WO1998010535A1 - Systeme de communications sans fil faisant appel a un multiplexage d'orientation du faisceau - Google Patents

Systeme de communications sans fil faisant appel a un multiplexage d'orientation du faisceau Download PDF

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Publication number
WO1998010535A1
WO1998010535A1 PCT/EP1996/003924 EP9603924W WO9810535A1 WO 1998010535 A1 WO1998010535 A1 WO 1998010535A1 EP 9603924 W EP9603924 W EP 9603924W WO 9810535 A1 WO9810535 A1 WO 9810535A1
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WO
WIPO (PCT)
Prior art keywords
time
station
base station
data
information
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Application number
PCT/EP1996/003924
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English (en)
Inventor
Karl Wagner
Brigitta Wagner
Original Assignee
Karl Wagner
Brigitta Wagner
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Karl Wagner, Brigitta Wagner filed Critical Karl Wagner
Priority to PCT/EP1996/003924 priority Critical patent/WO1998010535A1/fr
Priority to AU69878/96A priority patent/AU6987896A/en
Publication of WO1998010535A1 publication Critical patent/WO1998010535A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/14—Relay systems
    • H04B7/15—Active relay systems
    • H04B7/204—Multiple access
    • H04B7/2041—Spot beam multiple access
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
    • Y02D30/00—Reducing energy consumption in communication networks
    • Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks

Definitions

  • the invention is concerned with wireless communications systems, particularly digital communications networks.
  • Wireless communications systems have been in use for a long time for a variety of uses, principally for two classes of applications: fixed communications links and mobile communications systems.
  • Fixed links come in principally two flavours: terrestrial radio links and satellite radio links. Both serve as a low cost replacement for cables.
  • Satellite systems for fixed links principally come in two flavours: systems for linking ground stations that are at a large distance from each other and systems that are relatively close to each other.
  • the former corresponds to the classical configuration: a satellite with a small, wide aperture antenna and ground stations with large high gain antennas. Due to the large antenna required such systems are relatively expensive.
  • Satellite systems due to the fact that relatively large antennas are needed for useful bandwidths, only play a marginal role, particularly in the marine communications field.
  • antenna beam width of an antenna of a given area is also inversely proportional to the operating frequency.
  • the result is that the area being illuminated by said antenna is much smaller than the area to be served by said central station station.
  • the central station has packets to transfer to the individual stations and the individual stations packets to be transferred to the central station.
  • the antenna at the central station is an antenna the beam direction of which may be steered, e. g. a phased array antenna.
  • the central station may point its beam towards a station to which it has a packet to transfer, emit its packet, subsequently direct its beam towards the next station, emit the next packet, and so on until all packets have been delivered, and then start the whole process all over again in the next cycle.
  • the beam of the central station antenna hops from one station to the next, the central station may service an area as large as desired, the service area size being completely independent from the beam aperture of the antenna, although only one antenna for transmission is used.
  • transmission capacity has to be dynamically assigned as required.
  • Each end user station then has to indicate each time it transfers a packet whether and what size of packet it will transfer next time. It may also indicate the size of packets for a number of cycles beforehands or request not to be polled for a number of subsequent cycles, depending on the traffic situation it experiences. Provided with this information the central station sets up a scanning schedule for the next cycle and allots time slots to each of the stations with the next packets it downlinks to them, thereby also taking link delay into account.
  • Synchronisation and path delay measurement in general are done in the following way: Each time the central station sends a packet to an end user station it adds the result of the last cycle time delay measurement as well as the time the packet was sent. When the user station receives this packet, it time stamps the packet with the time of arrival. Then it uses the information about time of transmission and link delay contained in said packet to calculate the time indication of the central station at the time the packet was received. It then compares this value with the time stamp it provided itself at the time of arrival of said packet, and, should there be a difference, adjusts its own time scale accordingly.
  • the end user station When sending a packet to the central station, the end user station provides in this packet the time when the packet was sent as well as the time it received the last packet or the time that has already passed between reception of the last packet and the transmission of the current packet. At the time this packet arrives at the central station, it is time stamped. The central station now compares this time stamp with the one of the last packet it sent to said station and determines the time difference. Then it takes the information provided by the end user station in its last packet and subtracts the time that passed between the reception of the last packet at this station and the emission of the current packet received from it. The result is the round trip delay. Dividing this by two provides link delay to this particular station. Next time a packet is sent down to said end user station this value together with the time of transmission of said packet is included in it.
  • the base station is equipped with transmission and reception facilities that are independent from each other, i. e. the ransmitter as well as the receiver have either separate, independently steerable antennas and operate in different frequency bands. They may also operate with a combined antenna system, which is actually two antenna systems integrated into one and with independently steerable beams for reception and transmission.
  • a complete communications cycle that is, a cycle in which each user station exchanges data with the base station, may be subdivided in one section of "user stations send, base station receives" and a subsequent section "base station sends, user stations receive” or vice versa, the whole being carried out in the same frequency band.
  • a user station wants to log in, it sends its identification code as well as a time stamp to the central station.
  • the base station now requests the preparation of a time slot from the main communication unit and gets it assigned. Now the respective time slot data as well as the delay between reception of the request packet and the acknowledge packet are sent to the user station. Login communications then is terminated.
  • the user station switches into main communications mode. It receives packets from the base station and replies with short packets at the middle of its assigned transmit time slot.
  • the base station exactly determines the position of these packets with respect to its assigned time slots and sends the values back to the user station.
  • the user station uses these values to calculate link delay and thus to exactly position its packets, and, as soon as the time position at the base station is correct, starts sending full length packets containing actual user information.
  • each base station carries a separate beam steerable login antenna of typically a tenth of the linear dimensions of the main communications antenna.
  • the total area to be serviced by this antenna is subdivided into cells of the width of the beamwidth of this antenna. These cells are continuously scanned with the antenna.
  • the end user station there are two possibilities: the simpler one is that it has a conventional antenna, in which case it simply waits for a signal from the base station.
  • the more complex one is that the ground station also has a steerable beam antenna. If such a station wants to log into the network, it also scans the sky. The entire hemispherical area then is subdivided into cells with the beamwidth of the end user steerable beam antenna. Scanning now the area beyond the grazing angle cell by cell, it stops when it finds a signal.
  • both types of user stations Upon receipt of a signal from the base station both types of user stations turn their transmitter on and send their identification code plus a time stamp indicating the actual user station time cyclically for a predefined period of time.
  • the base station login receiver When the base station login receiver detects this signal, it logs the station identification and the time stamps. After the last identification and time stamp block has been received, the base station send a signal back.
  • This signal contains the last time stamp received from the user station, the delay until the base station sent an answering signal, its own time stamp indicating the begin of its own transmission as well as other organizational data. Furthermore, the signal is superposed with a low intensity ( ten to twenty decibles less ) PN - 2 - PSK signal. The origin time stamp of the PN signal is also included in the back signal from the base station.
  • the data received from the base station enable the user station to coarsely determine the round trip delay towards the base station. This information is used to calculate the occurrence of the origin of the PN signal at the user station. Presetting the PN correlator with this data, the correlator is capable of quickly locking onto the PN signal. After having locked the user station replies with a signal that is also PN modulated, indicating the time difference of the origin of its own signal with respect to the origin of the signal received from the base station. It also sends the results of the coarse round trip delay measurement. Using this, the base station PN correlator quickly can lock onto the uplinked PN signal. Having locked in, an exact round trip delay value can be determined.
  • the login unit requests putting the new active station into queue at the main antenna beam control and user access management unit. Now a time slot for transmission of the user station as well as a time slot for transmission of the base station is defined. After having received these data, the login unit downlinks them to the user access station. Then control is passed to the main communications facility at the base station.
  • a node control station in the system collects all this information and gains an overview of of the loading distribution in the system. From this it determines which station needs what percentage of the total available transfer time and accordingly constructs a transmission sequence schedule.
  • This transmission sequence schedule is sent to all nodes in the system together with a send time stamp.
  • the send time stamp serves for synchronizing all nodes in the system to the time scale of the node control station.
  • a control cycle is a complete round of data exchange operations, in which every station in the network sends and receives data one after the other.
  • a transmit permit is passed from one node to the next, it is passed circularly around, such a cycle constituting a data transfer cycles.
  • the individual time slots in such a cycle are different and adjusted on command of a transmission sequence schedule instruction from the node control station.
  • a transmission sequence schedule is sent. Only and if this packet is received correctly by a node transmission is continued according to the new instructions. If the reception of said packet is incorrect, transmission is stopped until the next correctly received transmission sequence schedule packet is received. This latter property is essential to avoid that nodes send with a different time schedule which could lead to systematic packet collisions with consequential communications breakdown in the system.
  • the power supply system constitutes a substantial part of the total mass of any satellite.
  • a reduction in power requirement directly translates into a lighter satellite, particularly, because any energy consumed by the satellite is converted into heat by a large fraction. This heat has to be removed by a cooling system, another considerable weight factor in a satellite. If power consumption of a satellite can be reduced, the cooling system also becomes smaller.
  • a typical satellite is powered by solar panels and a storage accumulator bridging times during which the satellite is in the earth's shadow. It also helps in making peak power for short periods of time available to the satellite, in other words, it levels out power consumption as seen by the solar panels.
  • Figure 1 shows a block diagram of the login unit
  • FIG. 2 shows the operation of the login unit
  • FIG. 3 shows a block diagram of the operations unit
  • Figure 4 shows a block diagram of the input data multiplexer
  • Figure 5 shows a block diagram of the output data demultiplexer
  • Figure 6 shows a block diagram of system control
  • FIG. 7 shows the operation of the time slot administation transmission
  • FIG. 8 shows the operation of the time slot administation reception
  • Figure 9 shows the operation of the transmit / receive collision avoidance at subscriber station
  • Figure 10 shows the operation of dither control
  • FIG. 11 shows a block diagram of the RF system
  • Figure 12 shows a block diagram
  • FIG. 13 shows the operation of the login unit
  • Figure 14 shows a block diagram of the input data multiplexer
  • Figure 15 shows a block diagram of the output data demultiplexer
  • Figure 16 shows a block diagram of system control 3.2. Satellite based subscriber access system
  • Figure 17 shows a block diagram of the login unit
  • Figure 18 shows the general layout of the login system
  • FIG. 19 shows the operation of the doppler compensation in the login unit
  • FIG. 20 shows the operation of the login unit
  • FIG. 21 shows a block diagram of the operations unit
  • Figure 22 shows a block diagram of the input data multiplexer
  • Figure 23 shows a block diagram of the output data demultiplexer
  • Figure 24 shows a block diagram of system control
  • FIG. 25 shows the operation of the time slot administation transmission
  • FIG. 26 shows the operation of the time slot administation reception
  • Figure 27 shows the operation of the transmit / receive collision avoidance at subscriber station
  • Figure 28 shows the operation of dither control
  • Figure 29 shows a block diagram of the RF system
  • Figure 30 shows an orbital arrangement with similar orbits all intersecting in one point
  • Figure 31 shows an orbital arrangement with different orbits intersecting at different points
  • Figure 32 shows the operation of the dynamic satellite turn on and turn off process
  • Figure 33 shows the operations associated with the reactivation of satellites.
  • Figure 34 shows a block diagram
  • Figure 35 shows a block diagram of the login unit
  • FIG. 36 shows the operation of the login unit
  • Figure 37 shows a block diagram of the input data multiplexer
  • Figure 38 shows a block diagram of the output data demultiplexer
  • Figure 39 shows a block diagram of system control 3.3.
  • a multistation wireless communications network
  • Figure 40 shows an example of a network topology
  • Figure 41 shows a block diagram
  • Figure 42 shows a block diagram of the input data multiplexer
  • Figure 43 shows a block diagram of the output data demultiplexer
  • Figure 44 shows a block diagram of system control
  • Figure 45 shows a block diagram
  • Figure 46 shows a block diagram of the input data multiplexer
  • Figure 47 shows a block diagram of the output data demultiplexer
  • Figure 48 shows a block diagram
  • Figure 49 shows a block diagram of the input data unit
  • Figure 50 shows a block diagram of the output data unit
  • Figure 51 shows a block diagram of system control
  • Figure 52 shows a block diagram
  • Figure 53 shows a block diagram of the input data multiplexer
  • Figure 54 shows a block diagram of the output data demultiplexer
  • Figure 55 shows a block diagram of the node control system
  • Figure 56 shows the operation of the time slot data administration as carried out by the node control system
  • the invention draws heavily on digital packet protocols and routing methods typical for digital communications systems. However, they are only dealt with as much as is necessary to render the operation of the embodiments clear and insofar as they touch the invention.
  • the wireless local loop system described in the following serves to link a multitude of subscribers scattered around a central point to said central point. It is further data transparent, that is, any data fed into the subscriber line will come out in the same way at the central point, without modification. This allows the system to be used in any thinkable protocol environment, be it ISDN, ATM or anything else.
  • the embodiment described here uses adjusts channel capacity to actual requirements.
  • a system that assigns a fixed channel capacity, that is fixed time slot widths, to each channel without any demand adjustment is also possible and would be simpler. It is, however, less efficient.
  • fixed channel width transfer protocols e. g. ISDN
  • the base station - generally speaking - is made up of an operations section handling normal communications operations and a login section accepting and handling login requests of stations having been inactive so far.
  • the login section serves to enable subscriber stations, which have been inactive so far, to set up an active communications link to the base station. Its block diagram is shown in Figure 1.
  • the login section may also send out a call to a subscriber station and request a particular subscriber station to issue a login request. This is necessary if there is an incoming communications request towards a particular subscriber station ( incoming call ) .
  • the antenna system is a large aperture low gain system. Together with the associated receiver it is designed in a way that the direction of an incident signal can be determined immediately ( mono pulse approach ) .
  • Transmitter and receiver are conventional, narrow band units.
  • the login communications unit handles login operations, accepts information received from subscriber stations, sends related reply information and communicates with system control.
  • Figure 2 shows the sequence of actions during a login at the base station. Explanation starts in the top left corner, where the login system waits for a login request packet. If such a packet is received, the time of reception and the angle of incidence are determined. Then the station identification code extracted from it. Using the station identification a time slot pair is requested from system control. If one is available, it is assigned. Otherwise a "no channel available" message is returned. In this latter case, a reply packet indicating non availability of a communications time slot pair is sent to the user station. Then communications is terminated.
  • system control If, however, a time slot is available, system control returns the time slot timing and duration information. This information, together with the time the request packet was received and the time the reply packet is to be sent are entered into the reply packet. This packet then is sent.
  • the operations section is made up of an input data multiplexer, an output data multiplexer, a system control, the beam controls for transmission and reception as well as the radio unit ( transmitter, receiver and antenna system ). Its block diagram is shown in Figure 3.
  • the input data multiplexer funnels a multitude of input data channels together and brings them into a form so that they can be sent out.
  • the processed data then are fed into the transmitter and sent out by the antenna system.
  • the output data demultiplexer accepts the data received by the receiver and the antenna system, processes them and splits them into the individual output channels.
  • the system control controls all operating sequences in the base station. It supplies time frame information for transmission and reception. It also administates beam control operations.
  • the beam controls control the beam directions of the antenna system in a way that the beam lobes of the transmit as well as the receive antenna system always point to the subscriber station to be addressd at the moment.
  • the antenna system is a system in which beam direction for transmission and reception can be controlled separately. This may be done either by diplexer elements at the individual antenna elements or by means of separate antenna systems for transmission and reception.
  • Transmitter and receiver are largely conventional and without any peculiarities.
  • the input data multiplexer of the base station appropriately processes the data coming from a plurality of input channels and funnels them together for transmission. Its block diagram is shown in Figure 4.
  • the data from the input channels first are fed into a unit that collects the data stream during defined time intervals, transforming it into raw packets. In a next step various control information is added to these raw packets.
  • time slot information for next packet to be sent by subscriber station ( time slot position and length, from system control )
  • the thus completed raw packets then are fed into the packet buffers, provided with ECC/CRC information and entered into the buffer queue.
  • the packet buffers receive acknowledge and error information from the output data demultiplexer. They serve to remove packets acknowledged by the subscriber station from the buffer queue. If packets sent out are not acknowledged within a predefined time span they are repeated.
  • the filling level of the individual buffers is constantly reported to system control.
  • System control therefrom determines the capacity requirements of the individual channels and correspondingly assigns narrower or wider time slots.
  • the completed packets now are available at the transmit multiplexer. It is completely under control of system control, selecting a packet from the selected packet buffer and feeding it in serialized form into the transmitter at high speed.
  • the block diagram of the output data demultiplexer is shown in Figure 5. It gets a - more or less continuous - stream of data packets from the receiver. They are first read into a buffer. From there they are fed into a checking unit checking the packets for errors and correcting them, if required. If this is impossible, they are discarded. Next the addresses is extracted and the time of reception is determined. Then various control information is extracted. They are:
  • This information serves to determine propagation delay
  • This information serves to exactly point the beam of the receive antenna to the subscriber station
  • This information serves to exactly point the beam of the transmit antenna to the subscriber station
  • This information indicates which packets have been correctly received by the subscriber station. Acknowledged packets are removed from the transmit queue. If transmitted packets are not acknowledged within due time, they are retransmitted.
  • This information serves to determine time slot width for the individual subscriber stations according to the respective traffic requirements.
  • the remaining "payload information" of the individual packets now is fed into the receive demultiplexer. It in turn feeds them - under control of the adress extraction stage - to the respective output channels.
  • System control handles all operational sequences in the system. Broadly speaking these are time slot administration and associated control operations as well as controlling the beam headings of the antenna system. Its block diagram is shown in Figure 6.
  • a central unit of system control is the central clock. This is a highly stable quartz crystal based clock generator. It should have a short term stability of 0.1 ppm or better.
  • the central clock controls the adress cycler which cycles through the addresses of the various registers. It is in fact a kind of real time clock that constantly provides system time.
  • the time slot data register transmission and time slot data register reception contain the time slot information needed for controlling the transmit multiplexer of the input data multiplexer and the receive demultiplexer of the output data demultiplexer, respectively.
  • the time slot data registers are organized in the form of a list in which the time positions of the start and the end of a transmission / reception are related to a subscriber station identification / channel identification (this is in the case a subscriber station runs more than one channel ) . In response to actual system time provided by the adress cycler these registers output the subscriber station identification belonging to the time slot the addresss cycler is pointing to.
  • the subscriber / channel identification provided by the time slot data register transmission is directly fed into the transmit multiplexer of the input data multiplexer.
  • the transmit multiplexer thus takes a packet from the selected packet buffer, serializes it and sends it at high speed to the radio set and from there to the respective subscriber station.
  • the data output by the time slot data register reception serve to check if the incoming packets arrive at exactly the scheduled time positions. Should there be any deviation, a correction value is calculated and sent to the respective subscriber station to enable it to fine tune its time scale.
  • the time slot data registers are controlled by two time slot data administration units. There are two, one for transmission, another for reception.
  • the time slot data administration transmission gets information from the input data multiplexer about the filling level of the various packet buffers. It determines from these values the transfer requirements of the respective channels and calculates therefrom the time slot width for the respective channels. The results then are written into the time slot data register transmission.
  • the time slot data administration transmission further obtains information from the login communications unit about subscriber stations wishing to start active communications and assigns them a transmit time slot. At the same time the received direction information is forwarded to the beam control data register transmission.
  • a calculation of the required packet lengths is carried out using the buffer size and login data. After having done this for the complete set of logged in stations, all packet lengths are summed up and compared with the total available transmit cycle. If the result is bigger, the packet lengths are adjusted until the sum fits into a complete transmit cycle.
  • a list of logged in subscriber stations / channels versus time slot data ( time position, length ) is created.
  • This list is now submitted to the transmit / receive collision avoidance at the subscriber stations to check whether there are any transmit / receive overlaps at any of the subscriber stations. Thsi must be avoided as the subscriber stations are equipped with a transmit / receive switch for simplicity reasons and cannot transmit and receive simultaneously. If there are any such overlaps, the transmit / receive collision avoidance at the subscriber stations suitably changes the sequence in the time slot data list. If no overlaps are detected, it changes nothing. Finally the list is sent to the time slot data register transmission and the beam steering data register transmission. The new schedule is put into effect at the beginning of the next transmit cycle.
  • the time slot data administration transmission then returns to checking all the packet buffers of the input data multiplexer for their filling level.
  • the time slot data administration reception gets information from the output data demultiplexer about thetraffic requirements of the subscriber stations. It determines from these values the transfer requirements of the respective channels and calculates therefrom the time slot width for them. The results then are sent to the packet buffers input data multiplexer to be sent to the subscriber stations.
  • the time slot data administration reception further obtains information from the login communications unit about subscriber stations wishing to start active communications and assigns them a receive time slot. At the same time the received direction information is forwarded to the beam control data register reception.
  • a list of logged in subscriber stations / channels versus time slot data ( time position, length ) is created.
  • This list is now submitted to the transmit / receive collision avoidance at the subscriber stations to check whether there are any transmit / receive overlaps at any of the subscriber stations. This must be avoided as the subscriber stations are equipped with a transmit / receive switch for simplicity reasons and cannot transmit and receive simultaneously. If there are any such overlaps, the transmit / receive collision avoidance at the subscriber stations suitably changes the sequence in the time slot data list. If no overlaps are detected, it changes nothing. Now a check is carried out whether there have been any successful login requests at the beginning of the sequence of actions. If any are detected, their respective data are sent to the login unit to be sent to the stations starting active communications.
  • the list is sent to the input data multiplexer to be sent to the subscriber stations logged in. It is also sent to the time slot data register reception and the beam steering data register reception. The new schedule is put into effect at the beginning of the next transmit cycle. The time slot data administration reception then returns to checking the traffic status information of the subscriber stations.
  • the two time slot data administration units are controlled by a transmit/receive collision prevention unit in a way that there is never an overlap of a transmit and a receive time slot at a subscriber station. This is required, because the subscriber stations use a transmit/receive switch.
  • the transmit / receive collision avoidance at subscriber station proceeds to comparing the two lists and checking if there are any overlaps between transmission and reception at any of the subscriber stations. If any are detected, the order of thetiraing sequences in the lists are interchanged so long until all overlaps have been removed. Then the reordering loop is left.
  • the transmit / receive collision avoidance at subscriber station After having done so, the transmit / receive collision avoidance at subscriber station returns to waiting for requests of either the time slot data administration transmission or the time slot data administration reception to carry out a schedule check.
  • the two beam control data registers contain the direction data of the individual subscriber stations needed for directing the beam lobes of the antenna system onto the subscriber stations. To eliminate possible deviations the dither controls are required. They constantly move the beam lobes around the exact heading. By constantly comparing the measured field strength with the respective beam heading deviations from the correct heading can be detected and eliminated.
  • the purpose of the dither controls is to move the beam lobes around the exact direction towards each subscriber station, collect the associated field strength values and use them in conjunction with the known properties of the steerable beam antennas to determine whether the beam heading values in the beam steering data registers are still correct, and, if there are any deviations, to correct them.
  • the dither table contains three dither vectors. They all have the same size, but a different direction. Each of them has an angle of 120 degrees to each other ( in fact any other arrangement with more vectors and different angles might also be used, but would be more complicated ) .
  • the next value from the dither table is added to the respective value in the beam steering data register, until all values in the dither table have been used. After that, the whole action is repeated.
  • the associated field strength value is collected and written into the dither table into the column of the associated dither value. There is, in fact, a row in the dither table for each subscriber station logged in.
  • the associated field strength values for each station can be taken and used to determine the necessary corrections to be entered into the beam steering data register.
  • the dither value associated with the cycle count is read from the dither table.
  • the dither value is then added to the beam heading value taken from the beam steering data register and the antenna beam lobe is set to the thus calculated direction.
  • the associated field strength value is read in. In case of the reception system, this value comes directly from the receiver, in case of the transmission system, it is a value returned by the associated subscriber station. In any case, the associated value is written into the appropriate position in the respective dither table.
  • the cycle count is incremented by one and the dither control waits for the next access to that particular subscriber station.
  • there are two dither controls one for transmission and one for reception. Their operation is, however, completely identical.
  • the block diagram of the RF system of the base station is shown in Figure 11. It consists of a largely conventional transmitter and receiver as well as a transmit and a receive antenna the beams of which can be controlled electronically ( phased array ) . As a peculiarity it should be mentioned that the demodulator should be designed to enable it to synchronize rapidly on an incoming signal.
  • QPSK is provided as its modulation, however, for extremely high data rates QAM may be used in order to save bandwidth.
  • any known design may be used: phased arrays, arrays of directional antennas where the antenna beaming in the required direction is activated etc.
  • a subscriber station is either designed for one subscriber or a number of them ( apartment block, company ) .
  • It is made up of a login unit, an input data multiplexer, an output data demultiplexer, a system control as well as a radio unit ( transmitter, receiver and antenna system ) .
  • the input data multiplexer funnels a multitude of input data channels together and brings them into a form so that they can be sent out.
  • the processed data then are fed into the transmitter and sent out by the antenna system.
  • the output data demultiplexer accepts the data received by the receiver and the antenna system, processes them and splits them into the individual output channels.
  • the system control controls all operating sequences in the base station. It supplies time frame information for transmission and reception.
  • Login control is activated if a communications link to the base station is to be established. In order to do so, it sends a login request. Due to the fact that login communications is carried out with low bandwidth, the login unit switches the radio unit to low bandwidth mode. It then carries out the login data exchange with the base station and forwards the login data to system control and passes control to it. The radio unit then is reset to wideband mode.
  • Figure 13 shows the sequence of actions at the user station side.
  • the user station wants to log in at the base station it sends a login request packet containing its station identification to the login unit of the base station. It then waits for a reply. If no reply is received within a certain time, the request is repeated.
  • the time slot assignment data are extracted from the reply packet.
  • the time of reception of the login request packet at the base station is extracted from the reply packet.
  • the time of transmission of the reply packet at the base station is extracted from the reply packet.
  • the propagation delay between the user station and the base station is calculated. This is done in the following way: First, the time when the login request packet was sent and when the reply packet from the base station was received are deducted from each other. This yields the time span between login request and reply from the base station. In order to obtain the round trip delay time between subscriber station and base station the time span between time of reception of the login request packet and the time of transmission of the reply packet at the base station must be deducted. In order to obtain this value, the time of reception of the login request packet and the time of transmission of the reply packet at the base station are deducted from each other. Then the resulting round trip delay time is divided by two to obtain the propagation delay between subscriber station and base station.
  • a relationship between the time bases of the subscriber station and base station can be established. Then the exact time position of the receive slot of the base station, as seen from the user station is determined. Then main communications is started. Now a short packet is sent at the center of the reception time slot of the base station. As the base station always sends deviation information between the assigned time slot and the actual arrival of a packet, the subscriber station now waits until the arrival of such a packet from the base station. After having received the reply from the base station, the deviation information contained therein is used to finely correct the timing relationship between the base station and the local time bases. Then final communications for actual data transfer is started.
  • the input data multiplexer of the subscriber station appropriately processes the data coming from a plurality of input channels and funnels them together for transmission. Its block diagram is shown in Figure 14.
  • the data from the input channels first are fed into a unit that collects the data stream during defined time intervals, transforming it into raw packets. In a next step various control information is added to these raw packets.
  • the thus completed raw packets then are fed into the packet buffers, provided with ECC/CRC information and entered into the buffer queue.
  • the packet buffers receive acknowledge and error information from the output data demultiplexer. They serve to remove packets acknowledged by the subscriber station from the buffer queue. If packets sent out are not acknowledged within a predefined time span they are repeated.
  • the filling level of the individual buffers is constantly reported to system control.
  • System control therefrom determines the capacity requirements of the individual channels and communicates it to the base station.
  • the completed packets now are available at the transmit multiplexer. It is completely under control of system control, selecting a packet from the selected packet buffer and feeding it in serialized form into the transmitter at high speed.
  • the block diagram of theoutput data demultiplexer is shown in Figure 15. It gets a - more or less continuous - stream of data packets from the receiver. They are first read into a buffer. From there they are fed into a checking unit checking the packets for errors and correcting them, if required. If this is impossible, they are discarded. Packets not addressd to the local subscriber station are also discarded. Next the addresses is extracted and the time of reception is determined. Then various control information is extracted. They are:
  • System control handles all operational sequences in the system. Its block diagram is shown in Figure 16. In the context of the subscriber station, these are relatively few, in fact providing real time clock, implementing the transmit time slot schedule and determining the traffic requirements .
  • a central unit of system control is the central clock. This is a highly stable quartz crystal based clock generator. It should have a short term stability of 0.1 ppm or better.
  • the central clock controls the adress cycler which cycles through the addresses of the time slot data register transmission. It is in fact a kind of real time clock that constantly provides system time.
  • the time slot data register transmission controls the transmit multiplexer of the input data multiplexer. It contains the time slot information required for this.
  • the time slot data register is organized in the form of a list in which the time positions of the start and the end of a transmission are related to a channel identification in the case a subscriber station runs more than one channel. In response to actual system time provided by the addresss cycler this register outputs the channel identification belonging to the time slot the addresss cycler is pointing to.
  • the channel identification provided by the time slot data register transmission is directly fed into the transmit multiplexer of the input data multiplexer.
  • the transmit multiplexer thus takes a packet from the selected packet buffer, serializes it and sends it at high speed to the radio set and from there to the respective subscriber station.
  • the data output by the time slot data register reception serve to check if the incoming packets arrive at exactly the scheduled time positions. Should there be any deviation, a correction value is calculated and sent to the respective subscriber station to enable it to fine tune its time scale.
  • the time slot data register transmission is controlled by the time slot data administration unit transmission.
  • This unit receives actualized transmit time window data from the base station and feeds them into the time slot data register transmission. It also stops transmission if for some reason the time slot window data - which are to arrive at regular intervals - do not arrive as they are to be expected until another time slot window data set can be received. This is crucial to avoid using an outdated timing scheme that could cause interference with other stations.
  • the traffic demand determination or short demand determination receives data about the filling levels of the individual packet buffers. It determines capacity requirement data for the individual channels and forwards this value to the base station.
  • the clock signal of the central clock is also used by the output data demultiplexer to determine the time of reception values of the individual packets.
  • the RF system subscriber station is designed similarly as the one of the base station, but has additionally a narrow band mode for login. 4.2.
  • a satellite based communications system
  • the satellite based communications system described in the following serves to link a multitude of subscribers scattered on the earths surface to a satellite.
  • Much like the ground based wireless local loop system just set out it is data transparent, that is, data fed into it one one end will come out on the other end unaffectedly.
  • the system can be used in any thinkable protocol environment, be it ISDN. ATM, or anything else.
  • the satellite based communications system is made up of a network of orbiting satellites, that are all connected with each other with radio inter satellite link systems. They each carry an onboad switching system that can either link a particular subscriber station with another one linked to the same satellite or - using the inter satellite link system - with a subscriber station linked to another satellite. There is also the possibility to link a subscriber into the conventional ground based communications infrastructure via gateway stations. These gateway stations are designed in exactly the same way as subscriber stations, except that they are connected with the ground based global telecommunications infrastructure.
  • the embodiment described here uses adjusts channel capacity to actual requirements.
  • a system that assigns a fixed channel capacity, that is fixed time slot widths, to each channel without any demand adjustment is also possible and would be simpler. It is, however, less efficient.
  • fixed channel width transfer protocols e. g. ISDN
  • the base station - generally speaking - is made up of an operations section handling normal communications operations and a login section accepting and handling login requests of stations having been inactive so far.
  • the login section serves to enable subscriber stations, which have been inactive so far, to set up an active communications link to the base station. Its block diagram is shown in Figure 17.
  • Figure 18 shows a base station scanning an area for stations wishing to log in with its login / search steerable beam antenna as well as a subscriber station scanning all the sky above a predefined grazing angle for a signal from a base station. Scanning is done in a sector - wise manner, that is, the total service area angular area of the login / search antenna is subdivided into individual sectors or cells the width of the beamwidth of the login / search antenna. Scanning is done by stepping from one sector to the next, until all sectors have been scanned, and then starting the same action all over again, thus constantly scanning the service area of the antenna.
  • the transmit and receive frequency of the radio set is varied in dependence of the pointing direction of the login antenna in order to alway provide the same transmit and receive frequency on the ground.
  • the service area of the antenna is subdivided in sectors or cells the beamwidth of the unit's antenna, whereby the service area is all the area above the grazing angle.
  • the base station scans with a certain speed and takes a certain time to complete a scan of its entire area, the subscriber station advances its beam from one sector to the next in that time. This is so in order to ensure that the beam of the subscriber station antenna stays pointed long enough onto the same spot that it gets at least one scan of the base station antenna onto it if it happens to point into the right direction.
  • FIG 17 shows the block diagram of the login unit in the base station.
  • the login system is a dedicated one and serves for login purposes exclusively. It is made up of a login communications unit that controls the login process itself. It is connected with a login radio set, a PN correlator, a PN sequencer and a login antenna beam steering unit.
  • the radio set is made up of a transmitter, a receiver, a diplexer and a beam steerable login antenna.
  • the beam direction of the login antenna is controlled by the beam steering unit.
  • There is also a doppler compensator that gets beam heading data from the beam steering unit as well as orbital data from system control and calculates therefrom doppler shift.
  • the doppler shift value is entered into the transmitter and the receiver and controls these units so that the apparent operating frequency at the subscriber station is everywhere the same.
  • the login communications unit also has a link to the main communications system that takes over when login operations are finished.
  • the base station as well as the subscriber station use beam steerable antennas and scan their respective areas of service.
  • the satellite station preferably does this at a high speed and sends a beacon signal all the time.
  • the subscriber station does this at a much lower speed, preferably one step per full scan of the base station antenna.
  • the subscriber station gets a signal from the base station it stops scanning and starts transmitting a login request signal.
  • This signal is repeated cyclically for a time span that is longer than the time span the base station login unit needs to scan its entire service area. This is necessary because at the time the ground station receives a signal from the base station the base station antenna already points somewhere else due to the long propagtation delays inherent in satellite links.
  • the only chance for the ground station to be heard is to transmit long enough that the base station detects its signal during its scan operation. As soon as the base station detects a signal it stops scanning and tries to recover information from the signal. Then information is exchanged to finally establish an active communications link.
  • the time span the subscriber stations sends is - within certain limits - determined stochastically. This is required, because, if there is more than one station wishing to login in the same sector they jam each other's transmission. If, however, their transmissions are of a length determined partly by chance and thus different, the one that happens to have the longest transmission can be decoded and gets a chance to login.
  • Figure 19 shows the operations carried out in the satellite base station for providing doppler shift compensation. Explanation starts on top.
  • the process starts by checking whether beam direction has been changed recently. If this is the case, the new beam direction is read in and the resulting slant speed vector calculated. Then the actual position over ground, the subsatellite point is calculated. From this the radial speed vector resulting from the earth rotation is determined. Now, using a vector addition between the slant speed vector and the radial earth rotation speed vector is calculated. After having done so the doppler deviation can be determined and the radio set instructed to change its frequency accordingly.
  • Figure 20 shows a complete login process as carried out by the base station login system. Explanation starts at top left.
  • the whole service area of the login antenna is scanned for login signals with the transmitter constantly on to allow subscriber stations to fix base station direction. However, if the presence of a signal is detected, the scanning is suspended and the beam stays in the respective sector. Now the packet being received is time stamped. Then the time of transmission and the subscriber station identification are extracted. This sequence of action is repeated until the transmission from the subscriber station ends.
  • the base station login transmitter now prepares a reply message into which it puts the identification code of the subscriber station, the last time stamp recovered from the subscriber station transmitter, its own time stamp at which the login request packet was received, the origin of the PN signal superimposed to the signal to be transmitted, as well as the time of transmission of the reply packet. Then the packet is sent and a PN signal superimposed onto the transmitted signal is applied.
  • the subscriber station transmitter starts sending a reply.
  • the coarse round trip delay time value evaluated by the subscriber station is extracted.
  • This value is used to preset the PN correlator which now attempts to lock onto the PN signal superimposed on the signal sent by the subscriber station.
  • the origin of the PN signal as received at the base station can be determined.
  • the time offset data between the origin of the PN signal as received onboard the subscriber station and the PN signal sent out by the subscriber station is read, from the uplinked signal and the exact round trip delay is calculated.
  • the receive beam is dithered and the exact beam heading determined.
  • the time slot data are sent to the login unit for transmission to the subscriber station.
  • the login unit adds the precise round trip delay data and the sends the information to the subscriber station. Then it terminates communications with the subscriber station as the main communications unit takes over .
  • the login unit now listens if there are more stations wishing to login in the same sector. If within a predetermined time none are heard the login unit returns to scanning its service area to detect other stations wishing to log in.
  • the operations section is made up of an input data multiplexer, an output data multiplexer, a system control, the beam controls for transmission and reception, the radio unit ( transmitter, receiver and antenna system ) , the onboard switching system, the interlink system and its associated interlink antenna system as well as the active / standby control unit. Its block diagram is shown in Figure 21.
  • the input data multiplexer funnels a multitude of input data channels together and brings them into a form so that they can be sent out.
  • the processed data then are fed into the transmitter and sent out by the antenna system.
  • the output data demultiplexer accepts the data received by the receiver and the antenna system, processes them and splits them into the individual output channels.
  • the system control controls all operating sequences in the base station. It supplies time frame information for transmission and reception. It also administates beam control operations.
  • the beam controls control the beam directions of the antenna system in a way that the beam lobes of the transmit as well as the receive antenna system always point to the subscriber station to be addressd at the moment.
  • the antenna system is a system in which beam direction for transmission and reception can be controlled separately. This may be done either by diplexer elements at the individual antenna elements or by means of separate antenna systems for transmission and reception.
  • Transmitter and receiver are largely conventional and without any peculiarities.
  • the onboard switching system principally serves the same system as switching systems do in any ground based communications infrastructure: it routes a data channel from a source to a destination. This may be a subscriber on the same satellite having connection to another subscriber on it, it may also happen that only the source or the destination is on the same satellite, or that they both are linked ot another satellite and are simply routed through in order to get together ( through traffic routing / switching ) .
  • the interlink system and its associated interlink antenna system serve to link the satellite to adjacent satellites in the satellite network, either to route communications links originating ( or ending ) at itself to other source /destination satellites or to link satellites that have no direct contact with each other.
  • the active / standby control serves a particular purpose peculiar to satellite networks: due to the laws of celestial mechanics all satellite orbits must have the center of the earth in their orbital plane. This results in the fact that the subsatellite points of any system of orbits intersect somewhere, in other words, an equal distribution of satellites that the subsatellite points at any time are evenly distributed is not attainable. There will always be the situation that in some areas satellite subsatellite points are far away from each other and that at others there is a surplus of them. In this latter areas it is possible to "thin out" subsatellite points by shutting down some of the satellites, thus saving onboard power consumption. As the latter is always at a premium, this is clearly desirable.
  • the input data multiplexer of the base station appropriately processes the data coming from a plurality of input channels and funnels them together for transmission. Its block diagram is shown in Figure 22.
  • the data from the input channels first are fed into a unit that collects the data stream during defined time intervals, transforming it into raw packets. In a next step various control information is added to these raw packets.
  • time slot information for next packet to be sent by subscriber station ( time slot position and length, from system control )
  • the thus completed raw packets then are fed into the packet buffers, provided with ECC/CRC information and entered into the buffer queue.
  • the packet buffers receive acknowledge and error information from the output data demultiplexer. They serve to remove packets acknowledged by the subscriber station from the buffer queue. If packets sent out are not acknowledged within a predefined time span they are repeated.
  • the filling level of the individual buffers is constantly reported to system control.
  • System control therefrom determines the capacity requirements of the individual channels and correspondingly assigns narrower or wider time slots.
  • the completed packets now are available at the transmit multiplexer. It is completely under control of system control, selecting a packet from the selected packet buffer and feeding it in serialized form into the transmitter at high speed.
  • the block diagram of the output data demultiplexer is shown in Figure 23. It gets a - more or less continuous - stream of data packets from the receiver. They are first read into a buffer. From there they are fed into a checking unit checking the packets for errors and correcting them, if required. If this is impossible, they are discarded. Next the addresses is extracted and the time of reception is determined. Then various control information is extracted. They are:
  • This information serves to determine propagation delay
  • This information serves to exactly point the beam of the receive antenna to the subscriber station
  • This information serves to exactly point the beam of the transmit antenna to the subscriber station - Acknowledge information ( to the packet buffer of the input data multiplexer )
  • This information indicates which packets have been correctly received by the subscriber station. Acknowledged packets are removed from the transmit queue. If transmitted packets are not acknowledged within due time, they are retransmitted.
  • This information serves to determine time slot width for the individual subscriber stations according to the respective traffic requirements.
  • the remaining "payload information" of the individual packets now is fed into the receive demultiplexer. It in turn feeds them - under control of the adress extraction stage - to the respective output channels.
  • System control handles all operational sequences in the system. Broadly speaking these are time slot administration and associated control operations as well as controlling the beam headings of the antenna system. Its block diagram is shown in Figure 24.
  • a central unit of system control is the central clock. This is a highly stable quartz crystal based clock generator. It should have a short term stability of 0.1 ppm or better.
  • the central clock controls the adress cycler which cycles through the addresses of the various registers. It is in fact a kind of real time clock that constantly provides system time.
  • the time slot data register transmission and time slot data register reception contain the time slot information needed for controlling the transmit multiplexer of the input data multiplexer and the receive demultiplexer of the output data demultiplexer, respectively.
  • the time slot data registers are organized in the form of a list in which the time positions of the start and the end of a transmission / reception are related to a subscriber station identification / channel identification (this is in the case a subscriber station runs more than one channel ) . In response to actual system time provided by the adress cycler these registers output the subscriber station identification belonging to the time slot the addresss cycler is pointing to.
  • the subscriber / channel identification provided by the time slot data register transmission is directly fed into the transmit multiplexer of the input data multiplexer.
  • the transmit multiplexer thus takes a packet from the selected packet buffer, serializes it and sends it at high speed to the radio set and from there to the respective subscriber station.
  • the data output by the time slot data register reception serve to check if the incoming packets arrive at exactly the scheduled time positions. Should there be any deviation, a correction value is calculated and sent to the respective subscriber station to enable it to fine tune its time scale.
  • the time slot data registers are controlled by two time slot data administration units. There are two, one for transmission, another for reception.
  • the time slot data administration transmission gets information from the input data multiplexer about the filling level of the various packet buffers. It determines from these values the transfer requirements of the respective channels and calculates therefrom the time slot width for the respective channels. The results then are written into the time slot data register transmission.
  • the time slot data administration transmission further obtains information from the login communications unit about subscriber stations wishing to start active communications and assigns them a transmit time slot. At the same time the received direction information is forwarded to the beam control data register transmission.
  • Fig. 25 The detailed operation is shown in Fig. 25. Explanation starts at the loop at top center. There the time slot data administation transmission checks all the packet buffers of the input data multiplexer for their filling level. It then checks whether there have been any login requests from the login unit. If there are any, the transmit schedule list is checked whether there is any space available for additional time slots. If there is none available, the login unit is informed that a login is not possible at the moment. The time slot data administration transmission then returns to checking all the packet buffers of the input data multiplexer for their filling level.
  • a calculation of the required packet lengths is carried out using the buffer size and login data. After having done this for the complete set of logged in stations, all packet lengths are summed up and compared with the total available transmit cycle. If the result is bigger, the packet lengths are adjusted until the sum fits into a complete transmit cycle.
  • a list of logged in subscriber stations / channels versus time slot data ( time position, length ) is created.
  • This list is now submitted to the transmit / receive collision avoidance at the subscriber stations to check whether there are any transmit / receive overlaps at any of the subscriber stations. Thsi must be avoided as the subscriber stations are equipped with a transmit / receive switch for simplicity reasons and cannot transmit and receive simultaneously. If there are any such overlaps, the transmit / receive collision avoidance at the subscriber stations suitably changes the sequence in the time slot data list. If no overlaps are detected, it changes nothing. Finally the list is sent to the time slot data register transmission and the beam steering data register transmission. The new schedule is put into effect at the beginning of the next transmit cycle.
  • the time slot data administration transmission then returns to checking all the packet buffers of the input data multiplexer for their filling level.
  • the time slot data administration reception gets information from the output data demultiplexer about thetraffic requirements of the subscriber stations. It determines from these values the transfer requirements of the respective channels and calculates therefrom the time slot width for them. The results then are sent to the packet buffers input data multiplexer to be sent to the subscriber stations.
  • time slot data administration reception further obtains information from the login communications unit about subscriber stations wishing to start active communications and assigns them a receive time slot. At the same time the received direction information is forwarded to the beam control data register reception.
  • Fig. 26 The detailed operation is shown in Fig. 26. Explanation starts at the loop at top center. There the time slot data administation reception checks the status information of the subscriber stations which is provided by the output data demultiplexer. It then checks whether there have been any login requests from the login unit. If there are any, the receive schedule list is checked whether there is any space available for additional time slots. If there is none available, the login unit is informed that a login is not possible at the moment. The time slot data administration reception then returns to checking the traffic status information of the subscriber stations.
  • a list of logged in subscriber stations / channels versus time slot data ( time position, length ) is created.
  • This list is now submitted to the transmit / receive collision avoidance at the subscriber stations to check whether there are any transmit / receive overlaps at any of the subscriber stations. This must be avoided as the subscriber stations are equipped with a transmit / receive switch for simplicity reasons and cannot transmit and receive simultaneously. If there are any such overlaps, the transmit / receive collision avoidance at the subscriber stations suitably changes the sequence in the time slot data list. If no overlaps are detected, it changes nothing. Now a check is carried out whether there have been any successful login requests at the beginning of the sequence of actions. If any are detected, their respective data are sent to the login unit to be sent to the stations starting active communications.
  • the list is sent to the input data multiplexer to be sent to the subscriber stations logged in. It is also sent to the time slot data register reception and the beam steering data register reception. The new schedule is put into effect at the beginning of the next transmit cycle.
  • the time slot data administration reception then returns to checking the traffic status information of the subscriber stations. 4.2.1.3.3.3.3.
  • the transmit / receive collision avoidance at subscriber station
  • the two time slot data administration units are controlled by a transmit/receive collision prevention unit in a way that there is never an overlap of a transmit and a receive time slot at a subscriber station. This is required, because the subscriber stations use a transmit/receive switch.
  • Fig. 27 The detailed operation is shown in Fig. 27. Explanation starts on top left.
  • the transmit / receive collision avoidance at subscriber station waits for requests of either the time slot data administration transmission or the time slot data administration reception to carry out a schedule check. If there is any, it proceeds to calculating a list of arrival time slots at the subscriber station from the transmit schedule list provided by the time slot data administration transmission and a list of transmission time slots at the subscriber station from the receive schedule list provided by the time slot data administration reception.
  • the transmit / receive collision avoidance at subscriber station proceeds to comparing the two lists and checking if there are any present or future overlaps between transmission and reception at any of the subscriber stations. If any are detected, the order of the timing sequences in the lists are interchanged so long until all overlaps have been removed. Then the reordering loop is left.
  • the transmit / receive collision avoidance at subscriber station After having done so, the transmit / receive collision avoidance at subscriber station returns to waiting for requests of either the time slot data administration transmission or the time slot data administration reception to carry out a schedule check.
  • the two beam control data registers contain the direction data of the individual subscriber stations needed for directing the beam lobes of the antenna system onto the subscriber stations. To eliminate possible deviations the dither controls are required. They constantly move the beam lobes around the exact heading. By constantly comparing the measured field strength with the respective beam heading deviations from the correct heading can be detected and eliminated. To determine the precise transmit beam heading the transmission of the field strength values by the subscriber stations is required. In case of the receive beam heading the required values are directly available.
  • the beam control data registers also get attitude data from the attitude sensor. Thus, if the satellite changes attitude, the beam directions of the antennas are automatically corrected, so that they always point in the right direction, independent of any attitude changes.
  • the purpose of the dither controls is to move the beam lobes around the exact direction towards each subscriber station, collect the associated field strength values and use them in conjunction with the known properties of the steerable beam antennas to determine whether the beam heading values in the beam steering data registers are still correct, and, if there are any deviations, to correct them.
  • the dither table contains three dither vectors. They all have the same size, but a different direction. Each of them has an angle of 120 degrees to each other ( in fact any other arrangement with more vectors and different angles might also be used, but would be more complicated ) .
  • the next value from the dither table is added to the respective value in the beam steering data register, until all values in the dither table have been used. After that, the whole action is repeated.
  • the associated field strength value is collected and written into the dither table into the column of the associated dither value. There is, in fact, a row in the dither table for each subscriber station logged in.
  • the associated field strength values for each station can be taken and used to determine the necessary corrections to be entered into the beam steering data register.
  • Fig. 28 The sequence of actions carried out for one subscriber station is shown in Fig. 28. Explanation starts at top left.
  • the cycle count value is checked. If it is larger than two, it is set to zero. Further, this means that all dither values have already been applies to this particular station and that a complete set of field strength values is available.
  • the dither control then proceeds to reading these three field strength values and to calculate the deviation of the beam direction from the correct value. Using this value it corrects the beam heading entry in the beam steering data register. It then proceeds to point C.
  • the dither value associated with the cycle count is read from the dither table.
  • the dither value is then added to the beam heading value taken from the beam steering data register and the antenna beam lobe is set to the thus calculated direction.
  • the associated field strength value is read in. In case of the reception system, this value comes directly from the receiver, in case of the transmission system, it is a value returned by the associated subscriber station. In any case, the associated value is written into the appropriate position in the respective dither table.
  • the cycle count is incremented by one and the dither control waits for the next access to that particular subscriber station.
  • the block diagram of the RF system of the base station is shown in Figure 29. It consists of a largely conventional transmitter and receiver as well as a transmit and a receive antenna the beams of which can be controlled electronically ( phased array ) . As a peculiarity it should be mentioned that the demodulator should be designed to enable it to synchronize rapidly on an incoming signal.
  • QPSK is provided as its modulation, however, for extremely high data rates QAM may be used in order to save bandwidth.
  • this may be a diplexer and two separate phase controller and power combiners at each element ( or subgroup of elements ) .
  • any known design may be used: phased arrays, arrays of directional antennas where the antenna beaming in the required direction is activated etc.
  • Figure 30 shows an orbital network with similar orbits and polar intersection of the orbital planes in a satellite communications network. For the sake of easier understanding, only two orbits are drawn. It is, however, evident to those skilled in the art that such an orbital network may be made up of just any number of of orbits and the common intersection point also needs not necessarily lie on the poles ( in fact, in a multi orbital system there may even be a multitude of intersection points, but for the sake of ease of understanding a system with only two intersection points has been selected ) .
  • the distance is simply the product of the maximum separation of the orbits, multiplied by the cosine of the latitude.
  • the distance between the remaining satellite must mot be greater than in the area where distance is largest, it becomes evident that a removal is possible in an area where - in this case latitudinal - distance is half of that at the equator. This means that the above cosine must become one half. The cosine becomes one half at a latitude of 60 degrees .
  • Figure 31 shows the more general case where there are dissimilar orbits with different orientations and different intersection points. For the sake of ease of understanding, only three are drawn. Hoever, it is easy to see, that there are intersections, too, and consequently areas where satellite density is higher than elsewhere. Her however, the algorithm for shutting down satellites is not so simple as in Figure 27. It depends on the individual orbits and their properties and has to be determined in detail for each individual arrangement of orbits.
  • Figure 32 shows the process in the network necessary to decide the shutdown of satellites. Explanation starts on top.
  • the first step is to asess the present network topology.
  • area dependent parameters have to be entered into the topology data. These data are particularly data about end user density and terrain flatness. In areas where end user density is low and the terrain is flat, grazing angle and satellite density may be reduced without affecting service quality. Typical areas of this kind are the oceans .
  • the local traffic situation is analyzed. In heavily loaded areas it may be unwise to deactivate satellites, because that might increase traffic loading for the remaining satellites to unacceptable levels and in this case is to be avoided.
  • the congested areas are determined. In these areas the most closeup station configuration is sought for. Then it is decided whether deactivating a satellite allows a new network configuration with all ensuing distances below the predetermined maximum. If this is not possible the process proceeds towards the next area and carries out the above process there. If it is possible consequences of a deactivation on the loading situation are determined. If they are unacceptable, the process also branches towards the next area. If, however, this test also ends positively, said satellite is deactivated and the network in the area is restructured. Then the next possible candidate for deactivation is probed. If there are no more candidates in a particular area, the process branches inevitably at one of the decisions to the next area. There the same process is repeated. If all areas are cared for, the process returns to the top and starts from the beginning again.
  • Figure 33 shows the operations associated with the reactivation of satellites. Explanation starts on top, where the process monitors the areas containing satellites in standby mode. Again, like above, the area topology data are parametrized with the area parameters. Equipped with these data an area is analyzed for the distances between the satellites in it. If a distance exceeding the predetermined limit is found the inactive satellite in the area is reactivated and the network around it restructured. If everything remains within limits, nothing is changed. The process then proceeds to the next area. After having examined all areas, the process returns to the top and restarts from the beginning. 4.2.2. Subscriber station
  • a subscriber station is either designed for one subscriber or a number of them ( apartment block, company ) .
  • It is made up of a login unit, an input data multiplexer, an output data demultiplexer, a system control, a beam control as well as a radio unit ( transmitter, receiver and antenna system ) .
  • the input data multiplexer funnels a multitude of input data channels together and brings them into a form so that they can be sent out.
  • the processed data then are fed into the transmitter and sent out by the antenna system.
  • the output data demultiplexer accepts the data received by the receiver and the antenna system, processes them and splits them into the individual output channels.
  • the system control controls all operating sequences in the base station. It supplies time frame information for transmission and reception.
  • Login control is activated if a communications link to the base station is to be established. In order to do so, it sends a login request. Due to the fact that login communications is carried out with low bandwidth, the login unit switches the radio unit to low bandwidth mode. It then carries out the login data exchange with the base station and forwards the login data to system control and passes control to it. The radio unit then is reset to wideband mode.
  • FIG. 35 A block diagram of the login control of the subscriber station is shown in Figure 35. It consists of the login control system itself, a PN correlator and a PN generator. These latter units are connected to the receiver and the transmitter, respectively. They serve to echo the PN signal back to the base station to precisely determine round trip delay.
  • Figure 36 shows the corresponding operations in the subscriber station required for login.
  • the antenna After having received an instruction to log into the network, the antenna starts scanning the sky above the grazing angle for a base station beacon signal. A soon as such a signal is detected, the scan operation is stopped and the transmitter activated. The transmitter now sends the station identification code together with a time stamp indicating the time of transmission of each instance of a login request packet. This transmission is carried out for at least a time span required by the base station to perform a complete scan of its service area. Then the transmission is stopped.
  • the receiver now waits until echo information comes back from the base station. If no reply is detected within a predetermined time, the above sequence of events is repeated.
  • the reply is for the local station, it is time stamped. Then the transmission time stamp of the original login request as received by the base station and the receive time stamp of the base station are extracted. Then the time of transmission of the reply packet and the origin time stamp of the superimposed PN signal are retrieved. Using this data, the coarse round trip delay can be calculated and the PN correlator be preset. After having locked onto the signal the origin of the PN signal is determined and the PN generator of the subscriber station synchronized to it. After the base station transmission is terminated, the subscriber station sends a reply signal with superimposed PN signal. In this signal the obtained coarse round trip data and the origin of the superimposed PN signal are included.
  • the receiver After the end of this transmission, the receiver again waits for an answer from the base station. First, a check is done whether a "no channel available" message is contained in the reply packet. If this is the case, login communications is terminated. If this is not the case, the results from the exact round trip delay measurement are extracted and a relationship is established between the base station and local time bases. Then the transmit and receive time slot data for main communications are extracted. After this contact with the login system of the base station is terminated and active communications is started.
  • a short packet is sent at the center of the reception time slot of the base station.
  • the base station always sends deviation information between the assigned time slot and the actual arrival of a packet
  • the subscriber station now waits until the arrival of such a packet from the base station.
  • the deviation information contained therein is used to finely correct the timing relationship between the base station and the local time bases. Then final communications for actual data transfer is started.
  • the input data multiplexer of the subscriber station appropriately processes the data coming from a plurality of input channels and funnels them together for transmission. Its block diagram is shown in Figure 37.
  • the data from the input channels first are fed into a unit that collects the data stream during defined time intervals, transforming it into raw packets. In a next step various control information is added to these raw packets.
  • the thus completed raw packets then are fed into the packet buffers, provided with ECC/CRC information and entered into the buffer queue.
  • the packet buffers receive acknowledge and error information from the output data demultiplexer. They serve to remove packets acknowledged by the subscriber station from the buffer queue. If packets sent out are not acknowledged within a predefined time span they are repeated.
  • the filling level of the individual buffers is constantly reported to system control.
  • System control therefrom determines the capacity requirements of the individual channels and communicates it to the base station.
  • the block diagram of theoutput data demultiplexer is shown in Figure 38. It gets a - more or less continuous - stream of data packets from the receiver. They are first read into a buffer. From there they are fed into a checking unit checking the packets for errors and correcting them, if required. If this is impossible, they are discarded. Packets not addressd to the local subscriber station are also discarded. Next the addresses is extracted and the time of reception is determined. Then various control information is extracted. They are:
  • the remaining "payload information" of the individual packets now is fed into the receive demultiplexer. It in turn feeds them - under control of the adress extraction stage - to the respective output channels.
  • System control handles all operational sequences in the system. Its block diagram is shown in Figure 39. In the context of the subscriber station, these are relatively few, in fact providing real time clock, calculating distance, speed and direction towards the base station ( satellite ) , implementing the transmit time slot schedule, controlling beam direction and determining the traffic requirements.
  • a central unit of system control is the central clock. This is a highly stable quartz crystal based clock generator. It should have a short term stability of 0.1 ppm or better.
  • the central clock controls the adress cycler which cycles through the addresses of the time slot data register transmission. It is in fact a kind of real time clock that constantly provides system time.
  • the time slot data register transmission controls the transmit multiplexer of the input data multiplexer. It contains the time slot information required for this.
  • the time slot data register is organized in the form of a list in which the time positions of the start and the end of a transmission are related to a channel identification in the case a subscriber station runs more than one channel. In response to actual system time provided by the addresss cycler this register outputs the channel identification belonging to the time slot the addresss cycler is pointing to.
  • the channel identification provided by the time slot data register transmission is directly fed into the transmit multiplexer of the input data multiplexer.
  • the transmit multiplexer thus takes a packet from the selected packet buffer, serializes it and sends it at high speed to the radio set and from there to the respective subscriber station.
  • the data output by the time slot data register reception serve to check if the incoming packets arrive at exactly the scheduled time positions. Should there be any deviation, a correction value is calculated and sent to the respective subscriber station to enable it to fine tune its time scale.
  • the time slot data register transmission is controlled by the time slot data administration unit transmission.
  • This unit receives actualized transmit time window data from the base station and feeds them into the time slot data register transmission.
  • time slot window data - which are to arrive at regular intervals - do not arrive as they are to be expected until another time slot window data set can be received. This is crucial to avoid using an outdated timing scheme that could cause interference with other stations.
  • the time slot data administration unit transmission receives data concerning this from the orbital data computer. This data is used in order to correct the transmit time positions so that the packets sent out always hit correctly in the assigned receive time slot onboard the base station.
  • the orbital data computer obtains Kepler data from the base station and uses them to predict the distance changes between subscriber and base station. From this it calculates the changes in propagation delay. Due to the high speed of orbiting satellites this is required, because propagation delay changes sufficiently even between two packets that without such a measure there would be a severe mismatch between the assigned reception time slot at the base station and the actual arrival of the uplinked packet.
  • Another task of the orbital data computer is the calculation of the beam direction towards the satellite to track it with the beam steered subscriber station antenna.
  • the subscriber station may be onboard a vehicle or aircraft or even be used portably, and consequently subscriber station attitude may change any time, a steerable beam antenna is required. Unlike at the base station, it only serves to track one base station, and its steering speed need not be very high.
  • the beam control system obtains the initial direction information during the login process, during which it is under control of the login unit.
  • the beam control system receives angular position data of the satellite from the orbital data computer and attitude data from the attitude sensor.
  • the angular position data are used to correct beam direction as the satellites moves over the sky.
  • the attitude data are required to steer the antenna beam so that in case of attitude changes the antenna beam keeps being pointed to the base station.
  • the beam cotrol system also does slow dithering much in the same way as the base station to recognize any beam misalignments and to correct them. As only one antenna is used, it is sufficient to do this only for the reception direction, as the transmission direction will automatically be the same. 4.2.2.4.6. Attitude sensor
  • the attitude sensor serves to determine the attitude of the transmit / receive antenna. This is required in mobile or portable systems where attitude may change any time to keep the antenna beam lobes pointed onto the satellite. It may be of any known design, e. g. gyroscopic, using the Sagnac effect ( optical ) or the Coriolis effect ( mechanical /acoustical ) . However, as the Coriolis type sensors are easily and cheaply to fabricate and have excellent sensitivity, they may be preferred for cost reasons.
  • the traffic demand determination or short demand determination receives data about the filling levels of the individual packet buffers. It determines capacity requirement data for the individual channels and forwards this value to the base station.
  • the clock signal of the central clock is also used by the output data demultiplexer to determine the time of reception values of the individual packets.
  • the RF system subscriber station is designed similarly as the one of the base station, but has additionally a narrow band mode for login. 4.3.
  • a multistation wireless communications network
  • the multistation wireless communications network shown in Figure 40 described in the following serves to provide wireless network infrastructure over a wide are with very efficient radio spectrum usage nad a minimum of network routers to be passed in order to transfer information from one node to another. Furthermore it is intended to be very flexible and easily adaptable to varying needs, it can be easily modified and upgraded while staying fully operating.
  • FIG. 19 The principal layout of such a system is shown in Figure 19. There are three principal system components used in the network: star transponders, transfer nodes and end nodes.
  • the central points in the network are the star transponders. These are fully transparent transponders that receive any data sent to them and send them out as they have been received. They also may contain some network management functions that manage network operations in their immediate neighbourhood.
  • the star transponders in fact are the connecting device connecting the devices around them with each other: in fact, any device connected to the star transponder can receive anything any other device sends out. This allows quasi direct data transfers from any device around the transponder to any other one. There is no principal limitation to what kind or how many devices may use a star transponder. There may even be changes while the system is in full operation.
  • end node functionality into the transition node or into the star transponder. In the following detailed description this has been done. It is, however, optional.
  • FIG. 41 A block diagram of the end node is shown in Figure 41. It is made up of an input data multiplexer, an output data multiplexer, a system control, a radio transmitter, radio receiver as well as associated diplexer and antenna.
  • the input data multiplexer funnels a multitude of input data channels together and brings them into a form so that they can be sent out.
  • the processed data then are fed into the transmitter and sent out by the antenna system.
  • the output data demultiplexer accepts the data received by the receiver and the antenna system, processes them and splits them into the individual output channels.
  • the system control controls all operating sequences in the base station. It supplies time frame information for transmission, measures round trip delay to the star transponder and back and reports traffic status data to the central node control system.
  • Transmitter, receiver, diplexer and antenna are largely conventional and without any peculiarities.
  • the input data multiplexer of the end node appropriately processes the data coming from a plurality of input channels and funnels them together for transmission. Its block diagram is shown in Figure 42.
  • the data from the input channels first are fed into a unit that collects the data stream during defined time intervals, transforming it into raw packets. Then address information is added.
  • the thus completed raw packets then are fed into the packet buffers, provided with ECC/CRC information and entered into the buffer queue.
  • the packet buffers receive acknowledge and error information from the output data demultiplexer. They serve to remove packets acknowledged by the subscriber station from the buffer queue. If packets sent out are not acknowledged within a predefined time span they are repeated.
  • the filling level of the individual buffers is constantly reported to system control.
  • System control therefrom determines the capacity requirements of the individual channels and communicates it to the central node control system.
  • control information a separate dedicated channel is used. First, all the control information to be sent out is collected.
  • the source addresses and CRC/ECC information is added. Then the completed packet is fed into the buffer queue and is also available at the transmit multiplexer.
  • the transmit multiplexer is completely under control of system control. It selects a packet from each packet buffer, serializes it, and feeds it into the transmitter at high speed .
  • Transmission is done in a packet train manner, that is, they are sent out one after the other, thus using up the entire assigned time slot.
  • the block diagram of the output data demultiplexer is shown in Figure 43. It gets a - more or less continuous - stream of data packets from the receiver. They are first read into a buffer. From there they are fed into a checking unit checking the packets for errors and correcting them, if required. If this is impossible, they are discarded. Next the addresses is extracted and the time of reception is determined. Then various control information is extracted.
  • the remaining "payload information" of the individual packets now is fed into the receive demultiplexer. It in turn feeds them - under control of the adress extraction stage - to the respective output channels.
  • FIG 44 The block diagram of system control is shown in Figure 44. It handles all operational sequences in the system. In the context of the subscriber station, these are relatively few, in fact providing real time clock, implementing the transmit time slot schedule, measuring round trip delay time and determining the traffic requirements.
  • a central unit of system control is the central clock. This is a highly stable quartz crystal based clock generator. It should have a short term stability of 0.1 ppm or better.
  • the central clock controls the adress cycler which cycles through the addresses of the time slot data register transmission. It is in fact a kind of real time clock that constantly provides system time.
  • the time slot data register transmission controls the transmit multiplexer of the input data multiplexer. It contains the time slot information required for this.
  • the time slot data register transmission is organized as a list containing the start of the transmit time slot and the lengths of the packets on all channels to be sent out next.
  • the beginning of the assigned transmission time slot occurs it starts at the first packet buffer and gates out the number of bits equivalent to the length of the packet to be sent out. Having finished this, it proceeds to the next one, until a packet from each buffer has been taken and sent out. Then it stops and waits until the next transmit time slot occurs.
  • the time slot data register transmission is controlled by the time slot data administration unit transmission.
  • This unit receives actualized transmit time window data from the node control system and feeds them into the time slot data register transmission.
  • Round trip delay measurement is responsible for measuring round trip delay of packets sent out and nad being re- received.
  • the value obtained in fact is the double propagation delay towards the star transponder plus all the internal delays. As the latter are normally known, propagation delay can be calculated precisely. This value is used to calculate the local transmit time slot from the time slot assigned by the node control system.
  • Round trip delay measurement further receives time stamped packets from the node control system. This is used to establish a time relationship between the node control system and the local time base, this relationship data is also passed to the time slot data administration.
  • the round trip delay measurement unit receives time stamps and packet source addresses from the output data demultiplexer. It also receives time of transmission information and source addresses of packets that have already been sent out from the time slot administation trannsmission. In comparing these, the round trip delay measurement unit finds the corresponding packets sent out and then received back and subtracts their respective time stamps. The difference is the round trip delay plus the delays of the star transponder and the local system. As those delays are known, the round trip delay can be calculated. Dividing it by two yields the propagation delay to the star transponder.
  • the round trip delay measurement unit further is provided with the control packet from the central node control system.
  • This packet in itself contains a time stap indicating when it was sent out. Like any other packet, it is also time stamped at reception. Now deducting propagation delay from the reception time stamp, this value and the time stamp provided with the packet can be compared. They directly establish a relationship between the time base of the central node control system and the local system. This relationship information, together with the propagation delay information, is sent to the time slot data administration transmission and serves there to correctly establish the local trannsmit time slot.
  • the traffic demand determination or short demand determination receives data about the filling levels of the individual packet buffers. It determines capacity requirement data for the individual channels and forwards this value to the node control system.
  • the login sequencer controls the sequence of events when logging into the system.
  • the central node control system reserves a login time slot from time to time and announces this fact and its data in the control packets it sends out. Due to the fact that the network will normally be used for fixed terrestrial or inter satellite link purposes, it can be assumed that the location of all units are known. This means that the distance of the end node to its related star transponder is also known. A sufficiently precise value of the propagation delay for login purposes can easily derived from this value.
  • the round trip delay measurement unit uses this value to establish a time base relationship with the node control system's time base. Using these values, demand determination feeds its demand requirement data into the input data multiplexer and time slot data administration transmission calculates the correct login time slot data for transmission.
  • the node control system As soon as the node control system receives this request, it allocates a time slot to the new end node. As soon as this request packet is received back at the end node, the round trip delay measurement unit is able to precisely determine propagation delay and consequently the time base relationship with the node control system's time base.
  • the login process is then finished and normal communications are started.
  • the transfer node serves to transfer data from one star cluster to another. Its block diagram is shown in Figure 45. It is made up of a link set A connected with antenna A, which links the transfer node to one cluster, link set B, which is connected to antenna B links it to another cluster. Both link sets are connected with a router. The router in fact decides which part of the traffic flow in both clusters is to be transferred to the other side and carries the necessary actions out.
  • a transfer node certainly also can fulfil its basic functions . that is routing traffic from one cluster to another - without having any local connect facilities.
  • the block diagram of the input data multiplexer is shown in Figure 46. Operation of this unit is rather simple. Input data on all channels are collected during constant time intervals, then addresses and CRC/ECC information is added and the packets are fed into packet buffers. The transmit multiplexer takes the subsequent packet out of the respective packet buffer and sends it in serialized form to the router. It then proceeds to the next packet buffer, doing the same. All packet buffers are scanned in this way, when all have been scanned, the whole process starts all over again.
  • the block diagram of the output data demultiplexer of transfer node is shown in Figure 47. It receives a stream of packet from the router. It is first fed into a packet buffer, then the addresss is extracted. Then the packets are fed into the receive demultiplexer, which distributes it to the appropriate output channels using the addresss information extracted by addresss extraction.
  • the block diagram of the link set is shown in Figure 48. It is made up of an input data multiplexer, an output data multiplexer, a system control, a radio transmitter, radio receiver as well as associated diplexer and antenna.
  • the input data unit receives input data from the router and brings them into a form so that they can be sent out.
  • the processed data then are fed into the transmitter and sent out by the antenna system.
  • the output data unit accepts the data received by the receiver and the antenna system, processes them and forwards them to the router.
  • the system control controls all operating sequences in the link set. It supplies time frame information for transmission, measures round trip delay to the star transponder and back and reports traffic status data to the central node control system.
  • Transmitter, receiver, diplexer and antenna are largely conventional and without any peculiarities.
  • the input data multiplexer of the end node appropriately processes the data coming from the router and finally sends them to the transmitter. Its block diagram is shown in Figure 49.
  • Data from the router is first fed into the input data buffer.
  • the input data buffer receives acknowledge and error information from the output data unit. They serve to remove packets acknowledged by the subscriber station from the buffer queue. If packets sent out are not acknowledged within a predefined time span they are repeated.
  • the filling level of the input data buffer is constantly reported to system control.
  • System control therefrom determines the capacity requirements and communicates it to the central node control system.
  • the packets are now ready to be fed to the transmit cache.
  • control information a separate dedicated packet is used. First, all the control information to be sent out is collected.
  • the transmit cache is completely under control of system control. It inputs the control packet and as many other packets as fit into the assigned time slot and forwards it in serial way to the transmitter when the next transmit time slot occurs.
  • the block diagram of the output data unit is shown in figure 50. It gets a - more or less continuous - stream of data packets from the receiver. They are first read into a buffer. From there they are fed into a checking unit checking the packets for errors and correcting them, if required. If this is impossible, they are discarded. Next the addresses is extracted and the time of reception is determined. Then various control information is extracted.
  • FIG. 51 The block diagram of system control is shown in Figure 51. It handles all operational sequences in the system. In the context of the subscriber station, these are relatively few, in fact providing real time clock, implementing the transmit time slot schedule, measuring round trip delay time and determining the traffic requirements.
  • a central unit of system control is the central clock. This is a highly stable quartz crystal based and should have a short term stability of 0.1 ppm or better.
  • the central clock controls the adress cycler which cycles through the addresses of the time slot data register transmission. It is in fact a kind of real time clock that constantly provides system time. 4.3.2.3.3.2.
  • the time slot data register transmission controls the transmit multiplexer of the input data multiplexer. It contains the time slot information required for this.
  • the time slot data register transmission is organized as a list containing the start of the transmit time slot and its length.
  • the information about length of the transmit time slot is used to load as many of the packets in the input data buffer into the transmit cache as can be sent out within the assigned time slot.
  • the beginning of the assigned transmission time slot occurs the number of bits equivalent to the lengths of the packets to be sent out is gated out from the transmit cache. Then the time slot data register transmission stops and waits until the next transmit time slot occurs.
  • the time slot data register transmission is controlled by the time slot data administration unit transmission.
  • This unit receives actualized transmit time window data from the node control system and feeds them into the time slot data register transmission.
  • Round trip delay measurement is responsible for measuring round trip delay of packets sent out and being re- received.
  • the value obtained in fact is the double propagation delay towards the star transponder plus all the internal delays. As the latter are normally known, propagation delay can be calculated precisely. This value is used to calculate the local transmit time slot from the time slot assigned by the node control system.
  • Round trip delay measurement further receives time stamped packets from the node control system. This is used to establish a time relationship between the node control system and the local time base, this relationship data is also passed to the time slot data administration.
  • the round trip delay measurement unit receives time stamps and packet source addresses from the output data demultiplexer. It also receives time of transmission information and source addresses of packets that have already been sent out from the time slot administation trannsmission. In comparing these, the round trip delay measurement unit finds the corresponding packets sent out and then received back and subtracts their respective time stamps. The difference is the round trip delay plus the delays of the star transponder and the local system. As those delays are known, the round trip delay can be calculated. Dividing it by two yields the propagation delay to the star transponder.
  • the round trip delay measurement unit further is provided with the control packet from the central node control system. This packet in itself contains a time stap indicating when it was sent out.
  • the round trip delay measurement unit further is provided with the control packet from the central node control system.
  • This packet in itself contains a time stap indicating when it was sent out. Like any other packet, it is also time stamped at reception. Now deducting propagation delay from the reception time stamp, this value and the time stamp provided with the packet can be compared. They directly establish a relationship between the time base of the central node control system and the local system. This relationship information, together with the propagation delay information, is sent to the time slot data administration transmission and serves there to correctly establish the local trannsmit time slot.
  • the traffic demand determination or short demand determination receives data about the filling level of the input data buffer. It determines capacity requirements and forwards this value to the node control system. 4.3.2.3.3.5. Login sequencer
  • the login sequencer controls the sequence of events when logging into the system.
  • the central node control system reserves a login time slot from time to time and announces this fact and its data in the control packets it sends out. Due to the fact that the network will normally be used for fixed terrestrial or inter satellite link purposes, it can be assumed that the location of all units are known. This means that the distance of the end node to its related star transponder is also known. A sufficiently precise value of the propagation delay for login purposes can easily derived from this value.
  • the round trip delay measurement unit uses this value to establish a time base relationship with the node control system's time base. Using these values, demand determination feeds its demand requirement data into the input data multiplexer and time slot data administration transmission calculates the correct login time slot data for transmission.
  • the node control system As soon as the node control system receives this request, it allocates a time slot to the new end node. As soon as this request packet is received back at the end node, the round trip delay measurement unit is able to precisely determine propagation delay and consequently the time base relationship with the node control system's time base.
  • the star transponder is the central link in the communications network being described. Its block diagram is shown in Figure 52. Its main purpose is to retransmit any data it receives in a fully transparent way. It may additionally also be used to link subscriber channels into the network. Furthermore it also hosts conveniently the node control system controlling the sequence of actions of the cluster centered around the star transponder.
  • the star transponder is made up of an antenna, a diplexer, a transmitter, a receiver, a bit regenerator, a feed in and a feed out tee, an input data multiplexer, an output data multiplexer and the node control system.
  • the receiver is coupled over a bit regenerator, a feed in and a feed out tee coupled to the transmitter.
  • Receiver and transmitter are coupled over a diplexer to the antenna.
  • Transmitter, receiver, diplexer, bit regenerator, the feed out and feed in tees and antenna are largely conventional and without any peculiarities.
  • the input data multiplexer funnels a multitude of input data channels together and brings them into a form so that they can be sent out.
  • the processed data then are fed into the transmitter and sent out by the antenna system.
  • the output data demultiplexer accepts the data received by the receiver and the antenna system, processes them and splits them into the individual output channels.
  • Input data multiplexer and output data demultiplexer are facultative, they may be left away when local access is not required. Only the part needed for the node control system has to stay in this case.
  • the node control system controls all operating sequences in the star transponder as well as in the stations attached to it. It receives all traffic status data from all stations around the star transponder. It also receives login requests. Using all these data it determines the time slot data for all attached stations and broadcasts it. It also monitors all emissions for compliance with the determined schedule. Any deviations are immediately broadcast to notify the respective stations and help them eliminate them.
  • the input data multiplexer of the star transponder appropriately processes the data coming from a plurality of input channels and funnels them together for transmission. Its block diagram is shown in Figure 53.
  • the data from the input channels first are fed into a unit that collects the data stream during defined time intervals, transforming it into raw packets. Then address information is added.
  • the thus completed raw packets then are fed into the packet buffers, provided with ECC/CRC information and entered into the buffer queue.
  • the packet buffers receive acknowledge and error information from the output data demultiplexer. They serve to remove packets acknowledged by the subscriber station from the buffer queue. If packets sent out are not acknowledged within a predefined time span they are repeated.
  • the filling level of the individual buffers is constantly reported to the central node control system.
  • control information a separate dedicated channel is used. First, all the control information to be sent out is collected.
  • the source addresses and CRC/ECC information is added. Then the completed packet is fed into the buffer queue and is also available at the transmit multiplexer.
  • the transmit multiplexer is completely under control of system control. It selects a packet from each packet buffer, serializes it, and feeds it into the transmitter at high speed. Transmission is done in a packet train manner, that is, they are sent out one after the other, thus using up the entire assigned time slot.
  • the block diagram of the output data demultiplexer is shown in Figure 54. It gets a - more or less continuous - stream of data packets from the receiver. They are first read into a buffer. From there they are fed into a checking unit checking the packets for errors and correcting them, if required. If this is impossible, they are discarded. Next the addresses is extracted and the time of reception is determined. Then various control information is extracted.
  • the block diagram of the node control system is shown in Figure 55. Basically it is responsible for managing the time slots of all stations linked to the star transponder. It also monitors how well the actual arrival times of the respective packets correspond to the assigned time slots. If there are any deviations, their sizes and signs are sent out to the respective stations in order to be corrected.
  • a central unit of system control is the central clock. This is a highly stable quartz crystal based clock generator. It should have a short term stability of 0.1 ppm or better.
  • the central clock controls the adress cycler which cycles through the addresses of the time slot data register transmission. It is in fact a kind of real time clock that constantly provides system time.
  • the time slot data register transmission controls the transmit multiplexer of the input data multiplexer. It contains the time slot information required for this.
  • the time slot data register transmission is organized as a list containing the start of the transmit time slot and the lengths of the packets on all channels to be sent out next.
  • the beginning of the assigned transmission time slot occurs it starts at the first packet buffer and gates out the number of bits equivalent to the length of the packet to be sent out. Having finished this, it proceeds to the next one, until a packet from each buffer has been taken and sent out. Then it stops and waits until the next transmit time slot occurs.
  • the time slot data register transmission is controlled by the time slot data administration unit.
  • the time slot data administration is the central part of the node control system. It receives the traffic status data from all linked station via the output data demultiplexer. It also receives login requests. It determines therefrom a list of time slot data ( time positions and durations ) and broadcasts them so that all stations linked to the star node receive them and use them.
  • Figure 56 shows its operation in detail. Explanation starts at top left. There the buffer sizes in the input data demultiplexer are checked. Then the incoming traffic status data is read.
  • time slot list is checked for available space. If there is available space, a preliminary time slot is assigned. Now the sequence of events has arrived at point E. - Ill -
  • the required packet lengths are calculated. They are then all summed up and compared with the available total communications cycle. If the sum is bigger than the total cycle, the packet lengths are readjusted and summed up again. The sum then again is compared to the total cycle, and, if it is still bigger, the packet lengths are readjusted again. This is done until the sum fits into the available cycle.
  • the packet timing monitor unit gets all source addresses as well as their time stamps from the output data demultiplexer. It compares the time stamps with the respective time slot data from the time slot data administration. Any differences detected are forwarded to the time slot data administration which in turn sends them to the input data multiplexer to be broadcast to all stations linked to the star transponder to help them eliminate these differences.
  • geometrical topology can be effectively largely decoupled from network topology, allowing p ⁇ eudo static network topologies behaving much in the same way as terrestrial networks.
  • the above possibility is particularly useful if satellites are deactivated at congestion points, thus allowing a smooth takeover of the functionality of the deactivated satellites by those staying in operation.
  • Cycle count Cycle count + 1 At
  • Base station e. g. satellite

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  • Radio Relay Systems (AREA)

Abstract

Les systèmes de communications multipoint sans fil, qui fonctionnent à des fréquences élevées et sur des largeurs de bande étendues, nécessitent des antennes hautement directionnelles afin d'assurer un budget de liaison acceptable. Ces systèmes entrent en contradiction avec la nécessité de pouvoir étaler les stations individuelles sur une surface angulaire importante. Ce problème peut être résolu à l'aide d'un système maître-esclave dans lequel l'élément maître est équipé d'une antenne à faisceau orientable. Cette dernière va balayer chaque station esclave et échanger des données avec celle sur laquelle elle est pointée. Les systèmes de communication multipoint connaissent des retards de liaison dans l'ordre de magnitude des longueurs de paquets. Le problème qui se pose vient du fait que, lorsqu'une station émet, les autres stations ne peuvent le savoir avant que le retard de liaison entre elles et la station émettrice ne soit écoulé. Ainsi, lorsqu'une autre station émet pendant cet intervalle, il se produit alors une collision. Ce problème peut être résolu en faisant diffuser par chaque station ses exigences concernant les transferts suivants, ce qui permet au système d'établir un horaire d'émission global. Lorsque l'on utilise des satellites en orbite basse, la densité des satellites permettant d'assurer une couverture suffisante doit être calculée en fonction du point le plus éloigné du point d'intersection commun. On assiste ainsi à un surplus de satellites à proximité du point d'intersection. Le système décrit dans cette invention peut être utilisé afin de désactiver les satellites excédentaires, et d'économiser ainsi l'énergie embarquée. Dans de tels systèmes de satellites, les stations au sol individuelles possèdent chacune un décalage Doppler différent qui est fonction de leur position par rapport au satellite. Etant donné que l'on connaît déjà la position angulaire permettant de pointer le faisceau et d'assurer un fonctionnement correct de ces réseaux, cette dernière peut également être utilisée afin de déterminer la vitesse d'inclinaison et, partant, le décalage Doppler.
PCT/EP1996/003924 1996-09-06 1996-09-06 Systeme de communications sans fil faisant appel a un multiplexage d'orientation du faisceau WO1998010535A1 (fr)

Priority Applications (2)

Application Number Priority Date Filing Date Title
PCT/EP1996/003924 WO1998010535A1 (fr) 1996-09-06 1996-09-06 Systeme de communications sans fil faisant appel a un multiplexage d'orientation du faisceau
AU69878/96A AU6987896A (en) 1996-09-06 1996-09-06 Wireless communications system using beam direction multiplexing

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
PCT/EP1996/003924 WO1998010535A1 (fr) 1996-09-06 1996-09-06 Systeme de communications sans fil faisant appel a un multiplexage d'orientation du faisceau

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WO1998010535A1 true WO1998010535A1 (fr) 1998-03-12

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WO2012094127A1 (fr) * 2011-01-07 2012-07-12 Anue Systems, Inc. Systèmes et procédés permettant de produire précisément une variation de phase dans des signaux numériques
US8533518B2 (en) 2011-01-07 2013-09-10 Anue Systems, Inc. Systems and methods for precise timing measurements using high-speed deserializers
US8683254B2 (en) 2011-01-07 2014-03-25 Anue Systems, Inc. Systems and methods for precise event timing measurements
US8788867B2 (en) 2011-01-07 2014-07-22 Anue Systems, Inc. Systems and methods for playback of detected timing events

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US8850259B2 (en) 2011-01-07 2014-09-30 Anue Systems, Inc. Systems and methods for precise generation of phase variation in digital signals

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