Presentation ENIC

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State of the Art: Peak-to-Average-Power-Ratio (PAPR) Reduction Methods for OFDM Systems Markus Muck Broadband Systems and Technologies Lab Motorola Labs CRM Paris, France [email protected]

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Overview of the Presentation Presentation ENIC

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Review of Orthogonal Frequency Division Multiplex (OFDM) basics G

Presentation of of (dis)advantages of OFDM systems G

Important Problem: Peak-to-Average-Power Ratio (PAPR) G

State of the Art: PAPR Reduction Methods G

PAPR Reduction by Tone Injection G

Conclusions

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Review of Classical OFDM Systems Presentation ENIC

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Characteristics of OFDM – No adjacent carrier interference despite of overlapping side-bands of the individual carriers (Orthogonality) – Robust to narrow-band interference, because only very few carriers are affected – Equalization is very simple compared to Single-Carrier systems – Data Rate per carrier can be adapted to Signal-to-Noise ratio of single carriers (High Efficiency) – Single Frequency networks are possible ! attractive for broadcasting – Efficient implementation using FFT/IFFT algorithms

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A basic OFDM system Presentation ENIC

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Efficient Implementation possible thanks to Fast-Fourier Transformation Algorithms G

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– Example: 64-Point RADIX-4 FFT requires 96 complex multiplications – FFT can be reused for IFFT

Equalization by a simple Division

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Drawbacks of OFDM Systems Presentation ENIC

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The major Drawbacks of OFDM systems compared to Single Carrier systems are

– OFDM is sensitive to frequency offset – OFDM is sensitive to clock offset – OFDM is sensitive to phase noise – The OFDM time-domain signal has a relatively large peak-to-average ratio

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tends to reduce the power efficiency of the RF amplifier non-linear amplification destroys the orthogonality of the OFDM signal and introduced out-of-band radiation

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Problems due to high PAPR values Presentation ENIC

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Clipping and/or non-linear Amplification – Clipping Hereby, the integral over the clipped range is important. Clipping a very narrow peak doesn’t do much damage. Clipping over a long period (even if the amplitudes are only slightly above the clipping level) will considerably degrade the orthogonality in the frequency domain and will introduce out-of-band-radiation. I

– Non-linear Amplification I

A general rule is to have a working point at a back-off (saturation-to-average level) at about 3dB. So, if very high peaks occur, the amplifier is used only very inefficiently.

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Analyzing the PAPR properties of a signal Presentation ENIC

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Over-sampling is crucial – Peaks in the analog signal may be very narrow. Therefore, they are often “hidden between the samples” when no over-sampling is applied – Min. 16-times over-sampling is recommended. In this example 32-times over-sampling is applied.

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A reference OFDM signal for the Analysis Presentation ENIC

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As a reference, we choose an OFDM signal based on the following parameters G

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– – – –

64 carriers BPSK constellations All carriers used Randomly created data are modulated onto the carriers

The signal in time and frequency domain (not low-pass-filtered)

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Effects of Clipping Presentation ENIC

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Clipping of very high peaks only (before clipping, the signal was low-pass filtered such that the in-band signal only is remaining): G

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Clipping

FFT

Important Out-Of-Band Radiation has been introduced

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Effects of Clipping Presentation ENIC

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Clipping at a lower level G

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Clipping

FFT

Important Out-Of-Band Radiation has been introduced, increasing rapidly when setting the clipping level lower Page 10

Effects of Non-Linear Amplification Presentation ENIC

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We use a simple 3rd order amplifier model. No phase distortion is introduced.

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Effects of Non-Linear Amplification Presentation ENIC

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Influence of the Non-Linear Amplifier G

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FFT Important Out-Of-Band Radiation has been introduced Page 12

PAPR Reduction Methods Presentation ENIC

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General Idea behind PAPR reduction G

– Perform Modification of the Signal in Time Domain

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– Perform Modification of the Signal in Frequency Domain

Two Directions of Research – Modifications require Signaling for the Decoder – Modifications are compensated blindly and do not require Signaling

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PAPR Reduction with Signaling Presentation ENIC

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Ideas of PAPR Reduction with Signaling G

– Modification of the Signal in Frequency Domain. A certain set of possible modifications is predefined (e.g. inverse the amplitude of certain carriers, exchange certain carriers, shift the positions of all carriers, etc.) and signaled to the receiver by reserving some carriers for the signaling. Disadvantage: Loss of Spectral Efficiency

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– A set of Time Domain signals is defined. The transmitter find the best set which - added to the symbol to be transmitted - leads to the lowest peak. The used set must be signaled to the receiver by reserving some carriers for the signaling. Disadvantage: Loss of Spectral Efficiency

Problem: All information is lost if the signaling carriers are not demodulated correctly. Therefore, much redundancy must be incorporated.

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PAPR Reduction without Signaling Presentation ENIC

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Ideas of PAPR Reduction without Signaling – Some carriers in Frequency Domain are reserved for PAPR reduction. The transmitter chooses the carrier amplitude that leads to the best tradeoff Power Increase - Peak Reduction. Reduction Disadvantage: Loss of Spectral Efficiency – Tone Injection (TI): (TI) The constellation is modified by adding additional constellation points allowing a modification of the Time Domain Signal (only disadvantage: Slight power increase and slight performance decrease depending on the guard distance):

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PAPR Reduction without Signaling Presentation ENIC

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How to choose the Tone-Injection Carriers ? – Time Domain Signal before correction 2π

j ⋅n⋅υ 1 N −1 ⋅ ∑ fυ ⋅ e N , n = 0,1,..., N − 1 tn = N υ =0

– Time Domain Signal corresponding to a Tone Injection Vector on Carrier υ 2π

j ⋅n⋅υ 1 tn = ⋅ kυ ⋅ e N , n = 0,1,..., N − 1 N

– Condition for Peak Reduction on Time Domain Sample m

φ (kυ ) +

2π ⋅ m ⋅υ ≈ φ (t m ) + π N

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Tone Injection: The Receiver Structure Presentation ENIC

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Receiver Structure for TONE INJECTION

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A Simple Modulo Operation is required before Decoding the Data

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Importance of the Guard Distance Presentation ENIC

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Influence of the guard distance

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A tradeoff has to be found

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– A large Guard Distance leads to an important Power Increase, but the additional burst error influence is less important. – A small Guard Interval leads to an negligible Power Increase, but the additional burst error influence is important.

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How to measure the achieved Gain ? Presentation ENIC

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Output Power Gain – A reasonable way to “quantify” the achieved gain by PAPR reducting is to look at the possible output power of the signal while keeping a constant level of out-of-band radiation. In practice, a level of signal power to max. out-of-band radiation power of 35dB is a common value. The total output power gain is approx. 1dB including the power gain due to the additional tone-injection points. After subtraction of the TI power increase, the resulting gain is approx. 0.7 dB. This corresponds to an possible increase of the transmission power of approx. 18%.

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Conclusions Presentation ENIC

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PAPR Reduction is important for G

– Reducing the out-of-band Radiation – Conserving the Orthogonality of the Signal

PAPR Reduction by Tone Injection (TI) seems to be one of the most interesting ideas. G

TI: No Signaling is required. The spectral efficiency is not reduced. G

TI: The “Intelligence” is in the transmitter. Here, very clever PAPR optimization strategies can be elaborated. The Decoding algorithms in the receiver is very simple and cheap in hardware. G

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TI: At a constant Out-of-Band Radiation level, the transmission power can be increased by approx. 18%

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