A Bandwidth Part is a contiguous set of physical resource blocks (PRBs) on a given carrier.
These RBs are selected from a contiguous subset of the common resource blocks for a given numerology (u). It is denoted by BWP. Each BWP defined for a numerology can have following three different parameters.
Subcarrier spacing
Symbol duration
Cyclic prefix (CP) length
Why BWP is Required?
A wider Bandwidth has direct impact on the peak and user experienced data rates, however users are not always demanding high data rate. The use of wide BW may imply higher idling power consumption both from RF and baseband signal processing perspectives. In regards to this, a new concept of BWP has been introduced for 5G-NR provides a means of operating UEs with smaller BW than the configured CBW, which makes NR an energy efficient solution despite the support of the wideband operation.
BWP concept reduce bandband processing requirement to transmit or receive narrow bandwidth
BWP enable RF-Baseband interface operation with a lower sampling rates
UE RF bandwidth adaptation can provide UE power saving at least if carrier bandwidth before adaptation is large.
While DRX reduces UE activity in the time domain, BWP aims at reducing it in the frequency domain. With BWP, it becomes possible to constrain the UE to operate within a limited portion (i.e. a part) of the frequency bandwidth of the cell. Consider a voice service in the 3.5 GHz spectrum and a cell bandwidth of 100 MHz; obviously the data flow only requires a small fraction of the allocated spectrum.With BWP, the UE does not have to monitor the full channel bandwidth and can therefore reduce its power consumption.
Several BWPs can be configured by RRC, but only one is active at a time (the active BWP). The configured BWPs can differ in width (to save power in periods of low activity), in location within the frequency domain (to increase scheduling flexibility), and in the subcarrier spacing offered (to allow different services). Switching between configured BWPs for a UE can be triggered by RRC signaling, physical layer signaling (DCI), or by the MAC sublayer itself when an inactivity timer expires, or upon initiating a random access procedure.
We distinguish between two kinds of BWPs: the initial one and the default one. The initial BWP is the BWP selected by the MAC sublayer when performing a random access, or configured by the network for an SCell in CA. The default BWP is the BWP associated with the inactivity timer by the network: when an inactivity timer is configured, the expiry of the timer switches the active BWP to the default BWP.
An example of BWP operation is depicted in Figure:
Bandwidth Parts (BWP) is a contiguous set of physical resource blocks, selected from a contiguous subset of the common resource blocks for a given numerology (µ) on a given carrier. The width of a BWP may smaller than or equal to the cell bandwidth. Every bandwidth part has a direction (uplink or downlink) and occupies a contiguous range of common resource blocks with a particular subcarrier spacing. Every cell has an initial downlink bandwidth part and an initial uplink bandwidth part. Initial BWPs are used for initial access, for example by mobiles that are accessing a primary cell from the states of RRC_INACTIVE and RRC_IDLE.
