Comparison of detection schemes for fast acquisition direct-sequence spread-spectrum receivers

By using parallel correlation receivers or code-matched filters, the acquisition time of direct-sequence spread-spectrum waveforms can be significantly reduced for any application where a wide code-uncertainty range must be searched. The author discusses parallel acquisition schemes that allow multiple code-phase offsets to be examined at each test. In many applications the presence of data modulation and unknown Doppler offsets must be accounted for in the acquisition process. The tradeoffs between noncoherent combining loss, data modulation effects, and Doppler losses often force the code-uncertainty region to be divided into subintervals, greatly influencing how much parallelism is required in the acquisition scheme. Three detection schemes are examined for use in code-matched, filter-based PN (pseudonoise) receivers that must operate at low signal-to-noise ratios and where the code uncertainty is divided into one or more subintervals. Gaussian approximations, found to be extremely accurate when a large noncoherent accumulation period is required, have been used for easy comparison of the three schemes. The results show that no one approach is optimum for all conditions, and the detection choice depends strongly on the received E_s/N₀ and the number of subintervals that must be searched