1. Shashwat Shriparv
dwivedishashwat@gmail.com
InfinitySoft

2.  Code Division Multiple Access(CDMA) is a
spread spectrum technique in which we have to
use a wide bandwidth for different applications.
But at the same time more than one users can
use the same frequency is all done by using a
code called pseudo random code (PN) . In
addition it can provide an extra data security and
it can prevent interference and jamming of
signals

3.  Multiple Access is a technique where by many
subscribers or local stations can share the use of
communication channel at the same time or nearly
so ,despite the fact that there individual
transmissions may originate from widely different
locations . Stated in other way a multiple access
technique permit the communication resources of
the channel to be shared by a large number of users
seeking to communicate with each other. There are
subtle difference between multiple access and
multiplexing

4.  Multiple Access refers to the remote sharing of a
communication channel such a satellite or radio
channel by users in highly dispersed locations. On
the other hand multiplexing refers to sharing of a
channel such as a telephone channel by users
confined to a local site

5.  FDMA
 TDMA
 CDMA

6.  In Frequency Division Multiple Access disjoined sub
bands of frequency are allocated to the different
users on a continuous time basis. In order to reduce
interference between users allocated adjacent
channel bands, guard bands are used to act as
buffer zones. These guard bands are necessary
because of the impossibility of achieving ideal
filtering for separating the different users

7.  In Time Division Multiple Access each users is
allocated the full spectral occupancy of the channel
but only for a short duration of time called time slot.
Buffer zones in the form of guard times are inserted
between the assigned time slots. This is done to
reduce interference between users by allowing for
time uncertainty that arises due to system in
perfections, especially in synchronizations schemes

8.  In CDMA there is no limitations in the use of
frequencies that is we have to use any frequencies
allowed at any time that we need. And the same time
so many users can also use this same frequency.
But it is free from interference and jamming and it
can provide an extra data security. CDMA is derived
from direct sequence spread spectrum.

9.  When CDMA was first proposed, the industry gave it
approximately the same reaction that Columbus first
got from Queen Isabella when he proposed reaching
India by sailing in wrong direction. However, through
the persistence of a single company , Qualcomm,
CDMA has matured to the point where it is not only
acceptable , it is now viewed as the best technical
solution around and the basis for the third-generation
mobile system. It is also widely used in the U.S

10.  An airport lounge with many pairs of people
conversing. TDM is comparable to all the people
being in the middle of the room but talking turns
speaking. FDM is comparable to the people being in
widely separated clumps, each clump holding its
own conversation at the same time as, but still
independent of the others. CDMA is comparable to
everybody being in the middle of the room talking at
once , but with each pair in a different language.
The French-speaking couple just hones in on the
French, rejecting everything that is not French as
noise. Thus, the key to CDMA is to be able to extract
the desired signal while rejecting everything else as
random noise

11.  In CDMA, each bit time is subdivided into m short
intervals called chips. Typically, there are 64 or 128
chips per bit, but in the example given below we will
use 8 chips/bit for simplicity.
 Each station is assigned a unique m-bit code called
a chip sequence. To transmit a 1 bit, a station
sends its chip sequence. To transmit a 0 bit, it sends
the one’s complement of its chip sequence. No other
patterns are permitted.

12.  Consider there are 4 stations A,B,C and D. The
chip sequences are given below
A: 0 0 0 1 1 0 1 1
B: 0 0 1 0 1 1 1 0
C: 0 1 0 1 1 1 0 0
D: 0 1 0 0 0 0 1 0

13. it is more convenient to use a bipolar notation, with
binary 0 being -1 and binary 1 being +1 . So the chip
sequence will like this
A: (-1 -1 -1 +1 +1 -1 +1 +1)
B: (-1 -1 +1 -1 +1 +1 +1 -1)
C: (-1 +1 -1 +1 +1 +1 -1 -1)
D: (-1 +1 -1 -1 -1 -1 +1 -1)
Each station has its own unique chip sequence.
Let us use the symbol S to indicate the m-chip vector
for station S, and S for its negation

14.  All chip sequences are pairwise orthogonal , by
which we mean that the normalized inner product of
any two distinct chip sequences, S and T (written
as S T ) is 0 .
 And we know that
S S = 1
S S = -1
During each bit time , a station can transmit a 1 by
sending its chip sequence , it can transmit a 0 by
sending the negative of its chip sequence or it can
be silent and transmit nothing

15.  For the moment we assume that all stations are
synchronized in time, so all chip sequence begin at
the same instant.
When two or more stations transmit simultaneously
their bipolar signals add linearly. For example if one
chip period three stations output +1 and one station
output -1 , the result is +2 . One can think of this as
adding voltages: three stations outputting +1 volts
and one station outputting -1 volts gives 2 volts

16.  To recover the bit stream of an individual station, the
receiver must know that station’s chip sequence in
advance. It does the recovery by computing the
normalized inner product of the received chip
sequence (the linear sum of all the stations that
transmitted ) and the chip sequence of the station
whose bit stream it is trying to recover. If the
received chip sequence is S and the receiver is
trying to listen to a station whose chip sequence is C
, it just computes the normalized inner product , S C

17.  Capacity increases of 8 to 10 times that of an AMPS
analog system and 4 to 5 times of an GSM system
 Improved call quality, with better and more consistent
sound as compared to AMPS systems
 Simplified system planning through the use of the
same frequency in every sector of every cell
 Enhanced privacy
 Improved coverage characteristics
 Increased talk time for portables
 Bandwidth on demand

18.  In an ideal, noiseless CDMA systems the
capacity(i.e. , number of stations) can be made
arbitrary large, in practice , physical limitations
reduce the capacity considerably. First we have
assumed that all chips are synchronized in time. In
reality such synchronization is not possible
 An implicit assumption in our discussion is that the
power levels of all stations are the same as
perceived by the receiver . CDMA is typically used
for wireless systems with a fixed base stations at
varying distance from it . The power levels received
at the base station depend on how far away the
transmitter are

19.  We have also assumed that the receiver knows who
the sender is . In principle, given enough computing
capacity, the receiver can listen to all the senders at
once by running the decoding algorithm for each of
them in parallel. In real life suffice it to say that this is
easier said than done

20.  The main application of CDMA is in the field of
mobile phone communications. Due to the increased
needs and limitations of frequencies. Due to the high
availability of bandwidth CDMA can use for
application like internet accessing in mobile phones.
So many protocols have been developing in CDMA
the currently used CDMA is called CDMA ONE. We
have to expect so many improvements in
communication by the use of CDMA

21. Shashwat Shriparv
dwivedishashwat@gmail.com
InfinitySoft