Time-frequency methods in communications

Abstract

The wide range of potential applications of time-frequency (t,f) methods made them an important tool in most fields of science and engineering. Telecommunications is one of the key industries where (t,f) methods started playing an important role. The topic is represented by five sections selected for this chapter.

Due to possible hostile jamming, broadband communication platforms use spread spectrum technology where interference protection is achieved by interference excision. By distributing the signature of received data over a (t,f) region, it is possible to attenuate strong interferences (Section 13.1). Linear dispersion in wireless communication channels distorts the transmitted signal in both time and frequency. This is accounted for by a (t,f) scattering function. In wireless communication systems with CDMA protocol, fading and multiaccess interference can be dealt with using (t,f) processing. A (t,f) RAKE receiver is described which implements correlations in a (t,f) domain and accounts for both spectral and temporal channel variations resulting from the use of spread spectrum techniques (Section 13.2). Eigenfunctions of linear systems can be modeled by signals with a (t,f) distribution well localized in the (t,f) plane. The knowledge of the eigenfunctions of time-varying transfer functions allows optimization of the transmission strategy and exploitation of the channel dispersive properties (Section 13.3). Detection and parameter estimation of chirps in communication systems may be implemented using the fractional Fourier transform (Section 13.4). The last section focuses on the (t,f) estimation of radio-signal modulation parameters and includes a discussion on cognitive radio, quality of service and communication channels from a (t,f) perspective (Section 13.5).