METHOD AND MOBILE STATION TO PERFORM THE INITIAL CELL SEARCHIN TIME SLOTTED SYSTEMS

摘要

A method is disclosed that a Mobile Station MS performs at switch-on to sear ch the most favorable target cell in UMTS systems like the 3GPP CDMA LCR (Low Chip Rate) option at (1.28) Mcps TDD (Time Division Duplex) mode and the equivalent TD-SCDMA (Time Division Synchronous CDMA). Signal at the MS anten na is the sum of different RF downlink frames coming from different carriers in the assigned frequency ranges. A DL synchronization timeslot and a BCCH TS0 are both transmitted with full power in the frames, the first one includes o ne out of (32) SYNC codes assigned on cell basis. Following a conventional approach the absence of a common downlink pilot and without prior knowledge of the used frequencies would force the MS, for all the frequencies of the channel raster stored in the SIM card, the correlation of the received frame with all the (32) sYNCs stored in the MS, in order to detect the BSIC of a cell to which associate the power measures. Following the two-step method of the invention the power measures are performed in towo-step scan of the PLMN band without interleaved correlation steps; once a final frequency is select ed the respective frame is the only correlated one. At least one frame duration about (5) ms long of the whole (15) MHz bandwidth is acquired, IF converted, A/D converted and the digital set is stored. A rough scan is performed multiplying the digital set by a digital IF tuned in steps wide as the chann el band (1.6 MHz) along the (15) MHz band, and filtering the baseband signal wi th a Root Raise Cosine low-pass filter. The (5) ms baseband signal is subdivide d into (15) blocks of half timeslot (337.5.mu.s) and the power of each block i s measured. The power of the strongest block indicates the priority of the respective frequency. The strongest power values are put in a Spectral Table together with respective frame load indicators. The load indicators is the percentage of timeslots in a frame almost equally loaded as the strongest block. The three strongest frequencies are selected for the successive scan. The second step search is performed like the first one but the IF steps are now (200) kHz wide and cover the only (1.6) MHz spectrum around a selected frequency. A final frequency is selected for the successive correlation step . Then the frequency error of the MS reference oscillator is corrected with da ta- aided techniques and a calibration value stored for successive connections.< /SDOAB>