GSM FREQUENCY PLANNING PROJECT NUMBER: PRJ070 BY NAME: MUTONGA JACKSON WAMBUA REG NO.: F17/2098/2004 SUPERVISOR: DR. CYRUS WEKESA EXAMINER: DR. MAURICE MANG OLI
Introduction GSM is a cellular mobile network made up of hexagonal cells with a BTS at the centre of each cell or at the corner boundaries. Each BTS is allocated a different carrier frequency because only a finite part of the radio spectrum is allocated to cellular radio.
Why Cellular System? To address requirement for greater capacity For efficient use of frequency To address the poor quality of non cellular mobile networks and increase coverage replaces a large transmitter with smaller ones in cells smaller transmitting power each cell serves a small geographical service area each cell is assigned a portion of the total frequency
Objective With a fixed number of carrier frequencies available, the capacity of the system can be increased by Using smaller cells Sectorization of the cells Re-using the carrier frequencies more often The objective of this project was to develop a strategy to facilitate a cost-effective frequency reuse in GSM networks.
Background GSM is a digital standard that uses Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) as a multiple access technologies. GSM has three dedicated bands that are used. The three bands are usually called GSM900, GSM1800 and GSM1900. The total available bandwidth of GSM900 is divided into 124 200 khz bands (FDMA) and each group of 8 users transmits through a 200 khz band sharing the transmission time (TDMA).
GSM System Architecture Some of the most important entities in GSM systems are the MS, BTS, BSC and MSC.
Radio Network Planning The objective of network planning is to provide wireless telephony services in a serving area in the most cost-effective manner. The design usually involves; determining the number of BTSs and their location Meet the desired grade of service Satisfy the required traffic growth Minimizing the total startup cost and maximizing rate of return
Network Planning Process
The Cellular Concept and Frequency reuse There is limited resource of transmission spectrum that must be shared by several users. Different cells can use the same frequency given that the cells are separated by minimum distance (reuse distance, D) otherwise intercellular or co-channel interference occurs. Cluster patterns come in fixed numbers and are derived from the formula N = i² + ij + j².
7-cell cluster pattern
The Mobile Environment and Interference Radio path in wireless systems corresponds to the radio link between the MS and the BTS with which it communicates Two main interferences in a mobile environment are Adjacent Channel Interference Co-channel Interference Current implementation of GSM has mechanisms of reducing the effect of interference such as slow frequency hopping (SFH), discontinuous transmission (DTX), and power control.
Channel Assignment Strategies There are three major categories for assigning channels to cells (or BTSs) ; Fixed Channel Allocation (FCA) Groups of channels are assigned permanently to given cells following a prescribed reuse pattern. Dynamic Channel Allocation (DCA) Channels are allocated to users in a cell in accordance with varying traffic demands. Hybrid Channel Allocation (HCA) Combines both FCA and DCA schemes.
Proposed Frequency Reuse Strategy The capacity per unit area is given by To maximize capacity, the minimum K that will provide acceptable SINR is used. A reuse pattern of 3 cells is proposed where each cell has three sectors and the reuse distance is 3R. Channel allocation is by DCA scheme. log 2 1 [ + SINR ] KΠR Directional antennas are used so that each cell sector sees interference from two of the six closest co-channel cells. 3.3 C 1 3 R In this case C/I is = I 2 = 18.77 (power ratio) or 12.73 R db, which is above the threshold of 9 db required for GSM systems. C A = B T 2
System model
Sectorized System Model
Algorithm Each cell has 6 neighbors. Consider a total of 18 channels available, hence each cell has 6 primary channels with the following channel numbers; Subset A: 1, 4, 7, 10, 13, 16 Subset B: 2, 5, 8, 11, 14, 17 Subset C: 3, 6, 9, 12, 15, 18 The following assumptions are made Flat terrain Neighbors of each cell are defined All BTSs are centrally located in the cell and transmit at the same power
Simulation Results and Analysis The blocking probability, PBL is the measure of performance and is calculated from the ratio P BL = blocked _ calls calls _ received The blocking probability curves of the proposed DCA scheme and FCA scheme which is currently being used by cellular network operators in the country were generated and presented in a graph. A channel allocation scheme is better than another in a given load region if its blocking probability load curve is below the others curve.
Blocking Probability Graph
Conclusion The proposed channel allocation algorithm makes efficient reuse of channels using DCA with reduction of cluster size. The divergence between simulation results and real network behavior is affected by the structure of the implementation algorithms and the corresponding research of real network conditions like new call arrival schemes, user movement, etc
Recommendations Comparison with other DCA algorithms is needed. Extra functions like mobile environment effects and new call arrival schemes could be added to the proposed algorithm. We could have all the BSs registering and saving the received SINRs during a period of time. Using this data, a more sophisticated algorithm could be developed that tunes the antennas to make the network close to self operating.
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