Code Division Multiple
Access (CDMA)
is a radically new concept in wireless communications. It has gained
widespread international acceptance by cellular radio system operators
as an upgrade that will dramatically increase both their system
capacity and the service quality. It has likewise been chosen for
deployment by the majority of the winners of the United States Personal
Communications System spectrum auctions. It may seem, however,
mysterious for those who aren't familiar with it. This site is provided
in an effort to dispel some of the mystery and to disseminate at least
a basic level of knowledge about the technology.
CDMA
is a form of spread-spectrum , a family of digital communication
techniques that have been used in military applications for many years.
The core principle of spread spectrum is the use of noise-like carrier
waves, and, as the name implies, bandwidths much wider than that
required for simple point-to-point communication at the same data rate.
Originally there were two motivations: either to resist enemy efforts
to jam the communications (anti-jam, or AJ), or to hide the fact that
communication was even taking place, sometimes called low probability
of intercept (LPI). It has a history that goes back to the early days
of World War II.
The use of CDMA for
civilian mobile radio applications is novel. It was proposed
theoretically in the late 1940's, but the practical application in the
civilian marketplace did not take place until 40 years later.
Commercial applications became possible because of two evolutionary
developments. One was the availability of very low cost, high density
digital integrated circuits, which reduce the size, weight, and cost of
the subscriber stations to an acceptably low level. The other was the
realization that optimal multiple access communication requires that
all user stations regulate their transmitter powers to the lowest that
will achieve adequate signal quality.
CDMA changes the nature of the subscriber station from a predominately
analog device to a predominately digital device. Old-fashioned radio
receivers separate stations or channels by filtering in the frequency
domain. CDMA receivers do not eliminate analog processing entirely, but
they separate communication channels by means of a pseudo-random
modulation that is applied and removed in the digital domain, not on
the basis of frequency. Multiple users occupy the same frequency band.
This universal frequency reuse is not fortuitous. On the contrary, it
is crucial to the very high spectral efficiency that is the hallmark of
CDMA. Other discussions in these pages show why this is true.
CDMA is altering the face
of cellular and PCS communication by:
-
Dramatically improving the
telephone traffic capacity
-
Dramatically improving the voice
quality and eliminating the audible effects of multipath fading
-
Reducing the incidence of dropped
calls due to handoff failures
-
Providing reliable transport
mechanism for data communications, such as facsimile and internet
traffic
-
Reducing the number of sites needed
to support any given amount of traffic
-
Simplifying site selection
-
Reducing deployment and operating
costs because fewer cell sites are needed
-
Reducing average transmitted power
-
Reducing interference to other
electronic devices
-
Reducing potential health risks
Commercially introduced in 1995, CDMA
quickly became one of the world's
fastest-growing wireless technologies. In 1999, the International
Telecommunications Union selected CDMA as the industry standard for new
"third-generation" (3G) wireless systems. Many leading wireless
carriers are now building or upgrading to 3G CDMA networks in order to
provide more capacity for voice traffic, along with high-speed data
capabilities.
CDMA is a form of Direct Sequence
Spread Spectrum communications. In general, Spread Spectrum
communications is distinguished by three key elements:
1. The signal occupies a bandwidth much
greater than that which is
necessary to send the information. This results in many benefits, such
as immunity to interference and jamming and multi-user access, which
we'll discuss later on.
2. The
bandwidth is spread by means of a code which is independent of the
data. The independence of the code distinguishes this from standard
modulation schemes in which the data modulation will always spread the
spectrum somewhat.
3. The
receiver synchronizes to the code to recover the data. The use of an
independent code and synchronous reception allows multiple users to
access the same frequency band at the same time.
In order to protect the signal, the
code used is pseudo-random. It
appears random, but is actually deterministic, so that the receiver can
reconstruct the code for synchronous detection. This pseudo-random code
is also called pseudo-noise (PN).
There are three ways to
spread the bandwidth of the signal:
-
Frequency hopping. The signal is
rapidly switched between different
frequencies within the hopping bandwidth pseudo-randomly, and the
receiver knows before hand where to find the signal at any given time.
-
Time hopping. The signal is
transmitted in short bursts
pseudo-randomly, and the receiver knows beforehand when to expect the
burst.
-
Direct sequence. The digital data
is directly coded at a much higher
frequency. The code is generated pseudo-randomly, the receiver knows
how to generate the same code, and correlates the received signal with
that code to extract the data.
How spread spectrum
works:
Spread
Spectrum uses wide band, noise-like signals. Because Spread Spectrum
signals are noise-like, they are hard to detect. Spread Spectrum
signals are also hard to Intercept or demodulate. Further, Spread
Spectrum signals are harder to jam (interfere with) than narrowband
signals. These Low Probability of Intercept (LPI) and anti-jam (AJ)
features are why the military has used Spread Spectrum for so many
years. Spread signals are intentionally made to be much wider band than
the information they are carrying to make them more noise-like.
Spread
Spectrum signals use fast codes that run many times the information
bandwidth or data rate. These special "Spreading" codes are called
"Pseudo Random" or "Pseudo Noise" codes. They are called "Pseudo"
because they are not real gaussian noise.
Spread
Spectrum transmitters use similar transmit power levels to narrow band
transmitters. Because Spread Spectrum signals are so wide, they
transmit at a much lower spectral power density, measured in Watts per
Hertz, than narrowband transmitters. This lower transmitted power
density characteristic gives spread signals a big plus. Spread and
narrow band signals can occupy the same band, with little or no
interference. This capability is the main reason for all the interest
in Spread Spectrum today.
CDMA / Spread Spectrum
Resources:
Introduction to CDMA
ABCs
of Spread Sprectum
CDMA Technology
CDMA Basics
Overview of CDMA
CDMA
vs TDMA
Comparision between
GSM IS-95
Book:
Spread Spectrum: Hedy Lamarr and the mobile phone
CDMA Discussion Forum
Comments
can anyone help me to find the latlong of a cellphone using C
where x1,x2,x3 is latitude of 3 BTS and y1,y2,y3 is longitude of 3 BTS.
rt1,rt2&rt3 is round trip delay which i am getting from system as 40,31,52
the 3 latlong are:
site a:11.0209(lat),76.9796(long);site b:11.0149(lat,76.9857(long);site c:11.0246(lat,77.0066(lon)
These above are 3 latlong of Bts.I am using Triangulation method to calculate the latlong of cellphone.
CAN ANYONE PLS HELP ME OUT???I am not getting errors in the pgm but my final latlong is wrong.
The x,y values i.e our latitude and longitude of the cellphone in the pgm i have got by solving wrt 3 BTS
#include
#include
#define pi 3.14159265358979323846
double deg2rad(double deg) ;
double rad2deg(double rad) ;
double distance(double lat1, double lon1, double lat2, double lon2, char unit);
main()
{
float r1,r2,r3;
float x1,x2,x3,x,y1,y2,y3,y;
float d1,d2,d3;
float rt1,rt2,rt3;
float a,b,c;
fflush(stdin);
printf("enter the latitude of BTS1 x1:");
scanf("%f",&x1);
printf("enter the latitude of BTS2 x2:");
scanf("%f",&x2);
printf("enter the latitude of BTS3 x3:");
scanf("%f",&x3);
printf("enter the longitude of BTS1 y1:");
scanf("%f",&y1);
printf("enter the longitude of BTS1 y2:");
scanf("%f",&y2);
printf("enter the longitude of BTS1 y3:");
scanf("%f",&y3);
d1=distance(x1,y1,x3,y3,'K');
d2=distance(x1,y1,x2,y2,'K');
d3=distance(x2,y2,x3,y3,'K');
printf("\nthe distance d1 is:%f",d1);
printf("\nthe distance d2 is:%f",d2);
printf("\nthe distance d3 is:%f",d3);
printf("\nenter the round trip delay1:");
scanf("%f",&rt1);
printf("\nenter the round trip delay2:");
scanf("%f",&rt2);
printf("\nenter the round trip delay3:");
scanf("%f",&rt3);
r1=(rt1)*(0.01525);
r2=(rt2)*(0.01525);
r3=(rt3)*(0.01525);
printf("\n the length1 is:%f",r1);
printf("\n the length2 is:%f",r2);
printf("\n the length3 is:%f",r3);
a=2*((y3-y1)*((r1*r1)-(r2*r2)));
b=(((x1*x1))+((y1*y1))-((x2*x2))-((y2*y2)))*(2*(y3-y1));
c=2*(((r1*r1)-(r3*r3))-(x1*x1)-(y1*y1)+(x3*x3)+(y3*y3))*((y2-y1));
x=(a-b-c)/((2*(x2-x1)*(y3-y1))-(4*(x3-x1)*(y2-y1)));
y=(((r1*r1)-(r3*r3))-(2*(x)*(x3-x1))-(x1*x1)-(y1*y1)+(x3*x3)+(y3*y3))/(2*(y3-y1));
printf("\nthe latitude of cellphone is %f\n",x);
printf("\nthe longitude of cellphone is %f\n",y);
}
double distance(double lat1, double lon1, double lat2, double lon2, char unit)
{
double theta, dist;
theta = lon1 - lon2;
dist = sin(deg2rad(lat1)) * sin(deg2rad(lat2)) + cos(deg2rad(lat1)) * cos(deg2rad(lat2)) * cos(deg2rad(theta));
dist = acos(dist);
dist = rad2deg(dist);
dist = dist * 60 * 1.1515;
switch(unit)
{
case 'M':
break;
case 'K':
dist = dist * 1.609344;
break;
case 'N':
dist = dist * 0.8684;
break;
}
return (dist);
}
/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
/*:: This function converts decimal degrees to radians :*/
/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
double deg2rad(double deg) {
return (deg * pi / 180);
}
/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
/*:: This function converts radians to decimal degrees :*/
/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
double rad2deg(double rad) {
return (rad * 180 / pi);
}
CDMA System Acquisition time
Hi,
Can anybody tell me the System acquisition timings in Sec/Min in CDMA till the mobile acquire the signal and display it on the Handset. How much time required for Pilot & Sync ...
Thanks!