A digital DS spread-spectrum receiver with joint channel andDoppler shift estimation

A digital spread-spectrum receiver design is presented for communication over multipath channels with severe Doppler shifts. The characteristics of the underwater channel relevant to spread-spectrum system design are discussed, and a channel model for short-range communications (less than 10 km) is defined. The receiver considered uses a digital coherent RAKE combiner, coupled with an extended Kalman filter (EKF)-based estimator for channel parameters and pseudonoise code delay. Receiver performance is evaluated by computing average bit-error rate (BER) versus iterations of the EKF joint estimator, using both fixed and time-varying channels. It is shown that the BER obtained using the EKF joint estimator closely tracks the optimum BER obtained when the channel, delay, and Doppler parameters are known exactly. Finally, the Cramer-Rao lower bound for time-invariant joint channel, delay, and Doppler estimation is derived, and compared with the ensemble averaged mean-squared error of the EKF estimator     

Adaptive parameter estimation using parallel Kalman filtering forspread spectrum code and Doppler tracking

We present a new digital direct-sequence (DS) receiver with joint estimation of code delay, multipath gains and Doppler shift. A parameter estimator consisting of a parallel bank of extended Kalman filters (EKF's) extracts estimates of the timing, τ and the multipath coefficients, f_l distorting the received signal. A “detected” estimate of the Doppler shift, v_r distorting the received signal is also provided by the estimator. We compute the bit error rate that results when a RAKE matched filter uses the estimated parameters to detect the DPSK encoded binary data in the received signal. The bit-error rate (BER) is evaluated, and successful performance of the proposed receiver in the presence of Doppler shift distortion is observed in many cases. We demonstrate that the receiver can operate when the multipath coefficients vary in time (Doppler spread)     

On the feasibility of IEEE 802.11 multi-channel multi-hop mesh networks

Several scientific works have considered the possibility to build Wireless Mesh Networks (WMN) using multi-channel IEEE 802.11 architectures. At the basis of these works is the notion of “non-overlapping” channels, i.e. with a frequency separation equal or greater than 25 MHz. It is now a common assumption that multiple independent transmissions over these channels can coexist without mutual interference even in physical proximity. In this work we demonstrate that this assumption does not hold in general. Through an extensive set of experiments we illustrate the presence of cross-channel interference between “non-overlapping” channels at relay nodes due to the “near-far” effect. We analyze in what manner the MAC layer reacts to such an interference and how this problem extends to higher layers, with detrimental effects on the global throughput. The central problem is that cross-channel interference is not handled adequately by the MAC layer, and in some cases single-channel multi-hop settings perform better than multi-channel. Our results highlight a serious mismatch between some routing and channel assignment schemes proposed recently by the research community, assuming full separation between non-overlapping channels, and what is achievable in practice. More generally, as the presence of cross-channel interference can not be neglected at relay nodes, our findings point to a fundamental problem in building Multi-channel Multi-hop WMN based on IEEE 802.11b/g technology.     

Acquisition of timing and Doppler-shift in a direct-sequencespread-spectrum system

The paper presents a model for acquisition of a spread-spectrum signal distorted by Doppler shifts. The authors attempt a joint acquisition of both the time delay and the distorting Doppler velocity. They consider the case where the ratio of platform to propagation velocity, v_r/c is large enough to generate time compression of the pseudo-noise (PN) sequence itself. A Doppler effect of this type may be encountered in underwater acoustic or intersatellite communications. The simultaneous presence of the unknown Doppler shift and timing necessitates a substantial modification of conventional acquisition strategies. A two-dwell system, using a short duration correlation to acquire the timing information, with a subsequent longer correlation to acquire the Doppler velocity is proposed. The joint acquisition process is modeled by a finite state Markov chain, and the performance of the system is characterized by the mean and variance of the acquisition time. Two systems of linear equations that can be solved for the mean and variance, respectively, are developed. The matrices specifying the systems can be solved efficiently. As a particular example, the authors analyze the performance of systems with spread-spectrum signals having sequence lengths of 15 and 63 chips. It is shown that the two-dwell system can acquire timing and Doppler shifts in a reasonable time period. Although the system studied is not well-suited to multiple-access or antijam applications, the analysis techniques developed may prove useful in evaluating the performance of more complex acquisition techniques