Cooperative Diversity in Wireless Networks: Efficient Protocols and Outage Behavior

<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>

Diversity, i.e. transmitting multiple replicas of a signal, may mitigate fading in wireless networks. Among other diversity techniques, the space diversity of multi-antenna systems is particularly interesting since it can complement other forms of diversity. The recent cooperative diversity paradigm brings the advantages of multi-antenna space diversity to single antenna networked devices, which, through cooperation and antenna sharing, form virtual antenna arrays. However, cooperative diversity is a complex technique and research on this topic is still in its early stages. This paper aims at providing a general survey on the theoretical framework; and the physical and medium access control proposals in the literature.

This paper presents the application of two spatial transmission techniques which can be used in a cooperative diversity wireless network, where each mobile terminal has two antenna elements. Initially the current body of work in single antenna cooperative diversity is expanded to include the effect of cooperative diversity upon the capacity of a link. This is to serve as a baseline model for comparison with the dual antenna methods. This paper then introduces two techniques which can be used for cooperative diversity where mobile terminals are each equipped with two antenna elements. Using the same metrics as the single antenna case, a comparison is made between the results from simulations of each technique. It is shown that dual antenna elements offer considerable advantage over the single antenna element case, and that two antenna element space-time transmit diversity (STTD) based cooperative diversity is preferred at low SNRs.

We study power allocation for the decode-and-forward cooperative diversity protocol in a wireless network under the assumption that only mean channel gains are available at the transmitters. In a Rayleigh fading channel with uniformly distributed node locations, we aim to find the power allocation that minimizes the outage probability under a short-term power constraint, wherein the total power for all nodes is less than a prescribed value during each two-stage transmission. Due to the computational and implementation complexity of the optimal solution, we derived a simple near-optimal solution. In this near-optimal scheme, a fixed fraction of the total power is allocated to the source node in stage I. In stage II, the remaining power is split equally among a set of selected nodes if the selected set is not empty, and otherwise is allocated to the source node. A node is selected if it can decode the message from the source and its mean channel gain to the destination is above a threshold. In this scheme, each node only needs to know its own mean channel gain to the destination and the number of selected nodes. Simulation results show that the proposed scheme achieves an outage probability close to that for the optimal scheme obtained by numerical search, and achieves significant performance gain over other schemes in the literature

An efficient cooperative transmission scheme has been recently proposed based on superposition modulation. Analytical results are necessary to fully understand the superiority of this new scheme, which is the main contribution of this paper. Tight upper and lower bounds of the outage probability of this scheme are first proposed and then the large SNR behavior of the outage probability is studied. Our analytical results prove that the superposition cooperative transmission scheme can achieve around 3 dB better performance and larger diversity order than the selection relaying scenario, which are consistent with the simulation results reported earlier. Finally, we provide outage, delay-limited and ergodic capacity results of cooperative techniques

Cooperative diversity can provide dramatic gains in slow fading wireless environments. In all the cooperative diversity protocols,Distributed Space-Time Coding (DSTC) is the most representative. Due to its complexity,the practical distributed space-time coding is still a big challenge. In this paper,the authors introduced a novel cooperative diversity protocol: Opportunistic Distributed Space-Time Coding (ODSTC),which could achieve the performance similar to distributed space-time coding,with lower complexity. In addition,the principle to select DSTC or ODSTC was analyzed based on the distribution of relay nodes.

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.

Link quality prediction in mesh networks

Orthogonal frequency division multiplexing (OFDM) is a practical technique for communicating over broadband channels in multi-path fading environments. It is well known that varying the power allocation from frequency tone-to-frequency tone, commonly called power loading, can improve capacity or error rate performance. Unfortunately, the implementation of power loading is complicated by the fact that the transmitter must have knowledge of the forwardlink channel. This assumption is often unrealistic, particularly in systems using frequency division duplexing (FDD). In this paper we present a limited feedback approach for power loading OFDM symbols. In this approach, the receiver, which is assumed to have full forwardlink channel knowledge, designs a power loading vector and conveys it to the transmitter over a limited rate feedback channel. The technique uses a codebook of power loading vectors known to both the transmitter and receiver. We design limited feedback schemes for error rate optimized and capacity optimized power loading. We show how to design quantizers using iterative optimization techniques from the theory of vector quantization for the case of error rate selection. In the capacity case, we characterize optimal feedback schemes for asymptotic signal-to-noise ratio (SNR) scenarios and present a simple method called multi-mode power loading that uses a codebook that switches subcarriers off and on. Simulation results show that the limited feedback techniques provide performance close to perfect channel knowledge power loading.

Multipath fading is one of the primary factors for degrading the performance in a wireless network. Information theoretic and past research suggest the use various diversity techniques to combat fading in wireless networks. Antenna diversity, a form of diversity technique, when incorporated in a wireless transceiver increases the system capacity and is one of the effective methods to combat fading in wireless systems. Also, recent research by Laneman et.al., Sendonaris et.al. suggests that cooperation among users in a wireless networks is an effective approach for a better signal reception in multipath fading environments. The diversity gains obtained by cooperation among the users of a wireless network is termed as cooperative diversity . Although, prior research in cooperative diversity considers users equipped with single antenna, in practical scenarios users may be able to accommodate multiple antennas due to the recent advanced research in semiconductor industry. Hence, the primary purpose of this thesis is to design, simulate and analyze an end-end performance of multi-antenna wireless systems employing cooperative multi antenna relay nodes so as to exploit the cooperative diversity and antenna diversity simultaneously in a wireless networks. Three main contributions to the area of cooperative multiple-input multiple-output (MIMO) wireless systems is presented in this thesis. First, we perform information theoretic analysis to study the impact of antenna arrays on cooperative wireless networks and propose the best possible distribution of antenna arrays among the three terminals of a simple three terminal cooperative relay network. Second, we design, simulate, and analyze a cooperative multiple-input multiple-output (MIMO) wireless systems employing orthogonal space-time block codes as proposed by Alamouti in 1998 with a decode-and-forward (DF) relay terminal. We implement a maximal ratio combining receiver that provides almost twice the diversity gain with respect to point-point multiple input multiple output link. Finally, we implement a practical receiver for cooperative reception using multiple antennas at all nodes based on Bell-Labs Layered Space Time architecture (BLAST). We incorporate a practical adaptive decode-and-forward (DF) relaying technique for reliable signal retransmission for both Alamouti space-time coding and the BLAST schemes. Results presented in terms of bit error rates and throughput show that remarkable performance gains are achievable by combining the concepts drawn from space-time coding, cooperative relaying and array processing.

We consider the use of cooperative diversity in a multiuser wireless data network. This paper provides an analysis of the interaction between cooperative diversity and multiuser diversity on downlink channels. By using approximation of the signal-to-noise ratio (SNR) distribution of each cooperative diversity link by Gamma distribution, an analytic expression is derived for the average throughput of a single-cell wireless system with multiple cooperative diversity links combined with a fair-access scheduler. The proposed analytic approach is verified through comparisons with simulated results and shows that cooperative diversity makes the detrimental impacts on multiuser diversity.

The UWA channel is characterized as a time-dispersive rapidly fading channel, which in addition exhibits Doppler instabilities. To eliminate inter-symbol interference caused by multipath propagation, spatial diversity equalization is the main technical means. This paper mainly uses the method of complex weighted coefficient to achieve high SNR signal and maximize array gain. This paper uses the passive-phase-conjugation (PPC) algorithm to calculate signal-to-noise ratio(SNR) and signal-to-interference ratio(SIR) with impulse response function and communication signal. This paper firstly calculates the signal-to-noise-plus-interference ratio (SNIR) and uses the Sigmoid function to optimize the weighted coefficients of each channel. Secondly, the optimized weighted coefficients need to be normalized. It implements space and time joint equalization. Thirdly, by introducing the second order phase locked loop, it can track signal phase change caused by UWA channel. Lastly, the paper adopts fractional-decision feedback diversity equalizer(FDFDE). It achieves diversity equalization by using different channel weighted coefficients. The simulation and lake trial data processing results show that, the optimized diversity receiving equalization algorithm can improve communication system's ability in tracking the change of underwater acoustic channel, offset the impact of multipath and noise, and improve the performance of communication system. The performance of the communication receiving system is better than that of the combination with equal gain. At the same time, the bit error rate (BER) reduces for 1.7%.

We provide a compact characterization of the outage rates for wideband wireless communication, quantifying the effects of frequency and spatial diversity. Our information-theoretic computations yield the following findings. (a) Complex models typically employed in simulations can be replaced by simple, bandwidth-dependent, tap delay line models without loss of generality. (b) The spectral efficiency (i.e., the achievable rate, divided by the bandwidth) is well approximated as a Gaussian random variable, so that it is only necessary to specify its mean and variance to obtain outage rates. (c) The mean spectral efficiency depends only on the spatial diversity, while the variance depends on the spatial and frequency diversity via a product. Our definitions of frequency and spatial diversity have physically motivated interpretations, and are not based on high signal-to-noise ratio (SNR) asymptotics as in prior work.

To improve the communication quality and reliability in the South China Sea (SCS), we proposed a method to exploit the combination of cooperative diversity and evaporation ducts. The simulation results show that the reliability of 300 km ranges at 12 GHz increases from the lowest 43.08% to 94.90% by using space diversity. Communication-link reliability for 100 km ranges may approach 100% or 98% for 200 km ranges. Furthermore, communication effects get significantly improved, and the ratio of signal-to-noise ratio (SNR) exceeds 40 dB that is up to 84.52% with diversity reception. By investigating the cooperation diversity for communication, we could find a favorable condition to realize optional-wide-frequency, long-range, and high-reliability transmission, which supports the scenario of sparse mobile communication in the SCS.

The wireless mesh network is a new emerging technology that will change the world of industrial networks connectivity to more efficient and profitable. Mesh networks consist of static wireless nodes and mobile customer; have emerged as a key technology fornew generation networks. The Quality of Service (QOS) is designed to promote and support multimedia applications (audio and video), real time. However guarantee of QoS on wireless networks is a difficult problem by comparison at its deployment in a wired IP network. In this paper, we present an efficient routing protocol named as “QoS- Cluster Based Routing Protocol” (Q-CBRP) to support QoS in Wireless Mesh Network.