Adaptive Subband Transforms in Time-Frequency SSS Communications Systems

Abs&act- A smart time-frequency exciser for DSSS communications is proposed in this correspondence. This technique utilizes the concept of uncertainty in time-frequency analysis of signals. It brings the novel concept of domain switchable signal processing. Hence, the adaptive he- frequency (ATF) exciser has the capability of deciding the domain of the interference cancellation. Additionally, adaptive subband transform are utilized for frequency domain excision. For time-domain excision, the ATF excision algorithm utilizes a sliding time window to rejett the nonstationary, pulsed (time-localized) interference. It is shown that the proposed adaptive time-frequency exciser-based DSSS communications receiver drastically outperforms the existing systems. Its performance is nearly optimal and very robust to the inter and intradomain variations of the undesired signal. I. JNTRODUC'ITON Spread spectrum modulation techniques produce a transmitted fre- quency spectrum that is much wider than the information bandwidth. This increase in bandwidth yields a processing gain that provides de- sired features such as interference suppression, code-division multiple access, energy density reduction, and good time resolution in digital communications systems ( 11. However, the interference rejection capability of spread spectrum systems is limited. Both deliberate and unintentional interference (jamming) can be rejected up to a certain jamming margin. Among different spreading techniques, the direct sequence spread spectrum (DSSS) or pseudo-noise (PN) modulation system is considered in this study. The DSSS transmitter, which is shown in Fig. 1, spreads the incoming data bit stream dl (dl 6 {-1, 1}, VZ) and, therefore, its spectrum by multiplying it with the spreading sequence c (ez E { - 1, 1} for i = 1, . . . IC). During the transmission, the channel adds a noise term 5, and other interferences i,. Therefore, the received signal T, can be written as