What is Layer 1?
Layer 1 is the Physical Layer in the Open Systems Interconnect (OSI) reference model. It governs the transmission of raw data bits over a physical medium — whether copper, fibre, or radio waves. In 5G NR, Layer 1 encompasses all baseband signal processing functions including channel coding (LDPC and Polar codes), modulation (QPSK through 256-QAM), OFDM waveform generation, MIMO precoding, beamforming, and HARQ processing. The PHY layer is the most computationally intensive part of the 5G protocol stack.
How Does Layer 1 Work?
Layer 1 in 5G NR handles the translation between transport blocks (data from higher layers) and the actual RF waveform transmitted over the air. On the transmit side, it performs CRC attachment, LDPC or Polar encoding, rate matching, scrambling, modulation mapping, layer mapping, precoding, resource element mapping, and OFDM signal generation (IFFT + CP insertion). On the receive side, it performs the inverse operations plus channel estimation, equalization, and soft-decision decoding. In O-RAN architecture, PHY layer processing is split between the O-RU (lower PHY: IFFT, beamforming, CP) and the O-DU (upper PHY: coding, modulation, HARQ).
Use Cases
5G NR baseband processing in gNB and UE, O-RAN lower-layer functional split implementation, FPGA/ASIC design for PHY acceleration, PHY layer performance benchmarking, and real-time signal processing for uRLLC applications.
3GPP / Standards Reference
3GPP TS 38.211/212/213/214 (NR physical layer specifications), 3GPP TS 38.201 (NR physical layer general description)
Related Terms
Layers 2/3 | OFDM | MIMO | Numerology | gNB
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This glossary entry is part of the 5GWorldPro Complete 5G Glossary. To go deeper into 5G architecture and technology, explore our 5G Training courses.
