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In spread spectrum systems the transmit signal occupies a wider spectrum than required by the symbol rate. Slow and fast Frequency Hopping (FH) changes the carrier frequencies, and can be used for multiple access in synchronized and unsynchronized systems. In Code Division Multiple Access (CDMA), the data sequences are multiplied by spreading sequences with short chip duration, thus increasing the occupied bandwidth by the ï¿¿ï¿¿ï¿¿spreading factorï¿¿ï¿¿ï¿¿. Spreading sequences with suitable crosscorrelation (and autocorrelation) properties allow separation of users: pseudonoise (PN) sequences like maximum-length (m-) sequences, Gold-, and Kasami-sequences; Walsh-Hadamard codes and Orthogonal Variable Spreading Factor (OVSF) codes. In delay-dispersive channels, Rake receivers consisting of multiple correlators (fingers) collect the energy contained in different multipath components. Synchronization of the receivers to the available signals occurs in two steps: acquisition and tracking. CDMA allows universal frequency reuse (i.e., reuse distance one). Inter-user interference (consisting of intra-cell and inter-cell interference) is noise-like. Randomization of inter-cell interference is effected by multiplying the signals in different cells with different scrambling codes. Power control ensures that no single user provides dominant intra-cell interference (in the uplink). Near the cell edge, soft handover ensures good transmission quality through macrodiversity. Multiuser detection (MUD) is based on exploiting the structure of interference to mitigate its effect on the desired signal. Linear MUDs include decorrelating receivers and MMSE receivers. Nonlinear MUDs include the (optimum) multiuser MLSE as well as successive interference cancellation (SIC) and parallel interference cancellation (PIC). Time hopping impulse radio, mostly used in ultrawideband (UWB) communicat ions, represents each symbol by a sequence of pulses. Transmitted-reference signals allow simple receivers.