Digital Communications with Emphasis on Data Modems

13SPREAD‐SPECTRUM COMMUNICATIONS

13.1 INTRODUCTION

The original motivation for spread‐spectrum (SS) communications [1–8] was to mitigate the effect of electronic counter measures (ECMs), generally referred to as jamming signals intended to disrupt the flow of information between cooperating terminals. Solutions to the ECM problem were so widespread that the applications of SS to communication systems became known as anti‐jam (AJ) communications [9]. The more general benefit is that SS techniques also provides for more robust communications under a variety of conditions including unintentional interference resulting from the increased demand on the available frequency spectrum. The key to successful SS communication systems is the design of the transmitted waveform and the demodulator matched filter. In effect, the relatively narrow information bandwidth is spread over a much wider bandwidth and transmitted. To provide an AJ capability, the transmitted waveform is spread in bandwidth using a unique code or signaling technique known only by the intended receivers. The receiver detection processing correlates or matched‐filters the desired waveform while decorrelating the jammer or interfering signals at the detector output to provide reliable detection of the information. Regardless of whether the jamming signal is designed to concentrate the power in a narrow portion of the SS bandwidth or over the entire bandwidth, the jammer transmitted energy must be inordinately high to ...