Data-aided fading estimation technique for shadowed mobile satellite fading channels

A novel data-aided fading estimation technique that employs both pilot and data symbols is proposed to significantly reduce the bandwidth redundancy of the pilot-symbol-aided (PSA) systems using receivers with low complexity and latency in the shadowed mobile satellite fading channels. The shadowed mobile satellite fading channels are modeled as the sum of a lognormally distributed direct component and a Rayleigh distributed multipath component, and the PSA system employs 16-ary quadrature-amplitude-modulation (16QAM) for transmission. Monte Carlo computer simulation has been used to assess the technique on the bit-error-rate (BER) performances of the system in the light shadowed, the average shadowed and the heavy shadowed Rician fading environments. The results have shown that the proposed technique requires a very low bandwidth redundancy to provide satisfactory BER performances, and can substantially lower the error floors of the PSA systems.postprin

Bandwidth-efficient pilot-symbol-aided technique for multipath-fading channels

Pilot-symbol-aided (PSA) transmission is one of the effective methods to combat multipath fading in digital mobile communications systems. In the PSA systems, redundant bandwidth and power are required to transmit the pilot symbols. In this paper, we propose a novel fading estimation technique that requires a very low bandwidth redundancy in the PSA systems. The proposed technique uses simple linear interpolation on the pilot symbols and the detected data symbols to obtain the estimates of the channel fading effects. Monte Carlo computer simulation has been carried out to study the effects of the technique on the bit error rate performance of 16-ary quadrature amplitude modulation in the flat and the frequency-selective Rayleigh-fading channels corrupted with cochannel interference and additive white Gaussian noise. Results have shown that the proposed technique can, at the expense of a little power efficiency, significantly improve the bandwidth efficiency of the PSA systems using receivers with low com plexity and latency.published_or_final_versio

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