Electronic Design

The WiMAX Wait Is Over

A parade of new chips helps accelerate the development and infiltration of this promising wireless broadband network.

Things tend to occur in threes. Add broadband access methods to the list, with WiMAX—the wireless broadband metropolitan networking technology—now bursting onto the scene. Competing with cable and DSL, WiMAX (Wireless interoperability for Microwave Access) passed through its early development and standardssetting phases and is emerging from the product design stage into the real world of applications.

Some pre-WiMAX and WiMAX-like systems are out there now, but look for real products later this year and early in 2006. Expectations run high, as is the case for any new and promising technology. But it remains to be seen whether this superior wireless system will be adopted by the mainstream or end up as a niche for select wireless needs.

Initially, WiMAX was developed as a broadband access technology for metro-area networks (MANs). The IEEE standardized it as 802.16-2004 (802.16d), primarily for fixed (not mobile) point-to-multipoint (PMP) and point-to-point (P2P) services between homes and/or businesses and a nearby basestation, such as a cell site.

The frequency of operation between 2 and 11 GHz depends on geography. The most common bands are 2.3 to 2.5 GHz and 5.8 GHz in the U.S. and 3.5 GHz in Europe and Asia. Both licensed and unlicensed spectra are available.

Maximum data rate is 75 Mbits/s. A majority of systems will allocate that in lower-rate segments, depending on bandwidth and application. Channel bandwidth can be set from 1.75 to 20 MHz. The standard uses time-division duplexing (TDD) or frequency-division duplexing (FDD), whichever best fits the application.

FDD provides full duplex, but it's most often found in licensed spectra where paired frequencies are available. TDD is used for unlicensed applications. An adaptive modulation scheme adjusts the modulation method, which relies on the distance from the basestation, noise, multipath, and other typical wireless conditions. It also includes binary phase-shift keying (BPSK) and quadrature phase-shift keying (QPSK) for longer-range connections and 16QAM or 64QAM for shorter ranges.

The basic access mode is 256-point orthogonal frequency-division multiplexing (OFDM). Using OFDM will mitigate the multipath problems and solve the line-of-sight (LOS) problem associated with earlier microwave access problems. In addition, consumers can use either an indoor antenna or a simple outdoor antenna that doesn't require precise orientation.

Maximum transmission range is about 30 miles, but typical basestation radii will be more like 2 to 10 km. Full power control at both the basestation and the customer premises equipment (CPE) is implemented to optimize the signal for each subscriber. The system is compatible with the newer sectorized, adaptive, and beamforming antennas.

In a fixed service, a single basestation is expected to handle a few dozen T1/E1-like connections for businesses and several hundred consumer connections that have rates comparable to existing DSL and cable lines. Though Internet access is the most likely application, WiMAX also can handle Voice over Internet Protocol (VoIP) and video.

Last-mile/first-mile service is expected to be WiMAX's main application, but whether it can compete head-on with the well-entrenched existing cable and DSL service is still unknown. Of course, it will be a boon to customers in urban areas beyond the reach of a DSL line or without cable service. Rural areas that lack both services also will benefit.

Another major application involves inexpensive backhaul for cell sites and Wi-Fi hot spots. Both of these typicallyuse expensive T1 lines. In addition to eliminating cables, the equipment and service will be much less expensive and easier to install and provision. Since that T1 connection is the biggest cost of an 802.11 access point, look for WiMAX to greatly expand the number of hot spots around the country.

While fixed services will be a welcome addition to the already sliced and diced wireless continuum, some experts predict that a mobile/portable version of WiMAX eventually will dominate. The IEEE is working to complete the 802.16e standard, which will provide mobile operation. So, WiMAXconnected computers may operate anywhere within the range of a WiMAX basestation.

Furthermore, such computers will be able to roam. That means handoffs will be implemented, making them more like cell phones. While range and data rate will be more limited than fixed sites, data rates up to 15 Mbits/s and a range of up to three miles are expected at traveling speeds up to about 60 mph. The access mode is orthogonal frequency-division multiplexing access (OFDMA) with up to 2048 points that can be divided into multiple mobile bands.

Laptops, PDAs, and cell phones will be targets for 802.16e, which is expected to compete with existing and forthcoming 3G data services. Some cell phones will feature WiMAX—yes, VoIP on a cell phone. Some competition with existing Wi-Fi systems is likely, too. Mostly, though, the two services will be complementary, with one service being available when the other is not.

A newer standard effort, potentially to be called 802.16f, should provide roaming and handoffs between Wi-Fi and WiMAX systems. Meanwhile, the 802.16e standard ratification is slated for late 2005 or early 2006. Certification testing of 16e products won't begin until late 2006. Fully operational systems aren't expected until 2007 and beyond. According to industry experts, the mobile version will be the key to WiMAX success.

Market research firm iSuppli Corp. projects conservative WiMAX equipment growth to $1 billion by the end of 2006 and $2 billion by 2009. Analyst Jagdish Rebello of iSuppli sees four key areas: last-mile broadband access, back-haul, and portable/mobile applications.

Rebello expects back-haul to be the initial winner, with broadband access to be determined. He also predicts growth and interest in the portable/mobile sector. And with a quickly encroaching saturation point, Steve Rago, iSuppli's broadband guru, expects broadband growth to slow this year and in the future. Nonetheless, WiMAX is sure to find a niche.

Roland Van der Meer of venture capital firm ComVentures says that a recent survey shows Wi-Fi/WiMAX eventually overtaking cellular by 2010. Asked which wireless access technology will dominate globally by 2010, telecom leaders' responses broke down as 46% WiMAX, 31% Wi-Fi, and 23% 3G. Van der Meer thinks the need for fixed-mobile wireless convergence will spur the development of an all-IP wireless infrastructure, i.e., 4G.

DESIGNING WIMAX RADIOS - Microwave broadband radios usually are designed with available monolithic microwave integrated circuits (MMICs) for the RF and ASICs or FPGAs for the baseband stuff. But with firm standards in place and the WiMAX Forum providing certific ation and interoperability, semiconductor companies responded with chip sets that make it a snap to design WiMAX CPEs and basestations.

For example, the Fujitsu MB87M3400 is a single-chip system-on-a-chip (SoC) media-access controller (MAC) and physical layer (PHY) that complies with the IEEE 802.16-2004 standard. It can be used in basestations or CPE-like settop boxes. Also, it works with existing RF chips and circuits in the 2.5-, 3.5-, and 5.8-GHz bands.

The I and Q analog signals to and from the RF chips go to on-board analog-to-digital and digital-to-analog converters (ADCs and DACs) for receive and transmit operations, respectively. Typical features include adaptive modulation schemes like BPSK, QPSK, 16QAM, 64QAM, and 256QAM. The chip supports all available channel bandwidths from 1.75 to 20 MHz.

Using 64QAM with all 192 OFDM subcarriers, the chip can hit a 100-Mbit/s raw data rate. The uplink subchannelization defined in the standard is supported. The chip, which can be set up to handle FDD or TDD applications, also features full on-chip security with DES/3DES or AES/CCM encryption/decryption.

At the heart of the device lies an ARM926 RISC processor, which implements the 802.16 upper-layer MAC, scheduler, drivers, and protocol stacks, as well as any special user application software. In addition, an ARC Tangent RISC processor handles all DSP functions, the lower-layer MAC functions, and the offloaded processing from the upper-layer MAC. Available interfaces include Ethernet, RS-232C, SPI, I2C, and GPIO. An integrated memory controller and DMA are also incorporated.

Intel's baseband chip, the PRO/Wireless 5116, targets the CPE market for licensed and unlicensed operation (Fig. 1). The modem segment conforms to the 802.16-2004 standard and supports the 256-point fast-Fourier-transform (FFT) OFDM. The chip can handle channels with bandwidths up to 10 MHz. TDD and half-duplex FDD are both implementable. All modulation modes are included, along with Reed-Solomon and convolutional encoding forward error correction (FEC).

Among the internal processing resources are dual-core ARM 946-S engines for PHY, MAC, and application protocol processing. A separate DSP engine integrates three ALUs that simultaneously handle the complex multiply and accumulate operations required for OFDM. Also in the mix are in-line encryption methods AES, 3DES, and RC-4.

As for the 5116's interfaces, there's a flexible IF as well as I/Q connections to almost any radio architecture. A pair of ADCs and DACs and a standard 10/100 Ethernet port are incorporated. A TDM interface for T1/E1 operation is standard.

Numerous semiconductor companies now offer WiMAX products for the RF front end. Long-time linear and RF supplier Analog Devices' range of products includes mixers, local oscillators (LOs), power detectors, ADCs, and DACs.

The AD8317, an RF logarithmic detector/controller, accurately measures the power of radio signals from 1 MHz to 10 GHz (Fig. 2). It also supports all cellular standards, including 3G. Power measurement accuracy runs better than ±1 dB over a dynamic range of 50 dB. The device can function as a power controller when its outputs are used to adjust a power amplifier or a VGA in the signal chain. Its fast 5-ns response time enables RF burst detection beyond 125 MHz.

The ADL5350 is a new mixer good for frequencies out to 3 GHz. Its high linearity, as measured by the third-order intercept (IIP3), is +26 dBm with a compression point of +17 dBm at 900 MHz. Noise figure is -6 dB.

Because it can be used for upconversion and downconversion, the ADL5350 makes a good choice for transmit and receive chains in WiMAX or cellular equipment. Its LO input buffers eliminate the need for any external LO amplification.

The AD9862 is part of ADI's Mixed-Signal Front End (MxFE) line, which is based on the company's "smart partitioning" methodology. This mixed-signal philosophy partitions the signal path according to performance rather than along analog-digital boundaries.

The device integrates two 12-bit, 64-Msample/s ADCs and two 14-bit, 128-Msample/s DACs. Other features in the transmit path include programmable gain amplifiers (PGAs), 2x and 4x interpolation filters, a digital Hilbert filter, and a digital mixer for complex or real-signal upconversions.

On the receive side, the ADCs can receive diversity or I/Q data at baseband or low IF. Input buffers, PGAs, and decimation filters are included, as is a programmable delay-locked loop clock multiplier. The AD9862 can be used in subscriber or basestation equipment.

Don't forget ADI's TigerSHARC DSP. It's available to handle the signal processing for the OFDM in WiMAX as well as all other baseband functions. ADI has partnered with Cygnus Communications to offer flexible software-defined radio designs for WiMAX.

FEELING SECURE - Security is essential in any wireless system. In fact, WiMAX mandates encryption. IC supplier Cavium Networks offers a range of security solutions for basestations and CPEs. Its Nitrox line of security processors is designed for these applications (Fig. 3). The Nitrox and Nitrox II processors match up with basestation designs, while the Nitrox Soho Secure Communications Processors fit CPE designs.

All of these products contain multiple MIPS32 processors, instruction and data caches, several security engines, 10/100 Ethernet MACs, 32-bit PCI interfaces, and a variety of general-purpose I/Os. They also support AES-256, DES, 3DES, and ARC4 algorithms for symmetric encryption. For asymmetric encryption, they provide RSA and Diffie-Hellman algorithms. Also supported are the SHA-1 and MD5 algorithms. These in-line security processors operate from 75 Mbits/s to 3 Gbits/s in basestations and from 54 kbits/s to 20 Mbytes/s in CPE. Another long-time RF chip supplier, Maxim Integrated Products, recently jumped into the WiMAX fray with its MAX2022 (Fig. 4). This direct upconversion quadrature modulator for baseband transmitters features a frequency range of 1500 to 2500 MHz. In addition to its WiMAX application, it can be used in UMTS/WCDMA, DCS/PCS, and CDMA2000 cell-phone basestations.

The MAX2022 consists of two matched passive mixers for modulation in-phase and quadrature signals, three LO mixer amplifier drivers, and an LO quadrature splitter. On-chip baluns are integrated to allow for single-ended RF and LO connections. To eliminate the need for costly I/Q buffers, the baseband inputs are matched to permit direct interfacing to the transmit DACs. Typical specs include +23.3-dBm OIP3, +51.5-dBm OIP2, 45.7-dBc sideband suppression, and -40-dBm LO leakage. Output power is -20.8 dBm.

SiGe Semiconductor's latest WiMAX chip set consists of the SE7051 IF transceiver, the SE7351L 3.5-GHz transceiver, the SE7251L 2.5-GHz transceiver, and the SE 7380L switch.

The SE7051L IF transceiver is a biCMOS device designed to interface directly to most commercially available baseband controllers. It features low noise and high linearity, and it contains selectable IQ or bandpass I/O interfaces. Operating range runs from 10 to 70 MHz, and there's need for only a single IF filter. Also included are dual IF and RF synthesizers as well as a high-speed digital VGA that delivers 50 dB of gain control.

Two versions of the RF transceiver are available, so designers can comply with most worldwide frequency assignments. The SE7351L covers the 3.3- to 3.7-GHz range used in Europe and some Asian areas, while the SE7251L handles the U.S.'s 2.3- to 2.7-GHz range. Both deliver low phase noise and high linearity necessary for a high-level modulation scheme like QAM in OFDM.

The receiver's IIP3 figure measures +5 dBm. The noise figure is less than 6 dB, including the switch and filter losses. Total RF gain is 100 dB with an automatic gain control (AGC) of 40-dB gain control range. The transmitter section is optimized for OFDM and supports HD-FDD as well as TDD operation.

These ICs are made with gallium-arsenide (GaAs) heterojunctionbipolar-transistor (HBT) technology. An external power amplifier, also available from SiGe, is required.

In this vein, the SE7380L is a GaAs pseudomorphic high electron mobility transistor (PHMET) single-pole doublethrow (SPDT) switch that's used for transmit-receive switching. It features low insertion loss and high linearity for maximum dynamic range with OFDM and high-level modulation schemes.

Another major RF chip supplier, Texas Instruments, recently announced a complete portfolio of RF chip sets for WiMAX and other wireless broadband applications (Fig. 5). They are the TRF11xx for 2.5 GHz, TRF12xx for 3.5 GHz, and TRF24xx for 5.8 GHz. They target basestation and CPE equipment for WiMAX and Korea's similar WiBro broadband wireless system.

The TRF1115 and TRF1216 receive (RX) chips include the low-noise amplifier (LNA) and downconverting mixer for 2.5- and 3.5-GHz receivers. These match up with the TRF1112 and TRF1212 second mixer, two LOs, and a programmable gain amplifier. The output at 44 MHz feeds the ADC.

On the transmit (TX) side, the TRF1121 or TRF1221 upconvert 25-MHz IF from the DAC at 2.5 and 3.5 GHz, respectively. Two LOs are included. The TRF1122 and TRF1222 provide a second upconversion. Separate power amplifiers (PAs) for higher output, the TRF1123 and TRF1223, also are available for 2.5 and 3.5 GHz. They can drive the antenna directly or serve as drivers for higher-power PAs.

The 5.8-GHz chip set is a bit different. This two-chip set consists of an RF and IF transceiver. The TRF2432 IF chip interfaces to the baseband ADCs and DACs. It works with the TRF2436 RF front end. A 374-MHz surface-acoustic-wave (SAW) IF filter separates the two chips. This chip set supports both the 5.8- and 4.9-GHz bands.

Other TI parts that fall into the WiMAX arena include the ADS5500 ADCs, the DAC5687 DACs, the GC5016 digital up/downconverter, and the GC1115 crest factor reduction chip. Moreover, TI's TMS320-TC16482 DSP chip runs at 1 GHz, making it a good fit with WiMAX signal processing.

No chips are currently available for 802.16e products, but some are on the way. Intel is working on such chips in cooperation with Nokia. Nokia sees some potential in incorporating WiMAX in some of its high-end cell phones. Adaptix, a basestation and CPE manufacturer, is rumored to be working on a mobile system that uses OFDMA-TDD. Finally, Philips Semiconductor has been focused on the mobile segment, with products expected next year.

NEED MORE INFORMATION

Analog Devices
www.analog.com

Cavium Networks
www.cavium.com

ComVentures
www.comventures.com

Fujitsu
www.fujitsu.com/us/services/edevices

Intel Corp.
www.intel.com

iSuppli Corp.
www.isuppli.com

Maxim Integrated Products
www.maxim-ic.com

SiGe Semiconductor
www.sige.com

Texas Instruments
www.ti.com

WiMAX Forum
www.wimaxforum.com

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