OVSF-CDMA code assignment in wireless ad hoc networks

Orthogonal Variable Spreading Factor (OVSF) CDMA code provides a means of support of variable rate data service at low hardware cost. In contrast to the conventional orthogonal fixed-spreading-factor CDMA code, OVSF-CDMA code consists of an infinite number of codewords with variable rates but not every pair of codewords are orthogonal to each other. In an OVSF-CDMA wireless ad hoc network, a code assignment has to be conflict-free, i.e., two nodes can be assigned the same codeword or two non-orthogonal codewords if and only if neither of them is within the transmission range of the other and no other node is located in the intersection of their transmission ranges. The throughput (resp., bottleneck) of a code assignment is the sum (resp., minimum) of the rates of the assigned codewords. The max-throughput (resp., max-bottleneck) conflict-free code assignment problem seeks a conflict-free code assignment which achieves the maximum throughput (resp., bottleneck). In this paper, we present several heuristics for conflict-free code assignment in OVSF-CDMA wireless ad hoc networks. Each heuristic is proved to be either a constant-approximation for max-throughput conflict-free code assignment problem, or a constant-approximation for max-bottleneck conflict-free code assignment problem, or constant-approximations for both problems simultaneously.

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