5G NR HARQ Vs ARQ Process

1. Introduction to Error Control in 5G NR

5G NR employs a layered error-control strategy to balance speed, reliability, and efficiency:

• HARQ (Hybrid Automatic Repeat Request): Operates at the MAC/Physical layer for rapid error correction.
• ARQ (Automatic Repeat Request): Functions at the RLC layer to handle residual errors and ensure end-to-end reliability.

These mechanisms work in tandem to meet the diverse requirements of 5G use cases, such as URLLC (ultra-reliable low latency) and eMBB (enhanced mobile broadband).

2. HARQ Process: Deep Dive

2.1 Core Principles

HARQ combines Forward Error Correction (FEC) with retransmissions. Key features include:

• Soft Combining: Stores corrupted data in a buffer and combines it with retransmissions to improve decoding (e.g., Chase Combining or Incremental Redundancy).
• Multiple Parallel Processes: Up to 16 HARQ processes in FDD and 32 in TDD to avoid stalling during transmission.

2.2 HARQ Types

• Synchronous HARQ: Retransmissions occur at fixed intervals (rare in 5G NR due to inflexibility).
• Asynchronous HARQ (5G default): Retransmissions are scheduled dynamically using explicit HARQ Process IDs and New Data Indicators (NDIs). Grants include HARQ Process IDs (0–15 or 0–31). NDI toggles to indicate new data vs. retransmission.

2.3 Adaptive vs. Non-Adaptive Retransmissions

• Adaptive: The gNB (base station) adjusts: Modulation and coding scheme (MCS). Resource allocation. Beamforming parameters.
• Non-Adaptive: Retransmissions use the same MCS and resources.

2.4 Timing and Feedback

• Downlink HARQ: K1 Offset: UE sends ACK/NACK after K1 slots post-PDSCH reception. K2 Offset: UE transmits uplink data K2 slots after receiving an UL grant.
• Uplink HARQ: gNB sends ACK/NACK via PHICH (in LTE) or DCI (in 5G NR).

2.5 HARQ Workflow Example

1. Initial Transmission: gNB sends Transport Block (TB) with a HARQ Process ID.
2. UE Feedback: UE decodes TB and sends ACK (success) or NACK (failure).
3. Retransmission: If NACK, gNB resends the same TB (non-adaptive) or a revised version (adaptive). UE combines the retransmission with the buffered TB using soft combining.
4. Maximum Retries: After 4–8 retransmissions (configurable), HARQ declares failure and passes the error to ARQ.

3. ARQ Process: RLC Layer Reliability

3.1 RLC Acknowledged Mode (AM)

ARQ operates in the RLC AM entity, providing:

• In-Order Delivery: Ensures packets are delivered sequentially.
• Sequence Numbering: Each RLC PDU (Protocol Data Unit) has a unique sequence number (SN).
• Sliding Window: Manages transmitter/receiver buffers (default window size = 512).

3.2 ARQ Mechanisms

• Status Reports: Receiver sends NACKs for missing PDUs. Triggered by missing SNs or periodic polling.
• Retransmission Buffer: Stores unacknowledged PDUs until ACK is received.
• Timer-Based Triggers: t-PollRetransmit: Triggers status reports if ACKs are delayed. t-Reordering: Reorders out-of-sequence PDUs before delivery to PDCP.

3.3 ARQ Workflow

1. Transmitter Side: Segments PDCP PDUs into RLC PDUs with SNs. Maintains a send window (e.g., SN 0–511).
2. Receiver Side: Detects gaps in SNs (e.g., missing SN=5). Sends a STATUS PDU listing missing SNs.
3. Retransmission: Transmitter resends missing RLC PDUs. Receiver updates the window and delivers in-order data to PDCP.

4. HARQ-ARQ Interaction: Synergy and Optimization

4.1 Layered Error Handling

• HARQ tackles short-term errors (e.g., fading, interference) with low latency.
• ARQ handles persistent errors (e.g., HARQ failure, handovers) and ensures end-to-end reliability.

4.2 Example Scenario

1. HARQ Failure: A TB fails after 4 retransmissions (HARQ limit).
2. ARQ Trigger: The RLC layer detects the gap via SNs and requests retransmission.
3. Recovery: ARQ retransmits the RLC PDU, ensuring application-layer integrity.

4.3 Efficiency Considerations

• HARQ Reduces ARQ Overhead: By resolving 90%+ errors at Layer 1, HARQ minimizes ARQ retransmissions.
• Configurable Parameters: Max HARQ Retries: Balancing latency and reliability (e.g., 4 for URLLC, 8 for eMBB). RLC Window Size: Larger windows improve throughput but increase buffer requirements.

5. Key Differences and Design Trade-offs

6. 5G NR Enhancements (3GPP Releases 16/17)

6.1 HARQ Improvements

• Codeblock Group (CBG) Retransmissions: Retransmit only failed codeblocks within a TB, reducing overhead.
• Dynamic HARQ-ACK Feedback: Configurable K1/K2 offsets for TDD and dynamic spectrum sharing.
• Pre-emption: URLLC traffic interrupts ongoing eMBB transmissions, with HARQ retransmissions resuming afterward.

6.2 ARQ Optimizations

• Reduced Status Report Size: Compressed SN feedback (e.g., bitmap formats).
• Enhanced Polling Mechanisms: Smarter triggers to avoid unnecessary signaling.