Bidirectional Decision Feedback Equalizer Research Articles

Transmission Over SSMF at 850 nm: Bimodal Propagation and Equalization

The combination of 850 nm vertical-cavity surface-emitting laser (VCSEL) with standard single-mode fiber (SSMF) presents an effective and low-cost interface to increase the reach provided by multi-mode fiber links. At 850 nm, SSMF propagates two modes, and in this study, it has been experimentally shown that the different commercially available SSMF's present dissimilar values of differential mode delay. To cope with this unequal behavior of modal dispersion, we propose a scheme based on bidirectional decision feedback equalization (BiDFE) to overcome limited performance of other solutions as mode filtering or classical equalizers. A single span SSMF cabling model, including a measurement-derived statistical characterization of optical connectors, is simulated to evaluate the reach provided by the equalizer attending to both the conditions of the fiber excitation and the characteristics of the VCSEL. A minimum 1.45 km link length at 10 Gb/s is achieved if a linear combining BiDFE (LC-BiDFE) equalizer is included in the receiver, whatever laser launching condition and employing a single-transverse mode VCSEL. If a multi-transverse mode VCSEL is used, the reach provided by LC-BiDFE is slightly reduced but assuring a minimum coverage of 1.15 km.

Experimental demonstration of phase-coherent underwater acoustic communications using a compact array

High-rate phase coherent acoustic communication systems generally require large arrays to obtain spatial diversity. Large arrays may not be suitable for small underwater platforms such as underwater unmanned vehicles (UUVs). In this paper, acoustic communication utilizing a compact array with closely spaced elements was demonstrated using the data collected during the August 2010 lake experiment. In addition, a bidirectional multichannel decision feedback equalizer (DFE) was developed to enhance the performance of communication systems. The recorded data from three communication ranges of 25 m, 920 m, and 1 300 m were processed to verify the system performance. The results showed an improvement in output symbol signal-to-noise ratio (OSNR) of approximately 3.4 dB, and 4.9 dB for bidirectional two-channel DFE and four-channel DFE over a single-channel DFE, respectively.

Bidirectional equalization for underwater acoustic communications

The bi-directional decision feedback equalizer (BiDFE) that combines the outputs of a conventional DFE and backward DFE can improve the performance of the conventional DFE by up to 1-2 dB based on simulations. In this paper, the BiDFE concept is extended to multi-channel time reversal communications involving a DFE as a post-processor. Experimental data collected in shallow water (10-20 kHz) show that the performance can be enhanced by 0.4-1.8 dB in terms of output SNR. In particular, a larger improvement (e.g., 1.8 dB) is achieved for time-varying channels where the channel diversity in opposite directions is more profound.

Bidirectional equalization for underwater acoustic communication

The bidirectional decision feedback equalizer (BiDFE) that combines the outputs of a conventional decision feedback equalizer (DFE) and backward DFE can improve the performance of the conventional DFE by up to 1-2 dB based on simulations. In this letter, the BiDFE concept is extended to multichannel time reversal communications involving a DFE as a post-processor. Experimental data collected in shallow water (10-20 kHz) show that the performance can be enhanced by 0.4-1.8 dB in terms of output signal-to-noise ratio. In particular, a larger improvement (e.g., 1.8 dB) is achieved for time-varying channels where the channel diversity in opposite directions is more profound.

A Variable-Rate Spread-Spectrum System for Underwater Acoustic Communications

A key research area in underwater acoustic (UWA) communication is the development of advanced modulation and detection schemes for improved performance and range-rate product. In this communication, we propose a variable-rate underwater data transmission system based on direct sequence spread spectrum (DSSS) and complementary code keying (CCK), particularly for shallow-water acoustic channels with severe multipath propagation. We provide a suboptimum receiver that consists of a bidirectional decision feedback equalizer (BiDFE) to cancel both postcursor and precursor intersymbol interference (ISI). We also develop iterative signal processing and time-reversal (TR) diversity processing to mitigate the effect of error propagation in BiDFE. We present performance analysis on bit error rate (BER) for different data rates. Our works show that this new variable-data-rate DSSS-CCK is a suitable candidate for UWA communications over varying channel conditions and distance.

Hard- and soft-output trellis-based conflict resolution for bidirectional decision feedback equalization

The decision feedback equalizer (DFE) is widely used in practice for intersymbol interference (ISI) mitigation because of its low complexity. The performance of the DFE can be enhanced by arbitrating between the output of a standard DFE and a second DFE that operates on the time-reversed received sequence. The combination is called a bidirectional DFE (BDFE). Several previous papers have discussed how to arbitrate between the outputs of the forward and reverse equalizers to make hard decisions. We show that the arbitration algorithms proposed by other researchers are inefficient and propose a trellis-based conflict resolution (TBCR) algorithm that simultaneously provides better performance and lower complexity. We further show how the TBCR approach can be used to create soft-output BDFE algorithms for use with soft-input decoding of an error-control code. The soft-output BDFE algorithms offer better performance than previously proposed algorithms while requiring lower complexity than the maximum a posteriori equalizer. We also propose a novel method to reduce the computational complexity of BDFE algorithms by simultaneously computing the coefficients of the forward and reverse equalizers.

Bidirectional equalizer arbitrated by Viterbi decoder path metrics

A bidirectional decision feedback equalizer (DFE) is proposed with the arbitration of the Viterbi decoder. The proposed DFE uses the minimum path metric between Viterbi decoders for the forward and reverse DFEs in the arbitration process. It has over 2 dB SNR gain against the conventional bidirectional DFE and the single DFE.

Efficient Bidirectional DFE for Doubly Selective Wireless Channels

The bidirectional decision feedback equalizer (DFE) performs two equalizations, one on the received signal and one on its time-reversed version. In this paper, we apply the bidirectional DFE to wireless transmissions on rapidly time-varying dispersive channels and we propose an efficient implementation obtained by implementing the feedforward filter in the frequency domain. The feedback filter is adapted to the channel variations within one block and we propose a simplified design of the feedback filter coefficients based on a polynomial model of channel variations. Simulations performed on time-varying channels show that the proposed structure significantly outperforms existing architectures.

Serially Connected Bi-Directional DFE Suitable for 8-VSB Modulation

A serially connected bi-directional decision feedback equalizer (SB-DFE) is proposed to improve the performance of channel equalization by exploiting implicit multipath diversity. The SB-DFE improves equalization performance by cascading normal and time-reversed DFEs. Conventional bi-directional DFEs obtain improved performance by combining the two DFEs in parallel. They, however, need accurate channel estimation and the performance is not guaranteed for multi-level modulation because they utilize each DFEs hard decided output symbols. On the other hand, the SB-DFE utilizes the soft output of the normal DFE at the first DFE as the input to the following time-reversed DFE without channel estimation. The performance of the SB-DFE is compared with that of the normal DFE and the bi-directional arbitrated DFE (BAD) in the 8-level vestigial sideband (8-VSB) modulation system with Brazilian digital high-definition television (HDTV) test channels. The SB-DFE has 1 /spl sim/ 1.8 dB signal-to-noise ratio (SNR) gains over the others.

A soft-output bidirectional decision feedback equalization technique for TDMA cellular ratio

Issues encountered in the design of reliable narrowband time-division multiple access (TDMA) digital cellular mobile communication systems are considered. In particular, the problem of compensating for the harsh multipath fading environment in systems whose transmission bandwidth is commensurate with the coherence bandwidth of the fading channel is considered. A TDMA channel characterization parameter, the slot-normalized fade rate, is introduced, and an adaptive bidirectional equalization technique, which estimates the location of a deep fade within a time slot, is proposed. The simulation results show that the carrier-to-noise ratio requirement is only 15.5 dB when this equalization technique is used. This is achieved without diversity, and with low complexity. An equivalent equalized land mobile radio channel model and the analytical solution for the optimal bit likelihood calculation for pi /4-shift quadrature differential phase-shift keying (QDPSK) modulation are also derived under certain channel conditions. The results are used as soft decisions for the convolutional decoder. >