7 Mobile Radio Environment The transmissions over the wireless link are in general very difficult to characterize.EM signals often encounter obstacles, causing reflection, diffraction, and scattering.Mobility introduces further complexity.We have focused on simple models to help gain basic insight and understanding of the wireless radio medium.Three main components: Path Loss, Shadow fading, Multipath fading (or fast fading).
8 Free Space lossTransmitted signal attenuates over distance because it is spread over larger and larger areaThis is known as free space loss and for isotropic antennasPt = power at the transmitting antennaPr = power at the receiving antennaλ = carrier wavelengthd = propagation distance between the antennasc = speed of light
9 Free Space loss For other antennas Gt = Gain of transmitting antenna Gr = Gain of receiving antennaAt = effective area of transmitting antennaAr = effective area of receiving antenna
10 Thermal NoiseThermal noise is introduced due to thermal agitation of electronsPresent in all transmission media and all electronic devicesa function of temperatureuniformly distributed across the frequency spectrum and hence is often referred to as white noiseamount of noise found in a bandwidth of 1 Hz isN0 = k TN0 = noise power density in watts per 1 Hz of bandwidthk = Boltzman’s constant = x J/KT = temperature, in KelvinsN = thermal noise in watts present in a bandwidth of B= kTB where
11 Free Space lossTransmitted signal attenuates over distance because it is spread over larger and larger areaThis is known as free space loss and for isotropic antennasPt = power at the transmitting antennaPr = power at the receiving antennaλ = carrier wavelengthd = propagation distance between the antennasc = speed of light
12 Free Space loss For other antennas Gt = Gain of transmitting antenna Gr = Gain of receiving antennaAt = effective area of transmitting antennaAr = effective area of receiving antenna
13 Thermal NoiseThermal noise is introduced due to thermal agitation of electronsPresent in all transmission media and all electronic devicesa function of temperatureuniformly distributed across the frequency spectrum and hence is often referred to as white noiseamount of noise found in a bandwidth of 1 Hz isN0 = k TN0 = noise power density in watts per 1 Hz of bandwidthk = Boltzman’s constant = x J/KT = temperature, in KelvinsN = thermal noise in watts present in a bandwidth of B= kTB where
14 Data rate and error rate Bit error rate is a decreasing function of Eb/N0.If bit rate R is to increase, then to keep bit error rate (or Eb/N0) same, the transmitted signal power must increase, relative to noiseEb/N0 is related to SNR as followsB = signal bandwidth(since N = N0 B)
15 Doppler’s ShiftWhen a client is mobile, the frequency of received signal could be less or more than that of the transmitted signal due to Doppler’s effectIf the mobile is moving towards the direction of arrival of the wave, the Doppler’s shift is positiveIf the mobile is moving away from the direction of arrival of the wave, the Doppler’s shift is negative
16 Doppler’s Shift where fd =change in frequency due to Doppler’s shift v = constant velocity of themobile receiverλ = wavelength of the transmissionθXY
17 Doppler’s shift f = fc + fd where f = the received carrier frequency fc = carrier frequency being transmittedfd = Doppler’s shift as per the formula in the previous slide.
18 Multipath Propagation Wireless signal can arrive at the receiver through different pathsLOSReflections from objectsDiffractionOccurs at the edge of an impenetrable body that is large compared to the wavelength of the signal
21 Limitations of Wireless Channel is unreliableSpectrum is scarce, and not all ranges are suitable for mobile communicationTransmission power is often limitedBatteryInterference to others
22 Advent of Cellular Systems Noting from the channel model, we know signal will attenuated with distance and have no interference to far users.In the late 1960s and early 1970s, work began on the first cellular telephone systems.The term cellular refers to dividing the service area into many small regions (cells) each served by a low-power transmitter with moderate antenna height.
23 Cell ConceptCellA cell is a small geographical area served by a singlebase station or a cluster of base stationsAreas divided into cellsEach served by its own antennaServed by base station consisting of transmitter, receiver, and control unitBand of frequencies allocatedCells set up such that antennas of all neighbors are equidistant
25 Cellular Network Organization Use multiple low-power transmittersAreas divided into cellsEach served by its own antennaServed by base station consisting of transmitter, receiver, and control unitBand of frequencies allocatedCells set up such that antennas of all neighbors are equidistant
26 ConsequencesTransmit frequencies are re-used across these cells and the system becomes interference rather than noise limitedthe need for careful radio frequency planning – colouring in hexagons!a mechanism for handling the call as the user crosses the cell boundary - call handoff (or handover)increased network complexity to route the call and track the users as they move aroundBut one significant benefit: very much increased traffic capacity, the ability to service many users
28 Cellular Systems Terms Mobile Stationusers transceiver terminal (handset, mobile)Base Station (BS)fixed transmitter usually at centre of cellincludes an antenna, a controller, and a number of receiversMobile Telecommunications Switching Office (MTSO) /Mobile Switch Center (MSC)handles routing of calls in a service areatracks userconnects to base stations and PSTN
29 Cellular Systems Terms (Cont’d) Two types of channels available between mobile unit and BSControl channels – used to exchange information for setting up and maintaining callsTraffic channels – carry voice or data connection between usersHandoff or handoverprocess of transferring mobile station from one base station to another, may also apply to change of radio channel within a cell
30 Cellular Systems Terms (Cont’d) Downlink or Forward Channelradio channel for transmission of information (e.g.speech) from base station to mobile stationUplink or Reverse Channelradio channel for transmission of information (e.g.speech) from mobile station to base stationPaginga message broadcast over an entire service area, includes use for mobile station alert (ringing)Roaminga mobile station operating in a service area other than the one to which it subscribes
31 Steps in an MTSO Controlled Call between Mobile Users Mobile unit initializationMobile-originated callPagingCall acceptedOngoing callHandoff
32 Frequency ReuseCellular relies on the intelligent allocation and re–use of radio channels throughout a coverage area.Each base station is allocated a group of radio channels to be used within the small geographic area of its cellNeighbouring base stations are given different channel allocation from each other
33 Frequency Reuse (Cont’d) If we limit the coverage area within the cell by design of the antennaswe can re-use that same group of frequencies to cover another cell separated by a large enough distancetransmission power controlled to limit power at that frequency to keep interference levels within tolerable limitsthe issue is to determine how many cells must intervene between two cells using the same frequency
34 Radio PlanningDesign process of selecting and allocating channel frequencies for all cellular base stations within a system is known as frequency re-use or frequency planning.Cell planning is carried out to find a geometric shape totessellate a 2D spacerepresent contours of equal transmit powerReal cells are never regular in shape
35 Two-Dimensional Cell Clusters Regular geometric shapes tessellating a 2D space: Square, triangle, and hexagon.‘Tessellating Hexagon’ is often used to model cells in wireless systems:Good approximation to a circle (useful when antennas radiate uniformly in the x-y directions).Also offer a wide variety of reuse patternSimple geometric properties help gain basic understanding and develop useful models.
38 Hexagonal cell geometry and axes Geometry of HexagonsHexagonal cell geometry and axes
39 Geometry of Hexagons (Cont’d) D = minimum distance between centers of cells that use the same band of frequencies (called co-channels)R = radius of a celld = distance between centers of adjacent cells (d = R√3)N = number of cells in repetitious pattern (Cluster) Reuse factorEach cell in pattern uses unique band of frequencies
40 Geometry of Hexagons (Cont’d) The distance between the nearest cochannel cells in a hexagonal area can be calculated from the previous figureThe distance between the two adjacent co-channel cells is D=√3R.(D/d)2 = j2 cos2(30) + (i+ jsin30)2= i2 + j2 +ij = ND=Dnorm x √3 R =(√3N)RIn general a candidate cell is surrounded by 6k cells in tier k.
41 Geometry of Hexagons (Cont’d) Using this equation to locate co-channel cells, we start from a reference cell and move i hexagons along the u-axis then j hexagons along the v-axis. Hence the distance between co–channel cells in adjacent clusters is given by:D = (i2 + ij + j2)1/2where D is the distance between co–channel cells in adjacent clusters (called frequency reuse distance).and the number of cells in a cluster, N is given by D2N = i2 + ij + j2
51 Co–channel Cell Location Method of locating co–channel cellsExample for N=19, i=3, j=2
52 Cell Planning ExampleSuppose you have 33 MHz bandwidth available, an FM system using 25 kHz channels, how many channels per cell for 4,7,12 cell re-use?total channels = 33,000/25 = 1320N=4 channels per cell = 1320/4 = 330N=7 channels per cell = 1320/7 = 188N=12 channels per cell = 1320/12 = 110Smaller clusters can carry more trafficHowever, smaller clusters result in larger co-channel interference
60 Remarks SIGNAL TO INTERFERENCE LEVEL IS INDEPENDENT OF CELL RADIUS! System performance (voice quality) only depends on cluster sizeWhat cell radius do we choose?Depends on traffic we wish to carry (smaller cell means more compact reuse or higher capacity)Limited by handoff
61 Adjacent channel interference So far, we assume adjacent channels to be orthogonal (i.e., they do not interfere with each other).Unfortunately, this is not true in practice, so users may also experience adjacent channel interference besides co-channel interference.This is especially serious when the near-far effect (in uplinks) is significantDesired mobile user is far from BSMany mobile users exist in the cell
64 Reduce Adjacent channel interference Use modulation schemes which have small out-of-band radiation (e.g., MSK is better than QPSK)Carefully design the receiver BPFUse proper channel interleaving by assigning adjacent channels to different cells, e.g., for N = 7
65 Reduce Adjacent channel interference (Cont’d) Furthermore, do not use adjacent channels in adjacent cells, which is possible only when N is very large. For example, if N =7, adjacent channels must be used in adjacent cellsUse FDD or TDD to separate the forward link and reverse link.
66 Improving Capacity in Cellular Systems Adding new channels – often expensive or impossibleFrequency borrowing (or DCA)– frequencies are taken from adjacent cells by congested cellsCell splitting – cells in areas of high usage can be split into smaller cells (microcells with antennas moved to buildings, hills, and lamp posts)Cell sectoring – cells are divided into a number of wedge-shaped sectors, each with their own set of channels
67 SectoringCo-channel interference reduction with the use of directional antennas (sectorization)Each cell is divided into sectors and uses directional antennas at the base station.Each sector is assigned a set of channels (frequencies).
77 Design TradeoffSmaller cell means higher capacity (frequency reused more).However, smaller cell also results in higher handoff probability, which also means higher overheadMoreover, cell splitting should not introduce too much interference to users in other cells
78 Handoff (Handover) Process Handoff: Changing physical radio channels of network connections involved in a call, while maintaining the callBasic reasons for a handoffMS moves out of the range of a BTS (signal level becomes too low or error rate becomes too high)Load balancing (traffic in one cell is too high, and shift some MSs to other cells with a lower load)GSM standard identifies about 40 reasons for a handoff!
79 Phases of Handoff MONITORING PHASE - measurement of the quality of the current and possible candidate radio links- initiation of a handover when necessaryHANDOVER HANDLING PHASE- determination of a new point of attachment- setting up of new links, release of old links- initiation of a possible re-routing procedure
80 Handoff Types Intra-cell handoff Inter-cell, intra-BSC handoff – narrow-band interference => change carrier frequency– controlled by BSCInter-cell, intra-BSC handoff– typical handover scenario– BSC performs the handover, assigns new radio channel in thenew cell, releases the old oneInter-BSC, intra-MSC handoff– handoff between cells controlled by different BSCs– controlled by the MSCInter-MSC handoff– handoff between cells belonging to different MSCs– controlled by both MSCs
82 Handoff Strategies Relative signal strength Relative signal strength with thresholdRelative signal strength with hysteresisRelative signal strength with hysteresis and thresholdPrediction techniques
85 Handoff Based on Receive Level How to avoid ping-pong problem?
86 Handoff – 1G (Analog) systems Signal strength measurements made by the BSs and supervised by the MSCBS constantly monitors the signal strengths of all the voice channelsLocator receiver measures signal strength of MSs in neighboring cellsMSC decides if a handover is necessary
87 Handoff – 2G (Digital) TDMA Handoff decisions are mobile assistedEvery MS measures the received power from surrounding BSs and sends reportsto its own BSHandoff is initiated when the power received from a neighbor BS begins to exceed the power from the current BS (by a certain level and/or for a certain period)
88 Handoff – 2G (Digital) CDMA CDMA uses code to differentiate users.Soft handoff: a user keeps records of several neighboring BSs.Soft handoff may decrease the handoff blocking probability and handoff delay
91 Handoff Prioritization The idea of reserving channels for handoff calls was introduced in the mid 1980s as a way of reducing the handoff call blocking probabilityMotivation: users find calls blocked in mid-progress a far greater irritant than unsuccessful call attempts.The basic idea is to reserve a certain portion of the total channel pool in a cell for handoff users only.
92 Performance MetricsCall blocking probability – probability of a new call being blockedCall dropping probability – probability that a call is terminated due to a handoffCall completion probability – probability that an admitted call is not dropped before it terminatesHandoff blocking probability – probability that a handoff cannot be successfully completed
93 Performance Metrics (Cont’d) Handoff probability – probability that a handoff occurs before call terminationRate of handoff – number of handoffs per unit timeInterruption duration – duration of time during a handoff in which a mobile is not connected to either base stationHandoff delay – distance the mobile moves from the point at which the handoff should occur to the point at which it does occur
94 Summary cellular mobile uses many small cells hexagonal planning, clusters of cellscell repeat patterns 3,7,12 etc...re-uses frequencies to obtain capacityis interference not noise (kTB) limitedS/I is independent of cell radiuschoose cell radius to meet traffic demandN=7 is a good compromise between S/I and capacity.handoff
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