How Wireless Works… Matthew C. Valenti Lane Department of CSEE.
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How Wireless Works…
Matthew C. Valenti
Lane Department of CSEE
2/14/2007
And Why Sometimes it Doesn’t…
The Wireless Revolution
0.1
10
1000
1983
1986
1989
1992
1995
1998
2001
2004
100
Mil
lion
s of
Sub
scri
bers
1 Wireles
s subscr
ibers (c
ellular
and PCS)
Wireline access lines
1st generationAnalog (FM)
2nd
generation
3rd
generation
wireless growth in US alone Source: www.ctia.org
231.8 million Feb. 07
2007
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: 824-894 MHz PCS: 1.85-2.0 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 #1 Ch #2 Ch #3 Ch #1
The Cellular Concept
Lower power transmitters provide coverage to a small portion of the service area. Frequency is reused
Set #1 Set #2 Set #3
Ch #1
Set #3
Set #4
Set #2
Set #1
Cell Patterns
Idealized Cells Idealized 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 #1Cluster #2
Cluster #3
AB
CD
E
FG
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 #1Ch #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 ofWireless
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.
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