NR Bandwidth Part
What is BWP?
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.
?BWP ::=????????????????????????????SEQUENCE {
???locationAndBandwidth???????????????INTEGER (0..37949),
???subcarrierSpacing????????????????????SubcarrierSpacing,
???cyclicPrefix?????????????????????????????ENUMERATED { extended }??????????OPTIONAL ???-- Need R
}
locationAndBandwidth
Frequency domain location and bandwidth of this bandwidth part. The value of the field shall be interpreted as resource indicator value (RIV).
subcarrierSpacing
Subcarrier spacing to be used in this BWP for all channels and reference signals unless explicitly configured elsewhere. The value kHz15 corresponds to μ=0, value kHz30 corresponds to μ=1, and so on.
cyclicPrefix
Indicates whether to use the extended cyclic prefix for this bandwidth part. If not set, the UE uses the normal cyclic prefix. Normal CP is supported for all subcarrier spacings and slot formats. Extended CP is supported only for 60 kHz subcarrier spacing.?
Why BWP?
1.???Power Saving:
UE may use a wide bandwidth when a large amount of data is scheduled, while being active on a narrow bandwidth for the remaining time.
2.???Spectrum Flexibility:
gNB may support a very wide channel bandwidth which may not be supported by some UEs. BWP provides a mechanism to flexibly assign radio resources to UE in a portion of BS channel bandwidth according to UE bandwidth capabilities.
What is the advantage of BWP?
BWP allows mobile to reduces power consumption. This eventually allows manufacturers to build inexpensive mobiles that are completely unable to communicate over the whole of the transmission band.
Types of BWP:
Initial BWP:
The initial DL and UL BWPs are used for initial access before radio resource control (RRC) connection is established. Initial BWPs are usually small/narrow and allow UE to save power.
?During the initial access, the UE performs cell search based on synchronization signal block (SSB) composed of primary synchronization signal (PSS), secondary synchronization signal (SSS), and physical broadcast channel (PBCH). To access the system, the UE needs to further read system information block 1 (SIB1) which carries important information including the initial DL/UL BWP configuration. The SIB1 is transmitted on the PDSCH, which is scheduled by downlink control information (DCI) on the PDCCH using the control resource set with index zero (CORESET #0).
Before the UE reads the SIB1, the UE’s initial DL BWP has the same frequency range and numerology as those of CORESET#0. After reading the SIB1, the UE follows the initial DL/UL BWP configuration in the SIB1 and uses them to carry out random-access procedure to request the setup of RRC connection. The network should configure the frequency domain location and bandwidth of the initial DL BWP in the SIB1 so that the initial DL BWP contains the entire CORESET #0 in the frequency domain.
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Default BWP:
For a serving cell, the network may configure the UE with a BWP inactivity timer (bwp-InactivityTimer).
The expiration of this timer indicate that the UE has no scheduled transmission and reception for a while on the currently active BWP. Thus, the UE can switch its active BWP to a default BWP to save power.
?If default DL BWP is not configured, the UE uses the initial DL BWP as the default DL BWP.
?
Dedicated BWP:
This is non-initial BWP assigned to individual UEs. Normally, these are much wider and used for payload (UL or DL data) transmission.
?When one BWP with one numerolgy is next to different BWP (of different UE) with different numerology, gNB scheduler should consider providing guardband between these BWPs to avoid Inter-Numerology-Interference.
?
First active BWP:
First active DL and UL BWPs are the active DL and UL BWPs upon RRC (re-)configuration for an SpCell or activation of an SCell.
?In a master cell group (MCG), the SpCell refers to the primary cell (PCell) in which the UE performs the connection (re-)establishment procedure.
In a secondary cell group (SCG), the SpCell refers to the primary SCG cell (PSCell) in which the UE performs random access for RRC (re-)configuration.
How many BWPs gNB can assign for a UE?
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.
The initial BWP is referred to by BWP-Id 0. The other BWPs are referred to by BWP-Id 1 to maxNrofBWPs.
downlinkBWP-ToAddModList??????????SEQUENCE (SIZE (1..maxNrofBWPs)) OF ?BWP-Downlink
uplinkBWP-ToAddModList??????????????SEQUENCE (SIZE (1..maxNrofBWPs)) OF ?BWP-Uplink
maxNrofBWPs?????????INTEGER ::= 4 ???????-- Maximum number of BWPs per serving cell
Note: At any given time, UE can communicates using one active uplink BWP and one active downlink BWP only.?
Are DL BWP and UL BWP defined together? Yes for TDD.
BWP (Non-Zero / Non-Initial BWP) Configuration:
For each DL BWP or UL BWP in a set of DL BWPs or UL BWPs, respectively, the UE is provided the following parameters for the serving cell.
- a SCS by subcarrierSpacing
- a cyclic prefix by cyclicPrefix
- a common RB and a number of contiguous RBs provided by locationAndBandwidth that indicates an offset RBstart and a length LRB as RIV
- an index in the set of DL BWPs or UL BWPs by respective BWP-Id
- a set of BWP-common and a set of BWP-dedicated parameters by BWP-DownlinkCommon and BWPDownlinkDedicated for the DL BWP, or BWP-UplinkCommon and BWP-UplinkDedicated for the UL BWP.
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The DL/UL BWP configurations are divided into common and dedicated parameters. The BWP-common parameters are cell specific, implying that the network needs to ensure that the corresponding parameters are appropriately aligned across the UEs. The BWP-dedicated parameters are UE specific.
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For the PCell, the BWP used for initial access is configured via system information. For the SCell(s), the BWP used after initial activation is configured via dedicated RRC signalling.
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Can BWPs be overlapped? In Frequency Domain, Yes. Configured UE BWPs can be overlapped in frequency domain.
Will there be any interference if BWPs are overlapped in frequency domain? No. There will not be any interference because at any given time, there would be only one active BWP for the UE.
?The BW of the BWP must be at least as large as one SS block BW, but the BWP may or may not contain SS block. The BWP cannot be larger than the maximum BW supported by the UE.
?
Each configured DL BWP includes at least one control resource set (CORESET) with UE-specific search space (USS). The USS is a searching space for UE to monitor possible reception of control information destined for the UE. In the primary carrier, at least one of the configured DL BWPs includes one CORESET with common search space (CSS). The CSS is a searching space for UE to monitor possible reception of control information common for all UEs or destined for the particular UE. If the CORESET of an active DL BWP is not configured with CSS, the UE is not required to monitor it.
Note:
The UE is not expected to receive PDSCH, PDCCH, or CSI-RS (except for RRM) outside an active bandwidth part.
The UE shall not transmit PUSCH or PUCCH, SRS outside an active bandwidth part.
BWP Adaption/Switching (BA) and Switching Delay:
The BWP switching for a Serving Cell is used to activate an inactive BWP and deactivate an active BWP at a time.
?To recap, For FDD paired spectrum, DL BWPs and UL BWPs can be switched separately. For TDD unpaired spectrum, the paired DL BWP and UL BWP are switched together.
?The BWP switching is triggered/controlled by either of below
1.???PDCCH (DCI) indicating DL assignment (DCI_1_1) or UL Grant (DCI_0_1) OR????????????????????==> via BWP indicator field
2.???bwp-InactivityTimer, OR
3.???RRC signalling, OR
4.???MAC entity upon initiation of RA (Random Access) procedure.
RRC Signaling based BWP 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.
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Total Dealy for BWP Switching by RRC?is from the time when UE receives RRC reconfiguration involving active BWP switching or parameter change of its active BWP ==> Till the time when UE shall be able to receive PDSCH/PDCCH (for DL active BWP switch) or transmit PUSCH (for UL active BWP switch) on the new BWP on the serving cell.
??
NR Slot Length is determined by the smaller SCS if the BWP switch involves changing of SCS.
TRRCProcessingDelay is the length of the RRC procedure delay in ms. ????????????????????==> Refer TS 38.331 Table 12.1-1.
TBWPSwitchDelayRRC = 6ms is the time used by the UE to perform BWP switch.
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DCI-based BWP Adaptation:
It is based on PDCCH channel where a specific BWP can be activated by BWP indicator field in DCI Format 0_1 (UL Grant) and DCI 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.
?Why Fallback DCI Formats are not used for BWP switching?
DCI format 1_1 and DCI format 0_1 are non-fallback DCI formats for downlink assignment and uplink grant, respectively. They support the full set of NR features and their fields are largely configurable. On the other hand, fallback DCI formats 1_0 and 0_0, used respectively for downlink assignment and uplink grant, do not contain BWP indicator field and thus do not support DCI-based BWP switch.
?Note: Why Fallback DCIs are called so? Fallback means less, decrease, or an alternative plan. Fallback DCI formats are more compact than the full format because it doesn’t?include all options and therefore trade off less scheduling flexibility for reduced control overhead.
?The switch delay denoted by TBWPswitchDelay for DCI-based BWP switch is defined as the slot offset between the DL slot in which the UE received switch request and the first slot in which the UE shall be able to receive PDSCH (for DL active BWP switch) or transmit PUSCH (for UL active BWP switch) on the new BWP.
?Depending on UE capability bwp-SwitchingDelay, UE shall finish BWP switch within the time duration TBWPswitchDelay defined in below table.
?
Phy-ParametersCommon ::=???????????SEQUENCE {
?????????...
bwp-SwitchingDelay??????????ENUMERATED {type1, type2}????????????????OPTIONAL,
?????????...
}
Note: The BWP switch delay is dependent on SCS. If the BWP switch happens between BWPs of different SCS values, the switch delay requirement is determined by the smaller SCS.
?Timer-based (bwp-InactivityTimer) BWP Adaption:
This is a mechanism designed to mitigate possible DCI errors. When an inactivity timer is configured for a serving cell, the expiry of the inactivity timer associated to that cell switches the active BWP to a default BWP configured by the network.
?What if default BWP is not configured for the UE? Initial BWP would become Default BWP.
A UE can be provided by defaultDownlinkBWP-Id a default DL BWP among the configured DL BWPs. If a UE is not provided a default DL BWP by defaultDownlinkBWP-Id, the default DL BWP is the initial DL BWP.
?bwp-InactivityTimer is the duration in ms after which the UE falls back to the default Bandwidth Part. When the network releases the timer configuration, the UE stops the timer without switching to the default BWP. This timer ensures that the base station can find the mobile in the event of communication problems (Ex: if the mobile has moved to the wrong downlink bandwidth part due to decode/bit errors in DCI).
?bwp-InactivityTimer???????????ENUMERATED {ms2, ms3, ms4, ms5, ms6, ms8, ms10, ms20, ms30, ms40,ms50, ms60, ms80,ms100, ms200,ms300, ms500, ms750, ms1280, ms1920, ms2560, spare10, spare9, spare8, spare7, spare6, spare5, spare4, spare3, spare2, spare1 }???OPTIONAL,??--Need R
?drx-InactivityTimer????????????????ENUMERATED {ms0, ms1, ms2, ms3, ms4, ms5, ms6, ms8, ms10, ms20, ms30, ms40, ms50, ms60, ms80, ms100, ms200, ms300, ms500, ms750, ms1280, ms1920, ms2560, spare9, spare8, spare7, spare6, spare5, spare4, spare3, spare2, spare1},
?Why bwp-InactivityTimer max value is same as max value of drx-InactivityTime?
This prevents the case where bwp-InactivityTimer expiring while drx-InactivityTimer is running.
?When bwp-InactivityTimer starts?
A UE starts the BWP inactivity timer of a serving cell, if configured, when it activates a DL BWP other than the default DL BWP.
?When bwp-InactivityTimer re-starts?
A UE restarts the BWP inactivity timer of the serving cell
1.???Paired Spectrum (FDD Mode): When it decodes a DCI with downlink assignment for the active DL BWP.
2.???Unpaired Spectrum (TDD Mode): When it decodes a DCI with downlink assignment or uplink grant for its active DL/UL BWP pair.
3.???A UE shall start/restart the BWP inactivity timer when a PDCCH for DCI-based BWP switch is received.
Note: If a UE is provided by bwp-InactivityTimer a timer value for the serving cell and the timer is running, the UE decrements the timer at the end of a subframe for FR1 or at the end of a half subframe for FR2 if the restarting conditions are not met during the interval of the subframe for FR1 or of the half subframe for FR2.
Note: BWP inactivity timer can only be started or restarted when there is no ongoing random-access procedure associated with the serving cell.
MAC Entity based BWP Adaptation:
Upon initiation of the Random Access procedure on a Serving Cell, after the selection of carrier for performing Random Access procedure, the MAC entity shall for the selected carrier of this Serving Cell:
1> if PRACH occasions are not configured for the active UL BWP:
?????????2> switch the active UL BWP to BWP indicated by initialUplinkBWP;
?????????2> if the Serving Cell is an SpCell:
??????????????????3> switch the active DL BWP to BWP indicated by initialDownlinkBWP.
1> else:
?????????2> if the Serving Cell is an SpCell:
??????????????????3> if the active DL BWP does not have the same bwp-Id as the active UL BWP:
????????????????????????????4> switch the active DL BWP to the DL BWP with the same bwp-Id as the active UL BWP.
1> stop the bwp-InactivityTimer associated with the active DL BWP of this Serving Cell, if running.
1> if the Serving Cell is SCell:
?????????2> stop the bwp-InactivityTimer associated with the active DL BWP of SpCell, if running.
1> perform the Random Access procedure on the active DL BWP of SpCell and active UL BWP of this Serving Cell.
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If the MAC entity receives a PDCCH for BWP switching of a Serving Cell, the MAC entity shall:
1> if there is no ongoing Random Access procedure associated with this Serving Cell; or
1> if the ongoing Random Access procedure associated with this Serving Cell is successfully completed upon
reception of this PDCCH addressed to C-RNTI (as specified in clauses 5.1.4 and 5.1.5):
?????????2> perform BWP switching to a BWP indicated by the PDCCH.
?
If the MAC entity receives a PDCCH for BWP switching for a Serving Cell while a Random Access procedure associated with that Serving Cell is ongoing in the MAC entity, it is up to UE implementation whether to switch BWP or ignore the PDCCH for BWP switching, except for the PDCCH reception for BWP switching addressed to the CRNTI for successful Random Access procedure completion in which case the UE shall perform BWP switching to a BWP indicated by the PDCCH.
Upon reception of RRC (re-)configuration for BWP switching for a Serving Cell while a Random Access procedure
associated with that Serving Cell is ongoing in the MAC entity, the MAC entity shall stop the ongoing Random Access
procedure and initiate a Random Access procedure after performing the BWP switching.
Note: Regardless of the BWP switching type, The UE is not required to transmit UL signals or receive DL signals during the BWP switching time.
UE BWP related capabilities:
RRC based BWP switch ==> Mandatory support by UE (by default).
If UE supports more than one RRC configured BWPs, then it is applicable to support DCI and timer-based BWP switching mechanisms.
DCI and timer based BWP adaption can be reported by bwp-SameNumerology and bwp-DiffNumerology. UE can also report the switch delay type requirements using bwp-SwitchingDelay.
BandNR ::=????????????SEQUENCE {
???...
???bwp-WithoutRestriction?????????????ENUMERATED {supported}???????????????????????OPTIONAL,
???bwp-SameNumerology????????????ENUMERATED {upto2, upto4}???????????????????OPTIONAL,
???bwp-DiffNumerology???????????????ENUMERATED {upto4}?????????????????????????????OPTIONAL,
???...
???channelBWs-DL / channelBWs-UL??????????????????CHOICE {
???????fr1????????????????SEQUENCE {
???????????scs-15kHz????????????BIT STRING (SIZE (10))???????????????OPTIONAL,
???????????scs-30kHz????????????BIT STRING (SIZE (10))???????????????OPTIONAL,
???????????scs-60kHz????????????BIT STRING (SIZE (10))???????????????OPTIONAL
???????},???????????????????????????
???????fr2????????????????SEQUENCE {????????????????????
???????????scs-60kHz????????????BIT STRING (SIZE (3)) ???????????????OPTIONAL,
???????????scs-120kHz??????????BIT STRING (SIZE (3)) ???????????????OPTIONAL
???????}
???}
?????????...
?????????channelBWs-DL-v1590 / channelBWs-UL-v1590?????????????CHOICE {
??????????????????fr1 ????SEQUENCE {
????????????????????????????scs-15kHz ?????????????BIT STRING (SIZE (16)) ????OPTIONAL,
????????????????????????????scs-30kHz ?????????????BIT STRING (SIZE (16)) ????OPTIONAL,
????????????????????????????scs-60kHz ?????????????BIT STRING (SIZE (16)) ????OPTIONAL
??????????????????},
??????????????????fr2 ????SEQUENCE {
????????????????????????????scs-60kHz?????????????BIT STRING (SIZE (8)) ??????OPTIONAL,
????????????????????????????scs-120kHz ???????????BIT STRING (SIZE (8)) ??????OPTIONAL
??????????????????}
?????????}
?????????...
}
Phy-ParametersCommon ::=???????????SEQUENCE {
?????????...
bwp-SwitchingDelay???????????ENUMERATED {type1, type2}?????????????????OPTIONAL,
?????????...
}
?
bwp-WithoutRestriction
Indicates support of BWP operation without bandwidth restriction.
So, what actually bandwidth restriction means?
The Bandwidth restriction in terms of DL BWP for PCell and PSCell means that the bandwidth of a UE-specific RRC configured DL BWP may not include the bandwidth of CORESET #0 (if configured) and SSB. For SCell(s), it means that the bandwidth of DL BWP may not include SSB.
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bwp-SameNumerology
Indicates whether UE supports BWP adaptation (up to 2/4 BWPs) with the same numerology, via DCI and timer (bwp-InactivityTimer).
For the UE capable of this feature, the bandwidth of a UE-specific RRC configured DL BWP includes the bandwidth of the CORESET#0 (if CORESET#0 is present) and SSB for PCell and PSCell (if configured). For SCell(s), the bandwidth of the UE-specific RRC configured DL BWP includes SSB, if there is SSB on SCell(s).
?
bwp-DiffNumerology
Indicates whether the UE supports BWP adaptation up to 4 BWPs with the different numerologies, via DCI and timer (bwp-InactivityTimer).
For the UE capable of this feature, the bandwidth of a UE-specific RRC configured DL BWP includes the bandwidth of the CORESET#0 (if CORESET#0 is present) and SSB for PCell and PSCell (if configured). For SCell(s), the bandwidth of the UE-specific RRC configured DL BWP includes SSB, if there is SSB on SCell(s).
?
bwp-SwitchingDelay
Defines whether the UE supports DCI and timer based active BWP switching delay type1 or type2. It is mandatory to report type 1 or type 2 when bwp-SameNumerology or bwp-DiffNumerology is supported on at least one band.
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channelBWs-DL / channelBWs-UL
Indicates the UE supported channel bandwidths for each subcarrier spacing per Band. Absence of the channelBWs-DL / channelBWs-UL (without suffix) for a band or absence of specific scs-XXkHz entry for a supported subcarrier spacing means that the UE supports the channel bandwidths among [5, 10, 15, 20, 25, 30, 40, 50, 60, 80, 100] and [50, 100, 200] that were defined in clause 5.3.5 of TS 38.101-1 and TS 38.101-2 for the given band or the specific SCS entry.
For FR1, the bits in channelBWs-DL / channelBWs-UL (without suffix) starting from the leading / leftmost bit indicate 5, 10, 15, 20, 25, 30, 40, 50, 60 and 80MHz. For FR2, the bits in channelBWs-DL / channelBWs-UL (without suffix) starting from the leading / leftmost bit indicate 50, 100 and 200MHz. The third / rightmost bit (for 200MHz) shall be set to 1.
For FR1, the leading/leftmost bit in channelBWs-DL-v1590 / channelBWs-UL-v1590 indicates 70MHz, the second leftmost bit indicates 45MHz, the third leftmost bit indicates 35MHz, the fourth leftmost bit indicates 100MHz and all the remaining bits in channelBWs-DLv1590 / channelBWs-UL-v1590 shall be set to 0. The fourth leftmost bit (for 100MHz) is not applicable for bands n41, n48, n77, n78, n79 and n90.
References:
A Primer on Bandwidth Parts in 5G New Radio (https://arxiv.org/ftp/arxiv/papers/2004/2004.00761.pdf)
Bandwidth Part Adaption – MediaTek White paper (https://newsletter.mediatek.com/hubfs/mwc/download/bandwidth-part-adaptation.pdf)
TS 38.211 4.4.5 Bandwidth part
TS 38.213 12 Bandwidth part operation
TS 38.321 5.15 Bandwidth Part (BWP) operation
TS 38.300 6.10 Bandwidth Adaptation & 7.8 Bandwidth Adaptation
TS 38.331 12 Processing delay requirements for RRC procedures
TS 38.133 8.6.2 DCI and timer based BWP switch delay & 8.6.3 RRC based BWP switch delay
TS 38.306 4.2.7.2 BandNR parameters & 4.2.7.10 Phy-Parameters
BandWidth Part (BWP): A 5G feature for improving spectrum flexibility and power savings (https://www.youtube.com/watch?v=bk9pTECFg8E)
DCI Formats in NR (https://howltestuffworks.blogspot.com/2019/09/dci-formats-in-5g-nr.html)