Unified Design of LLR Quantization and Joint Reception for Mobile Fronthaul Bandwidth Reduction

Abstract

This paper proposes a unified design of log likelihood ratio (LLR) quantization and joint reception (JR) to reduce the mobile fronthaul (MFH) bandwidth in the split-physical (PHY) processing (SPP). SPP is one of the schemes that redefine the base station (BS) functional split between a baseband unit (BBU) and a remote radio head (RRH) in cloud radio access network (C-RAN). SPP splits the BS functions between the channel encoder/decoder and the modulator/demodulator of the PHY layer functions. In the uplink scenarios, the RRH forwards demodulator output LLR to the channel decoder at the BBU through the MFH. To reduce the MFH bandwidth, the number of LLR quantization bits must be minimized while maintaining the requested quality after JR, for example in terms of bit error rate (BER) requirement. To deal with the minimization, the proposed scheme adaptively controls the LLR quantization threshold according to the received signal to noise ratio (SNR) as well as the capability of source coding for data compression of the quantized LLR. Numerical simulations show that the proposed scheme is capable of reducing the average number of LLR quantization bits to fewer than 2 bits and 1.1 bits at the minimum with a JR block error rate (BLER) performance superior to that of selection combining.