Two-dimensional orthogonal variable-spreading-factor codes for multichannel DS-UWB

We propose a recursive algorithm to construct ternary two-dimensional orthogonal variable-spreading-factor (2D OVSF) codes with a tree-structure for multirate multichannel DS-UWB system. The proposed ternary 2D OVSF codes, including binary codes as a special case, have a significantly reduced restriction on code lengths and ensure orthogonality between different code layers. 2D OVSF codes modulated by the same information bit are transmitted over orthogonal channels. We study the BER performance of 2D OVSF code based UWB system in Nakagami fading multipath channels. The results show that, if the orthogonal channels have identical channel coefficients, the system is with zero multiple access interference (MAI). Furthermore, if the sequence length is larger than the number of multipath components, there is no multipath interference.