LTE Buffer Status Report (BSR)

What is Buffer Status Report?

Buffer status report as the name indicates will carry the information on the amount of data available for each Logical Channel Group (LCG) from UE to eNB.

Why is BSR required?

When Scheduling Request Indication (link on SR) is send to eNB for UL grant, eNB has no idea on the amount of data available at UE due to this

1. eNB will not be knowing how much UL grant needs to allocate for the UE and 
2. also if the allocated grant was not sufficient, UE needs to keep sending SR's for UL grants till all the data is served. 

This will slow the UL transmissions as UE has to send SR and wait for UL grant for the next UL transmission.
To avoid this delay, BSR is send in the first UL grant allocation received after sending SR (based on trigger conditions explained in the next sections).
Now eNB with the BSR info has better understanding of the amount of data pending at UE and will send UL grant allocations based on the BSR without further waiting for SR's from UE.

Triggers for BSR?

Any of the below conditions will trigger BSR in UE:

1.  Data arrives for logical channel which belongs to a LCG and if
• no data is available on any of the logical channels belonging to any other LCG's or
• data is available only for the logical channels belonging to the lower priority LCG.
2. upon retxBSR-Timer expiry and UE has data available for transmission for any of the logical channels which belong to a LCG.
3. Number of padding bits after UL grant is allocated is equal to or larger than the size of the Buffer Status Report MAC CE and its subheader. 
4. periodicBSR-Timer expires

BSR triggered in the conditions (1) & (2) is referred as Regular BSR.
BSR triggered in the condition (3) is referred as Padding BSR.
BSR triggered in the condition (4) is referred as Periodic BSR.

Types of BSR's & LICD's of BSR's in MAC MAC CE?

1. Long BSR: When more than one LCG has data available for transmission long BSR MAC CE is used. Corresponding MAC CE LCID value is b'11110.
2. Short BSR: When only one LCG has data available for transmission short BSR MAC CE is used. Corresponding MAC CE LCID value is b'11101.
3. Truncated BSR: When more than one LCG has data available for transmission but remaining UL allocation is not sufficient to sent Long BSR then Truncated BSR MAC CE is used. Corresponding MAC CE LCID value is b'11100.

Regular & Periodic BSR  based on the data availability on LCG's can form either Long or Short BSR's MAC CE.

Padding BSR on the other hand can form either Long, Short or Truncated BSR's MAC CE based on the data availability on LCG's & the number of padding bits left after UL resources are allocated.

When to send BSR?

When at least one BSR has been triggered and not cancelled

1. If the UE has UL grant for new transmission for this TTI
• generate and include the BSR MAC CE in the MAC PDU.
• start or restart periodicBSR-Timer except when all the generated BSRs are Truncated BSRs
• start or restart retxBSR-Timer.
2. else if a Regular BSR has been triggered
• if a SemiPeristantScheduling UL grant is not configured or if the Regular BSR was not triggered for the data becoming available for a logical channel on which logical channel SR masking is setup
• a Scheduling Request is triggered for obtaining UL grant.
3. In all other cases, do nothing.

When are the BSR's cancelled?

If any of the below two conditions are satisfied, all the triggered BSR's are cancelled

1. if UL grant in this subframe can accommodate all pending data available for transmission but is not sufficient include BSR MAC CE and its subheader.
2. when a BSR is included in a MAC PDU for transmission.

BSR transmission for Multiple MAC PDU & other conditions

Below are the few important point to be considered while forming BSR's

1. Even when multiple events trigger BSR, at max only one MAC BSR CE can be transmitted with Regular & Periodic having more priority then Padding BSR's.
2. When forming multiple MAC PDU's, The UE shall transmit at most one Regular/Periodic BSR in a TTI. UE may include a padding BSR in any of the MAC PDUs which do not contain a Regular/Periodic BSR.
3. All BSRs transmitted in a TTI always reflect the buffer status after all MAC PDUs have been built for this TTI. 

Configuration parameters for BSR

RRC configures MAC with Periodic-BSR timer and retxBSR timer values (Mac-main-config IE).

Periodic-BSR timer: Periodic BSR timer is restarted every time either a long/short BSR is transmitted.

retxBSR timer: For every UL grant received, retxBSR Timer is restarted.

References

1. 3GPP 36.321