Radio Receivers for Systems of Fixed and Mobile Communications

Radio receivers are an element of a radio system. The first chapter of the book shows the changes in radio systems designed to transmit information, from the moment of their occurrence under the influence of the tasks set during its development. The issues related to the transition from the analog form of the transmitted signal to the digital form on the architecture of networks, the need for relay transmission of a mobile terminal when changing its position in space, and the use of modern technologies that implement these functions are considered. It is formulated how the properties of the transmission medium and interference in the radio channel influence the structure and circuit design of a radio receiving device as part of this system. It is noted that the variety of modern systems caused by their multi-functionality and an even larger number of manufacturers of radio equipment for solving specific problems and conditions of signal propagation have led to difficulties in the exchange of information.

The cardinal solution to this problem was the transition from the regulation of manufacturers’ equipment to control over the interface used, at all levels of interaction described by the reference model for open systems interconnection (OSI). This made it possible to actively develop microcircuitry and build multi-functional integrated circuits, and hence radio receivers, using various technologies and implementation principles for various frequency ranges and transmission media.

Currently, a huge number of radio engineering systems of fixed and mobile communications are used in the world, designed for transmitting information and using various types of signal modulation, unit design, multi-station access methods, and various network structures. The interaction of systems with all their diversity is effectively ensured at the interface level. In the second chapter, the main attention is paid to the architectural features of WPAN, WLAN, WMAN, and WWAN, grouped according to their purpose and the distance between terminal devices in the structure of the radio network. The standards describing the systems analyze in detail the structure of radio frames, the purpose of individual bits, and the organization of physical and logical channels at different levels. For the construction of radio receivers, the most important are the distribution of radio resources in the network, increasing its efficiency, and reducing the power consumption of network elements. All this refers to the layer of the physical channel of the OSI reference model, which provides radio communication between the BS and the MS, without which the structure of the entire network ceases to perform its main function – the transmission of information. The structures of radio access networks (WiFi, HIPERLAN, UWB, etc.) and mobile communication of digital systems, including 5G systems, as well as the applied multiple access technologies, which led to a change in the structural schemes of the basic types of modulation, are considered in detail. Modern methods of increasing the throughput of a radio channel by aggregating the spectra of active BS in the network structure, as well as algorithmic methods for controlling the dynamic spectrum of the network (software-defined radio (SDR) and cognitive radio (CR)), are considered.

Structural diagrams of modern receivers intended for signal transmission systems with digital modulation differ significantly from analog signal receivers. This is due to the fact that technologies that significantly complicate the processing of the received signal are used to increase the speed of signal transmission, and increase their noise immunity, for which not only traditional circuitry but also software methods can be used.

The architecture of networks designed for data transmission using a physical radio channel, only between two terminal devices of the network, with a complex interface, has also become significantly more complicated. This made it possible to use signals with different types of modulation in the allocated radio frequency band, to apply roaming technology, space-time coding, and aggregation of parts of the free spectrum. All this also affected the structure of the receiver.

The study of their features begins with the study of the basic structures of analog signal receivers with the gradual addition of units that provide, for example, CDMA or OFDM technologies. Integral technologies make it possible to form the structure of the RF front-end and the digital path on a single substrate, which, while maintaining its architecture, use a constructive implementation based on new principles, replacing passive AE components.

Free space as a medium for transmitting radio signals makes it possible to organize communications in any direction, both for an unlimited number of subscribers and with only a specific subscriber. This can be considered both its advantages and its disadvantages, depending on the structure and purpose of the network.

Thus, the organization of a large number of transmission directions increases the level of inter-channel and intra-channel interference at the point of reception of the radio signal, leading to nonlinear distortions and even blocking of the channel. When organizing a personal radio communication channel, it can also be accompanied by the occurrence of nonlinear distortions and inter-symbol interference caused by the phenomenon of signal multipath at the receiving point.

In this chapter, both the causes of interference and the ways to combat them are considered, based on the use of algorithmic and circuitry methods for increasing the noise immunity of signals in a radio channel: invariant reception, the construction of adaptive antenna systems based on space-time MIMO coding technology for fixed, mobile communication and radio access. As an example, we analyzed the influence of interference on the sensitivity and other parameters of the receiver.

Modern digital telecommunication systems have a complex architecture that includes a fixed network, ground equipment, and a mobile terminal that are part of the radio network structure. The interaction of systems and network equipment, implemented using different principles and technologies and meeting different standards, is provided at the interface level described in the reference model for the interaction of open systems OSI (Open System Interconnect). The radio interfaces of the physical channel are based on the technical indicators of radio receiving devices, including terminal equipment: the type of radio signal modulation, the method of access to the channel, the rate of transmitted signals, the S/N ratio at different points of the radio frequency front end, etc.

These characteristics form the requirements for specific devices that implement the interface and methods (algorithmic and circuitry) providing the specified properties.

The technical specifications for the terminal devices instruct the designers of the RF front-end architecture to meet the requirements for the amount of intermodulation distortion, signal blocking, and cross modulation. The reasons for the occurrence of distortions and ways to reduce them to acceptable values are considered. Simulation in the MicroCap environment of a resistor cascade on BT and calculation of its noise properties are performed.

The effectiveness of various methods for analyzing individual nodes of a RF front end based on the use of linear and nonlinear AE models is discussed and compared, limitations are indicated, and current trends in the design of radio engineering nodes and devices are considered. The expediency of using digital methods of analysis is confirmed while maintaining the physical description of the ongoing processes and the AE models used in the MicroCap environment, as well as the usual forms of presentation of the results: process diagrams at various points of the circuit in the time domain, spectral, and amplitude-frequency characteristics.

The high accuracy and efficiency of the results of the numerical analysis of devices based on the use of nonlinear AE models is shown, but only under the condition that the parameters of the Spice AE models coincide with the data published in the reference books for the corresponding components. Of course, in this case, the condition that the applied descriptions of the AE models fit into the admissible limits of its spread, given in reference books of average physical parameters, with real samples, must be met.

Such an analysis makes it possible to obtain information that is not attainable when using linear models of the AE, when its operating mode for direct current changes or the AE switches from a linear mode in terms of the input action level to a nonlinear one, for example, when assessing the harmonic factor and the level of intermodulation distortion.