CELLULAR COMMUNICATIONS Cellular Basics

Spectrum Reuse  Earlier systems: single central transmitter  Cover wide area  Single channel per user  25kHz for sufficient audio quality and guard interval  40 users in 1MHz, 400 users for 100MHz  Modern systems have millions of subscribers

Spectrum Reuse  Several transmitters, each having only certain coverage area  Cell==coverage area  Reuse same spectrum in many transmitters

Cells

 Often shown as hexagonal shapes  In reality, very irregular boundaries  Signal strength decreases gradually=>no exact cell edges  Some cell areas may overlap  Allocate different spectrum to adjacent cells  Can overlap without causing interference

Cells

Clusters  Cells with different spectrum grouped together as cluster  Often clusters of size 7

Cluster: set of different frequencies used in group of cells Cluster is repeated by linear shift isteps along one direction j steps in the other direction How many different frequencies does a cluster contain? Theoretical Network Planning Honeycomb (hexagonal) cell structure

Reuse Distance Distance between cell centers =× Cell Radius Reuse distance distance between the centers of two co-channel cells u 2 2 R = i + j +2ij 3 3 Rcos  where Ris Cell Radius R u is Reuse Distance andcos(  /3) = 1/2 3

Cluster Radius Radius of a cluster c u 2 2 R = R 3 = i + j +ij 3 R

Cluster Size C: number of channels needed for (i,j) grid is proportional to surface area of cluster Surface area of one hexagonal cell is R 2 S = 33 2 R Surface area of a (hexagonal) cluster of C cells is u R R 2 u S = C S = 33 2 R 3 { } Combining these two expressions gives u R = R3C

Possible Cluster Sizes We have seen u R = R3C and also u 2 2 R = i + j +ij 3 R Thus: C= i + j +ij 2 2 with integeriand j.

C = 1i= 1, j = 0} Cluster size for CDMA net C = 3i= 1, j = 1 C = 4i= 2, j = 0 C = 7i= 2, j = 1} Usual cluster sizes for TDMA C = 9i= 3, j = 0} cellular telephone nets C = 12i= 2, j = 2 · Cluster sizeC =i 2 +ij +j 2 = 1, 3, 4, 7, 9,... · Cellular Telephony Chose C to ensure acceptable link quality at cell boundary Typical Cluster Sizes

Reuse distance 2 – reuse pattern One frequency can be (re)used in all cells of the same color

Reuse distance 3 – reuse pattern

Design Objectives for Cluster Size High spectrum efficiency many users per cell Small cluster size gives much bandwidth per cell High performance Little interference Large cluster sizes

The effect of decreasing cell size Increased user capacity Increased number of handovers per call Increased complexity in locating the subscriber Lower power consumption in mobile terminal: · Longer talk time, · Safer operation Different propagation environment, shorter delay spreads Different cell layout, ·lower path loss exponent, more interference ·cells follow street pattern ·more difficult to predict and plan ·more flexible, self-organizing system needed (cf. DECT vs. GSM)

Cells  Macrocells  10km, sparsely populated area  Microcells  1km, densely populated area  Picocell  200m, particular buildings, streets

Reuse distance 3 – reuse pattern

Fixed and Dynamic assignment  Fixed frequency assignment: permanent  certain frequencies are assigned to a certain cell  problem: different traffic load in different cells  Dynamic frequency assignment: temporary  base station chooses frequencies depending on the frequencies already used in neighbor cells  more capacity in cells with more traffic  assignment can also be based on interference measurements

21 Increasing Capacity  Add new channels  Dynamic channel allocation – frequencies can be taken from adjacent cells by congested cells  Cell splitting – cells in areas of high usage can be split into smaller cells  Cell sectoring – cells are divided into a number of wedge-shaped sectors, each with their own set of channels (typical: 3)  Microcells – antennas move to buildings, hills, and lamp posts

Cell sectorization  Use directional antennas  Collocate cell antenna at the cell edges  Reduce cost

Handoff/Handover  Maintain call while moving

Basic Network Architecture

Basic Architecture  Base Station Controller (BSC)  Control each base station  Manage hand-off of a call from one base station to other  Mobile Switching Center(MSC)  Manages setup and tear down of calls to and from mobile subscribers  Home Location Register (HLR)  HLR subscriber database including location

Network  Base Transceiver Station (BTS)  Antenna Tower  Radio transceivers  Power Supply  Link to BSC (land lines or microwave)

Setting up calls/registration  Make a call originated from mobile handset  Allocate resources (channel)  Receive a call  Locate the current cell  After the telephone is switched on  Contact base station  Register to use a network

Registration  Authenticate (e.g. for billing)  Authentication Center (AuC)  Store my location  HLR for “home” subscribers  VLR for “visiting”/roaming subscribers  Mobile communicates with the network to update status/location  Network keeps last known location

Receiving a calls  Network should send a notification to a mobile  Network send to the area where mobile is located  Mobile listen to a “paging” channel  Examine each message on the paging channel and compares number with his own  Respond if match

Paging channel  Always listening to the paging channel drains the battery  Divide paging channel into 10 subgroups according to a last digit of mobile phone number  Mobile has to listen only 1/10 of time  Longer call setup time

Random Access Channel(RACH)  Respond to call /paging channel message  Initiate a call  “Access” message  Request a channel/slot/resources for further communications  Slotted ALOHA

Handover(EU)/Handoff(US)  Mobile monitor signal strength  Network knows about availability of channels  Mobile monitors strength of signal from current and adjacent cells and sends this information to network  When signal drops below certain level, network reserved new channel at adjacent cell  Mobile switch channel, network shuts down old channel

33 Handoff Region BS i Signal strength due to BS j E X1X1 Signal strength due to BS i BS j X3X3 X4X4 X2X2 X5X5 X th MS P min P i (x) P j (x) By looking at the variation of signal strength from either base station it is possible to decide on the optimum area where handoff can take place.

Types of Handoffs  Hard handoff  A hard handoff is a “break before make” connection.  MS is linked to no more than one BS at any given time.  Hard handoff is primarily used in FDMA and TDMA. Soft handoff It isn't a “ break before make ” transition. The call can be carried on both cells simultaneously. Soft handoff is used in CDMA.

Handoff Decisions  Decision-making process of handoff may be centralized or decentralized  Three different kinds of handoff decisions  Network-Controlled Handoff  Mobile-Assisted Handoff  Mobile-Controlled Handoff

Umbrella Cells

Operation Support Systems  Network Management Systems  Service Delivery  Service Fulfillment, including the Network Inventory, Activation and Provisioning  Service Assurance  Customer Care  Billing