ITS: Radio Channel Impulse Response Measurement Systems

Radio Channel Impulse Response Measurement Systems

RCIRMS Home

System Configurations

NTIA/ITS Impluse Measurement System Specifications

Applications:

Website maintained by ratzloff@its.bldrdoc.gov

DESCRIPTION:
Channel impulse response is a measurable response by a radio communication channel when an electromagnetic impulse is transmitted over the air. The radio propagation channel acts as a time and spacial varying filter which can distort the signal. The filter characteristics are determined by both the reflection of electromagnetic waves off of objects and the movement of reflective surfaces, as well as the movement of the transmitter and/or receiver. The purpose of multiple impulse measurements is to quantify the varying filter characteristics of the communication channel along both the time and spacial dimensions.

MISSION:
The mission of these measurements is to characterize different radio communication channels at various frequency bands in an effort to help private industry and government agencies evaluate the performance of their communication systems in specific environments. This is done through use of the impulse response measurements for statistical analysis, as well as modeling, and simulation. Click here for information about cooperative research and development agreements (CRADA).

OVERVIEW:
Impulse response measurements of a radio propagation channel provide important information for the design, development, and planning of radio communication systems. Over the years, much effort has been devoted to the correlation of radio system performance to various parameters determined through statistical analysis of impulse response data. Modeling of the impulse response has also been used for the simulation of the radio channel which, in turn, can determine the effects of the propagation channel on the signal itself. In addition, the impulse response, once determined, can be used to simulate the actual received signal as it is distorted through the propagation channel by convolving the impulse response with the transmitted signal.

The impulse response contains several important pieces of information, each of which can be analyzed for its effect on signal propagation through the communication channel.

Extent and nature of propagation delays (multipath) due to reflections from various surfaces. Multipath results in distortion of the signal and signal fades due to the summation of multiple delayed copies of the signal with different phase relationships.
Doppler effects due to the movement of reflective surfaces, and movement of the transmitter and/or receiver.
Absolute time of travel for the various delay paths.
Path loss between the transmitter and the receiver.

The measurement systems exploits the unique characteristics of pseudo-random noise (PN) codes to produce the impulse response. The transmitter modulates an RF carrier with a PN code to produces a binary phase-shift keyed (BPSK) signal. The signal is transmitted over the communication channel of interest and then downconverted at the receiver to produce baseband co-phase and quadrature-phase signals. The signals are then correlated with a local copy of the PN code to generate a complex impulse response (magnitude and phase).