The wagers are in and the personal communications services (PCS) race is on! Wireless service providers are betting on the future success of the new digital mobile communications services being promised by PCS in the United States. The initial bets were made via FCC auctioning of PCS channels in the 1.8- to 2-GHz range across the United States. Successful bidders put millions on the line just to own a piece of the PCS airways.
But owning a PCS channel is just one winning heat in an ongoing race. Successful channel bidders face a time line for bringing PCS on line for customers. To do this, they must place another bet–on an air-interface technology for providing efficient, effective and profitable mobile PCS.
A key contender in the air-interface race is the wideband, spread-spectrum technology referred to as code division multiple access (CDMA). An interim standard, IS-95, defines CDMA mobile-station/base-station compatibility for U.S. cellular services. This same standard is likely to be the cornerstone of CDMA-PCS.
Although not a new technology, CDMA is new to PCS and cellular, and testing CDMA base stations and mobile units is a critical concern. To address this concern and keep CDMA in a competitive position, IS-95 has been complemented by IS-97 and IS-98. IS-97 defines the minimum performance requirements for CDMA base stations, and IS-98 does the same for dual-mode cellular stations (CDMA phones). A brief overview of IS-98 alone is enough to give CDMA equipment manufacturers and network providers a taste of test and evaluation expectations.
The Test Environment
The IS-98 requirements for CDMA phones fall into four main categories: functional testing, receiver quality, transmitter quality and power measurements.
Key elements in the test setup are the CDMA phones to be tested, a verified CDMA base station or base station emulator, and various measurement instruments (Figure 1). Most tests are done with coaxial cable connections between the phone and base station. This provides a controlled transmission environment and eliminates free-space radiation. It also allows easy connection of instruments such as power meters and spectrum analyzers to monitor transmission. Controlled interferers, such as fader simulators and signal generators, can be injected as needed for in-depth performance testing.
Functional Testing
Functional tests include mobile registration and call processing. In basic terms, the question being asked is: “Can this mobile unit make a phone call?”
Functional testing can be done by putting the mobile unit into a special diagnostic mode or by using a base station to control the mobile unit. The latter is preferable since it’s a closer representation of actual operating conditions.
In mobile registration, which should be done first, the base station transmits pilot and sync channels so the mobile can synchronize to the network. The base station should be configured under various conditions to validate mobile-unit functionality. Typical variations are RF channel, system selection (A or B), pseudo noise offset, total power output, and relative power of the pilot and sync channels.
When the mobile is synched to the base station, the mobile can complete registration by using the paging and access channels to send specific messages to the base station. These messages, which contain information about the mobile unit, can be analyzed to ensure that the mobile is indeed sending the correct information.
After registration, the mobile unit is tested for call origination. Essentially, this verifies that the mobile unit can be used to make and receive phone calls.
Receiver Quality
While being able to set up a mobile unit call is important, the quality of the call is equally so. The mobile receiver should be tested under varying conditions to verify its operating capabilities. Receiver sensitivity and dynamic range are probably the most important, but certainly not the only, tests required.
To test mobile-unit sensitivity and dynamic range, a call must be made. The mobile unit’s receiver sensitivity is then measured as the minimum power that can be received at the antenna connector for which the frame error rate (FER) does not exceed a specified value. Dynamic range is the power range at the antenna connector over which the FER does not exceed a specific value. In all of these tests, FER is the actual measured value and should be less than 0.005 with 95% confidence.
Single-tone desensitization is an example of a receiver quality test done with an interferer. Again, the actual measurement is of FER. The measurement is made with a call in progress and in the presence of a single tone at a +900 kHz offset from the CDMA channel frequency. In the presence of this interfering tone, the FER is expected to be less than 0.01 with 95% confidence.
Transmitter Quality
Transmitter tests are designed to measure the quality of transmission from the mobile phone. The two main tests are waveform quality and coding accuracy, and both are performed with a call in progress.
The waveform quality test measures waveform quality factor, frequency error and transmit-time error. The measurements are made at the mobile unit’s antenna connector while using call setup with Service Option 2 and a 9,600-bps data rate. Waveform quality is expected to be greater than 0.944, frequency error is expected to be within +300 Hz, and transmit-time error is expected to be within +1 s.
Coding accuracy is also measured with a call in progress using Service Option 2 at 9,600 bps. The mobile unit’s ability to correctly encode data is tested by repeatedly sending a known data frame on the forward traffic channel. A frame of data is then demodulated at the mobile unit’s antenna connector and the I (in-phase) and Q (quadrature) elements are compared to a stored pattern. There should be no coding errors in the demodulated frame.
Power Measurements
IS-98 recommends a wide variety of mobile-unit power measurements. The goal is to verify the mobile unit’s capability to adjust to varying transmit power requirements as it moves toward or away from a base station site or encounters terrain-related power variations. Essentially, the mobile unit adjusts its transmit power based on both received power and power control bits sent from the base station.
The IS-98 mobile unit power measurements include–but are not limited to–open loop, closed loop, minimum, maximum and gated power. These measurements involve:
Range of Open-Loop Output Power
–This test verifies that the mobile unit responds properly to changing received power by adjusting its transmitter power.Time Response of Open-Loop Power Control
–This test measures the mobile unit’s open-loop time response to a step change in the mean input power. It involves making step changes in the input power (from the base station) and measuring the time for the mobile unit’s transmit power to make a corresponding change.Range of Closed-Loop Power Control
–In normal operation, the mobile unit provides a closed-loop adjustment to its transmit power. Testing this power control capability requires various adjustments of base-station power and transmission of various power-control bit patterns while measuring power response at the mobile unit’s antenna connector (Figure 2).
The base station begins the test by starting the mobile at a specific power level. It then transmits 100 power-control bits with a value of zero. The mobile should respond by powering up. The base station will then transmit 100 ones, followed by another 100 zeros. The power level of the mobile is monitored during these transmissions.
Maximum RF Output Power
–This is the maximum RF output power that the mobile station can transmit. In the test, specific parameters are set up in the access parameters message and continuous 0 power control bits are sent to the mobile base station. Output power is then measured at the mobile unit’s antenna connector. Spurious emission levels are also measured between 824 and 849 MHz and compared to a specified limits mask.Minimum Controlled Output Power
–This is the power measured at the mobile unit’s antenna connector when both closed-loop and open-loop power control indicate minimum output (power- control bits are a continuous stream of ones).Standby Output Power and Gated Output Power
–Standby output power occurs when the mobile unit’s transmit functions are disabled (for instance, during initialization, idle and access states). Gated output occurs when the mobile unit is operating in the variable data rate transmission mode. Measurements of gated output power are made at the antenna connector and are then compared to a limits mask (Figure 3).Meeting the CDMA Test Challenge
CDMA mobile-phone verification requires a variety of tests under a wide range of operating conditions. These tests can be done by using a base station to provide functional stimulus, response and control for the mobile unit under test. However, the speed and efficiency of testing will be limited by the accessibility of the base station’s control parameters and the flexibility of control allowed.
CDMA phones also must be tested for interoperability. CDMA mobile-phone manufacturers must verify that their product will operate with CDMA base-station implementations from various manufacturers (and vice versa). This raises the specter of having multiple base stations in the mobile-unit test bay. The system costs and complexities of such a test bay can have a significant negative impact on production-test efficiency and profits.
To hedge your bets in the race for PCS profits, it would be wise to consider a base-station emulator. Such a test resource would provide maximum accessibility and control of base-station parameters. A base-station emulator also could be set up to provide the differing implementations of various base-station manufacturers. This flexibility in a single unit would substantially cut the cost and complexity of CDMA phone test bays while increasing test efficiency.
About the Author
Jim Hebert is the Wireless Product Marketing Manager at Tektronix, and has spent more than seven years there as a design engineer. He holds an E.E. degree from Washington State University and an M.B.A. degree from the University of Oregon. Tektronix, Inc., Measurement Business Division, P.O. Box 500, M/S 58-699, Beaverton, OR 97077-0001, (503) 627-4697.
Communications Test
Copyright 1995 Nelson Publishing Inc.
September 1995
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