Mobile WiMAX Promises Portable Broadband Wireless

May 12, 2007
Growth of WiMAX as a mobile wireless broadband access solution will depend on limiting the number of design solutions and the amount of different components needed.

WiMAX may represent the "future" of wireless broadband connectivty, but it will not succeed without the support of many in the WiMAX Forum working with vendor companies. The WiMAX Forum is dedicated to making the WiMAX wireless communications standard a commercial reality and ensuring that WiMAX products will seamlessly work with each other.

The promise of broadband wireless access has existed for almost as long as cellular networks. Previous efforts at providing broadband wireless services have failed for various reasons, even though supported by a variety of well-funded equipment vendors. Generally, failure can be attributed to the lack of standardization and an excess of unique, proprietary solutions, both of which prevented a commonality of components for low-cost solutions. The failure of any effort to provide broadband wireless access can generally be traced to two factors, the cost and the risk of a proposed solution.

Since each broadband supplier had its own technology solution, dozens of different development programs were spawned to create different transceivers and baseband sections in each case. There was no economy of scale, as each program inevitably proposed multiple, expensive components with non-recurring-engineering (NRE) costs at each level that needed to be amortized as part of the development program. In addition to the hardware, each unique solution also called for a parallel software-development program to create unique firmware and interface software for operation, provisioning, and maintenance. These practices typically led to high equipment prices for the network operators. It should be remembered that even though these operators were well-funded, this didn't mean their investors were in business to lose money or take a chance on unproven technology.

Another key cost consideration was the fact that proprietary head-end radio solutions required proprietary customer premise equipment (CPE). Operators would be forced to supply (and likely subsidize) this equipment. The upshot was that all these costs associated with the lack of standardization eventually resulted in an untenable business case for network operators. The market of end users simply didn't have demand for broadband wireless (fixed or mobile) at prices that would make the entire venture worthwhile.

The majority of the equipment vendors selling high-cost, mid-1990s radios were startup companies. Network operators were understandably reticent to choose—and therefore be committed to— a single vendor's solution, especially when these were companies without previous histories. The possibility of supply interruptions, reliability problems with new technology, and the potential failure of a given supplier to stay in business were all factors that caused operators to delay their decisions to build out networks. In the end, very few operators were willing to be "captive" to a single vendor; the risk was simply too high.

Unlike industries that don't learn from their own mistakes, communications companies and the vendors that develop their networks are notoriously sensitive to reliving mistakes, especially when recent and painfully fresh in their minds. Standardization (i.e., interoperability) was clearly recognized as the key to fulfilling a broadband wireless dream, so the WiMAX Forum and its member companies work to ensure that service providers have choices, resulting in competition, better pricing, and less risk. This standardization drives down costs— both in terms of development (reuse of standard chip sets and associated reference designs) and ongoing component costs (economies of scale on a few parts, versus low volumes on numerous custom components). Risk is also mitigated through standardization because no operator is captive to a single vendor; and just as important, end-user equipment can be purchased directly by that user, instead of being supplied by the network operator. When costs are reasonable and risks are reduced, a profitable business case for the all members of the supply chain can be realized and the dream of true broadband wireless becomes a realistic goal.

The requirement for economies of scale and standardized equipment works its way all the way down to the component level. To that end, the need for component commonality has been a key part of TriQuint Semiconductor's WiMAX product-development philosophy. For example, high channel selectivity is critical to the front-end performance of both fixed and mobile WiMAX systems, implying the importance of the front end filter. In many cases, the spectrum allocated globally for WiMAX applications is precariously close to spectrum allocated for other applications. For example, there is no bandwidth buffer between the US WCS and satellite radio bands at 2.3 GHz. Because of the closely spaced frequency allocations and diversity of applications, there will be a need for superheterodyne transceiver solutions in the foreseeable future. Because of the need of highly selective filters in such transceiver achitectures, TriQuint Semiconductor has developed a broad portfolio of intermediate-frequency (IF) surface-acoustic-wave (SAW) filters for WiMAX applications, while also trying to maintain an economy of scale that will support economical WiMAX solutions.

Pursuing the goal of component commonality, TriQuint's engineering team has worked with key members of the WiMAX Forum ( and their design engineers to steer designs toward common IFs for WiMAX transceivers. In part because of these efforts, the industry has mainly converged to a handful of IF solutions at 380, 398, 456, 580, and 810 MHz. Although the idea of an ideal IF based on available mixers, local oscillators (LOs), analog-to-digital-converter (ADCs), and other components might have led to a single IF as the solution, this may not have been the most practical choice in terms of long-term WiMAX product development, and thus the multiple IF choices.

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Of course, in addition to the center frequency, there will be a choice of IF filter bandwidth, which is generally determined by the width of the WiMAX channels. The WiMAX Forum has created a number of certification profiles to guide equipment-supplier company members, each of which specifies a RF frequency range, a channel bandwidth, and a duplexing scheme (Tables 1 and 2). All of these profiles are not completed or released as of yet. In order to be released for certification, a given profile must have a minimum number of vendors that have committed to building equipment for it.

An RF designer may initially desired to procure an IF filter for each possible channel bandwidth. However, some creative system engineering can help everyone in the value chain by minimizing the number of discrete IF SAW filters that must be developed and supported. A greater diversity of filters only leads to less economy of scale and higher prices for each of a wide range of filters. Less diversity for WiMAX filters results in higher-volume production for less model numbers, resulting in lower prices for each filter.

A number of designers have solved this problem by using wider IF filters than necessary and then complete the channelization in the baseband. For example, a designer developing a WiMAX radio for 2.3-to-2.4-GHz operation could use a single 10-MHz IF SAW filter for both the 5- and 10-MHz channel products. By reusing a single filter product, the variability from printed-circuit board (PCB) to PCB will shrink, the volume on the single component (the filter) will increase, and the cost will decrease on the increased volume. Similarly, a 7-MHz filter can be used for both 3.5- and 7-MHz channels.

TriQuint Semiconductor and other standardization advocates have also focused development energies on active products with an eye toward component commonality. TriQuint's product line includes just three products focused on, but not limited to, the needs of the mobile WiMAX market. The models TGA2720 and TGA2730 power amplifiers, for example, are new high-gain, high-linearity components for the 2.3-to-2.7GHz and 3.3-to-3.9-GHz ranges, respectively (not currently shown on the TriQuint website). A quick review of Table 2 shows that all the various profiles proposed by the WiMAX Forum are covered by just these two components. In addition, the TGA2723 power amplifier combines the two discrete amps into a single 4 X 4-mm dual-band PA device.

The success of WiMAX will rely on factors that span across all links in the supply chain, as well as external factors like competing solutions that are seeking to satisfy the demand for broadband wireless. That said, the basic decisions of RF and system designers will play a part in determining whether WiMAX becomes the first successful attempt at delivering widespread personal broadband. Component commonality will drive economies of scale, reduce design cycles, and help create realistic and profitable business plans for all members of the supply chain. Vendor companies that want to see WiMAX succeed as the leading solution for fixed and mobile broadband communications should get behind the WiMAX Forum push for standardization because without it, as we've seen before, proprietary approaches won't succeed. At the same time other 3G/4G services will attempt to satisfy the growing demand for economical and affordable broadband service delivery if WiMAX does not.

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