Small-Scale Multipath Measurements
There are many wideband sounding technique for determining the small scale multipath. These techniques may be classified as direct pulse measurements, spread spectrum sliding correlator measurements, and swept frequency measurements.
Types of Sounding Techniques.
· Direct RF Pulse System
· Spread Spectrum Sliding Correlator Channel Sounding
· Frequency Domain Channel Sounding
Direct RF Pulse System:
· A wideband pulsed bistatic radar usually transmits a repetitive pulse of widthTbbs and uses a receiver with a wide bandpass ﬁlter (BW=2/Tbb Hz). The signal is then ampliﬁed, envelope detected, and displayed and stored on a high speed oscilloscope.(see figure 11.4)
· Immediate measurements of the square of the channel impulse response convolved with the probing pulse can be taken.
· If the oscilloscope is set on averaging mode then this system provides a local average power delay proﬁle.
· This system is subject to interference noise. If the ﬁrst arriving signal is blocked or fades, severe fading occurs, and it is possible the system may not trigger properly
Spread Spectrum Sliding Correlator Channel Sounding:
· The advantage of a spread spectrum system is that, while the probing signal may be wideband, it is possible to detect the transmitted signal using a narrowband receiver preceded by a wideband mixer, thus improving the dynamic range of the system as compared to the direct RF pulse system.
· In a spread spectrum channel sounder, a carrier signal is "spread" over a large bandwidth by mixing it with a binary pseudo-noise (PN) sequence having a chip durationTcand a chip rateRc equal to 1/Tc Hz
· The power spectrum envelope of the transmitted spread spectrum signal is given by