2 Introduction (1) Cellular technology is the foundation of mobile wireless communicationssupports users in locations that are not easily served by wired networks.is the underlying technology formobile telephones,personal communications systems,wireless Internet andWireless Web applications.
3 Introduction (2)Cellular technologies and standards are conveniently grouped into three generations.The first generation is analog based and while still widely used is a passing from the scene.The dominating technology today is the digital second-generation systems.Third generation high speed digital systems have begun to emerge.
4 Introduction (3) Cellular radio is a technique that was developed to increase the capacity available for mobile radio telephone service.The way to increase the capacity of the system isto use lower power systems with shorter radius andto use numerous transmitters/ receivers.
5 Principles of Cellular Networks Cellular Network OrganizationFrequency ReuseIncreasing CapacityOperation of Cellular SystemsMobile Radio Propagation EffectsHandoffPower ControlTraffic Engineering
6 Principles of Cellular Networks Cellular Network OrganizationFrequency ReuseIncreasing CapacityOperation of Cellular SystemsMobile Radio Propagation EffectsHandoffPower ControlTraffic Engineering
7 Cellular Network Organization (1) Use multiple low-power transmitters (100 W or less)Areas divided into cellsEach served by its own antennaA Band of frequencies allocated to each cell.Each cell is served by a Base Station (BS), consisting of transmitter, receiver and control unit.Adjacent cells are assigned different frequencies to avoid interference or crosstalk.Cells sufficiently distant from each other can use the same frequency band.
10 Cell shape Criteria/ recommendations Antennas in all adjacent cells must be equidistant (hexagonal pattern)This simplifies the task ofDetermining when to switch the user to another antennaChoosing another antennaHexagonal patternProvides for equidistant antennasRadius of hexagon=length of side of hexagon= RDistance b/w cells, d = (3/2)R + (3/2)R = 3RNOTE: A precise hexagonal pattern is not used
11 CellEach cellular base station is allocated a group of radio channels to be used within a small geographic area called a cell.
12 Principles of Cellular Networks Cellular Network OrganizationFrequency ReuseIncreasing CapacityOperation of Cellular SystemsMobile Radio Propagation EffectsHandoffPower ControlTraffic Engineering
13 Frequency Reuse OR Frequency Planning The design process of selecting and allocating channel groups for all of the cellular base stations within a system is called frequency reuse.
14 Frequency ReuseAdjacent cells assigned different frequencies to avoid interference or crosstalkIt is not practical to attempt to use same frequency band in two adjacent cells except CDMA, (Code Division Multiple Access)Objective is to reuse frequency in nearby cells10 to 50 frequencies assigned to each cellTransmission power controlled to limit power at that frequency escaping to adjacent cellsThe issue is to determine how many cells must intervene between two cells using the same frequency So that the cells do not interfere.
15 Possible Solution (1) We define N. N is number of cells in a repetitious pattern.In a hexagonal cell pattern, only the following values of N are possibleN = I2 + J2 + (I J), I,J = 0,1,2,3,…Possible values of N are 1,3,4,7,9,12,13,16,19,21, and so on
20 Possible Solution (2) Example: For AMPS (American Advanced Mobile Phone System)Total number of frequencies allotted to the system = K = 395N = 7Each cell can have K/ N = 57 frequencies per cell on average
21 Possible Solution (3) If D = minimum distance between centers of cells that use same frequency band (called co-channels)R = radius of a celld = distance b/w cells = 3RSince d = 3R[Rappa P-60]
22 To Find Nearest Co-Channel To find nearest co-channel neighbors of a particular cell, one must do the followingMove i cells along any chain of hexagons and thenTurn 60 degrees counter-clockwise and move j cells.This is illustrated in figure 3.2 for i = 3, and j = 2 (example, N = 19)[Rappa P-60]
24 Cluster size and Capacity (1) Let a cellular system has a total of S duplex channels available for use.If each cell is allocated a group of k channels (k < S), andIf the S channels are divided among N cells into unique and disjoint channel groups which each have the same number of channels,Total number of available radio channels S = k N[Rappa P-58-60]
25 Cluster size and Capacity (2) CLUSTER: The N cells which collectively use the complete set of available frequencies is called a cluster.Measure of capacity: If a cluster is repeated M times within the system, the total number of duplex channels, C, can be used as a measure of capacity and is given byC = M k N = M SFrom above, “the capacity of a cellular system”, is directly proportional to the number of times a cluster is replicated in a fixed service area.[Rappa P-58-60]
26 Cluster size and Capacity (3) If the cluster size N is reduced while the cell size is kept constant,more clusters are required to cover a given area, andhence more capacity (a large value of C) is achieved.A larger cluster size causes the ratio b/w the cell radius and the distance b/w co-channel cells (R/D) to decrease, leading to weaker co-channel interference.
27 Cluster size and Capacity (4) Conversely, a small cluster size indicates that co-channel cells are located much closer together.RESULT: Smaller N, gives max C but more interference.The value for N is a function of how much interference a mobile or base station can tolerate while maintaining a sufficient quality of communications.From a design point of view, the smallest possible value of N is desirable in order to maximize capacity over a given coverage area.
28 Cluster size and Capacity (5) Problem For an area of 1000 m2, total of 100 duplex channels are allocated. By keeping cell size constant equal to 20 m2, calculate capacity C for following cluster sizes.N = 4N = 8
29 Cellular Systems Terms (1) Frequency Reuse Factor: of a cellular system is given by 1/N, since each cell within a cluster is only assigned 1/N of the total available channels in the system.Cluster Size: The factor N is called the cluster size.Footprint: The actual radio coverage of a cell is known as the footprint and is determined from field measurements or propagation prediction models.[Rappa P-58-60]
30 Cellular Systems Terms (2) Base Station (BS) – includes an antenna, a controller, and a number of receiversMobile telecommunications switching office (MTSO) – connects calls between mobile unitsTwo types of channels available between mobile unit and BSControl channels – used to exchange information having to do with setting up and maintaining callsTraffic channels – carry voice or data connection between users[Rappa P-58-60]
31 Position of Base Station Transmitters When using hexagons to model coverage areas, BS transmitters are depicted as either being in the center of the cell (center-excited cells) or on three of the six cell vertices (edge-excited cells).Normally, omni-directional antennas are used in center-excited cells and sectored directional antennas are used in edge-excited cells.Practical considerations do not allow BS to be placed exactly as they appear in the hexagonal layout.Most system designs permit a BS to be positioned up to one-fourth the cell radius away from the ideal location.[Rappa P-58-60]
32 ExampleIf a total of 33 MHz of bandwidth is allocated to a particular FDD (Frequency Division Duplex, is the first variation of W-CDMA ) 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 usesfour cell reuseseven cell reuse andTwelve cell reuseIf 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.[Rappa P-61]
33 Principles of Cellular Networks Cellular Network OrganizationFrequency ReuseIncreasing CapacityOperation of Cellular SystemsMobile Radio Propagation EffectsHandoffPower ControlTraffic Engineering
34 Capacity Problem In time, as more customers use the system traffic may build upso that there are not enough frequency bands (channels) assigned to a cellto handle its calls.It is a Capacity Problem.
35 Approaches to Cope with Increasing Capacity Following approaches are used.Adding new channelsFrequency borrowingCell splittingCell sectoringMicrocellsA Microcell Zone ConceptFemto Cell[Rappa P-86]
36 Adding new channels Typically, when a system is set up in a region, Not all of the channels are used, andGrowth and expansion can be managed in an orderly fashionby adding new channels.
37 Frequency borrowingFrequencies are taken from adjacent cells by congested cellsDynamic assignment of frequencies to different cells
38 Cell splitting (1)Process of subdividing a congested cell into smaller cells (Same concept as of micro-cells),each with its own base station andcorresponding reduction in antenna height and transmitter power.It increases the capacity of a cellular system sinceit increases the number of times that channels are reused.The additional number of channels per unit area
39 Cell splitting (2)The increased number of cells would increase the number of clusters over the coverage region, which in turn would increase the number of channels, and the capacity, in the coverage area.It allows a system to grow byreplacing large cells with smaller cells,while not upsetting the channel allocation scheme required to maintain the minimum co-channel reuse ratio Q between co-channel cells.
40 Cell splitting (3)A small value of Q means small N provides large capacityA large value of Q means large N improves the transmission quality due to a smaller level of co-channel interference.A trade-off must be made b/w these two objectives in actual cellular design
41 Cell splitting (4)Base stations are placed at the corners of the cellsAssume that area served by BS A is saturated with traffic.New BSs are needed to increase the No. of channels in the area and to reduce the area served by single BSBS A has been surrounded by 6 new cells
42 Cell splitting (5)Cells addition also preserves the frequency reuse plan.Micro cell G has been placed half way b/w two larger stations utilizing the same channel set G.This is also case of other microcells.L/2L
43 Cell splitting (6)Distribution of traffic and topographic features is not uniformThis presents opportunities of capacity increaseOriginal cell 6.5 to 13 kmCells in areas of high usage can be split into smaller cells1.5 Km minimum recommendationFrequent handoffA radius reduction by a factor of F reduces the coverage area and increases the required number of base stations by a factor of F2.
44 Cell splitting (7) Conclusion: By decreasing the cell radius R, and keeping the co-channel reuse ratio D/R unchanged,cell splitting increases the number of channels per unit area.
45 ExampleConsider Figure 3.9. Assume each base station uses 60 channels, regardless of cell size. If each original cell has a radius of 1 Km and each microcell has a radius of 0.5 Km, find the number of channels contained in a 3 Km by 3 Km square centered around A under the following conditions:without the use of microcells;when the lettered microcells as shown in Figure 3.9 are used, also calculate increase in capacity; andif all the original base stations are replaced by microcells. Assume cells on the edge of the square to be contained within the square.
47 Cell sectoringCells are divided into a number of wedge-shaped sectors,Typically three or six sectors per cellEach with their own set of channelsEach sector is assigned a separate subset of the cell’s channelsDirectional antennas are used at the base stationto focus on each sector.
48 Cell sectoring To increase the capacity is to keep the cell radius unchanged andseek methods to decrease the D/R ratio.Sectoring increases SIR (Signal to Interference Ratio) so that cluster may be reduced.In this approach:First the SIR is improved using directional antennas,Then capacity improvement is achieved by reducing the number of cells in a cluster,Thus increasing the frequency reuse.However, in order to do this successfully, it is necessary to reduce the relative interference without decreasing the transmit power.
49 Cell sectoringThe co-channel interference in a cellular system may be decreasedby replacing a single omni directional antenna at the base station by several directional antennas,each radiating within a specified sector.By using directional antennas,a given cell will receive interference andtransmit with only a fraction of the available co-channel cells.
50 Cell sectoring The technique is called sectoring. for decreasing co-channel interference andthus increasing system performance by using directional antennasis called sectoring.
51 Sectoring improves S/I A cell is normally partitioned intothree 120 sectors orsix 60Fig. 2.12
52 Cell sectoring During sectoring Assume The channels used in a particular cell are broken down into sectored groups andare used only within a particular sectorAssumeSeven cell reuse120 sectors
53 Sectoring improves S/I Only two cells have sectors with antenna patterns which radiate into the center cell.Compare it with situation of 6 omni directional antennasIVIIIS/I has been increased from 17dB to 24.2 dB.VIIS/I improvement allows the wireless engineer tothen decrease the cluster size Nin order to improve the frequency reuseand thus system capacity.VIFig. 2.13I
54 Conclusion:Initially co-channels were placed at a distance D due to possible interference.Now we have reduced interference and increased SIR.So we can decrease D and bring co-channels near to each otherThus we decrease N and increase channel capacity.
55 Cell sectoring Further improvements in S/I: In practical systems, further improvement in S/I is achieved bydown tilting the sector antennassuch that the radiation pattern in the vertical (elevation )plane has a notch at the nearest co-channel cell distance
56 Microcells Microcells are used in As cells become smaller, city streetscongested areas,along highways, andinside large public buildingsAs cells become smaller,antennas move from the tops of tall buildings or hill to the top of small buildings or the sides of large buildings, and finally to lamp posts, where they form microcells.Decrease in cell sizeDecrease in radiated power by BS and mobile station
57 Typical ParametersThe average delay spread refers to multipath delay spread that isSame signal follows different paths andThere is a time delay between the earliest and latest arrival of the signal at the receiver.
58 ExampleAssume a system of 32 cells with a cell radius of 1.6 Km, a total frequency bandwidth that supports 336 traffic channels, and N = 7.If there are 32 total cells,what geographic area is covered?how many channels are there per cell?, andRepeat for a cell radius of 0.8 Km and 128 cells.
59 The area of hexagonA hexagon of radius 1.6Km has an area 6.65 km2.Total area covered = 6.65 km2 per cell x 32 cells= 213 Km2.No. of channels per cell = 336/7 = 48 channels per cellTotal channel capacity = 48 channels per cell x 32 cells = 1536 channelsFor second fig same 213 Km2 is covered but Channel Capacity = 6144 channels
61 Repeaters for Range Extension Often a wireless operator needs to provide dedicated coverage for hard-to-reach areas, such as within building, or in valleys or tunnels.Radio re-transmitters, known as repeaters are often used to provide such range extension capabilities.In practice, directional antennas or distributed antenna system (DAS) are connected to the inputs or outputs of repeaters for localized spot coverage, particularly in tunnels or buildings.
62 Principles of Cellular Networks Cellular Network OrganizationFrequency ReuseIncreasing CapacityOperation of Cellular SystemsMobile Radio Propagation EffectsHandoffPower ControlTraffic Engineering
64 Principal Elements of a Cellular systems Mobile Telecommunications Switching Office (MTSO)The MSC (Mobile Switching Centre ) and MTSO are one and the sameBase Transceiver Station (BTS)Public Telecommunications Switching Network
65 Base Station (BS/BTS)In the center of each cell there is BS that includesAn antenna,A controller,Processes call b/w the mobile unit (MU) and the rest of the networkNumber of transceivers,For communicating on the channels assigned to that cell.
67 MTSO One MTSO serving multiple BSs Typically the link b/w MTSO and BS is by wire line, although a wireless link is also possibleIt connects call b/w MUsIt connects public telephone or telecommunication networkIt assigns the voice channel to each call,performs handoffs andmonitors the call for billing information.
68 Operation of Cellular Systems Channels Type Two types of channels b/w BS and MUControl channelsTo exchange information having to do with setting up and maintaining calls and with establishing a relationship b/w MU and nearest BSTraffic channelsCarry a voice or data connection b/w users
70 A Nortel Networks base station deployed for the network.
71 Steps in an MTSO Controlled Call between Mobile Users Mobile unit initializationMobile-originated callPagingCall acceptedOngoing callHandoff
72 Mobile unit initialization 1. MU scans/selects the strongest setup control channel.2. MU selects the BS3. Handshake takes place b/w MU and the MTSO through BS4. Handshake is used to identify the user and to register its location5. Handshake is repeated periodically to care for handoff
73 Mobile-originated call A MU originates a call by sending the number of the called unit on the pre-selected setup channel.MU checks forward channel (from the BS) transmit on corresponding reverse channel (to BS)
74 PagingThe MTSO sends a paging message to certain BSs depending on the called mobile unit number.Each BS transmits the paging signal on its own assigned setup channel
75 Call acceptedThe called MU recognizes its number on the setup channel being monitored and responds to that BS.BS sends the response to MTSO and circuit is setupThe MTSO selects an available traffic channel and notifies.
77 Handoff While moving from one channel to another The traffic channel has to change to one assigned to the BS in the new cell.The system makes this change without either interrupting the call or alerting the user.
78 Additional Functions in an MTSO Controlled Call Call blockingIf all the traffic channels assigned to the nearest BS are busyMU makes a preconfigured number of repeated attempts.After a certain number of failed tries a busy tone is returned to the user.Call terminationWhen one of two users hangs up, the MTSO is informed and … two BSs are released
79 Additional Functions in an MTSO Controlled Call Call dropb/c of interference, weak signalsBS can not maintain minimum required SNR for a certain period of timeThe traffic channel to the user is dropped and the MTSO is informedCalls to/from fixed and remote mobile subscriberThe MTSO connects to the PSTN.MTSO can set up a connection b/w MU and fixed subscriber.MTSO can connect to a remote MTSO via the telephone network or via dedicated line and set up a connection
82 CAI: Common Air Interface FVC: Forward Voice Channels RVC: Reverse Voice ChannelsFCC: Forward Control ChannelsRCC: Reverse Control ChannelsMIN: Mobile identification number, which is the subscriber’s telephone numberMSC: Mobile Switching Center. It is sometime called a mobile telephone switching office (MTSO)NOTE: Control Channels are also called as Setup channels because they are only involved in setting up a call and moving it to an unused voice channel.[Rappa P-13-15]
83 Fig. 1.6Figure 1.6: Timing diagram illustrating how a call to a mobile user initiated by a landline subscriber is established.[Rappa P-16]
84 Fig. 1.7Figure 1.7 Timing diagram illustrating how a call initiated by a mobile is established.[Rappa P-17]
85 Principles of Cellular Networks Cellular Network OrganizationFrequency ReuseIncreasing CapacityOperation of Cellular SystemsMobile Radio Propagation EffectsHandoffPower ControlTraffic Engineering
86 Mobile Radio Propagation Effects Signal strength (SS)Must be strong enough between base station and mobile unit to maintain signal quality at the receiverMust not be so strong as to create too much co-channel interference with channels in another cell using the same frequency bandFadingSignal propagation effects may disrupt the signal and cause errors
87 Mobile Radio Propagation Effects In designing a cellular layoutThe desired maximum transmit power level at the BS and the MUThe typical height of the MU antennaThe available height of the BS antennaThese factors determine the size of Cell
88 Channel (Path Loss) Model Propagation effects are dynamic and difficult to predictFind a model based on empirical dataOne of the most famous widely used models was developed by Okumura et al.
89 Channel (Path Loss) Model Okumura Model was subsequently refined by Hata.Hata’s Model Empirical formulationTakes into account a variety of environments and conditions.Models are applied to a given environment to develop guidelines for Cell Size
91 ExampleFor a carrier frequency of 900 MHz, ht = 40m, hr = 5m, and d = 10 Km. Estimate the path loss for a medium size city.A(hr) = 8.95 dBLdB = dB
92 Principles of Cellular Networks Cellular Network OrganizationFrequency ReuseIncreasing CapacityOperation of Cellular SystemsMobile Radio Propagation EffectsHandoffPower ControlTraffic Engineering
93 Handoff/ Handover Both Handoff and Handover have same meaning. The term Handoff is used in U.S cellular standards documentITU documents use the term Handover.
94 Handoff What is? It is the procedure for changing the assignment of a MU from one BS to another as the MU.Handled in different ways in different systems.Involves a number of factors
95 Handoff Initiation May be Network Initiated Decision is made solely by the network measurements of received signals from the MU.Mobile Assisted Handoff (MAHO)MU provides feedback to the network concerning signals received at the MU.
96 Handoff Performance Metrics Call blocking probabilityProbability of a new call being blocked, due to heavy load on the BS traffic capacity.MU is handed off based not on signal quality but on traffic capacity.Call dropping probabilityprobability that a call is terminated due to a handoff
97 Handoff Performance Metrics Call completion probabilityProbability that an admitted call is not dropped before it terminatesProbability of unsuccessful handoffProbability that a handoff is executed while the reception conditions are inadequateExample: Time lapseHandoff blocking probabilityProbability that a handoff cannot be successfully completedExample: Other BS busy
98 Handoff Performance Metrics Handoff probabilityProbability that a handoff occurs before call terminationRate of handoffNumber of handoffs per unit timeInterruption durationDuration of time during a handoff in which a mobile is not connected to either base stationHandoff delayDistance the mobile moves from the point at which the handoff should occur to the point at which it does occur
99 Handoff Decision Principal parameter is SS. SS is measured from the MU BS averages signal over a moving window of time to remove the rapid fluctuations due to multi-path effects.
100 Handoff strategiesThese strategies are used to Determine Instant of HandoffRelative SSRelative SS with thresholdRelative SS with hysteresisRelative SS with hysteresis and thresholdPrediction techniques
101 Handoff decision as a function of handoff scheme
102 Relative SSThe MU is handed off from BS A to BS B when the Signal Strength (SS) at B first exceeds that at A.At L1, SS is adequate but decliningthere is hand off.b/c SS fluctuates due to multi-path effects, even with power averaging SS again may increase handoff back to A.Problem: This may result in ping-pong effect.
103 Choosing ThresholdIf high threshold Th1 is used system performs like Relative SS scheme.If low threshold Th3 is used, quite low as compared to the crossover SS (at L1) before handoff.MU will move far in into the new cellThis reduces the quality of the communication linkIt may result in a dropped call.A threshold should not be used aloneb/c its effectiveness depends on prior knowledge of the crossover SS b/w current and candidate BSs.
104 Relative SS with hysteresis (1) Handoff occurs only if the new BS is sufficiently stronger by a margin of H.Look at graph,After crossover pointPB > PAAt L3, PB - PA = HHandoff occurs at L3.
105 Relative SS with hysteresis (2) This scheme prevents the ping-pong effectHandoff mechanism has two statesWhile the MU is assigned to BS A, the mechanism will generate a handoff when the relative SS reaches or exceeds the H.Once the MU is assigned to B, it remains so until the relative SS strength falls below –H, at which point is handed back to A.Disadvantage: First handoff may still be unnecessary if BS A still has sufficient SS.
106 Relative SS with hysteresis and Threshold Handoff occurs only ifThe current signal level drops below a threshold, andThe target BS is stronger than the current one by a hysteresis margin H.In our example, handoff occurs at L3. if the threshold is either Th1 or Th2 and at L4 if the threshold is at Th3.
107 Prediction Technique The handoff decision is based on the expected future value of the received SS.
108 Handoff priorityMany handoff strategies prioritize handoff requests over call initiation requestswhen allocating unused channels in a cell site.Handoff must be performed successfully and infrequently as possible.Handoff must be imperceptible to the users.[Rappa P-62-67]
109 Proper signal level (1) Signal Level Specification for Handoff Once a particular signal level is specified as the minimum usable signal for acceptable voice quality at the BS receiverNormally taken as b/w -90 dBm and -100 dBma slightly stronger level is used as a threshold at which a handoff is made.[Rappa P-62-67]
110 = Pr handoff – P r minimum usable Proper signal level (2)The margin, given by = Pr handoff – P r minimum usable can not be too largeb/c unnecessary handoffs which burden the MTSO may occur can not be too smallThere may be insufficient time to complete a handoff before a call is lost due to weak signal conditions.[Rappa P-62-67]
112 Possible reasons of NO handoff When there is an excessive delay by the MSC (MTSO) in assigning a handoff orwhen the threshold is set too small for the handoff time in the system.Excessive delays may occur during high traffic conditions due to computational loading at the MSC ordue to the fact that no channels are available on any of the nearby BSs[Rappa P-62-67]
113 Dwell timeThe time over which a call may be maintained within a cell, without handoff, is called the dwell time.The dwell time, of a particular user is governed by a number of factors, includingPropagationInterferenceDistance between the subscriber and the BSOther time varying effects.
114 ImplementationsIn first generation cellular systems, SS measurement are made by the BSs and supervised by the MSC.In today’s second generation systems, handoff decisions are mobile assisted (MAHO)[Rappa P-62-67]
115 First Generation - Analog Each BS constantly monitors the SS of all of its reverse voice channels (RVC) to determine the relative location of each MU with respect to BS tower.Locator Receiver (LR): in addition to measuring the RSSI of calls in progress within the cell, a spare receiver in each BS, called LR is used to scan and determine SS of MUs which are in neighbouring cells.LR is controlled by MSCBased on the LR SS info from BS, the MSC decides handoff.
116 Second GenerationEach MU measures the received power from surrounding BS andcontinually reports the results of these measurements to the serving BS.A handoff is initiated whenthe power received from the BS of a neighboring cell begins to exceed the power received from the current base station by a certain level or for a certain period of time.
117 MAHOThe MAHO method enables the call to be handed over b/w BSs at a much faster rate than in first generation analog systemsSince the handoff measurements are made by each MU andThe MSC no longer constantly monitors SSMAHO is particularly suited for microcellular environmentswhere handoffs are more frequent.
118 Intersystem handoffIf a mobile moves from one cellular system to a different cellular system controlled by a different MSC Intersystem handoff becomes necessaryMany issuesLocal call may become a long distance callCompatibility between two MSCs before handoff
119 Managing Handoff (1)Different systems have different policies and methods for managing handoff requestsSome systems handle handoff request in the same way they handle originating calls.In such systems, the probability that a handoff request will not be served by a new BS is equal to the blocking probability of incoming calls.However, from the users point of view, having a call abruptly terminated while in the middle of a conversation is more annoying than being blocked occasionally on a new call attempt.
120 Managing Handoff (2)To improve the QoS as perceived by the users, various methods have been devised to prioritize handoff requests over call initiation requests when allocating voice channels.Prioritizing HandoffsGuard Channel ConceptsQueuing of handoff requestsPractical Handoff ConsiderationsUmbrella Cell Approach solution to a problemCell Dragging is a problem
121 Guard Channel Concepts Channel Reservation:Fraction of total available channels in a cell is reserved exclusively for handoff requestsDisadvantage:Reduces the total carried traffic (CT) as fewer channels are allocated for originating calls.Possible Solution:Use of dynamic channel assignment strategies Minimize the No. of required guard channels by efficient demand based allocationProvides efficient spectrum utilization
122 Queuing of handoff requests (1) Criteria:There must be finite time interval b/wthe time the RSS drops below the handoff threshold andthe time the call is terminated due to insufficient signal level.
123 Queuing of handoff requests (2) It is one of the methods to decrease the probability of forced termination (PFT) of a call due to lack of available channels.There is a tradeoff b/w CT & PFTQueuing does not guarantee zero probability of forced terminationSince large delays will cause the RSS to drop below the minimum required level to maintain communication andhence lead to forced termination
124 Practical Handoff Considerations During a callHigh speed vehicles need more handoffPedestrians may never need a handoffFor micro-cellsMSC become burdened if high speed users are constantly being passed b/w very small cells.ProblemHandling of high speed and low speed traffic simultaneously while minimizing the handoff intervention from MSCSolution:Umbrella Cell approach
125 Umbrella Cell approach “Large” and “Small” cells areProvidedby using different antenna heights (often on the same building and towers) anddifferent power levels andCo-located at a single locationProvideslarge area coverage to high speed users whilesmall area coverage to users traveling at low speeds.
127 Umbrella Cell Approach This approach ensures thatthe number of handoffs is minimized for high speed users andprovides additional micro-cell channels for pedestrian users.
128 Umbrella Cell Approach Scenario:A driver in a high speed car is attending a call on mobile telephone, present in a large cell enters in a micro cell. Discuss the responsibility of BS/MSC
129 Umbrella Cell Approach The speed of each user may be estimated by the BS or MSC byEvaluating how rapidly the short-term average signal strength on the RVC changes over time, ORMore sophisticated algorithms may be used o evaluate and partition users.If a high speed user in the large umbrella cell is approaching the BS and its velocity is rapidly decreasing,the BS may decide to hand the user into the co-located micro-cell, without MSC intervention.
130 Cell Dragging (1) Problem of Cell Dragging It occurs in an urban environment when there is a LOS radio path b/w the subscriber and the BS.As the user travels away from the BS at very slow speed, the average SS does not decay rapidly.Even when the user has traveled well beyond the designed range of the cell, the RSS at the BS may be above the handoff threshold,Thus a handoff may not be made
131 Cell Dragging (2)This creates a potential interference and traffic management problemSince the user has meanwhile traveled deep within a neighboring cell.Solution:Handoff threshold and radio coverage parameters must be adjusted carefully.
132 Handoff Time First generation- Analog Cellular Systems 10 Seconds GSM Requires that the value of be on the order of 6 dB to 12 dBGSMMAHO1or 2 Seconds, after decision is made is usually on the order of 0 dB to 6 dBAdvantage: Faster handoff greater range of options for handling high speed and low speed users
133 Recent Trends in handoff Two typesHard handoffChannelized wireless systems that assign different radio channels during a handoffSoft handoffIt is the ability to select b/w the instantaneous RSS from a variety of BS.
134 Soft handoffSpread Spectrum mobiles (Code Division Multiple Access, CDMA) share the same channel in every cell.By simultaneously evaluating the RSS from a single subscriber at several neighboring BSs,The MSC may actually decide which version of the user’s signal is the best at any moment in time.This technique exploits macroscopic space diversity provided by the different physical locations of the BS.
135 Principles of Cellular Networks Cellular Network OrganizationFrequency ReuseIncreasing CapacityOperation of Cellular SystemsMobile Radio Propagation EffectsHandoffPower ControlTraffic Engineering
136 Power ControlDesign issues making it desirable to include dynamic power control in a cellular systemReceived power must be sufficiently above the background noise for effective communicationDesirable to minimize power in the transmitted signal from the mobileReduce co-channel interference, alleviate health concerns, save battery powerIn Spread Spectrum systems using CDMA, it’s desirable to equalize the received power level from all mobile units at the BS
137 Types of Power Control (1) 1. Closed-loop power controlAdjusts signal strength in reverse channel (MU to BS) based on some metric of performance in that reverse channel, such asReceived signal power levelReceived signal-to-noise ratio, orReceived bit error rateBS makes power adjustment decision and communicates to MU on control channel
138 Types of Power Control (2) Closed loop power control is also used to adjust power in the forward channel.In this case MU provides information about received signal quality to the BS, which then adjusts transmitted power.GSM standard a TDMA standard defines, according to power outputEight classes of BS channels andFive classes of MU
139 GSM Transmitter Classes Adjustments in both directions are made using closed loop power control
140 Types of Power Control (3) 2. Open-loop power controlDepends solely on mobile unit with no feedback from BS used in some spread spectrum systems (SSS).In SSS, the BS continuously transmits an unmodulated signal known as a pilot.The pilot allows MU to acquire the timing of the forward (BS to mobile) CDMA channel.
141 Types of Power Control (4) It can also be used for power control.The MU monitors the RSS of the pilot and sets the transmitted power in the reverse (mobile to BS) channel inversely proportional to it.Assumption: Forward and Reverse link RSS are correlated.
142 Types of Power Control (5) Not as accurate as closed-loop,but can react quicker to fluctuations in signal strengthE.g when MU emerges from behind a large building.This fast action is required in the reverse link of a CDMA systems where the sudden increase in RSS at the BS may suppress all other signals.
144 Trunking Theory (2) Trunked Radio Systems share a small pool of frequencies among a large number of usersThe benefit of this technology to the agencies is thatmany more channels are available for specialized traffic than there are frequencies.Example, the Fort Worth trunked system has only 20 frequencies, but services over 400 channels.
145 Trunking Theory (3)Trunking is the aggregation of multiple user circuits into a single channel.The aggregation is achieved using some form of multiplexing.Trunking theory was developed by Agner Krarup Erlang,Erlang based his studies of the statistical nature of the arrival and the length of calls.The Erlang B formula allows for the calculation of the number of circuits required in a trunk based onthe Grade of Service (GoS) andthe amount of traffic in Erlangs the trunk needs cater for.
147 Principles of Cellular Networks Cellular Network OrganizationFrequency ReuseIncreasing CapacityOperation of Cellular SystemsMobile Radio Propagation EffectsHandoffPower ControlTraffic Engineering
148 Traffic EngineeringThe Danish mathematician Agner Krarup Erlang is the founder of teletraffic engineering.Ideally, available channels would equal number of subscribers active at one timeIn practice, not feasible to have capacity handle all possible loadFor N simultaneous user capacity and L subscribersL < N – nonblocking systemL > N – blocking system
149 Blocking System Performance Questions Probability that call request is blocked? ORWhat capacity is needed to achieve a certain upper bound on probability of blocking?If blocked calls are queued, what is the average delay? ORWhat capacity is needed to achieve a certain average delay?
150 Traffic Intensity (1) The basic measure of traffic intensity is A. It is measured in a dimensionless unit, the erlang = mean rate of calls attempted per unit timeh = mean holding time per successful callA = average number of calls arriving during average holding period, for normalized
151 Traffic Intensity (2) Cell as a Multiserver Queuing System: No. of servers equal to channel capacity, N.The average service time at a server is h.A basic relationship in a multiserver queue ish = NWhere is server utilization, or the fraction of time that a server is busy.Note: A = N, is considered as a measure of the average number of channels required.
152 Traffic Intensity (3) Example: What should be capacity of the cell to meet average demand, for average calling rate of 20 calls per minute and average holding time 3 minutes?To meet the fluctuations, what do you suggest about the capacity of the cell?
153 Traffic Intensity (4) Example: A cell having 10 channels is busy over a period of one hour as shown in the figure. Calculate mean number of channels busy for this time.
155 Traffic Modeling (1) Upper limit of traffic intensity? ITU-T “mean busy hour traffic”Average of the busy-hour-traffic on the 30 busiest dayA is measure of busy-hour-trafficA is input to traffic modelTraffic model answers the questions posed.
156 See detail on next slide Traffic Modeling (2)Two key factors/elements that determines the nature of traffic modelThe manner in which blocked calls are handledThen number of traffic sources.See detail on next slide
157 For Cellular systems, the LCC model is generally used most accurate Traffic Modeling (3)Manner in which blocked calls are handledLost calls delayed (LCD) – blocked calls put in a queue awaiting a free channelBlocked calls rejected and droppedLost calls cleared (LCC) – user waits before another attemptLost calls held (LCH) – user repeatedly attempts callingFor Cellular systems, the LCC model is generally used most accurateNumber of traffic sourcesWhether number of users is assumed to be finite or infinite
158 Traffic Modeling (4) Erlang B Infinite Source, LCC Model:The key parameter is the probability of loss, or grade of Service (GoS).GoS = 0.01 means that, during a busy hour, the probability that an attempted call is blocked is 0.01Values in the range 0.01 to are generally considered quite good.
159 Traffic Modeling (5) Erlang B Following equation is known as Erlang B.P = Probability of blocking (grade of service)A = Offered traffic, erlangsN = Number of servers
161 Illustrations of these points on next slides Traffic Modeling (7)Two important points can be deduced from the table:A larger-capacity system is more efficient than a smaller-capacity one for a given grade of service.A larger-capacity system is more susceptible (open) to an increase in traffic.Illustrations of these points on next slides
162 Traffic Modeling (8) Illustrations: First point Consider two cells, each with a capacity of 10 channelsJoint capacity of 20 channelsLook in column GoS=0.002Traffic intensity for 10 channels = 3.43Traffic intensity for 20 channels = 6.86For a single cell of 20 channelsTraffic intensity = 10.07
163 Traffic Modeling (9) Second point Consider a single cell of 10 channels,Look in column GoS=0.002Traffic intensity/ Load = 3.43 Erlangs30% increase in the traffic reduces the GoS to 0.01Consider a single cell of 70 channels,Traffic intensity/ Load = 51.0 ErlangsOnly 10% increase in the traffic reduces the GoS from to 0.01