Maximum entropy whitening algorithm with adaptive neuron slope for blind DFE

In this paper, a cyclic-prefixed (CP) system with pulse amplitude modulation (PAM) for uplink transmission in light-fidelity (LiFi) is discussed. LiFi supports bi-directional transmission, i.e., downlink and uplink transmissions. However, there have been only a few studies on uplink LiFi despite the fact that the uplink traffic demand increases due to frequent use of, for example, video call and live streaming services. The uplink transmission is designed such that a transmitter has a low peak-to-average ratio as well as a lower computational complexity using a CP in conjunction with PAM. We investigate three different schemes, namely linear equalization (LE) where only a feedforward filter is applied, decision feedback equalization (DFE) where a feedback filter is added at the receiver and Tomlinson-Harashima precoder (THP) where a feedback filter is added at the transmitter. Both feedforward and feedback filters are jointly optimized. In addition, we consider an optical wireless channel with reflections, randomly-located and randomly-oriented user equipment, a human body acting as a reflector and a limited linear dynamic range of a light emitting diode. We conclude that a DFE system is suitable in a low spectral efficiency region, where, compared to an LE system, up to 4 dB gain is achieved. A THP system is suitable for a high spectral efficiency region, where up to 5 dB gain compared to the LE and DFE systems is achieved.

The simplified Gaussian clustering algorithm (SGCA) adopting a reduced constellation is derived, then the dual-variance Gaussian clustering algorithm (DVGCA) is proposed, which makes the soft or hard decisions using the SGCA. Since the DVGCA is extremely reduced-complexity and operates in the soft or hard decisions mode according to the variance for error level of the equalizer output, it is effective for blind equalization of high-order quadrature amplitude modulation (QAM) signals. In addition, the blind decision feedback equalizer (DFE) using the DVGCA is presented, which leads to superior performance to ones without a feedback path over the channel with in-band spectral nulls.

Multiple-input multiple-output (MIMO) digital communication systems have received great attention due to their potential of increasing the overall system throughput. In such systems, MIMO decision feedback equalization (DFE) schemes are often used to mitigate intersymbol interference (ISI) resulting from channel multipath propagation. In this context, the existing computationally efficient methods for exact estimation of the DFE filters under minimum mean-square-error criteria (MMSE-DFE) rely on fast Cholesky decomposition and backsubstitution or Levinson techniques. These methods may still present several difficulties in implementation as the demand on higher transmission rates increases, and thus a simple solution is necessary. In this paper, new procedures for fast computation of the MIMO-MMSE-DFE are presented. The new algorithms are obtained from a simple observation, namely, that the optimal feedforward filter (FFF) is related to the well-known Kalman gain matrix, commonly encountered in fast recursive least squares adaptive algorithms-for which fast recursions exist and are readily applicable. Moreover, the feedback filter can be easily computed via stable fast MIMO convolution techniques. As a result, the proposed method is less complex, more structured, and can be as reliable in finite precision as known approaches in the literature.

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.

Computationally efficient methods for blind decision feedback equalization of QAM signals

We propose and numerically investigate a method for direct in-phase quadrature (IQ) modulation of spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) for coherent communications. In particular, an injection-locked spin-VCSEL is investigated by using spin-flip rate equations for generating quadrature amplitude modulation (QAM) signals. A polarizer 45° tilted to the polarization axis of the injected external light is used to extract the in-phase and quadrature-phase components of the output light from the injection-locked spin-VCSEL. It is confirmed that QAM signals can be generated with the proposed method successfully, and long electron spin relaxation time and small linear birefringence in the spin-VCSEL are suitable for efficiently generating clear QAM constellations with a low error vector magnitude. A detuning condition for the injection locking of the spin-VCSEL is found to be important for obtaining wide modulation bandwidth of the direct IQ modulation.

This paper presents a unifying technique for robust feedback filter design in the decision feedback equaliser (DFE). The robustness is needed for combatting error propagation effects which is the major limitation when using DFEs. Numerous constraint strategies for the design of robust DFEs are available in the literature, however, they have been presented in their own ad-hoc context. Consequently there is a desire to give a unified approach to robust feedback filter design. The approach in the current presentation is to first develop a mean square error (MSE) measure which is a function of the feedforward and feedback filters. This measure is then combined with a feedback filter constraint using four existing norm constraint approaches (the L/sub 1/, L/sub 2/, L/sub /spl infin// and the two tap norm). The use of second order cone programming allows these existing norm constraint strategies to be unified and solved in a computationally efficient manner. The evaluation is performed with respect to the impact on error propagation, through the mean and the variance of the burst error length.

Interpolated decision feedback equalization for long intersymbol interference channels

An important challenge for implementing optical signal processing functions such as wavelength conversion or wavelength data exchange (WDE) is to avoid the introduction of linear and nonlinear phase noise in the subsystem. This is particularly important for phase noise sensitive, high-order quadrature-amplitude modulation (QAM) signals. In this paper, we propose and experimentally demonstrate an optical data exchange scheme through cascaded 2nd-order nonlinearities in periodically-poled lithium niobate (PPLN) waveguides using coherent pumping. The proposed coherent pumping scheme enables noise from the coherent pumps to be cancelled out in the swapped data after WDE, even with broad linewidth distributed feedback (DFB) pump lasers. Hence, this scheme allows phase noise tolerant processing functions, enabling the low-cost implementation of WDE for high-order QAM signals. We experimentally demonstrate WDEs between 10-Gbaud 4QAM (4QAM) signal and 12.5-Gbaud 4QAM (16QAM) signal with 3.5-MHz linewidth DFB pump lasers and 50-GHz channel spacing. Error-free operation is observed for the swapped QAM signals with coherent DFB pumping whilst use of free-running DFB pumps leads to visible error floors and unrecoverable phase errors. The phase noise cancellation in the coherent pump scheme is further confirmed by study of the recovered carrier phase of the converted signals. In addition to pump phase noise, the influence of crosstalk caused by the finite extinction ratio in WDE is also experimentally investigated for the swapped QAM signals.

Remote monitoring of patients using wireless capabilities increasingly plays an important role in healthcare and the communication with implanted medical devices (IMDs). In the case of IMDs, transmission is characterized by low peak power and low duty cycle to reduce the potential for adverse bio-effects. Currently, most IMDs use radio-frequency (RF) electromagnetic waves to communicate through the body. The corresponding maximum bandwidth allowed is 300 kHz as regulated by the FCC, which inherently limits the communication rates of these devices (limiting to maximum data rates of 50 kb/s). Beyond bandwidth restrictions, the main limitation for using RF electromagnetic waves in the body is loss of signal because of attenuation in the body. For these reasons the output power of RF devices is limited to 25 µW, which also limits data rates to 50 kb/s. In this study, an ultrasonic communication channel is explored for IMD communications as an alternative to EM transmission. We demonstrate the ability to communicate through tissues using the ultrasonic communications channel while maintaining data rates necessary for streaming of high definition video. In our experiments we used a matched pair of 5-MHz f/3 single-element transducers operating in a pitch-catch configuration in a tank of degassed water. Between the transducers was placed either a slab of pork tenderloin or a slab of beef liver (each more than 2 cm thick). Quadrature amplitude modulation (QAM) and quadrature phase shift keying (QPSK) signals were coded for transmission into the transmitting transducer using an arbitrary waveform generator. Signals were transmitted and captured by sending 10 snapshots (packets) of 50,000 samples (10,000 training / 40,000 decision-directed) using a fractionally-spaced (2 samples per symbol) decision feedback equalizer with up to 40 taps in the feed forward section, and 40 taps in the feedback section. Signals were successfully decoded through the samples using QPSK, 16QAM and 64QAM codes with bit error rates of (BER) < 1e-4. A maximum data rate of 30 Mbps using the 64QAM was achieved through both the pork tenderloin and the beef livers. These experiments successfully demonstrate that communications through the ultrasonic channel can provide data rates capable of streaming high definition video.

We describe a new robust feedforward filter (FFF) for multi-level decision feedback equalization (MDFE) which has only four poles and one zero. This FFF has low complexity and low power consumption. We evaluate the performance degradation caused by the dispersion of the FFF. With updating the feedback filter (FBF), there is a 0.2-dB loss or less in the required SNR, even though the Qs or pole frequencies are degraded within +/-10%.

This paper analyzes the performance of space-time decision feedback equalization (STDFE) assisted multiuser detection (MUD) for multiple-antenna space division multiple access (SDMA) systems to improve system capacity over dispersive fading channels. The MUD-STDFE receiver consists of a bank of matched filters and symbol-spaced feedforward filters (FFF) that both spatially and temporally whitens noise and the precursive parts of multiple access interference (MAI), followed by a bank of causal feedback filters (FBF) to cancel postcursive ISI and MAI for each user. The algorithm for computing this multi-dimensional FFF and FBF coefficients is presented, based on minimum mean-squared error (MMSE) optimality criterion. The error probability is estimated accurately and efficiently using the Gauss quadrature rule (GQR). The proposed MUD-STDFE receiver is capable of improving the performance significantly, separating the users and offering a significant performance gain relative to linear MMSE detection in an SDMA system.

In this paper, we focus on the automatic modulation for unsynchronized quadrature amplitude modulation (QAM) signals in both time and frequency in the practical communication systems. In practice, the receiver has little prior knowledge about the transmitted signal such as the timing error and frequency offset, directly applying the log-likelihood estimation to recognize the QAM signal would cause the decline in the recognition performance. To resolve this challenging problem, the paper presents the improved recognition scheme: before recognition, apply the Gardner loop timing synchronization and differential approach to the received signal. Simulation result shows that the proposed scheme is robust to timing and frequency offset and when SNR≥12dB and the number of symbols N is more than 1000, the successful classification rate of the 16QAM signal reach 98%.

In this work, we base on multi-layered perceptron neural networks with backpropagation algorithm (MLP/BP) to construct soft decision feedback equalizers (DFEs). The proposal is used to recover distorted 16-point quadrature amplitude modulation (16-QAM) signal. For better performance, error control codes (ECC) are applied to enhance the accuracy of the transmitted data. From the simulations, we note that the MLP/BP-based soft DFEs with bit-interleaved TCM can recover severe distorted 16-QAM data as well as suppress intersymbol interference (ISI) and background additive white Gaussian noise (AWGN). As compared with the LMS DFE, the proposed scheme can provide better bit-error-rate (BER) and packet-error-rate (PER) performance.

The Costas loop can be considered an extension of the phase-locked loop (PLL). The PLL, invented 1932 by French engineer Henri de Belleszice, was first used for the synchronous reception of amplitude modulated radio signals with carrier (c). If the modulation index is less than 100%, the modulated signal is always in phase with the carrier signal (c), irrespective of the polarity of the modulating signal. The PLL then reconstructs a local carrier (loc) that is in phase the carrier (c) of the AM signal. The modulating signal is recovered by multiplying the AM signal with the local carrier (c) with subsequent lowpass filtering. This method of synchronous reception fails, however, when an AM signal with suppressed carrier is transmitted. Here the modulated signal is in phase with the (suppressed) carrier when the modulating signal is positive, but it is in antiphase with the carrier when the modulating signal is negative. When this AM signal is fed to the input of a PLL, it would create a local carrier (loc) that is in phase with the carrier when the modulating signal is positive, but in antiphase with the carrier when the modulating signal is negative. Recovery of the modulating signal is therefore not possible. The situation would be the same when the transmitted signal is a Binary Phase Shift Keying (BPSK) signal, a Quadrature Phase Shift Keying (QPSK) signal, or a Quadrature Amplitude Modulation (QAM) signal. This problem has been solved by the invention of the Costas loop by American engineer James P. Costas in 1956. The Costas loop is able to lock onto a modulated signal also in cases where the carrier is suppressed. When we compare the topology of a Costas loop with that of a PLL, we recognize that the PLL is built up from a cascade of three circuits: (1) a phase detector, (2) a loop filter, and (3) a voltage-controlled oscillator (VCO). There is a feedback path from the output of the VCO to the input of the phase detector. In case of the PLL, the input signal is propagated through this cascade of three blocks. There is only one signal path in the PLL. In contrast, there are two signal paths in the Costas loop: (1) a so-called “I branch” (in phase), (2) a so-called “Q branch” (quadrature phase). It is shown that with that configuration the loop is able to create a local carrier that is correctly locked to the carrier of the received signal even in cases when the carrier is suppressed. The invention of the Costas loop had a dramatic impact on any kind of communication receivers; one or more Costas loops are found in TV, radio, satellite communications, mobile phones, modems, etc.