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 Schemesare used?

IntroductionThe 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

Important Definitions:

Multiplexing and Compression? Carrier? Signal (Digital and Analog)? Bandwidth? Information? Transmission Medium? Frequency?

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.

Carrier: In information technology, a carrier signal is a transmitted electromagnetic pulse or wave at a steady base frequency of alternation on which information can be imposed by increasing signal strength, varying the base frequency, varying the wave phase. Carrier in telecommunication industry, a

telephone or other company that sells or rents a telecommunication transmission services.

Definitions Cont...

Compression means transmitting/storing the same amount of information using less amount of resources

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. Information Transfer: is the process of

moving messages containing user information from a source to a sink.

Multiplexing Methods

FDM : Frequency Division Multiplexing FDMA: Frequency Division Multiple

Access TDM : Time Division Multiplexing

FDMA: 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. Problem?!

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 and one user can have multiple frequencies Above problem is solved.

FDM

Frequency BandFrequency Band

Frequency Band Frequency Band

Frequency BandFrequency Band

Frequency BandFrequency 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 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

TDM

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ot 2

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ot 2

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ot 3

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ot 4

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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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.

Spread Spectrum Principle

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33

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1&2&31&2&31&2&31&2&3 11

2&32&3

SpreadingSpreading

De-spreadingDe-spreading

Basic SS Block diagram

Channel

encoder

Channel

encoderModulationModulation Channel DecoderChannel DecoderDemodulationDemodulationChannel Channel

PN Code GeneratorPN Code Generator PN Code GeneratorPN Code Generator

Information SequenceInformation Sequence

Recovered Information SequenceRecovered Information Sequence

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.

Code Tree

SF=1SF=1 SF=2SF=2 SF=4SF=4

C1,1 =(1)C1,1 =(1)

C2,1 =(1,1)C2,1 =(1,1)

C2,2 =(1,-1)C2,2 =(1,-1)

C4,1=(1,1,1,1)C4,1=(1,1,1,1)

C4,1=(1,1,-1,-1)C4,1=(1,1,-1,-1)

C4,1=(1,-1,1,-1)C4,1=(1,-1,1,-1)

C4,1=(1,-1,-1,1)C4,1=(1,-1,-1,1)

How to obtain the code? The first element in the tree is 1. For each element, there are two possible

sub-elements 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.

CDMACDMA

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 multi-user 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

Code assigned to each

user

Same Code in Bi-polar

form for demo

Same Code in Bi-polar

form for demo

Actual transmission on

the air

Actual transmission on

the air

Digital transmission

from user before

spreading

Digital transmission

from user before

spreading

Receiver decodes the transmission of C sender from the total signal

Receiver decodes the transmission of C sender from the total signal

Basic Access

Techniques!!

References?1) Mobile Radio Networks, Networking and Protocols,

Bernhard H. Walke. ISBN: 0-471-97595-8. WILEY.2) UMTS, Mobile Communications for future, Flavio

Muratore, ISBN: 0-471-498297. WILEY.3) Mobile and Wireless Networks, Uyless Black, Prentice Hall

Series in Advanced Communications Technologies.ISBN:0-13-440546-3.

4) Digital Communications, I. A. Glover, P. M. Grant, Prentice Hall. ISBN:0-13-565391-6.

5) Computer Networks, Andrew S. Tanenbaum, ISBN:0-13-394248-1.

6) CDMA, Principles of Spread Spectrum Communication, Andrew J. Viterbi. ISBN:0-201-63374-4.

7) Mobile IP, the internet Uplugged, James D. Solomon.ISBN:0-13-856246-6.

But still a very valuable But still a very valuable resource:resource:

Andrew Tanenbaum: Andrew Tanenbaum: Computer Networks, 4Computer Networks, 4thth Edition 2003, Prentice Edition 2003, Prentice

HallHall

But still a very valuable But still a very valuable resource:resource:

Andrew Tanenbaum: Andrew Tanenbaum: Computer Networks, 4Computer Networks, 4thth Edition 2003, Prentice Edition 2003, Prentice

HallHall