The Project
The project work is carried out as part of the CDIO course TSKS05 offered by Division of Communication Systems at ISY, Linköping University. The examiner of the course is Danyo Danev.
Background
The MIMO (Multiple Input Multiple Output) technology has gained an increasing interest in wireless communication due to its capability to increase the throughput of a communication system without an increased bandwidth or increased transmit power. The technology is based on the use of multiple antennas in both the transmitter and the receiver. By using equal transmit power on all antennas, a higher spectral efficiency is achieved (bits per second per Hertz of bandwidth) due to the resulting array gain of the antennas.
The MIMO technology also allows to direct signal power in a certain direction by altering the phase of the transmitted signal on each of the antennas in order to create constructive interference in a certain point or direction. This enables a transmitter, a base station for example, to direct the communication to a certain receiver without interfering with other receivers.
Project task
The main task of the project was to develop a system capable of demonstrating the effects of massive MIMO beamforming. Using a set of loudspeakers, which would also be used as microphones, the system would be able to measure sound transmitted from a source and from that sound extract characteristics of the channel between the transmitter and receiver. With this information the loudspeakers would be able to produce a sound, which would be constructively added at the point where the measured sound originated from. The original project idea was to place a set of four wine glasses next to each other, ringing one of them and letting the system crack this glass, leaving the remaining glasses unaffected. This would have been a spectacular way of showing the powers of massive MIMO, since it would become rather obvious that signal power is focused into one point. However, this idea boiled down to a more controlled and measurable setup where eight loudspeakers were used and where one of the pairs was used in point of focus and the other pairs was used in the MIMO array. One of the speakers in the pair in point of focus would be used for sending the pilot (simulating a glass) and the other speaker would be used as a reference point, a verifyer, to controll that the sound energy was focus to the speaker sending the pilot. Since the product was limited to use only eight pairs of low powered loudspeakers, the product would not be able to produce enough signal power to crack a wine glass.
Implementation
Hardware
The hardware consists of a computer, a custom made distribution box and eight loudspeaker pairs.
The loudspeakers (left) are simple computer loudspeakers modified to be able to both transmit and record
sound. The modification is a amplification circuit designed by
Mikael Olofsson.
The purpose of the distribution box (right) is to channel all the outgoing and ingoing signals to the right
speakers. It is developed by the project team and contains some simple amplification circuits together
with a transistor circuit for switching the mode of the speakers.
The picture to the left shows the complete setup of the system during a run.
Software
All functionality of the system is implemented in MATLAB. The software part consists of four modules: Controller, Spectral Analyzer, Channel Estimator and Signal Generator. The Controller is the main program were all the other modules, written as MATLAB functions, are executed.
The protocol of a run is shown in the picture below.
Results
As already stated the main goal of the product is to achieve constructive interference at the
point of focus and destructive interference everywhere else. For that reason, two microphones were
used and placed as shown in the previous image. The recorded voltage level is shown in the
following image. The blue line represent the voltage recorded by the microphone at the point of focus.
The red line represent the recorded signal from the second microphone, placed half a wavelength away
from the point of focus. The difference between the energy of the two signals has been proved to
lie in the region of 15-20 dB.
