Note: Descriptions are shown in the official language in which they were submitted.
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Method and Apparatus for Data Communication
Field of the Invention
The present invention relates to broadband communication networks and more
particularly to wireless broadband communications.
Background of the Invention
In the past, broadband communication was achieved by providing high bandwidth.
cable connections between a provider and a client. The cables are commonly in
the form
of coaxial cables, twisted pair copper telephone lines, or fibre optic cables.
A broadband
signal is transmitted down the cable toward the client end. For example, cable
t.elevision
involves a same signal transmitted to each client within a group of clients.
The signals are
sent in one direction only thus providing a one directional data link. Each
client then
decodes portions of the signal as desired.
Information being sent to the clients, commonly called the down link or
forward
link, is usually transmitted at a much higher capacity than the up link or
return link. Such
asymmetry in communications prevents clients from both generating and
distributing
substantial amounts of information into the network. As a consequence, the
distribution
of high content information by the network is exceedingly hierarchical,
requiring the
producers of information to access the hub nodes wherefrom the information is
transmitted to the clients. Generally, subscribers desire a high bandwidth
feed from a
service provider and lower bandwidth feed to the service provider. An
arrangement is
similar for television on demand where television programs are provided to a
client from
the service provider and clients only send small data packets including
ordering
information etc. to the service provider.
It is well known that one of the most expensive aspects of any broadband
communicatior.t
network is the cost of running cable from a service provider to each client.
For example,
fibre optic cables are estimated to cost tens of thousands of dollars per mile
of installed
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cable. This results from labour costs, costs of routing the cables and
obtaining rights to
bury the cables, costs of repairing damaged cables and so forth. It would be
highly
advantageous to obviate any need to lay new cable in order to support
broadband
communication over a wide area and more particularly bi-directional
communication
between a service provider and a client subscriber or between client
subscribers,
especially in view of the growing need by clients to be able to access and use
ever
increasing amounts of return link capacity. Additionally, adding the
capability to existing
wireline infrastructure is expensive and problematic, often requiring the
special
conditioning of old lines or the addition of new infrastructure. In many
areas,
infrastructure is simply not in place, forcing those desiring broadband
communications to
face the daunting task of not only installing links to clients, but also
installing totally new
switching systems.
The convergence of the Internet and television services has given rise to a
need to
provide high bandwidth bi-directional communication. Other applications such
as video
conferencing, require high bandwidth in both transmit and receive directions.
Conventional broadband data distribution services are not capable of
supporting such
requirements.
In an attempt to overcome this problem, cable service providers have released
a
cable modem for use in providing Internet services over a standard cable
connection.
Such a connection has many known problems. The bandwidth is limited by the
cable
itself and division of subscribers into groups requires the addition of new
hardware. The
bandwidth to any given client is limited by the number of active subscribers
in their
group and by the physical limitations on the information carrying capacity of
the coaxial
cable. Further, such a system is highly asymmetric and incapable of, for
example,
supporting extensive video interactivity between client subscribers.
Telephone system operators have also tried to overcome the above noted
problems with broadband data distribution by developing high speed modems that
condition signals for distribution over twisted pair telephone lines. These
modems, called
Digital Subscriber Line (DSL) modems, are most commonly represented by a class
of
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modems called Asymmetric DSL (ADSL). Some of the drawbacks of these systems
are
that they require the twisted pair telephone lines to be in good condition in
order that the
modems can be effectively used.
Also, networks employing ADSL are highly asymmetrical, having forward link
capacity significantly larger than return link capacity. In addition, unless
the ADSL links
are in good condition and relatively short, supporting high quality video
services
becomes problematic. Another significant problem faced with ADSL is the
requirement
for a public telephone subscriber line network to be in place in order to
facilitate
deployment. Such a requirement for an in-place infrastructure is simply
impossible in
many regions of the world, hence deployment of ADSL is limited to those
regions that
are well developed economically and have a solid technological base.
Another attempt to overcome the above known problems is known Local
Multipoint Distribution Service (LMDS) or Local Multipoint Communication
Systems
(LMCS). LMDS (LMCS) is a wireless, two-way broadband technology designed to
allow
network integrators and communication service providers to bring a wide range
of high-
value, quality services to homes and businesses. Services using LMDS
technology
include high-speed Internet access, real-time multimedia file transfer, remote
access to
corporate local area networks, interactive video, video-on-demand, video
conferencing,
and telephony among other potential applications.
In the United States, the FCC became interested in the possibility of LMDS
bringing long needed competition to the telecommunication marketplace, where
it has the
use of 1.3 GHz of RF spectrum to transmit voice, video and fast data to and
from homes
and businesses. With current LMDS technology, this roughly translates to a 1
Gbps
digital data pipeline. Canada already has 3 GHz of spectrum set aside for
LMCS. Many
other developing countries see this technology as a way to bypass the
expensive
implementation of cable or fiber optic networks.
LMDS uses low powered, high frequency (25 -31 GHz) signals transmitted over a
distance of 3-5 kilometers. These zones of coverage, or cells, are created by
sectorial
antennas and switching systems mounted on rooftops of urban buildings and
towers.
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These cells are typically spaced 4-5 kilometers (2.5 - 3.1 miles) apart. LMDS
cell layout
determines the cost of building transmitters and the number of households
covered. With
circular cells of 4 kilometers in radius, about 50 square kilometers falls
within a single
cell. In urban areas, this translates to about 80,000 homes within a single
cell.
Despite the promise of LMDS there are a number of issues that compromise its
acceptance as a widely deployed and ubiquitous wireless multimedia
distribution system.
One significant drawback to the deployment of LMDS systems is the large signal
attenuation that is faced by the frequencies used in transporting the data
between the hub
and subscriber terminals. This signal attenuation is exacerbated by rain,
foliage, building
blockage, and other factors related to the absorption, refraction, or
reflection of the signal
by obstacles in the propagation path. The underlying phenomena are well
understood but
are of such severity that current deployments of LMDS are limited to
situations where the
links are free from obstructions and where attenuation by rain or snow is
sufficiently
countered.
Another issue, also related to propagation characteristics, is that the
intended
polarization reuse schemes for increasing the capacity of LMDS systems are
also prone
to degradation by the same phenomena that attenuate the signal. As a
consequence there
are issues of deployment and capacity which bring into question the economic
viability of
LMDS systems.
Though there is much promise in LMDS systems operating at 28 GHz, the effect
of the propagation environment makes wide scale deployment problematic. Though
initial expectations assumed this technology could provide services to
everybody within a
coverage area, most current plans are for providing service only to those
locations where
a signal is received without even nominal propagation impairments. Also,
current trends
in LMDS are mainly for providing commercial access to broadband communication.
In view of the problems faced by LMDS due to its use of frequencies around a
28GHz band, the most obvious solution to the above noted problems is to
develop a
system using frequencies more robust to propagation degradation. Typically
these
frequencies are in bands substantially lower than 28GHz. The difficulty with
using bands
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at lower frequencies is that there is insufficient bandwidth to support the
density of
communications demanded by broadband digital services and what bandwidth is
available is often shared with other users such as satellite systems or
dedicated point to
point communications links. In order to effectively use other frequency bands
for
broadband wireless communications it is therefore necessary to devise a system
which
can co-exist with primary users and in addition, use limited bandwidth in such
a manner
that effective broadband communications can be established. At present, such a
solution
is unavailable for use with the types of data throughput anticipated and
currently
required.
A great deal of the early discussion of LMDS applications centered on the
transmission of video. With the recent surge of interest in the Internet and
the
accompanying demand for bandwidth, fast data appears to be the greatest
application for
this technology. With 1.3 GHz of spectrum, LMDS can provide a pipeline for a
great deal
of data. Homeowners pay about $30 per month for video, but businesses
regularly pay
over $1000/month for a high speed TI (1.544 Mbps) line from phone companies.
Using only the 850 MHz unrestricted bandwidth, along with a modulation scheme
such as quadrature phased shift keying (QPSK), well over 100 T1 equivalent
lines can be
provided in a cell formed by an omnidirectional LMDS transceiver even without
splitting
cells into separate sectors. Though it has been proposed that by using
horizontal and
vertical polarized sectors within a cell, LMDS providers can re-use bandwidth
and
multiply the number of T1 equivalents available, insufficient polarisation
isolation makes
such claims dubious especially in applications wherein subscribers are
individuals and
not businesses. A typical commercial LMDS application is believed to be able
to provide
a downlink throughput of 51.84 - 155.52 Mbp/s and a return link of 1.544 Mbp/s
(T1).
This capacity translates into potential to provide "full service network"
packages of
integrated voice, video and high-speed data services. Actual service carrying
capacity
depends on how much of the spectrum is allocated to video versus voice and
data
applications. Assuming that 1 GHz of spectrum is available, an all-video
system could
provide in the order of 275 channels of digital broadcast quality television
plus on-
demand video services. Unfortunately, LMDS has many known drawbacks and even
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though an LMDS television system was ilistalled in Brooklyn in the early
1990's, it has
failed to find wider acceptance.
It is an object of the present invention to provide a robust broadband
wireless
communication system for use in delivering data from a service provider to a
plurality of
clients. It is another object of the present invention to provide a system for
operation at
lower frequencies than LMDS, which provides many of the benefits of LMDS
without
many of the drawbacks.
Summary of the Invention
In accordance with the invention there is provided a wireless communication
hub
comprising: a plurality of radiators each associated with an oblong microcell
and for
radiating a narrow beam outward from the hub within the oblong microcell,
different
radiators for radiating within different oblong microcells, radiators
associated with
adjacent oblong microcells for radiating within different frequency ranges
such that
adjacent oblong microcells are frequency isolated and at least two spatially
isolated
oblong microcells within a same half of a rosette are associated with
radiators for
radiating within a same frequency range and are for radiating beams having
sufficiently
low side lobe levels for providing the spatial isolation; a plurality of
modulators each for
modulating a signal based on data received and for providing the modulated
signal to a
radiator from the plurality of radiators; and a processor for providing the
data to the
modulator.
Preferably, the hub comprises radiators associated with at least 16 microcells
disposed radially about the hub wherein radiators for radiating at a same
frequency are
for radiating with a same effective isotropic radiated power (EIRP). Also
preferably,
sidelobe levels of radiators within the hub are below a maximum level based on
beam
widths of the narrow beams, modulation techniques employed within the
modulator, and
environmental factors related to scattering of radiation within a cell
according to the
following equation
fi=10log(N_1)+S, +a o
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wherein C/Io is a carrier to interference ratio and is a threshold in dB for
operation of a
demodulator of the modulated signal at a known performance level with Io, the
interference noise, substantially greater than thermal noise No, N is equal to
a number of
like frequency petals within a rosette, SL is mean sidelobe level of the
radiators at angles
greater (M-O.5)X(BWh) away from peak free space main lobe of the beam where
BWh is
the azimuth width of the individual microcell, and af is dependent upon
environmental
factors associated with multipath scattering and represents a degradation of
sidelobe level
of radiation radiated by the radiators as a value expressed in dB.
In accordance with another aspect of the invention there is provided a
communication architecture comprising: a plurality of similar overlapping
rosettes each
rosette defined by radiation from an antenna hub comprising at least 16
directional
radiators for radiating power at frequencies associated with a microcell
forming a portion
of the rosette less than the whole, the rosette comprising: a number of
microcells greater
than 15, adjacent microcells within a same rosette associated with different
radiated
frequencies, some radiators within the hub corresponding to same frequencies
and for
radiating within different spatially isolated microcells wherein the at least
16 radiators are
for radiating signals having sufficiently low sidelobes to provide the spatial
isolation,
radiators associated with adjacent microcells for-radiating at different
frequencies such
that the adjacent microcells are frequency isolated; and, means for changing
the
orientation of the microcells within the rosette for limiting inter-rosette
interference.
In accordance with yet another aspect of the invention there is provided a
method
of arranging a plurality of overlapping rosettes, each rosette defined by
radiation from an
antenna hub comprising a plurality of directional radiators for radiating
power at
frequencies associated with a microcell forming a portion of the rosette less
than the
whole, the rosette comprising: a number of microcells, adjacent microcells
within a same
rosette associated with different radiated frequencies, some microcells within
the rosette
associated with same frequencies. The method comprises the step of: (a)
orienting
microcells of adjacent rosettes such that microcells of different rosettes and
associated
with a same frequency are offset by an angle other than a multiple of 180
degrees relative
one to another.
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There are significant advantages to wireless systems operating at lower than
28
GHz frequencies and preferably in the range of 2-7 GHz. First, the electronics
for driving
the system is less costly since semiconductor devices operating at lower
frequencies are
currently cheaper and plentiful. Second, the attenuation due to rain and other
obstacles is
reduced. Third, the antennas are physically smaller and are easily designed
having high
directivity and low sidelobe levels.
Brief Description of the Drawings
The invention will now be described with reference to the attached drawings in
which:
Fig. 1 a is a simplified diagram of a plurality of sectorised overlapping LMDS
cells,
sectorisation supported by frequency allocation within each sector using
different
frequencies in each sector according to prior art proposals;
Fig. lb is a simplified diagram of a plurality of sectorised overlapping LMDS
cells,
sectorisation supported by polarisation allocation within each sector
according to other
prior art proposals;
Fig. 2a is a simplified diagram of limited frequency reuse within a known
cellular system
according to the prior art and showing frequency reuse of a single frequency
every eighth
cell
Fig. 2b is a simplified diagram of limited frequency reuse within known
cellular systems
wherein frequency reuse occurs by using two frequency ranges within a single
cell and
arranging cells according to Fig. 2a;
Fig. 3 is a simplified diagram of a plurality of cells each comprising 16
oblong microcells
according to the invention and showing frequency channel reuse;
Fig. 4 is a transmitter according to the invention;
Fig. 5a is a simplified diagram of a single cell design used in simulating
results of the
application of the invention to a multicell configuration broadband
communication
network;
Fig. 5b is a graph of the PDF for co-channel interference within a rosette as
shown in Fig.
5a;
Fig. 5c is a graph showing how C/I within a rosette according to Fig. 5a is
invariant with
distance away from the hub;
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Fig. 5d is a graph showing measurements that were taken in a highly foliated
urban
environment where the af is about 3 dB; and
Fig. 6a is a simplified diagram of a constellation of rosettes wherein
microcells of like
frequency ranges within different rosettes are oriented 180 degrees relative
one to
another;
Fig. 6b is a graph of an estimated Probability Distribution Function (PDF)
based on C/I
calculations showing the probability of achieving a stated C/I in the
configuration of Fig.
6a wherein approximately 33% of terminals receive a C/I of 14 dB or better;
Fig. 6c is a graph of C/I for a typical receiver situated along the boresight
axis of the
main lobe defining a microcell as a function of radial distance from the
rosette centre;
Fig. 7a is a simplified diagram of a constellation of rosettes having
irregular offset
alignments between microcells within adjacent rosettes and associated with a
same
channel frequency range;
Fig. 7b is a graph of an estimated Probability Distribution Function (PDF)
based on C/I
calculations showing the probability of achieving a stated C/I in the
configuration of Fig.
7a wherein approximately 97% of terminals receive a C/I of 14 dB or better;
Fig. 7c is a graph of C/I for a typical receiver situated along the boresight
axis of the
main lobe defining a microcell as a function of radial distance from the
rosette.
Detailed Description of the Invention
The present invention presents a wireless communication architecture allowing
significant frequency reuse while mitigating the deleterious effects of radio
propagation
and systemic self interference. Further, the carrier to interference levels
(C/I) are limited
to improve system performance. This is achieved by dynamic monitoring of the
propagation environment, by dynamically assigning frequency bands, and by
controlling
the radiated power of all terminals.
In most wireless systems using fixed beam antennas, an effort is made to
mitigate
co-channel interference. The degree to which such interference is reduced
determines the
information carrying capacity of the contiguous wireless network deployed over
a
service area.
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Frequency re-use and co-channel interference reduction is undertaken with LMDS
systems. Referring to Figs. 1 a and 1 b, two proposed LMDS systems are shown
applying
sectorisation. In Fig. la, a central antenna 1 is disposed within a circular
transmission
area. The antenna comprises four sectorial transmitters operating at different
frequencies
with each sector operating with a different frequency. Directional receivers
are installed
at client sites to receive the signal from an associated transmitter. The
receivers are
positioned to receive a signal from a particular transmitter having a clear
signal at the
receiver location. Because there is no frequency reuse, the total bandwidth of
such a
system is no better than a same frequency use in all four sectors. With a same
frequency
used in each cell.
In Fig. 1 b, the antenna comprises four sectorial transmitters operating at a
same
frequency with adjacent sectors operating with different polarisations.
Directional
receivers are installed at client sites to receive the signal from an
associated transmitter.
The receivers are positioned to receive a signal from a particular transmitter
having a
clear signal at the receiver location. Because of polarisation usage, each
cell has twice the
bandwidth of other LMCS cells
Frequency re-use and co-channel interference mitigation is also carried out in
cellular and PCS telephony systems. Modeling the coverage areas of a cellular
system as
hexagons, as shown in Fig. 2a, like frequency hexagonal cells are separated by
a distance
D that is substantially greater than the nominal radius (R) of the hexagonal
cell.
Frequency re-use is thus achieved over the contiguous coverage area and co-
channel
interference is mitigated by counting on the propagation path loss between
cells of like
frequency to be sufficiently high as to result in significant attenuation of
co-channel
signals and isolation of cells.
In other embodiments of the cellular and.PCS telephony systems, the same
frequencies are used in a single hexagonal cell, as shown in Fig. 2b. In this
embodiment
the like frequencies are re-used 3 times per hexagonal cell as shown, for
instance, by the
re-use of (f'2) in cell number 2. Co-channel interference is mitigated by two
process
here; firstly by the sectoral isolation achieved by using sectorizing antennas
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secondly, by spacing cells containing like-frequencies by the cell spacing D
described
above.
In each instance shown in Figs. 2a and 2b, each hexagonal cell has 1/7t" the
bandwidth of the entire frequency range.
Information carrying capacity as a function of cell radius and coverage area
is an
issue with LMDS systems. This also relates to the frequency re-use
considerations. First,
the size of the area serviced by a single frequency range in an LMDS system is
large and
therefore bandwidth to each individual client is limited. Further, when two
channels are
used in accordance with standard LMDS frequency re-use practices (as shown for
instance in Fig. 2b), the LMDS bandwidth of approximately 1 GHz results in
each sector
having approximately 500MHz of bandwidth. With four quadrants, a total of 2
GHz of
bandwidth is supported in a single circular area. In essence, over 1000 T1
line equivalents
can be supported within each cell, assuming 1 Bit/S/Hz efficiencies in the
modem
technology.
In practical terms this means that with 80,000 homes in the coverage area of a
4 Km
radius LMDS cell, an overall bandwidth of 2GHz results in an average available
bandwidth of 25,000 bits/sec per home. There are some drawbacks to this
scenario. It is
impossible, for instance, to dynamically allocate bandwidth when a quadrant
has more
usage than a neighbouring quadrant, even when a client is able to receive
signals from the
transmitters associated with each quadrant, because in order to do this, the
receiver needs
to be physically moved and/or the antenna polarization must be changed. As a
consequence there is the possibility, because of the broad sectorization, of
not being able
to dynamically assign information carrying capacity as needed. To do so would
involve
installation of a more complex and dynamic interference monitoring system to
oversee
the operation of the contiguous network, and would require dynamic reporting
of
interference by the client terminals.
The issue of terminal complexity is already exacerbated by the high cost of
the 28
GHz component technology present in LMDS terminals. Large radii LMDS systems
furthermore, require higher dynamic range and higher power amplifiers further
increasing
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cost. Lowering the power amplifier output has the commensurate effect of
lowering the
information rate of the link. This becomes an issue, especially in the return
link from the
client when it is desirable to have high return link capacity, as would be the
case for a
client have a transmitting video server or providing internet service
connectivity to other
clients.
It has now been found that a plurality of low cost hubs each for communicating
with a substantially smaller area and each supporting frequency reuse provides
a more
effective broadband communication network. Use of radio frequency technology
operating in the 3-7 GHz range and preferably in the 5.2/5.8 GHz range is low
cost, and
will most likely remain so in comparison to 28 GHz technologies simply because
of the
issues of ease of volume manufacturing and availability of more diverse
semiconductor
technologies. Smaller service areas entail shorter path distances thereby
allowing the use
of lower power RF amplifiers. For those desiring high capacity return links,
increasing
return link amplifier output power and dynamic range is easily achievable with
this
invention.
It is evident to those of skill in the art that increasing frequency reuse
results in
increased overall bandwidth and the attendant problems of co-channel
interference.
However, it has been found that the co-channel interference can be controlled
by the
physical orientation of an antenna and a rosette according to the invention,
and
furthermore, by applying technology capable of universal addressing and
control of both
client and hub terminals by TCP/IP and other standard IP protocols, a wide
ranging and
dynamically effective modicum of system control are exercisable limiting co-
channel
interference. Furthermore, because of the non-reliance on polarization
discrimination as
with LMDS, and because of the overlap of non-alike oblong microcells of this
invention,
it is possible to dynamically assign and re-assign capacity. This latter
facility also is very
useful in contending with strong, singular multipath interferes that arise in
wireless
systems; ie a modicum of frequency and angular diversity becomes available to
the client
terminals with this invention.
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Referring to Fig. 3, two circular transmission areas in the form of cells
according
to the invention are shown. A single ce119 is broken into a number of oblong
microcells
8. The microcells 8 are also referred to as picocells and petals. Preferably,
each oblong
microcell 8 overlaps its neighbouring microcells. Microcells are assigned
different
frequencies with possible reuse of frequencies within a single cell. Because
of the
appearance of the microcells, petals, within a cell, the architecture of a
cell is referred to,
herein, as a rosette. Preferably, overlap of microcells is substantial such
that a majority of
subscribers receive signals from two or more microcell transmitters.
The centre of the rosette 9 contains a hub system 10 comprising a transceiver
shown in more detail in Fig. 4. In the hub system 10, RF wireless signals are
converted to
digital data and routed by a Carrier Sense Multiple Access System (CSMA) to
and from
individual microcells 8. Data is sent from one rosette 9 to another rosette 9
via dedicated
interconnecting links based on any of fibre optic,communication, dedicated
microwave
wireless communication, point-to-point atmospheric laser communication, and
wireline
communication techniques. One of skill in the art will know criteria for
selecting a point
to point communication method for use between cell hubs. As a result, it is
possible to
route data generated by a subscriber in one microcell 8 within a cell 9 to
another
subscriber in a microcell 8 of another ce119. This is particularly useful for
supporting
communication media such as telephones, video conferencing, and Internet
communications. The system accommodates non-wireless subscribers that are
connected
to the network via the CSMA system either at the hub or along the inter-
rosette link.
Optionally, non-wireless subscribers gain access to the system using Ethernet,
TCP/IP, a
modem or another communication system.
Typically, the physical configuration of each rosette 9 is a same
configuration, as
is the electronic system that defines the hub 10 and its antenna. Of course,
using a same
physical configuration and Hub allows employment of economies of scale in hub
production and facilitates maintenance, upgrading, use, installation, and
technical support
of the system. In practice, rosettes are placed over a coverage area in such a
manner that
microcells containing like-frequencies do not align with each other or
illuminate a same
coverage area. The microcells 8 in Fig. 3 are assigned a channel - frequency
range - from
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4 available frequency ranges A, B, C, and D. When the radiators have
sufficiently small
sidelobe levels by spacing the microcells that are within channel A by at
least three other
microcells, overlap between radiated signals is substantially eliminated.
Consequently, it
is possible to electromagnetically isolate signals of like frequency
microcells. Isolation is
further enhanced by having subscribers to the system use highly directional
antennas
having narrow beam widths. Preferably, antennas having a beamwidth of 7.5-15
degrees
at -3dB power contour are used. Preferably side lobe levels are less than -
35dB at more
than 45 degrees azimuth with respect to a peak for a rosette having 32
microcells with 4
frequency bands each reused 8 times.
It is, therefore, possible to reuse frequency ranges within a single cell and
to reuse
a same frequency pattern in adjacent cells. Frequency reuse is shown in Fig. 3
with
frequencies repeating within each quadrant of the cell 9. Thus, each frequency
is reused
four times. This provides sufficient isolation between cells of a same
frequency range
while supporting substantial overlap between adjacent microcells. Each
microcell
operates independently of all other microcells so that clients within a
microcell are
provided two way communication with the antenna in the hub 10. Of course,
because of
microcell overlap, a single client often has a choice between two or even
three different
microcells. This is more significant at distances further from the Hub where
signal
attenuation is a greater concern.
Though the diagram of Fig. 3 shows 16 petals in each rosette, it is preferred
that a
single rosette 9 comprise 32 or 48 petals. Four frequency channels are still
generally
sufficient. This allows for substantial frequency reuse and considerable
flexibility. It
allows sufficient flexibility in orienting adjacent cells to minimise
interference. It also
results in an amount of microcell overlap that is advantageous as set out
hereinbelow.
Rosettes do not have to have a same transmission range; alternatively stated,
cells
can vary in size. Typically, large rosettes are used in areas where subscriber
density is
low whilst rosettes having smaller radii are disposed in areas where
subscriber density is
high. Isolation between microcells is predicated on the propagation
environment. Because
of the high degree of frequency reuse within the system according to the
invention, it is
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important to ensure that the co-channel interference experienced by a
subscriber is kept at
or below such a level that a useable Signal to Noise plus Interference ratio
is maintained
at both the subscriber and hub ends of the communication link. In order to do
this a
number of system level design considerations are followed, which are listed
below.
A lower power frequency band is used for communication with users close to the
rosette hub or not substantially affected by propagation losses. The lower
power
band allows the use of lower power amplifiers for equivalent radius cells. The
lower frequency allows the use of lower power amplifiers for equivalent radii
of
cells. Higher power bands are provided for users more distant from the hub or
those who are substantially affected by propagation losses. Thus each
microcell is
effectively divided into a near-in microcell and a far-away microcell.
Effective isotropic radiated power (EIRP) Quantification: Subscribers within a
rosette are assigned frequencies having specific EIRPs. Like frequencies in
microcells of a same rosette are assigned the same EIRPs. For every rosette
and
its microcells, the EIRP assignments span the difference in EIRP between low
and
high power bands typically span the difference in EIRP between near-in and far-
away subscribers, which are respectively close to the hub or on the periphery
of
the cell's coverage area. The far-away EIRP is typically the highest EIRP
allowed
by licensing regulations and in essence, defines the extent of the rosette.
Dynamic Propagation and Interference Environment Monitoring: Every
subscriber terminal has the facility to monitor the signal environment and
report
back to a system controller the level and source of detected interference.
Every
microcell of a rosette contains a unique identifier in its downlink data
stream.
Every subscriber also has a unique identifier that is detected and decoded by
the
hub. As part of this monitoring system, the subscriber terminal reports to the
system controller the EIRP Quantification requirement data so that it is
assigned
an appropriate downlink data channel. Further, such a monitoring and feedback
system allows for dynamic assignment of a receiver of a particular subscriber
to
one of the antennas within the hub that transmits to the area where the
receiver is
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located. Thus, because of overlapping adjacent microcells, selection of a
microcell from a plurality of available microcells is possible. This allows
for
system redundancy in case of failure, environmental changes and so forth. It
also
supports load balancing between adjacent microcells because subscribers at an
edge of a microcell, or more accurately stated an area within two adjacent
microcells, are optionally served by the channels within that microcell or
within
an adjacent microcell.
Each microcell contains a number of high capacity, wideband RF data channels.
These channels use a modulation technique that is robust in face of multipath
and co-
channel interference. Typically QPSK is employed. Each channel has a
connection to the
CSMA data hub, typically built to the IEEE 802.3 standard and called
"Ethernet."
Data Processing and Delivery
Referring to Fig. 4, a simplified block diagram of a transmit antenna and a
receive
antenna for use in the hub 10 is shown. A wide area network in the form of an
Ethernet
network is in communication with an antenna driving circuit. Frames are
received and
buffered in volatile memory. Once complete, the frames are provided as
superframes to a
scrambling encoding processor for encoding. From the scrambling encoding
processor,
the data is provided to an RF modulator for modulating the data within the
frequency
range of the microcell. The RF modulator modulates the data and provides a
modulated
signal to the antenna,. The antenna is for radiating a signal that is highly
directional and
has low sidelobes. From the antenna is radiated a signal forming the microcell
transmission signal.
Since the system supports bi-directional communication, the receiver block
diagram is also shown. A receive antenna receives an RF signal from a client.
The signal
is demodulated to extract the signal data and the data is decoded by a
descrambling
decoding processor. The data is then arranged in frames and provided to an
Ethernet
encoder for transmission via the Ethernet. This same block diagram applies to
individual
subscriber antennas as well. In this fashion, the subscriber appears to be
connected via a
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LAN to the hub and the hub appears connected via the LAN to other hubs. The
resulting
network acts as if locally installed even though it may span many kilometers.
In use, Ethernet data frames identified as being for a remote wireless
subscriber
are identified, accepted by an Ethernet interface, and organised into
microcells based on
the subscriber location. Frames of data are labeled and stored in a temporary
volatile
memory. At instances these frames are retrieved en masse and assembled into
large
superframes, which typically not only contain the Ethernet information frames
but also
frames of data identifying the microcell and giving other system level
information.
Super frames of data are then scrambled, encoded, and encoded with
synchronization bytes. The data is then modulated and sent out on the RF
channel. At the
subscriber terminal, all data transmitted is demodulated and decoded. Data
identified for
the subscriber is retrieved, segmented as the original Ethernet frames and
provided to the
subscriber Ethernet interface as such. From the interface, the data is
provided to the
subscriber CPU. System level information is also decoded and provided to the
CPU and
specifically, to that application program running on the CPU that controls the
subscriber
terminal.
On the return link to the hub there are a number of techniques that mediate
the
flow of data from all of the subscribers registered on a channel. Receiving
lower
bandwidth data from subscribers at the hub, identifying the data and
performing actions
accordingly is well known in the art of communications and is not detailed
herein. For the
purpose of the present description, it is assumed that the subscriber data is
packaged as
Ethernet frames, and encoded, synchronized, and modulated in much the same
manner as
the down link data. Of course, it is evident that this need not be so.
Hub and Subscriber Antenna Characteristics
An important design issue to the system is the antenna used at both the
subscriber
and hub end. According to the present embodiment and because of the
significant
frequency reuse in the system it is important that antennas used in the system
reject
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interference. It has been found that a level of co-channel interference
experienced overall
is directly related to the side lobe levels of antennas used in the system.
The antennas, other than having low side lobe levels are designed with narrow,
highly collimated beams. Multipath delay spread is a problem in high-speed
wireless
delivery systems and one way to mitigate this problem is to only accept
signals having a
delay spread no greater than 10% of the bit duration. For rosettes operating
over 2 km
ranges with 5 Megabit per second data rates, this translates to a beam width
of about 8
degrees, after which a steep attenuation in the main lobe is desirable in
order to reject
multipath.
In accordance with a further embodiment polarised antennas are used and
signals
having different polarisations are transmitted to, in effect, double the
bandwidth of the
system. For example, if a rosette comprises 2 frequency ranges and two
polarisation
states - vertical and horizontal, the result is four frequency ranges each
having twice the
bandwidth of four separate frequency ranges each having like polarisation
covering a
same overall frequency range. Therefore, polarisation and other techniques may
be
employed within a rosette in order to increase bandwidth.
It is preferred that sidelobes from all antennas are limited according to the
equation
below:
~ = 101og(N -1) + S, +a j
0
wherein C/Io is a carrier to interference ratio and is a threshold in dB for
operation
of a demodulator of the modulated signal at a known performance level with Io,
the
interference noise, substantially greater than thermal noise No, N is equal to
a number of
like frequency petals within a rosette, SL is mean sidelobe level of the
radiators at angles
greater (M-O.5)X(BWh) away from the free space main lobe of the beam where BWh
is
the width of the individual microcell, and af is dependent upon the
environmental factors
associated with scattering of signals as expressed in dB.
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When radiators meeting this requirement are used, the receive antennas act to
filter out noise and the transmit antennas act to reduce transmitted inter-
microcell
interference. Therefore, the entire system acts to provide isolation between
microcells of
same carrier frequencies.
Examples
The Unlicensed National Information Infrastructure Frequency Allocations from
5.15GHz to 5.825GHz were used in a series of experiments conducted in an urban
residential and commercial environment characterized by high foliage and roof
top/building presence in the signal path. Tests were conducted with rosette
petals having
11.25 degrees beamwidths in azimuth and elevation. Furthermore, simulation
studies
were conducted to examine the behaviour of constellations of rosettes over a
contiguous
coverage area. The simulations, where possible, used data taken from the
experiments.
In both the experiments and simulation studies, frequencies were reused every
45
degrees thus requiring four different frequency ranges each reused 8 times. A
parabolic
Tx/Rx low sidelobe antenna having -30dB sidelobes and a gain of 25 dBiC was
used at
the hub with subscriber antennas having similar characteristics. 200 MHz of
spectrum
was assigned to the system for the transmit (forward link) direction and 100
MHz was
assigned to the return link. Therefore, each petal is assigned 50MHz of
bandwidth in the
forward link and 25 MHz in the return link.
Simulations where conducted on the carrying capacity of a 5.2/5.8 GHz rosette
system as
described and compared to a typical 28 GHz LMDS system. Assuming a cell size
of one
kilometer in radius, there are approximately 5000 homes within a cell and
approximately
160 homes within a petal. Thus the bandwidth to each home during maximum usage
(all
homes demanding as much bandwidth as possible) is 50/160 MHz or approximately
1/3
megabits/second per home in the forward direction when using a robust
modulation
scheme for DVBS/ QPSK (ETSI 300 421) generating an information density of 1
bit/sec/Hz. This is substantially more than the 16,000 bits/second for the
prior art LMDS
system. Since, at any time it is unlikely everyone within a same area is using
the
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communication medium to its fullest, most subscribers will always see a
substantially
greater amount of bandwidth.
Next, an analysis is presented below for demonstrating the behavior of
systemic
C/I in a broadband network according to the invention. The analysis considers
singular
rosettes and constellations of rosettes operating over a large contiguous
area.
The primary constraint on spectrum reuse in wireless systems is co-channel
interference. Most of the literature studying the channel assignment problem
has been
aimed at cellular phone systems where the antennas used are either
omnidirectional or
have provide for sectors within a cell having very wide sectorial beam widths.
When a
sector is assigned a frequency range or channel, the term frequency sector is
used. A
reused frequency sector is a repeated frequency assignment or channel within
two
different sectors. Reused frequency sectors are typically spaced by a number
of cell radii
apart as is the case with, for example, 800 and 1900 MHz wireless (cellular
and PCS)
telephony, and can be reused within a cell as shown in Figs. 2a and 2b. With
LMDS, it is
proposed to reuse frequencies in different sectors within a same cell by
employing
polarization isolation. Otherwise, LMDS makes use of a same frequency sector
only once
in the cell. Whatever the case, in LMDS the frequency is reused in the next
cell as shown
in Figs. 1 a and 1 b.
The channel assignment problem in rosette systems that utilise highly
directive
antennas is quite different. The same frequency sectors are used a
multiplicity of times in
a same cell. Adjacent cells also have exactly the same configuration of
directive
antennas, microcells, and frequencies, and spacing between adjacent rosettes
is often
smaller than the rosette radii. In fact, like frequency microcells commonly
overlap.
Further, the position and the configuration of interlacing of the cells is
changed by
rotating the microcells with respect to each other, and in so doing, the co-
channel
interference seen by the subscriber terminals within the cells is also
changed.
The high directionality of the microcell and subscriber antennas significantly
attenuates the co-channel interference originating from other like-frequency
microcells.
The degree to which co-channel interference is attenuated is dependent on a
number of
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factors, such as antenna height, average beamwidth and sidelobe level of both
the
transmitting and receiving antennas, and the propagation environment. This
last factor is
spatially variable, has a pronounced effect on the C/I that is at the
receiving terminal, and
ultimately places limits and constraints on narrowness and spatial separation
of like
frequency microcells in a rosette. Narrowing the beamwidth of a microcell and
reducing
the angular spacing between like frequency microcells beyond a certain point,
for
instance, results in no commensurate increase in the frequency reuse factor of
a rosette,
regardless of the sidelobe level, simply because multipath scattering rises to
significantly
deleterious levels under such conditions.
Experimentally, it has been found that in 5.2 GHz rosettes operating with a 45
degree spacing between like-frequency 11.25 degree width microcells,
scattering
degrades the isolation between like-frequency microcells by a nominal, af, of
8 dB above
what is predicted with free-space side-lobe level measurements of the
microcell antenna
patterns. This factor, af, is believed to be the aggregate of many multiple
reflections and
has not been separately identified. They are generated by radiation emanating
from the
sidelobes as well as the principle lobe of the microcell antenna. It is
believed that this
factor is strongly dependent on wavelength and polarisation of the frequency
and
dimensions of the objects situated within the propagation environment.
Additionally, it has been found that the propagation path loss exponent for
the
environment described above is typically between -2.4 and -3.4; and that it
varies both as
a function of antenna height and distance separating transmitting and
receiving antennas.
These findings are based on over 400 detailed measurements taken in highly
foliated
urban residential environments typical of proposed service areas for the
present
invention, and are consistent with other findings in current literature on
urban 5.2 GHz
propagation and such fields as C and Ku Band satellite communications where
antenna
side-lobe degradation by multipath scatter is noted.
While it is impossible to quantify the individual sources of the multipath
which
contributes to a f, it is possible to contend with strong singular reflections
that are often
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present and significantly deleterious because they are at levels within the
same order of
magnitude as direct and desired microcell signals
According to an embodiment, such strong direct scattered signals are monitored
by the receiving terminal. Analysis of the signals results in identification
of an originating
microcell. This function is undertaken by the Systemic Co-Channel Interference
Controller (SCIC), which oversees the operation of the rosettes forming a
contiguous
network. The SCIC is provided with an ability to switch radiation emanating
from any
microcell on and off, thereby facilitating the sampling and registration of
strong
interferers by receivers. Problematic radiators that generate strong multipath
interference
are turned off. Because of substantial overlap between adjacent microcells,
the resulting
rosette likely maintains coverage of most of the same area. Of course, when
two radiators
are used for a single microcell, one near-in and one far-away, only the
radiator that
results in the interference is affected. Alternatively, the terminals being
interfered with
are assigned interference free channels. For example, addition of a fifth
channel for use
when substantial interference exists is a possible solution.
Simulation results set out below are based on a rosette configuration with 8
petals
per frequency group as shown in Fig. 7. Each beam is 11.25 degrees in azimuth
with each
subsequent beam offset by 45 degrees. There are in total 4 groups of petals
with each
group offset from the next. Only one group is shown in Fig. 7.
Both intracellular and intercellular interference are analysed. Only
interference
from beams using a same frequency is considered. Adjacent channel interference
is
ignored. Experiments and simulations described below were conducted with the
following parameters:
Parameter Default value
Frequency 5.2 GHz
Maximum Cell Radius 2000m
Subscriber Height 11 m
Tower Height 25m
Subscriber Up tilt Angle 0 degrees
Tower Antenna Down Tilt Angle 0 degrees
Subscriber Antenna Beamwidth 11.25 degrees @ -3dB Contour
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Hub Antenna Beamwidth 11.25 degrees@ -3dB Contour
Sidelobe Levels (all antennas) -30 dB
Rosette Packing Architecture Hexagonal
EIRP Constant for all hubs
Environment: Propagation Path loss 2.9+/-0.5
The antenna pattern used in simulation, was
P(em) =(sin0 sin"~)
where P defines the equal gain contours of the antenna pattern, and the values
of m and n
are simulation factors adjusted to generate simulated main lobes, resulting in
patterns
with a3dB beam width of 11.25 degrees in azimuth (0) and in elevation (~).
This closely
follows that used in experimental and prototype systems. Such a pattern gives
a nominal
directive gain of 25 dBi. A sidelobe level of -30 dB was used for all angles
greater that
30 degrees away from the main lobe. Both subscriber and hub antennas were
alike in the
simulations and the experimental trials
A path loss exponent of 2.5 was used in the simulations. This is in line with
experimental results where the path loss for the average terminal has a mean
of about 2.9
with a variance of 0.5 for the urban environment
A power control strategy was utilised to minimize intracellular interference.
The
dynamic range of power produces circumferential zones of coverage around the
hub. A
user in a given zone is provided a transmit tower antenna transmitting enough
power to
reach the outermost subscriber in the zone. Users of a same channel in a
different
microcells are then constrained to being in a same zone, and thus at a same,
constant
EIRP. This power control strategy was used throughout the simulations. It is
assumed
that each cell is identical in how its dynamic range is divided into power
zones and
attributed channels to the respective zones. In the 5.15-5.35 GHz frequency
plan
embodied within the experimental system, typically two zones would arise; a
low power
zone, near-in, for communications within the immediate area of the hub
typically to a
radius of 700 meters, given a 25 dBln/MHz EIRP limit, and a high power zone,
far-away,
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for communications to the periphery typically 1500-2000 meters for a 34
dBm/MHz
EIRP limit.
Intracellular Interference
Intracellular Interference is the co-channel interference that a receiver
within a designated
microcell in a single rosette experiences due to the other like-frequency
microcells within
a same rosette. A single rosette used in the measurement and simulation is
shown in Fig.
5a. The PDF for such interference is shown in Fig. 5b.
The co-channel interference experienced is due solely to the azimuth position
of
the receiver within the microcell and the sidelobe level. In a case where
there is no
multipath scattering then:
C 1= 10log(N-1)+S, +af
Io
where SL is the ratio between the received power in the desired microcell and
the mean
sidelobe level; note that SL depends on the azimuth position of the receiver
with respect
to the main lobe; N is the number of like frequency microcells in the rosette;
and af is the
multipath scatter factor, which in ideal circumstances is equal to 0.
Fig. 5c shows how the C/I under such circumstances is invariant with distance
away from the hub. Fig. 5d shows measurements that were taken in a highly
foliated
urban environment where the af is about 3 dB. The measurements also
demonstrate the
invariance with distance of C/I.
Intercellular Interference
Intercellular interference is the co-channel interference that a receiver
within a
designated microcell in a single rosette experiences due to the other like-
frequency
microcells in adjacent rosettes.
In the simulation study, a constellation of 19 rosettes packed into a
hexagonal
architecture was assumed as shown in Fig. 6a. Interference as seen by a
subscriber in a
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randomly chosen target microcell was calculated; this interference is based on
radiated
signals simulated based on all other like-frequency microcells in the
constellation.
In the first case, microcells were aligned as shown in Fig. 6a. Typically 4000
random placements of a subscriber were undertaken and the resultant C/I was
calculated
based on the criteria given above. The resulting data was used to generate a
Probability
Distribution Function (PDF), which shows the probability of achieving a stated
C/I in the
given scenario shown in Fig. 6a. The resulting PDF is shown in Fig. 6b.
Accordingly,
approximately 33% of the terminals in this situation receive a C/I of 14 dB or
better;
which is close to the threshold of QPSK modem operation.
Fig. 6c shows the C/I that a typical receiver situated along the boresight
axis of
the main lobe defining a microcell would experience as a function of radial
distance from
the rosette centre. Accordingly, at distances greater than 1200 meters from
the hub the
C/I of the average user along the centre of the microcell is below 14 dB.
The effect of rosette rotation is demonstrated in Fig. 7a, where the rosettes
in the
19 element hexagonal constellation are changed in alignment with respect to
each other,
thereby no longer having regular alignment shown in Fig. 6a.
The resulting PDF of C/I is shown in Fig. 7b. Accordingly, in this situation
with
rosette rotation, about 97% of the users experience a C/I of 14 dB and
greater.
Fig. 7c shows the C/I that a typical receiver situated along the central axis
of a
microcell would experience as a function of radial distance from the rosette
centre under
circumstances of rosette rotation. Accordingly, even at 2000 meters from the
hub the C/I
of the average user is approximately 15 dB and greater.
These results demonstrate the beneficent effects that rosette rotation and
microcell
interlacing has on C/I performance, and ultimately, the information carrying
capacity of
such a highly organized wireless architecture.
Systemic Co-Channel Interference Control
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A brief description of the SCIC was given above. The algorithm operates in
controlling the overall co-channel interference generated by the microcells
and their
client subscriber terminals. This control is possible because every microcell
radio
terminal, be it hub or subscriber, has an address in the form of an Internet
Protocol (IP)
address. Similarly, the means of communicating between the SCIC computer and
the
rosette network is, for example, via TCP/IP (Transport Control
Protocol/Internet
Protocol).
The SCIC attempts to maintain a figure of merit for the constellation of
rosettes
forming a contiguous network. The Mean Systemic Co-channel Interference Ratio
(MSCIR) is preferably maintained, and it is the ratio of desired carrier power
radiated by
a hub of a rosette to a subscriber to the interferening co-channel carrier
power generated
by emissions from all other like frequency microcells within the rosette and
other
adjacent rosettes.
The MSCIR is affected by the following functions:
(a) maintaining a specific side lobe suppression level for microcell and
subscriber
antenna beams;
(b) positioning microcells of adjacent rosettes in such manner that like-
frequency zones
of illumination are geographically isolated from each other and do not
geometrically
align;
(c) creating isolation between subscribers and interfering zones of
illumination by
mechanically or electrically steering subscriber antenna beams away from such
zones;
(d) having the subscriber terminal actively monitor the propagation
environment and
report co-channel interference levels due to interfering signals from other
microcells;
(e) Adjusting the EIRP's of hub and subscriber terminals.
Rosette Frequency Grouping to effect the establishment of a MSCIR
Preferably, a compliment of frequencies divided into N distinct groupings of
channels is assigned; each grouping is assigned to a specific microcell and
repeatedly
assigned to the adjacent N+l; 2N+-1; 3N+1;... microcells of the rosette. In
this manner the
azimuth spacing of like-frequency oblong microcells in a rosette has a maximum
possible
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spacing for a selected frequency reuse ratio. Preferably when the number {M}
of oblong
microcells per full rosette is a multiple of the number {N}, the number of
microcells
having exactly the same channel band is equal to (M/N}.
The individual channels of a distinct grouping are blocked into sub-groups of
fixed EIRP, each sub-group stepped in EIRP with respect to the other.
Subscribers are
assigned channels only from a sub-group where the EIRP is sufficient to meet
the MSCIR
requirement. The stepping is of a dynamic range for meeting MSCIR requirements
of
near-in and far-away subscribers within the microcell.
Data Routing within the Rosette System
In encoding and modulating data within the directional forward link RF signal
transmitted from each radiator, any of a number of transmission schemes is
useful
provided they are robust enough to contend with the vagaries of the
propagation
environment and the co-channel interference Typically, a TDM divided stream in
which
concatenated blocks of data in the stream are identified for specific
subscribers resident
within a microcell is used. Each frequency range, channel, interfaces with a
local area
network (LAN) resident within the hub. For example, the LAN is a Carrier
Sensed
Multiple Access System using Collision Detection typically known as the
Ethernet or
IEEE802.3 Standard. Data on the Ethernet is received by an electronic
subsystem that
interfaces with an individual channel of a microcell. The data is received as
a standard
Ethernet Frame. This frame of data is buffered in a volatile memory, and then
buffered
frames are retrieved, concatenated with spacing between frames filled with
either dummy
bits or system specific bits to form super frames. The super frames are then
scrambled,
Viterbi encoded, Reed-Solomon encoded, and modulated for transmission. Super
frames
thus created are then TDM slotted and radiated into the microcell on a signal
at the
channel frequency. One standard that is suitable for this type of forward link
data
distribution is based on the ETSI (European Technical Standards Institute) 300-
421.
This standard also specifies the use of QPSK modulation, which is robust in
face
of co-channel interference.
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A subscriber receives data super frames modulated within an RF frequency
within
a microcell. The subscriber system demodulates all data contained in the super
frames
and detects Ethernet frames directed specifically- to the subscriber's
computer and
communication system. Ethernet data frames identified for the subscriber are
presented to
the subscriber computer as Ethernet Frames on the subscriber LAN. Therefore,
each
subscriber appears connected to a LAN. Lost or corrupted frames are ignored,
leaving it
to the higher levels of the serving protocol to either retransmit the lost
data in the case of,
for example, TCP/IP sessions between the subscriber and network or simply
continue the
streaming of data in the case, for example, of UDP packet streaming for video
services.
Each microcell channel is given a unique identifier imbedded in the
concatenated forward
link bit stream of super frames. Each subscriber transmission is given a
unique identifier,
which is imbedded in the return link data to the hub. There is a return link
channel
accessible by all subscribers within a microcell. Access to the channel is in
a TDM
slotted format, which is pre-arranged by the hub controller. Requests for
access or
transmission of short data messages are done on a random access, contention
basis within
available TDM return link slots
Using Ethernet as a wireline access protocol and TDM with reservation and
contention slots, it is possible to devise a system that supports efficient bi-
directional data
transmission over the wireless communication medium within a cell. The
resulting
communication is applicable to many fields including voice, video and data
communication.
A system according to the present invention is suitable for many communication
applications. These include Internet service provision, video on demand,
interactive
television, voice networks, and so forth. Because of the wireless technology
employed,
there is little installation cost, no cables to run, and very simple,
unintrusive user setup
procedures. Also, the chances that a communications link will be lost due to
digging and
so forth are non existent since the system is not based on a physical
connection to the
subscriber's location. The architecture provides a system that is modular,
scalable, and
transparent to data or protocols communicated through the architecture.
Because of the
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high bandwidth that is supported, there are many commercial, medical, and
educational
applications for the technology.
The system envisaged operates in the 5150-5350 MHz bands on the forward link
and 5725-5825 MHz bands on the return link, however it can be scaled to any
frequency.
Operating in these stated bands requires sharing on a co-channel basis of
frequencies
common to a number of Low Earth Orbiting (LEO) satellite systems, which are
primary
users of these bands. In these bands, the lower bands are used to transmit
signals from a
hub while the higher bands are used to transmit signals from subscribers to
the hub. This
limits interference with satellite bands to those signals emanating from the
hubs. To
further mitigate interference to the satellites it is necessary to use highly
directive
antennas, having the characteristics stated above, and orient these antennas
below the
horizon by 3 degrees so that the -30 dB sidelobes are maintained at elevation
angles of
22 degrees above the horizon. Further, the maximum EIRP's in the band 5150-
5250 MHz
must not exceed 25 dBm/MHz and in the band 5250-5250 it must not exceed 34
dBm/Mhz.
Numerous other embodiments may be envisaged without departing from the spirit
or scope of the invention.
29
