Code Acquisition and Timing-Frame Synchronization for Asynchronous DS-UWB Transmission Systems

We consider initial timing acquisition, frame synchronization and code acquisition for asynchronous DS-UWB transmissions in a distributed network topology, i.e., no common timing reference and neither synchronization among terminals are provided. Thus, when an arbitrary terminal in active mode needs to establish communication with reachable terminals, it listens for a common synchronization signal. Such a low-duty cycle synchronization signal is broadcasted by active terminals which accept a communication link. Thus, a simplified maximum likelihood (ML) algorithm is provided for initial timing acquisition. This estimator offers good performance in UWB radio channels independently of the multipath intensity profile for both fast and slow fading. Frame synchronization is achieved by using a secondary synchronization signal transmitted orthogonally to the common synchronization signal. Such a secondary synchronization signal is composed by appending consecutive slots such that those form a codeword that belongs to an optical orthogonal code (OOC) codebook. The OOC properties are exploited to identify the first slot number corresponding to a received codeword. For that purpose, the received signal is correlated with the OOC codeword components in GF(q) and cyclic shifts over some slots. Thus, a soft decision variable is formed by accumulating those correlations over those slots. So, the largest accumulation over all OOC codewords and slot numbers corresponds to a ML estimate of the first slot number of the received codeword. By knowing such a slot number, frame boundary identification is achieved. Once initial timing and frame synchronization are achieved, the spreading code employed by a given terminal can be identified by a simple correlation with a set of spreading code candidates