A cellular wireless system provides a wireless connexion to the populace telephone web for any endorser located within the coverage part by the system. Cellular webs provides legion advantages over other solutions like increased capacity, reduced power use, larger coverage country, reduced intervention from other signals which has resulted in the gradual addition in the Numberss of endorsers twenty-four hours by twenty-four hours. This consequences in inconvenient service, inefficient traffic and there is addition in barricading chance as figure of endorsers ‘ additions. Hence, in order to supply better service the web interior decorator should see all these parametric quantities. This undertaking chiefly emphasizes on resource use of the available wireless channels. Network public presentation is an indispensable factor and it should non be compromised at any cost.
One of the most of import public presentation steps of radio webs is the system capacity. One of the methods of increasing the system capacity is by increasing the sum of wireless spectrum to be used in wireless web. However, since wireless spectrum is a really scarce resource, the solution turns out to be really expensive and alterations in spectrum assignment have to be approved worldwide, which frequently takes old ages. Hence the possible manner out is to use the available limited resources efficaciously and expeditiously. Assorted factors like cell splitting, sectoring, frequence reuse enhances overall system. For the efficient use of wireless resource, different channel assignment algorithms such as Fixed Channel Assignment, Borrowing Channel Assignment and Hybrid Channel Assignment Strategies are used. In telecommunication system, big figure of user is connected to the system. It is hard for the service supplier to apportion single channel for each user. Hence telecommunication technology efficaciously allocates the channel for requested user.
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The undertaking mainly trades with the traffic behaviour of cellular system and efficient use of the available limited resources through different channel assignment schemes. The chief aim of the undertaking is to
- Determine the channel efficiency of different channel assignment schemes i.e. Fixed Channel Assignment and Borrowing Channel Assignment.
- The simulation determines the barricading chance of the system of assorted schemes.
- By comparing the channel efficiency of each scheme, the best scheme is determined.
To verify the public presentation of different channel assignment scheme, the MATLAB Programming has been used. MATLAB is a high-ranking linguistic communication and provides synergistic environment that enables to execute computationally intensive undertakings faster than any other scheduling linguistic communications like C, C++ , and Fortran. It allows matrix use, plotting of maps for a given information. It allows algorithm execution. It can besides expose information diagrammatically. It supports to plan user interface that gives a user friendly environment.
This undertaking requires the calculation of Grade of service ( GOS ) for fixed channel assignment scheme and adoption channel assignment scheme. The input informations for the computation are: mean keeping clip, inter reaching clip, figure of available channel and Number of users. The plan executes to supply the served calls and blocked call which is used to cipher the GOS. The deliberate GOS is the consequence of the simulation which can be compared with the theoretical GOS. The system that provides relatively little GOS is considered to be better.
A a wireless web made up of a figure of wireless cells ( or merely cells ) each served by at least one fixed-location transceiver known as a cell site or base station. These cells cover different land countries to supply wireless coverage over a wider country than the country of one cell, so that a variable figure of portable transceivers can be used in any one cell and moved through more than one cell during transmittal. [ 8 ] Many maps such as mobility direction, call set up and handover are performed by the web in order to better the quality and efficiency of the web.
The frequence reuse factor is the rate at which the same frequence can be used in the web. It is 1/K ( or K harmonizing to some books ) where K is the figure of cells which can non utilize the same frequences for transmittal. Common values for the frequence reuse factor are 1/3, 1/4, 1/7, 1/9 and 1/12 ( or 3, 4, 7, 9 and 12 depending on notation ) .
As the figure of nomadic user additions, the cell becomes more engorged and the available frequence spectrum is deficient to supply satisfactory service to client. In such a instance the congested cell should to subdivide into smaller cell. The procedure of spliting the congested cell into smaller cell to supply required class of service, is called cell splitting. Cell splitting increases the capacity of cellular system. As a consequence the transmit power for the split cell is reduced to fulfill the SNR required by the system. Cell dividing reduces the mean radius of cells in the cellular web, so is the decreased transmit distance from the subscriber terminus to the base station, and frailty versa. And because of the shorter distances involved, conveying power must be reduced at both the base station and the endorser terminuss to avoid intervention throughout the web.
In the figure, the original base station ‘A ‘ is surrounded by six other base Stationss after cell splitting and besides the radius of freshly formed cell is half of the original cell.
The term handoff refers to the procedure of reassigning an on-going call or information session from one channel connected to the nucleus web to another. Hand off automatically alterations frequences as the nomadic unit moves into a different frequence zone so that the conversation can be continued in a new frequence zone without redialing. Handoff must be performed successfully and every bit infrequently as possible, and be unperceivable to user. To run into these demands, we should stipulate an optimal signal degree at which handoff to be performed.
A difficult handoff occurs when users experienced an break during the handover procedure cause by frequence switching. Soft handover is the ability to choose between the instantaneous received signals from different base Stationss. If handover does non happen rapidly, the QOS may degrade below an acceptable degree and the connexion will be dropped. Handoff can be classified into network-controlled handoff, mobile-controlled handoff and mobile-controlled handoff. When a nomadic user move to the border of the cell boundary, handover procedure will raise. Handover induction occurs dependent on different standards and schemes used in the system. The most common standards are: Relative signal strength, Relative signal strength with threshold, Relative signal strength with hysteresis, Relative signal strength with hysteresis and threshold.
Co-channel Interference ( CCI )
Co-channel intervention is the factor that causes decrease in the capacity and public presentation of the cellular system. The distance between two cells holding same frequence is limited by the co-channel intervention and so the capacity of the cellular system is driven by the co-channel intervention. Co-channel intervention or CCI is crosstalk from two different wireless senders utilizing the same frequence.
When sing the size of cell about changeless and all the base station is conveying same power, in such a instance the co-channel intervention is independent of the familial power and is map of ‘D ‘ and ‘R ‘ . The relationship between carbon monoxide channel intervention distance ( D ) and radius ( R ) is given by
Adjacent channel Intervention
Adjacent-channel intervention or ACI is intervention caused by immaterial power from a signal in an next channel. ACI may be caused by unequal filtering, such as uncomplete filtering of unwanted transition merchandises in frequence transition ( FM ) systems, improper tuning, or hapless frequence control, in either the mention channel or the meddlesome channel, or both.
The undertaking of teletraffic theory is to plan systems as cost efficaciously as possible with a predefined class of service when we know the hereafter traffic demand and the capacity of system elements.
In teletraffic technology the word traffic is used to denote the traffic strength i.e traffic per unit clip. Traffic strength is the mean figure of calls at the same time in advancement during a peculiar period of clip. It is measured in Erlang or CCS.
Traffic technology is a method adapted to optimise the public presentation of a telecommunications web by dynamically analysing, foretelling and modulating the behaviour of informations transmitted over that web. Traffic technology is besides known as tele traffic technology and traffic direction.
The instantaneous traffic strength in a pool of resources is the figure of busy resources at a given blink of an eye of clip. Most normally used unit of traffic is Erlang. A traffic strength of one Erlang means uninterrupted tenancy of a installation during the clip period under consideration.
It is the volume of traffic carried by the switch. It refers to the maximal capacity of the web.
Offered traffic refers to the norm generated entire traffic including the traffic that is blocked in the system. So the capacity should be higher than offered traffic ; otherwise, many users would non be able to acquire service because all lines would be occupied all the clip on norm.
Relation between offered traffic and carried traffic
Carried traffic ( Ct ) = offered traffic ( 1- Blocking chance )
Lost or Rejected traffic or flood
The difference between offered traffic and carried traffic is equal to the rejected traffic. The value of this parametric quantity can be reduced by increasing the capacity of the system.
Overview of Taffic Flow
Inter reaching clip
Interarrival clip is a value used in line uping theory. Line uping theory utilizations theoretical accounts to analyse systems that involve waiting in lines for a service. The interarrival clip is the sum of clip between the reaching of one client and the reaching of the following client. It is calculated for each client after the first and is frequently averaged to acquire the average inter reaching clip, represented by? .
It is mean continuance of a typical call and denoted by ‘H ‘ .
The traffic strength offered by each user is equal to the call petition rate multiplied by the keeping clip. That is, each user generates a traffic strength of Au Erlang given by
Where? is mean figure of call petition per unit clip for each user and H is keeping clip.
For a system incorporating u users and an unspecified figure of channels the entire offered traffic strength A, is given by
Furthermore, in a C channel bole system, if the traffic is every bit distributed among the channel, so the traffic strength per channel, Ac, is given by
When the offered traffic exceeds the maximal capacity of the system, the carried traffic becomes limited due to the limited capacity. The maximal possible carried traffic is the entire figure of channels, C, in Erlang.
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
So the GOS is specified for the traffic at the busy hr. At other times it is much better. If it is excessively big, endorsers are unable to finish the call and are unsatisfied with the operator and besides this means congestion in the system may happen at the busy hr. So if the offered traffic increases the figure of short pantss must evidently be increased to turn out a given class of service.
There are two type of trunking system available. First one is if the channel is available, the requested user will acquire that channel instantly upon petition. If the channel is non available, the requested user is blocked without entree and is free to seek once more subsequently. This is called Loss Call Cleared ( LCD ) . Erlang B expression is used to mensurate the GOS or for measuring of barricading chance which is given by the undermentioned relation
Where C is the figure of trunked channels offered by a trunked wireless system and A is the entire offered traffic.
Different Channel Assignment Schemes
Fixed Channel Assignment Strategy
In this scheme the figure of channels is for good assigned to a base station. Any call effort within the cell can merely be served by the fresh channel in that peculiar cell. If channel is occupied, call is blocked and subscriber does non acquire the service. The same set of channels is used in other cells at some distance off. The distance between any two cells utilizing the same set of channels must be equal to or greater than the co-channel reuse distance, which is the minimal distance at which wireless frequence can be reused with acceptable signal/interference ratio.
Borrowing Channel Assignment Strategy
In this scheme, if the assign channel is to the full occupied, a channel is borrowed from a adjacent cell. While borrowing it should guarantee that the adoption channel does non interrupt any of the calls in advancement in the giver cell. In cellular system adoption of the channel from one cell to another cell is monitor by the MSC.
Hybrid Channel Assignment Strategy
In intercrossed channel allotment, all the available channels are divided into two groups, fixed and dynamic. Channelss in the fixed group are assigned to cells as nominal channels, as in fixed channel allotment methods. Channelss in the dynamic group are shared among all the cells. Nominal channels are preferred for usage in their several cells. When a call petition 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.
Dynamic Channel Assignment Strategy
Dynamic channel allotment methods do non delegate lasting channels to cells. The Mobile Switching centre, alternatively of the base station, is responsible for channel allotment. When there is a call petition, the base station notifies the nomadic shift centre, which searches for the most suited channel. While delegating the channel by the MSC, these factors are considered such as reuse distance, future call barricading chance, and channel use frequence.
The chief advantages of dynamic channel allotment are flexibleness and traffic adaptability. It is because channels are non assigned into the cells as nominal channel, and each channel assignment determination is made dynamically based on the web status.
The underlying scheme in fix channel assignment is the lasting assignment of a set of channels to both new calls and handoff calls. If the call is requested from the cells so the call will be served from the available channel that has been allocated as the nominal channel. If resources are non available so that call will be blocked. Same thing happens for handoff calls besides. GOS for freshly generated call and the handoff call are independent. In fixed channel assignment algorithm there is no proviso for sharing resources although the resources are free.
In the procedure of execution of the undertakings, the undermentioned input parametric quantities are assumed: the figure of calls that is to be processed, mean inter reaching clip, mean keeping clip and entire figure of available channel for both new calls and the Handoff calls. From this parametric quantity the call reaching clip and name terminating clip for all calls is determined. By comparing the reaching clip, name terminating clip and available free channel, the figure of served calls and blocked calls are determined. This method is applied for different channel assignment scheme.
In instance of fixed channel assignment scheme pre determined set of channel are allocated for both handoff calls and the New calls. And public presentation of this scheme is calculated. Where as in instance of borrowing scheme if the available channel is to the full occupied, so impart will be borrowed from the other cells to function the requested calls without interrupting call in advancement at that calls. This scheme considerable increases the channel efficiency.
For the confirmation of Erlang B tabular array, foremost we have to bring forth the N figure of calls. The input parametric quantities: Average keeping clip, calls arrival rate per hr and entire figure of calls are kept fixed for changing figure of channels. The plan calculates the offered traffic ( traffic strength ) . Further served calls and blocked calls are calculated. Barricading chance of the system is determined by happening the ratio of the out of use calls and the entire attempted calls.
For fixed channel assignment scheme, barricading chance for both freshly generated calls and handoff calls are determined independently utilizing above procedure.
In borrowing channel assignment scheme if the available channel is to the full occupied and new call is generated from the cells so the call will be served by borrowing the channel from other cells or the channel that has been assigned for manus off calls.
RESULT AND CONCLUSION
Simulation consequences for Erlang B confirmation
The above tabular array verifies the Erlang B expression. The Erlang B expression is used to mensurate the GOS or for measuring of barricading chance which is given by the undermentioned relation:
Simulation consequences for Fixed Channel Assignment Strategy
Simulation consequences for Borrowing Channel Assignment Strategy
Comparison between Fixed channel assignment scheme and adoption channel scheme
Comparing DATA 1 of fixed channel assignment scheme and DATA 2 of borrowing channel assignment scheme, we can detect that for the same values of input parametric quantities, the system barricading chance obtained from the simulation in instance of borrowing channel assignment scheme is less as comparisons to that of the fixed channel assignment scheme. Hence we can reason that the adoption channel scheme provides minimal blocking as compared to fixed channel assignment scheme.
Analysis based on simulation can be performed for intercrossed channel assignment scheme which will ensue in better blocking chance among other channel assignment schemes.
Decision and Recommendation
From the above simulation consequence of different channel assignment scheme, for given figure of channels and entire figure of the calls, the public presentation of the adoption channel assignment scheme is far better than that of fixed channel assignment scheme. In borrowing assignment scheme, the limited channels are expeditiously utilised hence increasing the channel efficiency. Besides, these scheme improves the GOS of the system while in the instance of fixed channel scheme it is non so.
- I. Katzela, M. Naghshineh, “ Channel Assignment Schemes for Cellular Mobile Telecommunication Systems: A Comprehensive Survey, ” IEEE Personal Comm. , June 1996.
- An adaptative Quality of Service Channel Borrowing Algorithm for Cellular Networks,Ibrahim Habib1, Mahmoud Sherif 2, Mahmoud Naghshineh3, and Parviz Kermani3.
- Rappaport, T.S. , Wireless Communication-Principles and Practice, Pearson Education, 2003
- Flood, J.E. , Telecommunication Switching, Traffic and Network, Pearson Education, 2002.
- Comparision of Channel-Assignment Stategies in Cellular Mobile Telephone Systems,Ming Zhang, Student Member, IEEE, and Tak-shing P. Yum, Senior Member, IEEE
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