Adaptive self-orthogonalizing per-tone decision feedback equalizer for single carrier modulations

Faster-than-Nyquist (FTN) signaling was originally proposed by Mazo, which provides the opportunity of increasing data transmission rate effectively. However, this induces unavoidable ISI effects at the receiver and leads to higher receiver complexity. So far, a number of time domain equalization receiver algorithms have been proposed, which have higher demodulation complexity in FTN system with infinite ISI. Against this background, people put forward low complexity iterative frequency domain decision feedback equalization and simulate to show that frequency domain equalization can reduce computational complexity of FTN. In order to promote the practical application of FTN, this paper expands low complexity IBDFE to frequency-selective fading channels. At the same time, simulation results verify its feasibility.

We proposed an adaptive two-stage nonlinear feedforward equalizer and decision feedback equalizer to minimize the negative influence of nonlinear distortions. A data rate of 960 Mbit/s is experimentally achieved on the guided visible-light transparent transmission over a 100-m ultra-large effective area pure silica fiber. The experimental results verified the effectiveness and improvement of the proposed method compared with the traditional feedforward post-equalizers.

In broadband wireless communication systems, single-carrier transmission with frequency domain equalization (SC-FDE) is regarded as an alternative modulation technique to the orthogonal frequency division multiplexing (OFDM). Compared with OFDM systems, SC-FDE has similar implementation complexity and performance. However, SC-FDE with linear equalizer will suffer from serious performance deterioration under deep frequency selective fading channels. An effective method to solve this problem is to introduce non-linear decision feedback equalization (DFE) to SC-FDE. In this paper, a low complexity iterative decision feedback equalizer in the frequency domain for single- carrier systems is proposed. Based on the minimum mean square error (MMSE) criterion, a simplified parameter estimation method is introduced to calculate the coefficients of the feed-forward and feedback filters, which reduces the implementation complexity of the equalizer. Simulations show that the performance of the proposed algorithm is similar to the traditional iterative block DFE (IBDFE) under various multipath fading channels but with lower complexity.

In this paper, a method for weight control of an adaptive array is proposed for a system using a cyclic prefix in single carrier wireless communications (SC‐CP). In the SC‐CP system, the delayed wave within the guard interval (GI) can be equalized by discrete frequency domain equalization. Therefore, it is to be expected that the characteristics can be improved with respect to the received signal power by reception of several arriving waves within the GI. On the other hand, reception of several arriving waves leads to generation of zero points in the frequency response so that noise enhancement may be a problem for a discrete frequency domain equalizer with a zero forcing (ZF) reference. Also caused in the SC‐CP system are the intersymbol interference (ISI) and interblock interference (IBI) due to the delayed wave exceeding the GI length. In this paper, a weight control method is proposed that simultaneously realizes enhanced received signal power by reception of several arriving waves and avoidance of noise enhancement due to ZF reference equalizer. At the same time, the weight control method to suppress delayed waves outside the GI is theoretically derived and its characteristics are evaluated by a computer simulation. © 2005 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 88(9): 38–48, 2005; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/ecja.20189

In impulse radio ultra-wideband (IR-UWB) systems where the channel lengths are on the order of a few hundred taps, conventional use of frequency-domain (FD) processing for channel estimation and equalization may not be feasible because the need to add a cyclic prefix (CP) to each block causes a significant reduction in the spectral efficiency. On the other hand, using no or short CP causes the interblock interference (IBI) and thus degradation in the receiver performance. Therefore, in order to utilize FD receiver processing UWB systems without a significant loss in the spectral efficiency and the performance, IBI cancellation mechanisms are needed in both the channel estimation and equalization operations. For this reason, in this paper, we consider the joint FD channel estimation and equalization for IR-UWB systems with short cyclic prefix (CP) and propose a novel iterative receiver employing soft IBI estimation and cancellation within both its FD channel estimator and FD equalizer components. We show by simulation results that the proposed FD receiver attains performances close to that of the full CP case in both line-of-sight (LOS) and non-line-of-sight (NLOS) UWB channels after only a few iterations.

Single-Carrier Frequency Division Multiple Access (SC-FDMA) has been selected as the uplink transmission scheme in the 3GPP Long Term Evolution standard. SC-FDMA has reduced sensitivity to phase noise and a lower Peak-to-Average Power Ratio (PAPR) compared to Orthogonal Frequency Division Multiple Access. In this paper we propose joint Tomlinson-Harashima Precoding and transmit power allocation for SC-FDMA. We derive the optimum power allocation for SC-FDMA transmission for both Zero-Forcing (ZF) and Minimum Mean-Square Error (MMSE) LE receivers in order to maximize the achievable data rate subject to constant transmit power. Although this improves the system's performance and offers a 1-2 dB improvement over Frequency-Domain Decision Feedback Equalization (FD-DFE), when the proposed transmit power allocation scheme is combined with decision feedback equalization the system incurs a performance degradation due to error propagation. In this paper we propose a joint implementation of the derived power allocation scheme with THP. Here we show that the system's performance is further improved over both FD-LE and FD-DFE when transmit power allocation is applied.

A 10 Gb/s adaptive analog decision feedback equalizer (DFE) with 6 taps is realized in 0.13 μm CMOS. A modified Cherry-Hooper stage is designed as the delay element in the feed-forward path. An analog implementation of the LMS algorithm is used to continuously adapt the feedback filter coefficients. A clock and data recovery (CDR) circuit is used to extract the clock from the DFE output. The adaptive DFE dissipates 452 mW and can equalize PRBS data corrupted by a 300m-long multimode fiber (MMF) achieving BER <; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-13</sup> .

A novel PAM4 receiver with adaptive threshold voltage, adaptive decision feedback equalizer and fixed linear equalizer has been presented. The proposed techniques enable threshold voltage to be adjusted automatically depending on data rate, signal swing and loss of channel. Consequently the receiver can be used in various situations without being manually calibrated. Adaptive decision feedback equalizer for PAM4 signaling is proposed, incorporated with sign-sign least mean square algorithm. Simulation results across lossy channel show proper convergence of threshold voltage and decision feedback equalizer values with the proposed receiver.

A parallel adaptive decision feedback equalizer (ADFE) architecture is proposed. It allows adaptation at every sample to effectively equalize intersymbol interference for applications that operate at high sampling rates and require fast tracking capability. It is shown that if the effect of error propagation is small, the filter coefficients of the parallel ADFE will be exactly the same as those of a serially adapting ADFE. Simulation confirms that the parallel ADFE effectively tracks a time-varying channel with typical signal-to-noise ratios at a sampling rate four times higher than that of a serial ADFE, while under the same channel condition a previously proposed block ADFE fails to converge and becomes unstable.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Among existing works of high-speed pipelined adaptive decision feedback equalizer (ADFE), the pipelined ADFE using relaxed look-ahead technique results in a substantial hardware saving than the parallel processing or Look-ahead approaches. However, it suffers from both the signal-to-noise ratio (SNR) degradation and slow convergence rate. In this paper, we employ the predictive parallel branch slicer (PPBS) to eliminate the dependencies of the present and past decisions so as to reduce the iteration bound of decision feedback loop of the ADFE. By adding negligible hardware complexity overheads, the proposed architecture can help to improve the output mean-square error (MSE) of the ADFE compared with the Relaxed Look-ahead ADFE architecture. Moreover, we show the superior performance of the proposed pipelined ADFE by using theoretical derivations and computer simulation results. A VLSI design example using Avant! 0.35-/spl mu/m CMOS standard cell library is also illustrated. From the post-layout simulation results, we can see that the PPBS scheme requires only 38.4% gate count overhead, but it can help to reduce the critical path from 7.06 to 4.69 ns so as to meet very high-speed data transmission systems.

The adaptive multichannel Decision Feedback Equalizer (DFE) has been shown to be an effective algorithm for enabling reliable high rate acoustic communications in complex and time-varying underwater environments. The choice of the number of channels used in an equalizer presents a performance trade-off. The minimal achievable error that can be realized by the equalizer decreases as the number of channels increases. However, an increase in the number of channels increases the number of filter weights that need to be adapted. Thus, the computational complexity of least squares and Kalman type adaptation algorithms is proportional to the number of channels squared. In addition, the averaging interval required by an adaptation algorithm in order to achieve good performance grows linearly with the number of parameters. Thus, an increase in the number of M-DFE channels can reduce the rate of channel fluctuation that can successfully tracked by the M-DFE. The partitioning of an array into sub-arrays which are each independently equalized before combining their outputs can both improve performance and reduce complexity in processing real-world signals. The choice of the size and sensor locations for the subarrays is analyzed. The resulting adaptive subarray multichannel DFE algorithm is compared to other multichannel equalization algorithms.

This paper presents a 6.25Gb/s adaptive decision feedback equalizer (DFE) for high speed backplane communication, which consists of a high-speed DFE and an adaptation engine based on Sign-Sign LMS algorithm. Half-rate structure and current mode logic are employed to pursue a high operating speed. The adaptation engine is composed of sense amplifiers (SA), a 5-bit up/down counter and a 5-bit DAC, which are implemented in CMOS logic at 781.25MHz. Designed in 0.18µm CMOS process, the equalizer has an area of 0.6×0.55mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> including I/O pads, and the static power consumption is 45.9mW under 1.8V power supply. Measurement results show that the proposed equalizer can adapt to signals achieved from PCB channels of different lengths, and the data rate can be up to 6.8Gb/s with horizontal eye opening of outputs larger than 0.8UI.

In this paper, we design a practical time-reversal quasi-orthogonal space-time block code (TR-QO-STBC) system for broadband multi-input multi-output (MIMO) communications. We first modify the TR-QO-STBC encoding structure so that the interference between the transmitted blocks can be completely removed by linear processing. Two low complex decision-feedback equalization (DFE) schemes are then proposed. One is built from the frequency-domain decision-feedback equalization (FD-DFE). The derived bi-directive FD-DFE (BiD-FD-DFE) cancels the interference among the successive symbols along the time axis. The other one is the enhanced V-BLAST, which cancels the interference between the real and imaginary parts of the spectral components. They have distinct performance characteristics due to the different interference-cancellation strategies. The underlying orthogonal and symmetric characters of TR-QO-STBC are exploited to reduce the computational complexity. Computer simulations confirm that the proposed equalizers can achieve better performance than the existing schemes.

Block transmission systems with guard intervals usually either use a cyclic prefix or introduce a sequence of zeros of length greater than or equal to an upper bound of the channel memory between blocks to prevent inter-block interference. Another possibility is to transmit known symbols in the guard interval. Although this has been considered before, the proposed channel identification algorithm was computationally intensive. We propose a simpler algorithm based on first order statistics. At the receiver, the channel is estimated by first computing the mean of the relevant parts of the received blocks. To enhance the performance at high signal-to-noise ratio, we combine the first order statistical algorithm with a simple iterative source-channel estimation algorithm. There is no extra loss of information rate as opposed to training sequence based methods for systems with guard intervals, no condition imposed on the channel and no scaling ambiguity as opposed to second (or higher) order statistical or finite alphabet-based methods. Just like zero guard intervals, known symbol guard intervals guarantee free inter-block interference and channel irrespective symbol detectability.

Optical wireless communication (OWC) systems use the atmosphere as a propagation medium. However, a common problem is that from time to time moderate cloud and fog emerge between the receiver and the transmitter. These adverse weather conditions impose temporal broadening and power loss on the optical signal, which reduces the digital signal-to-noise ratio (DSNR), produces significant intersymbol interference (ISI), and degrades the communication system's bit error rate (BER) and throughput. We propose and investigate the use of a combined adaptive bandwidth mechanism and decision feedback equalizer (DFE) to mitigate these atmospheric multipath effects. Based on theoretical analysis and simulations of DSNR penalties, BER, and optimum system bandwidths, we show that a DFE improves the outdoor OWC system immunity to ISI in foggy weather while maintaining high throughput and desired low BER.