How does a 5G network decide which modulation scheme to use?
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The network does not use the highest modulation order all the time.
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It continuously balances radio-link reliability with data capacity.
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This animation follows the complete journey from coded bits to radio symbols.
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01 — π/2-BPSK
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π/2-BPSK can be used as an uplink option with transform precoding in applicable configurations.
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It carries one coded bit per modulation symbol and supports power-efficient transmission under challenging radio conditions.
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02 — QPSK
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QPSK carries two coded bits per modulation symbol.
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Its constellation points are widely separated, making it relatively robust against noise and interference.
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It is commonly associated with difficult radio conditions, coverage-limited users and cell-edge operation.
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03 — 16-QAM
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16-QAM carries four coded bits per modulation symbol.
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It provides a practical balance between robustness and capacity when the radio channel becomes cleaner.
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04 — 64-QAM
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64-QAM carries six coded bits per modulation symbol.
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The constellation points are positioned closer together, so the receiver needs better signal quality to distinguish them reliably.
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05 — 256-QAM
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256-QAM carries eight coded bits per modulation symbol.
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It provides higher spectral efficiency, but the dense constellation requires strong SINR, low distortion and accurate channel estimation.
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FROM BITS TO RADIO SYMBOLS
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The simplified processing journey is:
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Coded bits
→ Modulation mapper
→ I/Q symbols
→ OFDM and IFFT processing
→ RF channel
→ UE demapper
→ Recovered bit information
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The modulation mapper groups coded bits and converts them into complex I/Q constellation points.
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These symbols are placed onto OFDM resource elements and transmitted across multiple orthogonal subcarriers.
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The radio channel then introduces:
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• Noise
• Interference
• Fading
• Phase errors
• Amplitude distortion
• Multipath propagation
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At the receiver, the UE estimates the channel, evaluates the received constellation and calculates soft information for the channel decoder.
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LINK ADAPTATION
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The scheduler uses channel-quality information and implementation-specific algorithms to select an appropriate Modulation and Coding Scheme.
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When radio quality improves:
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• Higher-order modulation becomes possible
• More bits can be carried per symbol
• Spectral efficiency can increase
• User throughput can improve
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When radio quality deteriorates:
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• Constellation points become harder to distinguish
• Error probability increases
• BLER can rise
• Retransmissions may increase
• The scheduler selects a more robust MCS
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The objective is not simply to maximize modulation order.
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