Recap of Last 2 Classes

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Beatrice Emily Hampton
6 years ago
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1 Recap of Last 2 Classes Transmission Media Analog versus Digital Signals Bandwidth Considerations Attentuation, Delay Distortion and Noise Nyquist and Shannon Analog Modulation Digital Modulation

2 What will we learn today Baseband, Passband Why upconvert and downconvert Pulse Shaping Filters Channel Coding

3 Baseband/Passband Baseband/DC Signal centered at 0 Hz (DC) IQ Signals Passband Signal down/up converted (moved up) to some nonzero frequency Non-zero frequency Low frequency (10-50MHz) ntermediate Frequency (IF) High frequency (Ghz) RF frequency

4 Why Downconvert? Nyquist Theorem: If a signal s bandwidth is B, it must be sampled at a frequency greater than 2B for accurate reconstruction Digitizing a 2.4 GHz signal would require a sampling rate greater than 4.8 GS/s

5 Why Downconvert? For most communication systems, only one (or a few) narrow-bandwidth channels matter The band of interest can be frequency shifted down closer to DC to reduce the necessary sampling rate

6 Shifting Frequency Mixing sin(x)sin(y) = (cos(x - y) cos(x + y)) / 2 Multiplication of the desired signal by a sinusoid at a slightly different frequency results in a difference term very close to DC, and a sum term at a higher frequency

7 Single-Stage Downconversion Uses a single mixer to mix a frequency band down to baseband or IF Simpler design means savings on hardware

8 Filtering No filtering before the mixer Unwanted signals can mix in to the desired band

9 Superheterodyne Downconversion Uses multiple mixing stages and filters to isolate the desired signal Rejects images as well as unwanted signals RF In Attenuat or Mixer Bandpass Filter Mixer Bandpass Filter 20 MHz BW Attenuat or Mixer Amp Lowpass Filter IF Out

10 IF Digitization After downconversion, the signal is now at a frequency that can be digitized relatively easily Signal can be downconverted again to baseband/dc using digital signal processing (DSP) 15 MHz IF, 20 MHz Bandwidth

the signal Complex baseband notation provides a

11 Complex Baseband Data RF signals often contain useful information in the phase as well as the amplitude of the signal Complex baseband notation provides a mathematical vector representation of a signal s phase and amplitude

12 Complex Baseband Data To obtain complex data, the signal must be quadrature sampled The signal is split, then mixed with an LO at orthogonal phases, then sampled IF in LO 0 90 ADC ADC I Q

13 Digital Downconversion Analog-domain quadrature sampling requires two ADCs and perfectly orthogonal LOs The quadrature downconversion can also occur in the digital domain IF in ADC LO 0 90 I Q

14 Digital Downconversion: Decimation Passband Digital Downconversion Decimation Baseband

15 Why Upconvert? Shannon s Theorem plays an important part Generating a signal at carrier frequency y with bandwith x requires a minimum sample rate of (y + x/2) 2 Frequency shifting or upconverting the data allows the waveform to be sampled at a much lower rate Upconversion Generation Upsampling PCI Transfer

16 Overview of RF Upconversion Very similar to downconversion, just in opposite direction from baseband up to a new carrier frequency Complex signal is mixed with LO at carrier frequency

17 Direct IQ Modulation Digital baseband waveforms are generated in software and converted to analog signals via AWG Analog differential I and Q signals are injected into the modulator. Baseband data directly mixed with LO at the carrier frequency in modulator and outputted as modulated signal at RF. I +/- L O Q +/- 90 deg. R F

18 Baseband Waveform Generation Discrete signals are generated in software as separate I and Q waveforms The combined signals of I and Q contain the modulating message signal centered around 0 Hz. Waveforms are sampled based on Shannon s Theorem and is a function of the signal bandwidth

19 IQ Impairments Direct upconversion process imperfect in practice I and Q Signals are not perfectly balanced Mixing process is not completely orthogonal

20 Baseband Waveform Generation Baseband waveform generation for the superheterodyne architecture is identical to the direct upconversion process:

21 IQ to IF Upconversion At the initial stage, the IQ signal is centered around 0 Hz and consequently has a lower sample rate Signal is upconverted or frequency shifted to an IF (intermediate frequency) using digital signal processing (DSP) in the AWG NI PXIe-5672: 25 MHz IF, 20 MHz Bandwidth

22 Downconversion Digitizer (DDC) Demodulation Channel Decoding Source Decoding Source Coding Channel Coding Modulation AWG (DUC) Upconversion A. Modern Digital Communication Systems Communications Channel Modulation and Demodulation

23 Digital Modulation Process Data starts out as bitstream Bit values are mapped to symbols. Ex: 2 bits per symbol value Symbols modulate carrier to form signal

Digital Modulation Review Message signal is discrete (binary or 2n-ary) Actual bit values are mapped to symbols Symbols used to modulate carrier Basic digital modulation

24 Digital Modulation Review Message signal is discrete (binary or 2n-ary) Actual bit values are mapped to symbols Symbols used to modulate carrier Basic digital modulation types are analogous to analog modulation Amplitude Shift Keying Amplitude Modulation (AM) Frequency Shift Keying Frequency Modulation (FM) Phase Shift Keying Phase Modulation (PM)

25 ASK B. Digital Modulation Methods Amplitude Shift Keying FSK Discrete Version of AM Frequency Shift Keying Discrete Version of FM PSK Phase Shift Keying Discrete Version of PM

26 Quadrature Amplitude Modulation Change both amplitude and phase of carrier based on symbol value (combination of ASK & PSK) Use MT Modulate QAM to generate QAM baseband data

27 Samples per Symbol Number of samples used to generate the waveform representation of each symbol.

28 Data Formatting Bit-packing Packet / frame definition and assembly Header (40) Coun t (16) Static (56) CRC (16) Header Static Bits Upper and lower 8 bits of count Upper and lower 8 bits of CRC

29 Channel Coding Adds redundant bits to the data stream to increase the receiver's immunity to noise and interference Example of spreading code: Spreading code Original bit stream Coded bit stream

30 Pulse Shape Filtering Bit-stream Symbols Modulated Carrier Symbols are filtered to form waveform

31 Synchronization Parameters Header (40) Coun t (16) Static (56) CRC (16) Header Static Bits Upper and lower 8 bits of count Upper and lower 8 bits of CRC

32 Continuous Data versus Burst

33 Modulation Impairments IQ Impairments I/Q Gain Imbalance DC Offset Quadrature Skew

34 Constellations Modulation quality diagnostic tool Visualize impairments (I/Q gain/offset errors, noise, skew)

35 Eye Diagrams Displays either I or Q data Illustrates quality of transition between digital states

36 Trellis Diagram Visually depicts the quality of the phase transitions between symbols

Spectral Monitoring/ SigInt

RF Test & Measurement Spectral Monitoring/ SigInt Radio Prototyping Horizontal Technologies LabVIEW RIO for RF (FPGA-based processing) PXI Platform (Chassis, controllers, baseband modules) RF hardware