5G NR BWP Types and BWP Operations
In 5G New Radio the cell bandwidth is expected to be large compared to LTE, but a UE’s receive
and transmit bandwidth is not necessarily required to be same as of cell bandwidth. As per 3GPP
specification 38.300, the receive and transmit bandwidth of a UE can be adjusted to a subset of
total cell bandwidth referred as BWP. This bandwidth can shrink during period of low activity to
save power; the bandwidth location can be changed to allow different services. The bandwidth
adaption can be achieved by configuring the UE with BWP(s) telling the UE which of the
configured BWPs is currently the active one.
3GPP 38.211 specifies Bandwidth Parts (BWP) as 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. To know basics about Bandwidth part please read our following post.
BWP Allocation Types
Figure below represents the different BWPs types available for a UE. Considering typical use
cases, Idle Mode BWP is smaller than Connected Mode BWPs.
Three types of BWP are available:
Initial BWP
Active BWP (UE Specific)
Default BWP (UE Specific)
Initial BWP is used to performs Initial Access Process. It includes Parameters like RMSI
(Requested Minimum System Information), CORESET* and RMSI Frequency
location/bandwidth/SCS. It can be 24~96 PRBs with different settings and relaxed to wider BWP
after RMSI decoding.
Active BWP is defined as UE specfic can also be used to BWP performs Initial Access Process. It
is the first BWP where UE starts data transfer after RRC configuration/reconfiguration. The very
first Active BWP should be different from the default BWP.
Default BWP is again UE specific BWP and configured during RRC reconfiguration, if it not
configured then it can be assumed that Intial BWP is the default BWP. Every UE would switch
back to default BWP when BWP timer expires.
Bandwidth Parts Operations during Initial Access
The BWP parameters are used to configure the mobile operator between the UE and the cell.
According to 3GPP TS
38.331 for each serving cell the network configures at least an initial bandwidth part, comprising
of downlink
bandwidth part and one (if the serving cell is configured with an uplink) or two (if using
supplementary uplink
– SUL) uplink bandwidth parts. Furthermore, the network may configure additional uplink and
downlink
bandwidth parts.
The bandwidth part configuration is split into uplink and downlink parameters as well as into
common and
dedicated parameters. Common parameters (in BWP-UplinkCommon and BWP-
DownlinkCommon) are “cell
specific” and the network ensures the necessary alignment with corresponding parameters of other
UEs. The
common parameters of the initial bandwidth part of the PCell are also provided via system
information. For all
other serving cells, the network provides the common parameters via dedicated signaling.
Step Stage DL BWP UL BWP Processing
PSS and SSS
0 DL Synchronization
Decode
UE decode MIB and get
1 MIB decode
CORESET #0 configuration
CORESET Get Initial DL-BWP and Initial UL-BWP
2 RMSI decode
#0 setting for RMSI decoding
Msg-1-UE >— Initial UL-
3 Random Access Request to gNB
—> gNB BWP
Msg-2-UE <—– CORESET
4 Random Access Response (RAR) gNB
< gNB #0
Msg-3-UE >— Initial UL-
5 RRC connection request
—> gNB BWP
RRC connection setup
Msg-4-UE <—– CORESET Configure UE specific BWP (default/1st
6
< gNB #0 active/ other) BWP
If not configured, still use initial BWP
RRC set-up completed
Msg-5-UE >— 1st Active 1st Active
7 Initial BWP is the 1st Active BWP if no
—> gNB BWP BWP
additional configuration carried in Msg4
BWP Activation/Deactivation and Switching
The traffic patterns within one active data session can change frequently as the data rate may
increase or decrease based on the type of service or the user behavior (accessing the internet and
answering a phone call for example). It becomes very important to quick switch between different
bandwidth parts to manage different power consumption for different data rates.
According to TS 38.321 BWP selection and switching can be done with different mechanisms as
listed below:
RRC-Based Adaptation: It is more suitable for semi-static cases since the processing of
RRC messages requires extra time, letting the latency reach ~10 msec. Due to longer
switching latency and signaling overhead, a RRC-based method can be used for
configuring a BWP set at any stage of the call, or for slow adaptation type services (e.g.,
voice) where the resource allocation is not changing rapidly within the same data session.
MAC CE (Control element):It is used upon initiation of Random Access procedure
DCI-Based Adaptation: It is based on PDCCH channel where a specific BWP can be
activated by BWP indicator in DCI Format 0_1 (UL Grant) and Format 1-1 (DL
scheduling). This method better fits on-thefly BWP switching as using this method the
latency is as low as 2 msec. However, this method requires additional considerations for
error handling as UE may fail to decode the DCI with BWP activation/deactivation
command.
Timer-Based implicit fallback to default BWP is a mechanism designed to mitigate
possible DCI errors. If the UE is not explicitly scheduled with a BWP after the timer
expires, it will automatically switch to the default BWP.
