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Lecture 4: Channel Correction Mechanisms
1. Forward Error Correction (FEC)
Allows the receiver to detect and correct errors without requesting retransmission (unlike ARQ).
- Redundancy: Adds parity bits to the original data.
- Common Codes:
- Hamming: Single-bit correction.
- Reed-Solomon: Burst error correction (CDs, DVDs).
- LDPC: Low-Density Parity-Check (Wi-Fi 6, 5G).
- Convolutional: Mobile and deep-space comms.
2. Adaptive Equalization
Signal processing to counteract Inter-Symbol Interference (ISI) and multipath fading.
- How: Dynamically adjusts a filter at the receiver to match the channel's changing state.
- Algorithms:
- LMS (Least Mean Squares): Low complexity, used in Wi-Fi.
- RLS (Recursive Least Squares): Fast convergence, used in Satellite.
- CMA (Constant Modulus Algorithm).
3. Diversity Techniques & MIMO
Reduces fading by sending multiple versions of the same signal.
- Time Diversity: Different time slots.
- Frequency Diversity: Multiple frequencies (OFDM, FHSS).
- Spatial Diversity (MIMO): Multiple antennas at TX and RX.
- Polarization Diversity: Horizontal/Vertical waves (Satellite TV).
MIMO (Multiple Input Multiple Output)
- 2x2 MIMO: 2 TX, 2 RX antennas.
- Massive MIMO: Large antenna arrays (5G).
- Beamforming: Focusing the signal towards the specific receiver to reduce interference.
4. Adaptive Modulation & Coding (AMC)
Dynamically switches Modulation and FEC rates based on Channel Quality Indicator (CQI).
| Channel Quality | Modulation | Data Rate | Reliability |
|---|---|---|---|
| High CQI (Good) | Very High | Low | |
| Medium CQI | Moderate | Moderate | |
| Low CQI (Bad) | Low | Very High |
Summary
- FEC: Corrects errors on the fly.
- Equalization: Fixes timing/shape (ISI).
- MIMO: Increases capacity and robustness via antennas.
- AMC: Optimizes speed based on real-time signal strength.