Reduced complexity ICI cancellation scheme for OFDM DVB-SH receivers

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Abstract

Mitigating the negative effects of Doppler spread is crucial in the future Digital Video Broadcasting Satellite to Handheld (DVB-SH) terminals. The implementation of conventional methods, such us the Decision Feedback Equalization (DFE), to estimate the intercarrier interference is not suitable for low cost DVB-SH terminals. To solve this problem, we propose a new technique, and its hardware architecture, that extremely reduces the consumption of hardware resources. We demonstrate that a reduced implementation of the proposed method, that requires only six multiplications and five additions, performs similar to the DFE method that needs high complexity cells.

Introduction

The interest in mobile television systems is growing in the last years. The demand of mobile television services and added value services supported by television systems have noticeably increased [1]. This demand has led to the development of multiple technologies that are changing the mobile television scenario [2]. Well known examples are the European standards: Digital Video Broadcasting for Handhelds (DVB-Hs) [3] and Digital Video Broadcasting Satellite to Handheld (DVB-SH) [4], [5].

DVB-H and, the more recently approved, the DVB-SH standard, were proposed to provide TV reception in mobile scenarios by means of battery-limited handheld devices. DVB-H is based on delivering the TV signal using a high cost terrestrial network in a congested UHF band. Its descendant, DVB-SH, proposes a cost-effective hybrid satellite/terrestrial network in a less congested frequency band (the S-band). DVB-SH is causing a great interest in TV operators and final users, and it is being seen as the alternative technology for mass broadcasting television.

Both standards, DVB-SH and DVB-H, use Orthogonal Frequency Division Multiplexing (OFDM) modulation technique. Jointly to OFDM, Time Division Multiplexing (TDM) is also specified for the DVB-SH satellite links. OFDM technique is widely recognized as an efficient modulation technique for modern high data rate wireless communication systems. Its robustness against frequency selective fading, its multipath immunity, and its high spectral efficiency are well known advantages of this modulation technique. However, in mobile scenarios, the OFDM systems are highly sensitive to rapid time variations. These time variations, also called Doppler shift, destroy the OFDM orthogonality, yielding to intercarrier interference (ICI). ICI is responsible for a critical degradation of the received signal quality, especially when the receiver is moving at high speeds. Therefore, in order to preserve an acceptable reception quality in mobile scenarios, it is necessary to cancel the intercarrier interference.

Several methods have been proposed in the literature to estimate and mitigate the ICI effects in OFDM systems. The ICI self-cancellation schemes addressed in [6], [7] require a modification of the transmitted signal and therefore they are not applicable to existing standards like DVB-SH. In [8], [9], [10] linear equalization techniques are presented that equalize the received signal taking into account the full frequency domain channel matrix. In [11], the authors present a technique called Decision Feedback Equalization (DFE) method that uses two simple FFT processors to cancel the ICI terms of the OFDM signal. However, we will show that the complexity required by the DFE and linear equalization methods are high when compared to our proposed scheme. Other methods like the one addressed in [12], applies an iterative process to cancel the ICI, which implies to perform a deinterlever–decoder–coder–interleaver loop that is prohibitively complex in real DVB-SH receivers.

This paper proposes an efficient hardware implementation of an ICI cancellation scheme for DVB-SH that achieves the same performance as other existing methods, but highly reduces the hardware complexity.

This paper is organized as follows. Section 2 presents an overview of the physical layer of a DVB-SH transmitter and receiver. Section 3 revises the expressions of a mobile OFDM system, and a mathematical expression for the ICI term is introduced. In Section 4, a new ICI cancellation technique for DVB-SH receivers is proposed, whose performance is evaluated. Section 5 presents the hardware architecture of the proposed ICI cancellation scheme. The synthesis results are shown in Section 6. Finally, some conclusions are drawn in Section 7.

Section snippets

Overview of DVB-SH standard

The DVB-SH standard specifies a transmission system designed to deliver multimedia and data services to small handheld devices using frequencies below 3 GHz. The DVB-SH standard proposes a hybrid satellite-terrestrial network that provides a large-scale coverage.

Modeling a mobile OFDM channel

The received OFDM symbol can be expressed as (1), where Y and X are the received and the transmitted symbols in the frequency domain, respectively [16]. The term diag(H0) is a diagonal matrix made of the channel transfer function H0, NG is the additive white Gaussian noise (AWGN), and IICI is the intercarrier interference (ICI) term. This term appears when the orthogonallity between subcarriers is lost due to, for example, temporal channel variations.Y=diag(H0)X+IICI+NG

Regardless of why the

ICI cancellation schemes and proposed method

Several methods have been proposed in the literature to mitigate the effect of the ICI. Among all the proposed methods, ICI cancellation schemes based on adaptive equalization methods have proven to be very effective, such as linear equalization techniques [8], [9], [10]. Although these techniques theoretically require the inversion of large matrices, the band structure of the frequency channel matrix can be exploited reducing the complexity [10]. Another well-known solution, the Decision

Hardware architecture

The objective of this section is twofold: we will address the hardware architecture for the proposed SLM ICI cancellation scheme, and we will compare this architecture to an implementation of the DFE cancellation scheme. Hence, we will demonstrate that our proposed technique reduces the hardware complexity in DVB-SH receivers.

As mentioned before, the main function of both ICI cancellation techniques is to estimate the ICI term (IICI). This estimation is used to compensate the equalized received

FPGA resources for the SLM ICI cancellation block

Sections 4 ICI cancellation schemes and proposed method, 5 Hardware architecture have shown the minimum accuracy required to the SLM ICI cancellation subsystem not to exceed a signal to noise degradation larger than 0.1 dB. As a result, the number of non-zero diagonals of the simplified leakage matrix L has been selected to 3, whereas the accuracy to represent, in term of number of bits, the channel first derivative H1 and the quotient H1/H0 are fixed to NB_H1 = 8 and NB_Z = 10, respectively. The

Conclusions

Intercarrier interference (ICI) is responsible for the degradation of quality reception in OFDM based mobile receivers. In this paper we have proposed an efficient method to estimate and compensate the intercarrier interference in DVB-SH receivers.

The proposed method is based on a simplification of the leakage matrix that is commonly used to model the ICI, concluding that using only 3 coefficients of the leakage matrix is enough to perform similar to the DFE method. Simulation results show an

Acknowledgment

This work was partially supported by the Spanish Government under the Project TEC2010-17029.

Ana Cinta Oria was born in Huelva, Spain. She received his Master and Ph.D. degrees in Telecommunications Engineering from the University of Seville (Spain) in 2005 and 2010. Since 2005, she has been with the Department of Electronic Engineering, Escuela Superior de Ingenieros, University of Seville. Her current research interest is in OFDM systems, especially in digital video broadcasting systems.

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    Ana Cinta Oria was born in Huelva, Spain. She received his Master and Ph.D. degrees in Telecommunications Engineering from the University of Seville (Spain) in 2005 and 2010. Since 2005, she has been with the Department of Electronic Engineering, Escuela Superior de Ingenieros, University of Seville. Her current research interest is in OFDM systems, especially in digital video broadcasting systems.

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    Jorge Chávez was born in Seville, Spain. He received his Master and Ph. D. degrees in Electrical Engineering from the University of Seville, (Spain) in 1992 and 1996 respectively. Since 1992 he has been with the Department of Electronic Engineering, Escuela Superior de Ingenieros, University of Seville. His current research interests include analogue and digital signal processing, communication systems, and broadband communications systems design including PLC, DVB standards.

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    Darío Pérez-Calderón received his Telecommunication Engineering degree from the University of Seville (Spain) in 2009. Since 2008, he has been with the Department of Electronic Engineering, Escuela Superior de Ingenieros, University of Seville. His current research interest is mainly in OFDM systems, especially in digital video broadcasting systems.

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