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

11.8 Modern Practical DSSS CDMA Systems 647
Similar to CDMA2000, 3GPP WCDMA also has a counterpart to EV-DO known as high-speed
packet access (HSPA). On downlink, the recent HSPA release (Release 6) achieves the peak
rate of 14.4 Mbit/s. However, existing deployments can support a peak rate of only 7.2 Mbit/s.
Still, at this high rate, most data users would be quite satisfied, with the exception perhaps of
high-definition TV viewers.
Power Control vs. MUD
It is interesting to note that despite intense academic research interest in multiuser CDMA
receivers (in the 1980s and 1990s), all cellular CDMA systems described here rely on power
control to combat the near-far problem. The reason lies in the fact that power control is quite
simple to implement and has proven to be very effective. On the other hand, MUD receivers
require more computational complexity. To be effective, MUD receivers also require too much
channel and signal information about all active users. Moreover, MUD receivers alone cannot
completely overcome the disparity of performance in a near-far environment.
11
.8.2 CDMA in the Global Positioning System (GPS)
What ls GPS?
The Global Positioning System (GPS) is the only fully functional global satellite navigation
system. Utilizing a constellation of at least 24 satellites in medium Earth orbit to transmit
precise RF signals, the system enables a GPS receiver to determine its location, speed. and
direction.
A GPS receiver calculates its position based on its distances to three or more GPS satellites.
Measuring the time delay between transmission and reception of each GPS microwave signal
gives the distance to each satellite, since the signal travels at a known speed. The signals also
carry information about the satellites' location. By determining the position of, and distance to,
at least three satellites, the receiver can compute its position using triangularization. Receivers
typically do not have perfectly accurate clocks and therefore track one or more additional
satellites to correct the receiver's clock error.
Each GPS satellite continuously broadcasts its (navigation) message via BPSK at the rate
of 50 bit/s. This message is transmitted by means of two CDMA spreading codes: one for the
coarse/acquisition (C/A) mode and one for the precise (P) mode (encrypted for military use).
The Cl A spreading code is a PN sequence with period of 1023 chips sent at 1.023 Mchip/s.
The spreading gain is L = 20, 460. Most commercial users access only the Cl A mode.*
Originally developed for the military, GPS is now finding many uses in civilian life such as
marine, aviation, and automotive navigation, as well as surveying and geological studies. GPS
allows a person to determine the time and the person's precise location (latitude, longitude, and
altitude) anywhere on earth with an accuracy of inches. The person can also find the velocity
with which he or she is moving. GPS receivers have become small and inexpensive enough to
be carried by just about everyone in cars and boats. Handheld GPS receivers are plentiful and
have even been incorporated into popular cellular phone units.
How Does GPS Work?
A GPS receiver operates by measuring its distance from a group of satellites in space, which
are acting as precise reference points. Since the GPS system consists of 24 satellites, there will
always be more than four orbiting bodies visible from anywhere on Earth. The 24 satellites
* The P spreading code rate is 1 0.23 Mchip/s with a spreading gain of L = 204,600. The P code period is
6.1 871 x 1 0 12 bits long. In fact, at the chip rate of 1 0.23 Mchip/s, the code period is one week long!