Cooperative Diversity (CD) networks have been receiving a lot of attention recently as a distributed means of improving error performance and capacity. For sufficiently large signal to noise ratio (SNR) this paper derives the average Symbol Error Probability (SEP) for Analog Forwarding CD links. The resulting expressions are general as they hold for an arbitrary number of cooperating branches, arbitrary number of cooperating hops per branch, and various channel fading models. Their simplicity provides valuable insights to the performance of CD networks and suggests means of optimizing them. Besides revealing the diversity, they clearly show from where this advantage comes from and prove that presence of diversity does not depend on the specific (e.g., Rayleigh) fading distribution. Finally, they explain how diversity is improved in multi-hop CD networks.

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 — Within a new paradigm, where wireless user cooperation is viewed as a form of (opportunistic) multipath, we exploit the unique capabilities of direct-sequence spread spectrum transmissions in handling multipath to design a novel spectrally efficient protocol for wireless cooperative networks. We show how and why our proposed system achieves diversity without increasing bandwidth. After analyzing its performance, we deduce that user capacity can be significantly improved with respect to existing third generation cellular systems in the uplink. Index Terms — Fading, diversity, cooperative diversity, CDMA. I.

We consider a wireless network with one source/destination pair and several linear amplify-and-forward relays. The influence of the gain allocation at the relays on the performance in cooperative relay communication links is analyzed. We present an optimal gain allocation which results in a coherent combining of all signal contributions at the destination and maximizes the instantaneous throughput of the link.

Abstract—This paper introduces a novel differential modulation scheme for a two-user cooperative diversity system which does not require channel state information at either the users or the destination. The performance of fixed decode-and-forward and selection relaying protocols is evaluated in both symmetric and asymmetric interuser-channel cases. The lower bound on the performance of the decode-and-forward protocol is given, while the exact bit-error probability of the selection relaying protocol is thoroughly derived. The decode-and-forward relaying protocol achieves a performance gain when the signal-to-noise ratios in the interuser channels are symmetric and sufficiently high. The selection relaying protocol shows a larger performance gain and does not exhibit an error floor like in the case of the decode-and-forward protocol. In addition, it is robust to the asymmetric interuser channels. Index Terms—Asymmetric interuser channels, cooperative diversity, decode-and-forward relaying, differential modulation, selection relaying. I.

Abstract — Cooperative relaying recently emerged as a viable option for future wireless networks. By simultaneously exploiting path loss savings known from relaying scenarios and the diversity inherent to any scheme involving spatially separated transmitters, this technique is able to leverage gains from both relaying and spatial diversity techniques. In this paper, we study different cooperative relaying protocols and compare their performance with that of direct transmission and conventional relaying. We investigate under which conditions the developed techniques provide gains over other approaches. Our results confirm that cooperative relaying is an effective means of enhancing the performance of wireless systems whenever temporal and frequency diversity is scarce. I.

Abstract — Distributed space-time coding (STC) gives rise to nonidentical channels between different transmit and receive antennas, which are not co-located. Therefore, the performance and the optimum power allocation strategy for distributed STC is different from conventional STC. In this paper, we derive the exact bit error probability for distributed space-time block codes over nonidentical Ricean channels, with an arbitrary number of relays. Based on the closed-form BEP results, we investigate the optimum power allocation strategies between different nodes and symbols. I.

Abstract — The aim of this paper is to show how cooperation among nodes of a wireless network can be useful to reduce the overall radiated power necessary to guarantee reliable links among the network nodes. The basic idea is that if the links between cooperating nodes are sufficiently reliable, the cooperating nodes can transmit in a coordinated manner in order to emulate a virtual MIMO system that can yield considerable gains in terms of diversity or capacity. In this paper, we provide first a theoretical analysis of a single-user scenario showing how the cooperation gain is related to the spatial density of the cooperating nodes. Then, we compare alternative distributed space-time coding strategies aimed at achieving the promised advantages in a multi-user context 1. I.

We consider a wireless network with one source/destination pair and several linear amplify-andforward relays. All nodes are equipped with one single antenna. To achieve cooperative diversity we propose time-variant and relay specific phase rotations induced at each relay to make the e#ective channel time-variant. This transformation of spatial diversity into temporal diversity can be exploited by an appropriate outer code. Furthermore we show that the allocation of the amplification gains at the relays has great influence on the diversity performance and we give a low complexity extension to existing gain allocations.

The application of distributed space-time coding schemes in a simulcast network is considered, and a key challenge is addressed which arises in the downlink: Since the local oscillators employed at the individual transmitting nodes may independently differ from the nominal carrier frequency, frequency offsets will occur between the individual transmission signals. The influence of these frequency offsets on the performance of a specific distributed space-time coding scheme is investigated, and both simulative and analytical results are presented. Appropriate receiversided counter measures are considered and possibilities are discussed to estimate the occurring frequency offsets at the receiver. Index Terms---Wireless communications, cooperative networks, distributed space-time coding techniques, frequency offsets.