Neighbor Cell Description

MS keeps on measuring the BCCH signal levels of the serving cell as well as the neighbor cells. In order to know the adjacent cells, neighbor cell description information will be broadcast periodically in system information of each cell. This information lists the BCCH of all neighbor cells. MS extracts the information from system information and use it as basis for neighbor cell  measurement.

For GSM network, the neighbor relationship between cells is finalized when designing the network topology. During the network construction, the neighbor cell relationship must be configured in accordance with the topology design that has been planned. Moreover, after the commission of network, neighbor relationship should be modified according to the data of driver test and traffic measurement. When network’s architecture is changed (e.g. Adding BTS or changing the network frequency configuration.), network operator must strictly follow the changed cell neighbor relationship, reset and verify it. Improper neighbor cell description is usually one of the important reasons of call drop. Besides, since the actual network topology structure is often greatly different from the theoretical calculation result, and network is in the ever-changing environment, network operator must modify the neighbor cell description information according to the real situation.   

Application of Neighbor Cell Description

 

In theoretical calculated neighbor cell relationship, cell A and cell C are not adjacent cells. Assume that one MS moves from cell A to cell C during the conversation, theoretically, MS needs twice of cross-cell handovers. Assume that the interference in cell D is rather serious, call drop is likely to occur during this period. But in fact, the coverage of A, B, C, and D is not the case as the theory. A and C have overlapping coverage. If A and C are regarded as adjacent cells here, that is to add BCCH of C and A respectively to the neighbor cell description of A and C, then when MS passes from A to C, only one handover happens. What’s more, call drop could be avoided because of the good quality of cell C signals.

Part of the signals from cell A leaks out and covers some areas far away from this cell. It is over shooting. If MS is in dedicated mode in the shady area and moves from this area towards B and C, since there are no BCCH of cell B and C in cell A’s neighbor cell description, call drop is unavoidable. If the antenna of BTS is located too high, or the transmitting power is too large, over shooting phenomenon will occur. BTS built at the early stage of GSM construction usually have this problem, because coverage is the major problem at that time and the antenna height is very high. The best solution for this phenomenon is to adjust the location and downtilt of the antenna, and adjust the transmitting power of the BTS to eliminate the BTS covering the shady area. In real situation, it is hard or even impossible to change the location of antenna. So one more simple and applicable method is to add BCCH of B and C to the neighbor cell description of cell A (no need to add A’s BCCH to B and C). But it must be ensured that there are no cells, which are neighbor cells of cell A, using the same frequency and same BSIC with cell B and C. Generally, this method is not recommended.