Martin – Page 231 – WirelessMoves

Handover: The Biggest Asset of Wireless Operators in an IP World

For my upcoming course on LTE services at the University of Oxford in April, I've been giving the voice over IP topic some more thought. Unlike UMTS, LTE is a pure IP based network so it doesn't have an inherent circuit switched voice capability. It's a bit like burning the bridges behind you so you can't go back.

From an operator point of view, potential solutions are the IMS (IP Multimedia Subsystem) or reusing the circuit switched MSC architecture over an IP based channel such as promoted by the VOLGA forum, an approach that I think has a great potential! But what are the benefits of network operator VoIP vs. other purely Internet based alternatives such as Skype or about Grand Central / Google Voice, about which Ajit Jaokar has written recently in conjunction with VOLGA? Well, handovers are!

It's handovers because Internet based Voice over IP services will work well while the network can handover a moving user to another LTE or UMTS cell. But as soon as UMTS and LTE coverage runs out and the system is forced to go to the GSM/GPRS/EDGE network that voice call is history.

Not so with network operator based voice systems. For both IMS and VOLGA, methods are in place to prepare a circuit switched channel in the GSM network before the handover and the mobile device is instructed to use this channel after the handover. In the case of VOLGA, it's a pretty straight forward thing to do. Here, higher layers of the protocol stack will not see a difference between the voice call being transported over an IP data stream over LTE and a circuit switched timeslot in the GSM network.

In my opinion, an invaluable advantage for wireless operators that Internet based voice services will not be able to mimick. And I agree with Ajit, in the future we will see application layer based voice services such as Grand Central and network layer based voice services of wireless operators working together instead of fighting with each other.

Book Review: Less Walk More Talk – How Celtel and the Mobile Phone Changed Africa

It doesn't happen often but every now and then I see a book, and without opening it, the title just does it and I have to buy it. It happened again at the Mobile World Congress. While doing my book presentation at the Wiley booth, I spotted "Less Walk More Talk – How Celtel and the Mobile Phone Changed Africa" by Russell Southwood.

In an instant I decided that the only thing I knew about mobile telephony in Africa came from a number of stories I heard at conferences and read on some web sites. So I thought no matter which approach the book takes it's probably very interesting and I will learn a lot. I was not disappointed.

While I first speculated that the book would tell me about all the things mobile telephony has done for people in Africa, it is actually the story of Mo Ibrahim, who, with comparatively little money he made by selling his first startup company, founded Celtel to bring mobile telephony to Africa, his birth continent. What started in one country soon spread to many and the book has many anecdotes from Mo and others who have worked in Celtel over the years.

The book clearly shows that Africa is one of the hardest places in the world to do business. Although stories of finding oneself in a a war zone and waiting for British marines to fly you out to an aircraft carrier off the coast to impossible negotiations with governments to take a microwave link across the Congo river into service to link mobile networks in two countries instead of routing a call between people only half a mile away from each other via a satellite link to London and back, most of the stories have a good ending and show that with persistence and sometimes also luck, things can turn for the better. The book also deals with corruption and how Celtel always went out of its way to steer clear of it, because there was the strong believe that nothing good would come out of it.

Another thing that surprised me were the timelines. Celtel started with its first network in 1999. I still remember that time in Europe, GSM was still quite early in its success measured by today's standards and yet, Celtel and others took it to Africa. The book also tells the story of how difficult it was to find investors who were willing to bring money into Celtel so it could spread across the continent to compete with its rivals, MTN and Vodacom. Superbly written and it helped me understand the business world a bit better, not only concerning Africa, but also in general.

In the end, Celtel was sold to MTC for over 3 billion US dollars and since then it has been renamed into Zain. Mo Ibrahim has taken his share and has retired from the company, now operating networks in 15 African countries. He now heads the Mo Ibrahim Foundation, which promotes good governance in Africa and has supporters such as Kofi Annan and Bill Clinton.

I've learnt a lot by reading this book, not only about how companies are created or about mobile telephony in Africa, but also a thing or two about Africa itself. It's changed my view on a number of things and I am very thankful for that. It has also triggered some background research which I will discuss in a future post or two.

HSPA Downlink From 1 to 80 MBit/s and Beyond

With the addition of ever more features into HSPA by 3GPP, peak data rates keep rising and one can find many different peak data rate numbers in the media. To bring some order into this, I've decided to put together a table with shows which features bring which rough peak data rates:

3.6 MBit/s : Baseline HSPA with 16QAM modulation
7.2 MBit/s : 16 QAM, more simultaneous channels)
14.4 MBit/s : 16 QAM, even more simultaneous channels
21 MBit/s : 64 QAM modulation
28 MBit/s : MIMO (Multiple Input Multiple Output = 2 antennas) + 16 QAM, (3GPP Rel 7)
42 MBit/s : MIMO + 64QAM (3GPP Release 8)
42 MBit/s : 64QAM + Dual Carrier (3GPP Rel 8)
82 MBit/s : MIMO + 64QAM + Dual Carrier (i.e. 2 x 5MHz) (3GPP Release 9)
+ more in case 3GPP decides to increase the number of carriers that can be bundled in Rel 9 or beyond.

In the near future, operators that can upgrade their base station to 64QAM modulation without a hardware replacement are probably tempted to do so. If they were smart with the backhaul in previous upgrades, they might already have the capacity at the base station to support the added traffic. All further steps require new hardware at the base station so operators will probably think a bit about it before actually deploying it.

Important: These are peak rates, i.e. they are only achieved under very favourable coverage circumstances and microcell environments. In most macro radio environments, speeds are much lower due to interference and less then optimal signal strength.

Dual Carrier HSDPA Potential for European Operators

Back in February I wrote about the new Dual Carrier HSDPA feature
in 3GPP Release 8 to push beyond the current 5 MHz single carrier limit
of HSDPA. In short, the feature allows bundling of two adjacent 5 MHz
carriers if supported by the mobile device for higher throughput or a
better scheduling gain due to the higher capacity of the resulting
channel. But do network operators today have adjacent 5 MHz channels to use the feature? I've had a look at the frequency band assignments for Germany, Austria and Switzerland and in all cases the network operators have at least 10 MHz of continuous spectrum in the 2.1 GHz band. Some even have 15 MHz, i.e. 3 adjacent 3G carriers.

In some countries, not all of the initial licensees have made it to the network operation phase so some bandwidth is lying dormant these days. In Germany, for example, Mobilkom and Hutch3G never made it out of the box. Their 10 MHz bands are currently unused and might at some point be offered for sale to the current four operators or new entrants. In the meantime, 3GPP is pushing forward in Release 9 to go beyond the dual carrier specification to enable the bundling of even more carriers. An interesting detail in this debate is that in Release 9 the bundled not all of the carriers will probably have to be adjacent anymore. This is quite important for T-Mobile in Germany, for example. They are a bit unfortunate when it comes to the third carrier as their frequency assignment is not adjacent to one of the assignments of the two parties that gave up.

And looking even further into the future, it might very well be that 3GPP will specify a multi-band/multi-carrier HSDPA operation in Release 9 as indicated here by 3G Americas. That would go far beyond the cooperative use of different frequency bands I discussed in a previous post. Quite a challenge for mobile device hardware designers as it's already a challenge to design a small device with half a dozen or more antennas. In the future, a future dimension is added to the equation by having to make sure that several antennas can be used for transmitting and receiving data simultaneously without interfering with each other.

And by the way: According to the 3G Americas paper above, similar things are happening with LTE-Advanced as well, the maximum 20 MHz channel bandwidth has become too narrow already in the hunt for ever higher speeds.

Radio and Baseband Now Integrated in a Single Chip

With the number of functionalities that require dedicated hardware in mobile phones rising and rising, it's important to integrate as many of them as possible on a single chip to save space and power. In the past mobile devices used a classic radio chain: A receiver/power amplifier, an analog modem chip and a baseband (CPU and DSL) chip. These days it has become possible to integrate the analog modem chip and the baseband chip on a single die and products such as the Broadcom BCM21331GSM/EDGE chip are now used by major phone manufacturers such as Samsung and Nokia for their 2G GSM/EDGE phones. I've looked a bit around and I haven't seen a similar product for combined 2G/3G phones yet. But that might just be a matter of time now. Maybe the current Qualcomm Snapdragon platform goes into that direction!? If I understand correctly, though, this platform still needs an external transciever chip, something that Broadcom has built into their single chip solution already.

If you have further info on that, please leave a comment, I'd be interested to lean more.

The Volga-Forum: Taking the Quest For Voice over LTE out of 3GPP

Over the past two years I've written numerous posts about different proposed options on how to do voice calls over LTE and the lack of a simple and straight voice solution. This is, in my opinion, a serious threat to the success of LTE if not resolved soon. A number of alternative solutions to the IP Multimedia Subsystem (IMS) have been analyzed in 3GPP, which is envisaged to be the successor to the circuit switched MSC voice architecture. However, even after many years of standardization, it has still not seen the light of day and some fear that it's become to complex. Instead, only fallback to GSM or UMTS for an incoming voice call has made it into the 3GPP standards. Looks like some parties in the game are not happy and have started to push things forward by going off stream with the newly formed Volga-Forum (Voice over LTE via Generic Access).

It seems that other solutions, which have been examined in 3GPP for connecting the current MSC (Mobile Switching Center) architecture to LTE have so far not found the necessary approval by the necessary majority of 3GPP members to become a work item, a necessity for becoming an official feature. I've discussed one of the alternatives back in August 2008 here for those who would like to have a closer look. In summary, the two MSC options examined suggest to replace the lower layers of the current voice protocol stack by IP, while leaving the higher layers of the protocol stack untouched. With relatively little work, the existing voice service can be reused for LTE, including the seamless handover of an ongoing call between LTE, UMTS and GSM and international roaming.

Even simple things don't happen over night and each day waiting for 3GPP to finally start working on this topic drags out the day when LTE can really go beyond USB dongles or built in laptop modems and compete with HSPA where voice calls work like a charm. This is where the Volga-Forum comes in. With a strong list of supporters such as T-Mobile and Ericsson, just to name the two biggest (some more operators in the boat would be nice though), they have pledged to take the task of defining an interoperable voice solution for LTE outside of 3GPP.

I would guess the aim is to put some pressure on 3GPP to move forward and the intention is to fold their work back into the standards later on. But even if they stay separate, the solution selected is based on UMA/GAN, as described in option 2 in 3GPP TR 23.879, so the Volga-Forum does not depend on the support of 3GPP to drive their implementation. The main parts of the approach require no modifications in the LTE radio network, the LTE core network or the MSC. All enhancements suggested would be implemented on the mobile device and a new gateway controller between the LTE core network and the MSC. The situation is thus very similar to the days when Kineto and others privately developed UMA, which tunnels GSM and GPRS over Wi-Fi, before it officially became part of the 3GPP standards some years later after being renamed to GAN (Generic Access Network).

A very interesting move, and I think it's good to see that things are moving forward! If you would like to add anything on the political or technical side of this, please leave a comment below.

Thanks to LTE Watch for the first pointer!

GSM and GPRS Coverage during the Mobile World Congress

The Mobile World Congress in Barcelona is long over for this year and it hasn't really been my focus, but I also had a look at how well the exhibition ground for the Mobile World Congress was covered for non-3G users.

While many if not the majority of the MWC attendees had 3G phones I am sure there was still a sizable percentage of people with 2G Blackberries and other GSM only phones. Even with my 3G phone I was mostly using the GSM/EDGE network due to Vodafone Spain's battery killing network configuration when being always-on. While I haven't checked how many cells Vodafone had deployed and if they had dedicated coverage in the exhibition halls, I can nevertheless report that unlike during other exhibitions I have been in the past, incoming and outgoing calls worked fine throughout the week and I saw no degradation in download speeds for my e-mails and while browsing via Opera Mini.

Especially when web browsing, one can immediately feel the difference between a loaded network in which only few timeslots are available for data transfers which on top are heavily used for example by Blackberries. But no, everything was fast and swift, despite probably more than 20.000 people being at the exhibition simultaneously.

So kudos to Vodafone Spain, your 2G network was working fine as well.

CS Voice Services over HSPA

3GPP is quite active in moving as many things as possible over to the HSPA high speed (packet based) shared channel on the UMTS air interface to save resources, increase capacity and to reduce mobile power consumption. An example I have reported on in the past is the Enhanced Cell-FACH feature. Looks like the traditional circuit switched voice service is now also set to migrate to HSPA with the CS (circuit switched) Voice Services over HSPA feature introduced in 3GPP Release 8.

This whitepaper from Qualcomm gives a good introduction. According to the paper, "all that is required" to put the circuit switched voice service on the packet switched high speed shared channel is a software upgrade in the radio network and the mobile. Like today, the MSC forwards the voice call to the Radio Network Controller (RNC) without any changes. On the RNC, the Adaptive Multi Rate (AMR) speech packets are then not put into a logical circuit switched bearer for a dedicated air interface channel but are instead put into PDCP packets which are then sent to the base station. The base station then schedules those AMR in PDCP packets on the high speed shared channel in the same way as IP in PDCP packets coming from the SGSN but gives them a higher priority.

On the mobile side a software enhancement is required to indicate to the network that CS voice over HSPA is supported. Further, the lower layer voice protocol stack needs to be enhanced to receive the AMR packets on the high speed shared channel instead of on a dedicated channel to the mobile.

As no IP layer is involved for transmitting the AMR speech packet in the radio network I would call this a Voice over Packet service. So be careful, this enhancement can't be counted as a wireless VoIP variant and is not related with the CS Voice over LTE proposal I reported on here, which is fully based on IP.

Will this feature make it into real networks? Time will tell. What do you think?

SAE Review Part 3: Keeping Track of Users – The GUTI and the GUMMEI

This is part 3 of my mini-series on the latest version of 3GPP TS 23.401 which describes how the LTE/SAE core network manages user mobility and routes data. In part 1, I've taken a look at the flexibility in terms of load balancing and network node distribution and part 2 featured a look at connection and mobility management. Part 3 now focuses at how the MME (Mobility Management Entity) keeps track of users while they are moving and helps handing over connections from one base station to the other.

As described in previous parts an LTE mobile always has an IP address assigned while it is switched on. To conserve battery and to reduce signaling there are two basic activity states are: While data is exchanged the mobile is seen as connected. If no data is transferred for some time, the network moves the connection to idle, which means the mobile has no physical connection over the air interface during that time. Data can still be sent and received but the air interface connection needs to be re-established first. For applications this is transparent, they will just notice some delay.

Mobility While Being Idle

Let's look at the idle state (to be exact RRC idle and ECM idle) first because that is the most simple from the network point of view. Here, the mobile is free to roam from one cell to another and only contact the network if it suddenly finds itself with a cell that is outside the group of cells to which its former cell belonged to. Such groups are referred to as Tracking Areas (TA) and the action performed when changing to a new cell in a different TA is referred to as a Tracking Area Update (TAU). For a mobile it is simple to detect a tracking area change because each cell broadcasts its Tracking Area ID as part of its general cell information. In case data arrives from the Internet for that user while the device is in idle state, the network has to search the mobile first and sends a 'paging' message via all cells belonging to the TA. The mobile then re-establishes the air interface connection and implicitly reports it's current location to the network.

Now let's have some fun with a couple of further abbreviations, because they are really cute. In GPRS and UMTS the mobile's temporary id was the Packet Temporary Mobile Identity, or the P-TMSI. This id is changed on a frequent basis and used instead of the IMSI (The International Mobile Subscriber Identification) in most air interface messages for security reasons. In LTE, the P-TMSI is now called the Globally Unique Temporary ID, or the GUTI. Some of the digits in the GUTI identify the Mobility Management Entity the mobile was last registered with and they are referred to as the Globally Unique MME Identifier, or the GUMMEI.

When contacting the network, the mobile sends the GUTI to the base station which then uses the parameter to identify the MME to which it will send the request to re-establish the communication session. It's also possible to roam between different radio technologies. If the mobile has reselected from a UMTS cell to an LTE cell, a TAU is made and since the mobile does not have a GUTI, the P-TMSI is sent instead. This way, the newly assigned MME can contact the 3G SGSN to request the subscribers current profile (IP address, PDP contexts, etc.). The same mechanisms apply when the mobile reselects from an LTE cell to a UMTS or GPRS cell. In this case the GUTI is sent in the P-TMSI parameter and the procedure is reffered to as Routeing Area Update (RAU) instead of TAU.

Handover between Two LTE Cells

3GPP TS 23.401 also describes the possible scenarios for handovers, i.e. the handing over of an active radio network connection. Unlike the cell reselection in idle state above, which is controlled by the mobile device, handovers are controlled by the network. Due to the flat network architecture of LTE, the handover is directly initiated by the base station and not, like in previous network architectures, by a higher network element. If implemented, two base stations can organize a handover between the two of them (over the X2 interface) and only report the successful execution to the MME afterwards. The MME then either just acknowledges the handover and the serving gateway is informed to redirect the downlink data traffic to the new cell. In case it makes sense from a network topology or traffic point of view, the MME can at this point also assign a new serving gateway. Note that in practice, the X2 interface is not a physical interface, i.e. base stations are only logically connected with each other over IP and not via a direct physical link.

If there is no direct interface between two base stations, or the base stations do not yet have that functionality implemented, the current base station can also ask the MME to coordinate the handover. This is called an S1 handover, due to the name of the interface between the base station on the MME. Again, there are a number of different variants such as with or without a change of the Serving Gateway.

Inter Radio Access Technology (Inter-RAT) Handovers

And last but not least, there is also the possibility to perform a handover from and to a different radio network architecture, i.e. from/to a GPRS or UMTS network. Three different variants exist:

To/from an SGSN that is aware how a LTE core network works and has the S4 interface implemented.
In case Direct Tunnel is used in a UMTS network, the S4 and S12 interfaces are used for the handover.
And for networks without upgraded SGSNs there is a backwards compatible variant. In this case the MME acts like an SGSN for the older network elements. This is probably the way how handovers to and from LTE will be made at first. This variant is described in Annex-D of the specification, which kind of marks it as a temporary solution.

It all sounds very complicated with lots of options and it probably is. It makes one appreciate network operators who have done their homework and have optimized their networks for seamless nationwide handovers without dropped calls and lost IP addresses. Yes, they do exist.

Nevertheless, to me this all looks a lot more straight forward compared to how things are done in UMTS. Here, the RNC complicates matters and in practice not all network elements can communicate with all others. In LTE/SAE, the removal of the RNC and using IP as the routing protocol for all network activities instead of ATM makes things a lot simpler. Those who don't believe me should have a look at the situation that requires a UMTS SRNS relocation for both the circuit switched and packet switched side simultaneously and how the messaging looks like…

For those who would like to know more about Inter-RAT handovers with CDMA networks I can warmly recommend the several hundred pages of specification in an additional document, 3GPP TS 23.402. Another tribute to CDMA that is also described there is the use alternative use of PMIP (Proxy Mobile IP) instead of the 3GPP GTP (GPRS Tunneling Protocol) on the Interface between the MME and the PDN-Gateway (to the Internet). Long live the options!

There we go, we are almost through with the main SAE features. Remains the Idle State Signaling Reduction (ISR) feature, but I keep that for part 4.

Escaping Future Bandwidth Bottlenecks: LTE and HSPA on Several Bands

I think everyone in the industry is pretty clear by now that the amount of data that cellular wireless networks will have to carry in the future is going to rise. In my recent book I’ve taken a closer look at theoretical and practical capacity on the cellular level in chapter 3 and I come to the conclusion that from a spectrum point of view, there is quite a lot of free space left in most parts of the world that will last for quite some time to come.

So while alternative approaches like integrating Wi-Fi and femtos into an overall solution will ultimately bring much more capacity, I think it is quite likely that network operators will over time deploy their cellular networks in ever more bands. In Europe, for example, I think it’s quite likely that operators at some point will have networks deployed on the 900, 1800, 2100 and 2600 MHz band simultaneously.

Quite an interesting challenge to solve for networks and especially for mobile devices as they have to support an ever growing number of frequency bands. Also, those bands should not also be used in tight cooperation instead of just aside each other. Ideally, the resources in the 900 MHz band could be reserved for in-house coverage as radio waves in this band penetrate walls quite well. But as soon as the network or the device detect that other bands can be received quite well, they should automatically switch over to them to leave more capacity for devices used indoors or under difficult radio conditions.

Switching between different frequencies and radio technologies during a call or a session is already done today but mostly based on deteriorating reception levels. So in the future, when using so many bands, I think this reactive mechanism has to be enhanced into a proactive mechanism and switch-overs need to be timed so that the user does not notice an interruption.