LTE (Long Term Evolution) integrates many individual technical solutions into a single comprehensive technology and is developed by the 3GPP consortium. The LTE technology is standardized and as the GSM it is identical all over the world and can be used by mobile providers as a third option to previously offered 2G and 3G. In Poland, LTE services have been available in the commercial offer since 2011.
LTE radio interface uses OFDM (Orthogonal Frequency-Division Multiplexing) to transmit data from the base station to the phone (downlink). Uplink transmission uses SC-FDMA (Single Carrier-Frequency Division Multiple Access – a modified OFDMA). It is one of the features not available in WCDMA-based UMTS.
As part of LTE, several solutions were developed allowing the end user to reach uplink rates up to 300 Mb/s, and up to 1 Gb/s with LTE-A (LTE Advanced). The operators can also use the currently allocated frequencies for quick network deployment without new concessions with an increase in network capacity, improved resource management, and customization.
LTE specification (according to 3GPP Release 8):
the technology can enable theoretical download speed (in radio layer) of 150 Mb/s with a channel width of 20 MHz,
4x4 MIMO, channel width of 20 MHz,
maximum upload speed – 50 Mb/s with a channel width of 20 MHz,
at least 200 user per cell,
small packet delay < 5 ms,
optimum cell radius up to 5 km,
FDD (Frequency Division Duplex) and TDD (Time Division Duplex) mode,
high quality transmission at a speed of the movement of 120 km/h (functional up to 350 km/h).
LTE uses three modulation types:
1. QPSK 2 bits per symbol, 2. 16QAM 4 bits per symbol, 3. 64QAM 6 bits per symbol.
Fig. 1. LTE modulation constellation
Uplink modulation type is QAM (Quadrature Amplitude Modulation). The higher the number of bits per symbol, the larger the volume of data sent and better bandwidth utilization. However, the higher the number of bits per symbol, the lower the noise immunity. Another modulation used in LTE is QPSK. It increases the effectiveness of bandwidth utilization without affecting the bit error rate (BER).
LTE was designed to provide high data rates. Unlike 2G and 3G, it does not support voice calls.
LTE means high speed - but there are several factors to consider. The main factor is the limitations of the technology available to the service providers - data rates depend on the bandwidth allocated for those services. Another factor is the network load. The data rates may also be affected by signal strength. In 2G, the voice calls are possible at a distance of up to 25 km from the base station. In 3G, the range is limited to 10 km. In LTE, with a decrease in signal quality and strength, the efficiency is significantly reduced. Often, when the device shows minimum coverage (one bar), the data rates will be lower compared to the 3G network in the same area.
3G also better handles low signal strength conditions. If you decide to use the LTE technology, check the coverage in your area. Weak LTE network signal can be amplified with compatible antennas included by some service providers with the equipment.
LTE is sensitive to network signal quality and strength. In many cases, it may be beneficial or even required to use external antennas. LTE technology requires two antennas (most modems and routers have two independent antenna sockets). The antennas have the same orientation, but the axis of polarization of one of them must be rotated by 90 degrees in relation to the other one due to simultaneous vertical and horizontal polarization used by the MIMO technology to ensure higher data rates and noise immunity. Antenna system manufacturers also offer integrated dual antennas for LTE technology.
LTE developers constantly work on increasing the network capacity by introducing new features, including carrier aggregation or simultaneous multiple transmissions.
Fig. 2. Carrier aggregation in LTE
The feature was implemented in LTE-A (LTE-Advanced). The operator can aggregate up to 5 component carriers, not necessarily adjacent, with a maximum bandwidth of 20 MHz to the maximum aggregated bandwidth of 100 MHz for uplink rates up to 1 Gbps.
Fig. 3. LTE Advanced logo
Another task is to optimize the network load and adapt it to the type of content driving the highest traffic - usually video transmission. LTE Broadcast (eMBMS) means accelerated network operation without a need to transmit the same data multiple times. The principle of operation is simple: the network broadcasts the video content and the clients can connect anytime to download it. This rule does not work for all videos but can be very efficient in live streaming.
Another service worth noting is VoLTE (Voice over LTE), i.e. voice calls over LTE. When the phone is connected to the LTE network, during a voice call, it must switch to an older 3G network standard. VoLTE eliminates switching between networks and improves call quality.