Subspace-based active user identification for a collision-free slotted ad hoc network

We propose a novel spreading code scheme, transmitter-receiver-based code, for wireless ad hoc networks. The design facilitates collision resolution using multiuser detection at each node, and is more bandwidth efficient than creating orthogonal channels in time or frequency. A subspace-based receiver structure is introduced, which identifies users of interest, or "active" users, with minimal prior information on the spreading code ensemble. A subspace-based blind multiuser detector can then be implemented to suppress multiaccess interference. The performance of the proposed active user identifier is studied by investigating its false alarm rate P_f and miss rate P_m. Tradeoffs between P_f and P_m are discussed, and a graphical method to determine the threshold value d_th of the decision statistic used in discriminating between active and inactive channels is introduced.