Introduction To Cellular System Computer Science Essay

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Introduction

The increase in demand for more number of communication channels within the limited frequency spectrum has led to the development of cellular system. In a cellular system, with the aid of the frequency reuse technique, the limited number of channels was able to be reused as many times as possible. This reuse of channels gave rise to some of the interferences such as co channel interferences and adjacent channel interferences.

There are certain interference reduction methods that are employed to reduce these interferences. Of these, cell sectoring and microcell zone concept are the basic methods to reduce co channel interference. Implementing special filters at the receiver end of a communication channel can be done for the cancellation of these types of interferences.

In this project, simulation for the cancellation of the co channel interference caused in a transmitting signal is done using the SIMULINK software. SIMULINK is a program developed upon the MATLAB. Models in SIMULINK are represented in block diagram form and can be assembled from block libraries and sub libraries. Furthermore, SIMULINK allows the display of results and changes of certain model parameters without interrupting the simulation.

A simulation model for co channel interference cancellation in PSK modulated signal is performed using SIMULINK. The model includes a PSK modulated transmitting signal, co-channel interference and PSK demodulator is used in the receiver end to cancel the interference. The simulation results are shown with the aid of several graphs that indicate the working of the model.

Introduction to Cellular System

The design purpose of early mobile radio system was to attain a large coverage area by using a single, high powered transmitter with an antenna mounted on a tall tower. Even though this approach achieved very good coverage, it was impossible to reuse the same frequencies throughout the system as it would result in interferences. With the reality that the government regulatory agencies could not make spectrum allocations in ratio to the high demand for mobile services, it was imperative to reorganize the radio telephone system to achieve high capacity with limited radio spectrum while at the same time covering very large areas.

Figure 2.1: Cellular system structure

The major breakthrough in solving the problem of spectral congestion and user capacity was the cellular concept. Without any major technical changes, it could offer high capacity in a limited spectrum allocation. The cellular concept is a system level design which calls for replacing a single, high power transmitter with many low power transmitters, and each on condition that coverage to only a small portion of the service area. The entire system was divided into portions with a base station, and close by base stations is given different groups of channels so that all the available channels are assigned relative to a very small number of neighboring base stations. The interference between base stations is minimized by assigning different channels to neighboring base stations. By methodically spacing base stations and their channel groups throughout a market, the available channels are dispersed all through the geographic region and is designed to be reused as many times as necessary so long as the interference between co channel stations is kept below satisfactory levels.

The number of base stations may be increased as the demand for services increases, thereby providing additional radio capacity with no additional increase in radio spectrum. This basic concept is the foundation for all current wireless communication systems, as it allows a fixed number of channels to give out an arbitrarily large number of subscribers by reutilizing the channels throughout the coverage region. Moreover, the concept of cellular communication allows any mobile to be used anywhere within a region by every element of subscriber equipment within a country or continent to be manufactured with the same set of channels.

Frequency Reuse

The core idea behind cellular mobile radio system is frequency reuse. The concept in the frequency reuse system is that in this, the users in different geographic locations may simultaneously use the same frequency channel. The spectrum efficiency is drastically increased with the frequency reuse system, but if the system is not designed properly, it may result in serious interference. Co channel interference which is a major concern in the concept of frequency reuse is the result of common use of the same channel.

Figure 3.1: Frequency reuse pattern

Frequency Reuse Schemes

The concept of frequency reuse can be used in the space domain and the time domain. Frequency reuse in the time domain results in the occupation of the same frequency in different time slots. It is termed as time division multiplexing (TDM). Frequency reuse in the space domain can be divided into two categories.

Same frequency allocated in two different geographic areas, such as in the case of AM or FM radio using the same frequency in different locations.

Same frequency frequently used in a common area in one system. The total frequency spectrum allocation is divided into K frequency reuse patterns for K= 4, 7, 12, and 19.

3.2 Frequency Reuse Distance

The number of co channel cells in the vicinity of the center cell, the type of geographic terrain contour, the antenna height and transmitted power at each cell site are the parameters on which the minimum distance which allows the same frequency to be reused will depend on.

The frequency reuse distance D can be found from

D = *R, where K is the frequency reuse pattern.

D = 3.46R for K= 4

D = 4.6R for K=7

D = 6R for K=12

D = 7.55R for K=19.

D increases with the increase in K for all the cell sites that transmit the same power. This improved D reduces possibility that co channel interference may occur.

Theoretically, a large K is preferred. However, the overall number of allocated channels does not change. The number of channels assigned to each of K cells becomes small with the increase in value of K. But trunking inefficiency is resulted if the total number of channels in K cells is divided as K increases. The same law applies to spectrum inefficiency. That is, if the total numbers of channels are divided into two network systems serving in the same area, spectrum efficiency increases.

But the challenge is to attain the smallest number K which can still meet the system performance necessities. This would entail estimating co channel interference and selecting the least frequency reuse distance D to decrease co channel interference. The smallest value of K is K=3, obtained by setting i =1, j =1 in the equation

Figure 3.2: Method of locating co channel cells

4. Interference

Interference is one of the major restraining factors in the performance of cellular radio systems. Some of the sources of interference include:

1. More than one mobile in the same cell

2. A call in progress in a neighboring cell

3. Base stations operating in the same frequency band, or

4. Any non-cellular system which accidentally leaks energy into the cellular frequency band.

Interference on voice channels causes cross talk due to an undesired transmission, where the subscriber hears interference in the background. Due to errors in the digital signaling, on control channels, interference leads to missed and blocked calls. Urban areas face more of interferences, due to the large number of base stations and mobiles and greater RF noise floor. Thus, interference has been recognized as one of the major drawback in increasing capacity and is seldom responsible for dropped calls.

Types of interferences

The two major types of system generated cellular interference are:

Co channel interference and

Adjacent channel interference

When in a given coverage area there is many cells that use the similar set of frequencies, it results in co channel interference. Co channel interference cannot be solved simply by increasing the carrier power of a transmitter, unlike thermal noise which can be overcome by increasing the signal to noise ratio. This is because by increasing the carrier transmit power increases the interference to neighboring co channel cells. To reduce co channel interference, co channel cells must be physically placed apart by a minimum distance to provide sufficient isolation due to propagation.

Interference caused from signals which are adjacent in frequency to the preferred signal is called adjacent channel interference. Adjacent channel interference results when nearby frequencies to leak into the passband due to the use of imperfect receiver filters. The problem turns to be serious if an adjacent channel user is transmitting in very close range to a subscriber's receiver, which the receiver attempts to receive a base station on the desired channel. This is called the near- far effect, where a nearby transmitter captures the receiver of the subscriber. Careful filtering and channel assignments can minimize adjacent channel interference. The adjacent channel interference may be reduced considerably by keeping the frequency separation between each channel in a given cell as large as possible.

Co Channel Interference

Frequency reuse is a concept in which in a given coverage area there is several cells use the same set of frequencies. These cells are termed as co channel cells, and the interference between signals from these cells is called co channel interference. Thermal noise can be overcome by increasing the signal to noise (SNR), whereas co channel interference cannot be combated by simply increasing the carrier power of the transmitter. This is for the reason that an increase in carrier transmit power increases the interference to the neighboring co channel cells. To diminish co channel interference, co channel cells must be physically separated by a minimum distance so that sufficient isolation due to propagation is provided.

When the size of each cell is approximately same and the base stations transmit the same power, the co channel interference ratio is independent of the transmitted power and becomes a function of the radius of the cell(R), and distance between the centers of the nearest co channel cells (D). Increasing the ratio of D/R results in the increase of spatial separation between co channel cells relative to the coverage distance of the cell. Thus interference is reduced from improved isolation of RF energy from the co channel cells. The constraint Q, called the co channel reuse ratio, is related to the cluster size. For a hexagonal geometry

A small rate of Q provides larger capacity since the cluster size N is small, whereas a large value of Q improves the transmission quality, due to a smaller level of co channel interference. A trade-off must be made between these two objectives in actual cellular design.

Table 5.1: Co-channel Reuse Ratio for some values of N

Cluster Size(N)

Co-channel Reuse Ratio(Q)

i=1, j=1

3

3

i=1, j=2

7

4.58

i=2, j=2

12

6

i=1, j=3

13

6.24

Let i0 be considered as the number of co channel interfering cells. Then, signal to interference ratio (S/I or S/R) for a mobile receiver which monitors a forward channel can be expressed as

Where S denotes the desired signal power from the desired base station and Ii is considered to be the interference power caused by the ith interfering co channel cell base station. The S/I ratio for the forward link can be found using the above equation if the signal levels of co channel cells are known.

The average strength of the received signal at any point decays as a power law of the distance of separation between averages received power Pr at a distance d from the transmitting antenna is approximated by

Here P0 denotes the power received at a close-in reference point in the far field region of the antenna at a small distance d0 from the transmitting antenna. The constraint n is the path loss exponent. Consider a forward link where the desired signal is the serving base station and where the interference is due to co channel base stations. Let Di denote the distance of the ith interferer from the mobile, the power received at a given mobile due to ith interfering cell will be proportional to (Di)-n. In urban cellular systems, the path loss exponent typically ranges between 2 and 4.

When the power transmitted by each base station is equal and the path loss exponent is the same throughout the coverage area, S/I for mobile can be approximated as

Considering only the first layer of interfering cells and if all the interfering base stations are equidistant from the desired base station and this distance is equal to distance D between cell centers then the above equation simplifies to

The above equation relates S/I to the cluster size N, which in turn determines the overall capacity of the system from C=M (kN) =MS

5.1. Co Channel Interference Reduction Methods

Cell splitting, sectoring and coverage zone are some of the technical approaches that are used to expand the capacity of cellular systems. Cell splitting allows an orderly increase of the cellular system. Directional antennas are used in sectoring technique to control the interference and frequency reuse of channels. The zone microcell concept widens the coverage of a cell and extends the cell boundary to hard to reach places. While cell splitting increases the number of base stations in order to increase capacity, sectoring and zone microcells rely on base station antenna placements to improve capacity by reducing co channel interference. The two main methods that reduce the co channel interference are discussed below.

5.1.1 Sectoring

In this technique, first the SIR is enhanced using directional antennas, and then capacity improvement is achieved by reducing the number of cells in a cluster, thus increasing the frequency reuse. The co channel interference in a cellular system may be decreased by replacing a single omnidirectional antenna at the base station by several directional antennas is called sectoring. The reason with which the co channel interference is reduced depends on the amount of sectoring used. Normally, a cell is partitioned into three 120 degree sector or six 60 degree sectors as shown below.

Figure 5.1: (a) 120 degree sectoring; (b) 60 degree sectoring

Assuming seven cell reuse, the number of interferers in the first tier is reduced from six to two, for the case of 120 degree sectors. Referring to the figure below, consider the interference experienced by a mobile located in the right most sectors in the center cell. There are three co channel cell sectors to the right of the center cell, and three to the left of the center cell. Out of those six co channel cells, only two cells have sectors with antenna patterns which radiate into the center cell, and hence a mobile in the center cell will experience interference on the forward link from only these two sectors. Further improvement in S/I is achieved by down tilting the sector antennas such that the radiation pattern in the vertical plane has a notch at the nearest co-channel cell distance. In the case of using 60 degree sectors, the signal to interference ratio is improved even more. In this case, the number of first tier interference is reduced from six to only one.

Figure 5.2: Illustration of how 120 degree sectoring reduces interference from co channel cells

5.1.2 Microcell Zone Concept

The microcell concept is a resolution to the difficulty of increased number of handoffs required when sectoring is employed. As the number of handoffs increases, it results in an increased load on the switching and control link elements of the mobile system. As illustrated in the figure, in this scheme, each of the three zone sites represented as Tx/Rx are connected to a single base station and share the same radio equipment. The zones are interconnected by coaxial cable, fiberoptic cable, or microwave link to the base station. As mobile moves within the cell, it is served by the zone with the strongest signal. This particular technique is superior to sectoring since antennas are placed at the outer edges of the cell, any base station channel may be assigned to any zone by the base station. A given channel is active only in the particular zone in which the mobile is traveling. Hence, the base station radiation is localized and interference is reduced. The main advantage of the zone cell technique is that while the cell maintains a particular coverage radius, the co-channel interference in the cellular system is reduced since a large central base station is replaced by several lower powered transmitters on the edges of the cell. Decreased co-channel interference improves the signal quality and also leads to an increase in capacity without the degradation in trunking efficiency caused by sectoring.

Figure 5.3: The microcell concept

SIMULINK AND THE COMMUNICATION TOOLBOX

SIMULINK is a program modeling linear and non-linear dynamic systems in the time-domain. Models in SIMULINK are represented in block diagram form and can be assembled from block libraries and sub libraries. Furthermore, SIMULINK allows the display of results and changes of certain model parameters without interrupting the simulation. Since SIMULINK is built upon the MATLAB numeric computation system, it offers direct access to the MATLAB workplace and MATLAB's mathematical and engineering functions. SIMULINK's main features are:

Modeling and analysis of dynamic systems, including linear, non linear, continuous, discrete and hybrid

Flexible 'open system' environment that allows addition of new blocks to SIMULINK.

Seamless interface with MATLAB's built-in math functions, 2-D and 3-D graphics, and add-on toolboxes for specialized applications.

Choice of methods of running a simulation(menu driven, on - screen or batch mode)

An optimized computer platform implementation that ensures fast and accurate results.

Unlimited model size

A typical SIMULINK session starts by either defining a new model or recalling a previously defined model and then proceeds to a simulation of the performance of that model. In practice these two steps are often performed iteratively as the model designer creates and modifies a model to achieve the desired behavior.

The communication toolbox is a collection of MATLAB functions and SIMULINK blocks for model development and simulation in the communications area. The functions/blocks are organized in the following sub categories: Data Source, Source coding and decoding, Error- control Coding, Modulation and Demodulation, Transmission and Reception filters, Transmitting Channel, Multiple Access, and Synchronization and Utilities.

Simulation for Co-Channel Interference Cancellation

A simulation model for co channel interference cancellation in PSK modulated signal is performed using SIMULINK in this session. The model includes a co-channel interference. The model shows the effect of co channel interference in PSK modulated signal with the effects on a spectrum plot.

The communication system in this model discussed includes these components:

A transmitter, that creates a PSK modulated signal. A square root raised cosine filter is applied with it. It results in the creation of the original signal to which the model adds interference.

The co channel interference introduced is similar to the transmitted signal but has a modifiable frequency offset and power gain. A sum block is provided in the model that adds the interfering signal to the original transmitted signal.

An Additive White Gaussian Noise (AWGN) channel.

A receiver, which filters, down-samples and demodulates the received signal to free it from the interference signal.

Figure 7.1: Co- Channel interference cancellation model

7.1. M-PSK Modulator

The M-PSK Modulator Baseband block modulates using M-ary phase shift keying method. The output is a baseband representation of the modulated signal. The M-ary number parameter, M is the number of points in the signal constellation.

Figure 7.2: M-PSK block

The output and input for this block are discrete time signals. For inputs that are integers, the block can accept the data types int8, uint8, int16, uint16, int32, uint32, single and double. For bit inputs, block can accept int8, uint8, int16, uint16, int32, uint32, Boolean, single and double.

Figure 7.3: M-PSK block parameter

M-ary number

It gives the number of points in the signal constellation.

Input type

It gives the indication whether the input consists of integers or group of bits. If this parameter is set to Bit, the M-ary number parameter must be 2k , where k is some positive integer. If it is set to Integer, it uses integers between 0 and M-1 as input values.

Constellation ordering

It determines how the block maps an integer or group of K bits to the corresponding symbol.

Phase offset

It is the phase of the zeroth point of the signal constellation.

Output data type

This can be set to double, single, fixed point, user defined, or inherit via back propagation.

7.2. Additive White Gaussian Noise (AWGN)

The AWGN channel block adds white Gaussian noise to a real or complex input signal. The block adds real Gaussian noise and produces a real output when the input signal is real. This block produces a complex output signal by adding a complex Gaussian noise, when the input signal is complex.

Figure 7.4: AWGN block

It inherits its sample time from the input signal. The noise is generated using the signal processing blockset random source block. The initial seed parameter in this block initializes the noise generator. The signal inputs can only be of type single or double. The port data types are taken from the signals that drive the block.

Figure 7.5: AWGN block parameter

Initial seed

The seed for the Additive Gaussian noise generator.

Mode

In this the mode by which the following specification for noise variance is set: Signal to noise ratio(Eb/No), Signal to noise ratio (Es/No), Signal to noise ratio(SNR), variance from port.

Eb/No (dB)

It is the ratio of bit energy per symbol to noise power spectral density in decibels.

Number of bit per symbol

It gives the number of bits in each symbol input.

Input signal power (Watts)

It is the mean square power of the input symbols or input samples in watts.

Symbol period

This parameter gives the duration of a channel symbol in seconds.

7.3. M-PSK Demodulator

The M-PSK Demodulator baseband block demodulates a signal that was modulated using the M-ary phase shift keying method. A baseband representation of the modulated signal is the input to this block. The output and input for this particular block are discrete time signals. The input can be a frame based column vector of data types single or double or it can be scalar.

Figure 7.6: MPSK demodulator block parameters

M-ary number

It gives the number of points in the signal constellation.

Constellation ordering

It determines how the block maps an integer or group of K bits to the corresponding symbol.

Phase offset

It is the phase of the zeroth point of the signal constellation.

Output data type

This can be set to double, single, fixed point, user defined, or inherit via back propagation.

7.4 Results and Displays

When the simulation model is run, the scope blocks in the model display these quantities:

The spectra of the original and interfering signals and the noisy transmitted signal

Figure 7.7: Transmitted Signal

The spectrum of the received signal

Figure 7.8: Received Signal

A scatter plot of the received signal constellation

Figure 7.9: Received Signal Constellation

8. Conclusion

The cellular system and the frequency reuse technique for efficient use of frequency spectrum has led to various communication interferences. Various interference reduction methods such as cell sectoring and microcell cell concept are employed to reduce co channel interference. Co channel interference cancellation can be done by implementing filter blocks and demodulators at the receiver. With the aid of SIMULINK model, cancellation of co channel interference in a PSK signal transmitted signal was done successfully. The results of the simulation model was obtained in the form of graphs that display the interference free received signal at the receiver.

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