Information Theory and Reliable Communication

Cooperative communication systems using various relay strategies can achieve spatial diversity gains, enhance coverage, and potentially increase capacity. For the practically attractive decode-and-forward (DF) relay strategy, we derive a high-performance low-complexity coherent demodulator at the destination in the form of a weighted combiner. The weights are selected adaptively to account for the quality of both source-relay-destination and source-destination links. Analysis proves that the novel coherent demodulator can achieve the maximum possible diversity, regardless of the underlying constellation. Its error performance tightly bounds that of maximum-likelihood (ML) demodulation, which provably quantifies the diversity gain of ML detection with DF relaying. Simulations corroborate the analysis and compare the performance of the novel decoder with existing diversity-achieving strategies including analog amplify-and-forward and selective-relaying.

https://www.dtc.umn.edu/s/resources/tcom07july.pdf

High-Performance Cooperative Demodulation With Decode-and-Forward Relays

This paper develops a general framework for maximum likelihood (ML) demodulation in cooperative wireless communication systems. Demodulators with piecewise-linear combining are proposed as an accurate approximation of the nonlinear ML detectors for coherent and noncoherent decode-and-forward (DF). The detectors with piecewise-linear combiner not only have certain implementation advantages over the nonlinear ML detectors, but also can lead to tight closed-form approximations for their error probabilities. High SNR approximations are derived based on the closed-form BER expressions. For noncoherent DF, the approximation suggests a different optimal location for the relay in DF than for the relay in amplify-and-forward (AF). A set of tight bounds of diversity order for coherent and noncoherent DF with multiple relays is also provided, and comparison between DF and AF suggests that DF with more than one relay loses about half of the diversity order of AF

Modulation and demodulation for cooperative diversity in wireless systems

We consider a general multiple-antenna network with multiple sources, multiple destinations, and multiple relays in terms of the diversity-multiplexing tradeoff (DMT). We examine several subcases of this most general problem taking into account the processing capability of the relays (half-duplex or full-duplex), and the network geometry (clustered or nonclustered). We first study the multiple-antenna relay channel with a full-duplex relay to understand the effect of increased degrees of freedom in the direct link. We find DMT upper bounds and investigate the achievable performance of decode-and-forward (DF), and compress-and-forward (CF) protocols. Our results suggest that while DF is DMT optimal when all terminals have one antenna each, it may not maintain its good performance when the degrees of freedom in the direct link are increased, whereas CF continues to perform optimally. We also study the multiple-antenna relay channel with a half-duplex relay. We show that the half-duplex DMT behavior can significantly be different from the full-duplex case. We find that CF is DMT optimal for half-duplex relaying as well, and is the first protocol known to achieve the half-duplex relay DMT. We next study the multiple-access relay channel (MARC) DMT. Finally, we investigate a system with a single source-destination pair and multiple relays, each node with a single antenna, and show that even under the ideal assumption of full-duplex relays and a clustered network, this virtual multiple-input multiple-output (MIMO) system can never fully mimic a real MIMO DMT. For cooperative systems with multiple sources and multiple destinations the same limitation remains in effect.

Multiple-Antenna Cooperative Wireless Systems: A Diversity–Multiplexing Tradeoff Perspective

The Third Generation Partnership Project's Long Term Evolution-Advanced is considering relaying for cost-effective throughput enhancement and coverage extension. While analog repeaters have been used to enhance coverage in commercial cellular networks, the use of more sophisticated fixed relays is relatively new. The main challenge faced by relay deployments in cellular systems is overcoming the extra interference added by the presence of relays. Most prior work on relaying does not consider interference, however. This paper analyzes the performance of several emerging half-duplex relay strategies in interference-limited cellular systems: one-way, two-way, and shared relays. The performance of each strategy as a function of location, sectoring, and frequency reuse are compared with localized base station coordination. One-way relaying is shown to provide modest gains over single-hop cellular networks in some regimes. Shared relaying is shown to approach the gains of local base station coordination at reduced complexity, while two-way relaying further reduces complexity but only works well when the relay is close to the handset. Frequency reuse of one, where each sector uses the same spectrum, is shown to have the highest network throughput. Simulations with realistic channel models provide performance comparisons that reveal the importance of interference mitigation in multihop cellular networks.

/pdf/relay-architectures-for-3gpp-lte-advanced-2kl7bbqz07.pdf

Relay architectures for 3GPP LTE-advanced

Relaying and cooperative diversity allow multiple wireless radios to effectively share their antennas and create a virtual antenna array, thereby leveraging the spatial diversity benefits of multiple-input, multiple-output (MIMO) antenna systems. This paper examines the benefits of cooperative diversity for scenarios in which the receivers cannot exploit accurate channel state information (CSI). In particular, noncoherent demodulation is explored for two classes of relay processing, namely, detect-and-forward and amplify-and-forward. A complete maximum likelihood (ML) framework for noncoherent demodulation is developed for detect-and-forward, and is shown to naturally extend the corresponding framework for coherent demodulation. By contrast, the intractability of ML demodulation for noncoherent amplify-and-forward is demonstrated, suggesting a disconnect from the well-developed framework for coherent demodulation. Simulation results exhibit the diversity benefits of the detect-and-forward algorithms.

Cooperative diversity for wireless fading channels without channel state information

This paper demonstrates the significant gains that multi-access users can achieve from sharing a single amplify-forward relay in slow fading environments. The proposed protocol, namely the multi-access relay amplify-forward, allows for a low-complexity relay and achieves the optimal diversity-multiplexing trade-off at high multiplexing gains. Analysis of the protocol reveals that it uniformly dominates the compress-forward strategy and further outperforms the dynamic decode-forward protocol at high multiplexing gains. An interesting feature of the proposed protocol is that, at high multiplexing gains, it resembles a multiple-input single-output system, and at low multiplexing gains, it provides each user with the same diversity-multiplexing trade-off as if there is no contention for the relay from the other users.

A Case For Amplify-Forward Relaying in the Block-Fading Multi-Access Channel

This paper applies spectral efficiency as a performance measure for routing schemes and considers how to obtain a good route in a wireless network. The objective for this study is to combine different perspectives from networking and information theory in the design of routing schemes. The problem of finding the optimum route with the maximum spectral efficiency is difficult to solve in a distributed fashion. Motivated by an information-theoretic analysis, this paper proposes two suboptimal alternatives, namely, the approximatelyideal- path routing (AIPR) scheme and the distributed spectrumefficient routing (DSER) scheme. AIPR finds a path to approximate an optimum regular path and requires location information. DSER is more amenable to distributed implementations based on the Bellman-Ford or Dijkstra?s algorithms. The spectral efficiencies of AIPR and DSER for random networks approach that of nearest-neighbor routing in the low signal-to-noise ratio (SNR) regime and that of single-hop routing in the high SNR regime. In the moderate SNR regime, the spectral efficiency of DSER is up to twice that of nearest-neighbor or single-hop routing.

Distributed spectrum-efficient routing algorithms in wireless networks

This work develops a general framework for maximum likelihood (ML) demodulation in cooperative wireless systems with a demodulate-and-forward (DF) protocol at the relays. Although this general structure admits both coherent and noncoherent modulation, we analyze the performance of this demodulator mainly for noncoherent demodulation and compare it to existing results for coherent demodulation. The ML demodulator consists of a band of square-law devices followed by a nonlinear combiner. We develop an accurate approximation for this demodulator using a band of square-law devices followed by a piecewise-linear (PL) combiner. This approximation not only has certain implementation advantages, but also leads to a tight closed form approximation for the bit error rate (BER) of the ML demodulator. Based on this closed-form result, we derive a high signal-noise-ratio (SNR) approximation, that provides a simpler but tight approximation of the BER of the ML demodulator.

Deqiang Chen

Papers

Modulation and demodulation for cooperative diversity in wireless systems

Cooperative diversity for wireless fading channels without channel state information

A Case For Amplify-Forward Relaying in the Block-Fading Multi-Access Channel

Distributed spectrum-efficient routing algorithms in wireless networks

Noncoherent demodulation for cooperative diversity in wireless systems