Virtual Resource Block

Description

The Virtual Resource Block (VRB) is a fundamental concept in the LTE (E-UTRA) and NR (New Radio) physical layer specifications for resource allocation. It acts as an intermediary logical indexing scheme between the higher-layer scheduling grants and the actual Physical Resource Blocks (PRBs) that carry modulated symbols over the air. The scheduler operates on VRBs, and a mapping function, defined in the specifications, translates these VRB indices to PRB indices for transmission. This abstraction is crucial for supporting different transmission schemes, primarily localized and distributed transmission. In localized transmission, a set of contiguous VRBs is mapped directly to a set of contiguous PRBs, which is efficient for frequency-selective scheduling where the channel quality is known. In distributed transmission, a set of VRBs is mapped to non-contiguous PRBs spread across the system bandwidth. This provides frequency diversity, which is beneficial for control channels, broadcast channels, and user data when channel quality information is unreliable or for high-mobility scenarios.

The mapping process is standardized and known to both the gNB/eNB and the UE. For the downlink, the specifications define two types of VRBs: Localized VRBs and Distributed VRBs. The type is indicated in the downlink control information (DCI). The mapping for distributed VRBs involves an interleaving process to achieve the frequency separation. For the uplink in LTE, a similar concept exists with some differences, such as the use of clustered DFT-s-OFDM where VRBs can map to non-contiguous clusters of PRBs. In NR, the concept is extended with even more flexible resource allocation structures, supporting both contiguous and non-contiguous allocations for various waveform types (CP-OFDM and DFT-s-OFDM).

The VRB's role is integral to the Medium Access Control (MAC) and Physical (PHY) layer interaction. The MAC layer receives or creates scheduling decisions in terms of VRBs. The physical layer then applies the precise VRB-to-PRB mapping, adds reference signals, and performs the necessary modulation and coding. This separation of concerns simplifies scheduler design and allows for advanced features like frequency-hopping, where the PRB mapping changes over time according to a predefined pattern, further enhancing diversity or interference randomization. The size of a VRB, like a PRB, is defined as 12 consecutive subcarriers in the frequency domain for one transmission time interval (e.g., a slot or subframe).

Purpose & Motivation

The VRB concept was introduced to decouple the logical resource allocation used for scheduling from the physical resource mapping on the radio interface. Prior to LTE, systems like UMTS used dedicated channels or code-based resource allocation, which lacked the fine-grained, flexible frequency-domain scheduling that OFDMA/SC-FDMA enables. The VRB abstraction solves the problem of efficiently supporting multiple resource allocation schemes under a unified framework. It allows the network to choose between frequency-selective scheduling (using localized mapping for peak throughput) and frequency-diversity scheduling (using distributed mapping for robustness) without changing the fundamental scheduling interface.

This flexibility was a key design goal for LTE to support a wide range of deployment scenarios, user mobility states, and service requirements. For control channels like the PDCCH, distributed transmission via VRBs is essential to ensure reliable reception across the cell. For user data, the scheduler can dynamically select the best mapping based on channel quality indicators (CQI) reported by the UE. The VRB mechanism also future-proofed the design, allowing for the introduction of new transmission modes in later releases (like Transmit Diversity or Multi-User MIMO) that rely on specific PRB mappings, without impacting higher-layer protocols. It is a cornerstone of the efficient resource utilization that defines 4G and 5G radio access networks.