PN Spreading and Despreading
Application: IS-95 uses DS PN spreading to achieve several signaling advantages. These advantages include increasing the bandwidth so more users can be accommodated, creating near-orthogonal segments of PN sequences which provide multiple access separation on the reverse link and universal frequency reuse, increasing tolerance to interference, and allowing the multi-path to be resolved and constructively combined by the RAKE receivers. Multipath can be resolved and constructively combined only when the multi-path delay between multipath component signals is greater than the reciprocal of the signal bandwidth. Spreading, and thus increasing the signal band-width, allows resolution of signals with relatively small delay differences.
PN Spreading and Despreading
Example:
Assume a signal s(t) has a symbol rate of 19,200 sym/sec. Then each symbol has a duration of 1/19200 or 52.0833 µsec. If s(t)is module 2 added to a PN sequence PN(t) with chips changing at a rate of 1.2288 Mchips/sec, each symbol will contain 1.2288x52.0833 or exactly 64 PN chips. The band-width of the signal is increased by a factor of 64 to 64x19,200 or 1.2288 MHz. The received spread signal has the form PN(t-t)s(t-t). At the receiver, a replica of the PN generator used at the transmitter produces the sequence PN(t-x) and forms the product . When the variable x is adjusted to equal t, PN(t-x)PN(t-t)s(t-t) equals PN(t-t)2s(t-t) which equals the desired symbol stream s(t-t) since PN(t-t)2 always equals one. This illustrates despreading.
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