Low probability of intercept R-Conjugate CDMA codes

Privacy and security have become increasingly important in code division multiple access (CDMA) systems, for both cellular and military applications. One way to improve security is to use spreading code diversity to transmit user information, a technique known as multi-code CDMA. In this paper, we present a new method of generating codes for multi-code CDMA that exhibit low probability of detection (LPD) and low probability of interception (LPI) while achieving near single code performance. The codes are calculated by first estimating the channel and interference plus noise covariance matrix, denoted Q. The eigenvectors of the resultant matrix are fed into a filter that performs a series of orthogonal projections to generate the codes. This results in codes that are R-conjugate, i.e. maintain orthogonality after propagating through the channel. The unknown channel must be estimated, but since it is time-varying, the resulting codes are dynamic. The LPD/LPI characteristics come about because the filter produces codes with random amplitudes. We demonstrate that these codes perform as well as the previously introduced R-conjugate codes based on eigen-decomposition of the interference plus noise covariance matrix, by maintaining the R-conjugate properties. Furthermore, if the channel can be accurately estimated, then we can achieve near ideal performance. This requires a slow fading channel which remains approximately constant over a block size M > 20N, where N is the length of the spreading code. We also illustrate the improved LPD/LPI properties of the codes.