LTE Test Network with 20 Base Stations in Austria

Heise news reports today about an LTE field trial T-Mobile is performing right now in Innsbruck, Austria and lists some interesting details about it:

  • Base stations: The outdoor test network consists of 20 base stations with three sectors each, supplied by Huawei.

    Note: That's a good network setup to test the impact of neighbor cell interference, a major factor that limits throughput in live networks. If the network vendor has implemented a test mode in the base stations, it might even be possible to simulate neighbor cell interference as there are probably not enough mobiles yet to generate meaningful load in all cells of the setup.

  • Backhaul: Fiber backhaul (200 MBit/s) is used. Looks like Innsbruck has good network infrastructure in place!
  • Frequency band: The 2.6 GHz frequency band is used.
  • Throughput: Downlink up to 35 MBit/s, uplink up to 31 MBit/s with a 20 MHz carrier.

    Note1: That sounds quite realistic as there's probably not much interference from neighboring cells yet due to the limited number of mobile devices used in the trial.

    Note 2: Broken down to a 5 MHz carrier for easy comparison with HSPA, the speed would have been 8.75 MBit/s (disregarding statistical multiplexing gains of a broader channel). I wonder what the speed would be today with HSPA+, 64QAM (no MIMO) and suitable devices. I suppose it would not be much less. Happy to hear your thoughts on this!

  • Round trip delay times: 21 ms. Very nice, current HSPA technology in live networks have a round trip time of around 100-110 ms.
  • Mobile device used: No names given but the picture in the original post is interesting!

    Note: Looks like the devices they used are still early proof of concepts. Side note: The LG dongle sized LTE mobile I've seen at the Mobile World Congress in Barcelona this year was already a lot smaller.

MIMO Testing Challenges

Over at Betavine Witherwire there's an interesting post on the challenges of consistently testing multi-antenna devices which will shortly appear on the market. The author of the post mentions that even without MIMO, 3G network capacity could increase by 50% if all devices are equipped with multiple receive antennas and sophisticated noise cancellation algorithms. Obviously that also translates in higher throughput per device. Consequently, network operators are likely to be very interested in these developments and accurate testing of the performance enhancements is a must.

While many tests with mobile devices today are performed with the air interface simulated over a cable, that won't work that easily anymore for MIMO and receive diversity as the antennas in the device are effectively bypassed. It's the antennas and their location and shape inside the device, however, that will make the big difference. More details in the post linked to above.

So I wonder if it's possible to model the impact of the antennas by simulating their characteristics in addition to the signal path with a simulator box that sits on the cable between a real base station and the mobile device)!?

A formidable challenge and I look forward to what the guys in 3GPP RAN4 come up with.

LTE and UMTS Air Interface Comparison

There's a very interesting blog entry over at the 3G and 4G Wireless Blog by Devendra Sharma on the differences between the LTE and UMTS air interface beyond just the physical layer. By and large he comes to the conclusion that the LTE air interface and its management is a lot simpler. I quite agree and hope that this translates into a significantly more efficient power management on the mobile side (see here) and improved handling of small bursts of data of background IP applications (see here and here). I guess only first implementations will tell how much it is really worth. I am looking forward to it.

LTE CS Fallback (CSFB) Issues Around SMS

While doing some research on CS Fallback (CSFB), the method currently favored by 3GPP to bring voice and SMS to LTE, I came across this discussion paper (SP-090429) initiated by Vodafone, China Mobile, Alcatel-Lucent, CATT, Huawei, Starent and ZTE. In the paper, the authors point to a number of issues with CSFB concerning SMS delivery and think about an LTE native implementation of SMS. According to the SA#44 meeting report, it looks like there was a heated discussion over it but no real agreement was reached on how to move forward.

A quick CSFB intro before moving forward: In its core, what CSFB does is to establish a signaling channel between a circuit switched MSC and the LTE core network. This way, mobiles currently attached to the packet switched LTE network can be informed about incoming circuit switched voice calls and SMS messages. In the case of voice calls, the mobile jumps back to 2G or 3G coverage and accepts the call. SMS messages can be directly delivered over the signaling link so no fallback is necessary. In that respect, CS fallback is not quite the right description for that part of the functionality.

The main issue with CSFB apart from having to fall back to a legacy technology for a core application is the increased call establishment time due to the fallback. It has been estimated that even in the best fallback case, this adds at least 1.5 seconds to the process. In many practical cases, it's likely to be longer. As call establishment times in mobile networks are already significantly longer than in fixed line networks, adding yet another source of delay is very undesirable.

In addition to this issue, the document lists a number of other issues, especially around SMS. Here are some examples:

  • Availability at launch: There is no guarantee that CSFB will be available at the launch of LTE as at least one circuit switched component, the Mobile Switching Center (MSC) has to be enhanced to support the new signaling interface (called SGs).
  • Roaming: It's not guaranteed that CSFB will be available in a foreign LTE network the mobile roams into when the user travels to another country.
  • Deferred Delivery: When an SMS can't be delivered immediately, like for example when the user has temporarily no coverage, an SMS waiting flag is set in the network. When the mobile is back in the network, the MSC is aware of the waiting message and delivers it. Unfortunately, this mechanism is not available with CSFB as the Mobility Management Entity (MME) in the LTE network does not inform the MSC that the mobile is back.
  • LBS: Location services based on SMS and Cell-ID interrogation are not possible with CSFB as LTE cell-id's can't be used in the circuit switched part of the network.
  • Resource Use: For SMS delivery the MME and the MSC are involved in addition to the SMSC. From a resource point of view, to many network nodes are used.
  • Specification Issues: It looks like there are some gaps in the current CSFB specification. One of them concerns SMS delivery during an ongoing fallback procedure due to an incoming call just before the SMS is delivered. Another, potentially even more significant issue, is concatenated SMS delivery, which is also still missing in the specification.
  • Test Scenarios: And finally, the current test plan for early LTE implementation does not include CSFB SMS test scenarios yet.

Quite a list of issues. While some can probably be fixed in short order, others are likely to be a bit more tricky. In combination with roaming and the EU mandate to inform users of roaming charges, hot-billing info, activation / deactivation of features and many other applications using SMS, it's clear that something has to be done to fix the issues. If native SMS over LTE is the solution, well, we'll see. Given the result of the discussions in the meeting report, this solution doesn't seem very likely to me.

Always-On LTE Roaming

I've been thinking a bit about LTE and international roaming lately and just realized that mobile network operators need to come up with a new billing scheme compared to current systems. Here's why:

2G and 3G devices only request the establishment of a data bearer (a PDP context in 3GPP talk, or getting an IP address in Internet talk) when an application requests it for the first time. Thus, from a billing point of view, nothing is charged until that point. With LTE, however, the device gets an IP address right when the device registers with the network after startup. In effect, the 2G / 3G packet call becomes history with LTE. While in the home network this can probably be managed quite well from a billing point of view, I wonder how network operators will proceed for roaming. After all, most users will probably not be too happy to be charged just for switching on their device.

For LTE USB dongles, this might not be a problem as the user can decide whether to plug it in or not. For notebooks with a built-in LTE modem, however, or an LTE capable smartphone, things are different. A user of an LTE capable smartphone probably wants to use it abroad as well, even if it is only for voice calls and the offline organizer functionalities without being charged if he doesn't actively use the Internet. I wonder how this will be solved in practice!?

I could imagine several solutions:

  • The device detects the roaming scenario and asks the user whether to attach to LTE and get an IP address and warns the user that this might be a chargeable event.
  • The device detects the roaming scenario and doesn't attach to the LTE network. Instead, a 2G or 3G network is selected where getting an IP address right away is not required. The question then is how the user could trigger this later-on. In case of a smartphone it could wait till an application tries to access the Internet and then reselect to LTE once the connection is established. That won't work for LTE capable notbooks, though, as there are always applications crying for IP connectivity…
  • The home network detects that the user is roaming and blocks initial access to the Internet. Then, via a web based landing page, the network informs the user that different rates will apply if he proceeds. The problem with this approach is that the user has to open the web browser first before his other applications can get access to the Internet.
  • A certain amount of data traffic while roaming  is already included in the subscription. When going beyond this amount, access is blocked until the user is informed (e.g. via SMS or a landing page) that further Internet access will be billed separately and the user has given his consent.

Hm, it all doesn't sound convincing yet. Better ideas, anyone?

How the LTE Core Network talks to UMTS and GSM

An important functionality that has to be in place when LTE networks are launched from day one is the ability for mobiles to roam from LTE to other types of radio access networks. In most parts of the world except for the US and Canada, that is UMTS and GSM. While doing some research on this topic as to how that works from a network point of view, all books I have come across so far point to the new S3, S4 and S12 interfaces between the 2G and 3G network nodes (the SGSN and RNC) and the LTE core network nodes (or the Evolved Packet Core (EPC) to be precise), i.e. the Mobility Management Entity (MME) and the Serving Gateway (S-GW).

One might be happy with this answer from a theoretical point of view but in practice this approach might be a bit problematic. As the functionality has to be there from day one, using the new interfaces means that the software of the 2G/3G SGSNs and RNCs need to be modified. Now one thing you don't want to do when introducing a new system is to fiddle with the system that is already in place as you've already go enough work at hand. So I was wondering if there was an alternative to introducing new interface, even if only for Inter-RAT (Inter Radio Access Technology) cell reselection triggered by measurements on the mobile side.

It turned out that there is. After some digging, annex D in 3GPP TS 23.401 provided the answer (sometimes I wonder what is more important, the specification text or the annexes…). Here, a network setup is described where the 2G and 3G SGSN is connected to the LTE world via the standard Gn interface (Gp in the roaming case) to the MME and the PDN-Gateway. To the SGSN, the MME looks like an SGSN and the PDN-Gatweay looks like the GGSN. No modifications are required on the 2G/3G side. On the LTE side, this means that both the MME and the PDN-Gateway have to implement the Gn / Gp interface. But that's something that has to be done on the new network nodes which means its not a problem from an real-live network introduction point of view. With the Gn / Gp interface support in place, the introduction of LTE and roaming between different radio access networks could be introduced as follows:

Cell Reselection Only at First

To make things simple, LTE networks are likely to be launched with only cell reselection mechanisms to 2G and 3G networks instead of full network controlled handover. That means that the mobile is responsible to monitor signal strengths of other radio networks when connected to LTE and autonomously decide to switch to GSM or UMTS when leaving the coverage area of the LTE network. When using the GSM or UMTS network the mobile also searches for neighboring LTE cells and switches back to the faster network once the opportunity presents itself (e.g. while no data is transmitted).

Handovers Follow Later

The advantage of cell reselection between different types of access networks is that they are simple and no additional functionality is required in the network. The downside is that when a network change is necessary while a data transfer is ongoing the mobile will either not attempt the change at all or the change results in an temporary interruption of the ongoing data transfer. The answer to the downside is to perform a network controlled handover between the different radio systems. This makes the change between access networks a lot smoother but requires changes in both the new and the old radio networks. On the GSM/UMTS side, the software of the base stations and radio network controllers have to be upgraded to instruct the mobile to also search for LTE cells while the mobile is active and to take the results into account in their existing handover mechanisms. As far as I can tell, no modifications are required in the SGSN, as transparent containers are used to transfer non-compatible radio network parameters between the different networks.

Packet Handovers Today

At this point I think it is interesting to note that packet handovers are already specified today for GPRS/EDGE to UMTS and vice versa. However, I haven't come across a network yet that has implemented this functionality. Maybe it is the speed difference between the two radio access networks that makes the effort undesirable. Between UMTS and LTE, however, such packet handovers might finally make sense as in many scenarios, the speed difference might not be that great.

The GGSN Oddity

One last thought: In annex D, the 2G/3G GGSN functionality is always taken over by the PDN-GW. That means that an LTE capable mobile should never use a 2G/3G only GGSN when first activating a PDP context in GPRS/EDGE or UMTS. If this was done I don't see how it would be possible to reselect to the LTE network later. This is due to the fact that the GGSN is the anchor point and can't change during the lifetime of the connection. If an "old" GGSN would be the anchor point, then the MME and S-GW would have to talk to the "old" GGSN after a cell reselection or handover from GPRS/EDGE or UMTS to LTE instead of a real PDN-GW. That's a bit odd and I don't see this described in the standards.

There are several ways how that could be achieved. Using a special APN for example that triggers the use of a combined GGSN/PDN-GW when the connection is established could be a possibility or the analysis of the IMEI (the equipment ID). While the first idea wouldn't require new software in the SGSN, the second one probably would and then there is always the chance that you miss some IMEI blocks in the list on the SGSN, especially for roamers, so it's probably not such a good idea after all. Another option would be to replace the GGSNs in the network or upgrade their software so they become combined GGSNs/PDN-GWs. However, there some risk involved in that so some network operators might be reluctant to do that at the beginning.

If you know more about this or have some other comments or questions in general, please leave a comment below.

VOLGA Forum Publishes Stage 2 Specification For Voice Over LTE

Regular readers of this blog probably remember that I'm a fan of Voice over LTE via GAN (VOLGA). For those who don't, have a look here on more details on why I think it has a good chance of becoming THE voice solution for LTE. It's amazing how fast the Volga-Forum is pushing out the specifications. In May, they published the stage 1 specification document, which contains a high level architecture and the requirements. Now only a month later, a first version of the stage 2 specification is available. Stage 2 specifications as per 3GPP contain a detailed architecture description and all procedures required from connecting to the network, originating and terminating calls, doing handovers, etc.

While their speed is incredible, maybe it should not be that surprising, because VOLGA is based on the already existing 3GPP GAN (Generic Access Network, i.e. GSM over Wi-Fi) specification. That's a good thing because that means that VOLGA could thus be developed quite quickly as it's likely that existing products can be modified instead of being designed from scratch. In addition, this should also mean that the first version of the standard is already quite mature as many areas were already verified during implementation and rollout of GAN in current networks.

I did a quick comparison between the two stage 2 specs and as I expected, many parts are very similar. While the GAN stage 2 specification has 126 pages, the current VOLGA stage 2 specification has 87 pages. This is probably because VOLGA is simpler than GAN. There are fewer handover procedures and most of the handover details are part of the 3GPP Single Radio Voice Call Continuity (SR-VCC) specification (for IMS) so they don't have to be included in the VOLGA spec. In addition to fewer handover scenarios, handovers are a bit more simple with LTE from a VOLGA perspective, as the network takes care of it unlike with GAN, where the mobile has to force the network into a handover. Also, there's no need to support the packet switched part of the network which also significantly lowers the complexity.

Well done, I am looking forward to the stage 3 specification which will contain the details on all messages and information elements used.

LTE TDD in China and Europe?

Even though LTE is a global standard there are two different air interface flavors: The first one is FDD and pretty much destined to be used around the world. And then there is TDD, for the moment mostly foreseen to be used in China. However, many European operators have acquired TDD spectrum in the 2.1 GHz band during the 3G auctions back in 2000. So while I haven't heard anyone talking about this so far, I wonder if LTE TDD might be something of interest to European carriers!? In that regard, does anyone know if the TDD band in China is anywhere near the TDD region that was auctioned in Europe? Might be a great push for dual mode FDD and TDD devices.

The Current LTE Spectrum Situation

I was recently asked by a friend how I see the current LTE spectrum assignments. I would have liked to give a simple answer but it is actually not quite straight forward. This is how I see it at the moment:


  • 2.1 GHz band, used for 3G today but still a lot of unused capacity: Most likely the first band where 5 MHz LTE carriers will be deployed. No limitations from regulators, LTE can be deployed straight away.
  • 2.5 GHz spectrum auctions still outstanding in most countries. Good for going beyond 5 MHz carriers
  • 900 MHz band: Maybe some deployments of 5 MHz carriers or less. Good for in-house coverage but the band is heavily used for GSM today so it's difficult to clean up enough space for a meaningful LTE deployment without running into congestion issues. It might get better as more people get 3G phones and some of voice and data traffic currently running over GSM in the 900 MHz band will start flowing over 3G in the 2.1 GHz band. That reduces the load, hence, it might be possible for carriers to clear some spectrum for LTE, if they haven't opted for UMTS 900 deployment, which is available today.
  • 800 MHz band (Digital Dividend): Strong push in Europe at the moment to free the same bandwidth in all member countries. First trials have started to bring high speed Internet to rural areas with HSPA and LTE.

North America:

  • Verizon will deploy LTE in the 700 MHz band and has a single 10 MHz carrier available. That's not much. The spectrum has been assigned, so it can be deployed straight away.


  • According to the presentation of NTT DoCoMo at the Mobile World Congress in Barcelona this year, they will also deploy LTE in the 2.1 GHz band, replacing one of their UMTS carriers with LTE at the beginning.


  • Not sure

Unfortunately, it does not end here, 3GPP has lots of additional frequency bands defined for LTE. So fracturization is likely to increase. As always, comments, additions, etc. are very welcome!

LTE – The First Global Cellular Standard – But Does it Matter?

Indeed on first thought, LTE will be the first global cellular standard in the future to which GSM, UMTS/HSPA, CDMA and potentially other cellular wireless technologies are likely to converge on. But does it really change anything?

Being a global standard does not necessarily mean all LTE capable devices can communicate with all networks around the globe. There are two main issues:

1) FDD and TDD Mode

While in most parts of the world, FDD (Frequency Division Duplex) will be the dominant air interface technology, TDD (Time Division Duplex) is pushed especially by China as an upgrade path for TD-SCDMA. So an FDD LTE device will not be able to use a TDD network and vice versa. With some luck, we might see devices that can do both FDD and TDD but nobody's really commenting on how feasible this really is. Only time will tell.

2) Two Dozen Different Frequency Bands

What's worse is the number of frequency bands are foreseen for LTE. In practice, this will mean that devices will be built for some but not all of those frequency bands. So it's nice to have a global standard but it's unlikely the mobile devices themselves will be usable on a global scale. The single 4G device working everywhere will remain a nice dream.

A Little Light At The End Of The Tunnel For Vendors

LTE being a global standard is a good thing for network equipment vendors. Most of the equipment will be the same including the base stations where only a few parts or modules are different to work on a different frequency band or operating mode (TDD/FDD).

Benefits For Network Operators

An economy of scale is created for networks operating on the
main LTE frequency bands (e.g. 900, 1800, 2100 and 2600 MHz). Most other frequency bands are only used by a few network operators so it's unlikely these will get the same prices from network vendors as their colleagues who use the mainstream bands. Also, the number of devices working outside the standard LTE bands are likely to be as limited as for UMTS/HSPA today. Just have a look of how many 3G devices are available for HSPA network in the U.S. compared to Europe.

Benefits For Users

For the users, I don't see a big change from the situation with HSPA today. Where the mainstream frequencies are used, there is a big choice of devices and this is likely to be the same with LTE. And network operators using less used frequency ranges will probably receive as few devices as those operating 3G network in such bands today.

What We Really Need

So what we really need is not only a global standard but also global frequency bands so everyone benefits the same. But, unfortunately, that's a dream that is very unlikely to come true anytime soon.