Long-Term Evolution (LTE) is the air interface supporting fourth generation cellular networks. LTE is specifically designed for packet data communications, where the emphasis of the technology is high spectral efficiency, high peak data rates, low latency, and frequency flexibility. The LTE specifications were developed by the Third Generation Partnership Project (3GPP).
GSM and UMTS are the predecessors of the LTE air interface and are referred to as second generation (2G) and third generation (3G) technologies, respectively. GSM was developed as a circuit switched network meaning that radio services are configured at the user’s request and resources remain allocated until terminated by the network controller. This type of operation is well suited to supporting voice calls. Eventually, GSM was enhanced to support low data rate services with packet switching capability but data rates were limited by GSM’s air interface, time division multiple access (TDMA). In TDMA, each user is assigned to a particular channel (frequency band) and time slot which serves to limit capacity as the channel spacing is only 200 kHz.
UMTS uses code division multiple access (CDMA) as its air interface. In CDMA, active users transmit simultaneously over the allocated bandwidth, typically 5 MHz. Signals are separated from each other by the use of orthogonal variable spreading factor (OVSF) spreading codes. The advantage of OVSF codes is that resources can be allocated asymmetrically among the active users. UMTS supports both circuit switched services for voice calls and packet switched for data sessions. Due to its larger bandwidth and superior spectral efficiency, UMTS can support higher data rates than GSM.
Unlike GSM and UMTS, LTE is a purely packet switched network in which both voice and data services are carried by IP. LTE uses orthogonal frequency division multiple access (OFDMA) in which the spectrum is divided into resource blocks (RB) that are composed of twelve 15 kHz subcarriers. By dividing the spectrum in such a way, complicated equalizers are no longer necessary to mitigate frequency selective fading. LTE supports higher order modulation schemes up to 64-QAM along with bandwidth allocations that can be as large as 20 MHz. In addition, LTE makes use of MIMO so that very high theoretical data rates can be achieved (75 Mbps in the uplink and 300 Mbps in the downlink for Release 8).
Second and third generation cellular networks consist of an interface to the public telephone or IP network, a radio network controller (RNC) that allocates radio resources among the users, a base station (referred to as a Node B in UMTS) that transmits and receives signals to and from the users, and user devices (MS for GSM and UE for UMTS). The LTE access network is similar with the exception that the RNC functionality has been pushed down into the enhanced Node B (eNB). The flatter architecture reduces the time required to establish data services resulting in lower latency. The architecture is shown below.
Initially standardized in 3GPP Release 8, the LTE standards continue to evolve over multiple releases to capture requirements that lead to improved data throughput, lower latencies, and increasingly flexible configurations. After the release is frozen, 3GPP continues revisions of the associated standards to correct errors and fill in omissions, but no new features are introduced.
Release 8 introduced LTE for the first time. The Release 8 functionality set was frozen in the March 2009 standards release (SA#43). The release consisted of a completely new radio interface and core network, which enabled substantially improved data performance compared with previous systems. Highlights from Release 8 include:
Up to 300 Mbps downlink and 75 Mbps uplink
Latency as low as 10 ms
Bandwidth sized in 1.4, 3, 5, 10, 15, or 20 MHz blocks to allow for a variety of deployment scenarios
Orthogonal frequency domain multiple access (OFDMA) downlink
Single-carrier frequency domain multiple access (SC-FDMA) uplink
Multiple-input multiple-output (MIMO) antennas
Flat radio network architecture, with no equivalent to the GSM base station controller (BSC) or UMTS radio network controller (RNC), and functionality distributed among the base stations (enhanced NodeBs)
All IP core network, the System Architecture Evolution (SAE)
The Release 9 functionality set was frozen in the March 2010 standards release (SA#47). LTE Release 9 brought refinements to LTE Release 8, and it introduced some new service features and network architecture improvements. Highlights from Release 9 include:
Evolved multimedia broadcast and multicast service (eMBMS) for the efficient delivery of the same multimedia content to multiple destinations
Location services (LCS) to pinpoint the location of a mobile device by using assisted GPS (A-GPS), observed time difference of arrival (OTDOA), enhanced cell-ID (E-CID).
Dual layer beamforming
The Release 10 functionality set was frozen in the June 2011 standards release (SA#52). LTE Release 10 is considered to be the start of LTE-Advanced. It significantly improved data throughput and extended cell coverage. Highlights from Release 10 include:
Higher order MIMO antenna configurations supporting up to 8×8 downlinks and 4×4 uplinks
Data throughput of up to 3 Gbps downlink and 1.5 Gbps uplink
Carrier aggregation (CA), allowing the combination of up to five separate carriers to enable bandwidths of up to 100 MHz
Relay nodes to support Heterogeneous Networks (HetNets) containing a wide variety of cell sizes
Enhanced intercell interference coordination (eICIC) to improve performance toward the edge of cells
The Release 11 functionality set was frozen in the March 2013 standards release (SA#59). LTE Release 11 included refinements to existing Release 10 capabilities, including:
Enhancements to Carrier Aggregation, MIMO, relay nodes, and eICIC.
Coordinated multipoint transmission and reception (CoMP) to enable simultaneous communication with multiple cells.
Enhanced PDCCH (EPDCCH), which uses PDSCH resources for transmitting control information. Previously, from Release 8, control information could only be transmitted in the PDCCH region of subframes.
Introduction of new frequency bands.
The Release 12 functionality set was frozen in the March 2015 standards release (SA#67). Highlights from Release 12 include:
Enhanced small cells for LTE, introducing a number of features to improve the support of HetNets.
Intersite carrier aggregation to coordinate the capabilities and backhaul of adjacent cells.
Machine-to-machine (M2M) communication, also referred to as machine-type communication (MTC).
Device-to-device (D2D) interface to support public safety communications systems, and proximity services (ProSe) for discovery and group communications. The LTE D2D interface is called a sidelink.
Interworking between LTE and WiFi or HSPDA.
Higher order modulation schemes of up to 64-QAM.
LTE operation in unlicensed spectrum.
The LTE standard releases from 13 onwards are known as LTE-Advanced Pro. Highlights from Release 13 include:
New working group introduced for programming of mission critical applications.
Carrier aggregation (CA), allowing the combination of up to 32 separate carriers to enable bandwidths of up to 640 MHz.
LTE operation in a combination of licensed and unlicensed spectrum.
Intersite carrier aggregation, to coordinate the capabilities and backhaul of adjacent cells.
Enhanced machine-to-machine (M2M) communication, also referred to as machine-type communication (MTC).
Interworking with Wi-Fi, licensed assisted access (at 5 GHz).
Further enhancements of public safety features, such as D2D and ProSe, which include small-cell dual-connectivity and architecture changes.
Single-cell point to multipoint.
New antenna techniques, such as 3D/FD-MIMO, which include study of high-order MIMO systems with up to 64 antenna ports.
Advanced receivers to maximize the potential of large cells.
Work on latency reduction.
The LTE radio access network is comprised of the following protocol entities.
Packet Data Convergence Protocol (PDCP)
Radio Link Control (RLC)
Medium Access Control (MAC)
The Physical Layer (PHY)
The first three protocol entities handle tasks such as header compression, ciphering, segmentation and concatenation, and multiplexing and demultiplexing. The physical layer handles coding and decoding, modulation and demodulation, and antenna mapping. The figure shows the delineation between the physical layer and higher layers.
LTE Toolbox™ focuses on the physical layer, which is highlighted in red in the preceding figure. It also supports interfacing with portions of the RLC and MAC layers, which are highlighted in blue. The primary features of the LTE physical layer are OFDM modulation, including the time-frequency structure of the resource blocks, adaptive modulation and coding, hybrid-ARQ, and MIMO.
Downlink Channel Mapping
System downlink data follows the indicated mapping between logical channels, transport channels, and physical channels. The red outline contains LTE Toolbox downlink functionality for physical channels, transport channels, and control information.
For more information, see Downlink Channels or the specific channel category of interest:
Uplink Channel Mapping
System uplink data follows the indicated mapping between logical channels, transport channels, and physical channels. The red outline contains LTE Toolbox uplink functionality for physical channels, transport channels, and control information.
For more information, see Uplink Channels or the specific channel category of interest:
Sidelink Channel Mapping
System sidelink data follows the indicated mapping between logical channels, transport channels, and physical channels. The red outline contains LTE Toolbox sidelink functionality for physical channels, transport channels, and control information.
For more information, see Sidelink Channels or the specific channel category of interest:
 Nohrborg, Magdalena, for 3GPP. LTE https://www.3gpp.org/technologies/keywords-acronyms/98-lte.
 Dahlman, E., Parkvall, S., and Sköld, J.. 4G LTE / LTE-Advanced for Mobile Broadband. Kidlington, Oxford: Academic Press, 2011. pp. 112–118.