1. Month Year
doc.: IEEE yy/xxxxr0
November 2012
DFT spreading OFDM optional specification proposal for 11ah low rate PHY
Date:
Authors:
Slide 1
John Doe, Some Company
John Doe, Some Company

2. November 2012
Background
TGah PHY supports 2/4/8/16MHz signals based on the down clocked versions of 11ac 20/40/80/160 MHz, and an 1MHz signals based on 32FFT.
Both 2MHz and 1MHz signals are mandatory in TGah PHY.
DFT spreading OFDM (DFTs-OFDM) is adopted for the uplink in IMT-2000 LTE for battery operated mobile terminal.
DFTs-OFDM application to 11ah PHY was suggested in IEEE /0753r0, and IEEE /0349r2 in 2012/3.
The proposal of IEEE /0349r2 was well-supported for its advantage of lower PAPR and lower ACPL (adjacent channel power leakage) in non-linear HPA operation.
Y N A 23
Slide 2

3. Abstract This contribution is a follow-up of IEEE802.11-12/0349r2.
November 2012
Abstract
This contribution is a follow-up of IEEE /0349r2.
We propose DFT-spreading OFDM based optional specification for 11ah low rate PHY, especially for 1MHz and 2MHz modes, aiming at sensor network applications which need high power efficiency.
Slide 3

4. Review of advantages of DFTs-OFDM
Battery driven wireless terminals are used for sensor network applications
Basic requirements for the use cases of 1a/1f and 2d/2e/2f
Low transmission speed: 2 MHz mode and 1 MHz mode
Long battery life time
Low power consumption at wireless sensor terminals
Basic Requirements for the use cases
Lower PAPR and lower ACPL (adjacent channel power leakage) with higher energy efficiency, i.e. low output power back-off operation at HPA.
Less BER performance degradation in non-linear HPA operation.
Slide 4

5. Null DC sub-carrier DFTs-OFDM
November 2011
Null DC sub-carrier DFTs-OFDM
57 DFT/IDFT is the simplest approach for DFTs-OFDM, however FFT algorithm can not be applied for DFT/IDFT implementation since 57 is neither “power of two” nor “even”.
It needs large amount of signal processing for DFT/IDFT.
56 DFT/IDFT is another simple approach for DFTs-OFDM.
56 DFT/IDFT needs less amount of signal processing than 57 DFT/IDFT.
DC tone is replaced with one data tone.
DC offset error at modulator and demodulator can degrade BER performance in the conventional DFTs-OFDM..
No BER performance degradation in OFDM since DC sub-carrier is not used.
DFTs-OFDM with null DC sub-carrier is proposed for 11ah low rate PHY optional specification, especially for sensor network applications where high power efficiency is required.
No BER performance degradation due to DC offset error in the proposed DFTs-OFDM since DC sub-carrier is made null.
Slide 5
Hongyuan Zhang, et. Al.

6. Block diagram of NDCS-DFTs-OFDM
January 2012
Block diagram of NDCS-DFTs-OFDM
: Additional blocks required for NDCS-DFTs-OFDM
Scrambler
FEC
Interleaver
BPSK/QPSK
mapper
DFT
IFFT
GI &
Window
Analog
& RF
(a) Transmitter side of NDCS-DFTs-OFDM
De-Scrambler
FEC
decoder
De-interleaver
BPSK/QPSK
De-mapper
IDFT
Frequency Domain Equalization
FFT
Remove GI
Analog
& RF
(b) Receiver side of NDCS-DFTs-OFDM
DFT on the transmission side
26 points DFT for 1MHz, 56 points DFT for 2MHz
IDFT and FDE on the reception side
26 points IDFT for 1MHz, 56 points IDFT for 2MHz
FDE is performed using CSI.
Slide 6

7. Details of DFT/IDFT in NDCS-DFTs-OFDM
M=56 and N=64 for 2MHz signals (52 Data tones, 4 Pilot tones, 7 Guard tones, and 1 DC tone)
M=26 and N=32 for 1MHz signals (24 Data tones, 2 Pilot tones, 5 Guard tones, and 1 DC tone)
Spectrum splitting M
M/2
M+1 N N
S/P
MOD
DFT
IFFT
P/S
windowing GI
...
D/A
Data
Output
...
...
...
M/2
(1) Transmitter side M
M/2
M+1 N N
P/S
DEM
IDFT
FDE
FFT
S/P
Remove GI
A/D …
Data
Input … … X … … …
M/2
(2) Receiver side
Slide 7

8. Preamble and signal design for NDCS-DFTs-OFDM
DFTs-OMDF optional specification for 1MH/2MHz BPSK/QPSK signals.
No change in the preamble and basic signal design of OFDM signals.
1MHz (32 FFT)
24 Data tones, 2 Pilot tones, 5 Guard tones, and 1 DC tone
2MHz(64 FFT)
52 Data tones, 4 Pilot tones, 7 Guard tones, and 1 DC tone
AH-STF
AH-LTF
AH-SIG
AH-LTF1
DFTS-OFDM 1
DFTS-OFDM 2 …
DFTS-OFDM n
OFDM based
DFT Spreading is applied in the data field only.
Slide 8

9. Modulated by random data.
Comparison of PAPR
Null DC subcarrier DFTs-OFDM (NDCS-DFTs-OFDM) achieves 2.0dB lower PAPR than OFDM, but 0.5dB higher than conventional DFTs-OFDM at CCDF=0.1%.
M=56
N=64
Modulated by random data.
2.0dB
Slide 9

10. Comparison of adjacent channel power leakage
NDCS-DFTs-OFDM has almost the same adjacent channel power leakage as the conventional DFTs-OFDM, which is 3-4dB lower than that of OFDM when OBO=3dB.
Rapp model
Slide 10

11. Estimated PAE improvement
DFTs-Spreading approach can reduce OBO from 6dB to 4.5dB to maintain ACLP.
1.5dB OBO reduction improves 20% in PAE (power-added-efficiency) from 15% to 18%. 5
56 sub-carriers / 2MHz mode / QPSK
6dB (OFDM)
6dB
5dB
4dB -5
-10
Power Density [dB]
-15
-20
-25
-30 1 2 3
Frequency (MHz)
Jeonghu Han et al, “A Fully-Integrated 900-MHz CMOS Power Amplifier for Mobile RFID Reader Applications,”　IEEE RFIC Symposium, 2006.
Slide 11

12. EVM according to DC offset error in NDCS-DFTs-OFDM
No EVM (Error Vector Magnitude) degradation due to DC offset error is observed in NDCS-DFTs-OFDM. 5 10 15 20 25 30 35 40 45 50 2 4 6 8
DC offset error [%]
EVM [%] :
DFTs -
OFDM
: NDSC-DFTs-OFDM
M=56
N=64
QPSK
Slide 12

13. Signal processing complexity
Estimation of required complex multiplications per OFDM symbol.
(1) 1MHz mode
DFT spreading(26 point)
IFFT(32 point)
Total
OFDM 80
DFTs-OFDM
338
418
NDCS-DFTs-OFDM
Note: 26 point DFT is decomposed to 2X13 point DFT.
(2) 2MHz mode
DFT spreading(56 point)
IFFT(64 point)
Total
OFDM
192
DFTs-OFDM
476
668
NDCS-DFTs-OFDM
Note: PFA (Prime Factor FFT algorithm) can be applied for 56 (8X7) points DFT.
Slide 13

14. November 2011
Summary
NDCS-DFTs-OFDM optional specification for 11ah low rate PHT is proposed.
26 point DFT and 32 point FFT for 1MHz mode QPSK/BPSK.
56 point DFT and 64 point FFT for 2MHz mode QPSK/BPSK.
DC sub-carrier is made null
Features of the proposed specification
No change in the preamble and basic signal design of OFDM signals.
Lower PAPR and lower ACPL than OFDM
No EVM degradation due to DC offset error
Less signal processing complexity by spectrum splitting
Slide 14

15. November 2011
References
[1] ah-dft-spread-ofdm-optimized-for ah
[2] ah-preamble-format-for-1-MHz
[3] ah-11ah-preamble-for-2MHz-and-beyond
[4] ah-11ah-phy-transmission-flow
[5] ah-dft-spreading-OFDM-options-for-11ah-phy-enhancement
Hongyuan Zhang, et. Al.

16. November 2011
Straw Poll
Do you support NDCS-DFTs-OFDM based optional specification in slide 7 and 8 for 11ah low rate PHY ?
Mainly for 1MHz/2MHz signals with BPSK/QPSK mode. Y N A
Hongyuan Zhang, et. Al.