This article presents a brief overview of common frequency synchronization techniques in OFDM systems. Frequency error in OFDM systems is often called carrier frequency offset (CFO). CFO can be caused by frequency differences between the transmitter and receiver oscillators, Doppler shift of mobile channels, or oscillator instabilities. All sub-carriers are affected by the same amount of CFO. CFO is classified into two categories: fractional sub-carrier spacing CFO, and integer sub-carrier spacing CFO

[1].

Fractional CFO introduces inter-carrier interference (ICI) between sub-carriers. It destroys the orthogonality of sub-carriers and results in bit error rate (BER) degradation. Integer CFO does not introduce ICI between sub-carriers, but does introduce a cyclic shift of data sub-carriers and a phase change proportional to OFDM symbol number. Therefore, it’s necessary to correct this CFO at the OFDM receiver.

There are several techniques to estimate and compensate for these errors using time-domain or frequency-domain approaches, sometimes called pre-FFT and post-FFT synchronization, respectively.

Pre-FFT Synchronization

Pre-FFT synchronization performs the estimation of CFO before OFDM demodulation (FFT processing). The pre-FFT approach provides fast synchronization and requires less computing power due to the fact that no FFT processing is needed.

Pre-FFT synchronization can be classified into two categories: non-data-aided (NDA) and data-aided (DA).

  • NDA methods exploit similarities between the cyclic prefix (CP) part and the corresponding data part of a received OFDM symbol to estimate fractional CFO [2, 3, 4, 5]. This can be done by correlating the CP and the corresponding OFDM symbol to estimate both timing and frequency offsets [2, 3]. NDA methods that use the CP can only estimate CFO in the range of [-0.5, +0.5] sub-carrier spacing [2, 3]. These methods require no additional OFDM training symbols, improving transmission efficiency. If the CP is heavily disturbed by severe multipath fading, part or all of the cyclic prefix of a given symbol will be interfered with by the previous symbol. As a result, the estimation accuracy is significantly degraded, causing degradation of the BER performance. In order to increase the frequency error estimation accuracy and compensate for the impact of multipath fading, NDA frequency synchronization requires a fine timing synchronization technique. 
  • DA methods exploit a known sequence of OFDM training symbols inserted at the start of every OFDM packet (widely used in 802.11 WLAN [5]) to estimate fractional CFO. The downside of DA pre-FFT synchronization is reduced transmission efficiency due to the insertion of the training symbols. However, this method provides better results and a wider CFO estimation range than the NDA algorithms do: [-1.0, +1.0] sub-carrier spacing [6].

Post-FFT Synchronization

Post-FFT synchronization methods usually perform the estimation of the remaining integer CFO left by pre-FFT frequency synchronization. Integer CFO can be estimated by correlating the received pilot sub-carriers with a shifted version of the known pilot sub-carriers [7]. Depending on spacing between pilot sub-carriers, this approach can estimate CFO range up to several multiple integers of sub-carrier spacing.

Using the pilot sub-carrier approach, one can also estimate sampling clock frequency offset by using a special pilot sub-carrier pattern. This integer CFO synchronization technique is only effectively performed after coarse timing synchronization and coarse frequency synchronization have been established (acquisition stage) to track the residual CFO errors, common phase error (CPE) left by pre-FFT frequency synchronization, and receiver local oscillator phase noise, respectively.

Conclusion

Both time-domain and frequency-domain frequency synchronization play important roles in correcting carrier frequency offset in OFDM systems. It’s up to the system designer to decide on the appropriate approach to use, depending on the requirements and specifications of the system.

References

  1. Lee, Jungwon, Hui-Ling Lou, Dimitris Toumpakaris, and John 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
  2. Van de Beek, Jan-Jaap, Magnus Sandell, Michael Isaksson, and Per Ola Borjesson. 1995. “Low-complex frame synchronization in OFDM systems.” IEEE International Conference on Universal Personal Communication 982-986. IEEE Tokyo. doi: 10.1109/ICUPC.1995.497156
  3. Van de Beek, Jan-Jaap, Magnus Sandell, Michael Isaksson, and Per Ola Borjesson. 1997. “ML estimation of timing and frequency offset in OFDM systems.” IEEE Transactions on Signal Processing 45(7):1800-1805. doi: 10.1109/78.599949
  4. Hsieh, Meng-Han and Che-Ho Ho Wei. 1999. “A low-complexity frame synchronization and frequency offset compensation scheme for OFDM systems over fading channels,” IEEE Transactions on Vehicular Technology 48(5):1596-1609. doi: 10.1109/25.790537
  5. Institute of Electrical and Electronics Engineers. 2007. “Part11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.” IEEE Std 802.11-2007. doi: 10.1109/IEEESTD.2005.339589
  6. K. Wang, Jugdutt Singh and Michael Faulkner. 2004. “FPGA Implementation of an OFDM WLAN Synchronizer.” IEEE International Conference on Field-Programmable Technology 89-94. doi: 10.1109/DELTA.2004.10039
  7. Zou, Hanli, Bruce McNair, and Babaik Daneshrad. 2001. “An integrated OFDM receiver for high-speed mobile data communications.” IEEE Global Telecommunications Conference 5:3090-309. doi: 10.1109/GLOCOM.2001.965995