Improving Performance of Higher
Layer Protocols with MIMO based
MAC
Guided By Prof. Anirudha Sahoo
Presented By Ankit Jindal
SYENRG MTP-3 Workshop
24th June 2009
Outline
• Introduction to MIMO
• MIMO at different layers
1.
Introduction
2.
Problem Definition
• Work at TCP layer
• Work at Routing layer
3.
MAC layer enhancements
• Basic concept
• Application
4.
TCP layer enhancements
5.
Simulation
• Basic concept
• Modifications for
MIMO based MAC
• Setup
• Results
6.
Routing layer enhancements
7.
Conclusion & Future work
• Basic concept
• Objective
• Different approaches
Introducing MIMO
h11
s1
User data stream
.
h12
s2
.
.
.
.
Channel
sM
Matrix H
.
r1
r2
User data stream
.
.
s
Transmitted vector
rM
r
Received vector
N
Where H =
N
…….. hM1
…….. hM2
h11
h12
h21
h22
.
.
…….. .
h2M
…….. hMM
h1M
hij is a Complex gain between
the ith transmit and jth
receive antenna
MIMO at different layers
Physical
Routing
MAC
TCP
Layer
Layer
Layer
Spatial
NoCategory
Modifications
Multiplexing
A
Category
DiversityB
Rate/Range/Reliable
Physical
Layer
Schemes
link
Rate
Links
Parallel
Communications
Range/Reliable
Links
Modified MAC for MIMO
Problem Definition
1. Design the TCP parameter with MIMO based
MAC to improve the performance.
2. Propose the modifications to the traditional adhoc routing layer protocols with MIMO based
MAC.
MIMO based MAC: Basic Concept
h11
s1
User data stream
.
h12
s2
.
.
.
.
Channel
sM
Matrix H
.
r1
r2
User data stream
.
.
s
Transmitted vector
rM
r
Received vector
N
Where H =
N
…….. hM1
…….. hM2
h11
h12
h21
h22
.
.
…….. .
h2M
…….. hMM
h1M
hij is a Complex gain between
the ith transmit and jth
receive antenna
Basic Concept
Signal received
at antenna i
Beamformer
Output
Beamformer
Output
Complex Gain
Basic Concept
Importance of Complex gain
Complex Gain = 0 implies Signal Nullified
Otherwise, Signal Received
Proper Selection of Weights enable the Nodes to
Receive Only Signal of Interest
Application on Multi Hop Network
Beamformer
Output
Node C can nullify signal of B by having Gain between B
and C = 0
Weight Selection Algorithm
Transmitter Weights Selection
Receiver Weights Selection
•If No CTS overheard then use •If No RTS overheard other
default weight vector
then intended one, then select
weight such that Gain = 1
•Else Solve the following set of •Else Solve the following set of
equations
equations
For all R belongs to set K
For all T belongs to set K
=0
Gain = 1 and
=0
Where WR is weight vector of Where WT is weight vector of
node R and K is set of all
node T and K is set of all other
nodes whose CTS have been nodes whose RTS have been
overheard
overheard
Problem Definition: Recall
TCP
No Modifications
Design TCP Parameter
Modified MAC for MIMO
TCP Layer Enhancements
CWL = Path’s BDP
Path’s BDP <= BWmin * Round Trip Delay
Forward Path Delay <= n * (S/BWmin)
Reverse Path Delay <= m * (S/BWmin)
Path’s BDP <= BWmin * (n * S/BWmin + m * S/BWmin)
Path’s BDP <= (n + m) * S
CWL = k * (n + m) * S
Where 0 <= k <= 1
TCP Layer Enhancements
CWL = k * (n + m) * S
Where 0 <= k < = 1
CWL can be set to constant multiple of round
trip hop count
Value of k depends upon the number of
packets that the path from source to
destination can accommodate
Implies k depends upon the MAC protocol
used
TCP Layer Enhancements
MIMO
IEEE 802.11
based MAC
Maximum h/2
h/3 packet can be in transit
Implies, k <= 1/2
1/3 for MIMO
IEEE 802.11
based MAC
TCP Layer Enhancements
Lower Bound on k
Limit the number of DATA packets in transit to half of
the maximum in order to accommodate ACK
Implies, k >= 1/6 for IEEE 802.11 MAC
k >= 1/4 for MIMO based MAC
TCP Layer Enhancements
CWL = k * (n + m) * S
Where k is a constant such that
1/4 <= k <= 1/2 For MIMO based MAC
1/6 <= k <=1/3 For IEEE 802.11 MAC
Appropriate value of k can be found out by
finding optimal CWL for different length chain
topology
Simulation: Setup
• 100,000 Packets
• Single TCP flow
• Buffer Size = 25 packets
• Shadowing model used
• Poisson Arrival
• Offered load 3 Mbps
• Two Antennae per node
• Each Channel Matrix entry = Gaussian
Distribution with mean 0 and variance 0.5
• Transmission range is assumed to be same
as Interference range
Simulation: Results
Throughput v/s Congestion Window Limit
There exists a CWL beyond which there is no
significant improvement in throughput
Simulation: Results
Average RTD v/s RTHC
There exists a CWL beyond which there is no
significant improvement in throughput but increase in
average round trip packet delay.
Simulation: Results
Optimal CWL v/s RTHC
Setting k = 1/2.8 approx the Optimal CWL found out
by simulations
Simulation: Results
Throughput v/s RTHC
Setting k = 1/2.8 approx the Optimal CWL found out
by simulations
Simulation: Results
Throughput v/s RTHC
CWL depends upon the underlying MAC protocol
used
Problem Definition: Recall
Routing Layer
Category A
Modify Routing Protocol
Rate/Range/Reliable link
E.g. MIR routing protocol
Parallel Communications
Routing Layer Enhancements: Basic Concept
Number of communications in the collision
domain depends upon the available degree
of freedom
Maximum N communications can happen in
same collision domain, where N is number of
antennae
Objective: To select path that allows more
number of parallel communications.
Objective
1
S1
D1
1
2
S2
D2
3
Example Topology: Two Antennae per node
Different Approaches
S1
Suppose each node knows its number of
active neighbors.
D1
Node append Cits active neighbor count in the
route request packet
Destination on receiving multiple route
B
request messages
has to rank the path
S2
We have different approaches in mind for
destination
to rank the path
A
D2
Different Approaches
Proposal 1
Sum of ANC along the path < Number of Antennas
Hop count
1
1
2
3
1
Use hop count to decide the path if above
equation is not satisfied for any path
Problem: A path is rejected because of
congestion at only few intermediate nodes
Different Approaches
Proposal 2
Count nodes satisfying, ANC < Number of Antennae
Select path having max value of C/h
1
1
2
2
1
Problem: A path having C/h = 1 of higher hop count say
20 nodes, is accepted compare to path having C/h = 0 of
low hop count say 5 is rejected
Different Approaches
Proposal 3
• Allow destination to entertain route request for only
some time in order to reject large hop count path
• Or, give some weight age to h and C
•Select the path which has maximum value of
alpha * C + (1 – alpha) * h,
where 0 <= alpha <= 1
Favorable value of alpha can be found out by
performing experiments for various topologies.
Different Approaches
Active Neighbor Count Calculation
A
B
B
Routing Table
ANC & Nodes
Conclusion and Future Work
• Traditional MAC protocols are not suitable for MIMO
based systems
• MIMO based MAC allows more number of parallel
communications
• For such MIMO based MAC, TCP CWL should be
appropriately modified
• Simulation data shows that TCP CWL can be
empirically set to 1/2.8 of round trip hop count to
improve performance
• For such MIMO based MAC, routing protocol can
also be designed to improve the performance.
• Knowing the active neighbor count, appropriate
routing protocol can be designed for such MIMO
based MAC
Conclusion and Future Work
• Propose the appropriate modifications to the AODV
routing protocol
• Detailed study of such routing protocol is required as
part of future work
• Performance evaluation of various approaches can
be done to design the efficient routing protocol for
MIMO based MAC
QUESTIONS???
Thank You !!!
[email protected]
[email protected]
Routing Layer Enhancements: Backup Slides
Y axis
Equality 1
(0,0)
Equality 0
X axis
Equality 0
Routing Layer Enhancements: Backup Slides
With N antennae, we want to have N + 1
communications in the same collision
domain.
Then, we have N + 1 equations each of N
variables, out of which N equation are of type
equality 0 and one equation of equality 1
With N + 1 equations in N dimension, there
can be only one point of intersection
Origin is the point of intersection of N
equations of type equality 0, which does not
satisfy last equation of type equality 1
Spatial Multiplexing: Backup Slides
Each receiver antenna receives the
superposition of all of the transmitted data
streams.
Receiver can retrieve the transmitted
streams, as each stream have different
spatial signature due to multi path.
Transmission of independent streams of
data through each antenna provides
linear increase in capacity
C = min(M;N)log2(1 + P)
Diversity: Backup Slides
Because of multi path, each stream is
independent & hence probability of each
stream facing poor channel is very less
Thus, Diversity helps in reducing BER or
increase in SNR at the output of combiner
• Reduce BER on the link.
• For the required BER on the link,
increase in SNR implies increased
communication range
• For the fixed BER and SNR, the transmit
power consumption can be minimized.
Shadowing Model
The free space model and the two-ray model
predict the received power as a deterministic
function of distance.
In reality, the received power at certain
distance is a random variable due to multi
path propagation effects, which is also known
as fading effects.
Shadowing Model
Beta is path loss exponent & alpha is
shadowing or standard deviation
Backup Slides: Beam Forming
• Controls the phase and relative amplitude
of the transmitted signal at each transmitter
• Create a pattern of constructive and
destructive interference in the wave front.
• Received streams are combined in such a
way that the expected pattern of radiation is
preferentially observed.
• Antenna should be separated by at least ½
of transmit signal wavelength
Backup Slides: Space Time Code
• Improve the reliability of data transmission
in wireless communications using multiple
transmit antennas.
• Transmit multiple, redundant copies of a
data stream to the receiver
• Probability of each stream facing poor
channel is very less
• Antenna should be separated by at least 4
to 10 times the wavelength to keep the signal
through each multi-path independent