Which modulation is best?

Depends on the channel. BPSK is most robust for noisy channels. 256-QAM maximizes throughput on clean channels. Modern systems adaptively switch based on conditions.

What is a constellation diagram?

Plot of all possible symbol states in the I-Q plane. BPSK: 2 points. QPSK: 4. 16-QAM: 16 in a grid. Distance between points determines noise tolerance.

Divides channel into thousands of narrow subcarriers, each independently modulated. Resists multipath, enables per-subcarrier adaptation, implemented via FFT. Used in Wi-Fi, 4G/5G, digital TV.

Digital Modulation

Quick Answer

Digital modulation maps bits onto carrier signal variations. Main types: ASK (varies amplitude), FSK (varies frequency), PSK (varies phase), QAM (varies both amplitude and phase). BPSK: 1 bit/symbol. QPSK: 2 bits/symbol. 16-QAM: 4 bits/symbol. 256-QAM: 8 bits/symbol. Higher-order = more data but needs better signal quality. Modern 5G uses OFDM with adaptive modulation.

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From Bits to Waveforms

Digital modulation converts binary data into carrier waveform variations. The modulator groups bits into symbols — each maps to a specific carrier state. BPSK: bit 0 → phase 0°, bit 1 → phase 180° (1 bit/symbol). QPSK groups pairs: 00 → 45°, 01 → 135°, 11 → 225°, 10 → 315° (2 bits/symbol, doubling data rate). The constellation diagram visualizes these mappings as points in the I-Q plane.

Key Formulas

ASK, FSK, and PSK: Three Basic Schemes

ASK varies carrier amplitude — OOK is the simplest form (carrier on/off). Easy but noise-sensitive. FSK switches between frequencies — used in Bluetooth and early modems, more robust. PSK switches phase angles — BPSK (2 phases, 1 bit), QPSK (4 phases, 2 bits), 8-PSK (8 phases, 3 bits). PSK offers the best balance and is the basis of most modern systems.

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QAM: Maximum Throughput

QAM uses both amplitude and phase, arranging constellation points in a grid. 16-QAM: 4 bits/symbol. 64-QAM: 6 bits/symbol. 256-QAM: 8 bits/symbol. 1024-QAM: 10 bits/symbol. Higher order packs points closer, needing higher SNR. 256-QAM needs ~27 dB SNR while BPSK works at 0 dB. Wi-Fi 6 uses up to 1024-QAM in good conditions, dropping to lower orders when noisy.

Bit Error Rate and Performance

BER = probability a received bit is wrong. For BPSK: BER = Q(√(2Eb/N0)). BPSK and QPSK have identical BER per bit. 16-QAM needs ~4 dB more than QPSK for same BER, 64-QAM ~4 dB more than 16-QAM. This predictable scaling helps designers choose modulation for their SNR budget.

OFDM: Modern Multi-Carrier Approach

OFDM divides a wide channel into thousands of narrow subcarriers, each independently modulated. Used in Wi-Fi, 4G/5G, digital TV. Handles multipath reflections gracefully — reflections that devastate single-carrier signals only affect a few OFDM subcarriers. The FFT efficiently implements modulation/demodulation of all subcarriers simultaneously. Each subcarrier adapts its modulation order based on individual SNR.

Frequently Asked Questions

Converting bits into carrier wave variations for transmission. Bits are grouped into symbols, each mapping to a specific amplitude, frequency, or phase state.

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