A Pragmatic Approach to Cooperative Communication

This paper presents the `Generalized Poor Man's SIMO System' (gPMSS) which combines two approaches, cooperative communication and diversity combination, to reduce packet losses over links in wireless sensor networks. The proposed gPMSS is distinct from previous cooperative communication architectures in wireless sensor networks which rely on a relay channel, and also distinct from implementations in 802.11 networks that require a wired infrastructure or hardware changes for cooperation. gPMSS foregoes the need for any changes to mote hardware and it works within the current IEEE 802.15.4 standard. We describe the gPMSS protocol that governs the cooperation between receivers. Three variants are evaluated including selection diversity, equal gain and maximal ratio combining. First, we demonstrate gPMSS on bit error traces in a fully reproducible manner. This is followed by an implementation of gPMSS in C# on the .NET Micro Framework edition of the recently released Imote2 mote platform. We demonstrate by means of experiments an increase in the packet reception rate from 22---30% to 73---76%, a relative increase of 150---245%. We also analyzed the power consumed by the transmitter per delivered packet and observe a reduction of up to 68%. We also take into account the retry limit of the IEEE 802.15.4 protocol and demonstrate that gPMSS is able to provide 99% packet delivery at the protocol's default retry parameters against 65---75% without it.

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Synchronization of relay nodes is an important and critical issue in exploiting cooperative diversity in wireless networks. In this paper, two asynchronous cooperative diversity schemes are proposed, namely, distributed delay diversity and asynchronous space–time coded cooperative diversity schemes. In terms of the overall diversity–multiplexing (DM) tradeoff function, we show that the proposed independent coding based distributed delay diversity and asynchronous space–time coded cooperative diversity schemes achieve the same performance as the synchronous space–time coded approach which requires an accurate symbol-level timing synchronization to ensure signals arriving at the destination from different relay nodes are perfectly synchronized. This demonstrates diversity order is maintained even at the presence of asynchronism between relay node. Moreover, when all relay nodes succeed in decoding the source information, the asynchronous space–time coded approach is capable of achieving better DM tradeoff than synchronous schemes and performs equivalently to transmitting information through a parallel fading channel as far as the DM tradeoff is concerned. Our results suggest the benefits of fully exploiting the space–time degrees of freedom in multiple antenna systems by employing asynchronous space–time codes even in a frequency-flat-fading channel. In addition, it is shown asynchronous space–time coded systems are able to achieve higher mutual information than synchronous space–time coded systems for any finite signal-to-noise ratio (SNR) when properly selected baseband waveforms are employed. </para>

In this paper, we investigate cooperative OFDM communications using amplify-and-forward (AF) protocol at the relays, in the presence of imperfect timing synchronization. In most studies on cooperative communications, perfect time synchronization among cooperating nodes is assumed. In practice, however, due to imperfect time synchronization, orthogonality among the subcarriers of the different nodes' signals at the destination receiver can be lost, causing inter-symbol interference (ISI). In this paper, we derive analytical expressions for the average SINR at the DFT output at the destination as a function of timing offset in cooperative OFDM with AF protocol, and illustrate the SINR degradation as a function of the timing offset. We also present an interference canceling (IC) receiver to mitigate the effects of ISI when there is timing offset. We show that the proposed IC receiver achieves improved BER performance even when timing offsets are large.

Cooperative communication has characteristics of spatial diversity and virtual antenna, thus it can improve communication efficiency significantly in cellular or wireless data networks. One of the essential design issues in cooperative communication is partner selection mechanism. The problem of selecting suitable partners for a cooperative strategy is more complex when prospective partners have links of dissimilar quality to a central destination, leading to the possibility of non-reciprocal cooperation. In this paper, we consider a coded cooperative system under Rayleigh fading and path loss. We present a novel algorithm for partner selection and optimal power allocation in the Decode-and-Forward (DF) cooperative diversity that minimizes the outage probability given required total transmission power constraint. We evaluate the proposed algorithm by conducting simulations. Simulation results show that the selection strategy can increase duration of cooperation, as well as decrease the probability of unsuccessful partner selection. The results demonstrate that the outcomes of the proposed algorithm are very close to results of full search for optimal set.

This paper presents an exhaustive study on the use of one-relay and two-relay cooperative communication schemes and 2-hop communication scheme for improving the energy efficiency and reliability of ultra-wideband based wireless body area networks (UWB WBANs). Various investigations have been performed to study the impact of the parameters like packet size, hop distance, transmit power and channel error rate on the energy efficiency and reliability. An optimal packet size is obtained for the maximization of energy efficiency for both on-body communication and in-body communication. The analytical and simulation results show enhanced reliability with cooperative communication than direct communication and 2-hop communication, for all values of source to destination distances. The results also depict a threshold behaviour for energy efficiency which separates the hoplength for direct transmission from the hoplength where cooperation and 2-hop communication will be useful. The simulation results reveal that if the channel conditions are poor, when the source to destination distance is larger than the threshold value, 2-hop communication gives higher energy efficiency compared with direct and cooperative communications.

In recent years cooperative diversity has been widely used in wireless networks. In particular, cooperative communication with a single relay is a simple, practical technology for wireless sensor networks. In this paper, we analyze several simple network topologies. Under the condition of equal power allocating, the optimum relay location of each network topology are respectively made sure by using symbol error rate (SER) formula. And these types of topologies are compared, the analysis results show that, linear network topology has the best system performance, the system performance of isosceles triangle topology is better than that of equilateral triangle topology.

It is widely recognized that half-duplex-relay-aided differential decode-and-forward (DDF) cooperative transmission schemes are capable of achieving a cooperative diversity gain, while circumventing the potentially excessive-complexity and yet inaccurate channel estimation, especially in mobile environments. However, when a cooperative wireless communication system is designed to approach the maximum achievable spectral efficiency by taking the cooperation-induced multiplexing loss into account, it is not obvious whether or not the relay-aided system becomes superior to its direct-transmission based counterpart, especially, when advanced channel coding techniques are employed. Furthermore, the optimization of the transmit-interval durations required by the source and relay is an open issue, which has not been well understood in the context of half-duplex relaying schemes. Hence, we first find the optimum transmission duration, which is proportional to the adaptive channel-code rate of the source and relay in the context of Code-Rate-Optimized (CRO) TDMA-based DDF-aided half-duplex systems for the sake of maximizing the achievable network throughput. Then, we investigate the benefits of introducing cooperative mechanisms into wireless networks, which may be approached in the context of the proposed CRO cooperative system both from a pure capacity perspective and from the practical perspective of approaching the Discrete-input Continuous-output Memoryless Channel (DCMC) capacity with the aid of the proposed Irregular Distributed Differential (IrDD) coding aided scheme. In order to achieve a near-capacity performance at a low-complexity, an adaptive-window-duration based Multiple-Symbol Differential Sphere Detection (MSDSD) scheme is employed in the iterative detection aided receiver. Specifically, upon using the proposed near-capacity system design, the IrDD coding scheme devised becomes capable of performing within about 1.8 dB from the corresponding single-relay-aided DDF cooperative system's DCMC capacity.

Wireless multi-hop networks are candidate communication infrastructures in developing regions because their installation cost is quite low comparing to that for conventional wired networks. Reliable and scalable dissemination mechanism is required in practical multi-hop networks. Cooperative communication is a new scheme to improve communication performance to realize a reliable dissemination mechanism. However, high mobility networks have a big issue for autonomous control for cooperative communication. Additionally, conventional cooperative schemes cannot reduce packet collisions even if packet collisions are a big degradation factor of performance in dense multi-hop networks. We have discussed a new cooperative communication scheme that employs Orthogonal Frequency-Division Multiplexing (OFDM). Our scheme can improve communication performance and reduce packet collisions based on distributed control matter. Our advance evaluation has shown a good potential of the proposed scheme to improve communication performance in multi-hop network simulations. On the contrary, the feasibility of the proposed scheme with a real hardware device was not shown. This paper evaluates the feasibility of our original cooperative communication scheme on IEEE 802.11a standard devices. Almost all conventional cooperative communication schemes require a special physical layer and a media access control mechanism. We have focused on a characteristic of OFDM, where a receiver can demodulate multiple signals within a guard interval period. The experimental evaluation has been performed with software defined radio devices (USRP B200). We have implemented multiple transmitters, a receiver and a fading simulator for each transmitter with two USRP B200 devices. The results can show that the proposed scheme works well on IEEE 802.11a standard devices and improves communication performance in practical situations. Hence, the proposed scheme will be a new candidate of cooperative communication mechanisms that can employ general consumer devices and can realize effective dissemination mechanism for multi-hop networks.

Cooperative communications have been proposed to exploit the spatial diversity gains inherent in multiuser wireless systems without the need of multiple antennas at each node. This is achieved by having the users relay each others messages and thus forming multiple transmission paths to the destination. In resource constrained networks, such as wireless sensor networks, the advantages of cooperation can be further exploited by optimally allocating the energy and bandwidth resources among users based on the available channel state information (CSI) at each node. In the first part of this article, we provide a tutorial survey on various power allocation strategies for cooperative networks based on different cooperation strategies, optimizing criteria, and CSI assumptions. In the second part, we identify the similarities between cooperative networks and several sensor network applications that utilize collaboration among distributed sensors to achieve the system goal. These applications include decentralized detection/estimation and data gathering. The techniques developed in cooperative communications can be used to solve many sensor network problems

In recent years, there has been an upsurge of interest in relay-based wireless networks in both industry and academia. With the introduction of relay node, which can forward packet for some other nodes which require relaying assistance, the network performance can be dramatically improved: the coverage of an infrastructure network is extended; spatial diversity is exploited and system capacity is increased; meanwhile, the transmission reliability is enhanced. In relay wireless networks, in order to fully exploit the benefit of relay transmissions, MAC and network layer schemes need to be redesigned. In this thesis, we will focus on the study of relay selection problem in MAC layer and optimal routing decision problem in network layer. Several relay and routing selection schemes are proposed for different wireless applications when concept of relaying is applied. In relay selection part, a cooperative MAC protocol is designed to dynamically select relay nodes according to real-time channel condition and contention relationship for broadcast traffics in vehicular networks; in respect to the cognitive radio networks, a relay-based cognitive radio architecture is proposed to balance the spectrum availability and traffic demand for secondary users, an experimental testbed is constructed to verify its effectiveness; we also propose an incentive relay set selection scheme to motivate secondary users to act as cooperative relays to forward packets for primary users. In routing selection part, a cooperative routing scheme which considers contention relationship among links is proposed in multi-source multi-destination multi-hop wireless networks; a cooperative network coding-aware routing scheme is given to enable relaying to further enhance the benefit of network coding in wireless networks. Finally, a software-defined radio-based cooperative communication testbed is built to evaluate the performance of various cooperative transmission schemes. The above studies demonstrate that with appropriate relay and routing schemes, relaying benefit and cooperative diversity is fully exploited and network performance is remarkably enhanced in relay-based wireless networks.

We develop a two-source–destination-pair cooperative communication scheme that can achieve multiuser diversity and cooperative diversity. In this scheme, we use hybrid decode–amplify–forward (HDAF) protocol to improve system performance. From practical point of view, HDAF protocol performs better than amplify-and-forward and decode-and-forward protocols by combining the merits of both. Meanwhile, we derive a closed-form expression of outage probability associated with HDAF protocol analyzed. Based on outage probability constraint, we investigate optimal power allocation that minimizes the total power. Numerical analysis is used to confirm the accuracy of the derived theory and to examine the performance of our proposed cooperative scheme, particularly as we analyze power savings in using optimal power allocation compared with equal power allocation, which is commonly assumed in previous studies. It is shown that significant power savings can be obtained by using the proposed power optimization method.

A hierarchical convergence mechanism for the heterogeneous wireless communication system via the heterogeneous cooperative relay node is presented in this paper, in which the techniques of cooperative communication and wireless relay are utilized to improve performances of the individual user and the overall converged networks. In order to evaluate the benefits of the proposal, a utility‐based capacity optimization framework for achieving the heterogeneous cooperative diversity gain is proposed. The heterogeneous cooperative capacity, relay selection and power allocation theoretical models are derived individually. The joint optimization model for relay selection and power allocation is presented as well. Owing to the computation complexity, the sub‐optimal cooperative relay selection algorithm, the sub‐optimal power allocation algorithm and the sub‐optimal joint algorithm are determined to approach the maximum overall networks' spectrum efficiency. These proposed algorithms are designed in conformance to guarantee the equivalent transmission rates of the different wireless access networks. The simulation results demonstrate that the utility‐based capacity model is available for the heterogeneous cooperative wireless communication system, and the proposed algorithms can improve performances by achieving the cooperative gain and taking full advantage of the cross‐layer design. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Cooperative communication is a promising technique for future wireless networks. It can be used in improving communication reliability, and enhancing power and spectrum efficiency. However, its performance gain degrades in the presence of co-channel interference which makes it essential to propose interference mitigation schemes. In this paper, we introduce two efficient cooperative communication schemes with interference management for multi-user cooperative wireless networks that are based on best relay and user selection (BRUS) technique. BRUS maximizes the received signal to noise ratio (SNR) while minimizing the interference by an optimal time slot allocation for the users. The first introduced scheme is using BRUS technique only if the user demands cooperation, and the second scheme is always using BRUS technique to enhance the system performance. We derive exact closed form expressions for the outage probability, ergodic capacity, average consumed power, and average bit error probability for the introduced cooperative schemes. Simulations are used to validate the analytical results and an agreement is observed. The results confirm the advantage of the introduced cooperation schemes in enhancing the system performance and improving the interference management.

On construction of quality virtual backbone in wireless networks using cooperative communication

In cooperative communications, due to the distributed nature, multiple different carrier frequency offsets (CFOs) may occur and make the channel time-varying. It is hard for the receiver to compensate multiple CFOs from multiple relay nodes simultaneously. Thus, the conventional space-time codes to collect cooperative diversity for co-located multi-input multi-output (MIMO) systems may not be applied directly. In this paper, we consider the cooperative transmission with multiple CFOs when the channels from relay nodes to destination node are flat (frequency-non-selective) fading. We approximate a flat fading channel with CFO as a block time-invariant intersymbol-interference (ISI) channel in the frequency domain. We then propose two distributed space-frequency codes (SFC) for such ISI channels in the frequency domain to achieve the cooperative full spatial diversity, where the space-frequency coding concept is different from the one in the literature and also has a different role. One is called frequency-reversal SFC and the other is called frequency-domain linear convolutive SFC. Furthermore, we show that, with only linear receivers, such as zero-forcing (ZF) and minimum mean square error (MMSE) receivers, our codes achieve the full cooperative diversity.