262 research outputs found

    NASA patent abstracts bibliography: A continuing bibliography. Section 1: Abstracts (supplement 26)

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    Abstracts are provided for 172 patents and patent applications entered into the NASA scientific and technical information system during the period July 1984 through December 1984. Each entry consists of a citation, an abstract, and in most cases, a key illustration selected from the patent or patent application

    Packet CDMA communication without preamble

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    Code-Division Multiple-Access (CDMA) is one of the leading digital wireless communication methods currently employed throughout the world. Third generation (3G) and future wireless CDMA systems are required to provide services to a large number of users where each user sends data burst only occasionally. The preferred approach is packet based CDMA so that many users share the same physical channel simultaneously. In CDMA, each user is assigned a pseudo-random (PN) code sequence. PN codephase synchronization between received signals and a locally generated replica by the receiver is one of the fundamental requirements for successful implementation of any CDMA technique. The customary approach is to start each CDMA packet with a synchronization preamble which consists of PN code without data modulation. Packets with preambles impose overheads for communications in CDMA systems especially for short packets such as mouse-clicks or ATM packets of a few hundred bits. Thus, it becomes desirable to perform PN codephase synchronization using the information-bearing signal without a preamble. This work uses a segmented matched filter (SMF) which is capable of acquiring PN codephase in the presence of data modulation. Hence the preamble can be eliminated, reducing the system overhead. Filter segmentation is also shown to increase the tolerance to Doppler shift and local carrier frequency offset. Computer simulations in MATLAB® were carried out to determine various performance measures of the acquisition system. Substantial improvement in probability of correct codephase detection in the presence of multiple-access interference and data modulation is obtained by accumulating matched filter samples over several code cycles prior to making the codephase decision. Correct detection probabilities exceeding 99% are indicated from simulations with 25 co-users and 10 kHz carrier frequency offset or Doppler shift by accumulating five or more PN code cycles, using maximum selection detection criterion. Analysis and simulation also shows that cyclic accumulation can improve packet throughput by 50% and by as much as 100% under conditions of high offered traffic and Doppler shift for both fixed capacity and infinite capacity systems

    LASER Tech Briefs, Spring 1994

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    Topics in this Laser Tech Brief include: Electronic Components and Circuits. Electronic Systems, Physical Sciences, Materials, Mechanics, Fabrication Technology, and books and reports

    Electroabsorption modulators used for all-optical signal processing and labelling

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    Real-time Digital Signal Processing for Software-defined Optical Transmitters and Receivers

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    A software-defined optical Tx is designed and demonstrated generating signals with various formats and pulse-shapes in real-time. Special pulse-shapes such as OFDM or Nyquist signaling were utilized resulting in a highly efficient usage of the available fiber channel bandwidth. This was achieved by parallel data processing with high-end FPGAs. Furthermore, highly efficient Rx algorithms for carrier and timing recovery as well as for polarization demultiplexing were developed and investigated
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