over time and frequency. Second is interference. Unlike the wired world where each transmitter and receiver pair can be thought as isolated point-to-point link, wireless users communicate over air and experience severe interference. How to deal with multipath fading and interference is central to Wireless Networks research.

Multipath fading Time diversity Coding and Interleaving Frequency diversity CDMA, OFDM Spatial diversity MISO, SIMO, MIMO First, let us focus on multipath fading. A straight forward idea to counter multipath fading is transmitting the same packet over independent paths or independent time periods. This idea gives rise to the time diversity, frequency diversity, and spatial diversity. The principle of time diversity is similar to the familiar error control codes for bursty errors. Deep fade corresponds to bursty errors. Coding and interleaving of codewords ensure that only a part of codeword experiences harmful deep fade and the whole codeword can still be successfully decoded. In order to gain Frequency diversity, we have CDMA and OFDM techniques. As for the spatial diversity techniques, we have MISO, SIMO, MIMO. They all belongs to multiple atennas techniques.

Interference TDMA Graph Coloring Contention-based mechanisms Aloha, CSMA, etc CDMA Multiuser Detection (SIC) What about interference? As for controlling interference in wireless networks, we have TDMA, a contention-free scheduling algorithms. At the same time we have contention-based mechanisms, such as Aloha, CSMA. Besides, we also have CDMA. I believe one biggest advantage of CDMA is that it provides an automatic solution to the problem of allocating the channel to busy users. The conventional Rake receiver based on matched filter is not optimal according to information theory of multiaccess channel. Therefore, we need multiuser detection receiver. One possible scheme is successive interference cancellation. However, complexity is a big problem for multiuser detection receiver, but still it might be a good way to control interference in wireless networks.

Summary Can we exploit the interference? Building better bit pipes Multipath fading Time diversity Frequency diversity Spatial diversity Interference TDMA Contention-based schemes CDMA+MD Building better bit pipes Allocating bit pipes wisely Multipath fading Opportunity Can we exploit the interference? To sum up, As for Multipath fading, we have diversity techniques; for interference, we have TDMA, contention-based and CDMA. By overcoming multipath fading, we aims to build better bit pipes. This corresponds to physical layer issue. By controlling Interference, we aims to allocate bit pipes wisely, this corresponds to MAC layer and Network layer issue. How about jointly consider physical layer issues, MAC layer issues and network layer issues? In multiuser setting, when jointly consider cross-layer issues, some smart guys find that multipath fading can be viewed as “opportunity” to be exploited: at any time in a large network, with high probability there is a user whose channel is near its peak. By allowing such a user to transmit at that time, the overall multiuser capacity can be achieved. This is the basic idea of opportunistic communication. In this context, there is no need to counter multipath fading, but rather we try to induce large multipath fading. Opportunistic communication can be extended to a more broad problem: cross-layer wireless resource allocation. The problem statement is simple. We want to allocate the resources, such as power level and information rate, among users according to the channel fades and buffer occupancy to optimize the network throughput and delay. Any question by now? Opportunistic Communication Cross-layer Wireless Resource Allocation

Can we exploit the interference? What is the basic characteristic of wireless medium which causes the interference? Cooperative communication Cooperative diversity Broadcast nature of wireless medium gives rise to the interference. Can we exploit the broadcast nature? The answer is yes. Consider the above figure, due to the wireless broadcast nature, information transmitted by s is received by nodes 1 and 2. After the first transmission, nodes s, 1, and 2 have the information and can cooperate in getting the information to 3, as shown in above figure. Acaturally, s,1,2 consitutes atenna array. So, s, 1, 2, and 3 constitutes MISO channel. This actually give rises to the idea of Cooperative MIMO, or more broadly speaking, cooperative communication. What is the benefits of cooperation. First, diversity. Cooperative diversity gain is easy to understand in terms of spatial diversity. Second, energy gain. Suppose node s,1 and 2’s power is P. Then the amplitude of the transmitting waveform is square root of P. By beamforing, we mean constructive addition. So at the receive node, the receive power is 9P. Now consider non cooperative context, we assum node 1 ‘s power is 3P. Because previously we suppose each of node s,1,2 ’s power is P. so the receive power is 3P. Here we have the engergy gain.

Summary Building better bit pipes Allocating bit pipes wisely Multipath fading Time diversity Frequency diversity Spatial diversity Interference TDMA Contention-based schemes CDMA+MD Building better bit pipes Allocating bit pipes wisely Multipath fading Opportunity Broadcasting Cooperation So, we see that Broadcasting nature makes the cooperation between nodes a reality. Cooperative Communication can be extended to a more broad problem: cooperative Networks. Nodes independently decide to cooperate or not according to own channel conditions and buffer occupancy and revenue can obtained from cooperation. This work is an interdisciplinary research between networks and economy. We also notice that, from multipath fading, we have opportunistiec communication and wireless resource allocation. Resource allocation has to deal with interference. Again, from inteference, we have cooperation. And the cooperation gain are mainly from diversity gain and energy gain, which centainly deal with multipath fading. So we see that Multipath fading problem and interference problem are interconnected and therefore, both the resource allocation problem and cooperative communication problem are cross-layer issues. Opportunistic Communication Cooperative Communication Cross-layer Wireless Resource Allocation Cooperative Networks

Cooperative Routing Cooperative communication is a cross-layer issues. Reduce power consumption Reduce Delay Because cooperative communication is a cross-layer issues. Some Smart guys begin to think that what will be the result if we consider the routing problem in cooperative communication context. This is the cooperative routing. My partners will introduce two results on cooperative routing. One is to reduce power consumption. Another is to reduce delay or increase throughput.

What we learn? Power and Delay trade-off in Wireless Networks Transmit the information to nodes far away low delay but high power consumption Transmit the information to nearby nodes High delay but low power consumption Transmitter makes a choice according to current channel state and buffer occupancy. What we learn from these two results. We see that with cooperation, we can transmit information to nodes far away. This gives rise to low delay but high power consumption compared to next scheme. On the other hand, we can transmit the information to nearby nodes. This gives rise to the high delay but low power consumption compared to previous scheme. So, actually, Transmitter can make a choice between these two schemes according to current channel state and buffer occupancy. When the network load is heavy, delay is the central concern. Transmit prefer to transmitting information to nodes far away. When network load is light and we want to prolong the network lifetime, we prefer to transmitting to nearby nodes. So what about this scenario:

Power and Delay Tradeoff Curve Time Savings Non-cooperative Cooperative The dark blue one correspondes to non-cooperative scenario. The shallow blue one is cooperative scenario. We see that there is a trade off between power and delay. And With cooperative communication, there is energy savings and Time Savings. Actually, the cooperative communication pushes the curve toward the origin. Energy Savings Delay

Conclusion Cooperative Communication. Research Trends Coded-based Cooperation Capacity of Cooperative networks Relay Coded-based Cooperation is a hot topic under extensive research. This concentrates on implementation of cooperation in the physical layer. Prof. Aazhang at Rice conduct the research which uses the LDPC code to implement the cooperation. I also know that Mr. Chen Xingyang at my group also conduct research in this topic. Another interesting and challenging topic is capacity of cooperative networks. For simplicity, we consider the simplest form of cooperative networks. Unfortunately, even the capacity of the simple three node relay networks is still unknown. Prof. David Tse provides a deterministic approach to make progress in solving this problem. Prof. Jeff Andrews put forward a novel transmission capacity concept. S D relay

Coded Cooperation BC mode MAC mode Relay can decode Destination may not MAC mode Relay re-encodes/transmits Source transmits New information Repeats previous information Destination decodes correlation

Modeling and Mathematics Mathematical Heavy We Capacity Modeling Mathematical Low New Capacity

Thank You!

Cooperative Routing Cooperative communication is a cross-layer issues. Cooperative Routing in static wireless networks. Information is routed from s to d in a sequence of time slots Each time slot Broadcasting mode Cooperation mode Multistage decision problem From the interference, we obtain the basic idea of cooperative communication which offers cooperative diversity (spatial diversity). Therefore, cooperative communication is a cross-layer issues. We want to elaborate this point by considering the cooperative routing problems, that is routing problems in the cooperative communication networks. Consider the above figure we have seen in the last slide. Information is routed from s to d in a sequence of time slots. the non-cooperative route is s-1-3-4-d. With cooperation communication, suppose s broadcasts the information to node 1 and 2. This is the broadcasting mode. After that, node s, 1, and 2 cooperatively transmit the information to 3. This is the cooperation mode. We do not consider the scenario that node s, 1, and 2 cooperatively transmit to more than one nodes because of difficult time synchronizaiton between different receive nodes. Therefore, each transmission is either a broadcast transmission or a cooperative transmission. Therefore, routing problem simplifies to multistage decision problem. First choice between broadcasting mode or cooperation mode. If Broadcasting mode, who broadcast? If cooperation mode, who transmit to?

Cooperation Graph reliable set of nodes that have information at kth stage set of nodes that will be added to reliable set in the next transmission Suppose the network has n+1 nodes. The cooperation graph has n layers. Attention that layer is not stage. Layer k consists of all the possible set of size k+1 which contains the node s. The arcs between the nodes in adjacent layers correspond to cooperative links, whereas the broadcast links are shown by cross layer arcs. Cooperative link and broadcast link has own link costs. We do not want to quantify the link cost due to time constraint. The optimal transmission policy is simply the shortest path between node s and node D. However, the computation complexity is exponential with n. So we have to resort to suboptimal policy.

Cooperative Routing benefits Restrict the cooperation to nodes along the optimal non-cooperative route. Energy savings One suboptimal policy is restricting the cooperation to nodes along the optimal non-cooperative route. Specifically, consider the figure we have seen before, at first, the node s transmit information to node 1. Secondly, node s and node 1 cooperatively transmit information to node 3. Thirdly, node s, 1 and 3 coopearatively transmit information to node 4. Follow this scheme until information arrives at node d. What is the benefits of cooperative routing. Energy savings. It is quite easy to understand. When node s and 1 cooperatively transmit information to node 3, due to spatial diversity, less power is required than non-coopearative scenario, where node 1 transmit information to node 3 by his own.

Alternative view toward Cooperative Routing Cooperative communication increases the transmission range. Consider the above figure. The source node A want to transmit the information to destination node d. The non-cooperative route is A-B-C-F-G-H-I-J-L-N-O-P. In our previous discussion, one cooperative routing policy is that node A transmit information to B, then node A and B cooperatively transmit information to C. Follow this until information arrives at destination node P. Now, we take another view toward cooperative routing. We know that cooperative communication increases the transmission range due to spatial diversity. Therefore, A can transmit information directly to node F, then to L, finally to P. This cooperation path is short than the non-cooperative one. Therefore, it is safe to conclude that this scheme can increase the throughput and decrease the delay. The next step is simply how to design a cross-layer protocol to achieve this scheme.