Microwave Engineering Europe - November 2008 - (Page 12) 12 COVER FEATURE: BASE STATION TESTING RF testing during the installation and maintenance of wireless Network Base Stations By Giovanni D’Amore, Agilent Technologies or today’s Wireless Service Provider (WSP), deploying wireless networks is no easy task. The network deployment process involves network design, site construction, base station configuration, equipment installation, optimization and troubleshooting. At each step in the process problems can occur which threaten the WSPs ability to deliver continuous and stable service at a high level of Quality of Service (QoS). Once deployed problems can arise requiring ongoing network maintenance and troubleshooting. Today’s increasingly complex wireless networks and soaring operating frequencies, from sub-GHz levels up to 5.8 GHz, complicate this task, forcing the WSP to deploy and maintain more cell sites to cover the same coverage area in the same amount of time. In addition the growing demand for wireless multimedia services, coupled with the increased complexity caused by the digital wireless evolution, has placed increasing pressure on the WSP. Much of this pressure comes from the fact that operating frequencies are getting higher, while base stations are getting more complex, supporting multiple technologies and incorporating such new technologies as Multiple-Input Multiple Output (MIMO). At the same time, base stations are migrating to a smaller, faster, cheaper design—all of which means that more functional tests are required to ensure optimal network operation. Faulty cables, connectors and antennas can cause 50 percent to 60 percent of base station problems. Interference can be another major cause of performance degradation. The routine testing of a base station’s cables, filters, antennas, amplifiers, and the troubleshooting of any internal or external interference is therefore absolutely critical. Effective installation routines are essential to ensure the rapid deployment of new cell sites. Effective preventative maintenance routines are essential to ensure the ongoing optimal performance of a base station in the field. Effective remedial maintenance routines are essential to ensure that when a fault occurs the base station is up and running as soon as possible in order to minimize customer disruption and degradation of QoS. F Figure 1: Typical cell site configuration. Cell site test requirements Figure 1 shows a typical cell site configuration. Any transmission system is made up of a transmitter, the transmission cables, the antenna and all of the connectors in between. In an ideal transmission system all of the signal power that is sent out of the transmitter is completely broadcast into the air. In reality the signal encounters disruptions so that reflections occur and signal power is lost. There are four separate areas where RF issues have to be addressed. The issues relate directly to the potential sources of disruption that can occur within the cell site and comprise: the cables and antenna; interference (external or internal); the filters and amplifiers and the transmitter power produced. At the top of the list are RF issues related to cable and antenna degradation, and antenna down tilt. Degraded feed lines cause poor coverage, unnecessary handovers, paging failures, and access failures on the uplink. Interference (e.g., from co-channel, adjacent channel, inter-modulation, external, and internal) is another common culprit. Downlink interference reduces coverage and results in dropped calls, while uplink interference causes access failure. Interference has a direct impact on the QoS of wireless communication services. Other common causes of failures in cell sites stem from damage to filters and towermounted-amplifiers (TMAs), errors in radio settings and configurations, transmitter performance degradation, and receiver sensitivity degradation. Problems can also occur when the reference clock is out of sync, resulting in island cells and hand-over failures. Backhaul transmission is another source of faults, with T1/E1 breakdowns being the most common defect in a cell. For effective installation and maintenance each of these areas have to be tested. The challenge is make the testing efficient and reliable. Traditionally the testing toolkit carried by the RF Installation and maintenance engineer comprised a wide range of test instruments, including dedicated cable testers, spectrum analysers, power meters etc. The engineer was faced with the task of carrying multiple boxes, learning how to use each one individually and ensuring that each was up to date with their calibration. Simply keeping track of the multiple instruments was a challenge in itself with the result that the RF field engineer’s speed, productivity and flexibility were severely compromised. Microwave Engineering Europe ● November 2008 ● www.mwee.com http://www.mwee.com
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