Tns lect 02

Taiz University, YEMEN 21 September 2014

Taiz University, YEMEN

• Some disturbances and noises are usually added with voice or data in channel. These unwanted signals are referred as transmission impairments.

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• If 𝑃2 > 𝑃1, so the gain G in deceibles (dB):

𝐺 = 10 log10

𝑃2

𝑃1 𝑑𝐵

• If 𝑃2 < 𝑃1, so the loss L or attenuation in dB:

𝐿 = 10 log10

𝑃1

𝑃2 𝑑𝐵

• The absolute power level in decibel depends on a reference level 1 mW and is defined as dBm by:

𝑃 𝑑𝐵𝑚 = 10 𝑙𝑜𝑔10(𝑃 (𝑚𝑊)

1 𝑚𝑊)

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Telecommunication

System

P1 P2

input power output power


• If the i/p and o/p circuits have the same impedance, then

𝑃2

𝑃1= (

𝑉2

𝑉1)2= (

𝐼2

𝐼1)2 𝑑𝐵

𝐺 = 20 log10𝑉2

𝑉1= 20 log10

𝐼1

𝐼2 𝑑𝐵

• If a passive element, such as an attenuator pad or a filter, is inserted in a system between its source and destination, the increase in the total loss of the system is called insertion loss of the element.

• If an active element, such as an amplifier, is inserted in a system between its source and destination, the received power by the destination may increase. There is thus an insertion gain of the element.

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1. An amplifier has an input resistance of 600 Ω and a resistive

load of 75 Ω. When it has an r.m.s. input voltage of 100 mV,

the r.m.s. output current is 20 mA. Find the gain in dB.

2. Calculate the received power Pr in dBm at the end of the system which is represented in the following block diagram.

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Examples

Channel

Lc=30dB

Filter

Lf=3dB

Amplifier

Ga=14dB

Pr Pt=10mW


• The minimum bandwidth (BW) needed to transmit a digital signal at

B bauds was shown by Nyquist to be 𝑊𝑚𝑖𝑛 =1

2𝐵; to avoid inter-

symbol interference (ISI).

• Digital transmission systems can use gain and phase equalization to obtain negligible ISI.

• The main advantage of using digital transmission (such as pulse code modulation (PCM)) is its good immunity for crosstalk and noise.

• In digital transmission it is possible to use regenerative repeaters instead of analog amplifier. A regenerative repeater samples the received waveform at intervals corresponding to the digit rate. If the received voltage at the sampling instant exceed a threshold voltage, this triggers a pulse generator which sends a pulse to the next section of line.

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• Variations in the extracted frequency at the regenerative repeater can cause a periodic variation of the times of the regenerated pulses, which is known as jitter.

• The variation in the times of the regenerated pulses due to changes in propagation time is known as wander.

• To estimate the performance of digital transmission system, the parameter bit error rate (BER) almost is considered versus the signal to noise ratio SNR.

• In digital transmission, the noise generated in the terminal and the noise encountered in the transmission line can be separated. By the proper choice of the code (coder-decoder), the quantization noise generated by the terminal equipment can be minimised. The line noise has little effect on the message signal because of the regenerative repeater.

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• Multiplexing defined as any process of sending of a number of separate signals together, over the same cable or bearer, simultaneously and without interference.

• Thus many speech channels are transmitted together as a single channel occupying the bandwidth of the physical facility.

• In multiplexing, the signals, which may be voice, video or data, are multiplexed together and the resulting signal is transmitted over a system with a suitably high bandwidth.

• When it is received, it is split up into the separate signals at which it is composed. This process is called demultiplexing.

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Multiplexing Types

Frequency division multiplexing (FDM)

Time division multiplexing (TDM)


• FDM is a broad band analog transmission technique in which multiple signals are transmitted over a single cable simultaneously as shown in the following Figure.

• FDM systems divides the available BW of the transmission medium into a number of narrow band or sub-channels. The channels are sent over a common path by modulation each channel to different carrier frequency (higher frequency).

• The signal thus occupies a relatively narrow bandwidth which is a part of a much wider bandwidth transmitted. Each speech channel occupies 4 kHz of the available bandwidth. The following Figure shows FDM in telephone transmission.

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FDM in telephone transmission


• To standardize the equipment in the broad band transmission system. ITU recommended the following FDM hierarchy.

• All multiplex equipment in the FDM hierarchy uses SSB modulation.

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• FDM for 12 telephone channels (group multiplex level) is considered for example.

• The signals pass through 12 low pass filters (LPF) to remove any high frequency components.

• The LPF outputs are mudulated on 12 separate carrier signals separated by 4 kHz. The modulations in all FDM hierarchy is single side band (SSB) modulation.

• The output of each of the 12 modulations must be filtered (band pass filter) to avoid the interference with each other.

• BPFs are used to restrict each signal to the allocated 4 kHz band. The principle of operation is shown in following Figure (Next slide).

• When the signal is received, converse process takes place as shown. In fact, the modulators and demodulators are combined into 12 single units to permit two way transmissions.


FDM principle

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FDM principle


• TDM is the sharing of a common transmission medium in time.

• In TDM, the time available is divided into small slots, and each of them occupied by a piece of one of the signals to be sent.

• Thus the multiplexing device should scan the input signal in round-robin fashion.

• TDM is a base band technology in which individual channels of data or voice are interleaved into a stream of framed bits across a communication channel.

• The following Figure shows the Principle of TDM.



TDM principle


• If analog signals are to be multiplexed, the signals should be sampled. The analog signals are digitized by CODEC (coder/decoder) device. It produces 7 or 8 bit number.

• Sampling is Ns= 8000 samples per sec (Ts=1/Ns=125 μsec/sample) to capture all the information from the 4 kHz telephone channel bandwidth.

• Pulse code modulation (PCM) technique is used in TDM.


Typical PCM system for TDM transmission


• T1 carrier is the widespread method (in North America and Japan) using PCM.

• T1 consists of 24 voice channels multiplexed together.

• Each channel has 8 bits, 7 for information and 1 for signalling.

• The output frame of 125 μsec consists of 24 × 8 = 192 bits plus one extra bit for framing.


The T1 carrier format


• Modems (modulator/demodulator) are data communication devices that convert digital signals to analog signals and vice versa.

• Modem allow digital transmission over analog telephone lines.

• Modems enables users to establish a link between computers using a telephone line.


Connection of two computers using modem


Modulation Types

Amplitude Shifts

Keying

(ASK)

Frequency Shift

Keying

(FSK)

Phase

Shift

Keying

(PSK)

Quadrature Amplitude

Modulation (QAM)

Taiz University, YEMEN 19 21 September 2014

Documents

Tns lect 02