Resource Block

Description

A Resource Block (RB) is the smallest element of radio resources that can be allocated to a user by the scheduler in the LTE and NR air interfaces. It represents a two-dimensional allocation in the time and frequency domains. In LTE, the definition is fixed: one Resource Block in the frequency domain is 12 consecutive subcarriers, each with a spacing of 15 kHz, resulting in a total bandwidth of 180 kHz. In the time domain, one Resource Block spans one slot, which is 0.5 ms (7 OFDM symbols for normal cyclic prefix). Therefore, the basic LTE Resource Block is a grid of 12 subcarriers x 7 symbols (84 Resource Elements for normal CP). The network scheduler allocates integer numbers of these RBs to different UEs in each 1 ms Transmission Time Interval (TTI), which comprises two slots.

In NR, the concept is more flexible to support diverse spectrum bands and use cases. An NR Resource Block is defined as 12 consecutive subcarriers in the frequency domain. However, the subcarrier spacing (SCS) is not fixed at 15 kHz; it can be 15, 30, 60, 120, or 240 kHz (with 480 and 960 kHz for future study). Therefore, the absolute bandwidth of an NR RB scales with the SCS (e.g., 180 kHz for 15 kHz SCS, 3.84 MHz for 240 kHz SCS). In the time domain, NR scheduling is based on slots, but the slot duration also scales inversely with the SCS (e.g., 1 ms for 15 kHz, 0.125 ms for 120 kHz). The NR physical layer is defined in terms of Resource Grids, composed of Resource Elements (one subcarrier for one OFDM symbol). A Resource Block is the grouping used for resource allocation signaling.

The allocation of RBs is dynamic and performed by the Medium Access Control (MAC) layer based on scheduling algorithms that consider channel quality indicators (CQI), buffer status, QoS requirements, and interference coordination. The Physical Downlink Shared Channel (PDSCH) and Physical Uplink Shared Channel (PUSCH) transport user data mapped onto allocated RBs. The control channels (PDCCH, PUCCH) are also mapped to specific Resource Elements, often at the edges of the carrier bandwidth. The number of RBs in a channel bandwidth defines the channel's transmission bandwidth configuration, which is always less than or equal to the total RF bandwidth to allow for guard bands.

Purpose & Motivation

The Resource Block was created to provide a standardized, granular unit for flexible and efficient spectrum sharing among multiple users in OFDMA-based systems like LTE and NR. Prior technologies like UMTS used code division multiple access (CDMA), where resources were primarily separated by spreading codes, making fine-grained frequency-domain scheduling difficult. The shift to OFDMA required a new fundamental resource unit that could be easily allocated in both time and frequency domains to exploit multi-user diversity and frequency-selective fading.

The RB solves the problem of how to partition the continuous time-frequency resource plane into manageable, allocatable chunks for scheduling, link adaptation, and signaling. It provides the building block for adaptive modulation and coding (MCS selection can be per RB group), interference management techniques like fractional frequency reuse, and carrier aggregation (where RBs can be allocated across multiple component carriers). Its fixed structure in LTE (12 subcarriers) was a design compromise to balance scheduling granularity, control signaling overhead, and implementation complexity. The more flexible RB definition in NR addresses the limitations of the LTE model, allowing efficient operation across a vast range of spectrum from sub-1 GHz to millimeter wave, and for services with vastly different latency and bandwidth requirements, such as massive IoT and ultra-reliable low-latency communications (URLLC).