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Comparison of 128QAM mappings/labelings for 802.11n

Comparison of 128QAM mappings/labelings for 802.11n. Ravi Mahadevappa, ravi@realtek-us.com Stephan ten Brink, stenbrink@realtek-us.com Realtek Semiconductors, Irvine, CA. Overview. 128QAM for increasing data rate of 802.11n MIMO 2xN: can achieve 2x54Mbps = 108Mbps in 20MHz

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Comparison of 128QAM mappings/labelings for 802.11n

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  1. Comparison of 128QAM mappings/labelings for 802.11n Ravi Mahadevappa, ravi@realtek-us.com Stephan ten Brink, stenbrink@realtek-us.com Realtek Semiconductors, Irvine, CA Ravi Mahadevappa, Stephan ten Brink, Realtek

  2. 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 128QAMconstellations/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) Ravi Mahadevappa, Stephan ten Brink, Realtek

  3. 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 Ravi Mahadevappa, Stephan ten Brink, Realtek

  4. 802.11a Transmitter • 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 Ravi Mahadevappa, Stephan ten Brink, Realtek

  5. 802.11a Receiver Ravi Mahadevappa, Stephan ten Brink, Realtek

  6. 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) Ravi Mahadevappa, Stephan ten Brink, Realtek

  7. 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 Ravi Mahadevappa, Stephan ten Brink, Realtek

  8. 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 Ravi Mahadevappa, Stephan ten Brink, Realtek

  9. 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 Ravi Mahadevappa, Stephan ten Brink, Realtek

  10. 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 Ravi Mahadevappa, Stephan ten Brink, Realtek

  11. 128QAM: Cross II, Gray-like • Generation by mirroring, flipping center 64QAM Gray constellation to outside and setting MSB from 0 to 1 Ravi Mahadevappa, Stephan ten Brink, Realtek

  12. Only start of curve relevant for good BICM performance(the higher, the better) 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 Ravi Mahadevappa, Stephan ten Brink, Realtek

  13. 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 Ravi Mahadevappa, Stephan ten Brink, Realtek

  14. 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 Ravi Mahadevappa, Stephan ten Brink, Realtek

  15. 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 Ravi Mahadevappa, Stephan ten Brink, Realtek

  16. 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 Ravi Mahadevappa, Stephan ten Brink, Realtek

  17. 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 Ravi Mahadevappa, Stephan ten Brink, Realtek

  18. Backup Slides Ravi Mahadevappa, Stephan ten Brink, Realtek

  19. 128QAM: Comparison, EXIT Chart • For Cross I, one needs to increase Eb/N0by 1dB to achieve the same starting point as Cross II • For Shifted I, one needs to increase Eb/N0by 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 3/4 Ravi Mahadevappa, Stephan ten Brink, Realtek

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