Orthogonal Code Usage – Coding & Decoding in WCDMA
How Orthogonal Code works in WCDMA?
- By spreading, each symbol is multiplied with all the chips in the orthogonal sequence assigned to the user. The resulting sequence is processed and is then transmitted over the physical channel along with other spread symbols. In this figure, 4-digit codes are used. The product of the user symbols and the spreading code is a sequence of digits that must be transmitted at 4 times the rate of the original encoded binary signal.
- The receiver dispreads the chips by using the same code used in the transmitter.
Notice that under no-noise conditions, the symbols or digits are completely recovered
without any error. In reality, the channel is not noise-free, but CDMA system employ
Forward Error Correction techniques to combat the effects of noise and enhance the
performance of the system.
- When the wrong code is used for dispreading, the resulting correlation yields an
average of zero. This is a clear demonstration of the advantage of the orthogonal
property of the codes. Whether the wrong code is mistakenly used by the target user
or other users attempting to decode the received signal, the resulting correlation is
always zero because of the orthogonal property of codes.