VWID: Variable-Width Channels for Interference Avoidance

1. VWID: Variable-Width Channels for Interference Avoidance Brad Karp UCL Computer Science CS M038 / GZ06 26th January, 2009

2. Alternatives to serializing transmissions by mutually interfering nodes? Alternatives to devoting entire channel to each node’s transmissions? Context: Sharing of Spectrum • Finite RF spectrum available for use by nodes in a wireless network • MACAW/802.11 approach to sharing of spectrum: • Each node uses entire channel (full width, in Hz) for each packet transmission • Try to schedule senders that interfere so that they don’t send concurrently • Interference plagues 802.11-style sharing in mesh networks • MAC can’t perfectly serialize interfering senders • Result: interference reduces throughput (evidence: Roofnet’s ETT overpredicts throughput…)

3. Another Way: Orthogonal Channels • 802.11a allows use of different channel widths: • 20 MHz (default): 54 Mbps nominal • 10 MHz: 27 Mbps nominal • 5 MHz: 13.5 Mbps nominal • Idea: assign non-overlapping (orthogonal) channels to mutually interfering links • In principle, should prevent interference • Under certain assumptions, increases total capacity vs. single-channel CSMA!

4. Modeling Capacity: Assumptions • Consider 2-client 802.11 network with one base station, all traffic from clients to base station • Base station has one radio, one antenna • Clients each have one radio, one antenna • All nodes in mutual range • Both clients send continuously • Client 1 received at BS with power P1, client 2 received at BS with power P2

5. P P ( ( ) ) l l 1 1 2 1 + + o o g g 2 2 N N Optimum sum-capacity Understanding Two-Node Capacity R2 (bits/s/Hz) R1 (bits/s/Hz) Transmitter 1’s Rate P 1 ( ) / / l b R H i 1 t + < o g s s z 1 2 ; N P Transmitter 2’s Rate 2 ( ) / / l b R H i 1 t + < o g s s z 2 2 ; N P P Sum of Rates + 1 2 ( ) / / l b R R H i 1 t + + < o g s s z 1 2 2 : N [Ramki Gummadi]

6. P P P P P ( ( ( ( ) ) ) ) l l l l 1 1 1 1 1 2 1 2 1 + + + + α o o o o g g g g = 2 2 2 2 P P N N N N 0 1 + 1 2 α α = = P 1 ( ) / / l b R H i 1 t + < α o g s s z 1 2 ; N α P 2 ( ) ( ) / / l b R H i 1 1 t - + < α o g s s z 2 2 : ( ) N 1 - α VWID throughput R2 (bits/s/Hz) Optimum throughput at A B R1 (bits/s/Hz) [Ramki Gummadi]

7. Finding Optimal Channel Widths • Want to maximize sum of two rates: R = R1 + R2 = • Setting gives maximum: • i.e., to maximize total throughput, assign each node channel width proportional to its share of total power received at AP

8. Example:CSMA vs. Orthogonal Channels • Two clients, each of which is received by base station with SNR of 1 • Under CSMA, one client alone achieves throughput: • so when alternating, each gets 0.5 bits/s/Hz • If we assign half of channel to each and allow concurrent transmissions, each gets: 0.79 bits/s/Hz

9. Exhaustive search: worst case cost is exponential in number of interfering links! VWID Prototype • Automated system for channel assignment • For each link, assign sender one of {5, 10, 20} MHz channel • Chooses assignment of channels that maximizes aggregate throughput across all links • Additional constraint: don’t decrease a sender’s channel width if doing so reduces that link’s throughput (vs. 20 MHz channel width)

10. VWID Experimental Evaluation • Outdoor 802.11a testbed: • 6 nodes; 10 links, 8 of which 1-2 km long • Bit-rate for each node fixed; chosen so that node gets reasonable throughput on its links • Results given only for UDP traffic; all nodes send as fast as they can • Experiments have carrier sense enabled because “gives higher throughput” (!?)

11. Link Throughput Improvement:Point-to-Point Links no VWID with VWID [Ramki Gummadi]