Chapter 3 Interpretation of lte mac protocol-buffer State Report (BSR)

Buffer Status Report (BSR)

   When introducing Sr, we know that the resources required for uplink data transmission are obtained through BSR, the buffer status report process is used to provide the service ENB with information about how much data the UE has to send in the upstream buffer. RRC uses the configured two timers.Periodicbsr-timerAndRetxbsr-timerAnd for each logical channel through signaling indicationLogicalchannelgroupWhich Logical Channel group (LCG) belongs to this logical channel? We have discussed above that uplink scheduling is for a logical channel group rather than a logical channel (RB.

When this happens, a BSR message is triggered:

- There is a logical channel belonging to a logical channel group. It has upstream data to be sent in the RLC or PDCP entity (such as the control information and business data of RLC/PDCP) or there is a logical channel with a higher priority than any channel belonging to a logical channel group and data needs to be sent, or any logical channel belonging to a logical channel group does not have any data to send (that is, the BSR with a buffer size of 0, it may be used to synchronize the buffer information between ENB and UE). The BSR triggered in these cases is called "regular BSR ";

-Upstream resources have been allocated, and the number of BITs is greater than or equal to the BSR control information, but with the addition of its sub-header, the BSR triggered at this time is "filled BSR ";

-Retxbsr-timerTimeout. For any logical channel that belongs to a logical channel group, data must be sent. BSR is called "conventional BSR". In this case, the BSR is sent mainly, but there is no corresponding upstream resources, the introduction of this timer is introduced in the ran2 63 Conference R2-83891, you can read it, its usage is, when the current conditions meet, allow ue to trigger a SR (scheduling request ):

   1. The BSR time has exceeded the bsr_retx time.

  2. Since then, no upstream authorization has been received

  3. ue has data to send

- Periodicbsr-timerTimeout. BSR is called periodic BSR ",

For regular and periodic BSR:

  -If more than one logical channel group (LCG) of the current TTI sending BSR has data to be sent, the report duration is BSR.

  -Otherwise, the short BSR is reported. For details about the long BSR, refer to the previous Mac format chapter ).

For BSR filling:

 -If the number of filled bits is greater than or equal to the short BSR control information, but with its child header but less than the long BSR control information, but with its child header:

 -If more than one logical channel group (LCG) of the current TTI sending BSR has data to be sent: Report truncation BSR, this BSR carries the Logical Channel group to which the highest priority Logical Channel with data to be sent;

 -Otherwise, the short BSR is reported.

 -If the number of bits to be filled is greater than or equal to the control information of long BSR, but with its Subheader added, the BSR is reported to be long.

If the BSR reporting process finds that at least one BSR has been triggered and has not been canceled:

 -If ue has upstream resources in this TTI to initiate a new transmission:

   -Indicates the generation of a BSR Control Information Unit;

   -Start or restart the timerPeriodicbsr-timer,Besides, BSR is truncated;

   -Start or restartRetxbsr-timer.

 -Otherwise, if the regular BSR has been triggered (if there are upstream resources for new transmission, the SR will not be triggered ):

   -Then an SR message is triggered.

A Mac PDU can only contain one Mac BSR control information unit. If multiple events trigger different BSRs at the same time, the regular BSR and periodic BSR should take precedence over the BSR filling.

 

When receiving the upstream authorization that initiates a new transmission (re-transmission is not required), ue should restartRetxbsr-timerTimer.

When a Mac PDU contains a BSR, all BSRs triggered are canceled, except in one case, that is, when UL authorization can accommodate all the data to be sent but cannot accommodate another BSR control information unit plus its sub-header.

 

Note: This section is mainly about the translation protocol, but it is important to note that the timer startup conditions and the timing of SR message triggering are not all BSR sending will trigger the SR signal. Also, pay attention to the definitions of different BSR messages.

 

 

There are several types of BSR: reg1_bsr, periodic BSR, and padding BSR;
Only the first method triggers the SR.

Periodic BSR reporting is based on pre-allocated resources and does not need to trigger SR;
Padding BSR reporting is a resource squeezed out of MAC Control Element and does not need to trigger SR

BSR itself cannot apply for resources, but the SR will notify ENB to apply for resources.
There are two kinds of Sr, DSR and RA-SR

• If the UE has resources allocated on pusch, then a buffer status report is transmitted.
• If a 'regular bsr' is triggered and the UE has no pusch allocation in the current TTI but has Sr resources allocated on pucch, then a SR is transmitted on pucch at the first opportunity.
• If a 'regular bsr' is triggered and the UE has neither Sr resources allocated on pucch nor pusch allocation, then a SR is issued using the random access procedure.

Trigger SR only when no pusch resource exists

 

The scheduling request (SR) is used to request upstream shared channel resources for sending upstream data. When Sr is triggered, it will remain suspended until it is canceled, that is, either when this request is met or this Sr is unnecessary. The UE can send upstream data only when upstream resources are available. The UE requires the scheduled buffer State Report (BSR), which is a MAC Control Information Unit sent over a shared channel, resources are also required for sending. How do I obtain the upstream resources used to send BSR? This requires sending SR on pucch or prach. Because the pucch allocated to the UE is a periodic exclusive resource, the UE should always have resources. However, if the SR sent on the pucch always fails, then, we need to compete with prach to obtain scheduling opportunities.