WirelessMoves

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.

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?

Mobile Devices Are Getting Ahead of the Networks

I still remember that in the early days of GPRS, the main problem was to get mobile devices that could actually make use of the new network service. The story repeated itself with UMTS where where things became even worse. When UMTS first started, there were lots of networks around but no or only clunky mobile phones available for at least a year or so.

In the meantime it looks like the situation has reversed. Quite a number of 7.2 MBit/s HSPA devices are available, but only few networks yet support ten simultaneous downlink spreading codes and have the required backhaul capacity to the base station. With HSUPA it is quite similar. A number of devices, mainly USB sticks, are available on the market today, but most networks still lack support. And it's not only in UMTS, where devices are far more capable then most networks today.

Even 2G mobiles now support features that most networks are lacking. The AMR (Adaptive Multi Rate) speech codec is a good example. Widely supported in handsets today, but only used in few networks today, despite the potential capacity increases the feature offers to operators. Or take DTM (Dual Transfer Mode), which enables simultaneous voice calls and Internet connectivity for GSM/GPRS/EDGE devices. Again, many mobiles support this today and it could be put into good use especially with feature phones. However, I haven't seen a single network that supports it in practice.

A worrying trend. Are the standards bodies specifying too much?

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:

• Good HSPA+ overview slide package by Dr. Anil M. Rao of AlcaLu
• Heavy technical HSPA+ background paper by Rohde & Schwarz
• My CPC intro I wrote some time ago (link to part 2 and part 3)

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.

Light Reading Webinar on Mobile Backhaul Evolution

With mobile networks getting faster and faster a growing pain for network operators is the backhaul connection between the base station sites and the next element in the network. Today, T-1 or E-1 connections are used with a line rate of 1.5 and 2 MBit/s. With HSDPA being put in place today,  backhaul capacity requirements of 3G base stations now reach 10 MBit/s or more. This means putting additional T-1 or E-1 lines in place. While this might still work today for HSDPA speeds despite the associated rising costs it certainly won't work tomorrow for WiMAX, LTE and other Beyond 3G technologies that require backhaul capacities of 60 MBit/s per base station and more.

So the big question is what comes after T-1/E-1 connections over copper, fiber or microwave!? The common answer these days seems to be:

IP over Ethernet with the capability to carry legacy GSM (TDM) and UMTS/HSDPA (ATM) links in IP pseudo-wires alongside native IP traffic generated by native WiMAX and LTE base stations.

But how do you connect the base station sites to Carrier Ethernet Networks? Can the last mile be done over copper, is fiber required or is next generation microwave an alternative? Questions over Questions :-)

I found some answers in a recent one hour Light Reading Webinar on the topic which is available for free at this link. If you are interested in the topic take a look.

Femtocell Thoughts - Part 3

In part one of this miniseries on femtocells I've been looking at the benefits for mobile operators and part two covered the question why users would put a femtocell into their home. This final part looks at the technical background and hurdles and gives a conclusion.

In practice it is extremely important to integrate femtocells with DSL or cable modems for several reasons. First, femtocells are installed by the user and such an approach therefore ensures that the installation is easy and is done properly.

Additionally, an integrated device is the only way to ensure quality of service for the femtocell since data traffic generated by 3G voice calls must be prioritized on the fixed line link over any other traffic. If a femtocell was attached to an already existing DSL or cable router which already serves other users, uplink data traffic of these users could severely impact 3G voice calls since ordinary DSL or cable routers do not have quality of service (QoS) features to ensure that traffic from the femtocell is prioritized. This behavior can already be observed in practice today in other situations. If an ordinary DSL or cable router is used for a VoIP call in addition to a simultaneous file upload, voice quality is usually very bad due to the packet delay and insufficient bandwidth availability caused by the file transfer.

Thus, a mobile operator deploying femtocells ideally owns DSL or cable access as well or is at least partnering with a company owning such assets. This way a single fixed line gateway could be deployed with Wifi for PCs and other devices and a femto radio module for 3G mobile devices. The single phone per user idea also benefits from such an approach since owning or partnering for DSL or cable access removes the competition between fixed and wireless voice. This also ensures that a femtocell is only used in locations where the mobile operator has licenses to operate femtocells since they use licensed 3G frequency bands.

In practice it can be observed today that a number of mobile operators are taking this route already by either buying DSL access provider companies or at least partnering with them (e.g. Vodafone/Arcor or O2/Telefonica in Germany). It’s unlikely that this is done specifically to roll out 3G femtocells at a later stage but it seems that such companies have understood that it is vital for the future of a telecommunication company to have both wireless and fixed assets in order to stand a chance to be more than a mere bit-pipe for services running over the network. On a side note it is interesting to see the trend of splitting up fixed and mobile access into separate companies several years ago seems to revert now and pains of separation are now followed by pains of re-unification.

Another technical aspect concerning femtocells is interference. In 3G networks, cells usually all transmit on the same frequency and interference is managed by having enough space between them and by adjusting output power and antenna angles. Most 3G operators have at least two frequencies they can use so femtos could for example use the mostly unused second frequency. However, there is still an issue with interference between femtocells of users which live in the same apartment building and have thus installed their equipment close to each other. Left on its own this will result in lower capacity of each cell and might impact quality of service.

Conclusion

When looking at the arguments presented above, femtocells are not likely to be an immediate and outright success. A number of hardware evolutions will probably be needed before form factor, usability and quality of service are adequate. This is likely to take a couple of years. Also, mobile operators need to continue their path of buying or partnering with companies owning fixed line DSL or cable access. This will surely also not happen overnight. However, there is currently still enough capacity available in the macro layer of the network so femtocells are not immediately needed to reduce the load on the network. Therefore, the major immediate benefit of femtocells is improving in-house coverage especially in rural regions, which thus remains a niche market for now, since 2G and 3G coverage and capacity for urban users is usually sufficient for in-house coverage. As such the story of femtocells might parallel the evolution of UMA (Universal Mobile Access) which has similar goals but a completely different concept. That’s a story for another day however…

As always, comments are welcome.

Femtocell Thoughts - Part 2

In part one of this miniseries on femtocells I've been looking at the benefits for mobile operators. This part deals with why users would put a femtocell into their home.

From the user's point of view the advantages of femtocells are less clear to me. While the user shares all of the operator advantages discussed in a previous blog entry, increasing customer retention and thus churn is not necessarily in the interest of users since it could reduce competition. Also, it is unlikely that all family members use the same mobile operator and thus could benefit from a single femto cell.

In addition, mobile multimedia users are usually still early adopters which tend to use sophisticated phones, of which many include Wifi. With such phones a femto cell for multimedia content is not required since Wifi offers a similar or better experience for Internet content. Multimedia services offered by mobile network operators, however, are usually not available over Wifi which, from the end user perspective, is not a huge loss since early adopters tend to use Internet services rather than multimedia services of operators that are usually more expensive or come with limitations not acceptable to such users.

An advantage not mentioned before is that better 3G in-house penetration would increase call establishment success rate for 3G video calls since mobiles reselecting to the 2G network because reception quality is better can not be used for incoming or outgoing video calls. Thus, femtocells could become an important element in the future to make video calls more popular as the service still fights with the famous hen/egg problem of 3G network availability and number of users with compatible handsets.

Monetary incentives could persuade users to install femto cells. Operators could for example offer cheaper prices for voice calls that are handled via the femto cell. Also, the operator could propose to share revenue with femto ‘owners’ if other subscribers use the cell for voice and data communication instead of a macro cell.

Often the argument is brought forward that femtocells allow to market single phone solutions in which the user no longer has a fixed line phone and uses his mobile phone both at home and on the go. However, such solutions which use the macro layer instead of femtocells have already been available for several years in countries such as Germany (O2’s famous home zone for example) and are already very popular. Also, it is unlikely that mobile network operators would have competitive prices for all types of calls so many users would still use a SIP phone or software client on a PC for such calls at home. Calling a mobile number is still more expensive in most parts of the world excluding the U.S.A. than calling fixed line phones so single phone offers have to include a fixed line number for the mobile phone in order. Again, this is already done in practice for example by O2 in Germany for a number of years but femtocells might enable the mobile network operator to deliver such services cheaper than how it is currently done over the macro layer.

It should alsobe mentioned that using a femtocell would have a configuration and usability advantage over SIP Wifi phones. However, it is likely that the configuration process for SIP and Wifi on handsets will improve over the next few years thus decreasing this advantage.

To be continued

So much for now on the user's point of view on femtocells. In the third part, to come soon, I will take a look at the technical background and hurdles.

As always, comments are welcome!

Femtocell Thoughts - Part 1

There is currently a lot of hype around Femtocells, tiny user installable 3G cells for homes and offices. Surely an interesting technical concept but still with many question marks attached such as why would users want a 3G cell at home or at the office and what the benefits are for the operator. Here’s what I think:

Operator Benefits

3G networks are operated on the 2100 MHz frequency band in Europe and Asia and in the 1900 MHz band in the U.S. which is far from ideal for in-house coverage. Even in cities it can be observed that dual mode 2G/3G mobiles frequently attach to the 2G network because many GSM operators use the 900 MHz band in Europe which is much better suited for in-house coverage as lower frequencies penetrate walls much better. Some proponents of Femtocells claim that in-house coverage for voice calls are greatly improved by Femtocells. In cities however, this benefit is rather small since GSM in-house coverage is usually not an issue. The user on the other hand does not really care if his voice call is handled by the 2G or 3G network.

An improvement could be seen however in cases where the mobile can’t decide to stick with either the 2G or the 3G network due to changing 3G signal levels. This creates small availability outages while the mobile selects the other network type. During these times, incoming voice calls are either rejected or forwarded to voicemail.

Also, it can often be observed in practice that a mobile device with weak 3G in-house coverage changes to the 2G network once a connection to the Internet is established (e.g. to retrieve eMails or to browse the web on the mobile phone) and sometimes changes back to the 3G network during the connection. The reason for these ping pong network selections are the changing reception levels due to the mobility of the user and changing environmental conditions. Such network changes result in outage times which the user notices since an eMail takes longer to be delivered or because it takes a long time for a web page to be loaded.

Another solution to the issues described above is the use of the 900 MHz frequency band for 3G in Europe and Asia and the 850 MHz band in the U.S. It is likely that this will happen over the next few years since regulators more and more tend to open the 900 MHz band for 3G networks in Europe. It will take a number of years however before network operators will have deployed their 3G networks in those lower frequency ranges and until devices for these bands are available. It’s also likely that 900 MHz cells would first be used to cover rural areas instead of enhancing coverage in areas already covered by 3G in the 2100 MHz band. In the meantime, Femtocells definitely have the advantage.

As the above weaknesses of 3G in higher frequency bands show, femtocells can increase customer satisfaction. Putting a femtocell in the user’s home would have the additional advantage for network operators of reducing churn, i.e. customers changing contracts and changing the network operator in the process. Customer retention is all the more reinforced if the Femto comes in a bundle with DSL access as further described below since changing wireless contracts also has consequences for the fixed line Internet access at home.

Another advantage of femtocells is to reduce the gradual load increase on the 3G macro network as more people start using 3G terminals for voice and data connections. This could result in a cost benefit since if the right balance of macro and femtocells are reached, fewer expensive macro cells would be necessary to handle overall network traffic.

The question is how much these advantages are worth to a network operator since Femtocells do not come for free!?

More to come

So much for now. Part 2 to come soon deals with why users would put a 3G femotcells into their home and part 3 will look at the technical background and hurdles for femtocells.

As always, comments are welcome!