| The Federal Communication Commission's 1985 ruling (rule modified in 1990) allowed unlicensed spread spectrum use of the three Industrial, Scientific and Medical (ISM) frequency bands. This ruling encouraged development of a number of wireless technologies. Today, many wireless local area network (WLAN) products occupy all three ISM bands and create a level of interference. In the next 5--10 years, even more ISM applications will emerge, creating more interference. Thus much more efficient utilization of the spectrum will be required. A promising modulation scheme adopted by the IEEE 802.11a WLAN is orthogonal frequency division multiplexing (OFDM), which provides high-speed data transfer with spectrum efficiency and reduced complexity. However, compared to single-carrier systems, OFDM has a larger peak-to-average power ratio (PAPR), which renders OFDM systems very sensitive to nonlinear characteristics in RF power amplifiers. The direct result is severe spectrum regrowth in OFDM-based WLAN systems, (i) conflicting directly with the goal of achieving high spectrum efficiency, (ii) increasing the bit error rate, and (iii) interfering with adjacent channels. Therefore, when working to limit spectrum regrowth in OFDM-based WLAN systems, analyzing the efficiency-linearity trade-off by modeling the nonlinearity is a critical design issue.; In this dissertation, the author develops a comprehensive spectrum approach to analyze OFDM-based WLAN systems. The author analyzes the spectrum regrowth caused by the effects of nonlinearity in RF power amplifiers in OFDM-based WLAN systems, presents an analytical relationship between spectrum regrowth levels and RF power amplifier intermodulation, proposes a theoretical method to predict the spectrum regrowth of OFDM-based WLAN systems, and derives explicit expressions relating the out-of-band power emission levels of an amplifier to its nonlinear parameters.; This research enables OFDM-based WLAN system developers to specify and measure spectrum regrowth effectively using simple RF power amplifier intermodulation descriptions and to enable spectrum administrators to plan and manage spectrum resources efficiently. This spectrum analysis can be further extended to other OFDM-based wireless systems, such as digital broadcasting systems, future fourth generation cellular systems, and ultra wideband systems. |
