We’ve reached part two of the series. So far we’ve looked at:

PAPR reduction techniques in MIMO-OFDM systems – Part 1: Impact of PAPR on the performance of MIMO OFDM systems

This blog post discusses techniques to reduce the peak to average power ratio (PAPR) in order to improve performance in OFDM or MIMO-OFDM systems. Like the previous blog post in this series, the terms OFDM and MIMO-OFDM are inter-changeable with each other in the context of PAPR.

Popular PAPR reduction techniques for MIMO-OFDM include clipping

[1], error correction [2,3], tone reservation [4], selected mapping (SLM) [5,6,7], and partial transmit sequences [8].

Time-domain signal clipping

Reduction of the PAPR can be achieved by clipping the time-domain of the OFDM signal in conjunction with signal filtering and distortion control. This technique does not require any redundancy or side information; hence no reduction in the system’s data throughput. However, the filter and distortion control must be well-tuned in order to avoid introducing excessive inter-symbol interference (ISI) between the transmit symbols.

Selected mapping

The selected mapping (SLM) technique is an efficient approach for PAPR reduction. SLM generates statistically independent sequences from the same input and side information in order to transmit with the lowest PAPR [7]. This leads to significant PAPR reductions. However, it turns out that a straight-forward application of SLM to OFDM/MIMO-OFDM systems requires a high level of complexity. Some variants of SLM have been proposed to avoid the SLM algorithm’s high level of complexity, such as decomposed individual SLM (D-ISLM) and decomposed concurrent SLM (D CSLM) [10].

The tone reservation technique

The tone reservation (TR) technique is a simple and efficient way to reduce PAPR. The main advantages of the TR technique are no side information and less complexity when compared to the SLM technique. The drawback of the TR is dedicated subcarriers are used for PAPR reduction, leading to lower bandwidth efficiency. In general, TR is considered better than SLM in terms of performance and implementation [4].

Error correction

This technique relies on redundancy information used by both the forward error correction and the PAPR reduction capabilities of the system. A PAPR reduction of about 3.5 dB can be obtained with a 3/4 rate block coding scheme for a 4-carrier OFDM system [3]. Combined turbo coding and selective mapping techniques can be used to significantly reduce PAPR with moderate additional complexity.

Partial transmit sequences

The partial transmit sequence technique provides an approach for dividing the total number of OFDM subcarriers into multiple subsets of subcarriers with the same size. Each subset of subcarriers is weighted by pure rotation factors in order to minimize PAPR in the time domain. For example, PAPR can easily be reduced by more than 3 dB using a 128-subcarrier OFDM transmitter with 4 subsets, and weighting factors limited to (±1, ± j) [6]. The receiver must know the transmit sequences generated at the transmitter in order to recover the data. To accomplish this, the weighting phase factors must be sent to the receiver as side information, leading to an increase in bandwidth overhead.

Conclusion

Multi-carrier systems like OFDM or MIMO-OFDM are very appealing for their high-throughput wireless data transmission. However, a major drawback is a high PAPR that degrades the efficiency of the analog parts of the system (e.g. the digital-to-analog converter and the power amplifier). Therefore, PAPR reduction techniques are required, as discussed in this blog post.

References

[1] Shang-Kang Deng and Mao-Chao Lin, “OFDM PAPR reduction using clipping with distortion control,” IEEE International Conference on Communications, vol. 4, pp. 2563 – 2567, 2005.

[2] T.A. Wilkinson, and S.K. Barton A.E. Jones, “Block coding scheme for reduction of peak to mean envelope power ratio of multicarrier transmission schemes,” IEEE Electronics Letters, pp. 2098-2099, 1994.

[3] A.A. Abouda, “PAPR reduction of OFDM signal using turbo coding and selective mapping,” Proceedings of the 6th Nordic Signal Processing Symposium, pp. 248-251, 2004.

[4] A.D.S. Jayalath and C. Tellambura, “Use of data permutation to reduce the peak-to-average power,” vol. 2, no. 2, March 2002.

[5] L.J. Cimini and N.R. Sollenberger, “Peak-to-average power ratio reduction of an OFDM signal using partial transmit sequences,” vol. 4, no. 3, March 2000.

[6] R.F.H. Fischer and M. Hoch, “Directed Selected Mapping for Peak-to-Average Power Ratio,” Electronics Letters, vol. 42, no. 22, pp. 1289-1290, 2006.

[7] Jing Gao and Jinkuan Wang and Zhibin Xie, “Peak to average power ration reduction for MIMO-OFDM systems with decomposed selected mapping,” Journal of Information and Systems Sciences, vol. 3, pp. 572-580, 2009.

[8] A. Alavi and C.Tellambura and I. Fair, “PAPR reduction of OFDM signals using partial transmit sequence: an optimal approach using sphere decoding,” IEEE Communications Letters, vol. 9, no. 11, pp. 982 – 984, 2005.

[9] E. Ahmeda, G. Abbas, S. Saleem, Q. Islam W. Aziz, “PAPR Reduction in OFDM using Clipping and Filtering,” World Applied Sciences Journal, vol. 18, pp. 1495-1500.