Tairan Wang

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

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.

Complex Field Network Coding for Multiuser Cooperative Communications

Multi-source relay-based cooperative communications can achieve spatial diversity gains, enhance coverage and potentially increase capacity when multiuser detection is used to effect maximum likelihood demodulation. If considered for large networks, traditional relaying entails loss in spectral efficiency that can be mitigated through network coding at the physical layer. These considerations motivate the complex field network coding (CFNC) approach introduced in this paper. Different from network coding over the Galois field, where wireless throughput is limited as the number of sources increases, CFNC always achieves throughput as high as 1/2 symbol per source per channel use. In addition to improved throughput, CFNC- based relaying achieves full diversity gain regardless of the underlying signal-to-noise-ratio (SNR) and the constellation used. Furthermore, the CFNC approach is general enough to allow for transmissions from sources to a common destination as well as simultaneous information exchanges among sources.

Smart regenerative relays for link-adaptive cooperative communications

Without being necessary to pack multiple antennas per terminal, cooperation among distributed single-antenna nodes offers resilience to shadowing and can, in principle, enhance the performance of wireless communication networks by exploiting the available space diversity. Enabling the latter however, calls for practically implementable protocols to cope with errors at relay nodes so that simple receiver processing can collect the diversity at the destination. To this end, we derive in this paper a class of strategies whereby decoded bits at relay nodes are scaled in power before being forwarded to the destination. The scale is adapted to the signal-to-noise-ratio (SNR) of the source-relay and the intended relay-destination links. With maximum ratio combining (MRC) at the destination, we prove that such link-adaptive regeneration (LAR) strategies effect the maximum possible diversity while requiring simple channel state information that can be pragmatically acquired at the relay. In addition, LAR exhibits robustness to quantization and feedback errors and leads to efficient use of power both at relay as well as destination nodes. Analysis and corroborating simulations demonstrate that LAR relays are attractive across the practical SNR range; they are universally applicable to multibranch and multi-hop uncoded or coded settings regardless of the underlying constellation; and outperform existing alternatives in terms of error performance, complexity and bandwidth efficiency.

Non-coherent distributed space-time processing for multiuser cooperative transmissions

User cooperation can provide spatial transmit diversity gains, enhance coverage and potentially increase capacity. Existing works have focused on two-user cooperative systems with perfect channel state information at the receivers. In this paper, we develop several distributed space-time processing schemes for general N-user cooperative systems, which do not require channel state information at either relays or destination. We prove that full spatial diversity gain can be achieved in such systems. Simulations demonstrate that these cooperative schemes achieve significant performance gain

Smart Regenerative Relays for Link-Adaptive Cooperative Communications

Without being necessary to pack multiple antennas per terminal, cooperation among distributed single-antenna nodes offers resilience to shadowing and can, in principle, enhance the performance of wireless communication networks by exploiting the available space diversity. Enabling the latter however, calls for practically implementable protocols to cope with errors at relay nodes so that simple receiver processing can collect the diversity at the destination. To this end, we derive in this paper a class of strategies whereby decoded bits at relay nodes are scaled in power before being forwarded to the destination. The scale is adapted to the signal-to-noise-ratio (SNR) of the source-relay and the intended relay-destination links. With maximum ratio combining (MRC) at the destination, we prove that such link-adaptive regeneration (LAR) strategies effect the maximum possible diversity while requiring simple channel state information that can be pragmatically available at the relay. In addition, LAR exhibits robustness to quantization and feedback errors and leads to efficient use of power both at relay as well as destination nodes. Analysis and corroborating simulations demonstrate that LAR relays are attractive across the practical SNR range; they are universally applicable to multi-branch and multi-hop uncoded or coded settings regardless of the underlying constellation; and outperform existing alternatives.

Mutual Information Jammer-Relay Games

We consider a two-person zero-sum mutual information game between one jammer (J) and one relay (Rfr) in both nonfading and fading scenarios. Assuming that the source (S) and the destination (D) are unaware of the game, we derive optimal pure or mixed strategies for J and Rfr depending on the link qualities and whether the players are active during the SrarrD channel training. In nonfading scenarios, when both J and Rfr have full knowledge of the source signal, linear jamming (LJ) and linear relaying (LR) are shown optimal in the sense of achieving Nash equilibrium. When the SrarrJ and SrarrRfr links are noisy, LJ strategies (pure or mixed) are still optimal under LR. In this case, instead of always transmitting with full power as when the SrarrRfr link is perfect, Rfr should adjust the transmit power according to its power constraint and the reliability of the source signal it receives. Furthermore, in fading scenarios, it is optimal for J to jam only with Gaussian noise if it cannot determine the phase difference between its signal and the source signal. When LR is considered with fading, Rfr should forward with full power when the SrarrRfr link is better than the jammed SrarrD link, and defer forwarding otherwise. Optimal parameters are derived based on exact Nash equilibrium solutions or upper and lower bounds when a closed-form solution cannot be found.

Efficient Demodulation in Cooperative Schemes Using Decode-and-Forward Relays

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 an efficient 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 demodulator can achieve the maximum possible diversity, regardless of the underlying constellation. Its error performance tightly bounds that of maximumlikelihood (ML) demodulation which provably quantifies the diversity gain of ML detection with DF relaying. Simulations corroborate our theoretical analyses and compare performance of the novel decoder with existing diversity-achieving strategies including analog amplify-and-forward and selective-relaying.

High-Throughput Cooperative Communications with Complex Field Network Coding

Relay-based cooperative communications can achieve spatial diversity gains, enhance coverage and potentially increase capacity. If considered for large networks, traditional relaying schemes suffer from spectral inefficiency that can be improved through network coding at a physical layer. These considerations motivate the complex field network coding (CFNC) scheme proposed in this paper. As opposed to network coding over Galois field, where wireless links cannot improve throughput limitations as the number of sources increases, CFNC achieves throughput as high as 1/2 symbol per source per time slot. With improved throughput, CFNC-based relaying achieves full diversity gain regardless of the channel signal-to-noise-ratio (SNR) and the underlying constellation, and is general enough to incorporate transmissions from sources to a common destination as well as information exchange among the sources at the same time.

Link-Adaptive Cooperative Communications Without Channel State Information

Link-adaptive regeneration (LAR) is a novel relaying strategy other than decode-and-forward (DF) or amplify-and-forward (AF) in user cooperative communications. Requiring simple channel state information (CSI) of both the source-relay and the relay-destination links, LAR has been shown to achieve full diversity using coherent modulations. In this paper, we generalize the idea of LAR into differential and non-coherent cooperative transmissions, which do not require CSI at either relays or destination. We prove that full spatial diversity gain can still be achieved in such systems, without incurring the overhead of cyclic redundancy check (CRC) codes. Simulations demonstrate that the proposed scheme is universally applicable to multi-branch and multi-hop cooperation regardless of the constellation size and outperforms existing alternatives.

Multi-Tier Cooperative Broadcasting with Hierarchical Modulations

We consider broadcasting to multiple destinations with uneven quality receivers. Based on their quality of reception, we group destinations in tiers and transmit using hierarchical modulations. These modulations are known to offer a practical means of achieving variable error protection of the broadcasted information to receivers of variable quality. After the initial broadcasting step, tiers successively re-broadcast part of the information they received from tiers of higher-quality to tiers with lower reception capabilities. This multi-tier cooperative broadcasting strategy can accommodate variable rate and error performance for different tiers but requires complex demodulation steps. To cope with this complexity in demodulation, we derive simplified per-tier detection schemes with performance close to maximum-likelihood and ability to collect the diversity provided as symbols propagate through diversified channels across successive broadcastings. Error performance is analyzed and compared to (non)-cooperative broadcasting strategies. Simulations corroborate our theoretical findings.