MCPA Active Combining

Introduction

Wireless operators have used multicarrier power amplifiers (MCPAs) to increase the coverage and capacity of existing base stations. Using MCPAs has increased operator revenue without the excessive costs of adding or swapping out equipment or the building of additional cell sites. 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?

Transitioning Problems

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.

MCPA Design Advantage

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 multi-purpose power amplifier, typically not limited to one particular modulation method, or restricted to single-carrier or narrow-band 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 re-combined 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 both difficult and expensive.

An MCPA is also very 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 typically 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
Combinations 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.

Site Saver

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 US operators have indeed 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.

The availability of suitable MCPAs that extend coverage and eliminate the dead service areas left by removing a site, along with the excess capacity present in current networks, give wireless operators the ability to actually remove a site without reducing customer service. The overall operating expenditure saving is considerable. This is similar to having a cell site in the network that provides good coverage, while consuming minimal additional power and requiring no additional costs for site rental.

MCPAs as Active Combiners

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 alternative, shown in Figure 1, is the use of a lossy combiner—or worse, to add new antennas, feed lines, or even new masts—to support the new air-interface. It is easy to see why OEMs and operators are looking for MCPAs to solve this migration problem cost-effectively.

(a) (b)

Figure 1: Seamless air-interface upgrading with an MCPA (a) in the conventional approach, requiring a lossy or filter-based combiner; or (b) with an MCPA acting as an active combiner

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

Spectrum Re-farming

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 that 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.

(a)

(b)

Figure 2: Two possible GSM/3G BTS OEM solutions: (a) the use of a band-split duplex filter, and (b) the use of an MCPA as an active combiner

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 2a).

This duplex filter is effectively a high-power filter-combiner, similar to those used in the early days of narrow-band cellular systems. However, this option is both 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 tuneable (ideally, remotely tuneable) filter. However, this filter would need reconfiguration each time frequency allocations are altered. The band-split duplex filter option is clearly expensive and difficult to implement.

The band-split duplex filter also wastes bandwidth because the filter-combiner pass-bands are required to not overlap, leaving a “dead-band” area between the two filters that consists of the roll-off 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 2b), 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 2b). Since 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.

Full Band Capability

Andrew Corporation’s recently announced MCPA system (Figure 3) offers a level of flexibility that will benefit both 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 US, 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 useable output power. Newer solutions, such as the active combiner solution, do not suffer from this drawback.

(a) (b)

Figure 3: Andrew’s outdoor cabinet solutions: (a) three-sector MCPA system cabinet with up to 12 MCPA modules, and (b) single-sector MCPA outdoor cabinet with a single MCPA module

Figure 4: RF architecture of MCPA.

Real World Deployments

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 Watts) 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—an extremely impressive result (Figure 5).

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 6).

(a)

(b)

Figure 5: Spectral plots from the multi-mode 1800 MHz MCPA: four GSM carriers and four EDGE carriers, with 400 kHz spacing, at 51.3 dBm (135 W) showing mean (a) input signal, and (b) output from the MCPA.

Figure 6: Output spectrum from the multi-mode MCPA showing active combining of eight GSM carriers (with 400 kHz spacing) and one W-CDMA carrier, at a total mean output power of 135 Watts

Application Flexibility

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 7). Both duplexed (TX & 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.

Figure 7: MCPA configuration for booster applications

Still another configuration of an MCPA as active combiner application arises when the BTS interface ports are low-power, radio based connections (Figure 8).

Figure 8: MCPA configuration for integrated applications

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.

Conclusion

The use of MCPA active combining—as both 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. Moreover, the MCPA active combining solution offers excellent performance for GSM-only, GSM-to-EDGE migration, and combined GSM/3G applications.

Reference

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

About the Authors

Christopher Zappala is director of engineering within the Wireless Network Systems segment at Andrew Corporation. During his 20 years in the industry, Zappala has been issued several US 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 John Hopkins University. He can be contacted at: chris.zappala@andrew.com.

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 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. He can be contacted at: jeff.strahler@andrew.com.