The LTE network has recently been approved by the 3GPP consortium. Thanks to the use of such a radio interface, it is possible to obtain a network with unprecedented operational parameters in terms of the maximum speed at which data is transmitted, the delay time when sending packets, and also the 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, scheduling mechanisms, organizing data retransmission and power control.
Background
Mobile broadband, which is based on the technology of transmitting high-speed data packets according to the HSPA standard, has already become quite widely recognized by users of cellular networks. However, it is necessary to further improve their services, for example, by using an increase in the speed of data transmission, minimizing the delay time, and also increasing the overall network capacity, as the requirements of users for such communication services are constantly increasing. It was for this purpose that the specification of the HSPA Evolution radio interfaces and the LTE consortium 3GPP was made.
Major differences from earlier versions
The LTE network differs from the previously developed 3G system in improved technical characteristics, including the maximum speed at which information is transmitted - more than 300 megabits per second, packet transfer delay does not exceed 10 milliseconds, and the spectral efficiency has become much higher. The construction of LTE networks can be carried out both in the new frequency bands, as well as in the operators already existing.
This radio interface is positioned as a solution to which operators will gradually switch from the standard systems that currently exist, such as 3GPP and 3GPP2. And the development of this interface is an important stage in the formation of the IMT-Advanced standard for 4G networks, that is, a new generation. In fact, the LTE specification already contains most of the features that were originally intended for 4G systems.
The principle of organization of the radio interface
Radio communication has a characteristic feature, which consists in the fact that the radio channel in quality is not constant in time and space, but depends on the frequency. Here it must be said that the communication parameters change relatively quickly as a result of multipath propagation of radio waves. In order to maintain a constant speed of information exchange over the radio channel, a number of methods are usually used to minimize such changes, namely, various methods of diversity transmission. At the same time, in the process of transmitting information packets, users may not always notice short-term bit rate fluctuations. The LTE network mode assumes as a basic principle of radio access not reduction, but the use of rapid changes in the quality of the radio channel in order to ensure the most efficient use of the radio resources available at any given time. This is implemented in the frequency and time domains through OFDM radio access technology.
LTE network device
What kind of system can be understood only by understanding how it is organized. It is based on the conventional OFDM technology, which involves the transmission of data over several narrowband subcarriers. The use of the latter in combination with a cyclic prefix makes it possible to make OFDM-based communications resistant to temporary dispersions of radio channel parameters, and also makes it possible to virtually eliminate the need for complex equalizers on the receiving side. This circumstance turns out to be very useful for the organization of a downward channel, since in this case it is possible to simplify the processing of signals by the receiver at the main frequency, which reduces the cost of the terminal device itself, as well as the power consumed by it. And this becomes especially important when using a 4G LTE network along with multi-stream transmission.
The uplink, where the radiated power is significantly lower than in the downstream, requires the mandatory inclusion 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 studies have led to the fact that now for the LTE uplink, a single-frequency information transmission technology in the form of OFDM with dispersion corresponding to the discrete Fourier transform law is used. This solution allows you to provide a lower ratio of the average and maximum power level in comparison with the use of traditional modulation, which can improve energy efficiency and simplify the design of terminal devices.
The basic resource used in transmitting information in accordance with ODFM technology can be demonstrated as a time-frequency network, which corresponds to a set of OFDM symbols, and subcarriers in the time and frequency domains. The LTE network mode assumes that two resource blocks are used here as the main element of data transmission, which correspond to a frequency band of 180 kilohertz and a time interval of one millisecond. A wide range of speeds for data transmission can be realized by combining frequency resources, adjusting communication parameters, including the coding rate and the choice of modulation order.
Specifications
If we consider the LTE network, what it is, it will become clear after some explanation. In order to achieve the high targets that are set for the radio interface of such a network, its developers organized a number of rather important points and functionality. Each of them will be described below with a detailed indication of what influence they have on such important indicators as network capacity, radio coverage, delay time and data transfer rate.
Radio spectrum flexibility
Legislation that applies in a given geographical region affects how mobile communications are organized. That is, they prescribe a radio spectrum allocated in different frequency ranges by unpaired or paired bands of different widths. Flexibility of use is one of the most important advantages of the LTE radio spectrum, which allows you to use it in different situations. The architecture of the LTE network allows not only working in different frequency ranges, but also using frequency bands having different widths: from 1.25 to 20 megahertz. In addition, such a system can operate in unpaired and paired frequency bands, supporting time and frequency duplex, respectively.
If we talk about terminal devices, then when using paired frequency bands, the device can operate in full-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 because it significantly reduces the requirements set for the characteristics of the duplex filter. Thanks to this, it is possible to reduce the cost of terminal devices. In addition, it becomes possible to introduce paired frequency bands with negligible duplex spacing. It turns out that LTE mobile networks can be organized with almost any distribution of the frequency spectrum.
The only problem with the development of radio access technology, which provides for the flexible use of the radio spectrum, is to make communication devices compatible. For this purpose, an identical frame structure is implemented in LTE technology in the case of using frequency bands of different widths and different duplex modes.
Multi Antenna Data Broadcast
The use of multi-antenna broadcasting in mobile communication systems can improve their technical characteristics, as well as expand their capabilities in terms of subscription services. Coverage of the LTE network involves the use of two methods of multi-antenna transmission: diversity and multi-threaded, as a special case of which the formation of a narrow radio beam is allocated. Spaced information can be considered as a way to equalize the signal level that comes from two antennas, which eliminates deep dips in the level of signals that are received from each antenna separately.
You can take a closer look at the LTE network: what is it and how does it use all of these modes? Diversity transmission here is based on the method of spatial-frequency coding of data blocks, which is supplemented by time diversity with frequency shift when using four antennas simultaneously. Diversity transmission is usually used on common downlink channels, where the scheduling function cannot be used depending on the state of the communication channel. In this case, the diversity transmission can be used to send user data, for example, VoIP traffic. Due to the relatively low intensity of such traffic, the additional overhead that is associated with the scheduling function mentioned earlier cannot be justified. Thanks to the diversity of data transmission, it is possible to increase the radius of cells and network capacity.
Multithreaded transmission for simultaneous transmission of a number of information streams over one radio channel involves the use of several receiving and transmitting antennas located in the terminal device and the base network station, respectively. This significantly increases the maximum speed of data translation. For example, if the terminal device is equipped with four antennas and such a quantity is available at the base station, then the simultaneous transmission of up to four data streams over one radio channel is quite realistic, which allows you to actually make its throughput four times larger.
If a network with a small workload or small cells is used, then thanks to multi-threaded transmission it will be possible to achieve a sufficiently high throughput for radio channels, as well as to efficiently use radio resources. If there are large cells and a load of a high degree of intensity, then the quality of the channel will not allow the use of transmission in multi-stream mode. In this case, the signal quality can be improved by using several transmitting antennas to form a narrow beam for transmitting data in
one stream.If we consider the LTE network - what this gives it to achieve greater efficiency - then it’s worth concluding that for high-quality operation under various operating conditions, this technology implements adaptive multi-stream transmission, which allows you to constantly adjust the number of streams transmitted simultaneously, in accordance with a constantly changing the state of the communication channel. When the channel is in good condition, up to four data streams can be transmitted simultaneously, which allows achieving a transmission speed of up to 300 megabits per second with a frequency bandwidth of 20 megahertz.
If the condition of the channel is not so favorable, then fewer streams are transmitted. In this situation, antennas can be used to form a narrow radiation pattern, increasing the overall reception quality, which ultimately leads to an increase in system capacity and expansion of the served area. In order to provide extensive radio coverage areas or data transmission at high speed, it is possible to transmit a single data stream with a narrow beam or use diversity transmission of data on common channels.
Adaptation and scheduling mechanism of the communication channel
The principle of operation of LTE networks implies that scheduling will mean the distribution of network resources between users for data transmission. It provides for dynamic scheduling in the downstream and upstream channels. LTE networks in Russia are currently configured to balance the communication channels and the overall performance of the entire system.
The LTE radio interface assumes the implementation of a scheduling function, depending on the state of the communication channel. With its help, data is transmitted at high speeds, which is achieved through the use of high-order modulation, transmission of additional information streams, reducing the degree of coding of channels, as well as reducing the number of retransmissions. For this, frequency and time resources are used, characterized by relatively good communication conditions. It turns out that the transfer of any specific amount of data is performed in a shorter period of time.
LTE networks in Russia, as in other countries, are built in such a way that traffic of services that are busy sending packets with a small payload after equal time intervals may necessitate an increase in the volume of signaling traffic that is required for dynamic scheduling. It may even exceed the amount of information broadcast by the user. That is why there is such a thing as static scheduling of an LTE network. What is it, it will become clear if we say that the user is allocated a radio frequency resource designed to transmit a specific number of subframes.
Thanks to adaptation mechanisms, it is possible to “squeeze everything possible” out of a channel with dynamic communication quality. It allows you to select a channel coding and modulation scheme in accordance with what communication conditions are characterized by LTE networks. What is it, it will become clear if we say that its operation affects the speed of data transmission, as well as the likelihood of any errors in the channel.
Power in the uplink and its regulation
This aspect concerns the control of the power level emitted by the terminals in order to increase the network capacity, improve communication quality, make the radio coverage area larger, and reduce energy consumption. To achieve these goals, power control mechanisms strive to maximize the level of the useful input signal while reducing radio interference.
Beeline LTE networks and other operators suggest that the signals in the uplink remain orthogonal, that is, there should be no mutual interference between users of the same cell, at least as far as ideal communication conditions are concerned. 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 decays on the way to the cell. The Megafon LTE network is designed exactly the same. It will be correct to say this: the closer the terminal is to a neighboring cell, the higher will be the level of interference that it creates in it. Terminals that are located at a greater distance from a neighboring cell are capable of transmitting signals of greater power in comparison with terminals located in close proximity to it.
Due to the orthogonality of the signals, the signals from terminals of different powers in the same channel on the same cell can be multiplexed in the uplink. This means that there is no need to compensate for signal level spikes that occur due to multipath propagation of radio waves, but they can be used to increase the speed of data transmission using adaptation mechanisms and scheduling communication channels.
Data relay
Almost any communication system, and LTE networks in Ukraine are no exception, from time to time makes mistakes in the process of sending data, for example, due to signal fading, interference or noise. Error protection is provided through retransmission techniques for lost or distorted pieces of information designed to guarantee high quality communications. The radio resource is used much more rationally if the data relay protocol is organized efficiently. To make the most of the high-speed radio interface, LTE technology has a dynamically efficient two-level data relay system that implements Hybrid ARQ. It is characterized by low overhead required to provide feedback and resend data, supplemented by a highly reliable selective retry protocol.
The HARQ protocol provides the receiver with redundant information, enabling it to correct any specific errors.
HARQ relaying leads to the formation of additional information redundancy, which may be required when retransmission was not sufficient to eliminate errors. Relays of packets that have not been patched by the HARQ protocol are relayed using the ARQ protocol. The LTE networks on the iPhone operate in accordance with the above principles., . HARQ , ARQ, , .
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The LTE air interface is high performance due to its main components. The flexibility of using the radio spectrum allows you to use this radio interface with any available frequency resource. LTE technology provides a number of features that enable the efficient application of rapidly changing communication conditions. Depending on the condition of the channel, the scheduling function provides the best resources to users. The use of multi-antenna technologies leads to a decrease in signal fading, and using channel adaptation mechanisms, signal coding and modulation methods can be used to guarantee optimal communication quality in specific conditions.