Proceedings of the 2nd International Conference on Engineering & Emerging Technologies (ICEET), Superior University, Lahore, PK, 26-27 March, 2015.
PERFORMANCE COMPARISON OF MULTI-CARRIER CDMA AND WAVELET TRANSFORM BASED MULTI-CARRIER CDMA A. Khalid COMSATS Institute of Information and Technology Lahore, Pakistan
M. R. K. Sial, W. Afzal, A. Afzal Faculty of Engineering and Technology Superior University Lahore, Pakistan Email:
[email protected]
Keywords-component; Discerete Wavelet Transform (DWT), Inter Symbol Interference (ISI), Multi-Carrier Code Division Multiple Access (MC-CDMA),
I.
INTRODUCTION
Evolution in wireless communication has reached the heights of fast data and voice transmission. Along with this, efficiency has also improved. 4G is in its early stages and opening up a whole new world of communication, in which users will have the freedom of doing what they want. For transmission of such high data rates, a multiple access technique called MC-CDMA has evolved. MC-CDMA gives advantages of OFDM and CDMA i.e. robustness of OFDM against multipath and flexibility of CDMA. Since MC-CDMA is the combination of OFDM and CDMA, so cyclic prefix (CP) is used, which intends to mitigate Inter Symbol Interference (ISI). It is always appended with every frame, at the end part of data symbol of OFDM at the transmission side [6]. The drawback of using cyclic prefix is that it uses excess bandwidth which leads to the spectral inefficiency as well as the loss of the data throughput [1]. OFDM has proved itself to be of great importance in 4G systems in recent days and is spectrum efficient. Since it is a part of MC-CDMA so, it becomes more efficient and increases the data rate further.
MC-CDMA uses Discrete Fourier Transform (DFT) which makes use of rectangular function as windowing waveform; this generates high side lobes which causes inter channel interference (ICI) when transmitted to non-ideal channel. As a solution to this problem, Wavelet transform Based MCCDMA technique for mobile radio communication is being suggested in this paper. In wavelet based MC-CDMA, orthogonality is maintained during transmission of data which prevents signals from Inter Symbol Interference (ISI). Orthogonality is maintained by making use of time and frequency localization property of the wavelet packets [3]. This property of wavelets can be used further to detect the received data either in time or frequency domain. This is something that cannot be done in case of simple MC-CDMA. In this paper, we have designed and simulated DFT-MCCDMA and DWT-MC-CDMA systems using MATLAB 10 and have compared their BERs. It is observed that DWT-MCCDMA outperforms DFT-MC-CDMA regarding BER using Rayleigh fading channel in air interface. In section 2, the characteristics of Rayleigh fading channel are discussed briefly, the reason for which it is used in our simulations. In section 3, system models are explained which are MC-CDMA and DWT-MC-CDMA, respectively. In this section systems are discussed by using their block model diagrams regarding which wavelet is used and why it is discussed. In section 4, when are two codes orthogonal to each other and which orthogonal codes may be used, are discussed? In section 5, a brief discussion about Wavelets and types they have. In section 6 Simulation results are discussed, recovered from MATLAB. Magnitude Response (dB)
Magnitude dB
Abstract: — With the increasing demand of fast and efficient wireless communication systems, the need for evolution in the communication systems has urged to its peak. In this continuum, 3G is playing its part in various countries already and now 4G is going to start its role in fulfilling the quest for high data rate and efficient communication. There is a technique called Multicarrier Code Division Multiple Access (MC-CDMA) which is a combination of Orthogonal Frequency Division Multiplexing (OFDM) and Code Division Multiple Access (CDMA) modulation techniques. It is an attractive modulation scheme for multipleusers high data rate wireless communication system. But since it uses guard band, extra bandwidth is utilized in data transmission. So there is another technique called Discrete Wavelet Transform Based MC-CDMA. This technique is free from guard band utilization and is bandwidth efficient. In our results shown at the end of this paper, this significance is enlightened by comparing their Bit Error Rates (BER), and showing drastic improvement in their BER with the usage of AWGN and Rayleigh fading channel.
Normalized Frequency
Figure 1. Rayleigh Channel Plot
II.
RAYLEIGH FADING CHANNEL IN WIRELESS COMMUNICATION
From general observation, we know that, in wireless communication, receiver is in motion with respect to the transmitter and has no line of sight path between transmitter and receiver. Due to the motion of receiver, small scale fading occurs which is caused by the multiple versions of the transmitted signal with different delays [4]. Rayleigh fading channel is considered as a good approximation for small scale fading. The Rayleigh distribution has the probability density function (PDF) given as [5]; 𝑟
𝑝(𝑟) = {
𝜎2
𝑒
−(
𝑟2 ) 2𝜎2
(0 ≤ 𝑟 ≤ ∞)}
(1)
(𝑟 < 0)
0
Where σ is the rms value of the received voltage signal before envelope detection, and 𝜎 2 is the time-average power of the received signal before envelope detection. Its plot is shown in Figure-1. III.
SYSTEM MODELS
A. DFT-MC-CDMA
MC-CDMA comprises of three sections namely transmitter, channel and the receiver. In MC-CDMA spreading occurs in Frequency domain. Multiple copies of same data are being made and multiplied by one chip of a user specific spreading code and then transmitted in parallel over overlapping carriers in the frequency domain. Here frequency diversity is being used. In MC-CDMA, the high rate data stream is first divided into low rate data streams. This would be done with the help of serial to parallel block. Now these low rate data streams are spread using the user defined spreading codes. PN sequence, Gold code or Walsh Hadamard codes are used for the spreading due to the reason that they have better crosscorrelation properties. The spreading is done in the frequency domain by simple multiplication of spreading codes with the data streams. After that the spread streams are then modulated by Quadrature Phase Shifting Keying (QPSK) or Quadrature Amplitude Modulation (QAM) and then passed through the IFFT block. After IFFT the data streams are combined using parallel to serial block. After combining the data a Cyclic Prefix (CP) is added. The purpose to use the cyclic prefix is to minimize the Inter Symbol Interference (ISI) and to ensure that distinct transmissions do not interfere with one another. Cyclic prefix (CP) is used in the guard time interval which is attached to each OFDM symbol. This is done in order to make the symbol
P-N Sequence Code S
IFFT
Add Cyclic Prefix
P
Modulation
Kth Use r Data in
P
S
Channel
P
S
Remove Cyclic Prefix
FFT
Demo dulatio n
Kth Use r Data out
S
P
P-N Sequence Code
Figure 2. Block diagram of DFT-MC-CDMA
duration longer than the channel delay. This is how addition of cyclic prefix combats ISI. Instead of reducing ISI it loses bandwidth and throughput is also reduced [4]. It also introduces immunity against propagation delays, echoes and reflections. Then transmit the signal in the channel as shown in the Figure-2.
B. DWT-MC-CDMA In Discrete Wavelet Transform based MC-CDMA, the high rate serial data stream is divided into low rate parallel data streams as shown in Figure-3 after passing through the serial to parallel block. These low rate data streams are spread through user defined spreading codes. Results in this paper are obtained by using Walsh code. The spreading is done by
simple multiplication of spreading codes with the data streams. After that, the spread streams are modulated by Quadrature Phase Shift Keying (QPSK). After modulation, Wavelet Transform is used instead of the Fourier Transform. Data is passed through Inverse Discrete Wavelet Transform (IDWT) block. The streams are then converted from parallel streams to a high data rate serial bit stream by using parallel to serial block. Here, cyclic prefix is not used due to the time and frequency localization property of Wavelet. The bit stream is then transmitted over the wireless communication channel. The block diagram of the wavelet based MC-CDMA is shown in Figure-3. At the receiver side, the signal is received and passed through the serial to parallel block. After that streams are passed into the Discrete Wavelet Transform (DWT) block and again converted to serial, before demodulation. After demodulation signal is dispread to give the original high rate data stream. IV.
ORTHOGONAL CODES
The codes that are used for the spreading data are orthogonal to each other and must have good cross-correlation properties, i.e., cross-correlation between the two codes must be very small or zero. For orthogonality the relation will be given as [5];
𝑘 𝑇 ∫0 𝑥𝑖 (𝑡) . 𝑥𝑗 (𝑡) = { 0
𝑖=𝑗 } 𝑖≠𝑗
(2)
The orthogonal codes which may be used for the spreading of data are; i) PN sequence. ii) Gold code. iii) Walsh Hadamard code. V.
WAVELETS
A waveform of limited duration that has an average value of zero is called wavelet. Wavelets are often non-symmetrical and irregular. Wavelets have pseudo frequency i.e. frequency varies slightly over the length of the wavelet. Wavelets are the mathematical functions that cut up the data into different frequency components. In wavelets the term scaling and dilation is use for stretching or shrinking the wavelet in time domain [8]. Following are some types of wavelets that are used for scaling and dilation: Shannon or Sinc Wavelet Daubechies Wavelet Gaussian Wavelet
Walsh code S
P
IDWT
Modulation
Kth User Data in
P
S
Channel
Kth User Data out
P
S
DWT
Demodul ation S
P
Walsh code
Figure 3. Block diagram of DWT-MC-CDMA
Biorthogonal Wavelet Mexican Hat Wavelet Coiflet Wavelet
But in the coding used for which the results are given, Daubechies wavelet filter is used due to its shorter Quadrature filter. Other wavelets have their own advantages which are useful in particular situations. Some are robust against timing errors while others may achieve lower interference level [7].
VI.
SIMULATION RESULTS
The Fourier transform based MC-CDMA and wavelet transform based MC-CDMA transceivers are designed and simulated using MATLAB 10. The transceiver is designed such that perfect reconstruction on the receiver side is assumed. The process by which the signal passes on the transmission side is applied exactly in reverse order at the receiver side.
In the graph shown in Figure-4, BERs of DFT-MC-CDMA and DWT-MC-CDMA with noise only, are being compared with respect to their Eb/No. There is a gain improvement of 25dB. DFT MC-CDMA WITH AWGN DWT-MC-CDMA WITH AWGN -1
10
-2
Bit Error Rate
10
In the simulation result, the graphs of DFT-MC-CDMA and DWT-MC-CDMA show a drastic improvement of 25dB, in which only noise is added. And there is an improvement of 26dB when Rayleigh fading channel and AWGN noise is used. It means higher data rate, faster and reliable communication. The spectrum of DWT-MC-CDMA is more efficient than the DFT-MC-CDMA’s spectrum. Spectrum is an important factor which vendor companies have to bother about and if they are made available with the better spectrum then it would be a step forward to the betterment of communication in wireless medium.
-3
10
Table 1. Parametric comparison of DFT-MC-CDMA and DWT-MC-CDMA -4
10
Parameters -5
10
0
5
10
15 20 Eb/No in dB
25
30
35
PARAMETRIC COMPARISON OF DFT-MCCDMA AND DWT-MC-CDMA DFT-MC-CDMA
40
Number of Users
Figure 4. BER comparison of DFT-MC-CDMA and DWT-MC-CDMA in AWGN
In the graph shown in Figure-5, BERs of DFT-MC-CDMA and DWT-MC-CDMA with Rayleigh Fading Channel + noise only, are being compared with respect to their Eb/No. There is a gain improvement of 26dB.The detail parameters of the coding are given in the Table 1.
BER for BPSK modulation in Rayleigh channel
-1
10
DWT-MC-CDMA
1
1
Data rate
9.6kbits/s
9.6kbits/s
Chip Rate
1.228800Mchips/s
1.228800Mchips/s
Spreading code
Walsh Hadamard
Walsh Hadamard
Code length
64
64
Modulation
QPSK
QPSK
64
64
No. of Subcarriers Subcarrier spacing
19200Hz
19200Hz
Channel Bandwidth
1.25MHz
1.25MHz
Eb/No for AWGN Eb/No for AWGN +Rayleigh channel
34dB
9dB
40dB
14dB
VIII. REFERENCES Bit Error Rate
-2
10
[1]
-3
10
[2] -4
10
[3] DFT MC-CDMA with rayleigh channel and AWGN DWT MC-CDMA with rayleigh channel and AWGN
-5
10
0
5
10
15 20 Eb/No in dB
25
30
35
40
Figure 5. BER comparison of DFT-MC-CDMA and DWT-MC-CDMA in Rayleigh fading channel+ AWGN
VII.
CONCLUSION
The efficiency of a discrete wavelet transform based MCCDMA technique has been observed from the resulting graphs of simulations. The results of DWT-MC-CDMA are better than simple DFT based MC-CDMA. Since there is no guard interval in DWT-MC-CDMA, which is an essential component in DFT-MC-CDMA, ideal spectral efficiency can be achieved by using the DWT-MC-CDMA.
[4] [5] [6]
[7] [8]
S. Baig, N.D. Gohar, F. Rehman: “An Efficient Wavelet Based MC-CDMA Transceiver for Wireless Communications”, IBCAST 2005. R. Prasad and S. Hara: “An Overview of Multi-Carrier CDMA”, IEEE 1996. A. K. Dubey, G. Vashistha and Parveen: “Reanalysis of BER for Wavelet Based MC-CDMA Communication”, IJCSMS, Vol. 11, May 2011. J. Li, A. Bose and Y. Q. Zhao: “Rayleigh Flat Fading Channels Capacity”, IEEE 2005. T. S. Rappaport: “Wireless Communications Principles and Practice”, Prentice Hall, Inc., 2002. Hassouna, S.I. Badran, E.F. Mohamed: “Wavelet Packet Modulation for Multi-Carrier CDMA Communications”, Arab Acad. for Sci. and Technol. And Maritime Transp., Alexandria, March 2007. H. Zhang, “Wavelet Packet Based Multicarrier CDMA Wireless Communication Systems”, Ph.D. Thesis, University of Cincinnati USA, 2004. D. L. Fugal, “Conceptual Wavelets in Digital Signal Processing”, Space and Signal Technologies LLC, 2009.
