Results from multiplication stage are accumulated over the symbol period. At the end of the period, the correlation sum is dumped. In the parallel correlator, data samples are held in a long shift register, and the pilot PN sequence is held in a reference pattern register. Each time a new sample is loaded into the data shift register, the contents of the two registers are multiplied and integrated to give a new correlation sum. You can use the Parallel Correlator function from Nova Engineering, Inc. to implement the multipath delay estimator.
Every user acts as an interference for every other user.
The more resistant the system is to interference, the more users can be served. However, it is too complex for practical DS-CDMA systems. Therefore, most research and development efforts are focused on developing sub-optimal schemes. Simulation has shown that groupwise successive IC is the most promising scheme. In this scheme, users are grouped according to their spreading factor and then IC or a decorrelator is applied within the group.