icar : an integrated cellular and ad-hoc relaying system *

iCAR : an Integrated Cellular and Ad-hoc Relaying System *Hongyi WuAdvisor: Dr. Chunming Qiao LANDER, SUNY at Buffalo

This project is supported by NSF under the contract ANIR-ITR 0082916 and Nokia.

OutlineMotivationsIntroduction of iCARARS Placement

Seed ARS Quality of Coverage

iCAR Performance Theorems Analysis Simulations

Signaling ProtocolsFuture Work and Conclusion

OutlineMotivationsIntroduction of iCARARS Placement

Seed ARS Quality of Coverage

iCAR Performance Theorems Analysis Simulations

Signaling ProtocolsFuture Work and Conclusion

What is a cellular system?The problem of scarce

frequency resourceBased on subdivision

of geographical areaOne Base Transceiver

Station (BTS) in each cell.

Frequency is reused in cells far away.

Problems in Cellular Systems A MH can only access the channels in one cell

(except soft-handoff). Unbalanced traffic among cells Variable locations of the Hot Spots (congested cells) Cell-splitting not flexible nor cost-effective enough Tremendous growth of wireless data/voice traffic Limited capacity

What is Mobile Ad hoc Network (MANET)?An autonomous system of mobile

nodes connected by wireless links.The nodes are routers.The nodes are organized in a

arbitrary graph.The nodes are free to move.

Objectives of Our Work

Balance traffic among cellsDecrease call blocking and dropping

probabilityIncrease system’s capacity cost-effectivelySupport heterogeneous networksProvide service for shadow areaReduce mobile host’s (MH) transmission

power and/or increase transmission rate

OutlineMotivationsIntroduction of iCARiCAR Placement

Seed ARS Quality of Coverage

iCAR Performance Theorems Analysis Simulations

Signaling ProtocolsFuture Work and Conclusion

Basic Idea : Integration of Cellular and Ad-hoc Relaying Technologies

ARS : Ad-hoc Relaying Stations

Each ARS and MH has two interfaces (celluar and relay)

ARSMH

One example of relaying

MH X moving into congested Cell B is relayed to Cell A

A B A Bx

x

(a) (b)

An ARS differs from a BTS and a MH

Compared to BTS Mobility Air interface

Compared to MH Mobility Security,authentication,privacy Billing

Basic Operations

Primary Relay : a strategy that establishs a relaying route between a MH (in congested cell) to a nearby non-congested cell. Failed Hand-off Blocked new call MH switches over from C-interface to R-interface

A Bx

Basic Operations (Cont’d)

Secondary Relay Primary relay

failedNot covered by ARSReachable BTS is

congested too Free the channel of

an active call which can be relayed to a neighbor cell

A Bx

y

A Bx

y

(a)

(b)

Basic Operations (Cont’d)Cascaded Relay

Cascade the above relays more multiple times if they are failed.

A Bx

C

A Bx

C

y

z

y

z

CI and NCI

Congestion-Induced (CI) Relaying Reduce call blocking or dropping

probability when congestion occures.

Noncongestion-Induced (NCI) Relaying Pro-actively balance load Shadowing Area Uncovered Area Transmission Power

OutlineMotivationsIntroduction of iCARARS Placement

Seed ARS Quality of Coverage

iCAR Performance Theorems Analysis Simulations

Signaling ProtocolsFuture Work and Conclusion

Full Coverage The maximum number of relay stations needed

so as to ensure that a relaying route can be established between any BTS and an MH located any where in the cell

2

2

2rRn

2 Km

350m200m

1.5 Km1 Km

500m

50188

1143818

2006632

Seed Growing ApproachWith fewer ARS’s, relaying can still be

effective. Some can be seeds (placed at each pair of shared edges), and others can grow from them (placed nearby).

Number of Seed ARSsFor a fix coverage area, the system with

fewer UN-SHARED edges needs more seed ARSs.

The max number is obtained by considering a circle area and count the number of shared edges.Proposition: For a n-cell

system, the maximum number of seed ARS’s is

443 nn

Quality of CoverageThe quality of ARS coverage (Q) is defined

to be the relay-able traffic in an iCAR system. The Q value depends on the traffic intensity,

the cell size, the ARS size, the system topology, etc.

The higher the Q value, the better the ARS placement

The Q value is not always proportional to the ARS coverage.

Seed ARS’s PlacementTwo approaches to place

the seed ARS Edge (ARS No.1)

S: ARS ceverage;TA, TB: Traffic intensity of cell A

and B.bA,bB: Blocking probability of

cell A and B.

AB B

B

B

BB

11'

2'2

)1(2

)1(2 ABBAEdge bTSbTSQ 3

3'

Seed ARS’s

Half of S covers cell A,but only unblocked part(1-bB) of them is relay-able

…

Seed ARS’s Placement Vertex (ARS No.1')

AB B

B

B

BB

11'

2'2)1(3

2)1(3

2BABBAVertex bbTSbTSQ

3

3'

Seed ARS’s

One third of S covers cell A. Note that, the Blocking probability isbB

2 because the call mayBe relayed to two cells.

Two third of S covers cell B. ..

Seed ARS’s: Edge v.s. VertexPreliminary results

Case1 : when TB<TA<50 Erlangs, Qvertex<QEdger.

Case2 : when TA, TB>50 Erlangs or TA<TB, QVertex>QEdge.

Case2 is out of normal operation range

Rule of Thumb 1 Place the seed

ARS's at edges of a hot spot cell.

Seed ARS v.s. Grown ARSPreliminary Results

Case1 : seed (ARS 2). Assuming edge placement of seed)

Case2 : grow (ARS 2’). The QoC value of the grown ARS is about 0.61•S •TA•(1-bB).

Rule of Thumb 2 Try to place an ARS

as a seed if it is possible.

Growing DirectionWhen there are

already sufficient seed ARS’s,

Additional ARS's can grow toward inside of a

hot cell A (ARS No.3) toward outside of cell

A (ARS No.3')Rule of Thumb 3

Place an ARS in the cell with a higher traffic intensity.

OutlineMotivationsIntroduction of iCARARS Placement

Seed ARS Quality of Coverage

iCAR Performance Theorems Analysis Simulations

Signaling ProtocolsFuture Work and Conclusion

TheoremsTheorems1

Assume that the total traffic in a n-cell system is T Erlangs, then the (system wide) call blocking probability is mininized when the traffic in each cell is T/n Erlangs.

Why?Assume there are M channels in each cell, and the traffic intensity in cell i

is Ti ( ). According to Erlang B formula, the blocking probability in each cell is

n

i

TiT1

Theorem (Cont’d) The average blocking probability of entire system is

In order to compute the minimum value of B, we derive the partial differentiation,

Solve a group of equations, we can get the critical points,

Theorems (Cont’d)Theorem2

For a given total traffic in a system, and a fixed number of data channels, an idea iCAR system has a lower blocking probability than any conventional cellular system (including a perfectly load-balanced one).

Why?An idea iCAR system can relay traffic from one cell to any other cells. So, it can be treated as a SUPER cell with nT traffic and nM channels. The blocking probability of the super cell is

We can prove that it is lower than B(M,T).

Analysis based on multi-dimensional Markov chains

Consider a system with only seed ARS’s

Analysis (Cont’d)For primary relaying

An approximate model (considering cell X in figure (b))To simplify the analysis, we assume that the

blocking probability of the neighboring cells of X is fixed, i.e. the traffic relayed to cell Bs won’t change their blocking probability. This assumption will be nullified in the accurate analysis model.

Analysis (Cont’d)For primary relaying

An approximate modelState diagram

Final result

Analysis (Cont’d)For primary relaying

Analysis (Cont’d)An

accurate model of primary relaying for a 2-cell system.

Analysis (Cont’d)Secondary

relaying An

approximate model

Analysis (Cont’d)An accurate model

SimulationsSimulation model

GloMoSim 25 cells Cell A is a hot spot Location dependent

traffic (ripple effect) 50 DCH’s per cell 56 seed ARS’s 25,600 MH’s Call arrive rate is in

poisson distribution Holding time is in

exponential

Simulations (cont’d)Results

Blocking rateBlocking rate can

be reduced by primary relaying, but not much

Secondary relaying reduces the call blocking rate further

Simulations (Cont’d)More results

Call Dropping Rate Throughput

OutlineMotivationsIntroduction of iCARARS Placement

Seed ARS Quality of Coverage

iCAR Performance Theorems Analysis Simulations

Signaling ProtocolsFuture Work and Conclusion

Signaling and routing protocols for QoS trafficWhy do we need signaling and routing protocols?

For iCAR to support real-time IP-based applications in wireless mobile environment, set up bandwidth guaranteed relaying path.

Candidates of protocols for iCAR

Protocol 1: a PSC-assisted protocolPrimary relaying

Protocol 1 (cont’d)Secondary relaying

Protocol 2: a link-state based protocolPrimary relaying

Protocol 2 (Cont’d)Secondary relaying

Protocol 3: an aggressive route-searching protocolPrimary relaying

Protocol 3 (cont’d)Secondary relaying

Performance ComparisonThree protocols have their own

advantages and disadvantages The PSC-assisted protocol will have the

lowest signaling overhead in terms of the number of signaling messages. But in this protocol, PSC becomes the performance bottle neck and a signal point of failure.

Performance Comparison (Cont’d)

The link-state based protocol is distributed. It requires the ARSs to flood the update messages. Also, the ARSs need large enough memory to maintain topology and bandwidth information, and high computation power to compute the relaying route.

The aggressive route searching protocol does not maintain the relaying bandwidth information of other ARSs. It is an on-demand and the simplest distributed protocol. It requires fewest memory and computing power.

SimulationWe evaluate the performance of the

proposed signaling protocols in terms of request rejection rate and signaling overhead via simulations.

Seven cells, 30~60 ARSs and 1600 MHs were simulated in the model we discussed before.

Simulation ResultsBlocking rate

Simulation results (cont’d)Signaling Overhead

OutlineMotivationsIntroduction of iCARARS Placement

Seed ARS Quality of Coverage

iCAR Performance Theorems Analysis Simulations

Signaling ProtocolsFuture Work and Conclusion

Future WorkMobility Tracking

With the help of GPS, we can keep track of the position of MHs and ARSs, so that we can move the ARSs to the best positions.

ARS Management/Moving With the movement of ARSs, issues such

as route reestablishment, etc., need to be addressed.

Future Works (Cont’d)MAC layer design

The iCAR system needs a novel MAC protocol to support relaying. The IEEE802.1X protocols may not be the optimized solutions for iCAR as the cellular infrastructure can help packet scheduling so as to avoid collisions.

Conclusion

A purely cellular or purely Ad-hoc network will not be scalable, nor versatile enough.

The integrated architecture can efficiently balance the traffic load dynamically, thus reduce the call blocking /hand-off dropping probability, and increase the effective capacity of a system.

Other benefits include shadow coverage, fault tolerance and reduced transmission power and/or increased transmission rate.

Publications “Integrated Cellular and Ad hoc Relaying (iCAR) System” Pushing the

Performance Limits of Conventional Wireless Networks”, HAWAII INTERNATIONAL CONFERENCE ON SYSTEM SCIENCES, HICSS-35, January 7-10, 2002, Big Island, Hawaii.

“Overcoming The Limits Imposed By Cellular Boundaries With iCAR", in Asia-Pacific Optical and Wireless Communications, November 12-16, 2001. Beijing, China.

"An Integrated Cellular and Ad hoc Relaying System : iCAR", in IEEE Journal on Selected Areas in Communication (JSAC) special issue on Mobility and Resource Management in Next Generation Wireless System, Oct., 2001.

"Distributed Signaling and Routing Protocols in iCAR (integrated Cellular and Ad hoc Relaying System)", in the Fourth International Symposium on Wireless Personal Multimedia Communications (WPMC'01), Sept. 9-12, 2001. Aalborg, Denmark.

Publications (Cont’d) "Quality of Coverage: A New Concept for Wireless Networks", in ACM

SIGCOMM 2001 conference student poster session, August 27-31, 2001, Mandeville Auditorium, UC San Diego, CA

"Performance Analysis Of iCAR (Integrated Cellular and Ad-hoc Relay System)", in IEEE International Conference on Communications (ICC'01), June 11-14, 2001. HELSINKI, FINLAND.

"An New Generation Wireless System with Integrated Cellular and Mobile Relaying Technologies", in International Conference on Broadband Wireless Access Systems (WAS'2000), Dec. 4-6, 2000. San Francisco, CA. 

"iCAR: an Integrated Cellular and Ad-hoc Relay System", in IEEE International  Conference on Computer Communications and Networks (ICCCN'2000), Oct, 2000. Las Vegas, NV.

"Load Balancing via Relay in Next Generation Wireless Systems" in IEEE Workshops on Mobile Ad Hoc Net Working and Computing (MobiHoc'2000), in conjunction with MobiCom'2000, Aug 7-11, Boston, MA. pp. 149-150.