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Cellular Communications
3. DSP: A crash course
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Signals
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DC Signal
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Unit Step Signal
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Sinusoidal Signal
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Stochastic Signal
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Some Signal Arithmetic
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Operational Symbols
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Time Delay Operator
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Vector Space of All Possible Signals
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Shifted Unit Impulse (SUI) signals are basis for the signal vector space
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Periodic Signals Periodic Signals have another basis signal: sinusoids
Example: Building square wave from sinusoids
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Fourier Series
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Another version Fourier Series
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Complex Representation
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Parseval Relationship
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Fourier Transform Works for all analog signals (not necessary periodic) Some properties
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Discrete Fourier Transform (DFT)
FT for discrete periodic signals
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Frequency vs. Time Domain Representation
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Power Spectral Density (PSD)
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Linear Time-Invariant(LTI) Systems
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Example of LTI
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Unit Response of LTI
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Convolution sum representation of LTI system
Mathematically
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Sum up all the responses for all K’s
Graphically Sum up all the responses for all K’s
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Sinusoidal and Complex Exponential Sequences
LTI h(n)
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Frequency Response eigenvalue eigenfunction
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Example: Bandpass filter
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Nyquist Limit on Bandwidth
Find the highest data rate possible for a given bandwidth, B Binary data (two states) Zero noise on channel Example shown with band from 0 Hz to B Hz (Bandwidth B) Maximum frequency is B Hz Period = 1/B 1 Nyquist: Max data rate is 2B (assuming two signal levels) Two signal events per cycle
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Nyquist Limit on Bandwidth (general)
If each signal point can be more than two states, we can have a higher data rate M states gives log2M bits per signal point Period = 1/B 4 signal levels: 2 bits/signal 10 00 11 01 General Nyquist: Max data rate is 2B log2M M signal levels, 2 signals per cycle
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Practical Limits Nyquist: Limit based on the number of signal levels and bandwidth Clever engineer: Use a huge number of signal levels and transmit at an arbitrarily large data rate The enemy: Noise As the number of signal levels grows, the differences between levels becomes very small Noise has an easier time corrupting bits 2 levels - better margins 4 levels - noise corrupts data
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Characterizing Noise Noise is only a problem when it corrupts data
Important characteristic is its size relative to the minimum signal information Signal-to-Noise Ratio SNR = signal power / noise power SNR(dB) = 10 log10(S/N) Shannon’s Formula for maximum capacity in bps C = B log2(1 + SNR) Capacity can be increased by: Increasing Bandwidth Increasing SNR (capacity is linear in SNR(dB) ) SNR in linear form Warning: Assumes uniform (white) noise!
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Shannon meets Nyquist From Nyquist: From Shannon: Equating: or
M is the number of levels needed to meet Shannon Limit SNR is the S/N ratio needed to support the M signal levels Example: To support 16 levels (4 bits), we need a SNR of 255 (24 dB) Example: To achieve Shannon limit with SNR of 30dB, we need 32 levels
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