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

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)

  • 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

Pure CSMA/CA

Carrier Sense? Clear


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

PHY

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

Physical Carrier Sensing

Virtual Carrier Sensing


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]

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

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

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.)

Throughput Performance for Multiple Receive Antennas

MIMA-MAC vs Conventional 802.11 MAC


[TPHN04] Simulation Results (contd.)

Fairness Performance for Multiple Receive Antennas

MIMA-MAC vs Conventional 802.11 MAC


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 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 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 …

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 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 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 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

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.)

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 (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

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.)

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

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

Moderate-High Network Densities

Low Network Densities


[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)

Rich Scattering:

Low Scattering:

Rich scattering does not degrade

MIMO links’ rate performance


[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)

Low Load:

High Load:

No logarithmic effect

for MIMO at low load


[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(Pt >> Pc: Power/Range Strategy)

Low Scattering & Large Elements:

Other Network Conditions:


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

Majority of Network Conditions:


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