1. EEE440 Modern Communication Systems Cellular Systems

2. Introduction The geographical area of coverage is organised into cells Each cell is controlled by a base station A common model of cellular structure in a two- dimensional case is to consider all cells to be hexagonal in shape and all of the same size In real systems, cells have complex shapes depending on antenna directivity and location, propagation conditions and terrain topography

3. Structure of a cellular system

4. BS-controls multiple MS MSC – controls multiple BS –Responsible for intercellular handover, mobile location, paging and mobility management HLR – contains reference and profile information for all mobile users registered with the MSC as their home location VLR- registers visiting MS Structure of a cellular system

5. Spectral allocation Radio spectrum allocation is made by authorities. e.g. in Malaysia, the MCMC allocates spectrum to mobile operators

6. Spectral allocation

7. Channel allocation The band is broken into a number of channels Channels in a wireless communication system typically consist of time slots in TDMA, frequency bands in FDMA and/or CDMA pseudo noise sequences, but in an abstract sense, they can represent any generic transmission resource The number of channels limit the number of simultaneous users To increase the capacity a given service area is divided into a number of cells The channels can be reused in different cells

8. Channel reuse Different cells can use the same frequency channel However, adjacent cells cannot be assigned the same frequency because of inter-channel inteference The assignment must be spaced far enough apart to keep interence to tolerable levels

9. Channel reuse For example in a one dimensional cell structure, the total number of channels can be divided into 4 groups (4-reuse) There are three-cells separating cells with the same set of frequencies

10. Channel reuse

11. The assignment strategy depends on the tolerable interference which is quantified by calculating the signal-to-interference ratio (SIR) or also called carrier-to-interference ratio (CIR) SIR = desired average signal power at a receiver total average interference power The SIR should be greater than a specified threshold for a proper signal operation For GSM the desired SIR is 7-12dB

12. SIR calculations Calculated on an average power basis Focus on the distance-dependent part of the received power equation (ignores shadow and multipath fading) Assume g(d)=kd -n ; n = 3 or 4

13. SIR calculations Consider 1-dimensional cell structure –D= spacing between interfering cells –R=the half width (center to edge) of each cell Consider downlink power receive at a mobile located at the edge of a cell (worst situation) at point P Say each base station located at the centre of its cell transmits with the same average power, P T

14. SIR calculations The average received power at distance d meter from a base station is given by P T d -n ; n = 3 or 4 The SIR at the mobile at point P is given by Sum of all interfering base stations

15. SIR calculations Theoretically all base stations transmitting at the same frequency will interfere with the home base station transmission However, in reality only a relatively small number of nearby interferes need be considered because of the rapidly decreasing received power as the distance, d increases

16. SIR calculations Consider the first tier interferers only The two interfering base stations closest to the mobile at point P are located at (D+R) and (D-R) respectively from the mobile The corresponding SIR is given by

17. SIR calculations Calculate the SIR in dB for different values of n (3 or 4) and different cell reuse (3 or 4) What can you conclude? Individual assignment questions.

18. SIR calculations Consider 2-dimensional cell structure All hexagonal cells of same size The number of cells for an area is given generally by, C=i 2 + j 2 + ij ; i, j = integers 1,2,3… For GSM C=3 or 4

19. SIR calculations Consider a typical hexagonal cell The distance from the center of the cell to any vertex is the radius R Each edge is of length R The distance across the cells = √3R

20. SIR calculations There are 6 interfering base stations around the home base station The spacing between the closest interfering base stations is given by D 3 =3R for 3-cell reuse (c=3) D 4 =2√3R for 4-cell reuse (c=4) –In general for C-cell reuse, D c =√3C R

21. SIR calculations Consider the case when the mobile is at the middle of the cell The SIR is given by SIR = P T / (6P T √3C R -n ) = 1/ (6√3C R -n ) At the edge of the cell, the are many proposed approximations

22. SIR calculations Estimate the appropriate C for GSM with minimum required SIR of 7dB.

23. Channel Allocation Schemes Allocate channels to base stations and access points and to avoid co-channel interference among nearby cells Fixed Channel Allocation –requires manual frequency planning to allocate specific channels to specific cells –This allocation is static and can not be changed –the number of channels in the cell remains constant irrespectively of the number of customers in that cell. –For efficient operation, FCA systems typically allocate channels in a manner that maximizes frequency reuse. –Thus, in a FCA system, the distance between cells using the same channel is the minimum reuse distance for that system. –The problem with FCA systems occurs whenever the offered traffic to a network of base stations is not uniform. –Consider a case in which two adjacent cells are allocated N channels each. There clearly can be situations in which one cell has a need for N+k channels while the adjacent cell only requires N-m channels (for positive integers k and m). –In such a case, k users in the first cell would be blocked from making calls while m channels in the second cell would go unused. –Clearly in this situation of non-uniform spatial offered traffic, the available channels are not being used efficiently. This result in traffic congestion and some calls being lost when traffic gets heavy in some cells, and idle capacity in other cells.

24. Channel Allocation Schemes Dynamic –handles bursty cell traffic and utilizes the cellular radio resources more efficiently. –DCA allows the number of channels in a cell to vary with the traffic load, hence increasing channel capacity with little costs. –In DCA systems, no set relationship exists between channels and cells. Instead, channels are part of a pool of resources. –Whenever a channel is needed by a cell, the channel is allocated under the constraint that frequency reuse requirements can not be violated. –There are two problems that typically 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.

25. Channel Allocation Schemes Hybrid –Combined FCA and DCA –Channel Borrowing is one of the most straightforward hybrid allocation schemes. –Here, channels are assigned to cells just as in fixed allocation schemes. –If a cell needs a channel in excess of the channels previously assigned to it, that cell may borrow a channel from one of its neighboring cells given that a channel is available and use of this channel won't violate frequency reuse requirements. –Note that since every channel has a predetermined relationship with a specific cell, channel borrowing is often categorized as a subclass of fixed allocation schemes. –The major problem with channel borrowing is that when a cell borrows a channel from a neighboring cell, other nearby cells are prohibited from using the borrowed channel because of co-channel interference. –This can lead to increased call blocking over time. To reduce this call blocking penalty, algorithms are necessary to ensure that the channels are borrowed from the most available neighboring cells; i.e., the neighboring cells with the most unassigned channels.

26. Channel Allocation Schemes Two extensions of the channel borrowing approach are Borrowing with Channel Ordering (BCO) and Borrowing with Directional Channel Locking (BDCL). Borrowing with Channel Ordering was designed as an improvement over the simpler Channel Borrowing approach BCO systems have two distinctive characteristics –The ratio of fixed to dynamic channels varies with traffic load. –Nominal channels are ordered such that the first nominal channel of a cell has the highest priority of being applied to a call within the cell. The last nominal channel is most likely to be borrowed by neighboring channels. Once a channel is borrowed, that channel is locked in the co-channel cells within the reuse distance of the cell in question. To be "locked" means that a channel can not be used or borrowed. From a frequency reuse standpoint, in a BCO system, a channel may be borrowed only if it is free in the neighboring cochannel cells.

27. Channel Allocation Schemes In Borrowing with Directional Channel Locking, borrowed channels are only locked in nearby cells that are affected by the borrowing. This differs from the BCO scheme in which a borrowed channel is locked in every cell within the reuse distance. The benefit of BDCL is that more channels are available in the presence of borrowing and subsequent call blocking is reduced. A disadvantage of BDCL is that the statement "borrowed channels are only locked in nearby cells that are affected by the borrowing" requires a clear understanding of the term "affected.“ This may require microscopic analysis of the area in which the cellular system will be located. Ideally, a system can be general enough that detailed analysis of specific propagation measurements is not necessary for implementation.

28. Channel Allocation Schemes A natural extension of channel borrowing is to set aside a portion of the channels in a system as dynamic channels with the remaining (nominal) channels being fixed to specified cells. If a cell requires an extra channel, instead of borrowing the channel from a neighboring cell, the channel is borrowed from the common "bank" of dynamic channels. An important consideration in hybrid systems of this type is the ratio of dynamic channels to fixed channels. The optimum ratio depends upon the traffic load Locally Optimized Dynamic Assignment Strategy (LODA), this method is best described as a purely dynamic channel allocation procedure as opposed to a hybrid method. In this strategy there are no nominal channels; all channels are dynamic. When a given cell needs to accommodate a call, it chooses from among the bank of available channels according to some cost criteria. The channel with minimum cost is assigned. In a general sense, the cost is a measure of the future blocking probability in the vicinity of the cell given that the candidate channel is assigned.

29. Traffic handling capacity The number of channels available per cell is given by the total number of channels divided by the cell reuse parameter, C System performance is measured by the probability of call blocking which describes the chance that a user attempting to place a call receives a busy signal. The measure depends on the number of channels available to handle simultaneous calls and the traffic expected to utilise the system With a specified call blocking probability (e.g. 1% or 5%) a limit must be put on the amount of traffic expected to use the cell

30. Traffic handling capacity Traffic intensity or traffic load is commonly defined as the product of the average number of call attempts per unit time(λ) and the average call length (1/µ) Traffic intensity, A = λ/µ in unit Erlangs The statistical model assumes that the pattern of call attempts or arrival obeys a Poisson distribution with average rate of arrival λ and the call lengths are exponentially distributed with average length 1/µ

31. Traffic handling capacity With N channels available, the cell blocking probability, P B is given by the Erlang-B formula A table or plot of P B vs A (Erlang-B function) is used to find the number of channels required for a given traffic load and P B

32. Cell size Asssume that users are uniformly distributed over the cell The area of the hexagonal cell of radius R is (3√3R 2 )/2 Say there is 1 call every 15 minutes and a typical call last for 200 seconds on average The load for 1 user is given by For a total cell load, A=101 the number of users is about 450 users

33. Cell size The user density for 450 users is given by 450 / (3√3R 2 )/2 = 173/R 2 mobiles per unit area Consider a rural area with density of mobile = 2 terminals per km 2. What is the cell radius For suburban = 100 mobiles per km2? For urban = 1000 mobiles per km2?