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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)

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)

Pure CSMA/CA

Carrier Sense? Clear

CSMA/CA with RTS/CTS

Virtual Carrier Sense? Busy

RTS

CTS

PHY

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

Physical Carrier Sensing

Virtual Carrier Sensing

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

Linear Topology:

2

3

1

0

200 m

200 m

D

Throughput reduction

(and unfairness) due to

spatial proximity

Hidden node effect

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

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

Throughput Performance for Multiple Receive Antennas

MIMA-MAC vs Conventional 802.11 MAC

Fairness Performance for Multiple Receive Antennas

MIMA-MAC vs Conventional 802.11 MAC

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

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

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

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

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)

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

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

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

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)

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

Switched Beam

Adaptive Array

MIMO Links

Setup

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

Exceptions

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

4 Antenna elements per node

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

Moderate-High Network Densities

Low Network Densities

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

Rich Scattering:

Low Scattering:

Rich scattering does not degrade

MIMO links’ rate performance

Fading impacts all rate strategies alike!

Low Load:

High Load:

No logarithmic effect

for MIMO at low load

Low Load & High Density:

Low Load & Moderate Density:

Low Load & Low Density:

Low Scattering & Large Elements:

Other Network Conditions:

Majority of Network Conditions: