LTE Optimization Handover (Intra , Inter and InterRAT)

Mobility Management Overview

-Mobility management is classified into mobility management in idle mode and mobility management in connected mode, based on RRC states. A UE with an RRC connection to the radio network is considered to be in connected mode. In the case of mobility management in connected mode, an eNodeB delivers the associated configuration through signaling on the control plane, and UEs perform measurements accordingly and complete the handover procedures under the control of the eNodeB, thus ensuring uninterrupted service provision.

-In the case of mobility management in connected mode, the mobile network ensures continuity of physical channels and provides uninterrupted communications service for UEs in connected mode through handovers when the UEs are moving in the network. A handover is a procedure where the serving cell of a UE in connected mode is changed. Handovers can be classified into intra-frequency handover, inter-frequency handover, and inter-RAT handover.

Classification of Handover

+Intra-frequency Handover

-Intra-frequency handover is performed between cells on the same frequency in the LTE system.

+Inter-frequency Handover

-Inter-frequency handover is performed between cells on different frequencies in the LTE system.

+Inter-RAT Handover

-Inter-RAT handover is performed from LTE to GSM/WCDMA/TD-SCDMA/CDMA2000

+All the handover involve 3 setups

Measurement step: UE strart intra-frequency or inter-frequency measurement according to the different triggers

Decision step: eNodeB make handover decision according to the resource state and related parameters

Execution step: eNodeB send execution command to UE to perform handover

Different types of handover measurement are triggered by different causes

Handover Type:

1-  Coverage   based:

Intra-frequency :The measurement configuration is performed when the UE establishes a radio bearer. The UE performs intra-frequency measurements by default.

Inter-frequency/ Inter-RAT :The eNodeB delivers the measurement configuration to the UE when the signal quality in the serving cell is lower than the associated threshold.

2-Load based(Intra-frequency / Inter-frequency / Inter-RAT) :The measurements are triggered by the Mobility Load Balancing (MLB) algorithm.

3-Frequency-priority-based (Inter-frequency) : The measurements are triggered by  A1 event

4- Service based (Inter-RAT/Inter-frequency) : The eNodeB triggers the measurements after it finds that only the voice service is running on the UE.

5-UL service quality Based(Inter-RAT/Inter-frequency) : The eNodeB triggers the measurement after it detects UL power insufficient at the UE

6-Distance based(Inter – frequency/Inter -RAT):The measurements are triggered by distance if it is more than the specific threshold 

Measurement Control Configuration : 

+eNodeB should deliver all the measurement control parameters to UE via RRC reconfiguration, including measurement object, report configuration, measurement ID and filter configuration .

+Besides, for inter-frequency/inter-RAT measurement, it also includes gap configuration for gap-assisted measurement.

When a UE establishes a radio bearer, the eNodeB delivers the intra-frequency measurement configuration to the UE through an RRC Connection Reconfiguration message by default. Then, the UE performs intra-frequency measurements by default. 

When measurement gaps need to be set up, the eNodeB delivers the inter-frequency and/or inter-RAT measurement configuration to the UE. After that, the UE performs gap-assisted inter-frequency and/or inter-RAT measurements. Inter-frequency and inter-RAT measurements can use the same gap pattern, but the eNodeB is able to differentiate between the gap configurations for inter-frequency and inter-RAT measurements. 

Events :

 EVENT A1

* Event A1 is triggered when the serving cell becomes better than a threshold. The event is triggered when the following condition is

true:

Meas serv - Hyst > Threshold

* Triggering of the event is subsequently cancelled when the following condition is true:

Me as serv + Hyst < Threshold

* The hysteresis can be configured with a value between 0 and 30 dB

* When using RSRP, the threshold can be configured with a value between -140 and -44 dBm. The value of the threshold is signalled

using the mapping presented in Table 242 (using a signalled value of between 0 and 97)

* When using RSRQ, the threshold can be configured with a value between -3 and -19.5 dB. The value of the threshold is signalled

using the mapping presented in Table 243 (using a signalled value of between 0 and 34)

 EVENT A2

* Event A2 is triggered when the serving cell becomes worse than a threshold. The event is triggered when the following condition is

true:

Meas serv + Hyst < Threshold

* Triggering of the event is subsequently cancelled when the following condition is true:

Me as serv - Hyst > Threshold

* The hysteresis can be configured with a value between 0 and 30 dB

* When using RSRP, the threshold can be configured with a value between -140 and -44 dBm. The value of the threshold is signalled

using the mapping presented in Table 242 (using a signalled value of between 0 and 97)

* When using RSRQ, the threshold can be configured with a value between -3 and -19.5 dB. The value of the threshold is signalled

using the mapping presented in Table 243 (using a signalled value of between 0 and 34)

 EVENT A3

* Event A3 is triggered when a neighbouring cell becomes better than the serving cell by an offset. The offset can be either positive or

negative. The event is triggered when the following condition is true:

Meas neigh + 0 neigh,Jreq + 0 neigh,cell - Hyst > Me as serv + 0 serv,Jreq + 0 serv,cell +Offset

* Triggering of the event is subsequently cancelled when the following condition is true:

Meas neigh + 0 neigh,freq + 0 neigh,cell + Hyst < Meas serv + 0 serv,freq + 0 serv,cell +Offset

* Both the neighbour and serving cell can have frequency specific and cell specific offsets applied to their measurements. Each of these

offsets can be configured with values between -24 and +24 dB

* The additional Offset added to the serving cell measurement can be configured with a value between -30 and +30 dB

* The hysteresis can be configured with a value between 0 and 30 dB

 EVENT A4

* Event A4 is triggered when a neighbouring cell becomes better than a threshold. The event is triggered when the following condition

is true:

Meas neigh + 0 neigh,freq + 0 neigh,ce/1 - Hyst > Threshold

* Triggering of the event is subsequently cancelled when the following condition is true:

Me as neigh + 0 neigh,freq + 0 neigh ,ce/1 + Hyst < Threshold

* The neighbour cell can have frequency specific and cell specific offsets applied to its measurements. Both offsets can be configured

with values between -24 and +24 dB

* The hysteresis can be configured with a value between 0 and 30 dB

* When using RSRP, the threshold can be configured with a value between -140 and -44 dBm. The value of the threshold is signalled

using the mapping presented in Table 242 (using a signalled value of between 0 and 97)

* When using RSRQ, the threshold can be configured with a value between -3 and -19.S dB. The value of the threshold is signalled

using the mapping presented in Table 243 (using a signalled value of between 0 and 34)

* UE support for event A4 is intended to be mandatory, but event A4 may not have been implemented and tested for some early devices.

UE use release 8 Feature Group Indicator (FGI) bit 14 to indicate whether or not event A4 has been implemented and tested. The FGI

bit string can be included within a UE Capability Information message

EVENT A5

* Event A5 is triggered when the serving cell becomes worse than threshold! while a neighboring cell becomes better than threshold2.

The event is triggered when both of the following conditions are true:

Me as serv + Hyst < Threshold!

Meas neigh + 0 neigh,freq + 0 neigh,cell - Hyst > Threshold2

* Triggering of the event is subsequently cancelled when either of the following conditions are true:

Me as serv - Hyst > Threshold!

Meas neigh + 0 neigh,freq + 0 neigh,cell + Hyst < Threshold2

* The neighbour cell can have frequency specific and cell specific offsets applied to its measurements. Both offsets can be configured

with a value between -24 and +24 dB

* The hysteresis can be configured with a value between 0 and 30 dB

* When using RSRP, the threshold can be configured with a value between -140 and -44 dBm. The value of the threshold is signalled

using the mapping presented in Table 242 (using a signalled value of between 0 and 97)

* When using RSRQ, the threshold can be configured with a value between -3 and -19.S dB. The value ofthe threshold is signalled

using the mapping presented in Table 243 (using a signalled value of between 0 and 34)

* UE support for event AS is intended to be mandatory, but event AS may not have been implemented and tested for some early devices.

UE use release 8 Feature Group Indicator (FGI) bit 14 to indicate whether or not event AS has been implemented and tested. The FGI

bit string can be included within a UE Capability Information message

EVENT A6

* Event A6 is triggered when a neighbouring cell becomes better than a secondary cell by an offset. The offset can be either positive or

negative.

* This measurement reporting event is applicable to L TE Advanced connections using Carrier Aggregation, i.e. connections which have

secondary serving cells (in addition to a primary serving cell)

* The event is triggered when the following condition is true:

Measneigh + Oneigh,cell - Hyst > Me as sec + Osec,cell +Offset

* Triggering of the event is subsequently cancelled when the following condition is true:

Measneigh + Oneigh,cell + Hyst < Meassec + Osec,cell +Offset

* The neighbour cell has to be on the same frequency as the secondary serving cell

* Both the neighbour and secondary serving cell can have cell specific offsets applied to their measurements. Each of these offsets can

be configured with values between -24 and +24 dB

* The additional Offset added to the secondary serving cell measurement can be configured with a value between -30 and +30 dB

* The hysteresis can be configured with a value between 0 and 30 dB

* UE support for event A6 is intended to be mandatory for all 3GPP release I 0 UE which support Carrier Aggregation. Nevertheless,

event A6 may not have been implemented and tested for some early release I 0 devices. UE use release I 0 Feature Group Indicator

(FGI) bit II to indicate whether or not event A6 has been implemented and tested. UE are only permitted to signal their support for

event A6 if they support Carrier Aggregation. The FGI bit string can be included within a UE Capability Information message

 EVENT B1

* Event B 1 is triggered when a neighbouring inter-system cell becomes better than a threshold. The event is triggered when the

following condition is true:

Me as neigh + 0 neigh,Jreq - Hyst > Threshold

* Triggering of the event is subsequently cancelled when the following condition is true:

Meas neigh + 0 neighJreq + Hyst < Threshold

* The inter-system neighbour cell can have a frequency specific offset applied to its measurements. Cell specific offsets are not

applicable to inter-system measurements. The offset can be configured with a value between -15 and + 15 dB

* The hysteresis can be configured with a value between 0 and 30 dB

* When using CPICH RSCP, the threshold can be configured with a value between -120 and -25 dBm. The value of the threshold is

signalled using the mapping specified in 3GPP TS 25.133 (using a signalled value of between -5 and 91)

* When using CPICH Ec/Io, the threshold can be configured with a value between -24 and 0 dB. The value of the threshold is signalled

using the mapping specified in 3GPP TS 25.133 (using a signalled value of between 0 and 49)

* When using GSM RSSI, the threshold can be configured with a value between -110 and -48 dB. The value of the threshold is signalled

using the mapping specified in 3GPP TS 45.008 (using a signalled value of between 0 and 63)

* UE support for event Bl is intended to be mandatory for UE which support inter-RAT measurements. Nevertheless, event Bl may not

have been implemented and tested for some early devices. UE use release 8 Feature Group Indicator (FGI) bit 15 to indicate whether

or not event B 1 has been implemented and tested. UE are only permitted to signal their support for event B 1 if they support inter-RAT

measurements for UTRAN, GERAN, CDMA2000 lxRTT or CDMA2000 HRPD (FGI bits 22, 23, 24 and 26 respectively). The FGI

bit string can be included within a UE Capability Information message

 EVENT B2

* Event B2 is triggered when the serving cell becomes worse than threshold! while a neighbouring inter-system cell becomes better than

threshold2. The event is triggered when both of the following conditions are true:

Meas serv + Hyst < Threshold!

Meas neigh + 0 neigh,Jreq - Hyst > Threshold2

* Triggering of the event is subsequently cancelled when either of the following conditions are true:

Me as serv - Hyst > Threshold!

Me as neigh + 0 neigh,Jreq + Hyst < Threshold2

* The inter-system neighbour cell can have a frequency specific offset applied to its measurements. Cell specific offsets are not

applicable to inter-system measurements. The offset can be configured with a value between -15 and+ 15 dB

* The hysteresis can be configured with a value between 0 and 30 dB

* When using CPICH RSCP, threshold2 can be configured with a value between -120 and -25 dBm. The value of the threshold is

signalled using the mapping specified in 3GPP TS 25.133 (using a signalled value of between -5 and 91)

* When using CPICH Ec/Io, threshold2 can be configured with a value between -24 and 0 dB. The value of the threshold is signalled

using the mapping specified in 3GPP TS 25.133 (using a signalled value of between 0 and 49)

* When using GSM RSSI, threshold2 can be configured with a value between -110 and -48 dB. The value of the threshold is signalled

using the mapping specified in 3GPP TS 45.008 (using a signalled value of between 0 and 63)

* UE support for event B2 is intended to be mandatory for UE which support inter-RAT measurements. Nevertheless, event B2 may not

have been implemented and tested for some early devices. UE use release 8 Feature Group Indicator (FGI) bits 22, 23, 24 and 26 to

indicate whether or not event B2 has been implemented and tested for UTRAN, GERAN, CDMA2000 lxRTT and CDMA2000

HRPD respectively. These FGI bits also indicate UE support for the relevant inter-RAT measurements. The FGI bit string can be

included within a UE Capability Information message

LTE Power Control ( Downlink power Allocation)

LTE Power Control 

Purpose of Power Control

+ Compensate path loss , including shadow fading and multiple path fading

+ Reduce interference on the edge cell

-LTE systems use the Orthogonal Frequency Division Multiple Access (OFDMA) technique on the downlink and the Single Carrier Frequency Division Multiple Access (SC-FDMA) technique on the uplink. With these techniques, the subcarriers of UEs in a cell are orthogonal. Power control compensates for path loss and shadow fading and counteracts interference between cells. In LTE systems, power control is performed on eNodeBs and UEs. 

DL Power Distribution

- Downlink power control is achieved through fixed power assignment or dynamic power control.

 + Fixed power assignment

Fixed power assignment is applicable to the cell-specific reference signal, synchronization signal, PBCH, PCFICH, and the PDCCH and PDSCH that carry common information of the cell. Users configure fixed power based on channel quality. The configured power must meet the requirements for the downlink coverage of the cell.

+ Dynamic power control

Dynamic power control is applicable to the PHICH and the PDCCH and PDSCH that carry dedicated information sent to UEs. Dynamic power control lowers interference, expands cell capacity, and increases coverage while meeting users' QoS requirements. However, these channels can also support fix power assignment, and in fact, this is our recommendation because the AMC function can also meet the requirement of QoS

Visit our Facebook Page 

Engineering-Academy

Cell Specific RS Power Setting

EPRE: Energy Per Resource Element

The RS power setting is based on EPRE

RS power is the reference power for the other channel

- The cell-specific reference signal is transmitted in all downlink subframes.

 - The signal serves as a basis for downlink channel estimation, which is used for data demodulation.

The power for the cell-specific reference signal is set through the ReferenceSignalPwr parameter, which indicates the Energy Per Resource Element (EPRE) of the cell-specific reference signal.

PB indicator the power ratio between type B symbol and type A symbol, which is specified by 3GPP protocol

RS Power ＝ Total power per channel(dbm) – 10lg(total subcarrier)+10lg(Pb + 1)

Pb determine the RS occupation  of total power, Pb=1 indicate the RS power is 9.4% of total power

Visit our Facebook Page 

Engineering-Academy

Synchronization Signal Power Setting

The synchronization signal is used for cell search and system synchronization. There are two types of synchronization signals, the Primary Synchronization Channel (P-SCH) and the Secondary Synchronization Channel (S-SCH).

The offset of the power for the P-SCH and S-SCH against the power for the cell-specific reference signal is set through the SchPwr parameter.

PowerSCH ＝ ReferenceSignalPwr +SchPwr

PBCH/PCFICH Power Setting

On the PBCH, broadcast messages are sent in each frame. The messages carry the basic system information of the cell, such as the cell bandwidth, antenna configuration, and frame number.

The offset of the power for the PBCH against the power for the cell-specific reference signal is set through the PbchPwr parameter.

The PCFICH carries the number of OFDM symbols used for PDCCH transmission in a subframe. The PCFICH is always mapped to the first OFDM symbol of each subframe.

The power for the PCFICH is set through the PcfichPwr parameter, which indicates an offset of the power for the PCFICH against the power for the cell-specific reference signal.

PowerPBCH ＝ReferenceSignalPwr + PbchPwr

PowerPCFICH ＝ ReferenceSignalPwr + PcfichPwr

PDCCH/PDSCH Power Setting

In the following two status, the power for PDCCH and PDSCH  power is fixed setting

When PDCCH carry scheduling information of common control information (RACH response /paging/SIBs indicator )

When PDSCH carry the common info (RACH response/SIB/paging message)

Dynamic Power Allocation - PHICH

The PHICH occupies very few resources, which means decreasing its transmit power cannot significantly reduce power consumption. In addition, the PHICH carries the ACK/NACK messages for the uplink data, which requires high accuracy. Decreasing the transmit power of the PHICH may reduce accuracy and uplink data rate. Therefore, the dynamic power allocation is not recommended in commercial network. 

Visit our Facebook Page 

Engineering-Academy

Dynamic Power Control - PDCCH

When PDCCH carry the following dedicate info,  power  control should be performed to ensure the receive reliability 

Uplink scheduling information (DCI format 0)

Downlink scheduling information (DCI format 1/1A/1B/2/2A)

PUSCH/PUCCH TPC commands (DCI format 3/3A)

PDSCH Power Presentation

The presentation of  PDCCH power

Regarding power control for the PDSCH, the OFDM symbols on one slot can be classified into two types. Above table shows the OFDM symbol indexes within a slot where the ratio of the EPRE to the EPRE of RS is denoted by ρA or ρB.

Power control for the PDSCH determines the EPREs of different OFDM symbols using ρA and ρB. ρA determines the power offset against the power for the RS when there is no reference signal on the PDSCH, and ρB determines the power offset against the power for the cell-specific reference signal when there is a reference signal on the PDSCH.

PPDSCH_A ＝ ρA + ReferenceSignalPwr

PPDSCH_B ＝ ρB + ReferenceSignalPwr

Visit our Facebook Page 

Engineering-Academy

LTE voice solutions CS fallback

CS Fallback 

overview:

lthere are two standard solutions to provide CS services for E-UTRAN UEs:

CS Fallback has a simpler network architecture compared with VoIP over IMS.

In LTE architecture, the circuit switched (CS) fallback in EPS enables the provisioning of voice and traditional CS-domain services (e.g. CS UDI video/ SMS/ LCS/ USSD). To provide these services LTE reuses CS infrastructure when the UE is served by E-UTRAN. 

A CS Fallback enabled terminal is redirected to 2G/3G network after it initiates a CS service such as voice calls.

CS Fallbcak procedure 

To support CS Fallback, the SGs interface is required, so as to let the MME perform a UE location update over the SGs interface so that the core network of the UTRAN or GERAN learns about UE location.

After a UE is powered on in the E-UTRAN, it initiates a combined EPS/IMSI attach procedure.

If a UE is camping on an E-UTRAN cell, it periodically initiates a combined TAU/LAU procedure, which allows for simultaneous UE location updates both in the MME and in the core network of the UTRAN or GERAN.

The Combined EPS/IMSI Attach Procedure is shown in the prvious snapshot:

After the RRC connection setup, the UE sends an Attach Request message to the MME, requesting a combined EPS/IMSI attach procedure. This message also indicates whether the CS Fallback or SMS over SGs function is required.
The MME allocates an LAI to the UE, and then it finds the MSC/VLR for the UE based on the TAI-LAI mapping. If multiple PLMNs are available for the CS domain, the MME selects a CS PLMN based on the selected PLMN information reported by the eNodeB. Then, the MME sends the MSC/VLR a Location Update Request message  over the SGs interface so that the core network of the UTRAN or GERAN learns about the UE location, which contains the new LAI, IMSI, MME name, and location update type.
The MSC/VLR performs the location update procedure in the CS domain.
The MSC/VLR responds with a “Location Update Accept” message that contains information about the VLR and temporary mobile subscriber identity (TMSI). The location update procedure is successful.
At last, the UE is informed that the combined EPS/IMSI attach procedure is successful by RRC Connection Reconfiguration message. (If the network supports SMS over SGs but not CS Fallback, the message transmitted to the UE contains the information element (IE) SMS-only. The message indicates that the combined EPS/IMSI attach procedure is successful but only SMS services are supported.)

Visit our Facebook Page 

Engineering-Academy

CS Fallback to UTRAN

Based on the capabilities of UEs and networks, the following mechanisms are available for an eNodeB to perform CS Fallback to UTRAN

-CS Fallback based on PS redirection

Flash CS Fallback(with RIM)

-CS Fallback based on PS handover

Flash CS Fallback(Blind handover)

CS Fallback Based on PS Redirection(UMTS)

- Once a LTE UE initiates a voice request, MME informs the eNodeB to perform a CS Fallback procedure:

- The UE sends the MME an NAS message Extended Service Request to initiate a CS service.

- The MME sends an S1-AP Request message to instruct the eNodeB to initiate a CS Fallback procedure (If the MME supports the LAI-related feature, the MME also delivers the LAI to the eNodeB).

- The eNodeB sends an RRC Connection Release message to instruct the UE to perform a redirection. The message contains information about a target UTRAN frequency. If flash CS Fallback is available, the RRC Connection release message includes information about a target UTRAN frequency,PSC and their associated system information, In this way, the UE can quickly access the target UTRAN without the need to perform the procedure for acquiring system information of the target UTRAN cell. Then, the UE can directly initiate a CS service in the UTRAN cell.

- Then, the eNodeB initiates an S1 UE context release procedure.

- The UE may initiate an LAU, a combined RAU/LAU, or both an RAU and an LAU in the target cell and initiates a CS call establishment procedure in the target UTRAN cell.

RAN Information Management (RIM) Procedure

To support Flash CS Fallback, eNodeB requires exchange information between E-UTRAN and GERAN/UTRAN through the core networks

- Flash CS Fallback is defined in 3Gpp R9 .With this function, SIB can be included into the ”RRC connection Release” during the redirection procedure. This is achived by the RIM procedure. with RIM, eNodeB can get information from GERAN/UMTS.

- The RIM procedure supports two information exchange modes:

 Single Report and Multiple Report. 

- In Single Report mode, the source sends a request, and then the target responds with a single report.

- In Multiple Report mode, the target responds with a report after receiving a request from the source, and it also sends a report to the source each time the system information changes.

- the RIM procedure in Multiple Report mode is performed as follows: After an E-UTRAN cell is set up, the eNodeB sends a request for system information to neighboring UTRAN cells. After a neighboring UTRAN cell receives a request or the system information changes, this cell sends the system information to the eNodeB.

- If an eNodeB supports flash CS Fallback, it requires the system information of neighboring UTRAN cells to perform a redirection. If the serving cell does not have that information, the eNodeB must initiate an RIM procedure in Single Report mode to acquire the system information.

CS Fallback Based on PS Handover(UMTS)

Once a LTE UE initiates a voice request, MME informs the eNodeB to perform a CS Fallback procedure:

The UE sends the MME an NAS message Extended Service Request to initiate a CS service.

The MME sends an S1-AP Request message to instruct the eNodeB to initiate a CS Fallback procedure (If the MME supports the LAI-related feature, the MME also delivers the LAI to the eNodeB).

The eNodeB initiates the preparation phase for a PS handover. If the preparation is successful, the eNodeB instructs the UE to perform a handover.

After the handover, the UE may initiate an LAU or combined RAU/LAU procedure in the UTRAN.

The UE’s context in EPS is released.

CS Fallback Procedure for terminated Calls(UMTS)

CS Fallback procedure for a terminated call is shown in the slide:
The MSC sends a Paging Request message from the CS domain to the MME over the SGs interface. Then, either of the following occurs:
-If the UE is in idle mode, the MME sends a Paging message to the eNodeB. Then the eNodeB sends a Paging message over the Uu interface to inform the UE of an incoming call from the CS domain, then UE initiates a connection establish procedure.
- If the UE is in active mode（connected）, the MME sends the UE an NAS message to inform the UE of an incoming call from the CS domain.
- The UE sends an Extended Service Request message containing a CS Fallback Indicator after receiving the paging message from the CS domain.
- The MME instructs the eNodeB over the S1 interface to perform CS Fallback.
- The subsequent steps are similar to the originated CS Fallback to UTRAN.
- The service request message from the UTRAN cell to UMTS CN is considered as the Paging Response message.

Visit our Facebook Page 

Engineering-Academy

CS Fallback to GERAN

Based on the capabilities of UEs and networks, the following mechanisms are available for an eNodeB to perform CS Fallback to GERAN

- CS Fallback based on PS redirection

Flash CS Fallback(with RIM)

- CS Fallback based on PS handover

Flash CS Fallback(Blind handover)

- CS Fallback based on CCO/NACC

-The procedures of CS Fallback to GERAN are similar with those of to UTRAN, just the CCO/NACC is particularly for GSM.

-During CS Fallback based on CCO/NACC, the eNodeB receives a CS Fallback Indicator from the MME, and then it sends a Mobility From EUTRA Command message to the UE over the Uu interface. The message contains information about the operating frequency, ID, and system information of a target GERAN cell. The UE searches for the target cell based on the information it received, and then it performs initial access to the cell to initiate a CS service.

CS Fallback Based on CCO/NACC(GERAN)

- The Cell Change order (CCO) procedure with Network Assisted Cell Change (NACC) is an alternative to the RRC Connection Release with Redirection procedure used for CS Fallback. The main difference is that the UE is moved to the target RAT whilst in RRC Connected Mode, also MME can get some response(UE Context Required) from GSM so as to trigger the UE context release procedure.

- In this CS Fallback procedure, the eNodeB sends a “Mobility From EUTRA Command” message over the Uu interface to indicate the operating frequency and ID of the target GERAN cell. If the source cell has the system information of the target cell, the system information is also carried in the message.

CSFB For SMS and LCS service

SMS :short message

LCS : location service

- SMS services are unknown to the eNodeB because SMS messages are encapsulated in NAS messages. During interworking with the UTRAN, SMS messages are exchanged between the MME and the MSC over the SGs interface. Because a UE does not require fallback to the UTRAN/GERAN to perform an SMS service, the SMS over SGs function can be used in a place covered only by the E-UTRAN.

- After a UE initiates an LCS request, the MME performs an attach or combined TAU/LAU procedure to inform the UE of the LCS capability of the EPS. If the EPS does not support LCS, the UE falls back to the UTRAN to initiate LCS under the control of the EPS. The CSFB procedure is the same as the procedure for CSFB to UTRAN for mobile-originated calls.

Visit our Facebook Page 

Engineering-Academy

Carrier Aggregation in LTE

3rd Generation Partnership Project (3GPP) requires LTE-Advanced networks to provide a downlink peak data rate of 1 Gbit/s. However, radio spectrum resources are so scarce that in most cases an operator owns only non-adjacent chunks of the spectrum. Due to the limited bandwidth of a single chunk of the spectrum, the 1 Gbit/s data rate requirement is hard to meet.

To deal with this situation, 3GPP TR 36.913 of Release 10 introduced carrier aggregation (CA) to LTE-Advanced networks, allowing aggregation of contiguous or non-contiguous carriers. CA achieves wider bandwidths (a maximum of 100 MHz) and higher spectral efficiency (especially in spectrum refarming scenarios).

During CA, upper-layer data streams are mapped to individual component carriers (CCs) at the Media Access Control (MAC) layer in LTE-Advanced networks. An eNodeB constructs one (two or more in the case of spatial multiplexing) transport block (TB) in each transmission time interval (TTI) for each CC. Each CC uses its own hybrid automatic repeat request (HARQ) entities and link adaptation mechanism. Therefore, the LTE-Advanced system can inherit single-carrier-based physical layer designs from the LTE system.

why we use the Carrier Aggregation :

Maximized Resource Utilization

A CA-capable UE (referred to as CA UE in this document) can use idle resource blocks (RBs) on up to five CCs to maximize utilization of resources.

Efficient Utilization of Non-contiguous Spectrum Chunks

With CA, an operator's non-contiguous spectrum chunks can be aggregated for efficient utilization.

Better User Experience

With CA enabled, a single UE can reach higher uplink and downlink peak data rates. On a network that serves a number of UEs, CA UEs with activated secondary serving cells (SCells) can use idle resources in their SCells and achieve increased throughput if the network is not overloaded.

Introduction of carrier aggregation influences mainly MAC and the physical layer protocol, but also some new RRC messages are introduced. In order to keep R8/R9 compatibility the protocol changes will be kept to a minimum. Basically each component carrier is treated as an R8 carrier. However some changes are required, such as new RRC messages in order to handle SCC, and MAC must be able to handle scheduling on a number of CCs. Major changes on the physical layer are for example that signaling information about scheduling on CCs must be provided DL as well as HARQ ACK/NACK per CC must be delivered UL and DL,as shown in the below figure:

The uplink and downlink air-interface protocol stack with CA enabled has the following characteristics:

    A single radio bearer has only one Packet Data Convergence Protocol (PDCP) entity and one Radio Link Control (RLC) entity. In addition, the number of CCs at the physical layer is invisible to the RLC layer.

    User-plane data scheduling at the MAC layer is performed separately for individual CCs.

    Each CC has an independent set of transport channels and separate HARQ entities and retransmission processes.

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, see figure 3. The RRC connection is only handled by one cell, the Primary serving cell, served by the Primary component carrier (DL and UL PCC). It is also on the DL PCC that the UE receives NAS information, such as security parameters. In idle mode the UE listens to system information on the DL PCC. On the UL PCC PUCCH is sent. The other component carriers are all referred to as Secondary component carriers (DL and UL SCC), serving the Secondary serving cells, see figure 3. The SCCs are added and removed as required, while the PCC is only changed at handover. 

there are a lot of scenarios that we can use the CA in it:

1- intra-eNodeB CO Coverage 

  2- intra-eNodeB different coverage carriers 

3- Intra-eNodeB carriers (one for macro coverage; another for edge coverage)

4-Intra-eNodeB carriers (one provided by the site; another provided by RRHs)

5-Intra-eNodeB carriers (one provided only by the site; another provided by the site and a repeater)

LTE Multipath propagation and channel coding(interleaving and SCRAMBLING)

Many radio propagation effects such as reflections can attenuate the transmitted radio signal

Visit our Facebook Page 

Engineering-Academy

Multipath Propagation gives rise to:

1. InterSymbol Interference (ISI)

2. Fast fading (Rayleigh fading)

This occurs when the propagation wave reflects on an object, which is large compared to the wavelength, for example, the surface of the earth, buildings, walls, etc. This phenomenon is called multipath propagation and it has several effects, these are:
- Rapid changes in signal strength over a small area or time interval
- Random frequency modulation due to varying Doppler shifts on different multipath signals.
- Time dispersion caused by multipath propagation delays

Previous symbol leaks into current symbol due to the different path delays
+ When the amount of ISI exceeds a certain level (~10%) bit errors occur
+ Can be reduced with equalizers, rake receivers or the use of OFDM

Multipath propagation yields signal paths of different lengths with different times of arrival at the receiver. Typical values of time delays (μs) are 0.2 in Open environment, 0.5 Suburban and 3 in

Urban.

This results in a varying received signal power as illustrated

This attenuation can result in bit errors that occur in consecutive blocks of data (burst errors). As a result the decoder fails to recover such errors.

Interleaving

The solution to overcome the problem with burst errors is to use a block interleaving technique

A radio channel produces bursty errors. Because continuous codes are most effective against random errors, interleaving is used to randomize the bursty errors. The interleaving scheme can be either block interleaving or convolutional interleaving. Typically, block interleaving is used in cellular applications. The first step of interleaving is determined by the delay requirements of the service.

Visit our Facebook Page 

Engineering-Academy

SCRAMBLING

In LTE, a frequency reuse of 1 will typically be used. This means that all cells use the same frequency band(s). For UEs close to the cell border, this will lead to massive interference in both UL and DL.

In order to reduce this inter cell interference, a cell specific bitlevel scrambling is applied for all transmissions in both UL and DL.

Other solutions for mitigating the inter-cell interference includes e.g. Inter Cell Interference Co ordination (ICIC). ICIC co-ordinates the radio resource allocations (scheduling) between neighboring

cells that experience problems.

Cell specific bit-level scrambling used in LTE for all datastreams in UL and DL

+ used in order to achieve interference randomization between cells

+ No frequency planning (freq reuse 1)

– massive inter-cell interference mitigated by scrambling and interference co-ordination techniques (e.g. ICIC)

+ Common scrambling used for cells in broadcast/multicast service transmissions (MBMS)

LTE Error detection and Correction - CRC and FEC Coding

Error detection and Correction - CRC and FEC Coding

In all radio systems the air interface will add noise to the signal . This will produce a distortion in the received signal. 

In the case of an analogue cellular system the human ear perform error correction of this received signal and noise. However in digital systems we do not have this luxury.

This noise will result in bit errors, that is what left the transmitter as a logic 1 could be interpreted as a logic 0 if the level of noise lowers the amplitude below the threshold for a logic 0. The same

could be the case for a transmitted logic 0 being interpreted as a logic 1.

All digital systems must have some method of overcoming these errors.

facebook page 

                                     Engineering-Academy

Cyclic Redundancy Check (CRC)

Cyclic Redundancy Check (CRC) is used to detect if there are any uncorrected errors left after error correction.

Blocks of data are passed through a CRC generator , which will perform a mathematical division on the data producing a remainder or checksum. This is added to the block of data and

transmitted.

The same division is performed on the data block in the receiver. If a different checksum is produced the receiver will know that there is an error in the block of data (alternatively there is an error in the

received checksum). This knowledge can be used to calculate Block Error Ratio (BLER) used in the outer loop power control.

The longer the checksum, the greater is the accuracy of the process. In the example the checksum is twelve bits long. 24 bits of binary information represents 16 777 216 (224) different combinations. It

could be imagined that various combinations of errors on the data and the checksum would produce the same checksum. The longer the checksum the less likely it is for this to happen. LTE uses a 24

bit CRC for the user data channels.

Forward Error Correction (FEC)

The next part in the transmitter is Forward Error Correction (FEC). The function of this block is to help the receiver correct bit errors caused by the air interface.

One method for correcting these errors would be to send the information a number of times  Provided this is more than twice, the receiver could select which message is most correct by a “best out of three” decision. The more times the data is transmitted the better is the error protection. However the

bandwidth is also increased proportionally What is required is a system that provides forward error correction with minimal increase in the bandwidth.

There are two basic types of FEC available, block or continuous codes.

Block Codes (Hamming Codes, BCH Codes, Reed-Solomon

Codes)

– Continuous Codes (Convolutional Codes, Turbo Codes)

+ Data is processed continuously through FEC generator

+ Resulting data stream has built-in redundancy that can be extracted to correct bit errors.

– LTE uses Turbo codes with rate 1/3 for DL-SCH transmissions.

– Convolutional coding used for BCH

Continuous codes, such as convolutional codes and turbo codes are continuously produced as the data is fed to the FEC. The result will contain redundant bits that help to correct errors.

LTE utilizes turbo coding for the user data, regardless of if a low latency and real time processing are required or not. This type of coding gives a much better error correction performance than

traditional methods, as for example convolutional coding. Some signaling, however, uses convolutional coding.

Convolutional coding

facebook page 

                                     Engineering-Academy

The original data is fed to the FEC generator, which in this case produces twice as much data. A coder that produces this increase, that is, two bits out for one bit in is known as a 1/2 rate coder. One

that produces three bits of information for one input is known as a 1/3 rate coder. This output is not simply the input data repeated; it will be subjected to noise superimposed by the RF transmission

path.

In the receiver, a device known as a ‘Viterbi Decoder’ can be used to correct these errors and recover the original data. This device works by taking the actual level of the data and estimating whether this was a 1 or a 0 when it left the transmitter, rather than use thresholds for 1 and 0.

facebook page 

                                     Engineering-Academy