Enhancing 802.11 Wireless Networks with Directional Antenna and Multiple Receivers

Chenxi Zhu, Fujitsu Laboratories of America

Tamer Nadeem, Siemens Corporate Research

Jonathan Agre, Fujitsu Laboratories of America

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:

• Diagonal terms:

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 and 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.