What Is TDMA and How Is It Tested?

The auctions for personal communication services (PCS) operating licenses are over and the winners are scrambling to obtain rights to erect antennas and base stations at strategic roof locations in key metropolitan areas. “Down the road, even resident telecommunications engineers may be located in these buildings,” said John Rosato, President of CS Technology, a company which already derives 40% of its business from managing roof space.¹

Such predictions indicate just how big the PCS market is expected to become. But even more important are the vast sums—in excess of $14 billion—that were spent on obtaining PCS licenses and the substantial equipment-related expenditures now being made. Developing, testing and maintaining all these new base stations and mobile equipment may create many job opportunities for test professionals.

The technologies used in many of these systems are novel and complex, and special instrumentation is needed to exercise and evaluate many of the functions. But, as always, to appreciate any underlying test rationale, it is necessary to understand the functionality of the system to be tested.

Cellular Communications

Early mobile radio systems used a single high-power base station with a tall antenna to cover as broad an area as possible, typically a 70-mile radius. Unfortunately, the few available channels were locked up over a large area by a few users. For instance, in 1970, the Bell System in New York City could support only 12 simultaneous mobile conversations.² To provide more usable channels, these broadcast implementations were replaced by cellular arrangements.

In a cellular system, many low-power transmitters cover a service area. Since these transmitters have only a limited range, it is possible to reuse frequencies in different cells as long as cells with identical frequency assignments are properly spaced. Each frequency can then be used simultaneously by multiple mobiles, facilitating a much higher subscriber density per megahertz.

Access Technologies

To establish and maintain communications, base stations and mobiles not only must access each other over agreed frequency channels, but also must use compatible frequency utilization and modulation/demodulation techniques. The three most prominent access technologies are:

Frequency division multiple access (FDMA).

Time division multiple access (TDMA).

Code division multiple access (CDMA).

The Advanced Mobile Phone Service (AMPS) system, the first cellular system widely used in the United States, uses FDMA. It partitions, or divides, the frequency spectrum and assigns each call a different subcarrier frequency. AMPS uses FM for speech transmission and frequency shift keying for signaling. Base stations transmit at frequencies between 869 and 894 MHz and mobiles between 824 and 849 MHz.

Systems using analog technology, such as AMPS, are referred to as first-generation cellular systems. Second-generation systems employ digital technologies and use primarily TDMA and CDMA, often in conjunction with FDMA.

TDMA operates on the principle that each radio channel is partitioned into multiple time slots and each user is assigned a specific frequency/time-slot combination. A user shares a single-frequency channel with others, but the privacy of the conversation is assured since the time-slot assignment is unique for the duration of the call.

To avoid early obsolescence of AMPS, still the dominant cellular system in the United States, the FCC has decided TDMA and CDMA mobiles operating in the 800-MHz band should provide at least dual-mode (TDMA/AMPS or CDMA/AMPS) operation. This also assures uninterrupted conversation when a mobile leaves a cell that provides TDMA or CDMA services and enters a cell which provides only AMPS access.

European countries initially used a variety of access systems but formed a committee in the early 1980s, the Groupe Mobile Speciale, to develop a common standard. The group recommended a system implementation, now known as the Global System Mobile (GSM), that uses TDMA, operates in the 900-MHz region and uses Gaussian Minimum Shift Keying (GMSK) modulation.

Cellular/PCS TDMA Applications

GSM is now widely used in Europe, providing voice, data and paging services. Although primarily intended for business users, it now also serves many individuals. To provide complete Personal Communication Network (PCN) services, the DCS1800 system, which also uses GSM technology and operates in the 1.8 GHz band, is being implemented in Europe.

The FCC defines PCS as a family of mobile or portable communications facilities which should offer a range of services to individuals and businesses and is integrated with a variety of communications and computing networks. With this charter, PCS encompass cellular implementations—plus more.

In the United States, the 1.9-GHz band was auctioned off for PCS use, giving licenses to two access providers for each geographical area. No mandates were issued on access technologies, but competitive forces are supposed to be the motivators to arrive at optimal technical solutions and standardization.

To avoid incompatibilities, the Telecommunication Industry Association established seven technical ad-hoc groups (TAG-1 through TAG-7) to draft standards for the most promising access technologies. TAG-4 and TAG-5 deal with the two TDMA standards that are drawing the most attention from PCS auction winners—an implementation based on the IS-136 standard and the GSM-derived PCS1900, respectively.

The IS-136 standard supersedes the IS-54 standard which governed the development of the digital North American TDMA cellular systems operating in the 800-MHz region. IS-54/IS-136 retains the 30-kHz channel spacing of AMPS, facilitating the evolution from analog to digital systems. IS-136 provides additional clarifications and definitions and covers PCS 1.9 GHz implementations.

AT&T uses the IS-136 TDMA implementation for the PCS networks it is installing. However, eight other auction winners, including Bell South Personal Communications and Pacific Bell Mobile Services, are opting for the GSM-derived PCS1900. This decision is based on the relative maturity of GSM. It is operational in Europe, its technology is proven and translating its design to a higher frequency, such as 1.9 GHz, is relatively easy.

Table 1 shows some of the characteristics of the major cellular and PCS systems using TDMA. Besides frequency allocations, channel spacing and number of speech channels per RF channel, a major difference exists in the modulation scheme used. GSM uses GMSK, while IS-54/IS-136 uses p /4 Differential Quadrature Phase Shift Keying (p /4 DQPSK).

Encoded voice, as used in digital systems, consists of ones and zeros with sharp transitions between them. Since such step-function events can produce near-infinite frequency spectrums, signal shaping is essential prior to modulation to minimize intersymbol interference.

GMSK uses Gaussian-shaped impulse response filters to modify the rectangular data pulses into pulses with low side lobes and narrow main lobes. To minimize the extent and abruptness of transitions, the phase changes of the modulating signal are limited to ± (p /2).

Systems using p /4 DQPSK employ baseband filters with a raised cosine frequency response. Phase-shift transitions are limited to ± (p /4) and ± (3p /4). While quadrature phase-shift keying provides high spectral efficiency, it requires coherent detection which is not ideal in a multipath fading environment. Using differential phase- shift keying, as employed in p /4 DQPSK, overcomes the detection problem.

The bandwidth efficiency of systems using p /4 DQPSK is 1.62 bps/Hz but it is only 1.35 bps/Hz for GMSK. However, the p /4 DQPSK output signal does not have a constant envelope and requires linear amplification. In contrast, systems employing GMSK modulation produce near constant envelope signals that use efficient and less costly nonlinear amplifiers.

TDMA Subsystem Test Issues

Verifying the performance of subentities of TDMA base-stations or mobiles is not much different than testing conventional communications equipment provided that appropriate signal sources, measurement instruments and access points exist.

“Traditional radio tests have always involved a stimulus-response method,” said Marty Gulseth, Product Manager at Hewlett-Packard. “To test a transmitter, you applied a known stimulus at the microphone or analog audio port and measured the RF signal to determine how accurately it represented the baseband signal. To test a receiver, you had to inject a known stimulus at the antenna input, then measure how accurately the receiver reconstructed the modulation from the test signal.

“However, in digital cellular base stations, the baseband signal is a shaped pulse train and a traditional stimulus-response approach may not be viable,” Mr. Gulseth continued. “To deal with this issue, some TDMA equipment manufacturers implement special test modes in their systems to cause the radio-under-test to provide either part of the stimulus or the response measurement capability.”

Required subsystem test instruments include signal sources, a power meter and a modulation analyzer—all TDMA-targeted. “For example, while it is normally possible to use a simple two-tone test to characterize intermodulation performance, this is not adequate for IS-54 or IS-136 amplifiers because the spectrum generated by the p /4 DQPSK digital modulation and the baseband filtering is complex,” said Bob Buxton, Product Marketing Manager at Tektronix. “The signal generator must be capable of generating a p /4 DQPSK signal and the spectrum analyzer must have appropriate modulation analysis features.”

Focused, rather than dedicated or general-purpose, instruments may provide an optimum solution, “A general-purpose spectrum analyzer could make many measurements required in the R&D phase, such as spurious emissions, but it is not easy to use for system-specific measurements,” Mr. Buxton continued. “A focused instrument would be a more useful tool because, in addition to being a general-purpose instrument, it can be preconfigured to make IS-54- or IS-136-related measurements.”

A focused-instrument approach is also appropriate for power meters. For instance, “Our Model 8540C power meter combines the general-purpose capabilities of a continuous wave and peak-pulse power meter with the application-specific capabilities required for TDMA testing,” commented Steve Reyes, Product Marketing Manager at Giga-tronics.

“The meter recognizes the beginning and end of a burst of RF power and takes an average of the power during the burst. The delay time and gate width also may be set for a specific time slot within the TDMA burst and the 8540C will provide an average power measurement of only the time slot of interest,” he said.

“The time-gating option can also be used to determine specification conformance of TDMA systems. GSM specification 11.10 requires that the burst average power is measured only during the useful portion of the burst,” Mr. Reyes added. “The useful portion of the GSM TDMA burst is determined to be 5% to 95% of the burst ‘on’ time.”

TDMA System Test

Overall performance testing of mobiles and base stations requires more than just evaluating receiver and transmitter functionality. For example, traffic-handling capabilities such as call initiation, user identification, handover to adjacent cells and system noise immunity as well as power control parameters must be tested.

Dedicated communications test sets, developed by companies such as Hewlett-Packard, Rohde & Schwarz, Noise/Com and Racal Instruments, are best suited for this task. Most of these sets have hardware and software options to suit a variety of applications, with some options unique to specific TDMA implementations.

For instance, the GSM-targeted Racal Instrument 6103 Digital Radio Test Set, available with a DCS1800 or a PCS1900 option, has system-specific code. “Every manufacturer has different codes for communicating between the base station and the telephone network (the T1/E1 interface),” stated Chris Foreman, Senior Marketing Manager at Racal. “To provide the needed functionality, we developed product-specific software in conjunction with most major GSM base-station manufacturers.”

System performance under expected air-link transmission-impairment conditions also must be determined. Multipath fading is a major problem in mobile communications. With this phenomenon, the amplitude of the signals arriving at the receiver is a vector summation of randomly phased signal components caused by reflection, such as bouncing off walls or buildings, in the multipath environment.³

Consequences of multipath fading include data loss during signal phase cancellations and excessive intersymbol interference. Other transmission impairments are caused by Doppler shift and atmospheric or location-dependent propagation losses. To determine the degree to which the equipment can remain functional in spite of these impairments, transmission-path emulators are used.

“Both IS-137 and IS-138, the radio interface minimum performance standards for TDMA cellular mobile and base stations designed per IS-136, require the use of an RF channel emulator in standard test setups,” said David Garrison, Development Group Leader at Telecom Analysis Systems. “The RF channel emulator must provide:

Multiple impairment paths per transmission channel.

Multipath fading with programmable vehicle velocity, simulating Doppler effects.

Programmable relative path delay.

Low RF output signal level.

RF channel emulators designed by Telecom Analysis Systems and Noise/Com fulfill these test and equipment-characterization requirements. Due to the number and varied nature of the required tests, programmability and automation are essential.

The specifications for TDMA systems are still evolving and, even after they are relatively firm, equipment implementations will depend on demands made by service providers. As a result, manufacturers of communications test sets will continue to expand their offerings, but whenever possible they will use a modular or add-a-feature-via-option approach to make the test equipment most versatile and cost-effective.

References

1. “Business Bulletin,” The Wall Street Journal, April 18, 1996, p. 1.

2. Garg, V.K. and Wilkes, J.E., Wireless and Personal Communications Systems, Prentice Hall, 1996.

3. Digital Wireless Seminar Notes, Tektronix, 1996.

4. Padgett, J.E., et al, “Overview of Wireless Personal Communication,” IEEE Communications Magazine, January 1995, pp. 28-41.

5. Falconer, D.D., et al, “Time Division Multiple Access Methods for Wireless Personal Communications,” IEEE Communications Magazine, January 1995, pp. 50-57.

TDMA Test Products

PCS1900 Test Capabilities

Added to GSM/DCS1800 Test Sets

The Model 6103 and 6113 Digital Radio Test Sets perform automated and manual receiver and transmitter tests of mobile telephones and base stations, respectively. Initially designed to satisfy GSM and DCS1800 test requirements, functionality has been extended to serve PCS 1900 testing needs. The 6103 includes a modulation analyzer and two PCMCIA memory card sockets for storing test sequences, instrument settings and results. Signaling procedures between the test sets and UUTs are selectable and fully automated. Racal Instruments, (714) 859-8999.

TDMA Adapter Extends Base-

Station Tester Capabilities

The HP 83204A TDMA Cellular Adapter adds TDMA (IS-136) digital measurement capabilities to the HP 8921A cell-site test set. The combination, also referred to as HP 8921A Option 500, retains analog functionality and adds the p /4 DQPSK signal generator, the p /4 DQPSK analyzer, BER and adjacent-channel power- measurement capabilities. It performs transmitter tests, such as RF power, frequency error, modulation accuracy and adjacent/alternate channel power measurements. Receiver tests include TDMA sensitivity (BER) and TDMA RSSI. Hewlett-Packard, (800) 452-4844.

Digital Mobile Transmitter

Test System Is Fast, Versatile

The MS8604A Digital Mobile Radio Transmitter Tester tests UUTs for compliance with worldwide communications system standards. The system combines a 100-Hz to 8.5-GHz spectrum analyzer, a 5.5-GHz true rms power meter and a DSP-based modulation analyzer. Software automates any digital mobile transmitter test. The typical modulation/frequency measurement time for a GSM system is <1 s. The system supports GSM, GMSK, GFSK, DCS1800 and PCS1900. An in-depth p /4 DQPSK analysis capability is provided. Anritsu Wiltron, (408) 776-8300.

Analyzers Cover Multiple

Digital Mobile Radio Standards

The Advantest R3462 and R3465 Modulation Spectrum Analyzers are used in digital mobile radio applications, including PCS. They combine spectrum-analyzer features with multi-standard modulation analysis, a color display and automated measurements. The R3465, with a 9-kHz to 8-GHz range, addresses NADC, PDC and PHS standards. The R3263, with a 9-kHz to 3-GHz range, is suited for systems using GSM technology. A single key accommodates switching from GSM, to DCS1800 to PCS1900 standards. Tektronix, (800) 426-2200.

Channel Emulator Covers

800-MHz and 1,900-MHz Bands

The TAS 4500 FLEX RF Channel Emulator, available in FLEX/GSM™, FLEX/Wideband™ and FLEX/Diversity™ versions, provides multiple impairment paths per channel, multipath fading with programmable vehicle velocity, and programmable relative path delays. The FLEX/GSM™ implements the six- and 12-path propagation models and attenuation functions required by the GSM equipment testing standards. Built-in test files enable the emulator to be configured as required by the IS-137/138 specification. It also is capable of testing at output levels specified by IS-137/138.

Telecom Analysis Systems

, (908) 544-8700.

Multipath Fading Emulator

Capabilities Expanded

Two new features have been added to the Multipath MP-2500 Fading Emulator:

the Nakagami fading statistics and a fourth Gaussian Doppler filter to cover “bad urban” conditions. They complement the existing family of Doppler, Rayleigh, Rician, Log-normal and Suzuki distribution fading statistics. The MP-2500 offers a 6-MHz bandwidth between 800 MHz and 2.5 GHz and contains up to 12 paths. It emulates one- or two-channel wireless communications between base stations and mobile transceivers. Noise/Com, (201) 261-8797.

Meter/Sensor Performs

TDMA-Targeted Measurements

The Series 8540C Power Meter combines general-purpose CW and peak-pulse power-measurement capabilities with TDMA-targeted test facilities. Used in conjunction with the 80400A Series Modulation Power Sensors, the 8540C performs 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. Giga-tronics, (800) 726-GIGA (4442).

Power Analyzer Has >60 dB

Dynamic Measurement Display

The Model 4500 Digital Sampling Power Analyzer has a peak-power dynamic range exceeding 60 dB and a statistical power measurement and plotting capability.

The peak-power range is -40 dB to +20 dB and sensors cover frequencies from 30 MHz to 40 GHz. The instrument provides power envelope analysis for digital telecom applications including cellular radio, spread spectrum and secure communications. Internal/external triggering helps examine modulated carriers and TDMA signals. The instrument features full-color menu-driven operation. Boonton Electronics, (201) 386-9696.

Systems, Usage

Characteristics

IS-54

2^nd Generation

Cellular, US

GSM

Cellular

Europe

DCS1800

PCS

Europe

IS-136