Mobile Communications Prepared by: R – THANDAIAH PRABU M.E., LECTURER / ECE Mobile Communications 1
Frequency ReuseEach cellular base station is allocated agroup of radio channels.Base stations in adjacent cells areassigned channel groups which containdifferent channels than neighboring cells.
Cellular Frequency Reuse ConceptCells with the same letter, usethe same set of frequencies. B G C AA cell cluster is outlined F Din bold, and replicated over E Bthe coverage area. B G C G C AIn this example, the A F D F D Ecluster size, N, is equal to 7; Eand the frequency reuse factor is 1/7,since each cell contains 1/7 of the total numberof available channels.
Choices of Hexagonal CellFactors:• Equal area• No overlap between cellsChoices: S S S A1 A2 A3
For a given SA3 > A1A3 > A2Here, A3 provides maximumcoverage area for a given value of S.Actual cellular footprint is determined by thecontour of a given transmitting antenna.By using hexagon geometry, the fewestnumber of cells covers a given geographicregion.
Frequency reuse A A 2 2 7 3 2 7 3 1 1 D7 3 A 6 4 1 A 6 4 A 5 5 6 4 5 A A D = 3N R D = distance between cells using the same frequency R = cell radius N = reuse pattern (the cluster size, which is 7). Thus, for a 7-cell group with cell radius R = 3 miles, the frequency reuse distance D is 13.74 miles.
Channel CapacityLet a cellular system have total ofS duplex channels for use.If S channels are divided into N cells (in a cluster) intounique and disjoint channel groups which each hasthe same number of channels, total number ofavailable radio channels is:S = KNWhere K is the number of channels / cell.
…Channel CapacityIf a cluster is replicated M times withinthe system, the total number ofduplex channels, C, or the capacity, isC = MKN = MS.Cluster size N = 4, 7 or 12
Design of cluster size NIn order to connect without gaps betweenadjacent cells (to tessellate)N = i2 + ij + j2Where i and j are non-negative integersExample i = 2, j = 1N = 22 + 2(1) + 12 = 4 + 2 + 1 = 7
To Find the NearestCo-channel Neighbor ofParticular Cell:• Move i cells along any chain or hexagon.• Then turn 60 degrees counterclockwise and move j cells.
ExampleIf a particular FDD (frequency duplex) cellular telephonesystem has a total bandwidth of 33 MHz,and if the phonesystem uses two 25 KHz simplex channels to provide fullduplex voice and control channels...compute the number ofchannels per cell ifN = 4, 7, 12.
Interference andSystem CapacityMajor limiting factor in performanceof cellular radio systems - two maintypes:– Co-channel interference– Adjacent channel interference
Co-Channel InterferenceCells that use the same set offrequencies are calledco-channel cells.Interference betweenthe cells is calledco-channel interference.
Co-Channel InterferenceSignal to interference ratio(SIR) or S/ I for a mobilereceiver is given by: ioS/ I = SIR = S /( ∑ Ii) i= 1S = signal power from designated base station
First Tier of Co-channel Cellsfor a Cluster Size of N = 7When the mobile is at the cell boundary (point A),it experiences worst case co-channel interference onthe forward channel.The marked distances between the mobile anddifferent co-channel cells are based onapproximations made for easy analysis.
AFirst Tier ofCo-ChannelCells for a A D+ ACluster Size R D Rof N = 7 D+Ii = Interference D- A Rpower caused by A R D Athe ith interfering D-co-channel cell R A
AssumptionsFor any given antenna(base station) the powerat a distance d is given by: Po Pr d -nPr = Po (d / do) ; n is path loss exponent
...Assumptions io D ∑ (/ i )-nHence, S / I = R -n i= 1io = total number of first layer interfacing cellsIf the mobile is at the center of the cell, Di = D io ∑ 1 -n i= 1 -n -nS/I =R / (D) = (R / D) /io
For a hexagonal geometryD / R =√(3N) = Q - co-channelreuse ratioS / I = [√(3N) ] n / io
Maximum co-channel interface –when mobile is at cell boundary.For N = 7S / I~R-4 / [ 2(D-R)-4+2(D+R)-4+ 2D-4]
Adjacent Channel Interference• Interference resulting from signals which are adjacent in frequency to the desired signal.• Due to imperfect receiver filters that allow nearby frequencies to leak into pass band.• Can be minimized by careful filtering and assignments; and, by keeping frequency separation between channels in a given cell as large as possible, the adjacent channel interference may be reduced considerably.
Channel Assignment Strategies: Fixed Channel Assignments• Each cell is allocated a pre-determined set of voice channels.• If all the channels in that cell are occupied, the call is blocked, and the subscriber does not receive service.• Variation includes a borrowing strategy: a cell is allowed to borrow channels from a neighboring cell if all its own channels are occupied. This is supervised by the MSC.
Channel Assignment Strategies: Dynamic Channel Assignments• Voice channels are not allocated to different cells permanently.• Each time a call request is made, the serving base station requests a channel from the MSC.• The switch then allocates a channel to the requested call, based on a decision algorithm taking into account different factors - frequency re-use of candidate channel, cost factors.
...Channel Assignment Strategies: Dynamic Channel AssignmentsDynamic channel assignment is morecomplex (real time), but reduces likelihoodof blocking.
Improving Capacity in Cellular Systems• As demand for wireless services increases, the number of channels assigned to a cell is not enough to support the required number of users.• Solution is to increase channels per unit coverage area.
CELL SPLITTING• The main aim of cellular mobile systems is to improve utilization of spectrum efficiency• The frequency reuse scheme is one concept, and cell splitting is another concept.• When traffic density is very high then each cell cannot provide enough mobile calls. Then the original cell can be split into smaller cells resulting in cell splitting.• Usually the new radius is one-half the original radius.
Cell Splitting• Subdivides a congested cell into smaller cells, each with its own base station.• Increases the capacity of a cellular system.
Sectoring• Achieves capacity improvement by essentially rescaling the system.• Cell radius R is unchanged but the co-channel ratio D / R is decreased.• Capacity improvement is achieved by reducing the number of cells in a cluster, and this increases frequency reuse.• Replacing a single omni-directional antenna at base station with several directional antennas, each radiating within a specified sector.
Micro Cell Zone Concept• Large control base station is replaced by several lower powered transmitters on the edge of the cell.• The mobile retains the same channel and the base station simply switches the channel to a different zone site and the mobile moves from zone to zone.• Since a given channel is active only in a particular zone in which mobile is traveling, base station radiation is localized and interference is reduced.
... Micro Cell Zone Concept• The channels are distributed in time and space by all three zones are reused in co-channel cells.• Advantage is that while the cell maintains a particular coverage radius, co-channel interference is reduced due to zone transmitters on edge of the cell.
Antennas• Antenna pattern, antenna gain, antenna tilting, and antenna height all affect the cellular system design• Different antenna patterns and antenna gains at the cell site and at the mobile units would affect the system performance and so must be considered in the system design.• Antenna tilting can reduce the interference to the neighboring cells and enhance the weak spots in the cell
Trunking and Grade of Service• Cellular radio system relies on trunking to accommodate a large number of users in a limited radio spectrum - How a large population can be accommodated by a limited number of services.• Trunking - each user is allocated a channel on a per-call basis; and upon termination of the call, the previously occupied channel is immediately returned to the pool of available channels• Initiated by Danish mathematician, Erlang.
Why 800 MHz frequency is selected for mobiles?• • Fixed Station Services - 30 MHz to 100 MHz• Television Broadcasting - 41 MHz to 960 MHz• FM Broadcasting - 100 MHz• Air to Ground system - 118 MHz to 136 MHz• Maritime mobile services - 160 MHz• Military Aircraft use - 225 MHz to 400 MHz• Frequency bands between 30 MHz to 400 MHz is crowded with large number of services and above 10 GHz is not used due to propagation path loss, multipath fading and improper medium due to rain activity. So 800 MHz is chosen for mobile communication Mobile Communications 38