Acquisition of frequency synchronization for GSM and its evolution systems

Abstract

In global system for mobile communication (GSM) and its evolution systems (such as general packet radio service, GPRS, and highspeed circuit switched data, HCSD), the cell related parameters are broadcast on a broadcast control channel (BCCH) to facilitate the mobile handsets in the area of coverage to acquire the frequency and timing synchronization as well as to obtain the cell specific parameters. The mobile handset (MS) in the cell needs to scan this BCCH (also known as 'beacon' or 'C/sub o/' channel) to find the strongest receive (Rx) carrier frequency based on the received signal strength. Once the Rx carrier frequency is known, the mobile handset's local oscillator (LO) is tuned to this frequency, which may be offset from the base station (BS) transmit (Tx) carrier frequency based on the specification of the local oscillator and operating environmental condition. The BS transmits periodically a frequency burst (FB) to allow the MS to establish the frequency synchronization with the BS. The characteristic of the FB is such that the received data stream exhibits a tone at 1/4 of the GSM symbol rate. The paper presents a novel approach based on a 2nd order adaptive LMS notch filter for the acquisition of this FB tone. Based on the local oscillator tolerance, the bandwidth of the notch filter is adjusted and the output power of the filter is compared with the received input power. The presence of a FB tone is declared, based on the ratio of the output to the input power of the filter. The threshold for FB detection is based on the signal-to-noise ratio (SNR), probability of detection and probability of false alarm. Once the FB tone is detected the notch filter shifts into the adaptive mode and iteratively converges to the actual FB tone frequency.