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Digital Cellular Techniques

Digital Cellular Techniques. ENGR 475 – Telecommunications October 26, 2006 Harding University Jonathan White. Outline. Negatives of AMPS TDMA / GSM CDMA 3G wireless systems. Negatives of AMPS. There were only 832 potential channels in a cell.

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Digital Cellular Techniques

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  1. Digital Cellular Techniques ENGR 475 – Telecommunications October 26, 2006 Harding University Jonathan White

  2. Outline • Negatives of AMPS • TDMA / GSM • CDMA • 3G wireless systems

  3. Negatives of AMPS • There were only 832 potential channels in a cell. • Meaning 416 conversations can be served by 1 cell phone tower. • Even by the mid 1990’s this wasn’t enough bandwidth. • This hurt growth potential. • Cloning / listening in on conversations was a minor problem. • Analog voice is slow to process.

  4. Positives about AMPS • Very good quality.

  5. More Bandwidth • The FCC allocated more bandwidth to cellular provider in 1995. • 890 – 915 MHz, 935 – 960 MHz • 25 MHz total • 1850 – 1910 MHz, 1930 – 1990 MHz • 120 MHz total • Companies bid billions of dollars to be the provider on these new frequencies. • This was supposed to provide competition that lowered prices. • This didn’t happen.

  6. More Bandwidth • The extra bandwidth gave carriers the ability to support many new customers. • By the mid 1990’s, technology had advanced enough to use vocoders in the cell phones. • DAMPS – Digital AMPS • Used TDMA to fit 3 conversations on 1 analog AMPS channel. • Still used the lower 800 MHz band

  7. GSM • GSM: Global System for Mobile Communications • This is what you have if you have a SIM card. • Over 2 billion people in the world use this. • Operates in the 900 / 1900 MHz band. • Uses TDMA, vocoders, and encrypted passing of keys. • Fits 8 conversations on 1 channel. • Enhanced by GPRS in 1997 and EDGE in 1999. • Allows for a few thousand customers to be served simultaneously by a cell site.

  8. CDMA • CDMA: Carrier Division Multiple Access • The future of wireless. • All users share the same 1.25 – 5 MHz channel. • Each user is assigned a unique code, and they essentially encrypt their conversation with that code. • The base station can then decode it. • This is like several AM stations broadcasting on the same station at the same time.

  9. CDMA Codes • However, these codes are very special in that they can be separated by using prime factoring by the base station. • Up to a large point, you can support many (theoretically hundreds of thousands) of calls in 1 cell tower. • The only limit is noise and the computing power that’s available at the cell tower. • This is the future of wireless.

  10. CDMA Current Benefits • 1. 5 times the capacity of GSM • 2. Improved call quality as compared to GSM • 3. Enhanced privacy • 4. Less power demands on the mobile.

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