Microwave Engineering Europe - December 2007 - (Page 18) 18 TEST & MEASUREMENT — OFDM Figure 8: OFDM can be implemented by using an Inverse Fast Fourier Transform (IFFT) in the transmitter and conversely an FFT in the receiver. In the transmitter, the IFFT converts the parallel input signals into the two modulated sine waves in the output. It’s as if the IFFT acts as a specialized multiplexer. Figure 9: An OFDM signal includes several sub-carriers designated as pilot carriers that are used as reference for phase and amplitude for synchronizing the receiver as it demodulates the data in the other subcarriers. The OFDM radio As you can see, a lot of complex math is involved in this. Many conventional instruments lack the signal processing capability to perform these measurements quickly. As shown in Figure 7, Keithley’s DSP enhanced architecture makes it possible to perform the analysis very quickly. OFDM is simple in concept, even though its implementation is complex. Mathematically, it can be implemented by using an Inverse Fast Fourier Transform (IFFT) in the transmitter and conversely an FFT in the receiver. Figure 8 shows the parallel symbols being converted to the two modulated sine waves in the output. It’s as if the IFFT acts as a specialized multiplexer. In order to keep things synchronized, an OFDM signal includes several sub-carriers (Figure 9) designated as pilot carriers that are used as reference for phase and amplitude for synchronizing the receiver as it demodulates the data in the other subcarriers. Key Measurements: Constellation and EVM Figure 10 shows the constellation of a WLAN signal conforming to the 802.11j standard. Note that even though the signal has been transmitted using many carriers, it is still essentially a QAM signal. There are also two extra symbols, representing the information modulated on the pilot carriers. OFDM is very pervasive, as shown in Table 1. WLAN WLAN is defined by the IEEE 802.11 standard, of which there are several variations, a through g, as shown in Table 2. Within a 16.25 MHz bandwidth are 52 carriers (Figure 11), numbered –26 to +26, spaced 312.5 kHz apart. Carriers 7 and 21 Figure 10: The constellation diagram of a WLAN signal conforming to the 802.11j standard. Note that even though the signal has been transmitted using many carriers, it is still essentially a QAM signal. There are also two extra symbols, representing the information modulated on the pilot carriers. Table 1: Communication services using OFDM. Figure 11: Each carrier within the modulation scheme is referred to as a sub-carrier. The sub-carriers are spaced at regular intervals called the sub-carrier frequency spacing (ΔF). The sub-carrier frequency relative to the center frequency is kΔF where k is the subcarrier number. Microwave Engineering Europe ● December 2007 ● www.mwee.com 016_017_018_020_022_MWEE.indd 18 22/11/07 16:53:36 http://www.mwee.com
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.