GSM Phaseout Architectures

Back in 2000, most of us in the industry thought that by 2012 or so, GSM would be on a good way to become history in Europe and elsewhere, having been replaced by 3G and whatever came afterward. Now in 2008, it's clear that this won't be the case. About a year ago, I published an article to look at the reasons why this has not happened. With LTE now at the doorstep, however, it has to be asked how mobile operators especially in Europe can support three radio technologies (GSM, UMTS/HSPA and LTE) into the foreseeable future.

While over the next few years, many network operators will transition their customer base to 3G handsets and thus might be able to switch off GSM from that point of view, there are a number of factors that will make them think twice:

  • There might still be a sizable market for customers who are not willing to spend a great deal on handsets. Fact is that additional hardware and licenses for combined GSM/UMTS prevent such handsets for becoming as cheap as very basic GSM only handsets.
  • Operators are keen on roaming charges from subscribers with 2G only handsets, this is a very profitable business.
  • Current 3G networks are transmitting on 2.1 GHz and as a result the inhouse coverage of 3G networks is much inferior to current GSM networks. Putting more base stations in place could help to some degree but it's unlikely to be a cost effective solution.

In other words, in order to switch GSM off (whenever that might be) a number of things need to fall in place first, i.e. needs to be part of an operator's strategy:

  • 3G must be used on a wide scale in the 900 MHz band (or 850 MHz respectively in the US and elsewhere). This, however, requires new mobile devices as only few models currently support this band. At this point in time it is not clear if national regulators will allow the use of 3G networks in the 900 MHz band in all European countries because it has significant implications on the competition with other technologies. Note: 4G deployment in 900/850 MHz is unlikely to help due to the voice gap discussed here.
  • An alternative could be that combined DSL/Wi-Fi/3G Femtos become very successful in the market, which could compensate for missing 900 MHz coverage. But I am a bit skeptical if they can become that successful.
  • Most roamers would suddenly pop-up with 3G capable handsets. I don't see that happening in the near- to mid-term either due to many countries not going down the 3G route and even for 900 MHz. Also, roamers with mobiles from places such as North America use different 3G frequencies and thus would not work in Europe and elsewhere and of course vice versa. Maybe this will change over the next couple of years two, but except for data cards, I haven't seen a big push for putting 3G on 850/900/1700/1900/2100 into handhelds.

At some point, however, it might become less and less economical to run a full blown GSM network alongside UMTS/HSPA and LTE networks despite lucrative 2G roamers and better inhouse coverage on 900 MHz. I see several solutions to this:

  • Since GSM traffic declines in favor of 3G it will be possible at some point to reduce the capacity of the GSM network. At this point, separate GSM, UMTS and LTE base station cabinets could be combined into a single box. Base station equipment keeps shrinking so it is conceivable that at some point the GSM portion of a base station will only take little space. By using a single antenna casing with several wideband antennas inside could keep the status quo in the number of antennas required to run three network technologies alongside each other. Cabling could also be kept fairly constant with techniques that combine the signal to/from the different antennas over a single feeder link. For details have a look at my post on the discussion I recently had with Kathrein.
  • Maybe advances in software defined radio (SDR) will lift the separation in base station cabinets between the different radio technologies. Should this happen, one could keep GSM alive indefinitely. SDR is discussed in the industry for many years now. Since I am not a hardware/radio expert I can't judge if and when this might become part of mainstream base stations.
  • And yet another interesting idea I heard recently is that at some point two or more operators in a country might think about combining their GSM activities and instead of running several networks, only a single GSM network is maintained by all parties involved . As this network is just in place to deal with the roamers and the super low ARPU users (and maybe still lacking inhouse coverage), it is unlikely that this network will be upgraded with new features over time, so it could be pretty much static. So running such a combined network might be a lot easier than running a combined 3G network to save costs.

So what is your opinion, which scenario is the likeliest?

HSUPA Deployment Status

In the past couple of months I’ve seen in the news that some mobile operators have started to deploy HSUPA (High Speed Uplink Packet Access), the uplink brother of HSDPA in their networks in selected areas. Today, T-Mobile announced that their nationwide HSUPA network roll-out is finished. And nationwide roll-out doesn’t mean just a couple of base stations, it means several thousands.

I guess it wasn’t too difficult to do since HSUPA is mainly a software update. Also, base station backhaul capacity has already been increased for HSDPA. The uplink part, which is usually increased as well in symmetric links such as E-1’s can now be put into good use.

This time, networks are a bit ahead of the mobiles. Except for data cards, I haven’t seen a lot of HSUPA mobiles yet. All right, Nokia and co. it’s about time now and I can’t wait to transfer my videos to YouTube right from the mobile:-)

WiMAX Frequency Implications

WiMAX world recently published an interesting article by Caroline Gabriel on spectrum and auction issues for Wimax (and other wireless technologies). A very good read!

I find it very funny how time changes opinions. Some years back, BT couldn't get rid of their mobile branch soon enough. Now, they can't wait to buy spectrum and to start from scratch. Total insanity, but it reflects the reality in my opinion that in the future, only operators being able to offer fixed (via Wifi) + cellular wireless access will remain relevant.

So far, I always thought refarming 900 MHz frequencies was a good idea. After this article I understand the political dimension of this a bit better. I guess some operators are hoping that they can use their current spectrum indefinitely and for a very low price if they can escape an auction.

I guess this would be a major disadvantage for potential new entrants. 900 MHz is great for indoor coverage especially in cities, as even 3G coverage at 2.1 GHz fades away very quickly indoors. So if new entrants wouldn't have a chance to get such bands in the future, they would be at a constant disadvantage everywhere, not only in the countryside.

As a user on the other hand I don't want to wait until 2020 before I get 3G and 4G deep indoors without Wifi. Ugh, a tough call for regulators.

Concerning the first mover advantage and the claimed 18 months WiMAX lead over LTE: First, I think this lead is not really a lead, as it is debatable how much faster WiMAX is compared to current HSPA networks.  Additionally I wonder if 802.16e is really ready for prime time. One year ago, three companies have bought nationwide licenses in the 3.6 GHz band in Germany. I haven't heard from them since doing anything beyond patchy deployments in a few places!?

In the meantime, 3G price plans have become available that give users several gigabytes of data per month for a couple of pounds. Should there be any first mover advantage, that's pretty much a show stopper in itself.

Sounds all a bit negative for WiMAX but I think there are still opportunities out there. The 3GPP operators are far away from doing everything right. Especially for those occasional users who just want to open their notebook no matter in which country they are and get access for some time without worrying about subscriptions, SIM cards, etc, this camp has not yet the right answer. And then, there are the countries that don't have 3G yet for various reasons such as India and China. In some countries, however, incumbents are starting to wake up. So hurry, WiMax before this one goes to them as well.

HSPA USB Stick – Autoinstall

I always wondered a bit how HSPA USB sticks would automatically install the required software on a Windows computer as advertised!? Today, I got a Huawei E220 HSPA USB modem into my hands and could try for myself. Surprisingly, the software really  installs itself as the device says. When plugged in it says it's a CD drive and, later on, a USB modem. Once the CD drive is auto-installed, the autostart program from the virtual CD drive is executed, which then installs the the drivers and control program for the modem. Very nice, quick and efficient. It also looks like the system is pretty much left in one piece which some people say can not be said from similar software from Vodafone and T-Mobile…

Power Consumption in 2G/3G Connected State

Some years ago, when I tested how long the battery of a mobile phone would last when a mobile device was connected to a 2G or 3G network (PDP context established) but not transferring any data for most of the time. At the time, the result was quite clear: I could almost watch almost in real time how the battery level decreased. Looks like things have changed pretty much in the meantime.

When repeating the test these days with a Nokia N95 and a Nokia N82, one being connected to an EDGE network and the other to a UMTS network over the course of the day while transferring almost no data, there seems no difference anymore to the device not being connected throughout the day. The picture on the left shows a screenshot of my N95 that was connected to an EDGE network throughout the day. Note that at the time the screenshot was taken, the mobile was also connected to a Wireless LAN network (i.e. some applications used the EDGE connection, others the Wifi connection). The same test with the N82 that was connected to a 3G network showed the same result.

Very good, one thing less to be concerned about! No more advice about disconnecting from the network due to the fear of running the battery into the ground quickly.

More HSPA+: Enhanced Cell-FACH

HSPA+ is about more than just higher data rates, it is also about enhancing the radio interface to allow more devices to simultaneously connect to the network in a more power efficient way. I’ve described most of those features in various blog entries in the past but it seems I have missed one feature: Enhanced Cell-FACH.

One of the challenges of always on Internet connectivity is that mobile devices or PCs running instant messaging applications, Voice over IP prgrams, push eMail and other connected programs are anything but silent even while these applications are just running in the background. Even if just one of those applications is running, the device transmits and receives several IP packets per minute to keep the connection to the servers on the Internet alive. This means that in most cases, the radio link to mobile devices is not in idle state for most of the time.

As keeping the mobile in a fully connected state while only little data is transfered is quite wasteful in terms of bandwidth and battery capacity. UMTS networks therefore usually set device into the so called Cell-FACH state, once they detect that there is only little activity. In this state, the device uses the random access channel to transmit IP packets in uplink and the Forward Access Channel (FACH) in downlink to receive IP packets.

This method is quite efficient for the mobile, since no power control is performed on those channels. Hence, there is no radio layer signaling overhead in this state, which leaves more air interface capacity for other devices and also saves battery capacity. For the network, however, managing more than a few mobiles per cell on the FACH is not as efficient, since the channel was never designed to function as an always on data pipe for a high number of devices.

This is where the Enhanced Cell-FACH extension comes in. Once mobiles support this feature and they are set into Cell-FACH state, their data packets are sent on a Highspeed Downlink Shared Channel (HS-DSCH) instead of the Forward Access Channel. This improves the efficiency of downlink transmissions and also speeds up a state transmission into dedicated state once more packets are transferred again. An application note by Rhode and Schwarz goes into the details in Chapter 6.

What puzzles me a bit at this point is two things:

  • When will the feature become available?
  • In Cell-FACH state, the mobile is identified via the Cell-Radio Network Temporary ID (C-RNTI). In theory, this is a 16 bit value, i.e. up to 65536 mobiles per cell could be in Cell-FACH state simultaneously. Strangely enough, most networks only seem to increase this value up to 0xFF (256) before the being reset back to 0. Anyone got any idea why?

MIMO Explained

Multiple Input Multiple Output, or MIMO for short, seems to make it into every wireless radio technology today from Wifi to WiMAX, HSPA+ and LTE. Yes, we know that it requires multiple antennas at the transmitter and the receiver and offers to multiply throughput by sending an individual data stream over each antenna in the same frequency band. But just how can the receivers at the other end separate the two data streams again as they have combined in the air to something completely incomprehensible!? National Instruments has a great introduction to MIMO on their web page which describes this in simple terms with just the right amount of maths that it is still understandable for someone out of college for a decade or more and who hasn’t touched maths since  🙂

HSPA+ Background Information

It looks like UMTS will not give way to LTE in the future just like that. The High Speed Packet Access (HSPA) extensions which are now used in most UMTS networks today might get another upgrade in the future with HSPA+. Features such as 64QAM modulation, MIMO and Continuous Packet Connectivity (CTC) are on the horizon. Here are some documents I found recently which go a bit deeper into the topics: