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In hybrid digital-analog (HDA) systems, resource allocation has been utilized to achieve the desired distortion performance. However, existing studies on this issue assume error-free digital transmission, which is not valid for fading channels. With time-varying channel fading, the exact channel state information is not available at the transmitter. Thus, random outage and resulting digital distortion cannot be ignored. Moreover, rate allocation should be considered in resource allocation, since it not only determines the amount of information for digital transmission and that for analog transmission but also affects the outage probability. Based on above-mentioned observations, in this paper, we attempt to perform joint rate and resource allocation strategies to optimize system distortion in HDA systems over fading channels. Consider a bandwidth expansion scenario where a memoryless Gaussian source is transmitted in an HDA system with the entropy-constrained scalar quantizer. First, we formulate the joint allocation problem as an expected system distortion minimization problem where both analog and digital distortions are considered. Then, in the limit of low outage probability, we decompose the problem into two coupled subproblems based on the block coordinate descent method and propose an iterative gradient algorithm to approach the optimal solution. Furthermore, we extend our work to the multivariate Gaussian source scenario where a two-stage fast algorithm integrating rounding and greedy strategies is proposed to optimize the joint rate and resource allocation problem. Finally, simulation results demonstrate that the proposed algorithms can achieve up to 2.3 dB gains in terms of the signal-to-distortion ratio over existing schemes under the single Gaussian source scenario, and up to 3.5 dB gains under the multivariate Gaussian source scenario.

Joint Rate and Resource Allocation in Hybrid Digital–Analog Transmission Over Fading Channels