Sidelink Shared Channel

Description

The Sidelink Shared Channel (SL-SCH) is a transport channel in the 3GPP LTE and NR protocol stacks dedicated to carrying user data and some control information for direct device-to-device communication, known as sidelink. It operates over the PC5 interface. Functionally analogous to the Downlink Shared Channel (DL-SCH) and Uplink Shared Channel (UL-SCH) used for network-to-device links, the SL-SCH is the workhorse for sidelink data transmission. In the transmitter's protocol stack, data from higher layers (e.g., IP packets for V2X messages) is processed by the Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), and Medium Access Control (MAC) layers. The MAC layer delivers transport blocks to the physical layer, which maps them onto the SL-SCH. The physical layer then applies channel coding (e.g., Turbo codes in LTE, LDPC in NR), modulation, and maps the coded bits to resource elements within the sidelink resource pool in the time-frequency grid. A key component associated with the SL-SCH is the Sidelink Control Information (SCI), transmitted on the PSCCH (Physical Sidelink Control Channel). The SCI carries the necessary demodulation and decoding information for the corresponding SL-SCH transmission, such as the resource allocation, modulation and coding scheme (MCS), and group destination ID. At the receiver, the process is reversed: the UE decodes the SCI first, then uses the information to locate, demodulate, and decode the SL-SCH transport block. The SL-SCH supports both broadcast and groupcast transmission modes and is designed for dynamic and efficient shared resource usage among multiple UEs in proximity.

Purpose & Motivation

The SL-SCH was created to provide a standardized, efficient, and scalable transport mechanism for user data in direct device-to-device communication, which was a foundational new capability introduced in 3GPP starting with LTE Release 12 for Proximity Services (ProSe). Prior to its introduction, device-to-device communication in cellular networks was non-standardized or relied on short-range technologies like Bluetooth or Wi-Fi Direct, which lacked the wide-area synchronization, quality of service, and seamless integration with cellular networks that 3GPP aimed to provide. The SL-SCH solves the problem of how to efficiently multiplex data from many potential transmitting UEs onto a shared radio resource pool in a controlled manner. It enabled critical use cases like public safety communication when cellular networks are unavailable (e.g., during disasters) and vehicle-to-everything (V2X) communication for automotive safety. Its evolution through subsequent releases has been driven by the need for higher reliability, lower latency, higher data rates, and more sophisticated resource allocation modes (mode 1 network-scheduled and mode 2 autonomous) to support increasingly demanding applications like autonomous driving and advanced industrial IoT.