Optimal Subcarrier Power Allocation for OFDM with Low Precision ADC at Receiver

Motivated by the need to reduce power consumption in the receiver analog-to-digital converter (ADC) in multi-Gbps communication systems, in this paper, we study subcarrier power allocation based on channel side information (CSI) at the transmitter. We derive a fixed point equation for subcarrier power allocation that minimizes uncoded symbol error rate (SER) when a finite precision ADC is used at the receiver. We study the sensitivity of the optimal power allocation with respect to a parameter that depends on the ADC precision. Based on this, we propose a simple analytical approximation for the optimal power allocation, which performs within 0.5 dB of the exact case over a wide range of signal-to-noise ratio (SNR). The proposed power allocation leads to only a small increase in the peak-to-average power ratio (PAPR) (< 0.3 dB) for channel models suggested in IEEE 802.15.3c. Further, for a 16-QAM input, 7/8 rate low density parity check (LDPC) code and 3-bit precision ADC, it attains a coded bit error rate (BER) of 10^{-5} at a SNR of 23.5 dB, which compares favorably with the 25 dB required by a traditional system with equal power allocation across the subcarriers and infinite sampling precision. We also show the robustness of the performance gain to channel estimation errors.