Comprehensive on-site testing of UMTS base stations using real-world scenarios, both at initial installation and then during ongoing maintenance, plays a vital role in preventing and solving performance problems before they impact subscribers. Poorly performing base stations will significantly impact the quality of service (QoS) experienced by users, particularly the higher data rate services available on 3G networks.
Whether poor network performance is caused by incorrect installation, hardware/software incompatibility, gradual degradation or complete failure of a particular module, the end result is that the subscriber experience will be less than satisfactory and network operator revenues adversely affected.
Making sure a cell site works from day one ensures that problems are isolated before the network goes 'live'. This is also the most cost effective testing solution as it is harder to trouble shoot when a cell site is active and some performance issues may not actually become visible until network capacity limits get tested.
So, it is important that base stations are maintained correctly to identify faulty components and avoid no-fault found return penalty charges at the same time as minimizing site visit costs. It is essential to understand what test capabilities are required to correctly confirm that actual base station performance matches planned performance.
As a consequence, base station test equipment needs to be chosen using three key performance criteria - test coverage, test speed and ease of use. Compromising on any of these key performance criteria is really a false economy as it will inevitably impact QoS and maintenance costs, which will have a knock-on effect for revenues.
Effects of site problems
A poorly operating cell can result in an increase in dropped calls, poor quality calls, poor data throughput, interference and actual network coverage not matching what was planned. Add to this the cell-breathing effect common to all CDMA systems, and it makes it all the more important that there are no differences between actual and planned cell site coverage. Unlike TDMA systems, the size of a W-CDMA cell is not fixed. With cell breathing, as more and more calls are set up, it becomes harder to separate weaker signals from the combined spread spectrum signal. As a result, a weak signal can be demodulated more easily when traffic levels are low and there are fewer interfering signals than when traffic levels are high. This means that the cell size shrinks as traffic levels increase and then grows again as they fall off.
In UMTS, channels are separated by frequency and coding allowing multiple channels in a single frequency band. The frequency bands are much wider than those used for GSM, approximately 5 MHz.
The requirements for testing a GSM or UMTS base station are much the same, in that what is wanted is the ability to make transmitter and receiver RF measurements. However, the methodology for undertaking these RF performance measurements is completely different. The whole nature of UMTS makes performance testing of base stations in the field a challenge - as propagation and cell loading conditions change dynamically so measurements recorded using purely on-air monitoring tools, or test mobiles, are inconclusive when trying to establish if a base station is operating to specification. These UMTS testing issues can be addressed by providing comprehensive on-site testing of base stations using repeatable scenarios, as defined by the 3GPP TS25.141 test specification.