Transcript Document

Cellular Networks
Cellular Networks
It is an infrastructure network that exploits
the frequency reuse concept, first
developed by Bell Labs in the 70s.
Frequency reuse: use the same spectrum
to support multiple users separated by a
distance.
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Cellular Networks (contd.)
 An area is divided into a number of cells.
 Each cell has a base station which has a lowpower transmitter of 100W or less covering a
limited area. The power of the transmitter is
carefully controlled to limit the power leaked into
neighboring cells.
 Adjacent cells are assigned different frequencies
to avoid interference (crosstalk).
 A CDMA network is somewhat different, as
discussed later.
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Frequency Reuse
 A number of frequencies (more precisely, chunks of small
frequency bands, or channels) are assigned to each cell.
 Cells are grouped into clusters. Each cluster uses the
entire available radio spectrum, but adjacent cells, whether
within a cluster or in different clusters, use different
frequencies, and thus they don’t interfere with each other.
 Therefore, the number of clusters is the number of times
the entire spectrum can be used. This is called frequency
reuse.
 In a CDMA network, adjacent cells may use the same
frequency band. It will be discussed later.
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Example 1—FDMA
 Assume we have a spectrum for 36 voice
channels to cover a 100 square km area.
 Scenario 1: A single high power transmitter is
used to cover the entire area.
 Scenario 2: Divide the area into seven cells each
covering a 14.3 square km area and having 12
channels.
 Scenario 1 supports 36 users and Scenario 2,
84 channels, a 2.3 time capacity increase.
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Example 1—FDMA (contd.)
 Cells 1 and 7 are far apart and therefore can use the
same frequency. So do pair 2 and 5 and pair 6 and 4.
 Thus cells 1, 2 and 6 form a cluster, and cells 7, 5 and 4
form another. Cell 3 belongs to a third cluster.
 The smaller the cells (thus more cells in the area to be
covered) the larger the capacity of the system (the more
users it can support, as explained later), and the smaller
the power required for phones, very important feature.
 But smaller cells will increase the installation and
maintenance costs, the complexity of the system due to
too many handoffs, etc.
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Example 1—FDMA (contd.)
If we divide the area into 15 cells, and
each group (cluster) of 3 cells can use the
entire spectrum, then the spectrum will be
used for 5 times.
Each time the entire spectrum is used, 36
users can be supported. All together
36x5=180 users will be supported. It’s a
five time increase of the capacity.
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Example 2—FDMA
 We have a total bandwidth of 25 MHz and each user
requires 30 KHz for voice communication. (This is
how spectrum was used for 1G cellular systems.)
 Scenario 1: one high power antenna to cover the
entire town. We can support 25MHz/30 KHz = 833
simultaneous users.
 Scenario 2: 20 low power antennas are used. That
is, to divide the area into 20 cells. We divide the
entire frequency band into 4 sub-bands and assign
one to each cell, which has a spectrum of 25 MHz/4
= 6.25 MHz.
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Example 2—FDMA (contd.)
 The number of users supported by each cell is 6.25
MHz/30 KHz = 208.
 In this example, 4 cells form a cluster. Since there
are total of 20 cells, the town is covered by 5
clusters (20/4=5).
 Each cluster will use the entire frequency band, so
the number of users per cluster is 833, as calculated
earlier, and the total number of simultaneous users
for 5 clusters is 833x5 = 4,165.
 The capacity is now 5 times the capacity with a
single antenna.
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Capacity calculation—FDMA
 n: capacity (number of total users)
 m: number of cells to cover the area
 N: frequency reuse factor (# cells/cluster)
 B: bandwidth per user
 W: total available bandwidth (spectrum)
mW
n
N B
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Capacity calculation—FDMA (contd.)
In the previous example,
 m=20,
 W=25 MHz,
 N=4, and
 B=30 KHz.
m W 20 25000
n

 4,166
N B
4 30
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Capacity calculation—TDMA
 n: capacity (number of total users)
 m: number of cells to cover the area
 N: frequency reuse factor (# cells/cluster)
 B: bandwidth per user
 W: total available bandwidth (spectrum)
 Nu: number of time slots per carrier
n
mW
Nu
N B
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Capacity calculation—TDMA (contd.)
Assuming again,
 m=20,
 W=25 MHz,
 N=4,
 B=200 KHz,
 Nu=4.
mW
20 25000
n
Nu 
4  2,500
N B
4 200
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Cell sizes
 The sizes of cells can be different.
 Cells designed to cover suburban areas have
antennas on tall towers and cover a large area.
 In urban areas antennas are low and
transmitting powers are also low. Therefore the
coverage areas are small for two reasons.
Since the population density is high and the number
of users per cell is limited, the cell size has to be
smaller.
Buildings may block radio wave transmission,
therefore more cells may needed to cover an area
in a city.
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Capacity of CDMA
 In CDMA users are separated by different codes but not by
frequencies or time slots as in TDMA and FDMA. In CDMA many
users can share the same frequency band and communicate at the
same time.
 A channel in TDMA or FDMA is a frequency and a time slot. There
is only a limited number of channels, which restrict the number of
simultaneous users. In CDMA a channel is a code. There is an
almost unlimited number of codes, and thus channels, but it doesn’t
mean an unlimited capacity.
 Each user is a source of noise to the receivers of other users (recall
the discussion we had on DSSS) or to the receiver in the base
station. This will limit the number of users.
 The number of user per cell (the capacity) is determined by the
signal to noise ratio. If there are too many users, the noise will be
high, the S/N (signal to noise) ratio will be low and reception quality
will be poor.
 This is different from TDMA/FDMA, where the capacity is
determined by the number of available channels.
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Capacity of CDMA (contd.)
n: number of users
W: total bandwidth
R: data rate
Sr: signal to noise ratio
W
n
R  Sr
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Capacity per cell (CDMA)
Assume:
W=1.25MHz=1,250,000 Hz
R=9600 bps
Sr should be larger than 3dB => 2 times
W
1250000
n

 65 users
R  S r 9600 2
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Capacity comparison: TDMA
Number of users per cell for IS-136 (a TDMA
standard used in the US)
Assume: W=1.25 MHz, B=30 kHz, N=4
Number of users per carrier (number of time
slots per frequency) Nu=3
W
1,250,000
n
 Nu 
 3  31.25
B N
30,000 4
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Capacity comparison: GSM
W=1.25 MHz, B=200 kHz, N=3
Number of users per carrier Nu=8
W
1,250,000
n
 Nu 
 8  16.7
B N
200,000 3
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Capacity expansion techniques
 Obtain additional spectrum. Can be very
expensive.
 Cell splitting, cell sectoring using directional
antennas
 Dynamically assigning channels to cells, instead
of fixed assignment of channels. If the demand
for a cell is high, more channels are assigned.
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Cell sectoring
 Several antennas are used in one cell.
 Each antenna, which is directional, only covers a
section of the cell.
 The interference will be reduced and thus a
lower frequency reuse factor can be used.
 Using three- and six-sector cells the frequency
reuse factor can be reduced from 7 to 4 or even
3, which means a capacity increase of 1.67 and
2.3, respectively.
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How Does Cellular System Work
 When a phone is turned on, it scans and selects
the strongest (control channel) signal sent by
adjacent BSs.
 Then a handshaking process takes place
between the mobile unit and the MTSO (Mobile
Telecommunications Switching Office) to identify
the user and register its location. This procedure
is repeated periodically as long as the mobile
unit is on to monitor the location of the MS.
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How is a call established
 Mobile unit initializes calls. The MS sends the number it
wants to reach to the BS, which sends the request to the
MTSO.
 The MTSO sends a paging message to certain BSs
depending on the called number. Each BS sends the
paging signal on its own setup channel.
 The called MS responds to the BS, which sends the
response to the MTSO.
 The MTSO sets up a circuit between the calling and
called BSs.
 The MTSO selects an available channel within each
BS’s cell and notifies each BS, which in turn notifies its
mobile units. The two MS tune to their respective
assigned channels.
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Location Management
Tracking the MS in order to deliver data to it.
Locate the MS.
Establish a dedicated channel before the
data change starts.
If the MS moves from one cell to another
during the data change, handoff
(disconnect the MS from the current BS
and connect it to a new BS) may be
required.
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Handoff
 Handoff: if a MS moves out of the range of one
cell and into the range of another, the MTSO
assign another channel to the MS.
 Soft handoff (for CDMA only): An MS moving
towards the edge of a cell will maintain
communication with two or more BSs for a short
while before decide which BS to select as its
point of attachment.
 Soft handoff makes the transition from one cell
to the next smooth.
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Location Management (contd.)
Periodic location updates by the MS
Paging the MS in a group of cells the MS
may be located in. (The location updates
are periodic but not continuous. Therefore
we may not know the exact location.)
Location information dissemination: store
and distribute the location information.
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1-G system: Advanced Mobile Phone
Service (AMPS)
Forward Reverse Channel Region
band
band
spacing
(MHz)
(MHz)
(kHz)
AMPS
824-849 869-894 30
USA
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2G systems
Freq (MHz)
GSM
IS-54 (US)
IS-95 (US)
TDMA/FDD
TDMA/FDD
CDMA/FDD
935-960
890-915
200
869-894
824-849
30
869-894
824-849
1,250
3
Variable
48
1,228
Carrier
spacing (kHz)
Bearer
8
chan/carrier
Bit rate
(kbps)
270
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3G systems
 International standard IMT-2000 (International
Mobile Telecommunications beyond 2000)
 A spectrum of 230 MHz is assigned globally to
IMT-2000
 Combine voice and data services
 Increase the quality of the voice, capacity of the
network, and data rate (384 kbps everywhere
and 2 Mbps indoor).
 W-CDMA (GSM) and CDMA2000 (IS-95) are
competing proposals.
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