Hybrid Automatic Repeat Request profile

Description

The HARQ profile is a configuration parameter set used in 3GPP systems to control the behavior of Hybrid Automatic Repeat Request (HARQ) operations. HARQ is a technique that combines forward error correction (FEC) with automatic repeat request (ARQ) to improve link reliability. When a transmission is received in error, the receiver can request a retransmission, and the transmitter may send additional redundancy to help the receiver decode the original message. The HARQ profile specifies key parameters that govern this process, allowing the network to tailor HARQ operation to different quality of service requirements, radio conditions, or service types.

The HARQ profile consists of at least two attributes: a power offset attribute and a maximum number of transmissions. The power offset attribute determines the amount of additional transmit power to be used for HARQ retransmissions relative to the initial transmission. This is important because retransmissions often require more power to overcome adverse channel conditions. The maximum number of transmissions specifies the maximum number of times a transport block can be transmitted (including the initial transmission and subsequent retransmissions) before the HARQ process declares failure and passes the responsibility to higher layers (e.g., RLC). For 1.28 Mcps TDD (also known as TD-SCDMA), the HARQ profile additionally includes a retransmission timer attribute, which controls the timing of retransmissions.

HARQ profiles are used in various 3GPP access technologies, including UTRAN (HSPA) and E-UTRAN (LTE). In HSPA, the HARQ profile is configured by the RNC and signaled to the NodeB and UE. It allows different HARQ settings for different radio bearers or flows, enabling differentiated treatment. For example, a conversational voice service might use a HARQ profile with a lower maximum number of transmissions to minimize delay, while a background download might use a higher maximum to ensure reliability. The power offset can be adjusted based on the desired block error rate (BLER) target. In LTE, the concept is similar, though the parameters may be configured differently (e.g., via RRC signaling).

The HARQ profile is part of the overall radio link configuration. It is typically established during radio bearer setup and can be reconfigured during handover or when radio conditions change. The network can use HARQ profiles to optimize the trade-off between reliability, latency, and resource efficiency. For instance, increasing the maximum number of transmissions improves the chances of successful decoding but increases delay and resource usage. The power offset affects the interference level and coverage. Therefore, careful selection of HARQ profile parameters is essential for meeting QoS requirements.

In 3GPP specifications, the HARQ profile is defined in documents such as 25.309 (FDD enhanced uplink), 25.319 (enhanced uplink), 25.321 (MAC), and 25.331 (RRC). These specifications detail the signaling, configuration, and operation of HARQ profiles. The term has evolved over releases, with the addition of the retransmission timer attribute for 1.28 Mcps TDD in Release 7. This reflects the ongoing enhancements to HARQ functionality to support new features and improve performance. The HARQ profile remains a fundamental concept in the design of reliable and efficient wireless links.

Purpose & Motivation

The HARQ profile exists to provide a flexible and standardized way to configure HARQ operation in 3GPP networks. HARQ is essential for achieving high reliability and efficient use of radio resources, but its performance depends on various parameters. Without a structured profile, each vendor might implement HARQ differently, leading to interoperability issues and suboptimal performance. The HARQ profile defines a common set of parameters that can be signaled and understood across network elements, ensuring consistent behavior and enabling operators to tune HARQ for different services and conditions.

The motivation for introducing HARQ profiles was to support the diverse requirements of different applications. For example, voice services require low latency and can tolerate some errors, while data services require high reliability but can tolerate more delay. By defining multiple HARQ profiles, the network can assign appropriate parameters to each radio bearer. This allows for efficient resource utilization and better user experience. The power offset attribute helps in adapting to changing channel conditions, while the maximum number of transmissions limits the delay and prevents excessive retransmissions. The retransmission timer for 1.28 Mcps TDD addresses the specific timing requirements of that technology.

Historically, HARQ was first introduced in 3GPP Release 5 for HSDPA, and later enhanced for HSUPA in Release 6. The concept of a HARQ profile emerged as a way to manage the increasing complexity of HARQ configurations. Over releases, the profile has been refined and extended to new technologies. The addition of the retransmission timer attribute in Release 7 for 1.28 Mcps TDD is an example of how the profile evolves to meet the needs of specific access technologies. Today, HARQ profiles are an integral part of radio resource management in 3GPP networks, contributing to the high performance and reliability of mobile broadband services.