948 research outputs found
Differential Amplify-and-Forward Relaying in Time-Varying Rayleigh Fading Channels
This paper considers the performance of differential amplify-and-forward
(D-AF) relaying over time-varying Rayleigh fading channels. Using the
auto-regressive time-series model to characterize the time-varying nature of
the wireless channels, new weights for the maximum ratio combining (MRC) of the
received signals at the destination are proposed. Expression for the pair-wise
error probability (PEP) is provided and used to obtain an approximation of the
total average bit error probability (BEP). The obtained BEP approximation
clearly shows how the system performance depends on the auto-correlation of the
direct and the cascaded channels and an irreducible error floor exists at high
signal-to-noise ratio (SNR). Simulation results also demonstrate that, for
fast-fading channels, the new MRC weights lead to a better performance when
compared to the classical combining scheme. Our analysis is verified with
simulation results in different fading scenarios
Differential Modulation and Non-Coherent Detection in Wireless Relay Networks
The technique of cooperative communications is finding its way in the next
generations of many wireless communication applications. Due to the distributed
nature of cooperative networks, acquiring fading channels information for
coherent detection is more challenging than in the traditional point-to-point
communications. To bypass the requirement of channel information, differential
modulation together with non-coherent detection can be deployed. This thesis is
concerned with various issues related to differential modulation and
non-coherent detection in cooperative networks. Specifically, the thesis
examines the behaviour and robustness of non-coherent detection in mobile
environments (i.e., time-varying channels). The amount of channel variation is
related to the normalized Doppler shift which is a function of user's mobility.
The Doppler shift is used to distinguish between slow time-varying
(slow-fading) and rapid time-varying (fast-fading) channels. The performance of
several important relay topologies, including single-branch and multi-branch
dual-hop relaying with/without a direct link that employ amplify-and-forward
relaying and two-symbol non-coherent detection, is analyzed. For this purpose,
a time-series model is developed for characterizing the time-varying nature of
the cascaded channel encountered in amplify-and-forward relaying.Comment: PhD Dissertatio
Communications over fading channels with partial channel information : performance and design criteria
The effects of system parameters upon the performance are quantified under the assumption that some statistical information of the wireless fading channels is available. These results are useful in determining the optimal design of system parameters. Suboptimal receivers are designed for systems that are constrained in terms of implementation complexity.
The achievable rates are investigated for a wireless communication system when neither the transmitter nor the receiver has prior knowledge of the channel state information (CSI). Quantitative results are provided for independent and identically distributed (i.i.d.) Gaussian signals. A simple, low-duty-cycle signaling scheme is proposed to improve the information rates for low signal-to-noise ratio (SNR), and the optimal duty cycle is expressed as a function of the fading rate and SNR. It is demonstrated that the resource allocations and duty cycles developed for Gaussian signals can also be applied to systems using other signaling formats.
The average SNR and outage probabilities are examined for amplify-and-forward cooperative relaying schemes in Rayleigh fading channels. Simple power allocation strategies are determined by using knowledge of the mean strengths of the channels.
Suboptimal algorithms are proposed for cases that optimal receivers are difficult to implement. For systems with multiple transmit antennas, an iterative method is used to avoid the inversion of a data-dependent matrix in decision-directed channel estimation. When CSI is not available, two noncoherent detection algorithms are formulated based on the generalized likelihood ratio test (GLRT). Numerical results are presented to demonstrate the use of GLRT-based detectors in systems with cooperative diversity
A differential ML combiner for differential amplify-and-forward system in time-selective fading channels
We propose a new differential maximum-likelihood (DML) combiner for noncoherent detection of the differential amplify-and-forward (D-AF) relaying system in the time-selective channel. The weights are computed based on both the average channel quality and the correlation coefficient of the direct and relay channels. Moreover, we derive a closed-form approximate expression for the average bit error rate (BER), which is applicable to any single-relay D-AF system with fixed weights. Both theoretical and simulated results are presented to show that the time-selective nature of the underlying channels tends to reduce the diversity gains at the low-signal-to-noise-ratio (SNR) region, resulting in an asymptotic BER floor at the high-SNR region. Moreover, the proposed DML combiner is capable of providing significant BER improvements compared with the conventional differential detection (CDD) and selection-combining (SC) schemes
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