中文说明：一种Matlab代码，用于生成三个接收天线的QPSK误码率与信噪比的关系图，以及三种不同的组合方法：MRC、等增益组合和选择组合。你应该用蒙特卡罗模拟生成图。此图中的信噪比应为单天线平均信噪比，范围应为-5 dB至15 dB。
English Description：
a Matlab code that produces a plot of probability of bit error rate for QPSK versus SNR for three receive antennas, and three different combining methods: MRC, equal gain combining, and selection combining. You should generate the plot using Monte Carlo simulation. The SNR in this plot should be the single-antenna average signal-to-noise ratio, and should range from -5 dB to 15 dB.