R&D 5G PHY

Introduction to the 5G Physical Layer (PHY)
  • The Foundation of Communication: The physical layer is the bedrock of wireless communication. It handles the raw transmission and reception of signals over the air interface. Think of it as the hardware and radio-wave-focused layer of the 5G system.

Key responsibilities of the 5G PHY
  • Modulation: Encoding digital data (bits) into radio wave patterns suitable for transmission. 5G uses advanced modulation schemes like OFDM and QAM to pack more data into the available spectrum.
  • Demodulation: Recovering the original digital data from received radio signals.
  • Channel Coding: Adding redundancy to data for error detection and correction, enhancing the robustness of transmissions.
  • MIMO and Beamforming: Using multiple antennas at both transmitter and receiver to increase capacity (MIMO) or to steer signals for improved reception (beamforming).
  • Resource Allocation: Managing how the radio spectrum is divided among multiple users in both time and frequency.

Why the 5G PHY is special
  • Wider Frequency Bands: 5G leverages new spectrum in both the sub-6GHz range (FR1) and the millimeter-wave range (FR2) to offer much more capacity.
  • Massive MIMO: Significantly increased antenna counts at base stations for enhanced spatial multiplexing and beamforming.
  • Flexible Design: The 5G PHY incorporates flexibility to adapt to diverse performance demands needed for different use cases (high speed, low latency, massive connections).
Layer 1: Physical Layer
  • Function: The physical layer is the foundation of the radio protocol stack. It directly interacts with the radio hardware to handle the actual transmission and reception of signals over the air interface. It's responsible for various critical functions including error correction coding, modulation and demodulation, OFDM (Orthogonal Frequency-Division Multiplexing), MIMO (Multiple Input Multiple Output) processing, and beamforming.
  • Transport Channels: These channels are the pathways for data as it moves across the physical layer. They define how data is processed and delivered to the higher layers and include channels such as the Broadcast Channel (BCH), Downlink Shared Channel (DL-SCH), Uplink Shared Channel (UL-SCH), etc.

Layer 2: Data Link LayerThis layer can be subdivided into three sublayers:
  • MAC (Medium Access Control): Manages protocol data units (PDUs) and handles multiplexing onto the physical channels, scheduling, error correction through HARQ (Hybrid Automatic Repeat Request), and dynamic allocation of radio resources.
  • RLC (Radio Link Control) ensures the reliable delivery of data. It is responsible for segmenting and reassembly of long SDUs (Service Data Units) and providing error correction through ARQ (Automatic Repeat Request).
  • PDCP (Packet Data Convergence Protocol) Compresses and decompresses the headers of IP packets and performs encryption and data integrity protection.

Layer 3: Network Layer
  • RRC (Radio Resource Control): This sublayer manages the control plane signalling between the UE (User Equipment) and the network, including the configuration of the lower layers, system information broadcasting, session management, and mobility management.
  • Multiple Access:Uses Orthogonal Frequency Division Multiplexing (OFDM) with a cyclic prefix (CP) for efficient data transmission.Supports Discrete Fourier Transform-spread OFDM (DFT-s-OFDM) for uplink.Employs both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) for paired and unpaired spectrum access.
  • Resource Blocks:Uses resource blocks, each spanning 12 subcarriers, as the basic unit for resource allocation.This agnostic approach adapts to various spectrum allocations.
  • Frame Structure:Radio frame has a duration of 10 milliseconds (ms) and consists of 10 sub-frames (each 1 ms).Sub-frames are further divided into slots (each with 14 symbols).
  • Physical Channels:Downlink channels: PDSCH (shared), PDCCH (control), PBCH (broadcast).Uplink channels: PRACH (random access), PUSCH (shared), PUCCH (control).Sidelink channels: PSBCH (broadcast), PSCCH (control), PSFCH (feedback), PSSCH (shared).
  • Modulation:Downlink: QPSK, 16QAM, 64QAM, 256QAM, 1024QAM.Uplink:OFDM with CP: QPSK, 16QAM, 64QAM, 256QAM.DFT-s-OFDM with CP: π/2-BPSK, QPSK, 16QAM, 64QAM, 256QAM.
  • Channel Coding:Uses quasi-cyclic LDPC codes for data channels (PDSCH, PUSCH).Employs Polar coding for control channels (PDCCH, PUCCH, PBCH).
  • Physical Layer Procedures:Handles various procedures like cell search, power control, synchronization, random access, HARQ (error correction), beam management, sidelink communication, and channel access.Implicitly supports interference coordination through resource control.
  • Physical Layer Measurements:UE and network measure radio characteristics (e.g., signal strength, timing) and report them to higher layers for tasks like handover and resource management.
38.202 - Services Provided by the Physical Layer This specification defines the interface and services offered by the physical layer to the higher layers of the 5G NR protocol stack. It outlines how data and signaling messages are conveyed to and from the physical layer, ensuring that higher-level processes can effectively utilize the lower-level functions for communication and control.38.212 - Multiplexing and Channel Coding 38.212 focuses on how data is organized (multiplexed) and protected (coded) for robust transmission across the air interface. It details the use of multiplexing techniques to combine multiple streams of data and channel coding schemes like LDPC (Low-Density Parity-Check) and Polar codes to correct errors that may occur during transmission, improving the reliability of data transfer.38.211 - Physical Channels and Modulation This section specifies the various physical channels used to carry data and control information in the downlink and uplink directions, as well as the sidelink for device-to-device communication. It also covers the modulation techniques employed, such as QPSK (Quadrature Phase-Shift Keying), 16QAM (16-Quadrature Amplitude Modulation), 64QAM, 256QAM, and even 1024QAM in the downlink, which define how the digital information is translated into radio waves.38.213 and 38.214 - Physical Layer Procedures for Data and Control These specifications lay out the procedures managed by the physical layer, including how the UE (User Equipment) searches for cells, adjusts its power levels, synchronizes its uplink transmission, and manages the timing of its signals. It also covers the procedures for random access (how devices initially connect to the network), HARQ (Hybrid Automatic Repeat Request for error correction), beam management (for directing signals in the most efficient way), and control of the physical layer resources for handling interference.38.215 - Physical Layer Measurements 38.215 describes the measurement protocols used by the physical layer. It details how the UE and network measure various radio conditions, such as signal quality, interference, and neighboring cell signals. These measurements are crucial for network functions like handover between cells or between different RATs (Radio Access Technologies), as well as for Radio Resource Management (RRM), which ensures that radio resources are used as efficiently as possible.
Data Flow and Processing
  1. Higher Layer Interaction: The physical layer receives data from higher layers (e.g., RRC - Radio Resource Control, PDCP - Packet Data Convergence Protocol) responsible for user plane protocol control and packet handling.
  2. Multiplexing and Channel Coding (Ref: 3GPP TS 38.211, 38.321):Multiplexing: Combines data streams from various users or logical channels onto a common physical channel for efficient transmission over the airinterface channel Coding: Applies error correction codes (e.g., quasi-cyclic LDPC codes for data channels, Polar codes for control channels) to the multiplexed data, adding redundancy to detect and potentially correct errors during transmission.

Transmission and Reception Procedures (Ref: 3GPP TS 38.213, 38.321)
  1. Physical Channel Selection: The physical layer selects appropriate physical channels based on factors like data type (control or user plane), QoS (Quality of Service) requirements, and channel conditions. Common physical channels include:Downlink:PDSCH (Physical Downlink Shared Channel): Carries user plane data from the network to the UE.PDCCH (Physical Downlink Control Channel): Transmits control information (scheduling, power control instructions) from the network to the UE.Uplink (not explicitly shown in most diagrams): PUSCH (Physical Uplink Shared Channel): Carries user plane data from the UE to the network.PUCCH (Physical Uplink Control Channel): Transmits control information from the UE to the network.
  2. Modulation (Ref: 3GPP TS 38.211, 38.321): The data undergoes modulation, converting digital bits into radio wave patterns suitable for transmission. 5G NR employs advanced modulation schemes like Orthogonal Frequency Division Multiplexing (OFDM) with various options for the number of modulation bits per symbol (QPSK, 16QAM, 64QAM, etc.).
  3. Resource Allocation: Physical layer procedures manage how radio resources (time, frequency, power) are allocated for different channels and users to optimize network efficiency and avoid interference.
  4. Synchronization: Synchronization signals are transmitted by the network to aid the UE in establishing and maintaining timing and frequency alignment for proper signal reception.

Measurement and Reporting (Ref: 3GPP TS 38.214, 38.331)
  1. Physical Layer Measurements: The UE continuously measures various aspects of the received signals, including:Reference signal received power (RSRP)Reference signal received quality (RSRQ)Signal-to-noise ratio (SINR)Other channel characteristics
  2. Reporting to Higher Layers: Measurement results are reported back to higher layers for further processing and network optimization tasks like cell selection, handover management, and radio resource control.

Definition of Uplink, Downlink Physical Channels: 38.211 
  • Uplink Channels: These channels carry data transmitted from the User Equipment (UE) to the network base station. Examples include:
  • Downlink Channels: These channels carry data transmitted from the network base station to the UE. Examples include:
  • Sidelink Channels: These channels enable direct communication between UEs without relying on the network base station. Examples include:

2. Frame Structure and Physical Resources:
  • The radio frame defines how data is organized over time in the 5G NR air interface. It's like a schedule dictating when specific physical channels can be used.
  • Physical resources refer to the basic units of time and frequency within a frame allocated to different physical channels. Think of them as building blocks for data transmission.

3. Modulation Mapping (BPSK, QPSK, etc.):
  • Modulation is the process of converting digital data (bits) into radio wave patterns suitable for transmission over the air.
  • TS 38.211 specifies various modulation schemes like Binary Phase-Shift Keying (BPSK), Quadrature Phase-Shift Keying (QPSK), 16-Quadrature Amplitude Modulation (16QAM), and higher-order QAM options.
  • These schemes influence the number of bits represented by each symbol in the radio wave, impacting data rate and transmission efficiency.

4. OFDM Signal Generation:
  • Orthogonal Frequency Division Multiplexing (OFDM) is a core technology used in 5G NR. It divides the available radio spectrum into numerous subcarriers, allowing efficient data transmission even in challenging channel conditions.
  • TS 38.211 details the generation of OFDM signals, which involves distributing data across these subcarriers.

5. Scrambling, Modulation, and Upconversion:
  • Scrambling is a technique that adds a pseudo-random sequence to the data before modulation. This helps to reduce the risk of errors caused by long sequences of identical bits.
  • Following scrambling, modulation converts the data into radio wave patterns using the chosen scheme (BPSK, QPSK, etc.).
  • Upconversion refers to shifting the modulated signal to a higher frequency range suitable for transmission over the air interface.

6. Layer Mapping and Precoding:
  • In scenarios with multiple antennas at both the transmitter and receiver (MIMO), layer mapping dictates how data streams are mapped to different antenna layers.
  • Precoding can be employed to optimize signal transmission on these multiple antennas. It involves manipulating the signals to improve channel conditions and potentially increase data rates or range.

7. Physical Shared Channel in Uplink, Downlink, and Sidelink:
  • This section of TS 38.211 defines the characteristics of the shared channels (PUSCH, PDSCH, PSSCH) used for user plane data transmission in uplink, downlink, and sidelink communication, respectively.

8. Reference Signal in Uplink, Downlink, and Sidelink:
  • Reference signals are particular known sequences embedded within the transmitted signal. They serve crucial purposes like synchronization, helping UEs and the network align their timing and frequency for proper signal reception. Channel Estimation: Allowing the network to estimate channel conditions for optimizing data transmission.

9. Physical Random Access Channel (PRACH):
  • The PRACH is a dedicated channel used by UEs to request access to the network, particularly for initial connection or after being in idle mode.

10. Primary and Secondary Synchronization Signals:
  • Primary synchronization signals are strong, readily detectable signals used for initial network discovery and coarse synchronization.
  • Secondary synchronization signals provide more precise timing information for robust signal reception.