• A Coherent Receiver for QPSK and SQPSK Signals

      Alem, W. K.; Weber, C. L.; Axiomatix; University of Southern California (International Foundation for Telemetering, 1978-11)
      A demod-remod type of coherent tracking loop for conventional QPSK and staggered QPSK (SQPSK) is presented. The phase detector characteristic (S-curve) is determined. The effects of power unbalance and arm gain unbalance on the S-curve are presented. The steady state rms phase error is shown as a function of the signal-to-noise ratio at the output of the arm filters.
    • A Fast Complex Integer Convolution Using a Hybrid Transform

      Reed, I. S.; Truong, T. K.; University of Southern California; California Institute of Technology (International Foundation for Telemetering, 1978-11)
      In this paper it is shown that the cyclic convolution of complex values can be performed by a hybrid transform. This transform is a combination of a Winograd transform, and a fast complex integer transform developed previously by the authors. This new hybrid algorithm requires fewer multiplications than any previously known algorithm.
    • Phase Referencing for MA Demultiplexing in the TDRSS

      Gagliardi, R. M.; University of Southern California (International Foundation for Telemetering, 1978-11)
      The TDRSS performance is based, to a large extent, on the ability to maintain phase coherency between user, satellite, and ground segment. This is especially true for the MA return subsystem, which uses coherent referencing for multiplexing and demultiplexing between the TDRS and ground processor. Phase noise appearing on these referencing waveforms destroy the phase coherency, and will degrade the overall MA return operation. In this paper the manner in which this phase referencing is achieved is described. In addition, the results of a preliminary study to distinguish the key MA return phase noise sources, and the manner in which each will ultimately influence performance, is presented. The results show that the return phase noise effects can be separated into "coherent" and "noncoherent" contributions, and each must be separately evaluated. The effect of the various tracking loop bandwidths throughout the link is shown, and the manner in which the specific phase noise spectra are eventually filtered is developed.
    • Pulse to Pulse Encoding in Optical Communications

      Prati, G.; Gagliardi, R.; University of Pisa; University of Southern California (International Foundation for Telemetering, 1978-11)
      Optical pulsing of a laser beam provides a convenient manner for transmitting digital information, and such procedures have led to well-accepted classes of pulse amplitude and pulse position modulation formats. However the excessive pulse dispersion and background light bursts that characterize several optical channels severely limit performance and achievable data rates. One procedure for combating these effects is to encode data over a multiple of pulse frames, and decode sequences of pulses rather than each pulse individually. In the paper pulse to pulse encoding and decoding of an optical beam is examined, considering both photodector gain effects and receiver thermal noise. Theoretical performance results are presented, and the relation between this type of decoding and "tree searching" is developed.
    • Spectral Shaping Without Subcarriers

      Welch, Lloyd R.; University of Southern California (International Foundation for Telemetering, 1978-11)
      For proper operation of the phase lock loop which tracks a carrier it is important to minimize the spectral energy at frequencies near the carrier. A traditional method is to modulate the data onto a subcarrier in such a way that there is little energy near D.C. The resulting signal then is used to modulate the carrier. The problem with such a scheme is that the total bandwidth is much larger than necessary to transmit the data. This paper proposes and analyzes a simpler scheme which increases the data bandwidth by a very small fraction, yet reduces the energy near D.C. to nearly zero.
    • TDRSS Telecommunications MA Return Channel

      Weber, C.; Halderman, D.; Blyth, R.; University of Southern California; TRW, Defense & Space Systems Group (International Foundation for Telemetering, 1978-11)
      The Tracking and Data Relay Satellite System (TDRSS) relays signals to and from ground terminal and user satellites. No signal processing is done on the satellite and as many functions as possible have been removed from the satellite and implemented in the ground terminal. The multiple access (MA) users serviced by the S-Band thirty element phased array on the satellite require the capability to form up to twenty simultaneous tracking antenna beams by phase and amplitude weighting the individual antenna elements in the ground terminal. Frequency division multiplexing is used to transmit the 30 antenna elements to the ground for beamforming. Phase and amplitude uncertainties build up over time between the antenna elements and the beam processing on the ground. To optimize and maintain required performance, a calibration technique is required to estimate the channel weight correction table for the MA return link.