Modern Wireless Communication: When Shannon Meets Marconi David Tse Wireless Foundations, University of California, Berkeley ICASSP 2006 Toulouse, France May 16, 2006
Modern Wireless Communication: When Shannon Meets Marconi David Tse Wireless Foundations, University of California, Berkeley ICASSP 2006 Toulouse, France May 16, 2006
Drive Towards Spectral Efficiency
Huge surge in air interface research in the past decade, driven by:
explosive demand for tetherless connectivity
dramatic progress in implementation technology
success of second-generation cellular standards, esp. CDMA (IS-95)
Significant impact has already been made.
Eg. IS-95 (mid 90’s) CDMA 2000 1x EV-DO (mid 00’s) 4 to 8 fold increase in spectral efficiency
…….and more to come.
Historical Perspective
Claude Shannon Gugliemo Marconi Information theory says every channel has a capacity.
Many recent advances based on understanding wireless channel capacity. New points of views arise. 1901 1948 Wireless communication has been around since 1900’s.
Ingenious system design techniques…….
but somewhat adhoc
Multipath Fading
Classical view: fading channels are unreliable
Modern view: multipath fading can be exploited to increase spectral efficiency. 16dB
Talk Outline
Real advances based on integration of theory and system
considerations. Two stories:
I: opportunistic communication (implemented in most 3G standards)
II multiple antenna (MIMO) communication (emerging standards)
I: Opportunistic Communication
Traditional Approach to Wireless System Design
Compensates for deep fades via diversity techniques over time, frequency and space.
(Glass is half empty.)
Example: CDMA
frequency diversity via Rake combining
time diversity via interleaving and coding
macro-diversity via soft handoff
transmit/receive antenna diversity
interference diversity: averaging of interference from many users.
Multipath Fading: Another Look
Multipath fading provides high peaks to exploit.
Channel capacity is achieved by such an opportunistic strategy. (Goldsmith & Varaiya 93)
Point-to-point performance benefits mainly in the energy-limited rather than the bandwidth-limited regime.
Multiuser Opportunistic Communication
Multiuser Diversity
In a large system with users fading independently, there is likely to be a user with a very good channel at any time.
Long term total throughput can be maximized by always serving the user with the strongest channel (Knopp&Humblet 95)
Application to CDMA 2000 1x EV-DO
Multiuser diversity provides a system-wide benefit.
Challenge is to share the benefit among the users in a fair way.
Symmetric Users
Serving the best user at each time is also fair in terms of
long term throughputs.
Asymmetric Users: Hitting the Peaks
Want to serve each user when it is at its peak.
A peak should be defined with respect to the latency time-scale tc
of the application.
Proportional Fair Scheduler
(Tse 99) De-facto scheduler in Ev-DO and similar algorithms
used in HSDPA. Schedule the user with the highest ratio
Rk = current requested rate of user k
Tk = average throughput of user k in the past tc time slots.
Performance
Channel Dynamics
Channel varies faster and has more dynamic range in mobile environments.
Inducing Randomness
Scheduling algorithm exploits the nature-given channel fluctuations by hitting the peaks.
If there are not enough fluctuations, why not purposely induce them?
Dumb Antennas
The information bearing signal at each of the transmit antenna
is multiplied by a time-varying phase. (Viswanath,Tse & Laroia 02)
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