Post on 31-Dec-2015
description
Wireless Networking & Mobile Computing
CS 752/852 - Spring 2012
Tamer NadeemDept. of Computer Science
Lec #8: MAC Sectorized Antennas
Enhancing 802.11 Wireless Networks Enhancing 802.11 Wireless Networks with Directional Antenna and Multiple with Directional Antenna and Multiple
ReceiversReceivers(Chenxi Zhu, Tamer Nadeem, and Jonathan Agre)
Introduction
• IEEE 802.11 WLANs have enjoyed tremendous popularity in recent years.
• RTS/CTS/DATA/ACK packets assume omni-directionality
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.
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
Spatial Fairness of 802.11
• Extend Bianchi’s discrete time Markov model to some simple multihop networks.
• Contention probability
Need to revisit Bianchi’s discrete time model
• 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.
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/pt slots, where pt is the probability that a data transmission terminates in a slot;
• Every node always has a packet to send to one of its neighbors.
Markov Model
•
•
•
•
•
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).
Markov Model
• Transitional Probabilities:
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)
• Contention probabilities 1; 2 of nodes in areas A/B and C
• Collision probabilities of the nodes in groups A,B and group C
Fairness Analysis (NA=Nc=NB=20)
• Throughput vs. Packet size • Stationary Probabilities
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)
Use of Directional Antenna
• Fairness relieved through interference reduction
• Directional antenna is a well known method to reduce the interference and to increase the range and the capacity for wireless networks.
S-MAC
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
s
R
I
N
#1
#8
#7#6
#5
#4
#3 #2
r
Addresses the hidden node problem and the deafness problem by continuously
monitoring the channel in all directions (sectors) at all time
S-MAC Architecture
TX2
TX1
RX3
RX2
RX1
switching fabric
DUX
DUX
DUX
TX symbol for self-interferencecancellation
S-MAC: SNAV=[NAVTX1,NAVTX2, NAVRX1, NAVRX2, NAVRX3]
TX
RX
DirectionalAntennas
…
Separate queues
Base Band
RXRF
TXRF
RF MAC and higher
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 Sr
i. ;
– 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:
Srik. = Sr
i - Gij* Sti.
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.
Operation of S-MAC (example I)
C
Example adopted from R. Choudhury, X. Yang, R. Ramanathan, andNH Vaidy, MobiCom 2002.
DMAC “Hidden Node due to asymmetric gain”D H
A
E B F G
RTSCTSRTS
Collision
Operation of S-MAC (example I)
Example adopted from R. Choudhury, X. Yang, R. Ramanathan, andNH Vaidy, MobiCom 2002.
SMAC: “Hidden Node due to asymmetric gain” avoidanceD H
A
C
E B F G
RTSCTS
CTS from F rcvdRTS not sent by A
Operation of S-MAC (example II)
Example adopted from R. Choudhury, X. Yang, R. Ramanathan, andNH Vaidy, MobiCom 2002.
“Hidden Node due to unheardRTS/CTS” avoidance
D H
A
C
E B F G
RTSCTS
E waits for B-F to finish
Operation of S-MAC (example II)
Example adopted from R. Choudhury, X. Yang, R. Ramanathan, andNH Vaidy, MobiCom 2002.
Deafness Prevention
D H
A
C
E B F G
E is aware C is Transmitting
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).
Fairness Analysis (NA=NB=20, Nc1=Nc2=10)
• Throughput vs. Packet size • Stationary Probabilities
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)
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.
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).
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.
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
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?
BACKUP SLIDES
Directional Antenna and DMAC (I)
• 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
N1
N2N3
Directional Antenna and DMAC (II)
• 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
N1 N2N3
N4
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.