luokai0 Communications Expert
Before Starting
- Wired or wireless channel?
- Physical layer or protocol design?
- System design or performance analysis?
Core Expertise Areas
Information Theory
Shannon capacity: C = B log2 of 1 plus SNR, maximum error-free rate. Entropy: measure of information content, H = negative sum of p log p. Channel capacity: depends on bandwidth and SNR, fundamental limit. Shannon limit: minimum Eb/N0 for reliable communication is negative 1.6 dB.
Modulation
ASK, FSK, PSK: amplitude, frequency, phase shift keying. QAM: quadrature amplitude modulation, combines amplitude and phase, high spectral efficiency. BPSK: binary PSK, most robust, low spectral efficiency. 16-QAM, 64-QAM, 256-QAM: higher order for more bits per symbol, needs higher SNR. OFDM: orthogonal frequency division multiplexing, divides band into subcarriers.
Error Correction
Hamming codes: detect and correct single bit errors. Convolutional codes: memory in encoder, Viterbi decoding. Turbo codes: near Shannon limit, iterative decoding. LDPC: low density parity check, near Shannon limit, used in 5G NR. Polar codes: provably capacity-achieving, used in 5G control channels.
Wireless Systems
Fading: multipath causes signal amplitude variations, Rayleigh distribution. Diversity: spatial, frequency, time diversity combat fading. MIMO: multiple input multiple output, spatial multiplexing increases capacity. Massive MIMO: hundreds of antennas at base station, beamforming to users. 5G NR: new radio, sub-6 GHz and mmWave, 100 MHz bandwidth, network slicing.
Best Practices
- Always analyze link budget before designing wireless system
- Account for implementation losses in real system vs theoretical
- Choose modulation order based on actual SNR not peak SNR
- Test error correction performance under expected channel conditions
Common Pitfalls
| Pitfall | Fix |
|---|---|
| Ignoring implementation losses | Practical systems lose 2-5 dB vs theory |
| Wrong SNR definition | Clarify whether Eb/N0, Es/N0, or SNR per bandwidth |
| Insufficient link margin | Add fade margin for wireless reliability requirement |
| OFDM PAPR issue | Use clipping, predistortion, or tone reservation |
Related Skills
- signal-processing-expert
- computer-networks-expert
- electronics-expert