iCAR : an Integrated
Cellular and Ad-hoc
Relaying System *
Hongyi Wu
Advisor: Dr. Chunming Qiao
LANDER, SUNY at Buffalo
This project is supported by NSF under the contract
ANIR-ITR 0082916 and Nokia.
Outline
Motivations
Introduction of iCAR
ARS Placement
Seed ARS
Quality of Coverage
iCAR Performance
Theorems
Analysis
Simulations
Signaling Protocols
Future Work and Conclusion
Outline
Motivations
Introduction of iCAR
ARS Placement
Seed ARS
Quality of Coverage
iCAR Performance
Theorems
Analysis
Simulations
Signaling Protocols
Future Work and Conclusion
What is a cellular system?
The problem of scarce
frequency resource
Based on subdivision
of geographical area
One 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 cells
Decrease call blocking and dropping probability
Increase system’s capacity cost-effectively
Support heterogeneous networks
Provide service for shadow area
Reduce mobile host’s (MH) transmission power
and/or increase transmission rate
Outline
Motivations
Introduction of iCAR
iCAR Placement
Seed ARS
Quality of Coverage
iCAR Performance
Theorems
Analysis
Simulations
Signaling Protocols
Future 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)
ARS
MH
One example of relaying
MH X moving into congested Cell B is relayed
to Cell A
A
x
B
(a)
A
x
(b)
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
x
B
Basic Operations (Cont’d)
Secondary Relay
Primary relay failed
A
Not covered by
ARS
Reachable BTS is
congested too
Free the channel
of an active call
which can be
relayed to a
neighbor cell
x
B
y
(a)
x
A
y
(b)
B
Basic Operations (Cont’d)
Cascaded Relay
Cascade the above relays more multiple times if
they are failed.
x
A
B
x
A
y
C
z
y
C
z
B
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
Outline
Motivations
Introduction of iCAR
ARS Placement
Seed ARS
Quality of Coverage
iCAR Performance
Theorems
Analysis
Simulations
Signaling Protocols
Future 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
 R2 
n  2  2 
r 
1 Km 1.5 Km
200m 50
114
350m 18
38
500m 8
18
2 Km
200
66
32
Seed Growing Approach
With 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 ARSs
For 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

3n  4 n  4

Quality of Coverage
The 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 Placement
Two approaches to place the
seed ARS
Edge (ARS No.1)
S
S
 TA  (1  bB )   TB  (1  bA )
2
2
Half of S covers cell A,
but only unblocked part
(1-bB) of them is relay-able
B
B
A
2
 S: ARS ceverage;
 TA, TB: Traffic intensity of cell A
and B.
 bA,bB: Blocking probability of cell A
and B.
QEdge 
B
…
B
2' 1
1'
B
B
3
3'
Seed
ARS’s
Seed ARS’s Placement
 Vertex (ARS No.1')
QVertex 
S
2S
 TA  (1  bB2 ) 
 TB  (1  bA  bB )
3
3
Two third of S covers
One third of S covers
cell B. ..
cell A. Note that, the
Blocking probability is
bB2 because the call may
Be relayed to two cells.
B
B
B
A
2
B
2' 1
1'
B
B
3
3'
Seed
ARS’s
Seed ARS’s: Edge v.s. Vertex
Preliminary 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 ARS
Preliminary 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•(1bB).
Rule of Thumb 2
Try to place an ARS as
a seed if it is possible.
Growing Direction
When 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.
Outline
Motivations
Introduction of iCAR
ARS Placement
Seed ARS
Quality of Coverage
iCAR Performance
Theorems
Analysis
Simulations
Signaling Protocols
Future Work and Conclusion
Theorems
Theorems1
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
n
(
T  Ti ). According to Erlang B formula, the blocking probability in each cell
is
i 1

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 multidimensional 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 model
State 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
Simulations
Simulation 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 rate
Blocking 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
Outline
Motivations
Introduction of iCAR
ARS Placement
Seed ARS
Quality of Coverage
iCAR Performance
Theorems
Analysis
Simulations
Signaling Protocols
Future Work and Conclusion
Signaling and routing
protocols for QoS traffic
Why 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
protocol
Primary relaying
Protocol 1 (cont’d)
Secondary relaying
Protocol 2: a link-state
based protocol
Primary relaying
Protocol 2 (Cont’d)
Secondary relaying
Protocol 3: an aggressive
route-searching protocol
Primary relaying
Protocol 3 (cont’d)
Secondary relaying
Performance Comparison
Three 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 ondemand and the simplest distributed protocol.
It requires fewest memory and computing
power.
Simulation
We 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 Results
Blocking rate
Simulation results (cont’d)
Signaling Overhead
Outline
Motivations
Introduction of iCAR
ARS Placement
Seed ARS
Quality of Coverage
iCAR Performance
Theorems
Analysis
Simulations
Signaling Protocols
Future Work and Conclusion
Future Work
Mobility 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, HICSS35, 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.