In wireless communications it is necessary for two devices to initially establish a communications link and then to maintain that link over time. These processes are generally called acquisition and tracking, respectively. What follows is a quick background into the concept of acquisition and tracking between two radios, more specifically regarding the acquisition portion of that process.
The terms radio, transceiver, and device are used freely throughout the disclosure to refer to an element that transmits and receives wireless signals such as ultrawide bandwidth (UWB) signals. They are not meant to be limiting, but are used for ease of description.
FIG. 1 is a block diagram of a wireless transmitter and receiver according to preferred embodiments of the present invention. As shown in FIG. 1, two radios 110 and 120 are provided that would like to talk to each other. In this description the first radio 110 will begin as a transmitter, and the second radio 120 will operate as a receiver.
In this embodiment the two radios 110 and 120 will communicate using packets. Alternate embodiments could use a different mechanism for passing data as desired. During communications, the first radio 110 will send a series data packets to the second radio 120 one after another, but not necessarily continuously. As a result, there will be a time when the first radio 110 is not transmitting and the two radios 110 and 120 are not in active communication with each other.
However, in order for the two radios 110 and 120 to talk to each other, they have to have each acquired the signal of the other. In this particular embodiment, acquisition is achieved by having the two radios synchronize their fundamental crystals, i.e., the fundamental clock that their radio runs off of. Generally this is accomplished by having the receiver 120 adjust a local clock to the phase of the clock used by the transmitter 110 to transmit signals.
Synchronization problems can arise from slight differences in the two radios 110 and 120, however. First, it is possible that the two radios 110 and 120 may not be operating at the same frequency. In this case it will be necessary to adjust the effective frequency of one of the radios to operate at the same frequency as the other radio. The amount of allowable frequency adjustment will depend on the accuracy of the crystals used in the device. Preferably crystals with an accuracy of 25 parts per million (ppm) are used, though other sorts of crystals can be used in alternate embodiments (e.g., crystals with an accuracy of 50 ppm). In the preferred embodiment the accuracy of frequency adjustment is about +/−3 MHz, although this may be varied in alternate embodiments. For the sake of the examples provided below, however, it will be assumed that both radios 110 and 120 operate at the same effective frequency.
Second, the two radios 110 and 120 may not necessarily be operating at the same phase. In other words, rather than operating in lock step with regard to phase, each may be operating at a different phase with respect to the other. In this case it will be necessary to adjust the phase of one of the radios so that it conforms with the phase of the other radio.
Third, although the effective frequencies of the two radios 110 and 120 may be the same, because of slight variations in manufacture, their actual frequencies may vary by a slight amount. For example, if the two radios 110 and 120 are both operating at an effective frequency of 100 MHz, the actual frequencies may be a fraction of a percent off from this effective frequency. Thus, the transmitter 110 might actually be transmitting at a frequency of 99.99 MHz, while the receiver 120 might operate at a frequency of 100.01 MHz. Overtime this could cause a slippage of phase between the two radios 110 and 120.
Therefore, it is desirable to provide a method of acquisition between wireless devices that is both fast and accurate.