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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