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Multi-band Modulation, Coding, and Medium Access Control

Multi-band Modulation, Coding, and Medium Access Control. Date: 2007-11-12. Abstract.

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Multi-band Modulation, Coding, and Medium Access Control

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  1. Multi-band Modulation, Coding, and Medium Access Control Date: 2007-11-12 R. C. Daniels, UT Austin

  2. Abstract Past IEEE 802.11 WLAN networks have used improvements in digital baseband algorithms (modulation, coding, etc.) and spatial multiplexing with multiple transmit and receive antennas to increase physical layer throughput. In this talk, we suggest that next generation WLAN systems must exploit large quantities of spectrum available at higher frequencies to achieve satisfactory throughput. In order to minimize MAC overhead and maximize PHY performance, we suggest some ideas for multi-band PHY and MAC implementation. R. C. Daniels, UT Austin

  3. VHT - Very High Throughput • Next Generation Wireless LANs • Stated Requirements (from previous VHT SG meetings): • Gigabit Throughput (5x Scaling) * • Extended Communication Range † • Improve MAC efficiency † * = critical requirement † = important requirement • Conflicting Requirements: • Backwards Compatibility with IEEE 802.11n • Interoperability and Coexistence R. C. Daniels, UT Austin

  4. Enhancing PHY Throughput • Exploitable dimensions in wireless (E-Mag) technology • Space » Higher Degree of Spatial Multiplexing • Polarization » Cross Polarized Multiplexing • Time » Broaden Bandwidth • Digital baseband improvements • Larger constellation sizes (256-QAM) • Advanced channel coding strategies (LDPC/Turbo) • Effective use of channel feedback (Digital Precoding) R. C. Daniels, UT Austin

  5. Enhancing PHY ThroughputExploiting the Spatial Dimension • We can always add more antennas, but will spatial multiplexing throughput gain scale? • Spatial multiplexing is limited by condition of the wireless channel • Throughput compromised by extra training in data and sounding* • Other drawbacks with large numbers of antennas • Cost • Size constraints on mobile devices R. C. Daniels, UT Austin

  6. Enhancing PHY ThroughputExploiting the Spatial Dimension • There exist information theoretic results that suggest maximum number of antennas [Hassibi ‘03] R. C. Daniels, UT Austin

  7. Enhancing PHY ThroughputExploiting the Time (Frequency) Dimension • Increasing the symbol time is the simplest way to increase throughput • Unfortunately, the necessary bandwidth (5x20 MHz = 100 MHz) allows for at most 1 channel at traditional frequencies (2.45 or 5 GHz) • Internationally available bandwidth to spare at higher frequencies [Daniels ‘07] R. C. Daniels, UT Austin

  8. Enhancing PHY ThroughputDigital Baseband Improvements • Higher constellation order (256-QAM) • Places more demands on the phase tracking and SNR • Advanced channel coding (LDPC/Turbo) • Already optionally present in IEEE 802.11n • More effective use of feedback • Present in IEEE 802.11n, doesn’t take advantage of recent limited feedback research [Choi ‘05], [Mondal ‘05], [Choi ‘06] 20 dB 30 dB 40 dB R. C. Daniels, UT Austin

  9. Enhancing PHY ThroughputSummary • Adding more antennas has limitations • Practical maximum spatial multiplexing gain (< 8) • More antennas is not the solution • Digital baseband additions only partially solve problem • Solution: Significantly more bandwidth needed R. C. Daniels, UT Austin

  10. The Multi-band Solution • Simple Idea • Lower frequencies for lower throughput • Higher frequencies for higher throughput • VHT focus • Range extension with lower frequencies • Throughput extension with higher frequencies • Both RF chains funnel data through digital baseband • Joint PHY and MAC for all carrier frequencies • Improves on IEEE 802.11n multi-RF approach R. C. Daniels, UT Austin

  11. Multi-band Modulation and Coding • This is an equivalent strategy used in past IEEE 802.11 standards • Now require a higher carrier frequency instead of higher SNR for enhanced throughput modulation and coding schemes • Can maintain backwards compatibility with IEEE 802.11n and just use higher frequencies for higher level MCSs R. C. Daniels, UT Austin

  12. Multi-band versus Multi-mode • Many have proposed 2.45/5/60 GHz multi-mode devices, or an IEEE 802.11n/802.15.3c combination R. C. Daniels, UT Austin

  13. Multi-band versus Multi-mode • Multi-band devices can be based off a single reference local oscillator • Concurrent multi-band operation [Hashemi ‘03] frequency, phase offsets and ADC or DAC consistent among all RF units R. C. Daniels, UT Austin

  14. The Multi-band Physical (M-PHY) Layer • Design Examples: A Preview • Training sent on one band, data on another Increase performance of higher frequency system, by performing synchronization, frequency offset at lower, more reliable symbol rate. R. C. Daniels, UT Austin

  15. Multi-band Synchronization Example • Multi-band frame synchronization and frequency offset estimation simulated on Hydra - an IEEE 802.11n prototype (http://hydra.ece.utexas.edu) • MCS 0/1/2 (BPSK/QPSK) • Dotted lines show improvement • Training at 20 dB • Data SNR shown on graph • Simulated multipath channel • Frequency offset added R. C. Daniels, UT Austin

  16. The Multi-band Medium Access Control (M-MAC) Layer • Design Examples: A Preview • Divide MAC functionality over each band to reduce contention • Short, low-latency packets (VoIP) use lower frequency channels • Throughput-demanding packets use higher frequency channels R. C. Daniels, UT Austin

  17. Summary • Inevitably more bandwidth necessary for next generation of WLAN (VHT) • Concurrent operation of PHY and MAC functions jointly on different bands reduces overhead and latency • Multi-band Modulation, Coding, and MAC moves WLAN into cognitive arena R. C. Daniels, UT Austin

  18. References • B. Hassibi and B.M. Hochwald, ``How much training is needed in a multiple-antenna wireless link,” IEEE Transactions on Information Theory, vol.49, no.4, Apr. 2003, pages 951-964. • H. Hashemi, ``Integrated Concurrent Multi-Band Radios and Multiple-Antenna Systems,” PhD Thesis, Caltech University, 2003. • J. Choi and R. W. Heath, Jr., ``Interpolation Based Transmit Beamforming for MIMO-OFDM with Limited Feedback,'' IEEE Trans. on Signal Processing, vol. 53, no. 11, pp. 4125-4135, Nov. 2005. • B. Mondal and R. W. Heath, Jr., ``Algorithms for Quantized Precoded MIMO-OFDM Systems,'' Proc. of the IEEE Asilomar Conf. on Signals, Systems, and Computers, pp. 381-385 Pacific Grove, CA, USA, Oct. 30 - Nov. 2, 2005. • J. Choi, B. Mondal, and R. W. Heath, Jr., ``Interpolation Based Unitary Precoding for Spatial Multiplexing MIMO-OFDM with Limited Feedback,'' IEEE Trans. on Signal Processing, vol. 54, no. 12, pp. 4730-4740, December 2006. • N. Devroye, P. Mitran, and V. Tarokh ``Achievable Rates in Cognitive Radio Channels,'’ IEEE Trans. Inform. Theory, vol.52, no.5, pp. 1813-1827, May 2006. • R. C. Daniels and R. W. Heath, Jr., ``60 GHz Wireless Communications: Emerging Requirements and Design Recommendations,'' submitted to the IEEE Vehicular Technology Magazine, April 2007. R. C. Daniels, UT Austin

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