BASIC COMMUNICATION SYSTEM ppt
To study & perform Phase Shift Keying technique
The transmission of digital signals is increasing at a rapid rate. Low-frequency analogue signal are often converted to digital format (PAM) before transmission. The source signals are generally referred to as baseband signals. Ofcourse, we can send analogue and digital signals directly over a medium. From electro-magnetic theory, for efficient radiation of electrical energy from an antenna it must be at least in the order of magnitude of a wavelength in size; c =fλ, where c is the velocity of light, f is the signal frequency and λ is the wavelength. For a 1 kHz audio signal, the wavelength is 300 km. An antenna of this size is not practical for efficient transmission. The law-frequency signal is often frequency-translated to a higher frequency range for efficient transmission. The process is called modulation. The use of a higher frequency range reduces antenna size. In the modulation process, the baseband signals constitute the modulating signal and the high-frequency carrier signal is a sinusoidal waveform. There are three basic ways of modulating a sine wave carrier. For binary digital modulation, they are called binary amplitude-shift keying (BASK), binary frequency-shift keying (BFSK) and binary phase shift keying (RPSK). Modulation also leads to the possibility of frequency multiplexing. In a frequency-multiplexed system, individual signals are transmitted over adjacent Non-overlapping frequency bands. They are therefore transmitted in parallel and simultaneously in time. If we operate at higher carrier frequencies, more bandwidth is available for frequency-multiplexing more signals.
Phase-shift keying (PSK) is a method of transmitting and receiving digital signals in which the phase of a transmitted signal is varied to convey information.
There are several schemes that can be used to accomplish PSK. The simplest method uses only two signal phases: 0 degrees and 180 degrees. The digital signal is broken up time wise into individual bits (binary digits). The state of each bit is determined according to the state of the preceding bit. If the phase of the wave does not change, then the signal state stays the same (low or high). If the phase of the wave changes by 180 degrees -- that is. if the phase reverses -- then the signal state changes (from law to high, or from high to low). Because there are two possible wave phases, this form of PSK is sometimes called biphasemodulation. More complex forms of PSK employ four or eight wave phases. This allows binary data to be transmitted at a faster rate per phase change than is possible with biphase modulation. In four-phase modulation, the possible phase angles are 0, +90, -90, and 180 degrees; each phase shift can represent two signal elements.