EP0602563A1 - Multichannel radio alarm system using code multiplexing - Google Patents

Abstract

Radio alarm systems having a multiplicity of information channels for transmitting signals (messages, information) between glass break detector, break (opening) detector and the like, and a microprocessor-controlled central unit are known. The individual information channels are in this case formed in accordance with the time-division-multiplexing method or by using the frequency-division-multiplexing method. To increase in a simple manner the interference immunity and to permit signals to be transmitted simultaneously and in an interference-free manner via the different information channels, the information channels are formed, in the transmission direction from the detectors to the central unit, by using the code-division-multiplexing method. The spread information channels thus formed are then superimposed on each other and converted to a single carrier frequency. <IMAGE>

Description

The invention relates to a radio alarm system according to the preamble of patent claim 1.

• a) Das Erkennen eines Einbruchsvorgangs erfolgt mit automatischen Detektoren (Meldern), die je nach Anwendungszweck nach verschiedenen Prinzipien arbeiten,
• b) die Auswertung der Meldungen geschieht in einer Zentraleinheit, abhängig von unterschiedlichen Zustandsbedingungen,
• c) die Alarmierung erfolgt entweder durch Weiterleitung des Alarms, etwa zu einer Polizeidienststelle oder in Form eines örtlichen Alarms durch Aktivieren von Alarmgebern wie Sirenen, Alarm-Blitzleuchten oder dgl..

The Security- (Sicherheit-) Electronic has developed into a valuable aid to immediately report fire, robbery, theft, burglary, robbery and kidnapping. The various components of an alarm system essentially have to perform the following tasks:
Detect, evaluate, alert.

• a) The detection of a break-in process takes place with automatic detectors (detectors), which work according to different principles depending on the application,
• b) the evaluation of the messages takes place in a central unit, depending on different status conditions,
• c) the alarm is given either by forwarding the alarm, for example to a police station, or in the form of a local alarm by activating alarm devices such as sirens, alarm flashing lights or the like.

Modern alarm systems usually consist of a central unit (including power supply), switching devices for arming / disarming, automatic intrusion detectors, manually operated hold-up alarms, transmission devices and / or local alarm devices and a network.

Depending on the object to be protected and the effort involved, the detectors used range from simple bolt or magnetic contacts to ultrasonic detectors of various types. Regardless of their mode of action, one can divide the detectors into three groups: detectors for outdoor surveillance, detectors for outer skin surveillance and detectors for indoor and property surveillance.

In conventional alarm systems, the automatic intrusion detectors are connected to the control center in series via so-called alarm lines and work according to the closed-circuit principle. A certain quiescent current flows through all detectors. If the current is interrupted by the triggering of a detector, an alarm message is issued. The alarm is also triggered if the detection line is destroyed or short-circuited at any point.

Various components, such as switching devices, local alarm devices, etc. are additionally protected via a so-called sabotage line, i.e. an alarm is triggered if these components are opened or damaged.

Conventional wired alarm systems thus meet the high requirements of the association of property insurers with regard to interference immunity. The advantages of using optical fibers are: high immunity to electromagnetic waves, high transmission capacity and galvanic isolation of the transmitter and receiver. However, it is disadvantageous that there is a great deal of effort involved in laying the line network, particularly if this is done subsequently.

From DE-OS 40 35 070 a radio alarm system is known, which offers adequate protection against interference by blocking the radio links and against manipulation from outside. The known radio alarm system consists of a central unit with a receiver unit for receiving radio signals from detectors in the form of data telegrams. The receiver units are designed so that only data telegrams from detectors belonging to the radio alarm system are processed in order to trigger an alarm or a function. In particular, in order to prevent manipulation from the outside with a radio transmitter of the same system, each detector of the radio alarm system sends out at least two radio signals of different carrier frequencies, which are coded the same or different (in the sense of an identifier). Accordingly, the central unit with its receiver unit receives at least two different radio signals from each detector and checks the affiliation of the detectors emitting the radio signals by checking the validity of the coding (identifier).

In addition, it is proposed in the radio alarm system known from DE-OS 40 35 070 that the radio signals emitted by the individual detectors are subjected to a field strength check in an evaluation unit, which further increases the interference immunity can be. In order to meet the VdS guidelines, the message channels are formed using the frequency division multiplex method in this radio alarm system.

Furthermore, a radio transmission method for an alarm system is known from EP-A1-0 293 627, in which the same radio frequency is used to transmit information between a central unit and the components of the alarm system. The individual message channels are formed using a time division multiplex method. These message channels are cyclical, i.e. queried sequentially in time in the central unit whether a message is transmitted via the respective time channel. In order to avoid useless and time-consuming polling cycles, a polling cycle is only carried out in the radio transmission method according to EP-A1-0 293 627 if it has been found in a previous, much shorter total poll, in which all components are polled at the same time, that Information has arisen for at least one component (eg a detector).

Finally, a radio alarm system is known from EP-A1-0 316 853, in which additional impulses, so-called pseudo impulses, are transmitted to protect against sabotage. The timely arrival of the additional pulses is recognized in the central unit and evaluated accordingly.

In principle, three basic methods are known for the transmission of messages via a transmission medium (e.g. line, radio link) used jointly by a large number of devices, namely the time division multiplex method, the frequency division multiplex method and the code multiplex method.

In the time-division multiplex method, the entire bandwidth of a single radio channel is available to each device, which, however, may only be used by the device for short periods of time. The characters or character strings of different devices are interleaved and are transmitted at a correspondingly higher bit rate in the single radio channel, the time channel (message channel) assigned to each device being repeated periodically with the frame period.

In the frequency division multiplex method, the total bandwidth available for message transmission is divided into narrow frequency bands, each of which corresponds to a message transmission channel. Such a narrow frequency band is available to each device for the duration of the radio transmission. In practice, frequency-division multiplex or time-division multiplex methods are used in radio alarm systems for message transmission, but applications of the code multiplex method are not known.

Applications of the code division multiplex method are known in car telephone systems (see also EP-A2-0 241 954 or EP-A2-0 211 460). In the case of code multiplexing, the various messages carried over the common transmission medium (e.g. radio link) are modulated onto a carrier by basic modulation and the resulting narrow-band signal compared to the channel bandwidth is converted to the channel bandwidth by multiplex modulation with the aid of a code word characterizing the receiver spectrally spread. A code generator is arranged in the transmitting devices of the radio stations, which generates a code identifying the transmitting device.

The invention has for its object to make the formation of message channels in a radio alarm system so that the interference immunity is increased in a simple manner and that the simultaneous interference-free transmission of messages is made possible.

This object is achieved in a radio alarm system with the features of claim 1.

The radio alarm system according to the invention has the advantage that no effort is required for time slot synchronization, as is essential for a TDMA system (Time Division Multiple Access). Only this enables the numerically predominant connections between the detectors and the central unit to be designed as unidirectional routes.

Furthermore, it is ensured in the radio alarm system according to the invention that the currently selected transmitter is reliably received even in the event of a superimposition of several messages.

The cost-effective implementation results from the fact that only one transmitter is required in the detectors. The high receiver effort arises only in the (few) central unit (s); there at least the power supply is not a problem. In the radio alarm system according to the invention, the message channels are formed using the code multiplex method, so that the messages can be sent asynchronously. On the other hand, the messages must be repeated at sufficiently small time intervals since these are listened to asynchronously by the receiver. The association of property insurers is currently stipulating a data telegram exchange every 10 seconds. If one assumes that there are no malfunctions during the majority of the time and thus the average time between two data telegrams is about 25 seconds, the energy consumption can be reduced by a factor of 2.5 by means of the safety queries, without accepting any restrictions in the operational safety of the alarm system have to.

The design of the radio alarm system according to claim 2 has the advantage that a blockage of the radio channel with high probability is a sabotage and not a "natural" event (e.g. co-users of the frequency band). By spreading with pseudo-random, orthogonal or quasi-orthogonal code words, any time shifts in the data telegrams are possible, but symbol interference is reliably avoided. The spreading factor used here is a compromise in order to combine the advantages of band spreading with the demand for frequency economy.

If, according to the embodiment according to claim 3, a single radio transmission channel is used to transmit information in the reverse direction, the quality of reception of one of the co-channel message transmission channels can be measured during the initial start-up and a corresponding adjustment to the channel properties (setting of the transmission field strength) can be carried out. As a result, the transmission security can be increased in a simple manner for all message transmission channels. There is also the possibility of storing the measured values determined (see also claim 5).

The embodiment according to claim 4 has proven to be particularly advantageous for increasing the transmission security if each data telegram comprises successive blocks with code words of the same length. If a signal is received outside of the given code words or time intervals, this is a first indication of sabotage. Transmission pauses could possibly be made in the case of routine reports if the interference field strength of the radio channel is monitored by the central unit. A transmission block between two pauses must be longer than the longest observation time T _K.

The embodiment of the radio alarm system according to claim 5 requires little circuitry without having to accept restrictions in the operational safety of the alarm system.

Fig. 1

das Blockschaltbild einer bevorzugten Ausführungsform und

Fig. 2

einen zeitlichen Verlauf am Beispiel einer Überlagerung von Sendebursts.

The radio alarm system according to the invention is described and explained in more detail below with reference to an embodiment shown in the drawing. It shows:

Fig. 1

the block diagram of a preferred embodiment and

Fig. 2

a time course using the example of an overlay of transmission bursts.

1 shows a radio alarm system with asynchronous transmission of messages via spread, superimposed message channels, the broadband sum signal obtained thereby being transmitted in a common frequency band. In particular, in the radio alarm system according to the invention, a large number of message channels for transmitting messages in the form of data telegrams between detectors M 1 ... Mn, in particular glass break detectors, infrared detectors, capacitive detectors, structure-borne sound detectors, opening detectors, microwave detectors, ultrasound detectors, etc., between switching devices S, in particular block lock, time-controlled switching devices and electronic switching devices, and between control units ST, in particular automatic dialing devices for alarm devices, in particular sirens and flash lamps or the like. and a microprocessor-controlled central unit ZE. In the transmission direction from the detectors M1 ... Mn to the central unit ZE, the message channels are formed using the code division multiplex method. The spread message channels thus formed are then superimposed on one another and converted to a single carrier frequency. Pseudo-random, orthogonal or quasi-orthogonal code words are preferably used for spreading.

A single common radio transmission channel is provided for the transmission of information in the backward direction, each data telegram having blocks with code words of the same length, as in the forwarding direction. These transmissions over the bidirectional radio transmission channels can be additionally secured by sending back an acknowledgment. Furthermore, in the radio alarm system according to the invention, the data telegrams from the detectors M are sent asynchronously and with one for all the same period duration of the message repetitions.

In a preferred embodiment of the radio alarm system according to the invention, L = 200 message channels are formed. The radio alarm system is operated in a frequency range of two to three GHz. With a transmission power of approx. 10 mW, the range (radio link) is approx. 30 m in the building and approx. 100 m in the free field. In normal operation, ie if no interference field strength (for example from other devices operating in the microwave range) is detected, a data telegram exchange is initiated, for example, every 20-30 seconds. In interference mode, ie when an interference field strength is detected, the data telegram is exchanged, for example, every 4 - 5 seconds, as is proposed in an unpublished application with the official file number P 42 39 702.2. The system delay T _s from Response of a detector M until the alarm is triggered (as well as for tamper detection) is 10s. With a net data volume of 32 bits (4 bytes per telegram) and a number of 2²⁴ (16 million) different encodings, less than one false alarm can be expected in 100 years. The high level of transmission security means that the requirements placed on operational security by the association of property insurers are more than met.

In the embodiment shown in FIG. 1, the central unit ZE has a correlator K which is set to the spreading code to be evaluated by means of a microprocessor MP and an evaluation unit AE connected to it and by means of a spreading code stored in a memory SP.

Dabei ist mit T_s die Systemverzögerung vom Melderansprechen bis zur Alarmauslösung bezeichnet.The connection sequence in the radio alarm system according to the invention with formation of the message channels according to the code multiplex method is described in more detail below with reference to FIG. 2. Each detector M sends asynchronously with a repetition period

${\text{T}}_{\text{M}}{\text{<T}}_{\text{s}}{\text{/ L - T}}_{\text{T}}\text{.}$

The system delay from detector response to alarm triggering is denoted by T _s .

Each radio connection uses its own CDMA channel (with its own PN sequence).

The receiver, in particular the central unit ZE, selects all transmitters (detectors) in turn using the respective PN sequence.

Ein Abfragen aller Sensoren benötigt L * T_K sec.Since each detector uses its own CDMA channel, the necessary observation time T _K for each channel is:

${\text{T}}_{\text{K}}{\text{= T}}_{\text{M}}{\text{+ T}}_{\text{T}}\text{= 49 ms + 1 ms = 50 ms}$

Querying all sensors requires L * T _K sec.

A large number L of message channels results in a small T _M and thus a relatively large transmission key.

With a net data volume of 32 bits per telegram, a transmission time T _T = 1 ms, a transmission data rate of 32 kbps follows. With a spread spectrum factor B _e = 1000, a CDMA bandwidth W = 32 Mbps results.

The transmitter duty cycle is

${\text{T}}_{\text{T}}{\text{= 1ms / d}}_{\text{M}}\text{= 49 ms = 0.02.}$

The transmission energy would thus be distributed fairly well over the 85 MHz broad frequency band.

At least 1000 PN sequences can be found which are suitable for CDMA in terms of their cross- and autocorrelation (eg according to Gold in the specialist book by Proakis, Digital Communications, Mc Graw Hill, p. 568). This means that min. 1000 CDMA channels can be encoded. It can be expected that the max. 200 assumed CDMA channels can be operated side by side without interference. Each CDMA channel is a co-user of the frequency band, which is superimposed on every other channel with the probability of 1 ms: 49 ms.

Claims (5)

Radio alarm system with a large number of message channels for the transmission of messages in the form of data telegrams between detectors, in particular glass break detectors, infrared detectors, capacitive detectors, structure-borne sound detectors, opening detectors, microwave detectors, ultrasound detectors, etc., between switching devices, in particular block locks, time-controlled switching devices and electronic switching devices, and between control units , in particular automatic dialing devices for alarm devices, in particular sirens and flash lamps or the like and a microprocessor-controlled central unit,
characterized in that in the direction of transmission from the detectors (M) to the central unit (ZE) the formation of the message channels using the Code multiplexing takes place and then the spread message channels thus formed are superimposed on one another and converted to a single carrier frequency. Radio alarm system according to claim 1
characterized in that pseudorandom, orthogonal or quasi-orthogonal code words are used for spreading. Radio alarm system according to claim 1,
characterized in that a single common radio transmission channel is used for the transmission of information in the reverse direction. Radio alarm system according to claim 1 or 3,
characterized in that each data telegram has blocks with code words of the same length and that the data telegrams are sent asynchronously by the detectors (M) and with a period of the message repetitions that is the same for all. Radio alarm system according to one or more of claims 1 to 4,
characterized in that the central unit (ZE) has a correlator (K) which is set to the spreading code to be evaluated by means of a microprocessor (MP) and an evaluation unit (AE) connected to it and by means of the spreading code stored in a memory (SP).

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