Parallel Cancellation Scheme Research Articles

Tutorial for Space-Time ICI Parallel Cancellation Techniques for OFDM Systems

Orthogonal frequency division multiplexing (OFDM) expects the subcarriers to be orthogonal. However, the factors, such as residual carrier frequency, offset time variations due to Doppler shift or phase noise leads to a loss in the orthogonality between subcarriers and results in inter-carrier interference (ICI). Further developing the parallel cancellation (PC) scheme to mitigate the ICI of OFDM systems, the authors expand this OFDM symbol-based PC scheme into a space-time (ST) coded system. This simple space-time parallel cancellation (STPC) scheme is a technique that combines the useful properties of ST and PC schemes together. Computer simulations indicate that OFDM systems using the STPC scheme outperform the regular PC and ST systems in slow and fast frequency selective fading channels, specifically at a high signal-noise ratio (SNR). Furthermore, the error floor of the STPC-OFDM system is significantly lower than that of the regular ST systems without increasing computational load.

Space-Frequency Parallel Transmitter Diversity Techniques for Three Pre-Coded OFDM Systems

This brief presents three space-frequency (SF) pre-coded orthogonal frequency division multiplexing (OFDM) systems to combat inter-carrier interference in mobile fading channels without knowing the channel state information at the transmitter. First, a $2{\times }1$ SF architecture is developed to form a $2{\times }1$ SF-OFDM system. Second, the parallel cancellation scheme is integrated with this SF-OFDM system to form a new SFPC-OFDM system. Third, three orthogonal transforms, Walsh–Hadamard (WH), Haar, and Slant, are employed as the pre-coder to form WHSFPC-, HAARSFPC-, and SLANTSFPC-OFDM systems. All three pre-coded systems improve bit error rate without expanding power or bandwidth in comparing with the SFPC scheme. Simulations indicate that both WHSFPC- and SLANTSFPC-OFDM systems outperform both SFPC- and HAARSFPC-OFDM systems, while SFPC-OFDM systems outperform SF-OFDM systems. All three pre-coded SFPC schemes are simple and backward compatible with the existing OFDM system. They may serve as baseband building blocks in OFDM transceivers and can be expanded to multiple input and multiple output systems.

Robust Multiuser Interference Cancellation for OFDM Systems With Frequency Offset

In orthogonal frequency division multiple access systems clusters of subcarriers are assigned to different users for parallel data transmissions. The subcarriers are overlapped, but orthogonal to each other such that there is no intercarrier interference (ICI). However, synchronization errors among users cause the loss of the orthogonality and introduce ICI resulting in multiple-access interference. Synchronization between users is particularly difficult in the uplink channel where the user signals are potentially asynchronous and affected by different frequency offsets due to misalignment in carrier frequencies and Doppler shifts. This paper proposes a method to lower the effects of different frequency offsets among user signals in an OFDMA uplink system. The multiple access interference due to the user frequency misalignments is reduced by reconstructing and removing the interfering signals in the frequency domain. An approach based on the selective cancellation method, is proposed and its performance is analyzed by means of theoretical analysis and computer simulations. The effectiveness of the proposed system has been evaluated in the case of ideal and no-ideal frequency offset estimation and has been compared with that of the classical successive and parallel cancellation schemes. Simulation results show that the proposed approach allows performance close to the ideal case, i.e., with ideal frequency synchronization among users, with a low increase of the implementation complexity. Moreover, it is also highlighted here, that the successive cancellation method slightly outperforms the selective scheme, at the expense of a higher computational complexity and processing delay

ICI cancellation for OFDM communication systems in time-varying multipath fading channels

In orthogonal frequency division multiplexing (OFDM) systems, time-varying multipath fading leads to the loss of subcarrier orthogonality and the occurrence of intercarrier interference (ICI). In this study, an efficient ICI suppression with less noise enhancement for multicarrier equalization is presented by using a parallel canceling scheme via frequency-domain equalization techniques, with the assumption that the channel impulse response (CIR) varies linearly during a block period. In order to avoid performance deterioration due to unreliable initial estimations in the parallel cancellation scheme, a cost function with proper weighting factor is introduced to improve the performance of the proposed equalizer. The proposed equalizer consists of a set of prefilters and a set of ICI cancellation filters, with two stages to perform different functions to achieve minimum mean square error (MMSE) equalization. The prefilters compensate for the multiplicative distortion at the first stage, and the ICI cancellation filters remove the effects of ICI by a parallel cancellation scheme at the second stage. Finally, the performance of the proposed equalizer is analyzed and compared with that of other equalizers, indicating significant performance improvement.

Performance Evaluation of Fiber-optic Synchronous Code Division Multiple Access Networks with Parallel Cancellation Scheme for PIN+OA receiver

In this paper, performance of PIN+OA receiver with inter-user interference cancellation technique based on parallel cancellation scheme for synchronous code division multiple access (S/CDMA) network has been analysed. The expression for probability of error is derived taking into consideration the inter-user interference, optical amplifier noise and various sources of receiver noise. The numerical results are computed for practical values of parameters and the results are compared with APD receivers. It is observed that the PIN+OA receiver provides an improvement of about 6.5 dB in the received optical power level over APD receiver for a given requirement.

Performance of an adaptive successive serial-parallel CDMA cancellation scheme in flat Rayleigh fading channels

The performance of a successive concatenated cancellation scheme for code-division multiple-access (CDMA) uplink transmission in cellular mobile radio is presented. Both serial and parallel cancellation stages are employed. The serial cancellation stage is first used to obtain initial data estimates followed by the parallel cancellation stages to enhance the accuracy of the estimates. The performance of this scheme is evaluated via analysis and simulation. In our analysis, we develop a model to consider the impact of wrongly estimating the phase and amplitude of the channel impulse response on the successive concatenated cancellation scheme. Analysis and simulation results in flat Rayleigh fading asynchronous channels with both perfect and nonperfect channel estimation and with perfect ranking confirms the accuracy of our analytical model as well as the significant improvement in performance compared to the conventional single-user matched filter (MF) detection and the stand-alone parallel cancellation schemes. Analytical results also show that the concatenated scheme has the potential to reach the single-user performance bound for a wide range of user base size, up to K=120 users, with a processing gain of 127 using one serial and three parallel cancellation stages. Finally, we propose a method whereby the receiver adapts with the number of users in order to retain the bit error ratio (BER) performance while decreasing the processing delay.

Improving the capacity of CDMA systems using convolutional coding and interference cancellation

The authors compare methods of reducing the co-channel interference on a DS-CDMA downlink signal by employing convolutional coding at the base station and a Viterbi decoder as the initial estimator for a parallel cancellation scheme. The combination of a matched filter and the Viterbi algorithm is shown to afford increased performance compared to equivalent bandwidth systems which do not use forward error correction. The use of a Wiener filter to collapse the spectrum of the spread signal leads to even greater capacity improvements. The systems discussed may be implemented with an affordable increase in receiver complexity and processing delay.