A RAKE receiver accomplishing joint blind multipath diversity combining and PN code timing recovery is proposed for direct-sequence spread-spectrum signaling over a frequency-selective fading channel. In this technique, an improved known modulus adaptive algorithm is exploited to perform multipath diversity combining in the blind mode, while a modified PN code timing recovery technique based on the timing error estimator extracts the finger error signals path by path independently. Taking the advantage of inherent diversity, this modified PN code timing recovery technique can efficiently combine finger error signals to avoid the problems with the drift or flutter effects in the timing error signals, and thus provide better code tracking performance as well. Extensive computer simulation results have verified the analysis and indicated very attractive performance of the proposed technique.

A modified fully-digital code tracking loop is proposed in this paper for direct-sequence spread-spectrum signaling on a frequency-selective fading channel. A data-modulated channel estimator is used to cope with the time-varying Rayleigh fading effect and the data modulation effect, and extract the desired error signal from each path independently in the multipath environments. By taking advantage of the inherent diversity with the maximal ratio combining (MRC) or a proposed Even/odd maximal ratio combining (EMRC) technique, this modified code tracking loop can avoid the problem due to the drift or flutter effects of the error characteristics, and provide better performance on frequency selective fading channels. Extensive computer simulation has verified the analysis and indicated very attractive behavior of the proposed digital tracking loop.

A modified sequential acquisition scheme is proposed in this letter to avoid the significant high error probabilities (false alarm and missing probabilities) occurring with the conventional sequential acquisition scheme in direct-sequence spread-spectrum systems while a high frequency offset is present. A new estimator of Ek/N0 is also designed to effectively solve the problems caused by the channel fading effects. Extensive computer simulation results have indicated that the proposed technique can achieve the desired low error probabilities, and furthermore its performance is very close to that with the perfect channel estimation.

This paper proposes a channel estimation technique which uses a postfixed pseudo-noise (PN) sequence combined with zero padding to accurately estimate the channel impulse response for mobile orthogonal frequency division multiplexing (OFDM) communications. The major advantage of the proposed techniques is the periodical insertion of PN sequences after each OFDM symbol within the original guard interval in conventional zero-padded OFDM or within the original cyclic prefix (CP) in conventional CP-OFDM. In addition, the proposed technique takes advantage of null samples padded after the PN sequences for reducing inter-symbol interference occurring with the information detection in conventional pseudo-random-postfix OFDM. The proposed technique successfully applies either (1) least-squares algorithm with decision-directed data-assistance, (2) approximate least-squares estimation, or (3) maximum-likelihood scheme with various observation windows for the purpose of improving channel estimation performance. Some comparative simulations are given to illustrate the excellent performance of the proposed channel estimation techniques in mobile environments.