Mobile Handset Design

9

GSM Mobile Phone Operations and Procedures

9.1 Initial Procedures after Mobile Power ON

When the mobile is switched ON, its first task is to find a suitable BTS through which it can gain access to the network. As discussed earlier, all BTSs broadcast their allocated BCCH carrier [various neighboring BTSs are allocated different radio beacon frequencies to transmit according to frequency planning and each BTS has a single broadcast (BCCH) frequency] in the respective cell. Again, in the same area there may be one or more BTSs installed by other cellular operators, and they also have different radio beacon frequencies. The mobile starts searching the relevant frequency band for BCCH carriers. This process is called cell selection. Generally, the upper layer of the protocol stack commands layer-1 to program the RF module to measure the RSSI (received signal strength indication – which is usually measured as the square root of I² and Q²) for different carrier frequencies, and then once it is performed, layer-1 indicates the result to the upper layer. On the basis of this result the upper layer decides on which carrier frequency it should search for the FB and SB burst (generally it selects the carrier frequency on the basis of the highest RSSI value).

9.1.1 Cell Selection

The mobile may implement the cell selection algorithm by one of two different search algorithms depending on its knowledge of the BCCH carriers in use.

The first algorithm is applied when the mobile has no knowledge ...