Yinman Lee

Performance of Decode-and-Forward Cooperative Communications Over Nakagami-

In this paper, we analyze the symbol error rate (SER) performance of decode-and-forward (DF) cooperative communications over Nakagami-m fading channels. For the classical three-node cooperation model, the exact SER expressions for both M phase shift keying (PSK) and M quadrature amplitude modulation (QAM) are derived. For easy usage, we also provide asymptotic approximations for the SER. In the analysis, we first consider the cooperation scenario in which all the channels are uncorrelated. In practice, it may happen that a correlation exists between the source-destination and relay-destination channels, e.g., for user cooperation in the uplink of a cellular network. We then examine the SER in this scenario. In addition, by varying the parameters of the Nakagami-m fading channels in the analytical results, we obtain some perceptions of choosing a good relay for enhancing the cooperation and make clear about how the included Nakagami fading figures and channel variances affect the SER performance. In addition, the optimal power allocation is investigated based on the derived asymptotic SER expressions. Simulations are finally provided to verify the correctness of our analysis.

m

Conventional generalized sidelobe canceller (GSC) is sensitive to a mismatch between the estimated and actual direction of arrival (DOA) of the desired signal. Such a mismatch induces signal cancellation in the GSC, and it severely degrades the beamforming performance. In this paper, we propose a new decision feedback (DF) technique to increase the robustness against the DOA mismatch. Our new scheme introduces a blind equalizer and a feedback filter in the GSC structure. We first derive Wiener solutions for the DF-GSC with perfectly matched and mismatched DOA and show that the problem of signal cancellation can be avoided. Then, we consider the adaptive GSC implementation in which the least-mean-square (LMS) algorithm is used for weight adaptation. In addition to the improved robustness, the proposed scheme also remedies the slow convergence problem inherent in the conventional adaptive GSC structure. The convergence behavior of the LMS-based DF-GSC is fully analyzed and the analytic signal-to-interference-plus-noise ratio (SINR) is also derived. Finally, simulation results demonstrate that while the proposed structure can considerably enhance the overall performance, it has greatly improved robustness as compared to other existing robust adaptive beamformers.

Fading Channels

A double space-time transmit diversity (DSTTD) system employs four transmit antennas to multiplex two Alamouti space-time block code (STBC) units. In this letter, a specially-designed canceling mechanism is presented, which can efficiently separate the two STBC units for such systems. We show that the orthogonal structure, and so the merit of Alamouti STBC, can be kept. Compared with some previous interference cancelation methods, the proposed one requires much fewer floating-point operations (flops) while achieving similar error-rate performance.

A robust adaptive generalized sidelobe canceller with decision feedback

In wireless communications, cochannel interference (CCI) and intersymbol interference (ISI) are two main factors that limit system performance. Conventionally, a beamformer is used to reduce CCI, whereas an equalizer is used to compensate for ISI. These two devices can be combined into one as space-time equalizer (STE). A training sequence is usually required to train the STE prior to its use. In some applications, however, spatial information corresponding to a desired user is available, but the training sequence is not. In this paper, we propose an adaptive decision feedback STE to cope with this problem. Our scheme consists of an adaptive decision feedback generalized sidelobe canceller (DFGSC), a blind decision feedback equalizer (DFE), and a channel estimator. Due to the feedback operation, the proposed DFGSC is not only superior to the conventional generalized sidelobe canceller but also robust to multipath channel propagation and spatial signature error. Theoretical results are derived for optimum solutions, convergence behavior, and robustness properties. With the special channel-aided architecture, the proposed blind DFE can reduce the error propagation effect and be more stable than the conventional blind DFE. Simulation results show that the proposed STE is effective in mitigating both CCI and ISI, even in severe channel environments.

A Simple Layered Space-Time Block Nulling Technique for DSTTD Systems

To take on challenges of communication in sparse VANET, we propose a Store-Carry-Broadcast (SCB) scheme to assist message dissemination by broadcasting over a specific road segment instead of a single vehicle. In the SCB scheme, an opposite vehicle helps to disseminate the safety messages to oncoming vehicles traveling on the reverse lane by broadcasting. Compare with the well-known store-carry-forward broadcasting in VANET, the SCB scheme consumes much less network bandwidth in terms of the number of broadcasting performed. Extensive simulation results are presented to demonstrate that when the traffic density is low or when the DSCR penetration rate is low, our proposed SCB scheme significantly reduced both broadcast overhead as well as delivery delay in sparse VANET.

Adaptive Decision Feedback Space–Time Equalization With Generalized Sidelobe Cancellation

In this paper, we analyze the symbol-error-rate (SER) performance of decode-and-forward (DF) cooperative communications over Nakagami-m fading channels. For the classical three-node cooperation model, which includes a source, a relay and a destination, the exact SER expressions for M-phase shift keying (PSK) and M-quadrature amplitude modulation (QAM) are derived. For easy usage, we provide asymptotic approximations for the SER as well. In addition, optimal power allocation is investigated based on the derived asymptotic SER expressions. Simulations are finally provided to verify the correctness of our analysis.

SCB: Store-Carry-Broadcast Scheme for Message Dissemination in Sparse VANET

It is known that the achievement of the decode-and-forward (DF) protocol for cooperative communications is critically based on the prerequisite that the relay forwards the source transmitted signals as faithfully as possible. This implies that the relay can mostly know the correctness of its decoded results. In this paper, we employ the idea of detection with erasure in relay to approach this prerequisite. An erasure region is set to minimize the cost function that is formulated based on a specially designed bit-error-rate (BER) expression for the cooperation network. Whenever the received signals fall into this region, the relay will not perform DF. Power allocation can be simultaneously optimized as well. Simulations show that its resultant BER can be quite close to that of the perfect DF scenario and better than the BER of most previously presented signal-to-noise ratio (SNR) threshold-based relaying schemes.

SER and Optimal Power Allocation for DF Cooperative Communications over Nakagami-m Fading Channels

In wireless communications, cochannel interference (CCI) and intersymbol interference (ISI) are two main factors that limit system performance. Conventionally, a beamformer is used to reduce CCI, whereas an equalizer is used to compensate for ISI. These two devices can be combined into one as space-time equalizer (STE). A training sequence is usually required to train the STE prior to its use. In some applications, however, spatial information corresponding to a desired user is available, but the training sequence is not. In this paper, we propose an adaptive decision feedback STE to cope with this problem. Our scheme consists of an adaptive decision feedback generalized sidelobe canceller (DFGSC), a blind decision feedback equalizer (DFE), and a channel estimator. Due to the feedback operation, the proposed DFGSC is not only superior to the conventional generalized sidelobe canceller but also robust to multipath channel propagation and spatial signature error. Theoretical results are derived for optimum solutions, convergence behavior, and robustness properties. With the special channel-aided architecture, the proposed blind DFE can reduce the error propagation effect and be more stable than the conventional blind DFE. Simulation results show that the proposed STE is effective in mitigating both CCI and ISI, even in severe channel environments.

Detection With Erasure in Relay for Decode-and-Forward Cooperative Communications

Incremental relaying (IR) was previously proposed to mitigate the spectral-efficiency loss in cooperative communications. In this letter, based on the amplify-and-forward (AF) protocol with M-ary phase-shift keying (M-PSK) signals, we propose a new way to implement IR with better spectral efficiency. Specifically, based on the end-to-end symbol error rate (e2e SER), an uncertainty region for detection is defined. Only if the processed signal at the destination falls into this region, the included relays will perform AF relaying. Simulations reveal that in terms of the e2e SER performance, the proposed IR-AF scheme performs quite well as compared with the conventional signal-to-noise ratio (SNR)-based IR-AF method, and, importantly, with significantly lower average relay usage.

Adaptive Decision Feedback Space-Time Equalization with Generalized Sidelobe Cancellation

In this paper, we present a low-complexity groupwise ordered successive interference canceler (OSIC) with the zero-forcing (ZF) criterion for N × N spatial multiplexing systems. The proposed detection is composed of a number of processing stages, and at each stage, a fraction of the transmitted data streams are nulled to zero. In this way, the original matrix-inverse operations are replaced by a series of inversions with smaller sizes, and the computational complexity can then be largely reduced. We show that this groupwise OSIC-ZF approach can provide a good tradeoff between the computational complexity and the error-rate performance and, therefore, is very attractive as a new detection method standing between the linear ZF detection and the optimal OSIC-ZF detection.