For several years, actress Lily Tomlin’s character Ernestine entertained millions by lampooning telephone operators. Of course, Ernestine didn’t have to cope with testing phones, especially not modern digital wireless ones.
A look at a few specifications quickly makes you aware that many tests must be performed while the phone is operating normally. These tests include power level and frequency, but also very specific performance aspects such as dynamic power control and soft handoff when moving from cell to cell. The immediate problem: The phone and test equipment must communicate using the proper protocols to set up the call in the first place.
For IS-95 code division multiple access (CDMA) systems, separate pilot, paging, and synchronization (sync) channels are required to set up a call. A channel in CDMA-speak is one of the 64 Walsh codes used to create the spread-spectrum signal. Code 0 is used as the pilot channel, code 32 for the sync channel, and codes 1 to 7 for paging. This leaves 55 codes, 8 through 31 and 33 through 63, for user traffic channels.
The pilot channel is transmitted by the base station and used to establish a phase reference for coherent detection of the data modulated signal—the traffic channel. A common pilot channel helps overcome the effects of fading on the traffic channel, and it is transmitted with higher power than the other channels.
The paging channel is used by the base station to notify a mobile phone of a call or answer a mobile origination and make an assignment to a traffic channel. The sync channel provides the data rate of the paging channel and the time of the base station’s pilot pseudonoise sequence with respect to the system time.
For testing, an orthogonal channel noise source is needed to simulate the noise from all the other users transmitting within the 1.25-MHz frequency band. Finally, there is the traffic channel itself that carries the desired data or conversation. When the mobile phone transmits back to the base station, an access channel is used for control.1, 2
Although these signals occur sequentially and not simultaneously, their relationships to each other are complex. The most efficient and convenient way to provide the very specialized signals required by CDMA and similar signals required by other digital second-generation (2G) mobile phone systems is to use a test set.
Steve Stanton, the wireless communications test manager at Tektronix, said, “In engineering, the one-box test set provides a ready-made connection for the development of either a phone or a base station. The knowledge that the device under test (DUT) is receiving the correct protocols and call-processing information eliminates the need to question outside factors when evaluating a design. The signaling capability of the single-box solution offers call-processing, setup, handoff, and call-clearing capabilities not provided by the traditional spectrum analyzer and signal generator.
“Additionally, tests such as sensitivity, bit error rate (BER), and frame error rate (FER) can be automatically performed without the need for custom software routines,” he continued. “All tests can be made according to the established standard, ensuring compliance with regulations. Compared to traditional test equipment, one-box testers are not necessarily more accurate on conventional tests. Instead, they handle the higher-level tests that require two-way communications with the DUT through the air interface.”
There are other areas besides engineering where test sets are used and other reasons for using them. “In production, factors include space and cost savings, speed of operation, and convenience,” according to Al Fisher, product marketing engineer for digital mobile radio products at Anritsu. “A one-box tester is enhanced by the capability to switch quickly between standards because, today, manufacturers may change the type of product they produce between shifts or even between breaks.
“Standards require specific tests to be performed which can be very repetitive,” he said. “A one-box tester is designed to execute these tests in the fastest, most efficient way. Although standards are changing, alterations usually can be implemented by firmware upgrades.”
Manufacturing test benefits greatly from the integrated user interface in a one-box tester. Instead of worrying about all the settings required for individual instruments, you select a test and the software automatically sets each instrument as required. This capability saves time and improves repeatability by minimizing human error.
And, there are cost and development time savings to be made through the use of a one-box test set. As Rob Van Brunt, Telecom Analysis Systems product manager of wireless test instruments, pointed out, you don’t have to spend the time and effort required to develop your own test bench. Nor do you have to worry about which vendor to blame for incompatibilities among the interconnected instruments.
A one-box test set is sold as an integrated package for a specific purpose. You have one vendor to deal with and only one piece of equipment to upgrade when the time comes.
Of course, not just any old test set will do. And unless the relevant standard for the type of system you are testing has been ratified, you may not find a test set at all. As Mr. Stanton of Tektronix explained, “Development of hardware is concurrent with the development of the standard, and one-box testers are not available prior to the final establishment of the operating parameters of the network. Without a test set, simply generating and analyzing the signal can be problematic.”
Test-Set Capabilities
There is hope for cutting-edge developers in the form of very flexible, multistandard testers. The first step is to determine exactly what type of system you are working with. See the sidebar for a discussion of personal communications systems (PCS) and cellular characteristics and terminology.
Most test sets handle only one major technology. The Hewlett-Packard HP 8935 Series Cellular/PCS Base Station Test Sets, the E6380A for CDMA and the E6381A for time division multiple access (TDMA), are typical of these products. Within a single technology, a test set may cover a wide range of specific standards. For example, the Rohde & Schwarz Models CTD 55 and CMD57 TDMA Communications Test Sets handle, global system for mobile communications (GSM), digital cellular system (DCS) 1800, and PCS 1900 standards.
Because software controls the specific characteristics of the test signals, it is possible for you to modify the operation in some one-box testers. In this way, changes to phone designs or standards can be accommodated.
The subsystems that constitute a one-box test set determine its capabilities. A typical test set might contain these measuring/generating systems:
A cellular/PCS base station simulator with a built-in additive Gaussian white noise (AWGN) generator.
A digital modulation analyzer covering 700- to 1,100-MHz and 1,400- to 2,300-MHz bands.
A 3-GHz RF spectrum analyzer.
A digital modulation signal generator covering 300 kHz to 3 GHz .
An audio analyzer/generator.
An analog modulation analyzer.
A digital modulation analyzer.
A power meter with a built-in thermocouple.
An FER/BER tester.
This list illustrates that one-box testers can have extensive capabilities.
On the other hand, as with individual instruments, the performance specifications of each module must be appropriate for the job. As Tek’s Mr. Stanton noted, “For some of the more demanding tests such as GSM power ramping and CDMA adjacent-channel power ratio, the performance and flexibility of traditional test equipment exceed that of the one-box tester.”
For example, adding a fading simulator and impairment generator to the list allows you to push the DUT to its limits. Varying some or all of the channel characteristics under a number of diverse conditions can expose marginal DUT performance. These very specialized instruments can be used in conjunction with a test set.
Fading simulators provide Rayleigh and Rician distribution models to simulate the effects of multipath signal recombination and a log-normal model for terrain-induced fading. A dynamic capability often is included to handle time-dependent fading. Impairment generators provide an AWGN source as well as provision for continuous wave (CW) signals or other externally generated interference.
Regarding the combined output signal from a CDMA base station, Ken Thompson, a product manager at Hewlett-Packard, agreed that conventional instruments were excellent for some measurements. Spectrum analyzers can display the occupied bandwidth, and a power meter can accurately measure the total transmitted power. However, specialized test sets are required to understand how the base station really is operating.
“To determine how many channels are in operation and their corresponding power levels, a code domain analyzer is required,” said Mr. Thompson. “The concept is similar to that of a conventional spectrum analyzer except the X-axis of the display corresponds to the digital codes in use instead of the frequencies in use. The code domain analyzer shows the number of active channels and their individual transmission powers.” See Figure 1.
Finally, having invested in a test set to deal with your wireless system performance measurements, you still may not be able to solve mixed-system problems. As Oded Agam, director of technical services at RADCOM in North America, pointed out, “Cellular systems internetwork with other technologies like Ethernet and asynchronous transfer mode (ATM). In addition to your test set, you may require a protocol analyzer that can monitor both sides of the network. Ideally, you should be able to automatically correlate the payloads on the two sides, measure latency, and get a histogram display of the latency distribution.”
References
1. Ojanperä, T., and Prasad, R., Wideband CDMA for Third Generation Mobile Communications, Artech House, Boston, 1998, pp. 51, 52.
2. Whipple, D., “New Technology for Cellular Phones Challenges Today’s Test Procedures,” EE-Evaluation Engineering, April 1995, p. 97.
NOTE: This article can be accessed on EE’s TestSite at www.evaluationengineering.com. Select EE Article Archives and use the key word search.
3. Garg, V. K., and Wilkes, J. E., Wireless and Personal Communications Systems, American Telephone & Telegraph Company, Bell Laboratories Division., Prentice Hall, 1996, p.12.
SIDEBAR
Characteristics of Cellular and PCS Services
PCS phones initially were intended as a lower-cost, higher-performance alternative to the cellular systems that existed in the early ‘90s. Cellular, in turn, had been seen in the late ‘70s and ‘80s as the answer to the very limited performance of traditional mobile radio. In contrast to cellular, which has earned much of its revenue from business users, PCS is intended to make mobile communications available to a very broad market.
In a 1960’s portable phone system, a single, high-power transmitter served a large area. Communicating with a mobile receiver was limited to a line-of-sight path, and only a few users could operate simultaneously.
Cellular systems use much lower power levels to cover smaller areas. Combinations of frequency or time multiplexing—frequency division multiple access (FDMA) or TDMA—have increased the number of users the system can support. Complex modulation schemes have further increased user capacity.
First-generation analog cellular radio in North America was called the advanced mobile phone system (AMPS). It was standardized throughout the United States, Canada, and Mexico.
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In contrast to this situation, first-generation communications in Europe were fragmented among several incompatible systems. 2G 800- to 900-MHz GSM stemmed from the decision to develop a new digital system in Europe.
Copyright 1999 Nelson Publishing Inc.
July 1999
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