Dual Carrier HSDPA Potential for European Operators

Back in February I wrote about the new Dual Carrier HSDPA feature
in 3GPP Release 8 to push beyond the current 5 MHz single carrier limit
of HSDPA. In short, the feature allows bundling of two adjacent 5 MHz
carriers if supported by the mobile device for higher throughput or a
better scheduling gain due to the higher capacity of the resulting
channel. But do network operators today have adjacent 5 MHz channels to use the feature? I've had a look at the frequency band assignments for Germany, Austria and Switzerland and in all cases the network operators have at least 10 MHz of continuous spectrum in the 2.1 GHz band. Some even have 15 MHz, i.e. 3 adjacent 3G carriers.

In some countries, not all of the initial licensees have made it to the network operation phase so some bandwidth is lying dormant these days. In Germany, for example, Mobilkom and Hutch3G never made it out of the box. Their 10 MHz bands are currently unused and might at some point be offered for sale to the current four operators or new entrants. In the meantime, 3GPP is pushing forward in Release 9 to go beyond the dual carrier specification to enable the bundling of even more carriers. An interesting detail in this debate is that in Release 9 the bundled not all of the carriers will probably have to be adjacent anymore. This is quite important for T-Mobile in Germany, for example. They are a bit unfortunate when it comes to the third carrier as their frequency assignment is not adjacent to one of the assignments of the two parties that gave up.

And looking even further into the future, it might very well be that 3GPP will specify a multi-band/multi-carrier HSDPA operation in Release 9 as indicated here by 3G Americas. That would go far beyond the cooperative use of different frequency bands I discussed in a previous post. Quite a challenge for mobile device hardware designers as it's already a challenge to design a small device with half a dozen or more antennas. In the future, a future dimension is added to the equation by having to make sure that several antennas can be used for transmitting and receiving data simultaneously without interfering with each other.

And by the way: According to the 3G Americas paper above, similar things are happening with LTE-Advanced as well, the maximum 20 MHz channel bandwidth has become too narrow already in the hunt for ever higher speeds.

CS Voice Services over HSPA

3GPP is quite active in moving as many things as possible over to the HSPA high speed (packet based) shared channel on the UMTS air interface to save resources, increase capacity and to reduce mobile power consumption. An example I have reported on in the past is the Enhanced Cell-FACH feature. Looks like the traditional circuit switched voice service is now also set to migrate to HSPA with the CS (circuit switched) Voice Services over HSPA feature introduced in 3GPP Release 8.

This whitepaper from Qualcomm gives a good introduction. According to the paper, "all that is required" to put the circuit switched voice service on the packet switched high speed shared channel is a software upgrade in the radio network and the mobile. Like today, the MSC forwards the voice call to the Radio Network Controller (RNC) without any changes. On the RNC, the Adaptive Multi Rate (AMR) speech packets are then not put into a logical circuit switched bearer for a dedicated air interface channel but are instead put into PDCP packets which are then sent to the base station. The base station then schedules those AMR in PDCP packets on the high speed shared channel in the same way as IP in PDCP packets coming from the SGSN but gives them a higher priority.

On the mobile side a software enhancement is required to indicate to the network that CS voice over HSPA is supported. Further, the lower layer voice protocol stack needs to be enhanced to receive the AMR packets on the high speed shared channel instead of on a dedicated channel to the mobile.

As no IP layer is involved for transmitting the AMR speech packet in the radio network I would call this a Voice over Packet service. So be careful, this enhancement can't be counted as a wireless VoIP variant and is not related with the CS Voice over LTE proposal I reported on here, which is fully based on IP.

Will this feature make it into real networks? Time will tell. What do you think?

Escaping Future Bandwidth Bottlenecks: LTE and HSPA on Several Bands

I think everyone in the industry is pretty clear by now that the amount of data that cellular wireless networks will have to carry in the future is going to rise. In my recent book I’ve taken a closer look at theoretical and practical capacity on the cellular level in chapter 3 and I come to the conclusion that from a spectrum point of view, there is quite a lot of free space left in most parts of the world that will last for quite some time to come.

So while alternative approaches like integrating Wi-Fi and femtos into an overall solution will ultimately bring much more capacity, I think it is quite likely that network operators will over time deploy their cellular networks in ever more bands. In Europe, for example, I think it’s quite likely that operators at some point will have networks deployed on the 900, 1800, 2100 and 2600 MHz band simultaneously.

Quite an interesting challenge to solve for networks and especially for mobile devices as they have to support an ever growing number of frequency bands.  Also, those bands should not also be used in tight cooperation instead of just aside each other. Ideally, the resources in the 900 MHz band could be reserved for in-house coverage as radio waves in this band penetrate walls quite well. But as soon as the network or the device detect that other bands can be received quite well, they should automatically switch over to them to leave more capacity for devices used indoors or under difficult radio conditions.

Switching between different frequencies and radio technologies during a call or a session is already done today but mostly based on deteriorating reception levels. So in the future, when using so many bands, I think this reactive mechanism has to be enhanced into a proactive mechanism and switch-overs need to be timed so that the user does not notice an interruption.

Dual Carrier HSDPA – The Push Beyond 5 MHz

Over at LinkedIn, Eiko Seidel recently published a link to a whitepaper by Nomor research on Dual Carrier HSDPA (or Dual Cell HSDPA operation as it is called in the standards), a new feature currently worked on in 3GPP. I've been waiting for this feature to come out for quite some time now as HSPA+ has already added 64QAM modulation and MIMO to HSPA. Consequently, not much can be done anymore to improve performance in a 5 MHz channel.

In practice, dual carrier (DC) HSDPA means that two adjacent 5 MHz carriers can be bundled by the network and DC capable HSPA mobiles can be assigned resources simultaneously on both carriers. In addition to the higher throughput, the 10 MHz bandwidth can also be used to schedule mobiles more efficiently around fading conditions, which according to the paper, results in an efficiency gain of up to 7% with 32 users and up to 25% with 2 users.

By increasing transmission speeds the round trip delay time is also further reduced, good news for online gamers. I have to note, however, that current round trip delay times of around 100ms are hardly distinguishable anymore from the delay of a DSL line. What's still distinguishable are the longer delay times caused by state changes after some time of inactivity. That's addressed by another feature, though, the enhanced Cell-FACH.

The enhancements also brings a number of new terminal categories. In addition to HSDPA terminal categories 1-20 which exist today (most people these days have a category 6 (3.6 MBit/s) or a category 7/8 (7.2 MBit/s) device), category 21-24 terminals will be able to use two adjacent carriers. The conserve energy on the mobile device side, the network can dynamically instruct such terminals to only listen to a single carrier if the amount of data to be transferred is low and doesn't warrant the use of two simultaneous carriers which requires more energy for decoding.

For the moment, multicarrier HSDPA is only for the downlink direction and while 64QAM is included, MIMO is not. Theoretical peak throughput in the combined 10 MHz carrier is around 42 MBit/s. But I guess this is not the end of the story yet, I think it is quite likely that in 3GPP Release 9, uplink dual carrier and MIMO is added to the feature list. The authors go a step further and speculate that in the future the standard could include further enhancements to go beyond two simultaneous carriers and to even include simultaneous transmission on carriers not adjacent to each other, even in different bands (e.g. 900 + 2100 MHz simultaneously).

While it looks good on paper, it remains to be seen which operators will go for it in practice. Some operators are determined to squeeze out as much as possible of their 3G networks before going to LTE. By the time these features are market ready, I'd say two to three years down the line, it's quite likely that many 3G base stations will already be used with two carriers per sector. If the feature can be done in software only, I guess it could become quite popular. In that time frame, however, many of today's 3G base stations will be end of live and might be replaced with triple mode GSM, UMTS and LTE base stations. If the feature is required when LTE is also in the cabinet, well, that remains to be seen.

But one way or the other, this new round of enhancements show that there is still a lot of life left in HSPA.

And here's some background as to where the technical details can be found in the specifications: First, 3GPP TR 25.825 contains an overview of the feature. Nomor's whitepaper lists a number of Change Requests (CR) to add the functionality to the relevant specification documents (TS 25.211, 25.212 and 25.214). I've had a look at the latest versions in 3GPP Release 8 and those CRs have been approved and are part of the specs now. So it looks like Dual Carrier HSDPA will be part of Release 8 which will be finalized in this quarter.

Let's see if there is already talk about this at the MWC in Barcelona in just a couple of days from now.

How Can LTE Reduce the Cost Per Bit?

Recently, a question was asked in the LTE forum on LinkedIn how LTE can reduce the cost per bit compared to todays broadband wireless systems such as HSPA. I found it quite interesting that a lot of people immediately jumped at the greater spectral efficiency as the means to reduce the overall cost. But I think there are also other innovations which will drive down cost:

  • There are no Radio Network Controllers (RNC) anymore, i.e. fewer network components
  • The backhaul network is radically different. While E-1/T-1 connections (cable, microwave) are still heavily used today, LTE will be rolled out with Ethernet over fiber / VDSL and microwave. Huge cost advantage here. It's not spectral efficiency operators worry about today, it's the rising E1/T1 backhaul costs.
  • In all fairness, it has to be said, that current HSPA networks are changing towards this as well in terms of backhaul and network element (e.g. one tunnel architecture) but it is not built in and the RNC is still required.
  • Another reason why LTE has a cost advantage over today's deployed networks is that technology has advanced and allows smaller base stations to be built which require less power, less space. These will be deployed from day 1 and in many cases will be put inside existing base station cabinets or mounted besides.
  •  Also count in remote radio head technology that will probably be used heavily with LTE to drive the cost down.
  • In the mid- to long term, I think LTE access will be the catalyst to have multi radio base stations with a common Ethernet based backhaul thus also driving down the cost of 2G and 3G systems to some extend that will remain in place for the time to come.

Anything else you can think of?

German Computer Magazine measures 5.76 MBit/s in HSDPA Downlink

Edition 25/2008 of the C't, a renowned German computer magazine, contains a number of interesting articles around mobile Internet access. In one of them, 3G USB dongles have been tested and those capable of 7.2 MBit/s in downlink (HSDPA category 7/8) have reached a maximum speed of 5.76 MBit/s. Impressive, that's even higher than what I experienced myself. The test were performed on the Hanover exibition ground, where both T-Mobile and Vodafone have upgraded their 3G network and their base station backhaul to support these speeds. I assume the tests were done while no exhibition was in progress, i.e. no traffic in the cell and also no or only little traffic in other cells in the neighborhood, which means only little inter-cell interference. They also tested HSUPA and achieved uplink data rates of around 1.8 MBit/s. Again, very impressive for a live network setup.


These days I was wondering if in the mid-term, femtocells might replace public Wi-Fi hotspots!?

With the rise of 3G USB keys and notebooks with built in 3G connectivity, the popularity of Wi-Fi hotspots, especially paid ones, is likely to degrade over time. Once people have a 3G card anyway and have instantaneous connectivity anywhere, people just won't bother anymore to search for a public Wi-Fi hotspot and go through the manual login process. In addition, femtos remove another shortcoming of public Wi-Fi, the missing air interface encryption which today leaves the door wide open for all kinds of attacks.

With rising demand for Internet access in hotspot areas such as hotels, airports, train stations, etc., HSPA or LTE femtocells might be the ideal replacement for aging Wi-Fi access points which at some point have to be replaced by new equipment anyway. So mobile operators such as T-Mobile, Orange and others, who have a dual 3G / Wi-Fi strategy today could at some point just make such a move if they see that use of their Wi-Fi systems is decreasing and use of their 3G/4G macro base stations in the neighborhoods of their Wi-Fi installations is significantly increasing.

Some 'dual-mode' operators might even have a database with the geographical location of their base stations and their Wi-Fi installations. Together with traffic statistics of both systems an automated system could document changes over time and could be used to help predict when and if a replacement of the Wi-Fi access points for femto cells might make financial sense. After all, femto cells are just as easily connected to a DSL line than a Wi-Fi installation.

Maybe some femto manufacturers even come up with integrated Wi-Fi/Femto boxes for public installations with the Wi-Fi being used to create a wireless mesh between several nodes in locations with only a single backhaul line and for access for those people not yet having 3G connectivity. Agreed, femto vendors today mainly position themselves around the femto base station for home networks but public femtos might be an interesting opportunity as well.

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

A1-on train
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 🙂