 Such a brute force acquisition procedure, whether implemented in parallel or serial form, clearly has the advantage that a definite decision will be made after only a single examination of all code phase positions, or a single search through the entire code period. Thus, multiple examination of each code phase posi- tion or multiple searches through the code are avoided by this procedure. However, this rudimentary advantage is overwhelmed by the obvious dis- advantage that a decision cannot be reached unless every code phase posi- tion has been examined or until the entire code period has been searched. For long codes, as are required in spread-spectrum systems with large pro- cessing gain, complexity of the parallel implementation or the time to search the entire code and thus reach a synchronization decision in the serial ver- sion is often prohibitive. Another scheme, which dates back more than two and a half decades, was first introduced by Ward [4] and is based on a sequential estimation of the shift register states of the PN generator. In particular, the RASE (Rapid Acquisition by Sequential Estimation) system of Figure 1.4 makes its best estimate of the first n received PN code chips (n is the number of stages in the code generator) and loads the receiver sequence generator with that estimate, thus defining a particular initial condition (starting state) from which the generator begins its operation. Since a PN sequence has the 4Note that the serial implementation of the maximum-likelihood algorithm is not true maxi- mum-likelihood since the various hypotheses (code phase positions) are tested using different observations of the received signal. This should not be confused with the method of sequential detection [10] associated with vari- able integration time acquisition systems. ACQUISITION SCHEMES FOR DS/SS SYSTEMS SEARCH STRATEGY MAXIMUM - LIKELIHOOD SERIAL SEARCH (PARALLEL) SEQUENTIAL ESTIMATION PERMITTED SWEEP STRATEGY ACQUISITION TIME DETECTOR FALSE ALARM STRUCTURE STATE UNCERTAINTY REGION Presiden