Microwave Engineering Europe - December 2007 - (Page 17) TEST & MEASUREMENT — OFDM 17 Figure 4: The main measurement for the quality of a received digital signal is Error Vector Magnitude, or EVM. This is the ratio of the received signal’s amplitude and phase compared to its ideal amplitude and phase. The best way to analyze the resulting signals is with a vector signal analyzer (VSA), such as the Model 2820, that processes all its data as quadrature pairs in a constellation diagram. Figure 3 shows constellation diagrams for several types of modulations: QPSK, 8PSK, and 16QAM. Modulation quality analysis A good measurement of the quality of a received digital signal is Error Vector Magnitude, or EVM (Figure 4). This is the ratio of the received signal’s amplitude and phase compared to its ideal amplitude and phase. Mathematically, EVM is given by EVM = √{Perror/Preference} • 100% or Figure 5: If the difference in path length between direct and reflected paths exceeds 1 microsecond, the receiver will receive the symbol in the next symbol period. EVM(dB) = 10log10 {Perror/Preference} Cellular technology specifications are usually stated in percent, while the Wireless LAN community tends to specify EVM using decibels. The multi-path problem Multi-path adds another layer of complexity to our EVM measurement. Figure 5 shows a Bluetooth signal with a symbol rate of 1M symbols per second. That means that the receiver will expect a specific symbol within a window of one microsecond. If multipath delays the signal by more than one microsecond, the receiver will receive the symbol in the next symbol period, causing a significant symbol error. The faster the data rate, the higher the chance that multi-path will cause Inter Symbol Interference (ISI). An obvious way to reduce the error rate would be to slow down the symbol rate; each symbol would last longer and be more resistant to multipath. Unfortunately, this reduces the data rate. What’s needed is a way to slow down the symbol rate without slowing the data rate — a seemingly impossible task. The answer to the puzzle is OFDM. OFDM transmits a large number of closely-spaced carrier waves, each modulated with a different signal. Figure 6 shows that the individual I and Q input signals are translated into separate carriers. The symbol rate for each carrier is low, making it resistant to multipath, but because there are so many carriers the overall data rate is high. Adjacent carriers are in phase quadrature with each other, which keeps crosstalk between them to a minimum without requiring a bank of narrow-band filters. Figure 6: Instead of transmitting a single symbol at a time, OFDM transmits multiple symbols simultaneously on a number of carriers. This is the Frequency Division Multiplex component. The sub-carriers are distributed in carefully chosen multiples of frequency so that they are “orthogonal” and the closely adjacent sub-carriers don’t interfere with each other. Figure 7: This block diagram shows the digital circuit in the Model 2810 Vector Signal Analyzer and the Model 2910 Vector Signal Generator. Microwave Engineering ● December 2007 ● www.mwee.com 016_017_018_020_022_MWEE.indd 17 22/11/07 16:53:16 http://www.mwee.com
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