Measurement Gap in LTE

What is measurement GAP?

Measurement GAP is introduced to measure the different frequencies of neighbor cell and inter RAT from the current cell in that mentioned GAP, so when neighboring inter frequency Cell has better signal than serving Cell then UE will measure the RSRP in that GAP and informs to current cell in measurement report. During this gap ue will not listen or transmit from/to serving cell.

In the active state, the eNB provides measurement gaps in the scheduling of the UE where no downlink or uplink scheduling occurs. Ultimately the network makes the decision, but the gap provides the UE sufficient time to change frequency, make a measurement, and switch back to the active channel. This can normally occur in a few TTIs. This has to be coordinated with the DRX, which also causes the system to shut off the radio for periods of time to save power.

UE will get the Measurement GAP configuration  from  RRC_Connection_Reconfiguration message while attaching to network and the Information Element(IE) is MeasGapConfig.

The MeasGapConfig consists of gap pattern type (gp0 or gp1) The gap patterns are to identify the periodicity and gapOffset

Figure 1:Reference from 36.331

Figure 2:Reference from 36.133

Each Measurement gap starts at an SFN and subframe meeting the following condition:

SFN mod T = FLOOR (gapOffset/10);

subframe =  gapOffset mod 10;

with T  = MGRP/10 

where MGRP is 40 for gp0 and 80 for gp1

1. For FDD, the UE shall not transmit in the subframe occurring immediately after the measurement gap.
2. For TDD, the UE shall not transmit in the uplink subframe occurring immediately after the measurement gap.
3.  If the subframe occurring immediately before the measurement gap is a downlink subframe

Basic LTE call flow

Basic LTE call flow:

Here i will explain the basic call flow of LTE, the full description of individual Resource Elements would be explained in later post:

Timers in LTE

RRC Timers

Timer	Start	Stop	At expiry
T300	Transmission ofRRCConnectionRequest	Reception of RRCConnectionSetup or RRCConnectionRejectmessage, cell re-selection and upon abortion of connection establishment by upper layers	reset MAC, release the MAC configuration and re-establish RLC for all RBs that are established;  inform upper layers about the failure to establish the RRC connection, upon which the procedure ends;
T301	Transmission ofRRCConnectionReestabilshmentRequest	Reception of RRCConnectionReestablishment orRRCConnectionReestablishmentReject message as well as when the selected cell becomes unsuitable	Go to RRC_IDLE
T302	Reception of RRCConnectionReject while performing RRC connection establishment	Upon entering RRC_CONNECTED and upon cell re-selection	if timer T302 expires or is stopped:  inform upper layers about barring alleviation for mobile terminating access; 2> if timer T303 is not running: 3> inform upper layers about barring alleviation for mobile originating calls; 2> if timer T305 is not running: 3> inform upper layers about barring alleviation for mobile originating signalling;
T303	Access barred while performing RRC connection establishment for mobile originating calls	Upon entering RRC_CONNECTED and upon cell re-selection	Inform upper layers
T304	Reception ofRRCConnectionReconfigurationmessage including the MobilityControl Info or reception ofMobilityFromEUTRACommand message including CellChangeOrder	Criterion for successful completion of handover to EUTRA or cell change order is met (the criterion is specified in the target RAT in case of inter-RAT)	In case of cell change order from E-UTRA or intra E-UTRA handover, initiate the RRC connection re-establishment procedure; In case of handover to E-UTRA, perform the actions defined in the specifications applicable for the source RAT.
T305	Access barred while performing RRC connection establishment for mobile originating signalling	Upon entering RRC_CONNECTED and upon cell re-selection	Inform upper layers
T310	Upon detecting physical layer problems i.e. upon receiving N310 consecutive out-of-sync indications from lower layers	Upon receiving N311 consecutive in-sync indications from lower layers, upon triggering the handover procedure and upon initiating the connection re-establishment procedure	If security is not activated: go to RRC_IDLE else: initiate the connection re-establishment procedure
T311	Upon initiating the RRC connection re-establishment procedure	Selection of a suitable E-UTRA cell or a cell using another RAT.	Enter RRC_IDLE
T320	Upon receiving t320 or upon cell (re)selection to E-UTRA from another RAT with validity time configured for dedicated priorities (in which case the remaining validity time is applied).	Upon entering RRC_CONNECTED, when PLMN selection is performed on request by NAS, or upon cell (re)selection to another RAT (in which case the timer is carried on to the other RAT).	Discard the cell reselection priority information provided by dedicated signalling.
T321	Upon receiving measConfig including areportConfig with the purpose set toreportCGI	Upon acquiring the information needed to set all fields ofcellGlobalId for the requested cell, upon receiving measConfigthat includes removal of the reportConfig with the purpose set toreportCGI	Initiate the measurement reporting procedure, stop performing the related measurements and remove the corresponding measId
NAS Timers
T3402	Found in Attach accept and Tracking Area Update Accept
T3412
T3423

RRC Constants:

Constant	Usage
N310	Maximum number of consecutive "out-of-sync" indications received from lower layers. Value in whole number
N311	Maximum number of consecutive "in-sync" indications received from lower layers. Value in whole number

Timer (EPS Mobile Management - UE Side)

Following table comes from 24.301 - 10.2 Timers of EPS mobility management (Table 10.2.1: EPS mobility management timers – UE side)

TIMER NUM.	TIMER VALUE	STATE	CAUSE OF START	NORMAL STOP	ON EXPIRY
T3402	Default 12 min. NOTE 1	EMM DEREGISTERED EMM REGISTERED	At attach failure and the attempt counter is equal to 5. At tracking area updating failure and the attempt counter is equal to 5.	ATTACH REQUEST sent TRACKING AREA UPDATE REQUEST sent	Initiation of the attach procedure or TAU procedure
T3410	15s	EMMREGISTEREDINITIATED	ATTACH REQUEST sent	ATTACH ACCEPT received ATTACH REJECT received	Start T3411 or T3402 as described in subclause 5.5.1.2.6
T3411	10s	EMM DEREGISTERED. ATTEMPTING TO-ATTACH EMM REGISTERED. ATTEMPTING TO-UPDATE	At attach failure due to lower layer failure, T3410 timeout or attach rejected with other EMM cause values than those treated in subclause 5.5.1.2.5. At tracking area updating failure due to lower layer failure, T3430 timeout or TAU rejected with other EMM cause values than those treated in subclause 5.5.3.2.5.	ATTACH REQUEST sent TRACKING AREA UPDATE REQUEST sent	Retransmission of the ATTACH REQUEST or TRACKING AREA UPDATE REQUEST
T3412	Default 54 min. NOTE 2 NOTE 5	EMM REGISTERED	In EMM-REGISTERED, when EMM-CONNECTED mode is left.	When entering state EMM DEREGISTERED or when entering EMM-CONNECTED mode.	Initiation of the periodic TAU procedure
T3416	30s	EMM REGISTERED INITIATED EMM REGISTERED EMM DEREGISTERED INITIATED EMM-TRACKINGAREA UPDATING INITIATED EMM-SERVICE REQUEST INITIATED	RAND and RES stored as a result of a UMTS authentication challenge	SECURITY MODE COMMAND received SERVICE REJECT received TRACKING AREA UPDATE ACCEPT received AUTHENTICATION REJECT received AUTHENTICATION FAILURE sent EMM DEREGISTERED or EMM-NULL entered	Delete the stored RAND and RES
T3417	5s	EMM-SERVICEREQUESTINITIATED	SERVICE REQUEST sent EXTENDED SERVICE REQUEST sent in case f and g in subclause 5.6.1.1	Bearers have been set up SERVICE REJECT received	Abort the procedure
T3417ext	10s	EMM-SERVICEREQUESTINITIATED	EXTENDED SERVICE REQUEST sent in case d in subclause 5.6.1.1 EXTENDED SERVICE REQUEST sent in case e in subclause 5.6.1.1 and the CSFB response was set to "CS fallback accepted by the UE"	Inter-system change from S1 mode to A/Gb mode or Iu mode is completed Inter-system change from S1 mode to A/Gb mode or Iu mode is failed SERVICE REJECT received	Abort the procedure
T3418	20s	EMM REGISTEREDINITIATED EMM REGISTERED EMM-TRACKINGARE AUPDATINGINITIATED EMM DEREGISTEREDINITIATED EMM-SERVICEREQUESTINITIATED	AUTHENTICATION FAILURE (EMM cause = #20 "MAC failure" or #26 "non-EPS authentication unacceptable") sent	AUTHENTICATION REQUEST received	On first expiry, the UE should consider the network as false
T3420	15s	EMM REGISTERED INITIATED EMM REGISTERED EMM DEREGISTERED INITIATED EMM-TRACKINGAREA UPDATING INITIATED EMM-SERVICE REQUEST INITIATED	AUTHENTICATION FAILURE (cause = #21 "synch failure") sent	AUTHENTICATION REQUEST received	On first expiry, the UE should consider the network as false
T3421	15s	EMM DEREGISTERED INITIATED	DETACH REQUEST sent	DETACH ACCEPT received	Retransmission of DETACH REQUEST
T3423	NOTE 3	EMM REGISTERED	T3412 expires while the UE is in EMM-REGISTERED.NO-CELLAVAILABLE and ISR is activated.	When entering state EMM DEREGISTERED or when entering EMM-CONNECTED mode.	Set TIN to "P-TMSI"
T3430	15s	EMM-TRACKING AREA UPDATING INITIATED	TRACKING AREA UPDATE REQUEST sent	TRACKING AREA UPDATE ACCEPT received TRACKING AREA UPDATE REJECT received	Start T3411 or T3402 as described in subclause 5.5.3.2.6
T3440	10s	EMM REGISTERED INITIATED EMM-TRACKING AREA UPDATING INITIATED EMM DEREGISTERED INITIATED EMM-SERVICE REQUEST INITIATED EMM REGISTERED	ATTACH REJECT, DETACH REQUEST, TRACKING AREA UPDATE REJECT with any of the EMM cause #11, #12, #13, #14 or #15 SERVICE REJECT received with any of the EMM cause #11,#12, #13 or #15 TRACKING AREA UPDATE ACCEPT received after the UE sent TRACKING AREA UPDATE REQUEST in EMMIDLE mode with no "active" flag	Signalling connection released Bearers have been set up	Release the signalling connection and proceed as described in subclause 5.3.1.2
T3442	NOTE 4	EMM REGISTERED	SERVICE REJECT received with EMM cause #39 "CS domain temporarily not available"	TRACKING AREA UPDATE REQUEST sent	None
Note 1	The default value of this timer is used if the network does not indicate another value in an EMM signalling procedure.
Note 2	The value of this timer is provided by the network operator during the attach and tracking area updating procedures. (This Timer value is set in Attach Accept message as well).
Note 3	The value of this timer may be provided by the network in the ATTACH ACCEPT message and TRACKING AREA UPDATE ACCEPT message. The default value of this timer is identical to the value of T3412.
Note 4	The value of this timer is provided by the network operator when a service request for CS fallback is rejected by the network with EMM cause #39 "CS domain temporarily not available".
Note 5	The default value of this timer is used if the network does not indicate a value in the TRACKING AREA UPDATE ACCEPT message and the UE does not have a stored value for this timer. (This Timer value is set in Attach Accept message as well).

System Information Messages in LTE

Query_1: Why SIBs are needed?

SIBs carry relevant information for the UE, which helps UE to access a cell, perform cell re-selection, information related to INTRA-frequency, INTER-frequency and INTER-RAT cell selections.

Query_2: How many types of SIBs are there?

In LTE there are 13 types of SIBs. Each SIB has its own job to do, which means, they are meant to carry information which are related to perform their assigned job.

Query_3: On which channels SIBs are transmitted?

All SIBs are transmitted on BCCH->DL-SCH->PDSCH.

Query_4: Minimum of how many SIBs are required for the UE to initiate Attach procedure?

In LTE, for a UE to access the eNB, at the most minimum 2 SIBs are required (SIB1 and SIB2).

Information regarding SIB2-SIB13 are carried in SI messages and are included in schedulingInfoList which is part of SIB1.

Scheduling of SIBs:

·         SIB1 uses a fixed schedule with a periodicity of 80 ms and repetitions made within 80 ms. 

·         The first transmission of SIB1 is scheduled in subframe #5 of radio frames for which the SFN mod 8 = 0

·         Repetitions are scheduled in subframe #5 of all other radio frames for which SFN mod 2 = 0.

·         Scheduling of other SIB's i.e. SIB-2 to SIB-9 is defined by si-Periodicity and si-WindowLength. Both these IEs are explained below. For the rest of the SIBs, Paging is received by UE in RRC_IDLE or RRC_CONNECTED mode for notification of an in-coming SI message.

Scenarios where UE acquire SIBs:
UE will initiate the SIB acquisition procedure in any of the following criteria's:

·         UE is powered on (selecting a cell)

·         Cell re-selection

·         After HO completion

·         After entering E-UTRAN from another RAT

·         coming out of OUT OF COVERAGE situation

·         receiving a notification that SYSTEM INFORMATION has changed

·         receiving an indication about the presence of ETWS (Primary/Secondary), CMAS notification

·         receiving a request from CDMA 2000 upper layers

·         exceeding the maximum validity duration of SIBs

HOW and WHEN UE start acquiring SIBs (other than SIB1):

There are two IE's which help the UE to calculate when to read SI messages. They are:

a. schedulingInfoList

b. si-WindowLengthd

schedulingInfoList informs the UE regarding the presence of SIB type; other than SIB1. It carriers two more parameters:

i.  si_Periodicity

ii. sib_MappingInfo

· si_Periodicity tells about the period in terms of radio frames which helps the UE to read the required SIB. The value of this parameter is used for calculating the Radio frame where SI will be received.

· sib_MappingInfo carries the type of SIBs mapped

si-WindowLength specifies that a SIB should be transmitted somewhere within the specified window length. Value is in ms. This window starts at the starting sub-frame of the mentioned si_periodicity. SIBs can be received in any of the sub-frame as mentioned in the WindowLength.

Example:

SIB1 is received by UE with the following information:

schedulingInfoList[0]		SchedulingInfo
	si_Periodicity	SchedulingInfosi_Periodicity_rf16
	sib_MappingInfo[0]	SIB_TypeSIB_Type_sibType3
schedulingInfoList[1]		SchedulingInfo
	si_Periodicity	SchedulingInfosi_Periodicity_rf32
	sib_MappingInfo[0]	SIB_TypeSIB_Type_sibType4
schedulingInfoList[2]		SchedulingInfo
	si_Periodicity	SchedulingInfosi_Periodicity_rf64
	sib_MappingInfo[0]	SIB_TypeSIB_Type_sibType5
si_WindowLength		ms20

(To look for the complete IE contents and explanation of SIB1 click here)

The above table states the following:

The first element of schedulingInfoList contains sib3	Periodicity= 16 radio frames or 160ms	Window length= 20ms
The seond element of schedulingInfoList contains sib4	Periodicity= 32 radio frames or 320ms	Window length= 20ms
The third element of schedulingInfoList contains sib5	Periodicity= 64 radio frames 640ms	Window length= 20ms

Now, UE needs two information to read SI. 
a. One is on which which Radio frame UE will receive the SIB. To determine which Radio frame, the formula is: 

SFN mod T = FLOOR(x/10)

b. on which sub-frame will the SI window start. To determine the start of the SI window at sub-frame, the formula is: 

a = x mod 10

Query: How to determine the value of x?

x = (n – 1)*w

where,

x	Integer value
n	a number which corrosponds to the order of entry in schedulingInfoList.For sib3 n is 1, for sib4 n is 2 and for sib5 n is 3
w	si-WindowLength
T	si-Periodicity of the concerned SI message

Let's start calulating the Radio frame and SI starting sub frame for SIB3, SIB4 and SIB5

Subframe from which the SI Window starts
For SIB-3	x = (n – 1)*w x=(1-1)*20 x=0	a = x mod 10 a= 0 mod 10 a=0	The SI Window will start at sub frame 0 of the Radio frame where SI will be acquired
For SIB-4	x = (n – 1)*w x=(2 - 1)*20 x=20	a = x mod 10 a= 20 mod 10 a=0	The SI Window will start at sub frame 0 of the Radio frame where SI will be acquired
For SIB-5	x = (n – 1)*w x=(3 - 1)*20 x=40	a = x mod 10 a= 40 mod 10 a=0	The SI Window will start at sub frame 0 of the Radio frame where SI will be acquired

Radio frame on which the UE will acquire designated SIB
For SIB-3	SFN mod T = FLOOR(x/10)	SFN mod 16 = FLOOR (0/10) SFN mod 16 = 0	Whenever SFN mod 16 will be 0. UE is expecting SIB3 and it will start looking from sub frame 0 till the end of the mentioned si-WindowLength
For SIB-4	SFN mod T = FLOOR(x/10)	SFN mod 32 = FLOOR (20/10) SFN mod 32 = 2	Whenever SFN mod 32 will be 2. UE is expecting SIB4 and it will start looking at sub frame 0 till the end of the mentioned si-WindowLength
For SIB-5	SFN mod T = FLOOR(x/10)	SFN mod 64 = FLOOR (40/10) SFN mod 64 = 4	Whenever SFN mod 64 will be 4. UE is expecting SIB5 and it will start looking at sub frame 0 till the end of the mentioned si-WindowLength

LTE system information blocks	Description
MIB	Carries physical layer information of LTE cell which in turn help receive further SIs, i.e. system bandwidth
SIB1	Contains information regarding whether or not UE is allowed to access the LTE cell. It also defines the scheduling of the other SIBs. carries cell ID, MCC, MNC, TAC, SIB mapping.
SIB2	Carries common channel as well as shared channel information. It also carries RRC, uplink power control, preamble power ramping, uplink Cyclic Prefix Length, sub-frame hopping, uplink EARFCN
SIB3	carries cell re-selection information as well as Intra frequency cell re-selection information
SIB4	carries Intra Frequency Neighbors(on same frequency); carries serving cell and neighbor cell frequencies required for cell reselection as well handover between same RAT base stations(GSM BTS1 to GSM BTS2) and different RAT base stations(GSM to WCDMA or GSM to LTE or between WCDMA to LTE etc.) . Covers E-UTRA and other RATs as mentioned
SIB5	Carries Inter Frequency Neighbors(on different frequency); carries E-UTRA LTE frequencies, other neighbor cell frequencies from other RATs. The purpose is cell reselection and handover.
SIB6	carries WCDMA neighbors information i.e. carries serving UTRA and neighbor cell frequencies useful for cell re-selection
SIB7	carries GSM neighbours information i.e. Carries GERAN frequencies as well as GERAN neighbor cell frequencies. It is used for cell re-selection as well as handover purpose.
SIB8	carries CDMA-2000 EVDO frequencies, CDMA-2000 neighbor cell frequencies.
SIB9	carries HNBID (Home eNodeB Identifier)
SIB10	carries ETWS prim. notification
SIB11	carries ETWS sec. notification