Random Notes on CDMA

1 CDMA The basic idea behind CDMA is to assign each user a signature (also called a code-word), s m (t). Users will use antipodal signaling (i.e., multiplying their signatures by ±1 to send data. Thus, what the receiver sees is (idealistically) r(t) = m b m s m (t) + w(t) where the b m are the users' bits (again, ±). We also generally assume that the system is interference limited so that the noise term is inconsequential. That is, we assume r(t) ≈ m b m s m (t) From the perspective of a receiver trying to decode, say, user 1 we have r(t) = b 1 s 1 (t) + m =1 b m s m (t) and we do the usual matched filtering (correlator receiver) where we multiply r(t) by s 1 (t) and integrate so that the decision paramater we use to guess what b 1 is looks like r 1 = b 1 E s 1 + m =1 b m T 0 s 1 (t)s m (t)dt where we've assmed the bit interval is (0, T). What's not necessarily obvious is that we've ALSO assumed that all the users signals arrive at the receiver in lock-step (their individual bit intervals start and stop at the same time). This actually 1