Multicarrier Interleave-Division Multiple Access Communication in Multipath Channels

Transcription

1 Multicarrier Interleave-Division Multiple Access Counication in Multipath Channels Habib ur Rehan *, Muhaad Naee **, Iran Zaa *, Syed Isail Shah ** * Center for Advanced Studies in Engineering (CASE) Islaabad ** Iqra University Islaabad Capus, H-9, Islaabad. Abstract In this paper a siple ultiuser Multicarrier syste is proposed for ultiuser counication, user separation and detection using Interleave Division Multiple Access (IDMA). The perforance of IDMA is analyzed in Gaussian and Qausi Static channels. In ulticarrier Interleave Division Multiple Access (MC-IDMA) users are separated and identified by interleaving instead of codes as in Code Division Multiple Access (CDMA). It eploys chip-by-chip interleaving and detection. As a sipler for of Multicarrier Code Division Multiple Access (MC-CDMA), it also includes the benefits of MC- CDMA such as itigation against fading, ultipath effects and asynchronous transission. The siulation results show that MC-IDMA not only iproves the perforance but also ore efficient in these channels as copared to MC-CDMA. Keywords: Multicarrier-CDMA, Multicarrier-IDMA Orthogonal requency Division Multiplexing (ODM).. Introduction Multicarrier-CDMA is a ulticarrier ulti-user technique, based on a cobination of Orthogonal requency Division Multiplexing ODM and CDMA [- 3]. The cobination benefit fro the features of both ODM and CDMA. ODM divides the total bandwidth into spectrally overlapping, narrow band sub-channels. The bandwidth of sub-channel is chosen uch less than the coherent bandwidth of the channel. It transfors high rate serial data into low rate parallel data fraes and aps these fraes on orthogonal sub-carriers. ODM transfors frequency selective fading into flat fading, has high spectru efficiency and enhanced anti-ultipath fading ability [2-3]. CDMA is a ulti user counication technique based on spread spectru. It assigns each user a unique spreading code; at the receiver the users are separated on the basis of low correlation of spreading codes. A drawbac of CDMA is that its perforance degrades rapidly as the nuber of users increases further it requires a coplex receiver. Multicarrier CDMA overcoe these difficulties. A MC- CDMA syste transits N chips siultaneously by assigning each chip to a separate carrier so that each input sybol is transitted on N carriers. Signal spreading in this schee is perfored in the frequency doain. The receiver extracts the transitted sybol by correlating the signal saples at the ODM output with the code sequence used for signal despreading. MC-CDMA syste lowers the sybol rate in each subcarrier increasing the sybol durations, which iniize the ultipath fading effects of the channel [4][6]. It uses ulti-user capability of CDMA syste, allowing ultiple users to interact siultaneously with low BER using standard receiver techniques [2][8]. In MC-CDMA, odulation and deodulation is achieved by using Inverse ast ourier Transfor and ast ourier Transfor (IT/ T) algoriths [5]. The idea of IDMA was first proposed in [9]. The technique presented was a chip-by-chip iterative user detection based on chip interleaving. The interleaving was used to separate users and a sei analytical technique is proposed to estiate the BER, which provide fast and accurate ethod to predict the perforance of the IDMA syste [0-]. Also [3] discuss the bit level and sybol level interleaving. A Multi User Interference (MUI) free, CDMA transceiver is presented in [4], which rely on Chip-Interleaving, and zero padded transission, as a result ultiuser separation is achieved with low coplexity code-atching filter. In [4] an IDMA syste in ultipath fading channels is presented. It suggests the use of a rae receiver to separate the ultiple paths and then apply the sae algorith to all the paths as in conventional CDMA. In this we paper present the idea of MC-IDMA. MC- IDMA siplifies the receiver architecture in ultipath channels by transforing the convolution of channel ipulse response with the transitted signal and recoputed it into ultiplication with the channel frequency response with the frequency-transitted signal. We analyze the chip-by-chip Interleaving schee with ulticarrier counication in Gaussian channel. We use Multi Carrier counication because it transfors frequency selective fading into flat fading and its convolutional effects of channel are converted into ultiplication. By adding interleaving effects at the chip level excellent results are achieved. Reainder of the paper is organized as follows. Next section describes the syste odel for IDMA. Section 3 describes the siulation results. The paper is concluded in Section 4.

2 X 0 Spreader Interleaver 0 X Spreader Interleaver S/P I T P/S Cyclic Prefix Insert X (M-) Spreader Interleaver M- igure : MC-IDMA Transitter with spreader for ultipath itigation, user specific interleaver for User separation. Decoder Deinterleaver 0 Interleaver 0 Decoder Deinterleaver Interleaver Eleentary Signal Estiator P/S T P/S C/P Reov e Decoder Deinterleaver M- Interleaver M- Syste Model The transitter odel is shown in figure for a MC- IDMA syste, which has two parts. irst part contains Spreader, orward Error Correction (EC) encoder and Interleaver [0] and the second part is ulticarrier transitter. We assue QPSK odulation. Sae spreading code is used to spread data of M users opposed to DS-CDMA where each user requires a unique spreading code for identification. In the first part EC encoder can be different for different users or it can be sae for all users [0]. The user specific interleaver acts at the chip level, used to separate and identify each user. The interleaved EC encoded chip data is the input of Multicarrier subsyste, which in first step converts the coposite signal of all users fro serial to parallel for followed by an IT operation. IT operation odulates each chip of data on a different subcarrier and provides frequency diversity. This odulated data is then reconverted to serial for and cyclic prefixes are added to igure 2: MC-IDMA receiver with decoder and deinterleaver each sybol. These prefixes are added to ae sybols appear periodic so that convolution of the signal and the channel becoes circular [3]. The length of the cyclic prefix ust be ore than the expected delay spread so that when the signal passes through the ultipath channel, it is circularly convolved with the channel ipulse response. The receiver structure of MC-IDMA is shown in figure 2. ro the down-sapled signal, cyclic prefixes are reoved and T coherently deodulates it. The signal is then fed to Eleentary Signal Estiator (ESE). ESE algorith is used to replace ultiuser detector. ESE is a low coplexity algorith as copared to ultiuser detection algoriths. Let c ( ) be the interleaved signal. Then the output of the T bloc is given by x( t) = N M i= n= 0 = X ( n) p( t T s ) e 2πn( t its ) j ( Ts ) ()

3 Where T, T is the chip duration and s = NTc c the duration of guard band. p(t) is pulse satisfying the following relations. 0 t T p( t) = (2) 0 otherwise The low pass equivalent ipulse response of a tie varying wide sense stationary uncorrelated scattering channel with L paths is given by L = (3) l= h(;) τ t ςl() t δ( τ τl) Where t and τ are tie and delay respectively. L is the nuber of ultipath, τ l is the delay in the j th path, ς l () t i th path gain which utually independent qausi static Gaussian variable and δ is the dirac delta function. The received signal y (t) is given by yt () = xt ()* h( τ;) t (4) The received signal after down sapling is given by K r( j) = H X ( j) + N( j) (5) = K r( j) = H X ( j) + H X ( j) + N( j) (6) = r( j) H X ( j) + ζ ( j) = (7) where ζ ( j) is the distortion due to all other users except the user plus the spectru of noise. If data fro all the users is independent then using central liit theore can be approxiated by a Gaussian ( j) ζ rando variable for large K. The ESE algorith is executed on this sequence. In the first step we estiate ean and variance of r(j) using (6). Then estiate ean and variance of ζ ( j) are estiated using (7). Now estiate X (j) fro r(j) using (2). This is the chip-by-chip ESE for a synchronized channel. 3. Siulation Results In this section, we provide siulation results to illustrate the perforance of newly proposed MC-IDMA on different channel conditions. In figure 3 we copare the perforance of an uncoded MC-IDMA in Gaussian channels with MC-CDMA. To illustrate the perforance gain offered by newly proposed MC-IDMA in a Gaussian channel, we provide siulation results in ter of bit error rate (BER) verses SNR with bloc length of B = 256 and spread length S = 64. The interleaver length is equal to Bloc length ultiply by spread length (I = B * S = 256 * 64 = 6384). The perforance copared by varying the nuber of users fro to 64. It is assue that all users are initially at the sae power level and sae spreading code. Siulation result show that at relatively high SNR perforance of MC-IDMA is approached that of a single user. Also for high nuber of users, its perforance is better than that of ulticarrier CDMA. Its potential for optial perforance is illustrated in figure 4 and figure 5 with BER plotted in ultipath static and ultipath Qausi static channels. igure 4 shows the perforance of MC-IDMA in ultipath static channel. Bloc length is fixed at 256 and spread length is 64 and Interleaver length is Siulation result reveals that 2dB ore SNR is required for MC-IDMA syste to approach single user perforance. igure 5 shows the perforance of MC-IDMA in ultipath static channel. In last two siulations perforance of new schee is siulated against different spread length and bloc length. igure 6 shows the perforance of MC-IDMA as a function of spread length and figure 7 analyze the perforance of MC-IDMA as a function of bloc length. or these two siulations nuber of users and SNR is fixed. The behavior of BER as a function of spread and bloc lengths is ipressive. 4. Conclusions In this paper we analyzed ulticarrier ultiuser counication technique MC-IDMA, in which interleaver is the only ean of user separation. It has been shown with siulation results that MC-IDMA has excellent BER perforance for higher nuber of users and is ore robust to ultipath effects, and at a relatively high SNR, it approaches to the perforance of single user. Also with increased interleaver length we can iprove the perforance of ultiuser counication.

4 igure 3. Perforance of MC-IDMA in Gaussian Channels igure 4. Perforance of MC-IDMA in ultipath static channels

5 igure 5. Perforance of MC-IDMA in ultipath Qausi static channels igure 6. Perforance of MC-IDMAas a function of spread length

6 igure 7. Perforance of MC-IDMA as a function of bloc Length. References [] S. Hara and R. Prasad, Overview of ulticarrier CDMA, IEEE Counications agazine, pp Dec [2] N. Yee, J.-P. Linnartz, and G. ettweis, "Multicarrier CDMA for Indoor Wireless Radio Networs", Proc. PIMRC '93, Yoohaa, pp. 09-3, Septeber 999. [3] M. K. Sion and M. S Alouini, Digital Counication over ading Channels, A unified approach to perforance Analysis, John Wiley & sons, inc., [4] R. D. J. Van Nee and R. Prasad, ODM for Wireless Multiedia Counications, Norwood, MA: Artech House, [5] S. B. Weinstein and P. M. Ebert, Data Transission by requency-division Multiplexing Using the Discrete ourier Transfor, IEEE Trans. Coun. Tech., Vol. COM-9, pp , Oct. 97. [6] J. A. C. Bingha, Multicarrier Modulation for Data Transission: An idea whose tie has coe, IEEE Co. Mag, pp. 5-4, May 990. [7] G. L. Turin, Introduction to Spread-Spectru Antiultipath Techniques and Their Application to Urban Digital Radio, Proc. IEEE, Vol. 68, pp , Mar., 980 [8] K. azel and L. Pape, "On the Perforance of Convolutionally-Coded CDMA/ODM for Mobile Counication Systes," Proc. PIMRC '93, Yoohaa, pp , Septeber 999. [9] Li Ping, L. Liu, K. Y. Wu, and W. K. Leung, A unified approach to ulti-user detection and spacetie coding with low coplexity and nearly optial perforance, in Proc. 40th Allerton Conference, Allerton House, USA, pp Oct [0] Li Ping, Interleave-Division Multiple Access and Chip-by-Chip Iterative Multi-User Detection, IEEE Radio Counications June 2005 [] L. Ping., On Interleave-division Multiple-Access, Proc. IEEE ICC 04, Paris, pp June 2004 [2] R. Singh and L. B. Milstein, Addaptive Interference Suppression for DS-CDMA, IEEE Trans. Co. Vol. 50 No. 2, pp , Dec [3] S. Zhou, G. B. Giannais, and C. Le Martret, ``Chip- Interleaved Bloc-Spread Code Division Multiple Access,'' IEEE Transactions on Counications, Vol. 50, No. 2, pp , ebruary [4] S. Zhou, P. Xia, G. Leus, and G. B. Giannais, Chip-Interleaved Bloc-Spread CDMA versus DS- CDMA for Cellular Downlin: A coparative study, IEEE Transactions on Wireless Counications, Vol. 3, No., pp , January [5] J. Luo, K. Pattipati, P. Willett, and G. Levchu, Optial grouping algorith for a group decision feedbac detector in synchronous CDMA counications, IEEE Trans. Co. Vol. 5 No. 3, pp , March 2003.