DE3118018A1 - Communications system - Google Patents

Abstract

The invention relates to a communications system with fixed transceiver stations and mobile transceiver stations, a plurality of fixed stations being controlled by a control station, which stations are interconnected and of which a plurality are in turn connected to a mobile switching centre (transfer facility, automatic telephone exchange, gateway) to form a dial-up telephone network. The radio traffic between the fixed transceiver stations (FS) and the mobile transceiver stations (MS) is carried in time division multiple access. The transmission can be performed using spread-spectrum technology in each time slot of the time division multiplex transmission, n bits in each case being able to be coded by an m-value code signal. <IMAGE>

Description

Messaging system

The invention relates to a message transmission system stationary transmitting / receiving stations and with movable transmitting / receiving stations, which can exchange messages via radio with the fixed stations, whereby several stationary stations are controlled by a control center, from which in turn several are connected to an overhead control system to form a telephone network.

State of the art From the article "Automobil Telephone Switching System" by T. Terashima et al in the journal NEC Research and Development, No. 57, April 1980, on pages 80 to 87, such a system is known. In this The cellular system consists of up to 32 fixed stations in a control center assigned, of which up to 6 are connected to a transfer device. Just There is a direct connection between the transfer devices. This system works in the frequency range at 800 MHz with frequency division multiplex as Multiple access procedure with a maximum of 600 channels in the area of a control center.

Only through the high frequency stability of the components used and steep channel filters allow adjacent channel filters Reduce, which requires a relatively high effort, as well as the avoidance of intermodulation products due to a non-linear amplitude characteristic in the receiving channel. Then there is the low interference immunity of the analog message transmission.

OBJECT The invention is based on the object of a message transmission system of the type mentioned at the beginning to indicate that a (n) inexpensive production and operation in particular of the mobile transmitting / receiving stations and a flexible one Expansion to a very high number of participants permitted and mutual interference and prevents interference from other radio services with secure transmission.

Solution This problem is solved with those specified in claim 1 Means. Refinements can be found in the subclaims.

Description will be made below the invention with reference to drawings of Embodiments explained in more detail. The figures show: FIG. 1 schematically the planes the messaging system; 2 schematically shows the level of a control center; Fig. 3 is a block diagram of a movable transmitter / Receiving station: 4 shows a block diagram of a stationary transceiver station; Fig. 5 is a diagram for a time frame in time division multiplexing; Fig. 6 pulse diagrams of the spread spectrum technique; 7 shows pulse diagrams when detour signals occur; Figure 8 is a list of code characters an m-valued alphabet; Fig. 9 is a block diagram of another movable one Sending / receiving station; 10 shows a block diagram of a further stationary transmitting / receiving station and FIG. 11 shows a block diagram of the signal processing in the receiver.

The communication system illustrated in Fig. 1 is in three Split levels. The lowest level is the radio level, the one with the movable ones Sending / receiving stations operated communication with the help of a stationary sending / receiving station is implemented on a fixed grid. The moving stations are through one thick point and the stationary stations are indicated by a filled square. The approximate range of a stationary transmitting / receiving station is indicated by a dash-dotted line Line indicated.

In the cell level above there are several stationary ones Transmitting / receiving stations connected to a control center. Every control center is supplied a cell. The control center organizes the radio traffic within its cell and mediates the calls to a gateway. The control centers are indicated by a filled square within a square. The delimitation of the Cells from each other is indicated by dashed lines.

All control centers are on the system level above connected and a transfer device is assigned to each of several control centers. The transfer device establishes the connection to the switched telephone network. The transfer device can be connected locally to a control center. The transfer devices are marked by a circle with dots and the demarcation between the areas of the transfer devices is indicated by a dashed line. The connections of the control centers with each other on the system level is used for data exchange between all control centers and thus the higher-level organization of the system, in particular the recording of the location of the moving stations and the transfer of calls.

The connections between the stationary transmitting / receiving stations and the control centers and between the control centers among each other are done using a broadband transmission medium. It can be through a cable connection, or a fiber optic connection be realized by radio relay.

In Fiq. 2 shows a control center with its cell.

There are six stationary transmitting / receiving stations on the Leitstejle LS FS connected, one of which at the site of the Control center set up can be. In the area of a stationary station FS is a movable transmitting / receiving station MS shown operating over the radio link, symbolized by a broken arrow, is connected to the stationary station FS. The connections between the stationary Stations FS and the control center LS are, for example, made of optical fiber-optic cables built up. The connection of the control center LS to the neighboring control centers LSp LSy and LS z are implemented, for example, by means of directional radio links.

The stationary transmitting / receiving stations are from the point of view a sufficient coverage in the entire cell is arranged accordingly also chosen their number. The radio transmission between the moving and the stationary Stations takes place in duplex traffic in the frequency range of, for example, 900 Mz in time division multiplex with multiple access. The Control center to ensure that no co-channel interference occurs. The fixed stations are able to communicate simultaneously on several channels in time division multiplex, however, they only act as a relay station for the control center in the course of the call. Of the fixed stations it is determined in which area of responsibility the moving stations are located. This information is evaluated in the Control center.

The LS control center takes care of the further processing of the calls and data from all stationary stations FS for the decentralized organization of all processes inside her cell.

It decides on the allocation of channels, on the change of serving stationary station and, if necessary, causes the transfer to the next cell. For this purpose, the control center has transmission facilities as well as input directions for data processing and memory for subscriber numbers in the cell area, allocated Channels, active and callable moving stations with local position and associated fixed station.

As already mentioned, the radio transmission takes place in time division multiplex. at With this method, messages are transmitted as a series of short pulses and the temporal interleaving of pulse sequences results in the different Time slots or channels. In Fig. 3 is the block diagram of a movable transmitting / receiving station, which works in time division multiplex shown. A microphone 1 includes a Analog / digital converter (A / D) 2, which converts an analog signal into binary characters. The binary characters arrive in a buffer memory 3 in which they are temporarily stored until they contain the carrier wave in compressed form in the assigned time slot modulate in a subsequent modulator 4. This is followed by a transmitter 5 on, which works via a transmit / receive switch (S / E) 6 on an antenna 7, the which emits the appropriately processed carrier oscillation.

At the modulator 4 and the receiver 8 is a frequency generation 15 connected, which is controlled by a reference oscillator 16.

A receiver 8 connects to the S / E switch 6, which receives the Converts signals into an intermediate frequency.

In the following demodulator 9, the signals are demodulated and cached in a subsequent buffer memory 10. From this become they are read out spread out and converted into an analog one by a digital / analog converter (D / A) 11 Signal converted back and re-f1exlefen through a listener or loudspeaker 12 Wi t-erhin is a microprocessor 13 and a Control unit 14 with Ads available. The microprocessor 13 takes over organizational tasks such as Synchronization to the time grid, setting of the time slots, sending of call, Identification and acknowledgment signals and processing of the from the fixed station coming control commands. To do this, it is connected to the buffer via control lines 3 and 10 and the S / E switch 6 connected.

In Fig. 4 is the block diagram of a stationary transmitting / receiving station shown. The stationary station, like the mobile stations, has an antenna 7, a receiver 8, a demodulator 9 in the receiving branch and a transmitter 5, a Modulator 4 in the transmission branch, as well as a reference oscillator 16 and a frequency generator 15 on.

There is also a microprocessor 13, which is controlled by control lines is connected to the corresponding assemblies.

In Sendezweiq a multiplexer 17 is arranged in front of the modulator 4, the multiple messages temporarily stored in N buffers 18 in the in the correct chronological order. N stands for the number of time slots or channels. The buffer memories 18 are connected to a device 19 for implementation and Forwarding of messages connected.

In the receiving branch, a demultiplexer 20 is connected to the demodulator 9, the received time-nested messages back on N buffers 21 distributed. The buffer stores 21 are connected to the device 19.

The type of implementation of the individual receive and transmit signals in of the facility 19 for the transfer to the guidance place and from the control center depends on which medium is used. At any point in time is always because of the selected multiple access in time division multiplex within a cell only one movable transceiver station with the stationary transceiver station tied together.

This means that mutual interference is not possible.

A time frame with N = 64 time slots is used for the time division multiplex method chosen which repeats itself periodically at a rate of 30 per second. In Fig. 5 is such a time frame and a single time slot, also called a channel is indicated. The timeframe is 1 / 30th of a second and contains 64 Time slots, of which only the first three time slots, marked with 1, 2, 3, and the last time slot, labeled 64, are shown.

The time slot number 2 is enlarged and with a possible occupancy shown. It begins with a synchronization preamble that enables recipients to to synchronize on the received message. A subsequent news preamble contains the data necessary for organization such as time slot and user number. The following message section contains approximately 2130 bits and provides the language information of an interval of 1/30 second duration. It follows that the speech digitization at 64 kbit / s. Each time slot ends with a guard time that has to be taken into account of runtimes. With these sizes, a bit rate of approximately is calculated 5 Mbit / s and a required bandwidth of depending on the type of mudulation used about 5 MHz.

Two such frequency ranges are required for duplex operation.

The following is the control of the communication system described above described. The processes of the control are very great because of the diverse tasks complex.

In addition to the pure call transfer, there are, for example, the following Functions: channel allocation and monitoring, handover to the next stationary Station, control center, recording of charge and call data, location recording of the moving stations.

For the area of a cell, the control center determines the various organizational measures taken. The communication path on the radio level is handled by only one fixed station for each moving station to avoid interference to avoid the signals. The responsibility of a fixed station for a mobile one Station is based on the amplitude or transit time of the signal from the moving station established. Decisions about a change, for example in terms of movement of a station only hits the control center.

The transfer between stationary stations is carried out by control commands to the two stations concerned triggered by the control center as soon as this recognizes the need as a result of the ongoing vehicle positioning. Usually it is not necessary to change the assigned channel.

When it is passed on to other cells, the control center always receives the information the connection to the public switched telephone network is maintained in whose cell the conversation started. Therefore there are two tasks to be performed here when making a call consists. First, the operating control center must be in cooperation organize the handover to a stationary neighboring station with the neighboring control center, where a new channel may have to be assigned and, secondly, the "new" control center forward the conversation to the "old" control center in the further course, so it continues to get into the telephone network from there.

The prerequisite for establishing a connection is the contact of the movable Station with a fixed station. This contact is when the device is switched on the mobile station, even if there is no conversation, always given, that automatically at certain time intervals (e.g. every 1 ... 2 min) the identifier broadcast on a special channel. This identifier is continuously recorded and evaluated. The control center maintains a list of all mobile devices that are ready to talk in a memory Stations within the cell and informs the memory in the home cell about the current location of mobile stations that are foreign to the cell. After changing cells the lists in the two control centers are corrected. Also, the change will reported to the home cell of the mobile station.

In the event of a call from the mobile station, the latter requests a special call channel over the fixed station the allocation of a call channel. The desired call number of the telephone subscriber is on this call channel transmitted and the control center establishes the connection. The subscriber picks up off, that's how the conversation takes place.

Otherwise the usual characters will sound (free / busy).

It is irrelevant for the moving station considered here, whether the call partner has a normal phone or a car phone, because in is connected via the telephone network in each case.

To call the mobile station, an inquiry is first made at the control center in the home cell recognizable from the phone number. This has saved whether the called mobile station is ready to talk, and if so, where it is is located. The currently responsible control center is dialed. She calls the moving station on the special call channel and assigns it a conversation channel to, on which the call is then handled.

Good language quality is required when using time division multiplex PCM technology limits the bit error probability to about 10 4. Um Achieving this goal is a ratio in the case of interference due to Gaussian noise alone from bit energy Eb and spectral noise power density No of Eb / No 8.5 dB required (Two-phase modulation, no coding used for error correction). Because of multipath effects in wave propagation result in considerable disturbances, which are based on signal power fading and the influence of neighboring characters. In both cases, superimpositions of waves are the cause, which the receiver on achieve different ways and due to phase differences to attenuations the signal amplitude and can lead to corruption of the phase information. Addicted From the statistics of the multipurpose propagation, mean values for E / NO are about 40 dB necessary to achieve the bit error probability of 10 4.

Frequently used countermeasures are diversity procedures, e.g. multiple transmission and coding for error correction, the former being comparatively low Effort, however, generally also a lower effectiveness. Coding for error correction is in contact with character rearrangement at suitable design very effective. In addition to the effort required for this However, it should be noted that error bundle due to the short duration the time slots of around 500 ts over large sections of the message telegrams and can only extend with long codes at a low code rate (= ratio the number of information characters for the code length). Both of these The common method is that they begin after over the received characters a - possibly incorrect - decision has been made. That in the following The described procedure tries to avoid incorrect decisions from the outset to suppress.

The starting point is the consideration that multipath signals are then not too Disturbances result if they can be detected separately. The spectrum spreading methods offer this favorable conditions. If signals are transmitted with bandwidth B, leave them alone determine their arrival times with an accuracy of the order 1 / B. By Spreading the bandwidth by the factor n shrinks the time interval for the discovery to one nth of the original value. The spread of the signals becomes less than 80 made that the digitized signals are multiplied by a code. If the clock frequency of the code corresponds to the width of the transmission band, in this way, the initially narrow-band signal is spread over the entire band. The received, spread signal is sent with the same code on the receiving side multiplied again after a synchronization in a correlation process has taken place.

This places the desired signal in the original narrow message band generated, while unwanted signals remain broadband and therefore not evaluated will.

Their interference depends on the cross-correlation of the turned Codes.

In Fig. 6a a digitized signal is shown and in Fig. 6b is a spreading code consisting of the same repetitive code words of the signal. The spread signal is shown in Fig. 6c, from which it can be seen that in the case of a binary zero in 6a, the code in 6b is inverted and is not inverted at a binary one.

In Fig. 7 is a received direct signal and two detour signals shown with different running times.

The characters of the direct signal are 1, 2, 3, 4 and those of the Detour signals marked with 'or ".

The detected signals are recorded underneath. It is clear can be seen how signals that overlap in time due to the spread of the band to be resolved.

However, Fig. 7 also shows that it is desirable to set the decision period for each character so far that as few detour signals as possible in the Decision period of the following character fall. So disturb the characters 1 " and 2 "the decision for characters 2 resp.

3. In contrast, the characters 1 ', 2' and 3 'are no longer mutually exclusive noticeable as disturbing. On the contrary: stand for the decision because of the Multipath propagation has two reception signals available, for example 1 and 1 '.

The gain lies in reducing the likelihood of being owed to perceive signals that cannot be correctly evaluated by fading.

At a bit rate of 5 Mbit / s, as stated above, the decision period for each bit is 200 ns. This would be detour signals with a path difference of more than 60 m to the direct signal already in the divorce period the following characters fall.

About the evaluation of the signal mixture from direct signal and detour signals To improve, n-bits are combined into an n-bit group and carried out through replaces an m-valued code character. In Fig. 8, n equals 4 in the left The possible variants of the 4-bit group are partly shown and in the column right column for m q equals 16 the code characters obtained by cyclic exchange partly shown.

This measure increases the bandwidth by a factor of 4 (= 16: 4) to around 20 MHz, but has two decisive advantages. First, the time interval is shortened for the discovery by the same factor, so that signals with path differences of more can be recognized and evaluated separately as 15 m. Second, lengthens the decision period for a message character from 200 ns to 800 ns, whereby only detour signals with a path difference of more than 240 m, subsequent signs can disturb.

Considerable disruptions should be expected even with larger detours these can be largely suppressed if the code is 16-valued Code characters for the consecutive 4-bit groups, e.g. according to a pseudo-random sequence changes from character to character.

Each individual cell should have the capacity of 64 time channels with any Be able to take full advantage of the distribution to the radio areas. But that means that im Transition area between the cells radio signals from different cells are received and a temporal separation is not always given. To avoid disruptions The cells are assigned different codes for the 16-value code characters. As the number of usable Codes with good correlation properties is limited, the codes repeat themselves to a certain extent, depending on the propagation conditions dependent distance.

In Fig. 9 is the block diagram of a movable transmitting / receiving station, the time division multiplex using the spread spectrum technique in each time slot works, shown. For this purpose, the movable station according to Fig.

3 has been expanded and all of the same circuitry is the same Provided reference numerals and are therefore no longer described in detail. At the buffer memory 3 is followed by a modulator 22, which is also from a code generator 23 and an IF generator 24 is controlled. The digital signal from the buffer memory 3 keys e.g.

in the case of two-phase modulation, the phase of the signal from the IF generator 24 um, then with the code of the code generator belonging to the respective time slot 23 is multiplied and thereby spread. The output of the modulator 22 reaches the transmitter 5, which with the frequency generation for conversion into a high frequency Signal is connected and from which via the S / E switch 6 to the antenna 7 for radiation.

After the receiver 8, a demodulator 25 is arranged, which is also of a code word synchronizer 26 is controlled. The received and in the receiver 8 converted signal is first synchronized with the code word in a correlation process and then multiplied to undo the spread. The received in the original signal lying in the narrow message band is given to the buffer memory and as already described, ultimately reproduced in the loudspeaker.

In Fig. 10 is the block diagram of a stationary transmit / Receiving station shown, which works in time division multiplex using spread spectrum technology. For this purpose, the stationary station according to FIG. 4 has been expanded and all are the same Circuits are given the same reference numbers and are therefore not explained in more detail. N modulators are connected to the multiplexer 17 27, which are also controlled by an IF generator 28 and one of the N codes. The N codes are generated in a code generator 29. The signals 1 to N of the multiplexer 17 key in the modulators 27, e.g. in the case of a two-phase modulation, the phase of the IF generator 28 around and are then with the respective code of the code generator 29 multiplied and thereby spread.

The signals from the modulators reach the antenna 7 as described and are radiated.

After the receiver 8 there are N demodulators 30 with code word synchronizers arranged to which the N codes of the code generator 29 are also applied. That from Receiver 8 output signal is in the demodulators 30 first with the respective Codeword synchronized in a correlation process and then multiplied to to cancel the spread again. Signals 1 to N, which are now narrow again, arrive in the demultiplexer 20 and are processed further as already described.

The radio link between the moving stations and the fixed ones Station is the part of the entire transmission path that is sensitive to interference. In addition to the Multipath propagation already described impair the useful signal here Noise, spectral components from neighboring frequency bands and simultaneous signals from other cells.

Therefore, a powerful signal processing in the receiver is one the essential fundamentals for the functional kidneys of the system. In Fig. 11 is therefore the unit provided after the receiver for recovery of the transmitted signal is shown.

From the receiver 8, the signal arrives at 17 correlators 31 if m is the same 16 for the code character according to FIG. 8, which is also controlled by a code generator 37 will.

One of the 17 correlators 31 is used to synchronize the time slots and is required for the sequence control 32 of the time slots. The other 16 correlators 31 compare the received signal with the 16 code words of the 16-value alphabet. Neighboring cells can use a different set of Use code words. If it is because of strong signals with long detours of more than 240 m is required, the code of this alphabet can move from one to the other 4-bit group Switch pseudo-randomly.

The correlators 31 are followed by 16 detectors 33, which have maxima in the autocorrelation function. A subsequent decision stage 34 selects the greatest out of all recognized maxima and defines the corresponding one Correlator assigned code word as the one sent with the highest probability Sign. A downstream converter 35 generates the associated 4-bit group and feeds them to the buffer. This delivers the during a time slot received signals at its output with a continuous current of 64 kbit / s.

Further processing depends on the device in which it is located Signal processing unit is located. In the moving station there is a conversion of the digital via an analog into an acoustic signal, while the fixed station converts the digital characters and, if forwarded via a fiber optic cable, electrically / optically converts.

L e r s e i t e

Claims (11)

Claims S message transmission system with stationary transmission / Receiving stations and with movable transmitting / receiving stations that use radio the stationary stations can exchange messages, with several stationary Stations are controlled by a control center, of which in turn several with a transfer device are connected to a switched telephone network, characterized in that that the control centers (LS) are interconnected and that at least the radio traffic takes place in time division multiplex with multiple access. 2. Message transmission system according to claim 1, characterized in that that the gateway is locally united with one of the control centers (LS). 3. Message transmission system according to claim 1, characterized in that that in each control center (LS) there are means that allocate channels in the time division multiplex to the stationary stations (FS), the change of the serving stationary stations Station (FS) and the transfer of a connection to a mobile transmitting / receiving station (MS) to a neighboring control center (LS). 4. Message transmission system according to claim 1, characterized in that that the mobile transmitting / receiving stations (MS) are switched on when the device is switched on synchronize to the received line grid and their identification at any time send on a dedicated channel, and that in the associated control center S (LS) from the running time of the identifiers the distance of the mobile stations (MS) from the associated stationary station can be determined. 5. Message transmission system according to claim 4, characterized in that that the current location areas of the active mobile transmitting / receiving stations (MS) are written into a memory in their Meimat control center. 6. Message transmission system according to claim 1, characterized in that that 64 time slots are provided for multiple access in time division multiplex. 7. Message transmission system according to claim 1, characterized in that that the stationary transmitting / receiving stations (MS) with their associated control centers (LS) through a broadband transmission medium, in particular an optical fiber optic connection, are connected. 8. Message transmission system according to claim 1, characterized in that that in each time slot of the time division multiplex transmission the transmission in spread spectrum technology succeeded. 9. Message transmission system according to claim 8, characterized in that that in each case n bits to be transmitted are combined into an n-bit group and are coded by an m-valued code character. 10. Message transmission system according to claim 9, characterized in that that in adjacent control centers (LS) different codes of the m-letter code characters to be used. 11. Message transmission system according to claim 9, characterized in that that the code of the m-valued code characters for successive n-bit groups changes from group to group according to a pseudo-random sequence.