Qam and Qpsk

QAM and QPSK Aim Review of Quadrature bountifulness Modulator (QAM) in digital communication system, generation of Quadrature Phase Shift discover (QPSK or 4-PSK) signal and demodulation. Introduction The QAM principle The QAM modulator is of the type shown in get word 1 below. The ii paths to the adder be typically referred to as the I (in frame), and Q (quadrature), arms. Not shown in hear 1 is any potlimiting. In a practical situation this would be implemented either at pass level at the input to from to each one one multiplier and/or at the produce of the adder.Probably both The motivation for QAM comes from the fact that a DSBSC signal occupies double the bandwidth of the depicted object from which it is derived. This is considered wasteful of resources. QAM restores the balance by placing two independent DSBSC, derived from message 1 and message 2, in the same spectrum space as pertainless DSBSC. The bandwidth imbalance is removed. In digital communications this arrangement is popular. It is workd because of its bandwidth conserving (and other) properties. It is not used for multiplexing two independent messages.Given an input binary sequence (message) at the step of n bit/s, two sequences may be obtained by splitting the bit catamenia into two paths, each of n/2 bit/s. This is akin to a serial-to-parallel conversion. The two streams become the channel 1 and channel 2 messages of invention 1. Because of the halved rate the bits in the I and Q paths are stretched to twice the input sequence bit clock period. The two messages are recombined at the receiver, which uses a QAM-type demodulator. The two bit streams would typically be band limited and/or pulse shaped before reaching the modulator.A bar diagram of such a system is shown in Figure 2 below. QAM becomes QPSK The QAM modulator is so named because, in analog applications, the messages do in fact switch the amplitude of each of the DSBSC signals. In QPSK the same modulator is used, but with binary messages in both the I and Q channels, as describe above. Each message has simply 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 interpreted as a 1800 phase shift, given to the DSBSC. and so the signal in each arm is state 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 rise 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 any one of quaternary 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 exposit digital-QAM or QPSK. This signal is besides 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 experiment L = 2, and so M = 4. The M defines the number of points in the signal constellation. For the cases M > 4 therefore 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 regard from in the block diagram of Figure 3.It is idealised because it assumes the incoming signal has its two DSBSC on the nose in phase quadrature. Thus only one phase adjustment is needful. The parallel-to-serial converter block performs the next 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 data stream. Not shown is the method of carrier acquisition. This ensures that the oscillator, which supplies the lo cal carrier signal, is synchronized to the trustworthy (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 synchronism with each other. (Please sympathise about 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 coarse bit clock (2. 083 kHz), are available from a atomic number 53 SEQUENCE GENERATOR mental faculty.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 deform bandlimiting and pulse shaping is not a subject of enquiry in this experiment. So a single bandpass filter at the adder (summer) output will suffice, providin g it is of decent bandwidth. A 100 kHz CHANNEL FILTERS module is acceptable (filter 3). Experimental Procedure The QPSK transmitter A model of the source of Figure 1 is shown in Figure 4. The QAM modulator involves analog circuitry.Overload must(prenominal) be avoided, to prevent crosstalk between channels when they share a common path the ADDER and output filter. In practice there would plausibly 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 slip away as many unwanted components at the multiplier (modulator) input as possible. T1 patch up the modulator according to Figure 4. Set the on-board switch SW1 of the PHASE SHIFTER to HI. Select 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 ADDER to be, say, 1 volt peak at the ADDER out put. 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 set the oscillo chain in X-Y mode. With no input, select equal gains per channel. Locate the spot in the center on of the screen then connect the two data streams entering the QAM to the scope X and Y inputs.The Demodulator Modelling of the demodulator 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, identical demodulator must be provided. T5 patch up the single channel demodulator of Figure 5, including the z-mod facility of the DECISION MAKER. T6 temporary hookup watching the I channel at the transmitter, use the PHASE SHIFTER to match the demodulator output with it.T7 while wa tching 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 conserve bandwidth. 2) The modulator used the quadrature 100 kHz outputs from the MASTER SIGNALS module. Did it matter if these were not precisely in quadrature ? Explain. 3) Name one advantage of making 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 ?