Digital Communication System Notes
**Chapter 1: Introduction to Digital Communication**
- **Digital vs. Analog Communication**: Digital systems are less prone to noise, offer better security, and support easier multiplexing and error correction.
- **Elements of Digital Communication System**: Includes Source, Encoder, Modulator, Channel, Demodulator, Decoder, and Destination.
- **Sampling Theorem**: States that a signal can be completely represented by samples taken at twice its highest frequency (Nyquist rate).
- **Quantization & Quantization Error**: Quantization maps analog values to a finite set of levels, introducing quantization noise.
- **Pulse Code Modulation (PCM)**: A method to digitally represent sampled analog signals. Steps include sampling, quantization, and encoding.
- **Differential PCM (DPCM)**: Encodes the difference between successive samples to reduce redundancy.
- **Delta Modulation (DM)**: Simplified form of DPCM where only the change (increase/decrease) is transmitted.
- **Adaptive Delta Modulation (ADM)**: Adjusts step size based on signal activity to improve performance.
**Chapter 2: Source Coding Techniques**
- **Entropy**: Measures the average information content per symbol.
- **Source Coding Theorem**: No source can be compressed to a rate below its entropy without losing information.
- **Huffman Coding**: Variable-length prefix code based on symbol probabilities to minimize average code length.
- **Shannon-Fano Coding**: Another prefix coding technique, less efficient than Huffman.
- **Run Length Encoding**: Compresses data by replacing sequences with value and count.
- **Arithmetic Coding**: Represents the entire message as a fraction, allowing high compression efficiency.
- **Lempel-Ziv Algorithm**: Dictionary-based compression without prior knowledge of source statistics.
**Chapter 3: Digital Modulation Techniques**
- **ASK, FSK, PSK**: Modulation techniques using amplitude, frequency, and phase changes respectively.
- **Binary PSK (BPSK)**: Represents data using two phases differing by 180 degrees.
- **Quadrature PSK (QPSK)**: Uses four phase shifts to transmit 2 bits per symbol.
- **Differential PSK (DPSK)**: Avoids phase ambiguity by encoding data as phase differences.
- **M-ary Modulation**: Techniques like M-PSK and M-QAM transmit multiple bits per symbol.
- **Coherent Detection**: Requires carrier phase synchronization, offers better performance.
- **Non-Coherent Detection**: Easier to implement but less efficient.
- **Bandwidth & Power Efficiency**: Trade-offs between data rate, signal bandwidth, and power consumption.
**Chapter 4: Transmission Over Bandlimited Channels**
- **Inter Symbol Interference (ISI)**: Overlapping of symbols in time due to band-limiting.
- **Nyquist Criterion**: A pulse satisfying this ensures zero ISI at sample points.
- **Raised Cosine Filter**: Common pulse shaping filter to control bandwidth and ISI.
- **Eye Diagrams**: Visual tool to analyze ISI, jitter, and timing errors.
- **Equalization**: Compensates for channel-induced distortion.
- **Linear Equalizer**: Uses linear filter to mitigate ISI.
- **Decision Feedback Equalizer**: Uses past detected symbols for better ISI cancellation.
**Chapter 5: Error Control Coding**
- **Error Types**: Single-bit, burst errors.
- **Linear Block Codes**: Encode k bits into n-bit codewords.
- **Hamming Code**: Detects and corrects single-bit errors.
- **Cyclic Redundancy Check (CRC)**: Polynomial-based method to detect burst errors.
- **Convolutional Codes**: Adds redundancy using shift registers and modulo-2 adders.
- **Viterbi Algorithm**: Optimum decoding for convolutional codes based on maximum likelihood.
**Chapter 6: Information Theory**
- **Information**: Measured in bits, indicates uncertainty removed.
- **Entropy**: H(X) = -Σp(x)log₂p(x), max when all symbols equally likely.
- **Mutual Information**: I(X;Y) = H(X) - H(X|Y), measures shared info between input/output.
- **Channel Capacity**: Maximum rate of error-free communication.
- **Shannon’s Theorem**: Capacity C = B log₂(1 + S/N) for AWGN channel.
**Chapter 7: Spread Spectrum Systems**
- **Spread Spectrum**: Signal bandwidth is spread wider than the minimum required.
- **FHSS**: Frequency changes according to pseudo-random sequence.
- **DSSS**: Spreads signal by multiplying with high-rate PN sequence.
- **CDMA**: Multiple users share same bandwidth using unique PN codes.
- **PN Sequences**: Deterministic sequences with random-like properties.
**Chapter 8: Digital Communication System Design**
- **Transmitter & Receiver Blocks**: Design includes source encoder, modulator, filter, etc.
- **Synchronization**: Aligns receiver with transmitter timing and carrier.
- **Carrier Recovery**: Extract carrier phase/frequency.
- **Timing Recovery**: Align sampling to symbol intervals.
- **BER Performance**: Bit Error Rate, a measure of system reliability.
**Chapter 9: Advanced Topics**
- **OFDM**: Divides bandwidth into many orthogonal subcarriers; resists multipath.
- **MIMO**: Uses multiple antennas to improve data rate and reliability.
- **Fading Channels**: Signal variations due to movement or obstacles.
- **Turbo Codes**: Iterative error correction with near Shannon-limit performance.
- **LDPC Codes**: Sparse graph-based error correcting codes.
- **Software Defined Radio**: Uses software for modulation/demodulation and other functions.
