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Cellular Networks

Cellular Networks. CPSC441, Winter 2010. Entire Coverage Area. First Mobile Telephone System. One and only one high power base station with which all users communicate. Normal Telephone System. Wired connection. Problem with Original Design.

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Cellular Networks

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  1. Cellular Networks CPSC441, Winter 2010

  2. Entire Coverage Area First Mobile Telephone System One and only one high power base station with which all users communicate. Normal Telephone System Wired connection

  3. Problem with Original Design • Original mobile telephone system could only support a handful of users at a time…over an entire city! • With only one high power base station, users phones also needed to be able to transmit at high powers • Car phones were therefore much more feasible than handheld phones.

  4. The Core Idea: Cellular Concept • The core idea that led to today’s system was the cellular concept. • The cellular concept: multiple lower-power base stations that service mobile users within their coverage area and handoff users to neighboring base stations as users move. Together base stations tessellate the system coverage area.

  5. Main Principles • Small cells tessellate overall coverage area. • Users handoff as they move from one cell to another. • Frequency reuse.

  6. Tessellation • Three regular polygons that always tessellate: • Equilateral triangle • Square • Regular Hexagon Triangles Squares Hexagons

  7. Circular Coverage Areas • Antennas are omni-directional (usually) • Circles don’t tesselate! • Closest shape - hexagon Users located outside some distance to the base station receive weak signals. Result: base station has circular coverage area. Weak signal Strong signal

  8. Thus the Name Cellular • With hexagonal coverage area, a cellular network is drawn as: • The network resembles cells from a honeycomb • thus the name cellular! Base Station

  9. Handoffs • Mobile users are “mobile” by definition! • Continuous access to network required • Not a problem within a cell • Moving between cells requires a handoff mechanism!

  10. A Handoff • At some point, user’s signal is • weak enough at B1 • strong enough at B2 • Messages between users and base stations required to coordinate handoff B1 B2

  11. Cellular Networks Components • PSTN – Public Switched Telephone Network • MSC – Mobile Switching Centre • Connects PSTN to BSS • BSS – Base Station System • Interface between cellular network and mobile user • MS – Mobile Stations • Cellphones, Carphones, etc.

  12. Frequency Reuse • Total spectrum allocated to the service provider is broken up into smaller bands • Adjacent cells assigned different frequencies to avoid interference or crosstalk • Spectrum is limited – need to reuse frequency • Optimal assignment is equivalent to graph colouring problem – NP-Hard!

  13. Example of Frequency Reuse Cells using the same frequencies

  14. Multiple Access Methods • Three widely-used policies: • Frequency Division Multiple Access (FDMA) • Time Division Multiple Access (TDMA) • Code Division Multiple Access (CDMA)

  15. FDMA • In FDMA, the band of frequency is broken up into smaller bands, i.e., subbands. • Each transmitter (user) transmits to the base station using radio waves in its own subband. Cell Phone User 1 Cell Phone User 2 : : Cell Phone User N Frequency Subbands Time

  16. TDMA • In pure TDMA, base station does not split up its allotted frequency band into smaller frequency subbands. • Rather it communicates with the users one-at-a-time, i.e., “round robin” access. … User 2 User 1 User 3 Frequency Bands User N Time

  17. Hybrid FDMA/TDMA • The TDMA used by real cellular systems (like AT&T’s) is actually a combination of FDMA/TDMA. • Base station breaks up its total frequency band into smaller subbands. • Base station also divides time into slots and frames. • Each user is now assigned a frequency and a time slot in the frame.

  18. … User 31 User 32 User 40 … User 21 User 22 User 30 … User 11 User 12 User 20 … User 1 User 2 User 10 Hybrid FDMA/TDMA (Cont’d) Assume a base station divides its frequency band into 4 subbands and time into 10 slots per frame. … … User 31 User 32 User 40 Frequency Subband 4 … … Frequency Subband 3 User 21 User 22 User 30 … … Frequency Subband 2 User 11 User 12 User 20 … … User 1 User 2 User 10 Frequency Subband 1 Frame Time

  19. CDMA • CDMA is a more complicated scheme. • Users communicate with BSS at the same time and using the same set of frequencies. • A desired user’s signal is deciphered using a unique code assigned to the user.

  20. References • LUCID Summer Workshop, July 27, 2004 http://www.ece.lehigh.edu/~skishore/research/lucid/lucid_2.ppt • Song Zhang, CPSC601 Fall 2008, project presentation

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