Self-cancellation Scheme Research Articles

A charge-pump and comparator based power-efficient pipelined ADC technique

A charge-pump and comparator based technique is presented for power-efficient pipelined analog-to-digital conversion. The technique takes advantage of a passive charge pump to implement the core function of residue voltage amplification and exploits a comparator-controlled charging circuit to buffer the residue voltage to the next stage. Unlike the conventional buffer circuit using source followers, no voltage headroom is sacrificed in this voltage buffering scheme. The comparator overshoot due to comparator delay is minimized by a self-cancellation scheme. The proposed pipelined ADC technique uses only capacitors, comparators and current sources with digital calibration to achieve low power consumption. Designed and fabricated in a 0.18μm CMOS technology, a proof-of-concept ADC has measured 39.1dB SNDR (6.2-bit ENOB) at 25MS/s while consuming 3.5mW from a 1.8V supply.

Repeated correlative coding scheme for mitigation of inter-carrier interference in an orthogonal frequency division multiplexing system

In this study, a repeated correlative coding scheme is proposed to combat the inter-carrier interference (ICI) caused by the frequency offset in orthogonal frequency division multiplexing (OFDM) communication systems. This proposed scheme combine two ideas of the well-known methods, which are the coding of adjacent subcarriers with antipodal of the same data symbol (ICI self-cancellation) and correlative coding. A mathematical expression for the carrier-to-interference ratio (CIR) by using this proposed repeated correlative coding scheme is derived. The simulated result of CIR for the proposed scheme is significantly improved compared to the correlative coding as well as a self-cancellation scheme. The bit-error-rate (BER) of the proposed scheme is also compared with the ICI self-cancellation scheme and correlative coding scheme, which is comparable to that of the ICI self-cancellation scheme and better than the correlative coding scheme.

Theoretical analysis of ICI self-cancellation in OFDM systems over Rayleigh flat-fading channels

Orthogonal frequency division multiplexing (OFDM) has been widely used for its robustness against multipath fading and low-complexity implementation. However, OFDM system, especially with large number of subcarriers and high modulation order, is severely affected by the phase noise of oscillators and carrier frequency offset (CFO). On the other hand, self-cancellation schemes have received a lot of attention due to their simple implementation and high efficiency to suppress inter-carrier interference (ICI) in OFDM systems. Among those ICI self-cancellation methods, symmetric conjugate symbol repetition (SCSR) has been proven to have the best bit error ratio (BER) performance for phase noise suppression. In this paper, the performance of OFDM systems with SCSR ICI self-cancellation in the presence of both phase noise (PHN) and CFO are investigated, and analytical expressions are derived to calculate error probability evaluated by symbol error ratio (SER) over additive white Gaussian noise (AWGN) and Rayleigh flat fading channels. An approach of second order approximation of PHN/CFO has been performed to estimate the residual ICI, which could provide more accurate results. Simulation results show perfect agreement with those obtained by theoretical analysis, which could be used to estimate OFDM system error probability, facilitating the design of the overall system.

A novel ICI self-cancellation scheme for OFDM systems

We propose a novel intercarrier interference (ICI) self-cancellation scheme for orthogonal frequency division multiplexing (OFDM) systems. The symmetric scheme is the best among all ICI self-cancellation scheme in the literature. Its coefficient pair is (1, − 1), and the loading subcarriers are the kth and N−k − 1th subcarriers, where N is the number of subcarriers. We propose to modify the symmetric scheme and change the coefficient pair from (1, − 1) to (1, −µ) where µis between 0 and 1. The proposed modified symmetric scheme has better carrier-to-interference-ratio (CIR) than all previous ICI self-cancellation schemes by at least 1.7 dB when the normalized frequency offset is 0.5. Copyright © 2011 John Wiley & Sons, Ltd.

Precoded OFDM System for ICI Mitigation over Time-Frequency Selective Fading Channels

In orthogonal frequency-division multiplexing (OFDM) systems, the capability to support high mobility is greatly limited by the intercarrier interference (ICI) caused by time-frequency selective fading channels. This paper presents a precoded OFDM system for ICI mitigation. A precoder is introduced to relieve the ICI by transmitting N-point composite information symbols at twice the subcarrier interval. A Hadamard-matrix-like pilot pattern is used to recover the composite information symbols in a postprocessor at the receiver. Simulations show that, compared to the conventional self-cancellation scheme, this scheme gives much better signal-to-interference-noise ratio performance with much less overhead. Furthermore, the scheme can support twice the vehicle speed in time-frequency selective fading channels than the standard OFDM systems without ICI mitigation.

Optimum weighting-coefficient-pair in inter-carrier interference self-cancellation scheme of OFDM system

As one of the most important components of the wideband wireless access technique, orthogonal frequency division multiplexing (OFDM) has a high usage rate of spectrum and combats inter-symbol interference (ISI) in multi-path fading channel. However, when there are frequency offsets during the signal transmission, the inter-carrier interference (ICI) is introduced, which significantly degrades the performance. The existing ICI self-cancellation schemes such as PCC-OFDM are not optimum to minimize the interference considering both noise and ICI. In this paper, a new metric named SINR (signal-to-interference-and-noise ratio) is proposed. We discuss the optimization issue when a constant frequency offset exists and in time-varying channels. The optimum weighting-coefficient-pair (OWCP) is obtained, which maximizes SINR theoretically through the alternant iteration algorithm. Simulations show that the performance of OWCP-OFDM is better than that of PCC-OFDM, especially when the frequency offset is large. Although the ICI self-cancellation scheme suffers bandwidth inefficiency, from the simulation results we can also see that the performance of OWCP-OFDM is much better than that of the standard OFDM systems with the same bandwidth efficiency when a frequency offset exists. Moreover, since the redundant modulation provides the capability to suppress ICI as well as a receiving SNR gain, it can be considered as exchanging the bandwidth for SNR.

Performance evaluation for PCC-OFDM systems impaired by carrier frequency offset over AWGN channels

Orthogonal frequency division multiplexing (OFDM) is sensitive to carrier frequency offset (CFO), which destroys the orthogonality and causes inter-carrier interference (ICI). ICI self-cancellation schemes based on polynomial cancellation coding (PCC-OFDM) can evidently reduce the sensitivity to CFO. In this paper, we analyze the performance of PCC-OFDM systems impaired by CFO over additive white gaussian noise (AWGN) channels. Two criteria are used to evaluate the effect of CFO on performance degradations. Firstly, the closed-form expressions of the average carrier-to-interference power ratio (CIR) and the statistical average ICI power, both of which reflect the desired power loss, are presented. Simulation and analytical results show that the theoretical expressions depend crucially on the normalized frequency offset and are hardly relevant to the number of subcarriers. Secondly, by exploiting the properties of the Beaulieu series, the effect of CFO on symbol error rate (SER) and bit error rate (BER) performance for PCC-OFDM systems are exactly expressed as the sum of an infinite series in terms of the characteristic function (CHF) of ICI. We consider the systems modulated with binary phase shift keying (BPSK), quadrature PSK (QPSK), 8-ary PSK (8-PSK), and 16-ary quadrature amplitude modulation (16-QAM), and all above modulation schemes are mapped with Gray codes for the evaluations of BER.

An Improved ICI Self-Cancellation Scheme for Multi-Carrier Communication Systems

For broadband wireless mobile communication systems the orthogonal frequency division multiplexing (OFDM) is a suitable modulation scheme. The frequency offset between transmitter and receiver local oscillator is main drawback of OFDM systems, which causes intercarrier interference (ICI) in the subcarriers of the OFDM system. This ICI degrades the bit error rate (BER) performance of the system. In this paper an improved self-ICI cancellation scheme is proposed to improve the system performance. The proposed scheme is based on discrete Fourier transform-inverse discrete Fourier transform (DFT-IDFT). The simulation results show that there is satisfactory improvement in the bit error rate (BER) performance of the present scheme. Keywords—OFDM, Intercarrier Interference, Interference Coefficients, DFT based Self-ICI Cancellation. I. INTRODUCTION FDM is an emerging modulation scheme in the current broadband wireless mobile communication system due to the high spectral efficiency and robustness to multi-path interference (1). A well-known disadvantage of the OFDM system is its sensitivity to frequency offset between transmitted and received signals, which may be caused by Doppler shift in the channel (2), or by the difference between the transmitter and receiver local oscillator frequencies. The carrier frequency offset causes loss of orthogonality and then the signals transmitted on each carrier are not independent of each other, thus leading to ICI (3). All the methods, available in the literature, that have been developed to reduce the effect of frequency offset can be divided into two groups. In first group, the frequency offset in OFDM systems is estimated and corrected at the receiver side and in the second group the frequency offset is reduced at the transmitter side. The first group is termed as the frequency offset estimation and correction, not considered in this paper. Here the second group, called frequency-offset reduction, is considered. In the frequency offset reduction approaches (2, 4-6), available in the literature, either a guard interval or redundant modulation is necessary in order to achieve a trade-

Decision Feedback Frequency Offset Estimation and Tracking for General ICI Self-Cancellation Based OFDM Systems

Inter-carrier interference (ICI) self-cancellation schemes were often employed in many OFDM systems as a simple and effective approach to suppress ICI caused by carrier frequency errors. The same procedure, however, can not perform very well at high frequency offsets. We propose a simple decision feedback scheme based on the general ICI self-cancellation scheme to perform estimation and tracking of the carrier frequency offsets. A system with the scheme does not consume additional bandwidth since it used the same data symbols employed for ICI cancellation for the estimation. After an initial estimation is completed, the scheme switches to the tracking mode to carry out the estimation of deviations in the frequency offsets. Finally this fine-tuned estimate is applied to the ICI self-cancellation scheme concurrently for frequency offset correction and hence improved the system performance greatly. Simulation results showed that our scheme allowed up to 9% of random variations in the frequency offset. The effectiveness of our scheme is further verified by calculating the bit error rate performance of various OFDM receivers.

ML Estimation of Frequency Offset for General ICI Self-Cancellation Based OFDM Systems

We develop a maximum likelihood estimation scheme for correcting the carrier frequency offsets prior to the general intercarrier interference (ICI) self-cancellation in the OFDM systems. Since the same data symbols employed for ICI self-cancellation are also used for frequency offset estimation, the proposed scheme does not consume additional bandwidth. The combined use of the estimation algorithm and ICI self-cancellation scheme provides both frequency offset compensation and ICI reduction hence improves the system performance greatly. The effectiveness of the proposed estimation-cancellation scheme is further verified by calculating the bit error rates of various OFDM receivers, and substantial improvements are found.

Pilot-Aided ICI Self-Cancellation Scheme for OFDM Systems

In this letter, a novel pilot-aided inter-carrier interference (ICI) self-cancellation scheme is proposed for use in orthogonal frequency division multiplexing (OFDM) systems. The proposed scheme maps both modulated data symbols and pre-defined pilot symbols onto non-neighboring sub-carriers with weighting coefficients of +1 and -1. With the aid of pilot symbols, a more accurate estimation of frequency offsets can be obtained, and the ICI self-cancellation demodulation can be operated properly.

Performance evaluation of an ICI self-cancellation coded transceiver for mobile DVB-T applications

Digital video broadcasting-terrestrial (DVB-T) is an important multimedia technology capable of high data-rate 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 induces 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 paper, 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 for the AWGN and the frequency non-selective fading channels, but both schemes have the similar performance in the frequency-selective fading channels.

A wide dynamic range analog predistortion-type linearizer using self-cancellation scheme

A new radio frequency linearized power amplifier (LPA) system for wideband-code-division-multiple-access (WCDMA) application is demonstrated. The main amplifier is a parallel combination of drive amplifiers, and the same type of drive amplifier is also used to generate the predistortion signal. Self-cancellation of the intermodulation-distortion (IMD) components is obtained by the generic identity between the nonlinear characteristics of the main amplifier and the predistortion signal, both of which are originated from the same type of devices. Such self-cancellation scheme exhibits the improvement of 13 dB in the adjacent channel leakage ratio (ACLR) for WCDMA signal excitation.

Robust ICI Self-cancellation OFDM Receiver with Dynamic Phase and Amplitude Estimations

Orthogonal frequency division multiplexing (OFDM) has been applied in the current wireless local-area networks and digital video broadcasting since it is robust against the frequency-selective channels. However, there is still a crucial intercarrier-interference (ICI) problem due to Doppler effect, local frequency drift and sampling clock offset, associated with OFDM systems. Recently ICI self-cancellation schemes have been proposed to significantly reduce the ICI and empirically they greatly outperform the convolutional coding schemes adopted by the IEEE 802.11 standard. However, all existing ICI self-cancellation receivers are still sensitive to the phase and amplitude ambiguities due to the phase offset, the local oscillator frequency drift and the multipath reflections. Therefore, in this paper, a novel receiver design integrating the ICI self-cancellation with a proposed dynamic phase and amplitude estimation mechanism is introduced, which can well solve the ambiguity problem. The Monte Carlo simulation results show that our phase and amplitude estimators can greatly decrease the error probability for the final symbol detection in the ICI self-cancellation OFDM receivers.

General ICI Self-Cancellation Scheme for OFDM Systems

One of the challenges in designing orthogonal frequency-division multiplexing (OFDM) systems is their inherent sensitivity to any frequency shift in the signal. A frequency offset between the local oscillators at the transmitter and receiver causes a single frequency shift in the signal, while a time-varying channel can cause a spread of frequency shifts known as the Doppler spread. Frequency shifts ruin the orthogonality of OFDM subcarriers and cause intercarrier interference (ICI); therefore, quickly diminishing the performance of the system. ICI self-cancellation schemes have been proposed to reduce the sensitivity of OFDM systems to frequency shifts. These schemes use signal processing and frequency domain coding to reduce the amount of ICI generated as a result of frequency shifts, with little additional computational complexity. These methods can be used as an alternative to the fine frequency-offset estimation methods to battle oscillator frequency offset or simply be used as an ICI mitigation technique when the system is operating over time-varying channels. We propose a general ICI self-cancellation scheme that can be implemented through windowing at the transmitter and receiver. We show that the previously proposed self-cancellation schemes are equivalent to special cases of this method. Through theoretical analysis of the signal-to-interference ratio and bit-error rate and the use of Monte Carlo simulations, we demonstrate that the proposed system considerably outperforms the existing systems in the presence of frequency offset or time variations in the channel. We consider both coherent and noncoherent systems.

Analysis of ICI Cancellation Scheme in OFDM Systems With Phase Noise

Phase noise in orthogonal frequency division multiplexing (OFDM) systems destroys the orthogonality of the subcarriers and inter-carrier interference (ICI) is caused. In this paper, the ICI self-cancellation scheme is adopted to combat the ICI caused by phase noise in OFDM systems. Moreover, the error coefficients are defined and the theoretical expressions of carrier to interference ratio (C/I) with and without the ICI self-cancellation scheme are separately derived. From the simulation results, it is verified that the ICI self-cancellation scheme obviously decreases the amount of the ICI caused by phase noise and the improvement of C/I could reach 10 dB when the normalized 3 dB bandwidth of phase noise is 0.4. However, the convolutional coding OFDM (COFDM) system could supply more performance gain at the expense of increasing decoder complexity compared to OFDM system with the ICI self-cancellation scheme in the frequency-selective channel.

Intercarrier interference self-cancellation scheme for OFDM mobile communication systems

For orthogonal frequency-division multiplexing (OFDM) communication systems, the frequency offsets in mobile radio channels distort the orthogonality between subcarriers resulting in intercarrier interference (ICI). This paper studies an efficient ICI cancellation method termed ICI self-cancellation scheme. The scheme works in two very simple steps. At the transmitter side, one data symbol is modulated onto a group of adjacent subcarriers with a group of weighting coefficients. The weighting coefficients are designed so that the ICI caused by the channel frequency errors can be minimized. 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 redundant modulation causes a reduction in bandwidth efficiency, it can be compensated, for example, by using larger signal alphabet sizes. Simulations show that OFDM systems using the proposed ICI self-cancellation scheme perform much better than standard systems while having the same bandwidth efficiency in multipath mobile radio channels with large Doppler frequencies.