Author: Martin

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.

Rise and Demise: Wind Italy vs. 3 UK

In my recent travels I have noticed that some wireless network operators have greatly improved their network for Internet access while others have consistently declined. To examples from both ends of the scale:

I haven't used the network of Wind in Italy for notebook access to the Internet a lot in the past anymore since they had no HSPA in their network. Even in the standard 3G mode, connections where slow due high packet loss. Recently, however, they have upgraded their network to HSPA, at least in Rome, and I have since gone back using their prepaid SIM for notebook Internet access. I consistently get around 1 MBit/s in downlink direction (most likely traffic shapped) and 384 kbit/s in uplink direction when sending eMails with file attachments and pictures. Well done Wind!

On the other end of the scale is 3UK. I've bought one of their SIM cards a couple of months ago and during that time, their performance in the UK was ok. These days however, both in the UK and abroad, I consistently get bad throughput and sometimes the connection doesn't work at all. During a week in London last week I got so frustrated with their service that I stopped using it at some point and replaced it with another prepaid SIM. Randomly blocked TCP and UDP ports to keep me from getting eMail and setting up my VPN connection in addition to slow throughput is not acceptable. This week in Rome it's also been pretty much unusable, data rates are just too slow. Well, I guess I will try again in half a year. Not earlier probably, because I don't see a reason to waste 10 pounds to activate the data option just to find out their service is anything but a service…

For alternatives and other countries take a look at the prepaid wireless Internet access Wiki.

Will There Be Enough Capacity for Evolved-EDGE?

In the past I've reported on activities in 3GPP on Evolved EDGE (here and here). Looks like standards are well on their way now and a number of network vendors and terminal manufacturers are working on the implementation.

According to analyst resources, especially China and India could be countries in which operators are interested in the new features, especially in the dual carrier functionality that promises to further increase data rates per device. In these countries, no or only little 3G has been deployed to date so it might well happen that operators in these countries will go directly to LTE, WiMAX and beyond and try to fill the gap in the meantime by evolving their GSM networks.

What I am wondering though is if networks in these countries really have enough capacity to assign timeslots on several carriers to a single mobile device? I would assume their network density and capacity is pretty well adapted to current traffic levels which doesn't leave much room for this. When looking at it from a different perspective the question is if they would be prepared to increase capacity (and thus CAPEX and OPEX) in their networks for the dual carrier feature?

If you have an opinion on this, please consider leaving a comment below.

50% Of The Traffic In 10% Of The Cells

In a recent press conference, Vodafone UK gave some numbers concerning the use of their 3G network. According to them, 50% of the data traffic is handled by only 10% of the cells. As I don't know what was said around that statement I wonder if they see this as good or bad!?

I can see several conclusions to draw from this statement:

I guess mobile operators would prefer a uniform traffic distribution in their network, both in space and time. But it doesn't even happen for voice calls as traffic is much higher in cities compared to the countryside and highly varies throughout the day. That's why operators use high capacity cells and increase base station density in cities.

So if most of the traffic is occuring in only few cells it could be good for Vodafone because they only need to upgrade those cells for higher data capacity on the air interface and the backhaul while while leaving the rest of their network as it is. And if that is not enough in high use areas additional base stations could be put in place, but again only in certain areas.That is simpler and maybe also cheapter than to densify the network throughout the country because traffic distribution is uniform.

On the negative side it could of course also mean that they can't keep up in the future adding capacity fast enough. For the moment, I hear that Vodafone's network is still doing fine, even in densly populated areas.

So if you are reading this and have some more background information or want to share your own thoughts around this statement, please consider leaving a comment.

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?

Does RAN Sharing Make Sense As Usage Grows?

In the UK Vodafone and Orange on the one hand and T-Mobile and 3 on the other are trying to find ways to decrease their CAPEX and OPEX of running their 2G and 3G radio access networks (RANs). Early announcements said that they intended to share not only the base station sites, antennas and feeders but also the base stations and the radio network controllers. Looks like in the meantime, Orange and Vodafone are likely to only share base station sites. This is not a particularly new approach, as in many countries sites and masts are often used by two or even more network operators.

With the quickly growing use of 3G networks for Internet access I even wonder if base station sharing makes sense at all!? After all, sharing a base station instead of using two individual ones decreases overall capacity. Sure, a single base station could be upgraded to have an individual transmitter for each operator, but not much beyond. Also, the backhaul link would have to be upgraded to support this. With usage increasing significantly these days I wonder if that wouldn't create a bottleneck rather sooner than later as operators can probably not add any additional hardware to such base stations to increase capacity later on.

So even if there should be some cost savings from this scheme, I wonder if the inflexibility this creates negates the effect. Cost for software and hardware upgrades to achieve higher speeds for example would have to be shared between the two companies. So if one is happy with the current performance of the network but the other is not, that would pose a serious issue for the party who's network is close to overload.

Maybe a better way to save costs is to open the radio networks for national roaming in areas which do not require a lot of bandwidth such as on the countryside for example. Operators could then agree who builds the RAN in which area and invites others to share the infrastructure. Not sure if national telecom regulators would be happy about such deals as it surely has an impact on competition, just as the original plans for RAN sharing.

Mobile Web Megatrends Conference in Berkeley

Mobile Web Megatrends
Do you know Ajit Jaokar and Michael Mace? If not I suggest to head over and check out their blogs (here and here) for great insight into the mobile domain. Or, even better, use the opportunity to meet them in person on the 8th of September at the University of Berkeley, California at the Mobile Web Megatrends Conference on September 8th, 2008. Topics of the conference range from mobile browser evolution, browser offline capabilites, advertising models, the iPhone (of course…), mass market impact with Nokia's S40 6th edition, cloud computing, etc. etc.

Lots of other great speakers, I let the conference's web site speak for itself.

Definitely a conference not to miss if you can make it. Unfortunately my calender is already booked for that date so I won't be able to make it. But good for you since I have one free ticket to give away! First come first served.