19-10-2016, 12:00 PM
1459832415-1a828240469d1c6e53797f897f85868334ce.pdf (Size: 227.46 KB / Downloads: 6)
Abstract—Underground is a challenging environment for wireless
communication since the propagation medium is no longer
air but soil, rock and water. The well established wireless
communication techniques using electromagnetic (EM) waves do
not work well in this environment due to three problems: high
path loss, dynamic channel condition and large antenna size.
New techniques using magnetic induction (MI) can solve two
of the three problems (dynamic channel condition and large
antenna size), but may still cause even higher path loss. In
this paper, a complete characterization of the underground MI
communication channel is provided. Based on the channel model,
the MI waveguide technique for communication is developed
in order to reduce the MI path loss. The performance of the
traditional EM wave systems, the current MI systems and our
improved MI waveguide system are quantitatively compared. The
results reveal that our MI waveguide system has much lower path
loss than the other two cases for any channel conditions.
I. INTRODUCTION
Underground wireless communication enables a wide variety
of novel applications, including soil condition monitoring,
earthquake and landslide prediction, underground infrastructure
monitoring, sports-field turf management, landscape
management, border patrol and security, and etc [1]. However,
underground is a challenging environment for wireless
communication [2]. The propagation medium is no longer air
but soil, rock and water, where the well established terrestrial
wireless communication techniques do not work well.
Traditional techniques using electromagnetic (EM) waves
encounter three major problems in underground environments:
high path loss, dynamic channel condition and large antenna
size [2]. First, EM waves experience high levels of attenuation
due to absorption by soil, rock, and water in the underground.
Second, the path loss is highly dependent on numerous soil
properties such as water content, soil makeup (sand, silt, or
clay) and density, and can change dramatically with time
(e.g., increased soil water content after a rainfall) and space
(soil properties change dramatically over short distances).
Consequently, the bit error rate (BER) of the communication
system also varies dramatically in different time or position.
The unreliable channel brings design challenges for the underground
devices and networks to achieve both satisfying
connectivity and energy efficiency. Third, there exist conflicts
in antenna design for underground communication using EM
waves. On the one hand, antenna size is expected to be as
small as possible to ease the deployment of the networks. On
the other hand, operating frequencies in MHz or lower ranges
are necessary to achieve practical transmission range [1]. To
efficiently transmit and receive signals at that frequency, the
antenna size is too large to be deployed in the soil.
Magnetic induction (MI) is a promising alternative physical
layer technique for underground wireless communication. It
solves the dynamic channel condition problem and large
antenna size problem of the EM wave techniques. Specifically,
the dense medium such as soil and water cause little variation
in the attenuation rate of magnetic fields from that of air,
since the magnetic permeabilities of each of these materials are
similar [1], [3], [4]. This fact guarantees that the MI channel
conditions remain constant. Moreover, the MI communication
solves the issue of antenna size since the transmission and
reception are accomplished with the use of a small coil of
wire. No lower limit of the coil size is required. However, MI
is generally unfavorable for terrestrial wireless communication
since magnetic field strength falls off much faster than the EM
waves [5], [6]. In underground environment, although the path
loss of MI caused by the soil absorption is much less than the
EM waves, the total path loss may still be higher.
In this paper, we first provide a complete characterization
of the underground MI communication channel. Based on the
analysis, we then present a new technique to effectively reduce
the path loss of the MI communication. In particular, the
MI transmitter and receiver are modeled as the primary coil
and secondary coil of a transformer. We derive the analytical
expression of the relationship between the transmitting power
and receiving power (i.e., path loss). Multiple factors are
considered in the analysis, including the soil properties, coil
size, the number of turns in the coil loop, coil resistance
and operating frequency. To reduce the high path loss and
extend the transmission range, we develop the MI waveguide
technique [7], [8], [9] for underground wireless communication.
In this case, some small coils are deployed between
the transmitter and the receiver as relay points, which form
a discontinuous waveguide. The MI waveguide has three
advantages in underground wireless communication: first, by
carefully designing the waveguide parameters, the path loss
can be greatly reduced. Second, the relay coils do not consume any energy and the cost is very small. Third, MI waveguide
is not a continuous structure hence is very flexible and easy
to deploy and maintain. We quantitatively compare the performance
of the traditional EM wave systems, the current MI
systems and our improved MI waveguide system. The results
reveal that our MI waveguide system has much lower path loss
than the other two systems for any channel conditions.
The remainder of this paper is organized as follows. In
Section II, the underground MI communication channel is
completely modeled. In Section III, the MI waveguide technique
for underground wireless communication is developed.
In Section IV, the performance of the EM wave systems, MI
systems and MI waveguide system is evaluated. Finally, the
paper is concluded in Section V.
II. MI CHANNEL MODEL
In MI communication, the transmission and reception are
accomplished with the use of a coil of wire, as shown in
Fig. 1(a), where at and ar are the radii of the transmission
coil and receiving coil, respectively; r is the distance between
the transmitter and the receiver; and (90◦ −α) is the angle
between the axes of two coupled coils.
Suppose the signal in the transmitter coil is a sinusoidal current,
i.e., I = I0 ·e−jωt, where ω is the angle frequency of the
transmitting signal. This current can induce another sinusoidal
current in the receiver then accomplish the communication.
The relationship between the two coupled coils is represented
by the mutual induction. Therefore, the MI transmitter and
receiver can be modeled as the primary coil and the secondary
coil of a transformer, respectively, as shown in Fig. 1(b), where
M is the mutual induction of the transmitter coil and receiver
coil; Us is the voltage of the transmitter’s battery; Lt and
Lr are the self inductions; Rt and Rr are the resistances of
the coil; ZL is the load impedance of the receiver. We use
its equivalent circuit to analyze the transformer, as shown in
Fig. 1©, where,
Zt =Rt + jωLt; Z
t = ω2M2
Rr + jωLr + ZL
;
Zr =Rr + jωLr; Z
r = ω2M2
Rt + jωLt
;
UM =−jωM Us
Rt + jωLt
. (1)
For wireless communication techniques using EM waves,
the Friis transmission equation [10] gives the power received
by one antenna, given another antenna some distance away
transmitting a known amount of power. In the MI communication
case, similarly, our goal is to work out the equation to
describe the relationship between the transmitting power Pt
and the receiving power (Pr). In the equivalent circuit, it is
equal to find the relationship between the power consumed
in the primary loop and the power consumed in the load
impedance ZL:
Pr
Pt
= ZL · U2
M
(Z
r + Zr + ZL)2 ·
Zt + Z
t
U2
s
(2)