PR-QMF based codes for multipath/multiuser communications

One common type of interference in a mobile communications channel is multipath interference. Traditionally, the DSSS/CDMA systems utilize maximal length sequences (m-sequences) as the spreading codes. The m-sequences were chosen because they possess the desired randomness properties and are easily generated via shift registers. The perfect reconstruction quadrature mirror filter (PR-QMF) based spread spectrum codes were proposed as an alternative to the m-sequences. A class of spreading codes based on PR-QMF subband trees is examined. After developing the properties of these codes as they relate to the multipath and multiuser channels, objective and constraint functions are formulated which incorporate these properties. These functions are then used in a multivariable optimization problem to determine the PR-QMF coefficients for use in the subband tree structure. Once the subband tree is complete, the spreading codes can then be extracted for use. It is shown that in the limiting cases, the optimal solutions tend towards TDMA and FDMA like structures.