Expect further WiMAX growth as the government pushes stimulus money into broadband access for the more remote, underserved areas in the U.S. Cable TV companies such as Comcast and Time Warner are also adopting WiMAX for BWA services in some parts of the country.
The U.S. uses the 2.3- and 2.5-GHz bands for WiMAX, and Korea uses 2.7 GHz. The rest of the world utilizes the 3.5-GHz bands, though the U.S. has an assignment in the 3.65-GHz band, and assignments in the newly opened 700-MHz spectrum are available. While most frequency assignments fall in the 2- to 11-GHz range, WiMAX has assignments in the 11- to 66-GHz range as well. These see limited use, though, and it will likely remain that way.
WiMAX uses OFDM and OFDMA with FFT sizes of 256 and 1024 subchannels on the 802.16d fixed version. For scaling in the 802.16 mobile version, it uses those sizes plus 128-, 512-, and 2048-subchannel versions. Typical bandwidth sizes are 3.5, 5, 7, 10, and 20 MHz for 802.16d, and 5, 8.75, 10, and 20 MHz for 802.16e.
Modulation methods include binary phase-shift keying (BPSK), QPSK, 16QAM, and 64QAM. Modulation is adaptive to the range and other environmental conditions. WiMAX also incorporates the MIMO capability of the 2x2 Tx/Rx format. Total cell capacity is in the 100- to 200-subscriber range. Typical spectral efficiency is 3.75 bits/Hz.
WiMAX is primarily a data service. Implementation of VoIP over WiMAX is occurring, but it’s more of a fixed service than an option for mobile handsets. The best way to think of WiMAX is as a super-long-range version of Wi-Fi. It’s not as fast, but it has a range of many miles. You will see WiMAX USB dongles, data cards, and even embedded WiMAX in laptops and netbooks. Development of WiMAX femtocells sits on the horizon, too.
Data rates depend on the service you buy, the bandwidth, the modulation, and other factors, but they typically run from about 1 to 2 Mbits/s in common consumer installations. Maximum rate is about 75 Mbits/s under maximum bandwidth and other conditions. Range extends from one to five miles depending on basestation placement and numbers.
For backhaul, maximum range is about 30 miles under ideal conditions. As for duplexing, either FDD or TDD can be used, but most WiMAX is TDD. Finally, the IEEE is currently working on a newer version called 802.16m that promises data rates from 100 Mbits/s to 1 Gbit/s. This is a good matchup with the ITU’s IMT Advanced requirements. WiMAX hopes to be part of that definition.
PRODUCTS OF NOTE
Analog Devices offers a line of RF products that can be used to implement LTE or WiMAX. Its AD935x transceivers are designed for both WiMAX and LTE customer premise equipment (CPE) and terminal implementations, as well as small basestations like femtocells and picocells.
The newest versions from the company, such as the AD9356 and AD9357, operate from 2.3 to 2.7 GHz and 3.3 to 3.8 GHz, respectively. Both feature dual transmit and receive chains (for MIMO) and include on-chip 12-bit analog-to-digital converters (ADCs), digital-to-analog converters (DACs), and frequency synthesizers. The ADRF670x and ADRF660x also address this space (Fig. 1).
The ADRF670x is an analog I/Q modulator with an RF output switch and phase-locked loop (PLL) with an integrated voltagecontrolled oscillator (VCO). Featuring a bandwidth to 500 MHz, it’s designed for use in IF upconversion transmit signal paths. It has high linear output power as well. The ADRF660x is a line of active RF mixers for receiver path downconversion. They include an RF input balun for single-ended 50-O input and a PLL synthesizer with integrated VCO. The differential IF output supports frequencies to 500 MHz.
The PC9608/9 LTE development system from picoChip targets small-form-factor LTE basestations optimized for metro, enterprise, and residential applications. It promises to expedite time-tomarket for basestation designs.
The system, which integrates baseband, software stack, and RF, allows designers and carriers to test the deployment of new network architectures. Many analysts and operators believe that LTE will require a dense network of small cells optimized for high-capacity data services to deliver on its full potential.
Most LTE implementations will be with standard macro basestations. However, operators realize that the higher-frequency assignments of most LTE will require more smaller basestations. That includes home femtocells.
The PC9068/9 incorporates a carrier-class software-defined LTE modem that complies with 3GPP standards and can be rapidly optimized and customized via an application programming interface. The system uses picoChip’s PC203 multicore picoArray silicon and supports the TDD and FDD versions of LTE (Fig. 2).
Wavesat’s Odyssey 9000 targets handsets, dongles, data cards, and other mobile wireless devices. The integrated systemon- a-chip (SoC) offers a fully programmable architecture that can easily adapt to LTE’s evolving standards. It also can easily implement WiMAX and the Japanese XGP equivalent. The hybrid chip combines highly efficient DSPs and hardware acceleration blocks that easily support the LTE 100-Mbit/s downlink and later 150-Mbit/s versions.
The first chip version, the OD9010, comes with a complete LTE protocol stack, including MAC, PDCP, RRC, and NAS layers. A reference design is also available. The OD9050 will offer 3G and LTE within the same chipset.
“We are working now with a few lead operators and OEMs to have fully interoperable LTE devices in place to meet the demand as LTE networks go operational worldwide beginning next year,” says Rah Singh, president and CEO of Wavesat.
REFERENCE
Agilent Technologies recently published a great new textbook on LTE called LTE and the Evolution to 4G Wireless, Design and Measurement Challenges. It was written by a group of 30 Agilent engineers and scientists. This up-to-date book addresses almost everything you need to know about LTE. Go to www.agilent.com to get a copy.