Partially Overlapped Channels Not
Considered Harmful
Arunesh Mishra, Vivek Shrivastava, Suman Banerjee, William Arbaugh
(ACM SIGMetrics 2006)
Slides adapted from Ashwin Wagadarikar, Duke
Spectral Bands and Channels
• Wireless communication uses emag signals over a range
of frequencies
• FCC has split the spectrum into spectral bands
• Each spectral band is split into channels
Example of a channel
Typical usage of spectral band
• Transmitter-receiver pairs use independent channels
that don’t overlap to avoid interference.
Channel A
Channel B
Channel C
Channel D
Fixed Block of Radio Frequency Spectrum
Ideal usage of channel bandwidth
• Should use entire range of freqs spanning a channel
• Usage drops down to 0 just outside channel boundary
Channel B
Channel C
Power
Channel A
Frequency
Channel D
Realistic usage of channel bandwidth
• Realistically, transmitter power output is NOT uniform
at all frequencies of the channel.
Channel B
Channel C
Channel D
Power
Channel A
Real Usage
Wastage of spectrum
• PROBLEM:
– Transmitted power of some freqs. < max. permissible limit
– Results in lower channel capacity and inefficient usage of the
spectrum
Consideration of the 802.11b standard
• Splits 2.4 GHz band into 11 channels of 22 MHz each
– Channels 1, 6 and 11 don’t overlap
• Can have 2 types of channel interferences:
– Co-channel interference
• Address by RTS/CTS handshakes etc.
– Adjacent channel interference over partially overlapping channels
• Cannot be handled by contention resolution techniques
 Wireless networks in the past have used only nonoverlapping channels
Focus of paper
• Paper examines approaches to use partially overlapped
channels efficiently to improve spectral utilization
Channel A
Channel B
Channel A’
Empirical proof of benefits of partial overlap
Link A Ch 1
Ch 1
Ch 3
Ch 6
Link B Ch 3
Link C Ch 6
Amount of Interference
• Can we use channels 1, 3 and 6 without interference ?
Empirical proof of benefits of partial overlap
Link A Ch 1
Ch 1
Ch 3
Ch 6
Link B Ch 3
Link C Ch 6
Virtually non-overlapping
• Typically partially overlapped channels are avoided
• With sufficient spatial separation, they can be used
Link A Ch 1
Link B Ch X
UDP Throughput (Mbps)
Empirical proof of benefits of partial overlap
6
5
4
3
0
10
20
30
40
50
Distance between the 2 links (meters)
LEGEND
Non-overlapping channels, A = 1, B = 6
Partially Overlapped Channels, A = 1, B = 3
Partially Overlapped Channels, A = 1, B = 2
Same channel, A = 1, B = 1
5
2
1
0
Channel Separation
• Partially overlapped channels can provide much greater spatial
re-use if used carefully!
60
Interference factor
• To model effects of partial overlap, define:
– Interference Factor or “I-factor”
• Transmitter is on channel j
• Pj denotes power received on channel j
• Pi denotes power received on channel i
I-factor(i,j) =
Pi
Pj
Theoretical Estimate for I-Factor
Channel B
Channel A
-30 dB
-50 dB
-22 Mhz
-11 Mhz
FcA
FcB
• Theoretically, I-factor = Area of intersection between
two spectrum masks of transmitters on channels A and
B
Normalized I-factor
Estimating I-Factor at a receiver on channel 6
1
0.8
I(theory)
0.6
I(measured)
0.4
0.2
0
0
2
4
6
8 10
Receiver Channel
12
WLAN Case study
• WLAN comparison between:
– 3 non-overlapping channels, and
– 11 partially overlapping channels
– over the same spectral band
• WLAN consists of access points (APs) and clients
– AP communicates with clients in its basic service set on a
single channel
• GOAL: allocate channels to AP’s to maximize
performance by reducing interference
Why use partial overlap?
Consider a case where you have 300 APs
Partial overlap
Non-overlap
5 channels, 60 APs each
3 channels, 100 APs each
60
100
100
60
60
60
60
100
Worst case
Worst case
Interference by all 100 APs
on same channel
Interference by all 60 APs
on same channel + little
interference from POV
channels
Why use partial overlap?
Consider a case where you have 300 APs
Partial overlap
Non-overlap
5 channels, 60 APs each
3 channels, 100 APs each
60
100
100
60
60
60
60
100
Worst case
Worst case
Interference by all 100 APs
on same channel
Interference by all 60 APs
on same channel + little
interference from POV
channels
Why use partial overlap?
Consider a case where you have 300 APs
Partial overlap
Non-overlap
5 channels, 60 APs each
3 channels, 100 APs each
60
100
100
60
60
60
60
100
Worst case
Worst case
Interference by all 100 APs
on same channel
Interference by all 60 APs
on same channel + some
interference from POV
channels
Channel assignment w/ non-overlap
• Mishra et al. previously proposed “client-driven”
approach for channel assignment to APs
• Use Randomized Compaction algorithm
– Optimization criterion: minimize the maximum interference
experienced by each client
• 2 distinct advantages over random channel assignment:
– Higher throughput over channels
– Load balancing of clients among available APs
Channel assignment w/ non-overlap
• (X,C) = WLAN
– X = set of APs and C = set of all clients
• How to assign APs to these 3 channels?
– MUST LISTEN TO THE CLIENTS!
• To evaluate a given channel assignment

– Compute interference for each client:
cf c  ( ( x)  1)
– Sum taken over APs on same channel since channels are independent
– Create vector of cfc’s (CF) and sort in non-increasing order
• Optimal channel assignment minimizes CF
Channel assignment w/ partial overlap
=
+
• Each client builds I-factor model using scan operation
• POV(x,xch,y,ych) = 1 if nodes x and y on their channels interfere
with each other
• To evaluate a given channel assignment
– Compute interference for each client:
cf c   ( ( x)  1)
– Sum taken over APs that interfere on own channel + all POV channels
– Create vector of cfc’s (CF) and sort in non-increasing order
• Optimal channel assignment minimizes CF
Results for high interference topologies
• 28 randomly generated topologies with 200 clients and 50 APs
– 14 high interference topologies (average of 8 APs in range for client)
– 14 low interference topologies (average of 4 APs in range for client)
Results for low interference topologies
• Using partially overlapped channels and I-factor, clients
can experience less contention at the link level.
 Higher layers have better throughput
Evaluating deployment strategy
•
•
•
•
square area, clients distributed uniformly at random
Clients can move around
Must ensure that APs cover full physical space
 APs must be distributed regularly
1
11
6
Avg. TCP throughput
Evaluating deployment strategy in non-overlap case
1.0
0.8
0.6
0.4
0.2
3 channels
0
400
600
800
Number of Clients
• 3 APs
– operating over independent channels 1 6 11
– arranged in equilateral triangle
1000
Channel separation vs. transmission range
• hard to deploy a new AP into one of the non-overlapping
channels without getting a lot of interference
• With channel separation, can get much lesser interference
1
4
7
11
Avg. TCP throughput
Evaluating deployment strategy in POV case
1.0
0.8
4 POV channels
0.6
0.4
0.2
0
400
3 channels
600
800
Number of Clients
• 4 APs
– Operating over partially overlapped channels 1 4 7 11
– arranged as a square
– Covering same spatial area as non-overlap case
• 4 APs can be placed closer  Get greater spatial re-use
1000
The Overall Methodology
Wireless Communication Technology
Such as 802.11, 802.16
Estimate I-Factor
Theory/Empirical
Estimated once per
wireless technology
I-Factor
Model
Algorithm for
Channel Assignment
Channel Assignment
with overlapped channels
Repeated for each wireless
network
Conclusion
• Efficient use of the spectrum can be made by using
partially overlapped channels
• Proper use provides:
– Higher throughput
– Greater spatial re-use