DSP Techniques for Software Radio DSP Front End Processing Dr. Jamil Ahmad

2 The DSP Front End DSP Front End for Software Radio Analog-to-Digital Conversion Techniques Direct Digital Synthesis

3 Up Conversion and Multiplex The DSP Front End DSP Front End Processing Digital Channel Selection Digital Multiplexing RF Front End IF Down Conversion And Digitization Frequency Band Channelization Direct Digital Synthesis ADC DAC DDC DDS RF Interface DUC Channelizer ( Base Station )

4 Data Conversion Sampling Reconstruction Voltage Mapping Analog Domain Continuous-Time Continuous Valued Signals ADC Discrete Time Continuous Valued Signals Quantization , , … Digital Domain Discrete-Time Discrete Valued Signals DAC Continuous Time Discrete Valued Signals

5 Sampling and Aliasing Why Filter Before Sampling 10KHz 0 40KHz80KHz -40KHz-20KHz20KHz60KHz

6 Sampling and Aliasing 0 10KHz-10KHz 0 10KHz 40KHz80KHz -40KHz 0 20KHz60KHz-20KHz Applying Anti-Aliasing Filter

7 Sampling Bandpass Signals Direct Sampling of BP Signals What should be the Sampling Rate? Images in Bandpass Sampling Fc-Fc 0 f

8 Bandpass Sampling Nyquist Second theorem fLfL fHfH B fcfc Minimum Sampling Rate is Twice the Bandwidth Nyquist rate achievable only at integer multiples of highest frequency and the Bandwidth of the BP Signal

9 Bandpass Sampling

10 Bandpass Sampling Consider the case where L = 5 for this case L is Odd here Whenever fH = LB, We can choose Fs = 2B to perfectly “interweave” the shifted spectral images

11 Bandpass Sampling Advantages of BP Sampling Bandpass Sampling ADC Baseband Sampling ADC LPF A BP-Sampling ADC Works like a Mixer and A Baseband-Sampling ADC

12 Hilbert Transformer (HT) What is Hilbert Transform? 90 o Phase Shifter All Pass Magnitude Characteristics

13 Hilbert Transformer -1/7 -1/5 -1/3 1 1/3 1/5 1/7

14 Hilbert Transform I/Q Conversion z -D h d (n) Discrete Hilbert Transform

15 Digital Conversion Practical System AAF Sampling Quantiza- tion Sampling Clock Analog Signal Bandlimited Analog Signal Coder Sampled Signal Quantized Signal Bit Stream

16 Digital Conversion Quantization Signal level divided into discrete steps Samples rounded to the nearest step value Introduces errors in the signal which is treated as ‘Quantization Noise’ or ‘Quantization Error’ Quantization Error depends upon the quantization step size

17 Digital Conversion - Quantization         Step Size Quantization error = Quantization Noise = Signal-to-Quantization Noise = B = Number of Bits

18 Digital Conversion Binary Coding        Two’s Comple ment Code Offset Binary Code Fraction Value / / / / / /

19 Digital Conversion - Quantization

20 Data Conversion Dynamic Range SDR utilizes wide band ADCs Critical for SDR Design to accommodate all type of analog input signals Full-Scale Range Utilization

21 Data Conversion Quantifying Dynamic Performance Harmonic Distortion Total Harmonic Distortion (THD) Thermal Noise Signal-to-Noise and Distortion Ratio (SINAD) Effective Number of Bits (ENOB) Signal-to-Noise Ratio Spurious-Free Dynamic Range (SFDR) Intermodulation Distortion (Two Tone and Multi- Tone)

22 Data Conversion Total Harmonic Distortion (THD) Signal-to-Noise and Distortion Ratio (SINAD)

23 Data Conversion Thermal Noise Effective Number of Bits (ENOB) T e = Effective Noise Temperature k = 1.38 × J/K B = Signal Bandwidth

24 Data Converters AD Bit 10MBPS ADC SINAD and ENOB at Different Input Signal Level

25 Data Conversion Spurious-Free Dynamic Range (SFDR)

26 Data Converters Improvement Techniques Dithering Out of Band Subtractive Automatic Gain Control (AGC) Response Time

27 Direct Digital Synthesis (DDS) The DSP Front-End

28 Direct Digital Synthesis (DDS) What is DDS? Digital Waveform Generator Flexibility in Control and Precision Fundamental Block in SDR No Manual Tuning

29 Direct Digital Synthesis DDS Approaches ROM Lookup Table Lookup Table Delay Frequency Word Accumulator DAC

30 DDS Approaches ROM Lookup To generate 1100Hz tone with Sampling Rate of 8KHz and 0.5Hz Resolution, Calculate i) Number of Points in LUT ii) Input Frequency Word Value

31 DDS Approaches ROM Lookup Approach Design Solution: In the Phase Wheel Required Frequency Resolution Frequency Word Value

32 DDS Approaches Issues with ROM LUT Method ROM Size directly proportional to Fs ROM Size Inversely Proportional to Frequency Resolution Memory Problem Phase Noise

33 DDS Approaches Taylor Series Approximation For small  Its true when

34 DDS Approaches Digital Oscillator Widely used in DTMF Tone Generation Issues with Higher Frequencies Z -1 a1a1