On the decoding of single delay STTC using filter bank based multicarrier modulation C. L´el´e , D. Le Ruyet, R. Zakaria Electronics and Communications Laboratory, CNAM, 292 rue Saint Martin, 75141, Paris, France [email protected], [email protected],[email protected]

carried only by the real component of the signal (assuming a 0 or π2 phase modulation term). Thus the imaginary part appears as an interference term. This interference term is source of problem in presence of a complex value channel since it destroys the real orthogonality. Therefore when combining FBMC with MIMO techniques such as STBC or STTC [8], [9], the decoding process cannot always be done in the same manner as with CP-OFDM modulation. In the case of a single delay STTC chain with 2 transmit antennas and 1 receive antennas a simple processing method to cancel the imaginary interference component has been introduced in [3]. The performance analysis of this scheme is done in [11] where we have shown a significant performance degradation due to the noise correlation compared to OFDM with STTC. In this paper, we propose a new decoding scheme based on an iterative interference estimation and cancelation. We will show that using this decoding scheme, FBMC with STTC reachs the same performance than OFDM. In section II, we give a short description of the discrete time FBMC modulation. Then, in section III, we provide an overview of the STTC single delay detection for FBMC. The decoding problem is presented and we give the details of the proposed detection technique. Performance of this decoding technique is evaluated assuming Raleigh channel between the antennas. Conclusion and perspectives are drawn in section IV.

Abstract—Filter bank based multi-carrier (FBMC) is a multicarrier modulation scheme that can be considered as an alternative to conventional OFDM with cyclic prefix (CP-OFDM) for transmission over multi-path fading channels. Compared to CP-OFDM, FBMC can provide a higher information bit rate, since it operates without the addition of a CP. Furthermore, FBMC enables the use of different pulse shapes therefore, it can provide a better frequency localization than CP-OFDM. Since FBMC is based on real orthogonality, the association with MIMO techniques needs specific decoding schemes. In this paper we consider the combination of FBMC with a single delay Space Time Trellis Code (STTC). We propose an iterative decoding based on interference estimation and cancelation which does not require any channel coding or decoding block. We show that if we omit the CP loss of CP-OFDM and by using this FBMC decoding scheme we outperform CP-OFDM in single delay STTC context, this with only a slight increase in complexity. 1

I. I NTRODUCTION Multi carrier modulations (MCM) attract a lot of attention for wire and wireless communications because of theirs capabilities to efficiently cope with frequency selective fading channels. Much of attention in the present literature emphasizes on the use of conventional Orthogonal Frequency Division Multiplexing (OFDM), which is able to fight against multi-path fading channels making use of a suitable cyclic prefix at the price of a reduction of spectral efficiency. As suggested in [1], [2], [4], Filter Bank Multi Carrier (FBMC) also called OFDM/OQAM is a MCM scheme which may be an appropriate alternative to OFDM. In FBMC each subcarrier is modulated with an Offset Quadrature Amplitude Modulation (OQAM). This principle has been introduced in [5], [6], but it is only recently [1] that FBMC has been presented as a viable alternative to OFDM. Furthermore, FBMC system is one of the most promising physical layer candidate for Cognitive Radio (CR). Compared to OFDM that transmits complex-valued symbols at a given symbol rate, FBMC transmits real-valued symbols at twice this symbol rate. Therefore a similar spectral efficiency is achieved by both systems. In practice, FBMC may provide a higher information bit rate, since it operates without the addition of a CP. All the nice features of FBMC come from the orthogonality conditions in the real field. Consequently, at the receiver side the data is

II. T HE OFDM/OQAM MODULATION We can write at the transmit antenna i the baseband equivalent of a discrete time FBMC signal as follows [1]: si [k] =

m=0 n∈Z

2π

D

dim,n g[k − nN ]ej M m(k− 2 ) ejφm,n | {z }

(1)

gm,n [k]

with M an even number of sub-carriers, F0 = 1/T0 = 1/2τ0 the subcarrier spacing, g the pulse shape and φm,n an additional phase term. The transmitted symbols dim,n are realvalued data transmitted by antenna i. They are obtained from a 22K -QAM constellation, taking the real and imaginary parts of these complex-valued symbols of duration T0 = 2τ0 , where τ0 denotes the time offset between the two parts [7], [1], [2], [4]. For a given subcarrier m and symbol time n, the real and

1 This work was supported in part by the European Commission under Project PHYDYAS (FP7-ICT-2007-1-211887)

978-1-4244-3584-5/09/$25.00 © 2009 IEEE

M −1 X X

86

ISWCS 2009

imaginary parts are driven by the phase term φm,n given by φm,n = φ0 +

π (n + m) (mod π) 2

FBMC MODULATOR

(2)

where φ0 can be arbitrarily chosen. Here, we set φ0 = 0 and g is assumed to be real-valued. Assuming a distortion-free channel, perfect reconstruction of real symbols is obtained owing to the following real orthogonality condition: ∞ X