The growing population of cell phones—today estimated at nearly 300,000,000 in service worldwide—has increased the focus on mobile-phone base stations. Without efficiently functioning stations, nobody talks. To avoid such problems, testing these stations is increasingly important.
Essentially, a mobile-phone cellular radio base station is a sophisticated transmitter and receiver that communicate with all of the subscribers’ mobile phones in close proximity. The area that the base station covers is referred to as the cell. Signals to and from the base station communicate with the fixed telephone network through T1 24-channel links.
Testing such a base station should be a straightforward job of applying a set of RF parametric measurements, for example, measuring output frequency, power, and modulation from the transmitter and determining sensitivity, tuned frequency, and distortion on the receiver. In reality, the testing is significantly more complex. Many transmitter and receiver channels and associated antennas on a base station and in digital systems, such as a global system for mobiles (GSM), code division multiple access (CDMA), digital advanced mobile phone service (D-AMPS), and parameters such as bit error rate (BER), must be measured as well as handover procedures and other housekeeping matters.
Numerous vital tests are carried out, especially during installation and commissioning, and they must be accurate and repeatable. The quality and scope of the tests performed at this stage determine the efficiency and, ultimately, the revenue earned from a base station.
GSM Base Stations
The correct title for a GSM/personal communications service (PCS) base station is a base transceiver station (BTS). It has evolved into a sophisticated assembly with embedded supplier-specific software used to control the functions of the base station. The software has a fundamental impact on testing because the tester cannot directly control the BTS to carry out tests; the BTS must be controlled through embedded software interfaces.
A BTS should not be considered in isolation to the other components that make up a network. Figure 1 shows a simplified GSM/PCS mobile phone network block diagram that focuses on three important functional blocks: the mobile switching center (MSC), the base station controller (BSC), and the BTS.
The MSC is closest to the core network infrastructure and responsible for switching and routing the traffic. Each MSC interfaces to a number of BSCs in the network over a 1.5-Mbit/s link using the A interface. The BSCs are themselves interconnected to a number of different BTSs that they control.
BTS Testing
Testing is an essential part of a base station’s installation and commissioning process. Even though the constituent parts of a BTS will have been thoroughly tested by the manufacturer, they are disassembled for transport and reassembled on site.
This can give rise to some faults. More significantly, other faults inevitably will arise during the complex installation process. For example, wiring errors will occur even in the best-managed working environment, connectors will not always be correctly crimped, backplane switches can be incorrectly set, and RF connectors and cables can easily introduce faults.
When contemplating test strategies for cellular radio testing and maintenance, consider the time taken for testing as time that could be used to generate revenue. Even one bad voice channel can affect numerous connections during the course of a day and produce large numbers of fault reports. A rapid, automated, high-integrity tester that is easy to operate could increase revenue significantly.
Portable, Automated BTS Tester
The BTS is controlled by the BSC through a 1.5-Mbit/s interface, known as the A-bis interface. A 1.5-Mbit/s link consists of 24 64-kbit/s time slots. In the A-bis interface, each of the 64-kbit/s slots is subdivided into four 16-kbit/s speech subslots. Each GSM carrier accommodates eight speech channels.
As a result, two 64-kbit/s time slots are needed for each carrier, meaning that, in theory, 12 carriers can be controlled and duplex speech connected for each 1.5-Mbit/s link. In practice, some time slots are needed for signaling, so only about 10 carriers can be serviced by one link.
Although the system from the BSC outward to the network is standardized, the A-bis interface differs for each manufacturer. This is a major challenge for the test-equipment supplier. Not only must vendor-specific software be developed for each manufacturer’s equipment, but ongoing development also is required to keep up with software enhancements.
As a result, it is necessary for the tester to interface with the A-bis connections to control it and carry out automated testing. The interface must communicate in both directions to read the messages returned from the base station. Interfaces also are required at the antenna connection ports of the BTS.
Figure 2 shows how the test set connects to the A-bis and RF ports of a BTS. With this arrangement, the tester can switch the BTS to different RF channels and check the control signals sent to the mobiles. The tester also should feature programmable go, no-go test limits to minimize operator errors, printing capabilities to support future trend analysis and maintenance tracking, embedded control software to eliminate the need for a PC, and a design conducive to portable applications.
About the Author
Steve Gledhill is the marketing manager for radio communications test at Racal Instruments. He has nearly 30 years of experience in RF test and measurement, initially with Marconi Instruments and more recently with IFR. Racal Instruments Ltd., 480 Bath Rd., Slough, Berkshire SL1 6BE, United Kingdom, 011 44 1628 604455, e-mail: [email protected].
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April 2001