Measurement of transmitter IQ parameters in HT and VHT wireless LAN systems

Orthogonal Frequency Division Multiplexing (OFDM) is widely used in the wireless local area network (WLAN) standards like IEEE 802.11a/n/ac. OFDM has high spectral efficiency, enabling the high data rates required for video streaming and sharing of large files. However, the impact of analog front-end non-idealities in an OFDM system limits its performance. One such non-ideality is IQ imbalance. At the transmitter, while analog baseband signal is being up-converted to passband signal, IQ imbalance gets introduced. We present a method to jointly estimate IQ gain, phase, and skew imbalances introduced at the transmitter. This method is applicable to the latest OFDM based single input single output (SISO) WLAN systems, especially in the context of WLAN test equipment design. We focus only on transmitter IQ parameters in the estimation since the IQ parameters present in the test equipment are considered to be negligibly small after calibration. The transmitter in this case is that of the device under test while the receiver is that of the test equipment. The estimation of IQ imbalance parameters is performed at the receiver in frequency domain using the channel estimates. Also a frequency domain compensation algorithm has been proposed. This compensation is applied after channel equalization.