Equalization Techniques and OFDM Troubleshooting for Wireless LANs-application notes
OFDM (orthogonal frequency-division multiplexing) signals used in 802.11a and 802.11g wireless LAN systems include provisions for equalization in the receiver to correct for some types of system and channel impairments. These features can be used by test engineers to find the source of system problems and to improve measurement performance. This note describes these equalization features and their use in analyzers or measuring receivers. It also describes other features of the OFDM signal and corresponding analyzer capabilities, which are useful complements to equalization. Many of those capabilities will be discussed here in terms of how they can be used together to make the highest quality measurements and yield the most powerful insights into signal or system problems. Like many digital communications signals, the OFDM wireless LAN standards 802.11a and 802.11g include specific provisions for adaptive equalization in the receiver. This equalization improves receiver performance by correcting for linear errors or impairments such as amplitude and delay distortion. These errors can arise from phenomena such as multipath in the signal path between the transmitter and the receiver, or by frequency response problems in the transmitter circuits. This adaptive equalization is particularly useful for broadband signals such as those used in OFDM wireless LAN standards, where the channels are nearly 20 MHz wide and the opportunity for frequency response problems is correspondingly greater than that for more narrowband signals.
The equalization features required by these standards can also be used by measuring receivers (such as Agilent’s vector signal analyzers) to identify the source of transmitter problems, as the measuring receiver can be configured to calculate and perform the adaptive equalization in different ways, and to explicitly display the results of the equalizer calculations or corrections. The discussions here apply in general terms to the use of equalizer tools for troubleshooting, but specifically to the Agilent 89600 Series vector signal analyzers, as they have the most flexible and comprehensive set of features for demodulation and other types of signal analysis