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Thursday, December 29, 2011

What is LTE?

One of the best posts out there on what LTE is. You can (and should) read the full post here: http://www.extremetech.com/mobile/110711-what-is-lte

So, what is LTE? Verizon Wireless, in its advertisements, will tell you that it is the fastest, most advanced network. AT&T will say it is the next generation of wireless technology. But that doesn’t say much, now does it? To put it simply, LTE is not just the next generation of wireless technology. LTE is an ongoing, living standard. LTE is a standard that will continuously improve over time. Many expect LTE to be the standard for cellular networks for at least the next decade, possibly even beyond!

LTE stands for Long Term Evolution. Its full name is 3GPP Long Term Evolution for the Universal Mobile Telecommunications System, or 3GPP UMTS LTE for short. Most refer to the standard as UMTS LTE or just plain LTE. While UMTS LTE is the more correct name, it will be called LTE for the remainder of this article for brevity.

But that doesn’t tell us what LTE actually is. LTE is what the 3rd Generation Partnership Project (the group responsible for standardizing and improving the Universal Mobile Telecommunications System, or UMTS) designates as their next step. UMTS is the group of standards that define 3G for GSM networks across the world, including AT&T and T-Mobile’s 3G networks. For those who use CDMA2000 (subscribers of Verizon, Sprint, etc.), then LTE is pretty much the replacement for your mediocre 3G network.

umts logo

LTE is a very good, easily deployable network technology, offering high speeds and low latencies over long distances. For example, Verizon LTE in Dallas, TX was rated with an average download speed of 15.75Mbps and an average upload speed of 1.49Mbps. Verizon’s 3G service was rated with an average download speed of 1.09Mbps and an average upload speed of 0.67Mbps. Its LTE service beat its competitors’ high speed service by a wide margin. Similar ratings followed in other cities as well. Unfortunately, AT&T’s LTE service is too new to take accurate measurements of. When the AT&T LTE network is more loaded with subscribers, then more accurate readings can be taken.

In this article, I will discuss what configurations LTE can be deployed in, why LTE is easily deployable, how LTE works as a radio technology, what types of LTE exist, how LTE affects battery life, what network operators want LTE to do, and the future of 4G as a whole. The most technical parts of the article are LTE can be deployed in, why LTE is easily deployable, how LTE works as a radio technology, and what types of LTE exist. For those who don’t want that information, you can skip to how LTE affects battery life and still get the gist of what I’m saying. But to get the complete picture, reading the whole article is advised.

How LTE is configured for deployment

LTE supports deployment on different frequency bandwidths. The current specification outlines the following bandwidth blocks: 1.4MHz, 3MHz, 5MHz, 10MHz, 15MHz, and 20MHz. Frequency bandwidth blocks are essentially the amount of space a network operator dedicates to a network. An operator may choose to deploy LTE in a smaller bandwidth and grow it to a larger one as it transitions subscribers off of its legacy networks (GSM, CDMA, etc.).

MetroPCS is an example of a network operator that has done this. A majority of its spectrum is still dedicated to CDMA, with 1.4MHz or 3MHz dedicated for LTE depending on the market. Leap Wireless has also done the same thing, except it’s using 3MHz or 5MHz instead of 1.4MHz or 3MHz. Neither of these carriers can afford to cut CDMA capacity by a significant degree just yet, so LTE operates on tiny bandwidths. Additionally, neither operator has enough backhaul (the core network infrastructure and connections to the internet) dedicated to LTE to make larger bandwidths worth it either.
verizon logo

On the other hand, Verizon Wireless has been dedicating 20MHz for LTE all across the board, since it has a nationwide 20MHz block of spectrum available for LTE. Combined with excellent backhaul, Verizon’s LTE service promises to be best in class. AT&T is dedicating 10MHz across the board because that’s all the free space it has, though it makes up for it with much better backhaul, so the performance differential between Verizon and AT&T isn’t noticeable right now. However, when AT&T gets more LTE customers, the difference will become clearer.

Less spectrum means that fewer customers can obtain the same high speeds that Verizon’s LTE customers get when connected to any particular cell. LTE can support up to 200 active data clients (smartphones, tablets, USB modems, mobile hotspots, etc.) at full speed for every 5MHz of spectrum allocated per cell. That means that if a particular tower has 20MHz of spectrum allocated to it, it can support up to 800 data clients at full speed. There are ways of supporting more data clients per 5MHz, but doing so requires sacrificing speed and capacity, as the 200-per-5MHz ratio is the optimal configuration. However, spectrum isn’t everything to LTE quality, as I will discuss later.

Why LTE is easier to deploy

The network architecture for LTE is greatly simplified from its predecessors because LTE is a packet-switched network only. It doesn’t have the capability to handle voice calls and text messages natively (which are typically handled by circuit-switched networks like GSM and CDMA). Anyway, the LTE SAE (System Architecture Evolution) is essentially a simplified version of the one used for UMTS networks today. An LTE network uses an eNodeB (evolved node B, essentially an LTE base station), a MME (mobile management entity), a HSS (home subscriber server), a SGW (serving gateway), and a PGW (a packet data network gateway). With the exception of the eNodeB, everything is considered as part of the EPC (evolved packet core) network. At the tower the eNodeB connects to the EPC.

evolved core packet framework
The MME and the HSS basically handle all duties regarding subscriber access to the network. It handles all the authentication, roaming rules for subscribers, etc. The SGW essentially acts like a giant router for subscribers, passing data back and forth from the subscriber to the network. The PGW provides the connection to external data networks. The most common data network the PGW provides a connection to is the internet. However, if the network operator desires handover with a non-UMTS network like CDMA2000, WiMAX, or a WiFi hotspot network run by the network operator, then an ePDG (evolved packet data gateway) and an ANDSF (Access Network Discovery and Selection Function) for the eNodeB can be installed to support those networks on the EPC.

Most operators around the world will use the basic network design. Verizon Wireless, Sprint-Nextel, Leap Wireless, MetroPCS, C Spire Wireless, and U.S. Cellular have installed or will install the same basic design with one major change: eHRPD will replace the core network connection to traditional UMTS networks.

Palm pre 1xrtt

They won’t be using the proper network design to handover to CDMA2000 because of eHRPD (Enhanced High Rate Packet Data, essentially an enhanced version of the core packet network for EV-DO), which plugs right into the network in a way that is supposed to replace a UMTS network. By its very nature, eHRPD is rather fragile because it attempts to emulate enough of what the LTE network core expects in a UMTS network to communicate and hand over. This is why Verizon’s LTE service has been breaking down at least once every quarter. LTE and CDMA handover wasn’t originally designed to work the way it does now, and the way they’ve implemented it is not officially supported in the standard (well, the 3GPP standard, anyway). Unexpected issues arise every time they do some network tweaking because of this. Sometimes the failure can spread to EV-DO and shut it down, leaving only 1xRTT available.

How LTE actually works

LTE uses two different types of air interfaces (radio links), one for downlink (from tower to device), and one for uplink (from device to tower). By using different types of interfaces for the downlink and uplink, LTE utilizes the optimal way to do wireless connections both ways, which makes a better optimized network and better battery life on LTE devices.

For the downlink, LTE uses an OFDMA (orthogonal frequency division multiple access) air interface as opposed to the CDMA (code division multiple access) and TDMA (time division multiple access) air interfaces we’ve been using since 1990. What does this mean? OFDMA (unlike CDMA and TDMA) mandates that MIMO (multiple in, multiple out) is used. Having MIMO means that devices have multiple connections to a single cell, which increases the stability of the connection and reduces latency tremendously. It also increases the total throughput of a connection. We’re already seeing the real-world benefits of MIMO on WiFi N routers and network adapters. MIMO is what lets 802.11n WiFi reach speeds of up to 600Mbps, though most advertise up to 300-400Mbps. There is a significant disadvantage though. MIMO works better the further apart the individual carrier antennae are. On smaller phones, the noise caused by the antennae being so close to each other will cause LTE performance to drop. WiMAX also mandates the usage of MIMO since it uses OFDMA as well. HSPA+, which uses W-CDMA (an improved wideband version of CDMA) for its air interface, can optionally use MIMO, too.

Verizon Cell Tower

For the uplink (from device to tower), LTE uses the DFTS-OFDMA (discrete Fourier transform spread orthogonal frequency division multiple access) scheme of generating a SC-FDMA (single carrier frequency division multiple access) signal. As opposed to regular OFDMA, SC-FDMA is better for uplink because it has a better peak-to-average power ratio over OFDMA for uplink. LTE-enabled devices, in order to conserve battery life, typically don’t have a strong and powerful signal going back to the tower, so a lot of the benefits of normal OFDMA would be lost with a weak signal. Despite the name, SC-FDMA is still a MIMO system. LTE uses a SC-FDMA 1×2 configuration, which means that for every one antenna on the transmitting device, there’s two antennae on the base station for receiving.

The major difference between the OFDMA signal for downlink and the SC-FDMA signal for uplink is that it uses a discrete Fourier transform function on the data to convert it into a form that can be used to transmit. Discrete Fourier transform functions are often used to convert digital data into analog waveforms for decoding audio and video, but it can be used for outputting the proper radio frequencies too.

The LTE technology itself also comes in two flavors: an FDD (frequency division duplex) variant and a TDD (time division duplex) variant. The most common variant being used is the FDD variant. The FDD variant uses separate frequencies for downlink and uplink in the form of a band pair. That means for every band that a phone supports, it actually uses two frequency ranges. These are known as paired frequency bands. The TDD variant uses one single range of frequencies in a frequency band, but that band is segmented to support transmit and receive signals in a single frequency range.

In the United States, Clearwire is the only network operator deploying LTE in the TDD variant. Everyone else is deploying in the FDD variant. The TDD variant becomes more important in Asia, as China Mobile (the largest network operator in the world in terms of subscriber count) uses TDD frequencies for their 3G network and it plans to upgrade to the TDD variant of LTE. Fortunately, LTE devices can support both variants on a single radio chip without too much trouble.

Enough about specs — what about battery life?

Now we lead to the part that most people care about: how it affects battery life. By itself, LTE devices should last roughly as long as their HSPA+ equivalents because of the optimized radios for both downlink and uplink operations. The reason why LTE devices right now eat batteries for breakfast is because the network operators are forcing these devices into active dual-mode operation.

For Verizon Wireless, this means that all of their LTE devices connect to both CDMA2000 and LTE simultaneously and stay connected to both. This means that you are eating twice the amount of battery for every minute you are connected than you would if you were connected only to CDMA2000 or LTE. Additionally, when you make calls on Verizon Wireless LTE phones, the CDMA2000 radio sucks down more power because you are talking. Sending and receiving text messages causes pulses of CDMA2000 activity, which cuts your battery life more. Arguably, constantly changing radio states could be worse for battery life than a switch into one mode for a period of time and switching back, so text messages may actually kill the batteries faster.

Then there is handover. Handover is the operation in which a device switches from one network to another or from one tower to another. Handover is the critical component that makes any cellular wireless network possible. Without handover, a user would have to manually select a new tower every time the user leaves the range of a tower. (WiFi is an example of a wireless network technology that doesn’t inherently support handover.) When the user travels outside the range of a WiFi network, the WiFi radio will just drop the connection. For cellular networks, this is even more critical because the range of a tower is not very predictable due to factors outside of anyone’s control (like the weather, etc.). LTE supports handover like all other cellular wireless networks, but it improves on it by doing it much faster when handing over to a supported type of network or cell.


However, Verizon is doing handover from LTE to EV-DO and back by plugging in a connection to an enhanced version of the EV-DO data network core called eHRPD. As discussed earlier, this isn’t a great solution by any means. It becomes more problematic when you consider that most LTE signals are very weak. Unfortunately, most customers have no idea because Verizon deceives them into believing it is stronger by using the EV-DO signal strength for the signal bars for LTE for all of their devices except the Galaxy Nexus.

The weak signal and the fragile link-up between EV-DO and LTE make handover occur a lot more than it is supposed to, which eats battery life even more. With AT&T using an HSPA+ network alongside LTE instead of CDMA2000, handover operation is a lot smoother. As far as battery life goes, it should be slightly better than Verizon LTE phones because LTE supports fast handover between UMTS and LTE. AT&T LTE phones are normally not forced into active dual-mode operation because HSPA+ lets you use data and talk at the same time. As a consequence, AT&T has no need to force the device into active dual-mode operation. However, battery life will still be pretty bad because LTE signals are still very weak in most AT&T LTE zones, and AT&T LTE devices default to connecting to LTE signals whenever possible.

C Spire Wireless, MetroPCS, Cricket Wireless, and U.S. Cellular will all have the same problem as Verizon Wireless with LTE battery life because they all plan to do the same thing as Verizon Wireless and force active dual-mode operation. As a result, turning off LTE will significantly improve battery life because the phone switches back to single-mode operation. Or in the case of AT&T phones, passive dual-mode operation (for GSM/HSPA+ handover) since they are typically in passive tri-mode operation for GSM/HSPA+/LTE handover. Passive multi-mode operation means that the device isn’t constantly connected to multiple networks, but will establish a connection and hand over the connection if the signal on the current network is too weak or snaps. This is ideal for multi-mode operation, but it isn’t possible for CDMA/LTE network operators until they make it possible for LTE to handle calls and text messaging.

The panacea

The ultimate goal of the network operators deploying LTE is to replace everything else they have with it. That means that it needs to become possible to handle voice calls, text messages, network alerts, etc. over the data network. However, no one developed the LTE specification with voice and text messaging in mind. It was designed as a data network only. So how do they solve the problem? By developing a VoIP solution that fits their needs. Two main standards came into existence: VoLGA (Voice over LTE via Generic Access) and VoLTE-IMS (Voice over LTE via IMS). VoLGA was based on GAN (Generic Access Network), which is also known as UMA (Unlicensed Mobile Access). Deutsche Telekom was the only network operator that wanted to use this method, as the design for VoLGA was heavily derived from T-Mobile USA’s implementation of UMA for its Wi-Fi Calling feature. No one else wanting to deploy LTE wanted to use it as a final or interim solution, as it would have meant keeping around the legacy GSM core network for this purpose.

Everyone else supported VoLTE-IMS (now referred to as VoLTE), which allowed them to fully discard their older networks and simplify their network design as they decommissioned legacy networks. However, IMS is much more expensive and difficult to deploy than VoLGA, at least for GSM network operators. But IMS also promised more flexibility. IMS could be used to make real-time video calling with all sorts of additional features possible. And so, Deutsche Telekom dropped VoLGA and joined everyone else in supporting VoLTE.

VoLTE uses an extended variant of SIP (Session Initiation Protocol) to handle voice calls and text messages. For voice calls, VoLTE uses the AMR (Adaptive Multi-Rate) codec, with the wideband version used if supported on the network and the device. The AMR codec has long since been used as the standard codec for GSM and UMTS voice calls. The wideband version supports higher quality speech encoding, which would allow for clearer voice calls. Text messages are supported using SIP MESSAGE requests. While video calling has been often discussed as a potential benefit of using VoLTE, no such standard for it exists yet.

t-mobile logo

In a somewhat ironic twist, T-Mobile USA became the first network in the world to commercially deploy IMS-based voice calling and text messaging by using it for an improved WiFi calling solution. An update to the T-Mobile Samsung Galaxy S II and an update to the T-Mobile HTC Amaze 4G both included the new Wi-Fi Calling solution.

Despite not having an LTE network, T-Mobile is the most prepared for deploying LTE to replace its existing networks. Once T-Mobile deploys LTE, it can easily modify the WiFi calling client software to allow it to work over the LTE network as well. Its advanced HSPA+ network architecture also means that it can easily (and relatively cheaply) plug in support for LTE, too. But, T-Mobile has no room in its spectrum nationwide to deploy LTE at the moment. It’ll need to repurpose some of the spectrum it currently uses, which means scaling back the bandwidth for HSPA+ or 2G GSM.

As for Verizon, AT&T, and Sprint deploying VoLTE? Well, Verizon stated at Mobile World Congress 2011 that it plans to begin offering handsets with VoLTE in mid 2012 to early 2013. AT&T has stated that it will deploy VoLTE in 2013. Sprint has not officially said anything about it yet.

The messy future of 4G

We’ve only scratched the surface of what LTE is all about, but this article includes pretty much everything that LTE subscribers would care about. Some of the other aspects of LTE include SON (self-organizing network) capabilities, which allows it to flexibly allocate capacity to parts of the cellular network as it is needed by redistributing connections to an optimal configuration at any given time. Handover to WiFi is another cool feature, too. However, most of the features like the former are pretty much only seen from a network operator’s side of things, and things like the latter may never actually be implemented.

LTE is a significant leap in optimized cellular wireless technology though. If you wish to get the highly-technical details of LTE and its ever-evolving specifications, check out the 3GPP’s specification series for LTE. Specifications for eHRPD and associated CDMA2000 specifications are available on the 3GPP2′s website. The VoLGA specifications are available on the VoLGA Forum’s website. The 3GPP hosts the IMS specifications, with the GSM Association hosting IMS Profile for Voice and SMS specifications on their website. We’ve covered the major highlights in this article, as there is way too much to cover. As the specifications detail, there were many improvements at every level of a cellular network that result in a high-performance, optimized network.

Whether LTE becomes the success story of the mobile industry remains to be seen. Network operators around the world are only now deploying it, and already it is turning into a mess. The 3GPP has already approved over forty frequency bands for LTE. Twenty-five of them are for LTE FDD and the rest are for LTE TDD. Roaming is going to be very difficult on LTE. In the United States and Canada alone, there are ten FDD bands and one TDD band for LTE. In Europe, there are three more bands for FDD LTE. In Asia and Oceania, there are the same three FDD bands for Europe, three more frequency bands for FDD, and the same TDD band as in the United States. The rest of the bands have yet to be used, but they are going to be used. Someone is going to have to figure out how to fit more bands on an LTE device without sacrificing portability.

WiMax is not LTE

And then there’s the 4G mess. Contrary to popular belief, LTE at the current stage was not always considered 4G. The International Telecommunications Union (or ITU) determines what can be considered 4G. Originally, the ITU declared that the collection of requirements known as IMT-Advanced determined what would be considered 4G. LTE did not make the cut (though a future version of it called LTE-Advanced did). Neither did WiMAX or HSPA+. However, the American and Canadian network operators’ collective influence made the ITU revise their specification on what 4G is to include any wireless technology significantly evolved from 3G technologies. Most technophiles are of the opinion that the IMT-Advanced specification determines what can be considered 4G, while most business people prefer the newer definition for 4G. For the purposes of this article, the revised standard is considered 4G. While this is out of the scope of this article (and also not really important either), I’m laying it out now to prevent any arguments. This means that LTE, HSPA+, and WiMAX are all considered 4G technologies, though WiMAX is still officially on the list of 3G technologies too.

I don’t know what the future holds for LTE, but it will certainly be very interesting. This is the most exciting time in the mobile industry since the switchover from analog to digital back in the early 1990s. LTE represents a paradigm shift from hybrid voice and data networks to data-only networks. Going forward, wireless network technology is likely to become more widely used because it will become easier to obtain than wire-based services (cable, DSL, etc.). It is doubtful that it would fully replace wire-based data services though. Hopefully, the issues we face with LTE now will go away over time. At the very least, it might jump-start development in more advanced battery and portable radio technologies that can handle more than what current ones can do