GB2227393A

GB2227393A - Communication arrangement

Info

Publication number: GB2227393A
Application number: GB8901467A
Authority: GB
Inventors: David Graham Pudsey
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1989-01-24
Filing date: 1989-01-24
Publication date: 1990-07-25
Also published as: GB8901467D0

Abstract

In a communication arrangement for enabling speech signals to be transmitted between a ground station and a plurality of mobile stations located on board aircraft, the ground station includes an aircraft allocation circuit 29 for controlling the transmissions from the aircraft to the ground stations. The control information is multiplexed onto an output channel 9 which also carries speech signals input at lines 3A, 3B ....3H. This enables the aircraft to communicate with the ground station even when they are not able to communicate with each other to determine the timing of their transmissions. <IMAGE>

Description

COMMUNICATION ARRANGEMENTS This invention relates to communication arrangements and- more particularly to arrangements in which a base station is arranged to communicate with a plurality of other stations. The invention is particularly, but not exclusively, applicable to arrangements in which the other stations are mobile.

Where it is wished to establish two-way communications between a base station and a plurality of other stations, problems may arise in controlling transmissions so that there is no interference between signals from the other stations or overloading of the base station receiving circuits. Such problems are particularly acute where the other stations are mobile.

For example, the base station may be a ground station and the other stations may be carried on board aircraft. Each aircraft station may have a plurality of terminals, for example to enable passengers to communicate verbally via telephone to the ground.

The ground station is able to control communications traffic from the ground in an upwards direction to designated terminals of the aircraft station. However, difficulties arise in communicating downwards from aircraft to the ground. Individual aircraft have no means of allocating time slots amongst themselves. Thus there is a danger of interference between the aircraft transmissions and of exceeding the handling capacity of the ground station. These problems may be reduced by including a large number of dedicated circuits at the ground station, each circuit being associated with a particular aircraft. However, such an arrangement is potentially unreliable because of the large number of circuits which are required and is inflexible in coping with changing demands.

The present invention arose in an attempt to provide an improved communication arrangement which is particularly applicable for communications between mobile stations and a base station.

According to the invention, there is provided a communication arrangement comprising a base station and a plurality of other stations in which transmissions from the plurality to the base station are controlled by the base station. By employing the invention, the base station is able to allocate transmission slots, for example, to meet the loading at each other station and thus requires fewer receiving lines than would otherwise be the case. The operation of the base station may also be flexible to enable it to meet varying demands. The invention may be particularly advantageously employed for applications in which the other stations are mobile, as in such cases it may be impossible to provide interconnections between them. The other stations may be carried by aircraft, automobiles or trains, for example.

Preferably, the base station includes a multiplexer for multiplexing a plurality of output signals for transmission to respective stations onto a single output channel for transmission. It is preferred that the multiplexer implements time division multiplexing but frequency multiplexing could also be used.

The invention is particularly applicable where communication between the base station and the other stations is in the form of speech signals and preferably the speech is digitised for transmission.

It is preferred that the base station includes allocation means for receiving input signals on a plurality of input lines for distribution to its output lines and means for multiplexing the signals on the output lines onto an output channel. In a preferred embodiment of the invention, the number of input lines is greater than the number of output lines. This is feasible where the communciation takes the form of conversational speech between two parties. It has been found that in such conversations, each participant speaks on average for only 40% of the time. If there are, for example, twice as many input lines to the allocation means as there are output lines, then for most of the time there will be no difficulty in allocating speech on the active input lines to the output lines.If there are more active input lines than output lines, this situation can be dealt with by digitising the input signals at two different rates, one being faster than the other. By accepting at least some lower rate inputs, all input signals may be distributed to and carried by the output lines.

One way in which the invention may be performed is now described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic block diagram of a ground station included in an air/ground communication arrangement in accordance with the invention; and Figure 2 is a schematic block diagram of communications equipment installed on an aircraft for inclusion in the arrangement of which the ground station shown in Figure 1 forms a part.

In an air/ground communications arrangement, communication between a number of aircraft and the ground is effected via a single ground station, illustrated in Figure 1. In the embodiment described, up to eight speech circuits may be transmitted by the ground station by means of time division multiplexing them onto a single channel, enabling communication links to be established with one or more aircraft.

Initial contact between an aircraft and the ground is made when an aircraft comes into the transmission range of the ground station and receives an invitation from the ground station. The aircraft responds to the invitation to form a communication link. The invitation from the ground station consists of a frame of data in its output which is designated as a "new entry slot" and which is transmitted at suitable intervals. The new entry slot is produced at a generator 1 in the ground station, as shown in Figure 1, and multiplexed onto the ground station's output. The aircraft transmits an individual short identification code (ID) in reply which is retained in a store 2 at the ground station. Once the identification code has been stored communication may take place between terminals on the aircraft and on the ground.

The ground station includes eight input lines, only some of which 3A, 3B ... 3H are shown, along which speech signals may be transmitted. Each line includes a detector 4 which permits only those signals consisting of speech to be passed onwards, blocking spurious signals and noise.

The outputs of the speech detectors 4 are applied to a plurality of codecs 5 which perform digitisation of the speech. Although in this embodiment of the invention the speech detectors are analogue filters they could of course operate digitally and be located after the codecs 5 in the transmission path. The output of each speech detector 4 is applied in parallel to two codecs 5' and 5''. One codec 5' has an output rate of 16 kBits per second which is twice that of the other codec 5'', the output rate of which is 8 kBits per second. Thus each set of speech data is available in two formats from the codecs 5, the frame length of the codec 5' outputs being twice that of the codec 5"' outputs.

The sixteen outputs from the codecs 5 are applied to an allocation circuit 6. This accepts signals on the sixteen lines and distributes them amongst the four available speech circuits 7 at its outputs. The signals on the lines 7 are then time division multiplexed at multiplexer 8 onto the output channel 9 of the ground station for transmission to aircraft. In this embodiment of the invention, the output channel is capable of transmitting 64 kBits per second and the speech circuits carry information at a rate of 16 kBits per second. If four or less of the input lines 3A, 3B ... 3H are active, i.e. carrying speech, then the allocation circuit 6 accepts digitised speech from the higher rate codecs 5' on the active lines. These signals are then applied to respective different speech circuits 7.If, however, there are more than four active speech lines then the allocation circuit 6 cannot take all its inputs from the higher rate codecs 5' because the speech circuits 7 are not capable of carrying that amount of information. Thus, where there are five active input lines for example, the allocation circuit 6 acts to accept two speech inputs from lower rate codecs 5' ', and three inputs from higher rate codecs 5'.

The inputs from the two lower rate codecs 5'' are applied on one of the speech circuits 7 and the three other signals are applied to the three remaining speech circuits 7. The allocation circuit 6 varies those lines from which it receives the lower rate codec outputs so that the transmitted speech signals are of uniform quality. It can be seen that this arrangement is capable of handling eight speech inputs simultaneously. At any instant, the output channel 9 is arranged to carry as many speech circuits as possible at the higher rate and the use of the lower rate is spread amongst the circuits so as to minimise the degradation resulting from the lower bit rate digitization.

Each frame transmitted on the output channel 9 includes an address which indicates the destination of the information, the address including the aircraft ID and the terminal within the aircraft to which the message is to be directed.

The communication station carried on board each aircraft is substantially identical to that illustrated in Figure 2. The aircraft receives the signal transmitted from the ground station on a line 10 and the incoming signal is applied to a demultiplexer 11. This monitors the aircraft ID codes and demultiplexes those frames intended for terminals on board that particular aircraft onto four speech circuits 12. The frames on the lines 12 are then routed at 13 to the appropriate terminals on the aircraft via codecs 14 which convert the digitised speech into audible form. Each line from the routing circuit 13 includes two codecs, one of which decodes the higher rate format and the other, the lower rate format.

The transmission path for speech from the aircraft to the ground station is similar to that for the transmission of speech upwards to the aircraft. Again, the arrangement includes eight input lines 15A, 15B ... 15H for the reception of speech signals which are passed via speech detectors 16 to codecs 17 for digitisation at two different rates. The outputs of the codecs 17 are applied to an allocation circuit 18 which distributes the applied input signals to four speech circuits 19. The signals carried by the speech circuits 19 are combined by a multiplexer 20 onto a single output channel 21.

The transmission from the aircraft is received by the ground station on line 22 and applied to a demultiplexer 23. The information is demultiplexed onto four speech circuits 24 at the ground station and passed to a routing circuit 25 which sends them to appropriate locations depending on the addresses attached to each frame. The digitised signals are translated into audible analogue form by codecs 26 on the output lines of the routing circuit 25.

The individual aircraft in communication with the ground station have no means of allocating time slots amongst themselves for transmission of their messages.

The ground station is arranged to control the transmissions from the aircraft. Each frame of information transmitted by an aircraft includes not only the speech data and the address to which that data must be sent but also the aircraft ID code and the number of active speech circuits which are being used by that aircraft. The number of active speech circuits is derived from the allocation circuit 18 on board the aircraft and is added via line 27 to the multiplexer 20 for transmission along channel 21. The aircraft ID is also added from its ID store 28.

When a signal is received from an aircraft by the ground station, the information regarding the number of speech circuits in use and the aircraft ID associated with that number of speech circuits is taken from frames of information on the speech circuits 24 in the receiving path of the ground station and applied to an aircraft allocation circuit 29. For each aircraft, the aircraft allocation circuit 29 determines the distribution of the active speech inputs to the aircraft's four speech circuits 19, if necessary, accepting signals from the lower bit rate codecs. It also determines the timing of frames transmitted from that aircraft with respect to transmissions from other aircraft so that the ground station does not become overloaded.Aircraft allocation information is added by the multiplexer 8 to each frame of speech information carried on the ground station output channel 9 together with the ID code of the aircraft for which the allocation information is intended. On reception of a frame of information from the ground station, the aircraft demultiplexer 11 passes the allocation information to an ID detector 30. If the aircraft ID is monitored, the allocation information is transmitted onwards along line 31 to the allocation circuit 18 to provide the necessary control.

If an aircraft temporarily ceases to require slots for communications downwards to the ground station, for example if a participant in a conversation pauses and there are no other active lines on that aircraft, communication cannot then be re-established by means of the aircraft allocation circuit 29 at a later time as it does not hold the ID codes of aircraft having no active lines. To enable communications to be re-established therefore, a re-entry slot is produced by the ground station at 32 and inserted onto the output channel 9 at suitable intervals. The re-entry slot is a frame containing a sequence of all the ID codes of aircraft having circuits established with the ground but not currently active in communicating in the downward direction. The "non-active" ID codes are derived from a comparator store 33. This compares the ID codes of those aircraft having currently active downwards communications with the ID codes of all aircraft in contact with the ground station to determine those aircraft which are not active. The re-entry slot acts as an invitation for those aircraft not in communication with the ground station to re-establish contact if required. Information contained within the re-entry slot is received by the aircraft and demultiplexed bydemultiplexerlionto a line 34 to an ID detector 35. The detector 35 monitors the ID codes contained within the re-entry slot and detects where that aircraft's own ID code is positioned in time in the slot.

This information is passed to a processing circuit 36 which also receives an input from the on-board allocation circuit 18. If it is necessary to re-establish downward communciation with the ground station because an input line has become active, this information is transmitted from the allocation circuit 18 to the processing circuit 36. Then, when the aircraft ID is detected within a reentry slot, the processing circuit 36 causes a burst of carrier to be inserted on the output channel 21 during the time slot designated as a response period to the re-entry slot. The carrier burst is timed to occur at a position within the response period which corresponds to the position of the ID code within the re-entry slot received from the ground station. The burst of carrier from that aircraft, and from any others, is detected at a detector 37 at the ground station. The detector 37 also receives the information contained in the transmitted re-entry slot. From the timing of the carrier bursts received and the positions of ID codes within the re-entry slot, the identities of those aicraft which wish to resume communication are established and passed to the aircraft allocation circuit 29.

In an alternative embodiment of the invention, a guard period is included between individual time slots transmitted from the ground station. If an aircraft wishes to re-establish communication with the ground station, it emits a carrier burst during the guard period.

The ground station detects this and generates a re-entry slot in response to which the aircraft wishing to resume communications replies.

Claims

1. A communication arrangement comprising a base station and a plurality of other stations in which transmissions from the plurality to the base station are controlled by the base station.

2. An arrangement as claimed in claim 1 wherein the timing of the transmissions from the plurality is controlled by the base station.

3. An arrangement as claimed in claim 1 or 2 wherein the other stations are mobile.

4. An arrangement as claimed in claim 3 wherein the mobile stations are located on board aircraft.

5. An arrangement as claimed in claim 3 or 4 wherein at least one of the mobile stations has a plurality of terminals.

6. An arrangement as claimed in any preceding claim wherein the base station includes a multiplexer for multiplexing a plurality of output signals for transmission to respective stations onto a single output channel for transmission.

7. An arrangement as claimed in claim 6 wherein the multiplexer implements time division multiplexing.

8. An arrangement as claimed in any preceding claim wherein transmissions between the base station and the other stations comprise speech signals.

9. An arrangement as claimed in claim 8 wherein the speech signals are digitised for transmission.

10. An arrangement as claimed in any preceding claim wherein the base station includes allocation means for receiving input signals on a plurality of input lines and distributing them onto output lines of the allocation means and means for multiplexing the signals on the output lines onto an output channel.

11. An arrangement as claimed in claim 10 wherein the number of input lines to the allocation means is greater than the number of its output lines.

12. An arrangement as claimed in claim 11 and including means for digitising the input signals on the input lines, each signal being digitised at two different rates and including means for selecting an appropriate rate for each signal to enable all input signals to be distributed to and carried by the output lines.

13. An arrangement as claimed in claim 10, 11 or 12 and including means at the base station for controlling the allocation of transmissions from the other stations to receiving circuits at the base station.

14. An arrangement as claimed in any preceding claim wherein at least one of the other stations includes allocations means for receiving input signals on a plurality of input lines and distributing them onto output lines of the allocation means and means for multiplexing the signals onto the output lines onto an output channel.

15. An arrangement as claimed in claim 14 wherein the operation of the allocation means is controlled by a control signal transmitted by the base station.

16. An arrangement as claimed in any preceding claim and including means for generating an entry time slot on the output of the base station which slot comprises an invitation for any of the other station to establish a communicaiton link with the base station.

17. An arrangement as claimed in claim 16 wherein each of the other stations includes means for responding to the entry time slot with an identification code.

18. An arrangement as claimed in any preceding claim and including means for generating a re-entry time slot on the output of the base station which slot includes identification data identifying those other stations not transmitting to the base station.

19. An arrangement as claimed in claim 18 and including means at each of the other stations for responding to a re-entry time slot including its identification data with a burst of carrier in a response time slot when it is necessary for that station to re-establish contact with the base station.

20. An arrangement as claimed in claim 19 wherein the burst of carrier occupies a position in the response time slot corresponding to the position of the station's identifying data in the re-entry slot.

21. A communication arrangement substantially as illustrated in and described with reference to the accompanying drawings.