Wireless Backhaul for Broadband Communication
Over Sea Khurram Shabih Zaidi
1, Varun Jeoti
2, Azlan Awang
3
Electrical & Electronics Engineering Department
Universiti Teknologi PETRONAS
Tronoh, Seri Iskandar, Perak, Malaysia [email protected],
Abstract— A comprehensive survey of different possible
solutions to provide wireless backhaul PTP links for
broadband communication over Sea is presented. The main
purpose of wireless backhaul network is to provide Long-
Distance Point-To-Point (PTP) broadband communication.
Wireless Backhaul gives a low cost solution for access to
remote areas with difficult terrain to install any wired link.
Long-range backhaul network with high capacity and
reliability is limited to line-of-sight (LOS) distances requiring
high antenna towers for further increase in range. A mirror
image of WiMax-like system used on land can be envisaged
on Sea to provide similar services at even non line-of-sight
(NLOS) distances. Satellite communication can also provide
large distance coverage for communication over Sea.
Tropospheric propagation using evaporation ducts over Sea
is also explored for long-range wireless communication over
Sea to achieve Trans-horizon NLOS distances. Current work
and future challenges regarding backhaul broadband
communication over Sea with some proposed solutions are
discussed at the end.
Keywords – Wireless Communication, Maritime, Wireless
Backhaul, Broadband Access, Over Sea Communication,
Evaporation Duct, Trans-horizon Communication, LOS & NLOS
Wireless Communication, Mesh Network.
I. INTRODUCTION
An increasing demand of high-capacity wireless
communications has driven an outstanding development
and innovations in Telecommunication industry in the last
few years. Advanced wireless access technologies such as
Worldwide Interoperability for Microwave Access
(WiMax) and Long Term Evolution (LTE) have come up
as a promising alternative to provide high-capacity reliable
broadband communications. Demand for high-speed
internet connections anywhere, has become a necessity for
everyone. This increasing demand has led to new
innovations for a reliable, high-speed broadband
infrastructure not only on land but also over Sea, with
millions of people travelling all around the World in Ships
and Ferries.
The importance of wireless communication between
ships and shore cannot, and should not be undervalued as
80 percent of world trade is transported on Sea. Maritime
communication is becoming more important in both
commercial and research fields especially in countries
which have economic dependence on an ocean area.
Numerous activities in the Ocean including oil
exploitation, maritime transportation, fish farming and
other activities make the maritime communications very
important. Some applications are near the shore and some
are at longer distance off-shore. Most of these needs can
be fulfilled by utilizing a 10Mbps data rate communication
system [1].
Fig. 1 Backhaul Links for Broadband wireless access
Fig. 1. Backhaul Links for Broadband wireless access
An overview of different backhaul connectivity options
is shown in Figure 1. Currently, Copper, Fiber Optic,
Microwave PTP & Satellite are the most popular choices
for backhaul links not only on land but also on Sea. Fiber
has always been the preferred choice due to its high
capacity and reliability, but its cost grows with the
distance and implementation over harsh terrain or deep
Sea can impose further challenges. Same goes for the
copper wired backhaul links. Microwave PTP Line of
Sight (LOS) Backhaul links over Sea can also provide
communication services, although LOS communication is
limited to visual distance for applications within a short
distance of 10km – 15km. Networks with WiMax-like
structure on land are being deployed around Sea-port areas
with clear LOS ranges.
Satellite communication has a cost model that is
insensitive to distance or location, requiring only a clear
path between the compact antennas dish and satellite. It
can also cover large distances over Sea. However, its high
cost data rate, latency and jitter, along with high
maintenance and replacement cost is still a hurdle for low-
cost, real-time applications at Sea [2].
Due to the curvature of the Earth the NLOS PTP
communication beyond visual horizon can be a problem.
2013 IEEE 11th Malaysia International Conference on Communications
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Recent studies reveal that using evaporation duct; the
communication range can be increased to Trans-horizon
distances in one hop. The signal at certain frequency and
angle gets trapped in the evaporation duct just above the
sea and therefore travels further achieving NLOS distances
[3].
This paper initially gives an overview of the current
maritime wireless communication implementations for
broadband access near Sea-shore and port LOS
environment. LOS is further discussed for PTP backhaul
links over Sea. Then covering larger NLOS distances over
Sea Satellite communication and tropospheric propagation
using evaporation duct over Sea is discussed. At the end
mesh topology MIMO techniques are discussed for high-
capacity and reliable backhaul links over Sea.
II. MARITIME WIRELESS COMMUNICATION
A novel framework for the simulation of maritime
wireless communication was introduced [4]. A high-speed
maritime ship-to-ship/shore mesh network in a project
called TRITON (Tri-media Telemetric Oceanographic
Networks) is proposed. A series of studies that form a
systematic approach to studying and establishing the
feasibility of developing a multi-hop communication
system for ships based on LOS 802.16d mesh technology.
Each mesh node can route and relay traffic. Field
measurements have been carried out at 2.3GHz and
5.8GHz. Maritime wireless communication challenges
include sea surface movement, channel property and the
effect of first Fresnel zone. There is plenty of spectrum
White Space (WS) at sea so the authors propose to use the
cognitive radio technology which in effect will solve the
spectrum scarcity issue, offer large bandwidth for wireless
maritime communications and reduce the cost. The system
can be implemented considering three network scenarios,
first is the mesh/ad hoc network alongside the coastline,
second is the ad hoc network in deep sea and third is mesh
network formed by the maritime facilities like the oil/gas
platforms, sea farms or small islands in between [5].
Measurements were taken to see the effect of
evaporation duct, present over Sea, for wireless
propagation at 10.5GHz [6]. Along a 9.9km low-altitude
path on Sea near Netherlands within the horizon
measurements were taken and compared with the
propagation prediction model program RPE (Radiation
Parabolic Equation). Further measurements for
Tropospheric propagation over the Sea at 2GHz were
reported in the British Channel Islands. The power
received was compared with ITU-R predictions for three
paths at various antennas heights [7].
A feasibility report of high speed radio link with
suitable frequency and receiver antenna height has been
studied over sea off Malaysian shores resulting in solution
of using 10.5GHz with low antenna height for beyond-the-
horizon radio wave propagation using evaporation duct
[8]. A high-speed wireless link with long offshore range
requires a design with optimal frequency to achieve such
range in one hop. The other issues such as Earth’s Bulge,
Sea wave motion, multiple reflections and diversity were
not addressed.
The backhaul links with single-hop and long-range
provide wireless services at larger distance without delay.
A framework for a sea-based network simulation along
with a specific routing protocol for maritime
communication network has been proposed in [9]. The
continuous motion of the ship on the waves causes the
swaying motion which changes the orientation and the up
and down movement of the ship alters the altitude and thus
varies the antenna gains as the height of the antenna varies
at every instant. The effect of wave motion on wireless
transmission, the strong two-ray path interference on sea
surface and the ship movement patterns in ship lanes have
been incorporated in [10]. The new model effectively
works for simple Sea conditions, with prior knowledge of
the location of ships.
Long-distance propagation measurements of mobile
radio channel over Sea at 2GHz have been taken in [11].
They took measurements along a 45km route, but the
distance between the Tx and Rx never exceeded LOS.
Another 5.8GHz fixed WiMax performance in a Sea Port
environment has been carried out in [12]. Measurements
access the performance of WiMax in the presence of
multipath, Doppler shift and boat’s rocking. Boat’s
rocking could increase BER especially when the link is
marginal. All these factors should be carefully taken into
consideration when deploying a fixed WiMax system in
sea ports. The distance was still at all times within the
LOS range. Other experimental measurements of
propagation characteristics for maritime radio links were
taken in [13]. WiMax performance was accessed at
3.5GHz and 5.8GHz. The experiments revealed strong
masking effect due to the presence of small islands
between the 14km range Tx and Rx. A propagation
channel measurement campaign in maritime environments
was carried out to investigate the impact of the wireless
channel in LOS and NLOS situations in [14]. An
empirical path loss model is obtained for NLOS. For
prediction of the average level of received power for a
given Tx-Rx separation Pr(d) is indicated as in [15].
! " !#$%& ' ()*+ ,-./%#$ $%0 & ' 1 , for $ 2 $% (1)
Using this path loss prediction equation for NLOS
groups all effects in to two main parameters; path loss
exponent n and the zero-mean Gaussian random variation
3, which represents the shadowing factor. The shadowing
value 3 is typically modeled as a normal random variable.
i.e:
1+4+5#)6 78& (2)
Where N(0, 2) is a Gaussian (normal) distribution
with mean 0 and standard deviation , in decibel units.
Analyzing experimental data and then comparing with the
predicted path loss model in Eq. 1 provide a good
similarity. The experimental results after analyzing key
wireless parameters can be compared to the free space and
two-ray theoretical models. The results showed that at
short distances the two-ray model fits measured large scale
2013 IEEE 11th Malaysia International Conference on Communications
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path loss reasonably well when LOS condition remains.
However, when the distance is very large, the received
signal is found to attenuate at a higher rate. This limits the
coverage zone of WiMax.
Many other experimental measurements are reported
for maritime wireless communication in [16]-[18].
III. LOS MICROWAVE PROPAGATION
Microwave Radio Links are an alternative choice for
wired backhaul links especially in geographically
challenging areas where wired connections are not
available or very difficult to install. Microwave
transmission can be carried out in various frequency bands
including licensed (6GHz to 38GHz) and unlicensed
(2.4GHz and 5.8GHz) bands [2]. The presence of Line of
Sight (LOS) between cell sites and aggregation points is
required and hence microwave is limited to short distance
transmission when used in metropolitan environments.
However, in rural environments, when a LOS is present,
microwave transmission can be quickly installed to cover
long distances. LOS distances are limited due to the
curvature of the Earth.
For any long-range backhaul wireless mesh network
the main features are extreme reliability, high-capacity,
security and easy network management. These features are
based on primary performance matrices such as
throughput and packet delay. In practice, the IEEE 1588
PTP may be used as an alternative due to its lower cost. For highly reliable synchronization in the network, IEEE1588 PTP can serve as a backup timing reference in base stations deployed with GPS receivers [19].
To obtain longer distances high antenna towers are
required with no obstruction in between. Microwave can
be implemented in the Point-to-Point (PTP), Point-to-
Multipoint (PMP). Whereas the PTP system requires a
radio and antenna at the end of every wireless link, in
PMP, one radio and antenna at an aggregation point are
sufficient to serve a number of cell sites [2]. Current
research is focused, for a microwave PTP link over Sea
which requires small height antenna towers. It is not
possible to construct very tall towers in deep Sea for an
off-shore long-range, PTP Backhaul Microwave link, to
clear Earth’s Bulge for LOS clearance, like on land.
Cambium Networks provides a free distribution of the
PTP LinkPlanner software [20] to help predict PTP fixed
Wireless Links using actual terrains from Google Earth.
Fig. 2. PTP LOS 20km Link over Sea
Availability defines how big portion of a certain time span a service should be up and running. With aggregation transport the number is usually four nines (99.99% availability) resulting in 52.56-minute downtime per year. Availability in general is impacted by equipment failure, power outages etc. and in wireless systems further reduced by weather conditions, Sea state level, rain and distance [21]. The software provides accurate prediction for LOS
links. Figure 2 shows a 20km PTP Backhaul Link over Sea
for Broadband communication. The Tx height is 10m
above sea-level and the Rx heights is only 5m. Greater
distance links might not be possible due to the curvature of
the Earth. Using PTP Link planner software based on the
above configurations an availability chart could be
obtained as in Figure 3. Four nines (99.99% availability)
resulting in only 52.56-minute downtime per year is quite efficient for this LOS PTP link path over the Sea.
Fig. 3. Availability for a 20km LOS link over Sea is 99.99% for 40Mbps
IV. SATELLITE COMMUNICATION
The Satellite communication by Inmarsat
(International Maritime Satellite) system, which is suitable
for ships far away from shore, is a NLOS, between the Tx
and Rx, solution for long-range wireless maritime
communication. Long distances can be covered by satellite
and its integration with a network on ground or Sea can
provide extra reliability and wireless access [22].
Satellite communication with its very wide coverage
range is very useful for cellular backhaul and has
significant advantages when expanding the network into
the remote rural areas [23]. The most expensive part in
satellite based backhaul solution is its bandwidth.
Optimization of this important resource in all
communication layers is still a challenge.
Satellite Communication solutions provide low data
rate and its high cost communication fee along with
maintenance and replacement cannot be afforded by
simple maritime users who require broadband
communication services at Sea. Satellite communication
induces unnecessary latency, which is not suitable for real-
time industrial and commercial application over Sea.
Recent studies have introduced an integrated wireless
communication architecture that tries to provide maritime
customers ubiquitous services by integrating
heterogeneous underlying wireless networks [24].
2013 IEEE 11th Malaysia International Conference on Communications
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V. NLOS TROPOSPHERIC PROPAGATION
Propagation Path through evaporation duct is one
solution to achieve trans-horizon distances. At a particular
frequency and antenna angle the signal can achieve long
distances using the evaporation duct. An experimental link
between Davies Reef and the AIMS was built to verify
this approach. This link operates at a frequency of
10.6GHz and provides a data rate of 10Mbps over a range
of about 78km using antennas located 7m above mean sea
level. This is the first ever reported use of the evaporation
duct to implement a high-capacity radio communication
link between the reef and the Australian mainland [3].
The radio refractive index (n) is caused due to the
molecular constituents of the air [17]. Normally, the
numerical difference in refractivity is a very small fraction
of unity. There are four refractive conditions which depend
upon refractive gradient. The relations of refractivity
gradient and related refractive condition are summarized in
Table 1. [25]. Trapping condition, often called ducting
phenomenon, causes anomalous radio wave propagation.
Well known tropospheric ducts are surface duct (ground-
based duct), surface-based duct and elevated ducts.
TABLE 1. REFRACTIVE CONDITION [8]
Condition N-Gradient
(N – Unit/km)
M-Gradient
(M – Unit/km)
Trapping dN/dh ! –157 dM/dh ! 0
Supper Refraction –157 < dN/dh ! –79 0< dM/dh ! 78
Standard –79 < dN/dh ! 0 78 < dM/dh ! 157
Sub Refraction dN/dh > 0 dM/dh > 157
Refractive conditions in Table 1, can be further explained by the tropospheric signal propagation shown in Figure 4.
Fig. 4. Four different refractive conditions
NLOS long distance communication using low height antenna transmitter and receiver is impossible to achieve without either relay or some other mechanism. Due to the changes in the refractive index above large portions of Sea water an evaporation layer is developed at an average height of 15m – 20m (Malaysian Region). Distances which are unachievable due to the Earth’s bulge can be achieved by propagating within the evaporation duct over Sea. Signal bends and is trapped between the duct layer and Sea surface, when refractive index dN/dh ! –157, as shown in
Figure 4. Therefore, signal can overcome the Earth’s bulge and travel longer distances, as shown in Figure 5.
Fig. 5. Signal path through Evaporation duct for a 50km NLOS Link
Tropospheric Multipath (one example of which is
ducting) is where there are many reflections arriving at the
antenna and the angles are not constant over time. In this
case larger separations are preferred and the availability
calculation will show the improvement which can be
achieved for a given antenna separation. In general
increasing the separation will improve the availability and
decreasing the separation will reduce the availability. This
will be more obvious in geographic locations which are
prone to high levels of tropospheric multipath. Figure 6
below shows a comparison of the predicted path loss using
evaporation duct and free space model using AREPS
simulation software [26]. Results clearly show 12dB less
path loss prediction as compared to free space model at a
50km link over Sea.
Fig. 6. PTP backhaul 50km NLOS link path loss comparison
Paths over the sea are subject to a special problem due
to the very strong reflection from the water. This reflection
can add an anti-phase signal to the direct wave and cancel
it out completely. This gradient can change and in certain
circumstances causes the signal to travel a long way in
ducts [18].
VI. MESH NETWORK
Backhaul applications can use the relatively simple
802.16-2004 standard for fixed connectivity applications,
in point-to-point, point-to-multipoint, and mesh
topologies. WiMax with new ratified 802.16a extension
uses a lower frequency range of 2GHz to 11GHz, and does
not require line of sight towers. It also boasts 70Mbps data
transfer rate that can support a large number of users.
WiMax Mesh networks support relatively high data
throughput [27] and communication over Sea requires
stable and highly reliable links. Mesh Network can
provide redundancy and for every wireless node there are
always more than two paths available. This provides
reliable high reliability as threshold SNR for one path goes
2013 IEEE 11th Malaysia International Conference on Communications
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below a certain level then there is always another path for
the continuity of high-speed services. Theoretically,
WiMax can provide single channel data rates up to
75Mbps and up to 350Mbps via multiple channel
aggregation [2].
Fig. 7. PTP Backhaul Mesh Network scenario over Sea
A WiMax-like PTP mesh network backhaul scenario
over Sea is assumed in Figure 7. It shows the possible
connections between trans-horizon wireless nodes.
Mobility, link quality and interference remain an issue and
needs a lot more research in this field. Routing protocols
rely on routing metrics for calculation of efficient routing
paths. The metric should provide stable, high throughput
with low delay, computationally efficient and loop-free
routing paths. The other thing is the Sea characteristics,
which can produce high variation in SNR. Sea conditions
vary with wind speeds and temperature. High wind speeds
and Sea waves can cause the received SNR level to fall
below the threshold required for high-speed
communication. Even during worst Sea conditions when
signal may breakup and cause one wireless path to
disconnect, there is always another wireless route
available in mesh network topology.
Based on information from routing tables, wireless
backhaul mesh network in necessary processes relating to
MIMO signal detection such as synchronization, channel
state acquisition and so on [28]. This feature in wireless
backhaul will deliver a larger benefit of MIMO adoption
to wireless backhaul compared with other wireless multi-
hop networks. Each link can be assigned with some weight
based on hop count, minimum delay, throughput, link
availability, traffic load, maximum bandwidth, etc. Based
on these weights best route with lowest weight can be
selected for packet forwarding. Routing topology can
change according to channel information from physical
layer. A threshold point for each route in a routing table is
made to decide the backhaul link.
Centralized and proactive routing algorithms are
feasible to achieve better network performance. This can
be implemented based on the network entry mechanism in
the 802.16 MAC layer. Such a routing algorithm
implementation can avoid the need for a separate routing
protocol and reduce network control overhead.
Determination of flow assignment and time slot allocation
by a scheduling algorithm depends largely on the routing
algorithms. The problems of routing and scheduling can be
solved either separately or jointly [29].
VII. IMPROVEMENT WITH MIMO
A key challenge in wireless communications is to
provide high data rate wireless communication services
with maximum reliability. Multiple-input multiple-output
(MIMO) wireless communication systems employing
multiple antennas at both the transmitter and the receiver
can provide higher data rates through multiplexing and
improve the system performance through diversity [28].
The combination of MIMO techniques with OFDM is
regarded as a promising solution for increasing data rates
and wireless access qualities of future 4th generation
wireless communication systems. MIMO multiplies data
throughput, and provides for a simultaneous increase in
range and reliability, all without consuming extra radio
frequency.
MIMO based wireless backhaul has a higher
throughput, lower average delay, and lower packet loss
rate than SISO based one [20]. MIMO is a multi-
dimensional approach that transmits and receives two or
more unique data streams through one radio channel
whereby the system delivers two or more times the data
rate per channel. A brief overview of MIMO-OFDM
wireless technology covering some key aspects of the
system design such as; channel modeling, ICI analysis,
channel estimation and space time block coding aimed at
increasing the transmission rate and providing reliable
QoS to users is presented in [24]. Multiple-antenna
transmission and reception techniques can include
transmitter beam-forming and receiver diversity. Beam-
forming and receiver diversity can improve range for
conventional one-dimensional signals, and are appropriate
for certain applications such as outdoor point-to-point
wireless backhaul, although they might not achieve
MIMO's capacity-multiplying effect, but still enhances the
coverage range using intelligent Beamforming [30].
VIII. CONCLUSION AND FUTURE CHALLENGES
This paper briefly summarises various techniques
being used for long-range wireless backhaul networks.
Whereas LOS might be readily available on land by
installing tall towers for transmission and therefore
achieving a long-range backhaul network. This might not
be possible on Sea, therefore an alternate solution is
required to achieve long-range distance for backhaul
networks. Satellite communication still is very expensive
and with its high delay it might not be the right choice for
real-time applications. For NLOS propagation based on
WiMax standard is one possible option to cover large
distance for backhaul links similar to its implementation
on land. The signal can achieve long-distances even by
using low-height antennas using evaporation duct present
over Sea. A complete Backhaul network solution for
Broadband communication still needs to be practically
implemented for beyond-the-horizon long range distances.
Future challenges for backhaul PTP links over Sea
mainly include the long-range with reliability. The Sea
2013 IEEE 11th Malaysia International Conference on Communications
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surface is not constant and with sometimes small and large
waves makes the surface irregular. The irregular surface of
the Sea causes scattering of signal and therefore induces
multipath which may or may not be useful. The main
attribute of backhaul links is reliability which might not be
so high with low SNR. Techniques such as MIMO can be
explored using the multipath available from sea surface
reflections. MIMO techniques can not only increase
capacity but also increase reliability. The resultant
complete system will be extremely useful not only for the
coast guards for security and protection but also for
commercial use in future for the maritime customers
demanding low-cost, high-speed internet access on board
ships and vessels.
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2013 IEEE 11th Malaysia International Conference on Communications
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978-1-4799-1532-3/13/$31.00 ©2013 IEEE 303
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