说明：  matlab生成m序列的函数 原来那个效率太低，主要是因为实际应用并不需要生成太长的m序列，加入了需求长度，能有效减少运行时间。25级的移位寄存器，运行了一个小时都没有出来一周期，我仅仅需要三千bit而已。改进版：
(matlab generate a function of the original m sequence that efficiency is too low, mainly because of the practical application does not require generation of long m sequences, joined the demand for length, can effectively reduce the running time. 25 of the shift register, run for an hour did not come out one cycle, I just need more than 3000 bit. Improved version:)