We present 27 geodetic VLBI maps of OJ 287 obtained from the archive of the Washington correlator. The observations presented here were made between 1990 October and 1996 December. During this period a sequence of six superluminal components has been identiÐed. We measured the proper motion of these components to be approximately 0.5 mas yr~1, which is about twice as high as that seen in previous VLBI observations. These results imply a higher component ejection rate than previously observed, in good agreement with the observed occurrences of radio outbursts. We have examined a possible connection between VLBI components and optical Ñares in the framework of a binary black hole system.
We present maps of BL Lac obtained from geodetic VLBI data from the archive of the Washington correlator. The observations were made from 1996 March to 1996 November, with periods from one experiment to another as short as 1 month. The dominant structure of the maps is given by a superluminal component (C2) moving predominantly at position angle D190¡. At the later epochs a new superluminal component (C3) emerges from the core and moves along a trajectory at a higher position angle (200¡). We also include maps from observations made in 1995 June and August showing the main component (C2) in the early stages of its evolution, as well as an older component (C1). The position angle of the component C1 is nearly the same as that of the new component C3. The component C2 shows indications of nonradial motion, which is discussed in terms of a helically distorted jet. The best Ðt was obtained for a halfÈopening angle of the jet of and an angle of the helical axis to the line of 2¡ .6 sight of 17¡.
We present 11 maps of BL Lacertae object 1803+784 derived from geodetic VLBI observations archived at the Washington Correlator. The observations were obtained during the years 1988-1999. In this period three superluminal components with an expansion rate of 0.084 mas yr À1 were identified. A remarkable feature of the VLBI images is that the superluminal components only become visible on the jet at a radius of 1 mas from the core, which was interpreted as a result of a helically curved jet. This feature and further evidence of helical patterns found in published high dynamic range VLBI maps at parsec scale and kiloparsec scale enabled us to obtain a well-constrained helical jet. The overall radio morphology of 1803+784 could be described by two dominant components given by a narrow helical jet extending east-west about 6 kpc from the core (deprojected size of 30 kpc) and a much larger north-south component perpendicular to the helical jet (cone axis) with a linear size of about 270 kpc.
A model for a communication system transmitting two nonorthogonal n-PAM signals with spectral overlapping through an AWGN band-limited channel is established. The system is shown to be equivalent to a discrete time-variant multi-user channel with strong interference among users. A receiver structure based on source separation concepts is proposed. The performance of signal detection in terms of symbol error rate is evaluated using Monte Carlo simulations..
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