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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.

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Spectral Bands and Channels

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  1. 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

  2. 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

  3. 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

  4. 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

  5. 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

  6. Channel A Channel B Channel A’ Focus of paper • Paper examines approaches to use partially overlapped channels efficiently to improve spectral utilization

  7. 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 ?

  8. 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

  9. 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!

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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 = +

  20. 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)

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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

  26. 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

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

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