How Wireless Works… Matthew C. Valenti Lane Department of CSEE

2/14/2007 And Why Sometimes it Doesn’t…

The Wireless Revolution Millions of Subscribers 1 Wireless subscribers (cellular and PCS) Wireline access lines 1 st generation Analog (FM) 2 nd generation 3 rd generation wireless growth in US alone Source: million Feb

2/14/2007 Wireless Is BIG Business In 2006, Americans used 857 billion minutes of talktime.  1.6 million years total.  Average of 10 minutes/day/customer.  42% of Americans use wireless as their primary phone. U.S. Revenues and Industry  Average bill of $49.30/month/customer.  Over $135 billion/year industry.  2.5% of US workforce in wireless industry  In the next 5 years, wireless will be a bigger US industry than automotive or agriculture. Worldwide Usage  2.3 billion wireless subscribers worldwide  986 million handsets sold in 2006.

Worldwide Wireless

2/14/2007 Cellular: MHz PCS: GHz

2/14/2007 How Crowded is the Spectrum? Wireless Spectrum  231 million subscribers in US  220 MHz available for Wireless (110 each direction)  So just give each subscriber about 1 Hz of dedicated bandwidth (0.5 Hz each direction), right? Problem: cellular signal occupies:  200 kHz (GSM)  550 distinct channel pairs.  Time division multiple access divides channels into 8 subchannels.  So 4400 “conversations”  1.25 MHz (CDMA)  88 distinct channel pairs.  Code division multiple access divides each channel into 64 subchannels  So 5632 “conversations”

2/14/2007 Something’s Got to Give…

The Cellular Concept Transmit power drops off with distance. When you are far-enough away you can re-use the channel. Similar concept to frequency re-use for radio and television stations. Low power transmitter, Frequency is re-used Ch #1Ch #2Ch #3Ch #1

The Cellular Concept Lower power transmitters provide coverage to a small portion of the service area. Frequency is reused Set #1Set #2Set #3 Ch #1 Set #3 Set #4 Set #2 Set #1

Cell Patterns Idealized CellsIdealized Coverage Reality! Footprint

The Cellular Concept Break the metropolitan area into small areas Each area is approximated with a hexagonal cell. A base station is located at the center of each cell. Each cell is assigned only a fraction of the total number of channels. Cells that are sufficiently far apart can reuse the same frequency. In the US, there are currently 200,000 base stations (cells). Cluster #1 Cluster #2 Cluster #3 A B C D E F G F F E E D D C C B B A A G G

2/14/2007 Sectorized Antennas Further interference reduction by using sectorized antennas.

Hand Off Ch #1 Ch #2 Mobile must be transferred between cells as it moves -Hard handoff -Soft handoff (CDMA) -Softer handoff (sectorized antennas) Possibility for a dropped call.

2/14/2007 The Challenges of Wireless Fading  Due to relative motion between TX and RX. Multipath  Due to signal reflections. Diffraction  Signal bending around objects (mountain, buildings) Shadowing  Obstructions that attenuate signal (foliage) Interference  Other signals  Main limitation in built-up areas. Noise  Thermal excitement of electrons in receiver.  Background noise in space.

2/14/2007 Facing the Challenges Source Coding  Companding: Reduces BW needed by voice. Channel Coding  Forward Error Correction Coding.  By adding parity bits to transmitted data, errors can be corrected. Spread Spectrum Communication Use of Multiple Antennas Advanced Receiver Processing  Equalizer: Undoes multipath This is the type of stuff that EE’s working in the communications industry work on!

2/14/2007 To Learn More (A Lot More)… Wireless Networking  CPE 462  11:00 AM – 12:15 PM, Tues-Thur.  Woerner  EE 327 and STAT 215 prereq’s. Wireless Communication Systems  EE 562  2:00 - 3:15 PM, Tues-Thur.  Valenti  EE 461 and/or EE 513 prereq’s.