The LTE network was recently approved by the 3GPP consortium. By using such an air interface, it is possible to obtain a network with unprecedented performance in terms of maximum data transfer rate, packet forwarding delay, and spectral efficiency. The authors say that the launch of the LTE network allows more flexible use of the radio spectrum, multi-antenna technology, channel adaptation, scheduling mechanisms, organization of data retransmission and power control.
Backstory
Mobile broadband, which is based on HSPA high-speed packet data technology, has already become widely accepted by cellular network users. However, it is necessary to further improve their service, for example, using an increase in the speed of data transmission, minimization of the delay time, as well as an increase in the overall network capacity, since the requirements of users toservices of such communication are constantly increasing. It was for this purpose that the specification of the HSPA Evolution and LTE radio interfaces was made by the 3GPP consortium.
Main differences from earlier versions
The LTE network differs from the previously developed 3G system by improved technical characteristics, including the maximum data transfer rate of more than 300 megabits per second, the packet forwarding 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 new frequency bands and in existing operators.
This radio interface is positioned as a solution to which operators will gradually switch from the systems of standards that currently exist, these are 3GPP and 3GPP2. And the development of this interface is a rather important step towards the formation of the IMT-Advanced 4G network standard, 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 is that the quality of the radio channel is not constant in time and space, but depends on the frequency. Here it is necessary to say that the communication parameters change relatively quickly as a result of the multipath propagation of radio waves. In order to maintain a constant rate of information exchange over the radio channel, a number of methods are usually used to minimizesimilar changes, namely different transmission diversity methods. At the same time, in the process of transmitting information packets, users cannot always notice short-term fluctuations in the bit rate. The LTE network mode assumes as a basic principle of radio access not to reduce, but to apply 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 this is can only be understood by understanding how it is organized. It is based on the conventional OFDM technology, which involves the transmission of data over several narrow-band subcarriers. The use of the latter in combination with a cyclic prefix makes it possible to make OFDM-based communication resistant to time dispersions of the radio channel parameters, and also makes it possible to practically eliminate the need for complex equalizers on the receiving side. This circumstance turns out to be very useful for organizing a downlink, since in this case it is possible to simplify the processing of signals by the receiver at the main frequency, which makes it possible to reduce the cost of the terminal device itself, as well as the power consumed by it. And this becomes especially important when using 4G LTE network together with multi-streaming.
The uplink, where the radiated power is significantly lower than in the downlink, requires mandatory inclusion in the workan energy-efficient method of information transmission to increase the coverage area, reduce the power consumption of the receiving device, as well as its cost. The conducted studies have led to the fact that now for the uplink LTE, a single-frequency technology for broadcasting information in the form of OFDM with a dispersion corresponding to the discrete Fourier transform law is used. This solution provides a lower ratio of average and maximum power levels compared to conventional modulation, which improves energy efficiency and simplifies the design of terminal devices.
The basic resource used in the transmission of information in accordance with ODFM technology can be shown as a time-frequency network that corresponds to the OFDM symbol set, 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 data rates can be realized by combining frequency resources, setting communication parameters including code rate and modulation order selection.
Specifications
If we consider LTE networks, what it is will become clear after certain explanations. In order to achieve the high targets set for the radio interface of such a network, its developers organized a number of rather importantmoments and functionality. Each of them will be described below, with a detailed indication of how they affect important indicators such as network capacity, radio coverage, delay time and data transfer rate.
Flexibility in the use of the radio spectrum
Legislative norms that operate in a particular geographical region affect how mobile communications will be organized. That is, they prescribe the 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 it to be used in different situations. The architecture of the LTE network allows not only to work in different frequency bands, but also to use frequency bands with 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 in that it significantly reduces the requirements 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 low duplex spacing. It turns out that networksLTE mobile communications can be organized in almost any distribution of the frequency spectrum.
The only challenge in developing a radio access technology that allows flexible use of the radio spectrum is to make communication devices compatible. To this end, the 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 allows improving their technical characteristics, as well as expanding their capabilities in terms of subscriber service. LTE network coverage involves the use of two methods of multi-antenna transmission: diversity and multi-stream, as a special case of which is the formation of a narrow radio beam. Diversity can be thought of as a way to equalize the level of the signal that comes from two antennas, which allows you to eliminate deep dips in the level of signals that are received from each antenna separately.
Let's take a closer look at the LTE network: what is it and how does it use all these modes? Transmission diversity here is based on the method of space-frequency coding of data blocks, which is supplemented by time diversity with a frequency shift when using four antennas simultaneously. Diversity is typically used on common downlinks where the scheduling function cannot be applied depending on the state of the link. Whereintransmit diversity can be used to send user data, such as 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. With data diversity, it is possible to increase the radius of cells and network capacity.
Multistream 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 transmission. For example, if the terminal device is equipped with four antennas and such a number is available at the base station, then it is quite possible to simultaneously transmit up to four data streams over one radio channel, which actually makes it possible to quadruple its throughput.
If you use a network with a small workload or small cells, then thanks to multi-streaming, you can achieve a sufficiently high throughput for radio channels, as well as efficiently use radio resources. If there are large cells and a high degree of load, the channel quality will not allow multistream transmission. In this case, the signal quality can be improved by using multiple transmit antennas to form a narrow beam for transmitting data in one stream.
If we considerLTE network - what this gives it to achieve greater efficiency - then it is worth concluding that for high-quality work 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 the constantly changing channel state connections. With good link conditions, up to four data streams can be transmitted simultaneously, achieving transmission rates of up to 300 megabits per second with a bandwidth of 20 megahertz.
If the channel condition is not so favorable, then the transmission is made by fewer streams. In this situation, antennas can be used to form a narrow beam, improving the overall reception quality, which ultimately leads to an increase in system capacity and an extension of the service area. To provide large radio coverage areas or data transmission at high speed, you can transmit a single data stream with a narrow beam or use data diversity on common channels.
Mechanism for adaptation and dispatching of the communication channel
The principle of operation of LTE networks assumes that scheduling will mean the distribution of network resources between users for data transmission. This provides for dynamic scheduling in the downstream and upstream channels. LTE networks in Russia are currently configured in such a way as to balance communication channels and overalloverall system performance.
The LTE radio interface assumes the implementation of the scheduling function depending on the state of the communication channel. It provides data transmission at high speeds, which is achieved through the use of high-order modulation, the transmission of additional information streams, a decrease in the degree of channel coding, and a decrease in the number of retransmissions. For this, frequency and time resources are used, which are characterized by relatively good communication conditions. It turns out that the transfer of any particular amount of data is made 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 forwarding packets with a small payload after the same time intervals may necessitate an increase in the amount 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 the LTE network. What this is, it will become clear if we say that the user is allocated an RF resource designed to transmit a certain number of subframes.
Thanks to adaptation mechanisms, it is possible to "squeeze everything possible" out of a channel with dynamic link quality. It allows you to select a channel coding and modulation scheme in accordance with what communication conditions are characterized by LTE networks. What this is will become clear if we say that his work affectson the speed of data transmission, as well as on the probability of any errors in the channel.
Uplink power and regulation
This aspect is about controlling the level of power emitted by the terminals to increase network capacity, improve communication quality, make the radio coverage area larger, reduce power consumption. To achieve these goals, power control mechanisms strive to maximize the level of a useful incoming signal while reducing radio interference.
LTE networks of Beeline and other operators assume that the signals in the uplink remain orthogonal, that is, there should be no mutual radio interference between users of the same cell, at least for ideal communication conditions. The level of interference that is created by users of neighboring cells depends on where the emitting terminal is located, that is, on how its signal attenuates on the way to the cell. The Megafon LTE network is arranged in exactly the same way. It would 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 further away from a neighboring cell are able to transmit stronger signals than terminals that are in close proximity to it.
Due to the orthogonality of the signals, the uplink can multiplex signals from terminals of different strengths in one channel on the same cell. This means that there is no need to compensate for signal level spikes,that arise due to the multipath propagation of radio waves, or you can use them to increase the speed of data transmission using the mechanisms of adaptation and scheduling of communication channels.
Data relays
Almost any communication system, and LTE networks in Ukraine are no exception, from time to time makes errors in the process of data transfer, for example, due to signal fading, interference or noise. Error protection is provided by methods of retransmission of lost or corrupted pieces of information, designed to ensure high quality communications. The radio resource is used much more rationally if the data relay protocol is organized efficiently. In order to make the most of the high speed air interface, LTE technology has a dynamically efficient two-layer data relay system that implements Hybrid ARQ. It features the low overhead needed to provide feedback and resend data, complete with a high reliability selective retry protocol.
The HARQ protocol provides the receiving device with redundant information, enabling it to correct any specific errors. Retransmission via the HARQ protocol leads to the formation of additional information redundancy, which may be required when retransmission was not enough to eliminate errors. Retransmission of packets that have not been corrected by the HARQ protocol is carried out withusing the ARQ protocol. LTE networks on iPhone work according to the above principles.
This solution allows you to guarantee the minimum delay of packet translation with low overhead, while the reliability of communication is guaranteed. The HARQ protocol allows you to detect and correct most of the errors, which leads to a rather rare use of the ARQ protocol, as this is associated with considerable overhead, as well as an increase in the delay time during packet translation.
The base station is an end node that supports both of these protocols, providing a tight link between the layers of the two protocols. Among the various advantages of such an architecture are the high speed of eliminating errors that remained after the operation of HARQ, as well as the adjustable amount of information transmitted using the ARQ protocol.
LTE radio interface has high performance due to its main components. The flexibility of using the radio spectrum makes it possible to use this radio interface with any available frequency resource. LTE technology provides a number of features that enable efficient use of rapidly changing communication conditions. Depending on the state of the channel, the scheduling function issues the best resources to the users. The use of multi-antenna technologies leads to a reduction in signal fading, and with the help of channel adaptation mechanisms, it is possible to use coding and signal modulation methods that guarantee optimal communication quality under specific conditions.
