This article discusses the effects of real-world signal impairments on the performance of the OFDM receiver. Some important signal impairments that can cause degradation to the performance of the system are I/Q imbalance, DC offset, phase noise, timing error, and carrier frequency offset. We’ll look at each of these in turn.

I/Q Related Impairments

I/Q imbalance is produced mainly by the RF front-end part of the receiver, especially in a direct conversion receiver architecture, because there is no requirement for an analog image rejection filter

[8]. Therefore, I/Q imbalance significantly degrades the performance of OFDM receivers. It applies the same amount of imperfection to all sub-carriers of the receiver.

I/Q gain imbalance causes the I signal to be smaller than the Q signal, or vice versa. This results in each sub-carrier being interfered with by its frequency mirror image sub-carrier. As a result, all sub-carriers are spread along the I axis of the constellation [2].

I/Q phase imbalance causes the phase shift between I and Q to be not exactly 90 degrees. This also causes all sub-carriers to be spread along the Q axis of the constellation [2].

Other OFDM Receiver Impairments

DC offset is caused by carrier leakage in the RF front-end, destroying the orthogonality of sub-carriers, which can result in severe inter-carrier interference (ICI) [7]. This effect degrades performance of the OFDM system.

Phase noise is caused by the oscillator in the RF front-end. It also causes ICI between sub-carriers. It generally rotates all sub-carriers by the same amount. This error is often called common phase error (CPE). Instead of a constellation that shows visible rotation, a small amount of CPE causes a fuzzy constellation display [2].

Timing error in the OFDM receiver is caused by wrong sample feeds to FFT windows after the removal of the cyclic prefix. Timing synchronization error introduces both ICI and inter-symbol interference (ISI) to OFDM systems [1, 5, 6]. A small amount of timing offset left by boundary detection proportionally rotates all sub-carriers in the frequency domain. This degrades the performance of the system. A large amount of timing synchronization error destroys orthogonality between sub-carriers due to the resulting ISI and additional ICI. In this case, performance of the system degrades significantly. A sampling clock frequency at the receiver that is different than that at the transmitter causes a slow drift of symbol timing and additional phase change to all sub-carriers.

Carrier frequency offset (CFO) is also caused by frequency differences between the transmitter and receiver oscillators. CFO is also caused by Doppler shift of mobile channels and oscillator instabilities. All sub-carriers of are affected by a same amount of CFO. CFO is classified into two categories: fractional sub-carrier spacing CFO, and integer sub-carrier spacing CFO [4]. Fractional CFO introduces ICI between sub-carriers. It destroys the orthogonality of sub-carriers and results in BER degradation. Integer CFO does not  introduce ICI between sub-carriers, but does introduce a cyclic shift of sub-carriers and a phase change proportional to OFDM symbol number. As a result, a complete OFDM symbol is wrong after demodulation.

Understanding signal impairment effects helps baseband receiver designers to identify the sources of performance degradation of the system. This also allows the study of how much they affect the performance by mathematical analysis or computer simulations. This analyses enables the designer to choose the best trade-off between implementation efforts and performance.

OFDM Receiver Impairment References

  1. Chang, Dah-Chung. 2008. “Effect and Compensation of Symbol Timing Offset in OFDM Systems With Channel Interpolation.” IEEE Transactions on Broadcasting 54(4): 761-770. doi 10.1109/TBC.2008.2002339
  2. Cutler, Bob. 2002. “Effects of Physical Layer Impairments on OFDM Systems.” RF design 25(5): 36-45. http://rfdesign.com/images/archive/0502Cutler36.pdf
  3. Krondorf, Marco, and Gerhard Fettweis. 2008. “OFDM Link Performance Analysis under Various Receiver Impairments”. EURASIP Journal on Wireless Communications and Networking. doi:10.1155/2008/145279
  4. Lee, Jungwon, Hui-Ling Lou, D. Toumpakaris, and J.M. Cioffi. 2004. “Effect of Carrier Frequency Offset on OFDM Systems for Multipath Fading Channels.” IEEE Global Telecommunications Conference 6: 3721-3725. doi: 10.1109/GLOCOM.2004.1379064
  5. Minn, Hlaing, V.K. Bhargava, and K.B. Letaief. 2003. “A Robust Timing and Frequency Synchronization for OFDM Systems.” IEEE Transactions on Communications 2(4): 822-839. doi: 10.1109/TWC.2003.814346
  6. Mostofi, Yasamin, and Donald C. Cox. 2003. “Analysis of the Effect of Timing Synchronization Errors on Pilot-Aided OFDM Systems.” Conference Record of the Thirty-Seventh Asilomar Conference on Signals, Systems and Computers 1:638-642. IEEE. doi: 10.1109/ACSSC.2003.1291989
  7. Tseng, Hsien-Yu, Wen-Jen Cho, Ting-Kang Chang, See-May Phoong, and Yuan-Pei Lin. 2008. “Compensation of IQ imbalance and DC offset for OFDM transmission over frequency selective channels.” 2008 IEEE International Conference on Communications  641-645. doi: 10.1109/ICC.2008.126
  8. Windisch, Marcus, and Gerhard Fettweis. 2006. “Performance Degradation due to I/Q Imbalance in Multi-Carrier Direct Conversion Receivers: A Theoretical Analysis.” 2006 IEEE International Conference on Communications 1:257-262. doi: 10.1109/ICC.2006.254737

 

A QAM64 MIMO OFDM PHY layer reference design