Recent trends in digital cellular radio testing for both mobile and fixed-end equipment show a rising price for test equipment relative to analog-only solutions. But, the costs and capabilities of the new instruments are not distributed evenly between two of the most popular TDMA technologies. Why, for example, does IS-136 test equipment cost more than GSM solutions?
If you have searched for cellular and PCS test equipment in the past two years, you probably have noticed a sharp fall in the price of GSM test solutions. The price contrast widens even more when you consider the features offered: The world of GSM test equipment is filled with a large collection of instruments, great and small, elaborate and simple. In contrast, the competing IS-136, CDMA, DECT, and PDC/PHP devices offer rather limited testing choices.
We can understand the origins of the anemic test equipment possibilities for CDMA systems if we consider that IS-95 (CDMA) is a relatively new standard compared to GSM. The CDMA test market is a tiny fraction of the GSM market. But anxious users are somewhat mystified when the same paucity of test solutions, relative to GSM solutions, seems to remain among the other more-established TDMA systems (IS-136, DECT, and PDC/PHP) after several years of success.
What is the origin of the apparent differences in TDMA test equipment and their prices? Are the differences in equipment illusions that obscure key distinctions in the underlying technologies, or are they a simple function of relative market sizes? A comparison of the test environments in two of the world’s most popular TDMA cellular/PCS radio technologies, GSM and IS-136, reveals some interesting contrasts in two otherwise similar technologies.
GSM and IS-136
Though GSM and IS-136 share similar market life and common TDMA radio techniques, each differs enough in technology, maintenance practice, and market realities to require very different test solutions. First, as Table 1 shows, the radio techniques are not the same.
Secondly, and much more importantly, market forces and the significant easing of control by regulatory bodies worldwide will exert increasing influence on the development of wireless systems and the equipment used to test these systems.
Standards Process vs Market Survival
The manner of GSM’s appearance on the market was completely different from the IS-136 experience. GSM was created within a traditional analysis environment for providing a single, digital, cellular system to replace a collection of incompatible analog systems within the European community.
The new system had to offer speech quality at least as good as the best analog phones and had to include new teleservices seldom seen in previous wireless systems. Advanced, spectrally efficient modulation and radio-access techniques were specified so that all the new features and high-quality services could be accommodated in the 900-MHz frequency band allocated throughout Europe for GSM.
Today, GSM is the only standard that describes an entire mobile radio network, not just a common air interface. Phase 2+, GSM’s third metamorphosis, supports a full load of PCS features in both the 1,800-MHz and 1,900-MHz bands.
Every detail of the system is committed to open documents published through ETSI with relatively few IPR restrictions. GSM enjoys the advantage of working from a clean start: a single, new, frequency-band allocation with no need to be backward compatible with many kinds of handsets or infrastructure.
GSM’s birth was as slow and tedious as the standards process that made it possible. Conversely, the IS-136 standard appeared under a very different American model. Under this model, the marketplace was the primary factor driving the standards debate. This environment produced an IS-136 standard much faster than its GSM brethren but shifted the costs of debugging the standard closer to the network operators and their customers.
Market Leadership
Today’s dominance of Microsoft’s DOS and Windows operating systems demonstrates the advantage of being first to market with a high-quality product. Such was the case with GSM’s open standards.
Currently, GSM systems are operational or under construction in more than 100 countries. In support of GSM, there are hundreds of different types of handsets available in enough shapes, colors, and sizes and with enough accessories to attract even the most demanding users. The early dominance of GSM on a global scale, its orderly and elegant architecture, its library of open standards, the large supply of chip sets and software packages, and the vast network of experts and specialists have driven the costs of GSM test solutions down while their variety expands.
Today, test manufacturers offer development systems, protocol analyzers, network optimizing aids, infrastructure testers, and handset testers for both factory and field use. The large variety of GSM test equipment drives the development of more specialized, faster, and cheaper solutions. As a result, the availability of test equipment contributes its own positive feedback to the growing acceptance of GSM.
Dual-Mode Accommodations
Figure 1 depicts the channel scheme IS-136 utilizes to support dual-mode service with AMPS. Until the advent of GSM in 1991, AMPS was the world’s premiere cellular standard with TACS and NMT variants assuming secondary roles.
When capacity problems began to appear in some large American AMPS markets, a pragmatic approach was required to accommodate the millions of 800-MHz AMPS phones already in the hands of happy subscribers. Replacing all the world’s AMPS systems was—and remains today—out of the question. Interim Standard 54 (IS-54) quickly appeared to increase the capacity of AMPS systems through a second, digital mode of service while still accommodating AMPS handsets.
The content of the original AMPS control channel, represented by the green forward (FOCC) and reverse (RECC) resources in Figure 1, was modified to direct new dual-mode phones to a new DTC defined in IS-54. The DTC supplements the analog FM voice channels, represented by the violet FVC and RVCs in Figure 1, with a TDMA technique employing p /4DQPSK modulation of two bits per symbol.
Both the uplink and downlink halves of the DTC are depicted with the orange resource in the bottom of Figure 1. The base station allots every third time slot to a mobile that responds with bursts back to the base site. The DTC fits in the same 30-kHz channel allotments reserved for the original AMPS channels.
No sooner were the IS-54 system enhancements deployed then did the PCS challenge emerge. Another pragmatic effort was launched that eventually added a DCCH to the IS-54 hybrid design. The DCCH, as defined in the current IS-136 standard, is depicted in Figure 1 as the orange and pale green resource.
One of the IS-54 DTC time slots is modified to hold a large catalog of control and services resources (denoted in Figure 1 as C) which the mobiles can respond to with bursts. IS-136 further defines how the same system, less the FOCC/RECC and FVC/RVC analog AMPS remnants, can be deployed in the 1,900-MHz PCS band. The IS-136 standard currently is going through one of several revisions to improve speech quality and fill out the subscriber services until they rival those offered in GSM.
By now, you may appreciate how much more complicated even routine field testing tasks can become in 800-MHz IS-136 systems relative to GSM. IS-136 systems must support millions of AMPS phones, most of which were designed and manufactured before IS-54 and IS-136 were considered.
With each new version of IS-136, yet another reserved bit in the old FOCC protocol is mustered into dual-mode service. Occasionally, the industry must sort through some sophisticated signaling issues to clear the confusion certain older AMPS phones may experience while reading the new FOCC messages.
Moreover, dual-mode systems have two very different types of modulation. In these systems, the infrastructure includes all manner of analog level setting chores.
As a result, field test equipment for IS-136 systems must handle more metrology tasks than GSM equipment, and some protocol analysis must be included in many situations, all without the tremendous market-size advantage of GSM.
Prospects for the Future
If we step back and view the history of both AMPS and GSM, we see that AMPS represents, in a sense, the first phase of IS-136, corresponding to GSM Phase 1. The AMPS phase preceded GSM by more than a decade. Today’s GSM Phase 2 can be equated to the IS-54 extension of AMPS. GSM Phase 2+, which is currently being defined, corresponds to IS-136.
Since the AMPS roaming backbone is left intact, IS-54 and IS-136 become advanced phases of AMPS. Given the large installed base of AMPS systems worldwide, we should expect a long life and continued growth for IS-136 systems, but not at the stunning rates we will see for GSM in all its flavors, both cellular and PCS.
The proliferation of technical alternatives for public wireless systems, such as CDMA for cellular and PCS, will continue. In response, test-equipment makers will lower costs by abandoning attempts to adapt general-purpose equipment to highly specialized digital applications. As the radios and sundry types of infrastructure become more integrated around similar chips and related designs and as the industry learns more about modern metrology, test solutions will become cheaper, faster, and much more powerful.
About the Author
Malcolm Oliphant is the technical support manager at IFR Systems. He is co-author of An Introduction to GSM and has seven years of field experience in testing modern GSM, IS-136, and CDMA systems and their components. IFR Systems, 10200 W. York St., Wichita, KS 67215-8999, (316) 522-4981.
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Glossary of Terms
AMPS Advanced Mobile Phone System
CDMA Code Division Multiple Access
DCCH Digital Control Channel
DECT Digital-Enhanced Cordless Phone
DTC Digital Traffic Channel
ETSI European Telecommunications Standards Institute
FDMA Frequency Division Multiple Access
FOCC Forward Control Channel
FVC Forward Channel
GSM Global System for Mobile Communications
IPR Intellectual Property Rights
NMT Norsk Mobile Telephone
PCS Personal Communications Services
PDC Personal Digital Communications
PHP Personal Handy Phone
RVC Reverse Channel
TACS Total Area Cellular System
TDMA Time Division Multiple Access
Table 1.
Aspect
GSM
IS-136
Frequencies
900, 1800, and 1,900 MHz bands
400, 800, and 1,900 MHz bands
Access
FDMA/TDMA, eight time slots per frame
digital access only
FDMA/TDMA, six time slots per frame
dual mode access with AMPS
Modulation
GMSK, BT product = 0.3
p /4DQPSK
Symbol Rate
271 ksps
24.3 ksps
Copyright 1997 Nelson Publishing Inc.
October 1997
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