Intersymbol Interference Cancellation Research Articles

Novel channel estimation method based on PN sequence reconstruction for Chinese DTTB system

Digital television/terrestrial multimedia broadcasting (DTMB), announced as the Chinese digital television terrestrial broadcasting (DTTB) standard, uses pseudo-noise (PN) sequence as guard interval (GI) and the training sequence for both multi- and single-carrier block transmissions. The conventional channel estimation methods can be performed through either the subtraction of the PN sequence or the cancellation of residual inter-symbol interference (ISI). In this paper, a novel iterative method is introduced to estimate the overlapping part of PN sequence, meanwhile the channel impulse response (CIR) estimation can be obtained and updated via the reconstruction of the PN sequence in an iterative manner. Simulations show that the proposed algorithm can effectively reduce the ISI and improve the symbol error rata (SER) of DTMB system over slowvarying broadcasting multipath channel even with long delay spread. In practice, the proposed algorithm can be applied to the DTMB receiver-chip directly with low-complexity cost.

A dual combining based decoding scheme for two-branched mobile STBC

In this paper, a dual combining based decoding scheme is proposed to enhance the detection performance of space-time block coding with two transmit antennas in a highly time-varying mobile channel. The proposed scheme analyzes the signal-to-noise ratio and signal-to interference plus noise ratio of decision variables for a zero-forcing combiner (ZFC) and a matched filter combiner (MFC), respectively. Based on the analysis, we derive a sophisticated decoding scheme which utilizes tentative detection symbols of the ZFC based decoder to cancel inter-symbol interference from the decision variables of the MFC. Computer simulations show that the proposed scheme outperforms the conventional decoding scheme with the ZFC or the MFC.

Bandwidth-efficient OFDM transmission with iterative cyclic prefix reconstruction

For orthogonal frequency division multiplexing (OFDM), cyclic prefix (CP) should be longer than the length of channel impulse response, resulting in a loss of bandwidth efficiency. In this paper, we describe a new technique to restore the cyclicity of the received signal when the CP is not sufficient for OFDM systems. The proposed technique efficiently restores the cyclicity of the current received symbol by adding the weighted next received symbol to the current received symbol. Iterative CP reconstruction (CPR) procedure, based on the residual intersymbol interference cancellation (RISIC) algorithm, is analyzed and compared to the RISIC. In addition, we apply the CPR method to Alamouti space-time block coded (STBC) OFDM system. It is shown that in the STBC OFDM, tail cancellation as well as cyclic reconstruction of the CPR procedure should be repeated. The computational complexities of the RISIC, the proposed CPR, the RISIC with STBC, and the proposed CPR with STBC are analyzed and their performances are evaluated in multipath fading environments. We also propose an iterative channel estimation (CE) method for OFDM with insufficient CP. Further, we discuss the CE method for the STBC OFDM system with the CPR. It is shown that the CPR technique with the proposed CE method minimizes the loss of bandwidth efficiency due to the use of CP, without sacrificing the diversity gain of the STBC OFDM system.

Interference Cancellation Schemes for Single-Carrier Block Transmission with Insufficient Cyclic Prefix

This paper proposes intersymbol interference (ISI) and interblock interference (IBI) cancellation schemes at the transmitter and the receiver for the single-carrier block transmission with insufficient cyclic prefix (CP). The proposed scheme at the transmitter can exterminate the interferences by only setting some signals in the transmitted signal block to be the same as those of the previous transmitted signal block. On the other hand, the proposed schemes at the receiver can cancel the interferences without any change in the transmitted signals compared to the conventional method. The IBI components are reduced by using previously detected data signals, while for the ISI cancellation, we firstly change the defective channel matrix into a circulant matrix by using the tentative decisions, which are obtained by our newly derived frequency domain equalization (FDE), and then the conventional FDE is performed to compensate the ISI. Moreover, we propose a pilot signal configuration, which enables us to estimate a channel impulse response whose order is greater than the guard interval (GI). Computer simulations show that the proposed interference cancellation schemes can significantly improve bit error rate (BER) performance, and the validity of the proposed channel estimation scheme is also demonstrated.

SVD-Based Techniques for Zero-Padded Block Transmission Over Fading Channels

In this paper, the performance of filter bank transceivers in the presence of a dispersive time-variant channel is investigated. It is well known that filter bank transceivers can be adapted to the channel transfer function to yield intersymbol interference (ISI) cancellation. When the channel is time variant, several problems arise, since the transceiver should be changed whenever the channel evolves. In this paper, we will allow both the transmitter and the receiver to change and satisfy the interference-free condition, under the assumption of a zero-padded block transmission. In this case, the optimum transmitter-receiver pair can be computed by using a singular value decomposition (SVD) of the channel matrix. A fast receiver adaptation based on SVD tracking is presented. Simulation results show that minimum performance loss with respect to the optimum receiver can be achieved for our reduced complexity receiver

Optimum ISI and ICI Cancellation Filter for OFDM Systems without CP

In this letter, we consider an Orthogonal Frequency Division Multiplexing (OFDM) system without cyclic prefix (CP), i.e. with full spectral efficiency and present an optimum frequency-domain filtering scheme for mitigating inter-symbol interference (ISI) and inter-carrier interference (ICI). Simulation results show the signal to interference and noise ratio (SINR) improvement.

Cancellation of linear intersymbol interference for two-dimensional storage systems

This paper discusses the cancellation of linear intersymbol interference (ISI) in two-dimensional (2-D) systems. It develops a theory for the error rate of receivers that use tentative decisions to cancel ISI. It also formulates precise conditions under which such ISI cancellation can be applied effectively. For many 2-D systems, these conditions are easily met, and therefore the application of ISI cancellation is of significant interest. The theory and the conditions are validated by simulation results for a 2-D channel model. Furthermore, results for an experimental 2-D optical storage system show that, for a single-layer disk with a capacity of 50 GB, a substantial performance improvement may be obtained by applying ISI cancellation.

Advanced detection and coding techniques for nonlinear intersymbol interference cancellation in 40 Gb/s systems

To mitigate the fibre nonlinearities in high-speed transmission (at 40 Gb/s or higher), the use of maximum a posteriori probability (MAP) symbol decoding supplemented with iterative decoding is proposed. The MAP detector operates on the channel trellis to correct the corrupted data and to provide soft outputs processed further in an iterative decoder. Impressive performance improvement is demonstrated.

Efficient ISI cancellation for STBC OFDM systems using successive interference cancellation

In this letter, we propose an efficient detection scheme for space-time block coded (STBC) orthogonal frequency-division multiplexing (OFDM) with insufficient cyclic prefix (CP). The proposed scheme employs successive interference cancellation (SIC) and cyclic prefix reconstruction (CPR) concepts. Simulation results present that the proposed scheme outperforms the conventional scheme for STBC OFDM systems.

Efficient Time-Domain Residual ISI Cancellation for OFDM-Based WLAN Systems

In orthogonal frequency-division multiplexing (OFDM) systems, the multipath components whose delays exceed cyclic prefix (CP) cause intersymbol interference (ISI) and intercarrier interference (ICI), which may degrade system performance severely. In this paper, we propose two novel iterative cyclicity restoration (CR) schemes to efficiently cancel these interferences for OFDM-based wireless local area network (WLAN) systems. The time-domain parallel CR (PCR) scheme is introduced first. Using time-domain iterative solution, PCR provides much lower computational complexity than conventional CR. Furthermore, in order to improve the performance of PCR, we present successive CR (SCR) method based on Gauss-Seidel iteration. SCR offers faster convergence speed than PCR without increasing the computational cost. It is shown that both PCR and SCR need only a few iterations to approach the system performance with sufficient CP. SCR has approximately the same performance as conventional CR with much lower computational complexity for WLAN applications.

The soft-feedback equalizer for turbo equalization of highly dispersive channels

The complexity of a turbo equalizer based on the Bahl-Cocke-Jelinek-Raviv (BCJR) algorithm is manageable only for mildly dispersive channels having a small amount of memory. To enable turbo equalization of highly dispersive channels, we propose the soft-feedback equalizer(SFE). The SFE combines linear equalization and soft intersymbol-interference cancellation. Its coefficients are chosen to minimize the mean-squared error(MSE) between the equalizer output and the transmitted sequence, under a Gaussian approximation to the a priori information and the SFE output. The resulting complexity grows only linearly with the number of coefficients, as opposed to the quadratic complexity of previously reported minimum-MSE structures. We will see that an SFE-based turbo equalizer consistently outperforms another structure of similar complexity, and can outperform a BCJR-based scheme when complexity is taken into account.

Bidirectional iterative ISI canceller for high-rate DSSS/CCK communications

This paper proposes a new RAKE receiver incorporated with a bidirectional iterative intersymbol interference (ISI) canceller in order to reinforce multipath robustness of high-rate direct-sequence spread-spectrum complementary code keying (DSSS/CCK) systems. The proposed RAKE receiver first combines multipath signal components through a channel matched filter (CMF) and removes postcursor-ISI by employing a codeword decision feedback equalizer (DFE). Then, a CCK codeword detector tentatively determines the current CCK codeword symbol and reuses it to subtract precursor-ISI from the previous symbol. Therefore, the ultimate symbol decision is made using the delayed signal with both postcursor-ISI and precursor-ISI cancelled. The detection performance can be more improved through an iterative refinement processing between the postcursor and the precursor components. Simulation results exhibit a significantly improved error rate performance of the proposed receiver compared with that of the legacy RAKE receiver employing only a postcursor DFE. The additional cost for realization of the proposed receiver is one symbol decision delay and reuse complexity of the DFE and the codeword detector.

A Novel Zero-Order FIR Zero-Forcing Filterbanks Equalizer Using Oblique Projector Approach for OFDM Systems

In conventional OFDM systems, the effect of inter-block-interference (IBI) can be completely removed by inserting sufficient redundant symbols between successive transmission blocks. In this paper, based on the reformulated received block symbols of the discrete multirate filterbanks model, a new transceiver model for the cyclic prefix (CP) OFDM systems is proposed, associated with the oblique projector technique (view as the pre-processor for achieving IBI-free). Consequently, a novel ISI-free receiver with the zero-order FIR zero-forcing (ZF) filterbanks equalizer can be devised, under noise-free environment. For performance comparison the bit-error-rate (BER) is investigated for the cases of noisy and noise-free channels. In all cases, viz., the length of CP is shorter or longer than the order of the channel impulse response, we show that the same BER performance compared with the one suggested in [3] can be achieved, under the same assumptions and conditions. Since a simple cascade configuration of the IBI cancellation using the oblique projector followed by the ISI cancellation using the zero-order FIR ZF filterbanks equalizer can be realized for OFDM systems with sufficient or insufficient CP, the complexity of transceiver design can be reduced.

Robust channel estimation and ISI cancellation for OFDM systems with suppressed features

A feature-suppressed orthogonal frequency-division multiplexing (OFDM) system and the corresponding channel estimation and intersymbol interference (ISI) mitigation techniques are investigated in this paper. Cyclic prefix (CP) and pilot tones, which are commonly used in civilian OFDM systems for ISI mitigation and channel estimation, create distinctive waveform features that can be easily used for synchronization and channel estimation purposes by intercepting receivers. As a result, CP and pilot tones are eliminated in the proposed feature suppressed OFDM system to reduce the interception probability. Instead, a set of specially designed OFDM symbols, driven by different pseudorandom sequences, are employed as preambles to avoid unique spectral signature. These preambles are inserted into the OFDM data symbol stream periodically and in a round-robin manner. In addition, a random frequency offset is introduced to each preamble to further mask the multicarrier signature. New challenges arising from these feature suppression efforts are studied, including robust channel estimation and demodulation techniques in the presence of frequency offset and severe interference. Based on our interference analysis, an iterative ISI and intercarrier interference (ICI) estimation-cancellation-based technique is proposed for both channel estimation and OFDM data demodulation. Our channel estimator performs joint frequency offset and channel impulse response estimation based on the maximum-likelihood (ML) principle. To reduce its complexity, we employ a number of techniques, which include approximation of the ML metrics, as well as fast Fourier transform pruning. The performances and feasibility of the proposed feature suppressed OFDM system and the channel estimator are analyzed and verified through numerical simulations.

Iterative cyclic prefix reconstruction and channel estimation for a STBC OFDM system

In order to prevent a loss of spectral efficiency due to the use of a cyclic prefix (CP), a CP reconstruction (CPR) method, such as residual intersymbol interference cancellation (RISIC), has recently been developed. In this letter, we apply the RISIC method to an Alamouti space-time block coded (STBC) orthogonal frequency division multiplexing (OFDM) system with insufficient CP. It is shown that in the STBC OFDM, tail cancellation as well as cyclic restoration of the RISIC should be repeated. An iterative channel estimation method for the STBC OFDM system with CPR is proposed. The performance of the proposed method is evaluated by computer simulation in a multipath fading environment.

Parameter Acquisition Techniques for Multiuser Detection with ISI Cancellation in a Quasi-Synchronous Reverse Link MC-CDMA System

In a quasi-synchronous reverse link multicarrier code division multiple access system, the signal detection is vulnerable to the interference due to the insufficient guard interval. A multiuser detection with intersymbol interference cancellation is a potential solution to overcome this problem. In this paper, we proposes a parameter acquisition technique based on a specially designed training sequence for the receiver. The concerned parameters are the transformed signature sequences and the ISI generation sequences. We analyze a criterion for the training sequences to achieve the minimum mean square error and propose the systematic generation of the optimum training sequences. We also propose the noise variance estimator for providing information about noise variance to some classes of multiuser detection. Simulation results prove relevant benefits of the proposed techniques and give useful insights into the system designs.

Bandwidth efficient single carrier systems with frequency domain equalisation

A new bandwidth efficient single carrier system with frequency domain equalisation is proposed. Turbo equalisation and cyclic prefix reconstruction are combined together to eliminate the normally required cyclic prefix for such a system. The proposed method achieves much better performance than the residual inter-symbol interference cancellation approach by utilising soft decoded information and introducing turbo equalisation. The performance of the proposed scheme is studied through simulations.

A MIMO-OFDM Receiver Employing the Low-Complexity Turbo Equalization in Multipath Environments with Delay Difference Greater than the Guard Interval

When the multipath delay difference exceeds the guard interval (GI), the performance of MIMO-OFDM transmission suffers severely from both the inter-symbol interference (ISI) from the adjacent OFDM symbols and the inter-carrier interference (ICI) within the same symbol. This paper therefore proposes a MIMO-OFDM receiver employing the low-complexity turbo equalization. The proposed receiver initially separates the data streams and suppresses ICI by linear processing. In the iterative processing, it cancels the other data streams as well as ISI and ICI. The MIMO-OFDM turbo equalizer consists of an ISI canceller, an ICI canceller, an optimal detection filter, and a MAP detector. The proposed receiver can improve the transmission performance by exploiting the log-likelihood ratio that the decoding process produces for canceling both ISI and ICI and separating of the spatially multiplexed streams. Computer simulations, which apply the wireless LAN to MIMO, demonstrate that the proposed receiver can provide excellent performance in the severe multipath channels where the delay difference is greater than GI.

Efficient Cyclic Prefix Reconstruction for Coded OFDM Systems

This letter describes an improved method to mitigate interference when the cyclic prefix is not sufficient for coded orthogonal frequency-division multiplexing (OFDM) systems. The proposed method, based on the residual intersymbol interference cancellation (RISIC), efficiently restores the cyclicity of the ith received symbol by adding the weighted (i+1)th received symbol to the ith received symbol. Simulation results show that the proposed scheme outperforms the conventional RISIC in the symbol error rate even when the channel impulse response is longer than a half of the symbol duration.

ICI and ISI analysis and mitigation for OFDM systems with insufficient cyclic prefix in time-varying channels

Channel variation during an OFDM block leads to the loss of orthogonality among subcarriers, resulting in inter-carrier interference (ICI) in orthogonal frequency division multiplexing (OFDM) systems. Many schemes have been proposed to suppress ICI, but they are computationally complex or at the price of sacrificing bandwidth. In some cases, such as high-density television (HDTV) broadcasting and satellite OFDM systems, the very long delay spreads pose the possibility that the channel length exceeds that of the moderate cyclic prefix (CP), resulting in intersymbol interference (ISI) and intercarrier interference (ICI). A time-domain equalizer (TEQ) is usually used in the receiver to reduce the duration of the overall response of the transmission system, and therefore minimize the ISI and ICI. However, the optimum design of TEQ turns out to be a difficult task. The ICI's introduced by insufficient CP and channels variations are different and usually exploit different methods to mitigate them. In this paper, we provide an ICI and ISI analysis in time- and frequency- domain for OFDM systems with insufficient CP over time-variant channels. Basing on this analysis, we propose an iterative method for joint ICI and ISI cancellation whose complexity is linear in the OFDM symbol length. Theoretical analysis and simulation results indicate that the proposed method can effectively mitigate ICI and ISI introduced by channel variation and insufficient CP with high bandwidth efficiency.