Spread spectrum communications--Myths and realities

Spread spectrum communication techniques date back to the early fifties. Since the earliest applications, system improvements have been more evolutionary than revolutionary. Like most improvements in electronic systems, these are due primarily to the availability of ever higher speed integrated circuit components, which translate in this case to wider spread spectra. In three decades the achievable spreading factor has grown by about three orders of magnitude’ to the point that we are now limited more by bandwidth allocations than by technology limitations. Before we xamine the quantitative effects of spreading, let us catalog briefly the multiple purposes of spread spectrum communications. First, we note that spreading here refers to expansion of the bandwidth well beyond what is required to transmit digital data. Thus, a system transmitting data at a rate ( R ) of 100 Mbits/s using approximately 100 MHz of bandwidth (W) is not spread at all, while a system transmitting at 100 bits/s spread over a spectrum of about 100 MHz has a factor W/R = 106, or 60 dB of so-calledprocessing gain.