PPROACH FOR ANTI-JAM SYSTEMS How can a receiver overcome the effects of intentional jamming, particu- larly when the jammer has much more power than the transmitted signal? Classical communication theoretic investigations of the additive white Gaussian noise channel suggest the answer. White Gaussian noise is a math- ematical model which has infinite power, spread uniformly over all fre- quencies. Effective communication is possible with this interfering noise of Basic Concepts and System Models infinite power because only the finite power noise components in the "sig- nal coordinates" can do any harm. Thus, as long as the noise components in the signal coordinates are not too large, reliable communication over an additive white Gaussian noise channel is achievable. For a typical narrow- band signal, this simply means that only the noise in the signal bandwidth can degrade performance. This classical theory suggests the following design approach in combatting intentional jamming: SELECT SIGNAL COORDINATES SUCH THAT THE JAMMER CANNOT ACHIEVE LARGE JAMMER-TO-SIGNAL POWER RATIO IN THESE COORDINATES. If many signal coordinates are available to a communication link and only a small subset of them are used at any time, and if the jammer cannot deter- mine the subset in use, then the jammer is forced to jam all coordinates with little power in each coordinate or to jam a few coordinates with more power in each of the jammed coordinates. Naturally, the more signal coordinates available, the better the protection against jamming. For signals of bandwidth W and duration T the number of coordinates is roughly [7] coherent signals (3.1) non-coherent signals. T typically represents the time to send a basic symbol. For fixed T, to make N large, W is commonly made large by one of two techniques: Direct Sequence Spreading (DS) Frequency Hopping (FH). The signals resulting from these basic forms are referred to as spread-spec- trum signals. Various hybrids of these two spreading techniques and other spread-spectrum signals are po