Communication Channel
Communication Channel

Typical communication system contains transmit system (called transmitter), receive system (called receiver) and transmission system, all together called telecommunication channel. Telecommunication channel then represents a set of technical devices allowing only one-way signal transmission between two points regardless of the used devices.

Telecommunication circuit is formed by a pair of channels allowing two-way communication. Communication can be either simplex (signal transmission circuit alternately in one direction or the other) or duplex (signal transmission circuit at the same time in both directions).

Based on the signal in channel two types of communication systems are known: analog and digital. In next chapters analog and digital transmission systems with basic function blocks will be introduced.

## Analog communication channel

Analog communication is a data transmitting technique in a format that utilizes continuous signals to transmit data including speech, image, video, electrons etc.

Analog transmission is inexpensive and enables information to be transmitted from point-to-point or from one point to many. Data in analog signal is generally carried by use of modulation. Therefore it is necessary to have that transmitter and receiver have compatible devices – a suitable modulator on the transmitter side to the demodulator on the receiver side.

A device used to both to send and receive signal that contains both modulator and demodulator is called MODEM (MOdulator+DEModulator).

Analog circuits do not involve quantization of information unlike the digital circuits and consequently have a primary disadvantage of random variation and signal degradation, particularly resulting in adding noise to the audio or video quality over a distance. Data is represented by physical quantities that are added or removed to alter data.

## Digital communication channel

Transmitter

Even digital communication system can work with analog signal. For this analog-digital convertor (ADC) is required and used. An analog signal is converted to the digital form in the analog-digital convertor. The detail process of signal digitalization is described in chapter Analog signal digitalization.

The next block working with digital signal is source encoder. The general description of source encoder is that it very effectively converts every number value into the digital representation (mainly the binary representation). Usually the coding process is already done in ADC. In the final digital form there is a big redundancy.

The main advantage of source encoder is reducing of the redundancy, it means less binary values (zeros and ones) to transmitting. Very simple example how to reduce redundancy: instead of “for example” we simply use “i.e.”

In many systems without a channel encoder is the output signal transmitted directly into communication channel. It is important to notice that output must be in correct form, one that is acceptable by the channel. As described in source encoding, one purpose of the source encoder is to eliminate redundant binary digits from the digitized signal. The strategy of the channel encoder, on the other hand, is to add intentional redundancy to the transmitted signal—in this case so that errors caused by noise during transmission can be detected and corrected at the receiver. This process is done by adding redundant information (we can say adding security information) to digital information of source encoder. The simplest forms of redundant information are repeating and parity.

Last device on transmitter side is modulator.

In digital modulation, an analog carrier signal is modulated by a discrete signal.

The type of modulation in use depends on channel and data that is transmitted. Some modulators are intended for noisy channels (such as radio, or WIFI), some are intended for clear channels (such as optical cable).

Receiver

A receiver is basically the same device as transmitter in reverse order. Very important task in design and optimization of digital communication system is minimizing failures in transmission.

Since the signal is usually corrupted because of noise in the communication channel, it has to be repaired by the receiver. This can be done with a device called demodulator which turns a numerical value into actual signal (e.g. number 1 represents the value 5 of the signal; the value 5 can be amperes, volt, etc). There are several ways of demodulation depending on how parameters of the base-band signal are transmitted in the carrier signal, such as amplitude, frequency or phase. For example, for a signal modulated with a linear modulation, like AM (Amplitude Modulation), we can use a synchronous detector. On the other hand, for a signal modulated with an angular modulation, we must use an FM (Frequency Modulation) demodulator or a PM (Phase Modulation) demodulator. Different kinds of circuits perform these functions.

The main task of channel decoder is to decode (or reconstruct) the output signal of channel encoder. In ideal condition the result is the same signal as was on the side of the transmitter. In real life there are always failures caused by noise in the communication channel. But because of the redundancy added by channel encoder the failures can be detected and corrected and we can get the accurate signal (which cannot be done using analog transmission).

The source decoder performs exactly an inverse function as source encoder. It means removed redundancy is added back to the signal. In case the output from receiver is digital, than the output from source decoder is output of the receiver. In other case, when an analog output is required, digital values are sent to digital-analog convertor (DAC). Digital-analog convertor converts the values to analog signal according the theory of sampling and quantization. The original signal is the result of these operations.

Block diagram of digital communication channel