Transmission bandwidth

Description

Transmission bandwidth is a fundamental parameter in 3GPP radio access networks, defined in specifications such as 21.905, 36.101, and 38.104. It refers to the RF bandwidth of an instantaneous E-UTRA (LTE) or NR (5G) transmission from a user equipment (UE) or base station (BS), measured in resource block (RB) units. This parameter characterizes the portion of the channel bandwidth that is actively used for transmission at any given moment. Unlike channel bandwidth, which is the total frequency range allocated to a network operator, transmission bandwidth is dynamic and can vary based on scheduling decisions, traffic load, and radio conditions.

In LTE, the transmission bandwidth is expressed as a number of resource blocks (RBs), where each RB occupies 180 kHz in the frequency domain (12 subcarriers of 15 kHz). The maximum transmission bandwidth depends on the channel bandwidth; for example, a 20 MHz LTE channel supports up to 100 RBs (18 MHz). In 5G NR, the concept is similar but more flexible due to scalable numerology. NR supports multiple subcarrier spacings (15, 30, 60, 120, 240 kHz), and the RB definition remains 12 subcarriers, but the subcarrier spacing determines the RB bandwidth. Therefore, the same number of RBs can occupy different absolute bandwidths depending on the numerology. Transmission bandwidth in NR is also measured in RBs and can be configured per bandwidth part (BWP).

The transmission bandwidth is a critical parameter for radio resource management (RRM) and scheduling. The base station (eNodeB in LTE, gNB in NR) allocates RBs to UEs in both uplink and downlink based on the required data rate, QoS, and channel conditions. The instantaneous transmission bandwidth is thus the sum of RBs allocated to all scheduled UEs at a given time. It directly impacts the achievable throughput and spectral efficiency. For example, a wider transmission bandwidth allows more data to be transmitted per unit time, but it also requires more spectrum and may be limited by UE capabilities (e.g., supported bandwidth classes).

In 3GPP specifications, transmission bandwidth is often used in the context of RF requirements. TS 36.101 and TS 38.104 define the transmission bandwidth configurations for different channel bandwidths, specifying the maximum number of RBs that can be used. These configurations ensure that transmissions stay within the channel bandwidth and meet spectral emission masks. The term is also used in testing and conformance (e.g., TS 36.141, TS 38.141) to verify that equipment operates correctly across supported bandwidths.

With the introduction of 5G NR, the concept of transmission bandwidth has become more nuanced due to bandwidth parts (BWPs). A BWP is a contiguous set of RBs within the channel bandwidth, and a UE may be configured with multiple BWPs, each with its own transmission bandwidth. This allows dynamic adaptation to traffic and power saving. Additionally, NR supports wider channel bandwidths (up to 400 MHz in FR2) and carrier aggregation, where multiple component carriers each have their own transmission bandwidth. The aggregation of these bandwidths provides higher peak data rates.

Overall, transmission bandwidth is a key enabler for flexible and efficient spectrum utilization in modern cellular networks. It bridges the gap between the static channel bandwidth and the dynamic allocation of resources to users, ensuring that transmissions are both spectrally efficient and compliant with regulatory constraints.

Purpose & Motivation

Transmission bandwidth was introduced to provide a precise, standardized way to describe the actual spectrum used by a transmission in LTE and later NR networks. Before its definition, bandwidth was often described in terms of channel bandwidth or gross data rates, which did not capture the dynamic nature of resource allocation in OFDMA systems. The parameter solves the problem of quantifying instantaneous bandwidth usage, which is essential for scheduling, link adaptation, and RF conformance testing.

The motivation for defining transmission bandwidth in Release 8 (the first LTE release) was the need for a common metric that could be used by network equipment vendors, UE manufacturers, and test houses. It allows the base station to communicate to the UE which RBs are allocated, and it enables the UE to adjust its RF front-end accordingly. It also facilitates coexistence studies and spectrum planning by clearly distinguishing between the occupied bandwidth of a transmission and the total channel bandwidth.

As networks evolved to 5G NR, the concept was extended to support multiple numerologies and bandwidth parts. This flexibility is crucial for meeting diverse service requirements, from enhanced mobile broadband (eMBB) to ultra-reliable low-latency communication (URLLC). Transmission bandwidth remains a cornerstone for efficient spectrum utilization and is integral to the design of 5G and future 6G systems.