4.3.1 Attenuation and Coverage
In this part we will discuss the communication distance and coverage area. In order to study these, we are going to introduce the attenuation of ultrasonic waves in air. Here, the ISO 9613-1 standard can be used for an estimation of the ultrasonic wave attenuation under specific conditions[50].
This standard is made for acoustic signal attenuation in audio range, but it can also be applied for a wider frequency range, including ultrasonic waves propagating in air. According to this standard, ultrasonic waves attenuation can be defined by this equation:
Lp Lw K Dim Ae
(4.1)In this equation, Lw is the sound power level which is produced by the point sound source. K is the attenuation due to geometrical divergence, Dim is directivity and
Ae
is air absorption. This is the basic equations of ultrasonic waves attenuation. In this equation, geometrical divergence and air absorption are the main reason for ultrasonic waves attenuation.However, the directivity of the sound source will also affect the attenuation.
When directivity is the more distinctness, the attenuation will become more less. The attenuation due to geometrical divergence can be defined by the following equation:
10lg
24 K Q
r
(4.2)In this equation,
r
is the distance from the ultrasonic source, and Q is the direction factor of the ultrasonic waves source.Air absorption is another major factor which will significantly affect the ultrasonic attenuation. The air 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:
2
7.4 10
8 eA f r
(4.3)Where f is the frequency of the ultrasonic wave, and
is relative humidity. The last factor Dim is defined by reflecting surface near ultrasonic source. Mainly, each reflecting surface can offer a 3dB gain increase.According to the above analysis, we can find that the main affecting factors of attenuation are air absorption, geometrical divergence and reflection. The air absorption and geometrical divergence are hard to change. Air absorption will increase with ultrasonic wave frequency. In our experiment, the frequency of ultrasonic waves is 40kHz. So we need to test the transition distance at this frequency. The equation also shows that high reflection can reduce attenuation.
0.0 0.5 1.0 1.5 2.0 2.5 3.0
-55 -50 -45 -40 -35 -30
receivepower[dBm]
distance[m]
Indoor Outdoor
Figure 28 Experimental result of transmission attenuation
So in the experiment we chose two different communication
environments. One is an open area with outdoor, where has almost no reflection. The other one is an indoor environment with many reflecting objects. The experimental result is shown in Figure 28. According to this result, the transmission attenuation for the indoor environment is better than the outdoor environment because of reflection. When used for the indoor communication, the ultrasonic signal can still be received when the transmission distance over 3 meters. This distance is not very long, but it is still enough for indoor short-range communication.
The next experiment is about coverage angle. In this thesis we chose to measure receiver power. The sensitivity of our receiver is about -68dBm, and the receiver power should be higher than this. In this experiment the transmitter was fixed, and the receiver revolved around it. The distance between the receiver and transmitter always maintained at 3 meters. The experiment result is shown in Figure 28. In this result, the signal was received successfully by the receiver side between about -60o and 60o, so the coverage angle is about 120o.
-100 -80 -60 -40 -20 0 20 40 60 80 100 -90
-85 -80 -75 -70 -65 -60
Power[dBm]
Angle[Degree]
Figure 29 Experimental result of coverage angle
The two experiments above have proved that ultrasonic waves can be used for wireless communication. The transmission distance and coverage angle are both enough to cover a 120 restricted space such as a common room.
4.3.2 Impact of Barrier
This chapter is going to discuss another case, while there is some block on the transmission route. In a real situation, there are many obstructions in the room and as we know, ultrasonic waves are hard to pass through thick blocks. For example, tables, computers or people in the room will become blocks for the wireless communications. Therefore, this thesis wants to test whether ultrasonic waves can be used in this case.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 -60
-58 -56 -54 -52 -50 -48
ReceivedPower[dBm]
Distance[m]
Figure 30 Influence of obstruction
Figure 30 shows the simulation and experimental result. Here, Figure30(a) is a schematic diagram of our experimental system. In this experiment, we set one pair of ultrasonic transducers in the area. A steel block was set between the transmitter and the receiver. We simulated and tested whether the receiver power depended on the distance between the receiver and the block. Figure30(b) shows the simulation result used for the
FDTD method and Figure30(c) shows the experimental result. In both simulation and experiment, the ultrasonic source was set at the left hand side of the area.
Table 2. Simulation parameters for Figure 5
Simulation area 150cm2
Distance between transducer and block
50cm
Length of block 40cm
In the simulation result, the ultrasonic wave did not pass through the block directly. So on the right hand side of the block a blank space can be found. However, ultrasonic waves can still arrive the area away from the block by diffraction and reflection. Figure 30(c) shows the experimental result. This experiment used a one square meter rubber slab as a block, and set it 50cm from the ultrasonic transmitter. Then the received power at different distances from the block was measured. The experiment result is the same as the simulation. In this result, the ultrasonic waves cannot pass through the block directly. Therefore, a blank space can be observed next to the block, both in the simulation and the experiment result. However, in the space that is away from the block, the reflection wave can be received again. So received power is very low when the receiver is near the block.
When the receiver comes away from the block, it can receive more reflected waves, so the received power is increasing. Finally, when the receiver comes too far away from the transmitter side, the received power comes down again because of the attenuation. Both the simulation and result show that ultrasonic waves can reach the place behind the block, by reflection and scattering. This experiment proved that ultrasonic waves can also be used for wireless communication when there are obstacles on the transmission path.