Abstract—We develop and analyze space–time coded cooperative diversity protocols for combating multipath fading across multiple protocol layers in a wireless network. The protocols exploit spatial diversity available among a collection of distributed terminals that relay messages for one another in such a manner that the destination terminal can average the fading, even though it is unknown a priori which terminals will be involved. In particular, a source initiates transmission to its destination, and many relays potentially receive the transmission. Those terminals that can fully decode the transmission utilize a space-time code to cooperatively relay to the destination. We demonstrate that these protocols achieve full spatial diversity in the number of cooperating terminals, not just the number of decoding relays, and can be used effectively for higher spectral efficiencies than repetition-based schemes. We discuss issues related to space–time code design for these protocols, emphasizing codes that readily allow for appealing distributed versions. Index Terms—Diversity techniques, fading channels, outage probability, relay channel, user cooperation, wireless networks. I.

Cooperative diversity is a transmission technique where multiple terminals pool their resources to form a virtual antenna array that realizes spatial diversity gain in a distributed fashion. In this paper, we examine the basic building block of cooperative diversity systems, a simple fading relay channel where the source, destination and relay terminals are each equipped with single antenna transceivers. We consider three different TDMA-based cooperative protocols that vary the degree of broadcasting and receive collision. The relay terminal operates in either the amplify-and-forward (AF) or decode-and-forward (DF) modes. For each protocol, we study the ergodic and outage capacity behavior (assuming Gaussian code books) under the AF and DF modes of relaying. We analyze the spatial diversity performance of the various protocols and find that full spatial diversity (second-order in this case) is achieved by certain protocols provided that appropriate power control is employed. Our analysis unifies previous results reported in the literature and establishes the superiority (both from a capacity as well as a diversity point-of-view) of a new protocol proposed in this paper. The second part of the paper is devoted to (distributed) space-time code design for fading relay channels operating in the AF mode. We show that the corresponding code design criteria consist of the traditional rank and determinant criteria for the case of co-located antennas as well as appropriate power control rules. Consequently space-time codes designed for the case of co-located multi-antenna channels can be used to realize cooperative diversity provided that appropriate power control is employed.

Cooperative diversity has been recently proposed as a way to form virtual antenna arrays that provide dramatic gains in slow fading wireless environments. However, most of the proposed solutions require distributed space–time coding algorithms, the careful design of which is left for future investigation if there is more than one cooperative relay. We propose a novel scheme that alleviates these problems and provides diversity gains on the order of the number of relays in the network. Our scheme first selects the best relay from a set of available relays and then uses this “best ” relay for cooperation between the source and the destination. We develop and analyze a distributed method to select the best relay that requires no topology information and is based on local measurements of the instantaneous channel conditions. This method also requires no explicit communication among the relays. The success (or failure) to select the best available path depends on the statistics of the wireless channel, and a methodology to evaluate performance for any kind of wireless channel statistics, is provided. Information theoretic analysis of outage probability shows that our scheme achieves the same diversity-multiplexing tradeoff as achieved by more complex protocols, where coordination and distributed space–time coding for relay nodes is required, such as those proposed by Laneman and Wornell (2003). The simplicity of the technique allows for immediate implementation in existing radio hardware and its adoption could provide for improved flexibility, reliability, and efficiency in future 4G wireless systems.

Abstract—When mobiles cannot support multiple antennas due to size or other constraints, conventional space-time coding cannot be used to provide uplink transmit diversity. To address this limitation, the concept of cooperation diversity has been introduced, where mobiles achieve uplink transmit diversity by relaying each other’s messages. A particularly powerful variation of this principle is coded cooperation. Instead of a simple repetition relay, coded cooperation partitions the codewords of each mobile and transmits portions of each codeword through independent fading channels. This paper presents two extensions to the coded cooperation framework. First, we increase the diversity of coded cooperation in the fast-fading scenario via ideas borrowed from space-time codes. We calculate bounds for the bit- and block-error rates to demonstrate the resulting gains. Second, since cooperative coding contains two code components, it is natural to apply turbo codes to this framework. We investigate the application of turbo codes in coded cooperation and demonstrate the resulting gains via error bounds and simulations. Index Terms—Channel coding, diversity, space-time coding, user cooperation, wireless communications.

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Abstract — Motivated by the recent works on the relay channel and cooperative diversity, this letter introduces coded cooperation, where cooperation is achieved through channel coding methods instead of a direct relay or repetition. Each codeword is partitioned into two subsets that are transmitted from the user’s and partner’s antennas, respectively. Coded cooperation achieves impressive gains compared to a non-cooperative system while maintaining the same information rate, transmit power, and bandwidth. We develop bounds on BER and FER and illustrate the advantage of coded cooperation under a number of different scenarios. Index Terms — User cooperation, diversity, transmit diversity, space-time coding.

Abstract—We consider a cooperative transmission scheme in which the collaborating nodes may have multiple antennas. We present the performance analysis and design of space–time codes that are capable of achieving the full diversity provided by user cooperation. Our codes use the principle of overlays in time and space, and ensure that cooperation takes place as often as possible. We show how cooperation among nodes with different numbers of antennas can be accomplished, and how the quality of the interuser link affects the cooperative performance. We illustrate that space–time cooperation can greatly reduce the error rates of all the nodes involved, even for poor interuser channel quality. Index Terms—Cooperative diversity, diversity methods, errorcorrection coding, fading channels, multiple-input multiple-output (MIMO) systems. I.

We examine the outage capacity of of coded cooperation. Coded cooperation is a wireless user cooperation protocol that integrates cooperative signaling with channel coding. Each user’s code word is partitioned into two subsets that are transmitted from the user’s and the partner’s antennas, respectively. A notable outcome of this research is that, unlike the decode-and-forward protocol that was shown by Laneman to have diversity of one, coded cooperation achieves diversity order in the number of cooperating users. Thus we show that coded cooperation is fundamentally distinct from decode-and-forward, despite their superficial similarities. We also apply our analysis to space-time cooperation and study the effects of cooperation resource allocation. Numerical evaluation of outage expressions show that coded cooperation has a performance advantage across a wide range of SNR over several other cooperation protocols. I.

Abstract—Relay links are expected to play a critical role in the design of wireless networks. This paper investigates the energy efficiency of relay communications in the low-power regime under two different scenarios: when the relay has unlimited power supply and when it has limited power supply. A system with a source node, a destination node, and a single relay operating in the time division duplex (TDD) mode was considered. Analysis and simulations are used to compare the energy required for transmitting one information bit in three different relay schemes: amplify and forward (AnF), decode and forward (DnF), and block Markov coding (BMC). Relative merits of these relay schemes in comparison with direct transmissions (direct Tx) are discussed. The optimal allocation of power and transmission time between source and relay is also studied. Index Terms—Amplify and forward, block Markov coding, decode and forward, energy efficiency, relay transmission. I.

We consider user cooperation in a wireless relay network that comprises a collection of transmitters sending data to a common receiver through inherently unreliable and constantly changing channels. Exploiting the recently developed technology of network coding, we propose a new framework, termed adaptive network coded cooperation (ANCC), as a practical, de-centralized, adaptive and efficient cooperative strategy for large-scale wireless networks. The key idea is to match network-on-graph, i.e. the instantaneous network topology described in graph, with the wellknown class of codes-on-graph, i.e. low-density parity-check (LDPC) codes and LDPC-like codes, to combat the lossy nature of wireless links and to adapt to the changing network topology. We demonstrate, through several ensembles of low-density generator matrix (LDGM) codes and lowertriangular LDPC codes, that the proposed framework can considerably increase the cooperation level and reduce the outage rate. Huge gains of some 20-40 dBs are achieved over the conventional repetition schemes!