X hits on this document

PDF document

DigitalCommons@University of Nebraska - Lincoln - page 12 / 69





12 / 69


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 [3]. 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 [5].

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[6]. 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

Document info
Document views210
Page views210
Page last viewedTue Jan 17 01:39:22 UTC 2017