What is Digital Modulation?

Transcription

1 What is Digital Modulation? Restricts modulating baseband signal to discrete states (Digital) Project Signals to I and Q Axes Polar to Rectangular Conversion IQ Plan Shows 2 Things What the modulated carrier is doing relative to the unmodulated carrier What baseband I and Q inputs are required to produce the modulated carrier Page 1

2 Digital Format Access Schemes One User Different time - different Users FDMA Different channel - different Users TDMA Different time - different Users CDMA Same channel many users OFDM Page 2

3 Measurements on wireless systems Swept Spectrum Analyzer (with span zero and enough ResBW) Time Domain (CCDF, pulse shaping, timing) Frequency Domain (Channel Power, spectrum mask, ) Modulation Domain Overall Modulation Quality, Modulation Quality on individual carriers Channel Response, Group Delay In Channel Spurious Search Vector Signal Analyzer Page 3

4 Time Domain Page 4

5 Measurements in the modulation domain: Modulation quality A 2 TRACE A: A 2 Ch1 IQ Meas Time I-Q Const 400 m /div /di 400 m v Vector diagram (p/4 DQPSK) Constellation (with noise contamination) Page 5

6 Eye Diagrams Raised cosine =1.0 Raised cosine =0.22 Eye closure as a result of filtering Smaller alpha requires higher tolerance on clock circuits Results above use noiseless signal Page 6

7 Vector Signal Analysis: a time-domain approach RF input Filtering ADC Block-mode Demodulation (with gaps) Display Engine Signal Analysis Algorithms LO Anti-alias Gap-free demodulation GAP-FREE Time Capture Fast RAM Memory Page 7

8 Signal formats: Supports >70 signal standards and modulation types Cellular Communication Stds opt BHE opt BHD LTE TDD analysis (MIMO) LTE FDD analysis (MIMO) opt B7T cdma2000/1xev-dv analysis opt B7U WCDMA/HSPA+ analysis (MIMO) opt B7W 1xEV-DO analysis opt B7X TD-SCDMA analysis Wireless Networking Stds opt B7R a/b/g opt B7Z n WLAN MIMO opt B7S WiMax fixed opt B7Y WiMax mobile (MIMO) Other Stds opt BHB MB-OFDM analysis opt BHC RFID analysis opt BHA TEDS AND AM/FM/PM analysis Carrier types Continuous, TDMA, CDMA, OFDM opt AYA: Flexible modulation analysis Demodulators FSK: 2 to 16 & GFSK CPM MSK (including GMSK) BPSK QPSK, OQPSK, DQPSK, pi/4 DQPSK 8PSK, D8PSK, 3pi/8 8PSK (EDGE), pi/8 D8PSK QAM: 16 to 1024, absolute encoding QAM: 16 to 256, differential encoding per DVB std Star QAM: 16, 32 APSK: 16, 16w/DVB, 32, 32w/DVB VSB: 8, 16 Filter types Raised cosine, square root cosine, IS-95 compatible, Gaussian, EDGE, low pass, rectangular, half sine (Zigbee) none User defined Adaptive equalizer alpha/bt Continuously adjustable: 0.05 to 10 Single button presets Cell NADC, GSM, EDGE, EDGE E, CDMA (base), CDMA (mobile), CDPD, PDC, PHP(PHS) Wireless Net Bluetooth, HyperLan1(HBR & LBR), Zigbee (868MHz, 915MHz, 2450MHz) Digital Video DTV (8, 16) DVB (16, 32, 64) Other APCO 25, APCO 25 P2(HCPM, HDQPSK), DECT, TETRA, VDL mode 3 8

9 Works with multiple platforms Test anywhere pick the platform that meets you needs SUPPORTED PLATFORMS Signal Analyzers Other Block diagram These platforms can measure: Bandwidths: Frequencies: Baseband formats: Channels: 10Hz to >10GHz DC to 50GHz Analog & Digital SISO to 4x4 MIMO CXA, EXA*, MXA*, PXA PSA ESA Scopes 90000X*, 90000*, 8000* 7000*, 6000* Logic Analyzers RDX Simulation Software ADS/SystemVue MathWorks Simulink** *MIMO PLATFORM ** Q S* SI-LXI N7109A Signal Generators ESG** MXG** PSG** File Compatibility Binary ASCII (.csv,.txt) MatLab (.mat) VSA (.sdf) 9

10 Modulation analysis Page 10

11 Important Concepts Page 11

12 Modulation Quality Analysis EVM[%] MER[dB] Modulation Error Basic Concept: Q Ideal point Ideal Signal at decision time Error Vector Magnitude Error Vector Measured point Measured Signal at decision time Measured signal is never equal to ideal signal, due to noise, transmitter impairments, propagation phenomena, I Page 12

13 Troubleshooting a digital transmitter 13

14 Transmitter design typical impairments DSP/DAC bugs Wrong filter implementation Clock errors I/Q path errors PA non-linearity LO instability Spurs generation Page 14

15 Power Amplifier (PA) Compression How to verify? Useful measurements: ACP, CCDF With compression Without compression Compare these measurements performed: - at the input and output of the PA - at the output for decreasing values of the input level Page 15

16 I/Q Impairments I/Q impairments are typically cause by matching problems due to component differences between the I side and Q side of the block diagram Page 16

17 I/Q Impairments: I/Q gain imbalance, Quadrature errors, I/Q offsets Significant measurements: constellation and EVM metrics Page 17

18 Clock impairments Incorrect symbol rate The effect of symbol rate errors on the different measurements depends on the the magnitude of the errors: - different methods to verify small or large symbol errors Page 18

19 Incorrect symbol rate: small errors (1) Measurement: EVM vs time transmitter s symbol period receiver s symbol period Error = 1 Error = Graph of error at each sample (looks like a V ) Page 19

20 Incorrect symbol rate: small errors (2) Measurement: EVM vs time Detection and troubleshooting hint: Verify the V shape of the magnitude of the error vector versus time display Page 20

21 Incorrect Baseband Filtering: errors in the value EVM vs time shows significant errors during signal transitions between symbols Page 21

22 Local Oscillator (LO) Instability Page 22

23 Local Oscillator (LO) Instability Phase error versus time Phase error vs time Constellation with LO instability Page 23

24 Interfering tone Interfering tones are typically caused by interactions of internal signals in active devices (such as mixers and amplifiers) Page 24

25 Troubleshooting examples: QPSK transmitter with a in-channel spur Page 25

26 OFDM: Orthogonal Frequency Division Multiplexing Orthogonal subcarriers Each carrier spectrum has a Sin(x)/x shape Nulls in each carrier s spectrum land at the center of all other carriers for zero inter-carrier interference Page 26

27 Motivation for OFDM Resistant to single carrier problems like multipath Data is carried by multiple carriers, not just one High rate data rate using low symbol rates Slower symbol rate x multiple carriers = high bits/sec Spectrally efficient Tightly spaced orthogonal carriers = high bits/sec/hz Page 27

28 Example: Measuring Center Frequency Leakage Using Time Gating, Band Power, C/N Marker Band power measures power during the short sync. Time gating selects just the short sync sequence. C/N marker computes leakage at carrier #zero. Page 28

29 Measuring Modulation Quality Page 29

30 OFDM Troubleshooting with the 89601A VSA Case Study #1: Amplitude & Phase Drift Page 30

31 Pilot Tracking Compensates (Hides) Impairments Page 31

32 OFDM Troubleshooting with the VSA Case Study #2: The V-Shaped EVM Plot Pilot phase track is on, so this can t be phase noise. Page 32

33 OFDM Troubleshooting with the VSA Case Study #3: Single-Frequency Interference Carrier -24 has much higher EVM. Overall impact on EVM isn t very high. Page 33

34 OFDM Troubleshooting with the VSA Case Study #4: Power Amplifier Saturation Before Amplification After Amplification Use Time-Gated CCDF to Investigate Different Modulation Types Page 34

35 Sub-Carrier (RB) LTE Downlink Frame Structure DL N symb OFDM symbols (= 7 OFDM Normal CP) (x Ts) CP 4 CP 5 6 CP CP CP CP 1 slot etc. = Ts The Cyclic Prefix is created by prepending each symbol with a copy of the end of the symbol Time (Symbol) sub-frame = 2 slots = 1 ms RS - Reference Signal (Pilot) P-SS - Primary Synchronization Signal S-SS - Secondary Synchronization Signal PBCH - Physical Broadcast Channel PCFICH Physical Control Channel Format Indicator Channel PHICH (Normal) Physical Hybrid ARQ Indicator Channel PDCCH (L=3) - Physical Downlink Control Channel PDSCH - Physical Downlink Shared Channel = 0.5 ms #0 #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #15 #16 #17 #18 #19

36 DVB - Modulation Accuracy Measurement Use can specify the measured subcarriers range from the 0 to 1704 (2K mode) 2. MER vs. Subcarriers can show the MER result at eachsubcarrier 3. More detailed modulation analysis results can be shown in the I/Q Error view like AI/QE/SNR/CS/Phase Jitter etc Page 36