LTE networks - what is it? Mode, structure and operating principle of LTE network

The LTE network was recently approved by the 3GPP consortium. Through the use of such a radio interface, it is possible to obtain a network with unprecedented operational parameters in terms of the maximum rate at which data is transmitted, the delay time for packet forwarding, and spectral efficiency. The authors say that the launch of the LTE network allows for more flexible use of the radio spectrum, multi-antenna technology, channel adaptation, dispatching mechanisms, organization of retransmission of data and power control.

Prehistory

Mobile broadband, which is based on the technology of transferring data packets at high speed according to the HSPA standard, has already become widely recognized by users of cellular networks. However, it is necessary to further improve their services, for example, by increasing the speed of data transmission, minimizing the delay time, as well as increasing the overall network capacity, as the requirements of users for the services of such communication are constantly increasing. It was with this in mind that the specification of the radio interfaces HSPA Evolution and LTE by the 3GPP consortium was made.

Major differences from earlier versions

The LTE network differs from the previously developed 3G system with improved technical characteristics, including the maximum speed with which information is transmitted - more than 300 megabits per second, the delay in sending packets does not exceed 10 milliseconds, and the spectral efficiency has become much higher. The construction of LTE networks can be carried out both in new frequency bands, and in those already available for operators.

This radio interface is positioned as a solution, to which operators will gradually move from the standards systems existing at the moment, this is 3GPP and 3GPP2. And the development of this interface is an important step in the development of the standard IMT-Advanced 4G networks, that is, a new generation. In fact, the LTE specification already contains most of the functions that were originally intended for 4G systems.

Principle of radio interface organization

Radio communication has a characteristic feature that is that the radio channel for quality is not constant in time and space, but depends on frequency. Here it is necessary to say that communication parameters change relatively quickly as a result of multipath propagation of radio waves. In order to maintain a constant rate of information exchange over a radio channel, there are usually a number of ways to minimize such changes, namely different methods of diversity transmission. At the same time, during the transmission of information packets, users may not always notice short-term fluctuations in the bit rate. LTE network mode assumes, as a basic principle of radio access, not reduction, but application of rapid changes in the quality of the radio channel in order to ensure the most efficient use of radio resources available at each time. This is implemented in the frequency and time domains through OFDM radio access technology.

LTE network device

What kind of system is it, you can understand, only figuring out how it is organized. It is based on the usual OFDM technology, which assumes the transmission of data over several narrow-band subcarriers. Application of the latter in combination with the cyclic prefix makes it possible to make the OFDM-based communication stable to temporal variances of radio channel parameters, and also makes it possible to practically eliminate the need for using complex equalizers on the receiving side. This circumstance is very useful for the organization of the downstream channel, since in this case it is possible to simplify the signal processing by the receiver at the main frequency, which allows reducing the cost of the terminal device itself, as well as the power consumed by it. And this becomes especially important when using the 4G LTE network together with multi-streaming.

The uplink, where the radiated power is significantly lower than in the descending one, requires the mandatory inclusion in the work of an energy-efficient method of transmitting information to increase the coverage area, reduce the power consumption of the receiving device, as well as its cost. The conducted researches have led to that now for the uplink LTE channel the single-frequency technology of information transmission in the form OFDM with the dispersion corresponding to the law of discrete Fourier transform is used. Such a solution allows to provide a smaller ratio of the average and maximum power level in comparison with the use of traditional modulation, which allows increasing energy efficiency and simplifying the construction of terminal devices.

The base resource used in the transmission of information in accordance with ODFM technology can be demonstrated in the form of a time-frequency network that corresponds to a set of OFDM symbols and sub-carriers in the time and frequency domains. LTE network mode assumes that two resource blocks are used as the main element of data transmission, which correspond to the frequency band of 180 kilohertz and the time interval of one millisecond. A wide range of data rates can be realized by combining frequency resources, setting communication parameters, including coding rate and modulation order selection.

Specifications

If we consider LTE networks, what it is, it becomes clear after certain explanations. To achieve the high targets that are established for the radio interface of such a network, its developers have organized a number of quite important points and functionalities. Each of them will be described in detail, with a detailed indication of the effect they have on such important indicators as network capacity, radio coverage area, delay time and data rate.

Flexibility of radio spectrum application

Legislative norms that operate in one or another geographic region affect how mobile communication will be organized. That is, they prescribe a radio spectrum, allocated in different frequency bands with unpaired or pairwise bands of different widths. Flexibility of use is one of the most important advantages of the LTE radio spectrum, which makes it possible to use it in different situations. The LTE network architecture allows not only to operate in different frequency bands, but also to use frequency bands having different widths: from 1.25 to 20 MHz. In addition, such a system can operate in unpaired and paired frequency bands, supporting the time and frequency duplex, respectively.

If we talk about terminal devices, then when using paired frequency bands, the device can operate in duplex or half-duplex mode. The second mode, in which the terminal receives and transmits data at different times and at different frequencies, is attractive in that it significantly reduces the requirements imposed on the characteristics of a duplex filter. Thanks to this, it is possible to reduce the cost of terminal devices. In addition, there is an opportunity for the introduction of paired frequency bands with a slight duplex separation. It turns out that LTE mobile networks can be organized almost for any frequency spectrum distribution.

The only problem in the development of radio access technology, which provides for the flexible use of radiospectrum, is to make communication devices compatible. For this purpose LTE technology implements an identical frame structure in the case of using frequency bands of different widths and different duplex modes.

Multi-antenna data transmission

The use of multi-antenna broadcasting in mobile communication systems makes it possible to improve their technical characteristics, as well as to expand their capabilities in terms of subscriber maintenance. Coverage of the LTE network involves the use of two methods of multi-antenna transmission: spaced and multithreaded, as a particular case of which the formation of a narrow radio beam is distinguished. The sparse information can be considered as a method of equalizing the signal level that comes from two antennas, which allows eliminating deep dips in the level of signals that are received from each antenna separately.

You can look more closely at the LTE network: what is it and how does it use all of the specified modes? The diversity transmission here is based on the method of space-frequency coding of data blocks, which is complemented by time diversity with a frequency shift when four antennas are used simultaneously. The diversity transmission is typically used on common downlink channels where the scheduling function can not be applied depending on the state of the communication channel. In this case, the diversity transmission can be used to transfer user data, for example, VoIP traffic. Due to the relatively low intensity of such traffic, it is impossible to justify the additional overhead that is associated with the dispatching function mentioned earlier. Due to the diversity of data transmission, it is possible to increase the cell radius and network capacity.

Multithreaded transmission for the simultaneous transmission of a number of information streams over a single radio channel involves the use of multiple receive and transmit antennas located in the terminal device and the base network station, respectively. This significantly increases the maximum data transfer speed. For example, if the terminal device is equipped with four antennas and such quantity is available at the base station, then it is quite realistic to transmit up to four data streams simultaneously on one radio channel, which in fact makes its bandwidth four times larger.

If a network with a small workload or small cells is used, then thanks to multithreaded transmission it will be possible to achieve a sufficiently high bandwidth for radio channels, and also to efficiently use radio resources. If there are large cells and a high intensity load, then the channel quality will not allow the use of the transmission in the multi-stream mode. In this case, the signal quality can be improved by using several transmit antennas to form a narrow beam for data transmission in one stream.

If we consider the LTE network - what this gives to it for achieving greater efficiency - then here it is necessary to conclude that for high-quality operation under various operational conditions, this technology implements an adaptive multithread transmission that allows to continuously regulate the number of streams transmitted simultaneously, according to the constantly changing State of the communication channel. With a good channel condition, up to four data streams can be transmitted simultaneously, which allows transmission speeds of up to 300 megabits per second with a bandwidth of 20 megahertz.

If the channel state is not so favorable, then the transmission is done in fewer threads. In this situation, the antennas can be used to form a narrow beam pattern, increasing the overall reception quality, which ultimately leads to an increase in the system capacity and the expansion of the serviced zone. In order to provide extensive coverage areas or data transmission at high speed, it is possible to transmit one data stream with a narrow beam or to use shared broadcasts on shared channels.

The mechanism of adaptation and dispatching of the communication channel

The principle of operation of LTE networks assumes that dispatching will be understood as the distribution among users of network resources for data transmission. It provides for dynamic dispatching in the descending and ascending channels. LTE networks in Russia are currently configured to balance the communication channels and overall performance of the entire system.

The LTE air interface assumes the implementation of the dispatch function, depending on the state of the communication channel. It provides data transfer at high speeds, which is achieved through the use of high-order modulation, the transfer of additional information flows, reducing the degree of coding channels, as well as reducing the number of repeated broadcasts. This involves frequency and time resources, characterized by relatively good communication conditions. It turns out that the transfer of any particular amount of data is done in a shorter period of time.

LTE networks in Russia, as in other countries, are built in such a way that the traffic of services that are busy sending packets with a small payload at the same time intervals may necessitate an increase in the amount of signaling traffic that is required for dynamic dispatching. It can even exceed the amount of information broadcast by the user. That's why there is such a thing as static dispatching of the LTE network. What this is, it becomes clear, if you say that the user is allocated a radio frequency resource, intended to transmit a specific number of subframes.

Thanks to the adaptation mechanisms, it is possible to "squeeze everything possible" out of the channel with a dynamic quality of communication. It allows you to select the channel coding and modulation scheme in accordance with what communication conditions are characterized by LTE networks. What is it will become clear if you say that his work affects the speed of data translation, as well as the probability of any errors in the channel.

Power in the uplink and its regulation

This aspect concerns the management of the power level emitted by the terminals in order to increase the network capacity, improve the communication quality, make the radio coverage area larger, reduce power consumption. In order to achieve these goals, the power control mechanisms tend to maximize the level of the useful incoming signal while simultaneously reducing radio interference.

LTE "Beeline" networks and other operators assume that signals in the uplink remain orthogonal, that is, there should be no mutual interference between users of one cell, at least, this concerns the ideal communication conditions. The level of interference that is created by users of neighboring cells depends on where the radiating terminal is located, that is, on how its signal attenuates on the way to the cell. LTE "Megaphone" network is arranged in exactly the same way. It will be correct to say this: the closer the terminal is to the neighboring cell, the higher the level of interference that it creates in it. Terminals that are located at a greater distance from the neighboring cell are able to transmit signals of greater power compared to terminals in close proximity to it.

Due to the orthogonality of the signals, in the uplink it is possible to multiplex signals from terminals of different power in the same channel on the same cell. This means that there is no need to compensate for the bursts of the signal level that arise from the multipath propagation of radio waves, and they can be used to increase the speed of data translation using adaptation mechanisms and dispatching communication channels.

Relay data

Almost any communication system, and LTE networks in Ukraine are no exception, from time to time makes mistakes in the process of data transfer, for example, due to signal fading, noise or noise. Error protection is provided by methods of retransmission of lost or distorted pieces of information intended to guarantee the provision of high quality communications. The radio resource is used much more rationally if the data relay protocol is organized efficiently. To maximize the use of the high-speed air interface, LTE technology has a dynamically efficient two-level data relay system that implements the Hybrid ARQ. It is characterized by a small overhead required to provide feedback and re-send data, supplemented by a protocol of selective repeat of a high degree of reliability.

The HARQ protocol provides the receiving device with redundant information, which enables it to correct any specific errors. HARQ relaying results in the generation of additional information redundancy, which may be required when insufficient retransmission is sufficient to eliminate errors. The retransmission of packets that have not been patched by the HARQ protocol is performed using the ARQ protocol. LTE networks on the iPhone work in accordance with the above principles.

This solution allows guaranteeing a minimum delay in the transmission of packets with low overhead costs, and reliability of communication is guaranteed. The HARQ protocol makes it possible to detect and correct most of the errors, which leads to a fairly rare use of the ARQ protocol, since this involves a lot of overhead, as well as an increase in the delay time for packet translation.

The base station is the end node that supports both of these protocols, providing a tight link between the levels of the two protocols. Among the various advantages of such an architecture is the high speed of error elimination, which remained after the HARQ operation, as well as the adjustable amount of information transmitted using the ARQ protocol.

The LTE air interface has high performance due to its main components. The flexibility of using a radio spectrum makes it possible to use this radio interface for any available frequency resource. LTE technology provides a number of functions that ensure the effective application of rapidly changing communication conditions. Depending on the status of the channel, the dispatch function gives the best resources to users. The use of multi-antenna technologies leads to a decrease in signal fading, and with the help of channel adaptation mechanisms it is possible to use methods of coding and modulation of the signal, which guarantee optimal communication quality under specific conditions.