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This work represents the performance evaluation of DS-CDMA in a Rayleigh fading environment. Performance analysis was carried out for a DS-CDMA system to evaluate the signal-to-noise ratio (SNR), signal-to-interference ratio (SIR) and bit error rate (BER) for different modulation schemes like ASK, PSK and FSK. Results were evaluated at different data rates and were presented in terms of SNR, SIR and BER. Computer simulation was also carried out by using Matlab to evaluate the signal waveforms with and without addition white Gaussian noise (AWGN). Butterworth filtered waveform was given to get better signal quality. Decoded output waveforms were also evaluated to confirm the simulation.

In digital communication system, selection of the most appropriate access method is a challenging task. To meet this challenge we have to be familiar with the technologies and system architectures of the CDMA digital cellular system. CDMA development started in early 1989 after the NA-TDMA standard was established. In early 1990, QUALCOM incorporated of San Diego, California pioneered to introduce the intensive system concepts [

The tremendous demand from the subscribers of mobile communication leads to the development of new standards for this technology. Various application areas are evolving in this field. To meet the future demands, the ITU, IMT-2000 plan was formulated [

In the recent years, wireless CDMA draws considerable interest of the communication engineers to be used as a technique for multiple access communications. CDMA has many advantages over the other traditional multiple access techniques such as TDMA and FDMA. CDMA can accommodate more users than TDMA and FDMA, interference rejection, suppression and multi-path mitigation are better in CDMA than TDMA and FDMA [

Direct sequence CDMA (DS-CDMA) and frequency hopping CDMA (FH-CDMA) are the two types of CDMA. DS-CDMA is the most popular between them for its simplicity, better performance and more reliability. Many research works have been done [

The BER performance of DS-CDMA system in a Rayleigh fading channel has been investigated in this research. So far, most of the researches on DS-CDMA mobile communication systems were done considering the mobile wireless channel either AWGN channel or Rayleigh distributed flat or frequency selective fading channel. The objectives of the current research work were: 1) To study a Direct Sequence Code Division Multiple Access (DS-CDMA) system in a Rayleigh fading channel, 2) To carry out the analysis for carrier to multiple access interference ratio and the compression for bit error rate (BER) of a DS-CDMA system in a Rayleigh fading channel, 3) To carry out numerical computations to evaluate the carrier to interference ratio (CIR) and to find the BER as a function of system parameters, 4) To carry out computer simulations to evaluate the signal waveforms and the amount of signal distortions due to fading and 5) To find the optimum system parameters in presence of fading.

In DS-CDMA is the most popular of the CDMA techniques. A DS-CDMA signal can be achieved when the original narrowband signal b(t) is directly multiplied by a wideband signal a(t) resulting in the signal b(t)a(t) as shown in

As mentioned earlier, in the DS-CDMA systems the modulated information bearing signal (the data signal) is directly modulated by a digital code signal. The data signal can be either an analog signal or a digital one. In most cases it will be a digital signal. What we often see in the case of a digital is that the data modulation is omitted and the data signal is directly multiplied by the code signal and the resulting signal modulates the wideband carrier. It is from this multiplication that the direct- sequence CSMA get its name. In

To obtain the desired spreading of the signal, the chip rate of the PN sequence must be much higher than the chip rate of the information signal. After transmission of the signal, the receiver uses coherent demodulation to de-spread the spread spectrum (SS) signal, using a locally generated code sequence. To be able to perform the de-spreading operation, the receiver must not only know the code sequence used to spread the signal but also the codes of the received signal and the locally generated code must also be synchronized. This synchronization must be accomplished at the beginning of the reception and maintained until the whole signal has been received. After de-spreading the modulated data signal and after demodulation the original data can be recovered.

Fading is the term used to describe the rapid fluctuations in the amplitude of the received radio signal over the short period of time fading is a common phenomenon in mobile communication channels caused by the interference between two or more versions of the transmitted signals which arrive at the receiver at slightly different times. The resultant received signal can vary widely in amplitude and phase, depending on various factors such as the intensity, relative propagation time of the waves, bandwidth of the transmitted signal etc. the performance of a system can be severely degraded by fading. A typical mobile radio environment is shown in

Rayleigh fading is a statistical model for the effect of a propagation environment on a radio signal, such as that used by wireless devices [

there is a dominant line of sight, Rician fading may be more applicable [

The performance of DS-CDMA wireless cellular and non cellular (single cell) mobile communication system in a Rayleigh fading environment has been analyzed in this research work. The numerical results were evaluated considering the near far problem. The performance results were evaluated in terms of Signal-to-Noise Ratio (SNR), Signal-to-Interference Ratio (SIR) and the Bit Error Rate (BER) of the DS-CDMA system.

Here Matlab programs have been used to get various graphs. Analytical approach of a wireless communication system was followed considering Rayleish fading channel. The results were represented in terms of Signal-to-Noise ratio (SNR) and the Bit Error Rate (BER) for practical values of system parameters. The plot of SNR versus received power at the receiver input, Pr (dbm) is shown in

Bit error rate (BER) is the number of erroneous bits divided by the total number of bits transmitted received or processed over some stipulated period. Example of bit error ratio are a) transmission BER, that is the number of erroneous bits received divided by the total number of bits transmitted and b) information BER, i.e. the number of erroneous decoded (corrected) bits divided by the total number of decoded (corrected) bits. The BER is usually expressed as a coefficient and a power of 10; for example, 2.5 erroneous bits out of 100,000 bits transmitted would be 2.5 out of 105 or 2.5 × 10^{−5}. The Bit Error Rate (BER) of a communications system is a measure of the ratio between the number of bit errors for an estimated received signal with respect to the transmitted

signal and the total number of bits transmitted. Therefore the lower the bit error is, the better the system performance is. Typical communications systems use BER as the primary metric in evaluating the system performance. The BER is evaluated in term of 1) BER Vs SNR (db) and 2) BER Vs No of user.

of users can be accommodated for PSK compared to ASK and FSK.

In this research, simulation was also carried out to evaluate the output waveforms for a given input data sequences. The waveforms are represented in Figures 7(a)-(d) through Figures 7 (a)-(d). The plot (

+1, +1].

Using theoretical analyses simulation in MATLAB environment is carried out to extract the numerical results and to draw the possible conclusions for the systems. Considering the acceptable BER is equal to or less than 10^{−3} for satisfactory performance of the systems, several conclusions have been made from the numerical results. The possible conclusions are summarized as: 1) For perfect power control in both the cellular and non-cellular DS-CDMA mobile communication systems, the processing gain and the SNR per bit E_{b}/N_{o} increases, the SINR increases and the BER decreases of the receiver which is an indication of the improvement of the system performance, 2) For perfect power control in both the cellular and non-cellular DS-CDMA mobile communication systems, the number of fading path or signal processing path increases, the SINR decreases and the BER

increases which means the degradation of the system performance, 3) In the perfect power control the performance of cellular and non-cellular DS-CDMA mobile communication systems degrades as the number of interfering cell increases and 4) For both the case of cellular and non-cellular (single cell) DS-CDMA mobile communication systems, the performance is the best for the PPC system and the worst for the absence of power control (APC) system.

We would like to thank all concerned with the APECE department for their all-out effort to support us for completing this research.

M. A.Rahman,M. M.Alam,Md. KhalidHossain,Md. KhairulIslam,Khan M. NasirUddin,Md.Shahinuzzaman, (2016) Performance Evaluation of a DS-CDMA System in a Rayleigh Fading Environment. World Journal of Engineering and Technology,04,1-9. doi: 10.4236/wjet.2016.41001