Half-Duplex for Sidelink Operation

Description

Half-Duplex for Sidelink Operation (HD) refers to a radio capability defined in 3GPP specifications where a User Equipment (UE) supporting sidelink communication (e.g., Proximity Services (ProSe), Vehicle-to-Everything (V2X)) is only able to either transmit or receive on the sidelink carrier at any given time, but not both simultaneously. This is in contrast to Full-Duplex (FD) operation, which would require advanced and costly self-interference cancellation techniques. The HD capability is a fundamental constraint that influences sidelink protocol design, resource allocation, and scheduling mechanisms.

Architecturally, HD operation impacts the physical layer and the Medium Access Control (MAC) layer of the sidelink interface. The UE's radio frequency (RF) and baseband processing chains are designed for a single direction at a time. This necessitates time-division between transmission and reception slots. In Mode 2 (UE autonomous resource selection), defined for V2X and later sidelink enhancements, the UE must incorporate a sensing procedure. During sensing, the UE listens to the sidelink channel to identify resources used by other UEs. Due to HD constraints, when the UE is transmitting its own data, it cannot simultaneously sense the channel, creating a 'deaf period' that must be accounted for in resource selection algorithms to avoid persistent collisions.

From a network perspective, when sidelink resource allocation is network-scheduled (Mode 1 for V2X, Mode 3 for LTE sidelink), the base station (eNodeB or gNB) must be aware of UEs' HD limitations. The scheduling grants must ensure that a UE is not scheduled to receive and transmit at overlapping times, unless advanced coordination or spatial separation is used. The specifications, particularly in TS 36.101 and 38.101 for RF requirements, define specific HD UE performance requirements for unwanted emissions, receiver sensitivity, and switching times between transmit and receive states. These requirements ensure that even with HD operation, reliable and low-latency sidelink communication is possible for critical services like autonomous driving and public safety.

Purpose & Motivation

HD sidelink operation was standardized primarily to enable low-cost, power-efficient device implementations for direct device-to-device (D2D) communication. The initial driver was Proximity Services (ProSe) in Release 12/13 for public safety and commercial discovery. Implementing full-duplex radios requires complex and expensive circuitry to manage the immense self-interference caused by the device's own transmitter overwhelming its nearby receiver. For mass-market IoT sensors, wearables, and even vehicles, this cost and complexity is prohibitive.

HD operation addresses this by accepting the limitation of non-simultaneous Tx/Rx, which is a common trait in many low-cost wireless systems (like WiFi). The challenge for 3GPP was to design a sidelink protocol within this constraint that still meets the stringent reliability and latency requirements, especially for later V2X applications. The specifications had to define new physical channels (PSCCH, PSSCH), synchronization signals (PSSS/SSSS, S-PSS/S-SSS), and resource allocation modes that function effectively with HD UEs. This design choice was crucial for the widespread adoption of 3GPP-based sidelink technology, making it feasible to embed in a vast array of devices without requiring breakthrough and expensive RF hardware.