Subcarrier Power Allocation in OFDM with Low Precision ADC at Receiver

Orthogonal frequency division multiplexing (OFDM) has been incorporated in standards/draft standards such as IEEE 80.15.3c, IEEE 802.11ad for building multi-Gigabit systems operating in a few GHz of bandwidth. The digital implementation of the receivers for such a system is challenging because high precision (6+ bits/sample) analog-to-digital conversion (ADC) at such high speeds is power hungry and expensive. In this paper, we show that by suitable subcarrier power allocation we can get good performance even with low precision ADC (1-4 bits/sample without oversampling). We derive an analytical expression for the uncoded SER of an M-QAM OFDM system with finite precision ADC. By accounting for automatic gain control (AGC), we show that equal received subcarrier power (ERSP) leads to less quantization noise power than equal transmit subcarrier power (ETSP). Furthermore, for high SNR, ERSP has a lower symbol error rate (SER) than ETSP. But for lower SNR, ETSP is better, and hence we also use convex combinations of ETSP and ERSP power allocations. We illustrate the accuracy of our analytical results with simulations for the Saleh-Valenzuela channel model with log-normal fading. Our results show that for 16-QAM, at SER of 0.01, with a 3-bit ADC and a combination of ERSP and ETSP, we can come within 1 dB of ETSP of the full precision case (while ETSP with 3-bit ADC has a SER floor above 0.04).