Page 1. Overview : Wireless Networks Lecture 7: Cellular Networks. The advent of cellular networks. The cellular network design.

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1 Overview : Wireless Networks Lecture 7: Cellular Networks Dina Papagiannaki & Peter Steenkiste Departments of Computer Science an Electrical an Computer Engineering Spring Semester Peter A. Steenkiste & Dina Papagiannaki 1 Cellular esign Frequency Reuse Capacity an Interference Elements of a cellular network How oes a mobile phone take place? Paging Hanoff Frequency Allocation Traffic Engineering Peter A. Steenkiste & Dina Papagiannaki 2 The avent of cellular networks The cellular network esign Mobile raio telephone system was base on:» High power transmitter/receivers» Coul support about 25 channels» in a raius of 80 Km To increase network capacity:» Multiple low-power transmitters (100W or less)» Small transmission raius -> area split in cells» Each cell with its own frequencies an base station» Ajacent cells use ifferent frequencies» The same frequency can be reuse at sufficient istance Implications on mobile hansets (per L1) Simplest layout» Ajacent antennas not equiistant how o you hanle users at the ege of the cell? Ieal layout 2 Peter A. Steenkiste & Dina Papagiannaki 3 Peter A. Steenkiste & Dina Papagiannaki 4 The hexagonal pattern Cell site A hexagon pattern can provie equiistant access to neighboring cell towers = 3R In practice, variations from ieal ue to topological reasons R Peter A. Steenkiste & Dina Papagiannaki 5 Peter A. Steenkiste & Dina Papagiannaki 6 Page 1

2 Frequency reuse Minimum separation? Each cell features one base transceiver Through power control cover the cell area while limiting the power leaking to other cofrequency cells Frequency reuse not possible for ajacent towers! The number of frequency bans assigne to a cell epenent on its traffic Peter A. Steenkiste & Dina Papagiannaki 7 Peter A. Steenkiste & Dina Papagiannaki 8 Frequency reuse characterization Capacity an Interference D = minimum istance between centers of cochannel cells R = raius of cell = istance between centers of ajacent cells N = number of cells in a repetitious pattern, i.e. reuse factor N = I 2 + J 2 + (I J), I,J = 0,1,2,3,... D R = 3N or Peter A. Steenkiste & Dina Papagiannaki 9 S = Total # of uplex channels available for use k = Total # of uplex channels per cell N = Cluster of cells which collectively use the complete set of available frequencies If a cluster is replicate M times within the system, the total # of uplex channels C can be use as a measure of capacity D = N Peter A. Steenkiste & Dina Papagiannaki 10 Traeoff If N k since S is a constant M for a fixe geographical area if the same cell raius is maintaine Capacity increases PRICE PAID: NOTE: To reuce co-channel interference Co-channel interference How to Increase Capacity? Aing new channels Frequency borrowing Microcells» Antennas on top of builings, even lamp posts» Form micro cells with reuce power» Goo for city streets, roas an insie builings Capacity since kn = S = fixe There is a trae-off between capacity an interference reuction Peter A. Steenkiste & Dina Papagiannaki 11 Peter A. Steenkiste & Dina Papagiannaki 12 Page 2

3 Cell splitting Cell sectoring Cell size ~ Km, Minimum ~ 1.5Km Requires careful power control an possibly more frequent hanoffs for mobile stations A raius reuction by a factor of F reuces the coverage area an increases the require number of base stations by a factor of F 2 Cell sectoring» Cell ivie into wege shape sectors» 3-6 sectors per cell, each with own channel set» Subset of cell s channel, use of irectional antennas Peter A. Steenkiste & Dina Papagiannaki 13 Peter A. Steenkiste & Dina Papagiannaki 14 Elements of a cellular system Channels Base Station (BS): inclues antenna, a controller, an a number of transceivers for communicating on the channels assigne to that cell Controller is use to hanle the call process between the mobile unit an the rest of the network MTSO: mobile telecommunications switching office, serving multiple BSs. Connects calls between mobiles an to the PSTN. Assigns the voice channel, performs hanoffs, billing Control channels to exchange information regaring setup an call maintenance. Establishing relationship between mobile an closet BS. Traffic channels carry voice an ata connections between users 5% of channels for control/95% for traffic Peter A. Steenkiste & Dina Papagiannaki 15 Peter A. Steenkiste & Dina Papagiannaki 16 Call progression Call progression (a) Monitor for strongest signal (b) Request for connection (c) Paging () Call accepte Peter A. Steenkiste & Dina Papagiannaki 17 Peter A. Steenkiste & Dina Papagiannaki 18 Page 3

4 Call progression Paging (e) Ongoing call (f) Hanoff Broacast mechanism to locate a target mobile unit Normally, there is knowlege on a limite number of cells where the mobile may be (Location Area in GSM, Routing Area if ata packet sessions) GSM: neighbor cells groupe in Location Area an subscriber only upates when moving across. Paging restricte to the Location Area itself.» How o we assign cells to LAs? Peter A. Steenkiste & Dina Papagiannaki 19 Peter A. Steenkiste & Dina Papagiannaki 20 Hanoff Hanoff between 2 cells Coul be network or client initiate, in which case the client provie the network with measurements Target performance metrics:» Cell blocking probability» Call ropping probability» Call completion probability» Probability of unsuccessful hanoff» Hanoff blocking probability» Hanoff probability» Rate of hanoff» Interruption uration» Hanoff elay Base station A Base station B Peter A. Steenkiste & Dina Papagiannaki 21 Peter A. Steenkiste & Dina Papagiannaki 22 Hanoff implementations Power Control GSM/W-CDMA» Inter-frequency hanovers will measure the target channel before moving over» Once the channel is confirme OK, the network will comman the mobile to move an start bi-irectional communication there CDMA2000/W-CDMA(same)» Both channels are use at the same time soft hanover IS-95 (inter-frequency)» Impossible to measure channel irectly while communicating. Nee to use pilot beacons. Almost always a brief isruption. Receive signal at mobile nees to be sufficiently above backgroun noise Mobile transmission power minimize to avoi co-channel interference, alleviate health concerns an save battery power In SS using CDMA, nee to equalize power from all mobiles are the BS Peter A. Steenkiste & Dina Papagiannaki 23 Peter A. Steenkiste & Dina Papagiannaki 24 Page 4

5 Open-Loop Power Control Close-Loop Power Control No feeback from the BS (some SS systems) BS transmits a pilot signal:» Mobile acquire timing an phase reference for emoulation Transmitte power in the reverse channel assume to be inversely proportional» Assumes forwar an reverse link signal strength closely correlate» Combats near-far problem in CDMA networks Features:» Not as accurate as close loop» Quick ajustment to rapi signal strength fluctuations Peter A. Steenkiste & Dina Papagiannaki 25 Signal strength from mobile to BS ajuste accoring to performance metric on the reverse channel» Reverse signal power level, receive signal-to-noise ratio, or receive bit error rate BS makes the ecision an communicates a power ajustment comman to the mobile on a control channel Mobile provies information about receive signal strength to the BS, an BS ajusts power accoringly Peter A. Steenkiste & Dina Papagiannaki 26 Fixe Channel Assignment (FCA) Dynamic Channel Assignment (DCA) Each cell is allocate a preetermine set of voice channels. Any call attempt within the cell can only be serve by the unuse channels in that cell If all the channels in that cell are being use the call is blocke user oes not get service A variation of FCA: the cell whose channels are all being use is allowe to borrow channels from the next cell. MTSO supervises this operation. Peter A. Steenkiste & Dina Papagiannaki 27 Voice channels are not assigne or allocate to ifferent cells permanently. Instea each time a request is mae, the serving BS requests a channel from the MTSO. MTSO allocates a channel to the requeste cell following an algorithm that takes into account the likelihoo of future blocking within the cell, the freq. of use of the caniate channel, the reuse istance of the channel, an other cost functions. MTSO only allocates a channel if it is available an not being use in the restricte istance for cochannel interference Peter A. Steenkiste & Dina Papagiannaki 28 FCA/DCA comparison Hybri Channel Assignment Avantage of DCA: Likelihoo of blocking ecreases an trunking efficiency increases Disavantage of DCA: MTSO shoul collect realtime ata on channel occupancy, traffic istribution an RSSI of all channels on a continuous basis. Overhea in terms of storage an computational loa on the system. In hybri channel assignment strategies, the concepts of fixe an ynamic channel assignment schemes are combine. In aition to the fixe set of channels assigne to each cell, there are channels in a global pool for use in case of shortage. Peter A. Steenkiste & Dina Papagiannaki 29 Peter A. Steenkiste & Dina Papagiannaki 30 Page 5

6 Hybri Channel Assignment Dynamic Loa balancing When a call requires service from a cell an all of its fixe channels are busy, a channel from the global pool is assigne to the call. The ratio of fixe to global (ynamic) channels is a significant parameter which efines the performance of the system. Ratio is a function of traffic loa an woul vary over time accoring to the offere loa estimations. Superior performance with non-uniform traffic Channel borrowing from neighboring cells. co-channel interference important When a channel is borrowe or ynamically assigne, the interference level in the cochannel cells within the reuse istance shoul be examine. If the interference level is going to be too high, the corresponing channels of those co-channel cells shoul also be locke. This not only ecreases the capacity of the overall system, but increases the computational loa on the system as well. Peter A. Steenkiste & Dina Papagiannaki 31 Peter A. Steenkiste & Dina Papagiannaki 32 Dynamic Loa Balancing with/without (irectional) locking Traffic Engineering If the cell has L subscribers.. an can support N simultaneous users. If L<=N, nonblocking system If L>N, blocking system If blocking:» What is the probability of a call being blocke?» What N o I nee to upper boun this probability?» If blocke calls are queue, what is the average elay?» What capacity is neee to achieve a certain average elay? Peter A. Steenkiste & Dina Papagiannaki 33 Peter A. Steenkiste & Dina Papagiannaki 34 Trunking theory terminology Thunking theory terminology Set-up Time: The time require to allocate a trunke raio channel to a requesting user. Blocke Call (Lost Call): Call which cannot be complete at time of request, ue to congestion. Holing Time: Average uration of a typical call. Denote by h (in secons). Traffic Intensity: Measure of channel time utilization, which is the average channel occupancy measure in Erlangs. Loa: Traffic intensity across the entire trunke raio system, measure in Erlangs. Grae of Service (GOS): A measure of congestion specifie as the probability of a call being blocke (for Erlang B), or the probability of a call being elaye beyon a certain amount of time (for Erlang C). Request Rate: The average number of call requests per unit time. Denote by λ calls per secon. Peter A. Steenkiste & Dina Papagiannaki 35 Peter A. Steenkiste & Dina Papagiannaki 36 Page 6

7 Trunking theory Simple example Traffic intensity: A = λh (average number of calls receive uring the average holing time) If channel capacity is N system can be seen as a multiserver queuing system λh = ρn ρ is server utilization, fraction of time server is busy A also average number of channels require A cell has a capacity of 10 channels In 1 hour it receive 97 calls lasting 294 minutes in total The rate of calls per min = 97/60 The average holing time = 294/97 A = (97/60) x (294/97) = 4.9 Erlangs Mean number of calls in progress is 4.9 Mean number of channels engage is 4.9 Peter A. Steenkiste & Dina Papagiannaki 37 Peter A. Steenkiste & Dina Papagiannaki 38 Cellular network esign Infinite source LLC Grae of Service Size to sustain the average eman in the busy hour (not peak eman!) Base on carrie traffic an not offere! Moel epens on:» How are blocke calls hanle? Coul be put in a queue (lost calls elaye) Rejecte or roppe 1. user may hang up an try again after some ranom time interval lost calls cleare (LCC) 2. User repeately attempts lost calls hel (LCH)» Number of traffic sources Finite or infinite? Infinite source assumption reasonable when sources at least 5 to 10 times the capacity of the system Peter A. Steenkiste & Dina Papagiannaki 39 Erlang B formula P = A = offere traffic, N = #servers, P = blocking probability Peter A. Steenkiste & Dina Papagiannaki 40 N A N N! A x x! x= 0 Example Erlang B Example Erlang B N N A larger capacity system is more efficient than a smallercapacity one for a given grae of service Peter A. Steenkiste & Dina Papagiannaki 41 Peter A. Steenkiste & Dina Papagiannaki 42 Page 7

8 Example Erlang B What s Next? N 30% Cellular Protocol Stanars Mobile IP Wireless Application Protocol (WAP) 10% A larger capacity system is more susceptible to an increase in traffic Peter A. Steenkiste & Dina Papagiannaki 43 Peter A. Steenkiste & Dina Papagiannaki 44 Not Use Peter A. Steenkiste & Dina Papagiannaki 45 FCA Performs better uner heavy traffic Low flexibility in channel assignment Maximum channel reusability Low computational effort Low call set up elay Low implementation complexity Low signaling loa Sensitive to time an spatial changes High force call termination probability Suitable for large cell environment FCA vs. DCA DCA Performs better uner light/ moerate traffic Flexible allocation of channels Not always maximum channel reusability High computational effort Moerate to high call set up elay Moerate to high implementation complexity Moerate to high signaling loa Insensitive to time an spatial changes Low to moerate force call termination probability Suitable for micro-cell environment Peter A. Steenkiste & Dina Papagiannaki 46 Hanoff Strategies First Generation Analog Systems (AMPS): Signal strength measure by BS an supervise by the MTSO Each BS constantly monitors the signal strengths of all of its reverse voice channels to etermine relative location of each MU w.r.t. BS. Locator Receiver at each BS measures RSS of users in neighboring cells Secon Generation Systems (TDMA IS-54/ GSM/ PHP): Mobile Assiste Hanoff (MAHO) Every MS measures the receive power from surrouning BS an continually reports these to the serving BS When the power receive from BS of other cells > power receive from the serving BS, then hanoff initiate avantage: Much faster than AMPS hanoffs Peter A. Steenkiste & Dina Papagiannaki 47 Page 8