Abstract:
In the digital multi-subscriber radio telecommunication system which operates using frequency hopping technology and is intended for use in the 2.4 GHz ISM (Industrial Scientific and Medical) frequency band, in which a 20 dB channel bandwidth of a maximum of 1 MHz is prescribed, the known, DECT radio system operating using TDMA/FDMA technology is matched, in RF terms, to the ISM band and is modified such that the data transfer rate is reduced, preferably halved, by comparison with the data transfer rate of 1152 kbit/s, which is prescribed as standard for DECT. The system according to the invention can be used in cordless telephones.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a digital multi-subscriber radio telecommunication system which operates using frequency hopping technology and is intended for use in the 2.4 GHz ISM (Industrial Scientific and Medical) frequency band, in which a 20 dB channel bandwidth of a maximum of 1 MHz is prescribed and rust be adhered to. 
     The ISM frequency band from 2400 MHz to 2485 MHz is available worldwide for industrial, scientific and medical purposes. This UHF frequency band car be used to provide radio services, that is to say including cordless telephones. However, in particular, exactly prescribed conditions must be adhered to for radio operation in this frequency band. 
     If a digital multi-subscriber radio telecommunication system operating using frequency hopping technology is to be produced in this 2.4 GHz ISM band, then it is a significant feature that a 20 dB bandwidth of a maximum of 1 MHz is prescribed for this purpose and must be strictly adhered to; this is known to be exceeded in the case of the DECT standard. 
     A digital multi-subscriber radio telecommunication system which operates using frequency hopping technology and is intended for use in the 2.4 GHz ISM (Industrial Scientific and Medical) frequency band is nor known to date. For the WLAN standard IEEE 802.11, a data transfer rate of 1 Mbit/s is stipulated for GFSK (Gaussian Frequency Shift Keying) modulation. However, this results in markedly higher demands on accuracy for the modulation deviation in the radio-frequency part of the radio sets and hence in a much higher technical and hence cost outlay than in DECT units, for example. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide, for the 2.4 GHz ISM frequency band, a digital multi-subscriber radio telecommunication system which operates using frequency hopping technology and is based on an already widespread radio system of this type, so that only relatively small alterations are required in the units used for this system when used as desired in the ISM band, and the extent of the alterations in the digital modules is so small that said modules can be incorporated in the units of the radio system which is already used without any significant additional technical and hence cost outlay. 
     In accordance with the invention, which is based on a digital multi-subscriber radio telecommunication system of the type mentioned in the introduction, this object is achieved by a modification to the known DECT (Digital European Cordless Telecommunications) radio system, which operates using TCM. (Time Division Multiple Access)/FDMA (Frequency Division Multiple Access) technology, such that the data transfer rate is reduced by comparison with the data transfer rate of 1152 kbit/s which is prescribed as standard for DECT, and that there is provision for the radio-frequency parts of the units used in the system to be matched to the ISM frequency band, which is altered by comparison with the DECT standard. 
     The invention thus provides a frequency hopping system in the 2.4 GHz ISM band. It has been recognized that the DECT system is frame duration of 10 ms means that the system is capable of making the required frequency changes, where a frequency should He maintained for a maximum of 0.4 s in 30 s. 
     Expedient embodiments, developments and one possible se for the system according to the invention are specified in the dependent claims. 
     According to one advantageous embodiment of the invention, the data transfer rate is reduced to half the data transfer rate prescribed in the DUCT standard, that is to say is reduced from 1152 kbit/s to 576 kbit/s. In this case, two time slots are expediently combined in order to maintain the net data transfer rate. The channel spacing is expediently likewise reduced to half the DECT channel spacing, that is to say is reduced from 1532 kHz to 864 kHz. 
     The Frequencies can thus be readily generated from a standard DECT system clock. In the radio-frequency part of the units used, only one new IF filter with half the bandwidth is required in addition to the front end&#39;s being matched to the new frequency range. 
     In principle, a reduction to another fraction of the DECT data transfer rate, e.g. to ⅔ or ¾ of the DECT data transfer rave, with another channel spacing is also possible. However, this may result in additional outlay by comparison with the reduction to half the DECT data transfer rate and to half the DECT channel spacing. 
     An illustrative embodiment of a subscriber unit for the digital multi-subscriber radio telecommunication system for use in the 2.4 GHz ISM frequency band is explained below and is shown in the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an entire block diagram of the transceiver unit for a subscriber. 
     FIG. 2 shows the block diagram of a PLL frequency synthesizer used in DECT units, and 
     FIG. 3 shows the block diagram of a PLL frequency synthesizer altered for use in ISM units. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The transmission path  1  of the highly integrated transceiver shown in FIG. 1 for the ISM band contains modules used in DECT units and operates using open-loop modulation, to which end the digital signal TXDATA which is to be transmitted is supplied via a Gaussian filter  2  to a voltage-controlled oscillator (VCO)  3  for the transmission mode, which has Win associated second voltage-controlled oscillator  4  for the reception mode. Also integrated are two voltage regulators  5  and  6  for setting the supply voltages of the analogue RF part or of the voltage-controlled oscillators  3  and  4 . 
     The voltage-controlled oscillators  3  and  4  generate differential signals, the signals from the active oscillator  3  or  4  being input into a frequency doubler  8  via isolating amplifiers  7 , and then also into a prescaler  11  in a PLL frequency synthesizer  19  in the path via a filter  9  and an amplifier  10 . 
     The output signal from the amplifier  10  is supplied in the transmission path  1  via a printed filter  12 , a preamplifier  13 , power amplifier  14  and a low-pass filter  15  to the transmission input of a duplexer switch  16 , whose antenna connection has an antenna  17  connected to it which is jointly us;ed for transmission and reception. 
     The desired output signal in the ISM band is thus produced in the path via a frequency doubler  8 , so that the voltage-controlled oscillators  3  and  4  can be operated at half the output frequency, which means that a higher level of isolation is attained for the open-loop modulation 
     An important component part of the integrated circuit called the transmission part chip  18  is the integrated PLL frequency synthesizer  19 , which can be programmed using a 3-wire bus interface  20  and can be set by the prescaler  11  and also by an arithmetic and logic unit with counters in the division ratios. Furthermore, the 3-wire bus interface  20  is used to control all chip functions, that is to say the various modes of operation, for example; in particular, the PLL arithmetic and logic unit and the mode of the voltage-controlled oscillators  3  and  4  are Controlled. 
     The operation of the PLL frequency synthesizer  19  also includes the loop low-pass filter  21 , which is not directly jointly integrated in the transmission part chip  18 . The number of interface wires to a baseband control device is thus kept at a minimum. 
     The PLL frequency synthesizer  19  is explained in more detail later in conjunction with FIGS. 2 and 3. The frequency doubling (carried out in the frequency doubler  8 ) of the output signal from the active differential voltage-controlled oscillator  3  or  4  is performed by multiplying the signal by its quadrature component. 
     An input of a down-converter  23  is connected via a first ceramic bandpass filter  24 , a low-noise amplifier  25  and a second ceramic bandpass filter  26  to the receiver connection of the duplexer switch  16  in a transceiver reception path  22 . The two bandpass filters  24  and  26  are designed to let through the ISM frequency band from 2400 MHz to 2485 MHz. The other input of the down-converter  23  is supplied with a signal which is output from the transmission path I and is derived by the voltage-controlled oscillator  4  for the reception mode. 
     The output signal taken from the down-converter  23  is passed via an IF filter  27  and a limiter  28 , is then demodulated in analogue form in a demodulator  29  and is subsequently subjected to analogue/digital conversion using amplifiers  30  and a sample and hold circuit  31 , so that a digital data signal RXDATA which has been is available. A large part of the reception path  22  is integrated in a reception part chip  32 . 
     The unit design originally provided for DECT has thus been altered in the radio-frequency part, whose front end is matched to the ISM frequency range, which is different from that for DECT, and in the IF filter  27 , for which half the DECT bandwidth is required. In addition, there are then also alterations, described in detail, in the PLL division ratios of the transmission part chip as a result of a modification to the counters in the arithmetic and logic unit for the PLL frequency synthesizer  19 . 
     FIG. 2 shows a PLL frequency synthesizer which is already used for DECT transceiver units and is contained together with other functional groups in the transmission part chip  18 , It contains the prescaler  11 , an N counter  33 , an A counter  34 , an R counter  35 , a phase detector  36 , a charge pump  37 , a tristate logic unit  38  and a PLL control logic unit  39 . The prescaler  11  and the counters form an arithmetic and logic unit which defines the PLL division ratios. 
     In DECT mode, the N counter  33  can be changed over between 34/35 in transmission mode and 32/33 in reception mode. The R counter  35  an be changed over between 6 and 12 in order to permit a system clock of 10368 MHz and 20376 MHz, respectively. The PLL frequency synthesizer  19  is able to address the DECT nominal channels given by f e =f 0 −c·1728 kHz, where f 0 1897.344 MHz and c=10, 9, 8, . . . , −53. 
     Hence, all the DECT nominal channels in the frequency band from 1880 MHz to 1990 MHz can be addressed by the slowly hopping frequency synthesizer (c=9, . . . , 0). The intermediate frequency is assumed to be 110.592 MHz. This gives an N counter value of 34/35 in transmission mode and of 32/33 in reception mode. 
     The phase detector  35  is phase- and frequency-sensitive. The prescaler  11  divides the signal frequency of the doubled output signal from the deactivated voltage-controlled oscillator  3  or  4  using a ratio of 1:32 or 1:33. The division ratio is controlled by means of the PLL control logic unit  39 . 
     The PLL frequency synthesizer for the ISM band (shown in FIG. 3) is modified in a few places by comparison with the PLL frequency synthesizer for DECT (shown in FIG.  2 ). The modified circuit blocks in the transmission part chip  18  in FIG. 1 are shown with a bold border in FIGS. 2 and 3. 
     The N counter  33  can be changed cover between 86-89 and 82-85, in each case based on the transmission mode or on the reception mode. The R counter  35  can be changed over between 12 and 24 in order to permit a system clock of either 10368 MHz or 20736 MHz. The PLL Ti frequency synthesizer is able to address the ISM nominal channels given by f e =f 0 +c·864 kHz, where f 0 2377.728 MHz and c=0, 1, 2, . . . , 127. 
     Hence, all ISM nominal channels in the frequency band from 2400 MHz to 2485 MHz can be addressed by a slowly hopping synthesizer (c =27, . . . , 121). The intermediate frequency is assumed to be 110.592 MHz, as in the DECT system. This gives an N counter value, for the ISM band, of 86-89 in transmission mode and of 82-85 in reception mode.