VoLTE: Semi-Persistent Scheduling (SPS) and TTI Bundling

The following is from the recently released 4G Americas paper '4G Mobile Broadband Evolution: 3GPP Release-10 and beyond:

With the support of emergency and location services in Rel-9, interest in Voice over LTE (VoLTE) has increased. This is because the Rel-9 enhancements to support e911 were the last step to enable VoLTE (at least in countries that mandate e911) since the Rel-8 specifications already included the key LTE features required to support good coverage, high capacity/quality VoLTE. There are two main features in Rel-8 that focus on the coverage, capacity and quality of VoLTE: Semi-Persistent Scheduling (SPS) and TTI Bundling.

SPS is a feature that significantly reduces control channel overhead for applications that require persistent radio resource allocations such as VoIP. In LTE, both the DL and UL are fully scheduled since the DL and UL traffic channels are dynamically shared channels. This means that the physical DL control channel (PDCCH) must provide access grant information to indicate which users should decode the physical DL shared channel (PDSCH) in each subframe and which users are allowed to transmit on the physical UL shared channel (PUSCH) in each subframe. Without SPS, every DL or UL physical resource block (PRB) allocation must be granted via an access grant message on the PDCCH. This is sufficient for most bursty best effort types of applications which generally have large packet sizes and thus typically only a few users must be scheduled each subframe. However, for applications that require persistent allocations of small packets (i.e. VoIP), the access grant control channel overhead can be greatly reduced with SPS.

SPS therefore introduces a persistent PRB allocation that a user should expect on the DL or can transmit on the UL. There are many different ways in which SPS can setup persistent allocations, and Figure below shows one way appropriate for VoLTE. Note that speech codecs typically generate a speech packet every 20 ms. In LTE, the HARQ interlace time is 8 ms which means retransmissions of PRBs that have failed to be decoded can occur every 8 ms. Figure below shows an example where a maximum of five total transmissions (initial transmission plus four retransmissions) is assumed for each 20 ms speech packet with two parallel HARQ processes. This figure clearly shows that every 20 ms a new “first transmission” of a new speech packet is sent. This example does require an additional 20 ms of buffering in the receiver to allow for four retransmissions, but this is generally viewed as a good tradeoff to maximize capacity/coverage (compared to only sending a maximum of two retransmissions).

The example in Figure above can be applied to both the DL and UL and note that as long as there are speech packets arriving (i.e. a talk spurt) at the transmitter, the SPS PRBs would be dedicated to the user. Once speech packets stop arriving (i.e. silence period), these PRB resources can be re-assigned to other users. When the user begins talking again, a new SPS set of PRBs would be assigned for the duration of the new talkspurt. Note that dynamic scheduling of best effort data can occur on top of SPS, but the SPS allocations would take precedent over any scheduling conflicts.

TTI bundling is another feature in Rel-8 that optimizes the UL coverage for VoLTE. LTE defined 1 ms subframes as the Transmission Time Interval (TTI) which means scheduling occurs every 1 ms. Small TTIs are good for reducing round trip latency, but do introduce challenges for UL VoIP coverage. This is because on the UL, the maximum coverage is realized when a user sends a single PRB spanning 180 kHz of tones. By using a single 180 kHz wide PRB on the UL, the user transmit power/Hz is maximized. This is critical on the UL since the user transmit power is limited, so maximizing the power/Hz improves coverage. The issue is that since the HARQ interlace time is 8 ms, the subframe utilization is very low (1/8). In other words, 7/8 of the time the user is not transmitting. Therefore, users in poor coverage areas could be transmitting more power when a concept termed TTI bundling (explained in the next paragraph) is deployed.

While it’s true that one fix to the problem is to just initiate several parallel HARQ processes to fill in more of the 7/8 idle time, this approach adds significant IP overhead since each HARQ process requires its own IP header. Therefore, TTI bundling was introduced in Rel-8 which combined four subframes spanning 4 ms. This allowed for a single IP header over a bundled 4 ms TTI that greatly improved the subframe utilization (from 1/8 to 1/2) and thus the coverage (by more than 3 dB).

Martin Sauter puts it in a simpler way in his blog as follows: The purpose of TTI Bundling is to improve cell edge coverage and in-house reception for voice. When the base station detects that the mobile can't increase it's transmission power and reception is getting worse it can instruct the device to activate TTI bundling and send the same packet but with different error detection and correction bits in 2, 3 or even 4 consecutive transmit time intervals. The advantage over sending the packet in a single TTI and then detecting that it wasn't received correctly which in turn would lead to one or more retransmissions is that it saves a lot of signaling overhead. Latency is also reduced as no waiting time is required between the retransmissions. In case the bundle is not received correctly, it is repeated in the same way as an ordinary transmission of a packet. Holma and Toskala anticipate a 4dB cell edge gain for VoIP with this feature which is quite a lot. For details how the feature is implemented have a look at 3GPP TS 36.321.

A whitepaper explaining the concepts of TTI Bundling is available on Slideshare here.

'EU-Alert' in Release-11

In the recently concluded 3GPP CT-50 in Istanbul, EU-Alert was adopted as part of Rel-11. The EU-Alert is introduced under Public Warning System (PWS) in parallel with Earthquake and Tsunami Warning System (ETWS).

PWS was introduced in Rel-9 and I blogged about it here. ETWS has been around since Rel-8 and was blogged here.

In fact EU-Alert is sent as part of the Cell Broadcast Message (CBS) using new identifiers. For more details see 3GPP TS 23.401.

The following is an old video from CHORIST project, which was instrumental in providing details of working of this EU-Alert system.

Also Read: Commercial Mobile Alert System (CMAS) here.

Dilbert Humour: Why your software contains bugs

Source: http://www.dilbert.com

4G Mobile Broadband Evolution: 3GPP Release-10 and Beyond

New Report from 4G Americas:

4G Mobile Broadband Evolution: 3GPP Release 10 and Beyond - HSPA+ SAE/LTE and LTE-Advanced provides detailed discussions of Release 10, including the significant new technology enhancements to LTE/EPC (called LTE-Advanced) that were determined in October 2010 to have successfully met all of the criteria established by the International Telecommunication Union Radiotelecommunication Sector (ITU-R) for the first release of IMT-Advanced. IMT-Advanced, which includes LTE-Advanced, provides a global platform on which to build next generations of interactive mobile services that will provide faster data access, enhanced roaming capabilities, unified messaging and broadband multimedia. The paper also provides detailed information on the introduction of LTE-Advanced and the planning for Release 11 and beyond. Release 10 is expected to be finalized in March 2011, while work on Release 11 will continue through the fourth quarter of 2012.

White paper embedded below and is available to view and download from the 3G4G website.

4G Mobile Broadband Evolution: 3GPP Release-10 and Beyond

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Making small purchases simpler with Ericsson IPX

Yesterday a colleague made me aware of this Ericsson's IPX SMS based payment system that looks like a competitor to the NFC technology and doesn't involve any additional chip/hardware. Here is a video:

From Ericsson's website:

Ericsson Internet Payment Exchange (IPX) is a leading mobile aggregator, providing delivery and billing services, via SMS, MMS, web and online mobile billing, to more than 2 billion mobile subscribers across 26 countries. Ericsson IPX also brokers location information in selected countries and Ericsson IPX Messaging provides reach to 96% of all mobile subscribers worldwide with SMS. Ericsson IPX customers are companies who offer digital content, mobile voting & directory information and enterprises offering mobile marketing, communities and banking.

Now, we all love SMS and we have to admit that its the simplest of technology and even the most primitive phone nowadays support it but there could be scenarios when this can be a bit of a problem:

1. SMS can sometimes be delayed if a particular cell is overloaded, etc. So how long do we have to stand in front of the machine?

2. If say for 2-3 mins we do not receive an indication that the machine has a cash, do we send another SMS to cancel the transaction?

3. If we have a problem, do we have a support number to call to? How much will that cost?

4. If there is a queue of people and someone else wants to purchase something as well, does the next person has to wait till the person before has received the item?

5. If two people have sent an SMS, how do they know whose cash is in the machine now? Do we start putting a Pin as well ?

I agree, this technology could be really useful if you have run out of cash (even if you have NFC chip) and you need to purchase something small.

The other obvious advantage is that you can target advertisement at regular users who are at a particular place at a particular time to make them buy something. Also you can get statistics like what time people tend to purchase, what do they purchase, where, etc.

Anyway, hard for me to see this take off big time.

6th ETSI Security Workshop

6th ETSI Security workshop was held last month. There were some very interesting areas of discussion including Wireless/Mobile Security, Smart Grids Security, etc.

Security in 3GPP

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Embedded SIMs and M2M Communications

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All presentations are available to download from here.

Wireless Friendly Buildings

Long back I wrote about problems with Radiation Proofed homes. Since then the wireless technologies have got more popular and the technologies in infancy have become mainstream.

Last week I heard Professor Richard Langley from Sheffield University speaking on the topic of wireless friendly buildings. The problem now is manifold rather than just keeping the wireless signal in or out.

Think about the WiFi that is installed in nearly every house. The signals from WiFi are best kept indoors to avoid the interference to neighbours. Wifi uses 2.4GHz ISM band. On the other hand we may want the mobile signal to penetrate the house so that we can get good reception. In Europe UMTS is mostly 2.1GHz and LTE may be mostly 2.6GHz. The intention of the building should be to keep the WiFi signal out and the UMTS/LTE signal in.

The problem we have to remember is that with the frequencies going higher, the penetration of signals are becoming a problem. This means that the construction of the buildings should be modified to keep the attenuation to minimum, higher the frequency.

With femtocells most likely to become more popular by the day, you may want to keep these frequencies from going out of the house but allowing them to come in. This presents a big challenge. The intention of the buildings design in the western world is to keep the cold/heat/radiation out. The concept of 'wireless friendly building design' is the least important in the mind of the architects and civil engineers.

The may change over the time due to effort by the organisations like the Wireless Friendly Building Forum (WFBF).

From an article in Building.co.uk:

At the moment, says Chris Yates, chairman of the Wireless Friendly Building Forum (WFBF), predicting the performance of a building to handle wireless signals is almost impossible. “There is a lot of spurious science around and software that purports to give plots of wireless coverage in a building. It’s utterly naive and doesn’t reflect reality,” he says.

One of the reasons the forum was set up at the end of last year was to co-ordinate research into the area of wireless systems in buildings. As the use of wireless devices increases, more and more issues over performance will arise, explains Yates - and there is nobody taking an interdisciplinary approach on how this performance can be improved.

With wireless looking set to be a mainstay of the way we work in the future, the WFBF ultimately wants to develop a way of defining and assessing its performance in any one building, similar to the way in which BREEAM or LEED rate a building’s environmental performance. “Then a value can be placed on it and developers and end users get interested and it becomes part of their decision-making process,” says Yates.

But until this is achieved, what should designers be doing? Here, we take a look at three wireless applications and the main implications for buildings.

Cellular signals are broadcast by public masts and are actually very difficult to keep out of a building. The main path in is through the glazing, but once inside, things can start to go haywire, with signals reflected or absorbed by the building’s structure.

Columns, lift shafts and risers in particular can create blackspots where reception becomes poor or non-existent. Concrete floors cast on lightweight metal decks will block most signals, as will materials such as lead roofing and the metal foils on the back of some insulation materials.

A common way to deal with this is to boost the signal or re-broadcast it using a repeater. These systems usually use an external antenna to collect the signal, which is transmitted to an amplifier and retransmitted locally. For multistorey buildings, several transmitters might be needed.

Of course, this equipment needs to be accommodated and installed, but when this should be done is a source of some confusion. The current BCO guide to specification does not outline at what stage ICT infrastructure should be installed, but according to Yates some sort of infrastructure provision should be made at the core and shell stage, even if the active equipment isn’t installed.

Signal strength can also be significantly decreased by the use of high-performance glazing and solar shading, which are becoming commonplace with the tightening of Part L of the Building Regulations.

Mitigating action can be taken. Buro Happold’s specialist facade division, for example, is now beginning to consider the effect that facade components have on wireless performance, while also considering trade-offs in acoustic, blast, thermal performance and aesthetics.

According to Yates these trade-offs need careful consideration. Some glass options might give marginally better performance in terms of thermal behaviour, but completely ruin the wireless service, whereas another option might give negligible degradation for a similar price. “

So it is something to think about. It’s no good handing the building over and then telling them there’s no wireless signal,” warns Yates.

The following is an interesting presentation on the related topic:

Wireless Friendly Buildings

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Making of the Qualcomm Museum

For Qualcomm, Baker created a corporate museum experience that gives visitors an understanding of what lies at the core of the company’s extraordinary success story.

Attocell: Smaller Femtocell

Over the last year Femtocells et al. have been slowly rebranded as Small cells and I agree that it is a much more generic term and easier for end users to understand.

Last year, around this time, magicJack announced a small USB Femtocell (a lot of Femto manufacturers disagree that its a Femtocell) that can help reduce the cost of call and just a few days back Picochip announced picoXcell, a USB form factor femtocell.

Yesterday, Ubiquisys announced the Attocell. As can be seen in the picture above, in mathematics, femto is 10^-15 and atto is 10^-18. I know that some of you may think that there is still scope for zeptocells and yoctocells but it may be difficult to see in the light of Attocells, what they can be.

A few years back, I blogged about how Femtocells can be used to steal the spectrum. Well, Attocell legally allows to do just that. The press release states the following:

The attocell connects to a user’s laptop via USB, which provides power and an internet connection. It then analyses the IP address and radio environment to determine which country it is in, and sets its 3G radio power accordingly to below the licenced level. In some countries its range will be just 5mm, in other countries, it could cover a whole room.

Like Ubiquisys femtocells, the attocell continuously monitors its radio environment to ensure that there is zero impact on existing mobile networks. This intelligence, combined with its tiny power output, is likely to make the attocell exempt from regulatory controls and the requirement for type approval.

In 5mm mode, the traveller simply lays the iPhone on top of the device and the phone connects automatically, just like a regular femtocell. Calls can be made using a Bluetooth or wired headset , or by using the iPhone’s speaker.

You can understand more by reading the official Attocell FAQ here.

It would be very interesting to see if any of the big operators support this device. Roaming revenues are big part of operators profit. Informa has predicted that this roaming revenue will keep rising. The roaming revenue comes from voice, sms as well as data. Some business users in one business trip frequently shell as much as a personal user would for a years contract. So its not really in operators interest to promote these devices.

On the other hand, small operators and MVNO may really like this as this can help them win more subscribers and can be the differentiators in the market.

Once more thing worth remembering is that since the arrival of Skype, the occasional and cautious mobile users have no hesitation on making VoIP calls and keeping friends, family and colleagues back home up to date with their status. The operators are aware of this and wouldnt mind the users instead using their mobiles via attocell.

Ubiquisys will show the Attocell in MWC in mid-Feb and we will have to wait and see the reaction.

Oops! Texting Fail: Woman Falls in Fountain

You can watch the CBS news video and that woman's interview here.