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. Meaning 416 conversations can be served by 1 cell phone tower. Meaning 416 conversations can be served by 1 cell phone tower. Even by the mid 1990’s this wasn’t enough bandwidth. Even by the mid 1990’s this wasn’t enough bandwidth. This hurt growth potential.This hurt growth potential.  Cloning / listening in on conversations was a minor problem.  Analog voice is slow to process.

Positives about AMPS  Very good quality.

More Bandwidth  The FCC allocated more bandwidth to cellular provider in – 915 MHz, 935 – 960 MHz 890 – 915 MHz, 935 – 960 MHz 25 MHz total25 MHz total 1850 – 1910 MHz, 1930 – 1990 MHz 1850 – 1910 MHz, 1930 – 1990 MHz 120 MHz total120 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 was supposed to provide competition that lowered prices. This didn’t happen.This didn’t happen.

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. Used TDMA to fit 3 conversations on 1 analog AMPS channel. Still used the lower 800 MHz band Still used the lower 800 MHz band

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  Allows for a few thousand customers to be served simultaneously by a cell site.

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. The base station can then decode it. This is like several AM stations broadcasting on the same station at the same time. This is like several AM stations broadcasting on the same station at the same time.

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. 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.The only limit is noise and the computing power that’s available at the cell tower.  This is the future of wireless.

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.