Number of Resource Blocks

Description

The Number of Resource Blocks (NRB) is a fundamental parameter in both LTE (E-UTRA) and 5G NR that defines the transmission bandwidth configuration of a carrier. A resource block (RB) is the smallest unit of resources that can be allocated to a user for a transmission time interval. In the frequency domain, one resource block consists of 12 consecutive subcarriers. Therefore, NRB specifies how many of these 12-subcarrier blocks are contiguously allocated within the carrier's channel bandwidth. The value of NRB is directly tied to the channel bandwidth; for example, in LTE, a 10 MHz channel typically corresponds to NRB = 50. The relationship is defined in the specifications, with tables mapping channel bandwidths (e.g., 1.4, 3, 5, 10, 15, 20 MHz for LTE) to their corresponding NRB values, accounting for necessary guard bands at the edges of the allocated spectrum.

Architecturally, NRB is a key input to the physical layer resource grid. The resource grid is a time-frequency matrix where the frequency dimension is defined by NRB * 12 subcarriers. This grid is where physical channels and signals, such as the Physical Downlink Shared Channel (PDSCH), Physical Uplink Shared Channel (PUSCH), and reference signals, are mapped. The base station scheduler uses NRB as a constraint, dynamically allocating subsets of these total resource blocks to different users based on their channel conditions and quality of service requirements. The total number of available resource blocks (NRB) fundamentally limits the peak data rate a carrier can support, as more RBs mean more frequency resources available for data transmission.

In 5G NR, the concept of NRB remains central but is extended with greater flexibility. While an NR resource block also comprises 12 subcarriers, the subcarrier spacing (SCS) can vary (15, 30, 60, 120, 240 kHz). The transmission bandwidth configuration NRB is defined for each SCS and channel bandwidth. Furthermore, NR introduces the concept of bandwidth parts (BWP), where a UE can be configured with a subset of the carrier's total NRB. This allows for power saving and support for devices with different bandwidth capabilities. Therefore, NRB operates at two levels: the overall carrier bandwidth configuration and the per-BWP configuration. The specifications (e.g., 38.101, 38.104) provide extensive tables defining the supported NRB values for every combination of frequency range, SCS, and channel bandwidth, ensuring global interoperability.

Purpose & Motivation

The NRB parameter exists to provide a standardized, scalable, and implementation-friendly way to define and configure the transmission bandwidth of a cellular carrier. Before such standardization, defining bandwidth in raw Hertz (Hz) would be ambiguous due to the need for guard bands and the discrete nature of OFDM subcarriers. NRB abstracts the bandwidth into discrete blocks of resources (12 subcarriers), which aligns perfectly with the scheduling and resource allocation mechanisms of OFDMA (used in downlink) and SC-FDMA (used in LTE uplink). This solves the problem of efficiently partitioning the continuous spectrum into manageable, allocable units for multiple users.

The historical context stems from the design of LTE in 3GPP Release 8, which adopted OFDMA and needed a fundamental resource unit. The resource block, and by extension NRB, was defined as this unit. It provided a common language for specifying UE RF requirements (like channel bandwidths for testing), defining physical channel structures, and implementing scheduling algorithms. As networks evolved to 5G NR, the principle remained vital but required adaptation. NR needed to support a much wider range of frequencies and bandwidths, from narrowband IoT to ultra-wideband millimeter wave channels. The NRB framework was extended with new tables and rules to cover these cases, maintaining backward compatibility in principle while enabling forward-looking flexibility. It addresses the limitation of a one-size-fits-all bandwidth definition, allowing the system to be optimized for diverse use cases from massive IoT to enhanced mobile broadband.