Quiz 7: Digital Communication

Trade & Technology

Here voice telephone signal is transmitted down the channel by means of digital mode. There are so many advantages of digital communication over analog communication. • In digital communication regeneration of digital signal is possible and hence for long distance communication digital communication is best suitable than the analog communication. More over the noise presented during digital transmission can be reduced greatly whereas in analog communication it is little bit difficult to eliminate noise. • Digital transmission is highly immune from the noise and hence very less percentage of noise is added during transmission but in case of analog communication huge amount of noise is added and amplified during transmission. • In digital communication More number of digital signals can be multiplexed one at a time known as time division multiplexing (TDM) but in analog communication it is not possible • Digital signal can be encoded or encrypted into different coding formats and hence high security can be provided. This cannot be possible in analog communication. • Processing and accessing of digital signal is more comfort than that of analog signal. Among all over the advantages a few disadvantages are also presented in digital transmission. • Digital systems involve greater complexity and more over it occupies more channel bandwidth for transmission.

The maximum information rate on the channel is restricted and limited by so many factors. Hartley's Law : According to Hartley's Law the rate of information on the channel depends on type of the coding used for data, time taken to transmit the data down the channel and available channel bandwidth. img Here, img is the amount of information to be sent in bits per second. img is the constant whose value is dependent on type of coding. img time taken to send the information in second. img available channel bandwidth in Hz From this relation if the available channel bandwidth is less than the time taken to send the data is more. Shannon-Hartley's Theorem : According to Shannon-Hartley theorem the maximum data rate down the channel is dependent on number of levels used for a signal digitization to transmit and the channel bandwidth. img img is the channel capacity for data transmission img is available channel bandwidth in Hz img is number of levels used for digitization. From this relation if the number of levels for digitization is increased then the maximum channel capacity for more information is increased for a particular channel. Similarly from the Shannon limit principle the maximum channel data rate also depends on signal-to-noise ratio presented at the receiver during signal transmission. img img is the channel capacity for data transmission img is available channel bandwidth in Hz img is signal-to-noise ratio. From the relation higher the signal-to-noise ratio if more comfort for high data rate. Therefore, the factors channel bandwidth, number of levels for signal digitization, signal coding, time to transmit the data and signal-to-noise ratio are going to limit the information rate down the channel. Because for a particular channel the bandwidth is fixed, if number of levels are increased it is less immune to noise, depending on available channel bandwidth the time taken to transmit the data is restricted and similarly the practical digital equipments restrict the signal-to-noise ratio. Hence all these factors are going to limit the information rate.

Sampling is a technique in which an analog signal is converted into digital form. In this technique a band limited analog signal is sampled at regular interval of time and these samples can be recovered and the original analog signal is retrieved at the receiver. The sampling of analog signal and its signal recovery at the receiver is shown in Figure 1. img Here the sampling interval img is high and is dependent of signal maximum frequency. Therefore, for efficient sampling and signal recovery the sampling frequency must be greater than or eqaul to the twice of the maximum frequency of the message signal. Efficient sampling process in frequency domain is illustrated in Figure 2. img Here, img is the sampling frequency img is the maximum signal frequency. img In Figure 2, the original signal (spectrum) centered at origin can be retrieved from its sampled signal (from the entire spectrum) by low pass filtering that utilizes the guard band for efficient filtering in order to safe retrieval of original message signal. Therefore if there exist a guard band in the spectrum then it gives a provision to low pass filter to adjust its cutoff frequency to extract the original signal without any of its frequency components. Suppose, unfortunately, if the sampling frequency is less than the twice of the maximum frequency then retrieval of original signal from its sampled signal is not possible since there exist no guard band or at least sharp edge cutoff frequency in between side band signal and the signal centered at origin as shown in Figure 3. img From Figure 3, as there is no guard band since img it is not at all possible to extract the original signal from its sampled signal. Therefore, for efficient sampling always maintain a guard band in between successive signal spectrums otherwise the signal cannot be filtered out by the low pass filter.

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