Volte call flow: Made simple

Dear Folks, today we will discuss about one of the basic things in Volte, which is Volte call flow.

The Normal Volte call flow is look like below:

We will discuss each step one by one:

Step 1: SIP Invite: Direction:- A Party---B Party
Calling party(A) informs IMS network and Called party(B) about New call. Session Description protocol(SDP) is used for carrying information such as Bandwidth and codec.

Step 2: 100 Trying: Direction:- B Party----A party
100 Trying is the response to the calling party to stop the retransmission of SIP invite. Every Node in IMS send the 100 trying message.

Step 3: 183 session in progress: Direction:- B Party----A Party
Called party informs about codec supported in response of SDP. Dedicated bearer(QCI=1) are created at both ends i.e. Calling end as well as called end.

Step 4: SIP PRACK(Provisional Response Acknowledgment):  Direction:-A Party----B Party
It is a provisional response acknowledgment to 183 session in progress message received. Calling party uses this PRACK to communicate final selected Voice codec.

Step 5: SIP 200 OK (PRACK): Direction:- B Party----A Party
Called party accepts final selected Voice codec by Calling party.

Step 6: SIP Update: Direction:- A Party----B Party
Calling party confirms the Resources to voice call.

Step 7: SIP 200 OK (Update):- Direction:- B Party----A Party
Called party confirms the resources to voice call.

Step 8: 180 Ringing:- Direction:- B Party----A Party
All pre conditions are satisfied now and called party alerts the Calling party about ringing of call.

Step 9: SIP 200 OK(INVITE): Direction:- B Party----A Party
Finally called party Answers the call. 

All About PDCCH channels in LTE

Hello folks, Back after long time. Today we will discuss about one of the most important channels in LTE i.e PDCCH.

PDCCH(Physical downlink Control Channel):- The PDCCH can be called as heart of the data transmission in LTE as its carries the control information about the data being transmitted on the current subframe and the information about the resources which UE need to used for the Uplink data. Which means for any data transmission and reception UE must need to decode the PDCCH channel.

So next important questions comes in my mind is
Information carried by PDCCH?
Where it carries the information?
And for Whom?

We will try to answers these questions one by one.

Information carried by PDCCH:- PDCCH carries a message called DCI ( Downlink Control Information ) which includes resource assignments for a UE or group of UE's. EnodeB can transmit many DCI's or PDCCH's in a subframe. 


we have following DCI formats:-


Format 0 for transmission of resources to UE for sending their uplink data, i.e Uplink grant.

Format 1 for downlink allocation of resources for Single Input Multiple Output (SIMO)case.

Format 1A for downlink allocation of resources for SIMO operation or allocating a dedicated preample signature to a UE for random access.

Format 1B for transmission control information of Multiple Input Multiple Output (MIMO) rank 1 based compact resource assignment

Format 1C for very compact transmission of PDSCH assignment

Format 1D same as format1B with additional information of power offset

Format 2 and Format2A for transmission of DL-SCH allocation for closed and open loop MIMO operation, respectively

Format 3 and format3A for transmission of TPC command for an uplink channel.

Where it carries the information?- 
Allocation of resources happens in terms of CCE ( Control Channel Elements ).



1 CCE = 9 continuous REG's ( Resource element Group )

1 REG = 4 RE ( Resource Element )

So 1 CCE = 36REs.


PDCCH uses the resources present in first n OFDM symbols where

n - Value present in PCFICH ( Number of OFDM symbols )



So the number of CCE's present to transmit the control information will be variable depending on the

1.PCFICH value

2.Bandwidth of the system from 1.4 Mhz to 20 Mhz.

3.Number of antenna ports present which in turn will effect the reference signals present.

For Whom?- 
eNodeB uses the PDCCH for sending the control information for a particular UE or a group of UE's. It means eNodeB uses the PDCCH  for some broadcast information also which is common for all the UE's. So to make that process easier eNodeB divided its CCE's into two parts which we call them as search space:


Common search space :- It consists of CCE's which are used for sending the control information which is common for all the UE's .Maximum number of CCE present in common search space is 16.
For Example:- Common search space CCE's are used by eNodeB for sending the control information of SIB's which is common for all UE's.  


UE specific search space :- CCE's belonging to UE specific space are used for sending the control information for a particular UE only. That means information present on UE specific CCE's can only be decoded by a specific UE.


Note:- eNodeB can also send the control information for a specific UE on the common search space.

We will discuss next blog that how eNodeB calculate CCE for UE. 

Different types of scheduler in LTE

Today we will discuss about the different types of schedulers in LTE.
There are mainly 3 types of schedulers in LTE, which are as below:
1. Round-Robin
2. Proportional Fair
3. Max CQI or Max C/I

We will discuss all one by one.

1. Round- Robin scheduler:- As it names suggest this type of scheduler schedules UE in a circular manner without considering the channel condition.

Pros:- Creating an equal resource share to all users.
Cons:- The UE with Sub-Optimal CQI will also allocate with PRB thus reducing the overall Cell/User throughput.

2. Proportional Fair Scheduler:- As it name suggest this scheduler tries to make balance between throughput and fairness among all the UEs. Means it tries to maximize the total throughput and at the same time tries to provides all users at least a minimal service.

Pros:- There is equal amount of trade off between throughput and fairness.
Cons:- Implementation is complex and we can not reach the highest cell throughput with this scheduler.

3. Max CQI scheduler:- As it name suggest this type of scheduler uses the strategy to assign the PRB to the user with the best channel condition means highest CQI.

Pros:- We can reach the maximum throughput.
Cons:- Cell edged UEs are starved of scheduling which leads to poor UE experiences.

So keeping in mind the throughput and fairness for users the proportional fair schedulers are most common to use.

Please do comment.. Your feedback are valuable for me.

Scheduling in LTE

Scheduling in LTE is a process in which the resources on the shared channel i.e. PDSCH and PUSCH are assigned to users on sub-frame basis. Scheduling done on the basis of below factors:
Users traffic demand, QOS requirements and estimated channel quality.


The scheduling is done by the Uplink and Downlink schedulers which both are situated in eNodeb.


The smallest time/frequency entity that the scheduler may assign is twelve sub carrier (180kHZ) in frequency domain and 1ms in time domain and this is called scheduling blocks.


Resources handled by scheduler in downlink are:

1.Physical resource blocks-PRB

2.PDCCH resource

3.DL power

4.TX Rank

5.Baseband module processing capability


Resources handled by scheduler in Uplink are:

1.PRB

2.PUCCH resources

3.Baseband module processing capability


Downlink scheduling process:

 1.UE provides a CQI report 

 2. DL scheduler assigns resources per RB based on QOS and CQI

 3.Resource allocation is transmitted in same TTI as data.


Uplink Scheduling process:

 1.UE request UL transmission via scheduling request

 2.Scheduler assigns initial resources without detailed knowledge of buffer count

 3.More detailed BSR(Buffer status report) follow in connection with data

Transmission Channels priority in scheduling:- For every cell in every TTI(1ms), the Scheduler determines the UEs that are assigned resources. Each radio bearer is given a certain scheduling priority, based on algorithms which take the QCI (QoS Class Identifier) related parameters as input.The UE with the highest priority is selected first for transmission.The different transmissions are prioritized in the following order:

Downlink:-
1.Common channels
2.HARQ retransmission of DCCH
3.Initial transmission of DCCH
4.HARQ retransmission of DTCH
5.Initial transmission of DTCH
Uplink:-
1.Transmission of random access msg3
2.HARQ retransmission
3.Initial transmission of DCCH
4.Initial transmission of DTCH.

Types of scheduling Algorithms: Below are the major types of Scheduling Algorithms:
1.Round Robin
2.Proportional Fair
3.Max C/I
4.Semi-persistant 

Will discuss in details about above scheduling algorithms in next blog.

VoLTE KPIs

Today we will discuss about the Various types of VoLTE KPIs. 
VoLTE KPI is mainly divided into three parts:

1.IMS KPI

2.EPC(Packet core) KPI

3.RAN KPI.


IMS KPI is further divided into 2 parts:- IMS Control plane KPI and IMS User Plan KPI.

IMS Control plane KPIs are as follows:

1.RSR:- Registration success rate.         

Formulae = Count of 200OK for Registration completed/Count of SIP register sent from UE excluding 401 error attempts*100

2.CSSR- Call setup success rate%           

Formulae = Count of (Normal end call+ call failed with user behaviour)/Sum of all call attempts*100

3.Call setup time:-          

Formulae= Avg of(Time(180 ringing)- Time(SIP INVITE request))


IMS User plane KPIs are as follows:

1.Mute rate(%)

Formulae= % of calls muted (samples >2 or 5s RTP loss in both direction)

2.MOS score

3.RTP packet loss %

Formulae = % of RTP packets lost in the uplink or downlink direction.

4.One way call %

Formulae = % of calls having no voice packets count for 2 or 5 Sec in either the downlink or uplink direction.


EPC(Packet core KPI):- This also classified further in 2 parts- MME generated and SGw/PGw generated.

MME generated KPI are as follows:-

1.VoLTE attach success rate% = % of PDN connect request responded successfully by MME for IMS APN.

2.VoLTE bearer Activation success rate:% of create request got successful.

3.Paging success rate%-= % of paging responses on QCI=5 received at MME.
SGw/PGw generated KPI are as follows:-
1. IMS IP Pool Utilization%


RAN KPI are as follows: 
1.Call drop rate % = % of calls getting dropped abnormally.
2.SRVCC Success Rate % = % of calls successfully transferred with SRVCC from VoLTE to legacy NW

3.Handover Success rate %(S1 and X2).

5G NR network Architecture

Below is the 5G NR architecture as per 3GPP standard.

Network nodes and their functions:

(UE)- User equipment

(gNB)- Next Generation Node Base station

Access and Mobility Management Function (AMF) – responsible for following

       – Termination of RAN Control Plane interface (NG2)

       – Termination of NAS (NG1), NAS ciphering and integrity protection

       – Mobility Management

       – Lawful intercept (for AMF events and interface to Lawful Intercept System)

       – Transparent proxy for routing access authentication and SM messages

       – Access Authentication

       – Access Authorization

       – Security Anchor Function (SEA): It interacts with the UDM and the UE, receives the       intermediate key that was established as a result of the UE authentication process; in case of USIM based authentication, the AMF retrieves the security material from the UDM

       – Security Context Management (SCM): it receives a key from the SEA that it uses to derive access-network specific keys

User plane Function (UPF) – functions are

       – QoS handling for User plane

       – Packet routing & forwarding

       – Packet inspection and Policy rule enforcement

       – Lawful intercept (User Plane)

       – Traffic accounting and reporting

       – Anchor point for Intra-/Inter-RAT mobility (when applicable)

       – Support for interaction with external DN for transport of signaling for PDU session authorization/authentication by external DN

Session Management Control Function (SMF) – supports following:

      – Session Management

      – UE IP address allocation & management (including optional Authorization).

      – Selection and control of User Plane function

      – Termination of interfaces towards Policy control and Charging functions

      – Control part of policy enforcement and QoS.

      – Lawful intercept (for Session Management events and interface to Lawful Intercept System)

      – Termination of Session Management parts of NAS messages

      – Downlink Data Notification

      – Initiator of Access Node specific Session Management information, sent via AMF over NG2 to Access Node

      – Roaming functionality

      – Handle local enforcement to apply QoS SLAs (VPLMN)

      – Charging data collection and charging interface (VPLMN)

      – Lawful intercept (in VPLMN for Session Management events and interface to Lawful Intercept System)

Data Network (DN): Operator services, Internet access or other services

Authentication Server Function (AUSF) – Performs authentication processes with the UE

Unified Data Management (UDM) – Supports:

      – Authentication Credential Repository and Processing Function (ARPF); this function stores the long-term security credentials used in authentication for AKA

      – Storing of Subscription information

Policy Control Function (PCF) – Provides:

     – Support of unified policy framework to govern network behavior

     – Policy rules to control plane function(s) that enforce them

Application Function (AF) – Requests dynamic policies and/or charging control

Rank Indicator(RI) in LTE

The basic advantage of MIMO(Multiple Input Multiple Output) technology is that in the MIMO technology different data streams are transmitted simultaneously on the same frequency and time.

https://mytechnew2019.blogspot.com/2019/01/benefits-of-mimo-technology.html

      Rank Indicator(RI) is an indicator which tells how well the multiple antenna works in the MIMO, i.e. the number of layers transmitted simultaneously on the same frequency and time.

Calculation of maximum RI:- Maximum RI is very close to number of antennas in Transmitting and receiving side. Suppose we have 2*2 MIMO( 2 Antennas in both transmitting and Receiving) then the value of RI are 1 and 2.

Note: If we have different number of  transmitting and receiving antennas then the maximum RI is equal to one of less antennas.

How RI works:- Lets take an example of 2*2 MIMO. The possibilities of RI are 1 and 2. 
                           The UE reports RI 2 when there is No Correlation(Interference) between both antennas i.e different data streams are being send by each TX(enhanced capacity).
                            The UE reports RI 1 when there is some interference between both antennas, i.e all the TX are sending same data streams(enhance coverage).

The feedback of RI by UE is based on the SINR value. For a UE to be able to support 2 layers (RI2) means that it has to be in a good SNR. The second thing in multipath rich environment, UEs prefer RI2 as it has embedded copies of the data streams. The third thing is the vendor's implementation. Some vendors switch on the basis of SINR only while other vendors switch on the basis of SINR plus FER while there are vendors that switch on the basis of SINR plus HARQ-Outs.

                        

Cell Reselection in LTE

Cell Reselection Procedure in LTE:- There are some procedure followed in cell reselection for LTE listed as below:-

1.Serving Cell Measurment
2.Cell Reselection Triggering
3. Cell Ranking 
4. Cell Reselection.
We will discuss one  by one.

1. Serving Cell Measurement:- UE, in Idle state, wakes up at the end of every DRX cycle to measure the signal of its serving cell (Qrxlevmeas) and calculate the received signal level (Srxlev) of the serving cell to decide whether it should stay or move to another cell.

2. Cell Reselection Triggering:- If the received signal level of the serving cell (Srxlev) is greater than the specified threshold value (s-IntraSearch for Intra Frequency and S-NonIntraSearch for Inter frequency)), the UE stays in the current serving cell. If not, it triggers a cell reselection procedure. The threshold values are delivered through SIB 3, and defined as s-IntraSearch in Release 8 and as s-IntraSearchP and s-IntraSearchQ in Release 9.



3. Cell Ranking:- The UE ranks each cell (Rs, Rn) based on the measured signal strength of the serving cell (Qmeas,s) and neighbor cells (Qmeas,n). Parameters required for cell ranking are delivered through SIBs 3 and 4. The serving cell is ranked using the hysteresis (q-Hyst) value stored in SIB 3 while the neighbor cells are ranked based on the offset (q-OffsetCell) value specified for each cell in SIB 4.

4.Cell Reselection:- After serving cell and neighbor cells are ranked, the UE decides whether the cell reselection criterion is satisfied (Rn > Rs) or not. If there are neighbor cell(s) that satisfy the criteria, the UE selects the best satisfying cell, and then reselct there. Cell reselection is performed only when the criterion is satisfied for a certain period of time (t-ReselectionEUTRA).



We can prevent too frequent cell reselection and make sure reselection is performed in proper manner by hysteresis and cell-specific offset values. In addition, we can control the q-Hyst and t-ReselectionEUTRA values by applying appropriate scaling factor (q-hystSF, t-ReselectionEUTRA-SF) depending on the traveling speed of the UE.

Below table is the parameter summary for the cell reselection.

Parametrs	Description	SIB Type
S-Intrasearch	Value that triggers Intra frequency measurment(dB)	3
q-Hyst	Hysteresis value for Serving cell ranking	3
q-RxlevMin	Minimum Rx Level required for UE to continue on serving Cell	3
p-Max	Maximum TX power allowed for UE	3
allowedMeasBandwidth	DL BW to be measured by UE	3
t-ReselectionEUtra	Cell Reselection timer Value	3
physcellID	PCI of neighbouring cell	4
q-offsetCell	offset value for serving cell ranking	4
intrafreqblackCelllist	List of neighbour cells that are black listed for reselection	4
physcellidrange	PCI Range	4

 

MRO Feature in LTE

MRO(Mobility Robustness Optimization):- MRO is a SON(Self Optimizing Network) feature in which they provide the solution for automatic detection and correction of errors(Radio Link Failure) in Handover.

Scenarios for MRO:- There are 3 scenarios where MRO feature can optimize in case of RLF in Handover.

1. Too Late Handover.
2. Too Early Handover.
3. Handover to Wrong Cell.

We will discuss one by one.

1. Too Late Handover:- In this case the UE does not receive the RRC Handover command, due to weak signal see figure below, the handover procedure in the source cell is initialized too late, since the UE is moving faster than the Handover (HO) parameter settings allow. Hence when the RRC HO command from the serving cell is transmitted the signal strength is too weak to reach the UE, which is now  located in the target cell, connection is lost. The UE attempts a connection re-establishment, containing PCID and C-RNTI belonging to the source cell, but received by the target cell. The target eNB will then inform the source cell about RLF to adjust Handover parameters.

2. Too Early Handover:-  In this scenario the signal strength in the target cell is too weak, and the connection is lost almost immediately after the Handover. see figure below. The UE has successfully been handed over from source cell to target cell , but since it was triggered too early the connection will drop almost immediately due to too poor radio conditions in the target cell . The UE will then re-established the connection in source cell.

3. Handover to Wrong Cell:- In this case RLF occurs in the target cell after a handover has been completed, and the UE attempts to re-establish its radio link in a cell which is not the source cell nor the target cell. See the fig below.

Working of MRO Feature:- Source eNodeB takes some appropriate actions to overcome the above type of failures by changing the threshold at which the handovers are triggered. Source eNodeB can use ‘Mobility Settings Change’ procedure to inform the neighbour eNodeB of the change of threshold it has performed. In this case the ‘MOBILITY CHANGE REQUEST’ message is sent with a cause value ‘handover optimization’ and indicates the change (in dB) of the handover trigger parameter change performed in the source cell. For instance, in the case of repeated handover to an

inappropriate cell, the source eNodeB could modify the way it builds its list of candidate

target cells.

The Parameters that can be optimized in connected mode: The majorly parameter modification are done for A3 events triggering as listed below.

1. A3 offset

2. A3 hysteresis.

3. Cell individual offset(CIO)

4. Time to trigger(TTT).

Ref: 3GPP.ORG

Carrier Aggregation in LTE

What is Aggregation?:- In Layman language the meaning of Aggregation is formation of number of things into cluster.

Carrier Aggregation in LTE:- 3GPP in Rel10 standardizes LTE-Advanced in addressing to meeting the IMT Advanced requirements. LTE-Advanced involves a set of features and one of them is carrier aggregation. In carrier aggregation it combines multiple LTE system bands or carriers with different bandwidth (1.5, 3 , 5 , 10 , 15 , 20 MHz) , thus increasing the overall capacity and the overall network throughput. Each aggregated carrier is referred to as a component carrier (CC) and the release 10 specifies that a maximum of five CC can be aggregated, hence a maximum aggregated bandwidth of 100 MHz, which is shown below.

 Carrier Aggregation scenario:- There are mainly 3 types of carrier aggregation depending on the component carriers.

1. Intra Band Contiguous(Continuos)
2. Intra Band Non Contiguous
3. Inter Band Non Contiguous

General Working Principal in Carrier Aggregation:- When carrier aggregation is used there are a number of serving cells, one for each component carrier. The coverage of the serving cells may differ, for example due to that CCs on different frequency bands will experience different pathloss. We have used only one carrier called primary carrier for coverage and all other carriers called secondary carriers are used for User data. The RRC connection is only handled by the Primary serving cell, Secondary serving cells,The SCCs are added and removed as required, while the PCC is only changed at handover.

Example: Suppose if we have 3 different Bands( 850,1800,2100MHZ) and we want to implement the CA(Carrier aggregation) then the principals as below:

       The primary cells/carriers must be 850 as we have less pathloss and more coverage and other 2 we can used in secondary cells/carriers for user data. 

Supervised Machine Learning - Overview and Examples

                                                        Supervised Machine Learning

Supervised Machine Learning is a type of machine learning in which we have both input and desired Output data are provided. Input and Output data are labelled for classification and regression to provide a learning  so that we can predict an output in future.

How does Supervised Machine Learning Work?- In Supervised learning we have an Input variable X and an Output variable Y and we use an algorithms to find the mapping function from the input(Input may be more than 1) to output as below:-
                                                        Y=f(X)
The Ultimate goal is to approximate the mapping function so that if we have new input data(X) we can predict an output(Y).

Why it is called Supervised?- Because the learning algorithm from the training data sets can be thought as a teacher supervising the class. The algorithms makes predictions from the training data sets and is corrected in case of wrong prediction. Learning stops when we achieved acceptable performance. 

Algorithms used in Supervised Machine Learning:- There are lots of Algorithms used. Some of are listed below:-
1. Linear Regression
2. Logistic Regression
3. Support Vector Machine
4. Decision tree
5. K-Nearest Neighbor
6. naive bayes.

Real world Examples of Supervised Machine Learning:-

1. To predict the price for House:- Suppose we have input data X(square footage, number of bedrooms, number of floors, Area, Year of build ) we can predict an output Y(price of that house).

2. To Predict the price of an specific stock.

3. To Predict the Loan Availability of an customer on basis of their credit history.

4. Spam email filter.

5. To predict an increase in Cell Traffic(In case of Telecom).

There are so many examples in real world. Wherever we have labelled input  and we have to predict an output we can use Supervised Machine Learning.

Machine Learning Overview and its Uses in Telecom

Today we are discussing about the very Hot topic for nowadays  which is Machine Learning. First we will find what is Machine Learning, why we need Machine Learning ,Types of Machine Learning and how Machine Learning can help Telecom.

What is Machine Learning?- Idea behind the machine learning is to make machines(computer) capable of taking decision without explicitly programmed means once machines are trained and new data comes in the system then machine can train themselves without reprogrammed again.

Why we need Machine Learning?- There are lots of data generates in every field nowadays and that data too is present in the form of structured and unstructured. With processing by Humans for these data are not so easy so these data can be used for training the machine and machine can take efficient and fast decision. Machine is also do fast computation and complex thing which human cant do.

How does Machine Learning Work?- Machine Learning is a technology in which machines learns through training( We have to train a model with training data sets often known as inputs) and processes specific input to predict an output. In order to get desired output we have to properly trained the model with help of some parameters/Algorithms.

Uses of Machine Learning in Telecom or how can Machine Learning will help Telecom?- The telecom operators have already the masses of data in terms of its customer data, Network performance data, network traffic data and social media data. So there are some Machine Learning applications which are helping telecom:-

1. Using Machine Learning to identify restart the sleeping cells:-  In this application we can analyse from network performance data to identify the sleeping cells and trigger an automatic restart.

2.To identify potential churners:- The customer churn is highest in the Telecom in comparison to other fields, so for the telecom operators it is very important to retain there customers. With help of Unsupervised Learning( We have customer behaviour data is already with us) we can identify the potential churners and plan accordingly.

3.Using Machine Learning to improve customer service application.

4.Using Machine Learning to identify fraud mitigation.

There are different types of Algorithms used in machine Learning as listed below. We will discuss each one in next blog.

What is NB-IoT(Narrowband IoT), its operation mode and features

In order to meet the requirements of IoT world, the 3GPP has designed the narrow band internet of things (NB-IoT) standard in its recent Release 13. The NB-IoT operates on the principal of LPWA(Low power consumption and wide coverage).

Uses of NB-IoT:- The potential uses of NB-IoT is communication(Machine 2 Machine) for IoT applications. Some of are as below:

1.Smart Metering(Electricity, Gas, Water)

2. Smart City

3. Security systems

4. Asset Tracking

5. Agriculture

Operating Mode for NB-IoT: NB-IoT is working on 180Khz Bandwidth(1 PRB in LTE).There are 3 operating modes which are explained below:

1. In-Band Operation:- Utilizing Resource blocks within an LTE carrier.( No Extra cost for Hardware upgradation but chances of increasing interference is high as we are using 1 PRB within the same band).

2. Guard-Band Operation:- Utilizing the unused resource blocks within an LTE carrier guard bands.(No Extra cost for hardware as well as chances of Interference is very less)

3. Standalone Operation:- Utilizing of currently used GSM frq.( Hardware upgradation required and chances of Interference is more, we have to do spectral refarming to overcome this).

Features of IoT:- There are 2 important features of NB-IoT which are listed below.

1. Wide Coverage Area:- NB-IoT offers 20dB of MCL(Maximum coupling loss) higher than LTE(up to 164dB). In order to meet this several modification have been deployed and one of the major is number of retransmissions(128 for Uplink and 2048 for Downlink). These repetitions are combined at receiver side to improve the SINR.

                                 There are 3 Coverage Enhancement(CE) mode in NB-IoT.

CE mode0:- Normal coverage with MCL~144dB and sub carrier spacing of 15khz.

CE mode1:- Robust coverage with MCL~154dB and  sub carrier spacing of 15Khz.

CE mode2:- Extreme coverage with MCL~164dB and sub carrier spacing of 3.75Khz.

The choice of the mode dependent upon channel conditions. Each mode determines the transmission parameters including the number of repetitions. Depending upon the coverage level the serving cell indicates the UE to repeat the transmission(in UL and UE informs to eNodeB in DL) using the same transmission power on each transmission. Combining the different retransmissions results in coverage enhancements.

2. Low Power consumption:- there are mainly 2 techniques used in NB-IoT to achieve this.

       Idle mode extended discontinuous reception (I-eDRX): this mode allows a discontinuous reception for maximum of 3 hours, which saves UE battery but still allows it to be reachable by the network through paging messages or downlink control channels.
Power saving mode (PSM): this power saving mode allows unconnected state for up to 13 days, where UE enters to a deep sleep. Unlike in I-eDRX, UE is unreachable while remains registered in the network.

The configurations and implementation of NB-IoT will discuss in next blogs..

Tracking area update(TAU) in LTE

Before jumping to Tracking area update procedure lets first understand what is Tracking Area(TA) and why we need TA(Tracking Area).

What is Tracking Area?- Tracking Area in LTE is geographical combination of several eNodeBs.
                       Each TA has two main identities:- Tracking Area code(TAC) and Tracking Area Identity(TAI).
    TAI= PLMN ID+TAC.

Why we need Tracking Area?- In Idle when there is no signalling and data bearers are associated with it the location of UE to MME is known by Tracking area(TA) only.

Now as we know what is TA and need of TA we are ready to discuss Tracking are update.

Tracking Area Update:- Tracking area update initiated by UE(direction is from UE to NW) only after the UE completes the attach procedure and moves into the EMM(EPS Mobility Management) state.

Note:- UE can trigger TAU in RRC idle state or in connected state but procedure ends only when the UE is in RRC connected state.

Reasons for TAU?- There are many reasons for Tracking Area Update(TAU). Some are listed below:-

> When a UE moves to a new Tracking Area.
> When the Periodic update timer T3412 expired. The value of T4312 is in the attach accept message.
> Registering with the Non EPS services(CS Domain) when the UE is already attached for EPS services. This includes completing an IMSI attach as part of TAU procedure.
> Registering for an EPS services after an inter RAT changed( From GERAN/UMTS to LTE).
> Re-Registering to LTE after CS fallback.
> In case of MME load balancing( UE initiates TAU procedure when eNodeB releases the RRC connection with cause "Load Balancing TAU required".

Note:- EPS update type should be as below:
"TA updating", "Combined LA/TA updating", "combined LA/TA updating with IMSI attach", "Periodic updating".

         

TTI Bundling in LTE

TTI Bundling:- Today we are going to discuss an LTE feature to improve mainly VoLTE performances at cell edges or in poor radio conditions.

Need to TTI Bundling?- TTI bundling as its name suggest is a bundling(together as a package) of TTI(smallest unit of time in which eNodeB is scheduling the UE).

       In normal scenario(without TTI bundling) at cell edges or in poor RF condition, the UE informs eNodeB about its power getting maximum value (UE has limited power 23dBm in LTE) therefore the chances of re-transmission of Voice Packets is increasing(Multiple NACK). Which further leads in delay of VoIP packets arriving(It is not acceptable in case of Voice call).

Note: Normally 1 HARQ retransmission is taking a delay of 8ms.

How it Works?- In TTI bundling we are sending a transport block multiple times in a consecutive subframe(Normally 4) without waiting for HARQ ACK/NACK messages. A combined ACK/NACK can be sent after processing all the transmissions of a transport block.

                     Actually, in TTI Bundling, we are sending multiple redundancy(4) versions of the same set of bits in consecutive(Bundled) TTI and eNodeB sends back ACK when it successfully decodes the data. This way we are avoiding delay (Which is mainly due to RTT of HARQ).

Note: TTI Bundling uses QPSK modulation.

SRVCC in LTE

SRVCC( Single Radio Voice call continuity):-  SRVCC is a feature/process by which a voice call(VoIP) in LTE can be continued in legacy RAT(2G/3G, CS call) via handover when UE moves out of LTE coverage. 

Why its called Single Radio?- The process is called single radio is due to only a single radio is required in the handset.

Call Flow for SRVCC:- Here below I m indicating the simplest way of SRVCC call flow.

The process:- When UE moving away from LTE coverage then UE notifies the eNodeB(By sending measurement reports). Then eNodeB informs MME to make a decision for handover to legacy RAT network.

We are defining thresholds of SRVCC for serving cell by Inter-RAT B1 and B2 thresholds.

Note: We can set a greater threshold in case of TTI bundling feature is enabled. As with TTI bundling feature VoLTE services can be improved at cell edges.

https://mytechnew2019.blogspot.com/2019/02/tti-bundling-in-lte.html

New Interface: For SRVCC implementation  3GPP introduces a new interface called Sv. The Sv interface is from MME to MSC.

Elements of SRVCC Network Architecture:- Here we are discussing about some important network elements which play a big role in SRVCC.

Sv interface:- Signalling interaction for SRVCC handover as an interface between MME and MSC server.

eNodeB:- SRVCC indication to MME with help of UE measurement reports.

MME:- Bearer splitting for Voice and Non-Voice services and MSC selection.