Download to read offline
cellular_network_concepts_in_mobile_computing
- 1. 1 Cellular concept Reference (TheodoreS. Rappaport, "Wireless Communications--Principles and Practice Book, Chapter 1 and Chapter 3)
- 2. • Transmitting andreceiving voice and data using electromagnetic waves in open space – wireless communication • Information from sender to receiver is carried over well defined frequency band – channel • Guard band • Multiple access scenario – FDMA TDMA CDMA 2
- 3. 3 Terms used inCellular Systems
- 4. 4 • In FDD, *a device, called a duplexer, is used inside the subscriber unit to enable the same antenna to be used for simultaneous transmission and reception. * to facilitate FDD, it is necessary to separate the transmitted and received frequencies by about 5% of the nominal RF frequency, so that the duplexer can provide sufficient isolation while being inexpensively manufactured. • In TDD, * only possible with digital transmission format and digital modulation. * very sensitive to timing. Consequently, only used for indoor or small area wireless applications.
- 5.
- 6. 6 How a CellularTelephone Call is Made The channels used for voice transmission from the base station to mobiles are called forward voice channels (FVC). The channels used for voice transmission from mobiles to the base station are called reverse voice channels (RVC). The two channels responsible for initiating mobile calls are the forward control channels (FCC) and reverse control channels (RCC). MIN (Mobile Identification Number) and ESN (Electronic Serial Number)
- 7.
- 8. Introduction to CellularSystems • Solves the problem of spectral congestion and user capacity. • Offer very high capacity in a limited spectrum without major technological changes. • Reuse of radio channel in different cells. • Enable a fix number of channels to serve an arbitrarily large number of users by reusing the channel throughout the coverage region.
- 9. Frequency Reuse • Eachcellular base station is allocated a group of radio channels within a small geographic area called a cell. • Neighboring cells are assigned different channel groups. • By limiting the coverage area to within the boundary of the cell, the channel groups may be reused to cover different cells. • Keep interference levels within tolerable limits. • Frequency reuse or frequency planning •seven groups of channel from A to G •footprint of a cell - actual radio coverage
- 10. Frequency Reuse • Replacinga single, high power transmitter with many low power transmitters, each providing coverage to only a small area. • Neighboring cells are assigned different groups of channels in order to minimize interference. • The same set of channels is then reused at different geographical locations.
- 11. • Cluster size: The N cells which collectively use the complete set of available frequency is called the cluster size. • Co-channel cell : The set of cells using the same set of frequencies as the target cell. • The set of closest co-channel cells is call the first tier. • Coordinates for hexagonal geometry • center of every cell falls on a point specified by a pair of integer coordinates. 11
- 12. • Consider acellular system which has a total of S duplex channels. • Each cell is allocated a group of k channels, . • The S channels are divided among N cells. • The total number of available radio channels • The N cells which use the complete set of channels is called cluster. • The cluster can be repeated M times within the system. The total number of channels, C, is used as a measure of capacity • The capacity is directly proportional to the number of replication M. • The cluster size, N, is typically equal to 4, 7, or 12. • Small N is desirable to maximize capacity. • The frequency reuse factor is given by S k < kN S = MS MkN C = = N / 1 Question If a total of 33 MHz of bandwidth is allocated to a particular FDD cellular telephone system which uses two 25 kHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses (a) four- cell reuse, (b) seven-cell reuse, and (c) 12-cell reuse. If 1 MHz of the allocated spectrum is dedicated to control channels, determine an equitable distribution of control channels and voice channels in each cell for each of the three systems.
- 13. • Hexagonal geometryhas – exactly six equidistance neighbors – the lines joining the centers of any cell and each of its neighbors are separated by multiples of 60 degrees. • Only certain cluster sizes and cell layout are possible. • The number of cells per cluster, N, can only have values which satisfy • Co-channel neighbors of a particular cell, ex, i=3 and j=2. 2 2 j ij i N + + =
- 14. Channel Assignment Strategies •Frequency reuse scheme – increases capacity – minimize interference • Channel assignment strategy – fixed channel assignment – dynamic channel assignment • Fixed channel assignment – each cell is allocated a predetermined set of voice channel – any new call attempt can only be served by the unused channels – the call will be blocked if all channels in that cell are occupied • Dynamic channel assignment – channels are not allocated to cells permanently. – allocate channels based on request. – reduce the likelihood of blocking, increase capacity.
- 15. Handoff Strategies • Whena mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. • Handoff operation – identifying a new base station – re-allocating the voice and control channels with the new base station. • Handoff Threshold – Minimum usable signal for acceptable voice quality – Handoff margin cannot be too large or too small. – If too large, unnecessary handoffs burden the MSC – If too small, there may be insufficient time to complete handoff before a call is lost. usable minimum , , r handoff r P P - = D D D
- 17. • Handoff mustensure that the drop in the measured signal is not due to momentary fading and that the mobile is actually moving away from the serving base station. • Running average measurement of signal strength should be optimized so that unnecessary handoffs are avoided. – Depends on the speed at which the vehicle is moving. – Steep short term average -> the hand off should be made quickly – The speed can be estimated from the statistics of the received short-term fading signal at the base station • Intersystem handoff: If a mobile moves from one cellular system to a different cellular system controlled by a different MSC. • Handoff requests is much important than handling a new call.
- 19. Interference and SystemCapacity • Sources of interference – another mobile in the same cell – a call in progress in the neighboring cell – other base stations operating in the same frequency band – noncellular system leaks energy into the cellular frequency band • Two major cellular interference – co-channel interference – adjacent channel interference
- 20. Co-channel Interference andSystem Capacity • Frequency reuse - there are several cells that use the same set of frequencies – co-channel cells – co-channel interference • To reduce co-channel interference, co-channel cell must be separated by a minimum distance. • When the size of the cell is approximately the same – co-channel interference is independent of the transmitted power – co-channel interference is a function of • R: Radius of the cell • D: distance to the center of the nearest co-channel cell • Increasing the ratio Q=D/R, the interference is reduced. • Q is called the co-channel reuse ratio
- 21. • For ahexagonal geometry • A small value of Q provides large capacity • A large value of Q improves the transmission quality - smaller level of co-channel interference • A tradeoff must be made between these two objectives N R D Q 3 = = Erlang: Dimensionless unit used in telephony as a measure of offered load or carried load on telephone circuits or telephone switching equipment. A single cord circuit has the capacity to be used for 60 minutes in one hour. Full utilization of that capacity, 60 minutes of traffic, constitutes 1 erlang.
- 22. 22 Question1 Total Bandwidth availableto a mobile service = 12.5 MHz Guard Band at each edge = 10KHz Each channel Bandwidth = 30 KHz Total no. of channels?
- 23. 23 Question2 Total Bandwidth availableto a mobile service = 50 MHz with 400 KHz duplex channels Guard Band at each edges = 100KHz Channels used as control channels= 04 KHz No. of voice channels available?
- 24. 24 Question3 If 20 MHzof total spectrum is allocated for a duplex wireless cellular system and each simplex channel has 25 kHz RF bandwidth. Find l ) The number of duplex channels 2) The total number of channels per cell size if N=4 reuse is used.
- 25. 25 Question4 Consider a cellularsystem in which total available voice channels to handle the traffic are 960. The area of each cell is 6km2 and the total coverage is 2000km2 . Find he system capacity if the cluster size is N=4. Find the system capacity if the cluster size is N=7.
- 26. 26 Question5 In order todetermine voice traffic on a line, we collected the following data during a period of 90 minutes. Calculate the traffic density in erlang.
- 27. 27 Question6 The average mobileuser has 500 minutes of use per month; 90% of traffic occurs during work days. There are 20 week days per month. Assuming that in a given day, 10% of traffic occurs during the busy hour (BH), determine the traffic per subscriber per BH.