Joint synchronization in the cdma reverse link using a two-bit window transpose domain filter
Seema Sud · 2012
The reverse links of 3G cellular systems, which are based on code division multiple access (CDMA), require that the base station synchronizes with each user's length N spreading code to despread and demodulate the information bits. A method of obtaining chip-level synchronization for all K users of the system simultaneously was recently presented [3]. The approach uses a modified transpose domain filter (TDF) implemented by a multistage Wiener filter (MWF), and the joint solution for all K users is implemented efficiently with a parallel bank of MWFs. The technique was shown to be effective in giving near 100% success in timing estimation for heavily loaded systems when Eb/N0≥ 0 dB, enough bits, M, e.g. M ≥ 16N, is used to obtain the estimate, and at least 20% of the chips in the spreading code are present. In this paper, we propose a modified TDF algorithm, in which processing occurs over a span of 2N chips vs. N chips. The proposed algorithm overcomes the limitations of the previous algorithm since the processing now spans two bits so the entire spreading sequence is always available for processing. We analyze the algorithm in AWGN and multipath, with known and unknown channels, and show that it achieves >; 90% success even in multipath with just M = 2N bits and near 100% with M = 4N bits over a wide range of Eb/N0and number of users K.