Abstract
Criteria for selecting spreading codes for a downlink transmission in a direct sequence code division multiple access (DS-CDMA) system are reviewed. For the most general case of a non-code inversion keying system (CIK) it turns out that the codes must have good partial aperiodic cross and autocorrelation properties as far as the optimisation of the RAKE receiver performance is concerned. Codes generally used such as the Gold or Kasami codes have good cross and auto correlation properties on their whole period but do not have any particular partial correlation properties. In order to solve this limitation, the author proposes a method for building a set of so-called extended structured codes. Each long code is obtained by concatenation of cyclically different short codes of period the processing gain, the short codes of the initial set presenting good correlation properties. This construction is an extension of the structured codes proposed by T. O'Farrell (1991) that overcomes some of their drawbacks such as a short period and a small number of long codes in the constructed set. Simulation results show that the 'extended structured codes' bring a large improvement with respect to Gold and m-sequences. >
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