Category: LTE and SAE

4G and Peak-Rate Marketing

Moray Rumney, Lead Technologist at Agilent Technology is quite outspoken about the negative impact of what he calls "Peak-Rate marketing in telecommunications", i.e. the gap between proclaimed (theoretical) data rates of wireless systems and realistic data rates and capacity achieved in practice. I fully agree with his arguments and will also discuss this topic in my next book. In the latest Agilent Measurement Journal, Moray looks at the topic again, this time from the point of view of how Femto cells could positively influence the data rate and capacity equation in the future. His argument is that the effect of adding a femto layer (Wifi or 3G/4G femto cells) in an overall network architecture increases throughput and overall capacity by orders of magnitude while increasing theoretic peak data rates of macro cells does relatively little in comparison. An article not to be missed, it starts on page 52! Also interesting from a wireless point of view is the article starting on page 25 about resolving design issues in HSPA mobile devices. Earlier issues of the journal can be found here.

LTE and the Voice Gap: CS Fallback

LTE surely is an exciting new radio technology and over time will become a worthy successor to GSM and UMTS. It will have a difficult start though, as it is lacking intrinsic support for the wireless killer application, voice calls. A number of different efforts are underway in 3GPP to fix this. I've reported before on IMS Centralized Services, which is elegant but unfortunately quite complex and CS Voice Services over LTE, which from my point of view would fix things with an acceptable amount of effort and complexity but is currently met with little love in the standards body.

A third initiative that, unlike CS Voice over LTE, has manged to become an active work item in 3GPP is to let the mobile device fall back to GSM or UMTS for incoming and outgoing voice calls. In 3GPP the technical specification that contains an overview of the feature is TS 23.272 'Circuit Switched (CS) fallback in Evolved Packet System (EPS)'.

From an overall architectural point of view I think this work item is rather a confession that there is still a long way to go until we have a real successor technology in place for voice. However, if it can do the job, who cares…

The technical specification of the feature is refreshingly clear how it is supposed to work. For those of you who don't want to go through the spec, here's a short overview of how things will work:

The Preparation Phase

  • When the GSM/UMTS/LTE capable device first connects to the EPS (the Evolved Packet System, i.e. to LTE), it indicates to the network that it wants to perform a "Combined Update". In practice this means that it requests from the network to also register its presence in the 2G/3G circuit switched network.
  • Registration of the mobile in the 2G/3G network is performed on behalf of the mobile device by the MME (Mobility Management Entity) network element which is part of the Access Gateway functionality. It connects back to a legacy Mobile Switching Center (MSC) via the SGs interface, which is an extension of the already known Gs interface between the SGSN and the MSC. In effect, the MME acts as an SGSN and the MSC thinks the device is attached to the 2G/3G network rather than the LTE network and performs a location update via the SGSN. This has been done for backwards compatibility so there are only few if any changes required on the MSC.
  • For the registration in the network, the MME has to give the MSC the 2G/3G Location Area ID (LAI) in which the mobile device is currently 'theoretically' located. Since the mobile can't tell the MME this value, it has to be computed out of the TAI, which is the corresponding identifier in LTE. In practice this creates a dependency between the TAI and the LAI, i.e. the location areas that describe a group of base stations in 2G/3G and LTE must be configured in a geographically similar way in order for the fallback to work later on.

The Execution Phase: Mobile Terminated Call

  • When a circuit switched call comes in for the subscriber it arrives at  MSC. The MSC will then signal the incoming call via the Gs/SGs interface to the MME which is, in it's eyes, a 2G or 3G SGSN that can forward the notification to the mobile device on its behalf. From the MSC point of view this is a legacy procedure that already exists today.
  • If the mobile is in active state, the MME forwards the request immediately to the mobile device. If the mobile wants to receive the call it signals to the MME that it would like to be handed over to the 2G or 3G network in which it can receive the call. The MME then informs the base station that the mobile has to be handed over to the 2G/3G network.
  • Since there might still be an IP data transfer ongoing at the time of the handover, the standard gives the two options: Either the data transfer is suspended or the packet switched connection is handed over to the 2G/3G network. Here, it starts to get a bit complicated as some 2G networks might not be able to handle voice and data connections simultaneously. As a matter of fact, most GSM networks don't have this ability, called Dual Transfer Mode (DTM) today.
  • If the mobile is in idle state when the voice call comes in, the MME pages the mobile to reestablish radio contact. Once contact has been re-established, it forwards the information about the call. Since there is no data transfer ongoing at this time, no handover of the IP connection is required since the mobile can re-establish the packet switched connection itself once it is in the 2G/3G network.
  • The eNodeB has the possibility to request 2G/3G measurements from the device to have a better idea to which cell to hand over the mobile or it can do so blindly by sending it information about a preconfigured cell.
  • Once the mobile device is in the 2G or 3G cell (and the packet switched handover is done if it was performed) it answers to the initial paging via the legacy cell. In case the MME has made a mistake and the legacy cell is in a different location area than where the device was registered in the preparation phase, the specification also contains a mechanism to first perform a location update and then reroute the waiting voice call to the new location area or even to an entirely different MSC.

The Execution Phase: Mobile Originated Call

  • This procedure is very similar to the mobile terminated call example above. The difference is that there is no paging coming from the network for an incoming call and of course no paging response to the MSC after the device is in the legacy cell.

SMS and CISS

  • For receiving text messages, the mobile device can remain the LTE nework, the SMS is forwarded by the MSC to the MME via the Gs/SGs interface and from there via RRC signaling over the LTE radio network to the mobile device. Sending text messages works in a similar way, there is no need to fall back to a legacy network.
  • For call independent supplementary services (CISS) such as changing call forwarding configuration, checking prepaid balance via USSD messaging, etc., a fallback to the legacy network is required.

Summary

When looking at the description above it becomes clear that falling back to GSM or UMTS for voice calls is not quite as straight forward a process as one might think at first. On the legacy side of the network, however, little or no work is required since the LTE network, and specifically the MME, acts like a 3G SGSN and therefore all procedures already existing for 3G to 3G/2G handovers can be reused.

One question that remains and is even asked in the technical specification is how much time is added by this fallback mechanism to the call establishment time and if this additional time will have a negative impact on the user's perception of the service.

And from my personal point of view I wished 3GPP would have rather invested the time it took to come up with this feature into the CS voice over LTE proposal. But better this feature than nothing at all the pragmatist in me says.

Beyond 3G: The Manuscript is Ready

Manuscript-sm
Some of you might have noticed that recent blog entries haven grown in size again and speculated that I have a bit more time at hand again. Well you have guessed right. Over the past months, I spent most of my free time working on my next book, to be published by John Wiley and Sons by the end of the year. Finally, the manuscript is ready and the title will be

"Beyond 3G: Bringing Networks, Terminals and the Web Together"

For the moment, people at Wiley's are now working on the cover, they are proof-reading the manuscript, and typesetting is starting soon. After that, I'll have to work a bit on the index, on the glossary, etc., etc. But that's a bit later in the year. I always find it amazing how many steps are necessary from finalizing the manuscript to having the finished book finally being shipped to the bookstores.

It's a long process. However, I strongly feel that all of this work is necessary and justified to produce something outstanding that has something which is missing in day to day online publishing: Depth and broadness.

That doesn't mean that online sources are less valuable, they are just different. I very much like my blog for example, because it catches spontaneous thoughts, thoughts about a clearly defined single topics, ideas, it's great for posting the latest news, for responding to someone else publishing something, and it is thus a great complement to my offline writing activities. Together, online and offline are hard to beat!

So what exactly will the book be about? Here's the current version of the back cover text:

Giving a sound technical introduction to 3GPP LTE and SAE, this book explains the decisions taken during standardization while also examining the likely competition for LTE such as HSPA+ and WiMAX. As well as looking at next generation network technologies, Beyond 3G – Bringing Networks, Terminals and the Web Together describes the latest mobile device developments, voice and multimedia services and the mobile web 2.0. It considers not only how the systems, devices and software work but also the reasons behind why they are designed in this particular way. How these elements strongly influence each other is discussed as well as how network capabilities, available bandwidth, mobile device capabilities and new application concepts will shape the way we communicate in the future.

  • Examines current and next-generation network technologies such as UMTS, HSPA+, WiMAX, LTE and Wifi
  • Analyses and explains performance and capacity in practice as well as future capacity requirements and how they can be fulfilled.
  • Introduces the reader to the current cellular telephony architecture and to voice over IP architectures such as SIP, IMS and TISPAN
  • Looks at mobile device hardware and mobile operating system evolution
  • Encompasses all major global wireless standards for application development and the latest state of the mobile web 2.0

As I said above, it's going to take until the end of the year until the book is finally shipped. If you would like to be informed when it's available, please send an eMail to "gsmumts at gmx.de", I'll be happy to keep you informed.

LTE Air Interface Primer

It's good to see whitepapers coming out from different companies taking a closer look at LTE. Lots of interesting material can be found at Agilent's LTE Network Testing web page. I especially like the LTE (Air Interface) introduction webcast and the LTE System Overview whitepaper.

Although I am already aware of many things discussed in the paper, its always good to read about things from an author that looks at the topic from a different perspective. Here are a few of the things I found outstanding (for me personally) in the whitepaper:

Good Air Interface Graphics: The paper has some awesome graphics on how the physical channels map onto the different sub-carriers (or more precisely resource blocks). It's interesting to note that the control channels can take up to 30% of the cell capacity in case lots of devices require to be scheduled (e.g. Figure 23). That's quite a lot of capacity wasted for signaling. I guess we have to wait and see if in practice that much signaling capacity is really required.

Fractional Frequency Re-use: To improve cell edge performance the paper mentions the use of only a fraction of the sub-carriers to be broadcast at higher power levels. I've written about this quite some time ago and it's good to finally see some papers who are going into this direction as well.

BCCH: Broadcast once every 10ms (one frame) around the center frequency.

HARQ ACK/NACK: Contains intereresting details on where, when and how the acknowledgments are sent in uplink and downlink direction.

CS Voice (And Other Services) over LTE

I've been speculating recently how voice calls could work in next generation 3GPP LTE networks. The politically and technically foreseen way is IMS, the IP Multimedia Subsystem, and a service platform running on top of IMS such as the IMS Centralized Services (ICS). ICS is quite promising as it includes a solution to bring GSM only handsets without any IMS extensions into an overall voice solution and can hand-over voice calls from LTE to GSM when leaving the coverage area. The major drawback of ICS is its complexity and its anyone's guess when we will see this in mobile devices for the general public.

In the meantime, there has been some support in 3GPP to investigate a different solution: How to extend the current circuit switched voice service of GSM to LTE. In 3GPP a number of companies started writing some proposals, which were gathered in 3GPP TR 23.879. In this paper, the main proposal is to connect the Media Gateway part of the Circuit Switched Mobile Switching Center (MSC) to the packet core and give the the MSC Server direct access to the Mobility Management (MME) Entity of the Access Gateway to the LTE Radio network.

This approach completely circumvents the IMS and reuses all upper signaling protocols already known from GSM. Only the lower protocol layers are replaced by TCP/IP. For the voice call itself, all higher layers of the voice transmission protocol are foreseen in the technical report to be kept, while the lower layers would be replaced by TCP/UDP/RTP between the mobile device and the MSC's media gateway.

Handovers would be supported via the interface between the LTE Access Gateway and the evolved MSC (eMSC). When the base station signals that a handover to a 2G GSM cell is required, the Access Gateway informs the evolved MSC via this interface of the handover and the intended target cell. The eMSC can then prepare a circuit switched channel in the 2G or 3G network and respond to the LTE Access Gateway with the necessary details which are then given to the mobile device by the base station in the handover command.

From a development point of view such an approach is much simpler than installing a full IMS system and put ICS on top. Also, all services available today, including SMS, are instantly available, without any further development.

Here are the main developments that I think would be required for such a solution:

  • Mobile device: The stack for voice telephony must be enhanced to put the signaling and voice data a packet switched connection while the device is attached to LTE. Also, the handover code must be enhance to not only support 2G to 2G, 3G to 2G, 2G to 3G handovers but also 4G to 2G or 3G and vice versa.
  • LTE Base station: The software needs a small enhancement to transparently forward the CS handover information it receives from the Access Gateway to the mobile device.
  • LTE Access Gateway: The MME in the Access Gateway needs to be enhanced to report a handover to the eMSC and to wait with the handover until the eMSC gives the go-ahead. Also, it would have to forward a transparent data container with information about the resources allocated in the circuit switched network to the mobile device.
  • MSC: Would have to be enhanced to communicate via DTAP over IP (instead of ATM, IuCS, BSSMAP and BSSAP) and to perform handovers from 4G to 3G or 2G and vice versa. Further, instead of assigning circuit switched traffic channels it would have to interact with the packet core to assign the correct QoS attributes which will ensure a smooth call and sets the scene for the MME to signal a 2G handover to the eMSC.

For further details, including how to deal with roaming subscribers, have a look at the technical report.

All in all, I would say that the enhancements required for 4G handovers are far less complicated than those required at the time to implement 3G to 2G handovers when UMTS was specified. 

From a technical point of view, this architecture has the drawback that voice calls to mobile clients would continue to use a protocol other than SIP, which is the dominant protocol in the fixed line VoIP world. The MSC and the Media Gateway would in effect act as a signaling protocol converter and, in case the call is handed over to a circuit switched 2G connection, as a voice codec transcoding function. Considering the comparatively small enhancements required in the handset and the newtork compared to a full IMS/ICS solution, this architectural imperfection could well be worth it.

From an IMS/ICS point of view the proposed solution looks of course "stone age". It would only support voice calls, i.e. no multimedia sessions, would not support several devices per account, no sessions, no instant messaging, you name it, just pure and simple voice and SMS (but with all the supplementary services that have been developed over the past 30 years!).

BUT: Due to its handover capabilities to 2G GSM and and seamless use over 2G, 3G and 4G networks, it might be the killer VoIP solution for operators to beat Internet based VoIP services (think Skype, etc) which are also pushing into wireless networks and devices today.

Strangely enough, the current work plan lists the technical report as "moved to Release 9" (look for "FS on CS Domain Services over evolved PS access"). I am not sure what that means but it sounds like it didn't meet the love of enough companies represented in 3GPP.

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.

Establishment of LTE Radio Bearers

Quite a number of whitepapers on the LTE Radio Layer and about the general architecture of LTE can be found these days on the web. A good one for example is the Agilent LTE Air Interface Introduction, about which I have reported previously. What's still missing, however, is a good description of how resources are dynamically assigned in the uplink and downlink direction of the air interface. This process is also referred to as Radio Resource Control (RRC) and has a big impact on how well the available bandwidth can be shared between all active subscribers in the network. The following blog entry aims at shedding some light on this process.

The description below is based on the following documents, which are an ideal source for further background reading:

Idle State

Let's assume the mobile device is already registered and the network has assigned an IP address to it. Further, no data has been exchanged with the network for some time, so the mobile has been put into Idle state. In this state the radio part of the mobile is mostly dormant but periodically performs the following main tasks:

  • Observes the paging channel in case the network needs to notify the device of incoming IP packets.
  • Observes the broadcast channel of the current cell to be informed of configuration changes.
  • Performs radio channel quality measurements on the current cell and neighboring cells. Neighboring cells can be other LTE cells but also UMTS, GSM and CDMA cells. For this purpose, the current cell broadcasts a list of neighboring cells. In case the radio coverage of the current cell becomes inferior to that of another cell, the mobile performs a cell reselection to a different cell.

Then let's assume the user starts the web browser on the mobile device and selects a web page from the bookmarks. This means that the mobile device has to request the establishment of a new radio bearer from the base station. It's important to note at this point that it's only the radio link that needs to be re-established, as an IP address is still assigned to the device. 

The Random Access (TS 36.300, 10.1.5)

The first step in re-establishing radio contact is to initiate a Random Access Procedure on the (uplink) Random Access Channel (RACH). Where in the frequency/time resource grid the RACH is located is known to the mobile via the (downlink) Broadcast Channel (BCH). The random access message itself only consists of 6 bits and the main content is a random 5 bit identity.

If the network receives the message correctly it sends a Random Access Response Message at a time and location on the Physical Downlink Shared Channel (PDSCH) that can be calculated from the time and location the random access message was sent. This message contains the random identity sent by the device, a Cell Radio Network Temporary ID (C-RNTI) which will be used for all further bandwidth assignments, and an initial uplink bandwidth assignment.

The mobile device then uses the bandwidth assignment to send a short (around 80 bits) RRC Connection Request message which includes it's identity which has previously been assigned to it by the core network (or the Non Access Stratum (NAS) in 3GPP talk).This NAS id is required as the base station can only establish a radio bearer with information stored in the access gateway as described below.

Establishment of Radio Bearers

As the mobile device is already known in the core network the following radio bearers are now established automatically:

  • A low priority signaling (message) bearer (SRB1)
  • A high priority signaling (message) bearer (SRB2)
  • A data radio bearer (DRB), i.e. a bearer for IP packets

Part of the bearer establishment procedure are authentication and activation of encryption. The required data for this process is retrieved by the base station (the eNodeB or eNB in 3GPP talk) from the Access Gateway (aGW), or more precisely from the Mobility Management Entity (MME). The MME also delivers all necessary information that is required to configure the data radio bearer, like for example min/max bandwidth, quality of service, etc. 

Setting up a radio connection is a pretty extensive task. For the user, however, the delay caused by these procedures should be as little noticeable as possible. The LTE requirements say that the whole procedure should not take more than 100 milliseconds. It's likely that this can be achieved in practice, as the transmit time interval (TTI) of LTE is 1 millisecond, which means there are enough opportunities to exchange messaging in uplink and downlink within this period to complete the task.

Resource Scheduling

Once the DRB is in place, everything is set up and the mobile then listens on the Physical Downlink Control Channel (PDCCH) for uplink/downlink bandwidth assignments. The PDCCH is broadcast every millisecond (i.e. 1000 times a second) in the first 1-3 symbols out of 14 symbols which are transmitted every millisecond. Figure 24 in the Agilent document visualizes this very nicely.

In downlink direction, resources are scheduled for the device on the PDCCH whenever data arrives from the network. How many Physical Downlink Shared Channel (PDSCH) ressources are scheduled and when mainly depends on the quality of service settings for the user and the current radio conditions.

In uplink direction, the mobile is only allowed to send data on the Physical Uplink Shared Channel (PUSCH) when the network schedules uplink transmission opportunities on the PDCCH. Uplink and downlink bandwidth assignments on the PDCCH are encapsulated in so called Control Channel Elements (CCE's, in case you come across this acronym in the specification documents), which are fixed length containers to simplify the search for the mobile on the PDCCH for its own bandwidth assignments)

Uplink resources are scheduled based on the amount of data in the uplink buffer of the mobile. The mobile device can signal the amount of data waiting to be transmitted in a the MAC header part of uplink packets. QoS and other parameters are of course also used by the base station in the uplink scheduling decision.

HARQ

As transmissions over the air interface are prone to transmission errors due to interference, each packet sent in uplink and downlink direction has to be acknowledged by the other end. This is done by sending Hybrid Automatic Repeat Request (HARQ) acknowledgments or non-acknowledgments on control channels.

In downlink direction, HARQ acks/nacks are for uplink transmissions are sent on the Physical HARQ Indicator Channel (PHICH), which is part of the PDCCH, i.e. they are transmitted in the first 1-3 symbols of each TTI. In uplink direction, HARQ ACK/NACKs are sent on the Physical Uplink Control Channel (PUCCH), which is implicitly scheduled shortly after a downlink transmission. Figure 27 in the Agilent document shows the PUCCH in orange.

CQI and Neighboring Cell Measurements

To be able to optimize downlink transmissions by adapting the modulation and coding scheme (MCS), the mobile device has to send channel quality indications (CQI) on the PUCCH and the PUSCH). In addition, the mobile also collects measurements on neighboring cells and reports them to the base station whenever a threshold is crossed (e.g. a neighboring cell is received better than the current cell).

DRX

With all of this work going on besides sending and receiving data, a discontinuous reception (DRX) mechanism has been designed so the mobile can switch off its transmitter periodically to conserve battery power. The activity and switch-off time can be set by the base station based on the QoS of the connection and current activity of the device. It's interesting to note at this point that the DRX cycle can range from a few milliseconds up to several seconds. More precisely the DRX cycle can be as long as the paging cycle while the mobile does not have a radio bearer. 

Many more details could be added, but I think this description covers the basic procedures for exchanging data of the LTE air interface. Hope you liked it.

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?

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.

3GPP Moves On: LTE-Advanced

LTE is not yet even deployed and the 3GPP  Third Generation Partnership Project) is already  thinking about how to further evolve the technology. A main driver is probably the ITU (International Telecommunication Union), who will in due time release their requirements for so called IMT-Advanced 4G wireless systems.

It is quite certain that in terms of bandwidth, LTE and all other beyond 3G wireless systems such as the current WiMAX 802.16-2005 (802.16e) specification will fall short of the ITU requirements, which will probably be in the range of 100 MBit/s to 1 GBit/s. A very ambitious goal. Earlier this month, 3GPP hosted an IMT-Advanced workshop in Shenzen to which 170 representatives of network vendors and network operators from all over the world attended.

Not much has been reported about it yet in the news or on blogs, so one could think they are working in the shadows. But far from that, the 3GPP website reported on it here and all papers presented during the workshop and a report can be downloaded from here. Tons of information! Compared to other standards bodies that keep their proceedings to themselves, it is great to see 3GPP is so openly distributing their information. They set a good example!

The following bullets list some of the first ideas for LTE-Advanced presented during the meeting to comply with the likely requirements of IMT-Advanced. During the meeting it was decided to officially gather and approve them in 3GPP TR 36.913 over the coming months:

  • LTE advanced shall be backwards compatible to LTE (i.e. like HSPA is backwards compatible to UMTS)
  • Primary focus should be on low mobility users in order to reach ITU-Advanced data rates.
  • Use of channel bandwidths beyond 20 MHz currently standardized for LTE (e.g. 50 MHz, 100 MHz).
  • Increase the number of antennas for MIMO beyond what is currently specified in LTE
  • Combine MIMO with beamforming.
  • Further increase in Voice over IP capacity
  • Further improved cell edge data rates
  • Improved self configuration of the network

Very ambitious goals, given that vendors are still working on the challenges of LTE. But then, what would the world be without ambitious goals?

Thanks to Zahid Ghadialy and his post on his ‘3G and 4G Wireless Blog’ for the pointer!