r self-interference due to multipath effects Making it difficult to detect the signal to achieve covert operation Making it difficult to demodulate the signal to achieve privacy Allowing multiple users to share one frequency band ("multiple access") in a way that provides flexibility in the number of users and the allocation of transmission capacity to different users. This is also known as Code Division Multiple Access (CDMA). Spread-Spectrum Systems Applications Until 1995, the main application of spread-spectrum techniques was in military systems to achieve the preceding first to third objectives. However, with the introduction of the IS-95 CDMA-based digital cellular system and the subsequent incorporation of spread-spectrum techniques in many standards, their dominant application is now in commercial digital cel- lular phone and wireless networking systems. The advantage of using spread-spectrum techniques is that they overcome problems due to multipath and that multiple users can be provided with a uniformly acceptable service rather than good service for some and bad for others, as can happen with older Frequency Division Multiple Access (FDMA) systems. Direct Sequence Spread Spectrum In Direct Sequence Spread Spectrum (DSSS), the transmitter and receiver contain identical pseudorandom sequence generators producing a pseudonoise (PN) stream of symbols at a rate (known as the chip rate) that is a multiple of the data symbol rate. The ratio of chip rate to data symbol rate is the spreading factor (L). In the transmitter, the input datastream is XORed with the PN signal before transmission, SO each data symbol is replaced by L chips. The bandwidth of the transmitted signal is therefore L times greater than that of the data- stream. In the receiver the received signal is XORed with the PN stream to recover the original datastream; this is equivalent to correlation with the known PN sequence. The use of DSSS gives LPI (Low Probability of Intercept) because the total signal power is spread over a wide bandwidth 