Rate Redundancy and Entropy Allocation for PAS-OFDM Based Mobile Fronthaul

From the perspective of an experimentalist, we discuss the comparison framework that ensures the same information rate in probabilistic shaping (PS) signals. Theoretically, the information rate can be tuned by adjusting the target entropy, scaling the signal bandwidth and adjusting FEC code rate. In practice, the information rate tuning should be based on the hardware feasibility (e.g., DAC resolution, available analog bandwidth, cost of rate adaptation). Adjusting a constellation entropy rate without considering FEC redundancy will cause an over-estimation of the PS system performance gain. To avoid the over-estimation issue, the modified pre-FEC threshold for PS signal is also proposed to estimate the system performance accurately. The modified pre-FEC threshold is obtained from the FEC code rate of PS signal. Moreover, we experimentally investigated the adaptive entropy allocation scheme in a millimeter-wave analog radio over fiber fronthaul using the modified pre-FEC threshold. By grouping the subcarriers into PS units and using the same QAM order, the proposed scheme can reduce the required data frame length as well as the processing complexity. Notably, this method requires neither dynamic FEC coding rate nor bandwidth adjusting method. Up to 2.5-dB power margins and a smoother pre-FEC performance among PS units are achieved in a frequency selective fading channel. If the modified FEC threshold were not used, it would induce up to 0.7-dB over-estimation in the received sensitivity. This indicates the importance of employing the proposed methods for comparison when FEC is used in combination with PS.