11 07 2780-01-0vht Multi Band Modulation Coding and Medium Access Control

7/30/2019 11 07 2780-01-0vht Multi Band Modulation Coding and Medium Access Control

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R. C. Daniels, UT AustinSlide 1

Multi-band Modulation, Coding, and

Medium Access ControlDate: 2007-11-12

Authors Affiliations Addre ss Phone email

Robert C. Daniels The University of Texas

at Austin

[email protected]

Robert W. Heath, Jr. The University of Texasat Austin

[email protected]


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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 tominimize MAC overhead and maximize PHY

performance, we suggest some ideas for multi-band

PHY and MAC implementation.


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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


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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)


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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


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Enhancing PHY ThroughputExploiting the Spatial Dimension

There exist information theoretic results that suggest

maximum number of antennas [Hassibi 03]


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Enhancing PHY ThroughputExploiting the Time (F requency) Dimension

Increasing the symbol time is the simplest way toincrease throughput

Unfortunately, the necessary bandwidth (5x20 MHz =

100 MHz) allows for at most 1 channel at traditionalfrequencies (2.45 or 5 GHz)

Internationally available bandwidth to spare at higherfrequencies [Daniels 07]


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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, doesnt take advantage of recent limitedfeedback research [Choi 05], [Mondal 05], [Choi 06]

30 dB20 dB 40 dB


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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


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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


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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


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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


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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


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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.


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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
http://hydra.ece.utexas.edu/http://hydra.ece.utexas.edu/


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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


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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 movesWLAN into cognitive arena


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References

B. Hassibi and B.M. Hochwald, ``How much training is needed in a multiple-antennawireless link, IEEE Transactions on Information Theory, vol.49, no.4, Apr. 2003, pages951-964.

H. Hashemi, `Ìntegrated Concurrent Multi-Band Radios and Multiple-AntennaSystems, PhD Thesis, Caltech University, 2003.

J. Choi and R. W. Heath, Jr., `Ìnterpolation 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., `Àlgorithms for Quantized Precoded MIMO-OFDMSystems,'' 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., `Ìnterpolation Based Unitary Precoding forSpatial Multiplexing MIMO-OFDM with Limited Feedback,'' IEEE Trans. on SignalProcessing, vol. 54, no. 12, pp. 4730-4740, December 2006.

N. Devroye, P. Mitran, and V. Tarokh `Àchievable Rates in Cognitive RadioChannels,' 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: EmergingRequirements and Design Recommendations,'' submitted to the IEEE VehicularTechnology Magazine, April 2007.

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11 07 2780-01-0vht Multi Band Modulation Coding and Medium Access Control