From Watson to W-CDMA

Communications News, May, 1998 by Cory Peichel

(Almsot) everything you wanted to know about the evolution of wireless technologies.

"Mr. Watson, come here, I need you" were the first words uttered by Alexander Graham Bell to his colleague Thomas Watson through their "electric speaking telegraph" invention. This device, later known as a telephone, became an instant success and the cornerstone of multi-billion dollar industry.

In 1970 the Federal Communications Commission (FCC) allocated spectrum in the 800 MHz band for cellular communications, and advanced mobile phone service (AMPS) was born. AMPS, a high-capacity analog standard introduced by AT&T, was the first widely deployed cellular technology. The first systems using this technology began operations in 1983 and are still used today.

Each AMPS provider is given half of the band of spectrum from 824849 MHz and half of the band from 869-894 MHz. These bands are broken up into 30-kHz sub-bands, called channels, with each provider receiving half of all available channels. All of the channels in the 824-849 MHz band are called reverse channels because they carry transmissions from the mobile cellular phone to the cell site. The channels in the 869-894 MHz range are called the forward channels because they carry transmissions from the cell sites to the cell phones.

The practice of dividing the larger bands into sub-bands is referred to as frequency division multiple access (FDMA) and is widely used in cellular communications.

AMPS does not try to cover one large area with one transmitter. Instead, an area is divided into smaller sections called cells. Each of these cells has its own low power transmitter, which uses a subset of the available channel pairs to communicate to mobile phones within the cell boundaries. Adjacent cells use a different subset of channels so that they do not interfere with the first set.

When the mobile unit begins to leave a cell, the network controller commands the first transmitter to hand off the call to another cell and simultaneously commands the transmitter in the second cell to use a new channel pair to take over the call.

A second generation of products began to emerge in the mid-1980s when cellular providers foresaw a need for additional capacity. These products are digital and provide flexible, secure systems. The analog human voice can be captured in an electromagnetic signal, sampled, and quantized into a digital binary stream.

Once digitized, the voice can be modeled and encoded using mathematical algorithms such as the vector sum excited linear predictive (VSELP) codec or enhanced full rate codec (EFRC). These algorithms effectively compress the amount of digital data needed in a voice transmission and open the door for more efficient spectrum utilization.

Interim Standard 54 (IS-54) by the Electronics Industries Association/Telecommunication Industries Association (EIA/TIA) laid the groundwork for North American time division multiple access (TDMA) digital cellular communications.

TDMA is an air interface that builds upon the foundation of AMPS. It, too, uses FDMA to divide the spectrum into 30 KHz reverse and forward channels, but it further divides each channel into three independent time slots. E-TDMA is a standard put forth to use half-rate voice coding and expands the number of time slots to six.

Each of these time slots is capable of carrying a voice transmission either from a mobile to a cell or from a cell to a mobile. By dividing each channel in time, the cellular providers are able to provide three times as many calls using the same bandwidth.

Besides increasing capacity, providers were able to reap the benefits of a wide array of revenue generating features like caller ID and short message service (SMS) with the implementation of TDMA cellular.

Since TDMA is a linear derivative of earlier architectures, it remains backward compatible with AMPS. This keeps the cost of implementing the technology relatively low because providers can migrate to TDMA using most of the same equipment that they use to service their AMPS customers.

Qualcomm Corp. holds most of the patents on a totally different method of digital communication for the commercial marketplace. The company proposed most of what later became Interim Standard 95 (IS-95) which defines a cellular system that uses direct sequence code division multiple access (CDMA-DS).

Direct sequence is an adaptation of the original technology developed for the U.S. military which used frequency hopping CDMA to provide very secure communications. It works by evenly spreading multiple conversations across a designated segment of spectrum.

CDMA-DS is a method whereby a range of spectrum such as that allocated for AMPS/TDMA is divided into 1.23 MHz sub-bands with 20 KHz gaps. These sub-bands are shared by multiple users, completely spreading their signals across the entire 1.23 MHz band. Users accomplish this by combining their voice data with a fast sequence of data called chips. The rate at which these combined data are transmitted is called the chip rate.