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The mystery of the multipath effect: How does it affect the propagation of wireless signals?
In the world of wireless communications, the quality of signal propagation is of vital importance, and multipath effect is one of the key factors affecting this quality. The multipath effect refers to the fact that during the propagation of wireless signals, they are affected by obstacles such as buildings and trees, forming multiple different propagation paths. This effect can be beneficial or a source of problems. The technology of "channel sounding" is a tool used to evaluate this environment and improve wireless communications.
Channel testing technology can effectively evaluate the wireless environment, especially in multiple-input multiple-output (MIMO) systems, providing a basis for signal optimization.
Motivation and Applications
The performance of mobile wireless communications is greatly affected by the wireless signal propagation environment. Due to obstructions from buildings and natural obstacles, multiple different propagation paths are formed between the transmitter and the receiver, and these paths differ in time, phase, and attenuation. In single-input single-output (SISO) systems, multiple propagation paths may cause difficulties in signal optimization. However, with the development of Multiple-Input Multiple-Output (MIMO) systems, these problems have been effectively solved and the channel capacity and service quality have been improved.
In order to evaluate the effectiveness of a multi-antenna system, the wireless environment needs to be measured, and channel testing is just such a technique.
Problem Statement and Basic Concepts
In a multipath system, the wireless channel depends on frequency, time, and location. Therefore, in addition to the direction and delay of the propagation path, there are parameters such as Doppler shift and complex polarization path weight matrix to describe the channel. To characterize the propagation path between each transmitter element and the receiver element, engineers send wideband, multi-tone test signals. After the receiver receives the continuous periodic test sequence from the transmitter, it will perform a correlation comparison with the original sequence and generate a channel impulse response (CIR). By obtaining the transfer function of CIR, we can estimate the channel environment and thus improve its performance.
Description of existing methods
MIMO Vector Channel Tester
Based on multiple antennas at the transmitter and receiver, the MIMO vector channel tester can effectively collect the propagation direction at both ends of the connection and significantly improve the resolution of multipath parameters.
K-D wave propagation model
Instead of a ray-tracing model, engineers modeled wave propagation as a finite set of discrete, locally planar waves. This method can reduce the amount of calculation and the requirement for optical knowledge. The wave is assumed to be planar between the transmitter and the receiver. There are a few other important assumptions: the relative bandwidth is small enough that the time delay can be simply converted into a phase shift between the antennas; the array aperture is small enough that no observable The amplitude changes. Based on this, the basic signal model can be described.
Real-time ultra-wideband MIMO channel testing
The goal of future test equipment is to increase the bandwidth of channel measurements. New real-time ultra-wideband channel testers can perform channel measurements in the range of nearly zero to 5 GHz. Real-time ultra-wideband MIMO channel testing greatly improves positioning and detection accuracy, further facilitating precise tracking of mobile devices.
Multi-tone signals as excitation signals can effectively support channel evaluation and enhance signal processing.
Data Post-Processing
Perform a discrete Fourier transform (DFT) on the K-1 (one waveform in each channel is lost due to array switching) waveforms to extract frequency domain samples for each tone frequency. In this way, an estimated channel transfer function can be obtained, which is the key to improving channel performance.
RUSK channel tester
The RUSK channel tester excites all frequencies simultaneously in order to measure the frequency response of all frequencies. The test signal is periodic and its period must be longer than the duration of the channel impulse response to capture all delayed multipath components.
End
In today's rapidly developing wireless communication field, the application of channel testing technology will provide us with more stable and efficient communication solutions. In the future, as technology develops further, can we find new ways to better utilize the multipath effect so that it becomes an aid to wireless communications rather than an obstacle?
