Wireless Networking Fundamentals

03

Antennas

A device that converts an electric current into an electromagnetic wave, as it was shown in Radio waves main characteristics, is called an antenna. In addition, antennas convert the energy of an electromagnetic wave into an electric current in the receiving mode. The possibility of using an antenna for both transmission and reception of electromagnetic waves is called the reciprocity.

Main characteristics

Radiation pattern

Radiation pattern is the main feature of antennas, is a diagram showing the intensity of the radiation field of the antenna at a given distance and angle of observation. Usually the radiation pattern is represented in polar coordinate systems,in vertical and horizontal planes in relation to the Earth plane. The radiation pattern demonstrates the distribution to the space of the energy supplied to the antenna. Examples of radiation patterns are shown in Figures 1-3:

Figure 1 Radiation pattern of isotropic antenna: a - vertical plane, b - horizontal plane, c - 3D image

Figure 2 Radiation pattern of omnidirectional antennaa - vertical plane, b - horizontal plane, c - 3D image

Figure 3 Radiation pattern of omnidirectional antennaa - vertical plane, b - horizontal plane, c - 3D image

Obvious maxima of the radiation pattern are called lobes. In real life it is difficult to realize an antenna directed only to the required side: usually, radiation within the main lobe will be accompanied by supplementary - side and back lobes:

Figure 4 Radiation pattern with obvious lobes

Main lobe beamwidth, azimuth, elevation angle

Main lobe beamwidth is the angular sector, inside the main lobe of the antenna radiation pattern, within which the largest part of the signal energy is located. The value is measured in point where the signal decreases by half power, which corresponds to decrease by 3 dB.

Effective operation of wireless communication systems is achieved when the quality alignment condition is fulfilled - the alignment of the antennas main lobes of the receiver and the transmitter. To achieve this, in the process of preliminary planning and installation following parameters are used:

  • azimuth - the angle formed between the direction of the antenna and the direction to the north in the horizontal plane;
  • elevation - antenna tilt in relation to horizon in the vertical plane.

Figure 5 Azimuth and elevation

Side lobes level

The "side lobes level" parameter shows how much side radiation level is weaker than the level of the main lobe, is calculated by the formula:

Front-to-back ratio

Front-to-back ratio of an antenna is a relation of the intensity of the field radiated by the antenna in the main direction to the intensity of the field radiated in the opposite direction:

Antenna gain

Antenna is a passive device and does not amplify the transmitted signal, however, due to a non-homogeneous radiation pattern, the device redistributes the energy. The redistribution of energy makes possible to increase the intensity of radiation on the transmitting side and to improve the sensitivity to some directions on the receiving side in comparison to the isotropic antenna. It is important to remember that the energy is redistributed not only in the main lobe direction to which the gain is calculated, but also in the direction of the side and back lobes.

Thus, the gain is the amount of increase in energy that an antenna adds to the RF signal; mathematically, it represents the power in the strongest direction divided by the power that would be transmitted by an isotropic antenna emitting the same total power:

Voltage standing wave ratio

When the electric current propagates through the feeder, some of the energy can be reflected from the load making by an antenna. It is a result of disarrangement of feeder resistance with load and the amount of reflected energy depends on the ratio of the resistances. voltage standing wave ratio shows how much energy supplied will be reflected back to the feeder.

For example, if an antenna with a 100 Ohms input impedance is connected to an RF cable with a 50 Ohms resistance, then VSWR=2:1, which means that half the energy during transmission will be reflected back to the RF cable.

Frequency range

In section Radio waves main characteristics the mechanism of an electromagnetic wave formation for a vibrator was considered. At the same time, the electric current flowing through the vibrator forms a standing wave the knot of which is located in the middle and current amplitude values are appeared at edges of the vibrator. Thus, the maximum values of the field density will appear if the length of the vibrator is a half of the oscillation wavelength, which indicates a relationship between the radiation wavelength and the dimensions of the antenna. According to the reciprocity effect, the connection between the wavelength and dimensions is also typical for the receiving antenna.

Generally, regardless of the antenna design, the wavelength of the radio signal is proportional to the size of the antenna, which is the one of recomendations for selecting the frequency range when designing a communication system. For this reason, one of the antenna characteristics is the operating frequency range for which the declared parameters are stored.

For example, the reason of the widespread of wireless data transmission systems in the 5 GHz band is the small size of the antenna - 6 cm and acceptable attenuation values.

Polarization

The polarization of the electromagnetic wave considered in Radio waves main characteristics, is determined by the design features and location of the antenna used. It is important to understand that the transmitting and receiving antennas must be aligned in the polarization: in Figure 6 are shown cases with alignment (a) and misalignment (b) in polarization on the receiving and transmitting sides:

Figure 6 Alignment (a) and misalignment (b) in polarization on the receiving and transmitting sides

Since electromagnetic waves of orthogonal polarizations do not interact, it is possible to use two radio signals of different polarizations in one frequency band. There are different cases for using this tool: increasing of the capacity, organization of a duplex channel, organization of multiple access, increasing of the link reliability.

Antennas type

Isotropic antenna

The isotropic antenna model is used in the antennas theory, it's radiation pattern is a sphere, is shown in Figure 1. Such antenna does not exist in real life.

Omnidirectional antenna

The antenna is used in practice, the most similar to the isotropic in the radiation pattern, is an omnidirectional antenna. An example of omnidirectional antenna radiation pattern is shown in Figure 2, and the structural design is in Figure 7:

Figure 7 An example of omnidirectional antenna

Directional antenna

Unlike the previous antennas, directional antennas differ in main lobe width and have a lot of design versions. An example of directional antenna radiation pattern is shown in Figure 3, and the structural design is in Figure 8.

It may seem that, due to the complicated design, the directional antennas exceed omnidirectional in their performance, but choosing of the antenna type directly depends on the problem need to be solved. For example, to establish wireless point-to-point link, better to use highly directional devices, to build multi-sector base station - antennas with a certain width of the main lobe up to omnidirectional in a single-sector configuration.

Figure 8 An example of directional antennas

The variety of constructional realizations of directional antennas is due to the popularity of directional antennas in wireless communication systems in compare to omnidirectional. For example, InfiNet Wireless product portfolio has solutions with connectors for external antennas and integrated antennas which are a microstrip array:

Figure 9 InfiNet units: a - with integrated antennas, b - with ablility to connect external antennas

Beamforming

One of antennas structural design is phased antenna array, which is a matrix of interconnected radiating elements, it is shown in Figure 10. The special feature of the phased array is possibility to form different radiation patterns by feeding signals to the radiating elements with different amplitude and phase parameters. Thus, the dynamical change of the signals supplied to the radiating surfaces allows to control the radiation pattern in real time.

Figure 10 An example of phased antenna array

Based on the ability of the phased antenna array to dynamically change the radiation pattern, the beam-forming technology was implemented, which has several advantages in compare with antennas with a fixed radiation pattern.

For example, two subscribers are connected to the base station sector and an interference has appered in the sector, Figure 11. If BS sector has a fixed radiation pattern, interference will have a negative impact on the performance of the base station. In the case of broadband interference, a frequency change will not help and the searching of a source of interference in the terrain is a complex task the solution of which can have a long-term nature.

Figure 11 Example of the interference effect on a sector with a fixed radiation pattern

Replace the sector antenna with a beamforming device, Figure 12.

Figure 12 Example of the interference effect on a sector with a beamforming technology: a - connection to CPE1, b - connection to CPE2

The beamforming technology allows to form a narrow radiation pattern in the direction of a particular subscriber, and to refocus the main lobe to other CPE's direction. An improvement in the performance of the wireless system occurs due to two factors: the susceptibility to local interference decreases and the link energy increases towards each of subscribers due to the sector energy redistribution into a narrower beam of the radiation pattern. An important advantage is that the using of beamforming technology does not require the support of this technology on the subscribers side.

MIMO technology

To increase the capacity of communication systems or improve reliability, it is possible to use scheme in which transmission and reception are performed by several antennas. Near-by antennas in such schemas need to be isolated to reduce the correlation by applying site, polarization, and other methods of separation. This technology is called MIMO (Multiple Input Multiple Output).

To explain the principles of MIMO technology, let's look at the scheme in Figure 13. Both transmitter and receiver use two antennas with different polarization - vertical and horizontal. On the transmitting side, the original data stream is divided into two substreams, each of them is sent to separated processing channel and transmitted through a separated antenna. The reverse process is realize on the receiver side - one data substream is received through a horizontal polarization antenna, the other through a vertical polarization antenna. The substreams are combined into one data stream and transferred to further processing stages.

Figure 13 Example of using of MIMO scheme with two streams

The scheme shown in Figure 13 doubles the bandwidth of the communication channel. The scenario of using several antennas is possible to increase the reliability of connection - in this case data substreams are transmitted at a lower speed, keeping the total system capacity, or the data stream is not divided into substreams, but is duplicated in each of communication channels.

In Figure 13 is shown MIMO scheme with two data streams, but it is necessary to keep in mind that number of data streams can be arbitrary and depends on number of antennas.

Scenarios for building wireless communication systems

To choose antenna devices for a wireless communication system, the conditions in which the communication system will be rolled out must be considered. There are three possible scenarios:

  • fixed;
  • mobile;
  • nomadic.

In fixed point-to-multipoint systems, a base station with one or more sectors is installed on a specified site. The radiation pattern of the sector depends on the configuration of the base station: to provide circular coverage, the beamwidth of the sector antennas is 360° for a single-sector configuration, 120° - with three sectors, 60° - with six and so on. Antennas with a wide radiation pattern are used, because the sector operates in the "point-to-multipoint" mode and must provide connection of fixed subscribers in a certain area. Subscriber devices are static and aligned to the sector. To decrease the level of parasitic radiation, the CPE radiation pattern width should be quite narrow.

Figure 14 Exapmle of fixed system

In mobile communication systems, unlike fixed systems, subscribers can change their location, so radiation patterns of client devices are usually selected as omnidirectional:

Figure 15 Examples of mobile system

A combination of these communication systems types is a wireless communication system with nomadic objects. In this scenario, the location of devices can change time to time. So in the case of using narrow-band antennas, it is necessary to make alignment when changing the location of the device, or to use systems with automatic tracking.

Figure 16 System with nomadic objects

These classification of wireless communication systems is valid for both point-to-multipoint and point-to-point architectures.

Unit of measurement

In accordance with the international system of units, power is measured in Watts:

In transmitting and receiving radio signals usually smaller values are used - milliwatts:

To estimate the link energy a dimensionless quantity - decibel is used, which is proportional to the decimal logarithm of the ratio of the two energy quantities:

Separately, the decibel-millivatt value is determined, in which the measured value is normalized to 1 mW:

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