1. Technical Field of the Invention
The present invention relates generally to the wireless telecommunications field and, in particular, to a method and apparatus for a mobile communications terminal to acquire and synchronize with a low duty-cycle beacon signal.
2. Description of Related Art
Mobile radio system base stations, such as the base stations used in wide area cellular phone systems or for indoor cordless phones, transmit a fixed frequency control signal on a beacon or control channel. A beacon channel serves the following purposes: (1) it provides a time, frequency, and signal power reference for mobile terminals; (2) it provides network broadcast information; and (3) it is used to facilitate access control. The beacon channel's reference function is particularly important for locking a mobile terminal to a fixed base station channel for communications therebetween.
Typically, when a mobile terminal is first turned on, it powers up with no prior knowledge about the frequency or timing of any radio base station. Consequently, the mobile terminal is required to search both in frequency and time for a proper reference signal pattern which the terminal can then lock onto. In prior analog communications systems, these time and frequency uncertainties at power up were separated by transmitting a continuous wave (CW) carrier from the base station. The mobile terminals were able to derive a frequency reference from the CW signal. Currently, in the more advanced digital radio systems, such as the Global System for Mobile communications (GSM) in Europe, the Personal Digital Communications (PDC) System in Japan, the Digital Advanced Mobile Phone System (D-AMPS) in North America, the Digital European Cordless Telephone (DECT) System, and the new Personal Communications System (PCS), the timing and frequency reference signals are combined in a single reference burst, which is broadcast periodically on a single carrier frequency. The mobile terminal has to find this burst, in both time and frequency, in order to synchronize (lock) with it.
Generally, two techniques are used in existing digital systems to acquire a reference burst and synchronize with it. With the first technique, a mobile terminal uses a plurality of on-line correlators to continuously search the received signal for a fixed reference pattern. Each correlator is used to search at a different frequency offset. With the second technique, a mobile terminal samples a reasonable portion of the received signal, and then processes the sample data off-line in order to search for the fixed reference pattern at the different frequency offsets.
The first of the above-described acquisition techniques is technically straightforward, but it requires a considerable amount of hardware to implement in case a large frequency uncertainty exists, because each correlator being used can cover only a limited frequency uncertainty range. Additionally, the first technique is relatively inflexible, because it requires major hardware changes in the terminals whenever a new base station reference signal is used.
The second acquisition technique is much more flexible than the first technique, primarily because the second technique can be implemented in software. Consequently, once the terminal has sampled a received signal, a digital processor in the terminal can store the samples in memory and test them during a relatively large time and frequency window. However, the second technique is still limited in the sense that by lowering the duty cycle of the reference signal, the signal sampling time is lengthened. Lengthening the sampling time places greater storage and processing requirements on the terminal hardware. Namely, for a reasonably fast signal acquisition time, the sampled data should contain at least one complete reference burst. In order to ensure that a complete reference burst is sampled, the duration of the sampled signal should be longer than the period between two successive reference bursts, plus the period of the reference burst itself.
U.S. Pat. Nos. 5,428,668 and 5,535,259 describe a recently developed private radio system that has been adapted for use with cellular mobile phones. The reference signal transmitted from the private radio system's base station has a very low duty-cycle and can have a very large frequency uncertainty. Consequently, a substantial amount of data can be sampled during the relatively long period between successive reference bursts. In that regard, existing terminal memory and processing constraints make the existing time and frequency synchronization techniques implausible.