Joong Lee

Adaptive feedback interference cancellation system (AF-ICS)

In this paper, the adaptive feedback interference cancellation system(AF-ICS) is suggested in order to cancel the feedback signal in the wireless communication system with the same frequency. AF-ICS prevents the oscillation of the receiver system and maintains the maximum output power of the power amplifier by the reduction of time-varying feedback signal. The cancellation ratio and the error ratio of the adaptive detector are achieved as simulation result of 40/spl sim/44 dB and the performance index (cancellation ratio and error ratio) of the adaptive detector is 13/spl sim/15 dB in the system implementation. AF-ICS improves total channel power about 4.11 dBm under CDMA-3FA and 90 dB feedback loss.

Frame-Based Recovery of Corrupted Video Files Using Video Codec Specifications

In digital forensics, recovery of a damaged or altered video file plays a crucial role in searching for evidences to resolve a criminal case. This paper presents a frame-based recovery technique of a corrupted video file using the specifications of a codec used to encode the video data. A video frame is the minimum meaningful unit of video data. Many existing approaches attempt to recover a video file using file structure rather than frame structure. In case a target video file is severely fragmented or even has a portion of video overwritten by other video content, however, video file recovery of existing approaches may fail. The proposed approach addresses how to extract video frames from a portion of video to be restored as well as how to connect extracted video frames together according to the codec specifications. Experiment results show that the proposed technique successfully restores fragmented video files regardless of the amount of fragmentations. For a corrupted video file containing overwritten segments, the proposed technique can recover most of the video content in non-overwritten segments of the video file.

RF predistortion of power amplifiers using 2/sup nd/ harmonic based technique for optimization of intermodulation products

An RF predistortion linearizer using a combination of a second harmonic and a difference frequency technique is proposed to reduce 3/sup rd/ and 5/sup th/ order intermodulation products in high power amplifiers. The second harmonic and difference frequency terms are generated using an envelope detector and two frequency multipliers. The RF predistorter has the advantage of low insertion loss and a short delay line requirement. From the experimental results, adjacent channel power ratio (ACPR) reduction of 11 dB is achieved for W-CDMA at 2140 MHz.

Forged seal detection based on the seal overlay metric

This paper describes a method for verifying the authenticity of a seal impression imprinted on a document based on the seal overlay metric, which refers to the ratio of an effective seal impression pattern and the noise in the neighborhood of the reference impression region. A reference seal pattern is obtained by taking the average of a number of high-quality impressions of a genuine seal. A target seal impression to be examined, often on paper with some background texts and lines, is segmented out from the background by an adaptive threshold applied to the histogram of color components. The segmented target seal impression is then spatially aligned with the reference by maximizing the count of matching pixels. Then the seal overlay metric is computed for the reference and the target. If the overlay metric of a target seal is below a predetermined limit for the similarity to the genuine, then the target is classified as a forged seal. To further reduce the misclassification rate, the seal overlay metric is adjusted by the filling rate, which reflects the quality of inked pattern of the target seal. Experiment results demonstrate that the proposed method can detect elaborate seal impressions created by advanced forgery techniques such as lithography and computer-aided manufacturing.

Sequence discrimination of heterogeneous crossing of seal impression and ink-printed text using adhesive tapes

This paper presents a technique to discriminate the sequence of stamped seal impression and ink-printed text in a document to detect falsely signed documents. In many Asian countries where a seal is widely used to endorse documents, a possibly forged document involves a seal impressed before the text is printed. The proposed method uses adhesive tapes with peel adhesion strength of approximately 25 oz/in. to exfoliate the top layer of the overlapping region of seal impression and ink-printed text in the document. A pair of digital images of the overlapping region, captured using an infinite focus microscope, is compared for color changes before and after the exfoliation with adhesive tapes. The proposed sequence discrimination index (SDI) measures the amount of color changes before and after the exfoliation to determine the sequence of seal impression and printed text. Experiment results show that the SDI successfully discriminates the sequence of seal impression and printed text for different types of ink cartridges and seal inkpads under various storage conditions, enabling forensic investigation of falsely signed documents with a seal.

Detection of Transcribed Seal Impressions Using 3-D Pressure Traces†

Seals have been frequently used to certify that individuals or organizations have authorized or approved a document that bears these impressions. Much attention has been focused on the detection of forged seal impressions to expose and prevent seal‐related frauds. This paper describes an image‐processing technique that detects seal impressions transferred from a genuine document to a target document using transcription media. The proposed method utilizes a three‐dimensional (3‐D) scanner to generate a pressure trace map of the suspect seal impression. After utilizing a noise‐reduction algorithm to improve image quality, the pressure map is aligned with a 2‐D image of the same seal impression. The pressure ratio, determined by comparing the pressure map and inked impression of a suspect seal, can be used to determine whether the seal is genuine or was transferred to the target document. The results show that the proposed technique successfully identified transcribed seal impressions with an error rate of <1%.

Nondestructive imaging of hidden figures on license plates by X-ray radiograph

In this case, we investigated the modified license plates. The evidences had new embossing pressed serial numbers after erasing the original numbers on the license plates by hammering. The X-ray radiograph could visualize the hidden figures; those were virtually unseen by naked eyes or undetectable by ordinary photography. To reveal the erased figures, we performed image processing with computer software after X-ray radiographs. It proved to be an efficient nondestructive way to visualize the hidden original figures on metals.

Invisible ink mark detection in the visible spectrum using absorption difference

One of popular techniques in gambling fraud involves the use of invisible ink marks printed on the back surface of playing cards. Such covert patterns are transparent in the visible spectrum and therefore invisible to unaided human eyes. Invisible patterns can be made visible with ultraviolet (UV) illumination or a CCD camera installed with an infrared (IR) filter depending on the type of ink materials used. Cheating gamers often wear contact lenses or eyeglasses made of IR or UV filters to recognize the secret marks on the playing cards. This paper presents an image processing technique to reveal invisible ink patterns in the visible spectrum without the aid of special equipment such as UV lighting or IR filters. A printed invisible ink pattern leaves a thin coating on the surface with different refractive index for different wavelengths of light, which results in color dispersion or absorption difference. The proposed method finds the differences of color components caused by absorption difference to detect invisible ink patterns on the surface. Experiment results show that the proposed scheme is effective for both UV-active and IR-active invisible ink materials.

An analog compensation method for asymmetric IMD characteristics of a power amplifier

The modulation frequency affects the asymmetric intermodulation distortion (IMD) products of a RF power amplifier. This effect reduces IMD cancellation performance of power amplifiers in connection with predistortion linearization. A phase extraction method to determine phase difference between upper and lower 3/sup rd/ order IMD products and a phase compensation circuit using an envelope injection technique is proposed. The experimental results demonstrate a significant improvement in the 3/sup rd/ order IMD cancellation performance.

Low power and small sized Ku-band MMIC low noise block downconverter design using InGaP/GaAs HBT process

In this paper, low noise block downconverter (LNB) is designed for Ku-band satellite communication system using commercially available 6 inch InGaP/GaAs HBT high linear process. Designed MMIC is composed of low noise amplifier, double balance mixer, and IF amplifier with a total chip area of 2.6/spl times/1.1 mm/sup 2/. Designed MMIC yields conversion gain of over 37.5 dB, noise figure of less than 14 dB, ripple of 3 dB, and output-referred P/sub 1dB/(1 dB compression power) of 2.5 dBm with total power dissipation of 3 V, 50 mA.