Cellular System Objectives Optimize Spectral efficiency Increase Affordability Increase Subscriber Capacity Provide Global Compatibility Provision Quality Service

Cellular networks: Why? Frequency reuse improves significantly the utilisation of the limited spectrum, and can offer high capacity to users. Use of low power transmitter can last the use of battery of mobile terminals, and reduce the danger of radiation to health and the weight of terminals.

Cellular coverage representation Ideal cell pattern  circular Fictious cell pattern  hexagonal Real cell pattern  footprint 2R R 2 2.6R R 2

Cellsite configurations Center excited cells Edge excited cells Omni-/Sectorized cells

Basic Components of a Cellular Telephone System  Cellular Mobile Phone:  A light-weight hand-held set which is an outcome of the marriage of Graham Bell’s Plain Old Telephone Technology [1876] and Marconi’s Radio Technology [1894] [although a very late delivery but very cute]  Control unit, transceiver unit (4W, 0.6 W, 1.6W)  Base Station:  A Low Power Transmitter, other Radio Equipment [Transceivers], interface equipment, power sources, antenna systems plus a small tower

Basic Components of a Cellular Telephone System  Mobile Switching Center [MSC] /Mobile Telephone Switching Office[MTSO]  An Interface between Base Stations and the PSTN  Controls all the Base Stations in the Region and Processes User ID and other Call Parameters  A typical MSC can handle up to 100,000 Mobiles, and 5000 Simultaneous Calls  Locates mobile subscribers, Handles Handoff Requests, Call Initiation Requests, and all Billing & System Maintenance Functions

Cellular Telephone System

HLRVLR AUC

Common Air Interface (CAI) Forward Channel Reverse Channel  n Specifies Four Channels [Voice Channels and Control / Setup Channels]  FVC: Forward Voice Channel  RVC: Reverse Voice Channel  FCC: Forward Control Channel  RCC: Reverse Control Channel

Call Setup Procedure  Cellular Phone Codes: Special Codes are associated with each Cell Phone to identify the phone, its owner, and service provider:  Electronic Serial Number(ESN) -A Unique 32-bit Code  Mobile Identification Number(MIN): A Subscriber’s Telephone Number  Station Class mark (SCM): Indicates the Max Tx Power for the User  When a Cellular Phone is turned on and Initiates a Call  Monitors the Control Channels and gets hold on to the strongest one  Makes a Call Initiation Request[Dialled Called party Number, MIN, ESN and SCM automatically transmitted]  Validation Procedure at MSC & Voice-Frequency pair Allocation  Base Station Pages the Information for the Mobile  MSC Connects the Mobile with the Called Party[Another mobile/Landline Phone]  Call is Established and Communication Starts

Call Setup – Mobile initiated  Call Setup Procedure (Cont’d)

Call Setup – Landline to Mobile  Call Setup Procedure (Cont’d) Receives call from PSTN. Sends MIN to all BSs Transmits page (MIN) for specified User Receives page and Matches the MIN With its own MIN Receives MIN, ESN, SCM and Passes to MSC Acknowledges receipt of MIN Sends ESN, SCM Verifies validity of MIN, ESN pair Requests BS to move Mobile to V F P Transmits info. For mobile to Move over to V F P Receives info. To tune to specified V F P Connects Mobile to Called party on PSTN

Cellular Mobile Telephony Cellular concept Bell Labs (1957 & 1960) Frequency reuse Typically every 7 cells Channel Assignment Handoff as caller moves Modified CO switch HLR, paging, handoffs Sectors improve reuse Every 3 cells possible

Frequency reuse concept A A B B C C E D D E F F G G A B C D E F G - T.S. Rappaport, “Wireless communications, principles & practice”, ISBN , Prentice-Hall, 1996, pp25-68.

Frequency Reuse Factor  Frequency Reuse Factor = N

Cluster formation (a group of N cells of radius ‘R’) Each cell of a cluster should have different RF channels Same cluster is repeated over remaining cells (FCA) Frequency reuse concept Distance between co-channel cells = Frequency reuse distance, D = R  (3N) Distance between adjacent cells d = R  (3)

Co-channel Reuse Ratio Q Area of larger hexagon = 2.6 D 2 Area of a cell = 2.6 R 2 No. of cells within the Region shown = N + 6. N/3 = 3 N = 2.6 D 2 / 2.6 R 2 Q = D/R = sqrt (3 N )

Cluster size (N)= i 2 +j 2 +i.j Reuse factor = 1/N There exists a tradeoff in selecting cluster size (N) A higher value is required to minimize the interference. A lower value ensures more channels over same network thus better radio capacity. Figure 3.2 Method of locating co-channel cells in a cellular system. In this example, N = 19 (i.e., i = 3, j = 2). (Adapted from [Oet83] © IEEE.) 19-cell reuse example (N=19)

An Example of Frequency Reuse  Suppose we have spectrum for 100 voice channels  Scenario 1: a high power base station covering entire area – system capacity = 100 channels  Scenario 2: divide spectrum into 4 groups of 25 channels each; cells (1, 7), (2, 4), (3, 5), 6 are assigned distinct channel groups – system capacity = 175 channels

 Suppose W = 25 MHz and B = 25 KHz/voice channel  W/B = 1000 voice channels can be supported over the spectrum  Scenario 1: a high power base station covering entire area (M = N = 1)  system capacity n = 1000 users  Scenario 2:  Coverage area divided into M = 20 cells with reuse factor N = 4  Each cluster accommodate 1000 active users  5 clusters in coverage area  system capacity n = 5000 users  Scenario 3:  M = 100 cells, N = 4  system capacity n = users  Scenario 4:  M = 100 cells, N = 1  system capacity n = users Frequency Reuse Example-2

Frequency Reuse - Summary each cell allocated a group k channels a cluster has N cells with unique and disjoint channel groups, N typically 4, 7, 12 total number of duplex channels S = kN Cluster repeated M times in a system Total number of channels that can be used (capacity) C = MkN = MS Smaller cells  higher M  higher C + Channel reuse  higher capacity + Lower power requirements for mobiles Additional base stations required More frequent handoffs Greater chance of ‘hot spots’

Effect of cluster size N channels unique in same cluster, repeated over clusters keep cell size same large N : weaker interference, but lower capacity small N: higher capacity, more interference need to maintain certain S/I level frequency reuse factor: 1/N each cell within a cluster assigned 1/N of the total available channels In most of the current networks, frequency reuse factor is 1.

Smaller N is greater capacity ? Q = D/R =  (3N)