Note: Descriptions are shown in the official language in which they were submitted.
19~-09.Oi AH/1ca P 12~
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METHOD AND SYSTEM IN A WIDE AREA RADIO COMMUNICATION
NETWORK
The invention relates to a method and a system
in a wide area radio communication network,
comprising at central stations, each cental station
being assigned to one or more peripheral station.
To make possible for a plurality of users of
wide area mobile or stationary radio systems to
utilize Gammon communication resources methods of
time and/or frequency sharing (e. g, TDMA, ALOF3A,
CSMA, Polling, FDMA) have been developed. The first
method developed was frequency sharing. The Nordic
mobile telephone system NMT is based on this method.
~5 According to the frequency sharing method the users
are allocated a certain channel at a certain
frequency during the communication. Time sharing is
a more modern method.
Digital wide area systems normally use time
0 sharing, some times in combination with frequency
sharing.
A drawback of the prior art methods for sharing
resources is that the central station disposed in
the center of the area must be able continuously to
~5 xeach the peripheral stations spread out in each
cell axea or sector (e.g, within 360° or 90°.;
see figs 1 and 2).
The quality of digitally transmitted signals is
effected by reflection and diffraction, see fig 3.
30 Dy using omni directional or sector directional
antennas the transmission capacity is limited. This
is a difficult problem especially in mobile systems.
Another drawback in current systems is that an
ongoing information process between two stations
35 results in transmission and reception in undesired
CA 02000529 1999-08-16
2
directions. Small cell configurations such as those shown in fig 4 are common
in radio area
networks.
EP,A3,0201254 discloses a more developed radio communication system which
utilizes spot beams, time division multiple access, and frequency-use to
provide
communication service from a central station to remote customers within a
system service
region. The central station provides multistage switching on intermediate
frequency level to
form a spotted beam in different directions so as to permit the respective
sharing of radio
transmitters and receivers over a major number of antenna transmitting and
receiving ports.
At the super central station, each section of the service area is covered by a
different one of a
raster of spot beams which are switched in accordance with a TDMA frame. A
small number
of transmission frequencies are re-used by different spot beams. The signal
output energy is
constant, regardless of the distance between the central station and the
customer stations.
An object of the present invention is to overcome limitations and drawbacks of
the
prior art systems mentioned above. Another object of the present invention is
to minimize
interference between stations and to minimize the total power consumption of
the system.
According to one aspect of the present invention there is provided a method of
communication in a wide area radio communication network, said network
comprising at
least two central stations, each central station being assigned to at least
one peripheral station,
each central station and each peripheral station having means for transmitting
radio energy
and receiving radio energy, means arranged in said central stations for
communicating to and
from said central stations radio signals in predetermined transmission
directions during
predetermined time intervals, said means for communicating including a
directed antenna
system, each central station transmitting and receiving radio signals to cover
a service area,
said method including the steps of:
(a) dividing a total service area coverage of each central station into
different
geographical sub-service area segments covered during selected time segments;
(b) operating each central station in different sub-areas by controlling said
directed
antenna system to operate in said sub-service area segment during
predetermined time
intervals;
(c) storing in each of said central stations information about distance and
direction to
each assigned peripheral station;
(d) distributing from each central station to each assigned peripheral station
CA 02000529 1999-08-16
2a
coordination signal information, said information including timing
compensation with respect
to the distance between said central station and said peripheral station
assigned thereto, and
including actual time intervals selected by said central station for sending
information to said
central station from said peripheral station; and
(e) coordinating in each central station reception of information transmitted
from each
assigned peripheral station at said time intervals by directing during said
time intervals the
antenna of said central station towards said peripheral station.
According to another aspect of the present invention there is provided a
system for
communicating in a wide area radio communication network, said network
comprising at
least two central stations, and a plurality of peripheral stations, each
central station being assigned to at least one peripheral station, each said
central station
comprising:
means for transmitting and receiving radio signals to and from any assigned
peripheral station assigned thereto, said means including a controllable
directional antenna
system,
means for controlling said directional antenna system in specific directions
during
selected specific time segments in at least two directions,
means for storing position data including distance and direction to at least
said
associated peripheral station; and
each of said peripheral stations comprising: .
means for transmitting and receiving radio signals to and from any associated
central
station,
means for receiving timing information and distance information, and
means for storing said timing and distance information in each peripheral
station for
controlling the timing of transmitting radio signals to said central station
on the basis of said
timing and distance information.
In order to explain the invention embodiments thereof will be described in
more detail
below with reference to the accompanying drawings, wherein
figs 1-6 are representations of prior art communication systems,
A
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fig 7 shows a timing sequency in the system
shown in figs 5 and 6,
fig 8 is a representation of the buffering
principle which takes place in the system
6 according to figs 5 and 6,
fig 9 is a view from above of an embodiment of
a system according to the invention during a
specific time interval,
fig 10 shows the system in fig 9 during another
time interval,
fig 11 shows a central station according to the
invention,
fig 12 shows another embodiment of the
invention utilizing phase array antennas,
~6 fig 13 is a representation of a timing
synchronizing system utilized in an embodiment
of the invention,
fig 14 is a representation of a phase
displacement system utilized in an embodiment
of the invention,
fig 15 is a representation of a system
according to the invention,
figs 16, 17 and 18 are representations of a
system according to the invention transmitting
a6 in duplex and semi duplex,
figs 19 and 20 are representations of different
time frames for different subsystems in duplex
and semiduplex,
fig 21 is a block diagram of a system
configuration,
fig 22 is a representation of the configuration
in fig 21
fig 23 is a block diagram of a sub-system, and
figs 24 to 26 are representations of diffferent
36 methods of connecting systems according to the
invention to external networks.
1939-09fi7 AHJkd P 12Z~
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The information transmission between two
stations in a prior art system using spotted beams
takes place during a specific time interval or time
slot, many slots constituting a frame, and in a
g specific direction. The transmission is normally
repeated continuously. With reference to figs 5 and
6 it is clear that when transmission is taking place
between two stations, the stations are directed
towards each other with regard to the transmitting
as well as the receiving direction.
The time diagram in fig 7 shows how the
transmission between stations is divided into frames
or time intervals.
When the information flow is continuous the
15 information is transmitted as packets in time slots
which are stored and rebuilt at the receiving unit
and then retransmitted from the system in the
original shape. The transmission is transparent to
the user. There is a certain delay in transmission
mainly caused by the package transmission.
Buffering and the rebuilding, see fig 8, in which 11
and 12 are references for peripheral stations and 10
for a central station: The time delay between each
package or the repeated time slot in a frame is
~5 adjusted to the time delay acceptable in connected
servir_es. In public telephone networks a delay of up
to 50 ms may be acceptable.
The wide area telecommunication, system
according to the invention is intended for all types
30 and combinations of telecommunication services, such
as analog and digital telephone transmission, high
quality sound and image transmission, low-speed
asynchronous data transmission and synchronous data
transmission, all services in corporation with other
35 types of networks and services.
mso.tn.o~ nty a i:es
Tt is also possible to implement the system
according to the invention for a specific service or
for a specific combination of services. During the
transmission phase the antenna of a central station
5 is always directed towards a specific peripheral
station, but a peripheral station may under certain
circumstances, such as in mobile applications,
utilize an omni directional antenna and/or a
adaptive directional antenna.
j0 Each station dynamically and continuously
redirects itself to the next station in turn for
transmission. To keep the system capacity high the
redirection delay is made very short in relation to
the time of the timeslots. In the system~according
to the invention the redirection time delays are
about or less than 1 microsecond which means that
the time losses due to redirection are marginal. At
a channel capacity of 2 Mbite/s and 50 active
stations less than 1 per cent of transmission time
~0 is lost due to the redirection.
The system according to the invention is
intended for stationary peripheral stations as well
as mobile type peripheral stations. In a system
comprising stationary peripheral stations the
a5 geographical position of each of the stations is
stored. Position data are utilized when the central
stations calculate and control the peripheral
stations transmitting and receiving direction, time
assignment according to traffic demand arid timing in
other subsystems, power and when applicable carrier
frequency in order to optimize capacity and quality
and minimize interference: When a new station is
connected or an active station is disconnected the
system automatically recalculates transmission
control data. Central stations use time slots in the
1959-09-W AH/kd P 12J~
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frames during which the area covered by that central
station is scanned to locate and identify any new
station added and any previous active station
disconnected or reconnected. Basically a central
station of the system has three basic states or
modes:
A) Identifying stations and compensating
for distance variations, for correct timing
and power control.
1p B) Controlling station traffic.
C) Transferring user and system information.
Optionally the system performs also:
D) Scanning for identifying non-active and new
stations and the locations thereof.
yB The system includes a central station antenna
which is directed towards one specific peripheral
station during short time intervals during which
information is transferred, so ws to achieve
optimized signal strengths and minimized
interference for the normal information exchange..
The information exchange which is well defined in
time as well as in direction makes possible a
simultaneous reuse of the frequency in the vicinity
of said stations, as long as the transimission
direction during that time interval is different.
From figs 9 and 10 it is apparent how three
different central.stations 10, 10'; 10 " can
transmit to three different peripheral stations 11,
11', 11 " and use the same frequency fl. This is
30 possible because of the highly directed antenna
system, of the central station and because of the
power, information transfer and timing control
applied to the system. fig 9 shows the transmission
taking place during time interval tl and fig 10
during time interval t2, During certain time
1939-p9.p7,OH/k5 P 1289
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periods the central stations search and scan for new
stations or movements of stations. The central
stations are not directed towards any specific
peripheral station during said time periods.
The system utilizes dynamically controlled
antennas 41 and the principle of this type of
antennas is shown in fig 11. In one implementation
of the system phase array antennas 42 are used, see
fig 12. The construction of the phase array antennas
y0 depends on the application. In fig 12 four plane
phase arxay antenna elements 17, each of which
covers 90°, are combined so as to cover a full
circle for one radio transmitter/receiver.
Alternatively, one plane phase array element is
combined with one transmitter/receiver in order to
allow the central station to serve a sector. Each of
the elements is constructed to have a well defined
transmission direction in space.
One central station 10 and the peripheral
2p stations 11 connected thereto comprise one subsystem
which combined with more subsystems forms a larger
communication system that cover a principally
unlimited area as illustrated in figs 21 and 22.
Different configuration types are possible, such as
those shown in figs 24, 25 and 26 with decentralized
or hirarcial,structures. Further, all or some of the
stations may be provided with dynamically controlled
directional antennas.
One implementation of timing control between
30 subsystems is shown in fig 13 where the central
stations 10 receive synchronizing signals from an
external reference source and communication channel
such as a satellite 21, radio/TV transmitter etc.
The timing control and traffic coordination between
35 Subsystems to avoid interference could also take
1989-09L.1 AH/kd P 1259
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8
place through the public telephone network or
through central stations which are set up to be
synchronized and exchange control data with adjacent
stations, 80, see figs 21 and 2,2. Compensation for
g and control of time reference differences, 34,
between different adjacent sites may be included in
advance by compensating for the relative time
difference between different geographically
separated stations. Alternatively, the central
~0 stations are provided with, a frame stability which
makes synchronization unnecessary. If
synchronization is not applied adaptive fault
detection methods are used to coordinate traffic in
the subsystems.
15 Fig 14 shows a simplified arrangement to which
there is applied a dynamic time slot allocation in
dependence of the traffic for each central station
indenpendent of traffic analyse on other central
stations. To minimize the interference between
20 central stations the scanning beam or beams at each.
central station can be phase delayed and/or combined
with a quality detection to perform communication as
mentioned above. Thus, in this way risk of
interference is minimized without a dynamic traffic
~5 coordination between none-adjacent central stations.
A way of avoiding interference between
different substations is simply not to allow
information exchange in some directions during some
time intervals. Areas 22 in fig 15a are referenced
as prohibited zone's and no transmission is allowed
in those areas. Th.e unit giving the reference number
23 is a source of interference, which can be
constant in time.
At certain time intervals a pair of station in
each subsystems are bound to interfere when they
19~-p9.p~,~~ P 1759
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9
transfer information at the same frequency. See
figure 15b. Two such stations are referenced as a
pair at risk. 24, 25, 26, 27, 28. However, for fixed
applications pairs at risk are known in advance and
the time intervals given to the stations in a pair
at risk are chosen by the control system so as to
minimize the risk of interference.
By adjusting the transmission power to a level
for each station pair that exchange information to
achieve a certain expected transmission qulity 'the
level of interference is minimized and the
information exchange is not performed with an
unnecessary high quality. Fig 15b shows power
diagrams 25, 26 for different stations situated at
y5 different distances from their central stations. In
order to maintain a desired transmission quality in
the system in unpredicted situations is fault
detection applied. The influence from interference
can be decreased by adapting forward error
?0 correction codes or by changing time slots or
frequency.
When a system according to the present
invention is used for duplex transmission
information packages to or from a central station
~5 are coordinated with respect to time intervals. In
that way losses due to directional changes in time
and capacity are minimized. Fig 16 and 17 show how
the antenna system of one central station for one
radio unit is directed in one specific direction at
30 the time T1 and transmits and receives information
packets to and from a peripheral station 20. At
point T2 another packet is received at the central
station from the peripheral station 19 and its
corresponding packet is sent out to 19, i.n duplex
35 mode.
1939-09.OL ~iFl/kd P 17~
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Fig 18 shows a similar process as described
with reference to fig 16 and 17 but at different
time slots T1, Tn, T5, Tk in semi duplex mode. The
transmission process is separated between the
central station. and the peripheral stations as
illustrated in the antenna diagram 38 and 39. 44 is
the information packets sent by a central station
and 45 is the information sent by the peripheral
station. In semi duplex mode the central station
coordinate the traffic flow to and from the
peripheral stations and direct the antennas towards
incoming packages from peripheral stations shortly
before the information 45 is received by the
antenna. Shortly after the transmission is completed
~5 the antenna is redirected. .A central station
controls each of the peripheral stations associated
therewith with respect to the time intervals during
which the peripheral station is allowed to transmit
information. In that way the central station knows
when information will be transmitted from each of
the peripheral stations and when the information
will be received. When such a package is received
the antenna lobe 38 has just taken the proper
direction. During the rest of the time period of
~5 each~frame the central station exchanges information
with other peripheral stations and in the next frame
the antenna is again directed towards the mentioned
station as long as the time slot is assigned.
Fig 19 shows the duplex frame structure for one
or several central stations l..n. The transmission
and receiving time are illustrated at 35 and 36. The
total amount of time available during one frame is
referenced by 32. One frame is devided into certain
slots 320. The relative time difference between
geographically separated central stations is
referenced by 34.
19AH/1cd P 12~
11
Fig 20 shows the semi duplex mode where each
subsystem or central station using the same radio
carrier is separated in a transmission time for the
central station and the peripheral stations 38 and
39. The total time for one frame for both directions
is referenced by 33. The time slot assigment in each
of the directions can be different which is shown at
380 and 390 in semi duplex mode the transmission
direction to and from the peripheral stations is
controlled by time slot allocation reservation which
makes it possible to handle traffic of different
capacity between ports in the system independently
of the transmission direction. The semi duplex frame
structure is separated in a transmission time period
and a receiving time period in order to operate
several different central stations at the same time
on the same frequency, see fig 22, with minimized
interference between subsystems. Said central
stations can be adjacent or non-adjacent.
Fig 21 and 22 show an exemple of an
implamentation of the system over a wide area. One
or several central stations 10 form subsystems 50
together with peripheral stations 11 and 12. One or
several central stations can be formed as one
station or super central station 40 at the same site
via a digital switch. The traffic within or between
different subsystems may take place through an
external digital switching device 90 in order to
increase redundancy or to use each super central
station more efficiently. Several subsystems 50 form
a system 70. Synchronisation and coordination
between subsystems and inter system data exchange is
shown at 80. Connection ports to other networks are
represented by 31 and 91 at the central or super
central stations and by 310 at the peripheral
W wt3~c a m
~v.~~.!~'~~~~
12
stations. Coexistance of two subsystems that cover
the same region is shown in figure 22, c.
Fig 23 shows one subsystem 50 and under the
central station 10 one or several separated
peripheral stations 11 and 12 with its ports 310
where user traffic is to be connected. Use of data
transmission is represented by 500. Signalling
between central and peripheral stations is
represented by 800 for the ports and by 900 for
1U other signalling. Interference is further possible
to minimize in the system by analysing the user
information which is required to be sent through
each subsystem. If so desired only information
packets that carries changes or certain types of
changes are transmitted through the system. When
information is considered redundant and is not
transmitted the information is instead reinserted at
the corresponding destination port of the system.
The information transfer between ports is controlled
an the signalling 800 which requires less band width
than usual data transfer 500. Data, such as silence
or "on hook condition" in a telephone system and/or
repaeted similar data streams or no data
transmission from a computer, LAN, PABX, IMAGES, etc
can be controlled by signalling between the
stations. Tn,this way the interference in the system
is decreased. Further, the system capacity will be
increased.
Fig 24, 25, 26 show examples of different
system configurations. In fig 24 each of the super
central stations or subsystems is connected to
another network 72. In fig 25 connection to another
network is made through one super central station or
subsystem only, inter traffic between stations shown
at 1000. Fig 26 shows a more complex and
1939 79-41 AH(kd P 11~
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13
decentralized structure where some super central
stations or central stations are terminated to other
networks and some are not.
3y allowing a plurality of geographically
adjacent subsystems each having a possibility to
cover 3600 horizontally while using the same
frequency, the frequency space effenciency is
increased by a factor from 7 to at least 1~ in
relation to other methods, see fig 4, of wide radio,
area networks. As the antenna system may also
include vertical space control and as different
users are separated in variable altitudes such as
sky scrapes the factor is further increased.
When each central station is equipped with an
antenna each covering a sector of 90° horizontally
the frequency efficiency is still further improved
by a factor of about 4 when interference is
controlled efficiently. Tf such an arrangement
includes two or more systems to cover the same area
from the same site 51 the efficiency is still
further improved. Each central and peripheral
station is equipped with ports that allow
multiplexed or unmultiplexed digital signals to be
transmitted through the system. The ports 31; 91;
310 form the connection points between the users and
the system. Said ports are designed in such a way
that it is possible to achieve a secure and if
required a compressed information exchange through
the system for each service connected. This is done
i~ order to avoid interference and to delete
unnecessary information transfer which at some time
intervals carries no significant data, see fig 23.
This further improves the frequency eff~.ciency and
the interference risks are further decreased.
1989-09.pt AH/kG p 125p
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14
The secrecy of the system according to the
present invention is also improved dramatically when
information is transmitted in packages in different
di~ections. By adjusting the transmission intensity
in dependence of the actual traffic and quality
needs the secrecy is further improved.
1p
20
~5
