MXPA98004633A - Method to improve the efficiency of the use of radio channels in areas of coverage that are transferred - Google Patents

Abstract

The present invention relates to a method for transmitting a signal in a wireless communication channel, the communication channel being common for primary and secondary communication systems having coverage areas that overlap at least partially, the primary system has a cycle of low work and transmits random instabilities that have a characteristic first duration, the method involves transmission instabilities in the secondary system, which has a duration at least equal to three times the first duration and contains forward error correction codes to allow that data contained in it be recovered in the event of a shock with an instability in the primary system, the instabilities in the secondary system have an intensity sufficiently lower than the intensity of the instabilities in the primary system to avoid interference with the primary system.

Description

METHOD TO IMPROVE THE EFFICIENCY OF THE USE OF RADIO CHANNELS IN AREAS OF COVERAGE THAT ARE TRANSFERRED This invention relates to a method and apparatus for improving the efficiency of the use of radio channels in overlapping coverage areas, for example, overlapping satellite rays. With the trend towards gradually smaller cells in cellular communication systems and the use of satellite beams in satellite systems, the coverage areas inevitably overlap. In such areas, the frequency can not be reused if the systems are sensitive to interference. For example »consider a primary system of many users distributed over a service area» transmitting information back in an individual channel to a common central receiving station. Assume that there is also a secondary communications system that wants to use the same communication channel »again from many users towards a common central receiving station, which has a coverage area that at least partially overlaps with the primary system. In order that the secondary system does not interfere with the primary system »conventionally under these circumstances the two systems would be different channel frequencies distributed. Since in case of satellite channels »the annual cost of bandwidth of 20 KHz can be more than one million dollars» this can be an inefficient solution when the load of the channels is not high. The increase in demand for wireless communication systems has made spectrum space an extremely valuable convenience. For mobile satellite services (SSM), this demand has supplied proposals for numerous second and third generation mobile satellite systems. The increase in demand for spectrum that can support the various wireless services has resulted in a desire to more efficiently use the existing spectrum. Clearly »the reuse of previously distributed spectrum without adversely affecting existing systems represents an increase in spectrum utilization and efficiency. Changes in the system design for the user have led to the deployment of satellite communications systems that have low antenna gain terminals and »with second and third generation satellite systems, high gain satellite tracking beams. As a consequence of terminal mobility, the orientation between the terminal and the satellite can change significantly and in short periods. An additional consequence of using a low gain antenna is that the frequency of reuse by other satellites and systems is severely limited. The transmissions of a low gain antenna terminal can be received by any satellite that covers the same frequency band and the same coverage area as an existing satellite system. Satellites and satellite systems that share a common spectrum may not share the same geographic coverage when serving terminals with low gain antennas. Transmissions of users of a first satellite system, although they are transmitted using a low antenna gain, may not be received by a second satellite system operating in the same frequency band if there are differences in the ground coverage of the antenna of satellite, and therefore do not interfere with users of the second satellite system. As a consequence of only partial overlap in geographical coverage between an existing satellite communication system and a second satellite communication system "a satellite communication channel and an existing satellite system that is widely used" in the sense that the channel is occupied for a fraction of time signi cat a »may seem to be lily charged for a second satellite system» in the sense that transmissions of users of the existing system only they are occasionally received by the second system. In order to increase efficiency in the use of responding bandwidth, many satellite communication systems operate in an environment of multiple access to allocated demand. The requested access channels are used by a communication system so that the subscribers are assigned satellite channel capacity for communications. For a randomly requested access channel »such as slotted ALOHA» message loss increases rapidly when the number of messages offered approaches the full load of the channel. As a result, the rated load of a grooved ALOHA channel is commonly maintained at approximately 20% of the full load. A requested access channel Aloha not slotted for similar reasons that for the slotted Aloha case, is designed to have a offered load of less than 0.1. The patent of E.U.A. Re. 32,905 discloses a satellite communication system in which spread spectrum means are incorporated to give a plurality of terminals, which characterize low gain antennas, to concurrently generate spread spectrum CDMA transmissions in the same spectrum as used in the existing systems, without interfering with users of the existing system. In addition »the system provides sufficient spectrum processing gain with its extended spectrum CDMA signal to substantially suppress the interference caused by transmissions from the existing system. As a consequence of whether processing gain, the system is capable of processing the CDMA extended spectrum transmissions generated concurrently by means of a plurality of terminals in the presence of interference of users of existing systems occupying the same spectrum, with a proportion of errors in the acceptable bits »and without adversely affecting the users of the existing system. The techniques described in the aforementioned patent can be applied for situations in which a substantially large processing gain can be achieved to substantially suppress the transmissions of the existing system. In the absence of sufficient gain of extended spectrum processing to remove the interference caused by the transmissions of users of an existing system, the approach provided in said patent is inefficient. A processing gain that is insufficient to remove the interference caused by users of existing systems may occur due to a limitation in the bandwidth for the extended spectrum signal "or a relatively high data rate index" so that the ratio of extended spectrum bandwidth to information index is too small to allow error proportions in the acceptable bits in the presence of transmissions of an existing system. An object of the invention is to overcome this problem in a way that does not interfere with the primary users but also provides acceptable service to the secondary users. In accordance with the present invention there is provided a method for transmitting a signal in a wireless communication channel said communication channel being common to primary and secondary communication systems having at least coverage areas that overlap at least partially. said primary system having a low duty cycle and transmitting random instabilities having a characteristic first duration "said method comprising transmission packages in instabilities in said second system» said instabilities in said second system having a duration at least equal to three times said first duration and containing forward error correction codes to allow the data contained therein to be recovered in the case of an impact with a stability in the primary system and said stabilities in the secondary system having a sufficient energy density below d e the intensity of the instabilities in the primary system to avoid interference with it. The operation can also be improved by using spacing information and channel status. Spacing is a procedure that scrambles the location of data symbols in a transmitted sequence in a predetermined manner and unravels the symbols to the appropriate order at the input of the receiver's FEC decoder. If a consecutive block of received symbols is corrupted due to interference, the decoder will cause these erroneous symbols to extend over time at the input of the FEC decoder. The symbols of time extension are much more tolerable for the decoder than a block of errors.

The quality of a received signal (or "channel state") can be used to the great advantage of the FEC decoder. Without information on channel status, the FEC deteriorates rapidly when the input error ratio exceeds 1%. With information on the status of the channel »and the dissemination of errors by means of the despacker, the FEC decoder can tolerate up to 30% of input error rate. In the case where the primary received signal (interference) is much stronger than the secondary (wanted) signal, a simple energy detector at the receiver output can provide the channel status information to the FEC, a large signal is equivalent to low desired signal quality. The overlapping coverage areas can be formed by overlapping satellite beams, although the invention can be applied to other situations in which the overlapping coverage areas occur, for example, in cellular radio. The duration of secondary instability commonly must be three times the duration of the primary instability and possibly up to ten times or more. Secondary instabilities are advantageously transmitted using extended spectrum techniques at a lower energy level than the background noise of the primary system. It is assumed that the primary and secondary systems each have their own service area, and at least there is partial overlap of these service areas. The users of these systems are distributed over their respective service areas. The primary system has many transmitters »but the overall duty cycle (or load) of transmissions on the selected channel is low. Both the primary and secondary users transmit in instabilities in the selected channel. With the help of forward error correction, and preferably channel spacing and status information, the decoder of a secondary user instability may occur if 1/3 of the stability has been corrupted by the simultaneous reception of a primary instability. . Although the system can be used to exploit in the case of overlap of 100% coverage, the system works best in the case of partial overlap. First, the ability of the secondary system to operate in the presence of the primary users must be considered. The work cycle of the primary instabilities in the channel is low. Because the service areas of the two systems only partially overlap, only a fraction of these primary user instabilities will be received in the secondary receiver »corresponding to the fraction of primary users" visible "to the secondary receiver. It is assumed that whenever a secondary user receives a primary instability, the received signal will be dominated (and therefore corrupted). The secondary receiver can tolerate up to 1/3 the time of overlap of primary instabilities in any instability, so the probability of irrecoverable corruption of a secondary instability becomes the probability that enough primary instabilities occur in a period of secondary instability to overlap more than 1/3 of the period. It is assumed that there are primary instabilities K per second of seconds in length L. The channel "on" in duty cycle for the primary system becomes K * L. It is further assumed that a fraction M of the primary instabilities is within the trace of the secondary receiver »so that K * L * M represents the fraction of time in which the primary instabilities are received by the secondary receiver. Secondary instabilities are assumed to be of length IM * L where l \ i > 3. The probability that more! \! * L / 3 of secondary instability overlaps by primary instability is given by means of: p.

As an example, primary instabilities of length L = 28 msec are assumed with K * L M = 0.02, and a length of secondary instability of N L = 0.5 seconds. The probability of more than 1S7 msec. of a secondary stability and overlap by primary instabilities is 3 x lO-ß. The second factor for channel sharing is the interference of secondary users in the primary system receiver. Secondary users are assumed to operate at an effective energy level that is sufficiently below that of the primary users so that no interference is below the noise level of the primary system. further, only a fraction of the secondary users will be received by the primary receiver, due to the limited overlap of the service areas. To continue the example "it is considered a satellite application" with the primary system being Inmarsat B "Atlantic ray" and the secondary system using the AMSC Eastern beam. The spectrum chosen for sharing is one of the Inmarsat access request channels »operated on slotted ALOHA (ie» random access to the channel of the request instabilities, with synchronized instabilities to start at specific times). The practical limit for the use of the slotted ALOHA channel is approximately 20% of the channel capacity. The connection footprint (received in satellite) of the AMSC Eastern beam overlaps approximately 10% of the Atlantic Inmarsat beam, so that the AMSC system would experience a binding interference of approximately 10% of the Inmarsat link transmissions. Thus the fraction of time in which the primary instabilities are received by means of the secondary receiver in this channel is 0.2 * 0.1 = 0.02% or 2% of the time, on average. Inmarsat B application instabilities are 28 ms long, while secondary system instabilities are 0.5 sec. long. The probability of irrecoverable corruption of secondary instability due to the primary instabilities received, as calculated previously, is 3 x 10- Inmarsat B transmitters operate with an EIRP that is 34 dB higher than that of the secondary system transmitters; however, since the secondary system uses multiple spectrum extended access with a nominal of 10 concurrent users, the energy of total secondary instability will be (on average) 24 dB lower than that of primary instability, which will be much lower than the level of noise of the primary receiver. The invention also provides a wireless communication system capable of sharing bandwidth with a primary system, said systems having at least coverage areas that overlap, said primary system having a low duty cycle and transmitting random instabilities having a first characteristic duration, said communication system comprising a plurality of terminals distributed to transmit packets in instabilities "said instabilities in said communication system having a duration at least equal to three times said first duration" and said packages containing error correction codes towards forward to allow data contained in them to be recovered in the case of a collision with an instability in the primary system "and said instabilities in said communication system having an intensity sufficiently lower than the intensity of the instabilities in the primary system to avoid interference with it. The invention will now be described in greater detail, by way of example, only with reference to the accompanying drawings, in which: FIG. 1 is a diagram showing overlapping satellite coverage areas; 0 Figure 2 shows a radio frequency pulse transmitted; Figure 3 is a functional block diagram of a satellite system; Figure 4 is a functional block diagram of a remote terminal; Figure 5 is a functional block diagram of a station on Earth; Figure G is a diagram showing the forward link TDM structure; ? Figure 7a is a diagram showing the structure CDMA grooved retraction joint; Figure 7b is a diagram showing the time control synchronization of the forward and backward link; Figure 8 is a block diagram of a second mode of a remote terminal; Figure 9 is a block diagram of a second mode of a station on Earth; Figure 10 illustrates the sleep clock synchronization algorithm for the remote terminal; and Figure 11 shows the synchronization algorithm of the local oscillator for the remote terminal. Referring now to Figure 1, the INMARSAT system covers a large area 1 and partially overlaps in the shaded area 2 with AMSC 3 rays. Traditionally, due to this overlap, the AMSC and INMARSAT satellites are needed to operate at different frequencies to avoid interference. This is a waste of bandwidth, especially when the INMARSAT system work cycle is smaller as it is in the request channels, the INMARSAT system uses separate random access request channels that are used when the client wishes to establish a call. The call itself is established in communication channels that have a much higher duty cycle and are generally not suitable for the willingness to share the channel proposed here. The effective work cycle of the INMARSAT request channels visible to the AMSC system is further reduced by virtue of the fact that requests originating away from the overlap area will not collide with messages originating within the AMSC system. Shocks will only occur when a request originates in or near the overlap area, thereby reducing the likelihood of shock to a much lower level than would be the case in the case of 100% overlap where all requests originating in the INMARSAT system would be seen by the AMSC system. The present system transmits data within basic units known as subpatterns as will be discussed in more detail below. The subpatterns, which are encoded CDMA »appear as instabilities of the radio frequency energy of O.5 sec. long. INMARSAT instabilities in the request channel are much shorter »in the order of 28 secs. Due to the low duty cycle in the request channel in the INMARSAT system and the partial overlap »which also reduces the effective duty cycle as it is visible for the AMSC system the probability that an instability INMARSAT collides with an AMSC instability is low so the chances are that the AMSC subpattern will be received without interference. However, in the case of an individual crash, due to the lengths that differ from the packets, only a small portion of the subpattern will be lost (see the shaded area in Figure 2) and this can be recovered using error correction techniques forward known. The probability that the packet will be lost beyond recovery due to the shock with INMARSAT instabilities with the help of FEC techniques is extremely low. To prevent AMSC signals from interfering with INMARSAT signals, the signal strength is typically maintained at more than 30 dB below the intensity of the INMARSAT signals. In one embodiment this is achieved using expanded spectrum techniques although other schemes are possible. It should be appreciated that the spread spectrum techniques themselves do not overcome the interference problem because the INMARSAT B request channel is only 20 KHz wide and the radio frequency instability when present fills the channel. In this way, it is not possible to obtain sufficient processing gain through the use of spread spectrum techniques alone to transmit data in the presence of the INMARSAT signal. The processing gain is only achieved with the expanded spectrum when a communication system contains a limited number of signal points, not when the interference signal covers the full amplitude of the channel. However, the use of expanded spectrum techniques allows the use of a low energy density signal, which can be recovered by narrowing in the receiver. This low level of signal ensures that in the case of a shock with an INMARSAT signal, the INMARSAT signal does not degrade, although during the collision the spread spectrum signal is actually lost. In this way, the spread-spectrum signal is not used to achieve processing gain, since it is not possible in the scenario contemplated by the invention, but instead ensures that the signal level is low enough that it does not occur. interference with the INMARSAT signal.

The satellite communication system shown in Figure 3 consists of a central station on Earth 11 that is connected over a wired connection 12, for example, through a public switched network »to a packet processing center 13» normally operated by means of a service provider, which in turn is connected in a wired connection path to a value-added retailer 14 and finally to one or more end clients 15. The processing center of package 13 includes a database 21 which stores the information pertaining to the subpatterns distributed in particular terminals in a way that is described in more detail below. The CPP 13 also stores lightning information when using multiple beam satellites. The station on Earth 11 is also coupled in the manner of satellite junctions 16, 17 to a geostationary multiple beam satellite IB that transmits the signals via junctions 19 to and from a plurality of remote terminals 20, commonly mounted on vehicles, such as helicopters, trailers, passenger cars and train cars. Satellite 18 allows coverage over a wide geographic area, such as, for example, all of North America. Satellite 18 can emit the entire coverage area although the techniques for directing the beam allow it to be subdivided if desired. It will be understood that the junctions 19 shown in Figure 3 represent point-to-point communications by virtue of the network access protocol to be described. The associated signals are emitted to a wide geographical area. The satellite communications link 17.19 originating from the station on Earth 11 and sent through a satellite 18 to terminals 20 'is referred to as the forward link. The satellite communications junction 19, 16 originating at the terminals 20 and sent through a satellite 18 to the station on Earth 11 »is referred to as the reverse junction. Those skilled in the art will recognize that the key elements of the present invention pertain to mobile and fixed terminals, to satellites in non-geostationary orbits and to terrestrial communication systems. The manner in which the forward binding transmission of the present invention is received and processed by means of the remote terminal is described with reference to Figure 4. A portion of the transmission energy sent by means of the geostationary satellite is captured. by means of an antenna 40 of the Antenna Unit 47 »and is applied to a bandpass filter 41 which rejects signals outside the desired frequency band. The transmissions passed by means of the bandpass filter 41 are applied to an amplifier 42 which is enabled by means of a Tx / Rx sense module 46 and connected by a coaxial cable 48 with a bandpass filter. 52 of the main electronic unit 50. The output signal of the bandpass filter 52 is supplied to a mixer 53 for downward conversion to a convenient intermediate frequency (Fl) by means of a frequency synthesizer 55 that is synchronized in phase to a local oscillator 64 and whose frequency is determined by a frequency controller 65. The function of the frequency controller 65 is provided by means of a microprocessor 57. The output of the mixer 53 is applied to a band pass filter Fl 54 for further reduce noise and electromagnetic interference near the frequency band of interest. The output of the bandpass filter 54 is applied to a quadrant detector 56 for downward conversion to the baseband by means of another output of the frequency synthesizer 55. The same phase (I) and the quadrature phase (C) 59 leaves the quadrature detector 68 supplied to analog-to-digital (CAD) converters 60. The digitized CAD signals 60 are converted into binary coded symbols by means of a demodulator 61 and then they are decoded into binary data by means of a decoder 62. The functions of the demodulator 61 of the CAD 60 and the decoder 62 are provided by means of a microprocessor 57. The output of the decoder 62 is written to an output regulator 77 which subsequently can provide digital signals to external data loads »such as computers and networks. The logic time controller 79 of the microprocessor 57 enables the terminal to operate in a sleep mode or to wake periodically to determine whether it will enter the transmission mode »the reception mode or the process mode. The DC power is kept to a minimum when the microprocessor 57 is in the sleep mode. The logic time controller unit 79 awakens the terminal during periods previously assigned as will be described in greater detail below. The Tx / Rx sense module 46 receives the DC signal provided in the coaxial cable 48 as provided by means of the control module Tx / Rx 51. The Tx / Rx sense module will rotate to the transmission amplifier 45 or the reception amplifier 42 or any that is determined by means of the DC level perceived in the coaxial cable 48. The control module Tx / Rx 51 in turn was controlled by the microprocessor 57 to be activated or deactivated according to its structure of access to the current network »Described in a later figure. Figure 4 also illustrates the spread-spectrum backbone CDMA transmission operated in the remote terminal. The source of the transmission can be analogous 67"as provided by an environmental detector" is digitized by means of a CDA 68 provided in the microprocessor 57. Alternatively "the source of the transmission could be digital signal 63" as a computer, which would be written to an input regulator 78. The output of the input regulator 78 is subsequently applied to a data formatter 69, which packs the backward link data signal and adds said information as the packet type, the fate and IO the package originator. The output of the data formatter 69 is supplied to a channel encoder 70 which provides for strong forward error correction and separates the symbols from the back-link packets. The output of the channel encoder 70 is then applied to a PN output which may also be referred to as a CDMA spread spectrum encoder. The output of the Pn 71 encoder is sent to a pattern processor 72 that spans the spread-spectrum packet encoded in the back-link subpattern structure. The pattern processor 72 supplies the binary PN coded signal 73 to a modulator 74 that converts the signal to a QPSK waveform. A frequency synthesizer 55 is used for upconversion of the baseband output of the modulator 74 in a mixer 75. The output of the mixer 75 is filtered using a bandpass filter 76 and connected by an axial cable 48 with the antenna unit 47, where a high power amplifier 45 is provided, in turn to a band pass filter 44 and finally to an antenna 43 for transmission. The high power amplifier 45 is enabled by means of the Tx / Rx sense module 46 under the control of the control unit Tx / Rx 51. Referring now to figure 5, the data to be sent to one or more remote terminals are supplied to the ground station using a wired connection 116 through a router 115 to a packet processing center 13. The packet processing center 13 then moves the data packets forward towards a data formatter 102. Each data packet supplied to the ground station, includes the address of the target mobile terminal or a group of mobile terminals. Since the mobile terminal may have an individual address or one or more address groups, the forward link of the present invention supports concurrent communications with multiple mobile terminals. The data formatter 102 places the data packets in subpatterns within a pattern structure > which will be described in more detail with reference to Fig. 6. The output of the data formatter 102 is passed to a channel encoder 103 for the forward error correction application to help compensate for the bit errors that may be caused by the error. satellite communications channel. The output of the channel encoder 103 is passed to a pattern processor 104 that adds the encoded packets to a TDM forward link structure. The pattern processor 104 silences forward binding data packets when there is no data to be transmitted during all or part of the forward binding subpatterns. The functions of the formatter 102 »the channel decoder 103 and the processor 104 are provided within a digital signal processor 101. The output of the pattern processor 104 is connected to a modulator 105 which converts the binary coded data into a form of modulated BPSK wave for subsequent upconversion in a mixer 106 by means of a frequency carrier signal supplied to a frequency synthesizer 109. The digital signal processor 101 and the frequency synthesizer 109 are synchronized in time to the same subpattern by means of a time controller and a control module 108. The time control signals are provided by means of a time controlling unit 108 for transition of the frequency synthesizer 109 at the correct time. The time control and control unit 108 also provides time-controlled control signals to the digital signal processor 101 »to ensure that the coded data signal of the pattern processor 104 is aligned in time with the carrier frequency generated by the synthesizer frequency 109. The output of the mixer 106 is applied to a bandpass filter 110 to minimize radio frequency emissions (RF) outside the expected satellite communications spectrum. The output of the bandpass filter 110 is then supplied to an amplifier 111 »which in turn supplies the RF signal to a diplexer 112 and an antenna 80 for transmission to a geostationary satellite where it is output to the remote terminal. For the reverse junction, an antenna 80 captures a portion of the back-link signal and passes it to a diplear 112 and then to a bandpass filter Bl that suppresses the spectral energy outside the frequency band of desired recoil union. The salt of the bandpass filter 81 is amplified using a low noise amplifier (ARB) 82 »and subsequently supplied to a mixer B4 for downconversion to a convenient intermediate frequency by means of a frequency synthesizer 85. The output of mixer 84 goes to >; j narrow bandpass filter 86 which further limits the spectral energy around the backward junction expanded spectrum signal. The output of the bandpass filter 86 is provided for a quadrature detector 87 for conversion to a baseband by means of a local oscillator 90. The signals in phase 88 and quadrature 89 are provided by means of the detector. quadrature 87 for digital conversion by means of a CAD 91. The digitized output of CAD 91 is provided for a regulator 93 and a spread spectrum processor 94. A time control and control unit 92 provides the CAD 91 with a conversion activator. »The frequency synthesizer 85 with a frequency control word» and the spread spectrum processor 94 with time controlling signals. Regulator 93 stores a subpattern of samples of the spread spectrum signal for a digital signal processor 95. Expanded digital signal processor 95 processes the digitized signal for the presence of CDMA transmissions of the remote terminals of the present invention. The spread spectrum processor 94 »consisting of multiple digital signal processors» provides concurrent processing of the digitized signal for all the misalignment assumptions of the time control and possibilities of the CDMA code. The spread spectrum processor 94 detects the presence of CDMA transmissions from the remote terminal and provides an indication for the digital signal processor 95 of the detection of a CDMA transmission »the associated CDMA code and the approximate start of the transmission. An interference detector 130 is provided to process the digitized signal for the presence of interferences of the existing satellite channels. It is considered, for example, to share the INMARSAT B backward request channels, which are slotted ALOHA channels with perhaps 20% nominal occupation, with the reverse link of the system operating through the Eastern beam of the AMSC system. Due to the limited overlap of the antenna traces of the two systems, the AMSC satellite receiver will receive less than 10% of the INMARSAT solar packets. In this way »the backward link will only need to compete with co-channel interference 2% of the time. Threaded encoding information, spacing and channel status can compete with 30% blocking per pack. The rf signals are 34 dB below the Inmarsat carriers, so the present system will not degrade the Inmarsat system when it is overloaded in Inmarsat carriers.

The fine synchronization unit 96 then provides calculations of the fine time and the frequency in detected CDMA transmissions. The output of the fine synchronization unit 96 consists of narrow QPSK signals which are then supplied to a demodulator 97 for conversion into a binary coded data signal "and in turn to a decoder 9B to zoom in and encode the forward error correction. The information on the state of the channel can be obtained on a per sample basis of the sample amplitude in the regulator 93"and passed to the decoder 9B. The output of the decoder 98 is provided for a data formatter 99 for conversion into a suitable format for the packet processing center 13. The packet processing center 13 then sends the back-link packets to a router supplying the packets. packages to end clients using wire means 116. The forward link TDM structure of the present invention is shown in Figure 6 as a function of time. The forward binding communications can be regulated to superpatterns 120 that are approximately one day long. Each superpattern 120 consists of several tipatrons of equal length I 121 having a length of about one hour. Each large pattern 121 consists of equal length patterns J 122 that have a length of approximately one minute. A pattern 122 comprises subpatterns of equal length K 123 with a repetition frequency of 0.5 seconds, each of which is subdivided into time periods having time-divided multiplex packets of time L 124 125 which are of equal duration. The first and second packets q 124 are referred to as synchronization / network packets or simply as a sync / network packet. The sync / network 124 packages provide the remote terminals with the help of synchronization and network status. You can transmit up to L-q data packets in the rest of a subpattern. When there are no communications to be provided during a subpattern or portions of it »the forward binding signal will be silenced. A number of data packets 125 are reserved on a master basis for the provision of a network bulletin board »which includes said information as a subpattern for satellite channel map formation. Each packet 125 may contain a number of separate fields such as an address field "an access control field" or an "overload mark" which is used to indicate that there is no more data to follow in a subsequent subpattern. The amount of data that can be sent in a subpattern is limited to N data packets. Depending on the number of terminals to be addressed and the amount of data to be sent, it may not be possible to send all the desired data in a subpattern. »After which the target terminal will normally return to sleep mode. The overload flag can be sent to tell the terminal to stay awake because there is more data to follow in a subsequent subpattern that would not commonly be distributed to those terminals. Of course »if the overload mark is given» the following packets for time lapses in the next subpattern that would normally be distributed to different terminals, so there is an exchange between system capacity and access. A potentially unlimited number of terminals can share a common active reception subpattern. The number of terminals that a shared active reception subpattern can feasibly share depends on the amount of data to be carried and the frequency with which it is desired to send the data to any particular terminal. The data packets L-q 125 can be directed to one or more of these terminals, or they can be silenced if there are no more forward binding packets to be transmitted. Each terminal, which has a unique address, which is active during a particular subpattern processes all data packets 125 and subsequently determines the terminal (s) (where each packet is addressed.) If a terminal does not detect its address between any of the data packets Lq »enters the sleep mode and will remain so until its next active reception subpattern or, if enabled for external interruption »until interrupted by a local source. If a terminal does not detect its address between the data packets 125, it processes the respective packets more and in effect responds. The backstop pattern structure shown in Fig. 7 (a) is similar to that of the forward joint described with reference to Fig. 6. The backspace data can be regulated in superpatterns 130 having a repetition frequency of a day. each superpattern 130 consists of many I 131 thipatrons having a repetition frequency of one hour and each multi-pattern 131 consists of J 132 patterns with a length of one minute and consists of K 133 subpatterns with a length of 0.5 seconds. Unlike the forward junction, the subpatterns 132 at the retraction junction are not subdivided. Each of the expanded data carrier packets in the entire subpattern that are multiplexed using CDMA multiplexing techniques. As shown in Figure 7 (b), the backward link subpatterns 133 at the remote terminals are synchronized in time with the forward link subpatterns 123, misaligned by a discrete number of subpatterns A. The forward link subpatterns 123 include up to Lq data packets that can be used to request a transmission from one or more remote terminals. For example, a request for a remote terminal transmission may be provided in a particular forward junction subpattern. The forward link packet is then processed by means of the terminal (s), and results in a terminal transmission during a subsequent backward link, misaligned in time by subpatterns A of the forward link subpattern having the request.

Figure 8 shows a second mode of the remote terminal. The signals of the reception antenna 300 are passed through a low noise amplifier 301 to a mixer 303 and conventional GPS receiver 302 which sends the current position coordinates to the microcontroller 310. The synthesized local oscillators 320, which is controlled by the reference oscillator 319 whose frequency is adjusted by means of microcontroller 310 by means of digital to analogue converter 318 in the manner to be described in a mixer 303 generates a signal Fl which is amplified in a first amplifier Fl 304 »is mixed in a mixer 305 to generate a second frequency Fl »which is then amplified in a second amplifier Fl 306, from which it is passed through mixers 307, 316 and digital latching circuits 308, 317 to microcontroller 310. The output of the oscillator of reference 319 is connected at 90 ° of the pass exchanger 309 »whose outputs is connected to the second inputs of mixers 307» 316 to generate 1 in phase I and the quadrature components C of the signal. The microcontroller 310 »for example a Phillips P80CL5S0 is connected to the external 1/0 ports 311, the memory 312 and the alarm clock 314, which periodically wakes the terminal to receive input signals. The microcontroller is connected to the power control circuitry 315. On the transmission side, the I and C components of the signal are passed separately from the microcontroller 310 to the QPSK modulator 321 »which is driven by the synthesized local oscillators 320 The output of the QPSK modulator is passed through the impeller 322 and the power amplifier 323 to transmit the antenna 324. The second mode of the ground station is shown in Figure 9. PPC / NOC 13 data is received through of modem 400 and passed to the data converter 401, which converts them into a suitable format for satellite transmission. From there, it is passed through a regulator 402, and a FEC unit 403 for forward-error correction »window unit 404 a digital to analog converter 405 and a BPSK 406 modulator. From there it is passed through the RF equipment of station on land 407 to the satellite. In the return path, the CDMA input signal is passed from the RF equipment 407 through the mixer 410, the amplifier Fl 411 to the mixers 412 and 414 to isolate the phase and quadrature components. These are digitalized into units 415 and 416 and fed to the CDMA / QPSK demodulator 417, the approach symbol 41S and the Viterbi decoder 419. The information on the state of the channel can be obtained on a per-sample basis from the sample amplitude of the digitalizers 415 »416 and passed to the decoder 419. After a CRC 420 check» the signals are passed through the data converter 410 and the modem back to PPC / NOC 13. The clock 422 is connected to the processor 421 »that performs the functions contained within the dotted drop. The frequency control unit 409 within the processor controls the synthesizer 40B »which drives the mixer Fl 410 in quadrature in phase and mixers 412» 414 through the hybrid of 90 ° 413 »and the modulator BPSK 406. As indicated above, the logical time controller or clock 79, 314 wakes up periodically to the remote terminal to hear if there is input data. In order to minimize the cost of the terminals, it is convenient to use a low cost oscillator, which can be forced to move. To correct this, the oscillator may awaken the terminal for a short period more frequently than the occurrence of its assigned subpattern, for the purposes of clock resynchronization. For example, if only a subpattern is assigned to a particular terminal in the hierarchical pattern structure, this will only be repeated every twenty-four hours, although the clock may need to be returned more frequently. In a convenient mode, the terminal can, for example, wake up once every hour only for synchronization purposes to ensure that when the next attention period approaches the clock will be properly synchronized with the ground station. Figure IO shows the algorithm to achieve it. In step 200, for example, after one hour, the sleep period of the end terminates and the time controller 79 »314 commonly a countdown time controller, which extracts a minimum current in the sleep mode» wakes up to the terminal just before the start of a front entry subpattern in the forward junction. The received signal is digitalized in a reception window of 0.625 sec. in step 201 to ensure that the window covers a subpattern of 0.5 sec. full. The reception window is processed in step 202 to identify a unique synchronization word having the subpattern. In step 205 »the difference between the actual arrival time and the predicted one of the single word is noted. Decision step 206 determines whether this difference is greater than a predetermined number of clock cycles »commonly 30» and whether it corrects clock synchronization by loading a new number into the internal register of the countdown time controller. The algorithm exits in step 208 and the terminal goes back to sleep until the next wake-up period. After processing of the reception window "other actions" such as synchronization of the local oscillator frequency can be taken in step 203. This process will be described with reference to figure 11. Commonly »the alarm clock 79» 314 works at 32 KHz and splits down to 8 Hz to generate a pulse every 125 sec. In one mode, the active state can be divided into two substrates, a fully active state in which the receiving circuitry is switched to a partially active state where only the microcontroller is turned on. Every 125 msecs, the clock can wake up only to the crocontroller 310, placing the terminal in the partially active state. The microcontroller checks to see if it is time to listen if there is a subpattern input. If not »go back to sleep. If there is a subpattern, it turns on the receiving circuit system to take an input subpattern »after which it turns off the receiving circuitry, which after the transmitter circuitry has the largest power drain. Then digitalize the subpattern »check the synchronization of the clock and look for data packets directed to it. Referring now to Figure 11, first a calculation of the frequency course is made with an FFT filter in step 210 and the digital samples corrected for the gross error in step 212. In step 213 a calculation is made of the fine frequency using a digital phase-synchronization circuit. The sum of the error calculations is obtained in step 211 and the corrected digital samples of total error in step 218. Step 219 decides whether the single synchronization word is detected and if the subpattern is not rejected. If the word "single" is detected, the total error calculation is transferred to a two-dimensional search box 215 for voltage correction 216 of the controlled voltage reference oscillator 64 »319.

A temperature detector 214 is connected to the search box to ensure that crystal oscillator 64, 319 is corrected for changes in temperature. The described system can coexist with a primary system without causing any system to interfere with the other, even if common frequencies are used.

Claims (9)

NOVELTY OF THE INVENTION CLAIMS

1. - A method for sending data in a wireless communication channel, said communication channel being common for primary and secondary communication systems having at least partially overlapping coverage areas, said primary system having a low duty cycle and transmitting packets in random instabilities having a first characteristic duration "said method comprising transmission packets in instabilities in said second system» said instabilities in the second system having a duration of at least three times the first duration and said packages containing error correction codes towards forward to allow the data contained therein to recover in the event of a shock with an instability in the primary system "and said stability in the secondary system having an intensity sufficiently lower than the intensity of the instabilities in the primary system. p to avoid interference with it.

2. A method according to claim 1, further characterized in that said packages employ spacing symbols to improve the improved error correction operation.

3. - A method according to claim 1 or 2 »further characterized in that the duration of the instabilities in the secondary system is 3 or more times the duration of the instabilities in the primary system.

4. A method according to any of claims 1 to 3 »further characterized in that said instabilities transmitted in said secondary system employ spread spectrum multiplexing techniques to bring the energy level to the primary receiver below the noise level of the primary system .

5. A method according to claim 4 »further characterized in that said secondary instabilities employ multiple code access access multiplexes.

6. A method according to any of the rei indications 1 to 5, further characterized in that said primary and secondary systems are satellite-based systems with overlapping rays.

7. A method according to claim 2, further characterized in that said reception packets are separated and the error is corrected taking into account the information on the state of the channel.

8. A wireless communication system capable of sharing bandwidth with a primary system, said systems having coverage areas that overlap at least partially, said primary system having a low duty cycle and transmitting random transmission instabilities that have a first characteristic duration »said communication system comprising a plurality of distributed terminals for transmission packets in instabilities, said instabilities in said communication system having a duration at least equal to three times said first duration, and said packages containing codes of forward error correction to allow the data contained therein to recover in the event of a shock with an instability in the primary system, and said instabilities in said communication system having an intensity sufficiently lower than the intensity of the instabilities in the primary system to avoid interference with it.

9. A wireless communication system according to claim 8 »further comprising means for symbol spacing in said packets to improve the error correction. IO.- A wireless communication system according to claim 9, further comprising means for monitoring the state of the channel »and a processor for approximating and correcting input packets» said processor being programmed to take into account the information on the state of the channel while performing the correction. 11. A wireless communication system according to any of claims 8 to 10 »further characterized in that the duration of instabilities 3B in said communication system at least three times the duration of the instabilities in the primary system. 12. A wireless communication system according to any of claims 8 to 11, further comprising means for transmitting instabilities in said communication system using spread spectrum techniques to bring the energy level to the receiver below the level of noise of the primary system. 13. A wireless communication system according to claim 12 »further characterized in that said instabilities in said communication system employs multiple code multiplex access multiplexing. 14. A wireless communication system according to claim 12 »further characterized in that said systems are based on satellite with overlapping rays. 15. A terminal for use in a wireless communication system capable of sharing bandwidth with a primary system "said systems having coverage areas that overlap at least partially" said primary system having a low duty cycle and transmitting instabilities randomly having a first characteristic duration said terminal comprising means for transmitting packets in said communication system having a duration of at least three times said first duration, said packets containing forward error correction codes to allow recoveries data therein in the case of a shock with a nestability in the primary system, and said instabilities in said 5-communications system having an intensity sufficiently lower than the intensity of the instabilities in the primary system to avoid interference therewith. 16. A terminal according to claim 15 »further characterized in that said packets -10 contain symbols spaced to improve the error correction. 17. A terminal in accordance with the rei indication 16 »further characterized in that said terminals transmit nestab lities that have a duration 15 of at least three times said first duration. 18. A terminal according to any of claims 15 to 17 adapted to communicate via satellite.