FEE Conference – Brussels – October 2013 An Introduction to Digital Modulation Presenter: Barry Hack, Aeroflex UK

1. FEE Conference – Brussels – October 2013An Introduction to Digital ModulationPresenter: Barry Hack, Aeroflex UK

2. f V= A(t) sin[2 f(t) + (t)] p V= A(t) sin[ (t)] q What do you know about Digital Comms ? Is it really different from analog modulation? Name some early systems using digital modulation? AM, Pulse PM FM (Sine wave modulation signal)

3. What do you know about Digital Comms ? Early systems employed: Sonar Morse code Semaphore Smoke signals

4. What do you know about Digital Comms ? • Other terms • Interleaving • Chipping Rate • Rake Receiver • Multi-path • Spreading Factor • Acronyms • GSM • TETRA • UMTS • BPSK • /4 DQPSK • CODEC • LTE • RBER • BCCH • MNC

5. Why do People want Digital Modulation ? • Security • Princess Diana and the Sun newspaper • Prevent eaves dropping and ‘spoof’ or ‘rogue’ users…. • Capacity • More users per piece of spectrum than analog • Less congestion • More revenue for operators • Cost -> Seller makes more money • Digital radios have less analog bits • Cheaper to produce, more reliable, easier to align

6. Why do People want Digital Modulation ? • Voice Quality • Works better where signal is weak • Roaming • Can speak over larger geographies • Emergency services can all communicate directly • GSM roaming across most countries (130+) • Immunity to interference • Capability to send voice and/or data

7. Low background noise Quality High background noise FM Range One ‘technical’ reason to move to ‘Digital’ Comparison with analog FM Digital

8. Real radio systems • They do not use one access method • They combine techniques and attempt to get the best of each (divide the users by space/location, time, frequency and/or code) • There is no “best” solution • It depends on what you are trying to achieve • voice, data • Geography • Regulatory constraints • Spectrum availability • Cost objectives • Services needed • User density • Politics

9. Baseband filter Modulator Power Amplifier Tx Filter Duplexer V.C.O. Baseband filter Demod-ulator Low Noise Amp Rx Filter The analog implementation • Information source modulates carrier directly Analog

10. A to D Speech Coder Channel Coder Baseband filters Modulator Tx Filter Duplexer V.C.O. D to A Speech Decoder Channel Decoder Filters & Equalizer Demod-ulator Rx Filter Simplified digital transceiver Digital Analog Note: TX output PA and RX LNA removed for clarity

11. f V= A(t) sin[2 f(t) + (t)] p V= A(t) sin[ (t)] q Modulation: where is the information AM, Pulse PM FM (Sine wave modulation signal)

12. Basic Digital Modulation Amplitude Frequency (FSK) Phase Both Amplitude and Phase

13. IQ Diagram: Phase and Amplitude Q+ 90° Reference Phase Mag Phase I+ 0° Origin • Magnitude is an absolute value from the origin • Phase is relative to a reference signal (from I + axis)

14. Mag Phase Phase 0 deg 0 deg Phase Modulation Amplitude Modulation 0 deg 0 deg Frequency Modulation Amplitude and Phase Digital Modulation: Signal vector

15. Modulation Measurements • Analog Systems • Power • Bandwidth • Frequency error • Modulation Accuracy (FM deviation / AM depth) • Digital Systems • Power • Bandwidth • Frequency error • Modulation accuracy (Error Vector Magnitude) • Burst Timing (Power) • Symbol Timing (Data)

16. Q Magnitude error (IQ error magnitude) Error vector Measured signal Ideal (reference signal)  Phase Error (IQ Phase error) I Modulation Accuracy • EVM is a good measure • Some systems (i.e. FSK) would use phase error only • Definition • EVM is the difference between the actual signal vector and an ideal signal vector. • Some causes of EVM • Component variations • PCB track layout • Phase Noise • Spurious signals • Modulator errors

17. Q I: Carrier p/2 I Q: IQ Modulator Causes of EVM - Example 1 • Carrier Leakage • Some of the un-modulated local oscillator bleeds across to the output • Poor screening • Poor PCB layout Carrier Leakage

18. skew Q I: Carrier p/2 I Q: IQ Modulator Causes of EVM – Example 2 • IQ Skew • The I and Q modulation paths are not exactly 90 degrees • Component tolerances • Different lengths for I and Q signal paths • Poor PCB layout Cos ( + 90 +skew)

19. Q I: Carrier p/2 I Q: IQ Modulator Causes of EVM – Example 3 • IQ Gain Imbalance • The I and Q modulation paths do not have the same gain • Component tolerances • Note: Gain and skew can be seen as AM modulation in the analog domain !

20. This angle is not 90 degrees showing skew Gain imbalance present because I and Q values are not symmetrical EVM • Example 1 Noise can be seen because of the spread of constellation points

21. EVM • Example 2 • Minor issues with carrier leak and phase noise • BUT the quality is well inside the measurement limits Vector Diagram Constellation Diagram Rotated Vector

22. Receiver Tests • Analog Systems • RSSI (Received Signal Strength Indicator) • Rx sensitivity (using SINAD measurement) • Digital Systems • RSSI (Received Signal Strength Indicator) • Rx sensitivity (using BER measurement)

23. Receiver Tests • Digital Systems • Bit Error Rate is a measure of the received bits in error as a ratio to the total received bits • Other measurements include… • RBER – Residual Bit Error Rate • FER – Frame Error Rate • MER – Message Error Rate • If you use a 1kHz test tone, SINAD and BER can give very similar answers for receiver sensitivity ….. Rx Sens = -119dBm

24. Q Q Q I I I unmodulated carrier, fc, arbitrary phase carrier, fc, with AM carrier, fc, with FM Vector representation of AM and FM • Remove carrier phase changes • Indicate relativephase changes only

25. Vector representation of AM - Slow

26. Vector representation of AM – Faster !

27. serial data stream A B C D Q 10 00 t Bit period serial/parallel conversion I I(t) 11 01 A B C D Q(t) vector phase states t Dibits Symbol period IQ Modulation Explained

28. I.sin(fc) ‘I’ signal A 1 0 Q.cos(fc) t ‘Q’ signal Bit period fc L.O. 90 º Q 10 00 1 I(t) A -sin(I), +cos(Q) I Q(t) 0 t Dibits Symbol period 11 01 IQ Modulation Explained

29. I.sin(fc) ‘I’ signal B 0 1 Q.cos(fc) t ‘Q’ signal Bit period fc L.O. 90 º sin(I), -cos(Q) Q 10 00 0 I(t) B I Q(t) 1 t Dibits Symbol period 11 01 IQ Modulation Explained

30. I.sin(fc) ‘I’ signal C 1 1 Q.cos(fc) ‘Q’ signal t Bit period fc L.O. 90 º -sin(I), -cos(Q) Q 10 00 1 I(t) C I Q(t) 1 t Dibits Symbol period 11 01 IQ Modulation Explained

31. I.sin(fc) D ‘I’ signal 0 0 t Bit period Q.cos(fc) ‘Q’ signal fc L.O. 90 º sin(I), cos(Q) Q 10 00 0 I(t) D I Q(t) 0 t Dibits Symbol period 11 01 IQ Modulation Explained

32. IQ Modulation Explained Q 10 00 I 11 01

33. Vector Timing and Synchronisation • To decide when the vector is at a symbol point you need to apply timing and synchronisation

34. QPSK Modulation – Noisy but perfect !

35. Phase offset QPSK Time offset QPSK Q Q I I Avoid zero crossings and so minimise AM Alternative Approaches to QPSK Real signals are filtered

36. A to D CODEC Audio 300-3.4kHz 8,000 13-bit samples / sec (103kbit / sec) 13kbit / sec Sampling and Speech Coding • Standard telecom data rates • 300 to 3.4kHz audio band • 8k samples / sec, 13 bits / sample (103kbit / sec) • Compressed to 8 bits / sample (64kbit / sec) with A-law or u-law compander • Even 64kbit / sec data is too high for wireless systems • CODEC • (COder-DECoder) reduces data rate by up to 80% • Several approaches: model vocal tract; code book & lookup table

37. Speech Coder Channel Coder A to D D to A Speech Decoder Channel Decoder Filters & Equalizer The Channel Coder

38. COLDFEETNEED HEAT Interleaving • Loss of a single data frame makes the entire message meaningless. How do we fix this?

39. Interleaving – Demonstration

40. Speech Coder Channel Coder A to D D to A Speech Decoder Channel Decoder Filters & Equalizer Equalisation

41. Equalisation • Mobile communications often rely on multi-path signals • How often do you actually have sight of the base-station when using a mobile phone? • The EQUALISER in the receiver • Overcomes the effects of delay spreading • Using real-time adaptive filtering implemented in DSP • Requires some prior knowledge of the received signal • i.e...... a training sequence

42. The End Any questions ?