Accurate synchronization using a full duplex DSSS channel

In the future, geolocation systems will incorporate the characteristics of communications networks with positioning technologies to create systems capable of performing location aware computing. Such systems have wide applications in troop movements, field hospitals and homeland defense, as well as dual-use areas such as firefighter safety and wireless healthcare systems. Of particular interest are systems which may be deployed in an ad hoc manner. By their very nature, these systems cannot make use of pre-existing infrastructure everything necessary to create a functional system must be self-contained, allowing a complete system to be deployed anywhere, at any time. In many of these applications, there are sub-meter accuracy requirements for indoor positioning. For example, it Is important to differentiate which side of a wall a firefighter is on. To achieve this level of accuracy, work is ongoing which exploits the properties of Orthogonal Frequency Division Multiplexing (OFDM) signals and signal coding to mitigate errors associated with multipath interference. However, even if multipath is eliminated, the ability of a system to provide accurate positioning is also critically related to the ability of the transmitters and receivers in the system to establish a reference time. Current simulations indicate that local synchronization must be within few hundred picoseconds. This paper describes a synchronization mechanism, which employs a duplex Direct Sequence Spread Spectrum (DSSS) signal on a single carrier frequency. Recent results indicate that a Maximum Likelihood Estimator (MLE)-based receiver for DSSS signals yields a significant improvement in receiver dynamic range. By using this increased headroom, and knowledge of the signal being transmitted by a node, it becomes possible to extract other DSSS signals on the same carrier frequency. Once a duplex channel is established, local synchronization can be established to an arbitrary accuracy using coding techniques.