IEEE 802.11 DCF Based MAC Protocols for Multiple Beam Antennas and their Limitations

Vivek Jain, Anurag Gupta Dharma P.

AgrawalECECS DepartmentUniversity of Cincinnati

{jainvk, guptaag, dpa}@ececs.uc.edu

Dhananjay Lal

Research and Technology Center

Robert Bosch CorporationDhananjay.Lal@RTC.Bosch.com

Outline

Introduction Multiple Beam Antennas IEEE 802.11 DCF Proposed Variants Performance Evaluation Concurrent Packet Reception

Bounds Protocol Guidelines Conclusions

Introduction

A B

C

DF

G

H

X

Nodes in Silent Zone

Omnidirectional Antenna – Low Throughput in Wireless Ad hoc networks due to poor spatial reuse

Omnidirectional Communication

A B

C

D

E

F

G

H

Directional Communication

Directional Antenna – Better Spatial reuse. But a node still unable to fully utilize “spatial bandwidth”.

Introduction Multiple Beam Antenna – Exploits spatial bandwidth fully A node can initiate more than one simultaneous

transmissions (or receptions).

DATA

DATA

DATA

A

B

C

D

E

F

G

DATADATA

DATA

Multiple Beam Antennas - Types

top view (horizontal)

Interferer 1

User 1

2

34

6

7

8

10

11

12

5User 3

9

User 2 Interferer 2

Interferer 3

1

Switched array

User 1

Interferer 1

top view (horizontal)

User 3

User 2

Interferer 3

Interferer 2

Adaptive array

Applications

Military networksCellular Communication NetworksWireless Local Area Networks

Multiple Beam Antennas - Beam Forming

Therefore, a node can either transmit or receive simultaneously but not both.

… …

Direction of Arrival Estimation Beam Formation

IEEE 802.11 DCF

TimeRTSDIFS

SIFS

DIFS RTS

Defer access

aSlotTime

RandomBackoff

Source

Destination ACK

Other

CTS

SIFS Data

SIFS

NAV (RTS)

NAV (CTS)

NAV (Data)

Physical Carrier Sensing

Virtual Carrier Sensing

De-facto medium access control for wireless LAN and ad hoc networks Originally designed for omnidirectional communication, its virtual

carrier sensing (VCS) mechanism is enhanced for directional communication to include directional of arrival also.

IEEE 802.11 DCF for Multiple Beam Antennas

Random Backoff after DIFS wait

Beam-based Node-based

Transmission Control Packets (RTS/CTS)

Directional Omnidirectional

All nodes employ IEEE 802.11 DCF with directional virtual carrier mechanism (DVCS).

MMAC-NBMDMAC-NBMDMAC-BB MMAC-BB

Performance Evaluation

1

23

4

8

7

Directional Coverage Area

Omnidirectional Coverage Area

5

6

The Antenna Model

Packet generation at each source node is modeled as Poisson process with specified mean arrival rate

Each packet has a fixed size of 2000 bytes and is transmitted at a rate of 2Mbps

Each node has maximum buffer of 30 packets Each packet has a lifetime of 30 packet durations Each simulation is run for 100 seconds.

Gains from spatial reuse only are

considered

Performance Evaluation Omnidirectional protocols are able to achieve concurrent data

communications between node pairs A-B and C-D. Directional protocols on the other hand suffer from deafness

problem while omni-directional antenna from poor spatial reuse.

B

A

C

D

Performance Evaluation None of the protocols are able to extract throughput of more

than 33% of the maximum possible value This implies only one route is active on an average and hence

concurrent packet reception is not occurring at node D.

A

B

C

D

E

G

F

Performance Evaluation

Concurrent packet reception at node D

Concurrent packet transmission by node D

A

B

C

D

E

G

F

Concurrent Packet Reception Bounds

N neighbors transmitting with probability p under saturation state

Protocol Guidelines Transmit control packets in beams with

transmitting nodes – suppress deafness Common backoff timer for all beams –

support concurrent packet transmission Employ local synchronization – support

concurrent packet reception Employ hot-potato type of buffering, i.e.,

successive CPR and CPT cycles – minimize delay

Conclusions Concurrent packet reception in multiple

beam antennas is highly improbable with IEEE 802.11 DCF based protocols

Asynchronous protocols are not suited for multiple beam antennas

A new MAC protocol based on the formulated guidelines is required.

Follow Up: IEEE Globecom 2005, St. Louis, Nov. 27-Dec.2.

V. Jain, A. Gupta, D. Lal, and D. P. Agrawal, "A Cross Layer MAC with Explicit Synchronization through Intelligent Feedback for Multiple Beam

Antennas,"

Thank You!!!

Questions ???

Performance Evaluation Deafness and route coupling do not affect omni-protocols,

but directional protocols experience performance degradation at higher loads.

A B

C D

Performance Evaluation Omnidirectional protocols overwhelms node C leading to

data loss when the packet lifetime expires.

A

B

C

D

Performance Evaluation Gains from omnidirectional communication

of control packets at medium loads Directional protocols perform better at

higher loads because of better spatial reuse.

D

H

I

J

C

G

B

FA

E

Performance Evaluation Node-based backoff protocols for multiple beam

antennas achieve maximum throughput due to gains from concurrent packet transmissions

A B

C

D

E

Performance EvaluationA B

C

D

E

Concurrent packet reception at any node

Concurrent packet transmission by any node

Performance Evaluation

Energy expended in random and compete-5 topologies

Multiple beam omni-directional protocols expend more energy due to omni-directional transmission of control messages.

Concurrent Packet Reception Bounds in Saturated State

The probability that a node receives data in b beams concurrently is given by ),(),()( MbQNbPbPCPR

bNb ppb

NNbP

)1(),(

P(b,N) is the probability that b out of N neighbors transmit in a slot concurrently and is given as

bM

bb

M

MbQ

!

),(

Q(b,M) is the probability of arranging b nodes in M beams such that CPR can happen, is given as

M

bCPRCPR bPP

2

)(

Thus the total probability of concurrent packet reception by a node is then given by