Semiblind Channel Estimation and Data Detection for OFDM Systems With Optimal Pilot Design

This paper considers semiblind channel estimation and data detection for orthogonal frequency-division multiplexing (OFDM) over frequency-selective fading channels. We show that the samples of an OFDM symbol are jointly complex Gaussian distributed, where the mean and covariance are determined by the locations and values of fixed pilot symbols. We exploit this distribution to derive a novel maximum-likelihood (ML) semiblind gradient-descent channel estimator. By exploiting the channel impulse response (CIR) statistics, we also derive a semiblind data detector for both Rayleigh and Ricean fading channels. Furthermore, we develop an enhanced data detector, which uses the estimator error statistics to mitigate the effect of channel estimation errors. Efficient implementation of both the semiblind and the improved data detectors is provided via sphere decoding and nulling-canceling detection. We also derive the Cramér-Rao bound (CRB) and design optimal pilots by minimizing the CRB. Our proposed channel estimator and data detector exhibit high bandwidth efficiency (requiring only a few pilot symbols), achieve the CRB, and also nearly reach the performance of an ideal reference receiver

Channel Estimation in Multicarrier Communication Systems

The data rate and spectrum efficiency of wireless mobile communications have been significantly improved over the last decade or so. Recently, the advanced systems such as 3GPP LTE and terrestrial digital TV broadcasting have been sophisticatedly developed using OFDM and CDMA technology. In general, most mobile communication systems transmit bits of information in the radio space to the receiver. The radio channels in mobile radio systems are usually multipath fading channels, which cause inter-symbol interference (ISI) in the received signal. To remove ISI from the signal, there is a need of strong equalizer which requires knowledge on the channel impulse response (CIR).This is primarily provided by a separate channel estimator. Usually the channel estimation is based on the known sequence of bits, which is unique for a certain transmitter and which is repeated in every transmission burst. Thus, the channel estimator is able to estimate CIR for each burst separately by exploiting the known transmitted bits and the corresponding received samples. In this thesis we investigate and compare various efficient channel estimation schemes for OFDM systems which can also be extended to MC DS-CDMA systems.The channel estimation can be performed by either inserting pilot tones into all subcarriers of OFDM symbols with a specific period or inserting pilot tones into each OFDM symbol. Two major types of pilot arrangement such as block type and comb type pilot have been focused employing Least Square Error (LSE) and Minimum Mean Square Error (MMSE) channel estimators. Block type pilot sub-carriers is especially suitable for slow-fading radio channels whereas comb type pilots provide better resistance to fast fading channels. Also comb type pilot arrangement is sensitive to frequency selectivity when comparing to block type arrangement. However, there is another supervised technique called Implicit Training (IT) based channel estimation which exploits the first order statistics in the received data, induced by superimposing periodic training sequences with good correlation properties, along with the information symbols. Hence, the need for additional time slots for training the equalizer is avoided. The performance of the estimators is presented in terms of the mean square estimation error (MSEE) and bit error rate (BER)

Channel Estimation in Multicarrier Code Division Multiple Access

The concepts of OFDM and MC-CDMA are not new but the new technologies to improve their functioning is an emerging area of research. In general, most mobile communication systems transmit bits of information in the radio space to the receiver. The radio channels in mobile radio systems are usually multipath fading channels, which cause inter-symbol interference (ISI) in the received signal. To remove ISI from the signal, there is a need of strong equalizer. In this thesis we have focused on simulating the OFDM and MC-CDMA systems in MATLAB and designed the channel estimation for them

Channel Estimation in OFDM systems

Orthogonal frequency division multiplexing (OFDM) provides an effective and low complexity means of eliminating intersymbol interference for transmission over frequency selective fading channels. This technique has received a lot of interest in mobile communication research as the radio channel is usually frequency selective and time variant. In OFDM system, modulation may be coherent or differential. Channel state information (CSI) is required for the OFDM receiver to perform coherent detection or diversity combining, if multiple transmit and receive antennas are deployed. In practice, CSI can be reliably estimated at the receiver by transmitting pilots along with data symbols. Pilot symbol assisted channel estimation is especially attractive for wireless links, where the channel is time-varying. When using differential modulation there is no need for a channel estimate but its performance is inferior to coherent system.In this thesis we investigate and compare various efficient pilot based channel estimation schemes for OFDM systems. The channel estimation can be performed by either inserting pilot tones into all subcarriers of OFDM symbols with a specific period or inserting pilot tones into each OFDM symbol. In this present study, two major types of pilot arrangement such as blocktype and comb-type pilot have been focused employing Least Square Error (LSE) and Minimum Mean Square Error (MMSE) channel estimators. Block type pilot sub-carriers is especially suitable for slow-fading radio channels whereas comb type pilots provide better resistance to fast fading channels. Also comb type pilot arrangement is sensitive to frequency selectivity when comparing to block type arrangement. The channel estimation algorithm based on comb type pilots is divided into pilot signal estimation and channel interpolation. The pilot signal estimation is based on LSE and MMSE criteria, together with channel interpolation using linear interpolation and spline cubic interpolation. The symbol error rate (SER) performances of OFDM system for both block type and comb type pilot subcarriers are presented in the thesis

Robust channel estimation for broadband satellite communications

Com o rápido crescimento das comunicações digitais nos últimos anos, a necessidade de transmissões de dados a alta velocidade aumentou significativamente. Além disso, espera-se que os futuros sistemas sem fios sejam capazes de suportar uma ampla gama de serviços que incluam vídeo, dados e voz. Orthogonal Frequency Division Multiplexing (OFDM) é um candidato promissor para conseguir taxas de transmissão elevadas em ambientes móveis, devido à sua resistência à Inter Symbol Interference (ISI), que consiste num problema comum em comunicações de dados de alta de velocidade. No OFDM, a modulação pode ser diferencial ou coerente. No caso da modulação diferencial não existe necessidade de obter estimação do canal mas o seu desempenho é inferior relativamente ao sistema coerente. A modulação coerente exige estimação de canal que proporciona um melhor desempenho mas apresenta uma estrutura do receptor mais complexa. Na dissertação vão ser estudadas estas duas técnicas: Pilot Symbol Assisted Modulation (PSAM) e pilotos implícitos. No primeiro caso os pilotos transmitidos são multiplexados com os dados, enquanto na segunda situação os pilotos são alocados em posições juntamente com os dados. Nesta dissertação, vamos analisar as diferentes técnicas de estimação em termos de Bit Error Rate (BER) e Mean Square Error (MSE). Para se poder fazer essa análise foi proposto um modelo de canal Land Mobile Satellite (LMS) que vai ser a base para o estudo da robustez e eficiência dos métodos de estimação apresentados.With the rapid growth of digital communication in recent years, the need for high speed data transmission is increased. Moreover, future wireless systems are expected to support a wide range of services which includes video, data and voice. Orthogonal Frequency Division Multiplexing (OFDM) is a promising candidate for achieving high data rates in mobile environment, due to its resistance Inter Symbol Interference (ISI), which is a common problem found in high speed data communication. In OFDM, modulation may be differential or coherent. When using differential modulation there is no need for a channel estimate but its performance is inferior than the coherent system. Coherent modulation requires the channel estimation which gives better performance but with relatively more complex receiver structure. In this thesis two channel estimation techniques will be studied: Pilot Symbol Assisted Modulation (PSAM) and implicit pilots. In the first case the pilots are multiplexed with the transmitted data, while in the second situation pilots are placed in positions along with the data. In this thesis, we consider different estimation techniques in terms of Bit Error Rate (BER) and Mean Square Error (MSE). In order to do this analysis a Land Mobile Satellite (LMS) channel model was proposed which will be the basis for the study of the robustness and efficiency of the estimation methods presented

An EM-based estimation of OFDM signals

Abstract -We propose an EM-based algorithm to efficiently detect transmitted data in an OFDM system as well as estimating the channel impulse response (CIR). The maximum likelihood estimate of CIR is obtained by using channel statistics (their means and covariances) via the expectation-maximization (EM) algorithm. This algorithm can improve signal detection and the channel estimation accuracy by making use of pilot symbols to obtain an initial estimate for the iteration. Simulation results show that the bit error rate (BER) can be significantly reduced by this algorithm, and validate its good convergence and robust properties