King F. Lee

A space-frequency transmitter diversity technique for OFDM systems

A transmitter diversity technique for wireless communications over frequency selective fading channels is presented. The proposed technique utilizes orthogonal frequency division multiplexing (OFDM) to transform a frequency selective fading channel into multiple flat fading subchannels on which space-frequency processing is applied. Simulation results verify that in a slow fading environment the proposed space-frequency OFDM (SF-OFDM) transmitter diversity technique has the same performance as a previously reported space-time OFDM (ST-OFDM) transmitter diversity system but shows better performance in the more difficult fast fading environments. Other implementation advantages of SF-OFDM over the ST-OFDM transmitter diversity technique are also discussed.

A space-time coded transmitter diversity technique for frequency selective fading channels

A simple space-time coded orthogonal frequency division multiplexing (OFDM) transmitter diversity technique for wireless communications over frequency selective fading channels is presented. The proposed technique utilizes OFDM to transform frequency selective fading channels into multiple flat fading subchannels on which space-time coding is applied. A two-branch transmitter diversity system is implemented without bandwidth expansion and with a small increase in complexity beyond that of a conventional OFDM system. Simulations verify that the two-branch transmitter diversity system achieves a diversity gain equivalent to that of the optimal maximal ratio combining (MRC) receiver diversity system.

First Demonstration of AC Gain From a Single-walled Carbon Nanotube Common-Source Amplifier

First demonstration of AC gain from a single-walled carbon nanotube transistor is presented. A top-gated carbon nanotube field-effect transistor (CNFET) is configured as a common-source amplifier and frequency response function of the amplifier is measured. Evidence of unambiguous signal amplification is observed in time domain as well as frequency domain up to a unity gain frequency of approximately 560 kHz. The observed roll-off in frequency is solely due to the RC time constant of the measurement apparatus. A specifically designed circuit compatible SPICE model for the CNFET is used to model both DC and AC characteristic with the same set of physical parameters for the first time. Good agreement between measurement and simulation is obtained. For a device without the parasitic load capacitance, the predicted intrinsic unity voltage gain frequency is 29 GHz and the cut-off frequency is ~ 50 GHz.

Pilot-symbol-assisted channel estimation for space-time coded OFDM systems

Space-time coded orthogonal frequency division multiplexing (OFDM) transmitter diversity techniques have been shown to provide an efficient means of achieving near optimal diversity gain in frequency-selective fading channels. For these systems, knowledge of the channel parameters is required at the receivers for diversity combining and decoding. In this paper, we propose a low complexity, bandwidth efficient, pilot-symbol-assisted (PSA) channel estimator for multiple transmitter OFDM systems. The pilot symbols are constructed to be nonoverlapping in frequency to allow simultaneous sounding of the multiple channels. The time-varying channel responses are tracked by interpolating a set of estimates obtained through periodically transmitted pilot symbols. Simulations are used to verify the effectiveness of the proposed estimator and to examine its limitations. It is also shown that the PSA channel estimator has a lower computational complexity and better performance than a previously proposed decision-directed minimum mean square error MMSE channel estimator for OFDM transmitter diversity systems.

Measuring Frequency Response of a Single-Walled Carbon Nanotube Common-Source Amplifier

Frequency response function (FRF) showing ac gain from a single-walled carbon nanotube transistor is presented. A top-gated carbon nanotube FET (CNFET) is configured as a common-source amplifier and the FRF of the amplifier is measured. Evidence of unambiguous signal amplification is observed in time domain as well as frequency domain up to a unity voltage gain frequency of approximately 560 kHz. The observed roll-off in frequency is solely due to the RC time constant of the measurement apparatus. A specifically designed circuit-compatible SPICE model for the CNFET is used to model both dc and ac characteristic with the same set of physical parameters. Good agreement between measurement and simulation is obtained. For a device without the parasitic load capacitance, we predict an intrinsic unity voltage gain frequency of 29 GHz and a cutoff frequency of ~ 50 GHz.

High dynamic range CMOS image sensor with pixel level ADC and in-situ image enhancement

We describe a CMOS image sensor with pixel level analog to digital conversion (ADC) having high dynamic range (>100db) and the capability of performing many image processing functions at the pixel level during image capture. The sensor has a 102x98 pixel array and is implemented in a 0.18um CMOS process technology. Each pixel is 15.5um x15.5um with 15% fill factor and is comprised of a comparator, two 10 bit memory registers and control logic. A digital to analog converter and system processor are located off-chip. The photodetector produces a photocurrent yielding a photo-voltage proportional to the impinging light intensity. Once the photo-voltage is less than a predetermined global reference voltage; a global code value is latched into the pixel data buffer. This process prevents voltage saturation resulting in high dynamic range imaging. Upon completion of image capture, a digital representation of the image exists at the pixel array, thereby, allowing image data to be accessed in a parallel fashion from the focal plane array. It is demonstrated that by appropriate variation of the global reference voltage with time, it is possible to perform, during image capture, thresholding and image enhancement operations, such as, contrast stretching in a parallel manner.

A multirate pilot-symbol-assisted channel estimator for OFDM transmitter diversity systems

Orthogonal frequency division multiplexing (OFDM) transmitter diversity techniques have been shown to be efficient means of achieving near optimal diversity gain in frequency-selective fading channels. For these systems, knowledge of the channel parameters is required at the receivers for diversity combining and decoding. In this paper, we propose a low complexity, bandwidth efficient, pilot-symbol-assisted channel estimator for multiple transmitter OFDM systems. The pilot symbols are constructed to be non-overlapping in frequency to allow for the simultaneous sounding of the multiple channels. The time-varying channel responses are tracked by interpolating a set of estimates obtained through periodically transmitted pilot symbols. The effectiveness and limitations of the proposed estimator are verified by simulations.

Image Processing for In-vehicle Smart Cameras

Smart cameras are increasingly being deployed in many automotive applications. The in-vehicle environment presents unique challenges to the camera functions and can significantly degrade the performance of smart cameras. In this paper, we present two image processing algorithms that address two of the most common challenges for in-vehicle smart cameras, namely exposure control and motion induced distortion. The proposed algorithms are very efficient and highly suitable for embedded real-time smart cameras. The algorithms were first developed for a real-time in-vehicle automatic license plate recognition smart camera but the techniques are also applicable to many other in-vehicle smart camera applications

An efficient color image acquisition system for wireless handheld devices

The paper presents a low-cost, low-power, and yet highly effective, color image acquisition system for wireless handheld devices such as cellular camera phones and PDAs. In common color image acquisition systems, Bayer pattern image data are captured from a CMOS/CCD color image sensor and processed to generate a YCbCr color signal for the image compression engine. The proposed system uses a novel vertex-centered color interpolation algorithm for parallel interpolation of groups of pixels within a window and directly processes the image from the Bayer pattern data to the YCbCr color space. The proposed system also uses pseudo-median filtering on the Cb and Cr color channels to enhance image quality with low hardware complexity.

Bandwidth efficient OFDM transmitter diversity techniques

Space-time coded orthogonal frequency division multiplexing (OFDM) transmitter diversity techniques have been shown to provide efficient means of achieving near optimal diversity gain in frequency-selective fading channels. However, these known techniques all require a cyclic prefix to be added to the transmitted symbols, resulting in significant bandwidth expansion. In this paper, bandwidth efficient space-time and space-frequency block-coded OFDM transmitter diversity techniques are proposed that exploit spatial diversity to eliminate the need for a cyclic prefix.