lec.2 Multiple Access.pptx

2024/1/3
2
Multiple Access
Technology
•Frequency Division Multiple Access (FDMA)
•Time Division Multiple Access (TDMA)
User
1
Time
Frequency
User
2
User
n
Code
.
.
.

2024/1/3 3
• Separation of whole spectrum into smaller frequency bands
• A channel gets a certain band of the spectrum for the whole
time
Advantages:
• no dynamic coordination
necessary
• works also for analog signals
Disadvantages:
• waste of bandwidth
if the traffic is
distributed unevenly
• Inflexible guard band
k2 k3 k4 k5 k6
k1
f
t
c
Frequency Division Multiple Access (FDMA)

Frequency Division Multiple Access (FDMA)
 The frequency band is divided into channels of equal
bandwidth such that each conversation is carried on a
different frequency.
 Best suited to analog mobile radio.
 Single channel per carrier.
 BS dynamically assigns a carrier frequency to each
active MS.
 Used in All first generation cellular systems and early
cordless telephones.
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FDMA Examples in Mobile Comm.
System
 Control channel
 Forward control channel
 Reverse control channel
 Traffic channel
 Forward traffic (traffic or information) channel
 Reverse traffic (traffic or information) channel
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Types of Channels
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BS
f1’
f2’
fn’
f ’
f
…
Reverse channels
Forward channels
f1
f2
fn
…
Control channels
Traffic channels
MS #1
MS #2
MS #n
…

FDMA: Channel Structure
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1 2 3 … N
Frequency
Total Bandwidth W=NWc
Guard Band Wg
4
Sub Band Wc
Frequency
Protecting bandwidth
…
f1’ f2’ fn’
…
f1 f2 fn
Reverse channels Forward channels

FDMA Transmitter
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ω1
ω2
mn(t)
m1(t)
m2(t) RF
Modulator
Modulator
1
Modulator
2
Modulator
n
…
FDMA Transmitter
ωn
∑

FDMA Receiver
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mn(t)
m1(t)
m2(t)
RF
Demodulator
Demodulator
1
BRF ω1
Demodulator
2
BRF ω2
Demodulator
n
BRF ωn
…
FDMA Receiver

In FDMA, the available bandwidth
of the common channel is divided into
bands that are separated by guard bands.
Note

f
t
c
k2 k3 k4 k5 k6
k1
• A channel gets the whole spectrum for a certain
amount of time
Advantages:
• only one carrier in the
medium at any time
• throughput high even
for many users
Disadvantages:
• precise
synchronization
necessary
Time Division Multiple Access (TDMA)

Time Division Multiple Access (TDMA)
 TDMA systems divide the radio spectrum into time slots,
and in each slot only one user is allowed to either transmit
or receive.
 TDMA systems transmit data in a buffer-and-burst method,
thus the transmission for any user is noncontinuous.
 TDMA is a more expensive technique
 needs a highly precise synchronization between transmitter
and receiver
 Most of second generation systems use TDMA
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TDMA: Channel Structure
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… t
f
#1
#2
#n
#1
#2
#n
…
(a). Forward channel
…
#1
#2
#n
Frame Frame
Frame
…
t
f ’
#1
#2
#n
#1
#2
#n
…
(b). Reverse channel
…
#1
#2
#n
Frame Frame
Frame
Channels in FDD Mode

TDMA
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MS #1
MS #2
MS #n
BS
…
…
Reverse channels Forward channels
t
Frequency f ’
#1
…
#1
…
Frame
Slot
…
#1
…
#1
Frame
…
t
Frequency f
Frame Frame
…
t
#2
…
#2
…
…
t
#n
… #n
…
…
#2
…
#2
…
t
…
#n
…
#n
…
t

TDMA: Frame Structure
2024/1/3 16
…
Time
Frequency
f = f ’
#1
#2
#n
#1
#2
#n
…
Forward
channel
Reverse
channel
…
#1
#2
#n
Forward
channel
Frame Frame
#1
#2
#n
…
Reverse
channel
Channels in Simplex Mode (TDD)

TDMA: Frame Structure (Cont’d)
2024/1/3 17
…
Time
Frequency
#1
#2
#n
#1
#2
#n
… …
#1
#2
#n
Frame Frame
Frame
Head Data
Guard
time

TDMA system
2024/1/3 18
M1(t)
M2(t)
Mn(t)
M1(t)
M2(t)
Mn(t)
RF
Modulator
RF
Demodulator
Timing and
synchronization
Timing and
synchronization
Simplified block diagram of a TDMA system

f
Time and frequency division multiplex combined
• Combination of both methods
• A channel gets a certain frequency band for a certain amount of time (e.g.
GSM)
Advantages:
– protection against frequency
selective interference
– higher data rates
Disadvantage:
• precise coordination
required
t
c
k2 k3 k4 k5 k6
k1

The features of TDMA
 TDMA shares a single carrier frequency with several users, where
each user makes use of nonoverlapping time slots．
 Data transmission for users of TDMA system occurs in bursts.
 Because of a discontinuous transmission, the handoff process is
simpler for a mobile unit, as it can listen to the base stations during
idle time slots.
 TDMA uses different time slots for transmission and the reception.
 The guard time between the slots is required.
 High synchronization overhead is required because of burst
transmissions.
2024/1/3 20

In TDMA, the bandwidth is just one
channel that is timeshared between
different stations.
Note

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F1
F2
F3
F4
F5
F6
F7 F1
F2
F3
F4
F5
F6
F7
F1
F2
F3
F4
F5
F6
F7 F1
F2
F3
F4
F5
F6
F7
F1
F1
F1
F1

2024/1/3 24
Introduction
 The cellular concept was a major breakthrough in
solving the problem of spectral congestion and user
capacity.
 It offered very high capacity in a limited spectrum
allocation without any major technological changes.
 It enables a fixed number of channels to serve an
arbitrarily large number of subscribers by reusing the
channels throughout the coverage region.

2024/1/3 25
Cellular Concept
 Cellular topology is a special case of an infrastructure
multi-BS network configuration that exploits the
frequency reuse concept.
 Why we need to reuse frequency?
Radio spectrum is one of the scarcest resources available.
 The cellular concept is a system-level idea
 which replacing a single, high power transmitter (large cell),
with many low power transmitters (small cells), each
providing coverage to only a small portion of the service
area.

2024/1/3 26
Cell
R
(a) Ideal cell (b) Actual cell
R
R R
R
(c) Different cell models
Cell Shape
The cell shapes need to cover a area without creating
ambiguous regions , there are many factors that cause
reflections and refractions of the signals

Cell Shape
hexagon is closest approximation to a circle

2024/1/3 28
Frequency Reuse Concept
 One of the major problem in cellular is to support a large number of
users with a limited frequency spectrum.
 In the 1970s, the Bell mobile system in New York could only support
12 simultaneous calls over a thousand square miles.
 The cellular concept can solve this problem to increase the system
capacity.
 Frequency Reuse is a major cellular concept. Two fundamental ideas:
 Cellular Topology: A large region cell is divided into small regions
called cells.
 Reuse the frequency spectrum.

2024/1/3 29
f
f
The same frequency can be
reused in different cells, if they
are far away from each other
Radio coverage,
called a cell.

2024/1/3 30
 A large service area is divided into many small regions called cells with
hexagonal shape.
 Each cell is one BS with a low power transmitter instead of high power
transmitter.
 Each cell is assigned a set of frequency channels. Neighboring cells are
assigned a different set of channels to avoid co-channel
 The same set of channels can be assigned to different cells that are
separated large enough to limit co-channel interference to a tolerable
level.
 The minimum distance between two co-channel cells (cells using the
same channel) is called reuse distance.

Cellular Architecture
• MS – Mobile Station
• BSC – Base Station Controller
• MSC – Mobile Switching Center
• PSTN – Public Switched
Telephone Network
segmentation
of the area
into cells

2024/1/3 32
Cellular Concept
 The fundamental principle of the cellular concept is
to divide the coverage area into a number of
contiguous smaller areas which are each served by
its own radio base station.
 Each of these smaller areas is called a cell.
 Cells are grouped into clusters.
 Each cluster utilizes the entire available radio spectrum.
 The number of cells in a cluster is called cluster size or
frequency reuse factor.

2024/1/3 34
Capacity of the network
 To implement frequency reuse:
 N cells are grouped together and called cluster. N is called a
frequency reuse factor or cluster size.
 Each cluster uses the all available, S channels.
 Each cell in a cluster is allocated S/N channels if using uniform fixed
channel assignment.
 The whole service area is divided into M clusters.
 The total number of channels, n, in the service area is
 n= M × N × S / N= MS = (m/N) × (W/B)
 With hexagonal cellular geometry, the possible values of N are given
 N= i2 + ij+ j2
 which are N=1, 3, 4, 7, 9, 12, 13

2024/1/3 35
Capacity of the network
The capacity of the network can be
increased by:
 increasing m
 decreasing the frequency reuse
factor N
Where
 m is the number of channles required
to cover an area
 N is the frequency reuse factor,

2024/1/3 36
Cell Reuse Example
A 7-cell reuse FDD cellular system has a total of 30 MHz of
bandwidth. Each simplex voice channel occupies 25 kHz
bandwidth. A total of 2 MHz bandwidth is allocated to
control channels. Determine the number of duplex voice
channels in the system and per cell. Assume that uniform
fixed channel assignment (UFCA) is used.

Solution:
 Total bandwidth for voice = 30 MHz –2MHz = 28 MHz
 Number of simplex voice channel in each link:
 14 MHz / 25 kHz = 14 000 / 25 = 560
 (assume each link has 14 MHz)
 Since voice communication requires a simplex channel from each
up/down link, total duplex voice channel in the system is 560.
 Number of channel per cell: 560 / 7 = 80 (using UFCA).

2024/1/3 38
Example : Importance of Cellular
Topology
 We want to provide a radio communication service to a
city.
 The total bandwidth available is 25 MHz, and each user
requires 30 KHz of bandwidth for voice communication.
 If we use omni antenna to cover the entire town, we can
only support 25 MHz/30 KHz = 833 simultaneous users.

Now let us employ a cellular topology where 20 lower power antennas are
opportunistically located to minimize both kinds of interference.
 We divide our frequency band into four sets and assign one set to each cell.
 Each cell has a spectrum of 25 MHz/4 = 6.25 MHz allocated to it.
 We have a cluster of four cells in this example.
 The number of simultaneous users supported per cell is 6.25 MHz/30 KHz =
208.
 The number of users per cluster is 4 x 208 = 832.
 The total number of simultaneous users is now 832 x 5 = 4,160 because we
have five clusters of four cells each.
 The new capacity is roughly five times the capacity with a single
antenna.

2024/1/3 40
Interference
 Two types of interference are important in such a
cellular architecture:
a) Cochannel interference
The interference due to using the same frequencies in
cells of different clusters.
b) Adjacent channel interference
The interference from different frequency channels used
within a cluster whose side lobes overlap.
 The allocation of channels within the cluster and
between clusters must be done so as to minimize
both of these.

How to calculate the distance?
2024/1/3 41
R D
o
ijR
jR
iR
C
ab
b
a
D
120
cos
3
*
2
)
(
3
)
(
3
cos
2
2
2
2
2
2






2
2
j
ij
i
K 


Where a =i3 R;
b= j3 R
Reuse factor
(i,j)
b
a
C

Reuse Distance Formula
2024/1/3 42
R
D
R
N
R
j
ij
i
D
3
)
(
3 2
2




2
2
j
ij
i
N 


Note: i and j are integers
where
Reuse factor
(i,j)

2024/1/3 43
Cellular Coordinate System
Use (i,j) to denote a
particular cell.
Example:
Cell A is represented
by (2,1).
A

lec.2 Multiple Access.pptx

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