Test of RF during the installation and maintenance of base stations for wireless networks

For wireless service provider (WSP, Wireless Service Wireless) current deployment of wireless networks is not an easy task. The network deployment process involves network design, construction in the locations, optimization and troubleshooting.

In each step, problems may arise that threaten the ability of WSP to provide a continuous and stable high quality of service (QoS, Quality of Service). Once the deployment can be problems that require network maintenance and correction of problems.
The current wireless networks becoming more complex and increasing operating frequencies, from sub-levels GHz to 5.8 GHz, complicate the task, forcing the WSP to deploy and maintain more cell sites to cover the same area of coverage in the same time. Moreover, the growing demand for wireless multimedia services, coupled with the increased complexity caused by digital wireless developments, has increased pressure on the WSP. Much of this pressure comes from the fact that the operating frequencies are higher, while base stations are more complex, supporting multiple technologies and incorporating new and multiple input / multiple output (MIMO, Multiple-Input Multiple Output). At the same time, base stations are migrating to design smaller and faster and cheaper, which leads to the need for more functional tests to ensure optimal network operation.
Defects in cables, connectors and antennas, may cause 50 to 60% of problems in the base stations. Interference may be other major cause of performance degradation. The routine tests on cables, filters, antennas, amplifiers, and detecting problems of any internal or external interference are therefore absolutely critical.


Requirements of test cell site
Figure 1 shows a typical cell site, where is located the antenna and associated equipment for wireless communication. A transmission system consists of a transmitter, power cables, antenna and all connections between them. In an ideal transmission system the power of signal is sent out of the transmitter is completely transmitted through the air. In fact, the meeting breaks originate signal reflections and signal power is lost. There are four distinct areas which should be directed toward the issues of RF. They relate directly to the sources of potential disruptions that may occur within the cell site and include: cables and antenna, interference (external or internal) filters and amplifiers and transmitting power produced.
At the top of the list are issues related to the degradation RF cable and antenna, and the tilt of the antenna. The power cords cause degraded poor coverage, control transfers unnecessary paging faults, and access failures in the ascendant. The interference (for example, the channel itself, adjacent channel, intermodulation, external and internal) is another common culprit. Downlink interference and reduced coverage because of dropped calls, while the uplink interference causing access failures. The interference has a direct impact on the QoS of wireless communications services.
Other common causes of failures in cell sites are the result of damage to filters and tower mounted amplifiers (TMA-Tower-Mounted Amplifiers), errors in radio equipment and configurations, transmitter performance degradation, and degradation of sensitivity receiver. Problems can also occur when the reference clock is out of sync, resulting in island cells and switching failures ongoing call. Backhaul transmission is another source of failures, disruptions T1/E1 being the most common defect in a cell.
For effective installation and maintenance routines each of these areas has to be checked. The challenge is to test these parts efficiently and reliably.
Traditionally, all testing tools carried by the technical staff of installation and maintenance of RF includes a wide range of test instruments, including dedicated cable testers, spectrum analyzers, power meters, etc.. The coach has faced the task of bringing multiple teams, learning the use of each and ensuring calibration date thereof. Just keep track of multiple instruments is a challenge in itself with the result that the speed, productivity and flexibility of the RF field technician can be seriously compromised.
Faced with this growing list of potential problems and the shortage of engineers and field technicians are experts in RF test requires an integrated solution that minimizes the number of necessary independent test instruments and to enable fast, accurate and easy numbers key measures.
The FieldFox handheld RF analyzer from Agilent Technologies (Figure 2) has been developed to meet the aforementioned need by providing an integrated solution for installing and maintaining wireless networks.


Check cable and antenna
Verification of the cable is required to detect imperfections or perturbations caused by reflection of incident energy over the whole length of it. The detection also has to include measures of distance to fault (DTF, Distance to Fault) to allow precise identification of the position of the ruling. Disturbances in the cable length can be formed by a dent or a change in cable diameter. Periodic effects in the cable can often be caused during the manufacturing process, eg towed by a wheel with a defect in the bearing. The cables may also contain one or more discrete defects, such as a driver due to bent or damaged, contaminated dielectric, a bad cut or a bad connector

.
The decoupling caused by failure or imperfections will in turn cause reflections. The reflections from individual imperfections that can be summarized in measures such as cable losses or return loss. With periodic failures reflected energy may appear to the extent of losses as a reflection peak at a frequency corresponding to the spacing of the imperfections. The spacing between periodic imperfections is half the wavelength of peak reflection.
The cable test techniques include measures of loss (return loss, insertion loss) and transfer measures (eg VSWR). Measures return loss are expressed in dB, 0dB remembering that they are open or short circuit and typically 40 to 60dB when measured under load conditions. With evidence of transmission the transmitted and reflected signals are combined to create a standing wave. The voltages of the peaks and troughs of the standing wave are measured and expressed in terms of rate of voltage standing waves (VSWR, Voltage Standing Wave Ratio). In the absence of reflections, for example, a perfect transmission system, VSWR has value unity. Reflections with high VSWR value is increased to the point where the reflections become unacceptable.
RF Analyzer Agilent FieldFox includes cable testing capabilities understandable. The instrument can be used to test antennas, cables, filters and amplifiers in order to make measurements of return loss, VSWR, insertion loss / transmission cable losses at a port and DTF. Measures return loss and DTF can be made at the same time helping to establish a correlation of the overall system degradation with specific faults in the cable and antenna system.
A key feature of FieldFox is QuickCal, built a calibration system that allows the user to calibrate the tester cable / antenna without having to take into the field calibration kit. This simplifies the cable and antenna test, ensures accuracy and repeatability in the measuring point and improves productivity. QuickCal corrects the error drift caused by temperature changes during operation of the instrument. FieldFox is also ready for calibration test port cable and antenna just connect.


RF Measurements
To identify the causes of potential problems of RF within a cell site requires an array of RF test instruments. By integrating all the key tools in a test of RF device, the RF analyzer Agilent FieldFox provides an integrated tool for RF field engineer.


Spectrum analyzer
FieldFox incorporates an optional spectrum analyzer covering a frequency range of 100 kHz to 6 GHz spectrum A quick scan detects and captures interference to measure the RF burst signals. It presents four traces at the same time and allows the user to choose different modes of detection.


Power meter with USB power sensor
FieldFox can connect to the power sensor Agilent U2000 Series USB power measurements of RF / microwave to 24 GHz provides real average power measurements with high dynamic range from -60 dBm to +20 dBm (depending on sensor). The sensor has an auto-zero function without external calibration.


Network Analyzer
FieldFox has an optional mode network analyzer that provides measures of vector network analyzer such as S11 magnitude and phase, size S11, Smith chart and in display.
The sweep speed reduces FieldFox called troubleshooting time with test times in excess of 50 percent faster. This allows engineers to address RF wireless networks becoming more complex in less time, dramatically improving productivity. Thanks to its 1001 points of resolution and excellent dynamic range is possible rapid troubleshooting.


Conclusion
Verification of antennas, cables, filters and amplifiers, and interference detection in cell sites are critical to ensure good quality of service (QoS) on a network. Although test solutions R & M of traditional hand can direct this work, can fail to deliver the speed, productivity and flexibility required to meet current RF engineers and technicians in the field. FieldFox RF analyzer with its high level of integration, calibration measures ready, and very short test times not only provides the functionality as demanded by today's wireless providers, but also a significant improvement in productivity. For wireless service provider (WSP), the result is the ability for a more effective deployment and maintenance of complex and rapid existing networks.


About the author
Giovanni D'amore entered Agilent Technologies (Hewlett-Packard) in 1999 as application engineer providing technical support for RF and microwave instrumentation such as network analyzers and spectrum. From 2005 he was part of the marketing organization to be responsible for implementing parametric and since 2007 happens to be responsible for developing business marketing network analyzer for the EMEA region.
Giovanni holds a degree in Electronic Engineering with specialization in Microwave and Telecommunication, University of Palermo (Italy).

Author:

Giovanni D'amore, Agilent Technologies