140 research outputs found

    ICI Cancellation in OFDM Systems by Frequency Offset Reduction

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    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. OFDM is a promising candidate for achieving high data rates in mobile environment because of its multicarrier modulation technique and ability to convert a frequency selective fading channel into several nearly flat fading channels. Now OFDM is being widely used in wireless communications standards, such as IEEE 802.11a, the multimedia mobile access communication (MMAC), and the HIPERLAN/2. However, one of the main disadvantages of OFDM is its sensitivity against carrier frequency offset which causes inter carrier interference (ICI). A well-known problem of orthogonal frequency division multiplexing (OFDM), however, is its sensitivity to frequency offset between the transmitted and received signals, which may be caused by Doppler shift in the channel, or by the difference between the transmitter and receiver local oscillator frequencies. This carrier frequency offset causes loss of orthogonality between sub-carriers and the signals transmitted on each carrier are not independent of each other. The orthogonality of the carriers is no longer maintained, which results in inter-carrier interference (ICI). The undesired ICI degrades the performance of the system. Depending on the Doppler spread in the channel and the block length chosen for transmission, ICI can potentially cause a severe deterioration of quality of service (QOS) in OFDM systems. ICI mitigation techniques are essential in improving the performance of an OFDM system in an environment which induces frequency offset error in the transmitted signal. The comparisons of these schemes in terms of various parameters will be useful in determining the choice of ICI mitigation techniques for different applications and mobile environments. This project investigates an efficient ICI cancellation method termed ICI self-cancellation scheme for combating the impact of ICI on OFDM systems. The ICI self-cancellation scheme is a technique in which redundant data is transmitted onto adjacent sub-carriers such that the ICI between adjacent sub-carriers cancels out at the receiver. The main idea is one data symbol is modulated onto a group of adjacent subcarriers with a group of weighting coefficients. By doing so, the ICI signals generated within a group can be self-cancelled each other. At the receiver side, by linearly combining the received signals on these subcarriers with proposed coefficients, the residual ICI contained in the received signals can then be further reduced. Although the proposed scheme causes a reduction in bandwidth efficiency, it can be compensated, by using larger signal alphabet sizes in modulation. The average carrier-to-interference power ratio (CIR) is used as the ICI level indicator, and a theoretical CIR expression is derived for the proposed scheme. The proposed scheme provides significant CIR improvement, which has been studied theoretically and supported by simulations. Simulation results show that under the condition of the same bandwidth efficiency and larger frequency offsets, the proposed OFDM system using the ICI self-cancellation scheme per- forms much better than standard OFDM systems in AWGN channel with large Doppler frequencies. In addition, since no channel equalization is needed for reducing ICI, the proposed scheme is therefore beneficial in implementation issue without increasing system complexit

    Suppression of Mutual Interference in OFDM Based Overlay Systems

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    A promising appraoch for overcoming spectrum scarcity are overlay systems that share a frequency band with already existing licensed systems by using the spectral gaps left by the licensed systems. Due to its spectral efficiency and flexibility orthogonal frequency-division multiplexing (OFDM) is an appropriate modulation technique for overlay systems. To enable a successful co-existence, techniques for suppressing mutual interferences between the overlay and the licensed system are proposed

    ICI Reduction Methods in OFDM Systems

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    Interference mitigation techniques for wireless OFDM

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    Orthogonal Frequency Division Multiplexing (OFDM) is a promising multicarrier wireless system for transmission of high-rate data stream with spectral efficiency and fading immunity. Conventional OFDM system use efficient IFFT and FFT to multiplex the signals in parallel at the transmitter and receiver respectively. On the other hand, wavelet based OFDM system uses orthonormal wavelets which are derived from a multistage tree-structured wavelet family. The Fourier based and wavelet based OFDM systems are studied in this dissertation. Two types of QAM schemes, circular and square modulations are used to compare the performance in both OFDM systems. A new approach of determining exact BER for optimal circular QAM is proposed. In addition, the presence of narrowband interference (NBI) degrades the performance of OFDM systems. Thus, a mitigation technique is necessary to suppress NBI in an OFDM system. Recent mitigation techniques can be broadly categorized into frequency domain cancellation, receiver windowing and excision filtering. However, none of the techniques considers wavelet based OFDM. Therefore, an interference cancelation algorithm has been proposed to work for both OFDM platforms. The performance results of two OFDM schemes applicable to digital video broadcasting (DVB)-terrestrial system and under the effect of impulsive noise interference are also studied. BER performances are obtained in all results. It has been shown that wavelet based OFDM system has outperformed Fourier based OFDM system in many cases

    A New Approach for Performance Improvement of OFDM System using Pulse Shaping

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    Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation technique, in which a single high rate data-stream is divided into multiple low rate data-streams and is modulated using sub-carriers, which are orthogonal to each other. Some of its main advantages are multipath delay spread tolerance, high spectral efficiency, efficient modulation and demodulation process using computationally efficient Inverse Fast Fourier Transform and Fast Fourier Transform operation respectively. The peak to average power ratio of the time domain envelope is an important parameter at the physical layer of the communication system using OFDM signaling. The signals must maintain a specified average energy level in the channel to obtain the desired Bit-error-rate. The peak signal level relative to that average defines the maximum dynamic range that must be accommodated by the components in the signal flow path to support the desired average. A secondary concern is the carrier frequency offset which disturbs the orthogonality among the carriers and results ICI. The undesired ICI degrades the performance of the system

    BER Analysis of OFDM Systems with Varying Frequency Offset Factor over AWGN and Rayleigh Channels

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    The progressively escalating demand for tremendously high rate data transmission over wireless mediums needsresourcefulconcord of electromagnetic resources considering restrictions like power incorporation, spectrum proficiency, robustness in disparity to multipath propagation and implementation complication. Orthogonal frequency division multiplexing (OFDM) is a favorable approach for upcoming generation wireless communication systems. However its susceptibility to the frequency offset triggered by frequency difference between local oscillator of transmitter and receiver or due to Doppler shift results to Inter Carrier Interference. This delinquent of ICI results inworsening performance of the wireless systems as bit error rate increaseswith increase in value of frequency offset. In this paper simulation results aredemonstratedfor analyzing the effect of varying frequency offset factor on system’s error rate performance

    An Efficient ICI Cancellation Technique for OFDM Communication Systems

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    A well known problem of orthogonal frequency division multiplexing (OFDM), however, is its sensitivity to frequency offset between the transmitted and received signals, which may be caused by Doppler shift in the channel, or by the difference between the transmitter and receiver local oscillator frequencies. This carrier frequency offset causes loss of orthogonality between sub carriers and the signals transmitted on each carrier are not independent of each other. The orthogonality of the carriers is no longer maintained, which results in inter-carrier interference (ICI). The undesired ICI degrades the performance of the system. Depending on the Doppler spread in the channel and the block length chosen for transmission, ICI can potentially cause a severe deterioration of quality of service (QOS) in OFDM systems. ICI mitigation techniques are essential in improving the performance of an OFDM system in an environment which induces frequency offset error in the transmitted signal. The comparisons of these schemes in terms of various parameters will be useful in determining the choice of ICI mitigation techniques for different applications and mobile environments. This project investigates an efficient ICI cancellation method termed ICI self-cancellation scheme for combating the impact of ICI on OFDM systems. The ICI self-cancellation scheme is a technique in which redundant data is transmitted onto adjacent sub-carriers such that the ICI between adjacent sub-carriers cancels out at the receiver. The main idea is one data symbol is modulated onto a group of adjacent subcarriers with a group of weighting coefficients. By doing so, the ICI signals generated within a group can be ―self-cancelled‖ each other. At the receiver side, by linearly combining the received signals on these subcarriers with proposed coefficients, the residual ICI contained in the received signals can then be further reduced. The carrier-to-interference power ratio (CIR) can be increased by 15 and 30 dB when the group size is two or three, respectively, for a channel with a constant frequency offset. Although the proposed scheme causes a reduction in bandwidth efficiency, it can be compensated, by using larger signal alphabet sizes in modulation. The average carrier-to-interference power ratio (CIR) is used as the ICI level indicator, and a theoretical CIR expression is derived for the proposed scheme. The proposed scheme provides significant CIR improvement, which has been studied theoretically and supported by simulations. Simulation results show that under the condition of the same bandwidth efficiency and larger frequency offsets, the proposed OFDM system using the ICI self-cancellation scheme performs much better than standard OFDM systems in AWGN channel with large Doppler frequencies. In addition, since no channel equalization is needed for reducing ICI, the proposed scheme is therefore beneficial in implementation issue without increasing system complexity
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