Remote monitoring of wireless base stations
The ability to monitor the RF performance of wireless communications systems, even detecting cable faults from a remote location, is becoming increasingly valuable as hundreds of new base stations come on line.
In an ideal world, hunters would not cut coaxial transmission lines, salt would not cause corrosion and connections would remain intact. There would always be enough people to inspect every base station on a regular basis, just to make sure everything is working fine. However, in the real world inhabited by wireless base stations, all sorts of intentional and unintentional acts can degrade RF system performance, and the number of base stations is increasing faster than the number of people trained to maintain them.
With this as the backdrop, the need to monitor RF performance from a remote site is obvious, as is the need to track the historical performance of the system in an attempt to predict gradual degradation. The alternative is to send service technicians into the field like bees, flitting from cell site to cell site, in order to perform regular maintenance.
Even in this expensive scenario, the possibility still exists that the system will begin to degrade shortly after the technician leaves, which means that the problem will remain undetected under the next visit-or until customer complaints raise a flag. Consequently, when compared to the costs of lost customers and hiring more technicians (from a shrinking pool of talent), the latest crop of RF monitoring systems is extremely cost-effective.
Base station RF performance monitoring systems have been available for several years. But only recently has a remote monitoring system become available that will monitor the vital signs of the RF system, including voltage standing wave ratio (VSWR) and the output power of a single channel in a multichannel system, without removing the base station from service. The latest version even provides the ability to detect a discontinuity in the transmission line with considerable accuracy, allowing the service provider to detect a problem and then isolate its source before sending someone to the site.
Big returns from VSWR The parameters that provide the most effective indication of overall system performance are VSWR and output power. VSWR, in particular, is the leading indicator of system health and provides advance warning of impending failure in both the transmit and receive sections. Transmitted power, especially if it can be measured on individual channels of an operating system, provides an equally important indication.
SMR, cellular and PCS systems use multiple transmit and receive antennas, each covering a sector of the desired coverage area. While three transmit and six receive antennas are probably the most common configuration, systems with six transmit and six receive antennas are in service as well. To determine RF performance, the VSWR of the transmission chain, from the transmitter and receiver through each transmission line to its respective antenna, must be measured.
VSWR is a measure of transmission line/antenna integrity. With a perfect VSWR, 1.0:1, all the transmitted power is directed at the antenna. Discontinuities in the transmission line or failure of the terminating antenna causes some of the transmitted power to be reflected back to the source. The more power reflected, the higher the VSWR. As VSWR increases, transmitted power decreases to the point at which no power is radiated. Beyond the obvious degradation of system performance caused by a high VSWR, the mismatch places an increased burden on transmitter components.
VSWR is equally important in monitoring the performance of the receive side of the base station, in which sensitivity decreases as VSWR increases. While the primary concerns in the receiver electronics are sensitivity and low-noise amplifier (LNA) noise figure, a poor antenna feed circuit with a high VSWR completely negates the best LNA noise figure.
Measuring receive VSWR is potentially more invasive than measuring its transmit counterpart, which can be sampled continuously if desired. In the receive link, a signal must be injected into the transmission line. There are two methods for performing this. The first is to remove the receive channel from service, make the measurement, and return it to service.
The second is to inject, through a coupler, a pulsed signal of extremely low power and short duration. This method allows the measurement to be performed at will, without removing the channel from service. Obviously, the second method is the one chosen by manufacturers of remote monitoring equipment.
Sweeps of antenna performance over the system’s full bandwidth have always required skilled technicians and expensive analysis equipment. In addition, the collected data have been compiled manually and presented to RF engineers for review. As a result, these measurements are made infrequently, and sometimes not at all.
In addition, a single set of measurements is of questionable value because transmission line conditions can change the moment the technician heads “back down the mountain.” Multiple measurements over time are required to produce enough data to generate a history of VSWR performance. This trend information is the most powerful exploitation of VSWR as a system evaluation tool. The latest equipment almost entirely eliminates operator intervention in making these measurements, and allows them to be conducted and analyzed in real time.
Measuring power on the fly
Ensuring that the specified output power reaches each antenna is a primary concern of every service provider, and the most direct way to do this is to make power measurements frequently, preferably without the need for a technician. The ability to measure a single, user-selected channel while the remainder of the system remains in operation is a powerful tool. It essentially allows the service provider to perform dynamic power verification and adjustment, from a remote location, in real time.
Another tool that provides significant benefits to the service provider is the distance-to-fault measurement, which can isolate the cause of degraded performance or a complete failure to within a few feet of its source. This tool allows the service provider to make a determination about the nature of an RF-oriented problem before allocating resources to solve it. It also indicates whether the technician can fix the problem on the ground or whether a rigger must be employed to climb the tower and inspect the line for damage.
The monitoring system
The CATS 76000 RF performance and VSWR monitoring system was designed to meet the need of service providers for monitoring the RF performance of every base station without actually visiting the site. It has its roots in precision RF measurement techniques and is embellished by the power of a PC to store measurement data, analyze results, alert the user when out-of-limit conditions occur and automatically perform routine checks at any interval.
The monitoring system (shown in the photo on page 11) consists of a main enclosure, which includes most of the RF electronics and onboard computer, and monitoring sensors that are installed in the feed line of each channel. The system communicates with other equipment via an RS-232D serial communications port. A direct communications link is not required. The system can be programmed to make measurements, and data can be stored locally. In the event of an alarm condition, a Form C relay is closed, which can be connected to the base station alarm system, notifying the provider of the problem.
The system is set up and operated from a PC with software called CellPro Plus that runs under Windows 3.1 or later versions. The hardware and software together essentially create a virtual instrument that provides information about all transmit and receive transmission chains in real time to a remote site.
On the transmit side, monitoring system will measure RF power on individual channels, while the other channels remain in operation. It can perform a “ping” test on each antenna in an emergency situation or for diagnostic purposes to ascertain the status of each antenna. In addition, it will perform VSWR sweeps on all antennas either on demand or as dictated by a schedule. VSWR and low-power alarms are enabled with Form C relays. The system will also measure the distance to a cable fault, with accuracy as fine as 5 ft.
The number of each active channel on each antenna is stored in the monitoring system’s memory, along with a power level alarm setting for each antenna. The system automatically steps through the measurement of each channel and records transmit power. It will perform these measurements at any interval set by the user. Results are displayed at the remote site as RF power versus channel number, as shown in Figure 1 on page 12.
If the measured power level falls below the set power level for any channel, an alarm situation is reported, various types of alarm indicators can be triggered and the condition is logged. The user can zoom to any channel to see its power level, which is displayed on a virtual power meter that is refreshed in real time. This display can be used as a reference for making adjustments to the transmit power level of each channel.
The antenna monitor portion of the instrument is a multiplexed, low-power synthesized signal source and tracking receiver. Antenna VSWR is measured by injecting a pulsed RF signal (of less than 100ms duration; typically 210dBm to 220dBm) into the feed line via a coupler. The short, low-power test signal is undetectable by the base station electronics. The service provider can select the frequencies on which VSWR measurements are made.
The instrument can make measurements either on a single frequency or over a swept range of frequencies selected by the user, as shown in Figure 2 on page 14. Swept measurements are made on as many as 25 frequencies across the operation bandwidth of the wireless system. The results are tabulated, plotted and displayed on the monitor, and can be exported to a spreadsheet program.The monitoring system provides preset frequencies and allows for user selection as well.
Distance-to-fault measurements determine where a problem occurs in the transmission line from the instrument to the antenna. The location of the fault, VSWR, and measurement parameters are displayed on the monitor, as shown in Figure 3 on page 16. Both the resolution and range of the measurement can be selected by the user. Resolution ranges from about 20 ft. with a narrow bandwidth measurement to 5 ft. over a broader bandwidth.
>From the past . the future
The CATS 76000 will schedule, control, and monitor power and VSWR measurement, antenna sweeps, and other parameters and store the results. The instrument will export its stored information by modem to the remote site for analysis.
The historical information provided by the monitoring system is potentially its most powerful capability, because by viewing trends in VSWR and output power, it is possible to identify performance degradation well before it results in a catastrophic failure. It also provides a stored record of system performance over time, with each time period recorded along with the measurement data for performance verification purposes.
In addition, the exported data can be analyzed with the many tools available in current spreadsheet programs. A good example, shown in Figure 4 above is this histogram of transmit power measurement created in Microsoft Excel. The power level of each channel on each antenna is displayed over five measurement periods. Channel 317 (furthest right) is the CCH channel and is always on.
In this example, the power level of channels 88, 109, and 130 are about half that of the other channels, a situation that the service provider had not detected until the monitoring system was installed and the histogram was created.
Summary When there are so many choices in wireless communications products, brand loyalty sometimes takes a back seat to new features, low monthly fees, and “a free phone.” However, several studies of cellular customers have shown that, given a satisfactory level of system performance and customer service, subscribers will remain with their current service provider, even when appealing new offers appear.
The message here for every service provider is that delivering good performance at every site is an essential component of overall customer service. Remote monitoring tools can go a long way toward ensuring that the transmit and receive sections of each base station are delivering optimum performance.
Crofut is product marketing manager at L3 Communications, Narda East, Hauppauge, NY, which manufactures the 76000 Series as well as other remote monitoring equipment for SMR, cellular and PCS systems.