ECE External Research Board Meeting Wireless Network and Communications

ECE External Research Board Meeting Wireless Network and Communications

Tan F. WongWireless Information and Networking Group

[email protected]://wireless.ece.ufl.edu/~twong

Ph: 352-392-2665Fax: 352-392-0044

Wireless Information Networking Group (WING)Wireless Information Networking Group (WING) http://wireless.ece.ufl.edu/

Current Research Interests• Communication and signal processing techniques to

support wireless ad hoc and sensor networks: – Channel and timing estimation in MIMO systems– Bit-interleaved space-frequency coded modulation for OFDM– Collaborative communications: coding, processing, and protocols– Interference detection & resource allocation for cognitive radios– Physics-based source localization using binary observations

• Spread spectrum and ultra-wideband research:– Interference avoidance & cancellation in spread spectrum– Joint power control and sequence optimization in CDMA – Power-efficient ARQ protocols for CDMA links– Acquisition and MAC protocols for UWB


Collaborative Communications1

Multiple antennas can be used to provide diversity against many impairments in radio networks:

Fading, Hostile Jamming, Collisions (especially for ad hoc networks)

However, multiple antennas are an unattractive choice in many systems because:

Antenna spacing needs to be several times the wavelength of the RF carrier

Array elements should be physically connected to a central combiner

Many systems, such as small handsets carried by infantry, cannot support these requirements

1Sponsored by the Office of Naval Research, National Science Foundation, and Harris Corporation. Joint work with Drs. Mike Fang and John Shea


Collaboration: A Cross-Layer Approach

Collaboration uses Network-Based Approaches to Achieve (Physical-Layer) Diversity

Antenna arrays are inherently present in any wireless network!

Different nodes in the network can act like elements of an antenna array

Since the nodes are not physically connected we refer to this as a Distributed Array

Information to be combined must be exchanged over wireless links!

There is no inherent central combiner, so distributed processing may be used.


Challenges Traditional combining techniques (MRC, EGC) require

a large amount of information to be sent to the combining node new physical-layer approaches to achieve diversity are required

Collaborating nodes must coordinate information exchanges; information exchanges should depend on physical layer signals new collaboration protocols are required

Phase coherence cannot usually be achieved between nodes because of the difficulty in synchronization new distributed signal processing techniques are required

In ad hoc networks, relay transmissions not only reduce spectral efficiency but also interfere with other transmissions practical collaborative protocols are required for use in ad hoc networks


Our Research

Cooperative communications is an active area of research in the Wireless Information Networking Group at the University of Florida

Our current research has three major thrusts

In Collaborative Reception/Distributed Decoding, we are developing efficient combining techniques and iterative, distributed decoding protocols

In Collaborative Jamming Mitigation, we are developing distributed signal processing techniques to detect jammed symbols and reduce the effect of the jamming before or during the decoding process.

In Network Diversity through Relaying in Ad Hoc Networks, we are developing efficient MAC protocols that use cross-layer information to enable efficient relaying in ad hoc networks


IDEA:

Nodes in the cluster form a virtual array

All nodes receive independent copies of the message from the transmitter

Collaborative Reception


Each node individually decodes received signal and estimates reliabilities of data bits using soft-input soft-output (SISO) decodersThe nodes use the reliability information in a process of smart information exchangeThe nodes perform decoding at the end of information exchange

Collaborative Decoding (Iterating between a process of information exchange and decoding) yields receive diversity

Collaborative Reception


Idea: more information needs to be combined for unreliable parts of a codeword than for reliable parts of the codeword

Technique: combine just the right amount of information such that the combined bit reliabilities exceed a pre-determined threshold T

Water filling in the reliability domain!

Reliability Filling


Collaborative Jamming Mitigation

Hostile jamming can severely disrupt communications networks

If multiple receive antennas with a coherent phase reference are available, jamming can be mitigated by beamforming

When multiple nodes collaborate in the presence of a jamming signal, phase coherence between nodes is not possible

Furthermore, in the presence of fading, each node receives the information and jamming signals at different amplitudes and phases


In Collaborative Jamming Mitigation, we develop distributed detection and estimation techniques to improve performance in the presence of hostile jammer

Nodes exchange received information to estimate and reject jammer

Jammer must use more power or transmit more of the time.

Collaborative Jamming Mitigation


Employ relay forwarding to achieve diversity or increase capacity in ad hoc networks

Example (see figure below)

The message has final destination H but intermediate destination B Nodes B and D suffer deep fades

Node C can act as a relay for the message; rather than relay it to B, node C sends it to F to move it on toward the destination H

Network Diversity through Relaying


Extend simple ALOHA protocol to support relay forwarding

Significant gains in end-to-end throughput and delay

Network Diversity through Relaying

Wireless Information Networking Group http://wireless.ece.ufl.edu

Reconfigurable Multi-node Wireless Communication Testbed

• Conduct physical and network layers experiments• Six-node cluster• Software processing baseband unit (A/D-D/A

board + PC)– Maximum reconfigurability, off-line processing

• Hardware processing baseband unit (FPGA board)

– Real-time implementation

• 250MHz Arbitrary waveform generator, 1GHz digital oscilloscope, spectrum analyzer

ISM-BandRF Frontend

LaptopComputer

DAQ(A/D-D/A)

FPGABoard

A/DD/A

ISM-BandRF Frontend

LaptopComputer

DAQ(A/D-D/A)

FPGABoard

A/DD/A


MIMO Processing and Coding • Employ physical antenna arrays to improve comm.

performance

• Interference mitigation using receive antenna array– Blind adaptive space-time interference rejection and multipath

diversity combining algorithm for DS-SS


MIMO Processing and Coding • Interference avoidance by selecting optimal spreading

codes for users in presence of jammers and MUI– More robust against jammers

– Higher user capacity with QoS control

– Lower Tx power better LPI/LPD

– Spreading codes depend on channel natural PHY security


MIMO Processing and Coding • Spatial multiplexing with multiple transmit antennas to

increase data rate– Bit-Interleaved Space-Time Coded-Modulation with Iterative

Decoding

• Channel and timing estimation in MIMO systems with jammers and interference

ECE External Research Board Meeting Wireless Network and Communications

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