Interpixel Interference Research Articles

Developing a comprehensive model for underwater MIMO OCC system.

Due to its spatial modulation feature and potential applications, optical camera communication (OCC) has gained significant attention in recent years for a range of applications including underwater. Nonetheless, due to the low frame rates of the camera, the OCC data rate is rather low, which is why multiple-input-multiple-output (MIMO) has been adopted to compensate. In MIMO systems, however, the signal from one light emitting diode (LED) may result in interference on the image sensor (i.e., the camera) resulting in inter-pixel interference (IPI). This paper presents a comprehensive model of the underwater OCC (UOCC) and experimentally verifies its performance under IPI by comparing signal to interference and noise ratio (SINR). The effect of distance between LEDs according to LED diameter D on signal to interference ratio (SIR) is presented and results indicate that coastal water has the SIR gain ∼2.5 dB for the link span of 1 to 6 m, and for harbor water channel length from 0.4 to 1.4 m the gain increased from ∼2 to ∼5 dB for d of 2D compared with d of 0.5D.

Investigating the effect of turbulence on IPI in a vehicular OCC system using PSF analysis

Optical camera communication (OCC) has emerged as a promising technology for wireless communication owing to its enormous potential benefits. However, turbulence conditions can restrict the feasibility of OCC systems that employ multiple-input multiple-output (MIMO) techniques. In this work, we consider a vehicular MIMO-OCC system model in which the traffic light LEDs transmit data streams separately in parallel channels to the camera of a vehicle. We analyze the effect of turbulence on the inter-pixel interference (IPI) and bit error rate (BER) of the low speed vehicular MIMO-OCC system using the point spread function (PSF) of the optical channel. We introduce two performance metrics, i.e., digital number difference (DND) and the percentage of separable LEDs (PSLED) for strong and very strong turbulence conditions, respectively. The parameters of the camera and traffic light LEDs can affect the IPI caused by turbulence. It is shown that the DND of MIMO-OCC systems with larger focal length and LED size less affected by turbulence. In addition, smaller f-number and pixel size reduce the effect of turbulence on DND. It is also demonstrated that very strong turbulence can reduce PSLED for links longer than 20 m. Moreover, if turbulence reduces PSLED, increasing the LEDs’ intensity cannot efficiently reduce the BER in the MIMO-OCC system.

Interpixel Interference Mitigation in Visible Light Communication Using Image Sensor

To improve the data rate in a camera-based visible light communication system, multiple LEDs in an LED array are used to transmit multiple bits in one frame. The problem is that LEDs are always captured in the image with some degree of blooming effect. This effect increases the intensity of pixels surrounding the origin LEDs in the image. When multiple LEDs are used, the blooming effect from all LEDs causes the interpixel interference (IPI) problem that greatly degrades the quality of the received signal. This paper analyzes the blooming effect and finds that the IPI problem increases when the number of “ON” LEDs in the array increases. Through the analysis, this paper proposes a differential coding to minimize the number of “ON” LEDs in every transmitted frame and thus minimize the IPI effect. The proposed scheme is then verified through simulations. The results show that a lower bit error rate, which indicates a better signal quality, can be achieved by applying the proposed scheme.

Two high-density recording methods with run-length limited turbo code for holographic data storage system

Two methods for increasing the data capacity of a holographic data storage system (HDSS) were developed. The first method is called “run-length-limited (RLL) high-density recording”. An RLL modulation has the same effect as enlarging the pixel pitch; namely, it optically reduces the hologram size. Accordingly, the method doubles the raw-data recording density. The second method is called “RLL turbo signal processing”. The RLL turbo code consists of trellis modulation and an optimized convolutional code. The remarkable point of the developed turbo code is that it employs the RLL modulator and demodulator as parts of the error-correction process. The turbo code improves the capability of error correction more than a conventional LDPC code, even though interpixel interference is generated. These two methods will increase the data density 1.78-fold. Moreover, by simulation and experiment, a data density of 2.4 Tbit/in.2 is confirmed.

3D image reconstruction with a controllable overlapping number of elemental images in computational integral imaging

In this Letter, we propose a three-dimensional (3D) image reconstruction method with a controllable overlapping number of elemental images in computational integral imaging. The proposed method can control the overlapping number of pixels coming from the elemental images by using the subpixel distance based on ray optics between a 3D object and an image sensor. The use of a controllable overlapping number enables us to provide an improved 3D image visualization by controlling the inter-pixel interference within the reconstructed pixels. To find the optimal overlapping number, we simulate the pickup and reconstruction processes and utilize the numerical reconstruction results using a peak signal-to-noise ratio (PSNR) metric. To demonstrate the feasibility of our work in optical experiments, we carry out the preliminary experiments and present the results.

Self-referential holography and its applications to data storage and phase-to-intensity conversion

Holographic recording methods require the use of a reference beam that is coherent with the signal beam carrying the information to be recorded. In this paper, we propose self-referential holography (SRH) for holographic recording without the use of a reference beam. SRH can realize purely one-beam holographic recording by considering the signal beam itself as the reference beam. The readout process in SRH is based on energy transfer by inter-pixel interference in holographic diffraction, which depends on the spatial phase difference between the recorded phase and the readout phase. The phase-modulated recorded signal is converted into an intensity-modulated beam that can be easily detected using a conventional image sensor. SRH can be used effectively for holographic data storage and phase-to-intensity conversion.

Hardware implementation of pixel detection in gray-scale holographic data storage systems

This paper presents hardware implementation of an efficient solution to recovering gray-scale data pixels of images that have undergone interpixel interference in holographic data storage systems. The adopted algorithm, called the turbo receiver using interference-aware dual-list (TRIDL) detection, enjoys benefits of low error rate performance and low complexity. To verify the functionality and feasibility, this paper implements TRIDL detection with some circuit design techniques such as resource sharing on a field-programmable gate array.

Neural Network Equalizer Matched to Recording Code in Holographic Data Storage

The neural network equalizer (NNE) matched to recording code is proposed to reduce the influence of interpixel interference in holographic data storage (HDS). A searching method is proposed to proceed simultaneously with the search of a conversion table of the 5–9 recording code and the search of simplified NNE by using hybrid genetic algorithm. Furthermore, the bit error rate performance of HDS system is obtained. The result shows that the proposed method is superior in the performance to the conventional method.

Measurement of the Jones matrix of liquid crystal displays using a common pathinterferometer

We propose a robust interferometric method to measure the Jones matrix of polarizationcomponents, especially liquid crystal displays. Phase values are measured by a simplecommon path interferometer containing a birefringent crystal as beam splitter and apolarizer as beam combiner. This solution eliminates the sensitivity of traditionalinterferometric techniques to vibration, temperature variation or wavefront distortion. Theproposed phase measurement method is applicable to the measurement of bothspatially homogeneous and binary modulated states, thus the modulation transferfunction and inter-pixel interference can also be studied. We demonstrate thistechnique by the measurement of a liquid crystal on silicon display. The resultingJones matrix, as a function of displayed gray level, can be efficiently embedded inany numeric model of an optical system containing the analyzed spatial lightmodulator.

Neuro-Interpolator for Holographic Data Storage

The neuro-interpolator (NI) integrating the interpolator with neural network equalizer is proposed to reduce the influence of interpixel interference in holographic data storage. The simplification of NI is performed by using a hybrid genetic algorithm based on a new mutation method. The bit error rate performance of the simplified NI is superior to that of the interpolator alone. Furthermore, the number of connections in the simplified NI is reduced to about half of that obtained by using a conventional mutation method.

MEMS thermal imager with optical readout

A cantilever-based uncooled IR imager was developed utilizing a novel optical readout scheme based on inter-pixel interference. A series of small arrays (approximately 100 × 100 pixels) were fabricated using 8-in. silicon MEMS processes. The array design and process development will be discussed and initial uniformity and imaging results presented. Future challenges in developing a direct-view IR imager will be addressed.

Low-complexity pixel detection for images with misalignment and interpixel interference in holographic data storage

This paper presents an efficient solution to recovering data pixels of images that have undergone optical and electrical channel impairments in holographic data storage systems. The channel impairments considered include interpixel interference, three types of misalignment, and noise. The proposed misalignment-compensation scheme, consisting of realignment and rate conversion, can effectively eliminate misalignment with more than 84% reduction in additions and 74% reduction in multiplications. In addition, several low-complexity techniques are introduced to reduce the complexity of a two-dimensional maximum a posteriori pixel detection method by up to 95% and do so with negligible degradation in detection performance.

Linear Reproduction of a Holographic Storage Channel using Coherent Addition of Optical DC Components

A holographic data storage channel is normally a nonlinear channel; however, it can be made linear. Using coherent addition of DC components in the reproduction process and calculating the square root of intensity, we can retrieve a linearly reproduced signal. Our simulation results revealed that a conventional equalizer works well to suppress interpixel interference so that a higher recording density can be achieved.

An Application of Neural Network Equalization to Polytopic Multiplexing Holography and Reduction of Interpixel Interference

The neural network equalization for polytopic multiplexing holography is studied to reduce interpixel interference. The bit error rate performance of the bilinear or bicubic interpolator followed by a neural network as an equalizer is obtained by computer simulation. The results show that the neural network equalizer provides an SNR improvement of about 1.0 dB over conventional equalization.

Resolution enhancement by interpixel interference elimination

This paper proposes a multiframe resolution enhancement algorithm by iteratively estimating and eliminating interpixel interference from neighboring pixels. The rationale is based on the observation that resolution reduction is caused by low-pass filtering nature of the image acquisition device as well as the digital zooming procedure. Both introduce interference between neighboring pixels. At a given location, the interpixel interference is from an integral effect of several low-pass filtering processes, each determined by one of its neighboring pixels. We propose using a Gaussian mixture to model the probability of the integral interpixel interference. The Gaussian mixture is determined by both the local image constraints and the local variation indicator (LVI). Local image constraints come from the image derivative priors, which evaluate the similarity between the current pixel and its neighborhood. A larger image derivative prior implies more interference. We use differences between the current pixel and its neighboring pixels as the image derivative priors. The LVI shows the reliability of the neighboring pixels. The LVI is obtained from both the temporal variation and the spatial variation. Neighboring pixels with larger LVIs are considered less reliable; hence, they provide less information for inferring the interpixel interference. Also, we show from the frequency domain representation that interpixel interference can actually be related to high-frequency loss. By estimating and compensating the missing high-frequency details iteratively, we can recover images with higher resolution. Experimental evaluation on varying inputs, including faces, synthetic text subjects, as well as license plates, validates the algorithm.

Iterative soft-minimum mean-square error equalization for digital nonlinear page-oriented memories

Digital page-oriented volume holographic memory (POVHM) is a promising candidate for next-generation ultrahigh capacity optical data storage technology. As the capacity of the POVHMs increases, the bit error rate performance of the system is degraded due to increased interpixel interference (IPI) and noise. To improve the system performance under these adverse effects and to increase the capacity, joint iterative soft equalization-detection and error correction decoding might be attractive. To address that, by considering the nonlinearity inherent in the channel, an iterative soft equalization method that is optimized in the minimum mean-square error (MMSE) sense, called the iterative soft-MMSE (ISMMSE) equalization, is devised. The performance of the ISMMSE is evaluated by use of numerical experiments under different amounts of IPI and optical noise. Simulation results suggest that the ISMMSE is a good candidate for an ultrahigh capacity POVHM, which employs joint iterative equalization-detection and decoding.

Soft-decision array decoding for volume holographic memory systems.

We study the use of soft-decision array decoding in a volume holographic memory (VHM) system that is corrupted by interpixel interference (IPI) and detector noise. Soft-decision methods can unify equalization and error decoding. A highly parallel array decoder is presented in the context of two-dimensional low-pass channel mitigation and error correction. The new decoding algorithm is motivated by iterative turbo-decoding methods and is capable of incorporating a priori knowledge of the corrupting IPI channel during decoding. The resulting joint detection decoding algorithm is shown to offer VHM capacity and density performance superior to that of hard-decision n = 255 Reed-Solomon codes in concatenation with a Wiener filter.

Improvement to holographic digital data-storage systems with random and pseudorandom phase masks

Various effects associated with using a random phase mask in holographic data storage are analyzed. It is shown that the nonlinear nature of recording a phase hologram coupled with the highly nonuniform profile of the object beam is the major source of interpixel cross talk. It is also shown that, although the nonlinear effects are reduced by an increase in the reference-to-object beam ratio, the scattering noise is increased. Thus an optimal beam ratio must be used to maximize the signal-to-noise ratio. It is demonstrated as well that the interpixel interference noise can be suppressed effectively by use of a multilevel pseudorandom phase mask, which also significantly reduces the nonlinear noise. These findings are supported by experimental results that show the signal-to-noise ratio and efficiency are improved significantly by use of a six-level pseudorandom phase mask.