RF System and Circuit Challenges for WiMAX

Broadband Wireless Access has occupied a niche in the market for about a decade, but with the signing of the 802.16d standard it could finally explode into the mass market. Intel’s baseband transceiver chip is flexible enough to accommodate Radio Frequency Integrated Circuit (RFIC) architectures of today and the future. With the emergence of this standard an ecosystem is developing that will allow multiple vendors to produce components that adhere to a standard specification and hence allow large-scale deployment. One of the major challenges of the 802.16d standard is the plethora of options that exist; Worldwide Interoperability Microwave Access (WiMAX) will address this issue by limiting options and hence ensure interoperability. The result will allow manufacturers of Radio Frequency (RF) components and test equipment to have their products used for mass deployment. In this paper, we focus on the various RF challenges that exist on a RF system-level and show how such challenges can translate into circuit designs. The RF is made more complicated by the fact that WiMAX indeed addresses wireless markets across the world both in licensed and unlicensed bands. Thus, solutions have to be flexible enough to allow for the many RF frequency bands and different regulations around the globe. Several major RF architectures are discussed and the implications for WiMAX specifications are explored, in particular both Intermediate Frequency (IF)and I/Qbased structures are investigated. Part of our discussion will provide insight into the cost and performance tradeoffs between Time Division Duplex (TDD) and Frequency Division Duplex (FDD) systems both in licensed and unlicensed bands. It is generally accepted that TDD systems offer cost advantages over their FDD counterparts; however, most licensed bands intended for data applications operate with FDD systems in mind. Some of the RF subsystem blocks that have stringent WiMAX specifications are also elaborated upon: these include synthesizers, power amplifiers, and filtering. These fundamental subsystem blocks are where most of the transceiver costs reside; the same blocks are also responsible for most of the RF performance. The industry is moving towards using Orthogonal Frequency Division Multiplexing Access (OFDMA) and either spatial diversity or beam forming techniques to enhance link margins. We touch on the RF challenges associated with these techniques. Finally, we view some of the important WiMAX specifications for RF and the implications for the design of RF circuits, which include SNDR, channel bandwidths, RF bands, noise figures, output power levels, and gain setting. Some important differences between WiMAX and 802.11 RF specifications are also highlighted.