A scheme for cancelling intercarrier interference using conjugate transmission in multicarrier communication systems

To mitigate intercarrier interference (ICI), a two-path algorithm is developed for multicarrier communication systems, including orthogonal frequency division multiplexing (OFDM) systems. The first path employs the regular OFDM algorithm. The second path uses the conjugate transmission of the first path. The combination of both paths forms a conjugate ICI cancellation scheme at the receiver. This conjugate cancellation (CC) scheme provides (1) a high signal to interference power ratio (SIR) in the presence of small frequency offsets (50 dB and 33 dB higher than that of the regular OFDM and linear self-cancellation algorithms [1], [2], respectively, at ΔfT = 0.1% of subcarrier frequency spacing); (2) better bit error rate (BER) performance in both additive white Gaussian noise (AWGN) and fading channels; (3) backward compatibility with the existing OFDM system; (4) no channel equalization is needed for reducing ICI, a simple low cost receiver without increasing system complexity. Although the two-path transmission reduces bandwidth efficiency, the disadvantage can be balanced by increasing signal alphabet sizes

Orthogonal Frequency Division Multiplexing modulation and inter-carrier interference cancellation

The Orthogonal Frequency Division Multiplexing (OFDM) technique, wireless channel models, and a pair of new intercarrier interference self-cancellation methods are investigated in this thesis. The first chapter addresses the history of OFDM, along with its principles and applications. Chapter two consists of three parts: the principal, the modern OFDM models, and the Peak to Average Power Ratio (PAPR) problem. Chapter two also summarizes possible PAPR solutions. Chapter three discusses a series of well-known wireless channel models, as well as the general formula for wireless channels. In Chapter four, ICI problem has been discussed, along with its existing solutions. Chapter five focuses on two new ICI self-cancellation schemes, namely the clustering method and the multi-codebook method. These two new methods show promising results through the simulations. A summary of this thesis and the discussion of future research are also provided in Chapter five

Comparative Analysis of Conventional and ICI-Self-Cancellation Digital Video Broadcasting Transceivers

Digital video broadcasting–terrestrial (DVB-T) is one of the important technologies in the communication area because of its capability of high data-rate multimedia transmission. Orthogonal frequency division multiplexing (OFDM) has been the backbone technique in the current DVB-T systems adopted by Europe and Japan. However, since the OFDM system is very sensitive to the frequency synchronization and phase errors, which will induce the intercarrier interference (ICI), the ongoing research has been dedicated to this ICI problem in the presence of the Doppler frequency drift and the fading channels. A means to deal with the ICI problem is called the ICI self-cancellation or polynomial cancellation coding scheme. In this thesis, we establish the complete simulation environment for the physical layer of the DVB-T system and then evaluate the effectiveness of the ICI self-cancellation coding scheme compared with the existing convolutional coding scheme for different fading channels and different Doppler frequencies. According to our simulation results, we conclude that the ICI self-cancellation scheme significantly outperforms the convolutional coding scheme which is adopted by the existing DVB-T standard in the AWGN and the frequency non-selective fading channels, but both schemes have the similar performance in the frequency selective fading channels

Iterative Receiver for MIMO-OFDM System with ICI Cancellation and Channel Estimation

As a multi-carrier modulation scheme, Orthogonal Frequency Division Multiplexing (OFDM) technique can achieve high data rate in frequency-selective fading channels by splitting a broadband signal into a number of narrowband signals over a number of subcarriers, where each subcarrier is more robust to multipath. The wireless communication system with multiple antennas at both the transmitter and receiver, known as multiple-input multiple-output (MIMO) system, achieves high capacity by transmitting independent information over different antennas simultaneously. The combination of OFDM with multiple antennas has been considered as one of most promising techniques for future wireless communication systems. The challenge in the detection of a space-time signal is to design a low-complexity detector, which can efficiently remove interference resulted from channel variations and approach the interference-free bound. The application of iterative parallel interference canceller (PIC) with joint detection and decoding has been a promising approach. However, the decision statistics of a linear PIC is biased toward the decision boundary after the first cancellation stage. In this thesis, we employ an iterative receiver with a decoder metric, which considerably reduces the bias effect in the second iteration, which is critical for the performance of the iterative algorithm. Channel state information is required in a MIMO-OFDM system signal detection at the receiver. Its accuracy directly affects the overall performance of MIMO-OFDM systems. In order to estimate the channel in high-delay-spread environments, pilot symbols should be inserted among subcarriers before transmission. To estimate the channel over all the subcarriers, various types of interpolators can be used. In this thesis, a linear interpolator and a trigonometric interpolator are compared. Then we propose a new interpolator called the multi-tap method, which has a much better system performance. In MIMO-OFDM systems, the time-varying fading channels can destroy the orthogonality of subcarriers. This causes serious intercarrier interference (ICI), thus leading to significant system performance degradation, which becomes more severe as the normalized Doppler frequency increases. In this thesis, we propose a low-complexity iterative receiver with joint frequency- domain ICI cancellation and pilot-assisted channel estimation to minimize the effect of time-varying fading channels. At the first stage of receiver, the interference between adjacent subcarriers is subtracted from received OFDM symbols. The parallel interference cancellation detection with decision statistics combining (DSC) is then performed to suppress the interference from other antennas. By restricting the interference to a limited number of neighboring subcarriers, the computational complexity of the proposed receiver can be significantly reduced. In order to construct the time variant channel matrix in the frequency domain, channel estimation is required. However, an accurate estimation requiring complete knowledge of channel time variations for each block, cannot be obtained. For time- varying frequency-selective fading channels, the placement of pilot tones also has a significant impact on the quality of the channel estimates. Under the assumption that channel variations can be approximated by a linear model, we can derive channel state information (CSI) in the frequency domain and estimate time-domain channel parameters. In this thesis, an iterative low-complexity channel estimation method is proposed to improve the system performance. Pilot symbols are inserted in the transmitted OFDM symbols to mitigate the effect of ICI and the channel estimates are used to update the results of both the frequency domain equalizer and the PICDSC detector in each iteration. The complexity of this algorithm can be reduced because the matrices are precalculated and stored in the receiver when the placement of pilots symbols is fixed in OFDM symbols before transmission. Finally, simulation results show that the proposed MIMO-OFDM iterative receiver can effectively mitigate the effect of ICI and approach the ICI-free performance over time-varying frequency-selective fading channels

Intercarrier Interference Suppression for the OFDM Systems in Time-Varying Multipath Fading Channels

Due to its spectral efficiency and robustness over the multipath channels, orthogonal frequency division multiplexing (OFDM) has served as one of the major modulation schemes for the modern communication systems. In the future, the wireless OFDM systems are expected to operate at high carrier-frequencies, high speed and high throughput mobile reception, where the fasting time-varying fading channels are encountered. The channel variation destroys the orthogonality among the subcarriers and leads to the intercarrier interference (ICI). ICI poses a significant limitation to the wireless OFDM systems. The aim of this dissertation is to find an efficient method of providing reliable communication using OFDM in the fast time-varying fading channel scenarios. First, we investigate the OFDM performance in the situation of time-varying mobile channels in the presence of multiple Doppler frequency shifts. A new mathematical framework of the ICI effect is derived. The simulation results show that ICI induces an irreducible error probability floor, which in proportional to the Doppler frequency shifts. Furthermore, it is observed that ICI power arises from a few adjacent subcarriers. This observation motivates us to design the low-complexity Q-tap equalizers, namely, Minimum Mean Square Error (MMSE) linear equalizer and Decision Feedback (DF) non-linear equalizer to mitigate the ICI. Simulation results show that both Q-tap equalizers can improve the system performance in the sense of symbol error rate (SER). To employ these equalizers, the channel state information is also required. In this dissertation, we also design a pilot-aided channel estimation via Wiener filtering for a time-varying Wide-sense Stationary Uncorrelated Scatterers (WSSUS) channel model. The channel estimator utilizes that channel statistical properties. Our proposed low-complexity ICI suppression scheme, which incorporates the Q-tap equalizer with our proposed channel estimator, can significantly improve the performance of the OFDM systems in a fast time-varying fading channels. At the last part of the dissertation, an alternative ICI mitigation approach, which is based on the ICI self-cancellation coding, is also discussed. The EM-based approach, which solves the phase and amplitude ambiguities associated with this approach, is also introduced

An Efficient ICI Cancellation Scheme to Mitigate the Effect of ICI on OFDM Systems

Inter-carrier interference (ICI) emerges in orthogonal frequency division multiplexing (OFDM) systems used for mobile communication as a consequence of the Doppler Effect\u27s loss of orthogonality among subcarriers. Inter-Carrier Interference (ICI), which affects every subcarrier, drastically lowers performance. The performance of OFDM systems may be enhanced using a variety of ICI mitigation strategies. Comparable subcarrier frequency offsets are guaranteed by the premise that the OFDM transmission bandwidth is suitably modest in the majority of ICI mitigation strategies, on the other hand. The frequency offsets between each subcarrier might change, hence a wideband OFDM system in a situation with high mobility is investigated. Furthermore, the suggested ICI cancellation approach, Total ICI Cancellation, does not reduce bandwidth efficiency or transmission rate. As an example, the Total ICI Cancellation approach uses the ICI matrix\u27s orthogonality to provide perfect ICI cancellation and a significant boost in BER at a linearly increasing cost. The suggested technique, which matches the BER performance of a wideband OFDM system without ICI, offers the best BER performance possible in the presence of frequency offset and time shifts in the channel, according to simulation findings in the AWGN and multipath fading channels

Factor Graph Based Detection Schemes for Mobile Terrestrial DVB Systems with Long OFDM Blocks

This PhD dissertation analyzes the performance of second generation digital video broadcasting (DVB) systems in mobile terrestrial environments and proposes an iterative detection algorithm based on factor graphs (FG) to reduce the distortion caused by the time variation of the channel, providing error-free communication in very severe mobile conditions. The research work focuses on mobile scenarios where the intercarrier interference (ICI) is very high: high vehicular speeds when long orthogonal frequency-division multiplexing (OFDM) blocks are used. As a starting point, we provide the theoretical background on the main topics behind the transmission and reception of terrestrial digital television signals in mobile environments, along with a general overview of the main signal processing techniques included in last generation terrestrial DVB systems. The proposed FG-based detector design is then assessed over a simpli ed bit-interleaved coded modulation (BICM)-OFDM communication scheme for a wide variety of mobile environments. Extensive simulation results show the e ectiveness of the proposed belief propagation (BP) algorithm over the channels of interest in this research work. Moreover, assuming that low density parity-check (LDPC) codes are decoded by means of FG-based algorithms, a high-order FG is de ned in order to accomplish joint signal detection and decoding into the same FG framework, o ering a fully parallel structure very suitable when long OFDM blocks are employed. Finally, the proposed algorithms are analyzed over the physical layer of DVB-T2 speci cation. Two reception schemes are proposed which exploit the frequency and time-diversity inherent in time-varying channels with the aim of achieving a reasonable trade-o among performance, complexity and latency.Doktoretza tesi honek bigarren belaunaldiko telebista digitalaren eraginkortasuna aztertzen du eskenatoki mugikorrean, eta faktoreen grafoetan oinarritzen den hartzaile iteratibo bat proposatzen du denboran aldakorra den kanalak sortzen duen distortsioa leundu eta seinalea errorerik gabe hartzea ahalbidetzen duena. Proposatutako detektorea BICM-OFDM komunikazio eskema orokor baten gainean ebaluatu da lurreko broadcasting kanalaren baldintzak kontutan hartuz. Simulazio emaitzek algoritmo honen eraginkortasuna frogatzen dute Doppler frekuentzia handietan. Ikerketa lanaren bigarren zatian, faktoreen grafoetan oinarritutako detektorea eskema turbo zabalago baten baitan txertatu da LDPC dekodi katzaile batekin batera. Hartzaile diseinu honen abantaila nagusia da OFDM simbolo luzeetara ondo egokitzen dela. Azkenik, proposatutako algoritmoa DVB-T2 katearen baitan inplementatu da, bi hartzaile eskema proposatu direlarik seinaleak duen dibertsitate tenporal eta frekuentziala probesteko, beti ere eraginkortasunaren, konplexutasunaren eta latentziaren arteko konpromisoa mantenduz.Este trabajo de tesis analiza el rendimiento de la segunda generación de la televisión digital terreste en escenarios móviles y propone un algoritmo iterativo basado en grafos de factores para la detección de la señal y la reducción de la distorsión causada por la variación temporal del canal, permitiendo así recibir la señal libre de errores. El detector basado en grafos de factores propuesto es evaluado sobre un esquema de comunicaciones general BICM-OFDM en condiciones de transmisión propios de canales de difusión terrestres. Los resultados de simulación presentados muestran la e ciencia del algoritmo de detección propuesto en presencia de frecuencias Doppler muy altas. En una segunda parte del trabajo de investigación, el detector propuesto es incorporado a un esquema turbo junto con un decodi cador LDPC, dando lugar a un receptor iterativo que presenta características especialmente apropiadas para su implementación en sistemas OFDM con longitudes de símbolo elevadas. Por último, se analiza la implementación del algoritmo propuesto sobre la cadena de recepción de DVB-T2. Se presentan dos esquemas de recepción que explotan la diversidad temporal y frecuencial presentes en la señal afectada por canales variantes en el tiempo, consiguiendo un compromiso razonable entre rendimiento, complejidad y latencia

Performance degradation due to multipath noise for narrowband OFDM systems: channel-based analysis and experimental determination

The performance of OFDM systems over a multipath channel can strongly degrade due to the propagation delay spread. The distortion of the received signal over the fast Fourier transform window is referred to as multipath noise. This work aims to analytically determine the performance loss due to multipath noise as a function of OFDM and channel parameters for narrowband OFDM systems. First, it is investigated whether it is possible to describe the multipath noise, varying over different OFDM packets due to the temporal variation of the channel, by an effective noise factor F-delay, from which the loss factor is directly determined. Second, the theory of room electromagnetics is applied to develop a closed-form expression for F-delay as a function of the OFDM and reverberation parameters. This analytical method is validated with excellent agreement. Finally, the loss factor is determined for IEEE 802.11 based on channel measurements in two large conference rooms, providing values up to 19 dB for an 800 ns cyclic prefix length

On Time-Variant Distortions in Multicarrier Transmission with Application to Frequency Offsets and Phase Noise

Phase noise and frequency offsets are due to their time-variant behavior one of the most limiting disturbances in practical OFDM designs and therefore intensively studied by many authors. In this paper we present a generalized framework for the prediction of uncoded system performance in the presence of time-variant distortions including the transmitter and receiver pulse shapes as well as the channel. Therefore, unlike existing studies, our approach can be employed for more general multicarrier schemes. To show the usefulness of our approach, we apply the results to OFDM in the context of frequency offset and Wiener phase noise, yielding improved bounds on the uncoded performance. In particular, we obtain exact formulas for the averaged performance in AWGN and time-invariant multipath channels.Comment: 10 pages (twocolumn), 5 figure