MOB Subject 3 Transmission Techniques

Master of Computer Science 1 - MOB Mobile Internet and Surrounding

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MOB Subject 3

Transmission Techniques

1. Signal transmission techniques - Practical

The local radio-electric networks use radio waves or infrared in order to transmit data. The

technique used with the origin for the radio transmissions is called narrow band transmission. It

consists in sharing the frequency band in relatively low sub-bandwidth in order to constitute

distinct channels. The different communications then passed on these channels called to narrow

band.

The radio transmissions however are subjected to many constraints which return the narrow

band transmission sometimes insufficient. It is the case in particular when the radio wave

propagation is effected by multi traffics. The transmission techniques like the spreading out of

spectrum dispersion and OFDM (Orthogonal Frequency Division Multiplexing) have been

developed to fight against the elimination problems which result of these multi traffics. These

techniques then use all the bandwidth available. One has then a transmission known as broad

band.

This practical aims to study these different aspects.

1.1. Definitions and basic concepts

1.1.1 Message, signal and of transmission modes

Message is constituted by the information elements or data that the user wishes to transmit.

One distinguishes two types of message:

- Analog message: data are analog. They are presented in the form of a function f(t)

continuous and with continuous time (ex: voice, video);

- Digital message: data are digital. They are presented in the form of a series {ik} of data

elements being able to take one among a discrete values unit called alphabet (ex:

characters of a text, entireties).

An analog message can be digitized by a sampling operation (discretization of the axis of times),

followed by a quantification operation (discretization of the values taken by the sampled analog

data).

Signals are the physic representation of the message to transmit. They are presented

generally in the form of a electric size (tension, current) which is then converted into an electric

or electromagnetic wave to be transmitted. Again, one makes the distinction between:

- Analog signal: signal associated an analog message;

- Digital signal: signal resulting of the setting in form of a digital message. A digital signal

is presented in the form of a succession of wave form being able to get one among

possibilities finished unit used to code information.

Transmission is the operation which consists in transmitting the signal of a machine towards

another, on a support given. It can be carried out in base band or on frequency carrier: