全双工系统能实现在同一时隙与同一频率传输数据,相比于半双工系统能大大地提升数据吞吐量和频谱效率。为了进一步提高全双工多输入多输出(Multi-input and multi-output,MIMO)中继系统传输速率,本文基于放大转发(Amplify-and-forward,AF)传输模式,在全双工双向中继系统中引入梯度下降算法,将用户发送端、接收端波束成形与中继端波束成形矩阵相结合设计一种最大化速率的交替迭代算法,并构造出一种最小均方误差(Minimum mean square error,MMSE)迭代算法作为初始条件,在此基础上推导出中继接收端与发射端的波束成形矩阵表达式。仿真结果表明,本文构造的交替迭代算法收敛速度快,而且相比于迫零、最小均方误差以及最大泄信噪比算法,和速率有显著提高。
大规模MIMO系统中,相对于其他基于信道矩阵分解的波束成形算法,如迫零、最小均方误差算法等,匹配滤波(Matched filter,MF)具有复杂度极低的优点,从而成为一种极具实用潜力的波束成形算法。鉴于此,本文推导了基站采用MF波束成形算法时,用户端信干噪比(Signal-to-interferenceand-noise ratio,SINR)的近似概率密度函数(Probability density function,PDF)。该函数对于推导与分析系统性能,如和速率、中断概率等至关重要。仿真表明:当基站天线数趋于大规模时,SINR公式的PDF曲线趋近于通过纯仿真得到的PDF曲线。
A pilot pattern across two orthogonal frequency division multiplexing OFDM symbols with a special structure is designed for the channel estimation of OFDM systems with inphase and quadrature IQ imbalances at the receiver.A high-efficiency time-domain TD least square LS channel estimator and a low-complexity frequency-domain Gaussian elimination GE equalizer are proposed to eliminate IQ distortion.The former estimator can significantly suppress channel noise by a factor N/L＋1 over the existing frequency-domain FD LS where N and L＋1 are the total number of subcarriers and the length of cyclic prefix and the proposed GE requires only 2N complex multiplications per OFDM symbol.Simulation results show that by exploiting the TD property of the channel the proposed TD-LS channel estimator obtains a significant signal-to-noise ratio gain over the existing FD-LS one whereas the proposed low-complexity GE compensation achieves the same bit error rate BER performance as the existing LS one.
为了提高稀疏信道环境下同相/正交(I/Q)不平衡正交频分复用(OFDM)系统的性能,该文提出了一种低复杂度的门限时域最小二乘信道估计算法。该算法通过估计噪声方差确定合适的门限过滤信道响应采样点内的噪声以提高估计精度。仿真结果表明,该文算法估计精度与现有频域和时域最小二乘信道估计方法相比分别提升了6 d B、2 d B,逼近基于压缩感知的时域迭代收缩算法,且计算复杂度低于后者。
为进一步提高多输入多输出(MIMO)双向中继系统性能,该文在梯度下降法的基础上,引入用户端功率分配和发射预编码与接收波束成形,结合中继站波束成形矩阵和分配功率相,构成一个完整的联合交替迭代结构(AIS);通过固定变量循环计算,逐个得到各个变量的最优值。仿真表明,与梯度下降法、迫零和最小均方误差加注水功率分配法相比,该文交替迭代结构方法的速率有所提高,误码性能提高较明显。当误码率等于10-2时,与3种算法中最优的梯度下降法相比,该文方法可获得2.5 d B的信噪比增益。
