BER Floor Research Articles

Design of multichannel adaptive filter by constructing multidimensional Wiener-Hopf equation

In the satellite-to-ground high-speed data transmission link, there are signal self-interference problems of symbols in the co-channel, as well as between orthogonal and polarized channels. A multichannel adaptive filter is designed by constructing a multichannel Wiener-Hopf equation, and the influence of five channel nonideal factors is suppressed to improve the BER performance. Experiments show that this method is effective to suppress the signal self-interference, and the BER floor is optimized from 1E-3 to 1E-7.

A Low-Complexity Message Passing Detector for OTFS Modulation With Probability Clipping

The recently proposed orthogonal time frequency space (OTFS) modulates the data in the delay-Doppler domain, thereby the 2-D diversity gain can be obtained to improve the communication performance over time-varying channels. However, the matrix-inversion based detectors have pretty high complexity in OTFS systems due to the large size of channel matrix. This letter proposes a low complexity matched filtering based message passing detector (MF-MPD), combined with a novel probability clipping (PC) solution. Benefit from matched filtering processing, the complexity and storage requirement of this solution can be reduced, and better BER performance in low SNR regimes can be achieved. Furthermore, the BER floor at high SNRs can be obviously improved via PC over time-varying channels according to the numerical results.

A Spatial–Temporal Subspace-Based Compressive Channel Estimation Technique in Unknown Interference MIMO Channels

Spatial–temporal (ST) subspace-based channel estimation techniques formulated with $\ell 2$ minimum mean square error (MMSE) criterion alleviate the multi-access interference (MAI) problem when the interested signals exhibit low-rank property. However, the conventional $\ell 2$ ST subspace-based methods suffer from mean squared error (MSE) deterioration in unknown interference channels, due to the difficulty to separate the interested signals from the channel covariance matrices (CCMs) contaminated with unknown interference. As a solution to the problem, we propose a new $\ell 1$ regularized ST channel estimation algorithm by applying the expectation-maximization (EM) algorithm to iteratively examine the signal subspace and the corresponding sparse-supports. The new algorithm updates the CCM independently of the slot-dependent $\ell 1$ regularization, which enables it to correctly perform the sparse-independent component analysis (ICA) with a reasonable complexity order. Simulation results shown in this paper verify that the proposed technique significantly improves MSE performance in unknown interference MIMO channels, and hence, solves the BER floor problems from which the conventional receivers suffer.

Experimental study of performance enhanced IM/DD transmissions combining 4D Trellis coded modulation with precoding.

We propose to combine 4D trellis-coded modulation (TCM) with transmitter-side Tomlinson-Harashima precoding (THP) in IM/DD transmissions, and experimentally investigate the achieved performance improvement. Theoretically, THP can approximately produce an end-to-end additive white Gaussian noise (AWGN) channel even with severe bandwidth limitation, allowing TCM to maintain its coding gain in the presence of inter-symbol interference. In our experiments with off-the-shelf commercial components, which limit the 3 dB bandwidth of the system to be ~3.5 GHz, the combination of TCM and THP shows a better receiver sensitivity for various system bit rates from 56 Gbit/s to 112 Gbit/s, considering the KP4 threshold of BER = 2 × 10-4. In the 112 Gbit/s back-to-back (B2B) transmission, with the help of THP the receiver sensitivity is improved by 3.3 dB using 4D-PAM4 TCM at the KP4 FEC threshold compared with using conventional PAM4. In addition, combining TCM and THP also helps to lower the BER floor.

Design and analysis of high-speed optical access networks in the O-band with DSP-free ONUs and low-bandwidth optics.

In this paper, we demonstrate and investigate high-speed passive optical networks (PON) in the O-band without using any digital signal processing (DSP) at the ONU side, meanwhile still adopting cost-effective low-bandwidth optics. We show that with commercial 10G-class optics including DML and PIN-TIA, 50Gb/s PAM4 PON with a power budget up to 29dB is achieved under the HD-FEC BER limit. We present the detailed design procedure of the DSP including the downlink Nyquist pulse shaping and pre-equalization and uplink post-equalization, and conduct a comprehensive study on this system including DSP complexity, DAC/ADC sampling rate requirement and tolerance to the response diversity of ONU receivers. We conclude that, 1) 55 and 35 taps are sufficient for downlink and uplink digital filters to approach the BER floor; 2) the minimum required DAC and ADC sampling rate are respectively 33.75GS/s and 32GS/s to achieve the optimal performance; and 3) with 1dB sensitivity penalty at the HD-FEC BER limit, the allowable response diversity of ONU receivers can be from -2.2dB/10GHz to 1.8dB/10GHz. Furthermore, we explore and demonstrate the suitable data rate based on on-off keying and duobinary modulation formats in this system and present corresponding results.

A New Prime Code for Synchronous Optical Code Division Multiple-Access Networks

A new spreading code based on a prime code for synchronous optical code-division multiple-access networks that can be used in monitoring applications has been proposed. The new code is referred to as “extended grouped new modified prime code.” This new code has the ability to support more terminal devices than other prime codes. In addition, it patches subsequences with “0s” leading to lower power consumption. The proposed code has an improved cross-correlation resulting in enhanced BER performance. The code construction and parameters are provided. The operating performance, using incoherent on-off keying modulation and incoherent pulse position modulation systems, has been analyzed. The performance of the code was compared with other prime codes. The results demonstrate an improved performance, and a BER floor of 10−9 was achieved.

Power-Division Non-Orthogonal Multiple Access (NOMA) in Flexible Optical Access With Synchronized Downlink/Asynchronous Uplink

The data traffic in optical access networks increases dramatically, driven by wired and wireless services. Limited by the available bandwidth with low-cost optical and electrical devices, modulations and multiple access schemes with higher spectral efficiency are desirable. The current time-division multiplexing and wavelength-division multiplexing cannot fulfill the requirement due to the low spectral efficiency and high cost. The multistack flexible optical access network shows its benefit by low cost, reduced fiber deployment, increased flexibility, and ultra-high network extension capability. With varying path losses to different users, the network performance is significantly limited by users with high path losses generated by more couplers or long fibers. These users cannot have good service qualities using the existing multiple access schemes. We propose to use the power-division non-orthogonal multiple access with synchronized downlink and asynchronous uplink. The performance and reliability are significantly improved for high path loss users, by optimizing all users as a group and adjusting the power ratio adaptively. In the uplink, the asynchronous transmission enables low latency and simple DSP in optical network units. The simulation shows a significantly better performance. And the proposed scheme is experimentally demonstrated by our testbed with 2.5-dB increased power margin in synchronized downlink and one-order-of-magnitude BER floor reduction in asynchronous uplink.

Low Complexity Compensation of Frequency Dependent I/Q Imbalance and Carrier Frequency Offset for Direct Conversion Receivers

Conventional algorithms for the joint estimation of carrier frequency offset (CFO) and I/Q imbalance no longer work when the I/Q imbalance depends on the frequency. In order to correct the imbalance across many frequencies, the compensator needed is a filter as opposed to a simple gain and phase compensator. Although, algorithms for estimating the optimal coefficients of this filter exist, their complexity is too high for hardware implementation. In this paper we present a new low complexity algorithm for joint estimation of CFO and frequency dependent I/Q imbalance. For the first part, we derive the estimation scheme using the linear least squares algorithm and examine its floating point performance compared to conventional algorithms. We show that the proposed algorithm can completely eliminate BER floor caused by CFO and I/Q imbalance at a lesser complexity compared to conventional algorithms. For the second part, we examine the hardware complexity in fixed point hardware and latency of the proposed algorithm. Based on BER performance, the circuit needs a wordlength of at least 16bits in order to properly estimate CFO and I/Q imbalance. In this configuration, the circuit is able to achieve a maximum speed of 115.9MHz in a Virtex 5 FPGA.

Characterization of Demapper EXIT Functions with BEC a priori Information with Applications to BICM-ID

This paper introduces a less is more approach to the modelling of iterative decoding for bit-interleaved coded modulation (BICM). It considers modelling the iterative exchange of soft a priori information between the demapper and decoder with a binary erasure channel model. This simplification, unlike the usual additive white Gaussian noise (AWGN) channel model, allows us to express the extrinsic information transfer (EXIT) chart curves for the demapper with polynomials whose order makes explicit the degrees of freedom available for design. The accuracy of our expressions simplifies the design process enabling us to compare in a more manageable manner the key performance parameters: the BER performance and the convergence threshold. This enabled us to find the pairing of memory-two recursive systematic convolutional (RSC) codes and mappers that achieve the earliest convergence threshold for a target BER floor at convergence: for 8-PSK semi-set partitioning (SSP) was optimal and converged at a signal-to-noise ratio (SNR)= 3.0 dB for a BER target of at least 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> ; two new mappings were the optimal pairings for 16-QAM, where the L4b2 mapping converged at 2.9 dB for a BER floor ≤ 3.46 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> and the H2b2 mapping converged at 3.1 dB for a BER floor ≤ 2.44 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> .

GPU-based DVB-S2 LDPC decoder with high throughput and fast error floor detection

A new strategy is proposed for implementing computationally intensive high-throughput decoders based on the long length irregular LDPC codes adopted in the DVB-S2 standard. It is supported on manycore graphics processing unit (GPU) architectures, for performing parallel multi-threaded decoding of multiple codewords with reduced accesses to global memory. This novel approach is flexible and scalable, and achieves throughputs superior to the 90 Mbit/s required by the DVB-S2 standard, while at the same time it improves error-correcting performances such as BER and error floors regarding conventional VLSI-based decoders.

Interference Cancellation Schemes in UWB Systems Used in Wireless Personal Area Network based on Wavelet based Pulse Spectral shaping and Transmitted Reference UWB using AWGN Channel Model

For high data rate ultra wideband communication system, performance study of mutual interference such as suppression of interference signal by performing pulse shaping from UWB to other narrowband system using wavelet technique and reduction of interference from other narrowband system to UWB system in conjunction with Transmitted-Reference (TR-UWB) signaling scheme with adaptive receivers is attempted in this paper. The TRUWB signal using wavelet is based on multi-resolution technique to avoid its interference to other wireless systems. Ultra Wideband (UWB) wireless systems supposed to coexist with other spectrally overlapping wireless systems such as narrowband interference (NBI), multiple NBI and wideband interference allocated from 3.1GHz to 10.6GHz. So suppression of UWB from other system can be performed by using Eigen value decomposition using wavelet technique. Mutual interference degrades the performance of the TRUWB system using auto-correlation receivers (AcR). In this paper adaptive receiver (using notch filter) is used for detection and suppression of mutual interference (narrowband and wideband signal) which do not require modification of the narrowband system. The bit error rate performances are investigated using MATLAB simulation under AWGN channel model. At 10-2 BER floor, TRUWB using adaptive receiver with NBI simulations under AWGN provides a more than of 6 dB SNR improvement than that of TR using AcR receiver with NBI. The performance degrades when coexistence of multiple NBI and wideband interference wireless system with UWB system is applied using the same receiver. KeywordsTR-UWB, AWGN Channel Model, NBI, multiple NBI , wideband interference ,Bit Error Rate .

An Innovative Receiver for Incoherent SAC-OCDMA Enabling SOA-Based Noise Cleaning: Experimental Validation

We propose a new low complexity receiver for spectral amplitude optical coded division multiple access (SAC OCDMA) that enables intensity noise reduction using semiconductor optical amplifiers (SOAs). Compared to the standard receiver requiring two optical filters at the receiver side, our receiver requires only one optical filter. While a 1.4-dB power penalty in incurred, network capacity is unchanged, i.e., BER floors due to intensity noise have the same level. The primary motivation for the low complexity receiver is not reduced component count, but rather modifying the receiver so that promising SOA noise mitigation techniques might be employed to increase system capacity. SOA noise cleaning suffers from a major limitation: filtering after the SOA can negate most of the signal enhancement, the so-called post SOA filtering issue. The only solution to date for the post-SOA filtering effect in SAC-OCDMA is prohibitively complex McCoy , <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J.</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Lightw.</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Technol.</i> , vol. 25, no. 1, pp. 394-401, Jan. 2007, i.e., requiring multiple SOAs per client. We demonstrate that our proposed receiver drastically limits the client side filtering, thus maintaining noise suppression and overcoming the post-SOA filtering effect. We compare BER at up to 10 Gb/s with and without noise cleaning. When a noise cleaning module is used, BER improvement of several orders-of-magnitude is observed when only a few users are active in the network. Examination of the noise properties, however, leads us to conclude that highly populated networks will have diminished improvement.

A Transmit Diversity Scheme for TDS-OFDM System

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Transmit diversity is especially important in broadcasting network. In this paper, a transmit diversity method is proposed for the Chinese National Digital Terrestrial Television Broadcasting (DTTB) Standard. The transmit diversity scheme is based on the design of orthogonal sequences which have perfect crosscorrelation and almost perfect autocorrelation characteristic. The characteristics of the proposed sequences can help for synchronization and channel estimation in time domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) system. Simulations using space-time codes (STCs) in Digital Terrestrial/Television Multimedia Broadcasting (DTMB) system with low density parity check coding (LDPC) show that the proposed transmit diversity method has about 1–2 dB signal-to-noise ratio (SNR) gain comparing with no diversity for QPSK and 16 QAM constellations at <formula formulatype="inline"> <tex Notation="TeX">${\rm BER}=10^{-4}$</tex></formula>, and it can avoid the appearance of BER floor for 64QAM constellation. </para>

Multiple-Antenna Receiving and Frequency Domain Equalization in Transmitted-Reference UWB Systems

A multiple-antenna receiving and combining scheme is proposed for high-data-rate transmitted-reference (TR) Ultra-Wideband (UWB) systems. The nonlinearity of the inter-symbol interference (IST) model is alleviated via simple antenna combining. Under the simplified ISI model, frequency domain equalization (FDE) is adopted and greatly reduces the complexity of the equalizer. A simple estimation algorithm for the simplified ISI model is presented. Simulation results demonstrate that compared to the single receive antenna scheme, the proposed method can obtain a significant diversity gain and eliminate the BER floor effect. Moreover, compared to the complex second-order time domain equalizer, FDE showed better performance robustness in the case of imperfect model estimation.

40 Gbps All-Optical 3R Regeneration and Format Conversion with Related InP-Based Semiconductor Devices

We report an experimental demonstration of 40 Gbps all-optical 3R regeneration with all-optical clock recovery based on InP semiconductor devices. We also obtain alloptical non-return-to-zero to return-to-zero (NRZ-to-RZ) format conversion using the recovered clock signal at 10 Gbps and 40 Gbps. It leads to a good performance using a Mach-Zehnder interferometric wavelength converter and a self-pulsating laser diode (LD). The self-pulsating LD serves a recovered clock, which has an rms timing jitter as low as sub-picosecond. In the case of 3R regeneration of RZ data, we achieve a 1.0 dB power penalty at 10 -9 BER after demultiplexing 40 Gbps to 10 Gbps with an eletroabsorption modulator. The regenerated 3R data shows stable error-free operation with no BER floor for all channels. The combination of these functional devices provides all-optical 3R regeneration with NRZ-to-RZ conversion.

Simultaneous all-optical 3R regeneration scheme with improved scalability using TOAD

A novel re-timing, re-amplifying, and re-shaping (3R) regeneration system is proposed to process multiple WDM (wavelengthdivision-multiplexing) channels simultaneously. Its re-timing capability is investigated by both simulation and experiment with polarizationscrambling method at 10 Gb/s bit rate. Jitter tolerance up to 0.8 UIpp is demonstrated with BER improvement and floor breaking ability.

Tunable dispersion slope compensation for 40-Gb/s WDM systems using broadband nonchannelized third-order chirped fiber Bragg gratings

We demonstrate tunable dispersion slope compensation using broadband nonchannelized third-order chirped fiber Bragg gratings (FBGs) for 40-Gb/s wavelength-division-multiplexed (WDM) systems. The slope is tuned by stretching as it has a third-order time delay variation with wavelength, and thus a second-order dispersion-wavelength curve. Calculation results include the examples of the slope tuning in addition to the interchannel dispersion differences and the slope tuning ranges with respect to the wavelength in a single grating. To verify the functionality of the third-order grating, we prepare a third-order grating with a dispersion variation of up to 400 ps/nm over the 3.5-nm usable bandwidth, and a dispersion slope tuning range of /spl sim/65 ps/nm/sup 2/ is achieved. For 40-Gb/s systems, two new gratings are designed and inversely cascaded to eliminate the inherent errors originating from the deleterious higher-order dispersion components in a single grating. The slope tuning range is -20 to +20 ps/nm/sup 2/ over the 5-nm usable bandwidth. A /spl sim/10-dB power penalty improvement is achieved and the BER floor is eliminated for the worst-case channel after transmission through a 170-km link using dispersion shifted fiber (DSF) with a 4/spl times/40-Gb/s RZ data stream.

Reduction of semiconductor laser amplifier induceddistortion and crosstalk for WDM systems using light injection

Semiconductor laser amplifiers cause both distortion and crosstalk in WDM systems due to gain saturation. A method using light injection is experimentally demonstrated to reduce distortion and crosstalk. In a two-channel experiment with a data rate of 2.5 Gbit/s, it is found that light injection can eliminate the BER floor due to distortion and crosstalk.

BER floors due to heterodyne coherent crosstalk in space photonic switches for WDM networks

The occurrence of bit-error-rate floors due to coherent heterodyne crosstalk in photonic space switches is studied. This source of errors occurs when a particular switch is used to route signals of nominally the same wavelength in a WDM network, DFB lasers are run at multigigabit rates, and wavelength referencing is held to a relatively tight tolerance to facilitate wavelength demultiplexing. In this letter we compare the dependence of switch architecture and size on the rate of coherently induced bit errors. Three regions of interest are identified: (1) random uniformly distributed laser phases with binomially distributed bit statistics in interfering channels, (2) random uniformly distributed laser phases with ones in interfering channels, and (3) worst-case laser phases with ones in interfering channels. Critical crosstalk values are identified for four different switch architectures and two different switch sizes in order to avoid coherently induced errors and therefore BER floors due to this mechanism. The minimum crosstalk required to avoid coherent errors can vary from -10 dB to -42 dB depending on the switch architecture, size, and requirement for short-term BER stability.

On Envelope Detection of Noisy Phase Signals with Asymptotically Small Linewidths

The impact of laser's phase noise on heterodyne and homodyne linewidth communications, as well as on direct detection systems with optical amplification is governed by intractable statistics induced by the phase sample path. Here we derive analytic asymptotic bounds on the underlying distribution inherited by the squared envelope of noisy phase signals. The approach taken invokes standard results from the theory of Large-Deviations and yields straightforward, closed-form expressions. This fundamental problem has many applications in linewidth communications, such as the determination of the BER floor in heterodyne ASK systems, which is hereby illustrated. The asymptotic theory predicts an IF bandwidth increase with the square root of the linewidth-to-bit ratio for a specified floor level. A comparison with the numerical solution of the underlying Fokker-Planck equation reveals a good agreement even at significant (non-asymptotic) laser's linewidths.