GSM and UMTS on trains

[inquisitor]

When travelling by ICE or IC-trains make sure you take your seat in a car equipped with repeaters so you get a better signal (but don't expect too much 3G coverage, especially on ICE trains which pass many tunnels). These cars are marked with the following sign:

[Amrando]

Originally Posted by inquisitor

When travelling by ICE or IC-trains make sure you take your seat in a car equipped with repeaters so you get a better signal (but don't expect too much 3G coverage, especially on ICE trains which pass many tunnels).

I am definitely familiar with poor coverage on trains. I am a frequent traveler to Japan, and phone and data service on the Shinkansen is similar. Unfortunately even on the vast majority of bullet trains JR and NTTdocomo do not offer network repeaters - only the very newest trains have them installed such as the brand-new N'Ex and Tohoku Hayabusa/Hayate. Otherwise it is very easy to see the train is passing through a city - you can watch signal strength climb from zero to full and fall back down in the space of a few seconds. Or you can try and make your calls during the lightning-fast station stops.

[kuba.g]

Using internet on trains is something with which I have a lot of experience as well, as I mainly travel by train in Europe.

The most interesting fact I have noticed is that the experience with it in Germany is very disappointing (if not the worst...). I have tried out MedionMobile (e-Plus = tragedy), Fonic (O2) and ja! Mobil (T-Mobile). O2 worked nice as long as it had 3G, but on most of the train routes this is not the case and then it jumped back to GPRS. I have found surfing on this GPRS not doable, so I hoped for a significant improvement with T-Mobile which is a 100% EDGE network. However, even on the EDGE network, and even though my mobile phone shows 50% signal, internet just doesn't load.... and this is the case for like 75% of the time.

I have no idea what causes this. I have found Poland's Plus EGDE network to work much better on trains (when there is at least 25% EDGE signal internet just loads, which somehow isn't in Germany.....). The same thing holds for TIM in Italy.

[Motel75]

Agreed, the service in Poland vs. Germany has always surprised me. Take the Berlin-Warszawa Express from Warsaw, and the signal is great all the way to the German border, after which it drops off completely, reappearing briefly only a couple of times for a few seconds until you reach central Berlin. It's possible there's a Poland-only repeater on board, but this has been my experience on normal trains as well.

[Amrando]

regarding mobile data on trains: (sorry to 'derail' the thread)

I have a friend who was formerly an engineer with Qualcomm. His explanation was fairly basic, but practical. The problem is that cellular networks' tower handoffs are not designed to happen seamlessly (fast enough) to handle 'driveby' transmissions at 300kmh. When driving at 100-150kmh, your phone is in contact with two towers simultaneously long enough for the two to converse with each other and 'handoff' the call from old to new. However at 300kmh (ICE), or 350kmh (TGV/Shinkansen) or god forbid 800kmh (aircraft overhead) your phone is in negotiation with too many towers at once. This is the 'real' reason why you are told to shut off your cellphone while flying. Because each phone creates a huge footprint of negotiations to transmit on the ground with the towers it passes over, and causes too much 'overhead' for the cellular providers.

[inquisitor]

@Amrando
What you say is correct for GSM, which indeed can handle handovers only up to 250km/h (GSM900) or 130 km/h (GSM1800), which is due to the so-called "timing advance": On GSM networks up to 8 terminals (= handsets) share the same frequency. In order to avoid interference each handset may only "talk" and "listen" during a time frame of 577μs, which is assigned to each terminal by the network. So practically each phone has just a small so-called timeslot of 577μs every 4616μs during which it may communicate. Now since the signal from a terminal next to the cell tower reaches the cell tower faster than the signal from a more distant terminal their communication would overlap and cause interference. That's why you need to consider the distance respectively the signal transit time between the terminal and the cell tower and adjust the timing so all signals arrive just in time to match the assigned timeslot. A more distant phone, whose signal has to travel longer hence would reach the celltower delayed and so miss it's timeslot, has to send it's signal a bit earlier in order to offset the delay of the longer transit time. This adjustment is called "timing advance" and the problem is simply, that if you travel too fast and approach the cell tower with high speeds you need to adjust timing advance faster than the GSM standard allows.

Btw the limited ability of timing advance also limits the range of GSM900 to 35km (allthough some network equipment manufacturers have "extended range" solutions, which double the range by recuding the number of timeslots to 4 at the expense of capacity halving, so only 4 terminals can share the same frequency).

Radio technologies which are based on all terminals sharing the same frequency by using it only during assigned timeslots are called Timing Divison Multiple Access (TDMA).

3G/UMTS in contrast is a Code Division Multiple Access (CDMA) technology where all terminals listen and talk simultaneously and the signal is later being subtracted out by a spreading code. This is a bit more complicated than TDMA and thus a detailed explanation would go beyond the scope of this discussion. However as a result UMTS can handle handovers up to 500km/h, so it would generally work even on high speed trains.

In the field however the main problem is the shielding from the metal frame of the train cars and additionally the metall coated windows on modern trains, which causes high attenuation (= weakening of the signal). Further those high-speed tracks lead often apart from populated areas, which have inferior cell coverage and then there are a lot of tunnels, which provide even worse to none reception.
Lastly at least in Germany we have the problem of UMTS networks only operating at 2100 MHz, which is a comparably high frequency, which has a way shorter range than our GSM networks, which operate at 900 and 1800 MHz and has worse signal dispersion properties, which basicly means signals at 2100 MHz are impacted even more by the attenuation of the metal frame and coated windows.

In other European countries they have UMTS also running at 900 MHz, which improves user experience significantly. The same applies for America, where AT&T, Rogers/Fido and Bell/Telus run UMTS at 850 MHz, while T-Mobile USA and Wind Mobile, Mobilicity, Videotron use the AWS band (1700MHz for uplink and 2100MHz for downlink) suffer from the physical disadvantages of the high frequencies they were awarded.

[dg7feq]

very good technical summary! thumbs up!

[kuba.g]

Thanks for the info, inquisitor!

This however kind of doesn't really explain to me why using the internet on German trains feels worse than in Poland or Italy. Consider only 2G (GSM, which is the same frequencies in all these countries). The experience for me as a user is that in Germany using T-Mobile I see some bars in the signal strength (sometimes even full) and still internet just doesn't load. Even though I have EDGE. This is also in regional trains (Nahverkehr) which in general don't travel faster than 140km/h - 160km/h. On trains in Italy (TIM) or Poland (Plus), travelling similar speeds, also using EDGE, internet just works. I can't seem to find a good explanation for this... I would have expected this to be exactly the other way round, Germany being a more developed and more ordered country. Would German trains be better isolated than Italian or Polish ones or something?

[inquisitor]

One reason could be the limited capacity of 2G networks. Usually a GSM basestation consists of 3 sector antennas (each covering a 120° angle) and each of the sectors can have not more than 4 channels, which again have 8 timeslots each. That means there are 3 * 4 * 8 = 96 timeslots at max, of which some are required as control channels (BCCH, SDCCH, etc.) reducing the total of possible voice calls per basestation to 90. So a GSM base station can handle not more than 90 voice calls.
Usually your phone won't be able to connect to all sectors, as at least one is oriented in the opposite direction. But on the other hand the cells (= the area covered by such a sector antenna) overlap, so your handset should always be able to connect to a neighbouring cell in case there's an overload on the serving cell. Further most European network operators have frequency spectrum at 900 and 1800 MHz of which the latter is used on top of 900 MHz cells in order to extend capacity. Anyway, wherever you are, the number of available channels/timeslots is limited, especially on trains, where due to attenuation the signal of many neighbouring cells will be too weak and only few cells can serve your handset (especially those at 1800 MHz, which suffer from the physical disadvantages of the higher frequency).
Now while a voice call requires just a single timeslot, an EDGE data connection with full bandwidth (236.8 kbit/s) occupies 4 timeslots, which corresponds to the capacity of 4 voice calls. So a smartphone downloading something at full speed consumes 4 times the capacity a voice call would do or in other words just 8 smartphones downloading at full EDGE speed could use a cell of 4 channels to the full.
Now imagine a train with several hundred passengers coming along, some of them making voice calls and some surfing on the web. They can quickly overload the network.
Of course network operators won't permit few heavy data users to practically block their networks and so they priorize voice calls over data and dynamically assign the number of timeslots available for EDGE, which results in fewer timeslots available for each EDGE-connected user and so fewer bandwidth.
This said I believe the higher smartphone penetration in the economical stronger countries in combination with the limited capacity of 2G networks (and the inability to use most of the GSM1800 cells while riding a train) cause the inferior user experience you see in Germany.
E.g. in Bulgaria I always get data rates of 150-200 KBit/s through EDGE, while here in Germany I rarely see rates above 100 KBit/s and in urban areas (indoors where you don't get a 3G signal) I often get even less.
Further consider the more difficult topographical properties of Germany with hilly uplands and the higher costs to build additional cell towers.
But there's hope for improvement due to the LTE networks (4G) on 800 MHz being rolled out in Germany, which will have much higher bandwidths than UMTS (3G) and have a very good signal dispersion due to the low frequencies used. The imminent roll out of 4G networks is also a reason why operators invest only little in the existing 2G and 3G networks here.
Another problem here in Germany concerns eplus and O2, who were awarded GSM900-licenses only in 2006 (earlier they used only GSM1800 besides UMTS2100). Further their 900MHz spectrum is in the so-called E-GSM band (E stands for extended), which uses frequencies below the original (P-)GSM band (P stands for primary). Allthough every GSM900 phone produced since 2005 supports E-GSM at the time when eplus and O2 got their new spectrum many trains were already equipped with repeaters, that did not support those E-GSM frequencies and so they did not work for eplus and O2 clients at all. According to this detailed article on Wikipedia Deutsche Bahn intends to replace those old repeaters until 2011 and 1495 train cars will be equipped with E-GSM capable repeaters. By September 2009 half of them had already been installed.

[Motel75]

Hmmm. I don't think this explains why when I take the train from Warsaw to Berlin, the very good service is completely lost at the German border. There could be several factors: there may be repeaters on the train that only work with Polish networks, or the providers in Poland have targeted railway lines as must-cover areas, which the Germans have not. Indeed, the service along many German railway lines is appalling, and it has nothing to do with fast trains or metallic coating on the windows. At the same time, service is always very good along the autobahn; it seems the German providers may have different priorities.

One thing that should be noted is that the vehicle speeds Inquisitor mentions apply if the vehicle is directly approaching the tower. This may be the case with cells in tunnels, but will not be so with towers placed some distance away (the relative speed can be figured out with the cosine of the angle of travel relative to the tower; if the train is perpedicular to the tower the relative speed is zero).