Abstract
Simultaneous wireless information and power transfer (SWIPT) has gained significant popularity in the recent past owing to its applications in a wide range of use cases. Although SWIPT has been fairly well investigated in the literature, the existing work has mainly focused on attaining the optimal rate energy (RE) tradeoff assuming Gaussian input alphabet. However, practical systems operate with finite input alphabets such as quadratic-amplitude modulation (QAM)/phase-shift keying. We characterize the attainable RE tradeoff in SWIPT systems employing finite input alphabet for transmission over parallel Gaussian channels of say orthogonal frequency-division multiplexing subcarriers or multiple-input multiple-output streams. Some of the key results in the literature that assume Gaussian input alphabet are shown to be special cases of our results. Furthermore, we provide insights into our results with the aid of graphical illustrations, which throw light on the optimal power allocation policy for various energy-harvesting constraints. Furthermore, we consider practically relevant time-sharing and power-splitting schemes operating with finite input alphabet and characterize their RE tradeoff. Their optimal solutions in the asymptotic regime are obtained, which serve as low-complexity solutions suitable for practical implementation. Our simulation studies have demonstrated that the Gaussian input assumption significantly over-estimates the attainable RE tradeoff, especially when the signal set employed is small. Furthermore, it is observed through numerical simulations that the proposed optimal power allocation performs significantly better than the power allocation based on the Gaussian input assumption. Specifically, as much as 30% rate improvement is observed when employing the classic 4-QAM signal set.
Highlights
The number of networked devices is expected to proliferate owing to a wide range of compelling applications [1], such as smart sensors in home applications, wearable devices, environmental sensors, warehouse management etc
We have characterised the RE trade-off in simultaneous wireless information and power transfer (SWIPT) systems operating with finite input alphabet over parallel Gaussian channels, which has hitherto not been studied in the literature
The RE trade-off of the SWIPT system operating with Gaussian input alphabet was shown to be a special case of our result
Summary
The number of networked devices is expected to proliferate owing to a wide range of compelling applications [1], such as smart sensors in home applications, wearable devices, environmental sensors, warehouse management etc. The existing SWIPT solutions discussed so far were conceived by considering Gaussian input alphabet, which do not model the practical communication systems, where finite signal sets such as QAM/PSK are employed. [24]-[26] provide some insights, the implications of having finite input alphabets on the RE trade-off remain largely unexplored Against this background, the following are the contributions of this paper: 1) Orthogonal frequency division multiplexing (OFDM) [27], MIMO systems employing singular value decomposition (SVD) aided beamforming [28], etc. We first characterise the RE trade-off of SWIPT systems operating with finite input alphabet over parallel Gaussian channels, and show that the seminal results on co-located EH and ID receivers presented in [7] are special cases of our results. The mutual information is measured in nats/channel use, unless stated otherwise
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