l higher capacity when it is mea- sured in the number of mobile units per high-density area. This slotted S-CDMA system has the advantage that only one set of inphase and quadrature matched filters is needed for all the mobile units assigned to the base station. The problem, however, is that with matched fil- ters, signals are not completely orthogonal. In general, each signal's matched filter output has some non-zero terms that contribute to interference at other time slots. This is illustrated in Figure 3.5. Ruprecht [25] noted that the matched filter is optimum for detecting sig- nals in white Gaussian noise but not necessarily for other situations. For example, he considered an application in which spread-spectrum signals function as sounders (or radar signals) used to determine propagation chan- nel characteristics (or target characteristics). In particular, if such spread- spectrum signals are used for estimating multipaths in a channel, it is important that pulses from different paths not interfere in the measure- ments. In the interest of minimizing mutual interference between pulses from dif- ferent multipath returns, Ruprecht determined that the maximum-likelihood estimation of multipath channels would use filters that are inverse to the transmitted spread-spectrum pulse signal rather than matched filters. Suppose we regard a codeword as an impulse response of a filter which, of course, is of finite duration. The ideal inverse filter is a real-valued wave- form of infinite duration. Naturally, by definition, when a codeword enters the inverse filter, the output is ideally a delta function. A similar discrete time version can also be shown to give only a single non-zero output sample for the discrete time-inverse filter. If such an ideal inverse filter is used in place of the matched filter, no inter- ference of the time-shifted signals is seen at the filter output. Thus, in the- ory, inverse filters in a time slotted S-CDMA system produce truly orthogonal signals at the output of the inverse filter. As 