Start and Length Indicator Value

Description

The Start and Length Indicator Value (SLIV) is a key Downlink Control Information (DCI) field in 5G New Radio (NR) used for dynamic time-domain resource allocation. Its primary function is to efficiently signal to a User Equipment (UE) the exact location and duration of its Physical Downlink Shared Channel (PDSCH) or Physical Uplink Shared Channel (PUSCH) transmission within a given slot. Instead of transmitting two separate values for the start symbol (S) and the length in symbols (L), the gNodeB encodes them into a single, compact integer value—the SLIV. The UE decodes this value using a predefined formula or table specified in 3GPP TS 38.214 to derive S and L.

The encoding rule ensures validity: the combination of S and L must define a resource that fits entirely within the slot's 14 symbols (for normal cyclic prefix). The formula is: if (L-1) ≤ 7 then SLIV = 14*(L-1) + S, else SLIV = 14*(14-L+1) + (14-1-S). The UE calculates S and L from the received SLIV by trying both possible interpretations. This joint encoding saves precious bits in the DCI payload, which is crucial for maintaining low control channel overhead, especially for smaller bandwidth parts and for scheduling many users.

SLIV is part of the Time Domain Resource Allocation (TDRA) table. The network can configure a UE with one or more TDRA entries via RRC signaling, where each entry includes parameters like mapping type, slot offset (K0/K2), and the SLIV. During dynamic scheduling, the DCI points to one of these pre-configured entries using a TDRA index field. This two-step process (RRC configuration + DCI indication) provides flexibility: common patterns can be configured semi-statically, while the DCI dynamically selects the appropriate one. SLIV directly determines the time-domain resources, which, combined with frequency-domain resource allocation (via the Frequency Domain Resource Assignment field), gives the UE its complete scheduled resource block set.

Purpose & Motivation

SLIV was created for 5G NR to address the need for highly flexible and efficient time-domain resource allocation. Unlike 4G LTE, which primarily scheduled resources in units of subframes (1 ms), 5G NR introduced a more flexible slot and mini-slot structure with variable numerology (subcarrier spacing). This allows transmissions to start at any symbol and have durations as short as one symbol. Signaling such flexibility with separate start and length fields would require too many bits in the DCI, increasing control channel overhead and reducing scheduling capacity.

The motivation was to minimize control signaling overhead while maximizing scheduling flexibility—a key requirement for 5G's diverse use cases. For enhanced Mobile Broadband (eMBB), it allows efficient packing of user data. For Ultra-Reliable Low-Latency Communications (URLLC), it enables immediate mini-slot scheduling to preempt ongoing eMBB traffic, which is facilitated by the ability to signal a transmission starting at any symbol. The joint encoding of SLIV is an elegant solution from information theory, using the constraint that S+L must be ≤ 14 to reduce the number of possible valid (S, L) combinations, thereby requiring fewer bits to signal them all. This design directly addresses the limitations of LTE's more rigid allocation and is fundamental to NR's dynamic TDD and low-latency capabilities.

Key Features

• Jointly encodes the start symbol (S) and allocation length (L) into a single integer value to save DCI bits.
• Supports flexible scheduling starting at any OFDM symbol within a slot for both downlink and uplink.
• Enables mini-slot transmissions (duration less than 14 symbols) crucial for URLLC and latency reduction.
• Uses a deterministic formula for encoding/decoding, standardized in 3GPP TS 38.214.
• Integrated into the Time Domain Resource Allocation (TDRA) framework with RRC-configured tables.
• Essential for dynamic resource sharing and preemption in 5G NR.

Evolution Across Releases

Defining Specifications