| Popis: |
Despite the significant advantages of direct sequence spread spectrum communications, whenever the number of users increases or the received signal is corrupted by an intentional jammer signal, it is necessary to model and estimate the channel effects in order to equalize the received signal, as well as to excise the jamming signals from it. Due to multi-path and Doppler effects in the transmission channels, they are modeled as random, time-varying systems. Considering a wide sense stationary channel during the transmission of a number of bits, a linear time-varying model characterized by a random number of paths, each being characterized by a delay, an attenuation factor and a Doppler frequency shift, is shown to be an appropriate channel model. It is shown that the estimation of the parameters of such models is possible by means of the spreading function, related to the time-varying frequency response of the system and the associated evolutionary kernels. Applying the time-frequency or frequency-frequency discrete evolutionary transforms, we show that a blind estimation procedure is possible by computing the spreading function from the discrete evolutionary transform of the received signal. The estimation also requires the synchronized pseudo-noise sequence for either of the users we are interested in. The estimation procedure requires to adaptively implementing the discrete evolutionary transform to estimate the spreading function and determine the channel parameters. Once the number of paths, delays, Doppler frequencies and attenuations characterizing the channel are found, a decision parameter can be obtained to determine the transmitted bit. We will show also that our estimation approach supports multiuser communication applications such as uplink and downlink in wireless communication transmissions. In the case of an intentional jamming, common in military applications, we consider a receiver based on non-stationary Wiener masking that excises such jammer as well as interference from other users. Both the mask and the optimal estimator are obtained from the discrete evolutionary transformation. The estimated parameters from the computed spreading function, corresponding to the closest to the line of sight signal path, provide an efficient detection scheme. Our procedures are illustrated with simulations, that display the bit-error rate for different levels of channel noise and jammer signals. |
