New PN code acquisition scheme for CDMA networks with low signal-to-noise ratios

Abstract

A new approach to PN code acquisition is presented and analyzed. A recirculation loop is used to improve probability of synchro cell detection P/sub D/ in each retrace of the code uncertainty region. To further improve P/sub D/ and probability of false alarm P/sub fa/ simultaneously, code diversity (a number of synchro channels in parallel) is used. This is especially effective in the channel with multiple access interference and near-far effect. Typical applications are networks with very low signal-to-noise ratio. Examples are code division multiple access (CDMA) LEO satellite systems experiencing high Doppler or any CDMA network where the number of users is approaching capacity limits. Even if the carrier Doppler is compensated, in any asynchronous LEO satellite network, compensation of code Doppler is not feasible. For this reason, in order to cope with code Doppler (D) and delay (/spl tau/), a modification based on transforming the two-dimensional uncertainty region (D, /spl tau/) into a new uncertainty region (T/sub c/, /spl tau/) will be introduced. Parameter T/sub c/ is the period of the correlation pulses at the output of a sliding correlator. When Doppler rate is present, the three-dimensional uncertainty region (D, R/sub d/, /spl tau/) is transformed into a new one (T/sub c/, R/sub t/, /spl tau/) where R/sub d/ and R/sub t/ are Doppler rate and correlation pulse period change rate, respectively. The main motivation for this work is to find new algorithms suitable for all digital receiver implementation and operation at low signal-to-noise ratios. These algorithms make CDMA techniques feasible for direct communication between LEO satellite and small ground-based user terminals (handsets). A comprehensive performance study of the new PN code acquisition system is presented and discussed. The results obtained demonstrate that, for low signal-to noise ratios, the acquisition time achieved with the new algorithm is one order of magnitude shorter compared with standard techniques known so far.