UNESCO/CISM SECOND ADVANCED SCHOOL OF
INFORMATICS
UNESCO PROJECT
Advanced Course on Networking
Introduction to Cellular Wireless Networks
Outline




Introduction (Resource Management and
Utilization).
Compression and Multiplexing (Other related
definitions).
The Most Important Access Utilization Schemes
(Techniques):
FDMA (Frequency Division Multiple Access)
TDMA (Time Division Multiple Access)
CDMA (Coded Division Multiple Access)
In which Systems the above Schemes
are used?
Introduction
 The idea behind the Resource Management in wireless
communications was how to assign channels or
frequencies to the radio cells in a way that the
probability of interference is sufficiently low.
 The utilization of the capacity of a transmission medium can
be improved through different methods that involved
transmitting several connections simultaneously in a
multiplex mode (Multiple Access Techniques).
 By using these techniques, the shared resource (spectrum)
can be divided/shared among users, ensuring Quality of
Service and the required amount of interference.
Frequency Spectrum
Frequency Spectrum in USA
Definitions

Multiplexing: is sending multiple signals or streams
of information on a carrier at the same time in the
form of a single, complex signal and then recovering
the separate signals at the receiving end.

Compression means transmitting/storing the same
amount of information using less amount of
resources
Definitions Cont...

Both Compression and Multiplexing are
attempts to make better use of resources: the
Communication Channel.

Multiplexing attempts to make better use of a
potentially underused resources. On the
Other hand, Compression attempts to make
better use of a potentially overused resources.
Definitions Cont…


A signal: In electronics, is an electromagnetic field
used to convey data from one place to another. In
information technology is simply “That which is
send or received”.
A signal can be analog or digital:


Analog technology refers to electronic transmission
accomplished by adding signals of varying frequency or
amplitude to carrier waves of a given frequency.
Digital: is a technology that generates, stores and process
data in terms of two states: Positive and Non-positive ( 1 or
0 bits).
Definitions Cont…

Frequency: for a periodic function, is the
number of cycles or events per unit time.

Bandwidth: (The width of a band of
electromagnetic frequencies) is used to mean:
(1) how fast data flows on a given
transmission path.
(2) more technically, the width of the range of
frequencies that an electronic signal
occupies on a given transmission medium.
Multiplexing Methods

FDM : Frequency Division Multiplexing
 FDMA:
Access

TDM : Time Division Multiplexing
 FDMA:

Frequency Division Multiple
Time Division Multiple Access
CDM : Code Division Multiplexing
 CDMA:
Code Division Multiple Access
FDMA





FDM divides the transmission frequency range
(Bandwidth) into narrower bands called
(subchannels).
The subchannels are smaller frequency bands and
each band is capable of carrying a separate voice or
data signals!
Guard bands are used to prevent interference on the
receiving end of the signal (Accommodate the
delay of the near-far-problem)
Disadvantage (FDMA): Full utilization of the
available frequency band is not possible!!
Advantage: Multiple callers can share the frequency
spectrum.
FDM
Frequency Band
Frequency Band
Frequency Band
Frequency Band
FDMA-Applications

FDMA is used in a variety of applications such as:
telephone systems, radio systems, cable TV at homes.

The first generation of Mobile networks. FD,TD-MA.

GSM (Global System of Mobile Telecommunication)
uses FDMA in combination with TDMA.

UMTS (Universal Mobile Telecommunication
Systems-3G) in combination with other multiplexing
techniques
TDMA : Time Division

Why TDM ?

FDM sometime offer less frequency for the communication
channel than the required amount

On the Other hand, in TDM the entire bandwidth of the radio
channel is used but is divided into time slots that are periodically
allocated to each station for the duration of the call

Pros and Cons :



It needs more synchronization between the sender and the receiver
It is more frequency-economic than FDM
This engagement of the transmission medium can cause using
asynchronous time slots instead of the synchronous ones
especially when the transmission pauses occurs
Slot 4
Slot 3
Slot 2
Slot 1
TDM
FDMA + TDMA
A combination of FDMA/TDMA can
be used. For example, in GSM systems.
The traffic is burst onto the channel at a
specific periods.
 With this combination, more channels
can be used with less interference.
 How?

FDMA+TDMA in GSM
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1
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Voice Channels
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1
Signalling Channels
CDMA (Code Division Multiple Access)



Like TDMA, in CDMA the analog speech is coded
into digital signals.
Unlike TDMA, in CDMA each conversation is
assigned a unique code (a signature for each
individual transmission).
The codes of different users are assigned to be
different from each other (e.g orthogonal to each
other).

The final signal at the receiver contains only the
relevant conversation. Any other signals are picked
up as a noise.
Spread Spectrum (SS) in CDMA


The idea of the SS is to transform an information data
signal into a transmission signal with much larger
bandwidth. This is achieved by encoding the
information data signal by a code signal called pseudo
random or pseudo noise (PN) which is independent
from the data signal and has a much larger bandwidth.
Example : Chip rate = 4MChip/s




Spreading Factor = 2  2 users with data rate = 2 Mbps
Spreading Factor = 4  4 users with data rate = 1 Mbps
Spreading Factor = 64  64 users with data rate = 64 kbps
Spreading Factor = 128  128 users with data rate = 32 kbps
Spread Spectrum Principle
1
Spreading
1
De-spreading
2
1&2&3
2
3
3
1
2&3
Basic SS Block diagram
Information Sequence
Recovered Information Sequence
Channel
Modulation
Channel
Demodulation
encoder
PN Code Generator
PN Code Generator
Channel Decoder
Spreading Sequence Assignment
(SSA)



The codes are said to be orthogonal when
their normalized inner product is equal to 0.
Code Ci is said to be orthogonal to code Cj if
Ci does not belong to the path from the root of
the tree to Cj or to the sub-tree below code Cj .
In W-CDMA spreading codes have variable
length: 4 to 512 chips in the downlink and 4
to 256 chips in the uplink.
C4,1=(1,1,1,1)
Code Tree
C2,1 =(1,1)
C4,1=(1,1,-1,-1)
C1,1 =(1)
C4,1=(1,-1,1,-1)
C2,2 =(1,-1)
C4,1=(1,-1,-1,1)
SF=1
SF=2
SF=4
How to obtain the code?



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The first element in the tree is 1.
For each element, there are two possible subelements that we will term the top and
bottom sub-element.
The top sub-element is generated by
repeating the root of the sub-element twice.
So that the top sub-element of 1 is (1,1).
The bottom sub-element can be constructed
by concatenating the root of the sub-element
with the inverse of itself. Thus, the bottom
sub-element of 1 is (1,-1) and so on.
CDMA
CDMA





CDMA technique is used in UMTS (Universal Mobile
Telecommunication Systems) (Called also W-CDMA)
CDMA provides better signal-to-noise ratio performance than
the conventional TDMA and FDMA. Which means that the
required high capacity can be approved!!
Advantage: It is easy to accommodate variable user capacity as
long as the user does not increase the whole energy of the multiuser signal.
Disadvantage: The power control issue which limits the max.
number of users in the cell.
There are different alternatives for CDMA in UMTS:
 W-CDMA
 TD-CDMA
 FD-CDMA
Multiple access in CDMA – An Example
Code
assigned
to each
user
Digital
transmission
from user
before
spreading
Same Code
in Bi-polar
form for
demo
Actual
transmission on
the air
Receiver decodes the transmission of
C sender from the total signal
References?
1)
2)
3)
4)
5)
6)
7)
Mobile Radio Networks, Networking and Protocols, Bernhard
H. Walke. ISBN: 0-471-97595-8. WILEY.
UMTS, Mobile Communications for future, Flavio Muratore,
ISBN: 0-471-498297. WILEY.
Mobile and Wireless Networks, Uyless Black, Prentice Hall
Series in Advanced Communications Technologies.
ISBN:0-13-440546-3.
Digital Communications, I. A. Glover, P. M. Grant, Prentice
Hall. ISBN:0-13-565391-6.
Computer Networks, Andrew S. Tanenbaum, ISBN:0-13394248-1.
CDMA, Principles of Spread Spectrum Communication,
Andrew J. Viterbi. ISBN:0-201-63374-4.
Mobile IP, the internet Uplugged, James D. Solomon.
ISBN:0-13-856246-6.
But one of the best:
Andrew Tanenbaum:
th
Computer Networks, 4
Edition 2003, Prentice Hall