Joint channel and phase noise compensation for OFDM in fast fading multipath applications

Orthogonal frequency-division multiplexing (OFDM) systems, such as the new wireless standards or the schemes proposed for third-generation (3G) evolution, exhibit great sensitivity to the effects of phase noise and the time-varying propagation channel, which can introduce interchannel interference (ICI) due to the loss of orthogonality among subcarriers. In this paper, joint channel estimation and ICI reduction schemes are investigated, which compensate the effects of phase noise and multipath channel in a realistic scenario, where the channel is not assumed perfectly known, whereas its estimation is obtained by combining a decision-feedback scheme and a pilot-aided estimator. We propose a technique for ICI compensation that has reduced complexity by only considering the most significant terms of ICIPublicad

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

Block-Type Pilot Arrangement with Alternating Polarity for ICI Mitigation in Mobile OFDM Systems

Improvement on Inter Carrier Interference (ICI)mitigation techniques for OFDM caused by Doppler effectsthrough minimizing channel estimation error and decreasingchannel time varying rate is investigated. The performanceof pilot-aided channel estimation techniques depends on pilot placement and arrangement and also on the channel time varying rate. The block-type and comb-type pilot arrangements are studied through different numbers of guard bands, with or without the involvement of the Doppler shift compensation. The estimation of channel at mid-point of each OFDM symbol is derived from pilot frequencies based on the least square algorithm while the channel interpolation is done using piecewise linear approximation. For ICI mitigation technique we implement frequency domain zero forcing equalizer. We compare the performance of schemes with different pilot arrangementsand Doppler shift compensations by measuring bit error rate with QPSK as sub-channel modulation scheme and with mobileto-fixed of single ring scattering as channel model. The results are in favour of block-type pilot arrangements with alternating polarity and Doppler compensation of 0:55 times the maximum Doppler shift

Compensation of Physical Impairments in Multi-Carrier Communications

Among various multi-carrier transmission techniques, orthogonal frequency-division multiplexing (OFDM) is currently a popular choice in many wireless communication systems. This is mainly due to its numerous advantages, including resistance to multi-path distortions by using the cyclic prefix (CP) and a simple one-tap channel equalization, and efficient implementations based on the fast Fourier and inverse Fourier transforms. However, OFDM also has disadvantages which limit its use in some applications. First, the high out-of-band (OOB) emission in OFDM due to the inherent rectangular shaping filters poses a challenge for opportunistic and dynamic spectrum access where multiple users are sharing a limited transmission bandwidth. Second, a strict orthogonal synchronization between sub-carriers makes OFDM less attractive in low-power communication systems. Furthermore, the use of the CP in OFDM reduces the spectral efficiency and thus it may not be suitable for short-packet and low-latency transmission applications. Generalized frequency division multiplexing (GFDM) and circular filter-bank multi-carrier offset quadrature amplitude modulation (CFBMC-OQAM) have recently been considered as alternatives to OFDM for the air interface of wireless communication systems because they can overcome certain disadvantages in OFDM. Specifically, these two systems offer a flexibility in choosing the shaping filters so that the high OOB emission in OFDM can be avoided. Moreover, the strict orthogonality requirement in OFDM is relaxed in GFDM and CFBMC-OQAM which are, respectively, non-orthogonal and real-field orthogonal systems. Although a CP is also used in these two systems, the CP is added for a block of many symbols instead of only one symbol as in OFDM, which, therefore, improves the spectral efficiency. Given that the performance of a wireless communication system is affected by various physical impairments such as phase noise (PN), in-phase and quadrature (IQ) imbalance and imperfect channel estimation, this thesis proposes a number of novel signal processing algorithms to compensate for physical impairments in multi-carrier communication systems, including OFDM, GFDM and CFBMC-OQAM. The first part of the thesis examines the use of OFDM in full-duplex (FD) communication under the presence of PN, IQ imbalance and nonlinearities. FD communication is a promising technique since it can potentially double the spectral efficiency of the conventional half-duplex (HD) technique. However, the main challenge in implementing an FD wireless device is to cope with the self-interference (SI) imposed by the device's own transmission. The implementation of SI cancellation (SIC) faces many technical issues due to the physical impairments. In this part of research, an iterative algorithm is proposed in which the SI cancellation and detection of the desired signal benefit from each other. Specifically, in each iteration, the SI cancellation performs a widely linear estimation of the SI channel and compensates for the physical impairments to improve the detection performance of the desired signal. The detected desired signal is in turn removed from the received signal to improve SI channel estimation and SI cancellation in the next iteration. Results obtained show that the proposed algorithm significantly outperforms existing algorithms in SI cancellation and detection of the desired signal. In the next part of the thesis, the impact of PN and its compensation for CFBMC-OQAM systems are considered. The sources of performance degradation are first quantified. Then, a two-stage PN compensation algorithm is proposed. In the first stage, the channel frequency response and PN are estimated based on the transmission of a preamble, which is designed to minimize the channel mean squared error (MSE). In the second stage the PN compensation is performed using the estimate obtained from the first stage together with the transmitted pilot symbols. Simulation results obtained under practical scenarios show that the proposed algorithm effectively estimates the channel frequency response and compensates for the PN. The proposed algorithm is also shown to outperform an existing algorithm that implements iterative PN compensation when the PN impact is high. As a further development from the second part, the third part of the thesis considers the impacts of both PN and IQ imbalance and proposes a unified two-stage compensation algorithm for a general multi-carrier system, which can include OFDM, GFDM and CFBMC-OQAM. Specifically, in the first stage, the channel impulse response and IQ imbalance parameters are first estimated based on the transmission of a preamble. Given the estimates obtained from the first stage, in the second stage the IQ imbalance and PN are compensated in that order based on the pilot symbols for the rest of data transmission blocks. The preamble is designed such that the estimation of IQ imbalance does not depend on the channel and PN estimation errors. The proposed algorithm is then further extended to a multiple-input multiple-output (MIMO) system. For such a MIMO system, the preamble design is generalized so that the multiple IQ imbalances as well as channel impulse responses can be effectively estimated based on a single preamble block. Simulation results are presented and discussed in a variety of scenarios to show the effectiveness of the proposed algorithm

Implementación de un sistema OFDM en un dispositivo SFF SDR

En los últimos años, el desarrollo en los sistemas de comunicaciones ha tenido un gran auge debido a las mejoras en los sistemas de procesado digital de señales. Entre ellos, destacan los sistemas basados en las técnicas de modulación OFDM con un gran crecimiento y una gran presencia en diferentes medios. Las mejoras en esta técnica de transmisión de información son constantes gracias a una gran cantidad de proyectos de investigación. Por este motivo, surge el interés de llevar a la práctica una implementación hardware de un sistema de comunicación basado en OFDM. En este proyecto, se han estudiado las características de los sistemas OFDM y se ha llevado a cabo un desarrollo de un sistema OFDM en código VHDL. De este modo, además de realizar un código general de un sistema OFDM que se implementa en módulos FPGA, se ha probado un transmisor OFDM en un dispositivo concreto pensado para las telecomunicaciones radio como es el “SFF SDR DP” de Lyrtech. Así, conjuntamente al desarrollo del código, el proyecto profundiza en el estudio del dispositivo SFF SDR y el proceso necesario para configurarlo y programarlo para llevar a cabo pruebas hardware de sistemas de telecomunicaciones. Y al mismo tiempo, se ha comprobado la mejora de la eficiencia en cuanto a recursos que tiene lugar al programar estos dispositivos mediante VHDL con respecto a otras alternativas basadas en programación a alto nivel. __________________________________________________________________________________________In recent years, the development of communications systems has been boosted due to improvements in the digital signal processing. Among them there are systems based on OFDM modulation techniques with strong growth and a strong presence in different media. Improvements in the techniques of transmission have been constant through a large number of research projects. For this reason it emerges an interest to produce a hardware implementation of a communications system based on OFDM. In this project, we have studied the characteristics of OFDM systems and we have carried out a development of an OFDM system in VHDL code. Thus, in addition to a general code of an OFDM system that is implemented in FPGA modules, we tested an OFDM transmitter on a specific device designed for radio telecommunications such as the SFF SDR DP of Lyrtech. Furthermore, the project extends the study of the "SFF SDR" devices in the process needed to set up and program it to execute tests of telecommunications systems hardware. And at the same time, it has been shown the improvement in the efficiency in resources that occurs when programming these devices using VHDL.Ingeniería de Telecomunicació