According to 3GPP specification, UE channel bandwidth is up to 100MHz for sub-6GHz bands and 400MHz for above 6GHz bands, which is much wider than the bandwidth of 20MHz in LTE.
It should be noted that UE capability of supported bandwidth varies and is often limited to the bandwidth less than the maximum bandwidth supported in the specification especially in the early stage of 5G deployment.
Besides, high UE power consumption can be a major issue if UE is required to perform transmission or reception in a wide bandwidth all the time regardless of how much the actual traffic load is.
To reduce UE power consumption and guarantee the data transmission rate, the concept of the bandwidth part (BWP) was adopted by 3GPP.
A BWP is comprised of a number of continuous physical resource blocks (PRB) with specific numerology.
For each serving cell, there are at most four BWPs can be configured for downlink (DL) or uplink (UL). Only one UL BWP and one DL BWP are active at a given time instant.
UE is not required to transmit or receive data outside an active BWP.
The BWP can be activated or de-activated by a timer, physical layer Downlink Control Information (DCI) signaling or higher layer RRC signaling.
When a large data packet needs to be transmitted, UE can be indicated to activate a BWP with a wide bandwidth. Otherwise, UE can be informed to switch to a BWP with a narrow bandwidth to save power
The above picture shows that a maximum of 4 BWP can be specified in DL and UL, but only one is active.
If UE is configured with a supplementary uplink (SUL), then the UE can be configured with four more BWPs (total 8), but only one will be active.
(Reason, in 5G Release 15, a UE can only receive one numerology at a time, therefor multiple bandwidth parts cannot be simultaneously active).
