The Cellular Concept-System Design Fundamentals

1. The Cellular Concept-System Design Fundamentals

2. Intro to Cellular concept • Divide the service area into small cells • To reduce the transmitting power, no of towers should be at least equal to no.of cells • Re-use the allocated frequency spectrum(channels) as many times as possible. • A cluster is group of cells which is replicated throughout the system • Advantages: • Spectral congestion • User capacity

3. Disadvantages • Greater capacity at the expense of large infrastructure • Optimal frequency utilization by making system more complicated • User equipment design made smarter at the cost of circuit complexity and processing power

4. Frequency Re-use • “Channel “- stands for a pair of radio frequencies used for two way connection between base and mobile subscriber • Cell- a small geographic area. • When considering geometric shapes which cover an entire region without overlap , there are 3 choices: • A square • Equilateral Triangle • Hexagon • Cells with the same number use the same set of frequencies

5. 3 4 3 1 1 2 2 3 3 1 4 3 4 2 3 1 1 2 2 1 2 Cluster Size 3 Cell Cluster 4 Cell Cluster

6. 3 4 1 2 5 7 6 3 4 1 2 5 7 6 3 4 1 2 5 7 6 Frequency Reuse for Area Coverage Base Station (BS)

7. Cellular Concept • S=total number of duplex channels available for use in a given area It depends on • K=Group of channels allocated to each cell • N=cluster size i.e. number of cells allocated to each cluster • Total number of available channels S=KN

8. Capacity • If cluster is replicated M times within the system, the total number of duplex channels, C, can be used as a measure of system capacity • C=MS where S=KN • The capacity of cellular system is directly proportional to the number of times a cluster is replicated in a fixed service area • The factor N is called cluster size • Typically N=4,7,12 • Larger cluster , means less replication and low capacity

9. Frequency Re-use Factor • Smallest possible value of N is desirable in order to maximize capacity over a given coverage area • The frequency reuse factor is given by 1/N of the total available channels in the system.

10. Locating Co-Channel Cells • In order to connect without gaps between adjacent cells , the geometry of hexagons is such that the number of cells per cluster, N, can have values which satisfy the following equation: N=i^2+i*j+J^2 • For typical values of N which are 3,4,7,12 : • (i, j)=(1,1)(2,0)(2,1)(2,2)

11. Locating nearest Co-channel Neighbor • To find the nearest co-channel neighbor of a particular cell, do the following : • Move i cells along any chain of hexagons and then • Turn 60 degrees counter-clockwise and • Move j cells

12. Channel Assignment Strategies • Channel Assignment Strategies are used for Efficient Utilization of Radio Spectrum with the main Objectives of: • Increasing Capacity • Minimizing Interference • Classification of Channel Assignment Strategies • Fixed channel assignment strategy • Dynamic channel assignment strategy

13. Cont • The choice of channel assignment strategy impacts the performance that includes: • Handoff i.e. how calls are managed when a mobile user is handed off from one cell to another • Call initialization load • MSC processing Load

14. Channel Assignment Strategies (Contd.) • Fixed Channel Assignment Strategy • Each cell allocated a predetermined set of voice channels • If all channels are occupied then calls are blocked • Several variations of the fixed assignment strategy exist such as Borrowing strategy to tackle call blockage • MSC supervises this borrowing ensuring that borrowing of channels does not disrupt or interfere with any other calls Or • Reserve some channels for handoff

15. Channel Assignment Strategies (Contd.) • Dynamic Channel Assignment Strategy • voice channels are not allocated to different cells permanently • at a call request, the serving base station requests the MSC for a channel • MSC allocates the channel following an algorithm that takes into account, • Frequency of use of the candidate channel • Reuse distance, etc.

16. Dynamic Allocation(Cont) • MSC only allocates a given frequency if that frequency is not presently in use in the cell or in any other cell which falls within the minimum restricted distance of frequency reuse to avoid co-channel interference. • Advantages/Disadvantages: • Reduction of blocking probability • It increases trunking capacity (traffic intensity/channel) of the system , since available channels in the market are accessible to all of the cells

17. Dynamic Allocation (Cont) • In dynamic allocation, real time data is needed for all channels on continuous basis : • Channel occupancy • Traffic distribution • RSSI(Radio signal strength indications) This increases computational load on the system but provides the advantage of increased channel utilization and decreased probability of blocked call

18. Handoff • Hand off enables a call to proceed un-interrupted when user moves from one cell to another. • Handoff is encouraged in order to maintain call quality as the subscriber moves in and out of range of base station • The HO operation not only involves identifying a new base station, but also requires that the voice and control signals be allocated to channels associated with the new base station

19. Cont.. • When mobile is on the edge of cell • RSL of the mobile in that cell falls below a set level • Base station of the cell originates a handoff request • MSC gets RSL information from all candidate cells • MSC asks the originating cell and strongest candidate cell to co-ordinate • In case the handoff is successful, the mobile is asked to switch to another Voice channel

21. Handoff Strategies (Contd..) • Many HO strategies prioritize HO requests over call initiation requests when allocating unused channels • HO must be performed • Successfully, infrequently and should be imperceptible to the users • Threshold signal level D D = Pr handoff – Pr minimum usable Pr handoff specifies the optimum signal level at which to initiate handoff Pr minimum usable minimum usable signal for acceptable voice quality at the base station receiver If D to large: unnecessary HO’s If D to small: calls may be lost due to insufficient time for HO

22. Handoff (Cont) • The call will be dropped when there is an excessive delay by MSC in assigning handoff or if D is too small for the handoff time in the system. • Excessive delay can occur during high traffic conditions due to computational loading at the MSC or the fact that no channels are available on any of the nearby stations

23. Handoff Strategies (Contd..) • BS monitors the signal level for a certain period of time before a HO is initiated • Average measurement of signal strength should be optimized so that unnecessary handoffs are avoided, while ensuring that necessary handoffs are completed before a call is terminated. • Dwell time • The time over which a call may be maintained within a cell, without HO is call Dwell time • Statistics of dwell time are important in the practical design of HO algorithms

24. Dwell time • The dwell time of a particular user is governed by a number of factors: • Propagation • Interference • Distance between the subscriber and base station • Time varying effects Dwell time vary greatly depending upon the speed of user and type of radio coverage • Vehicular Highway users concentrated around the mean dwell time • In cluttered microcell environment , there is a large variation around the mean

25. Handoff Strategies (Contd..) • In 1G analog cellular systems, • signal strengths measurements are made by BS and supervised by MSC. BS measures signal strength of all Reverse voice channels. • In each BS a spare receiver or locator receiver measures signal strengths of channels in neighboring cells • Based on the information from all the locator receivers the MSC decides if HO is necessary or not

26. Handoff Strategies (Contd..) • In 2G cellular systems that uses digital TDMA • HO decisions are mobile assisted; mobile assisted handoff (MAHO) • In MAHO every mobile measures the received power from the surrounding BS’s and continually reports the results to the serving BS • A handoff is initiated when the power received from the base station of a neighboring cell begins to exceed the power received from the current base station • MAHO allows much faster HOs, as the HO measurements are made by each mobile and the MSC no longer constantly monitors the signal strength • MAHO is particularly suited for microcellular environments where HOs are more frequent

27. Handoff Strategies (Contd..) • Intersystem Handoff During the course of a call if a mobile moves from one cellular system to a different cellular system controlled by a different MSC and intersystem handoff becomes necessary • Conditions for Intersystem HO • When a mobile signal becomes weak in a given cell and the MSC cannot find another cell within its system to which it can transfer the call in progress • Issues to be addressed when Implementing Intersystem HO • A local call may become long distance call (billing issue) • Compatibility between the two MSCs

28. Handoff (Cont) • Some systems handle handoff requests in the same way as they handle originating calls. • In such systems, the probability that a handoff request will not be served by a new base station is equal to the blocking probability of incoming cells • However from the user’s point of view, handoff requests are prioritized over call initiation requests.

29. Prioritizing Handoffs • Different systems have different policies and methods for managing handoff requests. Some systems give priority to HO over call initiation others deal them at same priority

30. Prioritizing Handoffs • Guard channel concept • Disadvantage – reduces total carried traffic • Advantage – efficient spectrum utilization with dynamic channel assignment • Queuing of Handoff requests • Possible because a finite time interval between the time the signal drops below the HO threshold and the time the call is terminated due to insufficient signal level • Queuing does not guarantee a zero probability of forced termination in case of large delays which will cause the received signal level to drop below the minimum required level to maintain communication.

31. Practical Handoff Considerations “Umbrella Cells”

32. Practical Handoff considerations • The umbrella cell approach ensures that the number of handoff’s is minimized for high speed users and provides additional microcell channels for pedestrian users • The speed of vehicle may be evaluated by MSC,by calculating how rapidly received signal strength varies for each reverse voice channel • If a high speed user is approaching base station , and its speed is decreasing , the base station will handover this user to microcell , without consulting MSc

33. Practical Handoff Considerations “Cell Dragging” • If a uses travels away from the base station at a slow speed , the average signal strength does not decay rapidly. • Even when the user has traveled well beyond the designed range of the cell, the received signal at the base station may be above the D, thus the HO may not be made. • It Creates potential Interference and traffic management problems because user has travelled down into neighboring cell. • To avoid cell dragging ,handoff thresholds and radio coverage parameters must be adjusted carefully.

34. Cont.. • In 1G cellular systems , the typical time to make a handoff is about 10 sec. • However in digital cellular systems such as GSM, mobile assists with the handoff procedure by determining the best handoff candidates and typically require 1 to 2 sec • The faster handoff process supports a much greater range of options for handling high speed and low speed users • It provides the MSC with substantial time to “rescue” a call that is in need of handoff

35. Interference • Interference in voice channels can cause Cross talk • Interference in control channels leads to missed and blocked calls • Interference is major bottleneck in increasing capacity and is often responsible for dropped calls • Interference is of two types: • Co-channel interference • Adjacent channel interference

36. Interference • Sources of interference : • Another mobile in the same cell • A call in progress in a neighboring cell • Other base stations operating in the same frequency band • Any non-cellular system which leaks energy into cellular frequency band

37. Interference(Cont) • The interference created by out of band users is very difficult to control • The transmitters from competing cellular carriers are often a significant source of out-of band interference because they often locate their base stations in close proximity to one another in order to provide comparable coverage

38. Co-Channel Interference • Cells which use the same set of frequencies are called as co-channel cells • The interference between signals from these cells is called as co-channel interference • The co-channel interference cannot be combated by increasing the carrier power of transmitter (as we do in case of thermal noise) • To reduce this, co-channel cells must be physically separated by minimum distance to provide isolation

39. Co-channel Reuse Ratio • The parameter Q, called as co-channel re-use ratio is defined as : Q=D/R • where D=distance between centers of the nearest co-channel cells • R=radius of the cell By increasing the D/R, the spatial separation between co-channel cells relative to the coverage distance of a cell is increased

40. Example • A total of 33MHz bandwidth is allocated to a particular FDD cellular phone system. If the simplex voice/control channel bandwidth s 25kHz,find the total number of channels available per cell if the system uses (a)4-cell frequency reuse (b)7-cel frequency reuse plan • If 1MHz out of total allocated bandwidth is used for control channels, determine an equitable distribution of the control and voice channels in each cell in case of frequency reuse plan

41. 33Mhz allocated to a particular FDD cellular phone system. If the simplex voice/control channel bandwidth s 50kHz,find the total number of channels available per cell if the system uses (a)4-cell frequency reuse (b)7-cel frequency reuse plan • If 1MHz out of total allocated bandwidth is used for control channels, determine an equitable distribution of the control and voice channels in each cell in case of frequency reuse plan