Super-resolution of pulsed multipath channels for delay spread characterization

For frequency selective channels, the delay spread is one measure for the channel distortion. This channel characterization is possible by either a CW frequency sweep, which gives directly the transfer function, or by sounding the channel with pulses which results in the impulse response. When the sounding pulse becomes a chirp-type waveform, the two approaches are distinguished more by implementation rather than by concept. Simple rectangular pulses can be used for partial, but usually sufficient, channel characterization. In this paper, the process is described for resolving impulse response components into bins smaller than the duration of the sounding pulse and smaller than the reciprocal of the channel bandwidth. From such "super-resolution", the delay spread can be established accurately even when it is much less than the sounding pulse duration, allowing longer, higher-energy pulses to be used for channel characterization. The process is demonstrated by generating the super-resolved real-only impulse responses from the real-only responses of an experimental multipath channel. Two algorithms are presented and compared, These are subtractive deconvolution where a loop gain of unity is shown to be stable; and a modified inverse filter technique, in which the modification caters for the spectral zeros of the sounding pulse. The experiments demonstrate explicitly how receiving antennas can be used to control the amount of multipath channel degradation.