When we say 'communication between UE and Network', we normally think about only signaling message (RRC or NAS message). When I say 'communication' in this case, it means 'control command exchange' between UE and network, not the data traffic.
In UMTS case, it is true that only RRC and NAS message functions as communication between UE and Network, but in LTE case there are several communication path at MAC layer. It implies that there are special MAC structure that carries special control information. These special MAC structure carrying the control information is called 'MAC CE', which means 'MAC Control Element'.
This special MAC structure is implemented as a special bit string in LCID field of MAC Header (Refer to LTE MAC page for the details of MAC header).
A MAC CE is therefore faster than an RRC message. It travels inside a MAC PDU on the DL-SCH or the UL-SCH, is protected by HARQ, and needs no RRC procedure around it. The MAC layer uses it for commands that must act within a few subframes, such as timing advance, DRX and buffer status.
Followings are the topics to be covered in this page.
MAC CE in Release 8
Which MAC CEs did the first LTE release define, and how does a receiver tell them apart from data? The answer is in the LCID field of each MAC subheader. The same 5-bit field names either a logical channel or a MAC CE, so the tables of LCID values double as the list of MAC CEs.
There are several MAC CE in downlink MAC and also several MAC CE in uplink MAC. Following table from 36.321 shows the LCID types of MAC header. The parts marked in red rectangle is LCID representing various MAC CE. These are relatively small set of MAC CE defined in 36.321 Rel 8 - 6.1.3 MAC Control Elements. You will see this gets longer as new release comes out (Refer to LTE-Advanced MAC for the snapshot of the evolution )
36.321 Release 8, Table 6.2.1-1 for the DL-SCH and Table 6.2.1-2 for the UL-SCH. The red rectangles mark the LCID values that identify a MAC CE.
The table below lists the MAC CEs of the red rectangles with their size and purpose. The sizes are those of 36.321 v19.3.0 clause 6.1.3, and they have not changed since Release 8, apart from the use of two bits in the Timing Advance Command.
Direction | MAC CE | LCID | Size | Purpose |
DL | UE Contention Resolution Identity | 11100 | 48 bits | Echoes the first 48 bits of the uplink CCCH SDU, so the UE knows its random access won the contention. |
DL | Timing Advance Command | 11101 | 8 bits | A 6-bit TA index that adjusts the uplink timing. The first 2 bits now carry the TAG Id. |
DL | DRX Command | 11110 | 0 bits | Has no payload. The subheader alone tells the UE to stop its on-duration and inactivity timers. |
UL | Power Headroom Report | 11010 | 8 bits | Two reserved bits and a 6-bit PH level, the room left below the maximum UE power. |
UL | C-RNTI | 11011 | 16 bits | The C-RNTI of the UE, sent in Msg3 when the UE already has one, for example after a handover. |
UL | Truncated BSR and Short BSR | 11100 and 11101 | 8 bits | A 2-bit LCG ID and a 6-bit Buffer Size index for one logical channel group. |
UL | Long BSR | 11110 | 24 bits | Four 6-bit Buffer Size fields, one for each of LCG 0 to 3. |
The same LCID value means different things in the two directions. For example, 11100 is the UE Contention Resolution Identity on the DL-SCH, but the Truncated BSR on the UL-SCH. There is no conflict, because the receiver knows which transport channel the MAC PDU arrived on. This reuse is what let each direction fit its MAC CEs into the few values left above the logical channel identities 00001 to 01010.
Most of these MAC CEs have a fixed size, so their subheader carries no L field. The DRX Command is the extreme case: its size is zero bits, and the subheader carries the whole command.
LCID identifies the MAC CE : the same 5-bit field that identifies a logical channel.Six MAC CEs in Release 8 : three on the DL-SCH, and PHR, C-RNTI and BSR in three formats on the UL-SCH.LCID values reused per direction : 11100 is Contention Resolution in the downlink and Truncated BSR in the uplink.Fixed sizes : from 0 bits for the DRX Command to 48 bits for Contention Resolution.
MAC CE for LTE Advanced
With the introduction of LTE Advanced, MAC CE has been extended a little bit further to meet the operations in LTE Advanced. Refer to MAC in LTE Advanced page for the details.
Carrier aggregation was the first reason for new MAC CEs. The eNB needed a fast way to switch secondary cells on and off, and the UE needed to report power headroom per cell. Later releases added MAC CEs for dual connectivity, sidelink, VoLTE bit rate adaptation and NB-IoT, and the reserved LCID range of Release 8 filled up.
The table below lists some of the MAC CEs that now occupy that range, from Tables 6.2.1-1 and 6.2.1-2 of 36.321 v19.3.0. On the DL-SCH, only 01011 and 01100 remain reserved. On the UL-SCH, no reserved value remains.
Direction | MAC CE | LCID | Purpose |
DL | Activation/Deactivation (1 octet) | 11011 | Seven C-fields that activate or deactivate SCells 1 to 7. |
DL | Activation/Deactivation (4 octets) | 11000 | 31 C-fields, used when an SCellIndex above 7 is configured. |
DL | Long DRX Command | 11010 | Moves the UE straight into the long DRX cycle. |
DL | Recommended bit rate | 10110 | The eNB recommends a bit rate for a logical channel, for example for VoLTE codec adaptation. |
UL | Extended Power Headroom Report | 11001 | Power headroom for each activated serving cell with carrier aggregation. |
UL | Dual Connectivity Power Headroom Report | 11000 | Power headroom for the serving cells of both the MCG and the SCG. |
UL | Sidelink BSR and Truncated Sidelink BSR | 10111 and 10110 | Buffer status for sidelink transmission. |
DL and UL | Extended logical channel ID field | 10000 | Points to a 6-bit eLCID in the subheader, which adds logical channels 32 to 38. |
The last row shows how the 5-bit LCID ran out of room. Instead of widening the field, 36.321 reserved LCID 10000 as an escape value. A subheader with that value carries an extra eLCID field, and the eLCID tables define its values. Every older subheader keeps its format, so an older receiver still reads every older subheader the same way.
Carrier aggregation started the growth : Activation/Deactivation and Extended PHR.The LCID space is now almost full : only 01011 and 01100 remain reserved on the DL-SCH.LCID 10000 is an escape value : the eLCID field adds logical channels 32 to 38.
Reference
[1] 3GPP TS 36.321 v19.3.0 - clause 6.1.3, MAC Control Elements, and clause 6.2.1, MAC header for DL-SCH, UL-SCH and MCH