Orthogonalization of Parallel Sequence Spread Spectrum Codes for High Order Modulation

Parallel Sequence Spread Spectrum (PSSS) is a potential physical layer (PHY) for a robust high rate wireless transmission. PSSS uses pseudo noise bipolar m-sequences with increasing cyclic temporal shift as spreading codes. These codes are used similar to the sinusoid carriers of an orthogonal frequency division multiplex (OFDM) system with a spectral efficiency of 1 bit/s/Hz for the simplest case of BPSK modulation. For optimal performance, these codes have to be in exact time synchronization. A disadvantage of unmodified PSSS is the limited orthogonality of its spreading codes, which leads to inter code interference resulting in a BER penalty. This penalty can be mitigated for BPSK and QPSK modulation by an iterative decoding algorithm. For higher modulation such as QAM, this iterative decoding will turn out as suboptimal. In this paper we present a method for the orthogonalization of the m-sequences, thus eliminating the inter code interference and enabling high order modulation for PSSS.