 Hello everyone, welcome to this session. I am Dr. Asha Tharange and today we are going to learn about binary phase shift keying that is BPSK modulation and demodulation techniques. At the end of this session, you will be able to represent given bit stream into BPSK signal, explain BPSK modulation and demodulation technique and state advantages, disadvantages and applications of BPSK. These are the contents we will be covering in this session. In the previous session, we studied ASK and FSK modulation techniques. Let us now study another digital modulation technique that is phase shift keying. In ASK amplitude and in FSK, the frequency of carrier is varied depending upon the input bit 1 or 0 of the bit stream. Similarly, phase shift keying popularly known as PSK is a type of digital modulation technique in which the phase angle of the carrier is varied with respect to the input bit 1 or 0 of the digital data. Thus PSK signal it consists of 2 phase angles which are 180 degree apart. For example, say 0 degree and 180 degree. This signal is thus also known as binary phase shift keying that is BPSK. Figure shows the PSK signal representation for a given input bit stream. As seen, whenever input bit is 1, output PSK signal is having 0 degree phase shift and whenever input bit is 0, output PSK signal is 180 degrees phase shifted. Thus 1 and 0 are represented with 2 phase angles separated by 180 degrees. The mathematical expression for PSK signal is expressed as shown. If carrier signal is given by a cos omega ct then VPSK of t is equal to a cos omega ct plus theta where theta is 0 degrees and 180 degrees for input bit 1 and 0 respectively. Thus VPSK of t is equal to a cos omega ct when input bit is 1 and minus a cos omega ct when input bit is 0. Also from the waveform you can observe that BPSK is similar to amplitude modulation with modulating signal being digital signal. Let us now see how BPSK signal is generated. Figure shows the BPSK modulator. It consists of bipolar NRZ encoder and product modulator. The binary input sequence is first applied to the bipolar NRZ encoder. This output bipolar signal is then applied to one of the two inputs of the product modulator. To another input of the product modulator a high frequency sinusoidal carrier signal is applied which is given by VC of t is equal to a cos omega ct. The output of the product modulator is BPSK signal as shown. Let us see its working in detail. When input bit is 1 the output of the bipolar NRZ is a positive constant voltage say plus 1 volt which is multiplied with the carrier a cos omega ct during the bit slot in product modulator. Thus the output during that bit slot is given by VPSK of t is equal to a cos omega ct. Thus the carrier signal and the BPSK signal they are in phase with each other giving 0 degrees phase shift. When the input bit is 0 the output of bipolar NRZ is negative constant voltage say minus 1 volt which is then multiplied by the carrier during the bit slot in the product modulator. The output is PSK signal and then is given by VPSK of t is equal to minus a cos omega ct. Thus the carrier signal and the BPSK signal are completely out of phase with a phase shift of 180 degrees in BPSK signal with respect to the carrier as shown. Thus in BPSK signal the phase shift of the carrier changes with respect to 1 and 0 bit of input data. Now before moving ahead pause this video and recall what is the major difference between coherent and non-coherent type receivers. Well the major difference is coherent receivers need synchronous carrier for demodulation which is either generated at the receiver or extracted from the incoming signal. Whereas the non-coherent receivers they do not need synchronous carrier for demodulation. Let us now see how BPSK signal is demodulated. Figure shows the block diagram of coherent BPSK demodulation. It simply consists of a synchronous detector that is a multiplier, low pass filter and threshold device. The incoming BPSK signal is equal to plus or minus a cos omega ct is applied to one of the input of synchronous detector. To another input synchronous local carrier is applied which is either generated at the receiver or extracted from the incoming BPSK signal. Finally the demodulated information bit stream is obtained at the output of the threshold device. Also the synchronous carrier signal in coherent BPSK is extracted from the received signal. For this a synchronizing circuit is used as shown. The synchronizing circuit consists of a square law device, band pass filter and a frequency divider. The received BPSK signal is applied to the square law device and the output of a frequency divider is the extracted synchronous carrier. Let us see its working in detail. During a bit slot when the incoming BPSK signal is plus a cos omega ct the output of square law device which is also known as squaring circuit is a square cos square omega ct which is equal to a square by 2 1 plus cos 2 omega ct. This signal is then passed through a band pass filter which is tuned to twice omega c angular frequency. Thus output is cos 2 omega ct which after division gives the required synchronous carrier cos of omega ct. Now in the synchronous detector the locally recovered synchronous carrier is multiplied with the received BPSK signal. The output of a synchronous detector is a cos square omega ct which is then passed to the low pass filter. The output of filter is thus a DC term plus a by 2. This voltage is then applied to a threshold device whose threshold value is set to 0 volt. As this voltage plus a by 2 is greater than the threshold value the device gives an output logic 1. Similarly, during the bit slot when the received BPSK signal is minus a cos omega ct the extracted local synchronous carrier is again cos of omega ct as shown. This received BPSK signal and the recovered local carrier is then multiplied in a synchronous detector. The output of synchronous detector is now minus a cos square omega ct which on passing through the low pass filter gives a DC term minus a by 2. Now this voltage is less than the threshold voltage of the threshold device. Thus the output of threshold device is now logic 0. Thus it can be seen that during the bit slot when input BPSK signal is a cos omega ct output is logic 1 and when input is minus a cos omega ct output is logic 0. This is how coherent BPSK demodulator works. Its major disadvantage is that it needs complex synchronizing circuit. Figure shows the constellation diagram of BPSK signal. It can be seen that BPSK signal is represented having two signals with amplitude a and phase angle 0 degree and 180 degree for bit 1 and 0 respectively. Let us see advantages of BPSK signal. Compared to other modulation techniques BPSK carries data over the radio frequency signal more efficiently. Even though it occupies same bandwidth as ASK compared to ASK it is less susceptible to noise. For a given probability of error, BPSK requires less transmitted power. High data rate of transmission can be achieved by using higher level of BPSK modulation. Let us now see disadvantages of BPSK signal. BPSK receiver is more complex than ASK and FSK. It has lower bandwidth efficiency. Multilevel BPSK modulation scheme such as QPSK, 16 QAM, etc. are more sensitive to phase variations. BPSK has a wide range of applications. It is widely used in military and commercial communication systems. Wireless LANs use BPSK. It is also used in biometric application and RFID and Bluetooth communication. Thus, in this section we discussed representation, generation and demodulation of BPSK signals with their advantages, disadvantages and applications. These are the references used. Thank you.