Category: Uncategorized

With the spectrum auctions currently ongoing in Germany these days and LTE being the hot topic a number of people have independently asked me recently when I think UMTS will be switched-off. A refreshing variant of the question when GSM will be switched-off. I find the question quite interesting and my answer is that I personally think that UMTS won't go away anytime soon. Having reached almost nationwide coverage in many countries, offering broadband speeds and continuing development ensuring competitiveness, the only reason I can see why to switch it off at some point is to save cost. But until it can be switched-off a number of things have to happen:

• LTE must reach a similar coverage as 3G networks today.

• Most mobile devices requiring a fast mobile and wireless Internet connection have to have LTE built in.

• A voice solution for LTE must be found as falling back to GSM (which is not switched-off either…) for voice calls is from my point of view not a viable option.

So when will those things have fallen into place? I seriously doubt that this will happen within the next 5 years. And once we get there, will there still be a need to switch 3G off or will multi-mode base stations that can generate GSM, UMTS and LTE signals just make it unnecessary?

I see a coexistence of GSM, UMTS and LTE for a very long time to come. So instead of working on phasing out UMTS, it might make more sense to work on solutions to integrate the different radio systems.

As always, comments are welcome!

One of the design principles of LTE was to streamline signaling as much as possible in order to simplify the system as much as possible and to execute procedures as quickly as possible. An interesting result of this is how messages of different protocol entities can be bundled into a single message that is sent over the air interface. Take the attach process as an example where the mobile device is ultimately assigned an IP address. In addition to reducing the number of steps required compared to GSM and UMTS, a single message is used to transmit the following towards the end of the procedure:

• An RRC Reconfiguration Message to establish a data channel (a DRB) for the user data;
• An Attach Accept message to tell the mobile that the attach was successful;
• An Activate Default Bearer Request message to tell the mobile to activate a logical bearer (for which a physical air interface bearer has just been configured with the RRC message above).

And all in one message on the air interface! In UMTS, those were all separate procedures with separate message exchanges. Pretty streamlined I would say! For details see 3GPP TS 23.401.

Self observation: It's interesting how even little things can have a big impact on usability and behavior. When I am in public places and use an unencrypted public Wi-Fi hotspot I want to be as secure as using a 3G connection with proper authentication and encryption enabled. So I use a VPN. However, I manually have to activate it and even though it's only 3 clicks I don't really like to do it. So I am sure if I had a 3G card inside my netbook instead of an external 3G USB dongle I would just not bother with the Wi-Fi and VPN and just get connected over 3G, despite a Wi-Fi hotspot being available. So if Wi-Fi is to become a way to offload traffic from the 3G macro network then a piece of software needs to be available that checks which options there are to connect, selects 3G or Wi-Fi without user interaction and in case of Wi-Fi automatically establishes a secure and encrypted tunnel. Without user interaction, though, that's the important point!

Here's a thought that I recently had when I looked at how the Hybrid Automatic Retransmission Request (HARQ) functionality works in HSPA and LTE: From a conceptual point of view HARQ is quite similar to the Acknowledgement mechanism of Wi-Fi. Here, the reception of each packet has to be confirmed by the receiver by returning a MAC Ack(nowledgement) frame back to the receiver. This is done in a way that the ACK package has precedence over any other packets that are waiting in the queues of other users of the system. If the ACK is not received, the sender automatically retransmits the packet with the same or a different modulation and coding scheme.

The HARQ mechanism of HSPA and LTE is pretty similar: Each transmission has to be immediately acknowledged on the MAC layer as well. If a NAK or nothing is received the transmission is repeated. When one goes into the details, of-course, there are fewer similarities. With HARQ, the system can use incremental redundancy to send a different version of the packet with different error detection and correction bits. In addition, several HARQ processes run concurrently so a transmission failure of a single packet does not stop the overall transmission. And then, HARQ uses an 'out of band' channel for the feedback, while the Wi-Fi Ack is a normal packet on the air interface.

For about one and a half weeks or so the German spectrum auction was not very exciting for external observers as little activity could be spotted. On Wednesday morning, though, things started to heat up.

Until that time, the total amount that was bid at this point was still below 400 million euros. Then suddenly, activity spiked and within a day the proceeds were almost up to one billion euros. The main activity for the moment is focused on the 800 MHz digital dividend band. By the end of Friday, the few MHz available there accounted for around €1.2 billion of the total €1.46 billion bid for everything so far.

While in other bands there is an ample amount of spectrum for everyone, only 30 MHz is available there, and consequently not enough space for the four contenders each wanting at least a 10 MHz chunk. So while a single 2x5MHz chunk is priced at over €200 million there at the moment, the same amount of spectrum can be had for 'as little' as €7 million in the 2.6 GHz band.

Every time I attend conference calls where people dial in from all over the world, my ears are usually suffering if there's more than one speaker. It's for various reasons:

• The volume different people have on the call is widely different. So while you have to listen very closely to understand some participants, others can be heard so loudly that your ear-drums almost pop out when the call goes from one extreme to the other.

• Most of these issues are caused not only because different telephone networks seem to interconnect on different volume levels. It's also because every participant has a different phone, some use the hands-free mode and, for international calls, some countries use a different voice codec that at some point is converted into the codec used in the country of the phone bridge.

• Add to that some echo when people are not muted, background noise such as babies crying, dogs barking and cars passing by and the perfect storm approaches.

• There are always people on a conference calls who are on the move and their mobile phones often try much too hard to filter out the background noise, resulting in shriek peaks and hard to understand participants.

• Automatic announcements that people are leaving and and re-joining the conference call due to patchy network coverage every couple of minutes doesn't make things much easier, either.

And on top of all of that put non-native speakers with sometimes heavy accents and after an hour your (or at least my) head starts spinning. So what's the solution to this?

I think it's wideband audio conference calls with heavy pre-processing of the individual call legs. As you can't get that over the standard telephone network, it must be Internet based, maybe, and that's already a big compromise, with telephone dial-in for those who for one reason or another can't access the Internet (on the move, stupid company firewall, etc.). I guess anyone who've once enjoyed the difference between wideband and narrowband speech knows what I am talking about. And on top of that the conference server or the clients should do some intelligent pre- or post-processing of the signal coming from the different participants. Is it really that hard to have everybody's voice arriving at the same level?

Anyone aware of such a system?

3G femtocells are an interesting topic but I haven't had much time yet to take a look at the details of how mobility management works in practice. There's lots of activity in 3GPP to standardize mobility management around femtocells (or Home NodeBs how they are called there) in Release 8 and beyond. However, there are already already femtos on the market today and they have to work together with pre-Release 8 mobiles. So I've had two fundamental questions: How can mobiles find the femtos when they are on the 3G macro layer and how does the femto get rid of users which do not belong to the subscriber group, i.e. everyone except the owner and his/her family and friends?

Then this book, "Femtocells – Technologies and Deployment" by Jie Zhan and Guillaume de la Roche came my way. I haven't had time yet to go through it in detail but it looks highly interesting and informative and I could answer my questions with it within minutes:

Cell-Reselection to a Femto: To make a cell reselection to another 3G cell, it needs to be part of the neighbor cell list of the cell. As there could be many femtos inside one macro network and provisioning them automatically might not be a straight forward approach, one option is to select a couple of Primary Scrambling Codes and declare them as neighbors in every macrocell or at least on those macrocells in which femtos are located. This works even if there are many femtos inside the coverage area of a macrocell as not all of the femots are overlapping and hence the PSCs can be reused. If the femtos scan their surroundings when they start up they can help to avoid the PSC overlapping issue.

How to get rid of non-femto subscribers: The femto deployments I have heard of so far are closed-subscriber-group femtos, i.e. only registered people have access. But since todays mobiles know nothing of femtos how can you ensure only those remain in the cell that are supposed to be there? The book gives this as one of the potential solutions: For the femtos a certain range of location area codes (LACs) are reserved. If a non-femto subscriber mobile finds the cell and tries to perform a location update it gets a location update reject with cause code #15 (no suitable cells in location area). The mobile then goes back to the macro layer and puts the LAC in the forbidden LAC list on the SIM. The 3GPP UMTS RRC spec says It's only removed when the mobile is switched-off or after a significant amount of time has passed (12-24h). A bit of a disadvantage here: If the user of femto-A passes femto-B during the course of a day, the mobile will try to register with femto-B and will be rejected. In case the LAC was the same as that of femto-A the mobile will not try to reselect to femto-A until the forbidden LAC list is cleared. In other words, the user comes home and the mobile will not use the femto.

Agreed, there's much much more to the topic, those where just my two most burning questions concerning femtos.

It's been a long time since I've last participated at the Carnival of the Mobilists so it was about time to send a post again. This week, the Carnival is hosted over at the Radvision Blog and Tsahi Levent-Levi has done a great job of assembling a great Carnival with the latest news on mobile from the blogosphere. So, head over and enjoy!

Looks like Apple has decided to let OperaMini into their AppStore and within just a couple of days it has become hugely successful according to Engadget here. Being a long time user of OperaMini and knowing about its strength and advantages in bandwidth constrained and high outage environments when moving in trains, cars, etc. I can imagine why everybody seems to rush to it. But not having an iPhone myself I'd be interested from you what your experiences are if you tried it!

Here's a little comparison of how UMTS and LTE deal with limited uplink power of mobile devices which I think it is quite interesting:

When uplink power for a UMTS E-DCH (HSUPA) transmission reaches a maximum, the number of simultaneously used codes can be reduced, a more conservative coding can be employed for additional redundancy and the modulation order can also be changed.

In LTE, modulation and coding can also changed as needed. And in addition, there's a third parameter: LTE uses an OFDM air interface, or to be more precise, SC-FDMA in the uplink direction. In other words, many subcarriers are used for the data transmission which are grouped into consecutive Resource Blocks (RBs) in case of uplink transmissions. When the mobile device reaches its maximum power level and the network detects this, it can reduce the number of RBs assigned in the uplink direction. This way the mobile can concentrate it's power on fewer RBs and hence it has more power available on the narrower channel it now uses. From a network point of view this is much better than leaving the number of RBs as they are and reduce modulation and coding as the RBs that are removed can be assigned to other devices also requesting resources in the uplink direction. For details see the power control section in this excellent book.