Qam and Qpsk

QAM and QPSK aim Review of Quadrature Amplitude Modulator (QAM) in digital communication system, generation of Quadrature var. Shift Keyed (QPSK or 4-PSK) signal and demodulation. Introduction The QAM principle The QAM modulator is of the compositors case shown in Figure 1 below. The ii paths to the adder are typically referred to as the I (in conformation), and Q (quadrature), arms. Not shown in Figure 1 is some(prenominal) bandlimiting. In a practical situation this would be implemented severally at inwardness level at the input to each(prenominal) multiplier and/or at the output of the adder.Probably some(prenominal) The motivation for QAM comes from the fact that a DSBSC signal occupies twice the bandwidth of the message from which it is derived. This is considered wasteful of resources. QAM restores the balance by placing devil independent DSBSC, derived from message 1 and message 2, in the same spectrum property as unmatched DSBSC. The bandwidth imbalance is rem oved. In digital communications this formation is popular. It is used because of its bandwidth conserving (and other) properties. It is non used for multiplexing two independent messages.Given an input binary program sequence (message) at the come in of n bit/s, two sequences may be obtained by splitting the bit stream into two paths, each of n/2 bit/s. This is akin to a serial-to-parallel conversion. The two streams bring into being the business 1 and channel 2 messages of Figure 1. Because of the halved rate the bits in the I and Q paths are stretched to twice the input sequence bit measure period. The two messages are recombined at the liquidator, which uses a QAM-type detector. The two bit streams would typically be band limited and/or pulse shaped out front reaching the modulator.A block plot of such a system is shown in Figure 2 below. QAM becomes QPSK The QAM modulator is so named because, in latitude applications, the messages do in fact vary the amplitude of eac h of the DSBSC signals. In QPSK the same modulator is used, besides with binary messages in both the I and Q channels, as describe above. separately message has provided two levels, V volt. For a non-bandlimited message this does not vary the amplitude of the output DSBSC. As the message changes polarity this is interpret as a 1800 phase shift, given to the DSBSC.Thus the signal in each arm is said to be undergoing a 1800 phase shift, or phase shift keying or PSK. Because there are two PSK signals combined, in quadrature, the twochannel modulator gives line up to a quadrature phase shift keyed QPSK signal. Constellation Viewed as a phasor diagram (and for a non-bandlimited message to each channel), the signal is seen to occupy some(prenominal) one of four point locations on the complex plane. These are at the corner of a square (a square lattice), at angles ? /4, 3? /4, 5? /4 and 7? /4 to the real axis.M-PSK and M-QAM The above has described digital-QAM or QPSK. This signal is also called 4-PSK or 4QAM. More generally signals can be generated which are described as M-QAM or MPSK. Here M = 2L, where L = the number of levels in each of the I and Q arms. For the present try out L = 2, and so M = 4. The M defines the number of points in the signal constellation. For the cases M 4 indeed M-PSK is not the same as M-QAM. The QAM Receiver The QAM receiver follows the similar principles to those at the transmitter, and is illustrated in idealised from in the block diagram of Figure 3.It is idealised because it assumes the incoming signal has its two DSBSC precisely in phase quadrature. Thus only one phase adjustment is required. The parallel-to-serial converter block performs the following operations 1. regenerates the bit clock from the incoming selective information. 2. regenerates a digital waveform from both the analog outputs of the I and Q arms. 3. re-combines the I and Q signals, and outputs a serial entropy stream. Not shown is the method of news boy wave acquisition. This ensures that the oscillator, which supplies the local carrier signal, is synchronized to the received (input) signal in both frequency and phase.In this experiment we will use a stole carrier to ensure that carrier signal in the transmitter and receiver are in synchronisation with each other. (Please read round Costas Receiver to understand more about carrier acquisition). In this experiment, two independent data sequences will be used at the input to the modulator, rather than having digital circuitry to split one data stream into two (the serialto-parallel converter). Two such independent data sequences, sharing a common bit clock (2. 083 kc), are getable from a single SEQUENCE GENERATOR module.The data stream from which these two channels are considered to have been derived would have been at a rate of twice this 4. 167 kHz. Lowpass filter bandlimiting and pulse shaping is not a receptive of enquiry in this experiment. So a single bandpass filter a t the ADDER (summer) output will suffice, providing it is of adequate bandwidth. A 100 kHz CHANNEL FILTERS module is acceptable (filter 3). Experimental Procedure The QPSK transmitter A model of the generator of Figure 1 is shown in Figure 4. The QAM modulator involves analog circuitry.Overload must be avoided, to prevent crosstalk between channels when they percent a common path the ADDER and output filter. In praxis there would probably be a filter in the message path to each multiplier. Although these filters would be included for pulse shaping and/or band limiting, a secondary purpose is to eliminate as many unwanted components at the multiplier (modulator) input as likely. T1 patch up the modulator according to Figure 4. Set the on-board switch SW1 of the flesh gearstick to HI. convey channel 3 of the 100 kHz CHANNEL FILTERS module (this is a bandpass filter of adequate bandwidth).T2 there are no critical adjustments to be made. Set the signals from each input of the ADD ER to be, say, 1 volt upper side at the ADDER output. T3 for interest predict the waveforms (amplitude and shape) at all interfaces, then confirm by inspection. Constellation You can display the four-point constellation for QPSK T4 coif the oscilloscope in X-Y mode. With no input, select equal gains per channel. Locate the pip in the centre of the screen then connect the two data streams entering the QAM to the scope X and Y inputs.The Demodulator Modelling of the sensor of Figure 3 is straightforward. But it consumes a lot of modules. Consequently only one of the two arms is shown in Figure 5. The PHASE SHIFTER can be used to select either channel from the QAM signal. If both channels required simultaneously, as in practice, then a second, undistinguishable demodulator must be provided. T5 patch up the single channel demodulator of Figure 5, including the z-mod facility of the DECISION MAKER. T6 while watching the I channel at the transmitter, use the PHASE SHIFTER to match the demodulator output with it.T7 while watching the Q channel at the transmitter, use the PHASE SHIFTER to match the demodulator output with it. Tutorial Questions 1) Explain how a QAM system conserves bandwidth. 2) The modulator used the quadrature 100 kHz outputs from the MASTER SIGNALS module. Did it be if these were not precisely in quadrature ? Explain. 3) Name one advantage of devising the bit rate a sub-multiple of the carrier frequency. 4) Why is there a need to eliminate as many unwanted components as possible into the modulator ?