We also did the simulation first this time. Figure 8 shows the simulation result with difference phase separation. When the phase difference in Figure 33(a) is 108°, the same to last simulation in Figure32. The phase difference in Figure 33(b) is 208°, in Figure 33(c) is 258°. In this result, an obvious difference can be found between each picture. The direction of the ultrasonic can be controlled by phase adjustment. Then this simulation was tested, and Figure34 shows the experiment result with the same phase separation. In the experimental result, it can also be observed that the direction of the ultrasonic wave is changing with the phase difference.
At last, we also compared the longest transmission distance between the phased array and single transmitter. When used one single transmitter, the transmission distance is about 3 meters. However, for the phased the longest transmission could reach 5 meters on the maximum direction. This is two meters longer than the single transmitter.
factors that will significantly affect ultrasonic waves attenuation.
0.0 0.2 0.4 0.6 0.8 1.0
-75 -70 -65 -60 -55 -50 -45
Receivepower[dBm]
distance[m]
60KHz 40KHz
Figure 36 Transmission attenuation for different frequency
This absorption will be affected by many things, for example air temperature, air compression, relative humidity and so on. Weather conditions will also affect it, such as wind and rain. In general the air absorption can be defined by the following equation, when air temperature is 20 degrees centigrade.
Figure36 shows the different ultrasonic attenuation between two frequencies. Obviously, the attenuation is much higher when the operating frequency is 60kHz. Because the transmission distance is very important for one indoor communication system, this thesis chose 40kHz as the operating frequency.
Because the final goal of this thesis is to build an ultrasonic wireless communication system, the next part will study the modulation method used for ultrasonic waves. Modulation is the addition of information to a signal. Mainly, the modulation method can be divided into three types.
They are amplitude modulation, frequency modulation and phase modulation. But as shown in Figure 35, the bandwidth of an ultrasonic transducer is very narrow, and the attenuation will change with the frequency. So the frequency modulation method is not suitable for the ultrasonic wireless communication in air.
Amplitude Shift Keying(ASK) was a very early method for wireless communications. In the early years, this method was used for wireless radio.
In the electronic communication area, it is also a familiar method. ASK sends digital data by changing the amplitude of the carry wave. Because it
is very easy to vary the amplitude of ultrasonic, ASK is the first modulation method studied in this thesis.
Figure 37 Transmission experiment used amplitude modulation
The experimental result is shown in Figure 37. Figure 37(a) is the digital data used for modulation and Figure 37(b) is the received ultrasonic signal.
The experimental system is shown in Figure 26 and the distance between the two transmitters is 1meter. On the receive side, a PC oscilloscope has been used to measure the received signal. As shown in this result, the ASK modulated signal can be sent and received by a pair of ultrasonic transmitters but there are also many disadvantages of ASK modulation.
One of the disadvantages is that the amplitude of the receiving signal is not stable. Both the transmission distance and the environment will significantly affect the received amplitude. Another disadvantage of all amplitude modulation techniques is that the noise will be received together with the signal. This will increase the BER of the transmission system.
Another disadvantage is the power usage. If we want to send an ASK signal, most of the energy will be used by the carrier wave. So the amplitude modulation method is not very suitable for ultrasonic wireless communication. In the next experiment, this thesis will test the phase modulation method.
Similar to the amplitude modulation, phase modulation can be used to send digital data. When the data is binary digits, phase shift keying(PSK) can be used for digital modulation. Usually, each phase of the signal encodes an equal number of bits.
In the phase shift keying modulation methods, Binary Phase Shift
Keying(BPSK) is the most common one. This modulation method is the most robust of all phase shift keying methods. This is because the anti-noise ability of BPSK is very good. We can get data from the received signal easily. However, the transmit speed is only able to 1 bit per symbol and not suitable for high speed applications.
The general form for BPSK can be described by the following equation:
2
bcos 2 1 , 0,1
n c
b
s t E f t n n
T
(4.6)Where Ebis the energy per bit, Tb is the bit duration, and fcis the frequency of the carrier wave.
Figure 38 Transmission experiment used BPSK modulation
This experiment used the BPSK method. The experimental system is the same as the previous one. Figure38 shows the received signal and spectrum.
From this result，it can be seen that the phase modulated 40kHz signal has been successfully received by the receiver side. However, in this result, we not only find that the phase of signal has changed, but also that the amplitude of the signal is being changed. In further tests, we find that this is caused by the ultrasonic transducer. When we change the phase of the signal, a great drop in amplitude can be observed. In order to make sure whether the change in amplitude affects the received signal, we choose to measure the constellation diagram and error vector magnitude(EVM) of this signal.
I Q
1 0
Figure 39 Example of constellation diagram for BPSK
The constellation diagram is a useful tool for wireless experiment, especially in the phase shift keying method. In this method, it shows the signal in a 2-dimensional area, and scat this area. The received data will be shown in this area, and we can watch whether the data is in the right place.
The example of a constellation diagram for BPSK is shown in Figure 39.
Figure 40 Constellation diagram for BPSK
0 1 2 3 4 5
0 10 20 30 40 50 60
EVM[%]
Transmit Speed[kpbs]
Figure 41 Transmit speed of BPSK
Figure 40 shows the experimental result of the constellation diagram of the BPSK signal. In this result, almost all constellation points are shown on the real axis, at 0 degrees and 180 degrees. This proves that the phase modulation method can be used for ultrasonic communications. In this experiment, the transmit speed is set as 1 kpbs. So in Figure 41, the EVM for different transmit speeds has been measured. The error vector magnitude, as conventionally defined for signal carrier modulation, should be lower than 20 percent. So the highest transmit speed is about 3 kpbs when we use the BPSK method.
I Q
01 11
00 10
Figure 42 Example of constellation diagram for QPSK
In order to increase the transmit speed another phase modulation method has been tested. This modulation method is Quadrature Phase Shift Keying(QPSK). This is also called 4PSK. This method uses four phases of the carrier. The example of the constellation diagram for QPSK is shown in Figure42. Because it uses four phases, this method is able to encode two bits per symbol. Generally, the date rate of QPSK is twice as much as BPSK when the bit error rate is the same.
Also, the general form for QPSK can be described by the following equation:
2 cos 2 2 1 , 1,2,3,4
4
s
n c
s
s t E f t n n
T
(4.7)Different from BPSK, in this equation Esmeans energy per symbol and Tsmeans symbol duration. Other parameters in this equation are the same as in the BPSK equation.
Figure 43 Constellation diagram for QPSK
0 2 4 6 8 10 12
10 20 30 40 50
EVM[%]
Transmit speed[kbps]
Figure 44 Transmit speed of QPSK
Figure43 shows the constellation diagram of the QPSK signal and figure44 shows the error vector magnitude for different transmit speed.
This experiment proves that the QPSK method can also be used for ultrasonic wireless communication, as with the BPSK method. The highest transmit speed for QPSK can reach 8 kbps, which is twice higher than BPSK.
Actually, phase shift keying not only contains BPSK and QPSK. All
numbers of different phases can be used for this method, just like 8PSK or 16PSK. Generally, these modulation methods are known as higher order PSK. Higher order PSK can offer a higher transmit speed, however the error rate may become too high and the requirements of the device will also increase. This thesis has tested the 8PSK method in order to make sure whether higher order PSK can be used for ultrasonic wireless communication.
I Q
011 110
000 101
010
111
100 001
Figure 45 Example of constellation diagram for 8PSK
The constellation diagram for 8PSK is shown in Figure45. This method uses four phases, and is able to encode three bits per symbol. Also, the general form for BPSK can be described by the following equation:
2 cos 2 ( 1) , 1,2, ,8
4
n s c
s
E n
s t f t n
T
(4.8)The parameters in this equation are the same as in the QPSK equation.
Figure 46 Constellation diagram for 8PSK
The experiment result is shown in Figure46. Obviously, no signal arrived at the receive side. In further experiments we tested many other cases, including different distance, different transmit speed and different power.
However, the 8PSK signal still cannot transmit successfully. This experiment proved higher order PSK was not suitable for this ultrasonic communication system. The QPSK method should be the best choice for the ultrasonic wireless communication system.