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. Fixed Block of Radio Frequency Spectrum Channel AChannel BChannel CChannel D

Ideal usage of channel bandwidth Should use entire range of freqs spanning a channel Usage drops down to 0 just outside channel boundary Channel AChannel B Frequency Power Channel CChannel D

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 Real Usage Channel AChannel B Power Channel CChannel D Wastage of spectrum

Consideration of the b 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 Paper examines approaches to use partially overlapped channels efficiently to improve spectral utilization Channel AChannel B Channel A’

Empirical proof of benefits of partial overlap Can we use channels 1, 3 and 6 without interference ? Ch 1Ch 6Ch 3 Amount of Interference Link A Ch 1 Link C Ch 6 Link B Ch 3

Empirical proof of benefits of partial overlap Typically partially overlapped channels are avoided With sufficient spatial separation, they can be used Link A Ch 1 Link C Ch 6 Link B Ch 3 Ch 1Ch 6Ch 3 Virtually non-overlapping

Empirical proof of benefits of partial overlap Link A Ch 1 Link B Ch X Partially overlapped channels can provide much greater spatial re-use if used carefully!

Interference factor To model effects of partial overlap, define: – Interference Factor or “I-factor” Transmitter is on channel j P j denotes power received on channel j P i denotes power received on channel i PiPjPiPj I-factor(i,j) =

FcAFcA-11 Mhz-22 Mhz -50 dB -30 dB FcBFcB Channel A Channel B 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 Normalized I-factor Receiver Channel I(theory) I(measured)

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

60 Why use partial overlap? Consider a case where you have 300 APs 100 Worst case Interference by all 100 APs on same channel Non-overlap 3 channels, 100 APs each Partial overlap 5 channels, 60 APs each Worst case Interference by all 60 APs on same channel + little interference from POV channels

60 Why use partial overlap? Consider a case where you have 300 APs 100 Worst case Interference by all 100 APs on same channel Non-overlap 3 channels, 100 APs each Partial overlap 5 channels, 60 APs each Worst case Interference by all 60 APs on same channel + little interference from POV channels

60 Why use partial overlap? Consider a case where you have 300 APs 100 Worst case Interference by all 100 APs on same channel Non-overlap 3 channels, 100 APs each Partial overlap 5 channels, 60 APs each Worst case 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: –Sum taken over APs on same channel since channels are independent –Create vector of cf c ’s (CF) and sort in non-increasing order Optimal channel assignment minimizes CF

Each client builds I-factor model using scan operation POV(x,x ch,y,y ch ) = 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 cf c ’s (CF) and sort in non-increasing order Optimal channel assignment minimizes CF Channel assignment w/ partial overlap = +

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

Evaluating deployment strategy in non-overlap case 3 APs –operating over independent channels –arranged in equilateral triangle channels Number of Clients Avg. TCP throughput

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 4 APs –Operating over partially overlapped channels –arranged as a square –Covering same spatial area as non-overlap case 4 APs can be placed closer  Get greater spatial re-use channels 4 POV channels Number of Clients Avg. TCP throughput

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