Types of BWP

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In the previous section we learned about BWP and its configuration. Now in this document we will learn more on BWP types and switching.

Now to begin with we know that carrier of one BWP is divided into several BWPs and each BWP has a continuous PRB(physical resource block). Some BWPs have SSB and associated RMSI while some BWPs have SSB but no associated RMSI and some BWPs don’t even have SSB.

Now, How many types of BWPs a network can configure?

Network can configure three different BWP – Initial BWP and two UE specific types, namely first active BWP and default BWP. 

Below IEs are used for BWP configuration:
(The BWP is split into Downlink and Uplink parameters and into common and dedicated parameters.)

1) DL BWP:

2) UL BWP:

Let's talk about BWP types:

1) Initial BWP:

When a device enters from idle mode to the connected state it obtains the information from the PBCH (MIB) a CORESET, where it can find the control channel used to schedule the remaining system information (RMSI decode).

So we can say that initial BWP is used for receiving information before the UE access, mainly used to receive SIB and RA related information, generally used in the idle state.

Also as we know and mentioned in previous part, BWPs that exist in connected states are generally larger than the initial BWP.

The CORESET configuration obtained from the PBCH not only defines but also activates the initial bandwidth part in the downlink. Using the downlink PDCCH the initial active bandwidth part is obtained from the system information. Initial BWP size might be configured approximately between 25 to 100 PRBs with different settings. These RBs are enough to decode initial access process and RMSI as well as frequency location and bandwidth.

The below IEs are used to identify initial DL BWP from PDCCH-configSIB1 which is present in MIB:

From MIB we find initial DL BWP (controlResourcesetzero) information and using this we can decode RMSI from that we get information of initial UL BWP setting. Using initial UL BWP information UE send the random access request to gNB (msg 1).

In PDCCH config SIB1 the value of this IE is 8 bits length, in which first four bits (MSB) will determine the “controlResourceSetZero” Index. This will indicate the number of symbols (RB) used to determine the CORESET seizes of the type0 PDCCH common search space.

And the last four bit (LSB) will determine the “searchSpaceZero” index. This will indicate the PDCCH monitoring occasions, meaning the system frame number and slot index that the UE need to monitor for the type0-PDCCH common search space.

Note:The common parameters of the initial BWP of the MSG-Pcell (master cell group) are also provided via system information.the network provides the common parameters via dedicated signalling for all other serving cell. as well as BWP ID=0 is always associated with the initial BWP.

How many BWP can configure for UE?

After the device is connected it can be configured with upto four downlink bandwidth parts and up to four uplink bandwidth parts for each serving cell but only one BWP is active for uplink and one for downlink at a given point of time.

Does UE can receive uplink & downlink outside BWP?

Thus UE cannot transmit PUCCH or PUSCH and also cannot receive PDCCH or PDSCH outside an active BWP.

2) First Active BWP:

At a given time instant on each serving cell, one of the configured downlink bandwidth parts is referred to as active downlink bandwidth part(DL-BWP) for that serving cell and the one configured as uplink bandwidth parts (UL-BWP) is referred as the active uplink bandwidth part for that serving cell.

For the first UE specific BWP, the UE can perform data transmission and reception and the PDCCH retrieval on this BWP.

Once DUT received RRC connection setup massages, from network side in case of SA from which DUT can decode  UE specific BWP configurartion. It could be default BWP/ First Active BWP/ other type BWP(Other BWP’s are defined under IE — downlinkBWP_ToAddModList with BWP-ID).

If first active BWP configuration is not present in RRC Connection setup massage then DUT should used initial BWP for further downlink or uplink DATA.

If UE received First Active BWP information in RRC connection setup(Msg4) message then now on word DUT should used First active BWP for further communication with network for DL-BWP or UL-BWP. We can Say that Msg5 (RRC connection setup completed) will used first active BWP for UL. 

Random access configuration to be used for the reconfiguration with sync (e.g. handover). The UE performs the RA according to these parameters in the firstActiveUplinkBWP  

First Active BWP is provided by IE: firstActiveDownlinkBWP-Id and firstActiveUplinkBWP-Id.

A DUT may assume that the active downlink bandwidth part and the active uplink bandwidth part of a serving cell have the same center frequency for unpaired spectra. This helps in simplifying the implementation as a single oscillator can be used for both directions.

3) Default BWP:

Now, the UE-specific BWP is configured for the UE in the RRC Reconfiguration. In case is it is not configured, then the Initial BWP is considered as the default BWP, After the BWP inactivity timer runs out, the UE is still not scheduled, and the UE is switched to the default BWP. Thus every UE would switch back to default BWP when BWP timer expires.

Default BWP is provided by the following IE – defaultDownlinkBWP-Id

The BWP configuration is divided into downlink and uplink parameters and into common and dedicated parameters.common parameters i.e. BWP-UplinkCommon and BWP-DownlinkCommon are cell specific and the network ensures the necessary alignment with corresponding parameters of the other UEs. 

Also, the common parameters of the initial BWP of the Pcell are provided through system information. And for all other serving cells, the network provides the common parameters through dedicated signaling.

For more info refer 3GPP TS 38.331.

The BWP parameters are used to configure the operatoer between the cell and the UE.As per 3GPP TS 38.331 for every serving cell the network configures at least an initial BWP, consisting of downlink BWP and one(if configured with uplink) or two(if configured using supplementary uplink) uplink BWP.Also network may configure additional uplink and downlink BWP.

Call flow of BWP configuration in NSA Case:

Call flow of BWP configuration in SA Case:

In next document we will learn more about BWP switching. …….. so stay tune and happy reading.

--Pinal Dobariya(your feedbacks are always welcome)

Reference:

3gpp 38.213

3gpp 38.214 

3gpp 38.321

3gpp 38.331

https://arxiv.org/ftp/arxiv/papers/1712/1712.09724.pdf 
white paper from MediaTek paper for BWP