LTE - Long Term Evolution
LTE stands for Long Term Evolution and it is commonly marketed as 4G LTE. This is the standard of mobile wireless communication which provides high transmission speeds, low latency and high network capacity. The 4G means that LTE is the replacement for older mobile telecommunication standards, i.e. 3G (UMTS and HSPA) and 2G (GSM and EDGE). The principle of operation and network architecture are very similar, so it is named as an evolution. However, the performance of LTE is much higher than older techniques.
LTE was proposed as the international standard in 2004 by NTT DoCoMo (Japanese mobile carrier). The first version of the standard was finalized in December 2008 by the 3GPP organization. The first commercial LTE network was launched by the Scandinavian carrier TeliaSonera a year later.
The first version of 3G network (i.e. UMTS) was supposed to provide 2 Mbps data transmission speed, however the achievable results were much lower, i.e. up to 384 kbps. Therefore, the main goal of LTE network was to achieve the assumed throughputs. The LTE network is able to provide up to 150 Mbps downlink throughput in 20 MHz cell. In case of 5 MHz cells (this bandwidth is used in 3G networks), the achievable throughput is 4 times lower, i.e. about 37 Mbps.
The uplink transmission is 3 times slower than in the downlink direction, because less efficient modulation (16 QAM instead of 64 QAM) and a single antenna are used. Therefore, the maximum uplink data transmission speed is 50 Mbps in 20 MHz channel.
The second advantage of LTE networks is much smaller latency in the comparison to 3G networks, including HSPA+. The 10 ms result was achieved thanks to simpler network architecture, i.e. most of the operations are implemented in the base station (called as eNodeB). Also, all data are sent using IP protocol, so the QoS (Quality of Service) solutions known from IP networks are applied.
Duplex transmission modes: FDD vs TDD
Generally, LTE system can operate in two transmission duplex modes, i.e.: FDD (Frequency Division Duplex) and TDD (Time Division Duplex). The LTE FDD requires paired frequency channels for uplink and downlink data transmission. In this mode, the operator has two channels with the same bandwidth, but they are placed in different frequency ranges. The main advantage of FDD mode is the possibility of downlink and uplink data transmission at the same time. Also, the FDD has larger cell range and higher system capacity than the TDD mode.
The LTE TDD is perfect solution for carriers who have the licence for unpaired spectrum. The main advantage of the TDD mode is the usage of the same frequencies for downlink and uplink data transmissions. However, it is not possible to transmit data in both direction at the same time, because the mobile phone has to switch between downlink and uplink mode. The operator can also adjust the link asymmetry ratio and select how many time slots are used for downlink and uplink transmissions. The most asymmetric configuration assumes 8 subframes for downlink data, 1 subframe for uplink data and 1 so-called special switching subframe as the guardian period. Therefore, the maximum downlink throughput in TDD mode is 120 Mbps in 20 MHz channel, but the uplink data speed is only 15 Mbps.
LTE is so-called all IP network, therefore the traditional voice calls can not be sent by this kind of network. However, this inconvenience was overcome by the solution called VoLTE. VoLTE stands for Voice over LTE, but in the practice it means VoIP over LTE. However, the Skype or the WhatsApp run via LTE network is not the VoLTE. This kind of transmission is treated the same as other packets and there is no guarantee of the service quality.
VoLTE requires the IMS (IP Multimedia Subsystem) in the carrier network and the mobile phone with special software (i.e. compatible with carrier’s IMS). If these preconditions are met then the voice call will use the resources reserved in the LTE network and all voice packet will be handled with the highest priority.
The main advantage of VoLTE solution is very high capacity. This is very important for mobile carriers, because they can serve 3 times more voice connection than it was possible in 3G networks using the same bandwidth.
Some people claim that LTE is not the true 4G network, because it does not provide 1 Gbps transmission speed. Therefore, 3GPP organization has standardized in March 2011 new communication standard (3GPP Release 10) which is called as LTE Advanced.
The main improvement is simultaneous transmission in up to five 20 MHz channels, so 100 MHz bandwidth is used thanks to Carrier Aggregation feature. This gives the downlink throughput up to 750 Mbps. However, the Release 10 predicts an increase in the number of special streams from 2 to 8. As the result, LTE Advanced provides up to 3 Gbps downlink throughput using 100 MHz (5x 20 MHz) bandwidth and MIMO 8x8.
The maximum uplink transmission is slower, because up to 4 spatial streams are used, so the maximum uplink throughput is 1.5 Gbps. However, the most important improvements of LTE Advanced standard are related to better performance at cell edges. This is introduced in the release 11 by CoMP (Coordinated Multi Point) operation functionalities. The idea is that the mobile phone receives the same signal from at least two base stations, whereby the network coverage and reliability is much better.