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Our
WiFi Cellular System enables the deployment of a cost
effective network that can support real time
applications such as voice, streaming video, and network
gaming while providing mobility to network users. The
main criteria taken into account while developing this
system are:
-
Coverage Microcellular Coverage
-
Capacity For Mixed Voice and Data Traffic in
City-Wide deployments
-
Mobility Driving Speed for Real-time
Applications
-
Quality of Service Real-time Applications
This network
is implemented using the following components:
A. Outdoor
Base Station
B. Outdoor
and Indoor Base Station
C.
Integrated Network Management System (INMS)
The
WiFi Cellular System is a network system developed based
on the cellular network principles it is a
micro-cellular network. The architecture of this system
is similar to that of a 3-layer cellular network. The
field layer is the base stations which are then
controlled by a base station controller and the overall
system is managed by a network management system.
In
order to support the applications that are mentioned
above, the network requires special technology
enhancements. The next few sections describe the major
wifi technology enhancements provided by our WiFi
Cellular over what is available in the market today.
Our
Base Station is the worlds first WiFi Cellular Base
Station optimized for Micro-Cellular networks. It has
been designed from the ground up to be an outdoor base
station for city-wide WiFi deployments. It forms the
platform that provides the extended range required to
implement a micro cellular network. Typical
microcellular networks have a cell radius of about 500m
is Non-Line of Sight (NLOS) conditions. We provides
this range which is typical range of metropolitan
cellular network deployment.
Achieving a 500m NLOS range is a difficult problem to
solve in 802.11 networks. The reason behind it is
two-fold:
1.An EIRP
restriction of 36 dBm by US FCC and various
other communications agencies around the world
2. Low
Power on terminals typical terminals available in the
market generally transmit at 16-20 dBm
3. The
power on the terminals is not adjustable the way
it is in the cellular world.
Each
of the above factors restricts the range of a WiFi
network. The last two in particular are major limiting
factors for developing extended access range on a WiFi
network. An analysis of this combination is as follows:
This
means that the transmit power on the Base Station (power
plus the antenna gain) is restricted to about 36 dBm.
To provide a perspective of the significance of this, in
the GSM world EIRP on base stations is about 55 dBm.
This means that the range achievable by WiFi base
stations is going to be considerably lower than that of
cellular base stations. Also, the wall penetration
capabilities of the WiFi base stations will also be
lower than that of cellular due to WiFis typically
higher operating frequency. This also means that the
density of WiFi base stations is going to be
considerably higher than that of conventional cellular
base stations.
This
is a major impediment in getting long ranges. With a
transmit power of 16-20 dBm on typical terminals, WiFi
base stations cannot be run on high power in order to
maintain a more or less symmetrical communication link
in terms of RF power. This asymmetry between the Base
Station and the terminal transmit power could prove
fatal to the network. The reasons behind this are as
follows.
Let
us assume that the transmit power on a base station is
set at 29 dBm and the terminal transmits at 18 dBm. The
difference in power is 11 dBm. When the base station
transmits, the terminal hears the transmission at a high
power so assumes that the rate at which it needs to
connect is high as well. However, when the terminal
responds using its lower transmit power, the base
station cannot hear the terminal at the same power so,
the link is not proper and the data rate for
communication is not set. The terminal then proceeds to
transmit at a lower data rate and keeps lowering it till
the link can be established for proper communication.
This process affects the throughput and the efficiency
of the base station and ultimately the network. In
general, asymmetry confuses the terminal and it is very
difficult to design a rate adaptation algorithm for this
scenario.
This
analysis shows that a base station should ideally
establish a symmetrical RF link between the terminal and
the base station. That is possible only when the base
station transmit power is close to the power on the
terminal. As we discussed before, the power on the base
station affects the coverage of the base station. The
solution lies in using high gain antennas to make the
most efficient base station. In addition to that, the
most optimal scenario would be one where the antennas
have multiple elements to provide coherent gain and have
diversity to provide diversity gain. This is optimal
since the gain provided by such antennas is symmetrical.
This
aspect of WiFi networks also has an impact on the base
station design. Since the power on the terminal is
typically fixed and it usually does not adjust based on
the power of the base station, it is important to design
a base station that meets the requirements of most
terminals. This means that the transmit power on a base
station should not be more than about 20 dBm in order to
keep the base station as close to the terminal transmit
power as possible.
This
analysis shows that the most ideal base station is one
which transmits at around 21 dBm and has an antenna gain
of about 15 dBi to achieve an EIRP of 36 dBm while
providing an efficient and effective base station.
This
problem however is not easy to solve. In 2.4 GHz
spectrum a 15 dBi omni antenna would be around 4m long.
Deployment of such an antenna is very challenging and
adds to the cost of the network. Also, the vertical
beam width of such an antenna will be so small that in
practical applications the coverage will suffer
heavily. For most cellular deployments, sector antennas
are ideal since they give the ability for coverage
optimization using down tilt and up tilt.
Our
product meets all the criteria of an ideal WiFi base
station. The main characteristics of our product
station are as follows:
Optimal Transmit Power = 21 dBm (max of 26 dBm)
Antenna Gain = 15 dBi
Type of Antenna = Cross Pol Diversity Sector Antennas
Number of Antennas = 8 per base station (arranged in 4
sectors)
With
this base configuration, we provide the following
features:
500m radius 360-degree coverage Micro-Cell in Non-Line
of
Sight (NLOS) environments which matches the
foot print of
most microcellular deployments in dense urban
environments
Significantly fewer BTS to cover same area and hence
low upfront cost to deploy network
Very high data rates with peaks at 54 mbps in the
Micro-Cell
Support for real-time applications such as voice and
interactive gaming
High-speed mobility in
urban environments for speeds up to
120 Kmph
Consistent coverage with minimal holes for busy urban
environments using external dual-diversity beam forming
smart antennas
Minimized hidden-node problems commonly seen in dense
urban networks due to innovative architecture
Auto
configuration through a management system
Automatic
cell shaping for interference minimization
Reduced
overhead because of fewer hand-offs
Multiple
radios on the platform to support the 360-degree
coverage
802.11
b/g access
Provision
for multiple types of backhaul
Ability to connect to another BTS using 802.11a based
protocol
Provision for external antenna
Sleek
design for easy installations
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