PTS with Non-uniform Phase Factors for PAPR Reduction in OFDM Systems

1. PTS with Non-uniform Phase Factors for PAPRReduction in OFDM Systems 2007.12.07 指導教授: 蔡育仁 博士 學生姓名: 黃信智 To appear in IEEE Communications Letters, Jan. 2008

2. Outline • Introduction • PTS Algorithm • PTS with Non-uniform Phase Set • Simulation Results • Conclusion • References

3. Introduction • OFDM Signal: Frequency domain {X} IFFT time domain {x}

4. Introduction • In OFDM systems, peak-to-average power ratio (PAPR) reduction is a very important issue in order to improve the efficiency of the transmitter power amplifier. • Numerous techniques have been proposed to reduce the PAPR of OFDM signals in the time-domain. • Partial transmit sequence (PTS) is believed to be one of the promising methods for PAPR reduction.

5. PAPR Definition • Definition of PAPR: Ex: High PAPR=16 Low PAPR=2.25

6. PAPR Reduction Techniques • Two categories: 1. Signal distortion ex: Clipping 2. Symbol scrambling ex: Coding (Golay complementary codes), Partial Transmit Sequence (PTS) Selected Mapping (SLM) Dummy Sequence Insertion (DSI)

7. S/P And Partition IFFT IFFT IFFT PTS Algorithm + Phase factor Optimization

8. Partition Methods • Three partition methods: (a) Adjacent (b) Interleaved (c) Random (a) (b) (c)

9. Notations • N: number of sub-carriers • M: number of disjoint sub-blocks • for k = 0, 1, …, N-1: the frequency-domain data sequence • for n = 0, 1, …, N-1: the time-domain data sequence • : the corresponding time-domain signal of sub-block i after N-point IDFT • W: number of available phase factors

10. OFDM Signal • Without applying PTS, the overall output signal can be represented as • Let • The PAPR is defined as

11. Relative Phases • The sample with the peak power, i.e. the k-th sample, can be re-written as • : the phase of sub-block i in the k-th sample • Taking the phase , , as a reference, we define

12. The pdf of relative phases N = 128 M = 4

13. Observations For a high power peak (such as the 1-st peak), the corresponding phases of different sub-blocks are more concentrated to compose a large amplitude. Hence, the distribution of is more concentrated for the samples with higher power.

14. Example Assume M=4 k

15. Phase Adjustments • To reduce the amplitude of a high power sample, one straightforward means is • To adjust the phases of some chosen sub-blocks to the reverse of the reference phase • To keep the phases of the other sub-blocks unchanged • Thus the amplitude of the composed signal vector in this sample can be substantially reduced.

16. The pdf of the phase adjustments (minimize the 1-st peak) N = 128 M = 4 Gaussian-like

17. Observations It is found that the phase adjustments of these M sub-blocks are not in uniform distribution. The distribution contains a delta function at zero, corresponding to the probability of maintaining the original phase in a sub-block, and a Gaussian-like distribution with the mean .

18. Example M=4, l=1, consider k

19. PTS with Non-uniform Phase Set • For M = 2, which leads to two rotational phase factors 1 and -1 being the same as that applied in the conventional PTS • However, for M > 2 we need to figure out the optimal set of phase factors • in the minimum mean square error (MMSE) sense

20. PTS with Non-uniform Phase Set The distribution of the phase adjustments can be approximated as where  is the probability of maintaining the original phase and  is the standard deviation that fit in with the Gaussian-like distribution

21. PTS with Non-uniform Phase Set Assume that the set of chosen phase factors is for i = 0, 1, …, W-1 Due to the delta function, is chosen as a phase factor The other W-1 factors are chosen to fit in with the Gaussian-like distribution

22. PTS with Non-uniform Phase Set Based on the MMSE sense, we define the error function as

23. PTS with Non-uniform Phase Set The phase factors that minimize J are By iterative calculations, we can figure out the optimal set of phase factors that minimizes J

24. PTS with Non-uniform Phase Set Apply  = 0.487 and = 0.92 The non-uniform phase factor sets for W = 4 and 8 are For conventional PTS

25. Performance (Interleaved Partition)

26. Performance (Adjacent Partition)

27. Conclusion In this work, we have proposed a modified PTS scheme by applying pre-determined non-uniform phase sets for PAPR reduction in OFDM systems. It is found that an improvement of 0.8 – 0.25 dB in PAPR reduction for different scenarios can be obtained when compared with the conventional PTS scheme with uniform phase sets.

28. Conclusion Equivalently, the outage probability can be improved by a factor of 10 for a specific PAPR threshold. It must be noted that the optimal set of phase factors can be determined in advance based on system parameters The required side information is the same as the conventional PTS scheme with uniform phase factors.

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32. Thank You~