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ABSTRACT. This work is aimed at investigating the use of Multi Carrier CDMA (MC-CDMA) techniques in variable bit-rate transmission over geo-stationary ...
USE OF MULTICARRIER-CDMA TECHNIQUES FOR VARIABLEBIT-RATE TRANSMISSION SYSTEM OVER GEO-STATIONARY SATELLITE NETWORKS Claudio Sacchi, Gianluca Gera, Carlo S. Regazzoni University of Genoa – Department of Biophysical and Electronic Engineering (DIBE) Signal Processing and Telecommunications Group (SP&T) Via Opera Pia 11/A I-16145 Genoa (Italy) Phone: +39-010-3532674 Fax: +39-010-3532134 E-mail: [email protected] ABSTRACT This work is aimed at investigating the use of Multi Carrier CDMA (MC-CDMA) techniques in variable bit-rate transmission over geo-stationary (GEO) satellite channels by means of realistic simulations. It is known by literature that MC-CDMA techniques are much more resilient with respect to multi-user interference effects in LEO satellite channels than singlecarrier DS/CDMA ones. Moreover, MC-CDMA exhibits a natural capability to deliver multirate services simply by assigning to each user a variable-cardinality set of subcarriers. The achieved simulation results clearly confirmed the expected improved robustness of MCCDMA techniques transmitting multirate data streams also in GEO satellite channels, with respect to state-of-the-art DS/CDMA transceivers. 1. INTRODUCTION In these last years, the a considerable amount of R&D activities were carried on about the actual applications of Spread Spectrum and CDMA techniques to satellite communication systems working both on geo-stationary (GEO) and low-earth-orbit (LEO) constellations [1]. The advantages involved by the use of CDMA techniques in wireless communications are well known. The main problem involved by the use of CDMA in the geo-stationary satellite environment consists in the heavy limitation of capacity due to multi-user interference (MUI) [2]. This work1 deals with the experimental study of a variable-bit-rate multimedia transmission link between a earth terminal and a GEO satellite, using a Multicarrier-CDMA (MC-CDMA) modulation and multiple access system. MC-CDMA techniques, whose basic concepts were introduced by Y.P. Linnartz in 1993 [4], are strictly derived by OFDM ones. OFDM transmission raised a great interest among researchers and developers due to its fading resistance, as it allows high bit-rate transmission over hostile radio channels [3]. MC-CDMA are classified as Spread Spectrum techniques, as a single data bit is modulated over orthogonally-spaced multiple carriers [4][5], with a consequential spectral spreading of the transmitted signal. A favourable aspect of OFDM and MC-CDMA consists in the possibility of providing full digital transceiver implementation by means of FFT and IFFT [3]. Moreover, MC-CDMA exhibits a natural inclination to variable-bit-rate (VBR) transmission. VBR services can be easily managed by assigning to each user a variable-cardinality set of subcarriers depending on each bit-rate request [10]. Results shown in [4] and [5] pointed out a significant improvement of the BER performances and capacity yielded by a MC-CDMA system, with respect to a DS/CDMA system using a rake receiver, working in the same condition of asynchronous transmission over an indoor wireless multipath channel and absence of power control. This improvement is due to the orthogonality inherent to MC1

This work has been partially supported by Italian National Research Council (CNR) and by Italian National Inter-University Consortium in Telecommunications (CNIT) within the framework of the “5% Multimedialità” research project.

CDMA techniques, which is retained also in multipath fading channels [5]. Actually a frequency-selective channel is split into a multiple set of flat-fading channels, thus making easier channel equalisation and information recovery. This fact is not verified for DS/CDMA systems, because multipath fading destroys the orthogonality of user codes and the resulting MUI may render symbol recovery impossible. In [10] an actual example of multi-code DS/CDMA satellite modem working over the Ka-band is dealt. Even though, the geostationary Ka-band channel exhibits a slow non-selective fading, the capacity of the system in terms of number of codes allowed to simultaneous transmission is strongly reduced by multi-user interference. The solution considered in [10] for improving system capacity was the introduction of a trained LMSE multi-user detection algorithm. Another solution considered in literature is to combine DS/CDMA with TDMA in order to assure the orthogonality among different users [11]. The use of MC-CDMA systems for variable-bit-rate (VBR) transmission has been already investigated for what concerns the upstream transmission over the L-band frequency rang in upstream LEO satellite networks. In particular, the BER performances of a VBR MC-CDMA system using state-of-the-art channel equalisation and orthogonality-restoring detection techniques has been investigated in [11]. The efficient detection of multi-user MC-CDMA signals in LEO satellite environments has been already dealt in [12], [13] and [14], by considering different approaches. Particularly, the use of adaptive MMSE/PIC multi-user detection is described in [12], whereas the application of advanced neural-network based receivers has been considered in [13] and [14] in order to improve the system performances in presence of different MUI levels and frequency selective channel distortions. In our analysis, the asynchronous, multi-user, and variable-bit-rate transmission over an upstream GEO satellite channel in the Ka-band frequency range was tested by means of laboratory simulations. In particular, we analysed the robustness of MC-CDMA techniques with respect to the multi-user interference, which is the most relevant factor of capacity limitation in Ka-band channels (at least in clear-sky conditions). The paper is structured as follows: Section 2 will contain the system description. Section 3 will deal with the simulation results and finally Section 4 will draw the paper conclusions. 2. SYSTEM DESCRIPTION The block diagram of the m-th user’s (m = 1..M) variable-bit-rate MC-SS transmitter considered for simulation is depicted in Figure 1. cm[1]

cm[2]

am(t)

sm(t) COPIER cm[Nm]

OFDM block

Figure 1. VBR MC-SS transmitter The transmitted binary data stream of the m-th user a m (t ) is copied into N m parallel streams. Each copied stream is multiplied by a binary pseudo-random coefficient c m [i] ∈ {−1,1}, i = 1..N m . The pseudo-random vector c m is actually the signature code of the m-th transmitter, distinguishing it by the other transmitters of the system. Then, the signed data streams are sent to an Orthogonal Frequency Division Multiplexing (OFDM) block, working at intermediate

frequency (IF). The number of subcarriers attributed to the m-th user for signal multiplexing in the frequency domain is equal to N m . Such a value can be regarded as the actual processing gain of the MC-SS transceiver. As a fixed amount of bandwidth is employed for transmission, each user’s MC-SS modulator is provided by a different number of orthogonal sub-carriers N m , depending on the user’s bit-rate. The users transmitting at the highest bit-rate will receive the smallest number of carriers, whereas the users transmitting at the lowest bit-rate will receive the highest number of carriers, thus complying with the usual trade-off between transmission speed and protection against channel noise [6]. Thus, the signal transmitted by the m-th user has the following formulation [11]:  +∞ N m −1  2πj ( f c + Fm ,i )t s m (t ) = Re ∑ ∑ c m [i ] a m [k ]e pTm (t − kTm ) (2.1) k = −∞ i =0  In a MC-CDMA VBR transmission, the orthogonal subcarrier set allocated to each user (and hence the term Fm ,i of equation 2.1) must be chosen so that the correct orthogonal spacing among subcarriers is ensured to the different user classes. Following a common approach (see e.g. [6]), C classes of variable-bit-rate users are assigned as follows: rc = 2 c −1 r c = 1,..C (2.2) where r is the symbol-rate of the slowest users (i.e. class 1) given as: r=

1 (2.3) T log 2 K

where log 2 K is the number of bits mapped in a data symbol, and T is the bit duration of slowest users. The formula providing the correct orthogonal spacing of the subcarriers for the m-th user of class c m is given in (2.4): f m,i =

(

)

F c m −1 2 − 1 r + iFrc m i = 0..N m 2

(2.4)

where f c is the intermediate frequency, F is the subcarrier spacing factor [4], Tm is the time of duration of a single bit transmitted by the m-th user, and pT (t ) is the pulse-shape m

waveform, assumed for simplicity as a rectangular pulse of amplitude 2 P (P is the transmission power) and duration Tm . In this paper, it has been hypothesised that the “slowest” users have to transmit data at a 512 Kb/s over 64 orthogonal carriers, other users at 1 Mb/s over 32 orthogonal carriers and the “fastest” users at 2 Mb/s over 16 orthogonal carriers (i.e. three different classes of VBR users). The signature codes c m [i ] has been chosen in the tree-structured mutually orthogonal and variable-length set described in [6]. The PN codes belonging to such a set are known as OVSF (Orthogonal Variable Spreading Factor) sequences; they are also used as short codes in 3rd generation W-CDMA wireless standard. The first four levels of binary sequence tree are depicted in Figure 2. The set of codes at each level of the tree is a mutually orthogonal Walsh set. Couples of codes belonging to different levels of the tree are also mutually orthogonal,

excepting in the case when one of the codes belonging to a higher level is father of one or more codes belonging to a lower level.

Figure 2. Tree structure of the OVSF codes set The asynchronous MC-CDMA VBR upstream transmission in is performed upon the modality shown in Figure 3. M MC-SS earth terminals send their multirate data-streams over the same bandwidth to a GEO satellite with random delays τ m m = 1..M , assumed as uniformly distributed within the bit duration time interval Tm . For sake of simplicity, we assume here that the GEO satellite is non-regenerative: this means that the overall tasks of demodulation are performed at the base-station.

s1(t-t 1) GEO VBR MC-SS transmitter #1

U P LINK

D O WNLINK

s2(t-t 2)

y(t)

VBR MC-SS transmitter #2

sM(t-t M)

BASE STATION

VBR MC-SS transmitter #M

Figure 3. VBR multi-user MC-CDMA transmission Two kinds of digital modulation have been used in the OFDM block of Figure 2: • Conventional BPSK modulation [8]; • Robust Quadrature Spreading (QS) modulation, proposed for MC-CDMA systems in [9]. The information bit is transmitted over two subcarrier components: the in-phase component (I) and the in-quadrature component (Q). In such a way, the robustness of the MC-CDMA modulation with respect to channel noise and MUI is improved. The cost to be paid is the doubling of the bandwidth with respect to the BPSK case. We considered the transmission over the 27-31GHz-bandwidth portion, usually referred to the Ka-band uplink [16]. From measurements shown in [16], we consider the “clear sky” conditions approximated with a single-ray AWGN channel. In fact, typical Ka-band

propagation impairments are related to rain fading and scintillation phenomena [1] [16], whose time variation is very slow as compared with the bit duration, so to be considered without losing generality as time-invariant. Moreover, such attenuation sources are not frequency-selective, at least referring to the transmitted signal bandwidth. The extension of the proposed analysis to a Ka-band channel working in generic atmospheric conditions should only consider the insertion of supplementary attenuation terms due to rain fading and scintillation in addition with the free-space loss. From the considerations made above, the received signal y(t) during a single bit period can be expressed as follows:  M +∞ N m −1  2πj { f c + Fm , i }t − 2πj { f c + Fm , i }τ m  y (t ) = 2 PR Re c m [i ]a m [k ]e e  + n(t ) t ∈ [kTm , (k + 1)Tm ] (2.5)  m =1 k = −∞ i = 0 

∑∑∑

where PR is the received power and n(t) is the Gaussian noise. One can notice from 2.3 that the transmission delay of each user is substantially translated in a phase delay. The detection of the wanted signal is performed by means of a coherent matched filters bank as shown in [4] and [15], one for each intended user. The asynchronous transmission delay of each user τ m is assumed to be known by the intended receiver. Also ideal carrier recovery is assumed as hypothesis for the receiver. Of course, asynchronous transmission delay will generate multiuser interference (MUI) due to uncorrelated users with respect to the wanted one. In particular, the generated MUI depends on the spectral correlation of the signature codes [15], mainly related to the phase delay involved by the asynchronous transmission delay, whereas the MUI in DS/CDMA systems is mainly related to the time correlation of the spreading codes. Each filter bank performs: a) re-multiplication of the received signal for the pseudo-random coefficient of the signature sequence; b) de-multiplexing of the data stream transmitted over the different orthogonal subchannels (i.e.: inverse OFDM operation); c) symbol recovery. Note that no channel estimation and equalisation is performed at the receiver stage, as the satellite channel is regarded as AWGN. In the next section we will show some simulation results in order to show what performances should be expected by the use of MC-CDMA physical protocol in GEO satellite environments. 3. SIMULATION RESULTS The system simulation has been performed by using a baseband equivalent model [17] of a MC-CDMA system working over AWGN channel. The simulator was implemented in MATLAB SIMULINK 5.3 environment. The configuration of the VBR user classes chosen for simulations is reported in Table 1. TOTAL

64 CARRIERS

32 CARRIERS

16 CARRIERS

NUMBER OF USERS 4 5 6 7 8

USERS

USERS

USERS

1 1 1 3 4

2 3 4 3 3

1 1 1 1 1

Table 1. Multi-user VBR transmission configuration

The tested user is the 1Mb/s one (i.e. Nm = 16). The first simulation results concerns with the equivalent baseband spectrum of the transmitted signals for each user class, characterised by different subcarrier numbers (see Figure 4a, 4b and 4c). It is easy to see the intrinsic diversity inherent to the MC-CDMA transmission, where data bits are orthogonally multiplexed among different subcarriers. In our simulation, we considered a subcarrier spacing factor F=2 in order to ensure the complete subchannel separation.

(a)

(b)

(c) Figure 4. Equivalent low-pass spectrums: a) signal transmitted by the 64 carriers users, b) signal transmitted by 32 carrier users, c) signal transmitted by 16 carrier users In Figure 5, the BER performances versus SNR for M=8 transmitting users are shown. In Figure 6, BER performances versus user number, supposing a SNR = 5dB are depicted.

Figure 5. BER performances versus SNR of a 8-users VBR MC-CDMA uplink transmission system over a geo-stationary satellite network (clear-sky conditions)

Figure 5. BER performances versus number of users for a VBR MC-CDMA uplink transmission system over a geo-stationary satellite network (SNR = 5dB, clear sky conditions) In all cases, the performances provided by the asynchronous MC-CDMA system are compared with the ones provided by a conventional DS/CDMA system with BPSK modulation, spreading codes like the one described in [6] and supposing ideal synchronisation. Line graphs of Figure 4 and Figure 5 clearly expose the considerable performance improvement involved by the exploitation of MC-CDMA techniques for asynchronous VBR transmission over geostationary upstream channels with respect to more conventional DS/CDMA techniques. This is due to the improved robustness of MC-CDMA techniques with respect to detrimental effects of MUI, even though spreading codes with notideal correlation properties are used in variable-bit-rate transmission applications. DS/CDMA systems can achieve comparable performances only by using high-complexity multi-user detection algorithms, as shown in [10]. 4. CONCLUSIONS AND FUTURE WORKS This paper presented a performance analysis of a variable-bit-rate asynchronous multi-user MC-CDMA transmission over a GEO satellite network. The achieved results show that MCCDMA techniques can provide an improved robustness with respect to the effects of the multi-user interference with respect to conventional DS/CDMA systems. For this reason, the use of multicarrier modulation can be proposed as a valuable alternative for multimedia satellite transmission in multi-user and variable-bit-rate applications. Future works should be related to the effects on system performances of non-linear distortions involved by highpower amplifiers (i.e.: the Peak-To-Average Power Ratio problem mentioned in [15]), nonideal behaviours of hardware components (e.g. phase noise in oscillators, timing unbalances, etc.), more efficient signature coding approaches for VBR transmission, efficient and lowcomplexity multi-user detection algorithms, etc. ACKNOWLEDGEMENTS Authors wish to thank Dr. Manuele Barbieri and Dr. Alberto Ghiglione for their valuable contribution in the collection of the paper results.

[1] [2]

[3] [4] [5] [6] [7]

[8] [9]

[10]

[11]

[12]

[13]

[14]

[15] [16] [17]

REFERENCES G. Maral, and M. Bousquet, “Satellite Communications Systems” (3rd Edition), John Wiley & Sons., Chichester (UK): 1998. S. Hwang, J. Ahn, T. Kim, K. Whang, “The effects of Rain Attenuation in a QuasiSynchronous CDMA Return Link for a Ka-Band Satellite Communication System”, IEICE Trans. on Fund. Electronics, Vol.E81-A, No.7, July 1998, pp.1436-1443. L. Hanzo, W. Webb, T. Keller, “Single and Multi-carrier Quadrature Amplitude Modulation”, Wiley, Chichester (UK): 2000. S. Hara, R. Prasad, “Overview of multicarrier CDMA”, IEEE Communications magazine, pp.126-133, December 1997. Z. Wang, G. B. Giannakis, “Wireless Multicarrier Communications, where Fourier meets Shannon”, IEEE Signal Processing Magazine, May 2000, pp. 29-48. E.H. Dinan, and B. Jabbari, “Spreading Codes for Direct Sequence CDMA and Wideband CDMA Cellular Networks”, IEEE Comm. Magazine, Sep. 1998, pp. 48-54. C. G. F. Valadon, G. A. Verelst, P. Taaghol, R. Tafazolli, B. G. Evans, “CodeDivision Multiple Access for Provision of Mobile Multimedia Services with a Geostazionary Regenerative Payload”, IEEE Journal on Selected Areas in Communication., Vol.17, No.2, February 1999, pp.223-237. J. G. Proakis, “Digital Communications”, 3rd Edition, McGraw-Hill: New York, 1995. S. B. Sliname, “MC-CDMA with quadrature spreading for wireless communications systems”. European Transactions on Telecommunications (ETT), vol. 9, no. 4 , JulyAugust 1998, pp. 371-378. C. Sacchi, L.S. Ronga, “ A DSP-based DS/CDMA modem for multimedia applications over geo-stationary satellite networks”, 2001 IEEE Intenat. Conf. on Acoust. Speech and Signal Proc. (ICASSP2001), available on CD-ROM. C. Sacchi, G. Gera, C. Regazzoni, “Performance evaluation of MC-CDMA techniques for variable bit-rate transmission in LEO satellite networks”, Proc. of 2001 IEEE International Conference on Communications (ICC 2001), Helsinki (SF) 5-11 June 2001, pp. 2650-2654. F. Petre, M. Engels, M. Moonen, B. Gyselinckx, H. De Man, “Adaptive MMSE/pcPIC-MMSE Multiuser Detector for MC-CDMA Satellite System”, Proc. of 2001 IEEE International Conference on Communications (ICC’01), Helsinki (SF), June 11-14 2001, pp. 2640-2644. G. Gera, C. Sacchi, C. Regazzoni, “A neural network-based receiver for synchronous MC-CDMA variable-bit-rate transmissions over LEO satellite channels”, 2001 IEEEEURASIP Non-linear Signal and Image Processing Worskshop (NSIP2001), in press. G. Gera, C. Sacchi, C. Regazzoni, “ Neural Network-based Techniques For Channel Equalisation in Asynchronous MC-CDMA Variable-Bit-Rate Transmissions over LEO Satellite Networks”, Proc. of 4th International Symposium on Wireless Personal Communications (WPCM01), Aalborg (DK), 9-12 September 2001, in press. B. M. Popovic, “Spreading Sequences for Multicarrier CDMA Systems”, IEEE Trans on Comm, Vol.47, No.6, June 1999, pp. 918-926. D. Rogers, L.J. Ippolito, F. Davarian, “System Requirements for Ka-band EarthSatellite Propagation Data”, Proc. of IEEE, Vol. 85, No. 5, June 1997, pp. 810-820. M.C. Jeruchim, P. Balaban, K. Shanmugan, “Simulation of Communication Systems”, 2nd Edition, Kluwer Academic Publishers: 2000.