Long-Term Evolution (LTE) is the project name of a new, high performance air interface for mobile communication systems. Developed by the Third Generation Partnership Project (3GPP), LTE is the evolution of the Universal Mobile Telecommunication System (UMTS) towards an all-IP broadband network. LTE's evolved radio access technology—the E-UTRA— provides a framework for increasing data rates and overall system capacity, reducing latency, and improving spectral efficiency and cell-edge performance. It is documented in the 3GPP Release 8 and Release 9 specifications. This LTE overview gives some of the highlights.
OFDMA-based: Unlike UMTS, which is based on wideband code division multiple access (W-CDMA) technology, LTE is based on orthogonal frequency-division multiple access (OFDMA). In the downlink, an OFDMA-based transmission scheme—together with multiple-access techniques—provides high data-rate capacity and high spectral efficiency. In this regard, LTE is similar in concept to Mobile WiMAX™, another emerging technology for wireless broadband access, although the systems operate with different frame structures, sub-carrier spacing and channel bandwidths.
A new, OFDMA-based scheme called single carrier frequency division multiple access (SC-FDMA) was developed for the LTE uplink. SC-FDMA enables a lower peak-to-average ratio (PAR) to conserve battery life in mobile devices.
Flexible modulation schemes: The downlink supports QPSK, 16QAM, and 64QAM data modulation formats, and the uplink supports BPSK, QPSK, 8PSK, and 16QAM.
MIMO: At present, LTE offers a 100-Mbps download rate and 50-Mbps upload rate for every 20 MHz of spectrum. Support is intended for even higher rates (up to a maximum of 326.4 Mbps in the downlink) using multiple antenna configurations. LTE supports single-user multiple input/multiple output (SU-MIMO) and multiple-user multiple input/multiple output (MU-MIMO) antenna configurations of up to 4 x 4 MIMO. These should enable up to 10 times as many users per cell as 3GPP's original W-CDMA technology. See Keysight's application note, LTE Operation and Measurement--Excerpts on MIMO Test.
Spectral efficiency: LTE also features a scalable bandwidth from 1.4 to 20 MHz in both the downlink and the uplink, with subcarrier spacing of 15 kHz and 7.5 kHz possible in the case of multimedia broadcast multicast service (MBMS). Targets for spectral efficiency over 3GPP Release 7 high-speed packet access (HSPA) are three to four times in the downlink and two to three times in the uplink. Sub 5-ms latency will be provided for small IP packets.
FDD and TDD modes: To support as many frequency band allocations as possible, both paired and unpaired spectrum operation is supported using frequency division duplex (FDD) and time division duplex (TDD) techniques, respectively. Paired spectrum operation is known as FD-LTE and unpaired spectrum as TD-LTE.
Co-existence with legacy systems: LTE is designed to support voice as well as data in the packet domain. However, as LTE evolves toward an all-IP network, it will co-exist with legacy systems including 3GPP HSPA, W-CDMA UMTS, and GSM/GPRS/EDGE. In conjunction with the 3GPP Evolved Packet Core (EPC) network, LTE will support inter-domain handovers between packet-switched and circuit-switched systems. Specifications for the EPC network are being developed in a concurrent project known as System Architecture Evolution (SAE).
For more LTE overview, download Keysight's free LTE application note, 3GPP Long Term Evolution: System Overview, Product Development, and Test Challenges.