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

A new approach to PIV code acquisition is presented and analyzed. A recircu lation loop is used to improve probability of synchro cell detection P-D in each retrace of the code uncertainty region. To further improve P-D and pr obability of false alarm P-Fa simultaneously, code diversity (a number of s ynchro channels in parallel) is used, This is especially effective in the c hannel with multiple access interference and near-far effect. Typical appli cations are networks with very low signal-to-noise ratio. Examples are code division multiple access (CDMA) LEO satellite systems experiencing high Do ppler or any CDMA network where the number of users is approaching capacity limits, Even if the carrier Doppler is compensated, in any asynchronous LE O satellite network, compensation of code Doppler is not feasible. For this reason, in order to cope with code Doppler (D) and delay (tau), a modifica tion based on transforming the two-dimensional uncertainty region (D, tau) into a new uncertainty region (T-c, tau) Will be introduced. Parameter T-c is the period of the correlation pulses at the output of a sliding correlat or, When Doppler rate is present, the three-dimensional uncertainty region (D, R-d, tau) is transformed into a new one (T-c, R-t, tau) where R-d and R -t are Doppler rate and correlation pulse period change rate, respectively. The main motivation for this work is to find new algorithms suitable for al l digital receiver implementation and operation at low signal-to-noise rati os. 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 pr esented and discussed, The results obtained demonstrate that, for low signa l-to-noise ratios, the acquisition time achieved with the new algorithm is one order of magnitude shorter compared with standard techniques known so f ar.