The principle of M-sequence used in CDMA mobile communication is: a code sequence generator consisting of N-level shift registers. The state of the register depends on the information ("0" or "1") input under clock control. For example, the state of the stage I shift register depends on the state of the stage I- 1 shift register after the previous clock pulse.

In the figure, C0, C 1, …, Cn are all feedback lines, where C0 = C 1 = 1 indicates feedback connection. Because the M sequence is generated by the cyclic sequence generator, C0 and Cn must be 1, that is, participate in the feedback.

And the feedback coefficients C 1, C2, …, cn- 1, if it is 1, participate in the feedback; If it is 0, the feedback line is disconnected, that is, the circuit is open and there is no feedback connection.

The function of m sequence is:

1, m sequence is the abbreviation of the longest linear shift register sequence. As the name implies, M sequence is the longest code sequence generated by multistage shift register or its delay element through linear feedback.

In a binary shift register, if n is the series of the shift register, the n-level shift register * * * has 2n states, and 2n- 1 states are left except all 0s, so the maximum length code sequence it can generate is 2n- 1 bit, that is to say, the longest period generated by the n-level linear feedback shift register is equal to 2n- 1 bit.

In CDMA system, two kinds of M sequences are mainly used: one is M sequence with a period of 2 15- 1, also known as short PN sequence; The other is an M sequence with a period of 242- 1, also known as a long PN code sequence.

2.m sequence is a basic and typical pseudo-random sequence. It is widely used in communication fields, such as spread spectrum communication, code division multiple access (CDMA) for satellite communication, encryption, scrambling, synchronization and error rate measurement of digital data.