PCS base station maintenance calls for a fresh approach
PCS networks will create a workload for technicians of 20 to 40 base stations each, in environments ranging from freezing cold to desert sun. New designs in equipment are required to characterize the RF performance of CDMA base stations.
Personal communications services (PCS) may be the “next big thing” in wireless communications, but the PCS infrastructure is throwing service providers and their technicians a curve. Compared to their cellular predecessors, PCS base stations are far less accommodating places to make measurements. Unlike the comparatively comfortable cellular equipment “bunkers,” there is often no enclosure to shelter the PCS technician and test equipment from the elements. Instead, the technician may have to work on a concrete slab or rooftop, fully exposed to sun, rain, snow and driving wind.
Next, there are new measurements to make, given the predominance of code-division multiple-access (CDMA) as the PCS access method. CDMA signals are wholly different from analog signals, such as FM, and significantly different even from those of other digital communication methods such as time-division multiple-access (TDMA). Traditional analog measurements of transmit power and receiver sensitivity are inapplicable to spread-spectrum modulation techniques. Not only are new measurements required, but new instruments are needed to make them.
Finally, there are simply more sites to maintain in PCS networks because the low transmit power of PCS requires more base stations than higher-power cellular systems require to serve the same geographic area. For technicians, this means a heavier load of site assignments and a reciprocal decrease in the time available to make measurements at each site.
Taken together, these requirements have created the need for portable test sets that vary in many ways from the test systems designed to evaluate the performance of cellular systems.
>From whence we came Early PCS test systems were either communications test sets sporting PCS “top boxes,” or they were “portable” RF measurement systems that consisted of several instruments connected by coaxial cables. While less than streamlined in the their setup and operation, they nevertheless served the urgent need of base station manufacturers and service providers for field test equipment. After all, CDMA-based communications systems had never been deployed before, much less tested, in commercial service, so the time frame in which test solutions were created by test equipment manufacturers was rather short.
However, now that PCS systems are more mature, and carriers have gained operation and maintenance experience, service providers are calling for a new standard of test equipment. Considerations are not just the measurements, but also the speed and ease with which these measurements can be performed, and the reliability and suitability of the test set for PCS environments in which technicians must operate.
To learn more about these requirements, our field application engineers spent many hours with PCS engineers and technicians who worked for base station manufacturers and service providers. The information gleaned from these experiences in the field lead to the development of the HP 8935 CDMA PCS test set (see sidebar on p.10) to satisfy the unique requirements of the PCS base station maintenance environment.
Base station maintenance wish list In general, technicians said they wanted a single instrument that could perform all of the tests required to ensure that a CDMA PCS base station was properly operating. They did not want to carry around several instruments and cables, and they did not want to perform tedious set-up at each site. Automation of instrument functions and test routines was a frequent request.
Perhaps most important was the technicians’ common desire to have an instrument that they did not have to treat with “kid gloves,” like the many instruments that were originally designed for the cozy bench-top environment and were pressed into service as “field test equipment” by virtue of handles bolted to their front panels. Field equipment needs to be reasonably weather-resistant and rugged, and it should have a display that is readable in direct sunlight. Cable interfaces that might obscure the display should not be on the front panel cables.
Service provider managers were consistent in their requests for an instrument that was extremely easy to use, so that people with a minimum level of technical knowledge and experience could confidently make measurements in the field. Finally, with an average load of 20 to 40 base stations per technician, the time required to verify base station performance had to be as short as possible.
While the wish lists from both groups were formidable, implementation of their requests was at least simplified by their similarities. Technicians and managers were seeking the same types of instruments, but for different reasons.
Technicians said they needed automation, ruggedness, and a “single-box” design to make their measurement “quota.” Managers said they needed simple, rugged, highly automated instruments that were simple to use, so that training time and costs could be reduced. They wanted instruments that could withstand heavy-duty service for long periods, with accurate measurement procedures that would be repeatable from base station to base station.
Similarities aside, creating an instrument to accommodate these needs required a completely new design because the test set would have to possess features and characteristics that had never before been assembled into a single instrument.
Requests become reality The instrument that emerged from the design process is traditional only in the sense that it performs RF measurements. The test set contains all of the hardware and software required to maintain the RF portion of a PCS CDMA base station. It also performs cable fault detection to identify transmission line problems right at the transmitter.
Unlike other PCS base station test sets, the test set eliminates the cable interfaces from the front panel where they can obscure visibility of the display. The interface panel is on the side of the instrument, which allows measurements to be made while the test set is facing upward. Because of the lack of open work space at most sites, “display-up” is likely to be the position in which the instrument will frequently be used. The test set is isolated from the ground by a protective shield that allows filtered air to enter, but nothing else.
The test set is also fully automated on two levels. Instrument-level automation allows test routines to run and measurement data to be taken and stored without operator attention. Manufacturer-specific automation performs test routines dedicated to specific base stations from manufacturers such as Lucent Technologies, Nortel and Samsung.
The test set will characterize CDMA signals for waveform quality (rho, r), average power, channel power, frequency error, carrier feedthrough, error vector magnitude, code domain power, code domain timing and code domain phase. It also measures VSWR, AM, FM, and single sideband (SSB) emissions, audio quality, signal plus noise and distortion (sinad), spectral purity (harmonic and spurious levels), frequency accuracy and interference.
Emphasis on hands-off operation Although the ability to make these measurements in a single instrument is important, the way in which the measurements are made is equally important. Software within the test set allows measurements to be run with little or no operator intervention and offers the technician on-line help to ensure that measurement conditions are correct.
More important, the test set has the ability to control operation of Lucent and Samsung base stations, configuring itself automatically for the conditions of each test. The instrument controller and computer that orchestrate operation of the test set, combined with software developed by Hewlett-Packard, reduce measurement time to a fraction of what would be required if each measurement were performed manually, even if all of the capabilities to perform the measurements were contained in a single instrument.
The measurement process is further streamlined by the use of standard PCMCIA cards. The PCMCIA interface makes it possible to store and retrieve measurement data and IBASIC program parameters, and to input calibration data and upgrade instrument firmware from the front panel. Firmware upgrades performed in this way take only two minutes.
Although the tests performed on PCS base stations vary little from manufacturer to manufacturer, the ways in which the base stations of each manufacturer must be configured and controlled are unique. To overcome this obstacle, we added (in addition to internal automation capabilities) the ability to run software that tailors the test configuration and test routines to those required by base stations from specific manufacturers.
The manufacturer-specific software reduces the level of technical and manufacturer-specific knowledge required by the technician to make accurate and complete measurements, reduces measurement error and reduces the time required to make a complete set of measurements.
Conclusion The sheer number of base stations required by PCS networks dictate the creation of techniques and instruments that save time, reduce maintenance costs, and ensure that customers experience good performance.
Hewlett-Packard realized that traditional field testing instruments whose capabilities were stretched to include PCS situations would at the best create an unpleasant test environment for the technician, and at worst make maintenance difficult or impossible to perform. As a result, a decision was made to make the HP 8935 test set as different as it needed to be to satisfy the needs of technicians who must maintain PCS base stations in conditions ranging from freezing cold to bright sunlight and desert heat.
Some of the unique operational features of this design effort include: * The test set is the size of an overnight bag and weighs 44 pounds. Because no additional instruments are needed to characterize the RF performance of the base station, this represents the total weight of RF test instrumentation that the technician must bring to the site.
* All connections are made on a recessed interface panel located on the side of the instrument. This allows it to be operated in an upright position, a frequent necessity because convenient waist-high tables or shelves are rarely present. The configuration also reduces the space needed and keeps cables from covering the display and keypad.
* When placed in its operating position, the bottom of the test set is protected by an ABS plastic shield that raises the body of the instrument and provides a space for airflow whileprotecting the unit from dirt, mud, snow and other contaminants. Air flow through the unit is kept free of contaminants by a removable filter.
* Although the instrument is not waterproof, it is designed so that dirt and moisture cannot enter the keypad, which can be wiped clean with a damp cloth. The display is also gasketed to resist the entrance of moisture. When in its operating position, the majority of the test set’s weight is present in the portion closest to the ground, making it difficult to tip over.
* Because LCDs are not well-suited to operation in even moderate sunlight, or over wide temperature ranges,
HP selected electroluminescent technology, which is visible even in the brightest direct sunlight and performs well over any temperature range likely to be encountered by a service technician.
* Finally, the test set was designed to withstand abuse in field operation. It was subjected to tests similar to those required of military test equipment, and it can withstand high levels of shock and vibration.