Abstract:
In the use of a transceiver based upon frequency modulation for signals coded by a method for spreading spectrums, a series of first data symbols having a valence M is used as a basis for assigning a series of L second binary data symbols to each of the first data symbols. In this case, the L-element series of second data symbols correspond to chip sequences used for the spectrum-spreading method. A signal containing the sequence of second data symbols is used by the transceiver for the frequency modulation.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation of copending International Application No. PCT/DE00/03984, filed Nov. 13, 2000, which designated the United States and which was not published in English. 
     
    
     
       BACKGROUND OF THE INVENTION  
       FIELD OF THE INVENTION  
         [0002]    The invention relates to the use of a transceiver configured for frequency modulation for signals that are coded by a spreading spectrum method, in particular signals that are coded by a direct-sequence code division multiple access (DSCDMA) method.  
           [0003]    For use in the industrial, medical or scientific sector, the industrial-scientific-medical (ISM) frequency band at 2.4 GHz is globally available. The frequency band can be accessed while conforming to the guidelines issued by the Federal Communications Commission (FCC) by spread spectrum technology. For example, this allows the creation of cordless phones or telecommunication systems based on radio transmission for the transmission of measured values using the ISM frequency band.  
           [0004]    For systems that use the ISM frequency band, direct-sequence code division multiple access or frequency-hopping code division multiple access (FH-CDMA) is predominantly used as the method of accessing the radio channel. In the case of systems with DS-CDMA, phase-shift keying (PSK) modulation and in the case of systems with FH-CDMA, frequency-shift keying (FSK) modulation is usually used as the modulation method.  
           [0005]    In many cordless telecommunication systems already on the market, such as for example digital enhanced cordless communication (DECT), worldwide digital cordless communication (WDCT) or in the case of Bluetooth systems, transceivers (transmitters/receivers) that are based on Gaussian-frequency shift keying (GFSK) modulation are used. GFSK modulation is a special form of FSK modulation in which a Gaussian low-pass filter with a prescribed bandwidth/symbol duration product BT, for example of 0.5, is used for the baseband prefiltering. Signals modulated by GFSK have a constant envelope, advantageously allowing simple transmitter amplifiers to be used. However, such telecommunication systems are not suitable, for example, for the DS-CDMA access method on account of the PSK modulation method required for this. Accordingly, in the case of such telecommunication systems, a transceiver that is suitable for the PSK modulation method, but in turn is not suitable for FSK modulation, must be used if the DS-CDMA access method is to be used.  
         SUMMARY OF THE INVENTION  
         [0006]    It is accordingly an object of the invention to provide for the use of a transceiver configured for frequency modulation for signals that are coded by a method for spreading spectrums which overcomes the above-mentioned disadvantages of the prior art methods of this general type, which makes it possible for a transceiver configured for a GFSK modulation method to be used for a DS-CDMA access method.  
           [0007]    With the foregoing and other objects in view there is provided, in accordance with the invention, a method for using a transceiver configured for frequency modulation for signals that are coded by a method for spreading spectrums. The method uses a series of first data symbols having a valence M as a basis for assigning a series of L second binary data symbols to each of the first data symbols. The series of the L second data symbols corresponds to chip sequences used in the method of spreading spectrums. A duration of a chip sequence is extended in comparison with a duration of the first data symbols by a factor L. A signal containing a sequence of the second binary data symbols is used by the transceiver for the frequency modulation.  
           [0008]    The idea on which the invention is based is that of coding the chip sequences used for the spectrum-spreading method by first data symbols, which are transmitted by frequency modulation. Each chip sequence in turn represents a data symbol from a multiplicity of first data symbols, the actual data to be transmitted by a spectrum-spreading method. As a result, the transceiver, which is configured for sending and receiving frequency-modulated signals, can be used without any circuit-related changes for signals coded by a spectrum-spreading method. Such transceivers are used, for example, in DECT, WDCT, SWAP or Bluetooth systems. However, in this case a reduction in the available transmission rate by a factor of L must be accepted, since the duration of a chip sequence is L-T bit  and precisely one first data symbol of the duration T bit  is coded by a chip sequence. The reduced data rate makes the invention suitable in particular for cordless phones with a TDD (Time-Division Duplex) speech connection or for the transmission of measured values with a low data rate.  
           [0009]    The transceiver preferably sends and receives signals that are modulated by the Gaussian-Frequency Shift Keying modulation method. This modulation method is used, for example, in the case of cordless phones conforming to the DECT standard and is therefore widespread and able to be implemented with inexpensive transmitting/receiving stages.  
           [0010]    The first data symbols are preferably binary data symbols or data symbols having the valence two, whereby the sequence assigned to a first data symbol d n  can, depending on its value, be given in the form c n,v  or 1−c n,v , where v=0, . . , L−1.  
           [0011]    The signals coded by a spectrum-spreading method are preferably coded by the direct-sequence CDMA method. As a result, a transceiver operating on the basis of the GFSK method can be used without great expenditure.  
           [0012]    In an alternative preferred use, the transceiver is suitable for frequency hopping and the signals coded by a spectrum-spreading method are coded by a frequency-hopping CDMA method (FH-CDMA). This provides the possibility of using it both for DS-CDMA-coded signals and for FH-CDMA-coded signals. Such a transceiver is then switched over according to the coding of the signals to be sent and received.  
           [0013]    The transceiver preferably sends and receives signals in the ISM frequency bands (ISM band: Industrial-Scientific-Medical band). As an example, mention may be made here of the 2.4 GHz frequency band, which is authorized by the Federal Communications Commission for use in industry, science and medicine, and can be used for example for the transmission of measured values by radio.  
           [0014]    For example, a transmitter from the DECT or WDCT system, which operates on the open-loop principle, may be used for the transceiver. Alternatively, however, transmitters operating on the closed-loop principle, such as for example a ΣΔ-modulated fractional-N PLL frequency synthesizer may also be used.  
           [0015]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0016]    Although the invention is illustrated and described herein as embodied in the use of a transceiver configured for frequency modulation for signals that are coded by a method for spreading spectrums, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
           [0017]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0018]    The single FIGURE of the drawing is a block diagram of a receiver of a transceiver used for explaining the method according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    Referring now to the single FIGURE of the drawing, there is shown a receiver that amplifies a radio-frequency received signal  1  with a low-noise amplifier  2  (LNA: Low-Noise Amplifier) and passes I and Q components of the amplified received signal to a first multiplier  3  and a second multiplier  4 , respectively. The first multiplier  3  multiplies the supplied signal (I component) by a carrier frequency  5 , to obtain from the received signal  1  a baseband received signal. By analogy, the second multiplier  4  multiplies the supplied signal (Q component) by a carrier frequency  6 , likewise to obtain from the received signal  1  a baseband received signal.  
         [0020]    The components of the received signal, “mixed down” in this way to the baseband received signals, are in each case fed to a first anti-aliasing filter  7  and a second anti-aliasing filter  8 , to satisfy the Shannon sampling theorem for a digitization of the baseband received signals.  
         [0021]    The low-pass-filtered signals are digitized, shifted in their frequency, filtered and fed to a differential demodulator by a first analog/digital converter  11  and a second analog/digital converter  12 . Alternatively, instead of a differential demodulator, an analog FM demodulator, used in DECT or WDCT systems, may be used on the basis of the limiter-discriminator principle. However, on account of the non-linearities of the analog FM demodulator, sacrifices in interference suppression must be expected.  
         [0022]    The frequency converter has third to sixth multipliers  13  to  16  and a first adder  17  and second adder  18 . The third multiplier  13  and the fourth multiplier  14  multiply the supplied digitized signal by a first control signal  9 , parallel to which the fifth multiplier  15  and the sixth multiplier  16  multiply the supplied digitized signal by a second control signal  10 . The first control signal  9  and the second control signal  10  correspond to the two signal components in the modulation of the transmitted signal on which the received signal is based in a transmitter, which signal components are used for generating the binary transmitted signal. The output signal of the third multiplier  13  and of the fifth multiplier  15  or the output signal of the fourth multiplier  14  and of the sixth multiplier  16  is added by a first adder  17  or by a second adder  18  and fed to a first filter  19  or a second filter  20 , respectively.  
         [0023]    The first filter  19  and the second filter  20  filter the demodulated signals and feed them to a post-processing device  21 , which normalizes and brings together the two signals. The output signal of the post-processing device  21  is fed to a differential demodulator, containing a multiplier  22  and a delay element  23 , disposed parallel to the latter, and also a downstream element  24  for forming the conjugate complex. The output signal of the element  24  is likewise fed to the multiplier  22  and multiplied by the output signal of the post-processing device  21 . The output signal of the seventh multiplier  22  is fed to an imaginary-part generator  25 , which filters the imaginary part out of the supplied signal.  
         [0024]    The use according to the invention is explained below on the basis of a simple exemplary embodiment.  
         [0025]    Binary data symbols d n ε{0, 1} are coded in the case of DSCDMA by chip sequences &lt;c n, 0 , . . . , c n,L−1 &gt; with chips c n,v ε{0, 1}. The chip sequences have in this case a length L. The binary data bit d n =1 is transmitted by sending out a chip sequence &lt;c n,0 , . . . , c n,L−1 &gt; and the binary data bit d n =0 is transmitted by sending out the inverted chip sequence &lt;1−c n,0 , . . . , 1−c n,L−1 &gt;. The use of such chip sequences in cordless telecommunications systems within the ISM frequency band serves for suppressing narrowband interference signals, which have a less disturbing influence in the case of the broadband CDMA method than in the case of a narrowband access method such as F/TDMA (Frequency/Time-Division Multiple Access). The FCC defines a minimum processing gain for suppressing interference signals, which has to be satisfied by telecommunications systems that use the ISM band, in order to avoid or suppress possible interference signals. To maintain the minimum processing gain stipulated by the FCC, transceivers for signals modulated by the (G)FSK modulation method may be used for the DS-CDMA access method.  
         [0026]    For this purpose, the bits d′ k  of a signal modulated by (G)FSK are merely assigned to the chips of the above chip sequences: d′ k =c n,v  for the binary data bit d n =1 and d′ k   =1−c   n,v  for the binary data bit d n =0, where k=n·L+v . Consequently, the bit rate T′ bit  of the signal modulated by (G)FSK corresponds to the chip bit rate T chip . By this simple mapping, a DS-CDMA system can be created. The bandwidth/duration product BT′bit of the Gaussian low-pass filter of the (G)FSK system consequently similarly applies to the DS-CDMA system. The resulting bit rate T bit  of the DS-CDMA system corresponds to a bit rate T′ bit  of the (G)FSK system: 1/T bit =1/(T′ bit  L), reduced by the length L of the chip sequence.  
         [0027]    In a hard decision, a detection of the chips c n,v  takes place in a receiver like a detection of the bits d′ k  of the signal modulated by (G)FSK. A decision for the sent bit d n =1 or d n =0 takes place in this case in the receiver by a comparison of the received chip sequence with the undisturbed chip sequences &lt;c n,0 , . . . , c n,L−1 &gt; for the bit d n =1 and &lt;1−c n,0 , . . . , 1−c n,L−1 &gt; for the bit d n =0. Alternatively, a soft decision may also take place in the case of an output signal of the demodulator available with higher accuracy.