Download slides - Dr. Fei Hu

Week 13 lecture 1
Cellular Communications (I)
Existing Network Infrastructure
 Public Switched
Telephone Network
(PSTN): Voice
 Internet: Data
 Hybrid Fiber Coax
(HFC): Cable TV
Market Sectors for Applications
 Four segments divided into two classes: voice-oriented
and data-oriented, further divided into local and widearea markets
 Voice:
– Local: low-power, low-mobility devices with higher
QoS – cordless phones, Personal Communication
Services (PCS)
– Wide area: high-power, comprehensive coverage, low
QoS - cellular mobile telephone service
 Data:
– Broadband Local and ad hoc: WLANs and WPANs
(WPAN-Wireless Personal Area Network)
– Wide area: Internet access for mobile users
Evolution of Voice-Oriented Services
Year
Event
Early 1970s
First generation of mobile radio at Bell Labs
Late 1970s
First generation of cordless phones
1982
First generation Nordic analog NMT
1983
Deployment of US AMPS
1988
Initiation of GSM development (new digital TDMA)
1991
Deployment of GSM
1993
Initiation of IS-95 standard for CDMA
1995
PCS band auction by FCC
1998
3G standardization started
FDMA – Frequency Division Multiple Access
NMT – Nordic Mobile Telephony
AMPS – Advanced Mobile Phone System
GSM – Global System for Mobile
Communication
TDMA – Time Division Multiple Access
IS-95 – Interim Standard 95
CDMA – Code Division Multiple Access
PCS – Personal Communication System
FCC – Federal Communication Commission
Evolution of Data-Oriented Services
Year
Event
1979
Diffused infrared (IBM Rueschlikon Lab - Switzerland
Early 1980s
Wireless modem (Data Radio)
1990
IEEE 802.11 for Wireless LANs standards
1990
Announcement of Wireless LAN products
1992
HIPERLAN in Europe
1993
CDPD (IBM and 9 operating companies)
1996
Wireless ATM Forum started
1997
U-NII bands released, IEEE 802.11 completed, GPRS started
1998
IEEE 802.11b and Bluetooth announcement
1999
IEEE 802.11a/HIPERLAN-2 started
HIPERLAN – High Performance Radio LAN
CDPD – Cellular Digital Packet Data
U-NII – Unlicensed National Information Infrastructure
GPRS – General Packet Radio Service
Different Generations: 1G
 1G Wireless Systems: Analog systems
– Use two separate frequency bands for forward
(base station to mobile) and reverse (mobile to
base station) links: Frequency Division Duplex
(FDD)
– AMPS: United States (also Australia, southeast
Asia, Africa)
– TACS: EU (later, bands were allocated to GSM)
– NMT-900: EU (also in Africa and southeast Asia)
– Typical allocated overall band was 25 MHz in
each direction; dominant spectra of operation
was 800 and 900 MHz bands.
AMPS – Advanced Mobile Phone System
TACS – Total Access Communication System
NMT – Nordic Mobile Telephony
2G
 2G Wireless Systems: Four sectors
– Digital cellular
 GSM (EU/Asia): TDMA
 IS-54 (US): TDMA
 IS-95 (US/Asia): CDMA
– PCS – residential applications
 CT-2 (EU,CA): TDMA/TDD
 DECT(EU):TDMA/TDD
 PACS (US): TDMA/FDD
CT-2 – Cordless Telephone 2
DECT – Digital Enhanced Cordless Telephone
PACS – Personal Access Communication System
2G (cont’d)
 2G Wireless Systems: Four sectors (cont’d)
– Mobile data
 CDPD shares AMPS bands and site infrastructure;
 GPRS shares GSM’s radio system - data rates suitable for
Internet
– WLAN – Unlicensed bands, free of charge and
rigorous regulations: very attractive!
 IEEE 802.11 and IEEE 802.11b use DSSS physical layer;
 HIPERLAN/1 uses GMSK;
 IEEE 802.11a and HIPERLAN/2 use OFDM: next generation
CDPD – Cellular Digital Packet Data
GPRS – General Packet Radio Service
DSSS – Direct Sequence Spread Spectrum
GMSK – Gaussian Minimum Shift Keying
OFDM – Orthogonal Frequency Division Multiplexing
3G
 3G and Beyond
– Purpose: develop an international standard that
combines and gradually replaces 2G digital cellular,
PCS, and mobile data services, at the same time
increasing the quality of voice, capacity of the
network, and data rate of the mobile data
services.
– Radio transmission technology of choice: W-CDMA
– 3G was envisioned to provide multimedia services to
users everywhere
Summary
Relative coverage, mobility, and data rates of generations of cellular systems
and local broadband and ad hoc networks.
Heterogeneous
Cellular Networks
Satellite
Regional Area
Low-tier
High-tier
Local Area
Wide Area
High Mobility
Low Mobility
 Seamless mobility across diverse overlay
networks
 “vertical” hand-offs
 software “agents” for heterogeneity management
 IP as the common denominator?
Design
Cellular
Networks
Cellular Network Architecture
Mobile
Switching
Center
Location
Register
(Database)
MSC
Radio
Network
Base Station
Controller
Backbone
Wireline Network
Mobile
Terminal
Base Station
Cell
BASIC ARCHITECTURE
Home Location Register
(HLR)
BACKBONE TELEPHONE NETWORK
Visitor Location Register
(VLR)
Mobile Switching Center
(MSC)
MSC
VLR
Mobile Terminal
(MT)
Local Signaling
Long Distance Signaling
Cellular Concept
 The most important factor is the size and the shape of a CELL.
 A cell is the radio coverage area by a transmitting station or a
BS.
 Ideally, the area covered a by a cell could be represented by a
circular cell with a radius R from the center of a BS.
 Many factors may cause reflections and refractions of the
signals, e.g., elevation of the terrain, presence of a hill or a
valley or a tall building and presence in the surrounding area.
 The actual shape of the cell is determined by the received signal
strength.
 Thus, the coverage area may be a little distorted.
 We need an appropriate model of a cell for the analysis and
evaluation.
 Many posible models: HEXAGON, SQUARE, EQUILATERAL
TRIANGLE.
Cell Shape
R
R
R
Cell
R
(a) Ideal Cell
(b) Actual Cell
R
(c) Different Cell Models
Size and Capacity of a Cell per Unit of Area and
Impact of the Cell Shape on System Characteristics
Cellular Concept Example
 Consider a high-power
transmitter that can support
35 voice channels over an area
of 100 km2 with the available
spectrum
 If 7 lower power transmitters
are used so that they support
30% of the channels over an
area of 14.3 km2 each.
 Then a total 7*(30% * 35) =
80 channels are available
instead of 35.
2
3
1
7
6
4
5
Cellular Concept
 If two cells are far away from enough that
the same set of frequencies can be used in
both cells, it is called frequency reuse.
 With frequency reuse, a large area can be
divided into small areas, each uses a subset of
frequencies and covers a small area.
 With frequency reuse, the system capacity can
be expanded without employing high power
transmitters.
Capacity Expansion by
Frequency Reuse
 Same frequency band or channel used in
a cell can be “REUSED’ in another cell as
long as the cells are far apart and the
signal strength do not interfere with
each other.
 This enhances the available bandwidth of
each cell.
 A group of cells that use a different set of
frequencies in each cell is called a cell cluster.
NUMBER OF CELLS IN A CLUSTER
FREQUENCY REUSE
Example: A typical cluster of 7 such cells and 4 such
clusters with no overlapping area
F7
F6
F7
F2
|------ F1
F5
F3
|
F4
|D
F7
|
F6
|---------- F1
F5
F4
F6
F5
F1
F4
F2
F3
F7
F2
F3
F6
F5
F1
F4
FREQUENCY
REUSE DISTANCE
D
F2
F3
RULE to Determine the Nearest Co-Channel
Neighbors (with the same frequency set):
Determining the Cluster Size
j
 To find nearest co-channel neighbors
of a particular cell
 Step 1: Move i cells along any
chain of hexagons;
 Step 2: Turn 60 degrees
counterclockwise and move j
cells AND REACH the cochannel.
 i and j measure the number
of nearest neighbors between
co-channel cells
 The cluster size, N,
N = i2+ij+j2
If i =2 and j = 0, then N = 4
If i = 2 and j = 1, then N =7
i
1
2
1
3
4
3
2
1
2
Frequency Reuse
 The distance between 2 cells using the same
channel is known as the REUSE DISTANCE D.
 There is a close relationship between D, R
(radius of each cell) and N (the number of
cells in a cluster) -- details later
D = (sqrt 3N) . R
 The REUSE FACTOR is then
D/R = sqrt (3N)
Frequency Reuse
 Let N be the cluster size in terms of
number of cells within it and K be the
total number of available channels
without frequency reuse.
 N cells in the cluster would then utilize
all K available channels.
 Each cell in the cluster then uses 1/Nth of the total available channels.
 N is also referred as the frequency
reuse factor of the cellular system.
Capacity Expansion by
Frequency Reuse
 Assume each cell is allocated J channels (J<=K).
If the K channels are divided among the N cells
into unique and disjoint channel groups, each with
J channels, then K = J N
 The N cells in a cluster use the complete set of
available frequencies.
 The cluster can be replicated many times.
 Let M be the number of replicated clusters and C
be the total number of channels in the entire
system with frequency reuse, then C is the system
capacity and computed by
C = M J N
Cellular System Capacity
- Example
Suppose there are K=1001 radio channels, and each cell is Acell = 6
km2 and the entire system covers an area of Asys = 2100km2.

1.
2.
3.
4.
Calculate the system capacity if the cluster size is N=7.
How many times would the cluster of size 4 have to be replicated in order
to approximately cover the entire cellular area?
Calculate the system capacity if the cluster size is 4.
Does decreasing the cluster size increase the system capacity?
Solution:
1. J=K/N=143, Acluster=N*6=42km2, M (# of clusters)=2100/42=50,
C=MK=50,050 chs.
2. N=4, Ac=4*6=24km2, M=2100/24=87.
3. N=4, J = 1001/4 = 250 chs/cell. C = 87 * K= 87,000 chs.
4.
Decrease in N from 7 to 4 increase in C from 50,050 to 87,000.
 Decreasing the cluster size increases system capacity. So the answer is YES!
Geometry of Hexagonal Cells (1)
How to determine the DISTANCE
between the nearest co-channel cells ?
 Planning for Co-channel cells
 D is the distance to the center of the nearest co-channel cell
 R is the radius of a cell
j
D
3R
i
R
30o
3R
0
Geometry of Hexagonal Cells (2)
D
 Let
norm be the distance from the center of a
candidate cell to the center of a nearest co-channel
cell, “normalized” with respect to the distance
between the centers of two adjacent cells, 3 R .
 Note that the normalized distance between two
adjacent cells either with (i=1,j=0) or (i=0,j=1) is 1.
 Let D be the “actual” distance between the centers of two adjacent
co-channel cells. D = Dnorm .
3R
Geometry of Hexagonal Cells (3)
 From the geometry we have
2
Dnorm
 j 2 cos 2 (30o )  (i  j sin( 30o )) 2
 i 2  j 2  ij  N
From
N and Dnorm equations
Dnorm  N
Geometry of Hexagonal Cells (4)
The actual distance between the center of the candidate cell and
the center of a nearest co-channel is then:
D  Dnorm 3R  3N R
For
hexagonal cells there are 6 nearest co-channel neighbors to
each cell (if cluster size = 7).
Co-channel cells are located in tiers.
In general, a candidate cell is surrounded by 6k cells in tier k.
For cells with the same size the co-channel cells in each tier lie on
the boundary of the hexagon that chains all the co-channel cells in
that tier.
Geometry of Hexagonal Cells (5)



As D is the radius between two nearest
co-channel cells, the radius of the hexagon chaining
the co-channel cells in the k-th tier is given by k.D.
For the frequency reuse pattern with i=2 and j=1 so
that N=7, the first two tiers of co-channel cells are
given in Figure (next slide).
It can be readily observed from Figure that the radius of
the first tier is D and the radius of the second tier is 2.D.
Calculate Number of Cells in A Cluster
A candidate cell has 6 nearest cochannel cells. Each of them in turn has 6
neighboring co-channel cells. So we can
have a large hexagon.
This large hexagon has radius equal to D
which is also the co-channel cell
separation.
The area of a hexagon is proportional to
the square of its radius, (let =2.598),
R
D
ASmall  R 2
AL arg e  D 2   [3(i 2  ij  j 2 ) R 2 ]
D  Dnorm 3R  3N R
Calculate Number of Cells in A
Cluster
 The number of cells in the large hexagon is then
AL arg e
AS ma ll


 3(i 2  ij  j 2 )
In general the large hexagon encloses the center cluster of N cells
plus 1/3 the number of the cells associated with 6 other
peripheral large hexagons.
Hence, the total number of cells enclosed by the large hexagon is
N  6( 13 N )  3 N ,
Thus , we _ get
N  (i 2  ij  j 2 )
Thus
we
proved N =
f(i,j)
mentioned
before
Frequency Reuse Ratio
 The frequency reuse ratio, q, is defined as
q = D/R
which is also referred to as the co-channel reuse
ratio.
D  Dnorm 3R  3N R
Thus  q = D/R = sqrt(3N)
Tradeoff
– q increases with N.
– However, a smaller value of N has the effect of
increasing the capacity of the cellular system
– But Smaller N can increase co-channel interference
– Tradeoff on N
We assume the size of all the cells is roughly the same, as
long as the cell size is fixed, co-channel interference will be
independent of transmitted power of each cell.
The co-channel interference will become a function of q where q
= D/R = sqrt (3N).
(q is the CO-CHANNEL REUSE RATIO and is related to the
cluster size).
A small value of q provides larger capacity since N is small.
For large q, the transmission quality is better, smaller level of
co-channel interference.
By increasing the ratio of D/R, spatial separation between cochannel cells relative to the coverage distance of a cell is
increased.Thus, interference is reduced from improved
isolation of RF energy from the number of cells per cluster N
co-channel cells.
Geometry of Hexagonal Cells (7)
Furthermore, D (distance to the center of the nearest
cochannel cell) is a function of NI and S/I (next
lecture)
in which NI is the number of co-channel interfering
cells in the first tier and S/I = received signal to
interference ratio at the desired mobile receiver.
Questions