 Hello everyone, welcome to this session. I am Dr. Asha Tharange and today we are going to learn MRE that is multilevel digital modulation technique. At the end of this session you will be able to explain MRE signaling scheme state the difference between binary and MRE signaling schemes and explicate MRE ASK, MRE FSK, MRE PSK modulation techniques and represent the given bit stream in this form. These are the contents we will be covering in this session. Before actually starting with MRE signaling scheme pause the video and think that in any communication system what are the two major resources that are used efficiently. Well channel bandwidth and transmit power are two primary resources that are used efficiently in any communication system. Let us now see what is MRE signaling scheme. We know that BASK, BFSK, BPSK and DPSK are binary digital modulation techniques. In all these techniques signal is represented using only two symbols consisting of single bit one and zero. As only two symbols are used two amplitudes, two frequencies or two phases are used to represent them respectively. The maximum bandwidth requirement of BASK is twice FB, BFSK is 4 FB, BPSK is twice FB and DPSK is FB. In order to reduce the bandwidth requirement two or more bits are combined and transmitted as one symbol. The number of symbols in this case is always greater than 2 expressed as M equal to 2 raise to N. Thus any signal representation scheme which uses the combination of two or more bits to represent a single symbol is known as MRE or multilevel signaling scheme. Figure shows the bit stream represented in binary form. Here the signal uses only two amplitude levels to represent two symbols one and zero. While this is a figure showing MRE signal representation for M equal to 4. In this two bits are combined to form four different symbols and as shown they are represented by four different amplitude levels. If this M level signal is used to modulate the carrier then this technique is known as MRE digital modulation technique. In this one of the M possible symbol is transmitted during each symbol duration TS which is given by TS is equals to TB log of M to the base 2 where TB is one bit duration. Now based on the type of modulation used MRE digital modulation techniques are classified as MRE ASK, MRE FSK, MRE PSK and MRE amplitude and phase shift KING which is also known as quadrature amplitude modulation that is QAM. Let us see MRE ASK modulation technique. In MRE amplitude shift KING known as MRE ASK M different amplitude levels of carrier are used. This signal can be represented by the expression SI of t is equals to AI cos of 2 pi fct present for one symbol duration and for i equal to 1 to M. Consider for example the signal for M equal to 4 and D equal to 1. Then the four amplitude levels will be minus 3 minus 1 1 and 3 volts. Thus the four symbols of MRE ASK signals will be given by S1 of t is equals to cos of 2 pi fct S2 of t is equals to minus cos 2 pi fct S3 of t is equals to 3 cos 2 pi fct and S4 of t is equals to minus 3 cos 2 pi fct. Figure shows the BASK signal representation. Here one bit is transmitted in one symbol duration and is represented by two amplitude levels of carrier. Whereas this figure shows the MRE ASK signal representation for M equal to 4. As shown here two bits are combined to form one symbol and four symbols are represented by four different amplitude levels of carrier. Also we can see that in BASK ten time slots or ten symbol durations are required to transmit ten bits. Whereas in MRE ASK the same ten bits are transmitted in only five time slots or five symbol duration thus increasing the bit rate. Figure shows the constellation diagram for binary and MRE ASK signal. We can see here as the level goes on increasing the separation between the two adjacent symbol amplitudes goes on decreasing. This makes demodulation difficult for higher M levels and put limitation on use of a higher value of M. MRE ASK is also used to represent PAM signal. The circuit needed to implement MRE ASK is simple. The major drawback of MRE ASK signal is it is susceptible to noise and distortion. Let us now see MRE FSK modulation technique. In MRE FSK M different carrier frequencies are used to represent M symbols. This signal is represented by si of t is equal to 2 cos 2 pi fci of t during one symbol duration for i equal to 1 to m. These frequencies are chosen so that they are orthogonal to each other and thus there is no interference between the adjacent carrier frequencies. Figure shows the BFSK signal representation. Here symbol 1 and 0 are represented by two different carrier frequencies whereas in MRE FSK as shown for m equal to 4 two bits are grouped and four symbols are represented by four different carrier frequencies. Figure shows the constellation diagram for BFSK signal and MRE FSK signal for m equal to 8. As shown all frequency components are orthogonal to each other in order to avoid the interference. The bandwidth for MRE FSK is given by B is equals to m upon 2 T s hertz. Bandwidth efficiency is thus given by rho is equals to 2 log of m to the base 2 upon m bits per second per hertz. Thus we can say that as m increases bandwidth efficiency for MRE FSK decreases. MRE FSK has better noise immunity than MRE ASK. All the transmitted m signals are equal in energy and duration. As the signals are orthogonal to each other there is no crowding in the signal space. The bandwidth efficiency of MRE FSK decreases and the power efficiency increases with increase in m. Let us now discuss MRE PSK modulation technique. In MRE PSK m different phase angles are used to represent m symbols mostly with a phase difference of 2 pi by m. MRE PSK signal is represented by si of t is equals to a cos of 2 pi fct plus phi i for one symbol duration for i equal to 1 to m. For example consider m equal to 4 and constant equal to 0. Then the four phases used will be 0 pi by 2 pi and 3 pi by 2. Also say for m equal to 4 and constant equal to pi by 4 then the four phases used will be pi by 4, 3 pi by 4, 5 pi by 4 and 7 pi by 4. Both of these are examples of four PSK and are popularly called as quadrature phase shift keying figure shows the PPSK signal representation. Here one is represented by 0 degree phase shift and zero is represented by 180 degree phase shift. This waveform represents MRE PSK signal for m equal to 4. As shown again two bits are combined to form a symbol and each of four symbols are represented by the carrier signal with four different phase shifts. Figure shows the constellation diagram for BPSK and MRE PSK for m equal to 4, 8 and 16. As shown as the level increases phase difference between the adjacent two signals goes on decreasing. This again puts limitation on using higher M levels as it becomes difficult to correctly recover the signals at the receiver as separation between them goes on decreasing. Bandwidth of MRE PSK is given by B is equals to 2 upon TS hertz. Bandwidth efficiency is given by rho is equals to log of M to the base 2 upon 2 bits per second per hertz. Thus as M increases bandwidth efficiency for MRE PSK increases. As MRE PSK uses constant envelope it is immune to noise. There is considerable reduction in bandwidth requirement. It has better performance than ASK and FSK. There is increase in probability of errors with increase in number of bits per symbol. Also design of MRE PSK modulator and demodulator is complex. These are the references used. Thank you.