28-06-2013, 02:12 PM
Underground Wireless Communication using Magnetic Induction
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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.
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).
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.
EVALUATION
In this section, we use MATLAB to compare the performance
of the traditional EM wave technique, the current MI
technique and the improved MI waveguide technique for wireless
underground communication. For EM wave propagation
in soil, we utilize the channel model developed in [2]. For MI
and MI waveguide systems, the models described in Section
II and Section III (equation (9) and (18)) are used.
Except studying the effects of certain parameters, the default
values are set as follows: the volumetric water content (VWC)
is 5% and the operating frequency is 300 MHz. The transmitter,
receiver and relay coil all have the same radius of 0.1 m.
The coil is made of copper wire with a 0.1 mm diameter (AWG
38). Hence the resistance of unit length R0 can be calculated
as 2.16 Ω/m. The permeability of soil medium is the same as
that in the air, which is 4π × 10−7 H/m.
CONCLUSION
In wireless underground communication, traditional techniques
using EM waves encounter three major problems:
high path loss, dynamic channel condition and large antenna
size. MI is an alternative technique that can solve two of
the three problems: the dynamic channel condition problem
and large antenna size problem, however, the high path loss
problem is even worse in the MI case. In this paper, we
provide an analytical model to characterize the underground
MI communication channel. Based on the channel model, we
develop the MI waveguide technique to solve the high path
loss problem.