GSM FREQUENCY PLANNINGPROJECT NUMBER: PRJ070

BYNAME: 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.� Directional antennas are used so that each cell sector sees

interference from two of the six closest co-channel cells. In this case C/I is = 18.77 (power ratio) or 12.73 dB, which is above the threshold of 9 dB required for GSM systems.

3.3321

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RR

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2 1logRK

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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

� 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.

receivedcallscallsblockedPBL _

_=

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.

THE END

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