This chapter rethinks the link abstraction for wireless networks in the context of coopera-tive communications, which has recently received interest as an untapped means for improv-ing performance of relay transmission systems operating over the ever-challenging wireless medium. The common theme of most research in this area is to optimize physical layer per-formance measures without considering in much detail how cooperation interacts with higher layers and improves network performance measures. Because these issues are important for enabling cooperative communications to practice in real-world networks, especially for the increasingly important class of mobile ad hoc networks (MANETs), the goals of this paper are to survey basic cooperative communications and outline two potential architectures for cooperative MANETs. The first architecture relies on an existing clustered infrastructure: cooperative relays are centrally controlled by cluster heads. In another without explicit clustering, cooperative links are formed by request of a source node in an ad hoc, decentralized fashion. In either case, cooperative communication considerably improves the network con-nectivity. Although far from a complete study, these architectures provide modified wireless link abstractions and suggest tradeoffs in complexity at the physical and higher layers.

Abstract—In a distributed wireless network, it is possible to employ several relays and mimic a multiple antenna transmission system. In this paper we propose a MAC layer solution that allows multiple relays to send information to the receiver at unison, using a randomized distributed space time code. The randomized space-time coding can recruit relays on the fly, thus significantly reducing signaling overhead. The cross-layer design between physical layer and MAC layer involves relay discovery and rate adaptation, and results in improvements in throughput and delay performance. The design is dynamic and can be adapted to changing network conditions. The proposed MAC scheme can be integrated into various wireless technologies such as distributed contention based networks (e.g., IEEE 802.11 BSS and ad hoc mode) as well as centralized multiple access networks (e.g., IEEE 802.16). I.

A cross-layer approach, called CoopMAC, has been proposed recently to integrate cooperation at the physical layer with the medium access control sublayer, thereby achieving substantial throughput and delay performance improvements. CoopMAC capitalizes on the broadcast nature of wireless channels, recruiting a relay opportunistically and on the fly to support the communication of a particular source-destination pair. Herein, we study a cross-layer framework that uses randomized cooperative coding, and allows the simultaneous recruitment of multiple relays which collectively act as the antennas of a distributed transmission array, by simply randomizing the antenna code selection rule. We show that randomized cooperation not only alleviates the burden associated with the relay selection process, but also fully exploits the available spatial diversity. 1.

Abstract — We consider distributed space-time coding for twoway wireless relay networks, where communication between two terminals is assisted by relay nodes. We compare existing and new protocols that operate over 2, 3 or 4 times slots. Particularly, a new class of relaying protocols, termed as partial decode-andforward (PDF), is proposed for the 2 time slots transmission. We show that the proposed amplify and ( forward (AF) ) protocols achieve the diversity order of min{N,T}, where N is the number of relays, P is the total power of the network, and T is the number of symbols transmitted during each time slot. When linear dispersion (LD) codes with random unitary matrices are used, the proposed PDF protocols resemble random linear network coding, where the former operates on unitary group and the latter works on finite field. I.

Abstract not found

Abstract — With the increased popularity of mobile multimedia services, efficient and robust video multicast strategies are of critical importance. In a conventional multicast system, the source station transmits at the base rate of the underlying network so that all the nodes can receive the data correctly. The performance of such a multicast system is limited by the node with the worst channel conditions, which usually corresponds to the nodes at the edge of the multicast coverage range. To overcome this problem, we propose a two-hop cooperative transmission scheme where in the first hop the source station transmits the packets and the nodes who receive the packets forward the packets simultaneously in the second hop using Randomized Distributed Space Time Codes (R-DSTC). We further integrate this randomized cooperative transmission with layered video coding to provide users different video quality based on their channel conditions. The performance of the system is evaluated and compared with a conventional multicast system. Our results show that the proposed cooperative system significantly improves the performance compared to conventional multicast. Index Terms: layered video coding, user cooperation, randomized distributed space time coding, wireless networks, video multicast I.

Abstract—Information theoretic studies have shown the significant performance improvements of cooperative communications. However, these studies ignore both the overheads incurred in real implementations of the cooperative techniques at the physical layer and their interactions with higher layer protocols in a networking context. In this paper, we study the performance of realistic networking scenarios facilitated by cooperation by taking overheads incurred at the physical, MAC, and network layers into account. In particular, (1) we modify the physical layer model of the QualNet network simulator to incorporate decentralized distributed space-time block coding into all SINR calculations and to combine signals transmitted concurrently from multiple relays, (2) we implement a path-centric MAC protocol to both reserve a multihop path between source and destination nodes and coordination relay nodes, and (3) we modify the DSR protocol to support path reservation at the network layer. Preliminary simulation results demonstrate that significant performance improvement can be achieved by employing cooperation. We also demonstrate the overheads which challenge their effectiveness in real networks. I.

is distributed space-time coding (DSTC) which achieves spatial diversity gain from multiple relays. A novel DSTC, called randomized distributed space-time coding (R-DSTC), shows considerable advantages over a regular DSTC in terms of system complexity. In this paper, we exploit the benefits of R-DSTC physical (PHY) layer and develop a distributed and opportunistic medium access control (MAC) layer protocol for R-DSTC deployment in an IEEE 802.11 wireless local area network (WLAN). Unlike other cooperative MAC designs, in our proposed PHY-MAC cross-layer framework, there is no need to decide which stations will serve as relays before each packet transmission. Instead, the MAC layer opportunistically recruits relay stations on the fly; any station that receives a packet from the source correctly forwards it to the destination. Through extensive simulations, we validate the efficiency of our MAC layer protocol and demonstrate that network capacity and delay performance is considerably improved with respect to legacy IEEE 802.11g network. I.

Abstract — Cooperative communication is a technique that can be employed to meet the increased throughput needs of nextgeneration WiMAX systems. In a cooperative scenario, multiple stations can jointly emulate the antenna elements of a multi-input multi-output (MIMO) system in a distributed fashion. Although distributed space-time coding (DSTC) is being considered by the IEEE 802.16j/16m standards for spatial diversity gain, it has several inherent drawbacks. These are addressed in the recently invented randomized distributed space-time coding, called R-DSTC. In this paper, we present the framework for the R-DSTC technique in the emerging relay-assisted WiMAX network, and develop a cooperative medium access control (MAC) layer protocol, called CoopMAX, for R-DSTC deployment in an IEEE 802.16 system. Our scheme couples the MAC layer with the physical (PHY) layer for performance optimization. The PHY layer yields significant diversity gain, while the MAC layer achieves a substantial end-to-end throughput gain. Through extensive simulations, we evaluate the performance of CoopMAX and show that it can generate capacity gains of up to about 77% for an IEEE 802.16 network. I.