Sili Lu

Coherent and Differential ICI Cancellation for Mobile OFDM with Application to DVB-H

We develop a reduced-complexity hybrid frequency/time-domain orthogonal frequency division multiplexing (OFDM) channel estimation algorithm for high-mobility scenarios where the channel varies significantly within each OFDM block, resulting in severe intercarrier interference (ICI). The algorithm exploits the banded and symmetric structure of the channel matrix in the frequency domain and its sparse structure in the time-domain to achieve significant complexity reductions, which we quantify for the Digital Video Broadcasting-Handheld (DVB-H) system. Furthermore, we compare coherent and differential mobile OFDM detection for DVB-H.

A Novel SFBC-OFDM Scheme for Doubly Selective Channels

Effective intercarrier interference (ICI) mitigation for multiple-input-multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) requires accurate channel estimation, which is very challenging due to the large number and fast time-varying nature of the channel parameters to be estimated using scattered pilots. We present a novel space-frequency block coding OFDM (SFBC-OFDM) scheme for doubly selective channels and a reduced-complexity channel-estimation algorithm, which exploits the banded and sparse structure of the channel in the frequency and time domains, respectively. Furthermore, we design a finite-impulse-response minimum-mean-square-error (FIR-MMSE) ICI cancellation algorithm for mobile SFBC-OFDM and demonstrate its effectiveness for digital video broadcasting-handheld (DVB-H) systems.

OFDM Interference Mitigation Algorithms for Doubly-Selective Channels

Frequency division multiplexing (OFDM) system may suffer from significant Inter-carrier Interference (ICI) and Inter-block Interference (IBI) under a time- and frequency- selective (or doubly-selective) channel. We propose a novel hybrid architecture where a time-domain FIR filter mitigates IBI by shortening the channel to the cyclic prefix length. This is followed by ICI cancellation in the frequency-domain using few FIR taps per subchannel combined with successive cancellation. We also propose an iterative joint data and channel estimation scheme to improve the channel estimation performance.

A novel CDD-OFDM scheme with pilot-aided channel estimation

Cyclic delay diversity (CDD) is a low-complexity standard-conformable transmit diversity scheme for coded orthogonal frequency division multiplexing (OFDM) systems. However, it makes channel estimation more challenging due to the increased frequency-selectivity of the equivalent single-input single-output channel. In this paper, we propose a novel CDD-OFDM scheme with pilot-aided channel estimation for any number of transmit antennas. By alternating and optimizing the cyclic delay parameter over adjacent OFDM symbols, we design a simple yet efficient channel estimation scheme and illustrate its excellent performance for the DVB-T application.

Digital Baseband Compensation of I/Q Imbalance in Mobile OFDM

I/Q imbalance and high mobility in OFDM systems result in performance-limiting intercarrier interference (ICI). However, the nature of ICI due to each of them is quite different. Unlike previous works which considered these two impairments separately, we develop a unified mathematical framework to characterize and mitigate ICI when both impairments are present. In addition, we exploit the special ICI structure to design efficient OFDM channel estimation and digital baseband compensation schemes for I/Q imbalance under high-mobility conditions.

Reduced-Complexity ICI Mitigation for Mobile SFBC-OFDM with Application to DVB-H

Effective inter-carrier interference (ICI) mitigation for MIMO-OFDM requires accurate channel estimation which is very challenging due to the large number and fast time-varying nature of the channel parameters to be estimated using scattered pilots. We present a novel SFBC-OFDM scheme for doubly-selective channels and a reduced complexity channel estimation algorithm which exploits the banded and sparse structure of the channel in the frequency and time-domains, respectively. Furthermore, we design an FIR-MMSE ICI cancellation algorithm for mobile SFBC-OFDM and demonstrate its effectiveness for digital video broadcasting-handheld (DVB-H) systems.

Reduced-complexity hybrid time/frequency channel estimation for DVB-H

Orthogonal frequency division multiplexing (OFDM) is the modulation scheme of choice for digital video broadcasting-handheld (DVB-H) which is required to operate under high mobility conditions resulting in significant Inter-Carrier Interference (ICI). Under these conditions, the conventional one-tap per subcarrier channel estimation and equalization is suboptimal since it does not account for ICI effects. A hybrid frequency and time-domain channel estimation method was developed in [1] to mitigate the ICI due to time-variation within one OFDM symbol. We show that significant additional complexity reductions can be achieved by exploiting the special structure of the channel matrix over both the time and frequency domains. Our simulation results show that the proposed method has much less complexity with small performance loss under fast-varying channel conditions for DVB-H system.

Receiver Design for Mobile OFDM with Application to DVB-H

In mobile reception, orthogonal frequency division multiplexing (OFDM) may experience significant inter-carrier interference (ICI) which introduces an error floor proportional to the normalized Doppler frequency. To mitigate the effect of ICI, many schemes have been proposed but they are computationally complex for systems using large OFDM symbol length such as digital video broadcasting system for handheld (DVB-H). In this paper, we propose a low-complexity ICI suppression and channel estimation scheme for DVB-H systems. The proposed scheme is applied to a typical DVB-H system. Simulation results demonstrate that the proposed scheme is robust to mobility and can be employed efficiently in a DVB-H system. Comparison of DVB-H system with OFDM modulation and single carrier modulation (SC) employing frequency domain equalization (FDE) is investigated. Simulation results show the superiority of the SC to OFDM modulation for DVB-H systems and for the simulated scenarios.

Optimum channel estimation pilot design for CDD-OFDM with application to DVB

Cyclic delay diversity (CDD) is a low-complexity standard-conformable transmit diversity technique for coded orthogonal frequency division multiplexing (OFDM) systems. However, it makes channel estimation more challenging due to the increased frequency-selectivity of the channel. In this paper, we propose a novel pilot-aided channel estimation scheme for CDD-OFDM. By alternating and optimizing the cyclic delay parameter for adjacent OFDM symbols, we design a simple yet efficient channel estimation scheme for frequency-selective channels and evaluate its effectiveness for a DVB-T system.

Comparison of time and frequency domain single and double differential schemes for OFDM UWB systems

Ultra-wideband (UWB) systems possess several distinct features such as very high data rate, large multipath or frequency diversity and improved ranging precision while requiring low-power low-complexity transceivers. Among the different modulation schemes, differential PSK is an attractive option because it does not require channel estimation, and hence reduces implementation complexity. This paper investigates an OFDM UWB receiver based on both time and frequency-domain differential PSK modulation and compares the performances of single and double differential schemes in the presence of frequency offset.