called Rake. The Rake processor uses the fine time-resolution capability of wideband signals to
resolve signals arriving over different propagation paths, and inserts them into a diversity
combiner to coherently construct a stronger received signal . These ideas lead to
developments in Code Division Multiple Access (CDMA) systems that would allow multiple
users to gain access to the channel.
Up until the 1970's spread spectrum systems where mainly military developed and
controlled. It was primarily used for satellite communications, and was developed by both the
Western and Russian military forces. The first major spin off for commercial applications was
the Global Positioning System (GPS) which uses CDMA based satellite technology. Before
CDMA was used in mobile phones in the late 1980‟s to early 1990‟s, traditional mobile phones
used FM and tone signaling with newly perfected microprocessor to enable automated calling by
a few select users in a given city. The Improved Mobile Telephone Service (IMTS) allowed 10
to 25 channels for a given region, with mobile transmit powers running nearly 100 Watts ERP.
This paved the way for a system developed at Bell Labs called Advanced Mobile Phone System
(AMPS), which shifted the channel control and call processing to land-side process and divided
coverage area into smaller cells, to increase coverage area .
In 1989, the first experiments using cellular CDMA at 800 MHz were conducted by
Qualcom Inc. CDMA system could provide high quality and a capacity greater than ten times
the capacity of the existing AMPS cellular system. This lead to the development of the 2G
standard called IS-95 wideband for 800 MHz cellular radio systems, which rested fully on the
spread spectrum CDMA platform. This 2G standard was widely accepted throughout the 1990‟s