1. Joint Coding and Modulation Diversity for 802.11ah
January 2010
doc.: IEEE /0084r3
Joint Coding and Modulation Diversity for ah
Date:
Authors:
Brian Hart, Cisco Systems

2. Background
802.11ah shall include support for 1 MHz, 2 MHz, 4 MHz, 8 MHz, and 16 MHz PHY transmissions. [1]
An ah STA shall support reception of 1 MHz and 2 MHz PHY transmissions. [1]
The 2 MHz PHY transmission shall be an OFDM based waveform consisting of a total of 64 tones (including tones allocated as pilot, guard and DC). This implies a tone spacing of kHz. The tone spacing for all other bandwidths PHY transmissions shall be same as the tone spacing in the 2 MHz PHY transmission.
Compared with ac, Tgah has smaller bandwidth and tone spacing .

3. 802.11ah Channel Model Based on IEEE 802.11-11/0883r1[2]
TGah outdoor channel model
3GPP/3GPP2 SCM (spatial channel model) shall be used to evaluate 11ah outdoor MIMO link and system performance.
TGah indoor channel model
The proposed indoor channel model for TGah is based on the n channel models, which have been widely used in the Standard development.

4. Abstract
Doc.IEEE /0069r0[3] proposed to use TGac features as a basis for Tgah.
Doc. IEEE /0336r0 [4] has demonstrated the performance advantages of Joint Coding and Modulation Diversity technique in ac system.
Based on our recent research, Joint Coding and Modulation Diversity technique can also improve the overall performance of ah system, especially in MIMO-OFDM scenario. Therefore, we recommend to introduce this technique to ah system.

5. Proposed Transmitter/Receiver Block Diagram
NOTES
— Red blocks are our proposed amendment.
—The number of transmit chains can be not equal to the number of space-time streams
— SVD-precoding, codebook-percoding and non-precoding are all supported.

6. In the amendment, we use rotational modulation method to combine with time diversity of channel coding, spatial diversity of MIMO and frequency diversity of OFDM, which is named joint coding and modulation diversity (JCMD) .
As compared with JCMD, the processing scheme in IEEE n/ac Standard is named bit interleaved coded modulation (BICM) for simplicity.
Amendment and simulation are based on the document IEEE n-2009 ,IEEE /0992r18 and Draft ac D1.1.

7. Basic principle of the rotational modulation
According to rotational matrix，rotate the conventional modulated symbol.
The relationship between conventional modulated complex symbol A + j*B and the rotational modulated complex symbol X + j*Y is shown in equation:
where A and B are the I (in-phase) and Q (quadrature) component of the normal QAM, respectively; X and Y are the I and Q component of rotated QAM, respectively

8. An example of rotated QPSK
It is already used in IEEE ad.

9. Our Proposed Rotation Matrix to 802.11ah system
Modulation
Proposed Rotation Matrix
QPSK
16QAM
64QAM
256QAM

10. Basic principle of the Spatial Interleaving
In this process , denotes rotated symbol on the stream at time sample. So the interleaving is usual spiral layer interleaving process among all streams at the same time. The method is as follow:
Where is the number of spatial streams.
For example, consider the case, =4:
Corresponding, uses the inverse algorithm at the receiver as follow:

11. Basic principle of the Spatial Q-Interleaving
In the spatial Q-interleaving process, I components of the complex signals are unchanged, while Q components of signals are changed as follows:
That is to say, Q component on stream i will be moved to the stream (Nss-i-1).
So, it is just a simple reverse interleaver, and the interleaver length is the number of the spatial streams.

12. Basic principle of the Frequency domain Q-interleaveing
In the frequency domain Q-interleaving process, the I components of the complex signals are unchanged, while Q components of signals are changed as follows:
is the number of subcarriers for data.
That is to say, Q component on subcarrier i will be moved to the subcarrier .
So, it is just a simple cyclic-shift interleaver, and the linterleaver length is the number of data subcarriers .

13. Basic principle of demodulation
Due to spatial Q-interleaving and frequency Q-interleaving, fading coefficient of I component is usually different from that of Q component

14. Basic principle of demodulation
For example, consider the R-QPSK（rotational quadrature phase-shift keying）:
The procedure for demodulation is shown as follows:
Compute the distance between the received point and each reference constellation point. The relationship between the reference constellation point and the rotational constellation point is shown as follows: so

15. Basic principle of demodulation
2) Compute the Likelihood ratio (LLR) for every bit. The LLR is the input of the decoder.
For the first bit :
For the second bit:
There is also a simplified algorithm to compute the LLRs:
Similarly, for M-ary QAM(Quadrature amplitude modulation), we should compute LLRs for bits.

16. Simulation Parameters
Values
PHY scheme
OFDM
Antenna scheme
2*2
Length of FFT 64
Number of subcarriers 56
Number of data subcarriers 52
Code Type
BCC
Channel Model
802.11n channel models (Indoor)
3GPP SCM channel model (Outdoor)
Code Rate
3/4, 5/6
Modulation Type
QPSK 16QAM 64QAM
Bandwidth
2 MHz
Sub-carrier spacing
31.25 kHZ
Channel estimation
Ideal channel estimation

17. MCS (modulation and coding scheme) （2*2）
CODE RATE
Number of OFDM symbols per frame
Block Size 2
QPSK
3/4 6
1248 4
16-QAM
1668 7
64-QAM
5/6

18. Indoor Channel Model LOS case E [2],BCC
Parameters
Value 2
Carrier Frequency
0.8 GHz
Code Rate
3/4, 5/6
Modulation Type
QPSK,16QAM,64QAM
Bandwidth
2.0 MHz
Sample Time
500 ns
FFT 64
Subcarrier Bandwidth
kHz CP 16
Speed
1.2 km/h
Gain dB (FER=0.1)
QPSK 6
16QAM
5.6
64QAM 5

19. Outdoor Channel Model 3GPP SCM[3],BCC
Parameters
Value 2
Carrier Frequency
0.8 GHz
Code Rate
3/4, 5/6
Modulation Type
QPSK,16QAM,64QAM
Bandwidth
2.0 MHz
Sample Time
500 ns
FFT 64
Subcarrier Bandwidth
kHz CP 16
Speed
1.2 km/h
Gain dB (FER=0.1)
QPSK 8
16QAM 7
64QAM 6

20. Hardware Platform The platform Rohde&Schwarz AMU (fading simulator)
2*FSV(signal analyzer)
2*SMBV(vector signal generator)
2*PicoChip PC203 Baseband Unit
2*RRU

21. Hardware Simulation Results
Parameters
Value
Code
Turbp
Modulation Type
QPSK
Code Rate
3/4
Code Length
768
Channel Model
TU 3Path
Speed
0km/h
Channel Estimation LS
SISO: JCMD obtains 2 dB SNR gain at FER=0.1 as compared with BICM.

22. Hardware Simulation Results
Parameters
Value
Channel Model
VA_MIMO_60km/h
Code
3GPP LTE Turbo
Code Rate
3/4
Modulation Type
QPSK
Scheme
BICM/JCMD
Precoding
No-Precoding
Channel Estimation
LS, LMMSE
MIMO detection
MMSE
2*2 MIMO: JCMD obtains 3.3dB SNR gain at FER=0.1 as compared with BICM.

23. June 2010
doc.: IEEE /xxxxr0
Conclusions
It is proved that the proposed scheme has the obvious SNR gains over the current scheme, which implies
Larger coverage area
Lower transmit power
The proposed scheme is easy to be implemented
Rotated QAM modulation
Q-components Interleaver within one OFDM symbol
In a word, the proposed scheme is very suitable for TGah to meet the requirement of PAR.
Brian Hart, Cisco Systems

24. References
[1] ah-spec-framework-text-of-11ah-bw-modes.pptx
[2] ah-Channel-Model-Text.docx
[3] 3GPP TR Technical Specification Group Radio Access Network; Spatial channel model for Multiple Input Multiple Output (MIMO) simulations
[4] ah-tgah-Introductory-proposal.ppt
[5] ac-joint-coding-and-modulation-diversity-to ac.ppt
[6] ah-specification-framework-for-tgah.docx

25. Strawpoll
Do you accept JCMD as an enhanced coded modulation scheme for ah?
-Yes
-No
-Abstain