Roadmap to Operational Excellence for Next Generation Mobile Networks

Roadmap to Operational Excellence for Next Generation Mobile Networks

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This presentation is from:

FP7 SOCRATES Final Workshop on Self-Organisation in Mobile Networks February 22, 2011 - Karlsruhe, Germany

This and all other presentations from this workshop are available to download from here.

Direct Communication between devices in case of disasters

Yesterday, a discussion started after I read this article on RCR Wireless News:

As in every major disaster, communications networks quickly showed their inherent weakness in times of greatest need. Japan's NTT Communications reported outages affecting Internet voice data that relies on IP-VPN technology.

In a brief statement, the operator apologized for the "trouble and inconvenience," following the string of earthquakes and significant aftershocks that rattled nerves and buildings throughout much of Japan. Some communication services are no longer available, NTT said, and telephone service, particularly long-distance service, is showing strain as well.

Service disruptions have been reported by all three of the major mobile operators in Japan, according to BusinessWeek.

This prompted me to ask on Twitter about which technologies are available that can help the mobile network cope with these problem.

Here are few approaches:

I blogged earlier about Multihop Cellular Networks (MCN) and ODMA. These technologies have their own limitations and problems and I have not heard of anything more about them being standardised or adopted.

Another post was on Ad-Hoc Networks that can be formed in case of failures resulting in Mobile devices being able to communicate directly without the need for network or base stations. The slight problem is that this approach replies on WiFi being available which may not always be the case.

A colleague suggested that in Tetra, Direct Mode of operation is available that is intended for situations like these. A presentation is embedded below:

Steven Crowley on twitter suggested that 802.16m has already started working in this direction. I got a related presentation on that which is embedded below:

Finally, Kit Kilgour mentioned about DSAC (Domain Specific Access Control) whose intention is to discontinue the voice service in emergency (to avoid congestion) but continue the packet domain normally. I have not looked at DSAC on this blog but in LTE instead Service Specific Access Control (SSAC) is used since LTE is PS only. See the blog entry here.

Please feel free to add any more information on this topic in the comments.

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.

1000th Blog post and I want your feedback

I started the 3G4G website 7 years back and in the next few years realised that maintaining website is very time consuming job. As a result I started the blog on the 3G4G website. Its been nearly 4 years since I started blogging. Initially I blogged on the 3G4G website and then moved to blogger. Over the time, the blog has become ever so popular and I regularly keep getting between 40,000 and 50,000 page views per month. In the next few months, I will touch the 1.5 million page views mark.

All this sounds great but I have not seen enough feedback and/or comments from you the readers. Some months back I added the feedback box at the bottom of the posts that you can use to provide me a quick feedback indicating if you found this post useful or not useful and if you would like more like these but I hardly get more than 1 or 2 feedbacks every post. The only one where I got some decent feedback was on the Dilbert post here. In the blog stats that was added last year by blogger, I can see that some of the posts even get good amount of views but not enough feedback. For example LTE-A UE categories has over 7000 views but just 3 very useful feedback. Another one on comparison of HSPA+ and LTE has over 4000 views since last May but the feedback is still not enough.

Over the last few years, a lot of my posts are being copied by others in entirety. Some of these blogs give credit to me but do not link my blog. Some of them do not even give me credit or link to the blog. In fact to stop some of these things, I started putting 'via 3g4g.blogspot.com' in the images and then I realised that some of these blogs, remove this and put the pictures up. Take for instance this post from TelecomDE, this blog post is copied from my blog post here. I created the picture from a presentation and that was from Huawei, so I added the Huawei logo in the picture. As you can see the Huawei picture is there but the 'via 3g4g.blogspot.com' has been removed. There are many instances of such things and I would like to thank some of my blog readers who point me out these things.

With my schedule being already extremely busy, I sometimes spend early mornings or late night, creating new blog posts with the things that are happening or about to happen in the wireless/telecom world. I think my blog covers some unique topics and I always add some useful pictures and images as it is said that 'A picture is worth a thousand words'. I would like to receive feedback from you, dear reader, on if you find this blog useful, how do you find it useful, what things you like most, what things you like least, how do you propose to change it for better.

I do get lots of personal mails from people saying how useful the blog and the website have been for them for Job hunting, etc. So If you found the Blog or the Website useful and you are a Linkedin user, can you please recommend the 3G4G website on Linkedin for me.

I will be deciding in the next few weeks, If I continue blogging and your feedback certainly will help. In the meantime, you can always follow me on Twitter where I am always on lookout for the latest in the field of wireless telecoms.

ETWS detailed in LTE and UMTS

Its been couple of years since the introductory post on 3GPP Earthquake and Tsunami Warning service (ETWS). The following is more detailed post on ETWS from the NTT Docomo technical journal.

3GPP Release 8 accepted the standard technical specification for warning message distribution platform such as Area Mail, which adopts pioneering technology for faster distribution, in order to fulfil the requirements concerning the distribution of emergency information e.g. earthquakes, tsunamis and so on in LTE/EPC. The standard specifies the delivery of emergency information in two levels. The Primary Notification contains the minimum, most urgently required information such as “An earthquake occurred”; the Secondary Notification includes supplementary information not contained in the Primary Notification, such as seismic intensity, epicentre, and so on. This separation allows implementation of excellent information distribution platforms that can achieve the theoretically fastest speed of the warning distribution.

The purpose of the ETWS is to broadcast emergency information such as earthquake warnings provided by a local or national governments to many mobile terminals as quickly as possible by making use of the characteristic of the widespread mobile communication networks.

The ETWS, in the same way as Area Mail, detects the initial slight tremor of an earthquake, the Primary Wave (P wave - The first tremor of an earthquake to arrive at a location), and sends a warning message that an earthquake is about to happen to the mobile terminals in the affected area. ETWS can deliver the first notification to mobile terminals in the shortest theoretical time possible in a mobile communication system (about four seconds after receiving the emergency information from the local or national government), which is specified as a requirement by 3GPP.

The biggest difference between Area Mail and the ETWS is the disaster notification method (Figure 1). Earthquake warnings in Area Mail have a fixed-length message configuration that notifies of an earthquake. ETWS, on the other hand, achieves distribution of the highest priority information in the shortest time by separating out the minimum information that is needed with the most urgency, such as “Earthquake about to happen,” for the fastest possible distribution as a Primary Notification; other supplementary information (seismic intensity, epicentre, etc.) is then distributed in a Secondary Notification. This distinction thus implements a flexible information distribution platform that prioritizes information distribution according to urgency.

The Primary Notification contains only simple patterned disaster information, such as “Earthquake.” When a mobile terminal receives a Primary Notification, it produces a pre-set alert sound and displays pre-determined text on the screen according to the message content to notify users of the danger. The types of disaster that a Primary Notification can inform about are specified as “Earthquake,” “Tsunami,” “Tsunami + Earthquake,” “Test” and “Other,” regardless of the type of radio access.

The Secondary Notification contains the same kind of message as does the existing Area Mail service, which is, for example, textual information distributed from the network to the mobile terminal to inform of the epicentre, seismic intensity and other such information. That message also contains, in addition to text, a Message Identifier and Serial Number that identifies the type of disaster.

A major feature of the ETWS is compatibility with international roaming. Through standardization, mobile terminals that can receive ETWS can receive local emergency information when in other countries if the local network provides the ETWS service. These services are provided in a manner that is common to all types of radio access (3G, LTE, etc.).

Network Architecture

The ETWS platform is designed based on the Cell Broadcast Service (CBS). The ETWS network architecture is shown in Figure 2. Fig. 2 also shows the architecture for 3G network to highlight the features differences between LTE and 3G.

In the ETWS architecture for 3G, a Cell Broadcast Centre (CBC), which is the information distribution server, is directly connected to the 3G Radio Network Controller (RNC). The CBC is also connected to the Cell Broadcast Entity (CBE), which distributes information from the Meteorological Agency and other such sources.

In an LTE radio access network, however, the eNodeB (eNB) is directly connected to the core network, and eNB does not have a centralized radio control function as the one provided by the RNC of 3G. Accordingly, if the same network configuration as used for 3G were to be adopted, the number of eNB connected to the CBC would increase and add to the load on the CBC. To overcome that issue, ETWS for LTE adopts a hierarchical architecture in which the CBC is connected to a Mobility Management Entity (MME).

The MME, which acts as a concentrator node, is connected to a number of eNBs. This architecture gives advantages to the network, such as reducing the load in the CBC and reducing the processing time, and, thus preventing delay in distribution.

Message Distribution Area

In the 3G ETWS and Area Mail systems, the distribution area can be specified only in cell units, which creates the issue of huge distribution area database in CBC. In LTE ETWS, however, the distribution area is specified in three different granularities (Figure 3). This allows the operator to perform area planning according to the characteristic of the warning/emergency occasions, e.g. notice of an earthquake with a certain magnitude needs to be distributed in a certain width of area, thus allowing efficient and more flexible broadcast of the warning message.

1) Cell Level Distribution Area: The CBC designates the cell-level distribution areas by sending a list of cell IDs. The emergency information is broadcasted only to the designated cells. Although this area designation has the advantage of being able to pinpoint broadcast distribution to particular areas, it necessitates a large processing load in the network node (CBC, MME and eNB) especially when the list is long.

2) TA Level Distribution Area: In this case, the distribution area is designated as a list of Tracking Area Identities (TAIs). TAI is an identifier of a Tracking Area (TA), which is an LTE mobility management area. The warning message broadcast goes out to all of the cells in the TAIs. This area designation has the advantage of less processing load when the warning message has to be broadcast to relatively wide areas.

3) EA Level Distribution Area: The Emergency Area (EA) can be freely defined by the operator. An EA ID can be assigned to each cell, and the warning message can be broadcasted to the relevant EA only. The EA can be larger than a cell and is independent of the TA. EA is a unit of mobility management. EA thus allows flexible design for optimization of the distribution area for the affected area according to the type of disaster.

Message Distribution

The method of distributing emergency information to LTE radio networks is shown in Figure 4. When the CBC receives a request for emergency information distribution from CBE, it creates the text to be sent to the terminals and specifies the distribution area from the information in the request message (Fig. 4 (1) (2)).

Next, the CBC sends a Write-Replace Warning Request message to the MME of the specified area. This message contains information such as disaster type, warning message text, message distribution area, Primary Notification information, etc. (Fig. 4 (3)). When the MME receives this message, it sends a response message to the CBC to notify that the message was correctly received. The CBC then notifies the CBE that the distribution request was received and the processing has begun (Fig. 4 (4) (5)). At the same time, the MME checks the distribution area information in the received message (Fig. 4 (6)) and, if a TAI list is included, it sends the Write-Replace Warning Request message only to the eNB that belong to the TAI in the list (Fig. 4 (7)). If the TAI list is not included, the message is sent to all of the eNB to which the MME is connected.

When the eNB receives the Write-Replace Warning Request message from the MME, it determines the message distribution area based on the information included in the Write-Replace Warning Request message (Fig. 4 (8)) and starts the broadcast (Fig. 4 (9) (10)). The following describes how the eNB processes each of the specified information elements.

1) Disaster Type Information (Message Identifier/Serial Number): If an on-going broadcast of a warning message exists, this information is used by the eNB to decide whether it shall discard the newly received message or overwrite the ongoing warning message broadcast with the newly received one. Specifically, if the received request message has the same type as the message currently being broadcasted, the received request message is discarded. If the type is different from the message currently being broadcast, the received request message shall overwrite the ongoing broadcast message and the new warning message is immediately broadcasted.

2) Message Distribution Area (Warning Area List): When a list of cells has been specified as the distribution area, the eNB scans the list for cells that it serves and starts warning message broadcast to those cells. If the message distribution area is a list of TAIs, the eNB scans the list for TAIs that it serves and starts the broadcast to the cells included in those TAIs. In the same way, if the distribution area is specified as an EA (or list of EAs), the eNB scans the EA ID list for EA IDs that it serves and starts the broadcast to the cells included in the EA ID.

If the received Write-Replace Warning Request message does not contain distribution area information, the eNB broadcasts the warning message to all of the cells it serves.

3) Primary Notification Information: If Primary Notification information indication exists, that information is mapped to a radio channel that is defined for the broadcast of Primary Notification.

4) Message Text: The eNB determines whether or not there is message text and thus whether or not a Secondary Notification needs to be broadcasted. If message text exists, that text is mapped to a radio channel that is defined for the broadcast of Secondary Notification. The Secondary Notification is broadcast according to the transmission intervals and number of transmissions specified by the CBC. Upon the completion of a broadcast, the eNB returns the result to the MME (Fig. 4 (11)).

Radio Function Specifications

Overview : In the previous Area Mail service, only mobile terminals in the standby state (RRC_IDLE) could receive emergency information, but in ETWS, emergency information can be received also by mobile terminals in the connected state (RRC_CONNECTED), and hence the information can be delivered to a broader range of users. In LTE, when delivering emergency information to mobile terminals, the eNB sends a bit in the paging message to notify that emergency information is to be sent (ETWS indication), and sends the emergency information itself as system information broadcast. In 3G, on the other hand, the emergency information is sent through the paging message and CBS messages.

Message Distribution method for LTE: When the eNB begins transmission of the emergency information, a paging message in which the ETWS indication is set is sent to the mobile terminal. ETWS-compatible terminals, whether in standby or connected, try to receive a paging message at least once per default paging cycle, whose value is specified by the system information broadcast and can be set to 320 ms, 640 ms, 1.28 s or 2.56 s according to the 3GPP specifications. If a paging message that contains an ETWS indication is received, the terminal begins receiving the system information broadcast that contains the emergency information. The paging message that has the ETWS indication set is sent out repeatedly at every paging opportunity, thus increasing the reception probability at the mobile terminal.

The ETWS message itself is sent as system information broadcast. Specifically, the Primary Notification is sent as the Warning Type in System Information Block Type 10 (SIB10) and the Secondary Notification is sent as a Warning Message in SIB11. By repeated sending of SIB10 and SIB11 (at an interval that can be set to 80 ms, 160 ms, 320 ms, 640 ms, 1.28 s, 2.56 s, or 5.12 s according to the 3GPP specifications), the probability of the information being received at the residing mobile terminal can be increased. In addition, the SIB10 and SIB11 scheduling information is included in SIB1 issued at 80-ms intervals, so mobile terminals that receive the ETWS indication try to receive SIB10 and SIB11 after first having received the SIB1. By checking the disaster type information (Message Identifier and Serial Number) contained in SIB10 and SIB11, the mobile terminal can prevent the receiving of multiple messages that contain the same emergency information.

3G Message Distribution Method: For faster information delivery and increased range of target uers in 3G also, the CBS message distribution control used in Area Mail was enhanced. An overview of the 3G radio system is shown in Figure 5.

In the Area Mail system, a Common Traffic Channel (CTCH) logical channel is set up in the radio link, and emergency information distribution is implemented by sending CBS messages over that channel. To inform the mobile terminals that the CTCH logical channel has been set up, the RNC orders the base station (BTS) to set the CTCH Indicator information element in the system information broadcast to TRUE, and transmits the paging message indicating a change in the system information broadcast to the mobile terminals. When the mobile terminal receives the CTCH Indicator, it begins monitoring the CTCH logical channel and can receive CBS messages.

In ETWS, by including the Warning Type in the paging message indicating a change in the system information broadcast, processing for a pop-up display and alert sound processing (Primary Notification) at the mobile terminals according to the Warning Type can be executed in parallel to the processing at the mobile terminals to start receiving the CBS messages. This enhancement allows users whose terminals are in the connected state (RRC_CONNECTED) to also receive emergency information. In the previous system, it was not possible for these users to receive emergency information. Also including disaster type information (Message Identifier and Serial Number) in this paging message makes it possible to prevent receiving multiple messages containing the same emergency information at the mobile terminal.

More detailed information (Secondary Notification) is provided in CBS messages in the same way as in the conventional Area Mail system, thus achieving an architecture that is common to ETWS users and Area Mail users.

Presentations on Femtocell Applications and Services from MWC2011

These three interesting presentations embedded below from Mobile World Congress 2011 are available to download from Femto Forum website here.

Femtocell Based Applications and Services

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Next generation remote management: bridging devices and applications

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Services and Femtocell Adoption

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Augmented Reality: Future Killer App?

Augmented Reality can be understood very easily with the two videos embedded below:

It may look cool and one may wonder how this can be useful practically, here is another video showing how this can be used:

So in future you may have quite a few people who can only look at you through the [phone rather than directly :)

The following is an extract from The Guardian article titled, "What is mobile augmented reality for?":

Mobile augmented reality is a relatively young technology, but it has already attracted a great deal of hype and scepticism in equal measure.

Overlaying digital information onto the real world, viewed through a cameraphone, is technically impressive, but the business models and usage patterns are still evolving.

That's a polite way of saying mobile AR is cool, but nobody really knows what it's for, or how it will make money. One of the more interesting conference sessions at this year's Mobile World Congress aimed to answer the key question: what is it for?

Tourism has been an early focus. Just this week, travel site TripAdvisor added an augmented reality feature to its iPad app (pictured above), while Lonely Planet has also used AR elements in several of its travel apps.

"You are most information-starved when you are in a completely new environment," said Jeremy Kreitler, vice-president of mobile at Lonely Planet. "Those are probably the environments where augmented reality will flourish the most."

The Layar chief executive, Raimo Van der Klein, pointed to the popularity of Twitter layers in his company's app, which allow people to see local tweets superimposed on their camera view of the world around them.

"In the future, it will be the physical world that will trigger usage," he said. "Your dynamic and changing context, as you interact with different media, products, packaging and people, and you would like to make sense of what you encounter."

Technology firm Qualcomm recently held an augmented reality contest for mobile developers, announcing three winners this week at Mobile World Congress. All three were games.

Qualcomm's vice-president of ventures, Nagraj Kashyap, took the view that games are often a good proving ground for new technologies in their early stages, with AR no different.

"It's just something that appeals to a wide cross-section of users," he said. "But to have augmented reality become mass, we need to move out of just the gaming context."

Qualcomm sees much potential in marketing, particularly when AR is used to add an interactive layer to print advertisements. Kashyap also thought educational and instructional AR content will be popular in the future. "Imagine pointing your phone at a newly bought washing machine and getting instructions for it on your phone."

However, Philipp Schloter, chief executive of developer Abukai, said that looking for individual killer apps is the wrong way to approach augmented reality.

"This is really more of an enabler that sits across many different areas," he said. He was backed up by Peter Meier, founder of Metaio, the company which makes the Junaio AR browser app. "I always see augmented reality as a new user interface technology, and less as something for which there's the killer app out there," said Meier.

"For me, this is about accessing and understanding information more easily, and enjoying information that is somehow related to the real world ... I don't think there's a killer app. This is more like the next touchscreen for mobile phones – more like the next user interface revolution."

David Marimon, who heads up mobile augmented reality and visual search for operator group Telefonica, suggested that new uses for AR will be found as different kinds of developers start to work with it, including visual and interaction designers.

He also said that Telefonica is keen to help developers find new uses for AR by providing them with technology and APIs to tap into the operator's customer data.

"We know where mobile phones are thanks to GPS and other sensors, which is a very intuitive starting point to get the context of the user," he said. "We are also working on visual recognition to acquire that context: we need to know what the user is looking at, for which we can use the camera."

In the recently concluded Mobile World Congress, there was a panel that discussed the options on Augmented Reality or AR as its better known. The slides are embedded below but only the initial slides provide some value.

Augmented Reality Panel - Mobile World Congress 2011

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I have heard of some and can can think of some more simple applications that can actually be very useful. Maybe some of them are already being developed.

1. reviews of Pubs/Clubs - If you planning to go to some Pub/club in an area you can just look at the places through your lens and immediately see the number of stars received in reviews.

2. Virtual tour guide - One of the apps Lonely Planet are working on is developing virtual tour guides that can tell you all the information about a place once in your mobile camera

3. In some countries where For Sale sign could not be put while selling houses, you can go in an area and look at the houses though your camera and it will tell you which house is for sale, which estate agent and what is the price

4. Some manufacturers have suggested that simple procedures required with gadgets like changing the toner or a printer can be done using AR apps.

5. Games is certainly and area that is going to be a major user of AR for effects and to get people excited

6. Dating apps could use AR to tell about the places where singles hangout in the real time.

7. Educational purpose.

8. CV's for Jobs - Personally, I think QR code can do the job in this case

9. AR could be used as your personal shopping assistant in the supermarket helping you do your shopping in the least amount of time - assuming you know all the things that need to be bought in advance

And many more uses of AR can be thought of and debated.

Finally, there is also a recent presentation titled "Augmented Research" embedded below:

Augmented Research

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Dilbert humour on Engineering budgets

Source: dilbert.com

Mobile Services Beyond Handsets

Interesting presentation by Ted Matsumoto from Softbank, Japan in Mobile World Congress 2011.

Mobile Services go beyond handsets

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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.