Digital Basics Introduction
This article discusses digital wireless basics. It covers wireless history along with basic radio principles and terms. Digital building blocks like bits, frames, slots, and channels are explained along with details of entire operating systems. Building on my analog cellular article, digital cellular gets treated along with the newest service: personal communication services or PCS.
Where we are now?
Wireless has gone digital, enabling services that analog couldn't easily provide. Like better eavesdropping protection, increased call capacity, decreased fraud, e-mail delivery, and text messaging. But digital has its drawbacks, especially poor coverage and often bad audio quality.We'll compare newer digital systems like GSM and PCS1900 with systems like analog and early digital cellular. We'll better understand where wireless is today and where it's headed.
New and existing wireless services share much in common. They all provide coverage using a cellular like network of radio base stations and antennas. They all use mobile switches to manage that network, allowing calls, arranging handoffs between cells, and so on. They all use use one of two microwave frequency bands. Sometimes both. They all use digital to some extent. But aside from providing basic voice and data handling, the many services differ greatly in features and how they provided. Here's a quick, completely oversimplified list to get us going. More information follows:
AMPS: Advanced Mobile Phone service. Conventional cellular service. Mostly analog, with some digital signals providing call setup and management. A first generation service, now only installed in remote regions.
IS-95: All digital cellular using CDMA, a spread spectrum technique. Sprint PCS uses this technology. Sometimes called by its trade name of PCS 1900. A second generation or early digital service.
IS-136: D-AMPS 1900. Feature rich cellular. Mostly digital, although backward compatible with analog based AMPS. AT&T uses it for their nationwide cellular network. Uses time division multiple access or TDMA. Incorporates the old standard IS-54, an early second generation system at the time. IS-136 operates at either 800 Mhz or 1900 Mhz. AT&T is moving to a transitional technology whereby three standards, in some form, will work together: IS-136, GSM, and the newer General Packet Radio Service or GPRS. Eventually AT&T will stop using IS-136, replace it with GSM, and eventually replace that with a wideband CDMA system.
GSM. European cellular come to North America at 1900 Mhz. Fully digital with advanced features. Each mobile has intelligence within the phone, using a smart card. Uses TDMA. Among others, Pacific Bell uses GSM. Will migrate in a few years to a wideband CDMA technology.
iDEN: Proprietary cellular scheme devised by Motorola and used nationwide by NEXTEL. Combines a cell phone with a business radio. TDMA based.
We'll look soon at each service. For right now, though, to give us some orientation, let's go over recent mobile telephone history. It is quite a L-O-N-G history, so feel free to skip over that series and go on to the next topic, which is about standards.
Click here for this free chapter from Professor Noll's book described below, the selection is an excellent, simple introduction to cellular. (32 pages, 204K in .pdf)
More info on Introduction to Telephones and Telephone Systems (external link to Amazon) (Artech House) Professor A. Michael Noll