Negative Acknowledgement

Description

A Negative Acknowledgement (NAK or NACK) is a fundamental feedback signal in data link layer and transport layer protocols, signifying the failure to successfully receive and decode a specific data unit. In the context of 3GPP systems, it is integral to error control mechanisms like the Radio Link Control (RLC) Acknowledged Mode (AM) and the Hybrid Automatic Repeat Request (HARQ) protocol used in the Medium Access Control (MAC) layer. When a receiver (e.g., a UE or base station) detects an error in a received transport block via a failed Cyclic Redundancy Check (CRC), or identifies a gap in the sequence of received packets, it generates and transmits a NAK for that specific block. This feedback is sent back to the transmitter, typically on a dedicated control channel like the Physical Uplink Control Channel (PUCCH) or Physical Downlink Control Channel (PDCCH) for HARQ, or within RLC status reports for ARQ.

The operation involves precise identification of the missing or erroneous data. In HARQ, each transport block is associated with a HARQ process ID and a sequence number (New Data Indicator). The NAK for a specific HARQ process instructs the transmitter to retransmit the same or a complementary version (incremental redundancy) of the failed block. The receiver then combines the soft bits from the original transmission and the retransmission(s) before attempting decoding again, improving the likelihood of success. In RLC AM, the NAK is part of a STATUS PDU that lists the sequence numbers of missing RLC Service Data Units (SDUs), triggering a selective retransmission. The timing of NAK feedback is critical; HARQ operates with very tight timelines (e.g., a few milliseconds) for low-latency retransmission, while RLC ARQ operates over longer timescales to recover from residual errors after HARQ.

NAK mechanisms are a cornerstone of reliable wireless communication, compensating for the inherently error-prone radio channel. They enable adaptive error correction by responding to actual channel conditions. The efficiency of the overall system depends heavily on the accuracy and speed of NAK feedback. False NAKs (reporting loss when data was received) or missed NAKs can lead to unnecessary retransmissions or permanent data loss, respectively. Therefore, the design of the feedback channel itself is robust, often using strong channel coding. The interplay between HARQ (fast, physical-layer centric) and RLC ARQ (slower, higher-layer) creates a multi-layered defense against errors, with NAKs acting as the crucial trigger at each layer to maintain data integrity and in-order delivery for upper-layer protocols like TCP.

Purpose & Motivation

The NAK exists to solve the problem of unreliable data transmission over error-prone channels, particularly the wireless medium in cellular networks. Without a mechanism to detect and correct errors, data would be corrupted or lost, making digital communication impractical. Simple forward error correction (FEC) adds redundancy to correct a limited number of errors, but it becomes inefficient for deep fades or severe interference. The NAK enables feedback-based retransmission (ARQ), which is highly efficient because it only consumes resources when errors actually occur.

Historically, ARQ protocols like Stop-and-Wait, Go-Back-N, and Selective Repeat used positive acknowledgements (ACKs) and NAKs for flow and error control. 3GPP systems evolved this concept with HARQ, which tightly integrates FEC and ARQ. The motivation for HARQ and its associated NAK/ACK signaling was to achieve very high reliability and low latency for packet data services in 3G (HSPA) and beyond. The fast PHY/MAC-layer HARQ with NAK feedback (on the order of milliseconds) addresses rapid channel variations, providing a first line of defense. The higher-layer RLC ARQ with NAKs then handles residual errors, ensuring end-to-end reliability. This two-layer approach addresses the limitations of pure FEC (bloated overhead in good conditions) and pure ARQ (high latency due to round-trip time), creating an adaptive and robust error control system fundamental to all modern 3GPP air interfaces.

Key Features

• Signals failure to decode a specific data block or sequence number
• Triggers retransmission in ARQ and HARQ protocols
• Integral part of RLC Acknowledged Mode status reports
• Sent via physical layer control channels (e.g., PUCCH) for HARQ
• Enables incremental redundancy combining in HARQ
• Essential for reliable in-sequence delivery of data

Evolution Across Releases

Defining Specifications