Overal LTE Sequence

Most of us would be specialized in one or a few specific area/layer as an engineer. Technically it is understandable because not a single person can master everything, but I strongly recommend you to try to have some big picture of the whole process. Whenever you have some issues or something for you to work, try to ask your self "Where is the current issue located in the whole picture ?".

You don't have to try rote memorization for these whole process, but if you keep trying to map your current issues to specific parts within the whole picture, these whole sequence would automatically pops up in your memory. I say so often to my kid or my juniors "Don't try to memorize it, just repeat it until you automatically recall" (They are looking at me seeminly saying "What is the difference ?" -:) I just smile -:).

Following is my version of the whole LTE procedures.. but if you are trying to describe it, you would have a little bit different version.. but I think overall logic would be similar. Every now and then, just try to recall these sequence in your mind and ask your self "how in detail I can explain about each of these steps ?". Actually each of these steps can be described in a volum of a thick book.

The Sequence, Step by Step

The list below runs from a UE with no power to a UE back in idle mode. Each step gets one line, and that is deliberate. The detail behind any single line fills a page of its own, so what the list carries is the order rather than the substance.

1) UE is Off

2) Power On UE

3) < Frequency Search >

4) < Timing Sync >

5) < Cell Search> : This includes MIB decoding and essential SIB decoding etc.

Normally a UE would find multiple cells in this process

6) < Cell Selection >

7) < Initial RACH Process >

8) < Registration/Authentication/Attach>

9) < Default EPS Bearer Setup >

10) Now UE is in IDLE Mode

11) <(If the current cell become weak or UE moves to another cell regisn) Cell Reselection>

12) <(When Paging message comes or User make a call) RACH Process>

13) < Setup Dedicated EPS Bearer >

14) Receive data

15) Transmit data

16) (If UE power is percieved too weak by the network) Network send TPC command to increase UE Tx Power

17) (If UE power is percieved too strong by the network) Network send TPC command to decrease UE Tx Power

18) < (If UE moves to another cell region) Network and UE perform Handover procedure >

19) User stop call and UE gets into IDLE mode

The list has two halves, and the join between them is easy to miss. Steps 1 to 9 run once, after the UE is powered on. Steps 10 to 19 repeat for as long as the UE stays on the network. Step 19 returns the UE to step 10 rather than to step 1, because the registration made in step 8 survives the release.

That is why the numbering is worth reading as two blocks rather than one run. A fault in the first block happens once per power cycle and is hard to catch again without rebooting the UE. A fault in the second block comes back on every call, which makes it far easier to reproduce and far more visible to a subscriber.

One more thing the numbers do not carry. A step is not a message. Step 8 alone covers an Attach Request, an authentication exchange, a NAS security mode exchange and an Attach Accept, and the Message Sequence page takes those one message at a time.

Steps 1 to 9 run once and steps 10 to 19 loop : the sequence returns to idle mode, and not to a powered off UE.
A step is not a message : one line of this list can hold a whole exchange, and step 8 holds four of them.

Notes on These Steps

Three notes follow the list, and they answer three different questions. The first says which steps vary between modems and which do not. The second states the assumption the whole sequence was written under. The third explains why a discussion about these steps goes wrong when nobody names a case.

NOTE 1 : The steps from 7) and onwards would be same or similar to every UE and every situation, but there can be many variations in procedure 3) ~ 6). Usually 3)~4) is determined by the internal algorithm implemented in Modem chipset. Even though overall logic is known to DSP engineers working on this area, there are a lot of variations in each modem manufacturer. It can be interplay of multiple factors - 3GPP requirement, Stored Information on UE and Modem specific angorithm. Step 6) is pretty complicated and shows various different behaviors depending on situation, but a good thing is that this step is relatively clearly defined in 3GPP.

NOTE 2 : When I was writing this note, I was thinking of the procedure of the cases of blind search assuming that there is almost no specific information stored on UE side and Test USIM with test PLMN (e.g, 001-01) and the device has never been attached to any network/test equipment. Just brand new and powered on for the first time. I took this assumption because that would common to most of the devices.

NOTE 3 : When you want to talk about the detailed variation of these procedure, you may need to a specific cases. Otherwise each person in the discussion may talk about different things based on their own assumption in their mind. Some of the specific cases you may set would be something like this (these are just some examples. There are huge set of different cases that you can think of and you may have over hundreds of test cases trying to check the detailed behavior if you are the person in charge of verification of cell selection / reselection / roaming).

Case 1 : Just bought a new device for testing purpose and put test USIM with PLMN 001-01 and I am using a protocol test equiment as Network.

Case 2 : I am using the same device as Case 1 and same test USIM, but this UE has been tested before to the same equipment and same band and frequency.

Case 3 : I bought a new commercial device from this vendor (e.g, SamSung, Apple etc) for this operator (e.g, AT&T, Verizon etc) and put the USIM provided by the network operator. I powered on the device for the first time within the area covered by the operator that the UE subscribes (e.g, within US)

Case 4 : Same device / USIM as in Case 3, but I just bought it in US and travelled to other country and powered on the UE.

Which Layer and Which Specification Owns Each Step

The opening paragraph above asks where an issue sits in the whole picture, and a step number on its own does not answer that. Two more columns do. One names the layer that owns the step. The other names the clause that defines it. Together they turn a step number into somewhere to look.

The layer column is the one that decides who to ask first. Steps 3 to 5 belong to the physical layer and to whatever the modem vendor built above it. Steps 6, 10, 11 and 18 belong to RRC. Steps 7 and 12 belong to MAC. Steps 8, 9 and 13 belong to NAS, and those never reach the radio on their own. They travel as payload inside an RRC message.

Step	Layer that owns it	Specification and clause
3) Frequency Search	PHY, on a band list from RRC	36.101 clause 5.5 for the operating bands and clause 5.7.2 for the channel raster
4) Timing Sync	PHY	36.211 clause 6.11, the synchronization signals
5) Cell Search	PHY and RRC	36.331 clause 5.2, system information
6) Cell Selection	RRC, in idle mode	36.304 clause 5.2.3
7) Initial RACH Process	MAC	36.321 clause 5.1
8) Registration / Authentication / Attach	NAS, the EMM sublayer	24.301 clause 5.5.1 for attach and clause 5.4.2 for authentication
9) Default EPS Bearer Setup	NAS, the ESM sublayer	24.301 clause 6.4.1
10) IDLE Mode	RRC and NAS	36.331 clause 4.2.1 and 23.401 clause 4.6.3.1
11) Cell Reselection	RRC, in idle mode	36.304 clause 5.2.4
12) Paging then RACH Process	RRC then MAC	36.331 clause 5.3.2, then 36.321 clause 5.1
13) Setup Dedicated EPS Bearer	NAS, the ESM sublayer	24.301 clause 6.4.2
14) Receive data	PDCP, RLC, MAC and PHY together	36.321 clause 5.3, DL-SCH data transfer
15) Transmit data	PDCP, RLC, MAC and PHY together	36.321 clause 5.4, UL-SCH data transfer
16) and 17) TPC commands	PHY	36.213 clause 5.1, uplink power control
18) Handover	RRC	36.331 clause 5.3.5 and 36.300 clause 10.1.2.1
19) Back to IDLE mode	RRC	36.331 clause 5.3.8, RRC connection release

Two rows cover more than one layer, and those are the rows where a fault is hardest to place. Steps 14 and 15 run the user plane, so PDCP, RLC, MAC and the physical layer all handle the same byte on its way through. A step 12 failure has two candidate layers rather than one, because the step starts in RRC with a paging message and continues in MAC with a random access.

One row is worth reading twice for a different reason. Steps 8, 9 and 13 are NAS procedures, and no NAS message has a channel of its own. Each one travels inside an RRC message as an octet string. A NAS fault and an RRC fault can therefore look alike in a log until the inner message is decoded.

A step number becomes useful once it carries a layer and a clause : the layer says who to ask and the clause says what the answer should be.
The hardest steps to place are the ones that span layers : the user plane steps touch four layers, and a paging failure could be RRC or MAC.

Where the Variation Lives, and Where It Does Not

NOTE 1 above divides the list in two. Steps 3 to 6 vary between modems, and the steps from 7 onward do not. 36.304 says why, and it says it in a clause short enough to quote. The division is not vagueness in the specification. It is a decision about what to fix and what to leave open.

The specification fixes the set and leaves the order

36.304 clause 5.2.3.1 gives the UE two cell selection procedures and no others. Initial Cell Selection requires no prior knowledge of which RF channels carry E-UTRA. The UE scans every RF channel in the bands it supports, limited only by its own capabilities. On each carrier frequency it need only find the strongest cell.

Two limits in that clause are where the variation lives. The capability limit fixes which channels are in scope and says nothing about the order they are tried in. The strongest cell rule sets a stopping condition and leaves open the search that reaches it. A modem that tries the home band first and a modem that works down from the widest bandwidth both satisfy the clause.

The other branch is what the cases below separate

Stored Information Cell Selection is the second procedure. It uses carrier frequencies the UE already holds, either from earlier measurement control information or from cells it detected before. If the UE finds no suitable cell, it starts the Initial Cell Selection procedure instead.

That branch is the one the cases above are pulling apart. NOTE 2 assumes a device that has never attached to anything, and that assumption is exactly the condition forcing the Initial branch. Case 2 describes the same device after an earlier test on the same band and frequency, which is the Stored Information branch. The two cases therefore run different clauses rather than merely different histories.

Step 6 leaves no room at all

Cell selection is where the specification stops offering choices. 36.304 clause 5.2.3.2 states the S criterion as arithmetic. Srxlev has to exceed zero, and Squal has to exceed zero as well unless the measurement uses RSS. Srxlev is Qrxlevmeas less Qrxlevmin and its offset, less Pcompensation and Qoffsettemp.

Nothing in that is open to interpretation. The inputs arrive in SIB1, SIB3 or SIB5, and the test is a subtraction compared against zero. So NOTE 1 above can call step 6 complicated and clearly defined in the same breath, and both halves hold. The behaviour varies because the inputs vary, and not because the rule does.

Step	What 3GPP fixes	What it leaves to the UE
3) Frequency Search	The bands and the channel raster, in 36.101	The order the channels are tried in
4) Timing Sync	The PSS and SSS waveforms, in 36.211	The detection algorithm and how long to dwell
5) Cell Search	Which SIBs exist and how they are scheduled, in 36.331	Which of them to read before moving on
6) Cell Selection	The S criterion, as arithmetic, in 36.304	Nothing in the test itself
7) onward	The message content and the procedure, clause by clause	Timing margins and error handling only

The specification fixes what to scan and leaves the order open : that gap is where two modems on the same band behave differently.
A clearly defined step can still behave in varied ways : the S criterion is fixed arithmetic, and its inputs arrive from the cell.
The cases above are not four histories but two branches : a device with nothing stored takes Initial Cell Selection, and a device with stored carriers takes the other branch.

Reference

The clause numbers in the tables above come from these specifications. Most were opened only at the clauses named, and each entry says how far it was read.

36.304 - Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode, v19.2.0. Clause 5.2.3.1 gives the two cell selection procedures quoted above. Clause 5.2.3.2 is the cell selection criterion S. Clause 5.2.4 is cell reselection evaluation. Clause 5.2 was read in full.
36.321 - Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification, v19.3.0. Clause 5.1 is the random access procedure. Clause 5.3 and clause 5.4 are DL-SCH and UL-SCH data transfer.
36.331 - Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification, v19.3.0. Clause 4.2.1 is the RRC states, clause 5.2 is system information, clause 5.3.2 is paging, clause 5.3.5 is RRC connection reconfiguration and clause 5.3.8 is RRC connection release.
24.301 - Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3, v20.0.0. Clause 5.4.2 is authentication, clause 5.5.1 is attach, clause 6.4.1 is default EPS bearer context activation and clause 6.4.2 is the dedicated one.
23.401 - General Packet Radio Service (GPRS) enhancements for E-UTRAN access, v20.0.0. Clause 4.6.3.1 is ECM-IDLE, which is the state step 10 above names as IDLE mode.
36.211 - Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation. Clause 6.11 defines the primary and secondary synchronization signals that step 4 above searches for. Only the clause headings and clause 6.1.1 were read for this note.
36.213 - Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures, v19.4.0. Clause 5.1 is uplink power control, which is where a TPC command changes the UE transmit power. The clause number was confirmed from the table of contents and the clause body was not read for this note.
36.101 - Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception. Clause 5.5 lists the operating bands and clause 5.7.2 is the channel raster. Both clause numbers were confirmed from the table of contents and the clause bodies were not read for this note.
36.300 - Evolved Universal Terrestrial Radio Access (E-UTRA) and E-UTRAN; Overall description; Stage 2, v19.2.0. Clause 10.1.2.1 is handover, which is step 18 above.