Electronic Design

WiMAX: The Real Deal

Broadband wireless gets ready for business.

Like any emerging technology, WiMAX (short for Worldwide Interoperability for Microwave Access) has taken years to become available to the public. Even so, it took less time than most new wireless technologies. It developed quickly from an idea to a formal standard to real products and services in just a few years. Many chips are now available, and products are beginning to flow. But best of all, several large carriers are beginning to offer broadband wireless services with big rollouts planned for 2008 and beyond.

The standard
WiMAX refers to a broadband radio technology defined by IEEE standard 802.16-2004 and 802.16e-2005. This standard defines an IP-based (Internet Protocol) wireless technology using orthogonal frequency-division multiplexing (OFDM) and multiple-input multiple-output (MIMO) in the 2- to 6-GHz microwave range to provide high-speed Internet access, e-mail, video, and other services. Its potential speed ranges from 1 to 20 Mbits/s, depending on the services offered by the provider. Using cell-phone-like basestations, coverage ranges over several miles.

The intended market for the original 802.16-2004 standard is a fixed service that provides high-speed Internet to areas lacking the usual DSL or cable TV access, like smaller cities and rural areas. It's also useful for backhaul applications like carrying Wi-Fi hotspot data back to the service provider. The mobile version, 802.16e-2005, provides equivalent broadband access while the user is in motion in a car or public transportation.

While Internet connectivity is the target application, WiMAX could carry Voice over IP (VoIP) phone calls, making WiMAX handsets or dual-mode cellular-WiMAX handsets a possibility. WiMAX also will be built into most new laptops, just like Wi-Fi. In fact, most laptops will probably have both wireless technologies.

The competition
WiMAX's main competition in the major metropolitan areas will be the DSL and cable TV companies. But in the wireless space, mobile competition will come from current cell-phone carriers offering 3G cell-phone services. AT&T, Sprint Nextel, and Verizon offer 3G (cdma2000 or WCDMA) plug-in modems for laptops, all of which can provide DSL-like speeds in a wireless setting.

As the interest in and adoption of WiMAX have grown, these carriers are now more aggressively selling their 3G data services, which run about $50 to $70 per month. Enhancements to 3G services include HSPA for AT&T's WCDMA systems and Rev. A and Rev. B versions of cdma2000 used by Sprint Nextel and Verizon, and these improvements will give WiMAX a run for the money.

Fourth-generation cell-phone services such as Long-Term Evolution (LTE) and the Ultra Mobile Broadband (UMB) enhancement to cdma2000 1xEV-DO will also compete with WiMAX, but they're a few years away (see "And They're Off! WiMAX, LTE, UMB, And The Race To 4GWireless," p. 18).

Wi-Fi is also a competitor-sort of. With hot spots and corporate access points everywhere these days, it's possible to connect to the Internet from most places where people congregate. Municipal Wi-Fi mesh networks have grown in number as well, giving Wi-Fi an edge in the broadband access space. But with that business slowing and Wi-Fi's inherently limited range, WiMAX is expected to find a sweet spot. A WiMAX link will allow far more range flexibility in addition to speed and mobility that Wi-Fi doesn't have.

Impediments to implementation
The key to launching any new wireless business is a good business model. Like cell-phone service, WiMAX requires a major business investment in infrastructure. While that investment is certainly less than a major cell-phone upgrade, it is still huge.

One of the main reasons WiMAX has been somewhat delayed is available spectrum, especially in the U.S. Most of the rest of the world has allocated 3.5 GHz for broadband wireless services. In the U.S., the primary spectrum availability is in the 2.3- to 2.5-GHz range, which is mainly held by the big cellphone carriers. Most WiMAX services will use that spectrum. But in January, the Federal Communications Commission will auction off 62 MHz in the 698- to 806-MHz range previously assigned to UHF TV.

The highest bidders will get this spectrum, and at least one of the winners is expected to offer a broadband wireless service of some kind, probably WiMAX. While WiMAX wasn't designed for this band, making it work there is relatively simple. Benefits include longer range and greater reliability, simply because physics says that the lower frequencies provide the greatest range and penetration. With the spectrum issues essentially resolved, WiMAX is ready for action.

Who offers what?
When you start seeing the promotions for WiMAX billing software that have been appearing recently, you can be sure the services are on their way. Perhaps the main WiMAX provider in the U.S. is going to be Sprint Nextel, which owns lots of 2.5-GHz spectrum for this service. The company is building out a nationwide network in the major markets. Known as XOHM, its mobile WiMAX service will begin next April.

Clearwire already is offering service in some parts of the country, including wireless VoIP. The company also has a joint agreement to work with Sprint Nextel to build out a nationwide network that subscribers of both companies can use. AT&T also owns spectrum in the 2.3-GHz range, covering areas in the southern U.S. that can be used for WiMAX. While AT&T won't confirm its commitment to WiMAX, it may likely offer fixed WiMAX service in areas where its traditional DSL service isn't implemented. AT&T also has a limited deployment of WiMAX in Alaska.

Surely AT&T will participate. But it may have some spectrum issues after selling its 2.5- GHz spectrum to Clearwire earlier this year. Another company, TowerStream, has been providing non-WiMAX broadband microwave access for years. It now plans to offer WiMAX services in its major market cities.

WiMAX has already been adopted in countries without the common Internet access services found in the U.S. India, Africa, and several other regions are seeing many new WiMAX installations. Korea's WiMAX equivalent, WiBro, is under way. Japan will move into the WiMAX space soon as well.

It seems that 2008 will be a WiMAX year. Some research firms are projecting a market for WiMAX between $3.3 billion and $3.9 billion by 2010.

Horn of plenty
Some WiMAX chips have been available for the past few years, but they target fixed applications. Most of these chips have now been redesigned and upgraded to achieve lower power consumption and include the features for the newer mobile WiMAX version. With lots of RF and baseband chips available, it's easier than ever to design WiMAX products.

Analog Devices introduced its AD9352 and AD9253 WiMAX transceivers last year. And recently, the company introduced its second-generation AD9354 and AD9355 WiMAX chips.

These transceivers target the potentially huge CPE market for modems, dongles, and PC cards. Both chips are made with 180-nm CMOS. The AD9354 targets the 2.3- to 2.7-GHz band, while the AD9355 serves the 3.3- to 3.7-GHz market. These transceivers use a zero intermediate frequency (ZIF) architecture that eliminates external surface acoustic wave (SAW) filters (Fig. 1). There are two full receive chains to take advantage of the MIMO feature that makes WiMAX so highly desirable. The noise figure is a low 3.25 dB, and the transmit chain has an error vector magnitude (EVM) of â??38 dBm with an output power of 0 dBm.

Interestingly, ADI has brought all the analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) normally in the baseband chip onto this RF chip. Instead of a standard analog I/Q interface to the baseband circuits, the interface is digital. This new digital interface was unique to Analog Devices, which called it ADI/Q. But JEDEC now has standardized it as JESD207.

Several other transceiver manufacturers have announced support for this interface, which greatly reduces power and chip sizes and simplifies the baseband chip itself. The on-chip data conversion and the second receiver chain help the mobile performance of these WiMAX devices.

Other features include automatic gain control (AGC), a power detector for automatic closed-loop transmit power control, an on-chip digitally compensated crystal oscillator (DCXO) with synthesizer and loop filter, and a temperature sensor. These devices are housed in an 8- by 8-mm, 56-pin lead-frame chipscale package (LFCSP). Samples are available now for $11.45 with full production expected in December.

ApaceWave Technologies' APW-2000 is a system-on-a-chip (SoC) baseband processor for mobile WiMAX customer premise equipment (CPE) products, PC cards, and the like. It includes the physical layer (PHY) and media access controller (MAC), an embedded CPU, and multiple interfaces to support the various external chips. It also includes full MIMO and beam forming. Apace- Wave's IP portfolio of over 20 patents includes algorithms and architecture features engineered for low power consumption and high data rate in full mobile mode.

Formerly Motorola Semiconductor, Freescale has a long history of making cell-phone basestation and other RF power amplifiers. Building on that experience, it announced three high-power LDMOS RFICs for WiMAX: the MW71C2725N and MW71C2750N for the 2.3- to 2.7-GHz band, and the MW71C3825N for the 3.4- to 3.7-GHz band (Fig. 2). These multistage amplifiers operate class AB and have a huge operating bandwidth out to about 200 MHz.

The MW71C2725N features 25 W of continuous-wave (CW) output power at the 1-dB compression point and at 4 W of average power. The overall gain is 28 dB with a 17% efficiency. Its adjacentchannel power ratio (ACPR) at 5.25- MHz offset is â??50 dBc with a 500-kHz channel bandwidth.

The MW71C2750N features 50 W of CW output power at the 1-dB compression point and at 8 W of average power. The overall gain is 26 dB with a 17% efficiency. Its ACPR at 5.25-MHz offset is â??49 dBc with a 500-kHz channel bandwidth.

The MW71C3825N for the 3.5-GHz band features 25 W of CW output power at the 1-dB compression point and at 5 W of average power. The overall gain is 23.5 dB with a 15% efficiency. ACPR at 5.25-MHz offset is â??49 dBc with a 500- kHz channel bandwidth.

The amplifiers have full interstage matching networks. They operate from 28 to 32 V dc and can handle a voltage standing-wave ratio (VSWR) up to 10 to 1 while delivering their rated CW output power. The devices can also be paralleled and their outputs combined to produce even higher power, or they may serve as a driver of an even higher-power discrete power amplifier.

Its innovative overmolded plastic package has very tight mechanical tolerances so designers can maintain the high manufacturing yields required at WiMAX frequencies. Electrostatic-discharge protection is built in. Samples are available now, and full production will be available in the first quarter of 2008.

Fujitsu's MB86K21 supports the new 802.16e mobile version of the standard. This SoC MAC and PHY is made with 90-nm CMOS. It includes 512/1024 fast Fourier transform (FFT) for the orthogonal frequency-division multiple access (OFDMA) PHY and support for 5- and 10-MHz channels with quadrature phase-shift keying (QPSK), 16QAM (quadrature amplitude modulation), and 64QAM.

Also, it implements a 2x2 MIMO scheme and includes AES-CCM security. A card bus/PCI host interface and USB 2.0 are provided. The chip is designed for mobile devices like laptops and PC cards. Samples are available now, as is a system design kit and a PC card reference platform kit that can greatly shorten time-to-market.

Intel's widespread investments have been key to ensuring WiMAX's success. For example, Intel recently supported KDDI in Japan to build WiMAX systems. Its early PRO/Wireless 5116 chip was for the original 802.16-2004 fixed standard. Intel's newer chip, the 2250, covers the fixed and mobile versions.

The big news is Intel's chip, code-named "Echo-Peak," which is the first fully integrated Wi-Fi/WiMAX solution. This embedded module will become available in selected Intel Centrino processor technology- based notebooks in 2008.

Navini Networks' Surfer 1000, 2000, and 3000 series of mobile WiMAX modems incorporate MIMO as well as Navini's unique beamforming technology. While most WiMAX products will use some form of MIMO, few will offer beamforming, an antenna technology that can greatly extend the range of signals for higher data rates as well as more reliable connections under difficult mobile conditions. Navini also makes WiMAX basestation equipment that includes the beamforming antennas.

PMC-Sierra also has some interesting WiMAX products that target both the femtocell basestation and CPE markets (Fig. 3). Its WiZIRD family includes several highly integrated MIMO RF ICs for WiMAX. The PM8800 is a 2Tx/2Rx MIMO chip that covers both the 2.3- to 2.7-GHz and 3.3- to 3.7-GHz bands in a single IC. That 2x2 MIMO feature provides a maximum downlink data rate of 63 Mbits/s per sector and a peak uplink data rate of 28 Mbits/s per sector in a 10-MHz channel.

This ZIF transceiver is compatible with both the fixed and mobile WiMAX standards. It has an integrated DCXO with automatic frequency control (AFC) and analog-to-digital and digital-to-analog converters. The PM8800 also supports the new digital parallel I/Q JEDEC JESD207 interface. Analog I/Q capability is provided as well. Its dual-band capability should aid carriers in offering global roaming with different spectrum.

The PM8801 IC is a smaller 1Tx/2Rx MIMO version with essentially the same features. With only one transmit path, it consumes less power, so it's more suitable for portable devices and laptops. Both chips are CMOS devices with 108-pin maximum load factor/quad flat no-lead (MLF/QFN) packages.

Sequans Communications' SQN2130 is the industry's first MAC/PHY chip for mobile WiMAX basestations. The ASIC features Sequans' patent-pending technology called mimoMAX that allows the implementation of full 2x2 MIMO. The chip easily fits any size basestation, from small femto cells to multisector macro cells. It accommodates time division duplexing (TDD) or frequency division duplexing (FDD) and can sustain the full line rate of more than 35 Mbits/s even with small packet sizes. Sequans also makes the SQN1130 chip for WiMAX subscriber products.

Sychip Inc., a subsidiary of muRata, has a new WiMAX embedded module. The WiMAX95xx is a chip-scale module with an 80-pin no-lead QFN package that allows direct attachment to a main pc board. This module makes it very easy for manufacturers of laptops, handsets, media players, navigation devices, and other portable products to incorporate WiMAX.

The WiMAX95xx is a complete WiMAX 802.16e-2005 mobile standard transceiver, offering an RF chip, MAC/ baseband chip, power amplifier, EEPROM, and DDR-SDRAM. It uses a 40-MHz clock with SDIO, USB, and GPIO interfaces.

Its software includes drivers and application layer interfaces that give content providers, manufacturers, and OEMs the flexibility to integrate and optimize their respective applications. The software also supports Linux, Windows Mobile, Nucleus, and VxWorks operating systems. Engineering samples are available now, and production is scheduled for the second quarter of 2008.

The newest member of Wavesat's portfolio of WiMAX products has a Mini- PCI design that targets the low-cost mass-market indoor and outdoor CPE space. The design is based on Wavesat's Evolutive NP7256 ASIC, which substantially reduces overall size and bill of materials (BOM) and includes all RF and baseband components in a small footprint with low power consumption.

This design addresses the fixed and nomadic standard but can be easily upgraded to support the 802.16e-2005 mobile standard. OEMs and ODMs can quickly build subscriber units with a minimum investment in time and money.

Based on silicon germanium (SiGe) instead of the usual gallium arsenide (GaAs), VT Silicon's latest RF power amplifiers for WiMAX incorporate the company's linear enhancement technology (LET), a distortion prevention technique that permits higher power levels with greater efficiency. WiMAX's broadband OFDM technique requires a wideband power amplifier that is as linear as technology permits. These new amplifiers, one for 2.5 GHz and another for 3.5 GHz, will provide a balance of high power, efficiency, and linearity. They are expected to be available in the near future.

Testing: one, two, three
You can't build a WiMAX product or get it certified by the WiMAX Forum without testing it. And, you couldn't face a tougher RF testing job than a WiMAX product. Thankfully, the test gear companies are on top of the challenge.

Agilent's one-box test set for mobile WiMAX can serve as the main test system for R&D, verification, and manufacturing. It's designed for ODMs testing mobile WiMAX subscriber devices, modules, and related products.

The N8300A wireless Networking Test Set is based on an architecture that integrates Agilent's state-of-the-art vector signal analysis and vector signal generator (VSG) hardware into a single package with an easy-to-use front-panel GUI and SCPI command set. The N6301A OFDMA Measurement Application software that accompanies the N8300A contains measurement algorithms and software leveraged from WiMAX R&D test tools and enables fast test development and measurement traceability throughout the product lifecycle.

Azimuth Systems' ACE 400WB mobile WiMAX test system implements a WiMAX channel emulator that permits comprehensive performance testing of WiMAX SoC products for 802.16e applications. Its sophisticated channelmodeling capabilities enable real-time testing of MIMO devices and speed the development of WiMAX chip sets, CPE products, and infrastructure equipment.

The mobility, high throughput, and extended range of mobile WiMAX depend on the multipath environment for MIMO spatial streaming and advanced antenna techniques. Azimuth's bi-directional and reciprocal channel modeling provides this real-world test scenario in the lab.

LitePoint's IQmax MIMO targets mobile WiMAX product testing from development to quality assurance. Two IQmax-500 hardware units are at the heart of the system (Fig. 4). Each contains a VSG and vector signal analyzer. Add to this the new IQsignal software, which includes a user-friendly GUI, and you have a full WiMAX 2x2 MIMO test system. You can add more units to get to a 4x4 system or beyond as needed.

The IQwave software is used to create arbitrary WiMAX waveforms, which generate actual baseband or RF signals after being loaded into the IQmax's VSG. IQwave can generate fixed or mobile WiMAX waveforms for both single input/single output (SISO) or MIMO testing.

TAGS: Mobile
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