A Look into 5G Virtual/Open RAN - Part 6: Inter-gNB CU Handover involving Xn

In previous blog posts I have discussed intra-gNB-DU handover and inter-gNB-DU handover scenarios.Now it is time to look at inter-gNB-CU handover that uses the Xn interface.

At the RRC protocol layer there will be the measurement setups and measurement reports as in the intra-gNB handover cases. And F1AP UE Context Setup and Release Procedures are identical with the ones discussed for inter-gNB-DU handover. Only the cause values are expected to be different, e.g. "successful handover".

Thus, I do not want to  focus here on la adder diagram call flow (that is by the way very well described in 3GPP 38.401, chapter 8.9.4), but invite you to have a look at a "big picture" that you see below.

(click image to enlarge)

What characterizes the inter-gNB handover is the transfer of the UE RRC/NGAP context form the source gNB-CU to the target gNB-CU. When the Xn interface is available to connect two neighbor gNBs this context transfer is executed using the XnAP Handover Preparation procedure. The Initiating Message of this procedure transfers the UE context parameters to the target gNB-CU. Then embedded in the Successful Outcome message the handover command is sent in return to the source gNB-CU that forwards it to the UE. In addition a temporary user plane transport tunnel for the purpose of data forwarding is established and later on released on the Xn user plane interface.

Once the UE performed the handover on the radio interface all the transport tunnels for the payload transmission need to be switched from the old gNB to the new one. This includes the tunnel to the UPF that is managed by the NGAP. Thus, the target gNB-CU starts the NGAP Path Switch procedure. 

In the target gNB environment it is necessary to establish a new F1AP UE context, new E1AP Bearer Context and new F1-U payload transport tunnel. All this happens BEFORE the Handover Command is sent to the source gNB/UE. And once there is an indication that the handover is completed all the radio and transport resources controlled by the source gNB will be released.

So the figure above looks complicated, but actually the underlying logic of context/data forwarding, radio resource allocation and transport tunnel switching is quite simple.

Special note: In case there is no Xn interface available the UE context/handover information can be transmitted using NGAP Handover Preparation procedure on the source side of the connection and NGAP Handover Resource Allocation procedure on the target side of the connection.

Related Posts:

• A Look into 5G Virtual/Open RAN - Part 7: Change of gNB-CU-UP without Handover
• 3G4G: Open RAN, OpenRAN and O-RAN

EN-DC SRB3 Demystified

3GPP 37.340 says that it is up the secondary node to establish "SRB3", but what exactly does this mean and how is it done?

Simple answer: The establishment of a signaling radio bearer (SRB) 3 in EN-DC mode means that RRC Measurement Reports for NR quality can be sent directly to the SgNB. This enables the 5G node to make intra-SgNB handover decisions and start the handover execution without involving the master eNodeB of the connection.

To prevent confusion the figure below shows a simplified scenario in which the Complete/Acknowledgement messages are not mentioned although they will be seen in the message flow.

A prerequisite is the successful addition of 5G radio resources as described in an earlier blog post. After this is completed the UE in the example transmits user plane information over the NR cell with the physical cell ID (PCI) = 12. In the transport network this cell is identified by NR CGI = xxxx52 (where „xxxx“ stands for a valid PLMN-ID and gNodeB-ID).

In the figure below the SgNB sends a X2AP SgNB Modification Required message that carries an embedded NR RRC cG-Config message. This cG-Config message is transparently forwarded by the MeNB to the UE. When arriving at the UE it activates CSI reference signal measurements on the 5G frequency including the serving 5G cell as well as its neighbors. It shall be noticed that here the concept of the Special Cell (SpCell) applies as it was defined for LTE-A CoMP scenarios. 

Instead of the X2AP SgNB Modification Required message the information for activating the CSI reference signal measurements can alternatively transported using the X2AP SgNB Addition Request Acknowledge or X2AP SgNB Change Required message.

In step 2 the UE sends a NR RRC (3GPP 38.331) Measurement Report that indicates a stronger 5G cell (the neighbor cell with PCI = 11) was measured. It might be a vendor-specific implementation to send this NR RRC Measurement Report simultaneously over uplink channels of the LTE radio link where it is carried by the LTE RRC Uplink Information Transfer MRDC (Multi-RAT Dual Connectivity) as well as over NR radio links where it is forwarded by the SgNB to the MeNB embedded in a X2AP RRC Transfer message.

Indeed, it is the SgNB that makes the handover decision, but since the MeNB is in charge of the signaling connection the handover command (here: another NR RRC cG-Config message that orders to switch the 5G radio link to the cell with PCI = 11) must be transmitted to the MeNB by using another X2AP SgNB Modification Required message.

After the UE received the NR CC cG-Config message sent by the SgNB the HO is executed and the 5G cell with PCI = 11 becomes the new primary secondary cell of the EN-DC connection.

Figure: Measurement Configuration, Reporting and Execution for intra-SgNB  Handover 

Guest Post: Exploring Network Convergence of Mobile, Broadband and Wi-Fi

This is a guest post by Ben Toner, Founder and Director, Numerous Networks

Are multiple networks better than one?

How many articles have you read with a title similar to "Which technology is better, 5G or Wi-Fi6?" If, like me, you regularly use Wi-Fi and cellular (I still use 4G though) then you might find it hard to take sides.

Enter Network Convergence - the concept of bringing multiple networks together to get the best of them all. Imagine, as an end user, not having to decide which network to use but instead feeling satisfied that your data was traversing the best combination of networks at that moment in time.

Imagine a business traveler being connected to Wi-Fi which is slow or busy while trying to take that all important conference call while sitting in an airport. Because you are roaming you want to use that Wi-Fi but you do not want to compromise the video call quality. If your network and device could work together to use just enough cellular data to supplement the slow Wi-Fi so that you stayed within your daily roaming quota but never lost a moment in the video call - then you would probably be very happy with that service. Better still, as you start walking off, if the call transitioned from Wi-Fi to cellular with no dropouts or hangup then you might be delighted!

Earlier I underlined best because that in itself is somewhat complicated.  The example above is easy to desribe but quite hard for to achieve within a framework where all possible scenarios are handled that well, for every user. The common questions which need to be factored into any such choice are:

• What do I as the end user want? 
• What performance can each network deliver. 
• How important is the transfer of content at that time and 
• How much am I willing to pay for it (how many MB of my data plan am I willing to use?). 

This is one of the challenges that we cannot easily solve today, but technology is being developed to help in that process. The operators and device vendors are working within standardisation to develop technology which can provide such a converged service. However at this time there is still a rules mechanism behind it all which does not really describe how user input and preference is going to be captured.

In the last 10 years I have witnessed many battles within service providers when deciding what "one size fits all" service to offer everyone when deciding how to make service provider Wi-Fi available to their customers; all fuelled by my points above.

A lot of concepts are well designed and somewhat mature but deciding exactly what will be implemented in standards is currently ongoing.

In the following slides and video I introduce this whole concept of Network Convergence. The following content introduces the concept and then takes a detailed look at the ATSSS; technology being defined in 3GPP. I also have highlighted the technologoies you can get hold of today to try out network convergence.

I encourage you all to download the example technologies and try convergence for yourself. I'm eager to hear opinions of what technologies work best for each of you. And better still, what is not being provided which you think should be...

Looking forward to your feedback and answering your questions...

Network Convergence of Mobile, Broadband and Wi-Fi from 3G4G

Ben Toner
Founder and Director, Numerous Networks

Related Posts:

• Introduction to 5G ATSSS - Access Traffic Steering, Switching and Splitting
• BT's Converged Core to Launch in 2023
• 5G and Fixed-Mobile Convergence (FMC)
• Tutorial: Service Based Architecture (SBA) for 5G Core (5GC)
• Updated 5G Terminology Presentation (Feb 2019)

RRC states in 5G

Looking back at my old post about UMTS & LTE (re)selection/handovers, I wonder how many different kinds of handovers and (re)selection options may be needed now.

In another earlier post, I talked about the 5G specifications. This can also be seen in the picture above and may be easy to remember. The 25 series for UMTS mapped the same way to 36 series for LTE. Now the same mapping will be applied to 38 series for 5G. RRC specs would thus be 38.331.

A simple comparison of 5G and LTE RRC states can be seen in the picture above. As can be seen, a new state 'RRC Inactive' has been introduced. The main aim is to maintain the RRC connection while at the same time minimize signalling and power consumption.

Looking at the RRC specs you can see how 5G RRC states will work with 4G RRC states. There are still for further studies (FFS) items. Hopefully we will get more details soon.

3GPP TS 22.261, Service requirements for the 5G system; Stage 1 suggests the following with regards to inter-working with 2G & 3G

5.1.2.2 Legacy service support
The 5G system shall support all EPS capabilities (e.g., from TSs 22.011, 22.101, 22.278, 22.185, 22.071, 22.115, 22.153, 22.173) with the following exceptions:
- CS voice service continuity and/or fallback to GERAN or UTRAN,
- seamless handover between NG-RAN and GERAN,
- seamless handover between NG-RAN and UTRAN, and
- access to a 5G core network via GERAN or UTRAN.

Connected and Autonomous Car Revolution

Last week we had the Automotive and Transport SIG event in Cambridge Wireless. There is already some good writeup on that event here and here. In this post my interest in looking at the technologies discussed.

R&S (who were the sponsors) gave their introduction presentation quite well highlighting the need and approaches for the connected car. He also introduced the IEEE 802.11p to the group.

As per Wikipedia, "IEEE 802.11p is an approved amendment to the IEEE 802.11 standard to add wireless access in vehicular environments (WAVE), a vehicular communication system. It defines enhancements to 802.11 (the basis of products marketed as Wi-Fi) required to support Intelligent Transportation Systems (ITS) applications. This includes data exchange between high-speed vehicles and between the vehicles and the roadside infrastructure in the licensed ITS band of 5.9 GHz (5.85-5.925 GHz). IEEE 1609 is a higher layer standard based on the IEEE 802.11p."

Back in December, Dr. Paul Martin did an equally useful presentation in the Mobile Broadband SIG and his presentation is equally relevant here as he introduced the different terms live V2X, V2i, V2V, V2P, etc. I have embedded his presentation below:

Roger Lanctot from Strategy Analytics, gave us some interesting facts and figures. Being based in the US, he was able to give us the view of both US as well as Europe. According to him, “LTE is the greatest source of change in value proposition and user experience for the customer and car maker. Bluetooth, Wi-Fi, NFC and satellite connectivity are all playing a role, but LTE deployment is the biggest wave sweeping the connected car, creating opportunities for new technologies and applications.” His officially released presentation is embedded below (which is much smaller than his presentation on that day.

There were also interesting presentations that I have not embedded but other may find useful. One was from Mike Short, VP of Telefonica and the other was from Dr. Ireri Ibarra of MIRA.

The final presentation by Martin Green of Visteon highlighted some interesting discussions regarding handovers that may be required when the vehicle (and the passengers inside) is moving between different access networks. I for one believe that this will not be an issue as there may be ways to work the priorities of access networks out. Anyway, his presentation included some useful nuggets and its embedded below:

Key challenges with automatic Wi-Fi / Cellular handover

Recently in a conference I mentioned that the 3GPP standards are working on standards that will allow automatic and seamless handovers between Cellular and Wi-Fi. At the same time operators may want to have a control where they can automatically switch on a users Wi-Fi radio (if switched off) and offload to Wi-Fi whenever possible. It upset quite a few people who were reasoning against the problems this could cause and the issues that need to be solved.

I have been meaning to list the possible issues which could be present in this scenario of automatically handing over between Wi-Fi and cellular, luckily I found that they have been listed very well in the recent 4G Americas whitepaper. The whitepaper is embedded below but here are the issues I had been wanting to discuss:

In particular, many of the challenges facing Wi-Fi/Cellular integration have to do with realizing a complete intelligent network selection solution that allows operators to steer traffic in a manner that maximizes user experience and addresses some of the challenges at the boundaries between RATs (2G, 3G, LTE and Wi-Fi).
Figure 1 (see above) below illustrates four of the key challenges at the Wi-Fi/Cellular boundary.
1) Premature Wi-Fi Selection: As devices with Wi-Fi enabled move into Wi-Fi coverage, they reselect to Wi-Fi without comparative evaluation of existing cellular and incoming Wi-Fi capabilities. This can result in degradation of end user experience due to premature reselection to Wi-Fi. Real time throughput based traffic steering can be used to mitigate this.
2) Unhealthy choices: In a mixed wireless network of LTE, HSPA and Wi-Fi, reselection may occur to a strong Wi-Fi network, which is under heavy load. The resulting ‘unhealthy’ choice results in a degradation of end user experience as performance on the cell edge of a lightly loaded cellular network may be superior to performance close to a heavily loaded Wi-Fi AP. Real time load based traffic steering can be used to mitigate this.
3) Lower capabilities: In some cases, reselection to a strong Wi-Fi AP may result in reduced performance (e.g. if the Wi-Fi AP is served by lower bandwidth in the backhaul than the cellular base station presently serving the device). Evaluation of criteria beyond wireless capabilities prior to access selection can be used to mitigate this.
4) Ping-Pong: This is an example of reduced end user experience due to ping-ponging between Wi-Fi and cellular accesses. This could be a result of premature Wi-Fi selection and mobility in a cellular environment with signal strengths very similar in both access types. Hysteresis concepts used in access selection similar to cellular IRAT, applied between Wi-Fi and cellular accesses can be used to mitigate this.

Here is the paper:

Integration of Cellular and Wi-Fi Networks from Zahid Ghadialy

Analysing the Network Sharing Opportunity

Interesting presentation by Thierry DUPONT, TDF in the LTE World Summit 2012.

LTE World Summit 2012 - Day 1 T1 Thierry Dupont TDF from ceobroadband

Overview of LTE Handovers

From the NTT Docomo Technical journal:


The LTE handover is broadly divided into a backward handover (PS handover) and forward handover. In the former, the network performs cell switching and notifies the mobile terminal of the destination cell, and in the latter, the mobile terminal performs autonomous switching to pick up the destination cell.


To control packet loss due to a momentary cutoff at the time of radio switching, PS handover supports a data forwarding process that transfers undelivered data from the switching-source eNodeB to the switching-destination eNodeB and a reordering process that corrects sequencing mistakes between forwarded data and new data.


The forward handover can be classified into Release with Redirection triggered by a cutoff signal from the network and Non Access Stratum (NAS) Recovery in which the mobile terminal autonomously performs a NAS recovery, either of which is accompanied by data loss due to a momentary cutoff. From a different perspective, handover can be classified in the following two ways according to whether it is accompanied by Radio Access Technology (RAT) or frequency switching or by eNodeB or EPC switching (Figure 7).


1) Intra-RAT handover: This is a handover that occurs within the LTE system in which node transition occurs between sectors within an eNodeB, between eNodeBs within an EPC switch, or between EPC switches. 


A handover between eNodeBs within an EPC switch may be an X2 or S1 handover. In an X2 handover, signal processing is performed by the X2 logical interface between eNodeBs, while in an S1 handover, signal processing is performed by the S1 logical interface between an eNodeB and the EPC switch. There is a tradeoff between the cost of maintaining an X2 link and the cost incurred by an S1 handover, and operations are configured accordingly.


Handover can also be classified by whether the center frequency is the same before and after handover, that is, whether the handover occurs within the same frequency or between frequencies.


2) Inter-RAT handover: This is a handover that occurs between RATs either as a transition from LTE to 3G or from 3G to LTE.

A detailed post on LTE to 3G Inter-RAT handover is available here.

Interoperability between LTE FDD/TDD network

In countries where FDD and TDD are both in use, it would be interesting to see Dual-mode LTE terminals that would support both TDD and FDD and it should be possible to do a reselection as well has handovers from one mode to another.

It should be noted that the Structure of TDD and FDD frames are different as shown above.

If you are wondering why we need both FDD and TDD modes in the same geographical location, its because of Spectrum being available as well as with TDD allows possibility of different UL/DL data rates which generally means more efficient use of spectrum.

Redirection, Reselection, Handovers and other Inter-RAT combinations in LTE

Another one from Qualcomm's 4G World presentation. You can see the number of scenarios that would have to be taken into account for; this was one of the reasons I believed SVLTE may be a good choice.

Related posts:

• Recap of Handover procedures in LTE
• LTE to 3G Handover Procedure and Signalling
• HeNBs (Femtocells) and eNBs Handovers
• Single Radio Voice Call Continuity (SR-VCC)

MRO: Handover failures signalling

Continuing on the Self-organising Network (SON) feature of Mobility Robust Optimisation, Handover failures.

Click on image to enlarge

One of the discussions I had with a colleague is how would the signalling happen in case of Handover failures I mentioned earlier.

After the handover failure, when the connection is successfully established again either as a normal Setup or Re-Establishment or RRC Reconfiguration then a new optional field is available:

rlf-InfoAvailable-r10 ENUMERATED {true} OPTIONAL,

This is used to indicate to the network that the UE has some information relating to the RL Failure that occurred.

The network will then use the UE Information Request I blogged about earlier to ask for this information. The UE will send the information back in the response.

It should be noted that this UEInformationRequest and Response messages were introduced part of Release-9 but there has been since some updates in Release-10. The Response message now looks as follows:

RLF-Report-r9 ::= SEQUENCE {

measResultLastServCell-r9 SEQUENCE {

rsrpResult-r9 RSRP-Range,

rsrqResult-r9 RSRQ-Range OPTIONAL

},

measResultNeighCells-r9 SEQUENCE {

measResultListEUTRA-r9 MeasResultList2EUTRA-r9 OPTIONAL,

measResultListUTRA-r9 MeasResultList2UTRA-r9 OPTIONAL,

measResultListGERAN-r9 MeasResultListGERAN OPTIONAL,

measResultsCDMA2000-r9 MeasResultList2CDMA2000-r9 OPTIONAL

} OPTIONAL,

...,

[[ locationInfo-r10 LocationInfo-r10 OPTIONAL,

failedPCellId-r10 CHOICE {

cellGlobalId-r10 CellGlobalIdEUTRA,

pci-arfcn-r10 SEQUENCE {

physCellId-r10 PhysCellId,

carrierFreq-r10 ARFCN-ValueEUTRA

}

} OPTIONAL,

reestablishmentCellId-r10 CellGlobalIdEUTRA OPTIONAL,

timeConnFailure-r10 INTEGER (0..1023) OPTIONAL,

connectionFailureType-r10 ENUMERATED {rlf, hof} OPTIONAL,

previousPCellId-r10 CellGlobalIdEUTRA OPTIONAL

]]

}

Everything after the extension marker ellipses (...) is added in release 10. More information in Release-10 RRC specs (36.331)

Mobility Robustness Optimization to avoid Handover failures

The following is from 4G Americas Whitepaper on SON:

Mobility Robustness Optimization (MRO) encompasses the automated optimization of parameters affecting active mode and idle mode handovers to ensure good end-user quality and performance, while considering possible competing interactions with other SON features such as, automatic neighbor relation and load balancing.

There is also some potential for interaction with Cell Outage Compensation and Energy Savings as these could also potentially adjust the handover boundaries in a way that conflicts with MRO. While the goal of MRO is the same regardless of radio technology namely, the optimization of end-user performance and system capacity, the specific algorithms and parameters vary with technology.

The objective of MRO is to dynamically improve the network performance of HO (Handovers) in order to provide improved end-user experience as well as increased network capacity. This is done by automatically adapting cell parameters to adjust handover boundaries based on feedback of performance indicators. Typically, the objective is to eliminate Radio Link Failures and reduce unnecessary handovers. Automation of MRO minimizes human intervention in the network management and optimization tasks.

The scope of mobility robustness optimization as described here assumes a well-designed network with overlapping RF coverage of neighboring sites. The optimization of handover parameters by system operators typically involves either focused drive-testing, detailed system log collection and postprocessing, or a combination of these manual and intensive tasks. Incorrect HO parameter settings can negatively affect user experience and waste network resources by causing HO ping-pongs, HO failures and Radio Link Failures (RLF). While HO failures that do not lead to RLFs are often recoverable and invisible to the user, RLFs caused by incorrect HO parameter settings have a combined impact on user experience and network resources. Therefore, the main objective of mobility robustness optimization should be the reduction of the number of HO-related radio link failures. Additionally, sub-optimal configuration of HO parameters may lead to degradation of service performance, even if it does not result in RLFs. One example is the incorrect setting of HO hysteresis, which may results in ping-pongs or excessively delayed handovers to a target cell. Therefore, the secondary objective of MRO is the reduction of the inefficient use of network resources due to unnecessary or missed handovers.

Most problems associated with HO failures or sub-optimal system performance can ultimately be categorized, as either too-early or too-late triggering of the handover, provided that the required fundamental network RF coverage exists. Thus, poor HO-related performance can generally be categorized by the following events:

* Intra-RAT late HO triggering

* Intra-RAT early HO triggering

* Intra-RAT HO to an incorrect cell

* Inter-RAT too late HO

* Inter RAT unnecessary HO

Up to Release 9, a UE is required to send RLF report only in case of successful RRC re-establishment after a connection failure. Release 10 allows support for RLF reports to be sent even when the RRC reestablishment does not succeed. The UE is required to report additional information to assist the eNB in determining if the problem is coverage related (no strong neighbors) or handover problems (too early, too late or wrong cell). Furthermore, Release 10 allows for precise detection of too early / wrong cell HO.

LTE Physical Layer Measurements of RSRP and RSRQ

One of the things on my mind for long time was to find a bit more about RSRP and RSRQ.

The following is from Agilent Whitepaper:

The UE and the eNB are required to make physical layer measurements of the radio characteristics. The measurement definitions are specified in 3GPP TS 36.214. Measurements are reported to the higher layers and are used for a variety of purposes including intra- and inter-frequency handover, inter-radio access technology (inter-RAT) handover, timing measurements, and other purposes in support of RRM.

Reference signal receive power (RSRP):

RSRP is the most basic of the UE physical layer measurements and is the linear average (in watts) of the downlink reference signals (RS) across the channel bandwidth. Since the RS exist only for one symbol at a time, the measurement is made only on those resource elements (RE) that contain cell-specific RS. It is not mandated for the UE to measure every RS symbol on the relevant subcarriers. Instead, accuracy requirements have to be met. There are requirements for both absolute and relative RSRP. The absolute requirements range from ±6 to ±11 dB depending on the noise level and environmental conditions. Measuring the difference in RSRP between two cells on the same frequency (intra-frequency measurement) is a more accurate operation for which the requirements vary from ±2 to ±3 dB. The requirements widen again to ±6 dB when the cells are on different frequencies (inter-frequency measurement).

Knowledge of absolute RSRP provides the UE with essential information about the strength of cells from which path loss can be calculated and used in the algorithms for determining the optimum power settings for operating the network. Reference signal receive power is used both in idle and connected states. The relative RSRP is used as a parameter in multi-cell scenarios.

Reference signal receive quality (RSRQ):

Although RSRP is an important measure, on its own it gives no indication of signal quality. RSRQ provides this measure and is defined as the ratio of RSRP to the E-UTRA carrier received signal strength indicator (RSSI). The RSSI parameter represents the entire received power including the wanted power from the serving cell as well as all cochannel power and other sources of noise. Measuring RSRQ becomes particularly important near the cell edge when decisions need to be made, regardless of absolute RSRP, to perform a handover to the next cell. Reference signal receive quality is used only during connected states. Intra- and inter-frequency absolute RSRQ accuracy varies from ±2.5 to ±4 dB, which is similar to the interfrequency relative RSRQ accuracy of ±3 to ±4 dB.

The following is from R&S white paper:

The RSRP is comparable to the CPICH RSCP measurement in WCDMA. This measurement of the signal strength of an LTE cell helps to rank between the different cells as input for handover and cell reselection decisions. The RSRP is the average of the power of all resource elements which carry cell-specific reference signals over the entire bandwidth. It can therefore only be measured in the OFDM symbols carrying reference symbols.

The RSRQ measurement provides additional information when RSRP is not sufficient to make a reliable handover or cell reselection decision. RSRQ is the ratio between the RSRP and the Received Signal Strength Indicator (RSSI), and depending on the measurement bandwidth, means the number of resource blocks. RSSI is the total received wideband power including all interference and thermal noise. As RSRQ combines signal strength as well as interference level, this measurement value provides additional help for mobility decisions.

Assume that only reference signals are transmitted in a resource block, and that data and noise and interference are not considered. In this case RSRQ is equal to -3 dB. If reference signals and subcarriers carrying data are equally powered, the ratio corresponds to 1/12 or -10.79 dB. At this point it is now important to prove that the UE is capable of detecting and decoding the downlink signal under bad channel conditions, including a high noise floor and different propagation conditions that can be simulated by using different fading profiles.

I will be adding some conformance test logs at the 3G4G website for Measurement and Cell Selection/Re-selection that will give some more information about this.

In case you can provide a much simpler explanation or reference please feel free to add in the comment.

LTE to 3G Handover Procedure and Signalling

It may be worthwhile brushing up the LTE/SAE Interfaces and Architecture before proceeding.

1) Overview of Handover Operation

With EPC, continuous communication is possible, even while the terminal switches from one type of radio access system to another.

Specifically, in order to achieve the internal network path switching required to change radio access systems, the S-GW provides a mobility management anchor function for handover between 3GPP radio access systems, and the P-GW provides the function for handover between 3GPP and non-3GPP radio access systems. In this way, the IP address does not change when the terminal switches radio access systems, and communications can continue after handover.

In handover between the 3GPP radio access systems, LTE and 3G, handover preparation is done before changing systems, including tasks such as securing resources on the target radio access system, through cooperation between the radio access systems (Figure 3 (a)(A)). Then, when the actual switch occurs, only the network path needs to be switched, reducing handover processing time (Fig.3 (a)(B)). Also, loss of data packets that arrive at the pre-switch access point during handover can be avoided using a data forwarding function (Fig.3 (b)).

In this way, through interaction between radio access systems, fast handover without packet loss is possible, even between radio access systems such as LTE and 3G which cannot be used simultaneously.

2) Handover Preparation Procedure (Fig.3 (a)(A))

The handover preparation procedure for switching radio access from LTE to 3G is shown in Figure 4.

Step (1):The terminal sends a radio quality report containing the handover candidate base-stations and other information to the eNodeB. The eNodeB decides whether handover shall be performed based on the information in the report, identifies the base station and RNC to switch to, and begins handover preparation.

Steps (2) to (3): The eNodeB sends a handover required to the MME, sending the RNC identifier and transmission control information for the target radio access system. The MME identifies the SGSN connected to the target RNC based on the received RNC identifier and sends the communication control and other information it received from the eNodeB to the SGSN in a forward relocation request signal. The information required to configure the communications path between the S-GW and SGSN, which is used for data transmission after the MME has completed the handover, is sent at the same time.

Steps (4) to (5): The SGSN forwards the relocation request to the RNC, notifying it of the communications control information transmitted from the eNodeB. The RNC performs the required radio configuration processing based on the received information and sends a relocation response to the SGSN. Note that through this process, a 3G radio access bearer is prepared between the SGSN and RNC.

Step (6): The SGSN sends a forward relocation response to the MME in order to notify it that relocation procedure has completed. This signal also includes data issued by the SSGN and required to configure a communications path from the S-GW to the SGSN, to be used for data forwarding.

Steps (7) to (8): The MME sends a create indirect data forwarding tunnel request to the S-GW, informing it of the information issued by the SSGN that it just received. From the information that the S-GW receives, it establishes a communications path from the S-GW to the SGSN for data forwarding and sends a create indirect data forwarding tunnel response to the MME.

Through this handover preparation, target 3G radio-access resources are readied, the radio access bearer between the SGSN and RNC is configured, and the data forwarding path from the

S-GW to the SGSN configuration is completed.

3) Handover Procedure for Radio Access System Switching (Fig. 3(a)(B)):

The handover process after switching radio access system is shown in Figure 5.

Steps (1) to (2): When the handover preparation described in Fig.4 is completed, the MME sends a handover command to the eNodeB. When it receives this signal, the eNodeB sends a handover from LTE command for the terminal to switch radio systems. Note that when the eNodeB receives the handover command from the MME, it begins forwarding data packets received from the S-GW. Thereafter, packets for the terminal that arrive at the S-GW are forwarded to the terminal by the path: S-GW, eNodeB, S-GW, SGSN, RNC.

Steps (3) to (6): The terminal switches to 3G and when the radio link configuration is completed, notification that it has connected to the 3G radio access system is sent over each of the links through to the MME: from terminal to RNC, from RNC to SGSN, and from SGSN to MME. This way, the MME can perform Step (10) described below to release the eNodeB resources after a set period of time has elapsed.

Step (7): The MME sends a forward relocation complete acknowledgement to the SGSN. A set period of time after receiving this signal, the SGSN releases the resources related to data forwarding.

Step (8): The SGSN sends a modify bearer request to the S-GW to change from the communications path before the handover, between the S-GW and eNodeB, to one between the S-GW and SGSN. This signal contains information elements required to configure the path from S-GW to SGSN, including those issued by the SGSN. When the S-GW receives this signal, it configures a communications path from the S-GW to the SGSN. In this way, the communications path becomes: S-GW, SGSN, RNC, terminal; and data transmission to the target 3G radio access system begins.

Note that after this point, data forwarding is no longer needed, so the S-GW sends a packet to the eNodeB with an “End Marker” attached, and when the eNodeB receives this packet, it releases its resources related to data forwarding.

Steps (9) to (10): The S-GW sends a modify bearer response to the SGSN, indicating that handover procedure has completed. The MME also releases eNodeB resources that are no longer needed.

Through this handover procedure, data is forwarded during the handover, the switch of radio access bearer is completed, and the communications path from the P-GW to the terminal is updated.

In the examples above, we described the handover procedure between 3GPP radio access systems in which the S-GW did not change, but handovers with S-GW relocation are also possible. In these cases, the P-GW provides the anchor function for path switching, as with switches to non-3GPP access systems.

TERMS

Anchor function: A function which switches the communications path according to the area where the terminal is located, and forwards packets for the terminal to that area.

Relocation: Switching communications equipment such as area switches during communication.

Voice over LTE – a step towards future telephony (Ericsson Whitepaper)

The Fact boxes in the Appendix give a very good quick overview of how different technologies work with VoLTE.

Voice over LTE – a step towards future telephony

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Proximity Indication - New RRC Uplink Message in Rel-9

The inbound handover from a Macro eNB to an HeNB (a.k.a. Femtocell) is not supported in Release 8. Before making a handover decision to a HeNB, the Macro eNB needs to acquire UE measurement information related to the so-called target CSG cell. Nevertheless, UEs cannot continuously make measurements and read the system information of lots of CSG cells in cases of large scale HeNB deployments.

In order to allow the UE to make those measurements efficiently, a newly defined proximity report can be configured within the RRC Reconfiguration message. This proximity report will allow the UE to send a so-called “proximity indication” to the source eNB in the uplink whenever it is entering or leaving the proximity of one or more cells with CSG IDs that the UEs has in its CSG Whitelist.

A UE that is able to determine that it is near its CSG cell can thus inform the network to take the necessary actions for handover preparation. The detection of proximity is based on an autonomous search function.

The source eNB, upon receiving the proximity indication, might ask the UE to perform measurements of the CSG cell, to read the System Information (SI) or, in case it already has all required information, it might already start the handover procedure. PCI (Physical Cell Identification) confusion is resolved in Release 9. The eNB will ask the UE to report the global cell identity. As usual the UE reporting is using the RRC measurement procedures. The ovell procedure is illustrated in Figure below.

In summary five basic steps can be identified:

1. Proximity configuration/reporting

2. HO measurement configuration/reporting

3. Resolution of PCI confusion by requesting and reporting System Information

4. Access Control in the network

5. HO execution

Since the CSG search can be very slow there are no strict requirements on the inbound handover performance, which can range from one to several 10’s of seconds.

Since the proximity information is based on UE signaling, the network might be receiving a lot of proximity indications, increasing the network load. Therefore, it was agreed to limit proximity indications a UE can send within a certain time frame. A timer, called the prohibit proximity timer, was introduced.

Source:

• 3GPP TS 36.331 v9.3.0 section 5.3.14
• Nomor Research Whitepaper - LTE Home Node Bs and its enhancements in Release 9

Single Radio Voice Call Continuity (SR‐VCC)

From a 3GPP presentation by Hannu Hietalahti

1. SR-VCC use case

1a. IMS call initiated in LTE can continue in CS domain after moving outside of LTE coverage area

1b. SR-VCC is invoked if no other VoIP capable PS system (e.g., HSPA/eHRPD) is available for VoIP PS-PS HO (Handovers)

1c. Only HO of a single voice bearer from PS to CS is specified

1d. Requires overlapping with 1xRTT/GSM/WCDMA coverage

2. SR-VCC allows a voice calls are anchored in IMS

2a. One-way HO from PS to CS systems (LTE to GSM/UMTS or LTE to 1xRTT)

2b. No simultaneous operation of different radio transceivers needed

3. Rel-9 SR-VCC improvements

3a. IMS support of mid call services (e.g., HOLD, MPTY)

3b. SR-VCC support for emergency calls

4. Video calls, reverse direction from CS call to IMS and optimisations are being studied in Rel-10

Recap of Handover procedures in LTE

Interoperability in LTE

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HeNBs (Femtocells) and eNBs Handovers

An excerpt from presentation by Dr. Doug Pulley, picoChip. The presentation is available to download from here.

Shunning mobiles in favour of Landlines

I guess its time to clean the cobwebs off the landlines. I was reading David Chambers analysis on Homezone tarrifs and it reminded me of the time when I would get big bundle of voice minutes to call using my mobile from home. In those days the voice quality seemed better, signal strength indicator was high and there were hardly any dropped calls.

Nowadays, the signal strength seems to have gone worse whether I am in the office or at home, the voice on the calls keeps breaking, there are too many dropped calls.

To give you an idea of what's going wrong; My phone kept stationary at the table has 4 bars strength of 3G/HSPA, it suddenly becomes 1 bar after 2-3 minutes then hands me over to what the phone says GPRS then the phone says EDGE. If the phone says EDGE then my calls drop within 2 minutes. If my phone says GPRS then I am worried that if it hands over to 3G then my call will drop. If the phone says 3G then unless there are 3 bars, the voice breaks.

Last week I used my landline phone after maybe a year or so and that reminded me how good the voice quality is. In theory the voice quality using mobile phone should be as good as the landline but in practice that may not be true. Of course the wideband AMR can offer much better HD voice but I need reliable voice more than HD voice.

So for the time being, I am going to be sticking with the landlines as far as possible due to reliable and clear communications and wait for the mobiles/networks to catch up.