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A Full Demo of C++ mechanism
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International Journal of Electronic Engineering Research
ISSN 0975- 6450 Volume 1 Number 2 (2009) pp. 155–168
© Research India Publications
http://www.ripublication.com/ijeer.htm
BER Vs SNR Performance Comparison of DSSS-
CDMA FPGA Based Hardware with AWGN,
Spreading Codes & Code Modulation Techniques
N. B. Kanirkar
1
and J. N. Sarvaiya
2
1,2
Lecturer (Selection Grade), Electronics Engineering Department,
S. V. National Institute of Technology, Surat-395 007, Gujarat, (INDIA).
Email :
1
nbk@eced.svnit.ac.in,
2
jns@eced.svnit.ac.in
Abstract
The Code Division Multiple Access (CDMA) is a multiple-access scheme
based on Spread Spectrum (SS) communication techniques. It spreads the
message signal to a relatively wide bandwidth by using a unique code that
reduces interference, enhances system processing, and differentiates users.
Basically in CDMA the DS-SS technique is given more importance. The
Direct Sequence Spread Spectrum (DS-SS) is a type of modulation that
spreads data transmission across available frequency band, in excess of
minimum bandwidth available. Because of its advantages, it is used in code
division multiple access systems to assign unique code to every user. The
research work includes real time processing of signal, BPSK & QPSK
modulation and multiplication with pseudo random code to generate spread
spectrum, transmitting it over a common frequency band and reproduced the
signal using the same pseudo-random code at the receiver.
With the increased popularity of DS-SS CDMA technology, it is obvious
that there is need for more comprehensive investigations of analysis and
design of this techniques with spreading codes and code modulation
techniques.
The hardware consist FPGA Transmitter, Receiver & BER module for
typical BER measurement for the transmission of different number of bits.
Again for the same performance check the GOLD, MLS and BARKER codes
are generated by typical mathematical analysis and calculations of register
values given for BER FPGA based module and then vigorous performance is
carried out for the BPSK and QPSK code modulation techniques.
The BER values are obtained under different code selection and its
transmission with either BPSK or QPSK modulation techniques. The main
stresses given on number of bits which are transmitted are changed for better
156 N. B. Kanirkar and J. N. Sarvaiya
BER vs SNR performance measurement with different signal value and
AWGN values.
Key Words: Code Division Multiple Access (CDMA), Direct Sequence
Spread Spectrum (DS-SS), Maximum Length Sequence(MLS), Binary Phase
Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), PN Sequence
Generator, Additive White Gaussian Noise (AWGN), BER (Bit Error Rate),
SNR (Signal to Noise Ratio).
Introduction
The wireless mobile communication systems provide access to the capabilities of the
global network at any time, irrespective of the location or mobility of the user. The
direct sequence spread spectrum (DS-SS) technique, incorporated into CDMA can
accommodate large number of users in one radio channel depending on the voice
activity level [1][2]. This feature also provides immunity to jamming signals and
enables resolution of multi-path components in a time dispersive radio propagation
channel [3].
Spread spectrum is a type of modulation that spreads data transmission across
available frequency band, in excess of minimum bandwidth required to send the
information. Spreading makes signal resistive to noise and other interference. Spread
spectrum is commonly used with personal communication devices such as cell phones
and LAN’s [1]. Spread spectrum has many unique properties that cannot be found in
other techniques like the ability to eliminate or alleviate multi-path interference,
communication privacy due to unknown random codes, multi user handling capacity
over a single frequency, low power spectral density since signal is spread over a large
frequency band [2]. There are two techniques to achieve spread spectrum. The most
common methods are:
(1) Direct Sequence Spread Spectrum (DS-SS)
(2) Frequency Hopping Spread Spectrum (FH-SS)
In case of DS-SS, the transmitter converts an incoming data (bit) stream into a
symbol stream. Using a digital modulation technique like BPSK or QPSK, a
transmitter multiplies symbols with a pseudo random noise code. This multiplication
operation artificially increases used bandwidth based on length of chip sequence. A
CDMA system is implemented via these coding. Each user over a CDMA system is
assigned a unique PN code sequence. Hence, more than one signal can be transmitted
on same frequency.
DS-SS Technique
The following are the important states of DS-CDMA system used for transmission
and reception of audio signals.
BER Vs SNR Performance Comparison of DSSS-CDMA 157
Spread Spectrum
The main principle of spread spectrum communication is that the bandwidth
occupancy is much higher than usual. Because of this much larger bandwidth the
power spectral density is lower, in the channel the signal just looks like noise [4]. The
spreading is done by combining the data signal with a code (code division multiple
access) which is independent of the transmitted data message [2]. The advantages of
the method are:
• As the signal is spread over a large frequency band, the power spectral density
is getting very small, so other communications systems do not suffer from this
kind of communications. However, the Gaussian noise level is increasing.
• Random access can be dealt with as a large number of codes can be generated
and as a result large number of users can be permitted.
• In this technique without knowing the spreading code, it is (nearly) impossible
to recover the transmitted data. Hence security is more.
There are couples of spread spectrum techniques which can be used. The most
famous one is Direct Sequence (DS) [7].
Direct Sequence Spread Spectrum (DS-SS)
In a DS-SS system, each user is assigned a unique code sequence that allows the user
to spread the information signal across the assigned frequency band [3]. Signals from
the various users are separated at the receiver by cross correlation of the received
signal with each of the possible user code sequences. Possible narrow band
interference is also suppressed in this process. By designing these code sequences to
have relatively small cross-correlation, the cross-talk inherent in the demodulation of
the signals received from multiple transmitters is minimized [7]. This multiple access
method is CDMA, which is a form of a DS-SS system. This modulation transforms an
information bearing signal into a transmission signal with a much larger bandwidth.
This transformation is achieved by encoding the information signal with a code signal
that is independent of the data and has much larger spectral width than the data signal.
This spreads the original signal power over a much broader bandwidth, resulting in a
lower power density [6].
The ratio of transmitted bandwidth to information bandwidth is called the
processing gain Gp of the DS-SS system:
t
i
B
G
p =
B
(1)
Where, B
t
is the transmission bandwidth and B
i
is the bandwidth of the
information bearing signal.
DS-SS CDMA Transceiver
Pseudo Noise Signals
Pseudo Noise (PN) signals are very important factor in DS-SS systems. A DS-SS
system spreads the base band data by directly multiplying the base band data pulse
with a pseudo-noise sequence that is produced by a pseudo-noise code generator [1].
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