Wireless Systems Design

Make Way For WiMAX Certified Products

This year, the market will welcome the first wave of broadband-wireless products that were built to comply with the Worldwide Interoperability for Microwave Access (WiMAX) IEEE 802.16-2004 standard. WiMAX was developed to support the compatibility and interoperability of broadband-wireless-access (BWA) equipment. It supports many wireless-broadband connections including the following: high-bandwidth metropolitan-area networks (MANs) for home and small-business users, backhaul networks for cellular base stations, and backhaul connections to the Internet for Wi-Fi hot spots.

Where they exist today, these applications use expensive, proprietary methods for broadband access. In contrast, WiMAX is based on interoperability-tested systems that were built using the IEEE 802.16-2004 standard-based silicon solutions. As a result, WiMAX will reduce costs. As the upcoming IEEE 802.16e becomes available for wireless MANs, WiMAX-certifiable mobile products will follow on a gradual basis. Using non-line-of-sight propagation, products like laptops, PDAs, and cell phones will deliver services directly to the end users in a point-to-multipoint architecture.

The WiMAX (IEEE 802.16) standard defines profiles for the media-access-control (MAC) and physical (PHY) layers. The MAC packs and unpacks raw data, while the PHY handles the air-interface and modulation schemes based on subscriber needs and radio-frequency (RF) link quality. The standard also includes flexibility to allow system vendors to customize their products in order to meet specific requirements.

Because variations in the RF interface will impact deployment, spectrum-governing authorities will determine the usable spectrum for various services. They also will allow portions of the spectrum to serve a specific segment. While a common RF ground exists, there is significant diversity in spectrum allocation and regulation. This diversity results in the demand for RF-diverse base stations and subscriber stations. A generic WiMAX subscriber system includes a control processor, MAC unit, baseband processor (BBP), and analog RF front end (FIG. 1). That front end places 802.16x into a specific licensed or unlicensed band.

Initial WiMAX implementations are focused on three bands in the RF spectrum (FIG. 2). The 3.5-GHz band is popular outside of the U.S. It is the most heavily allocated band, representing the largest global BWA market. It covers 300 MHz of bandwidth from 3.3 to 3.6 GHz. The spectrum supports large-pipeline backhauling to wide-area-network services.

The second band is the 5.8-GHz band, which has a range of 5725 to 5850 MHz. This band also is known as the upper Unlicensed National Information Infrastructure (U-NII) band. Many overlapping 5-GHz frequency bands are earmarked for BWA growth worldwide. The World Radio Conference's 5470-to-5725-MHz band adds significant license-exempt bandwidth. Yet most WiMAX activities are in the upper U-NII band, in which there are fewer competing services or interferences.

The third band is Multichannel Multipoint Distribution Service (MMDS). Two frequency ranges reside in MMDS: the 2500-to-2690-MHz band and the 2700-to-2900-MHz band. MMDS spectrum includes 31 channels of 6-MHz spacing in the first range. It also includes the Instructional Television Fixed Service (ITFS), which was underutilized and reallocated for BWA service in the U.S. In the longer term, other bands may be useful. Examples include the two Wireless Communications Service (WCS) bands and the 2.4-GHz Industrial, Scientific, and Medical (ISM) band.

Initially, the WiMAX Forum is focusing its profiling and certification on the MMDS, 3.5-GHz licensed, and unlicensed upper U-NII 5-GHz bands. The upper U-NII band boasts less interference, reasonable power levels, and adequate bandwidth. Frequency bands in the 2-to-6-GHz portion have relatively narrow allocated bandwidths. The microwave frequencies below 10 GHz are referred to as centimeter bands. Above 10 GHz, they're known as millimeter bands. The millimeter bands have much wider allocated channel bandwidths to accommodate the larger data capacities that are suitable for high-data-rate, line-of-sight backhauling applications. The centimeter bands are best for multipoint, near-line-of-sight (NLOS), tributary, and last-mile distribution.

The centimeter spectrum contains both tributary and last-mile potential. IEEE 802.16-2004 supports fixed-NLOS BWA to supplant or supplement DSL and cable access for last-mile service. For spectrums below the 6-GHz range, however, IEEE 802.16e will add mobility and portability to applications like notebooks and PDAs. Both licensed and unlicensed spectrums will be utilized in these deployments. 802.16e is tentatively scheduled to be approved in the second half of this year.

In any WiMAX network, power levels and control for both transmit and receive are important for system efficiency. To ensure successful communication, the levels must be actively managed. Power levels are dynamically adjusted on a per-subscriber basis, depending on the profile and distance from the base station.

Receive-level specifications are the same across the centimeter bands: 2 to 11 GHz. The receiver must be able to accurately decode an on-channel signal of −30 dBm (1 µW) maximum. It must be able to tolerate a signal as strong as 0 dBm (1 mW) at the input without damage to the front end.

In addition, the Rx should provide a minimum image rejection of 60 dB. The standard specifies that "the image-rejection requirement be inclusive of all image terms originating at the receiver RF and subsequent intermediate frequencies."

The subscriber stations that don't utilize subchannels (single carrier) must exhibit a minimal 30-dB range of monotonic power control. Those stations that do utilize subchannels (OFDM) comprise a category that will include all WiMAX-certified stations in the 2-to-11-GHz range. For those stations, the transmitter must have a dynamic-power-control range of at least 50 dB in no less than 1-dB steps. Power-control accuracy must be within +/−1.5 dB over a 30-dB range or +/−3 dB over ranges greater than 30 dB.

For the base-station transmitter, the output-power-level control must have at least a 10-dB range. The actual transmitted power will depend on the subscriber distance, propagation characteristics, channel bandwidth, and modulation scheme (BPSK, QPSK, 16QAM, or 64QAM). The least data-efficient method is BPSK. Because it is employed where the subscriber station is farthest from the base, BPSK requires additional transmit power. 64QAM offers high data efficiency, which is best when the subscriber station is closer to the base station.

The interface between the RF front end and the system-on-a-chip (SoC) incorporates control signals for transmit and receive operations and housekeeping. It also houses I/Q signals to interface with analog-to-digital and digital-to-analog data converters. The receive data that is delivered by the demodulator circuit to the SoC should be differential "I" and "Q" signals. Attenuators can be employed on the receive side to handle calibration and gain control. They will ensure maximum bit usage and the conversion efficiency of the ADCs.

For the SoC that is under development at Fujitsu, these aspects are being thoroughly modeled using a comprehensive set of tests at various stages of development. Based on the 802.16 specification, simulation models are generated to verify the functionality and performance of the chip from mathematical models to the final gate level. In parallel with system-level simulations, a complete WiMAX system based on multiple FPGAs is developed to provide real-life verification and performance measurements. This system's intended use is for WiMAX conformance and interoperability testing in conjunction with the SoC.

WiMAX-based systems deployment will begin later this year. Suppliers can expect steady, reliable growth that will focus initially on markets in China, south Asia, India, and some of South America. This growth will then move gradually into North America and Europe. With its potential to replace expensive, proprietary broadband wireless—and with an evolutionary pathway already established—WiMAX appears poised for success.

TAGS: Mobile
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