introduction to channel borrowing scheme in cellular networks


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introduction to channel borrowing scheme in cellular networks

  1. 1. Introduction to channel borrowingscheme cellular networksByTanmoy BarmanHALDIA INSTITUTE OF TECHNOLOGY1|Page
  2. 2. IntroductionAdvances in cellular mobile technology have engendered a newparadigm of computing, called mobile computing. The frequencyspectrum allocated to this service is not sufficient with respect toenormous growth of mobile communication users. Tracking down amobile user in a cellular network which is a collection of geometric areascalled cells each serviced by a base station is the other concern to thedesigner. This service also needs another problem to be solved, that isthe access of common information by mobile users. Other than theissues in existing technology, many other issues on developingtechnology need to be addressed. One main issue in cellular systemdesign reduces to one of economics. Essentially we have a limitedresource transmission spectrum that must be shared by several users.Unlike wired communications which benefits from isolation provided bycables, wireless users within close proximity of one another can causesignificant interference to one another. To address this issue, theconcept of cellular communications was introduced around in 1968 byresearchers at AT&T Bell Labs.2|Page
  3. 3. Channel AllocationThere are several challenges in mobile cellular environment which isgenerally conceived as a collection of geometric areas called cells, eachserviced by a base station (BS) located at its centre. A number of cellsare again linked to a mobile switching centre (MSC) which also acts as agateway of the cellular network to the existing wired network (shown inFigure 1). The problems in this area are clearly divided in two parts.Some of them are based on electronics and telecommunication andsome of them are information based.The basic concept being that a given geography is divided into polygons(hexagon) called cells. Each cell is allocated a portion of the totalfrequency spectrum. As users move into a given cell, they are thenpermitted to utilize the channel allocated to that cell. The virtue of thecellular system is that different cells can use the same channel giventhat the cells are separated by a minimum distance according to thesystem propagation characteristics; otherwise, intercellular or co-channel interference occurs. The minimum distance necessary to reduceco-channel interference is called the reuse distance. The reuse distanceis defined as the ratio of the distance, D, between cells that can use thesame channel without causing interference and the cell radius, R. Notethat R is the distance from the center of a cell to the outermost point ofthe cell in cases when the cells are not circular.3|Page
  4. 4. Figure 1A given radio spectrum is to be divided into a set of disjointed channelsthat can be used simultaneously while minimizing interference inadjacent channel by allocating channels appropriately (especially fortraffic channels).Channel allocation deals with the allocation of channels to cells in acellular network. Once the channels are allocated, cells may then allowusers within the cell to communicate via the available channels.Channels in a wireless communication system typically consist of timeslots, frequency bands and/or CDMA pseudo noise sequences, but in anabstract sense, they can represent any generic transmission resource.There are three major categories for assigning these channels to cells(or base-stations). They are Fixed Channel Allocation, Dynamic Channel Allocation and Hybrid Channel Allocation.4|Page
  5. 5. Fixed Channel AllocationFixed Channel Allocation (FCA) systems allocate specific channels tospecific cells. This allocation is static and cannot be changed. Forefficient operation, FCA systems typically allocate channels in a mannerthat maximizes frequency reuse. Thus, in a FCA system, the distancebetween cells using the same channel is the minimum reuse distance forthat system. The problem with FCA systems is quite simple and occurswhenever the offered traffic to a network of base stations is not uniform.Consider a case in which two adjacent cells are allocated N channelseach. There clearly can be situations in which one cell has a need forN+k channels while the adjacent cell only requires N-m channels (forpositive integers k and m). In such a case, k users in the first cell wouldbe blocked from making calls while m channels in the second cell wouldgo unused. Clearly in this situation of non-uniform spatial offered traffic,the available channels are not being used efficiently. FCA has beenimplemented on a widespread level to date(shown in Figure 2).In FCA schemes, a set of channels is permanently allocated to each cellin the network.5|Page
  6. 6. If the total number of available channels in the system S is divided intosets, the minimum number of channel sets N required to serve the entirecoverage area is related to the frequency reuse distance D as follows:N = D2 / 3R2Due to short term fluctuations in the traffic, FCA schemes are often notable to maintain high quality of service and capacity attainable with statictraffic demands. One approach to address this problem is to borrow freechannels from neighboring cells. Figure 2Dynamic Channel AllocationIn DCA schemes, all channels are kept in a central pool and areassigned dynamically to new calls as they arrive in the system. Aftereach call is completed, the channel is returned to the central pool. It isfairly straightforward to select the most appropriate channel for any callbased simply on current allocation and current traffic, with the aim ofminimizing the interference. DCA scheme can overcome the problem ofFCA scheme. However, variations in DCA schemes center around thedifferent cost functions used for selecting one of the candidate channelsfor assignment (shown in figure 3).6|Page
  7. 7. Figure 3DCA schemes can be centralized or distributed.The centralized DCA scheme involves a single controller selecting achannel for each cell;The distributed DCA scheme involves a number of controllers scatteredacross the network (MSCs).Centralized DCA schemes can theoretically provide the bestperformance. However, the enormous amount of computation andcommunication among BSs leads to excessive system latencies andrenders centralized DCA schemes impractical. Nevertheless, centralizedDCA schemes often provide a useful benchmark to compare practicaldecentralized DCA schemes.Dynamic Channel Allocation (DCA) attempts to alleviate the problemmentioned for FCA systems when offered traffic is non-uniform. In DCAsystems, no set relationship exists between channels and cells. Instead,channels are part of a pool of resources. Whenever a channel is neededby a cell, the channel is allocated under the constraint that frequency7|Page
  8. 8. reuse requirements cannot be violated. There are two problems thattypically occur with DCA based systems. • First, DCA methods typically have a degree of randomness associated with them and this leads to the fact that frequency reuse is often not maximized unlike the case for FCA systems in which cells using the same channel are separated by the minimum reuse distance. • Secondly, DCA methods often involve complex algorithms for deciding which available channel is most efficient. These algorithms can be very computationally intensive and may require large computing resources in order to be real-time.Centralised DCS Scheme  For a new call, a free channel from the central pool is selected that would maximize the number of members in its co-channel set.  Minimize the mean square of distance between cells using the same channel.Scheme DescriptionFirst Available Among the DCA schemes the simplest one is the(FA) FA strategy. In F A, the first available channel within the reuse distance encountered during a channel search is assigned to the call. The FA strategy minimizes the system computational time.Locally Optimized The channel selection is based on the futureDynamic blocking probability in the vicinity of the cell where aAssignment call is initiated.(LODA)8|Page
  9. 9. Scheme DescriptionMean Square The MSQ scheme selects the available channel that(MSQ), minimizes the mean square of the distance among the cells using the same channel.1-clique This scheme uses a set of graphs, one for each channel, expressing the non co-channel interference structure over the whole service area for that channel.Distributed DCA Scheme  Based on one of the three parameters:-  Co-channel distance - co-channel cells in the neighborhood not using the channel. - Sometimes adjacent channel interference taken in toaccount.  Signal strength measurement - anticipated CIR above threshold.  Signal to noise interference ratio - satisfy desired CIR ratio.9|Page
  10. 10. Hybrid Channel AllocationHCA schemes are the combination of both FCA and DCA techniques. InHCA schemes, the total number of channels available for service isdivided into fixed and dynamic sets. The fixed set contains a number ofnominal channels that are assigned to cells as in the FCA schemes and,in all cases, are to be preferred for use in their respective cells. Thedynamic set is shared by all users in the system to increase flexibility.Request for a channel from the dynamic set is initiated only when thecell has exhausted using all its channels from the fixed set (shown infigure 4).Extra features:- 3:1 (fixed to dynamic), provides better service than fixed scheme for 50% traffic. Beyond 50% fixed scheme perform better. For dynamic, with traffic load of 15% to 32%, better results are found with HCA.Example: When a call requires service from a cell and all of its nominalchannels are busy, a channel from the dynamic set is assigned to thecall.10 | P a g e
  11. 11. Figure 4Switching strategies11 | P a g e
  12. 12. Comparison between FCA and DCAFCA DCA  Performs better under heavy  Performs better under traffic light/moderate traffic  Low flexibility in channel  Flexible channel allocation assignment  Not always maximum channel12 | P a g e
  13. 13.  Maximum channel reusability reusability  Sensitive to time and spatial  Insensitive to time and time spatial changes changes  Not stable grade of service per  Stable grade of service per cell in cell in an interference cell an interference cell group group  Low to moderate forced call  High forced call termination termination probability probability  Suitable in microcellular  Suitable for large cell environment environment  High flexibility  Low flexibility  Radio equipment covers the  Radio equipment covers all temporary channel assigned to the channels assigned to the cell cell  Independent channel control  Fully centralized to fully distributed control dependent on the scheme  Low computational effort  High computational effort  Low call set up delay  Moderate to high call set up delay  Low implementation complexity  Moderate to high implementation complexity  Complex, labor intensive frequency planning  No frequency planning  Low signaling load  Moderate to high signaling load  Centralized control  Centralized, distributed control depending on the scheme13 | P a g e
  14. 14. Common Principles of Channel AllocationSchemesThe large array of possible channel allocation systems can becomecumbersome. However, all channel allocation methods operate undersimple, common principles. Throughout this report we have touched onthree points which an efficient channel allocation scheme shouldaddress: 1. Channel allocation schemes must not violate minimum frequency reuse conditions. 2. Channel allocation schemes should adapt to changing traffic conditions. 3. Channel allocation schemes should approach (from above) the minimum frequency reuse constraints so as to efficiently utilize available transmission resources.As the first requirement suggests, all channel allocation schemes adhereto condition 1. From a frequency reuse standpoint, a fixed channelallocation system distributes frequency (or other transmission) resources14 | P a g e
  15. 15. to the cells in an optimum manner; i.e., common channels are separatedby the minimum frequency reuse distance. Thus, a fixed channelallocation scheme perfectly satisfies condition 3 as well. However, afixed allocation scheme does not satisfy condition 2.Philosophically, any dynamic channel allocation scheme will meet therequirements of all of theabove three conditions to some degree. At the system architecture leveldynamic channel allocation schemes may differ widely, butfundamentally, their only difference is in the degree to which they satisfycondition 3. Different DCA schemes attempt to satisfy condition 3 (inaddition to conditions 1 and 2) by approaching the minimum frequencyreuse constraint arbitrarily closely, and by doing so in as short a timeperiod as possible. The above three conditions point to the fact thatdesign of dynamic channel allocation schemes falls within the generalclass of optimization problems. Furthermore, since we can alwaysassume that the available number of base stations is finite and thetransmission resources will always be countable (due to FCCrequirements if nothing else) then our problem can be reduced to thesubclass of combinatorial optimization problems. As with allcombinatorial optimization problems, there will exist a solution spaceand a cost function. A typical element of the solution space could be aparticular layout of frequency channels among the base-stations. Thecost function can be loosely characterized as the difference between thefrequency reuse of an arbitrary solution and the frequency reuse of theoptimized solution. The error associated with a non-optimized cost isrealized as a future increased blocking probability or an otherwiseunwarranted lack of channel availability. It is typically assumed that thesolution to the wireless dynamic channel allocation problem is NP-complete [Yue, Cox - 1971]. The definition of np-completeness followsfrom the conjecture made in the late 1960s that there exists a class ofcombinatorial optimization problems of such inherent complexity that anyalgorithm, solving each instance of such a problem to optimality, requiresa computational effort that grows super polynomially with the size of theproblem. In the case of dynamic channel allocation, the complexity isgenerally attributed to the required inclusion of co-channel interferencein any analysis of dynamic channel allocation schemes. The author isaware of one published article to date offering an analytical method(approximate) for calculating the performance of dynamic channelallocation. Recently, several approximation techniques have beenproposed as methods for solving condition 3 of the dynamic channelallocation problem. In particular there has been interest in applyingsimulated annealing techniques [Duque-Anton] and neural network15 | P a g e
  16. 16. methods to dynamic channel allocation.ConclusionThis document has been briefly discussed about the static and dynamicallocation techniques on cellular networks. These techniques have beenimplemented in different areas and each technique has its advantagesand disadvantages. Numbers of works are going on this field so far andfurthermore many researches are going to make these techniques morestable. Growing up mobile technologies must need a reliable techniqueto cope up with this matter.16 | P a g e
  17. 17. BibliographyThis document has been created with the help of Wireless communication department paper Challenges of computing in mobile cellular environment—a survey by S. DasBit*, S. Mitra Wireless telecommunication Lab Document17 | P a g e
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