Ultrawideband (UWB) radio is a communications technology which uses very high bandwidth radiofrequency (RF) signals. UWB radio technology is commonly used in location sensing applications because the UWB signals enable high location and ranging precision compared to traditional radio systems.
Typically, tags are attached to the objects to be located, and a network of sensors is placed at known points in the environment. UWB signals emitted by the tags are detected by the sensors, which use them to determine measurements such as the distance from the tag to the sensor. Processing logic can then combine the known sensor positions and the measurements to determine a 2D or 3D position for the tag. Typical UWB location systems are accurate to within a few tens of centimetres, even within environments that are normally challenging for radiolocation systems, for example those with many metal reflective surfaces.
U.S. Pat. No. 5,510,800 describes a UWB receiver used in position determination applications. The receiver is designed to detect a train of broadband radio pulses sent to it by a UWB transmitter. The receiver detects the presence or absence of the incoming train of UWB pulses, and measures the time-of-arrival of the pulses. It uses a sampling gate in the receiver to mix a replica of the expected incoming pulse with the incoming signal. The mixer has a high output response when it is triggered with a pulse replica at the exact moment when a pulse arrives at the receiver, and a low response if it is triggered when no pulse arrives at the receiver. The time-of-arrival of a received pulse is measured to be that at which the amplified output response of the sampling gate exceeds a predetermined threshold.
In the main embodiment of U.S. Pat. No. 5,510,800 the transmitter and receiver are tethered together by a cable, and the transmitted pulse train and replica pulse train are driven by the same clock located at the receiver. Consequently, the pulse repetition frequency (PRF) of the replica pulse train exactly matches the PRF of the transmitted pulse train. The receiver does not initially know at what instant an incoming pulse will arrive at the receiver since this depends on the distance between the transmitter and receiver. It therefore shifts the time at which the replica pulse train drives the sampling gate across the full pulse repetition period of the pulse train, thereby ensuring that the replica and incoming pulse trains coincide at the receiver at some time.
In applications where the transmitter and receiver are not physically connected, a clock at the transmitter drives the transmitted pulse train and a different clock at the receiver drives the replica pulse train. All clocks drift relative to each other to some degree. Consequently, a mismatch will develop between the PRF of the transmitted pulse train and the PRF of the replica pulse train dependent on the degree of drift between the transmitter and receiver clocks.
Additionally, the system described in U.S. Pat. No. 5,510,800 is limited in that the transmitted signal lacks the capacity to convey information about the transmitter. It is desirable to convey information such as the identity of the transmitter, particularly in cluttered environments in which the transmitter and receiver are not physically connected.
U.S. Pat. No. 5,901,172 describes another UWB receiver. The receiver is designed to detect single, individual pulses within a UWB pulse train as and when they are received, as opposed to only detecting pulses that match a replica pulse train as in U.S. Pat. No. 5,510,800. This type of receiver is known in the art as a non-coherent receiver. A non-coherent UWB receiver can be used with an appropriate transmitter to transfer data from the transmitter to the receiver. Since the receiver detects each pulse in isolation, only one pulse may be used to transfer each bit of data. This method of data transfer in UWB systems is problematic for the following reason. Broadband pulses used in UWB systems typically occupy a bandwidth of hundreds to thousands of Megahertz in regions of the radio spectrum below 10.6 GHz. These regions have already been allocated by regulators to other services. The output powers of UWB transmitters are consequently normally limited by regulatory restrictions to extremely low levels, so as to limit potential interference from a UWB transmitter to other users. However, a pulse of a UWB signal sent from a non-coherent receiver needs to have sufficient energy that a receiver can accurately detect and decode the bit transferred by the pulse. The range of distances over which the transmitter can successfully transmit data to a non-coherent receiver is therefore severely limited.
There is thus a need for an improved UWB receiver which is able to successfully convey data over longer distances and that does not suffer from mismatches between the clocking at either end of the communication channel.