This post will focus on the latter impairment, which is caused by the differences in the components used for frequency down conversion. These differences result in a phase and/or amplitude imbalance between the I and Q signals, an effect we will refer to hereafter as IQ imbalance or IQ mismatch. It has been shown that IQ imbalance results in limited image rejection.
In an earlier blog post, we addressed the effects of physical radio impairments on the quality of the transmitted OFDM signal. In particular, we saw how error vector magnitude (EVM) is linked to IQ gain and phase mismatch in a closed-form descent method formula, which helps radio designers figure out the achievable system specifications given these RF imperfections. In this short post we’ll focus on IQ imbalance on the receiver side and demystify its impact on multicarrier systems such as OFDM as mainly used in IEEE 802.11 and 802.16, and currently adopted in LTE and TV white space’s emerging IEEE standard 802.22.
Figure 1 depicts a kth OFDM subcarrier and its mirror image, –k. The shape of the transmitted signal is correct at the transmitter port (a), but the signal undergoes corruption on its way to the receiver due to fading (b), and within the receiver due to IQ imbalance (c).
Figure 1 a) kth OFDM subcarrier and its mirror image –k at the transmitter port. b) Received signal at RX antenna port.
c) Direct-conversion received signal with IQ mismatch.
It’s apparent from Figure 1 (c) that IQ imbalance will cause a subcarrier to suffer from interference from its mirror image. This can be supported by a widely modeled received signal with interference due to an imperfect direct conversion process with IQ imbalance, as follows:
The received signal includes the effect of the interference from the mirror sub-carrier which is usually quantified by the means of image rejection ratio (IRR) as follows:
The symbol error probability is intimately related to IRR, as shown below for a 64-QAM IEEE 802.11a signal .
Figure 2. A 64-QAM – IEEE 802.11a signal: performance as a function of I/Q mismatch for different image rejection ratios 
Several compensation methods have been devised. Some are standard dependent [3-4], while others rely on blind parameter estimation [5-6]. On the other hand, an interesting and simple scheme can be used in standard SDR radios such as Radio420X. This scheme is described in a short article by S. W. Ellingson .
- Razavi, B. 1997. “Design considerations for direct-conversion receivers.” IEEE Transactions on Circuits and Systems II: Analog and Digital Signal Processing 44:6(428-435). June 1997. doi: 10.1109/82.592569
- Windisch, Marcus, and Gerhard Fettweis. 2006 “Performance degradation due to I/Q imbalance in multi-carrier direct conversion receivers: A theoretical analysis.” Proc. IEEE International Conference on Communications (ICC’06) 1(257-262). IEEE. doi: 10.1109/ICC.2006.254737
- Schuchert A., R. Hasholzner, and P. Antoine. 2001. “A novel IQ imbalance compensation scheme for the reception of OFDM signals.” IEEE Transactions on Consumer Electronics 47:3(313–318). IEEE. doi: 10.1109/30.964115
- Tubbax, J., B. Come, L. Van der Perre, L. Deneire, S. Donnay, and M. Engels. 2003 “Compensation of IQ imbalance in OFDM systems.” Proc. IEEE International Conference on Communications (ICC’03) 5(3403–3407). IEEE. doi: 10.1109/ICC.2003.1204086
- Rykaczewski, Piotr, Volker Blaschke, and Friedrich K. Jondral. 2003. “I/Q Imbalance Compensation for Software Defined Radio OFDM Based Direct Conversion Receivers.” Proc. 8th International OFDM Workshop (279-283).
- Windisch, Marcus, and Gerhard Fettweis. 2004.”Standard-independent I/Q imbalance compensation in OFDM direct-conversion receivers.” Proc. 9th International OFDM Workshop (57-61).
- Ellingson, S. W. 2003. “Correcting I-Q Imbalance in Direct Conversion Receivers.” Virginia Polytechnic Institute and State University. http://www.ece.vt.edu/swe/argus/iqbal.pdf.