Data-intensive wireless applications have sped the development of multicarrier modulation techniques, such as orthogonal frequency-division multiplexing (OFDM). OFDM can overcome many problems that arise with high-bit-rate communications, such as time dispersion.^1-4

Because OFDM is becoming so widespread, with applications in European Hiperlan, US wireless local-area networks (WLANs, such as IEEE 802.11a/g standards), in wireless metropolitan-area networks (WMANs), and especially in emerging WiMAX applications, it would be useful to understand the workings of a WiMAX transceiver design and how to test it. What follows is the first part of a three-part series on WiMAX transceiver design and performance evaluation strategies.

In OFDM-based WiMAX systems, carrier frequencies are chosen to avoid interference with and from other carrier frequencies. The data-bearing symbol stream is split into several lower-rate streams and these are transmitted on different carriers. Since this increases the symbol period by the number of nonoverlapping carriers (subcarriers), multipath will only affect a small portion of the neighboring symbols. The remaining intersymbol interference (ISI) can be removed by cyclically extending the OFDM symbol.⁵ The length of the cyclic extension should be at least the maximum excess delay of the channel. In this way, OFDM reduces the effect of multipath channels encountered with high data rates and avoids the use of complex equalization schemes.

Recently, OFDM has been applied to WMAN systems for fixed wireless access. The IEEE 802.16-2004 WMAN standard,⁶ which operates in bands between 2 and 11 GHz and higher, specifies a metropolitan-area networking protocol that will enable a wireless alternative to cable, DSL, and T1 services for last-mile broadband access as well as providing backhaul for IEEE 802.11 WLAN hotspots. In addition, efforts are underway to adopt OFDM for mobile WMAN,⁷ fourth-generation (4G) cellular systems, and wireless personal-area networks (WPANs).

While OFDM is an effective modulation method, it has its deficiencies. Several impairments can degrade the performance of OFDM if the system and its transceivers are not properly designed. Practical integration of RF and baseband circuitry in these systems requires realistic testing and verification procedures and equipment. This involves incorporation of testing and measurement capabilities of the standard based WiMAX signals into the measurement receivers (such as vector signal analyzers). Representative test signals, including standards-based in-phase (I) and quadrature (Q) baseband samples, must be generated for use with test equipment such as the Signature signal analyzer from Anritsu Co. (www.us.anritsu.com).

More effective WiMAX transceivers can be designed by incorporating test mechanisms into the transceiver integrated circuits (ICs). For the purpose of this investigation, digital baseband transceivers for IEEE 802.16-2004 WMAN system will be studied. By examining the block diagram of a standard transmitter and a receiver proposed by the authors, measurement points can be identified and evaluated for their impact on performance. This study will review transceiver performance under various noise and impairment scenarios, and various measurement techniques for identifying different sources of noise and impairments.

The IEEE 802.16-2004 standard defines the physical-layer (PHY) and media-access-control (MAC) protocols for broadband wireless access (BWA). The standard introduces three major air-interface formats for frequencies below 11 GHz, namely: WirelessMAN-SC PHY (single carrier modulation), WirelessMAN-OFDM PHY (OFDM-256), and WirelessMAN-OFDMA PHY (OFDMA-2048). Among these three options, the WiMAX Forum has selected OFDM-256, which will be the focus of this study.

The standard defines different duplexing options, including time-division duplex (TDD), frequency-division duplex (FDD), or half-duplex frequency-division duplex FDD (H-FDD) formats. In licensed bands, the duplexing method shall be either TDD or FDD, while unlicensed operation is limited to the TDD format.

The OFDM-256 air interface offers many adaptive features as well as optional features. The adaptive features include the adaptation of modulation/coding format and adaptation of the cyclic prefix. A receiver should be able to demodulate a signal with different modulation formats as well as a signal with various transmission parameters. The standard includes some optional PHY features, such as space-time-coding (STC), adaptive antenna arrays, and subchannelization. It is unlikely that these optional features will be implemented in early products, and were not high in priority when developing a measurement strategy for OFDM-256. Therefore, this study includes only the required transceiver specifications, with optional features left for future study.