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What Is LTE?

Long-Term Evolution

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.

LTE Physical Layer

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:

References

[1] Dahlman, E., Parkvall, S., and Sköld, J.. 4G LTE / LTE-Advanced for Mobile Broadband. Kidlington, Oxford: Academic Press, 2011. pp. 112–118.