ORTHOGONAL FREQUENCY division multiplexing (OFDM) is at the root of WiMAX and many next-generation technologies. This form of digital modulation is spectrally efficient, as it carries more data per unit of bandwidth than modulation formats used in services like GSM and W-CDMA. It also tolerates environments with high RF interference. In addition, OFDM works well in hard multipath environments. In "Orthogonal Frequency Division Multiplexing," Keithley Instruments (www.keithley.com) defines OFDM and its uses while delving into the test instrumentation that is required to maintain it.
Most types of digital transmission involve modulating a pair of summed sine waves that differ in phase by 90 deg. The modulation signal can be represented by the vector sum of the inphase (I) and quadrature (Q) components. Digital information can be encoded in many ways. If the amplitude and phase of two sine waves is varied, the result is quadrature amplitude modulation (QAM). Optimally, the resulting signals will be measured with a vector signal analyzer (VSA) that processes all of its data as quadrature pairs in a constellation diagram.
To measure the quality of a received digital signal, designers often rely on a parameter known as error vector magnitude (EVM). Yet multipath can make it difficult to attain high levels of EVM performance. Luckily, OFDM transmits a large number of closely spaced carrier waves. Each wave is modulated with a different signal. The note depicts individual I and Q input signals translated into separate carriers. The symbol rate for each carrier is low, making it resistant to multipath. Because there are so many carriers, however, the overall data rate is high. Adjacent carriers are in phase quadrature with each other, which minimizes any crosstalk between them.
Many conventional instruments do not have the signal-processing capabilities needed to quickly make OFDM measurements. Mathematically, OFDM can be implemented using an inverse Fast Fourier Transform (FFT) in the transmitter andconverselyan FFT in the receiver. An OFDM signal includes several subcarriers that are designed as pilot carriers. They are used as references for phase and amplitude for synchronizing the receiver as it demodulates the data in other subcarriers.
The 14-page white paper ends by going over key measurements and test-equipment requirements for wireless local-area networks (WLANs) and WiMAX. Overall, it provides a solid overview of OFDM for those who are new to this technology.
Keithley Instruments, Inc., 28775 Aurora Rd., Cleveland, OH 44139; (440) 248-0400, FAX: (440) 248-6168, Internet: www.keithley.com.
