In the previous subsection we studied the baseband transmission of direct sequence spread spectrum modulated signal. For long distance transmission or transmission over a satellite channel, BPSK can be used. DS-SS BPSK Transmitter Fig. shows the transmitter of Direct Sequence Spread Spectrum with BPSK. The multiplier multiplies the two signals b(t) and c(t). The output

The pseudo-noise (or pseudo random) sequence is a noise like high frequency signal. This signal is binary in nature. Thus it looks like pulses. The sequence is not completely random but it is generated by a well defined logic. The same logic is used at transmitter and receiver. Since the sequence is generated by a

Necessity of Secure Communication There are some applications where it is necessary for the system to resist external interference and to make it difficult for unauthorized receivers to receive the message being transmitted. This type of communication is called secure communication such that noise interference and unwanted receivers should not detect the message. Such communication

A convolutional coding is done by combining the fixed number of input bits. The input bits are stored in the fixed length shift register and they are combined with the help of mod-2 adders. This operation is equivalent to binary convolution and hence it is called convolutional coding. This concept is illustrated with the help

The analog waveforms are converted to digital signals by PCM, DM, ADM, DPCM etc. techniques. This digital data can be represented by different formats or waveforms. These waveforms are commonly known as digital data formats or their representation is called as line coding. Digital symbols can amplitude modulate some carrier to generate amplitude modulated pulse

To overcome the quantization errors due to slope overload and granular noise, the step size (δ) is made adaptive to variations in the input signal x(t). Particularly in the steep segment of the signal x(t), the step size is increased. When the input is varying slowly, the step size is reduced. Then the method is

The automatic frequency correction (AFC) is incorporated in FM transmitter to keep carrier frequency stable. The discriminator reacts only to small changes in the carrier frequency but not to the frequency deviations in the carrier (since it is too fast). Suppose frequency of the carrier increases. This higher frequency is fed to the mixer for

Angle modulation has following advantages over amplitude modulation (AM): The amplitude of FM is constant. It is independent of depth of modulation. Hence transmitter power remains constant in FM whereas in FM it varies in AM. Since amplitude of FM is constant, the noise interference is minimum in FM. Any noise superimposing an amplitude can