The theoretical basis of spread spectrum technology:
shannon theorem: white Gaussian noise conditions, limits the transmission rate of the communication system (channel capacity) is:
Wherein; B is the signal bandwidth, S is average signal power, N is the noise power, C is the channel capacity.
In the channel capacity must be SNR and bandwidth are interchangeable.
Spread Spectrum Technology Category:
(1), Direct Sequence Spread Spectrum (DSSS)
CDMA chip rate: 1.2288MCPS CDMA signal rate: 9.6kbit / s
CDMA uses direct sequence spread spectrum system.
Expanding system information to be transmitted is spread with a pseudo-random (PN) sequence to a wide frequency band, at the receiving end, with the same pseudo-random sequence for the extension originating the received spread spectrum signal correlation processing, recovery the original signal. Since the interference signal uncorrelated with good pseudo-random sequence is extended at the receiving end, as falling within the frequency band of the interference signal power signal is greatly reduced, thereby increasing the output signal to noise ratio of the system, the purpose of anti-jamming.
In the CDMA system, spreading and scrambling in order to distinguish between the main channel
Distinguishing channels called orthogonal spreading code
as the scrambling code is called scrambling code, but does not completely orthogonal inter-code
M-sequence and a Walsh code:
Orthogonal spreading codes: a Walsh code
scrambling: m sequence of
Walsh codes is limited, the base station will use a different Walsh code, the scrambling code used to distinguish the base station, so that signals from different base stations can be seen as noise.
CDMA, two m-sequence used simultaneously - short code and the long code
Short code:
Period of 2 ^ 15-1
For the downlink, the orthogonal signals so that the different cells of (different cells use the same m-sequence, but different phase, i.e. different PN_OFFSET)
for the uplink, identical m sequence phase used, as spread use
generally have different phases 512, spaced 64 chips, with a different time offset of the correlation itself is very small, so that different phases can be distinguished from each other
Long code:
Period of 2 ^ 42-1
Used in downlink and uplink, each mobile phone according to the unique mask determines the phase of the long code to distinguish different users
each user has a unique mask, also a unique long code phase by and will not be repeated
CDMA中的Walsh码：
长度为64bit
在下行中用来区分不同信道
在上行中仅用作正交调制