Third-generation Wireless Test Equipment Requirements

Third-generation Wireless Test Equipment Requirements

Rob Van Brunt
Telecom Analysis Systems Inc.
Eatontown, NJ

Although the technology rollout is still several years away, development of third-generation (3G) wireless systems is well under way. During this critical development phase, manufacturers are making strategic decisions on the most appropriate modulation, protocol and coding designs needed to support the high transmission rates that are the cornerstone of 3G systems. These next-generation systems will be even more sensitive to propagation and interference effects as they look to squeeze greater throughput out of the available frequency spectrum allocations. To allow developers to fully evaluate their designs against these wideband channel models, suitable test equipment must be available to meet these new channel emulation requirements. This article reviews some of the new technology enhancements that will be found in 3G systems and the equipment needed to evaluate these emerging designs.

The rollout of second-generation (2G) digital cellular systems has reached full stride with several million subscribers currently on Global System for Mobile communications (GSM) and code-division multiple access (CDMA) networks in the US. These 2G systems offer significant advantages over first-generation analog systems including notable new user features and capacity gains for service providers. However, the feature set of 2G systems still pales in comparison to the offerings of its wired counterparts. In an attempt to maintain the industry's double-digit subscriber growth, manufacturers and service providers have begun defining advanced characteristics for 3G wireless networks. The characteristics of these next-generation systems will require enhancements to current test and measurement equipment.

2G Systems

2G digital cellular systems such as CDMA and GSM have been defined for more than five years, and test equipment targeted at these 2G systems has matured during this period. However, the deployment of these systems only began in earnest over the last two years, and early returns indicate that the popularity of these digital schemes and their new features is growing at a steady rate. It's clear that these 2G systems will cause the demise of the earlier analog-based network over the next several years. Yet, at the time of their deployment, 2G systems are already showing visible shortcomings.

These 2G deficiencies can be found in the areas of network capacity, roaming ability and data services. With three different major digital standards being deployed in the US, nationwide roaming is hampered by intersystem incompatibilities turning anytime, anywhere connectivity into just a tired marketing cliché. When 2G systems were defined, the popularity of the Internet was just beginning to take off. Although 2G systems have some limited data capabilities, they were not designed to provide the high speed Internet access users crave from wired networks. 2G systems also have failed to achieve the capacity gains suggested by initial paper designs. The failure to deliver on manufacturers' capacity promises (in combination with the rapid growth of cellular and PCS subscribers) has service providers looking ahead to further developments in order to enhance the spectral efficiency of their fledgling networks.

3G Systems

The key features touted for 3G systems (also known as IMT-2000) look to correct some of the shortcomings of 2G systems. The target goals for 3G systems include greater throughput, roaming and capacity.

The vision of the next-generation wireless handset is that of an Internet-ready terminal capable of a vast array of wireless innovations, including Web browsing and multimedia services such as full-motion video and videoconferencing. The ultimate goal is a virtual home environment where the subscriber can access all the same voice, data and audio/video services he or she would have while at home. Supporting broadband data connections will require at least an order-of-magnitude change in the data rates currently supported by 2G terminals.

When the Federal Communications Commission allowed multiple 2G digital standards to battle it out in the marketplace, it created at least a temporary problem for users looking to roam among wireless networks in North America. Internationally, the multiple North American standards in conjunction with incompatible standards from Europe and Asia have created a fragmented global coverage map that has resulted in consumers having little chance of utilizing a common handset across continents.

In an attempt to remedy this rogue roaming situation, the International Telecommunications Union (ITU) has set forth to develop specifications for a 3G system that incorporates global wireless network compatibility and increased network coverage for undeveloped areas. ITU's plan for this 3G system, formerly known as Future Public Land Mobile Telephone Service, is International Mobile Telecommunications 2000 (IMT-2000). IMT-2000 has provisions for a variety of wireless access methods ranging from in-building picocells to direct satellite communications. As shown in Figure 1 , a common spectrum around 2 GHz has been set aside for IMT-2000 deployment to facilitate seamless global roaming and includes spectra for both satellite and terrestrial access. Basically, IMT-2000 will use the frequency-division duplexing system with frequency intervals of 190 MHz. Currently, time-division duplexing is being utilized at 2010 to 2025 MHz.

If 3G systems can supply consumers with advanced data services and worry-free roaming, more subscribers will continue to hop onto the wireless bandwagon. This increase in users will put further pressure on service providers to maximize the capacity of their networks to accommodate the growing subscriber base. Since these service providers have not yet come close to recovering their investment in the 2G networks, they are already looking for ways to gracefully migrate to a 3G technology that can support more handsets in the existing frequency spectrum.

Development Hurdles

Finding a way to outfit existing infrastructure equipment with 3G technology while somehow preserving the immature investments in 2G systems will be a formidable task for developers. This practical constraint means manufacturers will look to enhance existing equipment with overlay technologies. For instance, base station manufacturers are applying advanced antenna technology to increase system capacity. These smart antenna systems are independent of the radio interface used and can be overlaid on legacy systems to boost coverage significantly and increase the re-use of precious frequency channels.

Although full of promise, smart antenna systems represent the cutting edge of both antenna and signal processing technology and still need to be evaluated under a variety of field conditions to prove their practical worth. Evaluating the performance of antenna array systems requires test equipment, such as RF channel emulators, to be outfitted with entirely new capabilities.

There is little question that with digital cellular technology still in its infancy, developers and network operators are positioned on the steep portion of the learning curve. Providing the high throughput connectivity required to adequately support data services will require modulation rates to increase by an order of magnitude. These high rate schemes are more susceptible to the harsh effects of radio channels. To be successful in the real world, further enhancements in signal processing and channel coding will be required to mitigate these propagation and interference impairments. This process involves testing the performance of different design parameters against RF channel and interference conditions in an iterative fashion.

Along with the obvious bit-level issues associated with high data rates, significant system issues arise from the desire to achieve seamless roaming. Although the ITU first envisioned a single global air interface for 3G systems, the international standards body has already realized that this single interface is a political and practical impossibility. As a compromise, the ITU has claimed it will accept a family-of-systems concept. This scheme provides for unique but compatible systems to be deployed around the world. Accomplishing this grand plan will require manufacturers to achieve an unprecedented level of interoperability. In addition, the integration of macro-, micro- and pico-cellular environments poses significant networking challenges and requires careful coordination of the frequency bands allocated for IMT-2000 systems. Developers now need test equipment designed for current cellular and PCS bands to cover the spectrum associated with IMT-2000.

Technology issues aside, development teams also are strapped for resources due to a lack of personnel resources in the wireless industry. In spite of this labor shortage, manufacturers have set aggressive timetables for 3G technology trials. Japanese developers have targeted the end of this year for field trials of wideband CDMA (W-CDMA) systems. Domestically, CDMA Development Group members are gearing up to deploy 3G networks based on the evolution of CDMAOne technology by 2000.

Achieving successful field trials will require extensive lab testing of these emerging technologies. In addition, development organizations do not have the time or resources to invest in home-brew test solutions, thus test equipment providers will play a key role in the realization of 3G networks.

3G Test Challenges

Currently, the foremost problem facing developers hoping to evaluate IMT-2000-type systems is that these 3G systems do not yet exist in the real world. As a result, designers need to find a way to recreate network, propagation and interference conditions on the lab bench. As they push the performance envelope, engineers need a way to benchmark new design iterations vs. real-world conditions. Although this technology phase is the most difficult for test equipment vendors to support, it can be the most critical time for testing. Difficult design decisions are being made based on the evaluation of candidate modulation, protocol and coding schemes.

Test equipment needs to provide accurate and repeatable conditions under which these candidate technologies can be evaluated. With little margin left for error, the successful evaluation of cutting-edge 3G technology requires test instrument settings to be accurate to within a few tenths of a decibel.

It is difficult for test equipment vendors to support these emerging test requirements since they involve new technologies and frequency bands. In contrast to wireless equipment manufacturers, test equipment vendors usually prefer to wait until the competitive dust has settled and a single 3G standard has emerged before dedicating resources for the design of new test gear. With more than a dozen different radio interface proposals submitted to the ITU for review, as listed in Table 1 , suppliers who make instruments to emulate base station equipment that is used to evaluate mobile handset performance must bide their time and wait for some semblance of a standard to emerge.

Even if all the necessary test equipment was available, it is a laborious task to evaluate new transceiver designs vs. all possible test cases. Next-generation wireless terminals are being designed to operate in both terrestrial and satellite environments, producing a virtually unlimited number of test scenarios. In an effort to maximize test efficiency, developers require test automation to increase the number of test conditions that can be executed during their evaluation period.

Unique Test Equipment Requirements

Analogous to the impact of the introduction of CDMA technology into the marketplace in the early 1990s, 3G technology once again demands unique testing solutions. Test and measurement vendors are already scrambling to retrofit existing instruments with the new features required by product developers. The instruments required to evaluate these new designs include traditional RF equipment (such as signal generators and spectrum analyzers) outfitted with new features and technology-specific equipment such as code-domain analyzers, RF channel emulators, additive white Gaussian noise (AWGN) and interference emulators, and base station emulators.

More than a dozen radio interface proposals have been thrown into the 3G fray and, in the majority of cases, the common thread that ties them together is the use of W-CDMA technology. CDMA is based on the premise that a user's communications signal is spread with a unique orthogonal code that distinguishes it from other users sharing the same frequency channel. In 2G systems, CDMA signals are spread at 1.2288 Mbps. This spreading rate was chosen to accommodate the 1.25 MHz of bandwidth allocated to QUALCOMM by PacTel during initial field trials. To achieve higher data rates and greater capacity, developers are increasing this spreading rate for 3G systems. This increase produces a greater coding gain, which translates into increased interference immunity.

W-CDMA, an alternate, but similar approach being adopted by developers, involves bonding multiple existing 1.23 MHz signals. This multicarrier approach increases effective throughput to a user and affords a graceful transition from current CDMA systems. In either case, signal generators must be equipped with new features to support the evaluation of these designs.

Traditional RF signal generators must be equipped with wideband digital modulation capabilities in order to mimic a W-CDMA transmitter. These signal sources are used in the evaluation of receiver performance and to provide the proper stimuli for in-channel and out-of-channel analyses of RF components slated for CDMA equipment. Since W-CDMA proposals include channel bandwidths up to 20 MHz, these signal generators must be flexible enough to handle multiple signal formats.

To prevent wideband signals from spilling energy into adjacent spectra, the components used in CDMA transmitters and receivers must adhere to stringent distortion specifications. The most notable of these specifications, adjacent-channel power ratio (ACPR), is defined as the ratio of channel power in the operating band to that in an adjacent channel. ACPR measurements are performed with spectrum analyzers equipped with CDMA-specific measurement algorithms. W-CDMA imposes a high demand on the measurement instrument and requires spectrum analyzers to have a wide dynamic range in order to accurately assess a module's performance.

CDMA technology has also introduced a new type of transmitter measurement significantly beyond the scope of the traditional power measurements associated with other transmission schemes. This code-domain analysis measurement characterizes the modulation capabilities of a CDMA transmitter by characterizing how well it assigns power to each of the desired orthogonal codes. Since some 3G systems will use a new flavor of orthogonal codes, changes will need to be made to code-domain analyzers available on the market today.

Characterizing W-CDMA Transmission Performance

Next-generation systems will be even more sensitive to propagation and interference effects as they look to increase data throughput and squeeze out additional capacity. Developers require ways to measure the performance of new receiver designs under the same adverse conditions they will likely encounter in the real world after deployment. It is desirable to begin evaluating the performance of transmission schemes early on in the development cycle to minimize time to market and to ensure that a product meets the desired performance criteria.

To accurately assess system performance, real-world conditions must be replicated in the lab under channel conditions that are both repeatable and easy to control. As shown in Figure 2 , RF channel emulators and AWGN and interference emulators utilized during the successful development of 2G systems are used to recreate the hostile conditions found in RF environments.

Just as the requirements placed on traditional RF test equipment have evolved, RF channel emulators also must now accommodate the characteristics associated with new W-CDMA systems. Existing W-CDMA proposals have channel bandwidths up to 20 MHz, which means RF channel emulators, as shown in Figure 3 , must handle carrier signals in excess of the bandwidth required to ensure that even the widest bandwidth signals are passed through the system without any undue parasitic effects. Wider bandwidth transmission schemes have the ability to resolve smaller multipath differences generated by the propagation channel. Therefore, it is imperative that channel emulators targeted at 3G systems have sufficient delay resolution between the multiple propagation paths they are emulating.

As discussed previously, next-generation technology will employ smart antenna systems to increase capacity by reducing interference. These smart antenna systems utilize antenna arrays on the transmit link to steer the signal to the desired user. On the receive link, they capitalize on the benefits of diversity reception.

Multi-antenna systems require simultaneous emulation of multiple transmission channels. A valid multichannel representation must recreate the fading conditions on each channel precisely and provide control over the relationships between the channels modeled in the system. As shown in Figure 4 , the RF channel emulator provides the capability to synchronously model up to eight independent propagation channels. Using this emulator, users can program the correlation of the fading conditions found between the channels in the overall model. In addition, this instrument is the first to offer a vector channel model to emulate the link characteristics generated by antenna array systems.

Cellular systems are inherently interference limited. Despite the fact that smart antenna systems attempt to minimize co-channel interference through beam forming, the effect of other users on wireless transmission must be assessed. This performance metric is critical for CDMA since link margin translates into capacity limits. With revenue-generating capacity on the line, developers and network designers are attempting to squeeze the last couple of tenths of a decibel out of the system. Therefore, accurate characterization of a transceiver's performance is critical and places stringent demands on the equipment used to emulate interference conditions.

The noise and interference emulator has been optimized to generate precise link conditions across cellular, PCS and IMT-2000 bands using AWGN and CW interference. By integrating all the components necessary for generating accurate co- and adjacent-channel interference conditions, the emulator provides a single instrument solution to a problem that previously required more than three instruments. The integration of functions such as a duplexer interface minimizes the need for excess cabling and allows unmatched calibration of carrier-to-noise, carrier-to-interference and carrier signal levels all the way to the antenna of the device under test.

Conclusion

The deployment of first- and second-generation systems was apparent due to their distinctive nature. The onset of 3G systems is less obvious since it is highly desirable for them to evolve gracefully from existing networks. Therefore, it is difficult for any pundit to predict the exact arrival of 3G systems. What is certain is that developers currently are creating these 3G systems and are already defining fourth-generation systems. During these development cycles, it is critical for engineers to have access to the necessary testing solutions that will aid them in making key design choices. Though often overlooked, communications test equipment will play an important role in determining the degree of success achieved by these next-generation systems.