Long Term Evolution: What is 4G LTE and how does it work?

LTE stands for Long Term Evolution, and it is a cellular technology that enables the fourth generation of mobile networks. Fourth-generation mobile networks, also known as 4G, are currently the most widely deployed mobile networks today. While the fifth generation of mobile networks (5G) has already been introduced, it hasn’t yet achieved the same penetration level as the 4G networks.

LTE or Long Term Evolution is a fourth-generation cellular technology standard that enables voice calls, text messages and mobile data on 4G LTE compatible mobile phones. 4G LTE uses bigger carrier bandwidths than 2G and 3G networks and delivers peak data rates of up to 3 Gbps with LTE Advanced Pro.

4G is the next evolutionary step after the third-generation (3G) mobile networks. 3G can be enabled by two key technologies, UMTS (Universal Mobile Telecommunications Systems) and CDMA2000, but 4G only uses LTE to provide a migration path to all 3G networks. LTE is the primary 4G technology that allows UMTS and CDMA2000 3G networks to migrate to 4G.

While WiMAX, Worldwide Interoperability for Microwave Access, is another technology that can address the requirements for 4G networks, LTE has been the primary technology path that is interoperable with key 2G and 3G technologies, including GSM, cdmaOne, UMTS, TD-SCDMA, and CDMA2000. If a mobile phone is connected to the LTE technology, it shows the LTE, LTE+, 4G or 4G+ symbols.

LTE is shown on a mobile phone as 4G, LTE, 4G+ or LTE+
— LTE is shown on a mobile phone as 4G, LTE, 4G+ or LTE+ —

What does LTE technology mean?

LTE is the next evolutionary step after 3G technologies UMTS, CDMA2000 and TD-SCDMA, and it supports voice calls, texts and significantly higher data rates than 3G. The latest LTE enhancement, LTE Advanced Pro, can deliver peak data rates of up to 3 Gbps and average data rates of around 80 to 100 Mbps.

4G is an umbrella terminology that consists of cellular technologies that meet the requirements specified by 3GPP (Third Generation Partnership Project) for the fourth generation of mobile networks. The original LTE technology was introduced as part of 3GPP Release 8 in 2009. Since then, LTE has seen two significant enhancements, including LTE Advanced and LTE Advanced Pro.

The latest LTE enhancement, LTE Advanced Pro, can offer download speeds of around 50 to 80 Mbps for an average mobile phone user. The peak download throughput for LTE Advanced Pro is up to 3 Gbps. LTE-Advanced was specified in 3GPP release 10, whereas LTE-Advanced Pro was specified in release 13. The fifth generation of mobile networks, 5G, is the successor of LTE and was specified in 3GPP Releases 15 and 16. Please look at the table below that provides a summary of how 4G LTE fits with all other technologies.

GenerationCellular TechnologyEnhancements
IS-95 (cdmaOne)
GPRS and EDGE for GSM networks

IS-95 A and B for cdmaOne
EVDO for CDMA2000
4GLTELTE Advanced
LTE Advanced Pro
Table showing how LTE fits with other cellular technologies

What is the maximum speed of 4G LTE?

On cell phones, including iPhone, Android, Windows or any other SIM-enabled devices, the fourth-generation mobile cellular technology is displayed by LTE, LTE+ or 4G and 4G+. If your cell phone shows the LTE or 4G symbol, it means the basic LTE network is serving your phone. However, if your phone shows 4G+ or LTE+, you are served by LTE-Advanced or LTE-Advanced Pro networks.

The maximum LTE mobile data speed depends on which flavour (release) of LTE a mobile operator offers. The maximum theoretical download speed of LTE networks is up to 300 Mbps which goes up to 1 Gbps for LTE Advanced and 3 Gbps for LTE Advanced Pro. However, in real life, we get average speeds that are considerably lower and result from factors like the number of simultaneous users, the distance between the user and the base station and obstacles like buildings.

Just because your phone shows a full 4G signal (signal bar) doesn’t guarantee super high 4G speeds. In busy hours, you may experience slower speeds even if you get a full signal. As a result, the average speeds are considerably lower, mainly in tens of Mbps. According to the tests we carried out for average LTE speeds, the download speeds are around 15 to 20 Mbps with LTE and 50 to 80 Mpbs with LTE Advanced.

In addition to high-speed internet on the mobile phone, 4G LTE technology also supports essential mobile services, including voice calls, text messaging (SMS), picture messaging (MMS) etc. Like its earlier 3G counterpart, 4G SIMs also exist in both voice/text & data and data-only formats. The former can be used in smartphones while the latter in mobile broadband dongles, tablets or any other mobile device. If you live in an area with decent 4G coverage, you can potentially use LTE as an alternative to fixed broadband service.

LTE is a packet-switched technology

Unlike earlier technologies that support both circuit-switching and packet-switching, LTE networks use packet-switching to deliver all services, including voice calls, text messages and mobile data. 4G LTE core network, EPC works alongside IMS to enable IP-based voice calls and SMS.

LTE or Long Term Evolution is the primary cellular technology that provides a 4G migration path to key 3G technologies, including UMTS (Universal Mobile Telecommunication Service) and CDMA2000. As a result, LTE streamlines the mobile network evolution by providing a single evolution path to all earlier technologies.

LTE uses the network resources efficiently, reduces the latency in data transfer, and reduces overall network complexity. Unlike 2G GSM and 3G UMTS networks, 4G LTE networks do not have a separate base station controller entity and use the radio base station, eNodeB, to perform the control tasks.

Have a look at the diagram below that shows that the radio network in 4G is a bit leaner compared to 3G UMTS. GSM and UMTS networks have BSC (Base Station Controller) and RNC (Radio Network Controller), respectively, to provide the radio network control function, but in 4G, this responsibility sits with the 4G base station Evolved Node B (eNodeB).

A simplified network diagram that shows how 4G LTE and 3G UMTS work together
— A simplified network diagram that shows how 4G LTE and 3G UMTS work together —

In 2G and 3G networks, conventional voice calls are only possible through the circuit-switched part of the network. LTE networks have the Voice over LTE (VoLTE) capability, which requires the LTE core network, Evolved Packet Core (EPC), to work with another network entity IP Multimedia Subsystem (IMS), to deliver IP-based voice calls and text messages. However, LTE also supports a 2G/3G circuit-switched fallback option (CSFB) that allows voice calls through a 3G or 2G network in areas where 4G is not available, or the user device does not support VoLTE. We have a dedicated post on Voice over LTE that can provide you with an in-depth view of VoLTE technology.

LTE supports both FDD and TDD duplex schemes

FDD stands for Frequency Division Duplex, and TDD stands for Time Division Duplex. FDD uses two separate frequency bands for downlink and uplink transmission. On the other hand, TDD uses the same frequency band, but downlink and uplink transmission take place at separate timeslots.

4G LTE networks support both FDD and TDD duplex schemes to support interoperability with 3G technologies using both FDD and TDD. While the FDD deployment is more common for technologies like UMTS and CDMA2000, the TDD duplex scheme is relevant for TD-SCDMA, which is TDD-based 3G technology.

Also, in addition to the support for full-duplex communication, i.e. simultaneous two-way communication, LTE networks also support half-duplex FDD deployment where the base station (eNodeB) can send and receive simultaneously, but the mobile phone cannot do the same. I have written a dedicated post on half and full-duplex FDD and TDD deployments in 4G LTE, which provides the details of what the radio frame structure looks like.

LTE uses OFDMA for downlink and SC-FDMA for uplink

The primary transmission scheme in 4G LTE networks is OFDM which stands for Orthogonal Frequency Division Multiplexing. OFDM is a multi-carrier transmission scheme and requires the available carrier to be divided into smaller sub-carriers of 15 kHz each. Each sub-carrier is individually modulated with the information signal. OFDM is robust, spectrally efficient and can utilise both time and frequency domains.

The multiple access techniques in LTE are based on the OFDM technology. Unlike GSM and UMTS, LTE uses separate multiple-access techniques for the uplink and the downlink. LTE employs OFDMA for downlink communication and SC-FDMA for uplink communication.

OFDMA or Orthogonal Frequency Division Multiple Access is a multi-user version of OFDM that enables simultaneous communication with multiple users. SC-FDMA or Single Carrier Frequency Division Multiple Access is based on OFDM, but it employs a single carrier instead of multiple carriers.

SC-FDMA is more power-efficient than OFDMA, which is why it is used in uplink communication to ensure a better battery life for mobile phone users. It achieves power efficiency through a lower Peak-to-Average Power Ration (PAPR) than OFDMA. Please have a look at my dedicated post on OFDM, OFDMA and SC-FDMA for details and visual representation of these concepts.

LTE uses QPSK and QAM for digital modulation

LTE is based on the OFDM transmission scheme, also known as multi-carrier modulation. OFDM itself is not a modulation technique, but it allows the modulation of multiple sub-carriers individually through the QPSK or QAM digital modulation schemes.

QAM stands for Quadrature Amplitude Modulation, and QPSK stands for Quadrature Phase Shift Keying. Higher modulation orders in LTE lead to improved throughputs by extracting a higher number of bits per symbol. LTE can use QPSK, 16 QAM, 64 QAM, whereas LTE Advanced and LTE Advanced Pro support 256 QAM.

In LTE, there can be multiple deployment combinations with variations to channel bandwidth and modulation techniques. For example, when an LTE deployment uses a 20 MHz channel with 64 QAM (Quadrature amplitude modulation), the network can offer up to 300 Mbps in the downlink and 75 Mbps in the uplink.

Have a look at my dedicated post on 4G LTE modulation techniques for details of what QPSK and QAM do in LTE, LTE Advanced and LTE Advanced Pro networks.

LTE employs spatial multiplexing through MIMO technology

MIMO stands for Multiple Input Multiple Output, and it is an antenna technology initially introduced in mobile networks when 3G UMTS saw the HSPA+ enhancement. The MIMO technology requires multiple antennas at the transmitter and the receiver to send and receive data in multiple streams. MIMO in 4G LTE networks is used primarily to improve data rates through spatial multiplexing.

Spatial Multiplexing, also known as Space Division Multiplexing, consists of spatially separated antenna elements that transmit the overall data payload (information) in multiple data streams. Each stream carries a unique piece of data. These separate streams are combined into a single data stream at the receiver to retrieve the overall data payload.

While spatial multiplexing is the key reason for using MIMO in 4G LTE, MIMO also delivers other benefits, including antenna diversity and beamforming. Diversity improves the signal quality against multipath fading, whereas beamforming offers directivity by pointing the signal towards particular user devices.

The original LTE launch, as per 3GPP release 8, employed a 4 x 4 MIMO configuration in the downlink and a 2 x 2 MIMO configuration in the uplink. LTE Advanced and LTE Advanced Pro use 8 x 8 in the downlink and 4 x 4 in the uplink. The higher the MIMO configuration, the higher the benefits in terms of bit rates and signal quality.

Have a look at our dedicated post on MIMO in 4G LTE, which dives into the details.

LTE can increase bandwidths through Carrier Aggregation

In 4G LTE networks, Carrier Aggregation or CA is a technique that allows the network to combine multiple frequency carriers into one. As per 3GPP release 8, the original LTE networks support flexible bandwidths of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz. Carrier Aggregation was introduced in LTE Advanced as per 3GPP release 10, allowing it to combine or aggregate up to five (5) carriers.

Since the largest bandwidth of an individual carrier in 4G LTE is 20 MHz, carrier aggregation allows LTE Advanced to deliver a maximum overall bandwidth of 5 x 20 MHz = 100 MHz. LTE Advanced Pro goes a step further and allows aggregation of up to 32 (thirty-two) frequency carriers to deliver a maximum carrier bandwidth of 32 x 20 = 640 MHz.

According to 3GPP, there are three carrier-aggregation scenarios: intra-band contiguous, which uses adjacent carriers within a single frequency band, intra-band non-contiguous, which uses non-adjacent carriers within the same band and inter-band non-contiguous, which uses carriers across separate frequency bands.

Have a look at our dedicated post on Carrier Aggregation (CA) for 4G LTE networks.

Here are some helpful downloads

Thank you for reading this post. I hope it helped you in developing a better understanding of cellular networks. Sometimes, we need extra support, especially when preparing for a new job, studying a new topic, or buying a new phone. Whatever you are trying to do, here are some downloads that can help you:

Students & fresh graduates: If you are just starting, the complexity of the cellular industry can be a bit overwhelming. But don’t worry, I have created this FREE ebook so you can familiarise yourself with the basics like 3G, 4G etc. As a next step, check out the latest edition of the same ebook with more details on 4G & 5G networks with diagrams. You can then read Mobile Networks Made Easy, which explains the network nodes, e.g., BTS, MSC, GGSN etc.

Professionals: If you are an experienced professional but new to mobile communications, it may seem hard to compete with someone who has a decade of experience in the cellular industry. But not everyone who works in this industry is always up to date on the bigger picture and the challenges considering how quickly the industry evolves. The bigger picture comes from experience, which is why I’ve carefully put together a few slides to get you started in no time. So if you work in sales, marketing, product, project or any other area of business where you need a high-level view, Introduction to Mobile Communications can give you a quick start. Also, here are some templates to help you prepare your own slides on the product overview and product roadmap.

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