Wireless digital telephone system

(54) NAME Wireless digital telephone system (56) QUOTE PUBLICATIONS: US A 4,882,770 (455-603), WO Al 86/06915 (H04Q 7/04) (57) SUMMARY:

Wireless digital telephone system comprising at least one emulated base station plus one or more subscriber stations, the emulated base station comprising a station similar to the subscriber station, but having the ability to initiate a synchronization process, thereby activating allocating time slots to the subscriber station within by monitoring for positive edges in the signal.

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Background of the invention

Today's telephone systems are generally increasingly using wireless technology for national calls and sometimes they have started to use digital technology. However, there is no system in general use today that has been able to provide efficient and powerful wireless, digital technology for local calls to and from individual subscribers. Such techniques have been described in various patents recently, all of which belong to the same applicant as for the present patent application, for example, U.S. Patent No. 4,644,561, dated February 17, 1987, and U.S. Patent No. 4,675,863, dated June 23, 1987. The technique described in these patents discloses base stations communicating with both a head office and a plurality of subscriber stations, in use, of digital wireless time division circuits, wherein there are repetitive, sequential time slot positions in a transmit channel bit stream, each time slot being associated with a particular subscriber.

The base stations used in the aforementioned time-sharing system are quite complex and costly, but economically feasible for a large system serving a large number of subscribers. However, for fairly small systems serving a fairly small number of subscribers, it may be financially impossible to realize. In addition, such a system utilizes a frequency pair, a transmit frequency and a receive frequency, and considering the limited amount of channels available within the spectrum, it would be very advantageous if only one frequency could be utilized effectively.

It is therefore an object of the present invention to provide what may be called a simulated or emulated base station which can effectively replace an actual base station in certain situations.

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Another object is to provide a system that can be utilized for a plurality of subscribers, but which only operates on a single frequency.

Other purposes will be apparent from the following description and claims:

Summary of the Invention

Briefly, the system of the present invention utilizes what is, in practice, a modified subscriber station for the purpose of operating as a simulated or emulated base station, thereby substantially reducing the overall cost and complexity of the system. This emulated base station differs from the subscriber station substantially only in that the base station can initiate the synchronization process, while the subscriber station only has the function of sweeping the RF signals emitted by the emulated base station until the subscriber station finds its assigned frequency and time slot. In the intervals between transmitting the RF signals, the emulated base station is intended to receive RF signals from the subscriber units. In this way, either the subscriber unit can communicate with the emulated base station, which thereby functions as another subscriber station, or it can communicate with another subscriber station which has been synchronized with it through the emulated base station.

Brief description of the drawings

Fig. 1 is a block diagram showing a general system in accordance with the present invention.

Fig. 2 is a schematic illustration of the RCC waveform used in the standard base station.

Fig. 3 is a schematic illustration of the RCC waveform used in the present invention.

Fig. 4 is a schematic illustration showing the positive edges of the amplitude of the received signal used in coarse synchronization according to the present invention.

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Fig. 5 is a block diagram of the coarse synchronization circuit of the present invention.

Fig. 6 is a block diagram of the received AGC receiving circuit used in accordance with the present invention.

Fig. 7 is a block diagram showing the frequency retrieval circuit used in accordance with the present invention.

Figure 8 is a schematic illustration of a wireless telephone system design exemplifying the present invention.

Fig. 9 is a schematic illustration, similar to Fig. 8, but showing a two-subscriber system.

Fig. 10 is a schematic illustration of the frame format of the two-subscriber system of Fig. 9.

Fig. 11 is a schematic illustration of the frame format with a plurality of two-subscriber systems.

Fig. 12 is a schematic illustration of a system embodying the present invention used for monitoring one or more functions.

Fig. 13 is a schematic illustration of a repetition system according to the present invention.

Fig. 14 is a schematic illustration of a system of the present invention using a plurality of repetition units.

Fig. 15 is a schematic illustration of a system according to the present invention, wherein a single repetition unit is used to operate both a plurality of other repetition units and subscriber units.

Detailed description of preferred embodiments

In Fig. 1, in block diagram form, the general internal function of a system generally denoted by 10. In this system, a person speaks during a telephone conversation in a telephone 12 and the voice signal is transmitted to a local telephone interface unit 14. The signal is digitized with an encoder. / decoder 16 and the resulting digital data stream is fed to a speech processor 18 which compresses speech data to a lower data rate. The compressed data is then fed to a modem 20 via line 22 and one

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dual switch 24, the modem having the function of converting the data stream into a spectrally effective analog signal. This analog signal is fed to a radio unit 26 via a line 28. The radio unit converts the signal into a radio frequency signal (RF signal) and transmits this RF signal via an antenna 30. *

The unit is intended to receive RF signals from a subscriber unit in the intervals between transmitting the RF signals. The radio unit 26 converts each of these received RF signals into an IF signal and feeds this IF signal to the modem 20 via a line 32. The modem 20 demodulates the IF signal to form a digital signal, which is then fed to the speech processor via switch 24 and a wire 36. The speech processor then acts to expand the signal to a digitally digitized speech signal and this digitized speech signal is then input into the encoder / decoder 16, which outputs an analog speech signal to the telephone 12 via the telephone interface 14.

The data transmit mode is similar to the mode described above except that the telephone is replaced by a computer terminal or a computer 38 and the telephone, encoder / decoder and speech processor bypassed by the alternative position of switch 24, which is thereby connected to terminal 38 via wires 40 and 42.

Both the modem 20 and the radio unit 26 are connected to a controller 44. The controller 44 is initially set to a predetermined time slot, in the modulation and training mode of the modem, and to a predetermined RF frequency and power level of the radio unit. However, these parameters can be set by the subscriber unit if they are not suitable to provide satisfactory reception at the subscriber station.

In a system utilizing an actual base station, such as, for example, the system described in the aforementioned patent No. 4,675,863, the transmitted waveform is divided into a plurality of frames, with a length of, for example, 45 ms. Each frame is in turn divided into four time slots of 11.25 ms. The base station transmits on all four time slots in order to provide a modulation waveform with 100% pulse ratio, the only exception being the radio control channel (RCC). The time slot of the radio control channel is slightly shorter than 11.25 ms and this causes a small gap in the modulation at the beginning of each frame. This gap is known as an AM hole. A diagram of the waveform of the radio control channel in the format of the actual base station is shown in Figure 2. However, in the system of the present invention there is no transmission of a waveform with 100% pulse ratio. Instead, the transmission occurs in only one time slot per frame (a waveform with 25% pulse ratio), as shown in Figure 3. This modified frame format necessitates changes in coarse synchronization, automatic gain control (AGC) and frequency retrieval. These changes are described in the following description:

coarse synchronization

Since the system of the present invention uses only a waveform with a 25% pulse ratio, it monitors the amplitude of the received signal and looks for positive edges in the amplitude signal. These positive edges are shown in Fig. 4. The subscriber unit sets its frame rate to align with the presence of these positive edges.

The circuit for obtaining the above-mentioned type of coarse synchronization is shown in block diagram form in Fig. 5, the received signal being displayed as it is fed into an amplitude calculation device 50 which produces a computer amplitude signal which is then fed to a comparator 52, where it is compared with a predetermined threshold signal. whereby a digital signal is generated (1 = signal is present, 0 = no signal is present). This digital signal is input to an edge detector 54 which outputs a sweep pulse to indicate the detection of a positive edge.

AGC

The 25% pulse ratio modulation requires a distinct type of AGC receiving circuit, which avoids tracking when there is no signal. An AGC with a slow rise467 720 and rapid decay is therefore recommended. This is shown in Fig. 6, where the received signal is input into an amplitude calculator 56, which can take the form of a preprogrammed ROM, from which a resulting amplitude signal is input into a comparator 58, in which the signal is subtracted from a predetermined threshold value to form a difference signal. This differential signal is applied through one of two scaling multipliers, shown by 60 and 62, to a low pass filter which includes an adder 64 and a delay means 66 connected in a loop 68. One or the other of the two multipliers is used in accordance with with the difference signal. If the difference signal is positive, the slow decay is implemented in the AGC control signal. If the difference signal is negative, the rapid increase in the AGC control signal is implemented. The output of the filter is the gain signal, which is then applied to the gain controller 44, shown in Figure 1.

Rough frequency retrieval

Since it is not necessary to perform frequency retrieval during the shutdown time (75% zero time) in the frame format with 25% pulse ratio and since the frame rate · is not known at the time when frequency retrieval is performed, a modified form of frequency retrieval circuit is shown, as shown in Fig. 7. In this circuit, the received signal is fed into a discrete Fourier Transform (DFT) 70 calculating means which outputs the high band energy (the energy in the frequency band above the center frequency) and the low band energy (the energy in the frequency band below the center frequency). The high-band energy output is subtracted from the low-band energy output at adder 72 and the output from it is applied to a mixer or multiplier 74. The received RF signal is also fed to a purifier 76 (stripping) which clears the signal's sign (negative or positive), only determining the amplitude of the signal. The purified signal is then fed to a filter 78, which smooths it by averaging the signal. The output of the filter 78

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U / / ά. A is fed via an amplifier 80 to the multiplier 74.

The basic meaning of the circuit of 76, 78 and 80 is to prevent the influence of noise on the output signal while the signal itself is amplified. Since noise is generally low in amplitude, noise in this regard will be effectively filtered out during the equalization process. Since, on the other hand, the actual signal generally has a rather large amplitude, this will in practice be amplified by adding the equalized or filtered signal to mixer 74.

The scaled signal leaving mixer 74 is balanced between high-energy and low-energy frequencies, and this balanced signal, which is proportional to the average amplitude of the received signal for a short time, is fed into a low-pass filter comprising an adder 82 and a delay means 84 connected a loop at 86. The delay means causes the output signal 88, to the VCXO controller, to represent the output signal immediately before the output signal which is in practice fed into the low pass filter. The VCXO controller is used to set the frequency of the system's master oscillator.

When initial or coarse synchronization has been performed, the system is in a dormant voice mode but is fully connected for voice operation. If the microphone at either end is lifted, the phone at the other end will ring until you answer the ringing phone or until the initiating microphone is hung up.

The calls are connected by a voice code word (VCW) at the beginning of each talk time slot, the code word indicating that the microphone is being lifted off at the initiating station. When this occurs, the station which functions as an emulated base station will itself go to the state with the microphone lifted relative to the head office (CO), whereby a connection is connected to the head office. The initiating subscriber station then proceeds to complete the call connection by dialing the desired number. When the initiating subscriber unit goes to the state

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with the microphone turned on, the emulated base station is informed with the corresponding voice code word VCW and presents a connection with the microphone put on to the head office.

When the emulated base station detects a ring signal from the head office, the subscriber unit is called, using the corresponding speech code word VCW, from the emulated base station. When the subscriber unit's microphone is then lifted off, the emulated base station is informed, via the corresponding voice code word VCW, and presents a connection with the microphone lifted to the head office.

The aforementioned type of wireless telephone system configuration is exemplified in Figure 8, wherein the subscriber unit 90 is shown in wireless communication through antennas 92 and 94 with the amulated base station 96. The station 96 is in wired connection through a line 98 and an interface 100 with the head office.

Two-subscriber system

The above system can be utilized with a two-subscriber arrangement, as shown in Fig. 9. In this system, each channel can carry two complete conversations without requiring the use of a duplex unit. In this regard, a two-subscriber unit 102 is connected by wires 104 and 106 to a subscriber telephone pair 108 and 110. The subscriber unit 102 is in wireless communication via antennas 112 and 114 with an emulated two-base station 116. Unit 16 is connected to head office by wire connections 118 and 120.

The two separate subscribers 108 and 110 utilize an arrangement with time slots as described in the aforementioned Patent No. 4,675,863, each subscriber being assigned a separate time slot. The frame format for this arrangement is shown in Fig. 10, showing four time slots numbered 1, 2, 3 and 4. The first two time slots are used for the emulated base station and the last two time slots are used for the two subscribers.

A plurality of two-subscriber systems can operate on separate channels without duplex units by synchronizing all broadcasts on the emulated base station. This is shown in frame format of Fig. 11, with channel 1 shown above and channel n (indicating any desired number of channels in between) shown below. Each channel uses the first two time slots for transmission and the last two for reception.

For remote search services

An emulated base station can be utilized with a plurality of different subscribers, one at a time. In such an arrangement, the subscribers continuously monitor the transmission of radio control channels (RCC), described more fully in said patent 4,675,863, for reception until the emulated base station searches for a particular subscriber by means of the subscriber ID number (SID). Upon receipt of a search, the subscriber initiates a retransmission to the emulated base station, using the synchronization process described above. To initiate a call, the subscriber of the radio control channel broadcasts RCC using the aforementioned synchronization process.

Monitoring Function

The present system can be utilized for monitoring one or more functions. In this regard, a plurality of subscribers may be periodically surveyed, using a computer as a control / data logger, for the purpose of reporting features such as temperature, weather, safety, water / flood alerts, fuel shortages, remote reading of gas, electricity or water meters etc. This is shown in Fig. 12, in which an emulated base station 122 is in wireless communication with a plurality of subscriber units indicated by 124, 126 and 128. The unit 122 is in wired connection with both a telephone 130 for voice communication and a computer or data terminal 132 for data entry. . Similarly, each subscriber unit is connected both to a telephone 134, 136 and 138 for voice communication as well as to a data means such as 140, 142 and 144 respectively.

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rehearsal System

An important use of the present system is in the form of a repetition unit in order to expand the area of ​​the system. In this arrangement, the emulated base station can be utilized to overcome interfering obstacles, such as rocks and the like. Fig. 13 shows this function, showing a subscriber unit 146 in wireless communication with an emulated base station 148 at the top of a mountain. The unit 148 is also wirelessly connected to a standard base station 150, which is connected to a main office.

The relative simplicity and cheap design of the emulated base station makes it very cost effective as a repetition unit. It can also be used as a rehearsal unit for the purpose of expanding the national area of ​​the system regardless of the presence or absence of obstacles. By utilizing the arrangement with time slots, the repeater unit, without the use of any duplex unit, fits into the overall system while remaining transparent to both the standard base station and the subscriber. Of course, it can also be placed between the subscriber and another emulated base station instead of a standard base station. This can be accomplished in a number of steps from one emulated base station to another, in order to greatly increase the area of ​​the system in a fairly inexpensive manner. This is shown in Fig. 14, in which a series of repetition units 152 are arranged between a subscriber 154 and a base station 156.

The repetition unit also works to, in addition to increasing the range of the system, purify the actual base station's signal by leveling before relaying to the subscriber.

A repetition unit can also be utilized in what may be called a repetition star system, for the purpose of operating multiple repetition units and / or subscribers. This is shown in Fig. 15, wherein a single repetition unit 158 ​​is in wireless communication with subordinate repetition units, 160 and 162, as well as with one or more subscribers, such as at 164. The subordinate repetition units themselves are in wireless communication with the subscriber as shown at 166.

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168, 170, 172 and 174, as with other minor repetition units, such as at 176. Any of these minor repetition units, such as a repetition unit 162, can be utilized as the final repetition unit which is in direct communication with the base station, as indicated at 178.

Multiple repetition units can be located in one location, on different channels and synchronized so that their broadcasts and receipts occur simultaneously, avoiding the use of duplex units. In such a configuration, a master repetition unit is utilized to monitor the RCC channel for the base station and transmits the monitored information to the various subscribers via the emulated base station radio control channel RCC. In such a configuration, subscribers are each assigned a call channel during call setup.

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