Slides adapted from Ashwin Wagadarikar, Duke

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Slides adapted from Ashwin Wagadarikar, Duke
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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

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Typical usage of spectral band

• Transmitter-receiver pairs use independent
  channels that dont overlap to avoid
  interference.

Channel A
Channel B
Channel C
Channel D
Fixed Block of Radio Frequency Spectrum
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Ideal usage of channel bandwidth

• Should use entire range of freqs spanning a
  channel
• Usage drops down to 0 just outside channel
  boundary

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Realistic usage of channel bandwidth

• Realistically, transmitter power output is NOT
  uniform at all frequencies of the channel.
• PROBLEM
• Transmitted power of some freqs. lt max.
  permissible limit
• Results in lower channel capacity and inefficient
  usage of the spectrum

Channel A
Channel B
Channel C
Channel D
Power
Real Usage
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Consideration of the 802.11b standard

• Splits 2.4 GHz band into 11 channels of 22 MHz
  each
• Channels 1, 6 and 11 dont 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
  non-overlapping channels

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Focus of paper

• Paper examines approaches to use partially
  overlapped channels efficiently to improve
  spectral utilization

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Empirical proof of benefits of partial overlap
Ch 1
Ch 6
Ch 3

• Can we use channels 1, 3 and 6 without
  interference ?

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Empirical proof of benefits of partial overlap
Ch 1
Ch 6
Ch 3

• Typically partially overlapped channels are
  avoided
• With sufficient spatial separation, they can be
  used

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Empirical proof of benefits of partial overlap

• Partially overlapped channels can provide much
  greater spatial re-use if used carefully!

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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

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Theoretical Estimate for I-Factor

• Theoretically, I-factor Area of intersection
  between two spectrum masks of transmitters on
  channels A and B

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Estimating I-Factor at a receiver on channel 6
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I(theory)
0.8
I(measured)
0.6
Normalized I-factor
0.4
0.2
0
0
2
4
6
8
10
12
Receiver Channel
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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 APs to maximize
  performance by reducing interference

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Why use partial overlap?

• Consider a case where you have 300 APs

Partial overlap 5 channels, 60 APs each
Non-overlap 3 channels, 100 APs each
100
100
100
Worst case Interference by all 60 APs on same
channel little interference from POV channels
Worst case Interference by all 100 APs on same
channel
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Why use partial overlap?

• Consider a case where you have 300 APs

Partial overlap 5 channels, 60 APs each
Non-overlap 3 channels, 100 APs each
100
100
100
Worst case Interference by all 60 APs on same
channel little interference from POV channels
Worst case Interference by all 100 APs on same
channel
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Why use partial overlap?

• Consider a case where you have 300 APs

Partial overlap 5 channels, 60 APs each
Non-overlap 3 channels, 100 APs each
100
100
100
Worst case Interference by all 60 APs on same
channel some interference from POV channels
Worst case Interference by all 100 APs on same
channel
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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

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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
• Sum taken over APs on same channel since channels
  are independent
• Create vector of cfcs (CF) and sort in
  non-increasing order
• Optimal channel assignment minimizes CF

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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
• Sum taken over APs that interfere on own channel
  all POV channels
• Create vector of cfcs (CF) and sort in
  non-increasing order
• Optimal channel assignment minimizes CF

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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)

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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

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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

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Evaluating deployment strategy in non-overlap case
1
Avg. TCP throughput
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6
Number of Clients

• 3 APs
• operating over independent channels 1 6 11
• arranged in equilateral triangle

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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

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Evaluating deployment strategy in POV case
1
7
Avg. TCP throughput
11
4
1000
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

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The Overall Methodology
Wireless Communication Technology Such as 802.11,
802.16
Estimate I-Factor Theory/Empirical
Algorithm for Channel Assignment
I-Factor Model
Estimated once per wireless technology
Channel Assignment with overlapped channels
Repeated for each wireless network
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Conclusion

• Efficient use of the spectrum can be made by
  using partially overlapped channels
• Proper use provides
• Higher throughput
• Greater spatial re-use