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Digital transmission is used to carry various types of information from one point to another. This information is transmitted in binary language, comprising a sequence of “0″ and “1″ logic states.

The information can be any of the following :

• Telephone channels
• Television signals
• Digital data
• Telegraphic message
• Fax, etc

The original information is converted to binary signal form, represented by a voltage/no voltage sequence in time.

Telephone Channel Digitization Techniques is used to convert telephone channel analog into binary signal form are :

• SAMPLING
• QUANTIZATION,
• CODING

Time-domain multiplexing is utilized to multiplex the different channels :

 Radio Link Transmission

 The equipment used can be grouped in two main families :

• Common coding equipment,
• Channel/channel coding equipment

Telephone Channel Digitalization Principle

To create a time division multiplex (TDM) each voice channel is deliberately limited to a frequency band from 300 to 3400 Hz (CCITT telephone band).

written by admin \\ tags: Microwave

The bit rate is equal to the number of bits transmitted or received in one second while the digital transmission quality is expressed by the bit error rate : b.e.r.

The bit error rate is equal to the total number of errored bits received in specific period of time divided by the total number of bits transmitted in that same period.

A comparison between the sensitivity characteristics of a digital and on analog microwave transmission system in terms of b.e.r. and noise power respectively, versus system loss is below :

It should be noted that the digital transmission system maintains its quality (ber) over a wide range of system loss whereas the analog transmission quality is directly proportional to the carrier to noise ratio.

The second reason is on economic reason.

If we analyze the relative costs of a telephone analog anf digital radio link transmission system according to table :

Can be seen that the relative cost of a digital radio link transmission system is cheaper on that of on analog transmission system by a ratio of about 6 to 9.

written by admin \\ tags: Microwave

The tremendous development of computers and the various kinds of data exchanged between major of human activity have led to the necessity to process data “digitally” and at high speed.

This tendency has resulted in the adoption of time division telephone switching and
naturally to digitizing of the transmitted signals. This is now a policy deliberately chosen by telecommunications administrations for technical and cost reasons, and to provide their subscribers with new service at the same as the telephone service and over the same public network.

Digital multiplexes are developed, leading to the standardization of the orders of transmission data rates and allowing easy interconnection of networks.

All these techniques, based on pulse code modulation (P.C.M) are contributing to the fact that telecommunications are rapidly reaching the age of telematics and videomatics.

In this general perspective, microwave digital links have on important role to play.

The beginning of modern digital transmission took place in Europe when CEPT and CCITT published the first recommendations for pulse coded modulation (P.C.M) techniques and the associated multiplex hierarchy, following the model existing for analog transmission.

The information to be transmitted can be any of the following:

• Telephone signals
• Television signals
• Digital data
• Telegraphic message, etc.

At the present time Telephone and Television signals represent the major part of the traffic dispatched by radio links. These signals being fundamentally analog signals, the question arises: why using digital modulation.

The first reason is a technical reason: The advantage of digital transmission over analog transmission can be shown reference to the signal and noise relationship with the distance.

The significant difference between the two types of transmission lies in the resultant noise level.

The cumulative noise characteristic of analog transmission and the non cumulative noise characteristic of digital transmission.

The noise contributions of the analog transmission stages or repeaters are additive that the signal to noise ratio become worse and worse with the distance and there is no known way to avoid this problem.

On the opposite in digital transmission the information is contained in the serial combination of <<0>> and <<1>> logic levels.

Once the decision has been mode whether the received signal information is a “0″ or “1″ logic level, the received signal can be regenerated without any noise contribution.

Therefore in digital transmission the noise is not additive with the number of repeaters and does not accumulate with increased distance.

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written by admin \\ tags: Microwave