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Wireless operators have used multicarrier power amplifiers (MCPAs) to increase the coverage and capacity of existing base stations. Until now, the traditional MCPA application has been used predominantly in countries that employ TDMA and CDMA access technologies, namely, the Americas (mostly North America) and some Asian countries. MCPA use in GSM systems in Europe, Australia, and some other areas of Asia, has not been as prevalent. However, now that “active combining” has emerged as a primary application for MCPAs, deployments are occurring in Europe and Australia with further expansions taking place in Asia. Why is this new MCPA “active combining” application becoming the optimum solution for migration to 3G systems?

In order to continually increase their subscriber base, today's wireless operators must keep pace with technology improvements by transitioning from 2G to 3G systems. However, while making this transition, wireless operators must eliminate the huge cost of adding or swapping-out major equipment. One example of the current situation involves single-carrier power amplifiers, which are usually restricted to one or two channels in GSM or PCS booster applications. These simple GSM or PCS configurations seem like an economically feasible migration solution in the short term. However, long-term life-cycle costs increase because of the likelihood that additional carriers or new modulation formats must be added during the lifetime of the cell site.

The lifetime of a cell site required to continuously service an increasing customer base is usually much longer than the lifetime of the equipment initially installed at the cell site or the lifetime of any particular multiple access technology. As a result, the ability to use add-on, flexible hardware becomes the optimum, cost-effective upgrade method and the goal of all wireless operators.

This goal of cost-effective, add-on, flexible hardware is achieved by the use of a quality MCPA system used in an active combiner application. At the heart of an MCPA active combiner application are the intrinsic design and inherent characteristics of the MCPA module itself.

The MCPA module is designed as an ideal multipurpose power amplifier, typically not limited to one particular modulation method, or restricted to single-carrier or narrowband operations. An MCPA module is capable of amplifying any number of carriers, at any power level, subject only to a specified instantaneous bandwidth capability and a specified mean power and peak power rating.

In order to meet performance and application requirements, an MCPA must possess a contiguous operational bandwidth. It must contain no filtering that separates carriers into individually amplified frequency bands because these separate carriers must then be recombined using a cavity or lossy combiner. If a contiguous operational bandwidth is not available, a significant loss of flexibility results that leads to severe frequency-planning restrictions on the network. The lack of a contiguous operational bandwidth also makes adding (and, in some cases, removing) carriers difficult and expensive.

An MCPA is also extremely linear. It must generate negligible intermodulation distortion (IMD) products such that emissions and air interface standards are met. Harmonic distortion is much less of an issue since system-level output filtering removes the harmonic distortion without sacrificing the amplifier's generic carrier capabilities. In order to minimize intermodulation distortion (performance typically -63 dBc or better), RF engineers use a “linearization scheme,” the most common of which are:

• feed-forward correction;

• cross-cancellation (a variation of feed-forward);

• analog pre-distortion correction;

• digital RF pre-distortion correction; and

• a combination of the above (for example, analog pre-distortion + feed forward).

Of the above linearization schemes, feed-forward (and its variants) is the most popular, since this approach combines a wide instantaneous bandwidth capability with excellent IMD performance.

The MCPA's intrinsic design advantages have already provided wireless operators an opportunity for cost savings. The goal to reduce the number of cell sites, or at least to not add new cell sites to the network, stems from the need to reduce operating and capital expenditures. This goal sometimes conflicts, however, with the need to improve network coverage.

To achieve both goals, operators use MCPAs. Some U.S. operators have found that it is not necessary to do cell-splitting in order to gain capacity since they already have plenty of spectrum to serve their needs (perhaps through acquisitions or successful auction bidding), and can even do the reverse (cell-amalgamation). Instead, wireless operators are in a position to reduce sites or implement a “net-zero” site growth strategy. Whenever a site needs to be added to the network for extending coverage, the wireless operator removes a site elsewhere in the network and deploys an MCPA at an adjacent site in order to overcome the coverage gap that would result from the cell site being removed.

In addition to providing wireless operators with “no growth” or cell-site reduction capabilities, MCPAs used in active combiner applications can benefit wireless operators even more.

Because of the MCPA's intrinsic design advantages, an MCPA system is extremely flexible. MCPA systems can handle any access technology, any generation of access technology (1G, 2G or 3G), and even multiple access technologies, simultaneously.

In particular, MCPAs can support the following air-interface protocols: analog, GSM, EDGE, W-CDMA, CDMA, CDMA2000, and OFDM. Also, MCPAs can support any carrier type, any number of carriers, any carrier spacing, and any carrier power level, subject only to a maximum overall mean output power rating and a given instantaneous bandwidth capability.

The MCPA's high degree of flexibility is essential in providing a seamless migration to future networks without a complete swapping out of all existing BTS equipment.

The concept of active combining can be applied in the United States, where the flexible use of spectrum is permitted and joint GSM/3G installations exist. This flexible use of spectrum is a technology advancement in which the United States is ahead of Europe.

The use of MCPAs as active combiners also works in OEM systems designed to take advantage of spectrum re-farming. Spectrum re-farming involving the 900 MHz band is currently under active discussion within European Telecommunications Standards Institute-based countries. The 900 MHz band was previously used for analog cellular systems and has already undergone a transition from these analog systems to GSM.

The present transition will be from GSM to 3G, which will be a markedly different transition than the previous one. The difference is that in the previous analog-to-GSM transition, there were relatively few subscribers compared to the current number of GSM subscribers. In the earlier scenario, wireless operators were committed to completing the transition as rapidly as possible, largely driven by the poor security of the analog phones. The difference in the present GSM-to-3G transition is that most wireless operators are planning a slow transition that will maintain GSM service for the foreseeable future as a complement to 3G service.

Because operators will maintain both GSM and 3G, this re-farming operation poses some challenges for the OEMs selling 3G equipment, which will be co-located potentially with existing GSM equipment. Planning restrictions and/or mast loading issues will likely dictate the use of the same antennas for both services in most cases. The real goal is for the OEMs to engineer elegant methods of allowing both GSM and 3G to co-exist.

The MCPA-based active combiner solution is a strong candidate for achieving this goal.

The filter-based GSM/3G combining solution consists of a band-split duplex filter as a means of combining GSM and W-CDMA signals (Figure 1a).

This duplex filter is effectively a high-power filter-combiner, similar to those used in the early days of narrowband cellular systems. However, this option is inflexible and wasteful of spectrum. It is inflexible because the duplex filter splits the band into two parts that are not necessarily equal, with one part of the band reserved for GSM and the second part reserved for W-CDMA. The exact frequency at which this split occurs is fixed in the filter design. Therefore, a cost-effective migration from GSM to W-CDMA is impossible without replacing the filter, perhaps often, in order to increase the W-CDMA bandwidth while simultaneously reducing the GSM bandwidth. An alternative solution is the use of a tunable (ideally, remotely tunable) filter. However, this filter would need reconfiguration each time frequency allocations are altered. The band-split duplex filter option is expensive and difficult to implement.

The band-split duplex filter also wastes bandwidth because the filter-combiner passbands are required to not overlap, leaving a “dead-band” area between the two filters that consists of the rolloff and drift characteristics of each of the separate bands. This dead-band area is bandwidth for which the wireless operator is paying but cannot use for revenue-generating traffic. In the already overcrowded cellular bands, this is a significant disadvantage.In the MCPA active combiner solution (Figure 1b), combining takes place in a broadband hybrid (or similar) combiner, and total frequency flexibility is maintained. Wireless operators do not even need to visit the cell site in order to reconfigure the ratio of GSM to W-CDMA bandwidth. The amount of bandwidth dedicated to GSM and W-CDMA can be remotely altered dynamically as the need for the W-CDMA technology increases.

The ideal OEM solution is to use the MCPA as an active combiner. This solution avoids the duplication of power amplifiers in the 3G BTS, and possibly also the 2G BTS (Figure 1b). Because the W-CDMA BTS would require some form of MCPA anyway, it is a relatively simple matter to incorporate the combiner within the 3G BTS product from the OEM vendor (possibly along with the receive multicoupler) and thereby provide a fully managed, warranted, end-to-end solution.

Andrew Corp.'s recently announced MCPA system offers a level of flexibility that will benefit the wireless operator's existing migration problem as well as the OEM's desire to meet re-farming issues. Andrew's MCPA system is composed of its variants for cellular, PCS, and GSM frequency bands, and its complete system rack includes filtering and low-noise amplifiers (LNAs).

The system is capable of supplying full output power over 60 MHz of instantaneous bandwidth for PCS applications — an unprecedented performance for a complex base station component. Not all MCPA systems offer such high degrees of versatility, however.

In the United States, some wireless operators have a split allocation in the PCS band, perhaps for historical reasons or through spectrum acquisition. This is a problem for most MCPAs because the large instantaneous bandwidth required to simultaneously cover these frequency allocations typically necessitates a reduction in the usable output power. Newer solutions, such as the active combiner solution, do not suffer from this drawback.

The Andrew MCPA applies advanced analog predistortion, high-efficiency techniques, and common feed-forward correction to achieve industry-leading RF performance. Feed-forward technology is chosen over more modern digital predistortion techniques because of the wide bandwidth performance. A mature feed-forward technology, coupled with proprietary high-efficiency techniques, provides top performance in a cost-effective solution.

Feed-forward-corrected MCPAs operate by isolating the distortion products of the main amplifier and adding them back into the output signal in such a manner that the distortion products are suppressed from the output signal. Generally speaking, a well-designed feed-forward amplifier can suppress the main amplifier's distortion products in excess of 30 dB across the operating bandwidth.

Figure 2 shows a high level view of the Andrew MCPA architecture. A pre-amplifier is used on the input side of the MCPA. After the pre-amplifier, the input signal is directed to the carrier loop of the feed-forward amplifier. The purpose of this correction loop is to isolate the distortion products, which are generated in the main amplifier. The feed-forward architecture first samples a portion of the input signal and uses it to drive the analog predistortion circuit. This predistortion circuit is aligned such that the amplified signal exiting the main amplifier has reduced distortion products, and the signal is conditioned in a manner that allows the error loop to operate most efficiently. The distortion products are isolated by adding a sample of the main amplifier's output signal with a portion of the original carrier input signal so that the carriers are subtracted from the resultant signal. The remaining signal, which contains a sample of the main amplifier's distortion products, is known as the error signal. Both paths of the carrier loop must be gain-, phase-, and delay-matched so that optimum carrier cancellation can be obtained over a wide bandwidth.

The resulting error signal is fed into the error loop. Adjustments are made to the amplitude, phase and delay of the error signal such that when added back into the amplifier output signal the distortion products are canceled over a wide operating bandwidth. Amplitude and phase adjusters are used in the carrier loop and error loop in order to align the two loops properly. Delay elements are used to guarantee the proper delay match for each of the two loops. A cavity filter is used to match the delay of the signal in the error path, thereby minimizing signal loss out of the main amplifier.

Efficiency enhancements in this MCPA are the result of the company's proprietary implementation of Doherty technology in the main amplifier. It allows for a 50% improvement in the main amplifier efficiency while maintaining the wide operating band-widths usually only available with lower-efficiency techniques. Similarly, the analog predistortion technique allows for wide operating bandwidth and improved amplifier efficiency in the main and error amplifiers. All of these technologies work together to provide an economical MCPA solution that is up to 18% efficient and operates over 60 MHz of instantaneous bandwidth at the full-rated power of the product.

Is it possible to realize such a near-perfect amplifier in practice? A typical scenario for an 1800 MHz MCPA active combiner application in Europe is the amplification of four GSM and four EDGE carriers at +51.3 dBm (135 W) composite power. In this application, the input and output operational spectra measurements indicate there is negligible degradation in the adjacent-channel emissions as the signal passes through the amplifier (Figure 3).

Still another real-world example of an MCPA as active combiner application is the 1900 MHz version of a single MCPA being used as a 2G/3G active combiner. In this 2G/3G application, eight GSM (2G) carriers are amplified adjacent to a single W-CDMA (3G) carrier. The output spectrum of this active combiner application also displays low adjacent-channel spectral emissions (Figure 4).

The MCPA active combining solution can be configured in a variety of ways to increase coverage and capacity. One such configuration involves the MCPA system interfacing with the BTS cabinet antenna ports directly (Figure 5). Both duplexed (Tx and Rx) and simplexed (Tx-only) BTS interfaces are available from the MCPA system. If the number of required antenna feed lines are to be minimized with the existing BTS (for example, four reduced to two), a loss in output power is realized due to an external lossy combining unit. Using the MCPA, the active combining is performed prior to amplification, while output power is set to the original configuration at the same time, minimizing required antenna feed lines. With a substitution to the BTS interfacing modules, the configuration is extended to include multiple BTS cabinets, which each potentially have unique air interfaces (such as 2G and 3G) that the MCPA processes simultaneously.

Still another configuration of an MCPA as active combiner application arises when the BTS interface ports are low-power, radio-based connections

In this scenario, the MCPA is configured to integrate with these connections through the implementation of low-power combiners and power dividers. In this way, the MCPA may be considered a complete RF front-end, handling all high-power amplification and antenna port interfacing functions such as integrated low-noise amplifiers for the receive path. Not typically considered a “high-power combiner” application, this so-called “integrated MCPA application” may be extended to multiple BTS cabinets with different air-interfaces and similar radio-based connections. The result is similar — multiple air-interfaces supported by equipment requiring a minimal number of antenna ports, producing output power characteristics that may be adjusted per customer/site requirements.

The use of MCPA active combining — as an after-market solution for existing wireless operator cell sites or as part of an OEM configuration for new systems to be installed — is a realistic, flexible, spectrum-efficient and cost-effective solution to the 2G to 3G migration problem. In particular, the MCPA active combining solution provides greater capital and operational cost savings than filter-based or high-power hybrid-based solutions.

1. P.B. Kenington, High Linearity RF Amplifier Design, Norwood, USA: Artech House, 2000 (ISBN: 1-58053-143-1).

Christopher Zappala is director of engineering in the Wireless Network Systems segment at Andrew Corp. During his 20 years in the industry, Zappala has been issued several U.S. patents related to RF circuits and systems. He received a bachelor's degree in electrical engineering from Lafayette College and a master's degree in electrical engineering from Johns Hopkins University.

Jeff Strahler is an Andrew Fellow RF engineer within the Wireless Network Systems segment, responsible for the design of cellular base station power amplifiers. Strahler was recently appointed an Andrew Fellow in recognition of his 18 years worth of research and product development. He received a bachelor's degree in electrical engineering from the University of Cincinnati and a master's degree in electrical engineering from The Ohio State University.

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