Power HeadRoom Report in LTE

Power Headroom Report(PHR) in LTE :- 

                             In Uplink, the UE has a maximum transmission power defined by their power class. Power Headroom indicates how much relative transmission power (With respect to UE maximum transmission power) can be further used by UE. In mathematical expression, Power Headroom can be expressed as below:

Power Headroom = UE max transmission Power - Power of PUSCH channel (Estimated or Scheduled not really transmitted).

Note:- Power Headroom Report is transmitted in same subframe in which PUSCH is transmitted.

By the mathematical expression there are 2 possibilities :

1. Power Headroom is +Ve ; Means UE is still having adequate transmission power left and thus network can assign more resource blocks to UE.

2. Power Headroom is -Ve ; Means UE did not have anymore additional power left and is transmitting at more than its maximum transmitting power already. In this case network will not assign more resource blocks to UE.

Power Headroom Report Mapping :- 
     The PHR ranges from -23.. to +40dB .

When UE send PHR ?- There are two triggering condition for UE through RRC messages( RRC configuration, RRC reconfiguration).

1. When Downlink Pathloss changed is greater than a certain threshold(in dB)
2. By some predefined periodic PHR timer (in Sub frames).

Note:- PHR can be efficiently used in UL power control to improve the throughput.

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.

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. 

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.

Types of DCI formats in LTE

DCI(Downlink control information) :- It tells the UE how to get its data which is transmitted on PDSCH in the same subframe. Means with the help of DCI only UE can decode the PDSCH from the resource grid and find its data.

     The DCI formats provides the UE details such as number of resource blocks, resource allocation type , Modulation scheme which UE has to use(QPSK,16QAM,64QAM), transport block size, redundancy version, coding rate( Amount of information bits over transmitted bits), For MIMO configuration PMI and RI.

There are 13 DCI formats but we are discussing only below important formats.

Format Name                                Use
DCI-0                                     UL Grant(Scheduling of PUSCH)  
DCI-1A                                  Used during the Random Access Procedure
DCI-1                                     Scheduling of the DL PDSCH for TX diversity
DCI-2A                                 Scheduling of DL PDSCH in Open loop MIMO(TM3)
DCI-2                                    Scheduling of DL PDSCH in closed loop MIMO(TM4)                                            

ENHANCED PDCCH LINK ADAPTATION IN LTE

Here we will discuss about a LTE Feature called Enhanced PDCCH Link Adaptation. The main aim of this feature is to increase the capacity to allow more users to be scheduled by improving the efficiency of PDCCH usages. The capacity can be improved by Utilization of CCE in more efficient way.

How it Works: In every period channel quality(CQI) is reported to eNodeB by UE. In basic PDCCH Link adaptation its uses the PDSCH successful transmission and its offset as an information for PDCCH to achieve any particular BLER.
                          Enhanced PDCCH Link Adaptation uses PDSCH HARQ-ACK from UE and also the PUSCH detection result to determine whether the PDCCH transmission is successful or not.

Algorithm Flow:

 1. SINR Estimation: Based on the CQI Report report that UE sent to eNodeB.

 2. PDCCH Outer Loop Adjustment: To Achieved the required BLER.

 Elements to determine the criteria:
   > Success or failure of the PDCCH transmission. The HARQ-ACK reports and PUSCH Detection results are used for this. As long as the eNodeB can detect the ACK, either positive or negative, PDCCH transmission considered successful. 

> Aggressive or Conservative PDCCH Allocation. The PDCCH allocation is considered conservative if the allocation of CCE exceeds the required number and the aggressive one is the other way around.

From the outer loop adjustment, the output is required number of CCE for the same.

 The result of the implementation can be seen in the below chart.

Based on the chart above it can be seen that the CCE used decreased by 27% and the cell throughput improved by 1.55%.

Conclusion: This feature is capable to occupy CCE more efficient which lead into the increased cell Throughput.

Benefits of MIMO technology

MIMO(Multiple Input Multiple Output) is technology which can provide significant advantage in system capabilities (Mostly in Throughput, Reliability and Capacity). Any MIMO implementation will take use of mostly below benefits.

Diversity Gain: Arises out of the provision of multiple antennas at the transmitting and/or receiving end of the radio link. This will create multiple transmission paths which minimizes fading effects. The result in an overall improvement in SINR and leading to increased Overall Channel Throughput.

Array Gain: Refers to the beamforming capability of a multiple antenna array. With Suitable signaling of feedback from the receiver , or with measurements made on a return link, it is possible to direct radiated energy towards the receiver in a steered beam. The results is improved channel performance and increased throughput.

Spatial Multiplexing Gain: Rises out of the orthogonality  Between the multiple transmission path created by the multiple antenna array. Since the receiver can resolve independent transmission paths it is possible to map different information streams into the transmission paths: differentiated by their spatial signature. This result in direct increase in the channel throughput in proportion to the number of separated transmission used.