Co Channel Interference Cci Computer Science Essay

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A cellular radio system provides a wireless connection to the public telephone network for any subscriber located within the coverage region by the system. Cellular networks provides numerous advantages over other solutions like increased capacity, reduced power usage, larger coverage area, reduced interference from other signals which has resulted in the gradual increase in the numbers of subscribers day by day. This results in inconvenient service, inefficient traffic and there is increase in blocking probability as number of subscribers' increases. Hence, in order to provide better service the network designer should consider all these parameters. This project mainly emphasizes on resource utilization of the available radio channels. Network performance is an essential factor and it should not be compromised at any cost.

1.2 Problem Definition

One of the most important performance measures of wireless networks is the system capacity. One of the methods of increasing the system capacity is by increasing the amount of radio spectrum to be used in wireless network. However, since radio spectrum is a very scarce resource, the solution turns out to be very expensive and changes in spectrum assignment have to be approved worldwide, which often takes years. Hence the possible way out is to utilize the available limited resources effectively and efficiently. Various factors like cell splitting, sectoring, frequency reuse enhances overall system. For the efficient utilization of radio resource, different channel assignment algorithms such as Fixed Channel Assignment, Borrowing Channel Assignment and Hybrid Channel Assignment Strategies are used. In telecommunication system, large number of user is connected to the system. It is difficult for the service provider to allocate individual channel for each user. Hence telecommunication engineering effectively allocates the channel for requested user.

1.3 Objectives

The project mainly deals with the traffic behavior of cellular system and efficient utilization of the available limited resources through different channel assignment strategies. The main objective of the project is to

Determine the channel efficiency of different channel assignment strategies i.e. Fixed Channel Assignment and Borrowing Channel Assignment.

The simulation determines the blocking probability of the system of various strategies.

By comparing the channel efficiency of each strategy, the best strategy is determined.

1.4 Methodology

To verify the performance of different channel assignment strategy, the MATLAB Programming has been used. MATLAB is a high-level language and provides interactive environment that enables to perform computationally intensive tasks faster than any other programming languages like C, C++, and Fortran. It allows matrix manipulation, plotting of functions for a given data. It allows algorithm implementation. It can also displays information graphically. It supports to design user interface that gives a user friendly environment.

This project requires the computation of Grade of service (GOS) for fixed channel assignment stategy and borrowing channel assignment stategy. The input data for the calculation are: average holding time, inter arrival time, number of available channel and Number of users. The program executes to provide the served calls and blocked call which is used to calculate the GOS. The calculated GOS is the result of the simulation which can be compared with the theoretical GOS. The system that provides comparatively small GOS is considered to be better.

Chapter 2 LITERATURE REVIEW

2.1 Cellular Network

A a radio network made up of a number of radio cells (or just cells) each served by at least one fixed-location transceiver known as a cell site or base station. These cells cover different land areas to provide radio coverage over a wider area than the area of one cell, so that a variable number of portable transceivers can be used in any one cell and moved through more than one cell during transmission. [8] Many functions such as mobility management, call set up and handover are performed by the network in order to improve the quality and efficiency of the network.

2.2 Frequency Reuse

The frequency reuse factor is the rate at which the same frequency can be used in the network. It is 1/K (or K according to some books) where K is the number of cells which cannot use the same frequencies for transmission. Common values for the frequency reuse factor are 1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12 depending on notation). [5]

f1

f2

f1

f4

f1

f2

f1

f3

f3

Fig.1 Frequency reuse factor or pattern 1/4

2.3 Cell splitting

As the number of mobile user increases, the cell becomes more congested and the available frequency spectrum is insufficient to provide satisfactory service to customer. In such a case the congested cell should to subdivide into smaller cell. The process of dividing the congested cell into smaller cell to provide required grade of service, is called cell splitting. Cell splitting increases the capacity of cellular system. As a result the transmit power for the split cell is reduced reduced to satisfy the SNR required by the system. Cell splitting reduces the average radius of cells in the cellular network, so is the reduced transmit distance

from the subscriber terminal to the base station, and vice versa. And because of the shorter

distances involved, transmitting power must be reduced at both the base station and the subscriber terminals to avoid interference throughout the network.

Fig.2 Cell Splitting

In the figure, the original base station 'A' is surrounded by six other base stations after cell splitting and also the radius of newly formed cell is half of the original cell.

2.4 Handoff

The term handoff refers to the process of transferring an ongoing call or data session from one channel connected to the core network to another. Hand off automatically changes frequencies as the mobile unit moves into a different frequency zone so that the conversation can be continued in a new frequency zone without redialing. Handoff must be performed successfully and as infrequently as possible, and be imperceptible to user. To meet these requirements, we should specify an optimum signal level at which handoff to be performed. [3]

A hard handoff occurs when users experienced an interruption during the handover process cause by frequency shifting. Soft handover is the ability to select between the instantaneous received signals from different base stations. If handover does not occur quickly, the QOS may degrade below an acceptable level and the connection will be dropped. Handoff can be classified into network-controlled handoff, mobile-controlled handoff and mobile-controlled handoff. When a mobile user move to the edge of the cell boundary, handover process will invoke. Handover initiation occurs dependent on different criteria and strategies used in the system. The most common criteria are: Relative signal strength, Relative signal strength with threshold, Relative signal strength with hysteresis, Relative signal strength with hysteresis and threshold. [3]

Signal Strength from base station 2

Signal Strength from base station 1

Threshold

Fig.2 Handoff Scenario

2.4 Interference

2.4.1 Co-channel Interference (CCI)

Co-channel interference is the factor that causes reduction in the capacity and performance of the cellular system. The distance between two cells having same frequency is limited by the co-channel interference and so the capacity of the cellular system is driven by the co-channel interference. Co-channel interference or CCI is crosstalk from two different radio transmitters using the same frequency.

When considering the size of cell almost constant and all the base station is transmitting same power, in such a case the co-channel interference is independent of the transmitted power and is function of 'D' and 'R'. The relationship between co channel interference distance (D) and radius (R) is given by

q = =

Where q is co-channel reuse ratio.

2.4.2 Adjacent channel Interference

Adjacent-channel interference or ACI is interference caused by extraneous power from a signal in an adjacent channel. ACI may be caused by inadequate filtering, such as incomplete filtering of unwanted modulation products in frequency modulation (FM) systems, improper tuning, or poor frequency control, in either the reference channel or the interfering channel, or both. [6]

2.5 Traffic Engineering

The task of teletraffic theory is to design systems as cost effectively as possible with a predefined grade of service when we know the future traffic demand and the capacity of system elements.

In teletraffic engineering the word traffic is used to denote the traffic intensity i.e traffic per unit time. Traffic intensity is the average number of calls simultaneously in progress during a particular period of time. It is measured in Erlang or CCS.

Relation Between CCS and Erlang

1 CCS = 100 Seconds of telephone time.

1 Erlang = 1 hour or 36 CCS of telephone line.

Traffic engineering is a method adapted to optimiz the performance of a telecommunications network by dynamically analyzing, predicting and regulating the behavior of data transmitted over that network. Traffic engineering is also known as teletraffic engineering and traffic management. [7]

Fig.3 Distribution of Traffic in 24 Hr

Fig. 3 shows that the traffic intensity varies with time. We can observe that the number of calls per minute is maximum between 9 - 11 Hr in a day. This time is called busy hour.

2.5.1 Traffic Intensity

The instantaneous traffic intensity in a pool of resources is the number of busy resources at a given instant of time. Most commonly used unit of traffic is Erlang. A traffic intensity of one Erlang means continuous occupancy of a facility during the time period under consideration.

2.5.2 Carried traffic

It is the volume of traffic carried by the switch. It refers to the maximum capacity of the network.

2.5.3 Offered traffic

Offered traffic refers to the average generated total traffic including the traffic that is blocked in the system. So the capacity should be higher than offered traffic; otherwise, many users would not be able to get service because all lines would be occupied all the time on average.

Relation between offered traffic and carried traffic

Carried traffic (Ct) = offered traffic (1- Blocking probability)

2.5.4 Lost or Rejected traffic or overflow

The difference between offered traffic and carried traffic is equal to the rejected traffic. The value of this parameter can be reduced by increasing the capacity of the system.

2.5.6 Overview of Taffic Flow

Overflow

Carried Traffic

SYSTEM

Offered Traffic

Fig 4. Traffic Flow

Fig 4 illustrates: offered traffic = Carried Traffic + Overflow

2.5.7 Inter arrival time

Interarrival time is a value used in queuing theory. Queuing theory uses models to analyze systems that involve waiting in lines for a service. The interarrival time is the amount of time between the arrival of one customer and the arrival of the next customer. It is calculated for each customer after the first and is often averaged to get the mean inter arrival time, represented by λ. [6]

2.5.8 Holding Time

It is average duration of a typical call and denoted by 'H'.

The traffic intensity offered by each user is equal to the call request rate multiplied by the holding time. That is, each user generates a traffic intensity of Au Erlang given by

Au = λH

Where λ is average number of call request per unit time for each user and H is holding time.

For a system containing u users and an unspecified number of channels the total offered traffic intensity A, is given by

A = UAu

Moreover, in a C channel trunk system, if the traffic is equally distributed among the channel, then the traffic intensity per channel, Ac, is given by

Ac=

When the offered traffic exceeds the maximum capacity of the system, the carried traffic becomes limited due to the limited capacity. The maximum possible carried traffic is the total number of channels, C, in Erlang.

2.5.9 Grade of Service (GOS)

It is the measured of proportion of calls that is lost due to congestion (Erlang B). In AMPS cellular system GOS of 2 % is specified which means 2 out of 100 called were rejected. Mathematically we defined GOS as

GOS =

So the GOS is specified for the traffic at the busy hour. At other times it is much better. If it is too large, subscribers are unable to complete the call and are unsatisfied with the operator and also this means congestion in the system may occurred at the busy hour. So if the offered traffic increases the number of trunks must obviously be increased to prove a given grade of service.

There are two type of trunking system available. First one is if the channel is available, the requested user will get that channel immediately upon request. If the channel is not available, the requested user is blocked without access and is free to try again later. This is called Loss Call Cleared (LCD). Erlang B formula is used to measure the GOS or for measurement of blocking probability which is given by the following relation

Pr [Blocking] =

Where C is the number of trunked channels offered by a trunked radio system and A is the total offered traffic.

Different Channel Assignment Strategies

2.6.1 Fixed Channel Assignment Strategy

In this strategy the number of channels is permanently assigned to a base station. Any call attempt within the cell can only be served by the unused channel in that particular cell. If channel is occupied, call is blocked and subscriber does not get the service. The same set of channels is used in other cells at some distance away. The distance between any two cells using the same set of channels must be equal to or greater than the co-channel reuse distance, which is the minimum distance at which radio frequency can be reused with acceptable signal/interference ratio.

2.6.2 Borrowing Channel Assignment Strategy

In this strategy, if the assign channel is fully occupied, a channel is borrowed from a neighboring cell. While borrowing it should ensure that the borrowing channel does not disrupt any of the calls in progress in the donor cell. In cellular system borrowing of the channel from one cell to another cell is monitor by the MSC.

2.6.3 Hybrid Channel Assignment Strategy

In hybrid channel allocation, all the available channels are divided into two groups, fixed and dynamic. Channels in the fixed group are assigned to cells as nominal channels, as in fixed channel allocation methods. Channels in the dynamic group are shared among all the cells. Nominal channels are preferred for use in their respective cells. When a call request comes in a cell, it is assigned a nominal channel if it is available. When no nominal channel is available, a channel from the dynamic group is assigned.

2.6.4 Dynamic Channel Assignment Strategy

Dynamic channel allocation methods do not assign permanent channels to cells. The Mobile Switching center, instead of the base station, is responsible for channel allocation. When there is a call request, the base station notifies the mobile switching center, which searches for the most suitable channel. While assigning the channel by the MSC, these factors are considered such as reuse distance, future call blocking probability, and channel use frequency.

The main advantages of dynamic channel allocation are flexibility and traffic adaptability. It is because channels are not assigned into the cells as nominal channel, and each channel assignment decision is made dynamically based on the network condition.

The underlying strategy in fix channel assignment is the permanent assignment of a set of channels to both new calls and handoff calls. If the call is requested from the cells then the call will be served from the available channel that has been allocated as the nominal channel. If resources are not available then that call will be blocked. Same thing happens for handoff calls also. GOS for newly generated call and the handoff call are independent. In fixed channel assignment algorithm there is no provision for sharing resources although the resources are free.

Chapter 3 RESEARCH METHODOLOGY

3.1 Implementation Issues

In the process of implementation of the projects, the following input parameters are assumed: the number of calls that is to be processed, average inter arrival time, average holding time and total number of available channel for both new calls and the Handoff calls. From this parameter the call arrival time and call terminating time for all calls is determined. By comparing the arrival time, call terminating time and available free channel, the number of served calls and blocked calls are determined. This method is applied for different channel assignment strategy.

In case of fixed channel assignment strategy pre determined set of channel are allocated for both handoff calls and the New calls. And performance of this strategy is calculated. Where as in case of borrowing strategy if the available channel is fully occupied, then channel will be borrowed from the other cells to serve the requested calls without disrupting call in progress at that calls. This strategy considerable increases the channel efficiency.

For the verification of Erlang B table, first we have to generate the N number of calls. The input parameters: Average holding time, calls arrival rate per hour and total number of calls are kept fixed for varying number of channels. The program calculates the offered traffic (traffic intensity). Further served calls and blocked calls are calculated. Blocking probability of the system is determined by finding the ratio of the blocked calls and the total attempted calls.

For fixed channel assignment strategy, blocking probability for both newly generated calls and handoff calls are determined independently using above process.

In borrowing channel assignment strategy if the available channel is fully occupied and new call is generated from the cells then the call will be served by borrowing the channel from other cells or the channel that has been assigned for hand off calls.

3.2 FLOWCHARTS

3.2.1 Flow Chart: Erlang B Verification

start

Available channel>1?

input avg holding time, inter arrival time, no. of calls, total no of channels

Determine call arrival time and call terminating time

Request call<total no of calls?

NO

GOS=

Calculate theoretical

Y

End

Block=block+1 No

Update block

Accept call

Yes

Served=served+1

Update served

Available channel= channel-1

Is call terminated?

Available channel= channel+1 Yes

Update channel

No

3.2.2 Flow Chart for Borrowing Channel Assignment Strategy

For New Call

start

For Handoff call

input avg holding time, inter arrival time, no. of calls, total no of channels

input avg holding time, inter arrival time, no. of calls, total no of channels

Determine call arrival time and call terminating time

Determine call arrival time and call terminating time

Request call<total no of calls?

Request call<total no of calls?

Update Hblock

GOS=

End

Update block

GOS=

No No

Block=block+1

Hblock=Hblock+1

Available channel>1?

End

Available channel>1?

Yes Y

Is channel available in neighboring cell?

Is channel available in neighboring cell? NO NO

Accept call

Accept call Yes NO Y N

Served=served+1

Served=served+1

Is GOS ≤set value?

Update served

Is GOS ≤set value?

Update served

Yes Y

N

Available channel= channel-1

Available channel= channel-1 No

Ch= Ch+1 Hch=Hch-1

Is cell cold cell?

Ch= Ch+1 Hch=Hch-1

Is cell cold cell?

Yes Y

Is call terminated?

Is call terminated? Y

Hch=Hch+11

Update channel

Yes Y

ch=cha+1

Update channel

N N

Chapter 4 RESULT AND CONCLUSION

4.1 Result

4.1.1 Simulation results for Erlang B verification

No. of Channel

Traffic Intensity

Blocking from Erlang B Formula

Blocking from Simulation

7

7.505

28.4115

28.43

10

7.5228

10.0525

9.98

12

7.5495

3.94228

3.53

15

7.4751

0.553763

0.38

Table 1. Erlang B Verification

The above table verifies the Erlang B formula. The Erlang B formula is used to measure the GOS or for measurement of blocking probability which is given by the following relation:

Pr [Blocking] =

Where C : number of trunked channels offered by a trunked radio system

A : total offered traffic.

Given Inputs:

Average Holding Time: 3

Call Arrival Rate per Hour: 150

Total Number of channels: X

Total number of channels: 20000

4.1.2 Simulation results for Fixed Channel Assignment Strategy

DATA 1:

INPUT PANEL:

OUTPUT PANEL:

4.1.3 Simulation results for Borrowing Channel Assignment Strategy

DATA 1:

INPUT PANEL:

OUTPUT PANEL:

DATA 2:

INPUT PANEL:

OUTPUT PANEL:

4.2 Comparison between Fixed channel assignment strategy and borrowing channel strategy

Comparing DATA 1 of fixed channel assignment strategy and DATA 2 of borrowing channel assignment strategy, we can observe that for the same values of input parameters, the system blocking probability obtained from the simulation in case of borrowing channel assignment strategy is less as compares to that of the fixed channel assignment strategy. Hence we can conclude that the borrowing channel strategy provides optimized and efficient method of radio resource utilization.

4.3 Conclusion

From the above simulation result of different channel assignment strategy, for given number of channels and total number of the calls the performance of the borrowing channel assignment strategy is far better than that of fixed channel assignment strategy. In borrowing assignment strategy the limited channels are efficiently utilized hence increasing the channel efficiency. Also, these strategy improves the GOS of the system while in the case of fixed channel strategy it is not so.

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