What is ISI?
ISI (Intersymbol Interference) is a form of signal distortion where energy from one transmitted symbol spreads into adjacent symbol periods, causing errors in detection and demodulation. ISI is primarily caused by multipath propagation, where reflected copies of the signal arrive at the receiver with different delays, causing the time-domain representations of consecutive symbols to overlap. OFDM — the foundation of both 4G LTE and 5G NR — was specifically designed to combat ISI through the use of cyclic prefixes.
How Does ISI Work?
In a multipath channel, the transmitted signal arrives at the receiver through multiple paths with different propagation delays. If the delay spread (difference between the longest and shortest path delays) exceeds the symbol duration, the tail of one symbol overlaps with the beginning of the next, corrupting the receiver’s ability to decode either symbol correctly. OFDM addresses ISI by dividing the wideband signal into many narrowband subcarriers, each with a much longer symbol duration, and prepending a cyclic prefix (CP) to each symbol. As long as the CP duration exceeds the channel’s maximum delay spread, ISI is completely eliminated. In 5G NR, different numerologies with varying CP lengths are available to match different propagation environments.
Use Cases
OFDM system design and cyclic prefix dimensioning, 5G NR numerology selection for different deployment scenarios, equalizer design for single-carrier systems, channel model characterisation, and propagation environment analysis for network planning.
3GPP / Standards Reference
3GPP TS 38.211 (NR physical channels — CP-OFDM configuration), 3GPP TR 38.901 (Channel model for 0.5–100 GHz)
Related Terms
OFDM | CP-OFDM | Numerology | ICI | CIR
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