Wireless signal analysis technique
Channel sounding is a technique that evaluates a radio environment for wireless communication, especially MIMO systems. Because of the effect of terrain and obstacles, wireless signals propagate in multiple paths (the "multipath" effect). To minimize or use the multipath effect, engineers use channel sounding to process the multidimensional spatial–temporal signal and estimate channel characteristics. This helps simulate and design wireless systems.
Motivation &amp; applications.
Mobile radio communication performance is significantly affected by the radio propagation environment. Blocking by buildings and natural obstacles creates multiple paths between the transmitter and the receiver, with different time variances, phases and attenuations. In a single-input, single-output (SISO) system, multiple propagation paths can create problems for signal optimization. However, based on the development of multiple input, multiple output (MIMO) systems, it can enhance channel capacity and improve QoS. In order to evaluate effectiveness of these multiple antenna systems, a measurement of the radio environment is needed. Channel sounding is such a technique that can estimate the channel characteristics for the simulation and design of antenna arrays.
Problem statement &amp; basics.
In a multipath system, the wireless channel is frequency dependent, time dependent, and position dependent. Therefore, the following parameters describe the channel:
To characterize the propagation path between each transmitter element and each receiver element, engineers transmit a broadband multi-tone test signal. The transmitter's continuous periodic test sequence arrives at the receiver, and is correlated with the original sequence. This impulse-like auto correlation function is called "channel impulse response (CIR)". By obtaining the transfer function of CIR, we can make an estimation of the channel environment and improve the performance.
Description of existing approaches.
MIMO vector channel sounder.
Based on multiple antennas at both transmitters and receivers, a MIMO vector channel sounder can effectively collect the propagation direction at both ends of the connection and significantly improve resolution of the multiple path parameters.
"K"–"D" model of wave propagation.
Engineers model wave propagation as a finite sum of discrete, locally planar waves instead of a ray tracing model. This reduces computation and lowers requirements for optics knowledge. The waves are considered planar between the transmitters and the receivers. Two other important assumptions are:
Based on such assumptions, the basic signal model is described as:
formula_1
where formula_2 is the TDOA (time difference of arrival) of the wave-front formula_3. formula_4 are DOA at the receiver and formula_5 are DOD at the transmitter,formula_6 is the Doppler shift.
Real-time ultra-wideband MIMO channel sounding.
A higher bandwidth for channel measurement is a goal for future sounding devices. The new real-time UWB channel sounder can measure the channel in a larger bandwidth from near zero to 5 GHz. The real time UWB MIMO channel sounding is greatly improving accuracy of localization and detection, which facilitates precisely tracking mobile devices.
Excitation signal.
A multitoned signal is chosen as the excitation signal.
formula_7
where formula_8 is the center frequency, formula_9 (formula_10 is Bandwidth, formula_11 is Number of multitones) is the tone spacing, and formula_12 is the phase of the formula_13 tone. we can obtain formula_12 by
formula_15
Data post-processing.
formula_18
where formula_19 is the noise power, formula_20 is a reference signal and formula_21 is the samples.
The scaling factor c is defined as
formula_22
RUSK channel sounder.
A RUSK channel sounder excites all frequencies simultaneously, so that the frequency response of all frequencies can be measured. The test signal is periodic in time with period formula_23. The period must be longer than the duration of the channel's impulse response formula_24 in order to capture all delayed multipath components at the receiver. The figure shows a typical channel impulse response (CIR) for a RUSK sounder. A secondary time variable is introduced so that the CIR is a function of the delay time formula_25 and the observation time formula_26. A delay-Doppler spectrum is obtained by Fourier transformation.