Short Questions and Answers on Performance Analysis of Analog Communication Systems

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Q.1. What is threshold effect in an envelope detector? Explain.

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Ans. When a noise is large compared to the signal at the input of the envelope detector, the detected output has a message signal completely mingled with noise. It means that if the input signal-to-noise ratio (Si /Ni) is below a certain level, called threshold level, the noise dominates over the message signal.

Threshold is defined as value of the input signal to noise ratio (So/No) below which the output signal to noise ratio (Si /Ni) deteriorates much more rapidly than the input signal to noise ratio.

The threshold effect starts in an envelope detector whenever the carrier power-to-noise power ratio approaches unity or less. It is important to state that the threshold effect is the property of an envelope detector, on the other hand, such an effect is not observed in a coherent detector.

Q.2. Define coded and uncoded system.

Ans. The modulation systems can be broadly classified into two classes as under:

(i) uncoded systems

(ii) coded systems

(i) In uncoded systems one symbol in message space is transformed into one symbol in the modulated signal space. Thus, for AM each possible amplitude of the original message is transformed into a particular amplitude of the modulated signal.Similarly, for PPM each amplitude of the message results in a particular displacement of the pulse position.

(ii) In coded systems (such as PCM), however, each message symbol or amplitude is transformed into a number of signal symbols.

It has been shown that only coded systems can achieve the most efficient exchange bandwidth for signal to noise ratio that is theoretically possible.

The of uncoded systems (such as FM or PPM) are inherently incapable of efficient exchange of the bandwidth and signal-to-noise ratio. We shall now consider PCM from the point of view of exchange of bandwidth and signal-to-noise ratio.

Q.3. Briefly compare Coded and Uncoded Systems.

Ans. (i) In uncoded system, (FM and PPM), we have observed that the improvement in S/N voltage ratio is linear with bandwidth. On the other hand for the coded system (PCM), the improvement of S/N voltage ratio was observed to be exponential with bandwidth.

Thus trading bandwidth and S/N ratio is much more efficient in coded systems than in uncoded systems. Coded systems are inherently capable of better transmission efficiency than uncoded systems.

(ii) For uncoded systems, doubling the bandwidth doubles the S/N voltage ratio. But in PCM, doubling the bandwidth allows twice the number of pulses to be transmitted. If we double the number of pulses, the possible patterns that can be formed increases as a square.

For example, a group of 2 binary pulses can form 4 patterns. But a group of 4 binary pulses can form 16 patterns. Hence, doubling the bandwidth squares the quantized levels. Similarly, tripling the bandwidth will allow three times the number of pulses. But, tripling the number of pulses increases the number of patterns to the power 3.