User Equipment Transmission Bandwidth Configuration

Description

The UE transmission bandwidth configuration is a fundamental concept in 3GPP NR (New Radio) specifications that defines the set of resource blocks (RBs) located within the UE channel bandwidth which may be used for transmitting or receiving by the UE. It represents the actual bandwidth occupied by the UE's transmissions, as opposed to the channel bandwidth which is the maximum bandwidth supported by the UE. This configuration is specified in 3GPP TS 38.101 (UE radio transmission and reception), TS 38.104 (BS radio transmission and reception), and TS 38.521 (UE conformance specification).

In the physical layer, the UE transmission bandwidth configuration is expressed in terms of resource blocks (RBs), where each RB consists of 12 subcarriers in the frequency domain. The number of RBs that can be configured depends on the subcarrier spacing (SCS) and the channel bandwidth. For example, with a 100 MHz channel bandwidth and 30 kHz SCS, the maximum transmission bandwidth configuration is 273 RBs. The configuration is signaled to the UE via higher layer parameters such as `carrierBandwidth` in the `SCS-SpecificCarrier` information element, which includes the `subcarrierSpacing` and the `carrierBandwidth` in RBs.

The UE transmission bandwidth configuration is used by the network to allocate resources for both downlink and uplink transmissions. It ensures that the UE operates within its supported bandwidth capabilities and that the transmissions do not exceed the channel bandwidth. The configuration also affects the RF requirements such as spectrum emission mask, adjacent channel leakage ratio (ACLR), and error vector magnitude (EVM), as these are defined relative to the occupied bandwidth.

In the context of carrier aggregation (CA) and bandwidth parts (BWPs), the UE transmission bandwidth configuration is per carrier and per BWP. Each BWP can have a different transmission bandwidth configuration, allowing the UE to operate with different bandwidths dynamically. This flexibility is crucial for supporting diverse services and optimizing power consumption.

The configuration is also important for the UE's RF front-end design, as it determines the filter requirements and the linearity performance needed. The 3GPP specifications define a set of supported transmission bandwidth configurations for each frequency band and SCS, ensuring interoperability between UEs and base stations from different vendors.

Overall, the UE transmission bandwidth configuration is a key parameter that bridges the physical layer resource allocation and the RF requirements, enabling efficient and reliable communication in 5G NR networks.

Purpose & Motivation

The UE transmission bandwidth configuration exists to define the exact set of resource blocks that a UE can use for transmission and reception within a given channel bandwidth. It solves the problem of ambiguity in resource allocation by providing a clear, standardized mapping between the channel bandwidth, subcarrier spacing, and the number of usable resource blocks. This ensures that UEs and base stations can communicate effectively without exceeding spectral boundaries or violating regulatory requirements.

Historically, in LTE, the transmission bandwidth configuration was similarly defined but with fixed subcarrier spacing of 15 kHz. With the introduction of NR in Release 15, the flexibility of multiple subcarrier spacings (15, 30, 60, 120 kHz) necessitated a more dynamic definition. The UE transmission bandwidth configuration allows the network to adapt the occupied bandwidth to the service requirements and the available spectrum, supporting diverse deployment scenarios from wide-area coverage to high-capacity hotspots.

Furthermore, the configuration enables efficient use of spectrum by allowing the network to configure the UE with a bandwidth that matches the traffic demands, thereby reducing power consumption and improving spectral efficiency. It also facilitates the implementation of bandwidth parts, which are essential for supporting reduced capability UEs and for dynamic spectrum sharing.