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# 电子工程代写|通讯系统代写Communication System代考|ENG436 Types of systems

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## 电子工程代写|通讯系统代写Communication System代考|Types of systems

The phrase wireless system refers to any system that uses electromagnetic waves to transfer information from one location to another without using wires. The applications can include transmitting voice between hand-held walkie-talkies, transmitting data from a satellite to ground or from one computer to another within a room, or using radar to sense rain. This handbook considers only the propagation of electromagnetic waves in the microwave through millimeter wave radio frequency spectrum, $0.3$ through 300 gigaHertz (GHz). These frequencies lie in the ultra high (UHF: $0.3$ to $3 \mathrm{GHz}$ ), super high (SHF: 3 to $30 \mathrm{GHz}$ ), and extra high (EHF: 30 to $300 \mathrm{GHz}$ ) communication bands. Frequency bands are often referenced by their radar band designations as shown in Table 1.1. Actual band identification is often less precise than that indicated in the table. For fixed satellite communication services, Ka band refers to the 20 – to $30-\mathrm{GHz}$ frequency range.

This handbook focuses on transmission in and through the lower atmosphere, the region of the atmosphere where weather phenomena occur. The properties of the lower atmosphere are highly variable and change hourly, daily, monthly, and yearly. Their effects on radio wave propagation produce random variations in the amplitude, phase, frequency, polarization, coherence bandwidth, delay spread, and propagation direction of the electromagnetic waves. Knowledge of the statistics of one or more of these effects may be necessary for system design.

A wireless system of considerable interest is the cellular system. For this system, the domain of interest is subdivided into a number of smaller cells with transmitters and receivers that handle communications within each cell or complex of cells. The organization and structure of a cellular system are not considered in this handbook, but the statistics of the properties of a transmission channel between a transmitter and receiver in a cell and the joint statistics for multiple transmission paths within a cell or between cells are. The context is the statistics for a single path and the joint statistics for multiple paths.

Much propagation data has been collected for use in the design of fixed service satellite and terrestrial communication systems. Fixed service means a communication system employing fixed terminals on the Earth’s surface. For satellite systems, the satellite can be in geostationary orbit or in any other orbit that produces a variation in the pointing direction from the fixed ground station to the satellite. Considerable data has also been collected for cellular systems and mobile satellite systems. Published annual attenuation statistics are available from many locations in Europe and North America. Some data are available from other locations too. Study Group 3 of the Radiocommunication Study Groups of the International Telecommunications Union (ITU-R) provides data banks for model development and verification and for use in system design. ${ }^1$ The empirical statistics in the data banks for fixed service systems are generally for observations of limited duration, that is, from records that span only 1 to 5 years. The data collected for mobile service systems are more limited. Models that summarize the data in the data banks will provide a better estimate of the expected statistics for a particular path than the empirical results from measurements of limited duration on that path.

## 电子工程代写|通讯系统代写Communication System代考|Design criteria

Communication systems are designed to specific availability requirements. For the simplest transmission path between a single transmitting antenna and a single receiving antenna, the amplitude of the received signal relative to the unwanted noise in the receiver may be the statistic of interest. If the received signal level is too low, the signal may not be detected in the noise; if too high, nonlinear receiver effects may distort the signal and render it unintelligible. The error rate for a digital communication link depends on the signal-to-noise ratio as well as other factors. The statistics of the signal-to-noise ratio are therefore important. The signal-to-noise ratio depends on the receiver design, the gains and losses of the transmitting and receiving antennas, the modulation and coding of the transmitted signal, the transmitted signal power, the path loss between the antennas, and the possibility of interference from other transmitters. Availability is the fraction of time that the communication link is available for use with a signal-to-noise that exceeds the design specification for a given error performance. The outage obtained.

The atmosphere may affect the performances of the antennas and transmission path (Figure 1.1). At frequencies above $10 \mathrm{GHz}$ and depending on antenna design, rainwater or wet snow on an antenna may reduce the magnitude of the received signal (increase the path loss). The geometric spreading of the electromagnetic energy transmitted by the antenna produces a change in signal strength with distance along the path to the receiving antenna. Water vapor and oxygen in the atmosphere may cause signal absorption on the path, producing a loss or attenuation relative to the geometric spreading. Scattering by clouds and rain produce an excess attenuation relative to the geometrical spreading and gaseous absorption. For a particular path, the total attenuation, gaseous absorption plus excess attenuation, changes with time as clouds and rain drift across the path and temperature and humidity change along the path. The statistics of changing path loss may therefore be important in the design of a system. Depending on carrier frequency and path length through the atmosphere, the total attenuation statistics may constrain system design.

## Matlab代写

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