Category: RAN

The FACH Power Consumption Problem

In UMTS and HSPA, there are a number of different activity states on the air interface while data is exchanged with the network. During phases of high activity, the mobile device is usually put into dedicated state (Cell_DCH) and transmits/receives data on the high speed downlink shared channels and a dedicated uplink channel. During times of lower activity or to keep a physical connection open to resume data transfers quickly (e.g. the user clicks on a link after some time of inactivity) the network puts the connection into Cell_FACH (Forward Access Channel) state. While the FACH is quite slow, it reduces power consumption somewhat. However, not enough for all kinds of applications.

eMail Polling in 3G mode

While in Austria recently, I noticed that when using 3's UMTS network and Profilmail with a POP3 eMail polling interval of 5 minutes, my battery ran dry within 6 hours. Quite devastating and very short compared to GSM/GPRS/EDGE where the battery easily lasts a full day under the same conditions. With the help of Nokia's Energy Profiler I dwelled down to the bottom of the problem. It turns out that 3 leaves the air interface in DCH state for 20-25 seconds after the last data packet has been sent before putting it into the Cell_FACH state for 1 minute and 45 seconds. Afterwards, the air interface connection is put into Idle state. In Cell_DCH state, even if no data is transmitted, power consumption is around 1.5 watts. In Cell_FACH state, power consumption is still around 0.8 watts, while in idle state and backlight off, power consumption is "almost zero". Even if no eMail is sent/received, these values result in the radio being active for almost half the time of each 5 minute interval, resulting in an average power consumption "in the pocket" (i.e. backlight always off) of 0.5 watts on average. As the battery capacity is 4.4 Wh (that is watt hours), the result is that the battery is empty in just a couple of hours.

If noticed this behavior in 3G networks before but never in such an extreme. This is because most other 3 G networks I usually use have different activity timers. In most other networks, the Cell_DCH state is left after about 15 seconds and Cell_FACH after about 30-45 seconds. This of course decreases the browsing comfort because it often takes longer than 30-60 seconds to read a web page in which case the transition to from idle to Cell_DCH state takes longer than from Cell_FACH to Cell_DCH. On the other side, however, it increases the autonomy on a single battery charge.

eMail Polling in 2G mode

Polling eMails every 5 minutes while the mobile is locked to GPRS is much more efficient. Here, the mobile takes about 1.5 watts while communication is ongoing. However, power consumption goes down almost immediately after no data is sent or received. As a result the average power consumption is only 0.1 watts or only a fifth of the power consumption while in 3G mode.

Remedies

Reducing the 3G timers to lower values is no option since it would have a negative impact on the users experience. Maybe the enhanced FACH, which is not yet implemented in devices and networks, will help somewhat in the future. When looking at the specifications, however, it looks like it mainly addresses capacity and not so much mobile device power consumption. So that remains to be seen. 

Another possibility is to switch from the POP3 pull approach to a push approach where the server starts communicating with the device only when a new eMail has been received or very infrequently to keep the TCP session open. Not sure how Blackberries receive their email, but it would be interesting to experiment a bit. IMAP push would be another option but unfortunately, Profimail does not support that extension.

Summary

An interesting case in which the 2G air interface is superior to 3G. How LTE and WiMAX fare in the same scenario is also in interesting question. LTE, for example, has a different air interface state model compared to 3G. Here, only active and idle state exist and active mode timers can be set by the network dynamically in a way to reduce the mobile's average radio activity time to almost the same values as when being in idle state. That should reduce power consumption somewhat if the base stations are clever enough to adapt the timers based on the traffic pattern observed. We shall see…

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.

MUROS – Packing 4 Voice Calls Into a Single GSM Timeslot

Back in 1992, the GSM world was simple. A carrier was divided into 8 timeslots and in those that were not used for broadcast information signaling, one voice call was carried. After some time, half rate channels were invented to put two calls in a single timeslot. At first, there was the HR codec which was inferior in speech quality but would use only use alternating instances of an assigned timeslot. But then, advances in coding technology gave rise to the Adaptive Multi Rate Codecs (AMR), and those codecs which fit into half a timeslot were, from a sound quality point of view, quite up to the Enhanced Full Rate voice quality. A nice move as voice capacity of the network effectively doubled.

And now, companies in 3GPP are attempting to again double the number of voice calls per timeslot to up to four! The work item is called MUROS (Multiple Users Reusing One Slot) and you can find some interesting papers about the concept HERE. The folder unfortunately also contains contributions to other topics discussed during this ad-hoc meeting so it’s a bit time consuming to find the papers on MUROS. For a start have a look at the following ones:

080007 – Details on Downlink DARP and Interference
080019 – Short Into to MUROS in Uplink and Downlink
080041 – A first draft of the 3GPP Technical Report which will result from this work

These papers give a first idea on how four voice calls could potentially be put into a single timeslot. Instead of further splitting up the timeslot in time, as was done for AMR half rate, two signals are to be emitted by the base station simultaneously. From a mobile station point of view one signal contains the user data and the other signal is perceived as noise. The mobile phone then has the task to filter out the noise (i.e. the data stream which is unwanted). In uplink direction, it is also foreseen that two mobiles transmit at the same time and that the base station uses interference cancellation and multi-user detection schemes to separate the two signals, potentially based on the use of different training sequences in the middle of the slot.

While at the moment different means are discussed of how the two signals can be told from each other on the receiving end it is clear that MUROS will not work with current mobile devices and also not with current mobile chipsets. Features like SAIC (Single Antenna Interference Cancellation) and DARP (Downlink Advanced Receiver Performance), both also discussed in 3GPP, will probably become the cornerstones for realizing MUROS.

While making this work on the radio layer is surely a formidable challenge, it will also prove to be tricky for radio resource management. Mobile can only tell the two signals apart under radio conditions that are favorable to them. The task of radio resource management will be to dynamically instruct the mobile to handover to ‘less used’ timeslots when signal conditions deteriorate. In the worst case, the mobile could even have to be handed over to a timeslot which it uses exclusively until radio conditions improve.

Should this feature be accepted and implemented it will keep GSM radio layer-, radio network, and chipset designers busy for quite some time to come.

Evolved EDGE Whitepaper by 3G Americas

Over the past 2 years I've written a number of blog entries on Evolved EDGE (here, here, here and here). Now that the feature set is mostly specified, vendors are moving into the implementation phase. A recent whitepaper of '3G Americas' is giving an interesting overview of the different features (without going to much into the implementation details), their potential, and different likely implementation phases. I quite like the paper as it takes a look at EEDGE not only from a technical but also from a deployment point of view.

From a technical point of view the paper mentions one thing in particular which I have not thought about before, and that is that there are two improvements brought by the dual carrier feature. The first improvement is that it improves throughput because timeslots can be assigned simultaneously on two carriers. That's quite obvious.

The second improvement is that even if some timeslots are busy on one carrier for voice calls, the same timeslots on the second carrier might be free and can thus be assigned. This doesn't result in the full theoretical speed but at least compensates for the fact that it is difficult in many situations to have 5 timeslots in sequence available on one carrier. In effect this statistically increases the number of timeslots available for a dual carrier mobile compared to a current single carrier mobile and thus increases the overall speed experienced by a subscriber by using timeslots that could otherwise not be used.

The Line Is Busy But You Don’t Notice

I recently surfed the web with my Nokia N95 over my home Wifi network using the OperaMini browser while simultaneously downloading a movie from my online video recorder at the full DSL line speed (7 MBit/s). What quite surprised me was that despite the line being 100% busy because of the file download, the browsing experience on the phone was perceptively not slower than if the DSL line was not used at all.

There are a number of conclusions that can be drawn from that:

  • It shows the huge difference in bandwidth requirements of different applications and due to the little bandwidth requirements of one application the user does not even notice that a network with a significantly higher bandwidth is already fully loaded.
  • From a technical point of view mobile web surfing with compressed page downloads costs almost nothing compared to the transmission costs incurred by other applications such as huge file downloads and full web browser use.

The situation obviously changes when even small screen devices download full web pages including high resolution images. Mobile web browsers such as Opera Mobile, the Nokia N- and Eseries web browser and the iPhone already do that today. However, for most web pages the experience is not as good not only due to the high amount of data that has to be transfered but because the processor is not being able to render the page as quickly as on a PC. Over time, this might well change but I guess there is still a lot of time left where the OperaMini compression approach will deliver suprerior results in most cases.

3G FACH capacity

With the rising number of push eMail devices in 3G networks and mobile applications such as instant messengers and voice over IP clients the number of small IP packets to keep the connections of such applications alive through network address translation routers is rising. For the network this means of lot of radio layer signaling and waste of bandwidth. For the mobile device, keep alive messaging means significantly increased battery consumption.

3G UMTS networks are thus putting devices that only send little data on the Forward Access Channel (FACH) which requires much less radio channel signaling overhead than if the device is instructed to remain or use the High Speed Downlink/Uplink channels for such kind of traffic. As more always-on devices are used in the networks, this will quickly become an issue since the total capacity of the FACH of a cell is limited to 32 kbit/s today. With the bandwidth so small I think most operators will be very thankful for the enhanced-FACH extension which reserves some capacity on the high speed downlink channels for FACH operation. Despite using the high speed channels, no additional radio layer signaling will be used so overhead and battery consumption remains limited at the expense of spectral efficiency. While networks and mobile devices do not support this feature today I expect that this is definitely a feature that will be implemented in the future.

For more on radio interface optimization for future devices and services have a look at my previous entries on continuous packet connectivity (here, here and here), some more background on enhanced FACH (here) and some thoughts on upcoming capacity issues due to keep alive messaging (here).

It seems there is now also an initiative in Release 8 of the 3GPP standards to improve the uplink behavior of the system while a device in in Cell_FACH state. More about that once I have taken a look at the details.

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.

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.