Capacity Enhancement of MIMO-OFDM System in Rayleigh Fading Channel

Transcription

1 IOSR Journal of Electronics and Counication Engineering (IOSR-JECE) e-issn: ,p- ISSN: Volue 6, Issue 1 (May. - Jun. 2013), PP Capacity Enhanceent of MIMO-OFDM Syste in Rayleigh Fading Channel Atul Gauta 1, Manisha Shara 2 1 Departent of Electronics & Counication, M.Tech Scholar, Lovely professional university, Punjab, India 2 Departent of Electronics & Counication, Faculty of Electronics & Counication Engineering, Punjab, India Abstract: MIMO-OFDM syste in Rayleigh Fading Channel is very popular technique for obile counication now a day s for research. Here we want increase the capacity of MIMO-OFDM of syste by using adaptive odulation, Algebraic Space-Tie Codes (ASTC) encoder for MIMO Systes are based on quaternion algebras.we found that ergodic capacity has soe liitation which reduce the syste s perforance to overcoe this we use ASTC code. ASTC code are full rank, full rate and non vanishing constant iniu deterinant for increasing spectral efficiency and reducing Peak to Average Power Ratio (PAPR). Keywords Adaptive odulation ASTC code, Capacity, BER, Ergodic capacity, PAPR, Spectral Efficiency and SNR I. INTRODUCTION NOW A day s integration of Orthogonal Frequency d i v i s i on Multiplexing (OFDM) technique with Multiple Input Multiple Output (MIMO) systes has been an area of interesting and challenging research in the field of broadband wireless counication. Multiple input ultiple output (MIMO) syste using ultiple transit and receive antennas are widely recognized as the vital breakthrough that will allow future wireless systes to achieve higher data rates with l i i t e d bandwidth and power resources, provided the propagation ediu is rich scattering or Rayleigh fading[1].on the other hand, traditionally, ultiple antennas have been used to increase diversity to cobat channel fading. Hence, A MIMO syste can provide two types of gains: spatial ultiplexing or capacity gain and diversity gain. If we need to use the advantage of MIMO diversity to overcoe the fading then we need to send the sae signals through the different MIMO antennae. If we want to use MIMO concept for increasing capacity then we need to send different set of data at the sae tie through the different MIMO antennae without the autoatic-repeat request of the transission [2]. OFDM has any advantages, which ake it an attractive schee for high-speed transission links. However, one ajor difficulty is OFDM s large Peak to Average Power Ratio (PAPR). Those are created by the coherent suation of the OFDM subcarriers. When N signals are added with the sae phase, they produce a peak power that is N ties the average power. These large peaks cause saturation in the power aplifiers, leading to inter odulation products aong the subcarrier and disturbing out of band energy. Hence, it becoes worth while reducing PAPR. Towards this end there are several proposals such as clipping, coding and peak windowing. Respectively, reduction of PAPR coes at a price of perforance degradation, ainly in ters of rate and BER. This paper proposes to use the ASTC codes as powerful coding techniques for IEEE x OFDM standard cobined with PAPR schee [7]. ASTC codes can for out a good solution first to overcoe the disadvantage of OFDM odulations and second to keep a robustness regarding the BER perforances. ASTC encoder is shaped fro two well known algebraic space tie codes. The first one is called the Golden code (GC), which was proposed in It is a 2 2STBC obtained using a division algebra, which is full rate, full diversity, and has a nonzero lower bound on its coding gain, which does not depend on the constellation size. The second code is the TAST code (TC) a2 2space tie algebraic code obtained using the integer algebra, with rate R=n t =2 Sybol/uc, and diversity D= n t n r =4, where uc denotes the used code word [6].Adaptive odulation is a proising technique to increase the spectral efficiency of a wireless counication syste. In this paper we investigate the effectiveness of adaptive odulation in axiizing the spectral efficiency of a MIMO ultiuser downlink channel [11].Under an average transit power and instantaneous bit error rate (BER) constraint, the transit paraeters including the sub channel transit power and/or spectral efficiency are optially adapted in the spatial and/or teporal doain to axiize the average spectral efficiency (ASE). Two categories, the continuous rate and discrete rate, of adaptive systes were considered. In the continuous rate category, we first consider the ASE optiization proble with both power and spectral efficiency to be jointly adapted, which is referred to as a variable rate variable power (VRVP) syste. The optial power and rate adaptation policy, as well as the ASE expression, are derived. Following that, two special cases are studied, the variable rate (VR) syste and the 49 Page

2 variable power (VP) syste. The VR syste fixes the power as constant while the VP syste fixes the rate. The closed for asyptotic expressions for the ASEs of these three adaptive systes are derived. The asyptotic ASEs for VRVP and VR systes are the sae and both achieve a full ultiplexing gain. Copared with the VRVP and VR systes, the asyptotic ASE for a VP syste with different nubers of transit and receive antennas shows a constant signal-to-noise ratio (SNR) penalty, while a VP syste equipped with the sae nuber of transit and receive antennas is unable to achieve the full ultiplexing gain. In the discrete rate category, the power and rate adaptation policy and the ASE are also derived for VRVP, VR, and VP systes. It is shown that the ASE results for the continuous rate systes act as tight upper bounds for their discrete rate counterparts. In particular, for the discrete rate VR syste, we obtained a closed for expression for the ASE and show that there is a 2 3 db SNR penalty copared to the continuous rate counterpart. However, the advantages include a uch sipler adaptation rule, a better BER perforance, and a preserved full ultiplexing gain [6].We will refer to this class of schees as adaptive QAM (A-QAM) with the following noenclature. We say an A-QAM schee is XY-Z-L for X and Y representing the type of variation for rate (equivalently, constellation size) and power, respectively.three options are possible for this variation: C (Continuous), D (Discrete) and K (Constant). The Z corresponds to the type of BER constraint, which can be I (Instantaneous) or A (Average). Finally, for discrete-power schees, L is the allowed nuber of power levels per constellation [11].The paper is organized as follows. Section II, syste odel is described. Section III, ASTC encoder is described. Section IV, frequency-selective correlated rayleigh fading channel.section V, Adaptive odulation is described. In section VI, we present siulation result for different scenarios. Finally, a conclusion is given in section VII. II. Syste Model A odel of MIMO-OFDM syste with N Tx transit antennas and N Rx receive antennas is depicted in the Figure 1. Let, x i, y i and r i be the transitted signal, received signal and the Additive White Gaussian Noise (AWGN) for the I th. The sub-carrier respectively and the syste uses frequency selective channel. Then the received signal can be given as, Y i =HiSi+r i ; 0 i N S (1) In Eq. (1), Ns represent the nuber of sub -carriers H i is the channel response atrix of I th the sub-carrier that is of size N Tx *N Rx. The H i is a Gaussian rando atrix whose realization is known at the receiver and it is given as L1 H h exp( j* 2 *i*1/ N ) (2) i l S l0 In Eq. (2) hl is assued to be an uncorrelated channel atrix where each eleent of the atrix follows the independently and identically distributed (IID) coplex Gaussian distribution and L represents the tap of the chosen channel (i.e. L-tap frequency selective channel).it is assued that a perfect channel state inforation (CSI) is available at the receiver but not at the transitter. The total available power is also assued to be allocated uniforly across all space-frequency sub-channels. 50 Page

3 In MIMO-OFDM syste Ergodic capacity is define as this is the tie average capacity of a channel. It is found by taking the ean of the capacity values obtained fro a nuber of independent channel realizations. Ergodic capacity is define by equation Where NS 1 1 c E log( I N.Q ) Rx NS i0 n Tx Q H H i H i (3) In above equation E (.) denotes the Ergodic capacity and I NRx is identity atrix of N Rx *N Rx. Ρ is SNR per sub carrier N Tx is nuber of transit antenna.figure 1 shows the block diagra of MIMO-OFDM syste. We use ASTC Encoder and Adaptive QAM (Quaderature Aplitude Modulation) for transission. CP (Control Prograing) is an operating syste originally created for 8 bit processor. FFT is an efficient algorith to copute the discrete Forier transfor and its inverse.rf switch generally called Radio Frequency switch. PIN Diode is generally used to ake it operate at very high frequency. In this switch input signal is fed at one end then this signal is split in no of output signal by deux. [1, 3] III. ASTC CODES IN A FREQUENCY-SELECTIVE CHANNEL CONTEXT We consider a coherent syste over a frequency-selective correlated Rayleigh fading MIMO channel. The overall scheatic diagra of ASTC-MIMO-OFDM transceiver is depicted in Fig.1.The transitted binary source sequence bi of length L is odulated using the adaptive QAM-4 odulator. Each inforation sequence at tie n i. T n, 1 (2 n 1) 3, (2 n 1) 2, (2 n 1) 1, i ni S i S S S i i (2 ni 1) (4) Is encoded by the ASTC encoder into two strea constellations represented by the code word X Nc*Nt where N t refers to nuber of transitted antennas and Nc is the nuber of used subcarriers. By their construction the channel was under the Quasi-Static Assuption, and does take into account neither the tie variation nor the selectivity channel case. To spread their power regarding the bit rate and the BER perforance into the selective channel case with tie variation, we introduce the best perfect algebraic code known as Golden codes with other tow well faous algebraic space tie codes, TAST and DAST [6]. A. Golden Encoder The code was proposed in 2004 by a STBC obtained using a division algebra, which is full rate, full diversity, and has a nonzero lower bound on its coding gain, which does not depend on the constellation size. The code word is written as: 1 ( n (1) (2)) ( (3) (4)) i n i n i n i X n (5) i 5 ( n (3) (4)) ( (1) (2)) i n i n i ni Where i i And, i i B. TAST Encoder As shown in [7] [9], the TAST code is a space tie algebraic code obtained using the integer algebra, with rate R = Nt = 2 Sybole/uc (used code word), and diversity D = Nt Nr = 4. Each space tie layer is associated with his proper algebraic space ' in order to alleviate the proble of ISI (Inter-Sybol-Interferences). The code word is expressed as: 1 ( (1) (2)) ( (3) (4)) X ni (6) 2 ( (3) (4)) ( (1) (2)) Where, 51 Page

4 exp(i ) 2 C. DAST Encoder The DAST code is a diagonal space tie algebraic code obtained using the turned constellations of integer algebra, with rate 1 Sybole/uc, and full diversity. The code word is described as X Dast H nt. diag(m nt) 1 1 M 2 1 (7) exp(i / 4) M is the rotation atrix of n t =2 degree. [7, 8] IV. Frequency-Selective Correlated Rayleigh Fading Channel Wide-Band systes are coonly a Frequency-Selective Correlated Rayleigh Fading Channels. However the ASTC requires a nonselective flat fading channels belonging to narrow-band systes. To alleviate this proble let focus on lattice representation of a Frequency-Selective Correlated Rayleigh Fading Channels. We adopt here the Clarke channel odel. The received signal is the su of q waves; we take into account the Doppler shifts effect. To obtain a correlated Rayleigh fading channel, the autocorrelation function of {h j k } process is given by: i j rh E[h kh kq ] (8) 2 r exp(j2 f q) j (2 f qt ) h c 0 s Where J o is the Bessel function with zero order, f is the axiu Doppler shift and j is the antenna s nuber. If we guess that we have Nt (Nc+Ng) subcarrier used and the channel length is L Nt(Nc+Ng) we can represent the channel in function of the correlated Rayleigh taps h k, where Ng refers to the nuber of guard subcarrier and Nt to the nuber of transitted antennas as follows (9) In order to use the ASTC codes properly, we need to convert the channel H into N t (Nc + Ng) non selective sub-channels, The core idea is that the wide-band frequency selective MIMO channel by eans of the MIMO- OFDM processing is transferred to a nuber of parallel flat fading MIMO channels. In fact each code word xp ni will be odulated within the NcNt sub-channels, without loss of generality, now we are assuing that all subcarriers are used: 1 z N ( F I ) x p N N (10) c Nt c t,1 This transfors the frequency doain vector x NcNt, 1 into the tie doain. Where x represents the Kronecker product and F 1 represent the IFFT Matrix defined as: (11) 52 Page

5 Where, Second, to shelter the signal fro the ISI (Inter-Sybol Interference) we add the cyclic prefix (CP) or what are coonly called the guard interval, we can express this step atheatically by ultiplying the signal z 0 I Ng I Nt (12) I NC Where I is i.i.d atrix. Eventually we transit a OFDM sybol xp NcNt,1 over a selective correlated Rayleigh fading channel H, thus: 1 p y H.. N.(F I ). x (13) 1 c Nt NcNt,1 Where w is an Nt (Nc + Ng) white Gaussian noise vector. This calculation fits either with Joint Coding (JC) or Per Antenna Coding (PAC) technique. In fact, in the (JC) ethod, the inforation bit strea is first encoded and then converted into Nt parallel sub-streas of which each is odulated and apped onto corresponding antenna. Fig 1 illustrates the (JC) schee. However in (PAC) schee, the incoing bit strea is first transfored to Nt parallel sub-streas and then encoding is perfored per sub-strea. So, basically, the transitter consists of Nt OFDM transitters aong which the inforation bits are ultiplexed. At the receiver we consider the syste is coherent over a selective correlated Rayleigh fading MIMO channel. First, the cyclic prefix is reoved. This is done by discarding the first NgNr saples of y, 1 y (F I N r ) 2 y (14) N c Where 2 is defined as [0NcNgI Ng ] atrix. Second, the FFT is perfored. Together, give results as x p 1 1 N,1 {(F I ) 3(F I )} c Nt Nt N r Nt N y r (15) Where 3 is coonly called the circulant atrix defined as: H (16) The decision vector for each four sybols is then decoded at tie (n i, n i+1 ) using a sub-optiu decoder like a Zero Forcing or MMSE decoder. In the optiu decoder for the algebraic space tie code was the Shnorr-Echnerr or Sphere-Decoder, but the the ZF or the MMSE still a good candidate for such codes, because they reduce the coputational load regarding the Shnorr-Echnerr or the Sphere-Decoder without significant perforance loss: 1 p p x4,1 (17) Where, In this case we decode each 2 sybols together, thus we slice the received x 4,1 Dast into x 2, 1 Dast [7,9,10]. V. Adaptive Modulation A. Continuous Policy To obtain the optiu CC-A adaptation policy for MIMO ultiplexing we have to tackle a calculus of variations proble with two isoperietric constraints. We denote by f Ri (ˆλ), f Si (ˆλ) : R R any nonnegative rate and power candidate adaptation laws for the ith eigen channel and by R i (ˆλ) and S i (ˆλ) the optiu laws i.e. 53 Page

6 those that axiize the ASE. The power laws are noralized to the target average power. ST. Matheatically, the MIMO ultiplexing design proble is expressed as follows. ax E [ fri ( )] (ASE) (18) {f Ri },{f Si } i1 Subject to 1 E [ f ( )] 0 i1 i1 Si E [ f ( )(1 (, f,..., f, f,..., f ))] 0 Ri i R1 R S1 S (19) Where the conditional BER (noralized to the target BER, BERT) for the ith Eigen channel is defined as 1 i EH[BER i(,h) ] BER (20) T With BERi (ˆλ, H) the instantaneous BER given the predicted and the true CSI. Under the Gaussian approxiation the conditional BER can be coputed fro the signal-to-noise plus interference ratio (SINR), thus, using the usual exponential expression for MQAM, 1 8 SINRi (,H) i E H exp( ) (21) 5BER 5 fri ( ) T 2 1 It will be shown at the end of this section that this Gaussian approxiation is quite accurate due to the particular for of the optiu adaptive policy. Introduction and after soe algebra it is straightforward to obtain 2 i ii Si ( ) SINRi (,H) 2 ij fsj ( ) 1/ ji f (22) With ˆΥ ˆΞ and the average SNR defined as y=st /σ2n.according to Appendix A, the conditional BER expression in (6) can be accurately coputed by (8) at the top of the page, where xk are the zeros of the NP theorder Laguerre polynoial and Lxk the associated weight factors used for Gauss-Laguerre quadrature integration. Specifically, in expression Λj = 1 for = 2 and for > 2 j fsj ( ) (23) l1, li, j fsj ( ) fsl ( ) Which ust be interpreted as a liit1 when fsl = fsj. To perfor expectation over the predicted channel gain ˆλ, note that ˆλ = (1 χ)ξ with ξ the -diensional vector ξ = (ξi) of unordered eigen values of 1/(1 χ)ˆh ˆH H HHH. Consequently, the joint probability density function (pdf) pˆλ(ˆλ) is easily obtained fro the Wishart pdf pξ(ξ) given in which can be expressed as: aibi 2d 1 per (a) per(b) j i p ( ) ( 1) Aj(a i,b i) i e! a, b i jd (24) Where d.= NT NR, a = (ai) and b = (bi ) represent perutation vectors of {1,..., }, the function per( ) is 0 or 1, respectively, depending on whether the perutation is even or odd, and Aj(ai, bi) is defined as the (j + 1)th coefficient of the following polynoial: (a 1)!(b 1)! x L (x) L (x) d i i d d ai 1 bi 1 (ai1 d)!(b i1 d)! (25) 54 Page

7 Where Ldn (x) is the generalized Laguerre polynoial. It is shown in that the arginal pdf pξ (ξ) can be represented as follows: 2(1) d j p ( ) e Bj (26) jd With Bj defined as the (j + 1) Th coefficient of the following polynoial 1 (i1)! d d 2 x (L i1 (x)) (27) i1 (i1 d)! In general, solving the proble stated is hard due to the coupling between Eigen channels introduced through both the conditional BER (iperfect CSI induced interference) and the statistical dependence between the coponents of ˆλ. However, under certain approxiations it is possible to find an accurate closed-for adaptation policy for MIMO ultiplexing with an average BER constraint and iperfect CSI. To analyze the behavior of optiu A-QAM MIMO ultiplexing we distinguish two scenarios according to the quality of the available CSI: good quality (χ relatively sall) and bad quality (χ relatively high). [11, 12] VI. Results And Disscussions As Fig 2 show how ergodic capacity change with respect to SNR value and nuber of transitting antenna (n t ). here we use MATLAB SIMULINK R2010 for calculating Ergodic capacity. If we copare our result with first reference paper result than there is good iproveent in Ergodic capacity when using less nuber of antenna but when we using ore nuber of transit antenna then at very sall value of SNR, ergodic capacity increase rapidly. Hence we are able to overcoe the liitation of ergodic capacity with sall nuber of antenna by using ASTC encoder. We see that at n t =1 when we increase SNR the value of ergodic capacity also increase w.r.t. SNR. It does not coe study state as in the result of first reference paper. Fig nuber 3 shows the individual variation of ergodic capacity with nuber of transit antenna. When nuber of transit antenna n t =30 its value above the 120 which ean that we enhance the channel capacity by using ASTC encoder and adaptive QAM Fig. 2 SNR versus Ergodic Capacity 55 Page

8 Fig. 3: Ergodic capacity versus no. of transit antenna In Fig 4. The y axis vertices variable 10-0 actually represent the 10-0 BER and so on. If we increases the SNR then BER is reduce.it also shows in figure when SNR increases the value of BER decrease. At the 20 SNR the value our BER is below the which tell us that we iprove the syste perforance. In Figure 5, we have a plot of the spectral efficiency of adaptive odulation versus average SNR in db. We do not take into account whether or not the bits are the correct ones that were sent or not. Because we have set the target BER to a value that we believe the syste ust operate under, the adaptation syste will try to achieve that level of perforance Note that at low SNR value, the syste achieves 2 bits per sybol, and QPSK is priarily used. However, when the SNR increases, the throughput also iprove steadily, which indicates that we are beginning to use ore spectrally efficient odulation schees. The curve begins to level out at close to 30 db, as 64QAM becoes the odulation schee used ost often and QPSK is rarely used. when SNR iproves, the syste is ore able to choose ore efficient odulation schees by using adaptive QAM. Fig. 6 shows the perforance of syste by using ASTC encoder and without also tell that by using ASTC encoder we increase our syste capacity w.r.t. SNR vs BER graph analysis. Fig. 4 Syste perforance w.r.t. SNR vs. BER 56 Page

9 Fig. 5 Spectral Efficiency for Perfect Adaptive Modulation vs. Average SNR for a Rayleigh Channel Fig. 6 Syste perforance by using ASTC encoder w.r.t. SNR vs BER VII. Conclusion Ergodic channel capacity has soe liitation in MIMO OFDM syste therefore it is necessary to iprove this because it affects the syste perforance. To iprove this we use ASTC encoder because it has properties full rank full, full rate, and non vanishing deterinant for increasing rate. ASTC is also able to reduce the ajor difficulty of OFDM s Large Peak to Average Power Ratio (PAPR).As a result we find that the ASTC codes like a good coproise between a PAPR reduction schee and BER perforance. Our results also show that adaptive odulation for MIMO OFDM syste is uch ore sensitive to iperfect CSI that MIMO bea foring. We can analyze MIMO-OFDM syste and use various algoriths to optiize channel capacity. Acknowldgeent I would like to thank y friends Anupa Kuar,Aandeep, Ajay Shara, Manoj Kuar, Kapil Shara, Pankaj Shara, Abhradip Paul and all y faily eber and y teachers who help e. 57 Page

10 References [1] Prof. Jaiswal and Er. Kuar, Anil. And Singh, Anand Prakash. (2012) perforance analysis of io-ofd in Rayleigh fading channel international journal of scientific and research publication vol.2 issue 5, May 2012 [2] Sali, Alouini Mohaed. And J Goldsith, Andrea. Meber IEEE (1999), Capacity of Rayleigh Fading Channel under Different Adaptive Transission and Diversity-Cobining Techniques. [3] Xiao, chengshan. and Zheng, Yahang R. Ergodic Capacity, Capacity Distribution and Outage Capacity of MIMO Tie-Varying and Frequency-Selective Rayleigh Fading Channels Departent of Electrical & Coputer Engineering University of Missouri, Colubia, MO 65211, USA. [4] Gupta, Jishu Das. And Ziri-castro, Karla. and Suzuki, Hajie.( 2007). "Capacity Analysis of MIMO OFDM Broadband Channels In Populated Indoor Environents," in proceedings of IEEE International Syposiu on Counications and Inforation Technologies, Oct ,Sydney, pp [5] Bolcskei, Helut. And Gesbert, David. and Paulraj, Arogyaswai J. Meber, IEEE. (2002) On the Capacity of OFDM-Based Spatial Multiplexing Systes IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 50, NO. 2, FEBRUARY [6] Ahed Bannour, Mohaed Lassaad Aari, Yichuang Sun, Ridha Bouallegue,2011 On the Capacity of ASTC-MIMO-OFDM Syste in a Correlated Rayleigh Frequency-Selective Channel /11,2011 IEEE. [7] Ahad Bannour, Mohaed Lassaad Anari and Ridha Bouallegue,2010 Adaptation of ASTC in a Correlated Rayleigh Frequency-Selective Fading Channels in OFDM systes with PAPR Reduction International Journal of Wireless & Mobile Networks (IJWMN), Vol.2, No.2, May [8] J. C. Belfiore, G. Rekaya, and E. Viterbo, The golden code: a 2 x 2 full-rate space-tie code with non-vanishing deterinants, IEEE Trans. Infor. Theory, vol. 13, pp , [9] A. Bannour, M. L. Aari, and R. Bouallegue, Analysis of ASTC in a correlated rayleigh fading channel with iperfect channel estiation, International Conference on Advanced Counication Technology, 2010[10] [10] M. L. Aari and F. Gagnon, Iterative channel estiation and decoding of turbo-coded OFDM sybols in selective Rayleigh channel, Canadian J. Elect. Coput. Eng., vol. 32, no. 1, pp. 9 18, [11] Jos e F. Paris and Andr [10] M. L. Aari and F. Gagnon, Iterative channel estiation and decoding of turbo-coded OFDM sybols in selective Rayleigh channel, [12] R. Weinstock, Calculus of Variations, 1st ed., McGraw-Hill, New York, Page