What is LTE?

LTE stands for Long Term Evolution. In order to understand why it's necessary, let's look at the evolution of mobile technologies, from the first generation (1G) to the fourth generation (4G).

The data speed set for 4G standard by ITU Radiocommunication Sector(ITU-R) is rather unreachable despite the amount of money manufacturers invested to achieve it. To get around this problem, 3GPP introduced LTE, which refers to the technology that is used in pursuit of 4G standard — the technology could be labeled 4G if it provides a substantial improvement over 3G. 4G LTE is a marketing term that allows service providers to claim next generation connectivity without having to achieve the 4G standard requirement.

Technology	1G	2G	2.5G	3G	4G
Design began	1970	1980	1985	1990	2000
Implementation	1981	1991	1999	2001	2010
Services	Analog voice	Digital voice, short message	Higher capacity, packetized data	Higher capacity data rates up to 2 Mbps	Higher capacity, completely IP-oriented, multimedia, data to hundreds of megabits
Access Technology	AMPS, TACS, NMT	TDMA, CDMA GSM	GPRS, EDGE	WCDMA, CDMA2000	One standard
Data rate	1.90 kbps	14.4 kbps	384 kbps	2 Mbps	> 200 Mbps
Multiplexing	FDMA	TDMA, CDMA	TDMA, CDMA	CDMA	OFDM
Core Network	PSTN	PSTN	PSTN, packet network	Packet network	Internet
Architecture		Circuit switch	Pacet switch	Circuit switch, packet switch	Packet switch

The inability to achieve 4G capacity is due to the lack of carrier aggregation and because phones do not have multiple antennae. Multiple Input Multiple Output (MIMO) is a technique for sending and receiving more than one data signal on the same channel at the same time by using more than one antenna. Combining carrier aggregation and MIMO brings us to LTE Advanced (LTE-A).

LTE supports scalable bandwidths of up to 20 MHz and provides peak data rates of 300 Mbps downlink and 75 Mbps uplink. LTE is also designed for reduced service latency, optimized packet transmission and simplified implementation complexity for cost reduction. LTE-A supports bandwidth of up to 100 MHz and provides peak data rate 3 Gbps downlink and 1.5 Gbps uplink.

Carrier aggregation allows operators to treat multiple radio channels in different or the same frequency bands as if they were one to produce higher speeds. In turn, this allows a user to perform activities like streaming HD video much faster than ever before. Imagine that your wireless connection is a pipe. You cannot increase the size of the pipe, but you can add a second and third pipe. By combining two or more pipes, you multiply the flow rate — this is the concept behind carrier aggregation. Another advantage of carrier aggregation is that speed does not decrease, no matter how far away from the base station you are.

LTE supports deployment in the following bandwidth blocks: 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz. LTE uses two types of air interfaces: one for downlink and another for uplink. This approach optimizes wireless connections both ways and helps extends the battery life on LTE devices.

For the downlink, LTE uses the orthogonal frequency division multiple access (OFDMA), which mandates the use of MIMO. Using MIMO means devices have multiple connections to the cell tower. This results in stability of the connection and reduces latency significantly while also increasing the total throughput of a connection.

A disadvantage of MIMO is that it works better the further apart the individual carrier antennae are. On smaller phones, the noise caused by the antennae being close to each other will cause LTE performance to drop.

For the uplink, LTE uses the discrete Fourier transform spread orthogonal frequency division multiple access (DFTS-OFDMA) scheme of generating a single carrier frequency division multiple access (SC-FDMA) signal. SC-FDMA is better for uplink because it has a better peak-to-average power ratio, which is an important feature for LTE devices that need to conserve battery life. The devices usually don't have a strong and powerful signal going back to the tower.

SC-FDMA is 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 antennae on the base station for receiving.

Radio spectrum for mobile communication is available in different frequency bands of different sizes in the form of paired frequency band (uplink and downlink transmissions are assigned different frequency bands) and unpaired frequency band (uplink and downlink transmissions share the same frequency band).

There are two LTE variants:

• Frequency division duplex (FDD) variant (the most common) uses paired frequency bands. There are two carrier frequncies, i.e., one for uplink transmission (f_UL) and one for downlink (f_DL). During each frame, there are ten uplink subframes and ten downlink subframes. Uplink and down link transmission can occur simultaneously within a cell. There is a one-to-one relation between downlink and uplink subframes.
• Time division duplex (TDD) variant (also known as TD-LTE) uses unpaired frequency band. Uplink and downlink transmission are separated in time within a cell. To meet different requirements on uplink-downlink traffic asymmetries, TDD supports seven uplink-downlink configurations, which provides downlink-uplink periodicity of 5 ms or 10 ms, and downlink-to-uplink ratios from 2:3 to 9:1.

  For example, an LTE TDD network deployed on 20 MHz of spectrum uses the whole chunk as one large block for frequency allocation purposes. For network bandwidth purposes, a LTE TDD network’s spectrum can be further divided to optimize for the type of network traffic (half up and half down, mostly down and a bit up, mostly up and a bit down, and so on).

Related links

• What is Multiplexing?
• Radio Frequency (RF) Modulation Techniques Basics
• OFDM Tutorial
• MIMO Tutorial
• "A Survey on 3GPP Heterogeneous Networks", IEEE Wireless Communications, Volume 18 Issue 3, 2011
• "LTE: The Evolution of Mobile Broadband", IEEE Communications Magazine, Volume 47 Issue 4
• "Overview of 3GPP LTE-Advanced Carrier Aggregation for 4G Wireless Communications", IEEE Communications Magazine, Volume 50 Issue 2