Non-Coherent Wideband Capacity of Sparse Multipath Channels with Limited Feedback
نویسندگان
چکیده
This paper investigates the ergodic capacity of multipath channels in the wideband regime with limited feedback. Our work builds on recent results that have established significant capacity gains achievable in the wideband/low-SNR regime, when there is perfect channel state information (CSI) at the transmitter. Furthermore, this benchmark gain can be obtained with just 1-bit of CSI at the transmitter about each channel coefficient. However, the capacity achieving signals are peaky; they have large instantaneous transmit power, especially in the non-coherent scenario. Signal peakiness is related to channel coherence and, in contrast to the prevalent assumption of rich multipath, we investigate the capacity of sparse multipath channels, motivated by recent experimental results. Sparsity naturally leads to coherence in time and frequency. With perfect receiver CSI, it is shown that limited feedback, even with an instantaneous power constraint, is sufficient to achieve the benchmark capacity. Our analysis reveals the benefits of channel sparsity in the non-coherent scenario, where we employ a training-based communication scheme. With an average power constraint, it is shown that the benchmark is achievable, provided the channel coherence scales at a sufficiently fast rate with signal space dimension. Furthermore, in contrast to peaky signaling schemes that violate instantaneous power constraints, we show that the benchmark is attainable in sparse channels with finite instantaneous transmit power. We present rules of thumb on choosing the signaling parameters as a function of the channel parameters so that the full benefits of sparsity can be realized.
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