The multiple access

1. LEC(2)
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2. 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
.
.
.

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• 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)

4. 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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5. 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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6. 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
…

7. 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

8. FDMA

9. 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
∑

10. 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

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

12. 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)

13. 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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14. 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

15. 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

16. TDMA: Frame Structure
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…
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)

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

18. TDMA system
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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

19. 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

20. 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.
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21. TDMA

22. 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

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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.

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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.

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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

27. Cell Shape
hexagon is closest approximation to a circle

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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.

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Frequency Reuse Concept
f
f
The same frequency can be
reused in different cells, if they
are far away from each other
Radio coverage,
called a cell.

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Frequency Reuse Concept
 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.

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

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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.

33. Cellular Concept

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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

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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,

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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.

37. 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).

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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.

39. 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.

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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.

41. How to calculate the distance?
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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

42. Reuse Distance Formula
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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)

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