CDMA code synchronization using segmented matched filter with accumulation and best match selection

This paper investigates codephase acquisition in direct-sequence spread spectrum (DS-SS) systems under conditions of high co-user noise and significant carrier frequency offset (Doppler). While conventional serial search acquisition is tolerant of these conditions, the process is quite slow. Conventional matched filters can provide fast acquisition but are unable to handle significant carrier frequency offset. This paper describes a segmented matched filter (SMF) that provides rapid acquisition under conditions of frequency offset and high noise. An accumulation and best match selection scheme is introduced and its performance is compared to the segmented matched filter with threshold detection and also to a conventional non-coherent correlator with serial search. Expected acquisition time is shown as a function of multiple access interference in the presence of carrier Doppler. Example calculations are based on GPS transmission parameters. In addition to calculated probability density junctions (pdf), the paper presents measured pdf from a mixed-signal ASIC that implements a 512 chip SMF with half-chip codephase resolution. With this acquisition method, the receiver carrier is assumed to be non-coherent, data modulation of the DS-SS signal is allowed and prior chip synchronization is not required.