Exploiting Physical Layer Advances in Wireless Networks Michael Honig Department of EECS Northwestern University.

Exploiting Physical Layer Advances in Wireless Networks

Michael HonigDepartment of EECS

Northwestern University

IEEE Spectrum Magazine, March 2004

Is spectrum scarce or abundant?

• Spectrum is scarce– Existing paradigm– Spectral efficiency is an important objective.– Physical layer advances play a crucial role in

improving wireless networks.

• Spectrum is abundant– Does spectral efficiency matter?– How should spectrum be allocated among services

(broadcast radio/TV, cellular, internet access,…)?– Should different wireless services share an

infrastructure of access points?

Physical Layer Advances

• Turbo (iterative message passing)

• MIMO

• Relay (cooperation)

• Adds degrees of freedom (DoFs) for diversity, multiplexing, interference mitigation and avoidance

• Two approaches:– Transmitters do not acquire Channel State Information

(e.g., space-time codes)– Transmitters acquire CSI, optimize resource allocation

(e.g., water-filling)

• Learning CSI can provide substantial benefits– Enables interference avoidance, can simplify coding– MIMO broadcast

• Overhead (feedback + channel estimation) is probably excessive for mobile users– e.g., 4x4 MIMO OFDM with 10 users, 100 sub-channels…

Exploiting MIMO

• Exchanging “interference prices” enables distributed power control in a spread spectrum network– Maximizes sum utility over links

• With MIMO/multi-carrier links, need an interference price for each signal dimension

• Tradeoff between information exchange (signaling overhead) and performance

Transmitter

Receiver

Distributed Resource Allocation

Exploiting Relays

• Objective: allocate power/time/bandwidth across links to maximize network objective (e.g., sum utility).– Difficult optimization problem due to half-duplex constraints.

• Is exchanging local state information (e.g., interference prices) good enough?

BST

R

M

RM

M

M

M

M

resources?

resources?

What if spectrum is abundant?

• Hinges on policy decisions– Spectrum “property rights” vs commons model– Vested interests

• Still need mechanisms for spectrum sharing– Sensing?– Incentives; spectrum markets?

(what’s the commodity?)– How should spectrum be allocated among services?

(broadcast radio/TV, cellular, internet access,…)– Should different wireless services share an infrastructure of

access points?

Will wireless nodes become generic commodities?

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