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
X
CTS
• Fairness: 802.11 MAC can unfairly favor one transmission over the
other as a function of the distance between the nodes
802.11 Simulated Performance [TPHN04]
Linear Topology:
Throughput reduction
(and unfairness) due to
spatial proximity
0
200 m
2
1
D
200 m
3
Hidden node effect
[TPHN04] Proposed Solution
PHY
• Single transmit/multiple receive antennas with OFDM modulation
MAC
• Mitigating Interference using Multiple Antennas MAC (MIMAMAC)
– 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-offreedom (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 sometimes 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 from MATLAB 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: