Microwave Engineering Europe - December 2007 - (Page 16) 16 TEST & MEASUREMENT — OFDM Orthogonal frequency division multiplexing By Mark Elo, Marketing Director of RF Products, Keithley Instruments rthogonal frequency division multiplexing (OFDM) is a form of digital modulation used in a wide array of communications systems. This paper will explain what OFDM is, why it’s important, where it’s used, and what test instrumentation is required to maintain it. Perhaps we should first explain what is so special about OFDM. Three things stand out. OFDM is spectrally efficient, carrying more data per unit of bandwidth than such services as GSM and W-CDMA. Figure 1 shows a comparison of the spectral efficiency of the leading cellular technologies and how they compare to WLAN and WiMAX. Fourth Generation technology, often referred to as the Long Term Evolution of wireless (LTE) and Ultra Mobile Broadband (UMB) for cellular devices, plans to use OFDM or OFDMA. OFDM tolerates environments with high RF interference. Some services that use OFDM — such as WLAN — operate in the unregulated ISM (Industrial Scientific Medical) bands, where they must co-exist with many unregulated devices, including analog cordless phones (900 MHz), microwave ovens (2.45 GHz), Bluetooth devices (2.45 GHz), digital cordless phones (2.45 GHz or 5.8 GHz) and Wireless LAN (2.45 GHz or 5.8 GHz). Finally, OFDM works well in harsh multi-path environments, as we shall see. Digital modulation overview Most forms of digital transmission involve modulating a pair of summed sine waves that differ in phase by 90°. The modulation signal can be represented by the vector sum of the in-phase (I) and quadrature (Q) components, as shown in Figure 2. There are many ways to encode digital information in this way. If you change the phase relationships between the two sine waves, the result is called phase shift keying (PSK). A common type of PSK is quadrature phase shift keying (QPSK), which uses four phases; if eight phases are used, the result is 8PSK. If you vary both the amplitude and phase of the two sine waves, the result is quadrature amplitude modulation (QAM). O Figure 1: The spectral efficiency of the leading cellular technologies and how they compare to WLAN and WiMAX. Fourth Generation technology, often referred to as LTE, or the Long Term Evolution of wireless for cellular devices, will use OFDM. Figure 2: Most forms of digital transmission involve modulating a pair of sine waves that differ in phase by 90°. The modulation signal can be represented by the vector sum of the in-phase (I) and quadrature (Q) components. Figure 3: Constellation diagrams for several types of modulation. Microwave Engineering Europe ● December 2007 ● www.mwee.com 016_017_018_020_022_MWEE.indd 16 22/11/07 16:53:00 http://www.mwee.com
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