Category: HSDPA

Efficiency: LTE vs. HSPA

Yesterday, I've been looking at how LTE Radio Bearers are established and how data is transferred over the air interface. It's now time to draw a conclusion with a comparison of how data is transferred over the HSPA air interface to see if there are improvements concerning the time it requires to establish a radio bearer and how efficient the interface is for transporting small amounts of data.

Establishment Time

In current UMTS/HSPA networks, mobile devices are put into RRC_idle state after about 30 seconds of inactivity. If data transfer resumes, e.g. because the user clicks on a link on a web page, it takes about 2-3 seconds to re-establish the full bearer on the High Speed Downlink Shared Channels. This is quite different, to say a fixed line DSL connection, which has no such noticeable delay. The 30 seconds delay to put the mobile into idle state is thus a compromise to give the user a good browsing experience in most situations at the expense of power consumption, as the device requires a significant amount of power to keep the radio connection open, even if no data is transferred (more about that in one of the next blog entries).

In LTE on the other hand, state switches between idle and connected (on the air interface) have been designed to be very short. The requirements list a time of 0.1 seconds. Given the air interface structure as described in the previous article and only a single inquiry by the base station to the access gateway node to get the subscribers subscription profile and authentication information, it is likely that this target can be fulfilled. This means there will be almost no noticeable delay when the mobile moves back from a mostly dormant air interface state to a fully active state.

Another advantage of LTE is that base stations manage the radio interface autonomously. This is a significant difference to current HSPA networks, in which centralized Radio Network Controllers (RNCs) manage the air interface activity state on behalf of the base station. As more mobile devices are connected to the packet switched part of the network, the signaling load keeps increasing for the RNC and might well become a limiting factor in the future.

Transferring Small Amounts of Data

Even while in active state on the air interface, the network can react
to reduced activity and instruct the mobile to only listen at certain
intervals for incoming packets. During all other times, the mobile can
switch off its transceiver and thus conserve power. This enables
resumption of data transfer from the mobile even quicker than going
through a full state change. This method is called Discontinuous
Reception (DRX) and is likely to be an important tool to conserve
battery power and reduce signaling. Again, the base station is free to set and change the DRX period for an ongoing connection at any time and no information has to be exchanged with other nodes in the network. 

In UMTS networks, the closest tool availble to handle connections in which only little data is exchanged is the Forward Access Channel (FACH). The FACH is also a a shared channel, but no power control is required. This saves a lot of energy at the mobile side as there is no dedicated channel for uplink power control required while data is exchanged on the FACH. With a capacity of only 32 kbit/s, however, the channels ability to handle many simultaneous connections is very limited. Furthermore, there are no DRX cycles, so the mobile has to continuously listen on the downlink for incoming data. Despite requiring much less energy than observing the high speed shared control channels and keeping a power control channel open in uplink direction, power requirements are still significant (as I will discuss in another blog post soon).

Why not sooner?

With all these advantages over UMTS, the question arises why UMTS has not been designed in this way in the first place? One of the answers probably is that when UMTS was specified back in the late 1990's, the world was still a circuit switched place for the end user. Most people still used dial-up modem connections and voice over IP up to the end user was not a hot topic. So the design of the radio network was strongly influenced by circuit switched thinking. HSPA was the first step into a fully packetized world with the introduction of the high speed downlink shared channels.

Today, however, even HSPA is still embedded into the concept of persistent radio channels and a hierarchical network structure in which control of the radio network is not fully at the base station but with radio network controllers. Also, there is still a high degree of communication with core network nodes such as the SGSN, e.g. whenever the RNC decides to put radio connection to the mobile into idle state. With HSPA, changes were made to give the base station more autonomy as data transfer over the high speed shared channels is controlled by the base station and no longer by the RNC.

To improve things further, companies organized in 3GPP have made many additional enhancements to the UMTS/HSPA air interface in recent versions of the standard. These are sometimes referred to as "Continued Packet Connectivity" CPC, and I reported about these new features here, here and here. Most of the CPC features are likely to be implemented and introduced in the next few years while LTE matures to help HSPA cope with the rising traffic until LTE can take over.

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?

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.

Idlemode
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.

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:

Enjoy!

T-Mobile Starts Using DSL for 3.5G HSPA Backhaul

Unstrung recently reported that T-Mobile has started to deploy RAD’s Ethernet over DSL solution for backhauling 3.5G traffic from their UMTS / HSPA base stations. I wondered in the past how soon we would see something like this happening since current 2 MBit/s E-1 line rental costs are prohibitively high and several are required for the bandwidth requirements of a 3.5G base station. The article says deployment starts in Germany, where T-Mobile is the incumbent and has surely made a favorable deal with their fixed line branch, T-Com, who is in the process of deploying a VDSL overlay network besides the already existing ADSL/ADSL2+ network. As VDSL only works over short distances, T-COM deploys curbside VDSL cabinets every several hundred meters. With 52 MBit/s in downlink and 11 MBit/s in uplink, a VDSL link offers more than enough bandwidth for a base station with multiple sectors. Backhaul from the cabinet is also not a problem since they are connected to the core network by fiber. The article doesn’t say if the base station continues to use E-1 links for voice traffic or if all data is backhauled via the DSL link.

T-Mobile And The Asus eeePC

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At the CeBIT kick-off press conference today, T-Mobile Germany and Asus announced an interesting cooperation: T-Mobile will start selling the eeePC in Germany and Austria with access to their Wifi and 3G networks. The 3G offer will include an HSPA USB stick. I’ve just recently bought an eeePC myself and time will tell how often it will be preferred over taking a full notebook with me. But I think chances are fairly high since it nicely fits into a bag and weighs almost nothing compared to the notebook.

For those who prefer using their mobile phone as a 3G "modem" for the eeePC (like me) instead of being locked to a single operator, here’s a link that explains how to do this as well. I tried with an N95, a Nokia 6680 and a Motorola V3xx and they all worked fine.

Your Nokia S60 phone as a WLAN Gateway to the Internet

Already back in 2006 I have speculated about Wifi/3G devices becoming gateways to the Internet for Wifi only devices (notebooks, etc.) by acting as Wifi access points. Looks like we are very close to such a  solution now with JoikuSpot. JoikuSpot is a piece of software for Nokia S60 phones which is able to relay HTTP and HTTPS web page requests from notebooks and other Wifi devices to the Internet via a 3G connection.

According to the description, the software sets the phone’s Wifi interface into unencrypted ad-hoc mode (not in Access Point mode) which means that everybody in range can use the gateway. This is a bit of an issue from a security and usage control point of view. Another current downside is the limit to HTTP and HTTPS which limits the use of the gateway to web surfing. eMail and other Internet applications. An interesting step, let’s hope they continue to work on the feature set in the future.

Via Teltarif

Wireless Now Accounts For A Third Of Austria’s Broadband Connections

Bad news for all of those who keep telling people that wireless broadband can’t compete with DSL and cable because networks couldn’t cope with the traffic: You are wrong! Arthur D. Little consulting published a study last week that last year 57% of new broadband connections were wireless (3.5G + WiMAX a bit I guess) compared to 36% of new connections via DSL and 7% cable.

In total 46% of broadband connections are now via DSL, 26% via cable and 28% wireless. As I am in in Austria from time to time and have a local SIM card for mobile broadband I think the reasons for this outright success are the following:

  • Very competitive pricing for wireless broadband
  • Prepaid offers. I for example have a prepaid SIM for 3.5G with 3GB worth of data which I can consume over 12 months. 1GB afterwards can be had 20 euros with a validity period of another 12 months. The same 20 euros buy 2 GBs with a validity period of 1 month. Even cheaper offers are available via postpaid.
  • So far pricing for DSL was very uncompetitive in Austria
  • All four wireless operators are advertising their broadband solutions heavily

I think these number show quite impressively that well designed 3.5G networks can cope with the load of broadband Internet access from a significant percentage of the population. I can confirm this myself as my wireless HSPA connection has always worked nice so far whenever I was in Austria. Therefore fears by some mobile network operators that their networks might be overloaded are unfunded, unless of course they have an under dimensioned network.

Also thoughts can be put at rest that wireless broadband is not profitable. With wireless voice minute prices down to 5 euro cents a minute in Austria and mobile broadband used heavily I haven’t heard anybody complain about losses.

Via heise.de news

UMTS pushes into the “Deep-Ruralness” of Austria

It’s been a year since I was last in what I would call a ‘deep rural’ spot in Austria in a little town in the region of Oberösterreich with around 2500 people living there and nothing but countryside and cows around. In the past months there have been press reports by a number of Austrian UMTS operators that they will now also start covering the Austrian countryside with UMTS. I didn’t give it much thought at the time as words in press statements usually have to be taken with a grain of salt.

Looks like, however, words have been followed by action. This time I was surprised when I saw UMTS networks of A1 and ‘3’ on my network selection dialog. Most excellent since A1/Mobilkom is the roaming partner of Vodafone Germany so I can use their Websessions here! A quick test revealed data rates of 1.2 MBit/s in downlink. Far below the capabilities of HSDPA but likely due to a limited backhaul. Never mind, it will do nicely.

First Signs of HSPA+ in the Standards

25306800table_51a
It looks like 3GPP member companies are moving quickly to standardize data rate enhancements for current 3.5G UMTS / HSPA networks as promised in diverse marketing slides floating around at conferences. The two main ideas to further increase transmission speeds are to use a higher modulation scheme (64QAM) and MIMO (Multiple Input Multiple Output) to send several data streams via different spacial paths simultaneously.

3GPP TS 25.306-800 has an interesting table (table 5.1a) that specifies current and future HSPA terminal categories for downlink. Today, mobile devices typically use on of the following terminal categories:

  • Category 12: Speeds up to 1.8 MBit/s (first HSDPA cards for notebooks)
  • Category 6: Speeds up to 3.6 MBit/s (most HSDPA cards sold for notebooks in 2007 and pretty much all mobile phones supporting HSDPA)
  • Category 7/8: Speeds up to 7.2 MBit/s (the latest HSDPA cards for notebooks)

I’ve tried all three types of HSDPA cards in practice and could reach around 1.4 MBit/s with a Cat-12 device, around 2.5 MBit/s with a Cat-6 device and around 4.2 MBit/s with a Cat-7/8 device.

For HSPA+, Release 8 of the 3GPP standards now introduce terminal categories 13 to 18. In Category 18, a terminal can receive up to 27952 Bits per 2ms TTI per MIMO channel (I assume). That would result in a speed of around 28 MBit/s. Interesting to see that in 2×2 MIMO mode, only 16QAM and not 64QAM is used. Category 14 terminals can receive 42.192 bits per 2ms TTI in non-MIMO mode. That’s still an impressive 21 MBit/s, even without MIMO.

In order not to get too excited about these numbers have a close look at the physical realities of a radio channel in the real world. Even though specified on paper such speeds are only achievable under the very best of radio conditions with little to no interference from neighboring cells. Having said that I nevertheless believe that we will see nice speed enhancements in practice. I am looking forward to see just how much!