Design Optimization of Feedforward Equalization for Mobile Fronthaul Based on Delta-Sigma Modulation With High-Order QAM Signals

Delta-sigma modulation can be used as a high spectral efficiency interface in place of conventional common public radio interface (CPRI) in mobile fronthaul (MFH) networks. However, inter-symbol interference (ISI) becomes more difficult to be mitigated in the networks due to bit quantization in delta-sigma modulation. In this paper, we propose a novel feedforward equalization (FFE) scheme based on the least mean square (LMS) algorithm for MFH networks employing 4th-order delta-sigma modulation technology with 1 and 2 bit quantization to alleviate the performance degradation caused by ISI. The performance of both 1-bit and 2-bit quantization systems employing the proposed FFE scheme with different tap lengths and step sizes has been systematically investigated. Our results show that, under the minimum computational complexity, the maximum transmission capacity can reach 8.98 Gbps when using the FFE scheme with 11 taps and step size of 3 × 10^-5 or 3.5 × 10^-5 and the average error vector magnitude (EVM) can be minimized to a level less than 0.032 with 19 taps and step size of 4 × 10^-5 or 4.5 × 10^-5 for 1-bit quantization system while for 2-bit quantization system, the maximum transmission capacity of 11.4 Gbps can be obtained when employing the FFE scheme with 19 taps and step size of 3.5 × 10^-5 and the minimal average EVM less than 0.01 can be achieved with 19 taps and step size of 4 × 10^-5. More hopefully, this work provides guidelines for optimizing the equalization scheme based on the requirements of EVMs, capacity, and computational complexity.