Fast acquisition in DS-CDMA using concatenated spreading sequences

We consider the problem of code acquisition in direct-sequence (DS) code-division multiple-access (CDMA). It has been shown that the acquisition in the presence of multiple-access interference may impose a significant limitation on capacity. It is known that by using concatenated spreading sequences (also known as Kronecker sequences) instead of conventional sequences (e.g. gold sequences), the acquisition time of serial-search acquisition schemes can be reduced for large signal-to-noise ratios (>15 dB). In this paper, we investigate the acquisition time of concatenated sequences for low signal-to-noise ratios. We consider different acquisition schemes employing the MAX criterion which are used for the acquisition of the inner sequence. Analytical expressions for mean and variance of the acquisition time are derived and verified through simulations. We compare the acquisition time using concatenated spreading sequences with the acquisition time using conventional spreading sequences. To reduce deteriorations due to non-vanishing auto-correlation sidelobes and multiple-access interference, we employ inverse filtering and interference cancellation. The results indicate that concatenated spreading sequences can considerably speed up code acquisition even for moderate signal-to-noise ratios.