 Hello everyone, welcome to this session. I am Dr. Asha Tharangi and today we are going to learn quadrature phase shift king demodulation technique. At the end of this session you will be able to explain QPSK demodulation technique and state advantages, disadvantages and applications of QPSK. These are the contents we will be covering in this session. In the last session, we discussed QPSK signal generation technique. Now pause this video and recall how QPSK signal is represented for its four symbols. Well, table shows the four symbols of QPSK and its representation. As shown, they are combined by phase shifting the carrier signal cos of 2 pi fct by pi by 4, 3 pi by 4, pi pi by 4 and 7 pi by 4. We know that QPSK is a type of M-ary PSK where M is equal to 4. The signal is represented with the expression as shown. V QPSK of t is equals to b o of t root of PS cos 2 pi fct plus b e of t root of PS sin of 2 pi fct where b o of t and b e of t are in phase signal and quadrature phase signal and cos of 2 pi fct and sin 2 pi fct are pair of quadrature carriers used for modulation. For a pi by 4 QPSK for one symbol duration the signal representation for all the four symbols is as shown. Figure shows the block diagram of QPSK demodulator. As shown, it consists of a carrier recovery circuit, two correlators and a parallel to serial converter. As it uses correlator, we can say this is the coherent or synchronous method of QPSK demodulation. Let us see in detail how it works. The incoming QPSK signal is first applied to a carrier recovery circuit which uses a squaring circuit to recover the carrier. The received QPSK signal is raised to its fourth power to generate a sinusoidal component cos raise to 4 2 pi fct. This signal is then passed through a band pass filter to pass only a sinusoidal signal of frequency 4 fc and suppress all other frequency components. Thus giving cos 4 2 pi fct signal at the output which in turn is applied to a divide by 4 frequency divider circuit to obtain the coherent carrier signal cos of 2 pi fct. A quadrature carrier sin 2 pi fct is also obtained by phase shifting the carrier by 90 degrees. These two coherent carriers are applied to two synchronous detectors. We know that the received QPSK signal is the sum of product of pair of quadrature carriers with in phase signal b o of t and quadrature phase signal b e of t as shown. The coherent carrier cos of 2 pi fct and the received QPSK signal are applied to a multiplier of a correlator 1 whose output is integrated in an integrator over 2 bit interval 2Tb which is equal to 1 symbol duration. This integrator produces an output equal to b o of t Tb root of PS which is dependent only on the in phase signal or odd bits stream b o of t and is independent of b e of t. Similarly, during the same symbol duration the quadrature carrier signal sin of 2 pi fct and the received QPSK signal is applied to multiplier of correlator 2 whose output is now integrated in an integrator over 2 bit interval 2Tb which is equal to again 1 symbol duration. This integrator now produces an output equal to b e of t Tb root of PS which is dependent only on the quadrature phase signal or even bit stream b e of t and independent of b o of t. The output of both the integrators are sampled after every 2Tb time interval. The sampled values are either positive or negative depends upon the bit 1 or 0 at b o of t and b e of t. For bit 1 it is positive and for bit 0 it is negative. These 2 sampled values are then applied to a decision device whose threshold value is set to 0 volt. The device produces an output bit 1 if the input to it is positive voltage and produces an output bit 0 if input is negative voltage. Thus the 2 correlators they act as 2 BPSKD modulators which extract in phase data bits that is odd data stream and quadrature phase data bits that is even bit stream. The output of both the decision device are then applied to parallel to serial converter. The bit obtained from the correlator 1 is in phase data bit or odd numbered data bit. Whereas the bit obtained from correlator 2 is quadrature phase data bit or even numbered data bit. Both these bits are obtained during each symbol duration. Parallel to serial converter the bits are then obtained as output bit stream with the data rate RB bits per second. This is same as the data rate of the input bit stream which is used at the transmitter or the modulator. In this way synchronous or coherent detection method is used for QPSK signal demodulation. Now pause this video and based on the demodulation process explained till now for the received QPSK signal write down what will be it is respective demodulated output bits for each symbol. Well let us now see depending upon the QPSK signal received during each symbol duration what are the demodulated outputs. When symbol S1 is received VQPSK of t is equal to root of PS cos 2 pi fct minus root of PS sin of 2 pi fct. This makes the output of in phase channel integrator as positive and the output of quadrature phase channel integrator as negative. Decoding the bits as one and zero respectively. Similarly when symbol S2 is received decoded bits are zero zero. When symbol S3 is received decoded bits are zero one and when symbol S4 is received decoded bits are one one. Thus during each symbol duration two bits are obtained at the output. Let us see the advantages of QPSK. Among all the MRE PSK signals QPSK is most often used as it does not suffer from bit error rate. For same data rate bandwidth requirement of QPSK is half of that of BPSK and DPSK. Therefore bandwidth efficiency is increased here. Also here the information transmission rate is higher. In the presence of the multipath spread and fading conditions pi by 4 QPSK performs best. Both coherent and non-coherent detection methods can be used which makes the receiver design flexible. Along with the advantages QPSK signal also have some disadvantages. The interchannel interference is significantly large in QPSK. To avoid this QPSK uses filtering but due to this the amplitude and phase of QPSK waveform changes. QPSK requires around 8 times its bandwidth to transmit the same power. Compared to BPSK QPSK is more sensitive to phase variations. Transmitter and receiver have to be synchronized very often. Example by using a special synchronizing patterns before user data arrives. As far as applications are concerned QPSK is used in various cellular wireless standards such as GSM, CDMA, LTE, 802.11 WLAN, 802.16 fixed and mobile YMAX, satellite and cable TV etc. Thus in this session we have seen how QPSK signal is demodulated using coherent detection method, what are its advantages, disadvantages and applications. These are the references used. Thank you.