CiiT International Journal of Wireless Communication, Vol 5, No 2, February 2013
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Multi Carrier-CDMA based SDR using Super Orthogonal Turbo Code K. Rasadurai, T. Ilavarasi and N. Kumaratharan
Abstract---Multi Carrier-CDMA, a combination of CDMA and OFDM has become a promising access technique for next generation wireless communication. It offers high speed data rate, flexible bandwidth allocation and enhanced performance to each user simultaneously. To further realize the benefits of MC-CDMA, it is made adaptable in the physical layer of software defined radio (SDR) where SDR is defined as a reconfigurable hardware with inbuilt software modules. Thus a MC-CDMA system based SDR is constructed which can be best suited for developing standards. However, as the number of users increases, due to overlapping of signals, orthogonality of the OFDM part is not maintained. It results in multiple access interference (MAI) which degrades the system performance in the downlink. To mitigate these effects, multi user detection techniques (MUD) is used. In this paper, MC-CDMA based SDR using super orthogonal turbo code (SOTC) is proposed and compared with MC-CDMA using Walsh Hadamard (WH) codes. The bit error rate (BER) is evaluated using the two codes and performance analysis is observed. Keywords---MC-CDMA, SDR, SOTC Codes and WH Codes I.
INTRODUCTION
T
HE integration of existing and future standards is the major goal of the next generation wireless communication. The advances in the digital technology enable the implementation of hybrid systems in the physical (PHY) and data link control (DLC) layers. Software defined radio (SDR) is one such emerging technology where functions are realized as programs running on components which are re-configurable [1]. It is open to adopt new technologies and standards that guarantee compatibility between several existing systems. To use its benefits and to provide a good quality of service (QoS), a hybrid system is enabled in the physical layer of SDR [1]. MCCDMA system based SDR would be a well-built candidate that supports existing and future standards. MC-CDMA is a multiple access system that is a combination of CDMA and OFDM systems. It offers high speed data rates and flexible bandwidth allocation to individual users with least interference. The concept of using orthogonal spreading codes with sub-carriers overcomes the effects of Manuscript received on February 13, 2013, review completed on February 20, 2013 and revised on March 01, 2013. K. Rasadurai, Research Scholar, Department of Information Technology, Sri Venkateswara College of Engineering, Sriperumbudur – 602105, Tamil Nadu, INDIA. T. Ilavarasi, PG Scholar, Department of Information Technology, Sri Venkateswara College of Engineering, Sriperumbudur – 602105, Tamil Nadu, INDIA. N. Kumaratharan, Associate Professor, Department of Information Technology, Sri Venkateswara College of Engineering, Sriperumbudur – 602105, Tamil Nadu, INDIA, E-Mail:
[email protected]. Digital Object Identifier No: WC022013006.
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multipath fading in MC-CDMA. In frequency domain, MCCDMA overcomes the effects of inter symbol interference (ISI) and inter carrier interference (ICI) [2], by introducing the wellknown guard time between the transmitted symbols to provide reduced signal processing complexity. The dual advantage of multi-carrier modulation and flexibility of spread spectrum technique made MC-CDMA system preferable for future wireless communication. Although MC-CDMA has many advantages, it suffers from multiple access interference (MAI) and peak to average power ratio (PAPR). When multiple users use the same frequency at the same time, overlapping of signals occurs that leads to loss of orthogonality. This loss results in MAI [3]. The interference becomes significant when the power level of the desired signal is low. To avoid the performance degradation due to MAI, efficient detection techniques can be used. Multi user detection (MUD) techniques are applied in MC-CDMA system to suppress the effects of MAI. In addition, the application of orthogonal codes for a synchronous system guarantees the absence of MAI in an ideal channel and minimum interference in fading channels. Several coding techniques were proposed to attain high performance in MC-CDMA system [8]. Effectual coding techniques like super orthogonal turbo codes (SOTC) [4], recursive systematic code (RSC) [5], walsh-hadamard codes (WH) [6] serve as good encoder and decoders for MC-CDMA system. The performance of CDMA and OFDM systems in SDR environment [7] is discussed to prove that MC-CDMA system is the notable technique. Channels such as AWGN and multipath fading channels [8] confirm excellent performance of interference cancellation [14] at the cost of lower transmission rate. Comparisons show that MC-CDMA outperforms CDMA over a Rayleigh fading channel [9]. To reduce complex detection, various MUD techniques [10] such as; parallel interference cancellation (PIC), serial interference cancellation (SIC), general minimum mean square error (GMMSE) in MCCDMA system was considered. Further, efficient decoding algorithms such as Maximum a posteriori (MAP) algorithm [15], log MAP algorithm and soft out viterbi algorithm (SOVA) were premeditated to proffer enhanced performance [11-12]. Hence to enhance the performance and offer high data rate, MC-CDMA system based SDR using super orthogonal turbo code is proposed in this work. The paper is organized as follows: Section II describes the system model to be constructed. The details of SOTC coding are explained in Section III. Simulation results and discussions are presented in Section IV, and the conclusion is arrived at Section V.
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CiiT International Journal of Wireless Communication, Vol 5, No 2, February 2013 II.
MC-CDMA BASED SDR
A. MC-CDMA based SDR Transmitter SDR consists of three layers; namely, physical layer, data link layer and application layer. The physical layer is enabled with existing systems so that it becomes adaptable to SDR technology. MC-CDMA system is made adaptable to SDR [1] to support future wireless communication systems. The data signal for MC-CDMA is generated by concatenating CDMA and OFDM. The input signal is spreaded using a spreading sequence and followed by inverse fast fourier transform (IFFT) modulation to confirm the orthogonality among the entire signals for excellent transmission with least interference.
The guard interval is a cyclic extension of each OFDM symbol by extending the duration of an OFDM symbol to The signal x(t) is upconverted and the RF signal is transmitted to the channel. B. MC-CDMA based SDR receiver The output of the channel, after RF down conversion is the received signal y(t) obtained from the convolution of x(t) with impulse response h(t) and addition of a noise signal n(t) as follows,
where r(k)(t) = x(k)(t)* h(t) is the noise-free received signal of user k, n(t) is the additive white Gaussian noise (AWGN) and * denotes the convolution operation. After doing analog to digital conversion of the received signal y(t), it is sampled at a rate 1/Td and generates the sequence ya , a=L1, . . . . , Nc – 1. The removal of ISI in the sequence is given by, a = 0, . . . . , Nc – 1 and it is demodulated by exploiting fast fourier transform (FFT). The output of the FFT demodulated sequence is
Fig 1 MC-CDMA based SDR using SOTC encoder
The input data is encoded using SOTC code that includes the Hadamard matrix. The principle of DS-CDMA is to spread the encoded data symbol with a spreading code c(k)(t) of length L,
assigned to user k, k = 0, . . . , K - 1, where K is the total number of active users. After spreading, the encoded signal x(k)(t) of user k is given by
for one data symbol duration Td = LTc, where Tc is the chip duration, d(k) is the transmitted data symbol of user k. The multiplication of the information sequence with the spreading sequence is done bit-synchronously and the overall transmitted signal x(t) of all k synchronous users results in
Since MAI strongly depends on cross-correlation function (CCF) of the spreading sequences, the CCF should be as small as possible. These multiple sub-carriers follow OFDM modulation with a spacing of symbol frequency
in order to achieve orthogonality among the signals. However, to completely avoid the effects of ISI and thus, to maintain orthogonality, a guard interval of duration is given by
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where, is the output of the FFT demodulated sequence, is the complex-valued symbols. Further, assuming that the fading on each sub-channel is flat, a received signal is represented in frequency domain accordingly, (10) where is the complex-valued flat-fading co-efficients, is the complex-valued source symbols and represents the noise of the nth sub-channel.
Fig 2 MC-CDMA based SDR using SOTC Decoder
After serial to parallel conversion, the signal is demodulated using FFT. Then it is decoded with SOTC decoder provided with the same spreading sequence used during transmission. Finally contented signal is received by appropriate users with high performance.
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CiiT International Journal of Wireless Communication, Vol 5, No 2, February 2013 C. Multiple Access Interference (MAI) In MC-CDMA system, the spreaded signal is passed through a frequency selective channel to the receiver. When the received codes are not orthogonal to each other, in other words, if the cross-correlation is non-zero, there arises multiple access interference (MAI). There are many techniques to overcome MAI among which multi user detection technique [17] is well suited for MC-CDMA systems. The design of spreading codes is considered to remove the effects of MAI completely [18]. In order to transmit MAI free signals [3], a basic concept is explained. Consider a Walsh-Hadamard (WH) sequence of length M, say WH = dk ; k = 1,2, . . . , M. Each user transmits a sequence of M samples which are uniformly spaced at a distance R = P/Q. These M samples are obtained by spreading the data by a specified element of WH sequence. It is calculated for frequency domain sequence as follows,
where is the spreaded signal without MAI, M is the different codes used for spreading and i is the user. III.
SUPER ORTHOGONAL TURBO CODE (SOTC)
SOTC is a low-rate convolutional turbo code [16] that can be applied as spreading codes in MC-CDMA system. It is well suited for spread spectrum applications. The idea behind SOTC is to concatenate two component codes i.e. recursive systematic code (RSC) and a WH generator. It helps to achieve good performance results for moderate users in uplink as well as in the downlink. This attempt of combining two codes is made to minimize the computational complexity [9] when the memory is greater than 2 in different multi-carrier multiple access schemes. A. SOTC Encoder An orthogonal convolutional encoder consists of orthogonal output sequences for each user. If the memory of the code is m, there are 2m different encoder states and a total of 2m+1 orthogonal sequences are required. Therefore, to get a set of orthogonal sequences, Walsh functions can be used. SOTC is simply a concatenation of recursive systematic code (RSC) and WH generator [4]. The inner encoder is the walshhadamard generator obtained from Hadamard matrices that can be generated as shown below, (12)
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Fig 3 Super Orthogonal Turbo Encoder
The initial data sequence is encoded using RSC code whose systematic output and interleaved output is given as input to the inner encoder i.e. WH generator. WH generator can simply be a look-up table that stores the output sequences [4]; it sends the signal for modulation. Thus the input data is coded using SOTC code and modulated using IFFT and transmitted through the channel [13]. B. SOTC Decoder After transmitting the signal over a fading or AWGN channel, the received signal is represented as Y and Y’, where Y is the systematic output and Y’ is the parity output. The channel outputs are (13) Where , i = 0,1, .... , n-1 is Gaussian random variables with equal variance. In general, for all i and t in an AWGN channel.
Fig 4 Super Orthogonal Turbo Decoder
The channel outputs Y and Y’ are decoded using inverse Walsh-Hadamard (IWH) and RSC decoder. The output from IWH is interleaved and sent to inner decoder for iterative decoding. A requirement for iterative decoding is the a priori information provided by the previous decoder must remain independent from the other inputs. This is accomplished by interleaving the data sequence before it enters the second component decoder. Ʌt represents the soft decision made by the decoder called log-likelihood ratio which is defined as,
Thus Hadamard matrices can be easily generated from (12). Fig-3 depicts an example of a SOTC encoder with memory m = 3. The log-likelihood ratio estimate [4] of a data bit can be expressed as the sum of
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CiiT International Journal of Wireless Communication, Vol 5, No 2, February 2013 is the a priori estimate provided by the
C. Walsh-Hadamard (WH) code Walsh-Hadamard codes are otherwise known as walsh codes. These are obtained by selecting the rows of Hadamard matrix as the code words. A Hadamard matrix is an n n matrix consisting of 1’s and -1’s, such that each row from any other row differs in exactly n/2 locations. A 4 4 Walsh code is given by C1 1 1 1 1 (16) C2 1 -1 1 -1 C3 = 1 1 -1 -1 C4 1 -1 -1 1 As a result of this matrix, a Walsh encoded signal appears as random noise to a terminal user unless the user knows the code that is used to encode the incoming signal. IV.
SIMULATION RESULTS
The proposed system is simulated with MATLAB version 7.14.0. The code rate used for SOTC is 1/8 and ½ for WH code. The number of frames chosen for SOTC is 1024 which is generally in multiple of 2 and the number of frames for WH code is 1000. The channels used are AWGN and Rayleigh fading channel for both the codes and among which AWGN channel shows improved performance. Since MC-CDMA system generally works with multi user detection to support many users, maximum likelihood detection technique is preferred. The simulation results are shown below with the proof of enhanced performance using SOTC compared to WH codes in MC-CDMA system. Table-1 shows the values of BER for the two codes using two different channels.
BER for BPSK on AWGN BER for BPSK on Rayleigh Channel
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10 Bit Error Rate
previous decoder. The next term, represents the extrinsic information provided by the current decoder, which in turn will be used as a priori information by the next decoder. The number of iterations determines the performance of decoder. The iterations can be completed with a predefined value or if a fixed threshold value has been reached.
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10 12 Eb/No, dB
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Fig 5 BER Performance using WH Code over AWGN and Rayleigh Fading Channels
B. Performance Using SOTC Code The code rate of RSC as well as WH generator is 1/8. The performance of MC-CDMA based SDR using SOTC code is evaluated in two channels; AWGN and Rayleigh fading channels. Fig-6 shows the results in terms of BER versus SNR for frame length n = 1024. The BER is approximately 0.0332 and SNR value is 1.423e-005 for AWGN channel which is far better compared to using WH code. Further, by applying an efficient decoding algorithm, the value of BER can still be improved using SOTC in MC-CDMA system. BER for BPSK on AWGN BER for BPSK on Rayleigh Channel
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10 Bit Error Rate
From Fig-4,
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TABLE-1 PERFORMANCE COMPARISON OF SOTC AND WH CODES Bit Error Rate Rayleigh Fading AWGN channel value channel SOTC code 0.0332 0.0976 WH code 0.1223 0.1464
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Fig 6 BER Performance using SOTC Code over AWGN and Rayleigh Fading Channels
V.
A. Performance using WH Code Based on the Hadamard matrix, the results are portrayed in terms of BER versus SNR as shown in Fig-5. The values of BER approximate to 0.1223 for AWGN channel and it is 0.1464 for Rayleigh fading channel. An increase in SNR value rapidly reduces the bit error rate but achieving low BER at low SNR values is the constraint of any multiple access system.
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CONCLUSION
MC-CDMA system based SDR using super orthogonal turbo code is proposed in this paper. Simulation results of SOTC and WH codes in MC-CDMA system were analyzed using MATLAB. It shows that the system performance in terms of BER using SOTC code outperforms WH codes. The improvement is more evident over the AWGN channel from Fig-5 and Fig-6. Therefore, high performance can be accomplished using SOTC code in MC-CDMA system for future wireless communication systems. Though the system shows enhanced performance with reduced effects of MAI, the process of decoding becomes complex with increased number of users. To overcome this problem in future, an efficient
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decoding algorithm such as Maximum a Posteriori algorithm is suggested using SOTC code to minimize the complexity and to offer high data rate to individual users.
[18] Kaiser, Stefan and Joachim Hagenauer, ‘Multi-Carrier CDMA with Iterative Decoding and Soft-Interference Cancellation,’ Global Telecommunications Conference, vol.1, pp. 6-10, November-1997.
REFERENCES
K. Rasadurai received his B.E degree in Electronics and Communication Engineering from Anna University, Chennai in 2006. He received his M.E Degree in Embedded System Technologies from Anna University, Chennai in 2009. He has three and half years of teaching experience. He is presently pursuing his research in the area of wireless mobile communication. He has published three papers in International Journals and presented four papers in International Conference. His area of interest includes mobile communications, wireless network.
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T. Ilavarasi received her B.Tech in Electronics and Communication Engineering from Pondicherry University, Pondicherry, in 2011. Currently she is pursuing Master of Engineering in Computer and Communication from Sri Venkateshwara College of Engineering, Anna University, Chennai. Her area of interest are Wireless Communication, MC-CDMA systems, Multiuser detection techniques.
Dr.N. Kumaratharan received B.E degree in Electrical and Electronics Engineering from the University of Madras in 2001 and M.E degree in Applied Electronics from College of Engineering, Guindy, in 2004 and a Ph.D in Wireless Communication from Pondicherry Engineering College in 2010. Currently he is working as Associate Professor, Department of Information Technology; Sri Venkateshwara College of Engineering, Sriperumbudur. He has published fifteen papers in International Journals and presented fourteen papers in IEEE International Conferences and a reviewer of some International Journals and Conferences. He is a recognized Ph.D Supervisor of Anna University, Chennai and is currently guiding seven research scholars. His area of interest includes Wireless Broadband Communication and Spread Spectrum Techniques.
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