Journal of Scientific & Industrial Research Vol. 73, July 2014, pp. 443-447 Cognitive Radio Transmission Based on Chip-level Space Time Block Coded MC-DS-CDMA over Fast-Fading Channel S. Mohandass * and G.Umamaheswari Department of Electronics and Communication Engg, PSG College of Technology, Coimbatore-641004, India Received 22 August 2013; revised 22 February 2014; accepted 03 May 2014 In this paper, a transmit diversity technique using chip level space time block coding (chip-stbc) is investigated for the downlink transmission of multicarrier direct sequence code division multiple access (MC-DS-CDMA) based cognitive radio base station. Synchronous transmission of multiple users signals over fast fading frequency selective Rayleigh channel is considered. Conventional symbol level space time block coding techniques cannot be applied in such channel conditions, since the coherence time of the channel will be less than the symbol period. Therefore, chip level STBC with chip level linear minimum mean square error frequency domain equalization (LMMSE-FDE) is utilized to achieve space-time diversity. The proposed cognitive radio system involves three adaptive techniques namely, variable-length-code spreading, adaptive modulation, and adaptive subcarrier deactivation, to achieve flexibility and improved performance in a dynamic wireless environment. Orthogonal Variable Spreading Factor (OVSF) codes are used to support users of different data bit rates. Adaptive modulation technique is used to improve the data throughput of the system. Adaptive subcarrier deactivation method is proposed for deactivating a set of subcarriers to avoid causing interference to the primary user. The simulation results show significant bit error ratio (BER) performance gain over conventional single antenna systems. Keywords: Transmit diversity, cognitive radio, adaptive modulation, adaptive subcarrier deactivation. Introduction Cognitive radio (CR) has been proposed to meet the growing demand for high data rate wireless applications, by efficient utilization of the available spectrum 1-2. Sarath et al 3 proposed Multicarrier Code Division Multiple Access (MC-CDMA) as a promising technique for cognitive radio systems, by exploring the reconfigurability options in MC-CDMA. Multicarrier direct sequence CDMA (MC-DS-CDMA) is a variant of MC-CDMA that spreads the serial to parallel converted data streams in time domain using a spreading code 4. Chang et al 5 proposed an interweave cognitive radio system based on a hierarchical wide-band two-dimensional spread MC-DS-CDMA. Chang 6 proposed an interference avoidance code assignment strategy for the hierarchical 2-D spread MC-DS-CDMA based cognitive femtocell system. In recent years, multiple antenna techniques have attracted the attention of researchers by improving the communication performance through diversity gain 7-8. In this work, cognitive radio architecture based on MC-DS-CDMA * Author for correspondence Email: mohandass25@yahoo.com with chip level space time block coding (chip-stbc) is considered to achieve antenna diversity. The space time spreading techniques 9-12 and symbol level space time block coding techniques 13-14 are suitable for slow fading channel conditions. The proposed chip-level STBC is applicable for fast fading channel conditions since it requires the channel to be static for only two chip periods. System model The cognitive radio system considered in this paper is based on orthogonal MC-DS-CDMA scheme using two transmitter antennas and one receiver antenna. Further, in this contribution a synchronous downlink transmission of cognitive radio base station is investigated, where K secondary users signals are transmitted synchronously. Transmitter model A stream of data symbols s i is serial to parallel converted to U parallel sub streams. Symbols in each substream are spreaded using OVSF spreading sequence C k of k th user with a variable spreading factor of SF. As a result, each symbol of k th user is changed into chip stream {d k,u (i), i=0,1,, SF-1; u=1,2,, U}. Then an STBC encoder codes the chip
444 J SCI IND RES VOL 73 JULY 2014 sequence {d k,u (i)} into two different sequences D k,u1 and D k,u2, one for each transmitter antenna. The U output signals from STBC encoders which will be transmitted using the same antenna are then interleaved by an S-depth interleaver, so that each signal is carried by S subcarriers. These subcarriers have maximum possible frequency spacing, so that they experience independent fading and hence achieve maximum frequency diversity. Thus, V=U.S number of subcarriers are used to carry the signal from each antenna. Finally, the inverse Fast Fourier Transform (IFFT) is performed to carry out multicarrier (MC) modulation. The IFFT block s output signal is transmitted using one of the transmitter antennas after adding cyclic prefix whose length is greater than the maximum delay spread of the channel. The k th user s transmitted signal is given by, where u=1,2,, U. (1) Adaptive Subcarrier Deactivation In dynamic spectrum access network, some portions of the spectrum are occupied by primary users. The proposed multicarrier CDMA based cognitive radio transceiver can deactivate or turn off those subcarriers overlapping with primary user s transmission in frequency domain, to avoid causing interference to incumbent users. The subcarriers are deactivated by assigning zero values to them and the active subcarriers are assigned with data symbols. In the proposed transmitter, out of U.S subcarriers, some of the subcarriers which are overlapping with the primary user s transmitted signal are deactivated or turned off. Variable spreading factor Emerging wireless systems must support variable rate transmission to accommodate multimedia services. Variable spreading factor is a technique used for supporting users of different data rates and services. In general, higher data rate services can be achieved by assigning a spreading code with smaller spreading factor and lower data rate services with larger spreading factor. The variable data rate is converted into a constant chip rate by the spreading operation. Orthogonal Variable Spreading Factor (OVSF) codes constructed based on Walsh code are used in the proposed forward link of synchronous MC-DS-CDMA systems to support multiple users with different transmission rates. The spreading codes are selected for different users such that they are orthogonal to each other. Adaptive modulation Adaptive modulation is used to improve the average throughput of the system by adaptively varying the modulation modes depending on the channel state conditions. A higher order modulation is used to send more bits per symbol when the channel condition is good and when the channel condition is poor, the modulation mode is switched to a lower order one, to reduce the BER. Instantaneous output SNR is used as the measure of channel quality 10. It is assumed that the instantaneous output SNR is known at the transmitter and by comparing it with the pre-determined thresholds, the transmitter decides on appropriate modulation mode. The threshold values for switching the modulation modes are chosen so that the BER remains below an acceptable level. Different levels of quadrature amplitude modulation (QAM) are used in the adaptive modulation scheme. Channel Model The channel is assumed to be frequency selective Rayleigh multipath fading channel. Let be the channel s complex impulse response between the i th transmitter antenna and the receiver antenna and n being the discrete time index. The coefficients 0,1,2, are independent and identically distributed (i.i.d.) random variables obeying the Rayleigh distribution. The phases 1, 2; =0,1,2, are introduced by the fading channels and are uniformly distributed in the interval. It is assumed that the maximum delay spread T m of the channel satisfies the condition T m << T c where T c is the chip period, such that each subchannel confirms the flat fading. Also, the channel is assumed to be fast fading channel i.e., the coherence time of the channel is less than the symbol period and the channel is constant over two chip periods (2T c ). Receiver model Assuming that K users signals in the form of (1) are transmitted synchronously over the Rayleigh
MOHANDASS & UMAMAHESWARI: COGNITIVE RADIO TRANSMISSION BASED ON CHIP-LEVEL SPACE 445 fading channel, the received complex low-pass equivalent signal can be expressed as, where, (2) and is the complex valued low-pass-equivalent additive white Gaussian noise (AWGN) having a double-sided spectral density of. Fast Fourier Transform (FFT) is performed after removing cyclic prefix and serial to parallel conversion of the received signal. Inverse STBC is done at each subcarrier to decode the space time block coded chips. It is assumed that the receiver has perfect knowledge of the fading parameters of (time index n is not mentioned for simplicity). The decision variables, and are found corresponding to the first two chips (l) for l = 0, 1} associated with the v th subcarrier, at the output of the Inverse STBC block. Finally, after combining the replicas of the same signal transmitted on the S subcarriers, the decision variables corresponding to the two chips { in the u th subblock can be expressed as, (3), the sum of K users first two chips values. N l is AWGN noise term. The term is the total diversity achieved through frequency interleaving and multiple antennas at the transmitter. Maximum frequency diversity can be achieved for a given number of combined subcarrier signals, if they experience independent fading. So the frequency domain spacing between the specific subcarriers that are combined must be higher than the maximum coherence bandwidth of. This condition is satisfied if (U/T c ) (1/T m ), i.e., U (T c /T m ) 12. Deactivation of the subcarriers that overlap with the primary user transmission affects the frequency diversity gain achieved at the receiver. If more number of subcarriers is deactivated, only a less frequency diversity gain can be achieved. Deactivation of subcarriers can also be done in a non-contiguous manner. We assume that at least one subcarrier will carry each of the U substream signals at the transmitter. The combined signals are given to LMMSE-FDE equalizers (E) 15. The LMMSE technique makes a trade-off between the ISI elimination and noise enhancement. Thus it performs better than the Zero Forcing Equalizer (ZFE), when the channel has spectral nulls or deep fading gains. After equalization, despreading is performed using the spreading sequence C k of the desired user k. The spreading sequences are assumed to be perfectly orthogonal. The despreaded symbol is given to a maximum likelihood detector (D) to detect the received symbols of the desired user. Results and Discussion The BER performance for BPSK data over Rayleigh fading channel using different transmit antennas (T) and frequency-interleaving (S) is shown in Fig. 1, with spreading factor SF=8. We observe that the BER performance is significantly improved by increasing the number of transmit antennas T from 1 to 2. For instance, about 4dB diversity gain can be achieved when T increases from 1 to 2, while setting BER equal to 10 4 and S=1, as shown in Fig. 1. To improve the BER performance of the system especially for high order modulations, the frequency diversity order is increased by increasing S. For instance, it is observed in the Fig. 1 that at
446 J SCI IND RES VOL 73 JULY 2014 Fig. 1 The BER performance of BPSK using different transmit antennas T and depth of frequency-interleaving S with SF=8 Table 1 The switching thresholds for adaptive modulation system Switching threshold (db) Modulation mode SNR 11 QPSK 11 < SNR 15 8-QAM SNR > 15 16-QAM BER = 10 4, the interleaver with S=3 and T=2 achieves 3dB gain in comparison with S=1 and T=2. However, when T=2 and S becomes large, the improvement rate starts dropping, which is because the system approaches to its maximum capacity. Thus, considering trade-off with the complexity of the system due to the increased S, Fig. 1 suggests that S need not be larger than 4. Adaptive modulation technique is used in the proposed method to improve the throughput of the system. In this work, a fixed threshold based adaptive QAM (AQAM) mode selection algorithm is used. The allowable BER is fixed to be 10-2 and a modulation mode is selected, if the instantaneous channel SNR perceived by the receiver exceeds the corresponding switching levels shown in the Table 1. The thresholds are selected from the BER curves of QPSK, 8-QAM and 16-QAM data over Rayleigh fading channel, so that the maximum allowable BER of the system is 10-2. In Fig. 2, the BER performance of the Chip level STBC-based MC DS-CDMA system is presented for QPSK, 8-QAM, 16-QAM and the Adaptive QAM data over Rayleigh fading channel for T=2 and S=2-depth frequency interleaving. From the Fig. 2 it is clear that the BER remains less than or equal to 10-2 for AQAM modulation. The average throughput (in bits per symbol) of the proposed adaptive modulated MC-DS-CDMA system is shown in the Fig. 3. From the Fig. 3 it is clear that the number of bits per symbol (bps) increases from 2 to 4 as the Fig. 2 The BER performance of the Adaptive-QAM modulated MC-DS-CDMA system Fig. 3 Average throughput of the Adaptive-QAM modulated MC-DS-CDMA system modulation scheme is adaptively switched from QPSK to 16-QAM. Thus using adaptive modulation, the throughput of the system is improved without affecting the required BER performance of the system by maintaining the BER less than or equal to 10-2. Conclusion In this work, a transmit diversity technique for the synchronous downlink transmission of MC-DS-CDMA based cognitive radio base station using chip level space time block coding is investigated. The proposed chip-stbc technique is suitable for fast fading channel conditions. Orthogonal variable length spreading codes, adaptive modulation, and adaptive subcarrier deactivation methods are used in the proposed cognitive radio system in order to achieve flexibility and improved performance in dynamic wireless environment. The simulation results show significant BER performance gain over conventional single antenna systems by using multiple transmitter antennas and S-depth frequency interleaving. If the number of receiver antennas is increased, better
MOHANDASS & UMAMAHESWARI: COGNITIVE RADIO TRANSMISSION BASED ON CHIP-LEVEL SPACE 447 BER performance can be achieved through maximal ratio combining (MRC). In the proposed scheme only one spreading code is used per user unlike other space time spreading (STS) schemes. Thus the total number of users supported is increased. The future work could be including interference cancellation (IC) based multi-user detection (MUD) techniques at the receiver to reduce multiple access interference (MAI) among various users, caused by multipath fading, timing offset error and subcarrier deactivation. Also, the performance of the proposed diversity technique could be analyzed for an uplink scenario. References 1 Mitola J III, Cognitive radio for flexible mobile multimedia communications, Proc IEEE MoMuC, (San Diego, CA) 1999, 3-10. 2 Farhang-Boroujeny B & Kempter R, Multicarrier Communication Techniques for Spectrum Sensing and Communication in Cognitive Radios, IEEE Commun Mag, 46 (2008) 80-85. 3 Sarath D, Nolan K E, Sutton P D & Doyle L E, Exploring The Reconfigurability Options of Multi-Carrier CDMA in Cognitive Radio Systems, Proc IEEE PIMRC, (Athens) 2007, 1-5. 4 Prasad R & Hara S, Overview of multicarrier CDMA, IEEE Commun Mag, 35 (1997) 126-133. 5 Chang C W & Kuo C C, An Interweave Cognitive Radio System Based on the Hierarchical 2D-Spread MC-DS-CDMA, Proc IEEE VTC Fall, (Ottawa, ON) 2010, 1-5. 6 Chang C W, An Interference-Avoidance Code Assignment Strategy for the Hierarchical Two-Dimensional-Spread MC-DS-CDMA System: A Prototype of Cognitive Radio Femtocell System, IEEE Trans Veh Technol, 61 (2012) 166-184. 7 Alamouti S M, A Simple Transmit Diversity Technique for Wireless Communications, IEEE J Select Areas Commun, 16 (1998) 1451-1458. 8 Hiwale A S & Ghatol A A, A reduced complexity MIMO system with antenna selection for high data rate wireless communications, J Sci Ind Res, 67 (2008) 498-504. 9 Hochwald B, Marzetta T & Papadias C, A Transmitter Diversity Scheme for Wideband CDMA Systems Based on Space-Time Spreading, IEEE J Select Areas Commun, 19 (2001) 48-60. 10 Vakil V & Aghaeinia H, Throughput improvement of STS-based MC DS-CDMA system with adaptive modulation, Elsevier Comput and Electr Eng, 36 (2010) 1147 1155. 11 Vakil V & Aghaeinia H, Throughput analysis of STS-based CDMA system with variable spreading factor in non-frequency-selective Rayleigh fading channel, Elsevier Comput and Electr Eng, 35 (2009) 528 535. 12 Yang L L & Hanzo L, Performance of Broadband Multicarrier DS-CDMA Using Space Time Spreading- Assisted Transmit Diversity, IEEE Trans on Wireless Commun, 4 (2005) 885-894. 13 D'orazio L, Sacchi C, Donelli M, Louveaux J & Vandendorpe L, A Near-Optimum Multiuser Receiver for STBC MC-CDMA Systems Based on Minimum Conditional BER Criterion and Genetic Algorithm-Assisted Channel Estimation, EURASIP J on Wireless Commun and Netw, 2011 (2011) 1-12. 14 Umair M, Khan M A & Choudry M A S, GA Backing to STBC Based MC-CDMA Systems, Proc IEEE ISMS, (Bangkok, Thailand) 2013, 503-506. 15 Al-Dhahir N, Single-Carrier Frequency-Domain Equalization for Space Time Block-Coded Transmissions Over Frequency-Selective Fading Channels, IEEE Commun Lett, 5 (2001) 304-306.