What is LTE?

Introduction

LTE (Long Term Evolution) is shorthand for 3GPP Long Term Evolution for the Universal Mobile Telecommunications System, or sometimes called 3GPP UMTS LTE, and more frequently UMTS LTE.

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.

LTE is an 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. Compare this to Verizon’s current 3G service with an average download speed of 1.09Mbps and an average upload speed of 0.67Mbps.

In other words, if the current network is a 5-lanes highway, then LTE represents a 20-lanes superhighway with multiple fast and high occupancy vehicle lanes, collectors and express lanes and many on- and off- ramps. LTE creates these lanes to allow data (video streaming, web browsing, file transfers, gaming data, etc.) to travel more efficiently, routing them to special, dedicated lanes that ease congestion and increase speed.

Because LTE offers significant improvements over older cellular communication standards, some refer to it as a 4G (fourth generation) technology along with WiMax. However, when looking at the standard proposed by the International Telecommunications Union (ITU), LTE does not technically correspond to 4G. In the paper, we will refer to LTE as an equivalent to 4G.

This overview of LTE will summarize what configurations LTE can be deployed in, why LTE is easy to deploy, how LTE works as a radio technology, what are the types of LTE, how network operators will use LTE, and the future of 4G as a whole.

How LTE is configured for deployment

LTE is flexible and supports deployment across different spectrums, which are the range of frequencies a network operator dedicates to a network. The current specification outlines the 1.4MHz, 3MHz, 5MHz, 10MHz, 15MHz, and 20MHz spectrum bands. An operator may choose to deploy LTE in a smaller spectrum and grow it to a larger one as it transitions subscribers off its legacy networks (GSM, CDMA, etc.).

MetroPCS LTE deployment is an example of this approach. A majority of its spectrum is still dedicated to CDMA, with 1.4MHz or 3MHz bands dedicated for LTE depending on the market. Leap Wireless took the same approach using 3MHz or 5MHz bands for LTE. Neither carrier can eliminate CDMA capacity yet, so LTE operates on tiny spectrums to continue operating legacy systems. Additionally, neither operator has enough backhaul (the core network infrastructure and connections to the internet) currently dedicated to LTE to make larger bandwidths worthwhile.

On the other hand, Verizon Wireless has been dedicating a 20MHz band for LTE across the board, since it has a nationwide 20MHz block of spectrum available for LTE. Combined with an excellent backhaul, Verizon’s LTE service promises to be best in class. AT&T is dedicating a 10MHz band because that is the only spectrum available it has for now. However, AT&T has a much better backhaul, so the difference in performance between Verizon and AT&T isn’t very noticeable now. As AT&T gains LTE customers and increases the spectrum, this difference will become clearer.

A smaller spectrum means that fewer customers will get the same performance as Verizon’s LTE customers 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.

Spectral flexibility means wider deployment options

A key characteristic of LTE technology is its suitability for deployment in scalable frequency bands ranging from 1.25 MHz to 20 MHz. Additionally, it can operate in all 3GPP frequency bands in paired and unpaired spectrum allocations.

The actual performance achieved with LTE depends on the bandwidth allocated for services, and not the choice of spectrum band itself. This gives operators flexibility in their commercial and technical strategies. Deployed at higher frequencies, LTE is attractive for strategies focused on network capacity, whereas at lower frequencies it can provide ubiquitous cost-effective coverage.

Why LTE is easy to deploy

The network architecture for LTE is simplified from its predecessors because LTE is a packet-switched network only. It does not have the capability to handle voice calls and text messages natively (which are typically handled by circuit-switched networks like GSM and CDMA). The LTE SAE (System Architecture Evolution) is a simplified version of the one currently used for UMTS networks. An LTE network uses an eNodeB (evolved Node B, – an LTE base station), an 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.

The MME and the HSS handle subscriber access to the network like authentication, roaming rules for subscribers. The SGW 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 one being the internet. However, if the network operator wants handover with a non-UMTS network like CDMA2000, WiMAX, or a WiFi hotspot 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 will use the basic network design.

How LTE actually works

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

For the downlink, LTE uses an OFDMA (Orthogonal Frequency Division Multiple Access) interface as opposed to the CDMA (Code Division Multiple Access) and TDMA (Time Division Multiple Access) interfaces used since 1990. OFDMA (unlike CDMA and TDMA) mandates that MIMO (Multiple In, Multiple Out) is used. This means devices have multiple connections to a single cell, which increases the stability of the connection, reduces latency tremendously, and increases the total throughput of a connection.

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. SC-FDMA is superior to regular OFDMA for uplink because it has a better peak-to-average power ratio. LTE-enabled devices, in order to conserve battery life, don’t typically have a powerful uplink signal, so many 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 are two receivers on the base station.

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 they can be used for outputting the proper radio frequencies too.

LTE technology uses two variations: an FDD (Frequency Division Duplex) variant and a TDD (Time Division Duplex) variant with FDD being the most common. FDD 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. However, the TDD variant becomes more important in Asia, as China Mobile (the network operator with the most subscribers in the world) uses TDD frequencies for their 3G network. It plans to upgrade to the TDD variant of LTE.

Fortunately, LTE devices can support both variants on a single radio chip.

How network operators will use LTE

The ultimate goal of the network operators deploying LTE is to replace all the existing technologies with it. That means LTE has to be capable of handling voice calls, text messages, network alerts, etc. over the data network. However, the LTE specification was not developed with voice and text messaging in mind. It was designed as a data network only. The solution involves implementing a VoIP approach that fits network operator’s needs. Two main standards exist for VoIP: VoLGA (Voice over LTE via Generic Access) and VoLTE-IMS (Voice over LTE via IMS). VoLGA is based on GAN (Generic Access Network), which is also known as UMA (Unlicensed Mobile Access).

Although IMS is much more expensive and difficult to deploy than VoLGA, at least for GSM network operators, it also has more flexibility. IMS can be used to make real-time video calling plus all kinds of additional features that could be made available in future. Most of the support is with 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 it is supported on the network and the device. The AMR codec has long been used as the standard codec for GSM and UMTS voice calls. The wideband version supports higher quality speech encoding, which allows for clearer voice calls. Text messages are supported using SIP MESSAGE requests. While video calling is often discussed as a potential benefit of using VoLTE, no standard for it currently exists.

Handover

Handover is when a device switches from one network to another or from one tower to another. Handover is a critical component that makes any cellular wireless network possible. Without it, users would have to manually select a new tower every time they leave the tower range or experience a drop of the connection – like WiFi, which does not support handover.

It is important for LTE users that handover can occur between network technologies. LTE users will expect uninterrupted, efficient and stable service starting from the day they buy their LTE device. It is, however, expected that the initial rollout of LTE networks will be in service hot spots that cover relatively small geographical areas. It is also evident that the full-scale rollout of LTE will take time, and the legacy systems will remain in use to serve the current mobile users. To provide seamless mobility and uninterrupted service, mobility across radio access technologies is required (i.e. moving from a LTE supported area to one that does not support LTE). Fortunately, there are solutions for network technologies to co-exist and LTE handles handover well and fast.

Frequency issues and 4G confusion

Although network operators around the world are just starting to deploy LTE, the approval by the 3GPP of over forty frequency bands is already causing confusion. Moreover, twenty-five of them are for LTE FDD and the rest are for LTE TDD. This could make roaming difficult on LTE. The rest of the bands have yet to be used, but will be used in the future. It will have to be determined how to fit more bands on an LTE device without sacrificing portability.

And then there is some confusion about 4G. Contrary to popular belief, the current version of LTE 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 called LTE-Advanced did. Neither did WiMAX or HSPA+. However, the American and Canadian network operator’s influence caused the ITU to revise their specification on 4G to include any wireless technology evolved significantly from 3G technologies. Most technophiles believe that the IMT-Advanced specification should determine what can be considered 4G, while most business people prefer the newer definition for 4G. In this overview we take the ITU’s revised specification as the standard for 4G.

Conclusion

LTE is a significant leap in optimized cellular wireless technology. If you are interested in learning the technical details of LTE and its 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.

Driven by the increasing use of smartphones and other advanced portable devices, mobile service providers are experiencing an unprecedented and continuing increase in mobile data traffic. But smartphones are just the tip of the iceberg. Consumer and enterprise end users have been just as quick to adopt advanced laptop and netbook computers and other handheld devices that can address a variety of mobile communications, information and entertainment needs.

As technology improves, new devices and transaction capabilities will emerge built on broadband ready notebooks and tablets that will also increase mobile data traffic. While others, such as connected cars, pervasive multimedia, alternative and virtual reality, and highly reliable machine-to-machine connections will further stretch mobile network capacity.

Whatever the device or business model, analysts and mobile service providers agree that mobile data traffic will continue to rise exponentially. The demand being placed on wireless networks requires leaps in network performance and capability. LTE is the standard to meet that demand.