B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

11. 8 Modern Practical DSSS CDMA Systems 649
Figure 11 .20
(a) Receiver
location from
one satellite
measurement.
(b) Location
narrowed down
by two satellite
measurements.
(c) Location
narrowed down
by three satellite
measurements.
(d) Practical
global positi
ning system
using four
satellites.
(e) Block diagram
of a GPS
receiver.
The user is somewhere
on this sphere
(a)
Somewhere on
this circle
(b)
At one of these
two points
(c)
Network control
(cl)
Receiver
(user)
Satellite
position
data
Receiver
Range
cakulalions
Position
estimator
(e)
spread spectrum. Despreading the received signal in the receiver, in tum, yields a significant
processing gain, thus allowing good reception at reasonable power levels. The third reason for
spread spectrum is that each satellite can use the same frequency band, yet there is no mutual
interference owing to the near orthogonality of each user's signal.
Each satellite circles the earth in 12 hours and emits two PN sequences modulated in phase
quadrature at two frequencies. Two frequencies are needed to correct for the delay introduced
by the ionosphere.
11 .8.3 IEEE 802.1 1 b Standard for Wireless LAN
IEEE 802. 11 b is a commercial standard developed for wireless local area networks (WLAN)
to provide high-speed wireless connection to (typically) laptop computers.