Different BTS configurations, depending on load, subscriber behavior, and morph structure, have to be considered to provide optimum radio coverage of an area. The most important BTS configurations of a BTS are presented next.
3.1.2.1 Standard Configuration
All BTSs are assigned different cell identities (CIs). A number of BTSs (in some cases, a single BTS) form a location area. Figure 3.2 shows three location areas with one, three, and five BTSs. The systems are usually not fine-synchronized (seesynchronized handoverin the Glossary), which prevents synchronized han- dover between them. That method of implementing BTSs is the one most fre- quently used. For urban areas with growing traffic density, that may change soon. For this situation, the configurations described in Sections 3.1.2.2 and 3.1.2.3 are more appropriate.
3.1.2.2 Umbrella Cell Configuration
The umbrella cell configuration consists of one BTS with high transmission power and an antenna installed high above the ground that serves as an
“umbrella” for a number of BTSs with low transmission power and small diameters (Figure 3.3).
Such a configuration appears to make no sense at first, because the fre- quency of the umbrella cell can not be reused in all the cells of that area due to interference. Interference even over a large distance was one of the reasons why the high radio and television towers were abandoned as sites for antennas shortly after they were brought into service at the initial network startup.
300 m–35 km
BTS TRX
BTS TRX
BTS TRX
BTS TRX BTS TRX
BTS TRX
BTS TRX
BTS TRX
Figure 3.2 BTSs in standard configuration.
BTS TRX
BTS TRX
BTS TRX
BTS TRX
BTS TRX
BTS
TRX
Figure 3.3 Umbrella cell with five smaller cells.
The umbrella cell configuration still has its merits in certain situations and therefore may result in relief from load and an improvement of the net- work. For example, when cars are moving at rather high speeds through a net- work of small cells, almost consecutive handovers from one cell to the next are necessary to maintain an active call. This situation is applicable in every urban environment that features city highways. Consequently, the handovers result in a substantial increase of the signaling load for the network as well as in an unbearable signal quality degradation for the end user. On the other hand, small cells are required to cope with the coverage demand in an urban environment.
The way out of this dilemma is to use both large and small cells at the same time, that is, the umbrella cell configuration. The umbrella cell can be protected from overload when traffic from only fast-moving users is assigned to it. This, on the other hand, reduces the signaling load of the small cells and improves the signal quality for the fast-moving traffic. The speed of a user can be determined to sufficient accuracy by the change of the timing advance (TA) parameter. Its value is updated in the BSC every 480 milliseconds (ms) by means of the data provided in the MEAS_RES message. The BSC decides whether to use the umbrella cell or one of the small cells. GSM has not speci- fied the umbrella cell configuration, which requires additional functionality in the BSC, a manufacturer’s proprietary function.
3.1.2.3 Sectorized (Collocated) Base Transceiver Stations
The term sectorized, or collocated, BTSs refers to a configuration in which several BTSs are collocated at one site but their antennas cover only an area of 120 or 180 degrees. Figure 3.4 illustrates the concept. Typically, it is implemented with BTSs with few TRXs and low transmission power. Like the umbrella cell configuration, this configuration is used mostly in highly popu- lated areas. A peculiarity is that it is fairly easy to fine-synchronize the cells with each other, which allows for synchronized handover between them. Even though in a collocated configuration, one channel per BTS has to be used for the generation of the BCCH, such a configuration has the following advantages:
• Sectorized, or collocated, BTSs are well suited for a serial connection of the Abis-interface (discussed in detail in Chapter 6). This configura- tion has the potential to save costs for access lines to the BSC. Other- wise, multiple sites require multiple (leased) lines.
• From the radio perspective, the advantage of using cells with a 120-degree angle is that it allows reuse of frequencies in one sector
(one direction), which otherwise would cause interference with neigh- bor cells if an omnidirectional cell were used.
• Sectorization eases the demand for frequencies, particularly in an urban environment.