+ Cellular Networks CPSC441, Winter 2010

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

+ 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.

+ The Core Idea: Cellular Concept The core idea that led to todays 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.

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

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

+ Circular Coverage Areas Antennas are omni-directional (usually) Circles dont 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

+ 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

+ 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!

+ A Handoff At some point, users signal is weak enough at B 1 strong enough at B 2 Messages between users and base stations required to coordinate handoff B1B1 B2B2

+ 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.

+ 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!

+ Example of Frequency Reuse Cells using the same frequencies

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

+ 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. Frequency Subbands Cell Phone User 1 Cell Phone User 2 : Cell Phone User N Time

+ 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. … Frequency Bands Time User 1 User 2 User 3 User N

+ Hybrid FDMA/TDMA The TDMA used by real cellular systems (like AT&Ts) 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.

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

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

+ References LUCID Summer Workshop, July 27, Song Zhang, CPSC601 Fall 2008, project presentation