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November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 1
doc.: IEEE 802.11-03/825r0
Submission
Comparison of 128QAM mappings/labelings for 802.11n
Ravi Mahadevappa, [email protected] ten Brink, [email protected]
Realtek Semiconductors, Irvine, CA
November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 2
doc.: IEEE 802.11-03/825r0
Submission
Overview• 128QAM for increasing data rate of 802.11n
– MIMO 2xN: can achieve 2x54Mbps = 108Mbps in 20MHz– 108Mbps peak too small to get 100Mbps MAC throughput– MIMO 2xN, 128QAM, R=7/8 code, 20MHz: 147Mbps
achievable
• Consider four 128QAM constellations/labelings– Determine the one which is most suitable
• Performance comparison using mutual information in bit-interleaved coded modulation (BICM) [1], [2]
• BER chart in AWGN and Rayleigh channel• PER chart in an 802.11a-like setting (2x2, 2x3 MIMO)
November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 3
doc.: IEEE 802.11-03/825r0
Submission
Introduction• Bit Interleaved Coded Modulation (BICM)
– Gray-labeling of the constellation points is best to achieve a low bit error rate (BER), if no iterative decoding is applied [2]
• For QPSK, 16QAM, 64QAM, 256QAM– True Gray-labeling possible, using a square constellation– Gray-labeling per I-/Q-channel– I-/Q-channel independent, can be demapped separately
• For 128QAM– no true Gray-labeling possible– e.g. 128QAM: 7 bits; one bit has to be “distributed” over I/Q
November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 4
doc.: IEEE 802.11-03/825r0
Submission
802.11a Transmitter
channelencoder
andpuncturer
QAMmapper iFFT
add cyclicextension(guard)
addtrainingsymbols
interpol.and filter,
limiter
bit interleaver
add pilotsymbols
D/A up-converter
amplifier
binary source
• Bit interleaved coded modulation– Channel encoder (error correcting coding) and QAM
symbol mapper are connected through a bit interleaver– The 802.11a WLAN system [3] exhibits this structure
November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 5
doc.: IEEE 802.11-03/825r0
Submission
802.11a Receiver
decimateandfilter
synchr.frequencycorrection FFT QAM
demapper
bit deinterleaver
de-punct.and
channeldecoder
down-converter
amplifier A/D
frequ.offset
estimator
channelestimator
andtracker
binary sink
pilotremoval
-1
November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 6
doc.: IEEE 802.11-03/825r0
Submission
64QAM Example: Gray-labeling• 64QAM with Gray-
labeling: bit labels of neighboring signal points differ by one binary digit
• Most systems with QAM modulation use Gray-labeling, e.g. 802.11a WLAN [3]
• Allows low-complexity bit detection (I/Q can be dealt with separately)
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111 111 111 111 111 111 111 111
101 101 101 101 101 101 101 101
100 100 100 100 100 100 100 100
November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 7
doc.: IEEE 802.11-03/825r0
Submission
128QAM Constellations: Shifted I• Based on two shifted
64QAM constellations
• Proposed for different systems, e.g. [4]
• Motivation: I&Q can be demapped separately
• Bit labels of neighboring signal points differ by two binary digits
• See later: Good for iterative BICM (demapper/decoder iterations), but not good for BICM
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1000000 1000011 1000110 1000101 1000100 1000111 1000010 1000001
1011000 1011011 1011110 1011101 1011100 1011111 1011010 1011001
1110000 1110011 1110110 1110101 1110100 1110111 1110010 1110001
1101000 1101011 1101110 1101101 1101100 1101111 1101010 1101001
0100000 0100011 0100110 0100101 0100100 0100111 0100010 0100001
0111000 0111011 0111110 0111101 0111100 0111111 0111010 0111001
0010000 0010011 0010110 0010101 0010100 0010111 0010010 0010001
0001000 0001011 0001110 0001101 0001100 0001111 0001010 0001001
10010101001001 1001111 1001100 1001101 1001110 1001011 1001000
10100101010001 1010111 1010100 1010101 1010110 1010011 1010000
11110101111001 1111111 1111100 1111101 1111110 1111011 1111000
11000101100001 1100111 1100100 1100101 1100110 1100011 1100000
01010100101001 0101111 0101100 0101101 0101110 0101011 0101000
01100100110001 0110111 0110100 0110101 0110110 0110011 0110000
00110100011001 0011111 0011100 0011101 0011110 0011011 0011000
00000100000001 0000111 0000100 0000101 0000110 0000011 0000000
November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 8
doc.: IEEE 802.11-03/825r0
Submission
128QAM: Shifted II, 64QAM/Gray• Based on two 64QAM
constellations, shifted
• Gray-labeling per 64QAM constellation
• Bit labels of neighboring signal points differ by more than one binary digit at several places
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001 001 001 001 001 001 001 001
011 011 011 011 011 011 011 011
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111 111 111 111 111 111 111 111
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001 001 001 001 001 001 001 001
011 011 011 011 011 011 011 011
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November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 9
doc.: IEEE 802.11-03/825r0
Submission
128QAM: Cross I, DVB-C• DVB-C(able) [5] uses
cross constellation and this bit labeling
• The two MSB’s are differentially encoded, to be rotationally invariant against 90degree flips
• “Almost” Gray-labeling within one quadrant, but bit labels differ by many bits along the zero I- and Q-axis
• Not designed for BICM
0000000
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01100 01110
01101 01111
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01001 01011
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IQ 00IQ 10
IQ 11 IQ 01
0000000010
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0010000110
1001010000
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1111011100
1111111101
0110001110
0110101111
1100011010
1100111011
0100101011
0100001010
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igits
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November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 10
doc.: IEEE 802.11-03/825r0
Submission
128QAM: Cross II, Gray-like• Center: 64QAM with
Gray-labeling as 802.11a; the 7th bit (most significant bit, MSB) is set to zero
• Borders: mirrored 64QAM; horizontally, vertically flipped from center, MSB set to one
• Labels of neighboring signal points differ by 3 digits at few places
• All other bit labels of neighboring signal points differ by only one binary digit
000000 000 000 000 000 000 000 000
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011 011 011 011 011 011 011 011
010 010 010 010 010 010 010 010
110 110 110 110 110 110 110 110
111 111 111 111 111 111 111 111
101 101 101 101 101 101 101 101
100 100 100 100 100 100 100 100
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0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0000001
001
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011 010 110 111101 101 101 1011 1 1 1
000 000 000 000011 010 110 1111 1 1 1
011 010 110 111001 001 001 0011 1 1 1
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3 digits 3 digits
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November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 11
doc.: IEEE 802.11-03/825r0
Submission
128QAM: Cross II, Gray-like• Generation by
mirroring, flipping center 64QAM Gray constellation to outside and setting MSB from 0 to 1
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111 111 111 111 111 111 111 111
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November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 12
doc.: IEEE 802.11-03/825r0
Submission
128QAM: Comparison, EXIT Chart• Extrinsic information
transfer (EXIT) chart [6] to predict performance
• AWGN, Eb/N0=9dB, at code rate 3/4
• For BICM, start of curve essential (this is the mutual information, that demapper “sees”)
• Cross II: highest start– best for BICM
• Cross I has “moderate” slope; Shifted II similar
– Mediocre for BICM
• Shifted I: lowest start– bad for BICM– but would be best for
iterative demapping and decoding
Only start of curve relevant for good BICM performance(the higher, the better)
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AM
AP
Pde
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outp
ut
a prio ri m utual info rm atio n at 128Q A M A P P dem apper input
cross II (Gray-like)
128Q A M , cro ss I (D V B -C labeling), 9dB
128Q A M , shif ted I, 9dB
co de rate 3/4
128Q A M , shif ted II (two shif ted 64Q A M G ray), 9dB
shifted I
November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 13
doc.: IEEE 802.11-03/825r0
Submission
128QAM: BER Chart, AWGN• AWGN, rate 3/4
memory 6 convolutional code
• 64QAM (Gray) as reference
• Best: Cross II
• Worst: Shifted I
• Difference about 2dB
0.0001
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Es/N0 [dB]
64QAM, rate 3/4 memory 6 code, reference128QAM, Cross II128QAM, Cross I
128QAM, Shifted II128QAM, Shifted I
64QA
M, referen
ce128Q
AM
, Cro
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128QA
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November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 14
doc.: IEEE 802.11-03/825r0
Submission
128QAM: BER Chart, Rayleigh• Rayleigh channel
(ergodic), rate 3/4 memory 6 convolutional code
• 64QAM as reference
• Best: Cross II
• Worst: Shifted I
• Difference about 2dB
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Es/N0 [dB]
64QAM, rate 3/4 memory 6 code, reference128QAM, Cross II128QAM, Cross I
128QAM, Shifted II128QAM, Shifted I
November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 15
doc.: IEEE 802.11-03/825r0
Submission
PER, 802.11a-like High-Rate System• MIMO-OFDM simulation,
with 11a parameters for symbol duration, guard time, 64FFT etc.
• M=2 TX antennas (spatial multiplexing), 128QAM rate 3/4 mem. 6 conv. code; PHY rate of 126Mbps
• MIMO sub-channels: independent fading, with exp. decay profile, Trms = 60ns
• MIMO ZF detection with soft post processing
• Ca. 1dB-advantage of Cross II over Shifted II
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Es/N0 [dB]
MIMO 2x3
MIMO 2x2
Shifted II
(2x64Gray)
Cross II
(Gray-like)
2x2, Cross II (Gray-like labeling)
2x2, Shifted II (2x64Gray)2x3, Cross II
2x3, Shifted II
128QAM constellations
1dB1.3dB
November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 16
doc.: IEEE 802.11-03/825r0
Submission
Conclusion
• To increase spectral efficiency, the use of 128QAM is a possible option
• 128QAM helps to smoothen the rate table• Constellation mapping and labeling important• Recommendation:
If 128QAM is to be considered for 11n, cross-constellation with Gray-like labeling (Cross II) should be used
November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 17
doc.: IEEE 802.11-03/825r0
Submission
References[1] E. Zehavi, “8-PSK trellis codes for a Rayleigh channel”, IEEE Trans. Commun.,
vol. 40, pp. 873-884, May 1992
[2] G. Caire, G. Taricco, E. Biglieri, “Bit-interleaved coded modulation”, IEEE Trans. Inf. Theory, vol. 44, no. 3, pp. 927-946, May 1998
[3] IEEE Std 802.11a-1999, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, High-speed Physical Layer in the 5 GHz Band
[4] IEEE P802.15-15-03-0311-00-003a, “Reasons to use non-squared QAM constellations with independent I&Q in PAN systems”, July 2003
[5] “Digital Video Broadcasting (DVB): Framing structure, channel coding and modulation for cable systems”, EN 300 429, V. 1.2.1 (1998-04), European Standard (Telecommunications series)
[6] S. ten Brink, “Convergence of iterative decoding,”, Electron. Lett., vol. 35, no. 10, pp. 806-808, May 1999
November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 18
doc.: IEEE 802.11-03/825r0
Submission
Backup Slides
November 2003
Ravi Mahadevappa, Stephan ten Brink, Realtek
Slide 19
doc.: IEEE 802.11-03/825r0
Submission
128QAM: Comparison, EXIT Chart• For Cross I, one needs
to increase Eb/N0 by 1dB to achieve the same starting point as Cross II
• For Shifted I, one needs to increase Eb/N0 by 2.2dB to achieve the same starting point as the Cross II constellation
• Shifted II: Increase by 1.4dB required
• Next step: Verify Eb/N0-offset predictions by BER simulations using memory 6 convolutional code of rate 3/40
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128Q A M , cro ss II (G ray-like labeling), 9dB128Q A M , cro ss I (D V B -C labeling), 9dB
128Q A M , shif ted I, 9dB
co de rate 3/4
128Q A M , shif ted II (two shif ted 64Q A M G ray), 9dB
128Q A M , cro ss I (D V B -C labeling), 10dB128Q A M , shif ted I, E b/N 0=11.2dB
128Q A M , shif ted II (two shif ted 64Q A M G ray), 10.4dB
