What is LTE-Advanced? ~ Technomanthra

Recently, several carriers around the world have begun talking about LTE-Advanced, or at least a few features of LTE-Advanced. Notably, Sprint and T-Mobile in the United States have been quick to mention that their systems for LTE being deployed offer an easy upgrade path to LTE-Advanced. But what exactly is it?

LTE-Advanced is essentially the next level of LTE. In particular, LTE-Advanced is LTE that fulfills the criteria to be considered 4G by the ITU. To be called 4G, it has to be able to fulfill the requirements set forth in the IMT-Advanced specification. To accomplish this, LTE-Advanced includes many advanced techniques, technologies, and capabilities that older wireless standards do not include.

The main goal of LTE-Advanced is to provide the IMT-Advanced functionality while retaining backward compatibility with current LTE user equipment that subscribers own. This is important because otherwise early adopters would be penalized when the carrier upgraded to LTE-Advanced on the infrastructure side. LTE-Advanced will provide 1Gbps downlink and about half that on the uplink for fixed wireless (that is, for services like Verizon Wireless’ HomeFusion), while mobile access will be about 100Mbps on the downlink and about 75Mbps on the uplink. Both forms of access will have latencies comparable to wireline (DSL, cable) broadband networks, making it an excellent substitute for wireline networks for many types of internet-based systems.

Assuming that you have already read our explanation of LTE, this will describe what LTE-Advanced adds to LTE.

3GPP Release 10 – The first stage of LTE-Advanced

The 3rd Generation Partnership Project (3GPP) develops telecommunications standards in concert with the European Telecommunications Standards Institute (ETSI), the GSM Association (GSMA), and the many telecommunications companies that are members to these organizations around the world. Newer versions of these standards are referred to as “releases” by the 3GPP. 3GPP Release 8 introduced the first final version of LTE. Release 9 adds basic voice over LTE through IMS capabilities and further enhancements to many other parts of the LTE standard.

Release 10, on the other hand, introduces all the basic features to qualify for IMT-Advanced. It also further refines other parts from Releases 8 and 9 and adds additional features. Because each specification release is quite large and broad, only the most visible or most dramatic features are being covered.

IMS Voice – Priority users and emergency service use

Priority users of multimedia on the IMS (IP Multimedia Subsystem) stack was added in order to bring support for first priority voice call sessions for emergencies, like with disaster response and emergency medical teams in the event of a natural disaster. Calls by those groups musthave priority over everyone else. This functionality already exists in circuit voice networks built into the 3G voice standard, it is just now being introduced ahead of VoLTE being commonly used.

IMS – Inter-Device Transfer

In Release 9, a feature called “inter-device transfer” was added to the specification. This feature added support for users who own multiple devices under a single subscriber account to be able to transfer IMS sessions among them through a form of handover that the user can control directly.

Release 10 extends this feature by adding support for transferring IMS sessions between subscriptions as well. For example: if you were watching TV on your smartphone, but arrived home and wanted to watch it on your TV instead, you could transfer the session from your mobile TV service under your mobile phone service subscription to your IPTV subscription and immediately be able to continue watching from your TV. This feature would be an invaluable feature for those subscribed to both AT&T Wireless and AT&T U-verse, in particular.

M2M – Network overload control

With 2G networks shutting down, the M2M (machine-to-machine, used for communication between specialized devices and systems) market is looking toward 3G and 4G to replace 2G communication systems.

M2M has a few requirements of its own: it must be able to handle large amounts of small data sessions, large amounts of continuous data sessions, and absolutely must minimize the amount of changes to hardware over a very long period of time.

Normally, the M2M market would just move to 3G systems and be done with it for the decade or so. However, there are several countries where 2G and 3G are being shut down, so the future of 3G is not quite so stable. Not to mention, many of these countries are ones with CDMA2000 networks, which have no future path anyway.

That being said, it becomes extremely important to ensure that M2M over LTE will not cause problems, so Release 10 introduces quite a few network-side improvements to rigorously control network quality and stability with M2M and normal subscriber usage of the network.

Femtocells

With the evolution of small cell networks (cellular networks that are comprised of large amounts of small radio stations with a short range and dedicated backhaul per cell) and hybrid macro/micro cell networks, a need has arisen for selective offloading and better management of sessions between the large network and the small cell network. Release 10 adds numerous enhancements to fix this.

Notably, Release 10 adds selective IP offload to the “Home eNode B” (the term used to describe a local femtocell used at home or in a business) so that certain types of traffic will go through the femtocell while the other traffic will go through the macrocell network or vice versa. Additionally, features to improve prioritization of neighbor cells for handover are included as well.

Handover to WiFi for offloading IP traffic

As we’ve discussed before, spectrum is a huge limiting factor to network capacity. Due to increasingly scarce amounts of unused spectrum, carriers around the world are starting to push WiFi usage more and more. In particular, AT&T in the United States offers free access to its repository of hotspots for mobile subscribers with cellular data plans.

With the addition of WiFi offload support in Release 10, carriers can choose to use selective offloading with WiFi to have non-IMS traffic run through the WiFi network while IMS still runs over LTE. Of course, it could all be offloaded to WiFi, but most carriers don’t seem willing to do that.

SU-MIMO for downlink and uplink

LTE uses OFDMA with a 2×2 or 4×4 MIMO configuration for downlink and SC-FDMA with a 1×2 MIMO configuration for uplink. However, this system is unacceptable to meet the LTE-Advanced efficiency requirements. In order to fix this, higher level MIMO configurations are necessary.

Release 10 extends the downlink air interface to support SU-MIMO (single user, multiple in, multiple out) with up to eight-layer spatial multiplexing, and the uplink air interface to support SU-MIMO with up to four-layer spatial multiplexing. This extension is intended for more complex (and suitably large) devices where the spatialization of the antennae is feasible. Simpler devices (like M2M and feature phones) can continue to use simpler configurations that use MU-MIMO (multiple user, multiple in, multiple out) specified in Release 8.

By using these more complex MIMO configurations, the reliability of the connection improves considerably. Not to mention, the average efficiency of the connection increases drastically with higher order MIMO.

Relays for LTE

Release 10 adds a new LTE coverage expansion option: relays. Think of relays as a more complex form of a repeater or signal booster. The idea here is to offer a way for carriers to extend coverage further more cheaply by not requiring the deployment of a full base station/tower node system. But, because this is a wide area network, security is a concern. Not to mention that frequency usage matters as well because cellular networks operate in specifically licensed bandwidths for a given area.

Relays for LTE will offer many of the same external features of regular towers, but will not have its own backhaul. Instead, it will pull in from a neighbor cell and then push out a new signal like that one. That will allow it to extend the range of a cell much further, and extend coverage. This is more useful in rural areas, where not as much backhaul is required to support a given area.

Lots of new TDD and FDD spectrum blocks approved

Clearwire’s 2.6GHz spectrum is now officially approved for usage with TD-LTE. It is defined as LTE band class 41. This is obviously required for Clearwire to migrate from WiMAX to LTE TDD.

Dish Network’s 2GHz S-band spectrum for North America is now approved for FD-LTE usage. It is defined as LTE band class 23. This is required for Dish Network to begin the process to procure equipment to deploy LTE, should it desire to do so.

LightSquared’s 1.6GHz L-band spectrum for North America is approved for FD-LTE usage. It is defined as LTE band class 24. While this is required for LightSquared to begin the process to procure equipment to deploy LTE, it will probably never happen because of interference issues.

Europe recently released 3.4GHz-3.6GHz and 3.6GHz-3.8GHz bands for wireless broadband usage. The 3GPP has approved TDD usage on both bands as LTE band classes 42 and 43, respectively.

Sprint’s LTE deployment required a new band class to be approved, so the 3GPP approved band class 25, which includes the existing PCS spectrum (previously approved as LTE band class 2) and adds the G-block PCS spectrum. While this is approved for Release 10, Sprint is using this with Release 8 and Release 9 level specification features. This does not mean Sprint is out of compliance or anything like that, though. Sprint just has to add support for the finished features in 3GPP Release 10 to upgrade to Release 10. This band essentially supersedes the previously defined band class for PCS (band class 2).

3GPP Release 11 – Coming soon to a carrier near you

3GPP Release 10 was completed at the end of 2011, giving plenty of time for telecommunications equipment vendors to make infrastructure equipment that is Release 10 compatible, and thus be able to advertise some degree of LTE-Advanced compatibility for equipment purchased in 2012. Release 11, on the other hand, is not scheduled to be finished until the very end of 2012. That being said, there are some features of the specification worth noting.

Extending FDD Upper 850MHz

The Cellular 850 band is unusual in that it is a American band that is used outside of the Americas as well. While PCS 1.9GHz has seen limited deployments in Asia due to CDMA One/CDMA2000 deployments there, the vast majority of CDMA One/CDMA2000 deployments were on Cellular 850. Currently, several separate band classes exist for portions of the 800MHz-900MHz range: band class 5 (Cellular 850 used in the Americas, Oceania, and South Korea), band class 6 (subset of Cellular 850 used in Japan), band class 18 (ESMR 800 used by Sprint in the US, Telus in Canada, and KDDI in Japan), and band class 19 (expanded version of band class 6 for Japan). Accordingly, the 3GPP merged band classes 5, 18, and 19 into a new global 850MHz band in order to improve economies of scale on that band. This works well with the FCC now allowing Sprint to deploy LTE on ESMR 800. This new band is LTE band class 26, and essentially supersedes all previously defined 850MHz frequency bands. No one has made equipment for the band yet.

Carrier aggregation

In order for LTE-Advanced to support larger amounts of data throughput, LTE-Advanced needs to support wider frequency bandwidths (notably 40MHz or more). In order to pull this off without losing backward compatibility, carrier aggregation was introduced in Release 10 to allow combining multiple pipelines together to create a larger single pipeline. It is essentially the same thing as channel bonding for wireline broadband networks.

While Release 10 did introduce the core specifications for this feature, Release 11 develops this feature further by defining potential band combinations and bandwidth sizes supported through aggregation. Consequently, carrier aggregation is the big target for Release 11.

AT&T, in particular, has pushed for approval of the following band combinations to be approved: Band 4 (AWS) with Band 17 (Lower 700 minus A block), Band 2 (PCS) with Band 17, Band 4 with Band 5 (Cellular 850), and Band 5 with Band 17. European network operators have pushed for Band 3 with Band 7 and Band 7 with Band 20. Verizon Wireless has pushed for Band 4 with Band 13 (Upper 700 C block) so that it can use carrier aggregation with its newly acquired AWS spectrum and its current LTE spectrum. American regional carriers are pushing for Band 4 with Band 12 (Lower 700) and Band 5 with Band 12.

AT&T is also working on getting the supplemental downlink channel in 700MHz it purchased from Qualcomm approved along with potential combinations. The current combinations suggested are with band 2, band 4, and band 5.

On the TDD side, intra-band carrier aggregation is being supported in order to allow more efficient utilization of the larger chunks of spectrum allocated in TDD bands. Europe and China have band class 38, Asia has band class 40, and Clearwire has band class 41. It is expected that 2x20MHz aggregations for LTE-Advanced will be common on TDD deployments.

Lower 850MHz band for Americas except the US

Carriers across the Americas (except the US) have spectrum on the SMR (specialized mobile radio) band and want to migrate from iDEN technology to LTE. Currently, the 3GPP is working on developing the band specification and will approve it as LTE band class 27. To be clear, this has nothing to do with the Cellular 850 spectrum, as these frequencies are below it. These were reserved quite some time ago for SMR usage, and have never been used for anything else.

Asia-Pacific Digital Dividend (700MHz)

Asia has finalized the spectrum to be freed up from the switchover to digital TV, and while the spectrum lies on the frequency range that the American bands do, it isn’t likely to be configured the same way because the 700MHz band plan for the USA is absolutely insane. No one wants to use that configuration if it can be helped. The 3GPP is working on figuring out the band plan with the various regulatory authorities in Asia.

LTE-Advanced is continuously evolving

Just like LTE, LTE-Advanced is continuously evolving to meet the needs of everyone who uses wireless broadband services. As the replacement for 2G and 3G networks that intends to unify the world under a single standard, it needs to be able to serve every possible use case. The 3GPP has a tough challenge ahead of itself. It needs to consider literally every market in the development of the standard, but it seems to be doing a good job right now.

There is more to LTE-Advanced, and more is always coming to the table. Expect far more improvements and features to come to LTE-Advanced. As an open standard with nearly infinite capabilities due to its all-IP system, anything is possible. As LTE-Advanced networks come online, we will see some truly innovative technologies develop that take advantage of these networks.

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Saturday 1 September 2012