AU5609599A

AU5609599A - A method of transmitting calls in a telecommunications system, in particular a system with moving satellites

Info

Publication number: AU5609599A
Application number: AU56095/99A
Authority: AU
Inventors: Cedric Baudoin; Guillaume Calot; Michel Cohen; Cedric Lapaille; Anne Magnier; Yves Peligry; Denis Rouffet
Original assignee: Alcatel CIT SA; Alcatel SA
Current assignee: Alcatel Lucent SAS
Priority date: 1998-10-26
Filing date: 1999-10-26
Publication date: 2000-05-04
Anticipated expiration: 2019-10-26
Also published as: KR20000029305A; AU727198B2; JP2000174687A; EP0998165A1; FR2785126B1; FR2785126A1; TW432819B

Description

P/00/01i1 Regulation 3.2

AUSTRALIA

Patents Act 1990

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C C *5 STANDARD PATENT Invention Title: 'A method of transmitting calls in a telecommunications system, in particular a system with moving satellites' The following statement is a full description of this invention, including the best method of performing it known to us: F1IPSYflCFNATP(f2fl929fC1nX.

A METHOD OF TRANSMITTING CALLS IN A TELECOMMUNICATIONS SYSTEM, IN PARTICULAR A SYSTEM WITH MOVING SATELLITES The invention relates to a method of transmitting digital calls in asynchronous mode for a telecommunications system in which the call resources of the terminals are allocated by a control station.

The invention relates more particularly, but not exclusively, to a transmission method for a system in which calls are relayed via equipment on board a satellite moving on an orbit.

BACKGROUND OF THE INVENTION To make the best advantage of a telecommunications system, it is preferable to manage the information that is transmitted in such a manner that it is possible at 15 any instant to transmit information at a data rate that is equal (or close) to the maximum rate acceptable by the system.

To this end, information is transmitted in digital form so as to limit noise and facilitate management.

Usually, digital information is in the form of cells or packets that can be transmitted during a given time slot, and said cells are transmitted in asynchronous. mode so as to enable the cells to be distributed over time in a manner that optimizes utilization of the system. For S 25 example, in a so-called asynchronous transfer mode (ATM) network, ceils are of constant length and they are given respective priorities; cells which correspond to realtime calls telephone calls or video conferences) have priority over cells corresponding to calls which do not take place in real time e-mail). In other words, the cells of the various calls are multiplexed and multiplexing control is based on the "quality of service" expected for each kind of call, i.e. a quality representative of information transfer from the transmitter to the receiver.

The parameters involved in quality of service include, in particular, the maximum delay time that can be accepted between transmissions and reception, the maximum rate at which cells can be lost, the binary error rate, the maximum data rate, the mean data rate, and the amount of time variation in cell transmission.

The invention results from the observation that in some communications networks, a decision to allocate resources based solely on quality of service is insufficient.

In particular, that kind of resource management turns out to be insufficient when relatively large variations of power can be involved, for example because of variations in ego signal propagation conditions, or because of changes of *distance between transmitters and receivers.

S.OBJECTS AND SUMMARY OF THE INVENTION eeee 15 In one embodiment, the amount of power to be allocated to each cell is determined at periodicity equal to the duration of the cell or to the duration of a group of cells.

Thus, given that the total power allocated to the various calls must not exceed a predetermined value Pmax, power can be managed like any other communications S.resource.

To determine the amount of power that should be allocated to each cell, the amount of power required is determined by conventional measurement means. For example, it is possible to measure the signal-to-noise ratio on reception and to compare it with a reference, and then to make transmitter power depend on the difference between the measured and the reference signal-to-noise ratios.

When the power of each cell is caused to depend on propagation conditions, the power to be transmitted when it is raining is greater than that required in fair weather.

A preferred embodiment of the present invention is particularly suitable for use in the case of telecommunications systems using moving satellites, in particular satellites in low or medium orbits. In such a system, a constellation of satellites is provided. At any FHPSYDCA/99298010.9 one instant, the antennas of a satellite can see a (fixed or moving) determined zone of the earth and the equipment on board the satellite is used for relaying calls within said zone. When the satellite loses sight of the zone in question, some other satellite in the constellation takes over.

In a system of that kind, the distance from each point in the zone to the satellite varies continuously because the satellite is moving. For calls, such variations in distance give rise to variations of power that can lead to disturbances. The power of a received signal depends on the distance between the transmitter and the receiver; the ***greater the distance the smaller the power. Power also varies as a function of the position of the transmitter and of the receiver relative to the reception or transmission pattern of the corresponding antennas of the satellite.

In this application, the power allocated to each cell can be made to depend on one or more of the following parameters: the position of the satellite on its orbit, the position of the transmitter member within its zone, the position of the receiver member, propagation conditions that exist between the transmitter member and the receiver member, the total power available from the equipment on board the satellite, and the total power acceptable on the ground.

Thus, when cell power is made to depend on the position of the receiver member within the zone, for calls between a transmitter station in a central region of the zone and a receiving terminal or member, transmitter power will be greater for a receiver member at the edge of the zone compared with transmitter power for a receiver member close to the central region. Under FHPSYDCA/99203012.8 such circumstances, power also depends on the position of the receiver terminal relative to the radiation pattern of the corresponding antenna on board the satellite.

The power available to the equipment on board the satellite is limited and in general it can also vary.

Such variation happens, for example, when the satellite is allocated to a plurality of zones, since under such circumstances, the way in which the total transmitter power of the satellite is shared between the zones covered in this way can be modified. In addition, environmental constraints can exist that make it necessary to limit the total power received at each point in the zone. Under such circumstances, the power of each cell must be adapted to comply with limitations and 15 variations of power, and on the constraints associated therewith.

In some cases, the power of each cell can depend also on the power allocated to the cells transmitted simultaneously on the same carrier but with different 20 codes, in particular orthogonal codes, and/or on other carrier frequencies. In particular, if orthogonal codes are used that are not perfectly orthogonal, it is then preferable to limit the dynamic range of powers amongst the various cells so that the reception of low power cells is not impeded by the noise that is produced by cells given codes that are not perfectly orthogonal thereto.

Furthermore, when codes are used that are not perfectly orthogonal, noise on reception increases with the number of different codes transmitted simultaneously.

Under such circumstances, in one embodiment, the power allocated to each cell is made to depend on the number of codes being transmitted simultaneously and on their orthogonality defects.

When a cell from a transmitter is being sent to a plurality of receiver members, the power allocated to the cell is the power required by the call going to the receiver member which requires the greatest power. By way of example, such cells are those which are transmitted by a management station for the purpose of transmitting signaling information to the terminals, in particular information for allocating resources to the various terminals, i.e. carrier frequencies and/or codes.

In an embodiment suitable for optimizing resource management, when provision is made to allocate a plurality of codes for simultaneous calls, the way in which the codes are allocated is independent from the call or the connection. In other words, the cells allocated to a given call or connection can themselves be multiplied by codes which differ from one time slot to another. This provides an additional degree of freedom which facilitates resource management. In addition, the time distribution of cells allocated to a given call can also vary.

The present invention provides a method of transmitting digital data made up of cells in asynchronous mode, in which the communications resources of terminals are allocated by a control station, and in which, when the total power allocated to the various oe*e calls should not exceed a determined value, the power S" allocated to each cell transmitted by a terminal or by the control station is determined periodically.

In an embodiment, the power allocated to each cell is determined on each appearance of a cell or on each appearance of a group of cells.

In an embodiment, the power allocated to each cell is made to depend on propagation conditions between the transmitter and the receiver.

In an embodiment, when a cell is transmitted simultaneously from a transmitter to a plurality of receivers, the power allocated to the cell is that required for the receiver which needs the greatest power.

In an embodiment, for each cell of a connection, a carrier frequency and/or code is allocated periodically, said carrier frequency and/or said code being independent of the connection.

In an embodiment, the carrier frequency and/or the code is allocated on the appearance of each cell or on each occasion that a group of cells appears.

In an embodiment, when a code is allocated to each cell, the codes used are mutually orthogonal.

In an embodiment, the power allocated to each cell is caused to depend on the number of codes transmitted simultaneously and on the orthogonality defects of said code.

In an embodiment, for transmission from the station to the terminals, each connection is transmitted on a single carrier and on a plurality of codes.

In an embodiment, the calls are of the ATM type.

In an application, calls are relayed by S"retransmission means on board a moving satellite, the terminals and the control station being in one terrestrial zone.

In an embodiment, the power of each cell is made to depend on the position of the satellite.

In an embodiment, the power allocated to each cell of a call is made to depend firstly on the position of the transmitter in the zone and secondly on the position of the receiver in the zone.

In an embodiment, the power allocated to each cell is made to depend on the total power of the retransmission means of the satellite.

In an embodiment, the power allocated to each cell is made to depend on the total power acceptable on the ground.

The present invention also provides the use of the method in a telecommunications system in which the station is connected to servers or users of a terrestrial telecommunications network, the terminals being designed to communicate with the servers or users of the terrestrial network via the satellite or the station.

BRIEF DESCRIPTION OF THE DRAWINGS Other characteristics and advantages of the invention appear from the description of various embodiments, which description is made with reference to the accompanying drawing, in which: SFigure 1 is a diagram of a telecommunications system to which the invention applies; Figure 2 is a diagram showing an aspect of the method of the invention; Figure 3 is a diagram of apparatus for implementing the method shown in Figure 2; and Figure 4 is a diagram showing the method of the invention.

.MORE DETAILED DESCRIPTION 15 The embodiment of the invention described below with reference to the figures relates mainly to a system for telecommunication via satellites forming a constellation in low or medium orbit.

A constellation comprises a set of satellites placed on a plurality of orbits, and with a plurality of satellites on each orbit. The set is such as to be capable of covering the entire globe of the earth or a g.

portion only thereof.

0 In such a system, the globe is subdivided into zones and each zone 10 (Figure 1) is associated with an orbit 12. At any instant, a zone 10 can "see" a satellite 14 on an orbit 12.

The equipment on board the satellite is designed to establish communication between terminals 16, 18, etc. to be found in the zone 10. In the description below, the reception and retransmission equipment on board satellites is sometimes referred to in simplified manner as the "satellite".

In the example, direct communication is not established between the terminals 16 and 18 via the satellite, but calls between the two terminals 16 and 18 are relayed via a management or control station, also referred to as a connection station 20. In other words, a call made from terminal 16 to terminal 18 is transmitted initially via satellite 14 to the station which subsequently retransmits the same call to the terminal 18, likewise via the satellite 14.

The satellite 14 also serves to establish calls between the terminals 16, 18, etc. and stations or terminals of a terrestrial telecommunications network 22 connected to the connection station 20. The terminals of the terrestrial network 22 are constituted, for example, by servers 24, 26, and 28 and by users 30, 32. In the example, the connection station 20 has an ATM switch 34 and the network 22 has a broad-band portion 36 and a narrow-band portion 38.

15 Compared with a telecommunications system using a geostationary satellite, a call passing via a satellite 14 in low or medium orbit has the advantage of minimizing the time required for propagation, which is particularly useful with calls that need to take place in real time.

For an orbit 12 at an altitude of about 1000 km and for a zone 10 having a diameter of about 700 km, the satellite 14 remains in view of the zone 10 for approximately 10 minutes. Thereafter, another satellite (not shown), on the same orbit or on another orbit, takes over; Uses for such a system are diverse: telephony, fax, consulting databases, interactive multimedia services, video conferences, and e-mail.

These various kinds of call require different qualities of service. The number of qualities of service required can vary as a function of the transmitter member. Thus, for example, for transmissions from the station 20, it is possible to provide four qualities of service, namely: a constant bit rate (CBR) quality of service used for telephone calls and fax calls; a variable bit rate-real time (VBR-RT) quality of service where this type of call corresponds, for example, to video conferences; a variable bit rate but not real time (VBR-nRT) quality of service; and finally an unspecified bit rate (UBR) quality of service which corresponds, for example, to transmitting electronic mail (Internet type).

Each of these qualities of service is associated with a maximum delay between transmission and reception, with the maximum acceptable delay being smaller for CBR quality, larger for VBR-RT quality, and larger still for VBR-nRT quality. In contrast, there is no required oo maximum value of delay on UBR quality of service.

15 In the example, for calls transmitted by terminals 16, 18, etc., three qualities of service are provided, of which the first two correspond to respective maximum delay times between transmission and reception that are fixed but of different values, and the third corresponds 20 to UBR quality.

Whatever the number of qualities of service, each quality of service is allocated a priority, with the highest priority being allocated to CBR quality, and the lowest priority to UBR quality.

The use of transmission in asynchronous mode makes it possible to share traffic resources in such a manner as to comply with.priorities.

In an ATM type asynchronous transmission, information is transmitted in the form of cells of invariable length, e.g. 1.5 ms, referred to as "time slots". Each cell comprises 384 payload bits and header bits.

Calls between the connection station 20 and the terminals 16, 18, etc. can also take place on a plurality of carrier frequencies. In addition, during each time slot, a cell is allocated to a code selected from N codes in the present example), all of which are mutually orthogonal.

The communication capacity of the system increases with increasing number of carriers and with increasing number of codes.

The diagram of Figure 2 has time plotted along the abscissa and codes Ci: C 1

C

2

C

3

C

80 plotted up the ordinate. The cells are referenced cell l cell 2 cell 80 with the subscripts given to each cell corresponding to the number of the code allocated thereto.

A signal S transmitted from station 20 to terminal 16 or from terminal 16 to station 20 is thus a signal having the following form: 15 S Cicell i i=1 To decode this signal on reception, the device shown in Figure 3 is used which has multipliers 421, 422, 4280 each of which has a first input receiving the signal S and a second input to which a respective code C 1

C

2

C

80 is applied.

Under such conditions, at the output from the :multiplier 42 i a cell cell i is obtained which, given that the codes are orthogonal, satisfies the following relationship:

C

i

C

j 0 if i j, and C.C j 1, if i =j.

In an aspect of the invention, to comply with the priorities, the program managing transmission decides for each time slot (1.5 ms) or for each group of time slots whether to allocate a determined carrier and a determined code to each cell, said allocation being independent of the connection. In other words, cells in any one connection can be on carriers and/or codes which differ from one time slot to another, or from one group of time slots to another.

This characteristic makes it possible to minimize use of available resources. If a given connection were to be continuously allocated the same code, then that code would not necessarily be used in optimum manner.

For example, if the connection in question contains periods of silence, the code would not be in use during said periods of silence, whereas using the invention, during periods of silence in a connection, it is possible to reuse the code for other calls or connections.

The telecommunications system must also take account of particular constraints associated with the fact that O calls take place via satellites 14 that are moving. The power requirement for each call therefore varies continuously, since the power necessary depends on a 15 plurality of factors.

A first factor is distance. Thus, when the satellite is at the nadir of the zone 10, distances are at a minimum and power requirement is at a minimum.

However, when the satellite comes into the field of view 20 of the zone, or when it is about to leave it, distances are much greater and the power required for a call increases. Furthermore, for a given position of the satellite, the distance between the station 20 to the :terminal depends on the position of the terminal within the zone; for example if the satellite is at the nadir of the zone, in general vertically above the station then the distance from the station to a terminal which is close thereto is significantly shorter than the distance between said station and a terminal that is close to the edge of the zone.

Another factor on which power depends for each call is constituted by propagation conditions which can vary over time, for example because of a change in the weather. Thus, for transmission passing through cloud or rain, more power is required than for fair weather transmission.

Yet another factor is constituted by the power that the transceiver means on board the satellite 14 can supply. In particular, if the satellite 14 is allocated to a plurality of zones, call management over the various zones can require variations in the allocations of power to the various zones. The power transmitted by the satellite can also be limited by environmental constraints; for example it can be required that signals received within the zone or within a portion of the zone must not present power in excess of a determined limit.

Thus, to take such constraints into account, in the method of the invention, each cell has a power allocated thereto, which power is determined periodically, e.g; for each time slot (duration of one cell) or for each group 15 of time slots, and in particular for each frame (every 24 ms) For transmission from a terminal 16, 18, etc. to a .*"'.connection station 20, the power can be determined by the terminal, or it can be determined by the connection station 20. If the connection station 20 is determining power, then it sends to the various terminals indications of the powers of each of the cells to be transmitted, e.g. simultaneously with sending the resources that the •cells are to use for the calls(carrier frequencies and codes).

The resource-allocation signals and optionally the power-allocation signals as issued by the station 20 for the attention of the terminals 16, 18, etc. are constituted in the present invention by cells which are sent simultaneously to a plurality of terminals or to all of the terminals. Under such circumstances, power requirement is not necessarilyidentical for all of the terminals 16, 18, etc. since firstly the distances involved can be different, and secondly, propagation conditions are not necessarily uniform over an entire zone. For example, part of the zone can be cloudless while another part of the zone is covered in clouds and rain.

Under such circumstances, the power used for a cell that is transmitted simultaneously to a plurality of terminals will be the power which corresponds to the least-favored call, i.e. to the call which requires the highest power.

Furthermore, the connection station transmits data proper to each of the terminals. Under such circumstances, the various signals are transmitted simultaneously to the individual terminals, with a respective carrier and code being allocated to each o• terminal.

Under such conditions, for transmission from the 15 connection station 20 to the terminals 16, 18, etc., the power requirement is determined for each terminal (depending on the position of the satellite, the position of the terminal, and propagation conditions), with said determination being performed periodically, e.g. for each time slot or for each group of time slots, and said power allocations are made for the cells that are to be transmitted by the station 20 to each predetermined terminal.

Thus, during a given time slot, the connection station 20 transmits cells at different powers. This property is represented in the diagram of Figure 4 where time is plotted along the abscissa and power up the ordinate. In this diagram, cells cell,, cell celli, occupy heights up the ordinate AP 1

AP

2 APi, which correspond to the respective powers they require.

Furthermore, the total power available, which is a function above all of the transmitter power of the means on board the satellite, has a limit value Pmax which can vary with time, as mentioned above. Under such conditions, it will be understood that for each time slot, or for each group of time slots, the number of cells transmitted by the connection station to the terminals is not constant. The number of cells per time slot or group of time slots will be smaller for bad propagation conditions or when the total available power Pmax is smaller.

In the example, the management of the data signals transmitted by the connection station to the terminals 16, 18, etc. is performed individually on each carrier.

In other words, a connection continues on the same carrier even if its cells might be distributed over different codes. Under such circumstances, the abovementioned power Pmax corresponds to the power Pmax allocated to the carrier. Nevertheless, in a variant, it is possible to perform management on an overall basis, 15 with the cells of any one connection then possibly being allocated not only to different codes, but also to different carriers.

."'.Furthermore, when the number of terminals is large, it is not possible in general during each time slot to serve all of the terminals simultaneously. Under such circumstances, during a given time slot, N first terminals are served and during the following time slot, the P following terminals are served (where N and P can S"vary from one time slot or group of time slots to another, given priorities or constraints on power), and so on. The order in which the terminals are served will also depend on priorities.

In an embodiment, a single code is allocated to each terminal during each time slot.

In another embodiment, each time slot is dedicated to transmission to a single terminal, and all of the codes are dedicated to that terminal.

In yet another variant, provision is made to serve only a limited number of-terminals in each time slot, and during that time slot, a plurality of codes are allocated to each terminal.

In the example, the connection cells for data signals that are to be transmitted from the station 20 to each of the terminals 16, 18, etc. are organized, as mentioned above, in four queues having different priorities. The queue with the highest priority guarantees a delay time of not more than 100 ms between transmission and reception, the second queue guarantees a delay time of not more than 200 ms, for example, the third queue a delay time of not more than 400 ms, while there is no minimum data rate guarantee minimum delay time guarantee) on the fourth queue.

Nevertheless, in order to avoid blocking, particularly for cells having the lowest priority, a maximum time is determined for which any one cell can remain in a queue, and if one or more cells have still not been transmitted when this time has elapsed, then the each cell is transferred to another queue of FIFO type (first-in, first-out). The determined length of time after which cells are transferred from a main queue into S. 20 a FIFO queue depends on priority; it is shortest for queues having highest priority.

Thus, for each priority, two queues are provided: a main queue and a FIFO type queue. FIFO type queues have priority over main queues. In other words, the FIFO queue of lowest priority has priority over the main queue of highest priority. However, priority order continues to be maintained between the various different FIFO queues.

The management of calls transmitted by the terminals 16, 18, etc. to the connection station is analogous to that which takes place in the opposite direction, i.e.

from'the connection station to the terminals.

Nevertheless, this management is facilitated by the fact that each terminal communicates only with the station Thus, at any instant, all of the cells from a given terminal have the same power allocated thereto, but the power of each cell is nevertheless variable as explained above for calls from the connection station towards the terminals.

In the example, the management of calls transmitted by the terminals to the station 20 differs from the management of calls transmitted by the station 20 firstly in that it relates to a plurality of carriers simultaneously, e.g. three, and secondly because the number of queues can be smaller. Thus, three queues are provided whereas four queues are provided for calls from the station The asynchronous telecommunications system having .":.cells of varying power and for which code resources and Spossibly also carrier resources are allocated as a function of priorities enables resources to be maximized, i.e. it enables data rate to be maximized, while complying with the constraints that affect a moving satellite system.

The invention is not limited to moving satellite telecommunications. It applies whenever power variations S 20 can occur in an asynchronous digital transmission system, in particular a system in which resources are periodically allocated by a control station. Such a station can also act in a system having geostationary satellite(s) or in a radio system.