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DFT-spread OFDM optimized for 802.11ah

DFT-spread OFDM optimized for 802.11ah. Date: 2011-03-11. Authors:. 11ah phy layer constraints. As claimed by recent presentations, 11ah phy layer will need to have a reduced transmission bandwidth  down-clocking provide low datarates  down-clocking, repetition [1]

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DFT-spread OFDM optimized for 802.11ah

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  1. DFT-spread OFDM optimized for 802.11ah Date: 2011-03-11 Authors:

  2. 11ah phy layer constraints • As claimed by recent presentations, 11ah phy layer will need to • have a reduced transmission bandwidth  down-clocking • provide low datarates  down-clocking, repetition [1] • be very robust to improve the range  repetition • On top of that, the power consumption need to be as low as possible, in order to ensure long battery life [2] • low duty cycle  good solution but not feasible for all use cases, and for APs • low power consumption in sleep mode • low power consumption in active mode • low output power • low envelope fluctuations (low PAPR) for a limited power backoff and a high power amplifier efficiency [5] • In this presentation, we address these constraints of a low PAPR and robustness by repetition

  3. Content • We propose a repetition mechanism based on the use of spreading/precoding prior to the OFDM modulation that enables: • to optimally exploit the frequency diversity (performance/range) • to have the PAPR of a single carrier technique (power efficiency)

  4. PAPR analysis • OFDM suffers from a very large peak to average power ratio (PAPR) • due to the fact that after the iDFT process, the time samples are all composed of the summation of the data symbols from each subcarriers with xm the time samples of indice m and Xk the data symbol transmitted on subcarrier k

  5. Classical OFDM modulation Subcarrier mapping Data symbol (BICM bit interleaved coded modulation) PSK or QAM OFDM iDFT Nc Subcarriers Σ … … + Nc Nc time samples each time sample is composed of the summation of the data symbols of Nc subcarriers Freq high PAPR Time

  6. PAPR analysis: DFT-spread OFDM solution • DFT-spread OFDM techniques propose a precoding/repetition mechanism based on a multiplication of data symbols by a spreading code before the mapping on subcarriers and the OFDM modulation [6] • On top of that, the spreading codes are Fourier codes (DFT), which can lead to a strong reduction of PAPR • the basic idea is that a DFT (precoding/spreading) followed by a iDFT (OFDM) leads to the identity. • Following this approach, a multiple access technique called SC-FDMA has been accepted in the uplink of LTE and LTE-advanced • Low power SC mode in 802.11ad can also be seen as a corner case of DFT-spread OFDM with a spreading length equal to the OFDM DFT size • DFT-spread OFDM is also a very good candidate for DVB-NGH

  7. DFT-spread OFDM modulation concept DFT matrix SFxSF (LxL) Subcarrier mapping k=0 OFDM iDFT SFx1 (Lx1) Data symbol PSK or QAM l=0 Nc Subcarriers x l=L n=0 m=Nc m=0 Nc time samples k=Nc This time sample is composed of a unique data symbol (no sommation) See [3] for calculations Spreading factor SF = L Number of subset of subcarriers=Q Number of subcarriers=Nc=QxL=QxSF frequency offset f column of DFT precoding matrix n

  8. DFT-spread OFDM modulation concept DFT matrix SFxSF (LxL) Subcarrier mapping k=0 OFDM iDFT SFx1 (Lx1) Data symbol PSK or QAM l=0 Nc Subcarriers x l=L n=0 m=0 m=Nc n x Nc time samples k=Nc n=1 the indice n of the DFT-matrix column used for spreading/precoding leads to a shift n of the time samples

  9. DFT-spread OFDM modulation concept DFT matrix SFxSF (LxL) Subcarrier mapping k=0 f = frequency offset of the subset of subcarriers OFDM iDFT SFx1 (Lx1) Data symbol PSK or QAM l=0 Nc Subcarriers x l=L n=0 m=Nc m=0 n x Nc time samples k=Nc n=1 a frequency shift f intruduces a simple phase shift on the time samples

  10. 11ah-optimized DFT-spread OFDM modulation DFT matrix SFxSF (LxL) Subcarrier mapping k=0 OFDM iDFT SFx1 (Lx1) Data symbol PSK or QAM l=0 Nc Subcarriers x l=L m=Nc m=0 x Nc time samples k=Nc no sommation: all time samples are composed of only one data symbol x lowest PAPR x

  11. 11ah-optimized DFT-spread OFDM solution • Benefits: • Best performance: • exploit all frequency diversity • no inter-code interference • Optimum PAPR reduction: • PAPR of a single carrier modulation • Low complexity • simple precoding and frequency interleaving prior to OFDM modulation) • possibility to build the signal in the time domain as a single carrier modulation, thanks to repetitions and phase shifts (no more iDFT at Tx) • On top of that, possibility for multiple access in downlink for up to L users, by reusing the groupID field and multiuser aggregation as defined in TGac without the need for feedback procedure. [3] and quick description in annex

  12. 11ah-optimized DFT-spread OFDM solution • Optimized mode: • FFT size Nc=64 • Spreading factor SF=L=8 (length of spreading codes, size of DFT precoding matrix) • Number of subsets of subcarriers Q=8 • Number of data symbols per subset of subcarriers=1 • Nb data symbols per OFDM symbol=8 11n MCS0

  13. Performance results • Repetition by spreading/precoding with a factor 8 leads to 8dB SNR gains for an equivalent PER • Results obtained without interleaving optimization for DFT-spread OFDM PER SNR 11n MCS0

  14. PAPR results • digital PAPR results before oversampling, windowing and pulse shaping for • 11ah-optimized DFT-spread OFDM • and classical OFDM CCDF=pr(PAPR>PAPR_abs) • With a BPSK, QPSK: the PAPR is nul, as a single carrier technique PAPR_abs

  15. PAPR results • The signal occupies the full band: Need to perform oversampling as usually done and pulse shaping in order to respect the spectral mask • compromise between low envelope fluctuations, low out-of-band radiations, efficient use of band (quasi rectangular PSD): work on progress • ideal low pass filter presents the worst degradation of PAPR, but the best filtering • on the contrary, rectangular pulse shape preserves the lowest PAPR but will exceed the mask • a compromise must be done and can easily be done in order to preserve a very low PAPR and respect the mask from [4], with IFDMA which is very close from our proposal

  16. Conclusion • 11ah Phy requires • low envelope fluctuations (low PAPR) for power savings • repetition for robustness and increase of range • DFT-precoded OFDM can be optimized in order to be very efficiency in respecting those constraints • full frequency diversity exploitation • PAPR of a single carrier scheme • it has proved its feasibility by having been accepted in 3GPP LTE uplink • on top of that, it opens the door to very simple downlink multi-user transmissions

  17. References Slide 17 Youhan Kim, Atheros [1] Porat R, Erceg V, Fisher M, Introductory TGah proposal, IEEE 802.11-11/0069r1, Jan. 2011 [2] Park M, Qi E, Low Power Consumption Opportunity in Sub 1 GHz, IEEE 802.11-11/1268r0, Nov 2010 [3] Cariou L, Hélard M, Frequency interleaved CDMA: a new multiple access scheme, IEEE VTC spring’09 [4] Tobias F, Klein A, Haustein T, A Survey on the Envelope Fluctuations of DFT Precoded OFDMA Signals, IEEE ICC’08 [5] Rohling H, May T, et al, “Broad-Band OFDM Radio Transmission for Multimedia Applications,” Proc. of the IEEE, vol. 87, October 1999. [6] P. Xia, S. Zhou, and G. B. Giannakis, “Bandwidth- and Power-Efficient Multicarrier Multiple Access,” IEEE Trans. on Communications, vol. 51, pp. 1828–1837, November 2003.

  18. DFT-spread OFDM calculations • Let’s consider a BICM symbol xn,f using as spreading code the nth column of a DFT matrix and transmitting on a subset of subcarriers with a frequency shift f • xn,f is first multiplied by a nth column of a L-sized DFT matrix to generate a vector of L chip symbols Xn,f that can beexpressed as follows:

  19. DFT-spread OFDM calculations • The frequency interleaving operation can be expressed thanks to the frequency shift f, and assuming the existence of Q subsets of L subcarriers. The symbol Xkn,f, transmitted on subcarrier k is then: • The iDFT operation leads to the Nc time samples xmn,f that will be transmitted after cyclic prefix insertion:

  20. DFT-spread OFDM calculations • After some straightforward calculations, the equation becomes: • By defining m = Lq + n, we finally get:

  21. Multiple access solution DFT matrix SFxSF (LxL) Subcarrier mapping k=0 using different codes per user per subset of subcarriers (CDMA on subset of subcarriers) user 1 OFDM iDFT DFT matrix SFxSF (LxL) Nc Subcarriers m=Nc m=0 user 2 Nc time samples k=Nc

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