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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 1 IEEE P802.22 Wireless RANs Date: 2007-01-11 Notice: This document has been prepared to assist IEEE 802.22. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEEs name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEEs sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.22. Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures http://standards.ieee.org/guides/bylaws/sb-bylaws.pdf including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chairhttp://standards.ieee.org/guides/bylaws/sb-bylaws.pdf Carl R. StevensonCarl R. Stevenson as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.22 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at patcom@iee.org.patcom@iee.org Authors: Modified CAZAC Sequences Based Low PAPR Preambles

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 2 Co-Authors:

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 3 Background (1) In Draft v0.2, two binary PN Sequences are used to generate the I and Q components of QPSK symbols which form preambles in the frequency domain (c.f. Section 8.3) Superframe and frame preambles currently specified have high PAPR (> 7.8 dB for 2K FFT mode) Preambles with high PAPR may be clipped by the power amplifier –lower synchronization and channel estimation accuracy –degraded detection performance minimized as much as possibleThe PAPR of preambles should be minimized as much as possible –Boosting up the transmission power of preambles avoid this performance degradation –Effective methods (e.g. clipping, coding and companding) for reducing the PAPR of the data modulation signals are available, and thus preambles with insufficiently low PAPR may limit the performance.

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 4 Background (2) When the effect of adjacent cell interference (ACI) on preambles has to be considered (c.f. Runcoms doc IEEE802.22-06/0223r0), a set of preambles with low time-domain cross-correlation energy is desirable. These requirements for preambles are very similar to those for channel sounding sequences (c.f. Section 8.10.5.4.4). Only one sequence for one preamble of a type is specified in Draft v0.2. i.e. the effect of ACI was not considered/may be insignificant for preambles. modifyIn this proposal, we modify the Constant Amplitude Zero Auot- Correlation (CAZAC) sequences to obtain preambles with very low PAPR.

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 5 Unified Construction of M-Phase CAZAC Sequences (1) unified perfect roots-of-unity sequences (PRUS)The preambles proposed in this contribution are based on a very general construction of M-phase CAZAC (in the M-PSK format), i.e. the unified perfect roots-of-unity sequences (PRUS) [1]. [1] W.H. Mow, A new unified construction of perfect root-or-unity sequences, Proc. IEEE 4th International Symposium on Spread Spectrum Techniques and Applications (ISSSTA'96), Germany, September 1996, pp. 955-959. It includes the Frank, Chu, Milewski, and GCL sequences and more. It was proved by an exhaustive search that the unified PRUS construction includes all M-phase CAZAC sequences with M 15, sequence length L 20 and LM 11 11. It was conjectured that no more unknown M-phase CAZAC sequences exist [2]. [2] H.D. Lüke, et al. Binary and quadriphase sequences with optimal autocorrelation properties: a survey, IEEE Transactions on Information Theory, Vol. 49, Dec. 2003, pp.3271-3282.

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 6 Unified Construction of M-Phase CAZAC Sequences (2) The unified CAZAC sequence s CAZAC of length L = sm 2 is By modifyingoptimizingBy modifying a properly selected s CAZAC and optimizing the parameters s, m, α(l), β(l), (l), low PAPR sequences can be obtained.

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 7 Generation of Modified CAZAC Sequences with M = 2 n phases store nN used bitsSimplest sequence generation: store nN used bits, N used = no. of usable subcarriers. More memory-efficient implementation –generate the integer phase indices based on Equation (1) –perform table lookup to obtain the corresponding I and Q representations. Only need to store M/4 = 2 n-2 pairs of I/Q values (i.e. phase angles in [0, π/4)) as multiplication by ±1 or ±j can be computed with little complexity. Generation of integer phase indices requires 1 multiplication and 3~4 additions per index. Proposed sequences can also be extended to form a sequence set with low cross-correlation energy for use as preambles (to resist ACI) or as channel sounding sequences. When these or other CAZAC-like sequences (e.g. the GCL sequences specified in the current draft) are used as sounding sequences, the lookup table and phase index computations can be shared to reduce the implementation cost.

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 8 Results on Low PAPR Preambles (1) Results are presented for the setting: –2K, 4K and 6K FFT modes, null subcarriers [L=184n, DC, R=184n-1] (n = no. of bonded TV channels) –Decimation factor = 2 or 4 –Number of bonded TV channels = 1, 2 or 3 Here, all PAPR values are estimated for continuous-time waveforms using an oversampling factor of 4. Without oversampling, the computed PAPR values may be over-optimistic.

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 9 Results on Low PAPR Preambles (2) The following table lists the PAPR values of the proposed modified CAZAC sequences for different modes of preambles in draft v0.2 with 32 and 128 phases, respectively. The proposed preambles can be used to replace preambles in the current draft with a PAPR gain of at least 5.87dB. FFT Size = 2048 Null subcarriers [L=184, DC, R=183] Modified CAZAC (128 phases) Modified CAZAC (32 phases) Frame Short Preamble (Decimation factor = 4) 1.88 dB2.03 dB Frame Long Preamble (Decimation factor = 2) 1.81 dB2.02 dB Superframe Short Preamble (Decimation factor = 4) 1.93 dB2.07 dB Superframe Long Preamble (Decimation factor = 2) 1.81 dB1.97 dB

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 10 Results on Low PAPR Preambles (3) The following table lists the PAPR values of the modified CAZAC sequences for frame preambles when the number of bonded TV channels n = 2 or 3. Null subcarriers [L=184n, DC, R=184n-1] Modified CAZAC (128 phases) Modified CAZAC (32 phases) Frame Short Preamble (2 bonded channels, FFT size = 4096, Decimation factor = 4) 1.79 dB2.08 dB Frame Long Preamble (2 bonded channels, FFT size = 4096, Decimation factor = 2) 1.69 dB2.09 dB Frame Short Preamble (3 bonded channels, FFT size = 6144, Decimation factor = 4) 1.75 dB2.04 dB Frame Long Preamble (3 bonded channels, FFT size = 6144, Decimation factor = 2) 1.75 dB2.14 dB

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 11 PAPR reduction as compared to the preambles based on PN sequences in the current Draft v0.2 is at least 5.87 dB. By reducing M from 128 to 32 and hence the lookup table size from 32 to 8 pairs of I/Q values, the resultant PAPR values are still very low and the worst-case PAPR is only increased mildly from 1.93dB to 2.14dB. Still, the memory requirement for the proposed 128-phase preambles are very affordable. Results on Low PAPR Preambles (4)

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 12 adjacent cell interferenceWhen adjacent cell interference is a concern, we propose a set of modified CAZAC sequences with low PAPR and low cross- correlation levels as preambles (and sounding sequences). The average energy of the time-domain cross-correlation functions is the same as that of the Chu set, leading to same adjacent cell interference power. Next, the PAPR values of a set of 114 modified CAZAC sequences are evaluated. The worst case PAPR of the proposed set is 2.55dB, which is about 2.2dB better than the Chu set. Modified CAZAC Sequence Set (1)

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 13 Modified CAZAC Sequence Set (2)

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 14 CDF of PAPR of Sequence #114 in the Modified CAZAC Set CDF

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 15 CDF of PAPRs of the 114 Sequences in the Modified CAZAC Set

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doc.: IEEE 802.22-07/0002r1 Submission January 2007 Edward Au, Huawei Technologies Slide 16 Summary 1.We propose the use of modified CAZAC sequences to replace the existing preambles specified in draft v0.2. –The proposed preambles can attain very low PAPR (1.93dB for 2K, 4K and 6K FFT) –It was demonstrated that sets of modified CAZAC sequences can also attain very low PAPR (2.55dB for 2K, 4K and 6K FFT & set size = 114), while having the same time-domain cross-correlation energy as that of the Chu set 2.The implementation cost of the preamble generators is very affordable, esp. when similar CAZAC-like (e.g. GCL) sequences are used as sounding sequences, the common lookup table and computation can be saved.

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