How Your Cellphone Works… Matthew C. Valenti Lane Department of CSEE

And Why Sometimes it Doesn’t… 9/17/2018

How many cellular subscribers are there worldwide? In the US?

The Wireless Revolution 1000 Wireless subscribers (cellular and PCS) 176.4 million Mar. 05 Wireline access lines 2nd generation 100 Millions of Subscribers 1st generation 10 3rd generation 1 0.1 1983 1986 1989 1992 1995 1998 2001 2004 wireless growth in US alone Source: www.ctia.org

Largest Carriers in US Cingular, 49.1 million (GSM) Verizon, 43.8 million (CDMA) Sprint, 24.8 million (CDMA) T-mobile, 17.3 million (GSM) Nextel, 16.2 million (iDen) In the US, the wireless industry brings in $100B/yr. 9/17/2018

Wireless Worldwide Over 1 billion subscibers. Penetration Rates for Select Countries Taiwan: 101% U.K: 100% Italy: 96% Netherlands: 94% Germany: 84% France: 76% Japan: 65% United States: 59% Russia: 50% India: 9% China: 38% Growing Fast 9/17/2018

Cellular: 824-894 MHz PCS: 1.85-2.0 GHz 9/17/2018

How Crowded is the spectrum 176.4 million subscribers 220 MHz available for Wireless So just give each subscriber 0.8 Hz of dedicated bandwidth (0.4 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” 9/17/2018

Something’s Got to Give… 9/17/2018

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. Ch #1 Ch #2 Ch #3 Ch #1 Low power transmitter, Frequency is re-used

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

175,368 cell towers in the US 9/17/2018

Cell Patterns Idealized Cells Idealized Coverage Footprint Reality!

The Cellular Concept A F B A G F B E C G D E C A D F B G E C D Cluster #1 Cluster #2 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. A F B A G F B E C G D E C A D F B G E C D Cluster #3

Frequency Reuse 2 2 1 5 1 5 4 4 3 7 3 7 6 6 2 1 5 4 3 7 N = 7 6

Cell Pattern N = 4 2 1 3 2 4 2 3 1 3 1 4 2 4 1 3 4

Sectorized Antennas Further interference reduction by using sectorized antennas. 9/17/2018

Hand Off Mobile must be transferred between cells as it moves Ch #2 Ch #1 Mobile must be transferred between cells as it moves Hard handoff Soft handoff (CDMA) Softer handoff (sectorized antennas)

Wireless Propagation Fading Multipath Diffraction Shadowing 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 Noise Thermal excitement of electrons in receiver. Background noise in space. 9/17/2018

But all is not lost… Source Coding Channel Coding Companding: Reduces BW needed by voice. Channel Coding Forward Error Correction Coding. By adding parity bits to transmitted data, errors can be corrected. Advanced Receiver Processing Equalizer: Undoes multipath This is the type of stuff that EE’s working in the communications industry work on! 9/17/2018

To Learn More (A Lot More)… Wireless Networking CPE 493g 9-10 AM, M-W-F ESB 801 Valenti EE 327 and STAT 215 prereq’s. Wireless Communication Systems EE 562 Reynolds 12 PM – 1 PM M-W-F ESB 201 EE 461 and/or EE 513 prereq’s. 9/17/2018