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Wireless Networking & Mobile Computing CS 752/852 - Spring 2012

Wireless Networking & Mobile Computing CS 752/852 - Spring 2012. Lec #8: MAC Sectorized Antennas. Tamer Nadeem Dept. of Computer Science. Enhancing 802.11 Wireless Networks with Directional Antenna and Multiple Receivers ( Chenxi Zhu, Tamer Nadeem, and Jonathan Agre ). Introduction.

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Wireless Networking & Mobile Computing CS 752/852 - Spring 2012

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  1. Wireless Networking & Mobile ComputingCS 752/852 - Spring 2012 Lec #8: MAC Sectorized Antennas Tamer Nadeem Dept. of Computer Science

  2. Enhancing 802.11 Wireless Networks with Directional Antenna and Multiple Receivers(Chenxi Zhu, Tamer Nadeem, and Jonathan Agre)

  3. Introduction • IEEE 802.11 WLANs have enjoyed tremendous popularity in recent years. • RTS/CTS/DATA/ACK packets assume omni-directionality

  4. Introduction (cont’d) • Channel reservation is made through carrier sensing • All neighbors of source and destination nodes need to be silent. • Limited number of channels and unlicensed spectrum usage Interference between transmissions is becoming a serious problem.

  5. Spatial Fairness of 802.11 • Different nodes have different neighbors •  experience different contention environments. • Nodes at the overlapping coverage area of the WLANs suffer from lower throughput Extend Bianchi’s discrete time Markov model to understand Spatial Fairness

  6. Spatial Fairness of 802.11 • Extend Bianchi’s discrete time Markov model to some simple multihop networks. • Contention probability  • conditional collision probability pc • Beyond a single hop  different nodes are attached to different ’spatial channels’  no longer share the same notion of discrete time. Need to revisit Bianchi’s discrete time model

  7. Assumptions • The carrier sensing range is the same as the communication range; • RTS/CTS messages are always used • A collision (duration of RTS/CTS) takes the same amount of time as an idle slot. DATA/ACK are free of collisions • Duration of the RTS/CTS/DATA/ACK four way handshake is a geometric random variable with average of 1/ptslots, where ptis the probability that a data transmission terminates in a slot; • Every node always has a packet to send to one of its neighbors.

  8. Markov Model

  9. Markov Model • The state (SA, SC, SB) represents the status of the nodes in group A,C,B in a slot, where • The Markov chain has 5 states: (0; 0; 0), (1; 0; 0), (1; 0; 1), (0; 0; 1), (0; 1; 0).

  10. Markov Model • Transitional Probabilities:

  11. Markov Model • Stationary State Probabilities: ps(0; 0; 0), ps(1; 0; 1), ps(0; 1; 0), and ps(1; 0; 0) = ps(0; 0; 1) • Collision probabilities of the nodes in groups A,B and group C • Contention probabilities 1; 2 of nodes in areas A/B and C

  12. Fairness Analysis (NA=Nc=NB=20) • Throughput vs. Packet size • Stationary Probabilities

  13. Fairness Analysis (NA=Nc=NB=20) • Node Contention/Collision • PaA= p*s(0; 0; 0) + p*s (0; 0; 1)PaC= p*s(0; 0; 0)

  14. Use of Directional Antenna • Directional antenna is a well known method to reduce the interference and to increase the range and the capacity for wireless networks. • Fairness relieved through interference reduction S-MAC

  15. S-MAC: Sectorized Antenna • Dedicated Rx per sector/antenna • Tx can switch to different antennas • Self-interference cancellation between Tx and Rx in different sectors • Consistent channel information at different nodes • No hidden nodes or deafness problem Addresses the hidden node problem and the deafness problem by continuously monitoring the channel in all directions (sectors) at all time

  16. S-MAC Architecture Directional Antennas Separate queues RX RF … DUX RX3 RX DUX RX2 S-MAC: SNAV=[NAVTX1,NAVTX2, NAVRX1, NAVRX2, NAVRX3] RX1 DUX TX symbol for self-interference cancellation TX RF TX2 switching fabric TX1 TX RF Base Band MAC and higher

  17. Self-interference Cancellation Scheme • Different TX and RX modules are all part of the same PHY • on-chip communication between them is possible. • When TXi transmits signal Sti, RXj receives Sri. ; • RXj cancels the interference caused by own TXi • RXj can then decode signal from another node k • This requires self-channel estimation from own i to j: Gij: Srik. = Sri - Gij* Sti.

  18. Sectorized NAV and Carrier Sensing • SNAV=[NAVTX1, NAVTX2, NAV1, NAV2, …, NAVM]. • NAVTXi: status of TXi (busy period). • Updated when S-MAC node is involved in a transmission using TXi • NAVj: status of medium in sector j. • Updated when S-MAC node senses a change of medium status in sector j (sending or receiving RTS/CTS/DATA). • Fully interoperable with regular omni 802.11 nodes.

  19. D H A RTS CTS G B F E RTS Collision Operation of S-MAC (example I) DMAC “Hidden Node due to asymmetric gain” C Example adopted from R. Choudhury, X. Yang, R. Ramanathan, and NH Vaidy, MobiCom 2002.

  20. D H A RTS CTS G B F E CTS from F rcvd RTS not sent by A C Operation of S-MAC (example I) SMAC: “Hidden Node due to asymmetric gain” avoidance Example adopted from R. Choudhury, X. Yang, R. Ramanathan, and NH Vaidy, MobiCom 2002.

  21. D H A RTS CTS G B F E E waits for B-F to finish C Operation of S-MAC (example II) “Hidden Node due to unheard RTS/CTS” avoidance Example adopted from R. Choudhury, X. Yang, R. Ramanathan, and NH Vaidy, MobiCom 2002.

  22. D H A G B F E E is aware C is Transmitting C Operation of S-MAC (example II) Deafness Prevention Example adopted from R. Choudhury, X. Yang, R. Ramanathan, and NH Vaidy, MobiCom 2002.

  23. Markov Model for S-MAC • The state (SA, SC1, SC2, SB) represents the status of the nodes in group A,C,B in a slot, where • SA + SC1 <= 1, SB + SC2 <= 1, SC1 + SC2 <= 1 • The Markov chain has 8 states: (0,0,0,0), (0,0,0,1), (0,0,1,0), (0,1,0,0), (0,1,0,1), (1,0,0,0), (1,0,0,1), (1,0,1,0).

  24. Fairness Analysis (NA=NB=20, Nc1=Nc2=10) • Throughput vs. Packet size • Stationary Probabilities

  25. Fairness Analysis (NA=NB=20, Nc1=Nc2=10) • Node Contention/Collision • PaAd= ps(0,0,0,0) + ps(0,0,0,1) +ps(0,0,1,0)PaCd= ps(0,0,0,0) + ps(0,0,0,1)

  26. Performance Evaluation • NS-2 simulator is used. • 802.11b with transmission rate 11 Mbps. • Transmission range of 250m and carrier sensing range is 550m. • All nodes are stationary. • UDP traffics packets with average packet size 1000 bytes. • Four way handshake (RTS/CTS/DATA/ACK) is used. • Simulated duration of 50 seconds and each point is averaged from 5 independent runs.

  27. Simulation Scenarios • Network of 2x2 grid of overlapping • Each AP has 40 clients that are distributed uniformly in its coverage area. • Infrastructure mode is used. • APs are upgraded with S-MAC of 4 sectors (1 Tx & 4 Rx). • All STAs still use omni directional antenna (regular 802.11 MAC).

  28. Simulation Results • Improvement arises from reduced interference with sector antennas and reduced collision from the S-MAC protocol. • Total throughput does not change significantly as the number of sectors increases from 2 to 4. • No significant change was found with different antenna orientations.

  29. Conclusion • S-MAC takes full advantage of directional antenna: • Avoids hidden node problem and deafness. • Multiple sectors can be used simultaneously. • Fully compatible with regular omni-antenna client nodes. • Easy to upgrade existing 802.11 networks with enhanced access. • Increase the network capacity with minimal cost. • Extendable to utilize smart antenna systems

  30. Ideas • For ad hoc networks: • Study effect of x% of nodes are S-MAC. • Study the effect of location of S-MAC node  find the optimum set of S-MAC nodes for best performance • For Infrastructure: • Best Carrier Sense Threshold for optimal performance • Mobility?

  31. BACKUP SLIDES

  32. Directional Antenna and DMAC (I) N2 N3 N1 • Conflict between increased spatial reuse (higher capacity) and increased collision (higher MAC overhead) • Collision caused by directional antenna • Hidden nodes due to asymmetry omni/directional gain • Hidden nodes due to unheard RTS or CTS packets • Deafness

  33. Directional Antenna and DMAC (II) N4 N1 N2 N3 • Conflict between increased spatial reuse (higher capacity) and increased collisions (higher MAC overhead) • Collisions caused by directional antenna • Hidden nodes due to asymmetry omni/directional gain • Hidden nodes due to unheard RTS or CTS packets • Deafness

  34. MAC Assisted Self-calibration • Self-calibration: • Estimate the channel from antenna i to antenna k, both of the same S-MAC node. • Applicable to all PHY (a/b/g). • Procedures • Step 1: send RTS in every sector to silence all neighbor nodes, so the SYNC sent next will not collide with other packets. • Step 2: send regular training symbols (SYNC) in every sector. • As SYNC is sent from antenna i, antenna k estimate the channel Gik. • Gik and Gki can be averaged: Gki= Gik:=(Gki+ Gik)/2.

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