What is Pseudo-Noise Code and M-Sequence in CDMA
Here I write about Pseudo-Noise Code and what its use Pseudo-Noise Code its two type and Pseudo-Noise Short code is identification for each BTS and Pseudo-Noise Long Code is Identification of each user. Let’s understand basic.
A pseudo-noise code (called PN code in short) has properties similar to a noise sequence. It is actually a regular periodical binary sequence though looking like a noise one. M-sequences are the most important and fundamental among all pseudo-noise codes.
“M-sequence” is the abbreviation for “Maximal Length Sequence of Linear Feedback Shift Register”. The definition of such a sequence is as follows:
If the period of an output sequence of a N-stage linear feedback shift register is P =2N -1, this sequence is called a m-sequence.
In a CDMA system, usually Galois generators are used to produce m-sequences each of which is comprised of the sequence of period 2N-1 (not all-0 state) and a mask. M-sequences with different masks have different output phases.
Pseudo-noise codes used in a CDMA2000 system are of two types, namely, m-sequence with length 215-1 and that with length 242-1.
On the forward link, the m-sequence with length 242-1 is used to scramble fundamental channels while the m-sequence of length 215-1 is used for quadrature modulation of the forward link (the period of the m-sequence is 215 with an all-0 state being added into it). Different sectors use different phases of the m-sequence for modulation, with the phase difference between adjacent phases required to be at least 64 bits. Hence, the maximum number of available phases is 512.
On the backward link, the m-sequence with length 242-1 is used for direct spreading. Each MS is assigned a m-sequence phase calculated by the ESN of the MS. Backward channels for these MSs are basically orthogonal to each other for the two-valued self-pertinency of m-sequence.