Wireless communications equipment encompasses the gamut of pagers and cordless phones to satellite communications systems¾ and obviously, each class of these uses different sets of technologies. So, it is not surprising that although some general-purpose test equipment is essential for testing any of these systems, additional application-specific test systems are being developed as fast as new wireless system implementations are being conceived.
Even a single superclass of wireless systems¾ cellular base stations and mobile transceivers¾ requires new special-purpose test sets as well as enhanced traditional test instruments. Cellular systems are drawing special attention since increased demands require better spectrum utilization, which can only be provided via newly developed digital implementations.
But most cellular communications systems installed during the last several years employ analog modulation to convey voice or data and FSK for signaling. Both old analog and new digital systems must be able to operate simultaneously without disturbing each other, while occupying identical frequency bands.
All newer (second-generation) cellular and cordless systems, except for one, use digital modulation techniques, primarily TDMA and CDMA. The exception is N-Amps, a hybrid developed by Motorola that triples analog (AMPS) channel capacity, which provides for cellular and paging applications and was standardized in 1992.1
TDMA systems exist in three dominant implementations: the European GSM, the North American IS-54 and the Japanese JDC systems. CDMA, standardized for cellular systems as IS-95, is based on QUALCOM developments and handles up to 20 times as many callers per channel as the analog AMPS. It operates relatively error-free in the presence of other signals.1
For many years, the testing needs of analog systems have been served well by conventional test equipment. But the digital-based wireless systems now in service require unique and, in many cases, application-specific test facilities.
Signal generators for digital applications must include appropriate digital modulation facilities; and similarly, test receivers must be equipped with corresponding decoders. But in addition, mobile systems require test facilities that verify compliance with signaling protocols, assess operability under noise and multipath fading conditions and check for proper power management functionality.
Contrary to broadcast systems, which always transmit at maximum power to cover the largest area possible, cellular base stations use low power transmitters to allow frequency reuse at smaller distances. Similarly, cellular mobiles use the lowest power possible for efficient channel utilization as well as battery power conservation. As a result, power levels are continually automatically readjusted to provide acceptable transmission quality under all conditions¾ while still minimizing power emission.
Of course, conventional test equipment is still needed and many companies have updated these instruments to provide features to test new wireless systems. But additional test equipment is still required to verify receiver performance as well as power management capabilities. Such equipment includes AWGN generators and multipath fading simulators. A typical interconnection arrangement using some of these instruments for CDMA testing is shown in Figure 1.
General-Purpose vs Application-Specific
Some test requirements are so unique that several companies have developed application-specific test sets targeted at a class of wireless communications systems. But the choice of general-purpose or applications-specific is not always obvious.
General-purpose and digitally enhanced instruments are most useful during design verification and testing of the modules that comprise the wireless communications system. “For example, when testing power amplifiers for intermodulation characteristics, the combination of a signal generator with complex digital modulation facilities together with a spectrum analyzer having modulation analysis functions provides the ideal solution,” said Bob Buxton, Product Marketing Manager at Tektronix. “These instruments measure spectral changes caused by intermodulation and associated parameters such as error vector magnitude. For transmitters, they measure modulation precision and spurious emissions.
“But general-purpose equipment will not suffice for testing fully assembled mobile phones or base stations,” Mr. Buxton continued. “At some stage, it will be necessary to establish an actual call and perform measurements under normal operating conditions. For base stations, this occurs when a network is commissioned and when in-service maintenance is required. In these cases, an application-specific radio test set is required.”
While each class of wireless communications system must adhere to a standard, technologies and implementations differ greatly among them. To use general-purpose instruments to efficiently perform a variety of unique tests, many companies provide communications system-specific application notes.
Some test equipment as well as complete system test sets can also be ordered with system-specific options or are software reconfigurable. Here are some examples for selected test situations:
Selected Transmitter Tests
A common test requirement for a transmitter is the verification of its basic frequency standard’s accuracy and stability, since data clock and carrier frequency are derived from it. A general-purpose counter/analyzer with a high enough resolution (typically one part in 108) and frequency range, such as the Fluke PM 6681 with TimeView™ software, is suited for this task. Application notes describe how the instrument can make these and other measurements for NADC or PDC system transceivers or perform FSK measurements on RF carriers.
“The PM 6681 and TimeView can also directly verify most FHSS system requirements defined in the 802.11 standard,” commented Staffan Johansson, Product Marketing Manager at Fluke. “These include channel frequency stability, the number of operating channels, hop rate, channel dwell time, channel switching time, channel settling, and frequency deviation of GFSK modulation.”
Output power is another essential transmitter measurement for which a general- purpose instrument is needed. But because of the large number of power levels to be measured for a digital system, it is advantageous if the power meter is not only fast and accurate, but also covers a wide dynamic range with a single sensor.
“It should also accurately determine the average power of a digitally modulated signal during a burst and a nonburst mode,” remarked Steve Reyes, of Giga-tronics. “Due to the complexity of the digital modulation, it is not practical to estimate average power during the on time by measuring the pulse signal average and performing a duty- cycle correction calculation. The preferred and more accurate method measures the burst signal only during the on time of the pulse.
“Software should also perform required signal evaluations, including adaptive averaging for different power levels, the PRF duty cycle, and modulations,” continued Mr. Reyes. “To provide valid readings over GPIB in the shortest possible time, the meter should also automatically determine when each measurement is settled.”
Selected Receiver Tests
The most frequently required instrument for receiver testing is the signal generator. Almost any conventional signal source can be used as the “interferer” required for some of the mobile receiver interference-rejection tests; but for performance testing, special modulation facilities are often needed.
AM, FM and phase modulation facilities are adequate to generate some burst, FSK or PSK signals. Producing many of the more complex signals transmitted and received by many wireless systems today requires an in-phase/quadrature (IQ) modulator plus signal shaping facilities.
In a way, any source that contains an IQ modulator may be looked upon as a digital system general-purpose signal generator. For instance, the Tektronix/Rohde & Schwarz SMHU58, which includes IQ modulation, could generate NADC signals today, and CDMA signals tomorrow.
But some tasks may be simplified by ordering implementation-specific options. In the case of the SMHU58, Option 06 provides the capability to test forward and reverse link components by generating an IS-95 standard channel-coded signal, including convolutional encoding, block interleaving and Walsh coding. The Anritsu Wiltron MG3671A accommodates expansion units, each tailored to generate a variety of signals for a class of wireless communications systems.
In addition to containing an IQ modulator, the Marconi 2050 Digital and Vector Generator includes Rayleigh and Rician fading simulation facilities. Applying fading simulation during product development enables early design verification and compliance testing. When testing under multipath fading conditions, a general-purpose multichannel fading emulator, such as the Noise Com MP2600-12, may be used.
“Our multipath fading emulator is used for CDMA, TDMA, GSM and AMPS equipment testing; and with its recently extended RF bandwidth (6 MHz to 20 MHz), it is capable of testing DCS-1800 and wireless LANs,” said Alex Kim, Marketing Manager at Noise Com. “Test files stored within the instrument allow easy setup and testing according to the individual application requirements.”
System Test Sets
Most communications system test sets include more than a dozen discrete functions within a single enclosure. The Hewlett-Packard HP 8920, for instance, integrates 22 complete instruments in a portable package. While originally designed as an AM/FM test set, it can now be configured to test both analog and TDMA dual-mode cellular telephones. Its most recent version, the HP 8920B, provides increased throughput. Most measurements are made quickly via software control.
For CDMA tests, the HP 8924C simulates full base-station operation. All CDMA transmitter tests are performed automatically, including high-speed DSP-based power measurements. The HP 8924C also measures transmitted waveform quality by the correlated power method recommended by IS-98, which reports waveform quality (Rho), frequency error, modulation phase and amplitude error, and carrier feedthrough.
A range of radiocommunications test sets and service monitors from Tektronix provides test facilities for a variety of systems and applications. The CMD55 and CMD57 support GSM, DCS1800 and PCS1900 base stations while the CMD80 simulates a CDMA base station. The systems provide all needed signaling and call test features.
A recent enhancement also lets the CMD57 behave just like a mobile phone and establish a call with a base station, reported Mr. Buxton. “Testing may now be carried out without taking a cell site out of service; this gives continuity of service for customers and revenue for operators.”
These and similar wireless system test sets provide mainly two advantages:
Because they are application-specific, they are tailored to perform all required tests in the shortest possible time.
Most contain all call setup and handling facilities needed to verify complete end-to-end system operation.
1. Garg, V. K., and Wilkes, J. E., Wireless and Personal Communications Systems, Prentice Hall, 1996.
RF/ Microwave Test Products
Enhancements to CDMA Test Set
Address Emerging PCS Needs
The CMD80 Digital Communications Test Set is now available in a single-band (800-MHz cellular or 1.8 to 2 GHz PCS ranges) or in a dual-band configuration. It also accommodates Rate Set 2 testing which, per JStd-008, encompasses a data rate of 14.4 kb/s. The CMD80 supports a full suite of CDMA automated measurements, including receiver quality measurements such as sensitivity, dynamic range and demodulation of traffic channels in AWGN, as well as transmitter quality measurements. Single band: $39,995; dual band: starting at $48,000. Tektronix, (800) 426-2200.
Systems Perform Wireless
Channel Impairment Tests
The WIS-98 CDMA Mobile Station Test System and the WIS-018 PCS Test System conduct tests under various emulated wireless channel impairments, such as AWGN, multipath fading and CW interference. The WIS-98/018 Wireless Impairment System performs simultaneous cellular and PCS tests. The three systems contain an AWGN generator, an RF channel simulator and an interface set with two synthesized CW interference generators and a duplexer. They perform tests that verify product performance in accordance with CDG Stage 1, IS-98 and ANSI-J-STD-018. From $97,000. Noise Com, (201) 261-8797.
Signals/Bursts to 4.2/3 GHz
The PM 6681 Timer/Counter/Analyzer features 50-ps single-shot resolution (10 to 20 Hz for 800/900 or 1,500-MHz bursts), self-synchronization and built-in arming delay as needed for measuring TDMA signal frequencies in digital cellular telephones. Together with TimeView software, it can verify many performance requirements for frequency-hopping spread-spectrum systems as outlined in the 802.11 standard. It measures and displays time-varying parameters, such as jitter, frequency stability and channel settling/switching time in cellular phone systems and wireless LANs. From $3,190. Fluke, (800) 44-FLUKE.
Upgradable Power Meter
Has 90-dB Dynamic Range
The Model 4220A Power Meter has a dynamic range of up to 90 dB when used with diode sensors. Frequency coverage extends from 10 kHz to 100 GHz and power measurements from -70 dBm to +44 dBm. The instrument meets EC standards, has a built-in power reference, stores complete calibration data for up to four sensors and emulates HP 437B power meters. The 4-1/2 digit readout displays power in dBm or W. RS-232-C and GPIB interfaces are optional. $2,450. Boonton Electronics, (201) 386-9696.
The 8540B Series Universal Power Meter and the 80400A Series Modulation Power Sensors perform high-speed power measurements of complex modulated signals over a wide dynamic range. Modulated average power measurements are performed on AM, pulse and digitally modulated signals, such as BPSK, p /4 DQPSK and 0.3 GMSK. Burst average power measurements are available for TDMA signals. The data capture rate is 4,000 readings/s over ranges from -60 dBm or -70 dBm to +20 dBm and frequencies from 10 MHz to 18 GHz. 8541B (single-channel): $3,095; 8542B (dual-channel): $4,595; 80401A: $1,295; 80402A: $1,475. Giga-tronics, (800) 726-GIGA.
Instrument Features Digital,
Vector Modulation, Fading
The 2050 Series Signal Generators cover ranges from 10 kHz to 1.35 GHz, 2.7 GHz and 5.4 GHz. In addition to conventional modulation capabilities, a wide-bandwidth I/Q modulator provides FSK, GMSK, PSK, DPSK, offset PSK and QAM formats, including signals required for NADC, PDC, PHP, CDMA and TFTS. PHP and DECT signals can be created using ancillary equipment. Programmable channel filters are provided. Broadband AM, burst and spread spectrum signals can be generated. Rayleigh and Rician fading simulation capabilities are built in. From $19,200. Marconi Instruments, (800) 233-2955.
Wide-Range Source Suited
For Wireless Test Systems
The PTS 3200 precision frequency generator supplies low-noise signals from 1 MHz to 3,200 MHz with a resolution of 1 Hz. The instrument employs direct digital synthesis and features fast switching (5 m s to 20 m s) and low phase noise. It has the agility needed for many wireless test applications. The output level is +3 to +13 dBm (1 V max into 50 W ) and the noise floor is -130 dBc/Hz. Remote programming is provided via high-speed BCD or GPIB. $14,850 including OCXO frequency standard. Programmed Test Sources, (508) 486-3008.
Expansion Modules Match
Communications System Needs
The MG3671A Digital Modulation Signal Generator is available with one to four expansion units. The units produce signals conforming to GSM, DCS1800, PCS1900, NADC, PDC, PHS, PACS, DECT, CT2, TETRA and TFTS communications system standards. An internal burst function unit provides frame/slot data formats for all TDMA signals. The MG3671A covers 300 kHz to 2.75 GHz, delivers low adjacent channel power leakage and stable level accuracy to 13 dBm maximum output power, and achieves a vector error of <1.8%. $49,000. Anritsu Wiltron, (408) 776-8300.
CDMA Test Set Includes Analog
Mobile Test Capabilities
The HP 8924C Mobile Station Test Set provides the measurement features needed to verify the performance of CDMA mobile telephones. Acting as a calibrated CDMA base station, it measures not only parametric characteristics but also the functional aspects of phone performance. In addition to its CDMA functionality, the HP 8924C includes AMPS, NAMPS and TACS analog phone test capability. The system performs transmitter/receiver tests to assure IS-98 conformance. The associated HP 83217A software automates all cellular mobile measurements. $55,000. Hewlett-Packard, (800) 452-4844.
Glossary of Terms
AMPS Advanced Mobile Phone System
AWGN Additive White Gaussian Noise
CDMA Code Division Multiple Access
CT Cordless Telephone
DCS Digital Cellular System
DECT Digital European Cordless Telecommunications
DQPSK Differential Quaternary Phase Shift Keying
FHSS Frequency Hop Spread Spectrum
FSK Frequency Shift Keying
GFSK Gaussian Filtered FSK
GMSK Gaussian Minimum Shift Keying
NADC North American Digital Cellular
N-AMPS Narrowband Advanced Mobile Phone Service
PACS Personal Access Communications System
PCS Personal Communications Services (U.S.)
PDC Personal Digital Cellular (Japan)
PHP Personal Handy Phone System
PHS Personal Handyphone System (Japan, formerly PHP)
PSK Phase Shift Keying
QAM Quadrature Amplitude Modulation
TACS Total Access Communications System
TDMA Time Division Multiple Access
TETRA Trans European Trunked Radio System
TFTS Terrestrial Flight Telephone System
Copyright 1996 Nelson Publishing Inc.