Joint phy mac designs and smart antennas for wireless ad hoc networks
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Joint PHY-MAC Designs and Smart Antennas for Wireless Ad-Hoc Networks. CS 838 - Mobile and Wireless Networking (Fall 2006). Review of IEEE 802.11a/b/g PHY/MAC. PHY Modulation: Orthogonal Frequency Division Multiplexing (OFDM) (11a/11g) or Direct Sequence Spread Spectrum (DS-SS) (11b/11g)

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Joint PHY-MAC Designs and Smart Antennas for Wireless Ad-Hoc Networks

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Joint phy mac designs and smart antennas for wireless ad hoc networks

Joint PHY-MAC Designs and Smart Antennas for Wireless Ad-Hoc Networks

CS 838 - Mobile and Wireless Networking

(Fall 2006)


Review of ieee 802 11a b g phy mac

Review of IEEE 802.11a/b/g PHY/MAC

PHY

  • Modulation: Orthogonal Frequency Division Multiplexing (OFDM) (11a/11g) or Direct Sequence Spread Spectrum (DS-SS) (11b/11g)

  • Antenna Technology: Single omni-directional antenna

    • 2 antenna Access Points (APs) ?

    • APs with directional antennas ?

MAC

  • Physical Carrier Sensing: Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

  • Virtual Carrier Sensing:Request-to-Send/Clear-to-Send (RTS/CTS) handshake (Hidden node avoidance)


Review of hidden node problem

Review of Hidden Node Problem

Pure CSMA/CA

Carrier Sense? Clear


802 11 solution of hidden node problem

802.11 Solution of Hidden Node Problem

CSMA/CA with RTS/CTS

Virtual Carrier Sense? Busy

RTS

CTS


Some limitations of 802 11 phy mac

Some Limitations of 802.11 PHY/MAC

PHY

  • Throughput (bits/sec): Antenna technology limits spatial re-use

Physical Carrier Sensing

Virtual Carrier Sensing


Some limitations of 802 11 phy mac contd

Some Limitations of 802.11 PHY/MAC (contd.)

MAC

  • Throughput: RTS/CTS handshake further limits the spatial re-use in the network

  • Fairness (and Throughput):RTS/CTS fails to completely take care of the hidden node problem, resulting in dropped packets for one transmission more than the other

    • Interference range is typically more than the successful reception range of CTS

  • Fairness: 802.11 MAC can unfairly favor one transmission over the other as a function of the distance between the nodes

X

CTS


802 11 simulated performance tphn04

802.11 Simulated Performance [TPHN04]

Linear Topology:

2

3

1

0

200 m

200 m

D

Throughput reduction

(and unfairness) due to

spatial proximity

Hidden node effect


Tphn04 proposed solution

[TPHN04] Proposed Solution

PHY

  • Single transmit/multiple receive antennas with OFDM modulation

MAC

  • Mitigating Interference using Multiple Antennas MAC (MIMA-MAC)

    • Built on top of 802.11 MAC with antenna awareness

    • N nodes in spatial proximity would be allowed to transmit simultaneously in a network of nodes with N receive antennas

    • PHY expected to cancel the interference of (N-1) unintended flows using advanced signal processing techniques


Tphn04 simulation results

[TPHN04] Simulation Results

Topology:

Simulation Technique

  • PHY Simulation: MATLAB (with channel bandwidth of 2 MHz and data rate of 1 Mbps)

  • MAC Simulation: ns-2 (fed with look-up tables mapping channel realizations to corresponding BER’s obtained from MATLAB)

Other Parameters

  • Input SNR: 12dB; Path-loss exponent: 4; Packet reception threshold BER: 10-5; Carrier sensing threshold BER: 10-1


Tphn04 simulation results contd

[TPHN04] Simulation Results (contd.)

Throughput Performance for Multiple Receive Antennas

MIMA-MAC vs Conventional 802.11 MAC


Tphn04 simulation results contd1

[TPHN04] Simulation Results (contd.)

Fairness Performance for Multiple Receive Antennas

MIMA-MAC vs Conventional 802.11 MAC


Smart antennas for wireless ad hoc networks

Smart Antennas for Wireless Ad-Hoc Networks

Switched Beam Antennas

  • Pre-determined set of weights applied to different antenna elements to form a fixed number of high-directionality beams

  • A K element array can form up to K beams

  • The directionality gain of each beam at the transmitter and the receiver is given by (assuming LOS/low angular spread)

  • Assuming that the transmitter and the receiver know each other’s direction, the total transmission gain (SNR gain) is bounded by


Smart antennas for wireless ad hoc networks1

Smart Antennas for Wireless Ad-Hoc Networks

Fully Adaptive Arrays

  • Fully adaptive set of weights applied to different antenna elements to adaptively change the radiation pattern

  • A K element array has K degrees-of-freedom (DOFs), and can adaptively null (K-1) uncorrelated interferers

  • Even in the presence of significant multipath scattering, the total transmission gain (SNR gain) of an adaptive array can be given by

  • Very high multipath scattering and low signal correlation can some- times limit the gain to


Smart antennas for wireless ad hoc networks2

Smart Antennas for Wireless Ad-Hoc Networks

MIMO Links

  • Digital adaptive arrays capable of operating in two modes: Spatial Multiplexing and Diversity

  • A rich set of multipath scattering between the transmitter and the receiver transforms a K element MIMO link into K independent links

  • In multiplexing mode, this can result in K fold increase in the data rate of the MIMO link

  • In diversity mode, this can result in a reduction in the variance of the received SNR. At high SNR, this results in


Smart antennas can be leveraged for

Smart Antennas can be leveraged for …

1) Higher Data Rate

  • For a given modulation scheme, the bit-error-rate (BER) on a link is determined by the link SNR

  • Switched Beam/Adaptive Array: Gain in SNR (G)  Perform adaptive modulation to increase bits transmitted per symbol and keep BER the same

  • MIMO Link: Operate the link in the spatial multiplexing mode


Smart antennas can be leveraged for1

Smart Antennas can be leveraged for …

2) Increased Transmission Range

  • The transmission range of a link is related to the link SNR by

  • Switched Beam/Adaptive Array: Gain in SNR (G)  Obtain a range extension factor given by

  • MIMO Link: Operate the link in the diversity mode. Not a straight forward relationship between the diversity order and the range extension, so resort to MATLAB simulations (diversity mode only reduces SNR variance)


Smart antennas can be leveraged for2

Smart Antennas can be leveraged for …

3) Increased Link Reliability

  • For a fixed data rate (modulation scheme), the bit-error-rate (BER) on a link is determined by the link SNR

  • Switched Beam/Adaptive Array: Gain in SNR (G)  For the same data rata, obtain a reduction in the BER by a factor of

  • MIMO Link: Operate the link in the diversity mode. For the same data rate, this can result in a reduction in the BER by a factor of


Smart antennas can be leveraged for3

Smart Antennas can be leveraged for …

4) Reduced Transmit Power

  • Switched Beam/Adaptive Array: Gain in SNR (G)  For the same BER, obtain a reduction in the transmit power by a factor of

  • MIMO Link: Operate the link in the diversity mode. For the same BER, this can result in a reduction in the transmit power given by


Sls06 simulation model

[SLS06] Simulation Model

Antenna Model

  • Switched Beam Array: Pre-determined, fixed beam pattern

  • Adaptive Array/MIMO Link: Dynamically tunable beam pattern

Channel Model

  • PHY: BER obtained fromMATLAB simulations by assuming a fast Rayleigh fading collision channel model (per location, antenna technology and strategy), with data rate of 2 Mbps, transmit power of 20 dBm, SINR of 10 dB and fade margin of 0-10 dB

  • Link: Packet loss probability obtained from ns-2 (fed with look-up tables of PHY simulations), with packet size of 1000 bytes


Sls06 simulation model contd

[SLS06] Simulation Model (contd.)

Network and Traffic Model

  • 100 nodes over a rectangular grid of 400x400 m to 1000x1000 m

  • Number of simultaneous flows in the network varied from 1 to 50

  • Multipath scattering varied from LOS to 180 degrees (rich scatter)

  • Number of antenna elements per node varied from 1 to 12

  • Initial transmission range of each node set to 100 m

Metrics

  • Throughput (T): Bits per second, normalized by the number of flows

  • Throughput/Energy (TE): Bits per unit of Joule consumed (consisting of communication circuit power Pc, transmit power Pt and computational power)


Sls06 simulation model contd1

[SLS06] Simulation Model (contd.)

Protocols and Algorithm

  • Goal: Obtain fundamental tradeoffs in the operation of different antenna technologies

  • Requires: Suppressing the inefficiencies of other factors

  • Solution: Centralized algorithm for finding routes, scheduling slotted transmissions, ensuring fairness, taking care of interferences etc.

  • Routing Strategy: Djikstra’s algorithm

Caveat

  • Simulation results are not indicative of how things might perform in a distributed setting


Sls06 strategy comparison t metric

[SLS06] Strategy Comparison: T Metric

Switched Beam

Adaptive Array

MIMO Links

Setup

  • High density network, load of 50 flows, fading loss of 5%, scattering angle of 90 degrees


Sls06 strategy comparison t metric contd

[SLS06] Strategy Comparison: T Metric (contd.)

Exceptions

  • Under low node density and small number of flows, range works better (better connectivity)

4 Antenna elements per node


Sls06 strategy comparison te metric

[SLS06] Strategy Comparison: TE Metric

Switched Beam

Adaptive Array

MIMO Links

Setup

  • High density network, load of 50 flows, fading loss of 5%, scattering angle of 90 degrees and Pt >> Pc


Sls06 strategy comparison inferences

[SLS06] Strategy Comparison: Inferences

Moderate-High Network Densities

Low Network Densities


Sls06 antenna technology comparison

[SLS06] Antenna Technology Comparison

Parameters of Interest

  • Network node density

  • Number of antenna elements

  • Number of network flows

  • Scattering angle

  • Fading loss

Components Impacting T and TE Metrics

  • Number of independent contention regions  density

  • Number of active links/contention region flows and density

  • Number of resources/contention region  elements and scattering


Sls06 technology comparison t metric scattering and elements under rate strategy

[SLS06] Technology Comparison: T Metric(Scattering and Elements under Rate Strategy)

Rich Scattering:

Low Scattering:

Rich scattering does not degrade

MIMO links’ rate performance


Sls06 technology comparison t metric scattering and fading under rate strategy

[SLS06] Technology Comparison: T Metric(Scattering and Fading under Rate Strategy)

Fading impacts all rate strategies alike!


Sls06 technology comparison t metric flows and elements under rate strategy

[SLS06] Technology Comparison: T Metric(Flows and Elements under Rate Strategy)

Low Load:

High Load:

No logarithmic effect

for MIMO at low load


Sls06 technology comparison t metric flows and density under rate range strategy

[SLS06] Technology Comparison: T Metric(Flows and Density under Rate/Range Strategy)

Low Load & High Density:

Low Load & Moderate Density:

Low Load & Low Density:


Sls06 technology comparison te metric p t p c power range strategy

[SLS06] Technology Comparison: TE Metric(Pt >> Pc: Power/Range Strategy)

Low Scattering & Large Elements:

Other Network Conditions:


Sls06 technology comparison te metric p t p c rate range strategy

[SLS06] Technology Comparison: TE Metric(Pt ~< Pc: Rate/Range Strategy)

Majority of Network Conditions:


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