HSDPA – Page 2 – WirelessMoves

Dongle Upgrade Incentives

Here's a thought experiment about whether and how operators should encourage users to upgrade their 3G dongles to a newer model:

Most HSPA dongles currently 'in the wild' are are HSDPA category 6, i.e. they are capable of theoretical speeds of up to 3.6 MBit/s. Category 7 dongles with a maximum theoretical speed of 7.2 MBit/s are now also available and currently going over the counter. The speed increase between the two is mainly due to an increase of the number of spreading codes the device can handle simultaneously. In other words, from an overall network capacity point of view it does not matter a lot whether most of the devices used for high speed Internet access are category 6 or 7. In the future however, this is going to change.

Pretty soon, higher speeds in HSPA networks will be mainly achieved by more sophisticated 3G devices and networks. Receive diversity with several antennas helps during weak signal conditions (this Ericsson paper is a good starting point for further research) and MIMO while reception is good. In addition, more sophisticated mathematical approaches to separate noise from useful data will also help to increase data speeds. From a network point of view, this means that the more of those newer devices are in the network compared to the number of older devices, the higher the overall throughput of the network.

So should it be in the opreators interest to encourage users to upgrade to newer devices? And if so, how could that be done best? Is the higher speed achived with those devices incentive enough or should the base station scheduler also take the UE category into account to further boost data rates of newer devices? I could also imagine to offer a reduced rate to users with newer hardware as they use the air interface more economically than users with older hardware. Kind of a similar approach to taxing older cars with higher emissions higher than new cars (don't take the analogy too far…). Or maybe this is all overkill and the normal equipment replacement cycle of 2-4 years will do the job anyway!?

3G Coverage on a Train Ride to Vienna

Recently I took the train from Linz to Vienna and I was quite surprised that Mobilkom Austria (A1) must have put a more or less dedicated 3G coverage alongside the railway track even in very rural areas. I've had 3G coverage for most parts of the trip and in the few places 3G coverage was lost, their EDGE network kicked in. I've reported on my experiences with non-optimized 3G HSDPA coverage on board of trains before (here and here), but this time, the experience was even better. The connection I established was maintained throughout the trip and high speed data transfers taking several minutes were performing very well as shown on the image on the left. I even dared to launch my IM client as connectivity was simply always there. I stepped out of the train very impressed by what is possible when operators decide to do a proper network planing and deployment.

3G Connection Sharing – Part 3: The D100

While the software on the Fritzbox for 3G connection sharing over Wi-Fi is still an early beta, I have recently also tested the Huawei D100, a box dedicated for 3G connection sharing. So far, I've seen it advertised by 3 in the UK (with contract only) or by 3 in Austria, here unlocked for 99 euros. Needless to say I preferred the Austrian variant and went into a shop while in Austria recently to buy one. I have now used it for several days with a E220 3G USB stick in the Mobilkom A1network and I have to say I am very impressed by stability of both the Wi-Fi and the 3G connection and the ease of use of the box.

Once power is switched on and the power on button has been pressed, the box automatically attaches to the 3G network and establishes an Internet connection. What I've found a bit odd is that the power button has to be pressed for the box to start. This is a bit inconvenient after a power outage for example. Should the 3G connection drop while being online, the box is smart enough to figure out something is wrong and automatically re-establishes the connection within a couple of seconds. It happened two or three times in the course of a couple of days but it's hard to say if that was due to the D100, the 3G stick or the network.

The Austrian D100 comes preconfigured for all Austrian networks, but it's also possible to add configurations for SIM cards of other operators and countries. The page for this is a bit hidden but one found, it's pretty much straight forward.

While most people will probably use it for Internet connection sharing, Dean Bubley recently had a number of other interesting ideas of how to use what he calls 'the dongle dock'. Have a look here, especially the last paragraph. Now I need to get a femto to test his idea 🙂

HSPA State Change Measurements

Last week I did some measurements to get an idea of the time required when switching between different HSPA air interface states. While data is transferred, the mobile is usually in Cell-DCH state on a High Speed Shared Channel. When only little or no data is transferred, the connection transferred to the Forward Access Channel, which is slow but has little overhead for both the network and the mobile device in terms of control measurements and power adjustments commands. If no data is transmitted for a longer duration (e.g. 30 seconds) the connection is put into Idle state. While the IP address is retained, the physical connection between the mobile and the network is severed.

As can be seen in the picture on the left, the round trip time to the first hop in the network of a ping packet is around 100 to 120 milliseconds while the mobile is using a high speed shared channel. While on the slower forward access channel, round trip time increases to 240 to 260 milliseconds. Moving from the high speed shared channel to the forward access channel is relatively quick, it takes around 550 to 600 ms (minus the actual round trip time of the packet itself). Going back to the high speed shared channel takes a little bit more time, around 1000 to 1500 milliseconds.

When using a 3G dongle with a notebook, a connection is rarely set into idle state as there is always one program or another such as an instant messenger, VoIP client, etc., that feels it needs to send a keep alive message to a server in the network before the idle time can expire. Therefore I haven't measured it this time. In the past, I've seen values around 2500 to 2800 milliseconds.

Some say that the effect of this state switching is that web browsing feels a bit more sluggish over HSPA than over a DSL line, which always offers Internet connectivity at full speed without the need of state switching. I use 3G connectivity a lot and quite frankly, while I can feel a difference, it's absolutely no problem to work and live with it.

And here's a quick overview of the test setup: Mobilkom Austria 3.5G HSPA network, a notebook connected via Wi-Fi to a D100 Wi-Fi/3G gateway, connected to a Huawei E220 3G USB stick, HSDPA category 6, no HSUPA.

Telstra to Upgrate to HSPA+

A tip from a reader brought me to this article on Telstra in Australia saying that they intend to upgrade their 3.5G network in Australia first to 21 MBit/s in 2009 and later on to 42 MBit/s. The step to 21 MBit/s seems logical. According to the 3GPP standards, that's an upgrade to 64QAM modulation. If they have the latest base stations from Ericsson, they might be able to do this without a hardware upgrade.

Concerning the 42 MBit/s, that sounds like the 28 MBit/s one gets with MIMO plus 64QAM modulation on top. When I last had a look at the standards document referenced above, there was not yet a terminal class for this maximum speed.

A note of caution: Such speeds can only be reached under very special circumstances, i.e. no other subscribers in the cell and the base station antenna very close by.

HSDPA Alongside A CS Voice Call

Back a year ago I noticed that an incoming circuit-switched voice call during a 3.5G HSDPA packet-switched data session forced the packet connection to go back to 64 kbit/s dedicated bearer while the call was ongoing. After the call the bearer was upgraded to 384 kbit/s but was only put back on the High Speed Shared Channels once the download was finished. Looks like the software on the network side has advanced a bit in the meantime as I recently noticed that even during a phone call an ongoing download continued at HSDPA speeds. Very nice!

Note: The test a year earlier was performed in the German Vodafone network while my latest observation is from the Orange France network. The RAN vendors might not necessarily be the same and it's even likely that they are not.

Mobile Broadband Use in Sweden – Interesting Statistics

I have never been to Sweden before, but judging from Ram Krishnan's report on his blog and the pages on Sweden on the prepaid wireless internet wiki, it seems the situation there concerning mobile broadband use is similar as in Austria: Affordable prices and prepaid SIMs that make it easy to go online with a 3G USB stick have made the mobile broadband market surge in the past 18 months. Ram quotes from the 2007 Swedish Telecommunications Market Report of the National Post and Telecom Agency (PTS), available in English here, on the uptake of mobile Internet access in Sweden. Here's my interpretation of the facts and figures:

Only 1% use mobile broadband as their only Internet access

If you have some time, take a look at diagram 8 in the report and Chapter 5 in general, there are some very interesting facts: The diagram shows that at the end of 2007 there were about 375.000 mobile broadband users, up from only 90.000 a year earlier. That compares to about 3 million DSL users and a population of around 10 million. The report says on page 39 that of 2.000 people questioned, only 40 said that they used the 3G dongle at home to complement another access method and only 20 (i.e. = 1%) said that the 3G dongle was their only Internet connectivity at home. So these numbers clearly point out that 3G Internet access is currently used mainly as a supplement to fixed line Internet connectivity. The report also says that since this is a relatively new phenomenon and that it remains to be seen if 3G will remain mainly an add-on to fixed line connectivity at home or if it will seriously start to compete with DSL.

500MB on average per user per month

Diagram 8 says that 375.000 users generated a traffic of 2.200 Terabyte (in the whole of 2007 ?). If you do the maths that amounts to (2.200.000 GByte / 375.000 users / 12 months = 493 MBytes / month.

How Close Are We To Saturation?

So how much is this in practice, how close are we to network saturation? I guess that's quite easy to say for someone who has access to the data of mobile operators. But I don't have that, so let's do a little extrapolating to get an idea of where we might be here with a bunch of assumptions: Let's say the number of people covered by a single 3G base station is roughly the same as a 2G base station, 2000 people. 375.000 broadband users compared to a population of 10 million is 3.75% of the population. Let's double that value to 7% to account for unequal 3G network distribution. 7% of 2000 people are 140 people per cell with a 3G card. Let's say the base stations usually have 3 sectors, and each sector gives an average throughput of 2 MBit/s. That's 6 MBit/s in total. Let's say busy hour accounts for 10% of the daily traffic of 500 MB * 140 people / 30 days / 10 = 233 MB/hour/base station. The capacity of the base station is 6 MBit/s * 60 seconds * 60 minutes / 8 bytes = 2700 MB/hour (minus the capacity used for voice calls).

The above calculation brings us at less than 10% of total available capacity of a base station today. But the input parameters used are highly speculative so the number could easily be half of that our it could be double. If anyone has a different opinion, please let me know.

How Much Traffic Growth in 2008 And Beyond?

This of course opens up the big question of how the growth will continue. When taking the 2006 numbers from the report, an average user consumed 200.000 GByte / 90.000 users / 12 months = 185 MByte / month, i.e. less than half of that of 2007. So this year's traffic and that of the following years will depend on:

How many additional users can be signed up
Do these users have the same usage patterns as the users today, or more, or less? Potentially, falling prices could attract users with less usage and only very occasional use but also other groups with little money but high Youtube desire.

I think the numbers are hard to predict since the user behavior and clientele will change. While I think that at this point Generation Youtube is not yet on a 3G stick, I wonder what will happen once they do?

The report references a forecast for 2008,
which reports another 140.000 3G USB sticks have been sold in the first
quarter 2008 and which expects 600.000 users by the end of 2008, an
increase of 40% over 2007. This is impressive but definitely a slowdown over the growth observed between 2006/07.

If we stick to the numbers above, we should still be way clear of the capacity limit for 2008. Same for the year after if another 300.000 subscribers are added. If the amount of traffic per user grows by a factor of two in that time frame and the network stays the same, data traffic would grow to 933 MB/base station/h in 2010, still clear of the capacity limit.

Once the limit of current deployments are reached, operators have a number of options:

Deploy a second carrier frequency
Densify the network, i.e. install more macro, micro or pico base stations to reduce the coverage per site and thus the number of users per base station
Work on DSL/mobile broadband conversion and offer interesting packages to users to offload traffic from the mobile network. This of course only helps if people start using wireless broadband as an alternative to DSL. This doesn't seem to be the case so far.

Anyway, one thing is for sure: In countries where broadband wireless access is priced attractively, base stations are no longer just stitting around idling and producing only heat.

T-Mobile USA and the HTC G1 Google Phone – An Interesting Couple

O.k. the HTC G1, or the first Google Android phone, is about to launch and everybody is looking at the Google side of things. But have a look on the other side of the equation: That phone has to use a network. And this network is going to be T-Mobile USA. The interesting thing about this is that this is one of the two networks on this planet that is using 1700 MHz UMTS. For the moment, they only have three very low end 3G phones (according to Wikipedia, see here, here and here) which must sell very well against insignificant competition such as the iPhone.

The HTC G1 will be even more than a quantum leap for T-Mobile USA, it will be the first phone which will use their 3G network in a meaningful way. The HTC page doesn't yet list a lot of network specifications on the device yet. I wonder if it will be dual band 3G, 1700 MHz for T-Mobile USA and 2100 MHz for the rest of the world or if the version announced for the UK will be a different hardware. But then, how about positively surprising me and delivering a Quad band UMTS device with 850, 1700, 1900 and 2100 MHz UMTS built in? Now that would be something, but I'd be really surprised.

I just had a look around which other phones do/will support 1700 MHz. Interestingly the Sony Ericsson X1 came up as 1700 Mhz + quad-band 3G capable. I wonder if T-Mobile USA will pick it up sooner or later!? Also interesting is the Wikipedia link on UMTS quad-band and UMTS tri-band. They give a pretty interesting overview which 3G phones work on more than one continent on speeds faster then EDGE. Nice to see that the list is growing. But what would really be nice for true world roamers are 5 bands. How would that be called? Quinband?

The FACH Power Consumption Problem

In UMTS and HSPA, there are a number of different activity states on the air interface while data is exchanged with the network. During phases of high activity, the mobile device is usually put into dedicated state (Cell_DCH) and transmits/receives data on the high speed downlink shared channels and a dedicated uplink channel. During times of lower activity or to keep a physical connection open to resume data transfers quickly (e.g. the user clicks on a link after some time of inactivity) the network puts the connection into Cell_FACH (Forward Access Channel) state. While the FACH is quite slow, it reduces power consumption somewhat. However, not enough for all kinds of applications.

eMail Polling in 3G mode

While in Austria recently, I noticed that when using 3's UMTS network and Profilmail with a POP3 eMail polling interval of 5 minutes, my battery ran dry within 6 hours. Quite devastating and very short compared to GSM/GPRS/EDGE where the battery easily lasts a full day under the same conditions. With the help of Nokia's Energy Profiler I dwelled down to the bottom of the problem. It turns out that 3 leaves the air interface in DCH state for 20-25 seconds after the last data packet has been sent before putting it into the Cell_FACH state for 1 minute and 45 seconds. Afterwards, the air interface connection is put into Idle state. In Cell_DCH state, even if no data is transmitted, power consumption is around 1.5 watts. In Cell_FACH state, power consumption is still around 0.8 watts, while in idle state and backlight off, power consumption is "almost zero". Even if no eMail is sent/received, these values result in the radio being active for almost half the time of each 5 minute interval, resulting in an average power consumption "in the pocket" (i.e. backlight always off) of 0.5 watts on average. As the battery capacity is 4.4 Wh (that is watt hours), the result is that the battery is empty in just a couple of hours.

If noticed this behavior in 3G networks before but never in such an extreme. This is because most other 3 G networks I usually use have different activity timers. In most other networks, the Cell_DCH state is left after about 15 seconds and Cell_FACH after about 30-45 seconds. This of course decreases the browsing comfort because it often takes longer than 30-60 seconds to read a web page in which case the transition to from idle to Cell_DCH state takes longer than from Cell_FACH to Cell_DCH. On the other side, however, it increases the autonomy on a single battery charge.

eMail Polling in 2G mode

Polling eMails every 5 minutes while the mobile is locked to GPRS is much more efficient. Here, the mobile takes about 1.5 watts while communication is ongoing. However, power consumption goes down almost immediately after no data is sent or received. As a result the average power consumption is only 0.1 watts or only a fifth of the power consumption while in 3G mode.

Remedies

Reducing the 3G timers to lower values is no option since it would have a negative impact on the users experience. Maybe the enhanced FACH, which is not yet implemented in devices and networks, will help somewhat in the future. When looking at the specifications, however, it looks like it mainly addresses capacity and not so much mobile device power consumption. So that remains to be seen.

Another possibility is to switch from the POP3 pull approach to a push approach where the server starts communicating with the device only when a new eMail has been received or very infrequently to keep the TCP session open. Not sure how Blackberries receive their email, but it would be interesting to experiment a bit. IMAP push would be another option but unfortunately, Profimail does not support that extension.

Summary

An interesting case in which the 2G air interface is superior to 3G. How LTE and WiMAX fare in the same scenario is also in interesting question. LTE, for example, has a different air interface state model compared to 3G. Here, only active and idle state exist and active mode timers can be set by the network dynamically in a way to reduce the mobile's average radio activity time to almost the same values as when being in idle state. That should reduce power consumption somewhat if the base stations are clever enough to adapt the timers based on the traffic pattern observed. We shall see…

3G FACH capacity

With the rising number of push eMail devices in 3G networks and mobile applications such as instant messengers and voice over IP clients the number of small IP packets to keep the connections of such applications alive through network address translation routers is rising. For the network this means of lot of radio layer signaling and waste of bandwidth. For the mobile device, keep alive messaging means significantly increased battery consumption.

3G UMTS networks are thus putting devices that only send little data on the Forward Access Channel (FACH) which requires much less radio channel signaling overhead than if the device is instructed to remain or use the High Speed Downlink/Uplink channels for such kind of traffic. As more always-on devices are used in the networks, this will quickly become an issue since the total capacity of the FACH of a cell is limited to 32 kbit/s today. With the bandwidth so small I think most operators will be very thankful for the enhanced-FACH extension which reserves some capacity on the high speed downlink channels for FACH operation. Despite using the high speed channels, no additional radio layer signaling will be used so overhead and battery consumption remains limited at the expense of spectral efficiency. While networks and mobile devices do not support this feature today I expect that this is definitely a feature that will be implemented in the future.

For more on radio interface optimization for future devices and services have a look at my previous entries on continuous packet connectivity (here, here and here), some more background on enhanced FACH (here) and some thoughts on upcoming capacity issues due to keep alive messaging (here).

It seems there is now also an initiative in Release 8 of the 3GPP standards to improve the uplink behavior of the system while a device in in Cell_FACH state. More about that once I have taken a look at the details.