Peak-to-Average Power Ratio Reduction in

MIMO-OFDM Systemsby

Hardik Patel

1 Introduction

Orthogonal Frequency Division Multiplexing (OFDM) is a fundamental tech-nique for high-speed optical communications, next-generation communica-tion systems, the third-generation partnership project (3GPP) in the form ofits long-term evolution (LTE) and many other communication systems[2][1].

We focus OFDM-based downlink transmission scheme for large-scale multi-user (MU) multiple-input multiple-output (MIMO) wireless systems. Or-thogonal frequency-division multiplexing (OFDM) [3] is an efficient and well-established way of dealing with frequencyselective channels. In addition tosimplifying the equalization at the receiver, OFDM also facilitates per-tonepower and bit allocation, scheduling in the frequency domain, and spectrumshaping. However, OFDM is known to suffer from a high Peak to AveragePower Ratio (PAR) [4], which necessitates the use of linear RF components(e.g., power amplifiers) to avoid out-of-band radiation and signal distortions.Unfortunately, linear RF components are, in general, expensive and powerinefficient than their non-linear counterparts, which would eventually resultin large costs for large-scale BS implementations having hundreds of anten-nas. Therefore, it is of major importance to reduce the PAR of OFDM-basedlarge-scale MU-MIMO systems to facilitate corresponding low-cost and low-power BS implementations.

1.1 Previous Works

To improve the difficult linearity requirements of OFDM, an extremely abun-dant PAR-reduction schemes have been proposed for point-to-point single-antenna and MIMO wireless systems [5] - [8]. For MU-MIMO systems, how-ever, a straightforward adaptation of these schemes is non-trivial, mainlybecause MU systems require the removal of MUI using a precoder [9].

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1.2 Refered Work

In this work[11], a novel downlink transmission scheme for large-scale MU-MIMO-OFDM wireless systems is developed, that only affects the signalprocessing at the BS while leaving the processing required at each terminaluntouched. The main idea of the scheme proposed in this work is to exploitthe excesss degree of freedom which is offered by using the BS with multipleantennas and jointly performing MU precoding OFDM modulation and PARreduction i.e, PMP.

We formulate PMP as a convex optimization problem, which jointly per-forms MU precoding, OFDM modulation, and PAR reduction at the BS.We develop and analyze a novel optimization algorithm, referred to as fastiterative truncation algorithm (FITRA), which is able to find the solution toPMP efficiently for the (typically large) dimensions arising in large-scale MU-MIMO-OFDM systems. The efficacy of the proposed MU-MIMO-OFDMsceme can be observed by simulation results.

2 Future Work

In the following we shall lay out a road map for our future work:

Further work on reducing the computational complexity of FITRA us-ing continuation strategies that is vital for a practical realization ofPMP in hardware.

References

[1] N. Cvijetic, Ofdm for next-generation optical access networks Journalof Lightwave Technology. Lightwave Technology, vol. 30, no. 4, pp. 384-398, Feb. 2012.

[2] Z. Zhang, J. Liu, and K. Long, Low-Complexity Cell Search with FastPSS Identification in LTE, IEEE Trans. Veil. Tecllnol., vol. 61, no. 4,pp. 1719-1728, May. 2012.

[3] R. van Nee and R. Prasad, OFDM for wireless multimedia communica-tions. Artech House Publ., 2000.

[4] S. H. Han and J. H. Lee, An overview of peak-to-average power ra-tio reduction techniques for multicarrier transmission, IEEE WirelessComm., vol. 12, no. 2, pp. 1536-1284, Apr 2005.

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[5] R. W. Bauml, R. F. H. Fischer, and J. B. Huber, Reducing the peak-toaverage power ratio of multicarrier modulation by selected mapping,IEE Elec. Letters, vol. 32, no. 22, pp. 20562057, Oct. 1996.

[6] B. S. Krongold and D. L. Jones, PAR reduction in OFDM via activeconstellation extension, in IEEE Int. Conf. on Acoustics, Speech, andSig. Proc. (ICASSP), vol. 4, Hong Kong, China, Apr. 2003, pp. 525528.

[7] R. F. H. Fischer and M. Hoch, Directed selected mapping for peakto-average power ratio reduction in MIMO OFDM, IEE Elec. Letters, vol.42, no. 2, pp. 12891290, Oct. 2006.

[8] J. Illic and T. Strohmer, PAPR reduction in OFDM using Kashinsrepresentation, in Proc. IEEE 10th Workshop on Sig. Proc. Advances inWireless Comm. (SPAWC), Perugia, Italy, June 2009, pp. 444448.

[9] S. K. Mohammed, A. Chockalingam, and B. S. Rajan, A lowcomplexityprecoder for large multiuser MISO systems, in IEEE Vehicular Tech.Conf (VTC), vol. Spring, Marina Bay, Singapore, May 2008, pp. 797801.

[10] C. Studer and E. G. Larsson, PAR-aware multi-user precoder for thelarge-scale MIMO-OFDM downlink, in Proc. 9th International Sym-posium on Wireless Communication Systems (ISWCS), Paris, France,August 2012.

[11] C. Studer and E. G. Larsson, PAR-aware large-scale Multiuser MIMO-OFDM downlink, in Proc. 9th International Symposium on WirelessCommunication Systems (ISWCS), Paris, France, August 2012.

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