Qpsk modulation for dsss cdma transmitter and receiver using fpga

1. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30 – 31, December 2014, Ernakulam, India
167
QPSK MODULATION FOR DSSS-CDMA TRANSMITTER
AND RECEIVER USING FPGA
Susan Kuriakose1
, Ms. Miji Jacob2
1, 2
ECE, SNGCE, Kadayiruppu, India
ABSTRACT
Direct sequence spread spectrum technique, which also known as direct sequence code division multiple access
(DS-CDMA), is one of the two approaches to spread spectrum modulation for digital signal transmission. Direct
sequence CDMA is the most popular of CDMA techniques. The DS-CDMA transmitter multiplies each of user signals
by a distinct code waveform. The detector receives a signal composed of the sum of all user signals. In a conventional
DS-CDMA system a particular user’s signal is detected by correlating the entire received signal with the user’s code
waveform. DS-CDMA transmitter and receiver are implemented in Verilog for FPGA .Transmitter module mainly
consist of user data generator, programmable chip sequence generator (PN sequence generator) and QPSK modulator.
The receiver module mainly consists of QPSK demodulator, PN sequence generator, matched filter and threshold
detector block. QPSK is a form of phase shift keying in which 2 bits are modulated at once. QPSK allows the signal to
carry twice as much information as ordinary PSK using same bandwidth. Modelsim tool will be used for functional and
logic verification. Xilinx synthesis technology will be used for synthesis of transmitter and receiver on FPGA Spartan
3E.
Keywords: DS-CDMA, PN, QPSK, BPSK, DSSS.
I. INTRODUCTION
Continuous growing demands from end user for more data have encouraged the engineers to develop many new
types of modulation scheme in satellite communication system. New types of modulation technique were introduced to
increase the efficiency in data transmitting and receiving rate within the same bandwidth. One of the common
modulation method used in satellite communication system is QPSK which is one form of PSK (Phase shift keying)
modulation scheme. In PSK modulation, the phase changed according to the base band data while the frequency and
amplitude remain unchanged. In QPSK quadrature means 4 different states that are used to represent a group of 2 bits
input data. The different inputs are 00, 01, 10 and 11 and each pair takes one form of QPSK state as shown in table1.
Mobile communications are rapidly becoming more and more necessary for everyday activities. With lot many
users to accommodate, more efficient and better use of bandwidth is the priority among cellular phone system
operators.DS-CDMA is a type of spread spectrum communication system in which multiple signal channels occupy same
frequency bandwidth, which is distinguished by the use of different spreading codes. CDMA base stations must be able
to discriminate this different code sequences to distinguish one transmission from other which is done by a matched
filter. Matched filter generates pseudo noise code and detects the user data.
In the recent years the CDMA on FPGA platform has attracted attention of academic research and industry.
Spartan-3E FPGA’s are best suited to a wide range of consumer electronics application.
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II. PROPOSED METHOD
A. Digital communication system
The role of a digital communication system is to transport digital data between two nodes i.e. the transmitter
receiver. A digital communication system is made of digital and analog parts. The message to be sent is from a digital
source. A digital modulator translates the discrete symbols into an analog waveform that can be transmitted over the
channel [3], [4]. In the receiver, reverse signal processing happens. The digital data is transmitted between the
transmitted and receiver by varying a physical characteristic of a sinusoidal carrier, either the frequency or phase or
amplitude. This operation is performed with a modulator at the transmitting end to impose the physical change to the
carrier and a demodulator at the receiving end to detect the resultant modulation on reception [6],
B. Design of DS-CDMA Transmitter
Code Division Multiple Access [2] is a
timeslots. With CDMA, the narrow band message is multiplied by a large bandwidth signal that is a pseudo random noise
code (PN code) as shown in fig 1.
All users in a CDMA system use the same frequency band and transmit them simultaneously. The transmitted
signal will be recovered by correlating the received signal with the PN code used by the transmitter.After spreading, the
Data signal is modulated and transmitted. There are several types available for modulation, viz. BPSK, QPSK, M
etc. In this design, QPSK modulation is used to modulate and transmit the spread signal [7].
The basic building blocks of a DS CDMA transmitter system are shown in Figu
C. QPSK Modulation and Demodulation
The signals of the QPSK modulation are defined in (1), where f
International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30 – 31, December 2014, Ernakulam, India
168
Table 1: Different phases of QPSK
The role of a digital communication system is to transport digital data between two nodes i.e. the transmitter
receiver. A digital communication system is made of digital and analog parts. The message to be sent is from a digital
source. A digital modulator translates the discrete symbols into an analog waveform that can be transmitted over the
. In the receiver, reverse signal processing happens. The digital data is transmitted between the
transmitted and receiver by varying a physical characteristic of a sinusoidal carrier, either the frequency or phase or
d with a modulator at the transmitting end to impose the physical change to the
carrier and a demodulator at the receiving end to detect the resultant modulation on reception [6],
Code Division Multiple Access [2] is a spread spectrum technique that uses neither frequency channels nor
timeslots. With CDMA, the narrow band message is multiplied by a large bandwidth signal that is a pseudo random noise
Figure 1: Direct sequence signals
All users in a CDMA system use the same frequency band and transmit them simultaneously. The transmitted
signal will be recovered by correlating the received signal with the PN code used by the transmitter.After spreading, the
transmitted. There are several types available for modulation, viz. BPSK, QPSK, M
etc. In this design, QPSK modulation is used to modulate and transmit the spread signal [7].
The basic building blocks of a DS CDMA transmitter system are shown in Figure 2.
Figure 2: DS-CDMA Transmitter
QPSK Modulation and Demodulation
The signals of the QPSK modulation are defined in (1), where fc is the frequency of the carrier and 0
Trends in Engineering and Management (ICETEM14)
31, December 2014, Ernakulam, India
The role of a digital communication system is to transport digital data between two nodes i.e. the transmitter and
receiver. A digital communication system is made of digital and analog parts. The message to be sent is from a digital
source. A digital modulator translates the discrete symbols into an analog waveform that can be transmitted over the
. In the receiver, reverse signal processing happens. The digital data is transmitted between the
transmitted and receiver by varying a physical characteristic of a sinusoidal carrier, either the frequency or phase or
d with a modulator at the transmitting end to impose the physical change to the
carrier and a demodulator at the receiving end to detect the resultant modulation on reception [6], [5].
spread spectrum technique that uses neither frequency channels nor
timeslots. With CDMA, the narrow band message is multiplied by a large bandwidth signal that is a pseudo random noise
All users in a CDMA system use the same frequency band and transmit them simultaneously. The transmitted
signal will be recovered by correlating the received signal with the PN code used by the transmitter.After spreading, the
transmitted. There are several types available for modulation, viz. BPSK, QPSK, M-QAM
is the frequency of the carrier and 0 <t <T [8], [10].

3. Proceedings of the International Conference on Emerging
Equation (1) can also be written as
where is the symbol energy
In QPSK, 2 bits are grouped together to form signals, and one of the 4 possible cases of dibits: 00, 01, 10 and 11
is transmitted. Also the odd bits were mapped to Si1 and even bits into Si2 [11].
Equation (2) of entire QPSK signal can be written as:
Where I (t) and Q (t) are pulse trains determined by odd and even bits. Since each bit occupies T
signals corresponding to the dibits last for T
The QPSK modulator is illustrated in fig 3
In the demodulator, the reverse operation happens. The I
as two different BPSK signals. The I-channel is demodulated with a cosine waveform and the Q
waveform. A parallel-to-serial-converter is used to combine the two sequences into a single one which represents the
modulating signal.
The QPSK Demodulator is illustrated in fig 4
International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30 – 31, December 2014, Ernakulam, India
169
In QPSK, 2 bits are grouped together to form signals, and one of the 4 possible cases of dibits: 00, 01, 10 and 11
is transmitted. Also the odd bits were mapped to Si1 and even bits into Si2 [11].
ire QPSK signal can be written as:
Where I (t) and Q (t) are pulse trains determined by odd and even bits. Since each bit occupies T
signals corresponding to the dibits last for Ts = 2Tb.
The QPSK modulator is illustrated in fig 3
Figure 3: QPSK Modulator
In the demodulator, the reverse operation happens. The I-channel and the Q-channel are demodulated separately
channel is demodulated with a cosine waveform and the Q
converter is used to combine the two sequences into a single one which represents the
The QPSK Demodulator is illustrated in fig 4
Figure 4: QPSK Demodulator
Trends in Engineering and Management (ICETEM14)
31, December 2014, Ernakulam, India
In QPSK, 2 bits are grouped together to form signals, and one of the 4 possible cases of dibits: 00, 01, 10 and 11
Where I (t) and Q (t) are pulse trains determined by odd and even bits. Since each bit occupies Tb seconds, the
channel are demodulated separately
channel is demodulated with a cosine waveform and the Q-channel, with a sine
converter is used to combine the two sequences into a single one which represents the

4. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30 – 31, December 2014, Ernakulam, India
170
D. Design of spreader device
Spreader device used at transmitter side consists of XOR gate which performs XOR operation between data
input and locally generated PN sequences.
E. Design of DS-CDMA Receiver
In DS-CDMA receiver, the input to the system is the QPSK modulated signal. The signal would have affected
by noise and other interference in the communication channel. The DS-SS CDMA receiver should be designed carefully
to reproduce the data signal with least error.
The QPSK modulated input signal is multiplied by the locally generated carrier wave by the oscillator. The
multiplied signal is thereafter passed through the low pass filter to get low frequency components only. A decision device
is used to approximate the signal to binary sequence. This binary sequence obtained is the spread sequence of the data
signal. The main sensitive part of the DS-SS receiver is the synchronization of the locally generated PN sequence and the
sequence obtained from the decision device. Even a single bit mismatch may give rise to noise instead receiving the
actual data signal. Suitable method is used to achieve synchronization and multiply the local PN sequence code with that
of the received PN code. The Data signal is obtained after carrying out the multiplication process.
Figure 5: DS-CDMA Receiver
F. Threshold Detector
The threshold detector compares the magnitude of the correlated output with the threshold value. If the
magnitude of the correlated output is higher than the threshold value, then it is interpreted as ‘1’. Otherwise it will be
declared as ‘0’. This is the detected information bits.
III. SIMULATION RESULTS OF ALL MODULES
A. Simulation of sine wave
B. Simulation of cos wave

5. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30 – 31, December 2014, Ernakulam, India
171
C. Simulation of QPSK wave
D. Generation of user data
E. Modulated output multiplied with oscillator signal at receiver
F. Simulation of Integrator output

6. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30 – 31, December 2014, Ernakulam, India
172
G. Simulation of Threshold Detector output
H. Simulation of Final output
It can be observed that received data is same as user data with some amount of delay.
IV. CONCLUSION
In this project I have implemented various modules of Direct Sequence Code Multiple Access Communication
System using QPSK Modulation. This design is completely reconfigurable. System developed is implemented with
Pseudo noise code sequence; it can be implemented with any length sequence very easily like Gold code. The developed
DS CDMA system provides efficient area utilization on FPGA. CDMA is one of the most important multiple access
technique. In this project the designed transmitter and receiver has been tested using an arbitrary chosen data stream,
where these data have been transmitted through implemented transmitter and then received by our implemented receiver.
Implementation of a CDMA communication system with DSSS technique in Verilog has the following
advantages
• The design is fully reconfigurable
• The number of bits and PN sequence can be changed very easily
• Useful for both FPGA and ASIC implementations.
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7. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30 – 31, December 2014, Ernakulam, India
173
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