Reduction of Interference with Linear Equalizer Using Quarter Subcarrier Mapping Scheme

Transcription

1 Reduction of Interference with Linear Equalizer Using Quarter Subcarrier Mapping Scheme Abstract MIMO - O FDM is well-known for its capability and reliability for higher data rate transmission and has been an effective multicarrier transmission technique over frequency selective channels. However, its vulnerability to frequency offset(fo ), has wiped out the orthogonality of the subcarriers and lead to inter-carrier interference(ici).besides, a time delay at the receiver which result in the Inter-symbol interference(isi) caused by severe frequency selective fading (FSF) from multipath channel between transmitter and receiver will intensely degrade the system performance. In order to overcome this, ICI-SC technique with quarter subcarrier mapping is adapted and equalization technique is appended at the receiver to filter and compensate the ISI. Thus, the objective of this paper is to evaluate PEP performance using STFBC MIMO -O FDM ICI-SC scheme with equalizer by using quarter subcarrier mapping for ICI and ISI reduction. The result indicates that the Pair-wise error probability (PEP) has improved better over STFBC MIMO -STFBC system by using quarter subcarrier mapping scheme (ICI-SC) with equalizer compared to existing subcarrier mapping scheme, and efficien t bandwidth with maximum diversity can be achieved. Index Terms PEP(Pair-Wise Error Probability),ICI-SC(Inter-carrier Interference Self-Cancellation), Inter-symbol interference (ISI),STFBC(Space Time Frequency Block Code), FO(frequency offset ) I. INTRODUCTION The major requirement in wireless communication system is to provide high data rate wireless access and high quality digital communications for various types of applications. MIMO is a fundamental technology which has attracted so much attention due to its potential of achieving extraordinary data rates and for its capability to achieve high capacity and link reliability within a given bandwidth without the need for additional transmitting power[1]. OFDM can achieve high data rate over frequency selective fading channel by converting a wideband frequency selective fading channel into a set of parallel narrowband frequency flat fading channels[].the combination of multiple input multiple output (MIMO) and Orthogonal Frequency Division Multiplexing (OFDM) is a dynamic techniques which can provides high data rates over frequency selective fading channels and is among of the top candidates in the next generation of wireless communication systems.[1] At present, OFDM is used for Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), and Local Area Networks (LANs) (IEEE 80.11a, IEEE 80.11g, wireless local area networks (e.g IEEE 80.11x and HIPERLAN/), ultra wideband based systems for short ranged wireless and the latest Long Term Evolution (LTE) for Fourth Generation (4G) in mobile network [3].MIMO-OFDM systems can enhance the data rates in frequency-selective fading channels and exhibit high spectral efficiency by concurrently exploiting the spatial, temporal, and frequency (STF) domains [5]. In this paper, a space-time-frequency block coding (STFBC) transmit diversity technique in MIMO technology, has rapidly become one of the most active research fields in wireless since it is capable of achieving maximum diversity thus, the signal quality and spectral efficiency can be obtained. [5].Nevertheless, one of the most challenging problems in OFDM is the instability to frequency offsets (FO) mismatching between transmitted and received signals, time variations due to Doppler shift or phase noise which eliminate the orthogonality of the subcarriers. This will result in inter-carrier interference (ICI) which expressively defect the performance of OFDM systems [5]. Therefore, previous researchers have proposed intercarrier interference self-cancellation (ICI-SC) to conceal the ICI problem [6]. The data symbol to be transmitted is mapped on to a pair of adjacent or non-adjacent adjacent carriers with a phase shift of π/ [7] with using the desired subcarrier mapping method. ICI is a combination of ICI cancellation modulation (ICM) and ICI cancellation demodulation (ICD) in which they will selfcancelled each other. The same data twice with the same magnitude with different polarization ( k, k+1= - k ) will be modulated or mapped over a pair of subcarriers [8].ICI cancellation demodulation operates by subtracting these two signals at the receiver. The existing subcarrier mapping such as symmetric method or adjacent method proposed by [9][10] has faced problems whereby the distance between the subcarriers and repeated subcarriers are either too far or too short and has difficulties to obtain frequency diversity gain. Apart from that, MIMO-OFDM also suffers from intersymbol interference (ISI)[11]. In order to achieve the best performance in wireless communications, an equalization technique is used to combat ISI. For this project, the linear equalizer which has only feed-forward element in the filter is used to combat ISI and to compensate the interference during transmission at the receiver. By using these filters, it is expected that PEP could be lessen due to the reducing number of ISI. ICI can be reduced and PEP will be lessen and systems become better when applying a quarter method data conversion with STFBC compared with the median subcarrier mapping scheme proposed by [4], adjacent and symmetric subcarrier mapping proposed by[9][10]. Consequently, the effect of FO in the system can be compensated, as well as ICI

2 and ISI can be reduced and maximum diversity order with an efficient bandwidth can be achieved. II SYSTEM MODEL Fig 1 :OFDM System Block Diagram with Equalizer ICI-SC technique proposed by [5] is exerted into this system whereby interference cancellation modulation (ICM) is applied to STFBC encoder with a repeating scheme in which r= indicates the symbols are being repeated twice but the repeated symbols are signed-reversed. In this paper, the PEP performance will be evaluated by using the quarter method as proposed as below: 0) ) 4 8 ) 4 8 0) N 1) 8 N ) 4 N 1) 4 ) 4 8 1) 4 8 ) 4 4 (0) ) 4 8 ( ) 4 8 (0) N ( 1) 8 N ( ) 4 N ( 1) 4 ( ) 4 8 ( 1) 4 8 ( ) 4 4 (1) Firstly, the data input at the transmitter side will be generated in serial format to perform serial to parallel conversion. The serial data streams represent the data information to be transmitted. Using the ICI-SC scheme, the binary bits are first mapped onto constellation points, and then the serial symbols are converted into a block of N/ parallel symbols.n is the total number of subcarriers of OFDM system. Then, the parallel symbols with the width N/ are mapped onto N subcarriers of OFDM system by the ICI-SC scheme.inverse Fast Fourier Transform (IFFT) as demodulator at receiver will modulate or transfer the symbols that already mapped into STF codewords. The IFFT at transmitter converts frequency domain data into time domain signal while maintaining the orthogonality of subcarriers. STF codeword will spread all the symbols along space,time and frequency domains. At receiver, the role of FFT is to recover the data symbols from the orthogonal subcarriers. The receiver converts serial input data to parallel form and the symbol transformation is performed by FFT [].The output from FFT will be accompanied with distortions or interferences including ISI.Figure 1 above shows system model of a new subcarrier mapping scheme (ICI-SC Technique) for STFBC MIMO-OFDM with equalizer. Multipath distortion can be cancelled effectively by using equalization with large filter length. It is basically a transversal filter which can be used at either pass band or base band []. For delay intervals in equalization, it can be less than or equal to the maximum delay within the filter structure []. Equalizer needs to be implemented in order to eliminate ISI. The equalized output is de-mapped, de-interleaved, and then convolutionaly decoded to recover the original data words [].The data words will then be multiplexed to gain the original message information. It is important that the equalization is needed at the receiver. By continuous updating its filter coefficients, the equalizer can minimize the error between actual output and desired output. The PEP performance will be evaluated in presence of a linear equalizer in MIMO-OFDM system. The equalizer is expected to reduce the ISI and reduce ICI thus improving the PEP performance. This research is very beneficial to our telecommunication industry as many practical systems in wireless communication can employ this technique to achieve high data rates transmission and spectral efficiencies with significantly best performance. The paper is organized as follows. In section II, the MIMO-OFDM system with STFBC and FO effect, with a linear equalizer appended at receiver is shown and will be discussed whereas in section III the derivation description of the PEP algorithm takes place. In section IV, simulation results will be analyzed by comparing different types of subcarriers mapping scheme (ICI-SC technique) with linear equalizer in terms of PEP performance with FO effect. Last but not least, conclusions are given in section IV. III. DESIGN STYLE OF PEP There are two kinds of techniques being considered in this research which are: A.Pair-wise error probability (PEP) Pair-wise error probability (PEP) is a vital in studying the reliability of a communication system. PEP is defined as the probability of detecting one symbol when another (different) symbol is transmitted.thus,it is is the transmitted symbol and the detected symbol. It is used when determining the union bound to sum the overall pairwise error probability. By assuming D and D are two different matrices related to two different STFB codewords and therefore the pair-wise error

3 probability (PEP) between D and D can be upper bounded as [11].The detected symbol is depends on the previous symbol, and possibly the following symbol. For example, given that,d is the transmit codeword, and D is the decoded P is the probability the codeword containing faulty. D D received signal vector is close to the D, the decoded codeword containing faulty. Therefore we need to minimize the upper bound of this PEP equation to get a minimum error and thus, a better system performance. The PEP performance will be evaluated with MIMO-OFDM system with equalizer. The baseband model of OFDM can be illustrated as shown above in Figure 1. N N 1 N D P D N i1 () B. Space Time Frequency Block Coding Schemes Space Time Frequency Block (STFB) coding schemes are used to enhance the reliability and system performance by exploiting from the diversity of space, time and frequency inherent in MIMO-OFDM system. The coding distributes symbols along transmit antennas, time slots and OFDM sub channels. A STFB codeword may occupied with several OFDM symbols which can increase the diversity order [9][10]. TABLE I: CODING IN STFBC METHOD [8] Antenna and Ant 1 Ant time slot Frequency (T1) i (T) f1 k k+1 f - k+1 * k * C. Linear Equalizer The most common type of channel equalizer used in practice to reduce ISI is a linear transversal filter with adjustable coefficients {C i }. Adaptive equalizers update their parameters on a periodic basis during the transmission of data, thus, they are capable of tracking a slowly timing-varying channel response. Figure below shows a block diagram of a system that employs a linear filter as a channel equalizer [1]. Fig : Block diagram of a system with linear equalizer IV. SIMULATION RESULTS TABLE II: SIMULATION PARAMETERS PARAMETERS VALUES IFFT size 18 Mapping scheme Numbers of OFDM symbols Bits per OFDM symbols 64-QAM 100 N*log(M) Number of carriers 7 Channel Multipath Rayleigh Fading Frequency offset 0.05 The STFBC design methods are simulated for different subcarrier mapping scheme ICI-SC technique such as quarter, median, symmetric and adjacent method for ICI-SC in MIMO- OFDM system. The results are compared in terms of their PEP performance in MIMO-OFDM system with linear equalizer appended at the receiver. The simulation conditions are selected as in the Table 1.By using OFDM base, which the systems to be examined have 7 subcarriers and the simulation is using MATLAB programming

4 PEP PEP PERFORMANCE FOR STFBC MIMO-OFDM SYSTEM PEP-fo=5%-quarter method with linear equalizer PEP-fo=5%-median method with linear equalizer PEP-fo=5%-symmetric method with linear equalizer PEP-fo=5%-adjacent method with linear equalizer performance as compared to other subcarrier mapping scheme (ICI-SC technique).the ICI effects due to FO can be reduced by using a STFBC ICI-SC and by adding equalizer at the receiver can help to diminish the ISI effect. V. CONCLUSION Eb/No (db) Fig 3. PEP comparison using different types of mapping with linear equalizer From figure 3 above, PEP with data conversion using different subcarrier mapping scheme (ICI-SC techniques) is evaluated. In this paper, the simulation using FO=5% is performed. PEP for STFBC with FO=5% with the new data subcarrier mapping scheme (ICI-SC technique) outperformed STFBC with others. At E b /N o =1db, the PEP for quarter method is the best which is PEP=110-6.On the other hand, the median method yields in PEP=810-5 whereas the symmetric PEP=110-5 and the adjacent method yields in PEP=9x10-4.The median method proposed by [8] can give a good PEP performance since the distance between subcarriers using median method is not too far nor too short, yet it could not challenge the quarter method, which provides the optimal distance for subcarriers and achieve maximum frequency diversity with maximum diversity order and thus results in the best PEP performance. The ICI is most likely to occur especially at the adjacent positions since the subcarriers are closer to each other. The ICI coefficient gradually changes with respect to the position of subcarriers.this explains why adjacent method results in the worst PEP performance. The distance between subcarriers using symmetric method is too far and using adjacent method is too short, therefore in order to produce a system with low ICI and high diversity order, the subcarrier distance must achieve optimal distance with maximum frequency diversity system so that it will have a better PEP performance with less interference.besides, linear equalizer which uses a linear transversal filter with adjustable coefficient {C i } has helped to reduce ISI as well [11]. When the E b /N o is increased, PEP will be decreased. This equation proves that pairwise error probability is inversely proportional to the noise spectral density. The greater the value of E b /N o, the smaller the value of PEP, means that when E b /N o is higher, the lesser PEP value which indicates the system is efficient. For overall results, it can be seen that the quarter subcarrier mapping scheme offers an optimal distance with maximum frequency diversity with the best PEP In this paper, quarter subcarrier mapping (ICI-SC technique) codes is used to evaluate the PEP performance in STFBC MIMO-OFDM system to reduce the ICI due to FO. As a result, the PEP for quarter subcarrier mapping scheme (ICI-SC) technique for STFBC MIMO-OFDM is the best. To sum up, the system achieves a maximum frequency diversity gain with optimal distance, a higher SNR, and less noise which contributes to the best PEP performance when using a quarter subcarrier mapping method with linear equalizer. In future, the equalization techniques can be performed by using adaptive equalizer and turbo equalizer. REFERENCES [1] Jaya Kumari, J., MIMO-OFDM for 4G Wireless Systems, International Journal of Engineering Science and Technology, (7), , 010 [] Miller, L. E., & Lee, J. SCDMA Systems Engineering Handbook. London, United Kingdom: Artech House Inc.,1998 [3] Bhasker Gupta and Davinder S. Saini, BER Performance Improvement in Coded OFDM Systems using Equalization Algorithms,Advanced Materials Research,Vol ,pp ,Department of Electronics and Communication, Jaypee University of Information Technology,Waknaghat,Solan H.P.,INDIA 17315,01 [4]A.Idris,K.Dimyati,S.K.Syed Yusof,D.Ali, Pairwise Error Probability of a New Subcarrier Mapping Scheme (ICI-SC Technique) for STFBC MIMO-OFDM System, Australian Journal of Basic and Applied Sciences, 5(5): 73-86, 011 [5]A.Idris,K.Dimyati,S.K. Syed Yusof,D.Ali, Interference Self- Cancellation Schemes for Space Time Frequency Block Codes MIMO-OFDM system, IJCSNS International Journal of Computer Science and Network Security, Vol.8 No.9, September 008 [6] Y.Gong and K.B Letaif, Space frequency time coded OFDM for Broadband wireless communications, in Proc.IEEE Global Telecom Communication, Conf.(GLOBECOM),San Antonio,pp ,001 [7] Zhao, Y. and S.G. Haggman, Intercarrier interference selfcancellation scheme for OFDM mobile communication systems, IEEE Transactions on Communications, 49: , 001 [8] A.Idris,K.Dimyati,S.Kamilah, A new data conjugate Intercarrier(ICI) Self-Cancellation for ICI Reduction in Space Time Frequency Block Codes MIMO-OFDM System, in Proc.IEEE Second International Conference on Computer and Network Technology,010. [9] Slimane Ben Slimane, K.Sathanathan, R.M.A.P. Rajatheva, Analysis of OFDM in the Presence of Frequency Offset, and A

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