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Mobile communications require flexible networks which can accommodate variable numbers of users under adverse propagation environments. Multiplexing by frequency or time division is inefficient and ineffective in guarding against multipath and mutual interference, noise and jamming. Code Division Multiple Access (CDMA), frequency hopping, Orthogonal Frequency Division Multiplexing (OFDM), and Ultra Wide Band (UWB) techniques have been developed to overcome these shortcomings. These have been enhanced by Multiple Input, Multiple Output (MIMO), polarization and space-time coding. All rely on digital sequences with good correlation. Ensembles of sequences with low off-peak autocorrelation and low cross-correlation are used in asynchronous CDMA systems.
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