Presentation on theme: "How Wireless Works… Matthew C. Valenti Lane Department of CSEE."— Presentation transcript:
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.
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
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.