and the signal is noiselike, making it hard to detect. In antijamming (AJ) applications, the transmitter introduces an unpredictable element into the modulation of the signal, known to the receiver but kept secret from opponents, as in stream ciphering. This, together with the wide bandwidth of the transmitted signal, makes jamming more difficult than for conventional signals. DSSS is used in a large number of applications, including CDMA cellular (mobile) phones (IS-95, CDMA2000, W-CDMA, UMTS, FOMA) Some cordless phones The 802.11b/g Wi-Fi standards Satellite navigation systems (GPS, Glonass, and Galileo) ZigBee/802.15.4 (for low-power wireless networked controls) In CDMA applications of DSSS (DS-CDMA), the receiver synchronizes to the received signal and correlates it with its particular PN pattern; this is called despreading. Despreading increases the power of the wanted signal by a factor of L relative to the signals from other users (and noise). The transmitters that are sharing the channel use different PN sequences, chosen SO that their cross-correlation is low, to reduce the interference between the received signals. The PN sequences must also have sharply peaked autocorrelation functions to help the receiver synchronize correctly to the partially unknown timing of the received signal. Some often used sets of sequences with these properties are called Gold and Kasami sequences (Proakis, 1989). Frequency Hopping Spread Spectrum In Frequency Hopping Spread Spectrum (FHSS), the available channel bandwidth, LW, is divided into L slots (channels) of bandwidth W. In any signaling interval the signal occupies only one slot or a few (<<L). Chapter 4 Coding The spreading of the signal spectrum arises because the active slot frequency is "hopped" around pseudorandomly. Because of the difficulty of maintaining phase references as the frequency hops, FSK modulation and noncoherent demodulation are normally used in FHSS, rather than PSK and coherent demodulation. In block-hopping FHSS, the input signal is first modula