Faculty FARULTI KEIURUTERAAN ELERTRIK, ‘Subject MAKMAL KEJURUTERAAN Review aa ELEKTRIK Release Date: 2012 Last Amendment: 2012 Subject code: SETISEW/SECSEL 3742 Procedure number: PK-UTM-FKE(O)10 SET/SEW/SEC/SEL 3742 FAKULTI KEJURUTERAAN ELEKTRIK UNIVERSITI TEKNOLOGI MALAYSIA KAMPUS SKUDAI JOHOR EXPERIMENT 8 DIGITAL COMMUNICATION LABORATORY QUADRATURE PHASE SHIFT KEYING (QPSK) Prepared by: HEAD OF LABORATORY Prepared by: HEAD OF DEPARTMENT Name : DR SHARIFAH KAMILAH BT. ‘Name: PROF. DR. ABU SAHMAH SUPAAT SYEDYUSUF Signature Ketua Makmal hat sksatm.mytesbabigtomm & Samp: Makmal Perhubungan Digit Fakulti Kejuruteraan Elektrik 81310 UTM Skudai, Johor Melaysia co Ketua Makmal Maimai Perhubungen Digt Fault Kejuruterean Elektnic 81310 UTM Skudal, Johor MalaysiaExperiment 18 - Quadrature Phase Shift Keying Preliminary discussion As its name implies, quadrature phase shift keying (QPSK) is a variation of binary phase shift keying (BPSK). Recall that BPSK is basically a DSBSC modulation scheme with digital information for the message. Importantly though, the digital information is sent one bit at a time, QPSK is a DSBSC modulation scheme also but it sends two bits of digital information a time (without the use of another carrier frequency). ‘As QPSK sends two bits of data ot a time, it’s tempting to think that QPSK is twice as fast as BPSK but this is not so. Converting the digital data from a series of individual bits to a series of bit-pairs necessarily halves the data's bit-rate, This cancels the speed advantage of sending two bits at a time. ‘So why bother with QPSK? Well, halving the data bit rate does have one significant advantage. “The amount of the radio-frequency spectrum required to transmit QPSK reliably is half thet required for BPSK signals, This in turn makes room for more users on the channel. Figure 1 below shows the block diagram of the mathematical implementation of QPSK. QPsk output Digital lata 2-bit serial-to- parallel converter Figure 1 182 1008 EnaraTnetramerts Experiment‘At the input to the medulator, the digital data's even bits (that is, bits 0, 2, 4 and so on) are. stripped from the data stream by a “bit-splitter® and are multiplied with a carrier to generate 0 BPSK signal (called PSkK:). At the same time, the data's odd bits (that is, bits 1, 3, 5 and so on) cre stripped from the data stream and are multiplied with the same carrier to generate a second BPSK signal (called PSka). However, the PSKq signal's carrier is phase-shifted by 90° before being modulated, This is the secret to QPSK operation. ‘The two BPSK signals are then simply added together for transmission and, as they have the same carrier frequency, they occupy the same portion of the radio-frequency spectrum. While ‘this suggests that the two sets of signals would be irretrievably mixed, the required 90° of phase separation between the carriers allows the sidebands to be separated by the receiver Using phase discrimination (introduced in Experiment 8) Figure 2 below shows the block diagram of the mathematical implementation of QPSK demodulation XR bo et eo] [x] EDD [4 oe i ese KX KAY I pk, |W Figure 2 Notice the arrangement uses two product detectors to simultaneously demodulate the two BPSK signals, This simultaneously recovers the pairs of bits in the original data, The two signals are cleaned-up using a comparator or some other signal conditioner then the bits are put back in order using a 2-bit parallel-to-serial converter. ‘Experiment 18 - Quadrature Phase Shift Keying © 2008 Emone Tstruments| a‘To understand how each detector picks out only one of the BPSK signals and not both of them, recall that the preduct detection of DSBSC signals is “phase sensitive". That is, recovery of the sessage is optimal ifthe transmitted and local carriers are in phase with each another. But the recovered message is attenuated if the two carriers are not exactly in phase, Importantly, if ‘the phase error is 90° the amplitude of the recovered message is zero, In other words, the message is completely rejected (this issue is discussed in Part E of Experiment 7). The QPSK demodulator takes advantage of this fact. Notice that the product detectors in Figure 2 share the carrier but one of them is phase shifted 90°, That being the case, once the phase of the local carrier for one of the product detectors matches the phase of the: ‘transmission carrier for one of the BPSK signals, there is automatically a 90° phase error between that detector's local carrier and the transmission carrier of the other BPSK signal. So, ‘the detector recovers the data on the BPSK signal that it's matched to and rejects the other BPSK signal ‘The experiment In this experiment youll use the Emona Telecoms-Trainer 101 to generate a QPSK signal by implementing the mathematical model of QPSK. Once generated, you'll examine the QPSK signal using the scope. Then, youll examine how phase discrimination using a product detector can be used to pick-out the data an one BPSK signal ar the other. Tt should take you about 1 hour to complete this experiment. Equipment + Emona Telecoms-Trainer 101 (plus power-pack) + Duol channel 20MHiz oscilloscope + three Emona Telecoms-Trainer 101 oscilloscope leads + assorted Emona Telecoms-Treiner 101 patch leads 1e4 (©2008 Enone Intromenta Experiment 18 Quadrature Phase Shift KeyingProcedure Port A - Generating a QPSK signal 1. Gather a set of the equipment listed on the previous page. 2, Set up the scope per the instructions in Experiment 1 3. Set the scope's Trigger Source control to the EXT position 4, Set the scope's Trigger Source Coupling control to the HF REJ position. 5. Set the scope's Channel 1 and Channel 2 Znput Coupling controls to the DC position. 6. Set the scope's Timebase control to the 0.5ms/diy position. 7. Locate the Divider module and set it up to divide by 2 by pushing the left-side switch up and the right-side switch down, Tip: The Divider module is underneath the Sequence Generator module. 8, Connect the set-up shown in Figure 3 below. Note: Insert the black plugs of the oscilloscope leads into a ground (GND) socket. aaoooo c@lss Figure 3 Experiment 18 - Quadrature Phase Shift Keying © 2008 Emona Tnstruments 185‘The set-up in Figure 3 can be represented by the block diagram in Figure 4 below. The Sequence Generator module is used to model digital data. The 2-bit Serial-to-Parallel Converter module is used to split the data bits up into a stream of even bit and odd bits. ‘As mentioned in the preliminary discussion, splitting the data up this way halves the bit rate of the two new sets of data. That being the case, the Sequence Generator module's SYNC output must be halved for it to be used as the triggering signal for the scope. Digital signal modelling I Bit-splitter \ i Master Sequence | 2-bit serial-to- Signals Generator | parallel converter \ P Even bits i Ee ee yt Odd bits Toch2 Divide +2 + ric wider | #2 ToEXT. Figure 4 ‘Set the scope's Mode control to the DUAL position to view the Serial-to-Paralll Converter module's two outputs. 10. Compare the signals. You should see two digital signals that are different to each other. Question 1 What is the relationship between the bit rate of these two digital signals and the bit rate of the Sequence Generator module's output? Tip: If youre not sure, see the preliminary discussion 186 "©2008 Enona Ietroments Experiment 18 Quadrature Phese Swift Keying By Rud oO) FE Lit Tapade‘Ask the instructor to check ‘your work before continuing 11, Modify the set-up as shown in Figure 5 below. Remember: Dotted lines show leads already in place, Fame si’ |°@led WF loses 2Pt * = wee a | 29 Figure 5 Excluding the digital data modelling, this set-up can be represented by the block diagram in Figure 6 on the next page, Notice that the bit-splitter's two outputs are connected to independent Multiplier madules. The other input to the Multiplier modules is a 100kHz sinewave. However, the signals are out of phase with each other by 90° which is a requirement of QPSK. Quodrature Phase Shift Keying © 2008 Emons Instruments a7 Experiment2 13, 14, 15, 16 Even,bits echt Multiplier 2-bit Serial-to- Parallel Converter 100KHz Digital fata Figure 6 Compare the even bits of data with the Multiplier module's output (PSKz). Set the scope's Timebase control to the O2ms/div postion, ‘Activate the scope's Sweep Multiplier to view the signals more closely. Use the scope's Horizontal Position control to locate a transition in the data sequence. Examine the carrier and note in what way it changes at the sequence’s transition. Question 2 What feature of the Multipliers output suggests that it's a BPSK signal? Deactivate the scope's Sweep Multiplier ‘Move the scopes connections to the set-up as shown in Figure 7 on the next page. “©2008 Emona Tnetraments Experiment 18 - Quadrature Phase Shift Keyingaaaaaa a c@l*d “Air cee? = moe) eo Fos t t 900 % 8 al Ba ae ao i Zo 8 Figure 7 ‘This change can be represented by the block diagram in Figure 8 below. Multiplier > KX Le osx, y 100kHz SINE 2-bit Serial-to- Parallel Converter Digital Master data NZ | signals 100kHz cos y XL To Ch.2 Odd bits Multiplier To Chi Figure 8 ‘Experiment 18 Quodrature Phase Shift Keying © 2008 Ene Tstruments 18919. Activate the scope's Sweep Multiplier to view the signals more closely 20. Use the scope's Horizontal Position control to locate a transition in the data sequence. 21. Examine the carrier and note in what way it changes at the sequence's transition Question 3 ‘What type of signal is present on the Multipier’s output? ‘Ask the instructor to check your work before continuing. 2a, Deactivate the scope's Sweep Multiplier and return the scope’s Timebase control to the O.5ms/div setting 23. Modify the set-up as shown in Figure 9 below. aaaaoa + c@lst i: 2 Ba — = Ht loses ta S o tet Frais | “O ao \| 2 eg: Figure 9 ia 1 2008 Enos eters Experiment 18 ~ Quadrature Phase Shift Keying‘The set-up in Figure 9 can be represented by the block diagram in Figure 10 below. The Adder module is used to add the PSKr and PSKq signals. This tums the set-up into a complete QPSK modulator. 2-bit Serial-to- Parallel Converter = £ psk q Sonal £ Techs = 100kH2 * PSK igure 10 24, Turn the Adder module's & control fully anti-clockwise. Note: This removes the BPSK; signl from the signal on the Adder module's cutput. 25, Adjust the Adder's g control to obtain a 4Vp-p output. 26, Disconnect the patch lead to the Adder module's 8 input Note: This removes the BPSkq signal from the signal on the Adder module's output: 27, Adjust the Adder's Gcontrel to obtain a 4Vp-p output. 28. Reconnect the patch lead to the Adder‘s B input Question 4 According to the theory, what type of digital signal transmission is now present on the ‘Adder's output? Experiment 18 Quadrature Phase Shift Keying © 2008 Emona rstruments|29, Set the scope's Timebase control to the O.2ms/div position. 30, Activate the scope's Sweep Multiplier to view the signal more closely. 31. Use the scope's Horizontal Pasition control to examine the signal from beginning to end, Question 5 ‘Why is there only one sinewave when the QPSK signal is made up of two BPSK signals? ‘Ask the instructor to check ‘your work before continuing, lee (©2008 Emena Instrumente Experiment 18 - Quadrature Pase Shift KeyingPart B ~ Using phase discrimination to pick-out one of the QPSK signal's BPSK signals Tr’s not possible to implement both a QPSK modulator and demodulator with one Emona ‘Telecoms-Trainer 101. However, itis possible to demonstrate how phase discrimination is used by a QPSK demadulater to pick-out one or other of the two BPSK signals that make up the ‘QPSK signal, The next part of the experiment lets you do this. 32, Deactivate the scope's Sweep Multiplier and return the scope's Timebase control to the Ims/div setting, 33. Locate the Tuneable LPF module and turn its Cut-off Frequency Adjust control fully clockwise. 34, Set the Tuneable LPF module's Gain control to about the middle of its travel 35, Locate the Phase Shifter module and set its Phase Change control to the O° position. 36. Modify the set-up as shown in Figure 11 below. Note: As there are a lot of connections, you may find it helpful to tick them off as you ‘add them, t 5 FIG Of # ae aaoo0o +lo@let th rat 1 t Figure 11 ‘Experiment 18 - Quodrature Phase Shift Keying © 2008 Emone Tstruments aaThe additions to the set-up can be represented by the block diagram in Figure 12 below. If you ‘compare it to Figure 2 in the preliminary discussion, youll notice that it implements most of one arm of a QPSK demodulator (either T or Q). Multiplier Tuneable module LPF @Psk ‘input “Stolen' local carrier | o 100kHz Phase Master shifter Signals Figure 12 37. Compare the even data bits on the Serial-to-Parallel Converter module's X/ output with ‘the Bandpass LPF module's output. 38. Vary the Phase Shifter module's Phase Adjust control eft and right and observe the effect on the demodulated signal. You are aiming to recover a bipolar (2-level) signal like ‘the original Xt or X2 signals from the Sericl-to-Parallel Converter module, 39. Set the Phase Shifter module's Phase Change control to the 180° position and repeat step 36, Question 6 What is the cause of the 3 and 4 level signals out of the Tuneable LPF during the phase adjustments above ? How many different Phase Adjust contral positions will give you a bipolar signal ? ‘Ask the instructor to check your work before continuing, ei (©2008 Emone Tstraments Experiment 18 - Guodrature Phase Shift Keying40. Modify the set-up as shown in Figure 13 below. Figure 13 ‘The addition of the Comparator on the Utilities module can be represented by the block diagram in Figure 14 below. If you compare this block diagram with Figure 2 in the preliminary discussion, youll notice that this change completes one arm of a QPSK demodulator. Utilities apsk ~ rout — X Even or wv seeps "Stolen" local carrier 100kHz] © ~N Figure 14 Erperiment 18 - Quadrature Phase Shift Keying © 2008 Emana Instruments 61841. Set the Phase Shifter module's Phase Change control to the O° position. 42, Compare the even data bits on the Serial-to-Parallel Converter module's X/ output with ‘the Bandpass LPF module's output. 43, Adjust the Phase Shifter module's Phase Adjust control until you have recovered the even data bits (ignoring any phase shift) Quest What is the present phase relationship between the local carrier and the carrier signals uged to generate the PSkr and PSka signals? ‘Ask the instructor to check your work before continuing 44, Unplug the scope's Channel 1 input from the Serial-to-Parallel Converter module's XI output and connect it to its X2 output to view the odd data bits. 45, Compare the odd data bits with the recovered data. They should be different. 46, Set the Phase Shifter module's Phase Change control to the 180° position. 47. Adjust the Phase Shifter module's Phase Adjust control until you have recovered the odd data bits (ignoring any phase shift). Question 8 What is the new phase relationship between the local carrier and the carrier signals used ‘to generate the PSkr and PSKa signals? 816 (©2008 Emons Tnetramente Experiment 18 Quadrature Phase SHY KeyingQuestion 9 Why is your demadulator considered to be only one half of a full QPSK receiver? Mi ‘Ask the instructor to check your work before finishing, Experiment 18 - Quodrature Phase Shift Keying © 2008 Enena Instruments waar