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Cellular Communications

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.

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Cellular Communications

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  1. Cellular Communications Cellular Basics

  2. 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

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

  4. Cells

  5. 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

  6. Cells

  7. 3 3 2 = S R R 2 Cell Footprint

  8. Clusters • Cells with different spectrum grouped together as cluster • Often clusters of size 7

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

  10. Co-ordinates for hexagonal cellular geometry • With these co-ordinates, an array of cells can be laid out so that the center of every cell falls on a point specified by a pair of integer co-ordinates.

  11. Cosine Rule

  12. 3 Reuse Distance Distance between cell centers = × Cell Radius Reuse distance distance between the centers of two co-channel cells p 2 2 = + + 2ij cos 3 R j R i u 3 where R is Cell Radius R is Reuse Distance u p and cos( /3) = 1/2

  13. R u = R c 3 Cluster Radius Radius of a cluster

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

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

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

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

  18. Reuse distance 3 – reuse pattern

  19. 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

  20. 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

  21. Cells • Macrocells • 10km, sparsely populated area • Microcells • 1km, densely populated area • Picocell • 200m, particular buildings, streets

  22. Umbrella Cells

  23. 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

  24. 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

  25. Cell sectorization • Use directional antennas • Collocate cell antenna at the cell edges • Reduce cost

  26. Handoff/Handover • Maintain call while moving

  27. Basic Network Architecture

  28. 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

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

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

  31. 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

  32. 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

  33. 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

  34. 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

  35. 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

  36. Handoff Region Signal strength due to BSi Signal strength due to BSj Pj(x) Pi(x) E Pmin BSi MS BSj X1 X3 X5 Xth X4 X2 • 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.

  37. 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.

  38. 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

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

  40. GSM

  41. GroupeSpeciale Mobile/Global System for Mobile

  42. GSM Air Interface • TDMA with FDD • 200Khz channels with 200KHz guard bands • GSM 900 has 124 carriers • GMSK modulation, 270kbps per carrier • Up to 8 users, 24.8kbps per user • FEC reduces to 13kbps per user for voice

  43. Physical Channel • RF carrier divided into 8 slots, numbered 0..7 • Timeslots carrying data • At most 8 traffic channels • Control messages • At least 1 control channels • More control (logical) channels • Packed into RF carrier

  44. Single Burst/Slot

  45. Frame Structure

  46. Logical Channel List TCH/F: Full-rate Traffic Channel Traffic channels (TCH) Two-way TCH/H: Half-rate Traffic Channel FCCH: Frequency correction Base-to- mobile SCH: Synchronization BCH BCCH: Broadcast control PCH: Paging Signaling channel CCCH AGCH: Access grant RACH: Random access SDCCH: Stand-alone dedicated control DCCH SACCH: Slow associated control Two-way FACCH: Fast associated control

  47. Broadcast Control Channels

  48. Common Control Channels

  49. Dedicated Control Channels

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