3RD SEM MC UNIT_1 CELLULAR SYSTEM

Cellular System(P4U1)

Cellular network is an underlying technology for mobile phones, personal communication systems, wireless networking etc. The technology is developed for mobile radio telephone to replace high power transmitter/receiver systems. Cellular networks use lower power, shorter range and more transmitters for data transmission.

Features of Cellular Systems:

Wireless Cellular Systems solves the problem of spectral congestion and increases user capacity. The features of cellular systems are as follows −

• Offer very high capacity in a limited spectrum.
• Reuse of radio channel in different cells.
• Enable a fixed number of channels to serve an arbitrarily large number of users by reusing the channel throughout the coverage region.
• Communication is always between mobile and base station (not directly between mobiles).
• Each cellular base station is allocated a group of radio channels within a small geographic area called a cell.
• Neighboring cells are assigned different channel groups.
• By limiting the coverage area to within the boundary of the cell, the channel groups may be reused to cover different cells.
• Keep interference levels within tolerable limits.
• Frequency reuse or frequency planning.
• Organization of Wireless Cellular Network.

Cellular network is organized into multiple low power transmitters each 100w or less.

Shape of Cells:

The coverage area of cellular networks are divided into cells, each cell having its own antenna for transmitting the signals. Each cell has its own frequencies. Data communication in cellular networks is served by its base station transmitter, receiver and its control unit.

The shape of cells can be either square or hexagon −

Square:

A square cell has four neighbors at distance d and four at distance Root 2 d

• Better if all adjacent antennas equidistant
• Simplifies choosing and switching to new antenna

Hexagon:

A hexagon cell shape is highly recommended for its easy coverage and calculations. It offers the following advantages −

• Provides equidistant antennas
• Distance from center to vertex equals length of side

Frequency Reuse:

Frequency reusing is the concept of using the same radio frequencies within a given area, that are separated by considerable distance, with minimal interference, to establish communication.

Frequency reuse offers the following benefits −

• Allows communications within cell on a given frequency
• Limits escaping power to adjacent cells
• Allows re-use of frequencies in nearby cells
• Uses same frequency for multiple conversations
• 10 to 50 frequencies per cell

For example, when N cells are using the same number of frequencies and K be the total number of frequencies used in systems. Then each cell frequency is calculated by using the formulae K/N.

In Advanced Mobile Phone Services (AMPS) when K = 395 and N = 7, then frequencies per cell on an average will be 395/7 = 56. Here, cell frequency is 56.

Propagational effects of signals

Antenna and Wave propagation plays a vital role in wireless communication networks. An antenna is an electrical conductor or a system of conductors that radiates/collects (transmits or receives) electromagnetic energy into/from space. An idealized isotropic antenna radiates equally in all directions.

Propagation Mechanisms:

Wireless transmissions propagate in three modes. They are −

• Ground-wave propagation
• Sky-wave propagation
• Line-of-sight propagation

Ground wave propagation follows the contour of the earth, while sky wave propagation uses reflection by both earth and ionosphere.

Line of sight requires the transmitting and receiving antennas to be within the line of sight of each other.

Depending upon the frequency of the underlying signal, the particular mode of propagation is followed.

Examples of ground wave and sky wave communication are AM radio and international broadcasts such as BBC. Above 30 MHz, neither ground wave nor sky wave propagation operates and the communication is through line of sight.

Transmission Limitations:

In this section, we will discuss the various limitations that affect electromagnetic wave transmissions. Let us start with attenuation.

Attenuation-

The strength of signal falls with distance over transmission medium. The extent of attenuation is a function of distance, transmission medium, as well as the frequency of the underlying transmission.

Distortion-

Since signals at different frequencies attenuate to different extents, a signal comprising of components over a range of frequencies gets distorted, i.e., the shape of the received signal changes.

A standard method of resolving this problem (and recovering the original shape) is to amplify higher frequencies and thus equalize attenuation over a band of frequencies.

Dispersion-

Dispersion is the phenomenon of spreading of a burst of electromagnetic energy during propagation. Bursts of data sent in rapid succession tend to merge due to dispersion.

Noise-

The most pervasive form of noise is thermal noise, which is often modeled using an additive Gaussian model. Thermal noise is due to thermal agitation of electrons and is uniformly distributed across the frequency spectrum.

Other forms of noise include −

• Inter modulation noise (caused by signals produced at frequencies that are sums or differences of carrier frequencies)
• Crosstalk (interference between two signals)
• Impulse noise (irregular pulses of high energy caused by external electromagnetic disturbances).

While an impulse noise may not have a significant impact on analog data, it has a noticeable effect on digital data, causing burst errors.

The above figure clearly illustrates how the noise signal overlaps the original signal and tries to change its characteristics.

Fading-

Fading refers to the variation of the signal strength with respect to time/distance and is widely prevalent in wireless transmissions. The most common causes of fading in the wireless environment are multipath propagation and mobility (of objects as well as the communicating devices).

Multipath propagation-

In wireless media, signals propagate using three principles, which are reflection, scattering, and diffraction.

• Reflection occurs when the signal encounters a large solid surface, whose size is much larger than the wavelength of the signal, e.g., a solid wall.
• Diffraction occurs when the signal encounters an edge or a corner, whose size is larger than the wavelength of the signal, e.g., an edge of a wall.
• Scattering occurs when the signal encounters small objects of size smaller than the wavelength of the signal.

One consequence of multipath propagation is that multiple copies of a signal propagation along multiple different paths, arrive at any point at different times. So the signal received at a point is not only affected by the inherent noise, distortion, attenuation, and dispersion in the channel but also the interaction of signals propagated along multiple paths.

Delay spread-

Suppose we transmit a probing pulse from a location and measure the received signal at the recipient location as a function of time. The signal power of the received signal spreads over time due to multipath propagation.

The delay spread is determined by the density function of the resulting spread of the delay over time. Average delay spread and root mean square delay spread are the two parameters that can be calculated.

Doppler spread-

This is a measure of spectral broadening caused by the rate of change of the mobile radio channel. It is caused by either relative motion between the mobile and base station or by the movement of objects in the channel.

When the velocity of the mobile is high, the Doppler spread is high, and the resulting channel variations are faster than that of the baseband signal, this is referred to as fast fading. When channel variations are slower than the baseband signal variations, then the resulting fading is referred to as slow fading.