 Hello everyone, welcome to this session. I am Dr. Asha Tharangi and today we are going to learn differential phase shift keying that is DPSK modulation technique. At the end of this session you will be able to explain DPSK modulation technique, obtain DPSK signal for a given bit stream and state how DPSK is different with respect to BPSK. These are the contents we will be covering in this session. Now, before actually starting with DPSK pause the video and recall what are the major disadvantages of BPSK system. Well, the major disadvantages of BPSK are it has lower bandwidth efficiency, it uses synchronous method for detection. Therefore, additional circuitry is needed to recover the carrier or to generate the synchronous carrier at the receiver. This makes BPSK receiver more complex than BASK and BFSK. Let us now see what is differential phase shift keying. As just discussed, BPSK uses coherent detection method. This increases the circuit complexity of demodulation. To overcome this differential phase shift keying which is popularly known as DPSK is used. It is said to be a non-coherent method of BPSK detection. Thus, it does not need synchronous carrier for demodulation. The major difference is that instead of transmitting the original bit stream as in BPSK in DPSK modulation technique, the input bit stream is modified or encoded to a new bit stream and transmitted. It is encoded in such a way that each bit transmitted depends upon its previous transmitted bit. This encoding is expressed in an equation form as shown. Here the modified bit D of t is obtained by X-noring input bit B of t with the previous transmitted bit D of t minus tb which is obtained by delaying the previous transmitted bit by one bit duration tb. To this encoded bit stream, BPSK is applied which gives V DPSK of t which is equal to plus or minus a cos omega ct. Here shows the DPSK modulator. It consists of two parts differential encoder and PSK modulator. Now this PSK modulator is same as in BPSK modulator. Here the encoded bit D of t is obtained at the output of a logic circuit on which the PSK is performed. The two inputs to the logic circuit are input bit B of t and previously transmitted bit delayed by one bit duration that is D of t minus tb. Let us see how it works in detail. Now in order to perform X-noring operation, initially one reference bit or known bit is transmitted. This bit can either be 1 or 0. Let us consider this initial reference bit transmitted as 1. This bit is converted to bipolar NRZ format and then applied to product modulator. The second input to product modulator is high frequency sinusoidal carrier given by a cos omega ct. The output is product of these two signals which is here a cos omega ct. This transmitted bit is delayed by one bit duration tb and then applied to a logic circuit X-nore along with the actual input bit to be transmitted. Thus after X-noring this produces the next encoded bit to be transmitted. This bit is again passed to product modulator through bipolar NRZ which on multiplying with carrier produces next signal for transmission. This bit is again delayed by one bit duration and X-nored with the next incoming input bit. To generate the next encoded bit as shown and this process continues. Thus we can see here for a given input bit stream B of t, the encoded bit stream D of t is obtained which is in unipolar NRZ format. This is then converted to bipolar NRZ form and applied to product modulator where it gets multiplied by the carrier signal a cos omega ct to generate the final modulated signal which is actually transmitted. Thus the encoded bit 1 is transmitted as a cos omega ct which is in phase with the carrier and the encoded bit 0 is transmitted as minus a cos omega ct which is 180 degrees phase shifted with respect to carrier. Thus we can say V dpsk of t is equals to plus or minus a cos omega ct. This can also be easily understood using tabular representation. Here the given input bit stream B of t which is to be transmitted. Here the initial reference bit transmitted is 1. This bit appears at D of t minus tB after one bit delay. This bit then gets X-nored with input bit at that instant and generates output bit 1 as shown. This again gets delayed by one bit duration and gets X-nored with the input bit 1 at that instant to produce new encoded bit as 1. This process continues and we get final encoded bits D of t as shown. By observing these bits we can say that whenever the input bit and the previous transmitted bit are not same encoded output is 0. And whenever the input bit and the previous transmitted bit are same the encoded output bit is 1. Thus we can say that in Dpsk the encoding is done in such a way that the transition in the input bit sequence with respect to the previous transmitted bit is represented by a symbol 0 and no transition is represented by a symbol 1. Also if we consider the phase angle of the transmitted signal we can see that each bit 1 of the input bit stream is represented by sending a signal with the same phase as that of its previous bit. Also each bit 0 of the input stream is represented by sending a signal with a phase angle 180 degrees phase shifted as that of its previous transmitted bit. Thus we can say that in Dpsk the information of the input bit stream is present in the difference between the phases of two successive bits transmitted instead of the actual phase angle in that bit slot as in binary phase shift keying method. This is how Dpsk signal is generated. Let us get the bandwidth of a Dpsk signal. In Dpsk the output phase shift depends upon the current bit and the previous bit. Thus a symbol consists of two bits and symbol duration TS is equals to 2TB. The input is given by 2 upon TS which comes to be FB. Thus minimum bandwidth in Dpsk is equal to maximum baseband frequency FB. With this background pause the video and complete this table. Assume the initial reference bit transmitted as 0. So for the given bit stream the encoded bit obtained is as shown in DFT. So the phase of the transmitted signal is as shown. Thus we can again observe that for each input bit 1 the transmitted signals phase is same as that of its previous transmitted bit and for each bit 0 of the input bit stream the transmitted signals phase is 180 degrees shifted with respect to its previous transmitted bit. Thus we can state that irrespective of initial reference bit transmitted either 0 or 1 the encoding remains the same. These are the references you can refer.