GSM mobile communication systems have the largest user base in the world; however, with the number of users increases continuously, the frequency resource becomes tight increasingly. To reduce frequency interference and improve traffic quality across the entire GSM network, mobile communication equipment manufacturers have introduced application solutions based on concentric cell technology in succession. FIG. 1 is a diagram of a concentric cell. As shown in FIG. 1, the cell is divided into two coverage zones (Overlaid subcell 2 and Underlaid subcell 3) according to the distance to the center (i.e., the base station 1) of the cell. Different carrier frequencies are allocated to the coverage zones 2 and 3 in consideration of strong uplink and downlink power and strong anti-interference ability near the base station 1, herein, frequencies or channels sensitive to interferences are allocated to Overlaid subcell 2, whereas frequencies or channels insensitive to interferences are allocated to Underlaid subcell 3. If the mobile station is in Underlaid subcell 3 (i.e., the mobile station is far from the base station 1), it will occupy a carrier frequency of the Underlaid subcell 3; if it is in Overlaid subcell 2 (i.e., the mobile station is near the base station 1), it will occupy a carrier frequency of the Overlaid subcell 2 first, however, if all of the channels of Overlaid subcell 2 are occupied, it will occupy a channel of the Underlaid subcell 3.
Because concentric cell technology achieves optimal combination of inner and underlaid subcells for all frequencies in the cell and even all frequencies across the entire network, it is helpful to reduce frequency interference and enhance frequency multiplexing.
In order to allocate carrier frequency resource reasonably and quickly for a voice call, the base station 1 usually allocates a carrier frequency of Underlaid subcell to the TCH first, and then switches the call to a carrier frequency of overlaid subcell or keeps it at the current carrier frequency according to the values of the measured reports in the TCH. The measured reports contain measured up-link level values and measured down-link level values, which are measured by the base station 1 and the mobile station for the up-link channel and the down-link channel, respectively; each measured level value comprises up-link (down-link) signal intensity, up-link (down-link) signal quality and timing advance.
In concentric cell technology, the carrier frequency resource of Overlaid subcell is usually used for telephone traffic; whereas the carrier frequency resource of Underlaid subcell is used to expand the coverage as much as possible, i.e., most calls should be allocated with channels of Overlaid subcell. However, in the conventional application solution, as shown above, calls are allocated to channels of Underlaid subcell first, and then they are switched to channels of Overlaid subcell or kept at channels of Underlaid subcell according to the values of measured reports in the TCH, thus the solution will result in unnecessary switching operations; in addition, because the calls always stay at channels of Underlaid subcell for some time first, the probability of channel congestion in the Underlaid subcell will increase.