Transmission bandwidth configuration

Description

Transmission bandwidth configuration is a key parameter in 3GPP radio specifications, defined in documents such as 21.905, 36.101, and 38.104. It specifies the highest NR or E-UTRA transmission bandwidth allowed for uplink or downlink in a given channel bandwidth, measured in resource block (RB) units. This parameter sets the upper limit on the number of RBs that can be allocated for transmission within a specific channel bandwidth. It is a static configuration that depends on the channel bandwidth and, in NR, on the subcarrier spacing (numerology). The transmission bandwidth configuration ensures that transmissions do not exceed the channel bandwidth and comply with spectral emission requirements.

In LTE, the transmission bandwidth configuration is defined for each channel bandwidth (e.g., 1.4, 3, 5, 10, 15, 20 MHz) and specifies the maximum number of RBs that can be used. For example, a 20 MHz LTE channel has a transmission bandwidth configuration of 100 RBs, corresponding to 18 MHz of occupied bandwidth, leaving guard bands to prevent interference with adjacent channels. The configuration is used by the base station (eNodeB) to schedule resources and by the UE to determine the maximum bandwidth it can support. It is also used in RF testing to verify that equipment meets the required adjacent channel leakage ratio (ACLR) and spectrum emission mask.

In 5G NR, the transmission bandwidth configuration is more flexible due to multiple numerologies. For each combination of channel bandwidth and subcarrier spacing, there is a specific maximum number of RBs. For instance, a 100 MHz NR channel with 30 kHz subcarrier spacing supports a maximum of 273 RBs (each RB is 360 kHz, so 273 * 360 kHz = 98.28 MHz). The configuration is defined in TS 38.104 for different frequency ranges (FR1 and FR2). NR also introduces the concept of bandwidth parts (BWPs), where each BWP can have its own transmission bandwidth configuration, but the maximum is still bounded by the channel bandwidth. The transmission bandwidth configuration is critical for network planning, as it determines the peak data rates and spectral efficiency achievable in a given channel.

The parameter is also used in carrier aggregation scenarios. When multiple component carriers are aggregated, each carrier has its own transmission bandwidth configuration. The total transmission bandwidth is the sum of the configurations of all aggregated carriers, but each individual carrier must adhere to its own configuration. This allows flexible aggregation of different channel bandwidths and numerologies.

In terms of implementation, the transmission bandwidth configuration is used by the scheduler to allocate RBs. The scheduler cannot allocate more RBs than the configuration allows. It also affects the design of the RF front-end, as the UE must be capable of transmitting and receiving across the entire configuration. The configuration is signaled to the UE via system information or dedicated signaling, depending on the scenario. For example, in LTE, the UE learns the channel bandwidth from the Master Information Block (MIB), and the transmission bandwidth configuration is derived from it.

The transmission bandwidth configuration is closely related to the concept of transmission bandwidth, but while transmission bandwidth refers to the instantaneous bandwidth used, the configuration refers to the maximum allowed. This distinction is important for understanding resource allocation and RF requirements. The configuration is a static parameter that ensures predictable performance and regulatory compliance, while the instantaneous transmission bandwidth can vary dynamically based on traffic and scheduling decisions.

Purpose & Motivation

The transmission bandwidth configuration was introduced to provide a clear, standardized limit on the maximum number of resource blocks that can be used for transmission within a given channel bandwidth. It solves the problem of defining the boundary between usable spectrum and guard bands, ensuring that transmissions do not cause undue interference to adjacent channels. Without such a configuration, network equipment might attempt to use more bandwidth than allowed, leading to spectral emissions that violate regulatory requirements.

In the early days of LTE (Release 8), the configuration was essential for establishing a common understanding between base stations and UEs regarding the maximum bandwidth capabilities. It allowed UE manufacturers to design RF front-ends that could support specific channel bandwidths and configurations, and it enabled operators to plan their networks with confidence. The configuration also facilitated coexistence with legacy systems and other operators in adjacent bands.

As 5G NR was developed, the transmission bandwidth configuration was extended to support multiple numerologies and wider channel bandwidths. This flexibility is crucial for meeting the diverse requirements of 5G services, from enhanced mobile broadband to ultra-reliable low-latency communication. The configuration remains a cornerstone for spectrum management and RF conformance, ensuring that 5G networks can operate efficiently and without interference.