Wireless Communication Chapter: Wireless Propagation Channels Section: Channel Models, Indoor Propagation

A Channel Sounder for the SHF and EHF Range

Contributed by Peter E. Leuthold and Pascal Truffer

The wideband channel sounder called ECHO 24 (Enhanced Channel Sounder operating at 4 GHz) measures highly time-variant CIR's by employing a correlation method. It achieves a 2 ns path delay resolution. For a coherent demodulation of the transmitted signal the receiver has to be provided with a reference signal at the carrier frequency . The intolerable frequency offset which occurs when using different local oscillators in the transmitter and receiver can be overcome by a remote antenna feeding in conjunction with a common carrier generated in the control station. The problem of line attenuation in coaxial cables is circumvented by the generation of appropriate optical signals which are fed to the transmitter and receiver via thin and flexible fibers. ECHO 24 uses a novel optical microwave generation principle [8]. Furthermore, the absence of mixers and frequency multipliers yields rather small and handy units which diminish the distortion of the electromagnetic field that is to be investigated.

The block diagram of the sounder system which consists of a transmitter (Tx) and a receiver (Rx) module as well as of a control station, is depicted in Figure 7. The optical part of the control station contains a laser diode (LD) and two Mach-Zehnder interferometers (MZI), which are used as light wave modulators. A quadrature modulation (I/Q Mod) is performed by means of a PN code impressed on the local oscillator signal (LO). The resulting signal controls the first MZI which generates a double sideband suppressed carrier (DSSC) modulation of the laser light for the Tx module. The laser light supplied to the Rx module is modulated at the second MZI by the local oscillator signal only. Real and imaginary parts of the received signal are returned to the control station by means of two standard coaxial cables where they are sampled for data acquisition purposes. The subsequent offline data processing requires the knowledge of the PN sequence. Both, the Rx and the Tx module are equipped with a wideband monopole antenna (A1 and A2).

Figure 7

The measured CIR of a time-variant indoor channel is shown in Figure 8. The Tx and the Rx module are set up in a distance of five meters. After one minute a person enters in between for another minute. The line of sight (LOS) path is attenuated, while the indirect paths are not influenced by that person.

A detailed description of ECHO 24 is given in [9]. You can view 3 pictures of the hardware here.

  
Figure: The CIR of a time-variant indoor channel at 24 GHz

Wireless Communication © Peter E. Leuthold and Pascal Truffer, 1999