1 / 11

VWID: Variable-Width Channels for Interference Avoidance

VWID: Variable-Width Channels for Interference Avoidance. Brad Karp UCL Computer Science. CS M038 / GZ06 26 th January, 2009. Alternatives to serializing transmissions by mutually interfering nodes? Alternatives to devoting entire channel to each node’s transmissions?.

corina
Download Presentation

VWID: Variable-Width Channels for Interference Avoidance

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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]

More Related