PGT313 Digital Communication Technology Lab 3 Quadrature Phase Shift Keying (QPSK) and 8-Phase Shift Keying (8-PSK) Objectives i) To study the digitally modulated quadrature phase shift keying (QPSK) and 8-PSK waveforms using an oscilloscope ii) To measure the bandwidth and power of the modulated QPSK and 8-PSK signals using an oscilloscope and the VSA software iii) To demodulate the modulated QPSK and 8-PSK signals using the VSA software iv) To measure the error vector magnitude (EVM) of the modulated QPSK and 8-PSK signals using an oscilloscope and the VSA software Equipment Required i) ME1110 Digital Modulation Training Kit ii) Keysight MSOX 3000 series 4 Channels Mixed Signal Oscilloscope with a minimum of 100 MHz bandwidth* *Note: For users with the Keysight DSOX 3000 series oscilloscope, self-customization on certain experiment procedures is required to obtain the expected experiment results. Accessories Required i) A PC running Microsoft Windows XP/Vista, pre-installed with the following: a. Agilent 89601A VSA software version 16.0 or higher [with option 200, 300, AYA] b. Agilent IO Libraries Suite software version 14.1 or higher for instrument connection ii) 1 SMA(m)-to-BNC(m) coaxial cable, 1.0 m iii) 3 Oscilloscope Probes iv) 1 Oscilloscope Digital Logic Probe v) 1 USB cable PGT313 Digital Communication Technology Lab 3-1/19
1. Introduction Quadrature Phase Shift Keying (QPSK) In Quadrature Phase Shift Keying (QPSK) modulation, a sinusoidal carrier is varied in phase while keeping a constant amplitude and frequency. Sometimes QPSK is also known as quaternary PSK, quadri-phase PSK, 4-PSK, or 4-QAM. (Although the root concepts of QPSK and 4-QAM are different, the resulting modulated radio waves are exactly the same.). QPSK uses four points on the constellation diagram, equi-spaced around a circle. A QPSK signal can be represented in a two-dimensional constellation diagram. The horizontal axis is called In-phase, and the vertical axis is named Quadrature. The points represent the four phases (45, 135, 225, and 315 degrees) which correspond to four symbols. The term "quadrature" implies that there are four possible phases (4-PSK or QPSK) which the carrier can have at a given time, as shown at right on the characteristic constellation for this modulation type. In PSK, information in conveyed through phase variations, since absolute phase cannot be established. In each time period, the phase can change once while the amplitude remains constant. In QPSK there are four possible phases, and therefore two bits of information conveyed within each time slot. The rate of change (baud) in this signal determines the signal bandwidth, but the throughput or bit rate for QPSK is twice the baud rate. The constellation and IQ amplitude mappings are shown in Figure 1. Quadrature S2 01 A 00 S1 45º Symbol Bit S(t) Phase I Q S1 00 A cos ( t+45º) 45º 0.707A 0.707A S2 01 A cos ( t+135º) 135º 0.707A 0.707A S3 10 A cos( t+225º) 225º 0.707A 0.707A S4 11 A cos( t+315º) 315º 0.707A 0.707A In phase S3 S4 10 11 Figure 1 QPSK Constellation and IQ Amplitude Mapping Assuming equal bit rates and rectangular pulse shaping on the I and Q components, the frequency spectrum of QPSK is identical to that of BPSK (as shown in Figure 2.2 in Lab 2), but different scale factor due to two equal power carriers forming the QPSK signal. For rectangular pulse shaping the main lobe transmission bandwidth of QPSK (reduction of 2 compared to BPSK) is now given as: BTX 2R S, where R s is the symbol rate. For QPSK, 2 bit per symbol, the symbol rate is given as: 8-Phase Shift Keying (8-PSK) Rb RS, where R b is the transmission bit rate. 2 Any number of phases may be used to construct a PSK constellation but 8-PSK is usually the highest order PSK constellation deployed with optimum error rate. The error-rate becomes too high when more than eight phases are used (you can imagine that the points in the constellation diagram get more closer to each other when more phases are used) and there are better, though more complex, modulations scheme available PGT313 Digital Communication Technology Lab 3-2/19
such as quadrature amplitude modulation (QAM). Although any number of phases may be used, the fact that the constellation represents binary data means that the number of symbols is usually a power of 2 this allows an equal number of bits-per-symbol. The constellation and IQ amplitude mappings for 8-PSK signals are shown in Figure 2. Q 010 S4 011 S5 100 101 S6 110 S3 001 S2 A 45º S1 000 111 S8 S7 I Symbol Bit S(t) Phase I Q S1 000 Acos ( t+0º) 0º A 0 S2 001 Acos ( t+45º) 45º 0.707A 0.707A S3 010 Acos( t+90º) 90º 0 A S4 011 Acos( t+135º) 135º 0.707A 0.707A S5 100 Acos ( t+180º) 180º A 0 S6 101 Acos ( t+225º) 225º 0.707A 0.707A S7 110 Acos( t+270º) 270º 0 A S8 111 Acos( t+315º) 315º 0.707A 0.707A Figure 2 8-PSK constellation and IQ amplitude mapping For the rectangular pulse shaping, the main lobe transmission bandwidth of the 8-PSK (reduction of 3 compared to BPSK) is now given as: BTX 2R S, where R s is the symbol rate For 8-PSK (3 bit per symbol), the symbol rate is given as: R S Rb 3, where R b is the transmission bit rate. PGT313 Digital Communication Technology Lab 3-3/19
2. Measuring and Verifying IQ and QPSK Modulated Signals 1. Make the following connections (as shown in Figure 3). By using three oscilloscope probes, connect the I, Q, and Output RF ports of the ME1110 training kits to the respective Channel 2, Channel 3, and Channel 1 of an oscilloscope. Note that Channel 1 represents I-baseband signal (TP1), Channel 2 represents Q-baseband signal (TP2), and Channel 3 represents the modulated ASK signal (J1A). At the rear panel of the oscilloscope, connect the digital channel D1 to Baseband Data port (J8B), D0 to the Data Clock (J8A), and D2 to the Symbol Clock port (J8C). Set the training kit to Mode 6 for the QPSK modulation. IQ modulator Digital Baseband Baseband Data FPGA Symbol Clock DAC I-Signal Q-Signal DAC Data Clock Low Pass Filter LO 90 0 Low Pass Filter I Q Filter Amplifier ME1110 Training Kit Output RF D1 D0 MSOX 3000 Series Oscilloscope Ch 1 D2 I-Signal Q-Signal Ch 2 Ch 3 Figure 3 Basic Setup for IQ Baseband Signal Measurement and Verification 2. Verify the generated IQ waveform with the recommended oscilloscope. Figure 4 shows a sample of the measured IQ waveform using the MSOX 3000 series oscilloscope. Note that you will have to use an edge positive (rising) trigger in the oscilloscope with the clock port (D1) in order to see a stable waveform, as shown in Figure 5. Note that you will have to press the [Edge] key in the Trigger section of the front panel to display the Edge Trigger Menu, and then select D1 as source and rising as slope. PGT313 Digital Communication Technology Lab 3-4/19
Modulated QPSK signal I Figure 4 Measured IQ and Modulated QPSK Waveform in Oscilloscope Q Symbol clock Baseband data Data clock Trigger Source - D1 Positive Edge or Rising Figure 5 Setting Trigger in Oscilloscope 3. Based on the baseband data in Figure 4, what are the binary bit streams going into the modulator? How many bits are represented in a symbol in the QPSK modulation? 4. Use the mapping table in Figure 1 to verify the measured I and Q signals based on the baseband data identified in step 3. PGT313 Digital Communication Technology Lab 3-5/19
The bit streams are: 10 10 01 01 00 10 00 11 1/2 symbol delay S3 S3 S2 S2 S1 S3 S1 S4 I 0.7A 0.7A 0.7A Q 0.7A 0.7A 0.7A 0.7A 0.7A 1 0 1 0 0 1 Figure 6 Measured I and Q Signals Based on QPSK Mapping in 2 Bits per Symbol Note that the measured I and Q have a 1 symbol delay. 5. Change the time base of the oscilloscope to view the phase changes. An example of the RF Output showing the phase change is illustrated in Figure 7. Acos( t+45º) Acos( t+225º) I = 0.7A I = 0.7A Q= 0.7A Q= 0.7A Bit 1 Bit 2 Figure 7 Measured IQ and QPSK Signals in Oscilloscope with Zoomed Time Base PGT313 Digital Communication Technology Lab 3-6/19
3. Measuring and Verifying IQ and 8-PSK Modulated Signals 1. Using the same setup as shown in Figure 3, measure the 8-PSK modulated signal together with its IQ signals. Set the training kit to Mode 7 for the 8-PSK modulation. The measured signal is shown below. Modulated 8-PSK Signal Figure 8 Measured IQ and Modulated 8-PSK Waveform in Oscilloscope 2. Based on the baseband data in Figure 8, what are the binary bit streams going into the modulator? How many bits are represented in a symbol in the 8-PSK modulation? 3. Using the mapping table in Figure 2, verify the measured I and Q signals based on the baseband data identified in step 2. The bit streams are: 010 110 001 010 011 101 100 111 000 101 111 110 101 000 101 Q I Symbol clock Baseband data Data clock 1 and half symbol delay S3 S7 S2 S3 S4 S6 S5 S8 S1 S6 S8 S7 S6 S1 S6 I 0 0 0.7A 0 0.7A -A 0.7A A 0.7A 0 0.7A A 0.7A Q A -A 0.7A A 0.7A 0.7A 0 0 A 0.7A 0 0.7A 0 1 0 1 1 0 Figure 9 Measured I and Q Signals Based on 8-PSK Mapping in 3 Bits per Symbol Note that the measured I and Q have a 1 symbol delay PGT313 Digital Communication Technology Lab 3-7/19
4. In the oscilloscope, try to zoom into the waveforms to correlate the phase change with the baseband IQ data. An example is shown in Figure 10 to illustrate the phase change with the I and Q data. I = A Q = 0 Acos( t+90º) I = 0 Q = A Acos( t+45º) I = 0.7A Q = 0.7A Bit 1 Bit 2 Bit 3 Figure 10 Measured IQ and 8-PSK Signals in Oscilloscope with Zoomed Time Base PGT313 Digital Communication Technology Lab 3-8/19
4. Measuring the QPSK modulated signal in 89600 VSA Software 1. Make the following connections in the ME1110 training kit. Connect the Output RF port of the training kit to the Channel 1 in the oscilloscope. At the rear panel of the oscilloscope, use an USB-to-USB cable to connect a PC to the oscilloscope. Set the training kit to Mode 6 for the QPSK modulation. PC running 89600 VSA software MSOX 3000 Series Oscilloscope USB-USB connection Ch 1 IQ modulator Digital Baseband Data in FPGA DAC I-Signal Q-Signal DAC Symbol Clock Low Pass Filter LO 90 0 Low Pass Filter I Q Filter ME1110 Training Kit Amplifier Output RF Figure 11 Test Setup for Displaying the Modulated Signal in VSA Software 2. Configure the recommended oscilloscope to interface with the 89600 VSA software (via USB port). Refer to Lab 2 (Section 3, Step 2) for the detailed procedure on how to select the USB controller in the MSOX 3000 series oscilloscope. 3. Verify the oscilloscope connection by using the Agilent Connection Expert and ensure that the instrument and its VISA address are detected. Refer to Lab 2 (Section 3, Step 3). 4. Launch the 89600 VSA software. The VSA will be initialized and the detected hardware is shown below. The oscilloscope front panel is now disabled. PGT313 Digital Communication Technology Lab 3-9/19
Figure 12 Detected Hardware Connected to VSA Software 5. Configure the VSA as follows (Refer to Lab 1, Section 3, Step 5-12): VSA Software Window Utilities > Hardware > Discovered Instruments Utilities > Hardware > Configurations Display format MeasSetup > Frequency Action Check whether Keysight MSOX 3000 series scope is detected Select Keysight MSOX 3000 series scope to the assigned Analyzer (Analyzer 1, 2, or 3) Set the display layout to Single and right-click to enable the Y Auto Scale Set the center frequency to 10 MHz and the frequency span to 3 MHz 6. Change the default Vector measurement type to Digital Demod. Select MeasSetup > Measurement Type > Digital Demod to demodulate the QPSK signal. 7. Next, select MeasSetup > Digital Demod Properties to set the digital demodulation properties. In the Format tab, set the following parameters: Format = QPSK, Symbol Rate = 1 MHz, Points/Symbol = 4, and Result Length = 80 Symbols. The software will occasionally stop and prompt a warning message regarding insufficient data for digital demodulation (e.g. it cannot synchronize to carrier and bit). Ignore the warning and continue with the setup. Click the Run button. PGT313 Digital Communication Technology Lab 3-10/19
The persistence Set this to 10x to 30x the number of symbols. For BPSK we have 2 symbols. Figure 13 Setting the Demodulation Format 8. In the Filter tab, set the following parameters: Measurement filter = Rectangular and Reference Filter = Rectangular. Leave the other settings as default. Figure 14 Setting the Demodulation Filter 9. Note that we need to turn off the Pulse Search option, as the carrier is present continuously. Otherwise the software may limit the maximum symbols in the Result Length. PGT313 Digital Communication Technology Lab 3-11/19
Figure 15 Turning Off the Pulse Search Option 10. After setting the digital demodulation properties, the VSA software may override the frequency span selection. Go to the MeasSetup window to change it to the preferred value. 11. Change the display to Grid 2 2 to display the demodulated signal as shown below. Note that the EVM is approximately 3.72% with a magnitude error of approximately 2.45% and a phase error of approximately 1.60 degrees. The IQ offset is about -39.39 db. These represent the errors of the IQ modulator for the QPSK modulation. IQ constellation Symbol/error summary table Spectrum of QPSK signal Figure 16 Demodulation Display in Grid 2 2 after the QPSK Modulator 12. Set the frequency span to 1.5 MHz (with the center frequency at 10 MHz) and set the averaging to RMS (Video) by 10 counts. Measure the OBW on Trace B (Ch1 Spectrum) window. Set the percentage of the bandwidth to around 97% in order to measure the first null bandwidth or main lobe bandwidth of the spectrum. The bandwidth is approximately 1.09 MHz, as shown in Figure 17. PGT313 Digital Communication Technology Lab 3-12/19
Measuring OBW in demodulation window Figure 17 Measuring OBW for the Main Lobe of the Modulated QPSK Signal 13. Note that the bandwidth is calculated using B = 2R S, where R S is the symbol rate used in this modulation (note that for QPSK, 2 bits per symbol, symbol rate is half of the data rate). Using this formula, calculate the bandwidth of the modulated signal. Assume no pulse shaping is used in this modulation. Symbol rate of the modulated QPSK signal = MSps Bandwidth of the modulated QPSK signal = MHz Does the calculated bandwidth correlate with the measured OBW? 14. Next, on the same trace window (Trace B: Ch1 Spectrum), measure the band or channel power for the main lobe of the QPSK signal. Select Markers > Calculation, and click the check box for band power. Set the band power center frequency to 10 MHz and the span to 1 MHz (as calculated previously). Note that averaging is set to RMS (Video) by 10 counts. What is the measured band power? Band power of the modulated QPSK signal = dbm PGT313 Digital Communication Technology Lab 3-13/19
Measuring Band Power in demodulation window Main lobe bandwidth or first null to null bandwidth Figure 18 Measuring Band Power of Main Lobe for QPSK Signal 15. Set the frequency span back to 3 MHz and average to 10 counts. Change the Measurement Filter to Root Raised Cosine (RRC) and Reference Filter to Raised Cosine (RC), with Alpha/BT of 0.05. Did you notice the EVM value change? Determine the channel power, EVM, magnitude error, and phase error. Repeat the measurement for Gaussian filters. Meas Filter: Rectangular Ref Filter: Rectangular Channel Power (dbm) EVM (% rms) Magnitude Error (% rms) Phase Error (deg) Meas Filter: RRC Ref Filter: RC Alpha/BT: 0.05 Meas Filter: Gaussian Ref Filter: Gaussian Alpha/BT: 0.05 16. The simulated waveform can be recorded by selecting Control > Record. To playback the captured data, select Control > Restart. To save the captured data, select File > Save > Save Recording. Refer to Lab 1, Appendix for the detailed procedure. PGT313 Digital Communication Technology Lab 3-14/19
Exercise 1 a. Use the following settings to display the IQ modulated QPSK signal in VSA: Frequency Span 6 MHz Frequency Points 1601 Averaging Type RMS (video) with 50 counts Result Length 1000 symbols Points/symbol 5 Will the above settings provide a better accuracy for the measurement? b. What is the effect of using different filter in the measurement on the demodulated EVM? PGT313 Digital Communication Technology Lab 3-15/19
5. Measuring the 8-PSK modulated signal in 89600 VSA software 1. Using the same setup as shown in Figure 9, measure the 8-PSK modulated signal in the 89600 VSA software. Set the training kit to Mode 3 for the 8-PSK modulation. 2. Configure the VSA as follows (Refer to step 4-10): VSA Software Window Utilities > Hardware > Discovered Instruments Utilities > Hardware > Configurations Display format MeasSetup > Frequency MeasSetup > Measurement Type > Digital Demod MeasSetup > Digital Demod Properties Display format Grid 2 2 Action Check whether Keysight MSOX 3000 series scope is detected Select Keysight MSOX 3000 series scope to the assigned Analyzer (Analyzer 1, 2, or 3) Set the display layout to Single and right-click to enable the Y Auto Scale Set the center frequency to 10 MHz and the frequency span to 3 MHz To demodulate the 8-PSK signal Set Format to 8PSK, Symbol Rate to 1 MHz, Points/Symbol to 5, Result Length to 200 Symbols, Measurement filter and Reference Filter, both to Rectangular. Disable the pulse search option. 3. In Grid 2 2 display, the demodulated 8-PSK signal is shown below. Note that the EVM is approximately 3.3%, with a magnitude error of approximately 2.44% and a phase error of approximately 1.28 degrees. The IQ offset is about -47 db. These represent the errors of the IQ modulator for the 8-PSK modulation. Figure 19 Demodulation Display in Grid 2 2 after the 8-PSK Modulator 4. Set the frequency span to 1 MHz (with center frequency at 10 MHz) and set the averaging to RMS (Video) by 10 counts. Measure the OBW on Trace B (Ch1 Spectrum) window. Set the percentage of the bandwidth to around 97% in order to measure the first null bandwidth or the main lobe bandwidth of the spectrum. The bandwidth is approximately 700.674 khz as shown in Figure 20. PGT313 Digital Communication Technology Lab 3-16/19
Main lobe bandwidth or first null to null bandwidth Figure 20 Measuring OBW for the Main Lobe of the Modulated 8-PSK Signal 5. Note that the bandwidth is calculated using B = 2R S = 2(R b /3), where R S is symbol rate used in this modulation (note that for 8-PSKwith 3 bits per symbol, the symbol rate is one third of the data rate). Using this formula, calculate the bandwidth of the modulated signal. Assume no pulse shaping is used in this modulation. Symbol rate of the modulated 8-PSK signal = Mbps Bandwidth of the modulated 8-PSK signal = MHz Does the calculated bandwidth correlate with the measured OBW? 6. Next, on the same trace window (Trace B: Ch1 Spectrum), measure the band or channel power for the main lobe of the 8-PSK signal. Click Markers > Calculation, and select the check box for band power. Set the band power center frequency to 10 MHz and the span to 666 khz (as calculated previously). Note that averaging is set to RMS (Video) by 10 counts. What is the measured band power? Band power of the modulated QPSK signal = dbm PGT313 Digital Communication Technology Lab 3-17/19
Measuring Band Power in demodulation window Figure 21 Measuring Band Power of Main Lobe for 8-PSK Signal 7. Set the frequency span back to 3 MHz and average to 10 counts. Change the Measurement Filter to Root Raised Cosine (RRC) and Reference Filter to Raised Cosine (RC), with Alpha/BT of 0.05. Did you notice the EVM value change? Determine the channel power, EVM, magnitude error, and phase error. Repeat the measurement for Gaussian filters. Meas Filter: Rectangular Ref Filter: Rectangular Channel Power (dbm) EVM (% rms) Magnitude Error (% rms) Phase Error (deg) Meas Filter: RRC Ref Filter: RC Alpha/BT: 0.05 Meas Filter: Gaussian Ref Filter: Gaussian Alpha/BT: 0.05 8. The simulated waveform can be recorded by selecting Control > Record. To playback the captured data, select Control > Restart. To save the captured data, select File > Save > Save Recording. Refer to Lab 2, Appendix for detailed procedure. PGT313 Digital Communication Technology Lab 3-18/19
Exercise 2 a. Use the following settings to display the IQ modulated PSK signal in VSA: Frequency Span 6 MHz Frequency Points 1601 Averaging Type RMS (video) with 50 counts Result Length 1000 symbols Points/Symbol 5 Will the above settings provide a better accuracy for the measurement? b. What is the effect of different filter used in measurement on the demodulated EVM? References [1] Digital Communications : Fundamentals and Applications, Bernard Sklar, Prentice Hall, 2 nd Edition, 2001. [2] Connected Simulations and Test Solutions Using the Advanced Design System, Application Note 1394, Literature Number 5988-6044E. [3] Agilent 6000 Series Oscilloscopes Performance Guide Using 89600 Vector Signal Analyzer Software, Application Note, Literature Number 5989-4523E. [4] Agilent Technologies Parameter Interactions, Agilent 89600 Series Online Help Documentation, Tutorial Section, Theory of Operation. Available free of charge using the 89600 VSA demo CD, Literature Number 5980-1989E. [5] 89600 Series Vector Signal Analyzers Installation and Service Guide, Publication Number 8900-90015. [6] Understanding Time and Frequency Domain Interactions in the Agilent Technologies 89400 Series Vector Signal Analyzers, Literature Number 5962-9217E. [7] Testing and Troubleshooting Digital RF Communications Transmitter Designs, Application Note 1313, Literature Number 5968-3578E. PGT313 Digital Communication Technology Lab 3-19/19