While designing a communication system, an engineer generally faces several constraints or limitations. These are noise limitation, bandwidth limitation and equipment limitation.
1.14.1. Noise Limitation
          The noise may be defined as an unwanted form of energy which tend to interfere with th, transmission and reception of the desired signals in a communication system. The noise cannot be eliminated completely. However, the effect of noise on desired signals can be minimised with the help of several techniques. Noise can be classified into two broad categories depending upon its source: external noise and internal noise. External noise is that type of noise
whose sources are external to a communication system. The examples of external noise are atmospheric noise, galactic noise and industrial noise. On the other hand, internal noise is that type of noise whose sources are internal to a communication system. The examples of internal noise are thermal noise and shot noise. This type of noise is inevitable in every communication system and forms a basic limitation on transmission and reception of signals.
In fact, the noise limits our ability to identify the intended or desired message correctly and thus limits information transmission. Typical noise variations are measured in microvolts. If the signal variations are quite large in comparison, then the effect of noise is very small and may be ignored. Infact, in many practical communication systems, the effect of noise is generally unnoticeable. However, in long-distance communication systems operating with a limited amount of signal power, the signal may be as small as noise or even smaller than the noise. Thus, in such cases, the presence of noise severally limits the capabilities of a communication system.
1.14.2. Bandwidth Limitation
          In addition to noise, the bandwidth limitation is another major constraint in a communication system. The frequency range or the band of frequency needed for a particular given transmission is known as bandwidth. This band of frequencies required for a particular transmission is also called channel. This band of frequencies or bandwidth for a particular transmission is always allocated by some International regulatory agencies. This type of regulation is essential to avoid interference among the signals having same frequency. But, for a given transmission, this allocated bandwidth may not be sufficient to convey the entire information.
Infact, the information theory states that the greater is the transmission bandwidth of a communication system, the more is the information that can be
transmitted. Let us consider a simple example. Suppose one is listening to a music in an AM radio. The complete amount of information available to the human ear is contained in a frequency range upto 15 kHz, i.e., musical information extends upto a frequency of 15 kHz. However, in AM radio the maximum modulating frequency is restricted upto 5 kHz and hence the maximum bandwidth of AM transmission is 10 kHz.
Therefore, an AM radio receiver cannot reproduce all the information contained in the music because this will require a bandwidth of 30 kHz. On the other hand, the bandwidth allocated to a FM transmission is about 200 kHz. Thus on FM receiver can easily reproduce the transmitted information without any distortion. This means that a FM system has a better fidelity than an AM system. Also, it may be observed in common life that one prefers to listen to a FM radio than an AM radio. Thus, we can conclude that bandwidth is a major fundamental limitation of a communication system.
1.14.3. Equipment Limitation
          The noise and bandwidth limitations dictate theoretically what can or cannot be achieved in terms of performance in a communication system. However, this theoretical limit may not be realised in a practical system due to equipment limitations. For example, theory might require a bandpass filter with a quality factor of 100 at a centre frequency of 1 kHz. Such a filter cannot be realised in practice. Even if a filter with nearly identical characteristics is built, the cost may exceed what the user of the communication system wants to pay. Thus equipment limitation is another major problem in a communication system.