低精度ADC下无小区大规模MIMO系统的频谱效率研究

Spectral Efficiency Analysis of Cell-Free Massive MIMO Systems with Low-Resolution ADCs

无小区大规模多输入多输出(cell-free massive multiple-input multiple-output,CF-mMIMO)系统的覆盖区域内随机部署了大量分布式接入点(access points,APs)在同一时间频率资源中服务所有的用户,可显著提升系统通信容量,是6G网络中最具潜力的使能技术之一。然而,大量AP处配备高精度模数转换器(analog-to-digital converters,ADCs)导致的高功耗与硬件成本,限制了CF-mMIMO系统的实际部署。为了有效地降低硬件成本,本文研究了低精度ADCs下CF-mMIMO系统的上行链路频谱效率(spectral efficiency,SE)。在不完美的信道估计下,利用加性量化噪声(additive quantization noise model,AQNM)模型和最大比合并(maximal ratio combining,MRC)接收机滤波器,推导了CF-mMIMO系统中用户上行可达速率的闭式表达式,并基于该表达式分析了AP数量、用户传输功率以及ADCs精度等系统参数对SE的影响。为了最大化CF-mMIMO系统的SE,提出了一种低精度ADCs下贪婪导频分配算法抑制导频污染。将导频分配建模为最大-最小导频优化问题,通过迭代更新速率最小用户的导频序列,使其所受导频污染的影响最小,从而最大化该用户的可达速率。最后,将配备低精度ADC的CFmMIMO系统与传统完美精度ADC系统进行性能比较。数值仿真结果表明,系统配备5位低精度ADCs时的SE逼近完美精度ADCs,增加AP端天线数可以弥补低精度ADCs导致的性能退化。此外,所提算法不仅有效抑制了导频污染,还缩小了用户之间的速率差距,提升了系统的95%用户SE。

Cell-free massive multiple-input multiple-output(CF-mMIMO)systems randomly deploy numerous distributed access points(APs)within the coverage area to simultaneously serve all users and frequency resources,which can significantly increase the system communication capacity and is one of the most potentially enabling technologies in 6G networks.However,the high power consumption and hardware cost caused by equipping perfect-resolution analog-todigital converters(ADCs)at numerous APs restrict the practical deployment of CF-mMIMO systems.To effectively reduce the hardware costs,the uplink spectral efficiency(SE)of CF-mMIMO systems with low-resolution ADCs was investigated.Under imperfect channel estimation,a closed-form expression for the user uplink achievable rate in CFmMIMO systems was derived utilizing the additive quantization noise(AQNM)model and the maximal ratio combining(MRC)receiver filter,and the effect of system parameters such as the number of APs,user transmission power,and ADCs resolution on the SE was analyzed based on this expression.To maximize the SE of the CF-mMIMO system,a greedy pilot assignment algorithm under low-resolution ADCs was proposed to alleviate the pilot contamination.The pilot assignment was modeled as a max-min pilot optimization problem,where the pilot sequence of the user with the lowest rate was iteratively updated to minimize its pilot contamination effect,thus maximizing the achievable rate of this user.Finally,the performance of the CF-mMIMO system with low-resolution ADCs was compared with that of the conventional perfect-resolution ADC system.Numerical simulations reveal that 5-bit low-resolution ADCs nearly match the spectral efficiency(SE)of perfect-resolution ADCs.Furthermore,augmenting the number of antennas at the AP can mitigate the performance loss due to low-resolution ADCs.Moreover,the proposed algorithm not only effectively alleviates the pilot contamination,but also reduces the rate gap among different users and improves the 95%-likely per-user throughput of the system.