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Wideband code division multiple access

WIDEBAND CODE DIVISION MULTIPLE ACCESS [WCDMA]

SIMULATION USING MATLAB

                                           James Agajo , Avazi Isaac

       Dept. of Electrical and Electronics Engineering, Federal Polytechnic, Auchi, Edo state Nigeria

        Dept. of Electrical and Electronics Engineering, University of Abuja, Nigeria

                                 Phone: +2348053312732 , agajojul@yahoo.com

This project models part of the frequency division duplex(FDD) downlink physical layer of the third generation(3G) wireless communication system known as Wideband Code Division Multiple Access(WCDMA) using matlab 7.0.1.WCDMA is a wideband spread spectrum 3G Mobile Telecommunication air interface that utilizes code multiple access. It provides a simultaneous support for a wide range of services with different characteristics on a common 5MHz carrier (that is, transmits on a pair of 5MHz wideband channels).WCDMA is the technology behind the 3G UMTs standard. It provides new service capabilities, increased network capacity and reduced cost for voice and data services. WCDMA is four times wider than the current channels that are typically used in 2G networks. It was adopted as a standard by the ITU under the name ‘IMT- 2000 direct spectrum'. It has data rates of up to 2Mbits which makes it easy to handle bandwidth-intensive application like video, data and image transmission. WCDMA supports two basic modes of duplex-frequency division and Time division. Current system uses frequency division, one frequency for uplink and one for downlink.

Keywords: UMST,WCDMA,GMSK,MULTIPATH

 

1.0  INTRODUCTION

1.1 BACKGROUND

W-CDMA is being used by Universal Mobile Telecommunication System (UMTS) as platform of the 3rd generation cellular communication system. W-CDMA uses noise-like broadband frequency spectrum where it has high resistance to multipath fading where as this was not present in conventional narrowband signal of 2nd generation (2G) communication system. High data rate signal transmission can be transmitted over the air by using W-CDMA system, thus enabling of multimedia rich applications such as video streams and high resolution pictures to end users. Thus, we need suitable modulation technique and error correction mechanism to be used in W-CDMA system.

In 2G networks, GMSK modulation scheme is widely used in GSM (Global System for Mobile Communication). This modulation can only transmit data rate of 1 bit per symbol. So it is quite sure that this kind of modulation scheme is not suitable for the next generation communication system. So, there is a need to study the performance of new modulation technique that could deliver higher data rate effectively in a multipath fading channel.[1]

1.2 OBJECTIVE.

The work is focused on the study and the performance measurement of high data rate modulation schemes at those channels which are subjected to Multipath Rayleigh Fading and Additive White Gaussian Noise (AWGN). Modulation Schemes that will be studied are 16-ary QAM (Quadrature Amplitude Modulation) and QPSK (Quadrature Phase Shift Keying).The performance study will be carried out by varying the chip rate of pseudo noise generator. W-CDMA (Wideband Code Division Multiple Access) scheme will also be studied by comparing some certain number of users under static and dynamic environment that are subjected to AWGN and multipath Rayleigh fading. The performance of fading channels in W-CDMA system are based on Bit Error Rate (BER) W-CDMA system at downlink transmission and Signal-to-Noise ratio (SNR).There will be three W-CDMA wireless cellular system models that will used in this project. The models are

2.0  RESEARCH METHODOLOGY

The research work  first review the high speed data rate modulation schemes, DSSS W-CDMA and fading effects on the channels. Then, we develop a generic model of DSSS W-CDMA as it is shown in figure 3.1 and is being simulated by MatLab modulation schemes 16-QAM and QPSK. [4]

2.1 SIMULATION METHODOLOGY.

As computer based simulations are the most fitting, powerful and proficient means to stand for the actual or real time scenarios of mobile radio system. Thus, MATLAB 7.0.1 has been used to simulate W-CDMA model based on associated parameters, theories and formulae. So we use the MatLab 7.0.1 for simulation using m files. Throughout this project, we set the bit rate of 384Kbps for the signal generator.[4]

There will be three W-CDMA wireless cellular system models that will be used in this research. The models are

1. W-CDMA system in AWGN channel

2. W-CDMA system in AWGN and Multipath Rayleigh Fading.

3. Multi-user W-CDMA system in AWGN and Multipath Rayleigh Fading (static and mobile).

2.2  BIT ERROR RATE (BER).

BER is a performance measurement that specifies the number of bit corrupted or destroyed as they are transmitted from its source to its destination. Several factors that affect BER include bandwidth, SNR, transmission speed and transmission medium.

2.3  SIGNALS TO NOISE RATIO (SNR).

SNR is defined as the ratio of a signal power to noise power and it is normally expressed in decibel (dB). The mathematical expression of SNR is

SNR = 10log10 (Signal Power) dB/Noise Power[5]

 

3.0  NOISE AND INTERFERENCE.

SNR is defined as the ratio of a signal power to noise power and it is normally expressed in decibel (dB). The mathematical expression of SNR is

SNR = 10log10 (Signal Power/Noise Power}……………….3.1

 

3.1 Additive White Noise Gaussian (AWGN).

The term thermal noise refers to unwanted electrical signals that are always present in electrical systems. The term additive means the noise is superimposed or added to the signal where it will limit the receiver ability to make correct symbol decisions and limit the rate of information. Thus, AWGN is the effect of thermal noise generated by thermal motion of electron in all dissipative electrical components i.e. resistors, wires and so on.

Mathematically, thermal noise is described by a zero-mean Gaussian random process where the random signal is a sum of Gaussian noise random variable and a dc signal that is  

                                       z = a +n        ………………………..3.2                                             

When noise power has a uniform spectral density, it is referred as white noise. The adjective "white" is used in the same sense as it is with white light, which contains equal amounts of all frequencies within the visible band of electromagnetic (EM) radiation.

Since thermal noise is present in all communication systems and is a prominent noise source for most system, the thermal noise characteristics that are additive, white and Gaussian are most often used to model the noise in communication systems.[6]

 

3.2 Rayleigh Fading.

Since signal propagation takes place in the atmosphere and near the ground, apart from the effect of free path loss, Ls, the most notable effect of signal degradation is multipath propagation. The effect can cause fluctuations in the received signal's amplitude, phase and angle of arrival, giving rise to terminology multipath fading.

 

3.3  Direct Sequence Spread Spectrum (DSSS).

DSSS is normally used in Code Division Multiple Access (CDMA) scheme. The received DSSS signal for a single user can be represented as    

                        Sn(t) = √2Es/Tsm(t)p(t)(Cos(2πfC+θ)              ………3.3                                           Where m (t) is the data sequence, p (t) is the PN spreading sequence, fc is the carrier frequency and θ is the carrier phase angle at t = 0.

 

3.4  DSSS CDMA BIT ERROR PROBABILITY CALCULATIONS.

There are two approaches to calculate BER for DSSS-CDMA operating under AWGN channel. The first approach uses accurate BER approximations because it is presumed that BER evaluation is numerically cumbersome. [8]

 

3.5 CONFIGURATION OF TRANSMITTER AND RECEIVER.

The system is configured based on synchronous W-CDMA system. Each user employs their own sequence to spread the information data. In the downlink transmission, the information data are modulated by the modulation scheme. After, the modulated data are spread by code that is M-sequence. The ‘spreaded' data of all users in the system are transmitted to the mobile users at the same time. The mobile user detects the information data of each user by correlating the received signal with a code sequence allocated to each user.[9]

The performance of the W-CDMA system is studied based on QPSK and 16-QAM modulation techniques that will be used in this simulation.

 

3.6  STEPS TO REALIZE THE SIMULATION IN WCDMA.M FILE.

The simulations for QPSK and 16-QAM modulation techniques are done by simulating the value of Eb/No at a fixed interval. For example, if the range of Eb/No is from 0 to 10 with interval of 1, the value of BER will be obtained for Eb/No at 1 interval. (See appendix).

This means the simulation to get the value of BERs has to be done 11 times. The range of Eb/No is determined by the behavior of the BER at that Eb/No's range. To realize the simulation of W-CDMA in LOS scenario, the value of fade is initialize to 0. Otherwise, it can be assigned to 1. When rfade=1, the channel of W-CDMA system is subjected to AWGN and multipath fading channel. The Doppler shift, on the other hand, is defined in fd. It represents the value of Doppler shift in Hertz (Hz).

Furthermore, the simulation of 16-QAM can be achieved by swapping the functions of modulator and demodulator from qpskmod and qpskdemod to qammod and qamdemod respectively.[9]

 

 

4.0   SIMULATION USING SIMULINK.

The WCDMA air interface is a direct spread technology. This means that it spreads encoded user data at a relatively low rate over a much wider bandwidth (5MHz), using a sequence of pseudorandom units called chips at a much higher rate (3.84 Mbps). By assigning a unique code to each user, the receiver, which has knowledge of the code of the intended user, can successfully separate the desired signal from the received waveform.

4.1 SUBSYSTEM FUNCTION.

 

WCDMA DL Tx Channel Coding Scheme: This is used for Transport channel encoding and multiplexing.

WCDMA Tx Physical Channel Mapping and WCDMA BS Tx Antenna are used for Modulation and spreading WCDMA Channel.

 

4.2  WCDMA BS Tx Antenna Spreading and Modulation.

The WCDMA BS Tx Antenna Spreading and Modulation subsystem performs the following functions:

a. Modulation

b. Spreading by a real-valued orthogonal variable spreading factor (OVSF) code.

c. Scrambling by a complex-valued Gold code sequence

d. Power weighting

e. Pulse shaping

4.3  WCDMA Channel Model.  

The WCDMA Channel Model subsystem simulates a wireless link channel containing additive white Gaussian noise (AWGN) and, if selected, a set of multipath propagation conditions. You can modify the multipath profile with the Propagation conditions environment parameter, as described under Parameters in the model.[10]

 

4.4 WCDMA Rx Antenna.  

The received signal at the WCDMA Rx Antenna subsystem is the sum of attenuated and delayed versions of the transmitted signals due to the so-called multipath propagation introduced by the channel. At the receiver side, a Rake receiver is implemented to resolve and compensate for such effect. A Rake receiver consists of several rake fingers, each associated with a different received component. Each rake finger is made of chip correlates to perform the dispreading, channel estimation to gauge the channel, and a derogator that, using the knowledge provided by the channel estimator, corrects the phase of the data symbol. The subsystem coherently combines the output of the different rake fingers to recover the energy across the different delays.[11]

 

4.5  WCDMA RX Physical Channel Demapping and Channel Decoding Scheme.   

The WCDMA RX Physical Channel Demapping and the WCDMA DL Rx Channel Decoding Scheme subsystem decode the signal by performing the inverse of the functions of the WCDMA DL Tx Channel Coding Scheme subsystem, as described above.

4.9.2 SIMULATION USING M FILES.

4.9.3 Performance Analysis of QPSK modulation technique of WCDMA in AWGN.

Table 4.1: Simulation result for evaluation on BER vs. SNR for ray tracing (also called 2-ray, one is LOS and other is reflected or NLOS) AWGN channel for 1 user when the number of data is 200,000.

4.9.3 Performance Analysis of QPSK modulation technique of WCDMA in AWGN and Multipath Fading Channel.

The simulation of BER is done in the range of 0 to 20 of Eb/No. The BER graphs of various Doppler shifts are simulated on the same graph as it is shown in figure 4.2.

The y axis of BER is blown up to depict the behavior in Doppler shift environment.

 

4.9.4 Performance Analysis Comparison of QPSK modulation technique of WCDMA between AWGN and Rayleigh Fading Channel.

 

4.5 Performance Analysis of 16QAM modulation technique of WCDMA in AWGN and Multipath Fading Channel.

We cannot obtain any results in this scenario as the results are inconsistent and uncertain. Therefore, we cannot investigate the performance of W-CDMA for this scenario.[13]

4.9.5 ANALYSIS AND DISCUSSION.

Simulation using m files shows that each QPSK and 16-QAM modulation techniques in AWGN channel has good performance when it is compared to that of Multipath Rayleigh channel. Also, the performance of QPSK and 16-QAM degrades when the channel is subjected to Multipath fading with increasing value of Doppler shift (Hz). In other words, it performs poorly as the speed of mobile terminal is increased. Moreover, the system performs badly as the number of users is increased. Comparison between QPSK and 16-QAM modulation schemes shows that 16-QAM performs very poorly in both AWGN (LOS channel) and AWGN with Multipath fading channel. The simulation of 16-QAM modulation technique using m files cannot be done because it is suspected that the variation of amplitude with phase causes errors in the constellation of 16-QAM signal.

 

5.0  SUMMARY AND CONCLUSION

In telecommunication field the major challenges is to convey the information as efficiently as possible through limited bandwidth, though some of the information bits are lost in most of the cases and signal which is sent originally will face fading. To reduce the bit error rate the loss of information and signal fading should be minimized.

In our thesis we analyze two modulation techniques, QPSK and 16-QAM to reduce the error performance of the signal and compare which technique is better through Rayleigh Fading Channel in the presence of AWGN.

The performance of W-CDMA system in AWGN channel shows that QPSK modulation technique has a better performance compared to that of 16-QAM. Furthermore, similar trend is found when the channel is subjected to multipath Rayleigh fading with Doppler shift. The performance of QPSK and 16-QAM modulation technique in W-CDMA system degrades as the mobility is increased from 60kmph to 120kmph for both QPSK and 16-QAM. However,

QPSK shows better performance compared to that of 16-QAM in LOS channel and multipath Rayleigh fading channel. In other words, 16-QAM suffers signal degradation and error proned when the simulations are done in these channels. As the number of users is increased, the QPSK modulation technique performs poorly in W-CDMA system. Unfortunately, the simulation for 16-QAM has failed to show the expected results in both Simulink and m files.

This is because the 16-QAM modulation scheme experiences adjacent carrier interference when the simulation is carried out. Therefore, it results in inconsistence of data or signal throughput causing abnormal values of BER and eventually affecting the performance of WCDMA system. It is expected that 16-QAM will show performance degradation similar like QPSK as the number of users is increased but with lower performance compared to that of QPSK.

In general, the reason that causes poor performance of W-CDMA system when the number of users is increased is because the value of cross correlation between the codes is not 0 and thus it causes interference. Many studies and researches have showed that 16-QAM modulation technique is a primary candidate for high speed data transmission in 3G mobile communication. High Speed Downlink Packet Access (HSDPA) is considered as a 3.5G where it has the capability to boost up the data rates of up to 10.7 Mbps using 16-QAM in a static environment. However, higher data rate modulation scheme (e.g.16-QAM) suffers significant degradation in noise and Multipath Rayleigh fading channel compared to lower data rate modulation technique (e.g. QPSK). The errors are resulted from interference between adjacent carriers phase in constellation of M-ary QAM. Larger value of M of M-ary QAM suffers more signal degradation. Thus, it is suggested that high data rate modulation technique such as 16-QAM needs an error correction coding such as convolutional coding or turbo coding so that the interference from the adjacent carrier phase in the constellation of 16-QAM can be eliminated if not minimized.

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[2]. Victor Wen-Kai Cheng, Wayne E. Stark, "Adaptive Coding and Modulation for Spread Spectrum", IEEE Journal, 1997.

[3]. Troels E. Kolding, Frank Frederiksen, Preben E. Mogensen, "Performance Aspects of WCDMA Systems with High Speed Downlink Packet Access (HSDPA)", Nokia Network R&D, Denmark, 2003.

[4]. Min-yan Song, Yang Xiao, Joachim Habermann, "High Data Rate Wireless System", IEEE, pp. 1344-1350.

[5]. Y. Rosmansyah, P. Sweeney, R. Tafazolli, "Air-Interface Techniques for Achieving High Data Rates for UMTS", IEEE 3G Mobile Communication Technologies, Conference Publication No. 477, pp. 368-372, 26-28 March 2001.

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[7]. Min-yan Song, Yang Xiao, Joachim Habermann, "High Data Rate Wireless System", IEEE, pp. 1344-1350.

[8]. Haifeng Wang, Zhenhong Li, "Novel Soft-bit Demodulator with Multi-dimensional Projection for High-order Modulation", IEEE, pp. 2051-2054, 2002.

[9]. Troels Emil Kolding, Klaus ingemann Pedersen, Jeroen Wigard, Frank Frederiksen, Preben Elgaard. Mogensen, "High Speed Downlink Packet Access (HSDPA): W-CDMA

Evolution", IEEE Vehicular Technology Society News, February, 2003.

[10]. E. Hossain, T. Issariyakul, "Performance bound of dynamic forward link adaptation in cellular W-CDMA networks using high-order modulation and multicode formats", IEEE Electronics Letters, Vol.40, No. 2, January 2004.

[11]. Bernard Sklar, "Digital Communications: Fundamentals and Applications", Prentice-Hall, 2nd Edition, pp. 30-33.

[12]. Julian Cheng, Norman C. Beaulieu, "Accurate DS-CDMA Bit-Error Probability Calculation in Rayleigh Fading", IEEE Transactions on Wireless Communications, Vol. 1, No. 1, January 2002.

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[14]. Michael B. Pursley, "Performance Evaluation for Phase-Coded Spread-Spectrum Multiple-Access Communication-Part 2: Code Sequence Analysis", IEEE Transaction on Communications, Vol. Com-25, No. 8, August 1977.

[15]. Dong In Kim, Ekram Hossain, Vijay K. Bhargava, "Dynamic Rate and Power Adaptation for Forward Link Transmission Using High-Order Modulation and Multicode Formats in Cellular W-CDMA Network", IEEE Journal, 2003.

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About the Author

Engr. James Agajo  is into a PH.D Programme in the field of Electronic and Computer Engineering, He has a Master's Degree in Electronic and telecommunication Engineering and also possesses a Bachelor degree in Electronics and Computer Engineering from the Federal University of Technology Minna Nigeria. His interest is in intelligent system development with a high flare for Engineering and Scientific research.He has Designed and implemented the most resent computer controlled robotic arm with a working grip mechanism 2006 which was aired on a national television , he has carried out work on using blue tooth technology to communicate with  microcontroller. Has also worked on thumb print technology  to develop high tech security systems with many more  He is presently on secondment with UNESCO TVE as a supervisor and a resource person. James is presently a member of the following association with the Nigeria Society of Engineers, International Association of Engineers(IAENG) UK, REAGON, MIRDA,MIJICT.

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