Digital Communications

10Broadband Transmission Techniques

Ever increasing demands for higher and differing data rates resulted in broadband transmission of multimedia signals. Broadband transmission has to cope with frequency‐selective fading and other issues related to, amongst others, multiple access and multiplexing. In addition, associated time‐synchronization problems and complex equalization techniques may be required. Spread spectrum (SS) and orthogonal frequency‐division multiplexing (OFDM) represent the two main technologies for transmission of broadband signals in such adverse conditions.

Historically, direct sequence (DS) and frequency hopping (FH) SS systems are developed and used for military applications, for example, for anti‐jamming (AJ) and low‐probability of intercept (LPI). SS is a transmission technique in which a data sequence is transmitted in a bandwidth much in excess of the minimum bandwidth necessary to send it, with a correspondingly reduced PSD. The spectrum spreading is accomplished before transmission through the use of a code that is independent of the data sequence. The same code is used at the receiver, operating in synchronism with the transmitter, to despread the received signal for recovering the original data sequence. Spreading the user signals accross a wide spreading bandwidth improves the anti‐jamming (AJ) capability of these signals. Since SS signals appear like random noise, they are difficult to detect and/or decode by receivers other than the intended ...