Electronic Design

Ready For WiMAX?

This wireless metropolitan-area network is making great progress as a superior alternative broadband access method.

It seems like it takes forever for any new wireless technology to work its way from initial idea through standards adoption to useful products. After the long development cycles of 3G cell phones, Bluetooth, Zig-Bee, Ultra-Wideband (UWB), and Wi-Fi (including 802.11n), you have to wonder if something is wrong with the system.

When you dig below the veneer of hype, though, you can see that WiMAX is moving forward just fine. It isn't a perfect technology, but it fills the metropolitan-area network (MAN) niche. And if the technology continues at a steady pace toward adoption, it should find its place alongside the many complementary wireless and non-wireless systems already in place.

Short for Worldwide Interoperability for Microwave Access, WiMAX refers to a MAN that promises high-speed data access over a major geographic sector. WiMAX is similar to Wi-Fi, but the key difference is that WiMAX has a maximum range of a few miles instead of Wi-Fi's few hundred feet.

WiMAX is based upon the IEEE's 802.16 MAN standard. Earlier versions targeted the 10- to 66-GHz bands with a line of sight (LOS) system capable of data rates of 34 to 134 Mbits/s using quadrature phase-shift keying (QPSK), 16QAM (quadrature amplitude modulation), or 64QAM at a range up to about 3 miles. More recent versions started with 802.16a, which defined an orthogonal frequency-division multiplexing (OFDM) physical layer (PHY) with 256 or 2048 subcarriers and a single carrier version.

This was subsequently revised and updated in September 2004 for the 2- to 11-GHz range with a non-line of sight (NLOS) objective, the so-called 802.16-2004 standard for fixed wireless service. The 802.16e-2005 standard for mobile service, which was ratified in December 2005, uses orthogonal frequency-division multiple access (OFDMA) and can potentially achieve a mobile data rate of 15 Mbits/s max at a range to 3 miles.

The primary WiMAX application is high-speed Internet access. It would compete head on with entrenched broadband systems from cable TV and DSL suppliers, but the chances are good that the technology will win out in the end. You'll hear the expression broadband wireless access (BWA) applied to WiMAX and any similar broadband wireless system.

WiMAX also promises to be an alternative 3G mobile Internet access service. It makes a good low-cost backhaul technology for cell-phone basestations or Wi-Fi access points as well, meaning that it should be a lower-cost wireless T1 line replacement.

The fixed version of WiMAX has been around for a few years, but with limited adoption—at least in the U.S. More WiMAX silicon and equipment is sold in other parts of the world, but that's expected to change. Developing countries, as well as those with huge swaths of area in need of high-speed Internet coverage, are picking up on the technology.

That's because WiMAX is a faster, lower-cost method for offering broadband connections, rather than stringing or burying cable. More and more, telephone systems using Voice over Internet Protocol (VoIP) over WiMAX are being installed over more conventional wired local service. India is just one of many countries where WiMAX is beginning to supply not only high-speed Internet access, but also phone service.

In the U.S., the hope is that WiMAX will provide broadband services to rural areas and small towns that are underserved by cable TV and DSL providers. And it's even happening in some areas. Bell South is currently testing services in rural Mississippi, Tennessee, Louisiana, Georgia, and Florida.

WiMAX is designed for service in the 2- to 6-GHz microwave range, where licensed or unlicensed spectrum is available. The outer limit of its capability is a 75-Mbit/s data rate at a range to 30 miles. But it's doubtful anyone will use it in that form. Instead, WiMAX will be deployed by building a network of basestations with coverage radii in the 2- to 6-mile range. All that speed will be divided up among many users, with most getting a solid data rate in the 1-to 3-Mbit/s range. Businesses can pay for more speed as needed.

WiMAX uses 256 OFDM and adaptive modulation (binary PSK, QPSK, 16QAM, and 64QAM—depending upon range, noise, multipath, etc.) as well as a wide mix of bandwidths (1.25 to 20 MHz) and data rates under differing conditions.

As for spectrum, WiMAX is agnostic and may use whatever is available locally. Throughout the world the 3.5-GHz band is the most popular, but the military occupies that space in the U.S. The most likely WiMAX band in the U.S. is 2.5 GHz, where a batch of licensed spectrum is available. The unlicensed 5.8-GHz band is also available, but it's unlikely this spectrum will be used for anything more than some backhaul applications.

The newest version of WiMAX is the mobile version, IEEE standard 802.16e-2005, ratified last December. The fixed version of WiMAX addresses applications where the customer's terminal modem is fixed, like a set-top box or a laptop in some fixed location. In the latter, the terms portable and nomadic describe a terminal in a laptop that can be moved from place to place but is fixed during operation.

With the mobile version, the terminal can move at up to 75 mph in a car, train, or bus. To achieve mobile operation, with all the roaming and handoff capability that cell phones have, a new standard had to be created. It uses the basic fixed WiMAX standard as a base, but it adopts OFDMA as its modulation/access method. It isn't compatible with fixed WiMAX, but that doesn't seem to be a problem.

With any mobile wireless system, some method of multiple user access is needed. Time-division multiple access (TDMA) or code-division multiple access (CDMA) could work with OFDM, but the standard uses the OFDMA access methodology. Multiple access is provided by assigning each user a group or subset of the total available OFDM carriers in a given bandwidth.

In this standard, the total number of subcarriers could be 128, 512, 1024, or 2048. Each user in the band typically will get 64 subcarriers, also called a subchannel. The subcarriers usually will be contiguous, but they don't have to be. Combined with a three-sector basestation antenna similar to that used with cell phones, each basestation can cover many hundreds of mobile users. Because of the flexibility and robustness of the mobile version, some experts believe it will become the de facto WiMAX mode for fixed or mobile applications.

The main issue with fixed and mobile WiMAX, and a good reason to worry, is whether or not the technology will provide the service it promises. WiMAX is a sophisticated microwave technology, and LOS between terminals is essential. Yet by using OFDM, some non-LOS situations can be tolerated. The OFDM broadband modulation scheme is robust in the presence of multipath, the bane of any microwave signal. But just how robust it is remains to be seen.

Will OFDM see through trees? Will it maintain a link at speed when the environment is changing second by second? The standard was designed for quality of service, so everyone is expecting it to do what the hype says. If implemented, a 2-by-2 multiple-input/multiple-output (MIMO) feature (that's part of the standard) should help provide the reliability that's expected of it.

At this point, WiMAX is on the verge of more widespread adoption. There are chips and equipment on the ready. However, few services exist at this point—though they're on the way.

According to sources at the WiMAX Forum, half of the group's 300 or so members are service providers that are currently considering or testing the technology. AT&T and Bell South already have deployed WiMAX in some rural areas, and there's no doubt that it will serve some subset of their subscribers.

The big Bells aren't the only ones showing interest in WiMAX. Intel and Motorola both recently announced major investments in Clearwire, a company that offers non-WiMAX broadband wireless. Clearwire is definitely committed to converting to WiMAX, and with Intel's $600 million and Motorola's $300 million, that conversion won't be so painful. The company is expected to roll out WiMAX in selected areas nationwide starting next year.

Sprint Nextel also recently announced plans to build out a WiMAX network starting next year. It expects to spend $1 billion to $3 billion to build a nationwide mobile WiMAX network. Some analysts question the idea of Sprint competing with its own recently expanded 3G cell phone network, whose services overlap with WiMAX broadband. Perhaps Sprint feels that it's a better strategy to compete with itself, because if it doesn't, other companies will.

Besides, Sprint is in a better position than its competition, namely Cingular, Verizon, and T-Mobile, to build a nationwide network because it already owns some viable 2.5-GHz spectrum that the others had to bid on during the Federal Communications Commissions' (FCC's) August spectrum auction. With this kind of backing, other companies will hop on the bandwagon. Intel, Motorola, and Samsung are expected to help Sprint build the system.

While a number of factors will play in to the success of WiMAX, most experts consider spectrum availability and competition essential. Spectrum continues to be tight in most parts of the world, but it's even more so in the U.S. The 3.5-GHz band is available worldwide, except in the U.S.— a real drawback for this country.

Some spectrum is available in the 2.5-GHz region, and that's what Clearwire and Sprint plan to use. For the most part, the spectrum is available. But it's tied up by others, and in many cases, it isn't used. The solution would be to track down who owns it and attempt to buy it (or otherwise find a way to use it). The 5.8-GHz unlicensed spectrum is also available, but few serious service providers plan to use it (except for backhaul).

WiMAX's most direct competition comes from 3G cell phones, including cdma2000 with its EVDO and WCDMA with its HSDPA. These standards only compete with the mobile version of WiMAX. They're all available in the form of plug-in cards for laptops with service from all the major carriers, but the service is a bit pricey. It does work well, however, with average speeds of 700 kbits/s to 1 Mbit/s and up to 2 Mbits/s. And you can get it now.

WiMAX Forum tests show that mobile WiMAX has greater spectral efficiency, handles more users, and produces higher data rates at speeds to 120 kph (about 75 mph) than the available cell-phone data services. On top of that, fewer basestations are required for equivalent coverage.

WiMAX wins hands down, and right now, it can offer what has taken 3G a decade of work to reach. Also, 3G offers no fixed equivalent. Yes, WiMAX will have some competition. Yet it would seem to have the upper hand, even if the other IEEE mobile standard 802.20 gets back on track or the Third Generation Partnership Project (3GPP) rolls out its encore to 3G, Long Term Evolution (LTE).

WiMAX is going to take another year or so before it comes into full sunlight. Fixed WiMAX will show up in set-top boxes. Meanwhile, mobile versions will end up fully embedded in some laptops and inside PCMCIA cards and USB dongles. Even VoIP over WiMAX cell-phone handsets are possible. These products and other equipment and services will emerge in 2007, but look for 2008 to be the big rollout year (see "Broadband's Evolution," p. 50).

Adaptix is a pioneer in the OFDMA business, so it's a natural fit with WiMAX. The company recently announced its BX-3000 Micro basestation and SX-300 mobile terminals. Adaptix VP Byron Young believes that the model for WiMAX mobile basestations will use many less expensive micro or pico cells (Fig. 1).

The BX-3000 forms the base while it connects to several RF modules with antennas by fiber-optic cable up to 2 km long. Also, a mobile terminal (SC-300) uses an Ethernet connection to a laptop. A more advanced MIMO customer terminal will be available in the near future.

Analog Devices doesn't have WiMAX-specific products yet, but you can expect them someday. Meanwhile, the company has created a special digital interface called ADI/Q that might develop into a standard. It's a digital interface between the RF front end and the baseband part of a WiMAX product.

The goal is to move the analog-to-digital converter (ADC) and digital-to-analog converter (DAC) functions onto the RF chips to create an all-digital interface to baseband. The interface includes all the lines for control loops, such as automatic gain control (AGC), transmit power control, and RF calibration. The design should result in interchangeability and ease of application, as well as the reduction of realtime software control.

Aperto recently achieved WiMAX Forum certification for its PacketMAX 100 and 300 customer-premise-equipment (CPE) units. The company is one of the first manufacturers to achieve such certification. WiMAX Forum's Jeff Orr said only a handful of products has been certified so far, but many more are on the way. He expects the first mobile products to be certified in the first quarter of 2007.

Fujitsu has been a long-time player in WiMAX with its MB87M3550 802.16-2004 compatible system-on-a-chip (SoC). The company recently announced its Mobile WiMAX SoC and related roadmap. Look for samples to be available in the first quarter of 2007.

Intel, a major WiMAX supporter, is investing heavily in the technology. It also offers the 5116, a digital baseband chip for fixed WiMAX. Intel's recently announced Rosedale 2 chip addresses the baseband needs of both the fixed and mobile WiMAX standards. Already, 10 key equipment suppliers have selected Rosedale 2 for their next-generation equipment. Also, look for a single-chip multiband Wi-Fi/WiMAX radio code-named Ofer that will facilitate the incorporation of both wireless technologies into future laptops.

Redline Communications, a maker of pre-WiMAX as well as true WiMAX equipment, recently teamed up with Intel and system integrator Nomad Digital to install a WiMAX network in California (Fig. 2). The system is built along 16 miles of the Caltrain commuter line between San Jose and Palo Alto.

Passengers on the train use Wi-Fi in their laptops to talk to a standard Wi-Fi router that in turn uses a WiMAX access point for backhaul. Seven WiMAX terminals along the tracks handle the handoffs and connection back to the server. Nomad, a U.K. company, has built other successful train systems and is expected to do many more here and in Europe.

Texas Instruments is a major supplier of WiMAX chips. In addition to the TRF11xx, TRF12xx, and TRF24xx RF up/downconverters, TI offers the TMS320TCI6482 1-GHz DSP chip and software library, which covers the physical-layer functions. Also, TI offers reference designs as well as support with various partners. TI works with Array-Comm to support the MIMO Smart Antenna technology. And, TI recently partnered with Wavesat on a Mini-PCI design for 5.8 GHz.

FPGA maker Xilinx is into WiMAX as well. Its Vertex4-SX, popular in implementing WiMAX baseband solutions, is a flexible base to design with, as today's FPGAs can handle the adaptive modulation/demodulation, forward error correction and other coding, basestation power control, and the OFDMA. Xilinx offers reference designs as well.

Despite the proliferation of wireless standards, WiMAX fills a need and a gap. It should succeed in delivering broadband services such as IPTV and VoIP, in addition to the usual Internet access. And it could become a great 4G cell phone option. Look for WiMAX in your cell phone, laptop, and set-top box in the future.


Analog Devices Inc.
Aptero, Texas Instruments
Fujitsu Microelectronics America
Intel Corp.
Redline Communications
National Science Foundation
WiMAX Forum

TAGS: Mobile
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