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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. Typical usage of spectral band.

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spectral bands and channels
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
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

Channel A

Channel B

Channel C

Channel D

Power

Frequency

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

Wastage of spectrum

Realistic usage of channel bandwidth
  • Realistically, transmitter power output is NOT uniform at all frequencies of the channel.
  • PROBLEM:
    • Transmitted power of some freqs. < 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

consideration of the 802 11b standard
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 non-overlapping channels

focus of paper

Channel A

Channel B

Channel A’

Focus of paper
  • Paper examines approaches to use partially overlapped channels efficiently to improve spectral utilization
empirical proof of benefits of partial overlap

Link A Ch 1

Link B Ch 3

Link C Ch 6

Amount of Interference

Empirical proof of benefits of partial overlap

Ch 1

Ch 3

Ch 6

  • Can we use channels 1, 3 and 6 without interference ?
empirical proof of benefits of partial overlap1

Link A Ch 1

Link B Ch 3

Link C Ch 6

Virtually non-overlapping

Empirical proof of benefits of partial overlap
  • Typically partially overlapped channels are avoided
  • With sufficient spatial separation, they can be used

Ch 1

Ch 3

Ch 6

empirical proof of benefits of partial overlap2

6

5

UDP Throughput (Mbps)

Link A Ch 1

4

Link B Ch X

3

0

10

20

30

40

50

60

Distance between the 2 links (meters)

LEGEND

Non-overlapping channels, A = 1, B = 6

5

Partially Overlapped Channels, A = 1, B = 3

2

Partially Overlapped Channels, A = 1, B = 2

1

0

Same channel, A = 1, B = 1

Channel Separation

Empirical proof of benefits of partial overlap
  • Partially overlapped channels can provide much greater spatial re-use if used carefully!
interference factor

I-factor(i,j) =

Pi

Pj

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
theoretical estimate for i factor

Channel B

Channel A

-30 dB

-50 dB

-22 Mhz

-11 Mhz

FcA

FcB

Theoretical Estimate for I-Factor
  • Theoretically, I-factor = Area of intersection between two spectrum masks of transmitters on channels A and B
estimating i factor at a receiver on channel 6
Estimating I-Factor at a receiver on channel 6

1

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

wlan case study
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

60

60

60

60

60

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

why use partial overlap1

60

60

60

60

60

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

why use partial overlap2

60

60

60

60

60

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

channel assignment w non overlap
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 overlap1
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 cfc’s (CF) and sort in non-increasing order
  • Optimal channel assignment minimizes CF
channel assignment w partial overlap
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 cfc’s (CF) and sort in non-increasing order
  • Optimal channel assignment minimizes CF

=

+

results for high interference topologies
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
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
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
evaluating deployment strategy in non overlap case

1.0

0.8

0.6

0.4

3 channels

0.2

0

400

600

800

1000

Evaluating deployment strategy in non-overlap case
  • 3 APs
    • operating over independent channels 1 6 11
    • arranged in equilateral triangle

1

Avg. TCP throughput

11

6

Number of Clients

channel separation vs transmission range
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
evaluating deployment strategy in pov case

1.0

0.8

4 POV channels

0.6

0.4

3 channels

0.2

0

400

600

800

Evaluating deployment strategy in POV case
  • 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

1

7

Avg. TCP throughput

4

11

1000

Number of Clients

the overall methodology
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

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