In this paper, we investigated the overfitting characteristics of nonlinear equalizers based on an artificial neural network (ANN) and the Volterra series transfer function (VSTF), which were designed to compensate for optical nonlinear waveform distortion in optical fiber communication systems. Linear waveform distortion caused by, e.g., chromatic dispersion (CD) is commonly compensated by linear equalizers using digital signal processing (DSP) in digital coherent receivers. However, mitigation of nonlinear waveform distortion is considered to be one of the next important issues. An ANN-based nonlinear equalizer is one possible candidate for solving this problem. However, the risk of overfitting of ANNs is one obstacle in using the technology in practical applications. We evaluated and compared the overfitting of ANN- and conventional VSTF-based nonlinear equalizers used to compensate for optical nonlinear distortion. The equalizers were trained on repeated random bit sequences (RRBSs), while varying the length of the bit sequences. When the number of hidden-layer units of the ANN was as large as 100 or 1000, the overfitting characteristics were comparable to those of the VSTF. However, when the number of hidden-layer units was 10, which is usually enough to compensate for optical nonlinear distortion, the overfitting was weaker than that of the VSTF. Furthermore, we confirmed that even commonly used finite impulse response (FIR) filters showed overfitting to the RRBS when the length of the RRBS was equal to or shorter than the length of the tapped delay line of the filters. Conversely, when the RRBS used for the training was sufficiently longer than the tapped delay line, the overfitting could be suppressed, even when using an ANN-based nonlinear equalizer with 10 hidden-layer units.

This paper presents the average block error rate (BLER) performance of circular 32QAM and 64QAM schemes employing a frequency domain equalizer (FDE) for discrete Fourier transform (DFT)-precoded orthogonal frequency division multiplexing (OFDM) in multipath Rayleigh fading channels. The circular QAM scheme has an advantageous feature in that the fluctuation in the amplitude component is smaller than that for the cross or rectangular QAM scheme. Hence, focusing on the actual received signal-to-noise power ratio (SNR) taking into account a realistic peak-to-average power ratio (PAPR) measure called the cubic metric (CM), we compare the average BLER of the circular 32QAM and 64QAM schemes with those of cross 32QAM and rectangular 64QAM schemes, respectively. We investigate the theoretical throughput of various circular 32QAM and 64QAM schemes based on mutual information from the viewpoint of the minimum Euclidean distance. Link-level simulation results show that the circular 32QAM and 64QAM schemes with independent bit mapping for the phase and amplitude modulations achieves a lower required average received SNR considering the CM than that with the minimum Euclidean distance but with composite mapping of the phase and amplitude modulations. Through extensive link-level simulations, we show the potential benefit of the circular 32QAM and 64QAM schemes in terms of reducing the required average received SNR considering the CM that satisfies the target average BLER compared to the cross 32QAM or rectangular 64QAM scheme.

In this paper, we address the problem of detector design in severely frequency-selective channels for spatial multiplexing systems that adopt filter bank multicarrier based on offset quadrature amplitude modulation (FBMC/OQAM) as the communication waveforms. We consider decision feedback equalizers (DFEs) that use multiple feedback filters to jointly cancel the post-cursor components of inter-symbol interference, inter-antenna interference, and, in some configuration, inter-subchannel interference. By exploiting the special structures of the correlation matrix and the staggered property of the FBMC/OQAM signals, we obtain an efficient method of computing the DFE coefficients that requires a smaller number of multiplications than the linear equalizer (LE) and conventional DFE do. The simulation results show that the proposed detectors considerably outperform the LE and conventional DFE at moderate-to-high signal-to-noise ratios.

Silicon photonics technology is a promising candidate for small form factor transceivers that can be used in data-center applications. This technology has a small footprint, a low fabrication cost, and good temperature immunity. However, its main challenge is due to the high baud rate operation for optical modulators with a low power consumption. This paper investigates an all-Silicon Mach-Zehnder modulator based on the lumped-electrode optical phase shifters. These phase shifters are driven by a complementary metal oxide semiconductor (CMOS) inverter driver to achieve a low power optical transmitter. This architecture improves the power efficiency because an electrical digital-to-analog converter (DAC) and a linear driver are not required. In addition, the current only flows at the time of data transition. For this purpose, we use a PIN-diode phase shifter. These phase shifters have a large capacitance so the driving voltage can be reduced while maintaining an optical phase shift. On the other hand, this study integrates a passive resistance-capacitance (RC) equalizer with a PIN-phase shifter to expand the electro-optic (EO) bandwidth of a modulator. Therefore, the modulation efficiency and the EO bandwidth can be optimized by designing the capacitor of the RC equalizer. This paper reviews the recent progress for the high-speed operation of an all-Si PIN-RC modulator. This study introduces a metal-insulator-metal (MIM) structure for a capacitor with a passive RC equalizer to obtain a wider EO bandwidth. As a result, this investigation achieves an EO bandwidth of 35.7-37 GHz and a 70 Gbaud NRZ operation is confirmed.

This paper introduces a new multi-satellite multi-beam system with single frequency reuse; it uses the MIMO (Multi Input Multi Output) technique to improve the frequency efficiency as the satellite communication band is limited. MIMO is the one of the most important approaches to improve the spectral efficiency in support of broadband communications. Since it is difficult to achieve high spectral efficiency by simply combining conventional MIMO satellite techniques, i.e. combining a multi-beam system with single frequency reuse with a multiple satellite system, this paper proposes transmitter pre-coding and receiver equalization techniques to enhance the channel capacity even under time/frequency asynchronous conditions. A channel capacity comparison shows that the proposed system is superior to conventional alternatives.

This paper proposed an iterative soft interference canceller (IC) referred to as turbo equalizer for the self-coherent detection, and extrinsic information transfer (EXIT) chart based irregular low density parity check (LDPC) code optimization for the turbo equalizer in optical fiber short-reach transmissions. The self-coherent detection system is capable of linear demodulation by a single photodiode receiver. However, the self-coherent detection suffers from the interference induced by signal-signal beat components, and the suppression of the interference is a vital goal of self-coherent detection. For improving the error-free signal detection performance of the self-coherent detection, we proposed an iterative soft IC with the aid of forward error correction (FEC) decoder. Furthermore, typical FEC code is no longer appropriate for the iterative detection of the turbo equalizer. Therefore, we designed an appropriate LDPC code by using EXIT chart aided code design. The validity of the proposed turbo equalizer with the appropriate LDPC is confirmed by computer simulations.

A summer-embedded sense amplifier (SE SA) is proposed to reduce feedback loop delay (TFB) in a decision feedback equalizer (DFE). In the SE SA, the position of the ISI compensator is changed from the latch input to the latch output, and hence the TFB is reduced. The simulated DFE achieves 32Gb/s and 66fJ/b with a 1-V 65-nm CMOS process.

This paper presents a programmable wideband low pass filter (LPF) with Continuous-Time (CT)/Discrete-Time (DT) hybrid architecture. Unlike the conventional DT LPF, the proposed LPF eliminates sample & hold circuits, enabling to expand available bandwidth. The transfer function and the influence of the circuit imperfection are derived from CT/DT hybrid analysis. A prototype has been fabricated in 40 nm CMOS process. The proposed LPF achieves 2.5 GHz bandwidth by wideband equalization, which offers capacitance ratio (Cratio) and clock frequency (fCK) programmability. The proposed LPF occupies only 0.048 mm2 of active area.

This paper proposes approximated log likelihood ratios (LLRs) for single carrier millimeter-wave (mmW) transmission systems in the presence of phase noise. In mmW systems, phase noise on carrier wave signals in very high frequency bands causes severe performance degradation. In order to mitigate the impairments of phase noise, forward error correction (FEC) techniques, such as low density parity check (LDPC) code, are effective. However, if the probabilistic model does not capture the exact behavior of the random process present in the received signal, FEC performance is severely degraded, especially in higher order modulation or high coding rate cases. To address this issue, we carefully examine the probabilistic model of minimum mean square error (MMSE) equalizer output including phase noise component. Based on the derived probabilistic model, approximated LLR computation methods with low computational burden are proposed. Computer simulations confirm that the approximated LLR computations on the basis of the derived probabilistic model are capable of improving bit error rate (BER) performance without sacrificing computational simplicity in the presence of phase noise.

This paper proposes novel nonlinear precoding for XOR-physical layer network coding (XOR-PNC) to improve the performance of bi-directional MIMO relay systems. The proposed precoder comprises a pre-equalizer and a nonlinear filter, which we also propose in the paper. We theoretically analyze the performance of the XOR-PNC with the proposed nonlinear precoding. As a result, it is shown that the proposed pre-equalizer improves the distribution of the received signals at relays, while the nonlinear precoder not only improves the transmission power efficiency but also simplifies the receiver at the relays. The performance is confirmed by computer simulation. The XOR-PNC with the proposed precoding achieves almost the lower bound in BER performance, which is much better than the amplify-and-forward physical layer network coding (AF-PNC).

In this paper, we consider filter-and-forward relay beamforming using orthogonal frequency-division multiplexing (OFDM) in the presence of inter-block interference (IBI). We propose a filter design method based on a constrained max-min problem, which aims to suppress IBI and also avoid deep nulls in the frequency domain. It is shown that IBI can be suppressed completely owing to the employment of beamforming with multiple relays or multiple receive antennas at each relay when perfect channel state information (CSI) is available. In addition, we modify the proposed method to cover the case where only the partial CSI for relay-receiver channels is available. Numerical simulation results show that the proposed method significantly improves the performance as the number of relays and antennas increases due to spatial diversity, and the modified method can make use of the channel correlation to improve the performance.

Open-loop (OL) transmit diversity is more subject to the influence of channel estimation error than closed-loop (CL) transmit diversity, although it has the merit of providing better performance in fast Doppler frequency environments because it doesn't require a feedback signal. This paper proposes an OL transmit diversity scheme combined with intra-subframe frequency hopping (FH) and iterative decision-feedback channel estimation (DFCE) in a shared channel for discrete Fourier transform (DFT)-precoded orthogonal frequency division multiple access (OFDMA). We apply intra-subframe FH to OL transmit diversity to mitigate the reduction in the diversity gain under high fading correlation conditions among antennas and iterative DFCE to improve the channel estimation accuracy. Computer simulation results show that the required average received signal-to-noise power ratio at the average block error rate (BLER) of 10-2 of the space-time block code (STBC) with intra-subframe FH is reduced to within approximately 0.8dB compared to codebook-based CL transmit diversity when using iterative DFCE at the maximum Doppler frequency of fD =5.55Hz. Moreover, it is shown that STBC with intra-subframe FH and iterative DFCE achieves much better BLER performance compared to CL transmit diversity when fD is higher than approximately 30Hz since the tracking ability of the latter degrades due to the fast fading variation in its feedback loop.

This paper presents comprehensive comparisons on the block error rate (BLER) performance of rate-one open-loop (OL) transmit diversity schemes with four antennas for discrete Fourier transform (DFT)-precoded Orthogonal Frequency Division Multiple Access (OFDMA). One candidate scheme employs a quasi-orthogonal (QO) - space-time block code (STBC) in which four-branch minimum mean-square error (MMSE) combining is achieved at the cost of residual inter-code interference (ICI). Another candidate employs a combination of the STBC and selection transmit diversity called time switched transmit diversity (TSTD) (or frequency switched transmit diversity (FSTD)). We apply a turbo frequency domain equalizer (FDE) associated with iterative decision-feedback channel estimation (DFCE) using soft-symbol estimation to reduce channel estimation (CE) error. The turbo FDE includes an ICI canceller to reduce the influence of the residual ICI for the QO-STBC. Based on link-level simulation results, we show that a combination of the STBC and TSTD (or FSTD) is suitable as a four-antenna OL transmit diversity scheme for DFT-precoded OFDMA using the turbo FDE and iterative DFCE.

Cell voltage equalizers are necessary to ensure years of operation and maximize the chargeable/dischargeable energy of series-connected supercapacitors (SCs). A two-switch voltage equalizer using a series-resonant voltage multiplier operating in frequency-multiplied discontinuous conduction mode (DCM) is proposed for series-connected SCs in this paper. The frequency-multiplied mode virtually increases the operation frequency and hence mitigates the negative impact of the impedance mismatch of capacitors on equalization performance, allowing multi-layer ceramic capacitors (MLCCs) to be used instead of bulky and costly tantalum capacitors, the conventional approach when using voltage multipliers in equalizers. Furthermore, the DCM operation inherently provides the constant current characteristic, realizing the excessive current protection that is desirable for SCs, which experience 0V and equivalently become an equivalent short-circuit load. Experimental equalization tests were performed for eight SCs connected in series under two frequency conditions to verify the improved equalization performance at the increased virtual operation frequencies. The standard deviation of cell voltages under the higher-frequency condition was lower than that under the lower-frequency condition, demonstrating superior equalization performance at higher frequencies.

This paper presents frequency diversity effects of localized transmission, clustered transmission, and intra-subframe frequency hopping (FH) using a frequency domain equalizer (FDE) for discrete Fourier transform (DFT)-precoded Orthogonal Frequency Division Multiple Access (OFDMA). In the evaluations, we employ the normalized frequency mean square covariance (NFMSV) as a measure of the frequency diversity effect, i.e., randomization level of the frequency domain interleaving associated with turbo coding. Link-level computer simulation results show that frequency diversity is very effective in decreasing the required average received signal-to-noise power ratio (SNR) at the target average block error rate (BLER) using a linear minimum mean-square error (LMMSE) based FDE according to the increase in the entire transmission bandwidth for DFT-precoded OFDMA. Moreover, we show that the NFMSV is an accurate measure of the frequency diversity effect for the 3 transmission schemes for DFT-precoded OFDMA. We also clarify the frequency diversity effects of the 3 transmission schemes from the viewpoint of the required average received SNR satisfying the target average BLER for the various key radio parameters for DFT-precoded OFDMA in frequency-selective Rayleigh fading channels.

In this letter, we present a joint blind adaptive scheme to suppress inter-block interference and estimate a carrier frequency offset (CFO) in downlink OFDMA systems. The proposed scheme is a combination of a channel shortening method and a CFO estimator, both based on the carrier nulling criterion. Simulation results demonstrate the effectiveness of the proposed scheme.

In this paper, a symbol-rate clock recovery scheme for a receiver that uses an integrating decision feedback equalizer (DFE) is proposed. The proposed clock recovery using expected received signal amplitudes as the criterion realizes minimum mean square error (MMSE) clock recovery. A receiver architecture using an integrating DFE with the proposed symbol-rate clock recovery is also proposed. The proposed clock recovery algorithm successfully recovered the clock phase in a system level simulation only with a DFE. Higher jitter tolerance than 0.26 UIPP at 10 Gb/s operation was also confirmed in the simulation with an 11 dB channel loss at 5 GHz.

In this paper, a new systolic array structure for the extended QR decomposition based recursive least-square (QRD-RLS) equalizer is proposed. The fact that the vectoring and rotation mode coordinate rotation digital computer (CORDIC) processors rotate in the same direction is used to show that the hardware complexity of the systolic array can be reduced. Furthermore, since the vectoring and rotation mode CORDIC processors in the proposed structure rotate simultaneously, operation time is also reduced. The performance of the proposed equalizer is analyzed by observing the flatness obtained by multiplying the frequency responses of the unknown channel with the proposed equalizer. Simulation results through hardware description language (HDL) coding and synthesis show that 23.8% of the chip implementation area can be reduced.

This paper proposes a spectrum-overlapped resource management (SORM) technique where each user equipment (UE) can ideally obtain the frequency selection diversity gain under multi-user environments. In the SORM technique for cellular systems, under assumption of adopting a soft canceller with minimum mean square error (SC/MMSE) turbo equalizer, an evolved node B (eNB) accepts overlapped frequency resource allocation. As a result, each UE can use the frequency bins having the highest channel gain. However, the SORM becomes non-orthogonal access when the frequency bins having high channel gain for UEs are partially identical. In this case, the inter-user interference (IUI) caused by overlapping spectra among UEs is eventually canceled out by using the SC/MMSE turbo equalizer. Therefore, SORM can achieve better performance than orthogonal access e.g. FDMA when the IUI is completely canceled. This paper demonstrates that SORM has the potential to improve transmission performance, by extrinsic information transfer (EXIT) analysis. Moreover, this paper evaluates the block error rate (BLER) performance of the SORM and the FDMA. Consequently, this paper shows that the SORM outperforms the FDMA.

This paper investigates the dominant impact on the interference rejection combining (IRC) receiver due to the downlink reference signal (RS) based covariance matrix estimation scheme. When the transmission modes using the cell-specific RS (CRS) in LTE/LTE-Advanced are assumed, the property of the non-precoded CRS is different from that of the data signals. This difference poses two problems to the IRC receiver. First, it results in different levels of accuracy for the RS based covariance matrix estimation. Second, assuming the case where the CRS from the interfering cell collides with the desired data signals of the serving cell, the IRC receiver cannot perfectly suppress this CRS interference. The results of simulations assuming two transmitter and receiver antenna branches show that the impact of the CRS-to-CRS collision among cells is greater than that for the CRS interference on the desired data signals especially in closed-loop multiple-input multiple-output (MIMO) systems, from the viewpoint of the output signal-to-interference-plus-noise power ratio (SINR). However, the IRC receiver improves the user throughput by more than 20% compared to the conventional maximal ratio combining (MRC) receiver under the simulation assumptions made in this paper even when the CRS-to-CRS collision is assumed. Furthermore, the results verify the observations made in regard to the impact of inter-cell interference of the CRS for various average received signal-to-noise power ratio (SNR) and signal-to-interference power ratio (SIR) environments.