Cancelling the signals removes the interference due to correlation with other spreading codes. Evaluating the cross-correlation matrix is a computationally intensive task. A new matrix must be calculated and inverted every time the user profile changes or multipath tap changes. With short spreading code, this implementation is more feasible. Altera provides the FFT MegaCore function, which is also suited for this application.
Decoders There are two types of decoders that can be used in the receiver.
The Viterbi algorithm is the optimal algorithm to decode convolutionally encoded data. The complexity of Viterbi decoding is an exponential function of the constraint length. W-CDMA requires a decoder with a constraint length of 9, which poses an implementation challenge. There are two different Viterbi implementations depending on the throughput requirements. See Table 1.