OFDMA Femtocells: A Roadmap on Interference Avoidance

Earlier, I have blogged about LTE femtocells being starting point of LTE and how LTE can be better technology than HSPA. In this months IEEE Communications magazine, there is a series of articles on Femtocells. I will try and cover some of these (unless I wander off in some other direction). The first one is titled 'OFDMA Femtocells: A Roadmap on Interference Avoidance'. At the end of this post, I have provided links to the research and the actual paper (in a legal way ;) so if you are not interested in the post and want to directly jump on the actual paper see the end of this post.

There are all kinds of statistics about the number of Femtocells worldwide. There could be upto 70million by 2012. If this happens the big problem would be the interference between Macro and Femtocells and also between Femtos. OFDMA (used in LTE and WiMAX both) Femtocells can handle the interference better than CDMA (UMTS and CDMA2000) Femtocells due to its Intracell interference avoiding properties and robustness to multipath.

So what are the main problems that the operators will face when deploying femtocells? Lets look at some of them:

• Access method: Three different approaches exist namely, Open access, Closed access and Hybrid access which is a mix of both of them. The first two approach has some problems and I have suggested a solution before ;) but the best solution may be to go for Hybrid approach where limited connectivity is available to non-subscribers of the femto.
• Time Synchronisation is another important aspect of OFDMA Femtos. To minimise multi-access interference and for successful handovers, synchronisation between all the Femtos and between Femto and Macro is a must. This should be acheived without any complicated hardware so as to keep the cost down.
• Physical Cell Idendities (PCI) could be a problem because of limited numbers
• Neighbouring cell list, which is restricted to 32 in LTE, could be a problem if too many Femtos are around
• Handovers could also be a problem if the UE keeps jumping between Femtos and macro. One solution could be the use of HCS.

Interference analysis will definitelty play an important part in the rollouts. If not properly managed, could result in dead zones within Macro. Power control Algorithms and Radio Resource Management strategy will help but effective Spectrum allocation technique is needed as well. The diagram above shows different approaches for subchannel allocation in OFDMA femtocells.


The Femtocells would need to be self-configurable and self-optimising. I tried to explain the SON concept earlier which is similar. Self-configuration comes into picture when the Femto is switched on. Once the parameters are adjusted then Self-Optimisation tries to optimise these defaults into something better and more suited to the current environment. Sensing of the environment plays an important part in this. The diagram above shows different approaches being used by different Femtocells. The cheapest approach would ofcourse be the measurement report approach where the phone is made to report the environment. The only problem being that whichever phone was used (automatically selected) will have considerable amount of its battery power used up :)

The team behind this IEEE paper has been doing some excellent research work in the field of femtocells.

There is a book that is under publication and will be available early next year. At the same time if it interests you, you can look at some of their publications including the IEEE one that has been quoted here. Here are all the necessary links:

• The Centre for Wireless Network Design
• Professor Jie Zhang homepage
• David López-Pérez, Research Fellow, homepage
• Femtocells World Summit presentation on Self-organisation of the Subchannel Assignment in OFDMA Femtocells
• OFDMA Femtocells: A Roadmap on Interference Avoidance - IEEE Communications magazine, Sep. 2009
• ICC 2009: Femtocells panel presentation slides

Hope someone finds all this info useful :)

ICC 2009: 3G to 4G: towards full mobility IP services

There are some interesting slides and presentation from the IEEE Communications Society ICC 2009 conference in Dresden, Germany in June 2009. Here is the link to slides from the topic "3G to 4G: towards full mobility IP services".

Let me know if you found it useful.

Paging Permission with Access Control (PPAC) Study in Release 8

A new feature that was studied part of 3GPP Release 8 was PPAC (Paging Permission with Access Control). The aim of this feature was that in an emergency situation, the network can get congested and as a result all access is barred except for emergency services. This can cause problem when the user requires to be contacted but is unreachable.

Lets take Case 1: Disaster risk management office in government calls to emergency responder within disaster areas in order to supply temporary service to the disaster areas.

This should not be a problem because the emergency responder is an authorised user with higher priority of access class and will be able to make and receive calls in the disaster area.



Case2: Ambulance attendant reaches a rescue site in the disaster area but cannot find the person who asked for help originally because of unexpected destruction. The attendant should be able to call him/her in order to make sure where he/she is.

Case3: Firefighter is at a scene of fire of high-rise apartment in the disaster area and calls to a person who asked for help in order to give out directives on the evacuation.

These scenarios as such are no problem except when there is congestion on the receiving side. In that case either the emergency attendant or the risk management office should be able to get in touch and establish the call.

In technical terms, the people like emergency attendants and disaster risk management office attendants are called authorised users and the ordinary people who need help are known as unauthorised users.

It should also be possible to make a small duration call between unauthorised users so people can check each others safety. This can be controlled by changing the permission of different access class for small durations so that people can trigger calls for small duration.

The study found that eMLPP (Enhanced Multi-Level Precedence and Pre-emption) that is already available since GSM days can resolve the problem of prioritisation in resource allocation. A new capability will be required to allow UEs with indications from the network to perform location registration and respond to a paging request even though it is under access class barring conditions to complete certain classes of calls or messages (e.g. calls from emergency personnel, …).

This new capability will be available probably when Release 9 is finalised in December this year.

As far as understanding this eMLPP is concerned, the following book has quite a lot of details on this topic. If you can get hold of it then do go through it.

Preferential Emergency Communications: From Telecommunications to the Internet (The Springer International Series in Engineering and Computer Science)

Here is the google books link for that.

Flexible RLC in Release 7 and Release 8

In R99, RLC packets had to be relatively small to avoid the retransmission of very large packets in case of transmission errors. Another reason for the relatively small RLC packet size was the need to provide sufficiently small step sizes for adjusting the data rates for Release 99 channels.

The RLC packet size in Release 99 is not only small, but it is also fixed for Acknowledged Mode Data and there are just a limited number of block sizes in UM Data. This limitation is due to transport channel data rate limitations in Release 99. The RLC payload size is fixed to 40 bytes in Release 99 for Acknowledged Mode Data. The same RLC solution is applied to HSDPA Release 5 and HSUPA Release 6 as well: the 40-byte packets are transmitted from RNC to the base station for HSDPA. An additional confi guration option to use an 80-byte RLC packet size was introduced in Release 5 to avoid extensive RLC protocol overhead, L2 processing and RLC transmission window stalling. With the 2 ms TTI used with HSDPA this leads to possible data rates being multiples of 160 kbps and 320 kbps respectively.

As the data rates are further increased in Release 7, increasing the RLC packet size even further would significantly impact on the granularity of the data rates available for HSDPA scheduling and the possible minimum data rates.

3GPP HSDPA and HSUPA allow the optimization of the L2 operation since L1 retransmissions are used and the probability of L2 retransmissions is very low. Also, the Release 99 transport channel limitation does not apply to HSDPA/HSUPA since the L2 block sizes are independent of the transport formats. Therefore, it is possible to use fl exible and considerably larger RLC sizes and introduce segmentation to the Medium Access Control (MAC) layer in the base station.

This optimization is included for downlink in Release 7 and for uplink in Release 8 and it is called flexible RLC and MAC segmentation solution. The RLC block size in fl exible RLC solution can be as large as an Internet Protocol (IP) packet, which is typically 1500 bytes for download. There is no need for packet segmentation in RNC. By introducing the segmentation to the MAC, the MAC can perform the segmentation of the large RLC PDU based on physical layer requirements when needed. The fl exible RLC concept in downlink is illustrated in Figure above.


There is a lot of interesting information in R&S presentation on HSPA. See here.

Main source of the content above and for further information see: LTE for UMTS: OFDMA and SC-FDMA Based Radio Access

Enhanced UL for CELL_FACH state in Release 8

Users should always be kept in the state that gives the best trade-off between data rate availability, latency, battery consumption and usage of network resources. As a complement to the data rate enhancements made to the dedicated state (CELL_DCH), 3GPP has also made significant enhancements to the common states (URA_PCH, CELL_PCH and CELL_FACH). Release 7 introduced HSDPA mechanisms in the common states in order to improve their data rates, latency and code usage. Release 8 introduces corresponding enhancements in the uplink, allowing base stations to configure and dynamically manage up to 32 common Enhanced Uplink resources in each cell.



This enhancement improves latency and data rates for keep-alive messages (for example, from VPN or messenger applications) as well as web-browsing events, providing a seamless transition from EUL in common state to EUL in dedicated state.

As a further improvement of the CELL_FACH state, Release 8 introduces discontinuous reception (DRX), which significantly reduces battery consumption. DRX is now supported in all common and dedicated states.



Enhanced FACH and RACH bring a few performance benefits:

• RACH and FACH data rates can be increased beyond 1 Mbps. The end user could get immediate access to relatively high data rates without the latency of channel allocation.
• The state transition from Cell_FACH to Cell_DCH would be practically seamless. Once the network resources for the channel allocation are available, a seamless transition can take place to Cell_DCH since the physical channel is not changed.
• Unnecessary state transitions to Cell_DCH can be avoided when more data can be transmitted in Cell_FACH state. Many applications create some background traffic that is today carried on Cell_DCH. Therefore, Enhanced RACH and FACH can reduce the channel element consumption in NodeB.
• Discontinuous reception could be used in Cell_FACH to reduce the power consumption. The discontinuous reception can be implemented since Enhanced FACH uses short 2 ms TTI instead of 10 ms as in Release 99. The discontinuous reception in Cell_FACH state is introduced in 3GPP Release 8.

For more information see: LTE for UMTS: OFDMA and SC-FDMA Based Radio Access

Net Neutrality: Good or Bad?

I am not sure what the right answer to this question is? There will be winners and losers in either case.

FCC (Federal Communications Commission) chairman Julius Genachowski has just outlined his much-awaited plan for Internet neutrality. If the plan is approved it would drag the wireless operators in the US into the public regulatory arena occupied by their wired cousins who have recently had to account for their neutrality policies to the FCC.

The proposed policy outlined today by Genachowski will mean the FCC will get to poke and pry into mobile operators' business policies and rule on how well they conform to FCC guidelines on neutrality in the same way that wiredtelcos must. The FCC will also impose new and tighter neutrality behaviour on the big phone companies including Verizon and AT&T.

In detail: Genachowski has reaffirmed the long-standing (since 2005) broadband principles that will now be formalised by the FCC.

• That consumers are entitled to access the lawful Internet content of their choice.
• That they are also entitled to run applications and use services of their choice, subject to the needs of law enforcement.
• That they are entitled to connect their choice of legal devices that do not harm the network.
• And that they are entitled to competition among network providers, application and service providers, and content providers.

Genachowski has added two extra principles. Internet access providers can't discriminate against particular Internet content or applications: and they must ensure that Internet access providers are transparent about the network management practises they implement.

"The rule-making process will enable the commission to analyse fully the implications of the principles for mobile network architectures and practises, and how, as a practical matter, they can be fairly and appropriately implemented," Genachowski said today.

U.S. phone companies may be forced to open their wireless networks to rival Internet services like Skype and Google Voice under the proposal. The proposal, if adopted, would be a victory for consumer advocates and big Internet companies like Google Inc at the expense of telecom operators like AT&T Inc, Verizon Communications and Sprint Nextel Corp.

"The risk to the wireless carriers is that they won't be able to stop customers from using free voice and text services like Skype or Google voice," said Bernstein analyst Craig Moffett. "Voice and text are where they make all of their money."

The FCC has already been examining why Apple Inc rejected Google Voice for use on iPhone, sold by AT&T.

The new proposal could result in mobile customers cutting their phone bills by opting for minimum carrier voice plans and doing without text-messaging plans if they use mobile voice and text services from Skype and Google.

Piper Jaffray analyst Christopher Larsen downplayed the risk, saying that if they have to, operators would be sure to find a way to change their fees in order to maintain profits.
Advocates of Net neutrality have long argued that service providers must be barred from blocking or slowing Internet traffic based on the content being sent or downloaded.

But service providers say the increasing volume of bandwidth-hogging services -- such as video sharing -- puts pressure on them as it requires active network management, and some argue that Net neutrality could stifle innovation.

AT&T, the No. 2 U.S. mobile service, said it was concerned about an extension of Net neutrality rules to the competitive mobile industry.

The new regulations would limit consumer choices and "affect content providers, application developers, device manufacturers and network builders," said an executive at Verizon, which owns the No. 1 mobile service with Vodafone Group Plc.

Wireless trade group CTIA, whose members include AT&T, Verizon Wireless and Sprint Nextel, said it was concerned the proposal would have "unintended consequences." Leading Cable provider Comcast Corp said it was pleased Genachowski "recognized that networks need to be managed."

Exactly my thoughts (but with proper technical terms, language and analysis ;) by Gary Kim in IP Communications:

In the communications business, rationing is a fact of network life. Since virtually every part of a communications network uses shared resources, and in a market where users do not want to pay too much for access to those resources, rationing of network resources is necessary.

Shared finite resources always pose a usage problem. Known as the "tragedy of the commons," the economic problem is that multiple individuals, acting independently, solely and rationally when using a common resource can ultimately destroy the shared limited resource.

Some people argue that this problem cannot exist with the Internet, which is virtually infinitely expansible. But that misses the point. In looking at shared resources, the "commons" is the access network's resources, primarily. In other words, the "choke point" is the homeowner's garden hose, not the reservoir.

Some might argue that IP technology, optics, Moore's Law and competition upend the traditional "scarcity" value of access bandwidth. Certainly it helps. Currently, most consumers have access to two terrestrial broadband providers, two satellite networks, three, possibly four mobile networks. Then, there are broadband pipes where people work, at school and at many retail locations.

Still, there are some physical and capital investment limits, at least at retail prices consumers seem willing to pay. If consumers are willing to pay much more, they can get almost any arbitrarily-defined amount of access bandwidth. That, after all, is what businesses do.

If consumers resist paying business prices, network investment has to be shared more robustly than it otherwise might.

Given that all network resources are shared, resources are finite. To support retail prices that require such sharing, networks are designed in ways that "underprovision" resources ranging from radio ports to multiplexers to backhaul bandwidth. Based on experience, network designers engineer networks to work without blocking or degradation most of the time, but not necessarily always. Unusual events that place unexpected load on any part of the access network will cause blocking.

Blocking, in other words, is a network management technique. And that's the problem the Federal Communications Commission is going to have as it looks at additional "network freedoms" rules commonly known as "network neutrality." The term itself is imprecise and in fact already covered by the existing FCC rules. One might argue the issue is more the definitions and applications of existing rules that require clarification.

The ostensible purpose of the new rules is to prevent access provider blocking or slowing of any lawful applications, but a rule exists for that. Instead, it appears a primary effect of the rules will be to extend wired network rules to wired providers.

Beyond that, policymakers will have to contend with tragedy of the commons effects. If, in forbidding any traffic shaping (a network management technique) in the guise of "permitting the free flow of bits," rulemakers might set the stage for dramatic changes in industry packaging and prices of Internet access and other applications and services.

U.S. consumers prefer "flat rate billing" in large part because of its predictability of cost. But highly differentiated usage, in a scenario where networks cannot be technically managed by any traffic prioritization rules, will lead to some form of metered billing.

If metered billing is not instituted, and if service providers cannot shape traffic at peak hours to preserve network access for all users, then heavy users either have to pay more for their usage patterns, they will have to change their usage patterns, or they might experience some equivalent of "busy hour blocking."

Application providers and "public policy advocates" seem to be happy that new network neutrality rules might be adopted. They might not be so happy if ISPs lose the ability to deny or slow access to network resources. On the voice networks, some actual call blocking is allowed at times of peak usage. Forcing users to redial might be considered a form of traffic shaping, allowing access, but at the cost of additional time, or time-shifted connections.

To the extent that such blocking rules already are impermissible, some other network management techniques must be used. And one way to manage demand is to raise its price, either by increases in flat-rate package prices, by instituting usage-based billing or some other functionally-similar policy.

To avoid the tragedy of the commons problem, in other words, requires raising the end user's understanding of cost to use the shared resource.

Prioritized traffic handling, which assigns users a lower priority in the network once they have reached their fair use level, might be a preferable traffic management technique to slowing any single user's connection, once their individual usage caps have been reached.

When that is done, heavy users experience degradation in service only when competing for resources in a congested situation. For peer-to-peer users, the experienced reduction in throughput will be limited over time.

Only in heavily loaded cells or areas will a peer-to-peer user experience serious issues. Prioritized traffic handling enables operators to focus on dimensioning their networks for normal usage, while still permitting unlimited or "all you can eat" traffic.

Perhaps there are other ways of handling the "rationing," but on a shared network with network congestion, available to users paying a relatively modest amount of money, while a highly-differentiated load being placed on the network by a small number of users, some form of rationing is going to happen.

Perhaps flat rate packaging might still be possible if rationing affects end user credentials, rather than bits and applications or protocols. In other words, instead of "throttling" a user's bandwidth when a pre-set usage cap is exceeded, what is throttled is access to the network itself.

CVs and Jobs via Souktel

A non-profit group in the occupied West Bank has started a scheme that uses mobile phone text messaging to help young Palestinians find work.

The group, based in Ramallah, has already registered 8,000 Palestinians on its Souktel system, most of them recent graduates. The system connects them to about 150 leading employers who are looking for staff.

Internet access in the West Bank remains low, reaching about one-third of the population. Most computer use is at internet cafes, which are largely male-dominated domains in what is still a conservative society.

Souktel enables young people looking for work to register by answering a series of simple questions in Arabic through text messages, which are used to create a mini-CV. They then receive regular information about relevant jobs on offer.

It costs little to use apart from a slight premium charged on each text sent. In the same way, employers can post notices about job vacancies and filter applications.

The project comes at a time when despite forecasts of improved economic growth in the West Bank, unemployment still stands at around 20%, with that figure even higher among young people.

The Palestinians are a highly educated population but the Israeli occupation in the West Bank, with checkpoints, roadblocks and frequent restrictions, makes it costly and difficult to travel and do business. Universities rarely offer careers advice.

You don't need an iPhone or to download software. It's just messaging and it works on a phone from 1995.

Souktel, an SMS service based in the Middle East and East Africa, is all about connections. The service, launched in 2006, uses SMS to connect users to everything from jobs and internships to humanitarian aid and youth leadership programs.

The name comes from "souk," the Arabic word for "marketplace," and "tel," or "telephone. Although at least 80 percent of people in Palestine have access to mobile phones, most people have Internet access only in cafés or public places, said Jacob Korenblum, co-founder of Souktel. "Getting information about medical care, jobs, and food bank services can be difficult," he said. And even at Internet cafes, Korenblum said that many people, especially women, lack access to these services. "We wanted to develop a very simple service," he said. "That's how Souktel started."

Korenblum who is Canadian, said that although he has been working in the aid sector since 2000, his personal interest in Palestine began in 2005. "I came to the West Bank to work for an NGO. The main things I realized was that there wasn't so much a lack of aid, but rather a lack of good ways to find out about it." Currently, Souktel is run by a team of six people, four of whom are Palestinian.

Souktel is a combination of two services -- JobMatch and AidLink. JobMatch is an SMS service that connects people seeking jobs with employers. Job seekers can register via SMS with Souktel, and then, through a series of text messages, enter details about themselves into the system. These include location, skills, career interests, and level of education. Whenever the job seeker is looking for a job, he/she can text "match me" to Souktel to receive an instant list of jobs that matches the resume that is already stored in the Souktel system. The job listings include phone numbers so that the job seeker can call potential employers to set up an interview.

Korenblum said that at least 2,000 people use the service each month and the service has about 8,000 total users. In the past year, JobMatch has connected about 500 people with jobs. Users tend to be between the ages of 18 and 25, and the system recently expanded to include internships and volunteer opportunities. In June, about 170 people found jobs using Souktel, but the service’s success is partially reliant on the economy.

Earlier this year, Souktel launched services in the Iraq and Somaliland. In the future, Korenblum hopes that Souktel continues to grow, and could be used to connect people not only with jobs, but with educational programs or health and social services. "SMS is pervasive,” he said. “It is also by far the most cost-effective way for people to get the information they need." He also hopes to continue to share Souktel’s platform. “We've been struggling with it for three years now, and we've arrived at something that works,” he said. “We want to save someone else time in trying to develop it, so they have something that is useful for them.”

If you find this interesting, check out a Souktel presentation here.

SMS for Emergency Services

I blogged few months back about SMS for emergency services in USA now the same is being tried in UK.

Ofcom is trialling a new system to let deaf people access 999 services using text messaging.

The system lets users who can’t speak send a text message to emergency services. Their text is received by 999 assistants and read out to fire, police or other emergency service. A reply is also sent back via SMS.

The trial kicked off earlier this month, with Ofcom asking people to register to test the service. As the trial will use actual emergency messages, it needs enough people to register to get a good feel for how the system is working as most won’t actually have cause to use it.

To register, text “register” to 999; anyone not registered will not be able to use the service.
Ofcom noted that users shouldn’t assume their message has been received until they’ve received a reply, and that anyone sending hoax messages will be prosecuted.

If the trial goes well, the texting system could be in place as early as next year, Ofcom said. It’s being supported by the major telecoms companies, as well as emergency services and the Royal National Institute for Deaf People (RNID).

SMS to the emergency website here.

HSPA Functions and Benefits

Very interesting diagram summarising HSPA Functions and Benefits

Source: 3G Americas Whitepaper, HSPA to LTE-Advanced: 3GPP Broadband Evolution to IMT-Advanced (4G)

Network Interfaces for Applications using Parlay and OneAPI

Here is an old posting on Parlay/OSA that might be useful to put things in context.

An important development related to service evolution is operators making interfaces available to external applications for information and control. Two widely deployed capabilities today include location queries and short message service. With location, mobile devices or external applications (e.g., applications operating on computers outside of the network) can query the location of a user, subject to privacy restrictions. This can significantly enhance many applications including navigation, supplying location of nearby destinations (e.g., restaurants, stores), location of friends for social networking, and worker dispatch. With SMS, external applications can send user requested content such as flight updates.

Until now, the interfaces for such functions have either been proprietary, or specific to that function. However, there are now interfaces that span multiple functions using a consistent set of programming methods. One set is the Parlay X Web Services, a set of functions specified through a joint project of the Parlay Group, the European Telecommunications Standards Institute (ETSI) and 3GPP. The Open Mobile Alliance (OMA) now manages the Parlay X specifications. Parlay X Web Services include support for location and SMS, as well as many other functions with which developers will be able to build innovative applications.

Table 4 (above) summarizes the available Parlay X specifications. Operators are beginning to selectively deploy these functions. The advantage of this approach is that developers can build applications that are compatible with multiple operator networks.

A related project is GSMA OneAPI, a GSM Association project to also define network interfaces, but that prioritizes implementation based on expected market demand. OneAPI defines a simplified Web service for most functions that is essentially a subset of the related Parlay X Web service. It also defines a REST (Representational State Transfer) interface for most functions as an alternative to using the Web service. RESTful interfaces are simpler for developers to work with and experiment with than Web services.

Regardless of whether operators deploy with Parlay X or OneAPI, these are mainstream interfaces that will open wireless networks to thousands of Internet programmers who will be able to build applications that leverage the latent information and capabilities of wireless networks.

Source: 3G Americas Whitepaper '3GPP Broadband Evolution to IMT-Advanced (4G)'