Abstract:
A method, and corresponding modem, of transmitting data. The method including: modulating the data onto a plurality of carriers with different carrier frequencies, resulting in a transmission signal; notching predetermined frequency bands of the transmission signal, which predetermined frequency bands would otherwise disturb external transmissions; and boosting carriers that are adjacent to the predetermined frequency bands and that would be attenuated because of the notching.

Description:
TECHNICAL FIELD 
     The invention relates to a method for transmitting data and to a modem. 
     BACKGROUND OF THE INVENTION 
     Powerline communication (PLC) might have interferences to fixed radio broadcasting or other external transmissions. Today, PLC modems have fixed notch filters for amateur radio bands. Filters for fixed notches can be implemented with a high suppression and very steep slopes. Concepts of dynamic or smart notching enables PLC modems to detect an ingress of fixed radio broadcast stations. The frequencies where radio stations have been detected shall be omitted by PLC. In order to realize this in a PLC transmitter, adaptive notch filters for suppressing a transmission signal in a predetermined frequency band might be used. These adaptive notch filters also attenuate carriers adjacent or neighbored to said predetermined frequency bands, because notch filters for dynamic suppression of frequencies have weaker slopes, especially if there are many individual frequencies to be notched. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for transmitting data and a modem, which enhances properties of adaptive notch filtering for the purpose of PLC systems. 
     The object is solved by a method of transmitting data, comprising:
         modulating said data onto a plurality of carriers with different carrier frequencies, resulting in a transmission signal;   notching predetermined frequency bands of said transmission signal, which predetermined frequency bands would otherwise disturb external transmissions; and   boosting carriers which are adjacent to said predetermined frequency bands and which would be attenuated because of said notching step.       

     In a further aspect the object is solved by a modem, comprising:
         a plurality of modulators each configured to modulate input data onto one of a plurality of carriers;   a plurality of amplifiers each connected to one of said plurality of modulators;   a transformation unit, connected to said amplifiers and configured to sum up signals from each of said amplifiers to generate a transmission signal;   a notch filter connected to said transformation unit, said notch filter being configured to notch predetermined frequency bands of said transmission signal;   a notch control unit connected to said notch filter and connected to said amplifiers, said notch control unit being configured to adjust the amplification of said amplifiers in correspondence to said predetermined frequency bands.       

    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent from the following description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which 
         FIG. 1  is showing a spectrum of a transmitted signal with multiple notches, 
         FIG. 2  is showing schematically a block diagram of a modem with an adaptive notch filter; and 
         FIG. 3  is showing schematically a block diagram of features of a method for transmitting data. 
     
    
    
     DETAILED DESCRIPTION 
     In order to implement flexible notch filters Infinite Impulse Response (IIR)-Filters are used. A corresponding filter structure is implemented fixed in hardware. The filter coefficients are programmed during operation. The number of notches that have to be implemented and the frequency bands that have to be notched are decided after an ingress detection of radio services by an antenna and a corresponding measuring unit of the modem or by snooping noise ingress at mains of powerline systems. The hardware filter structure has to be general for all filter frequencies and filter bandwidth. Usually this generic filters have weak slopes and attenuate the neighboring or adjacent carriers of an Orthogonal Frequency Division Multiplexing (OFDM)-scheme used within PLC modems. 
       FIG. 1  shows an example of a transmission spectrum of a transmission signal, where some flexible notch filters omit frequencies. The original Power Spectral Density (PSD) shall be −40 dBm measured with a resolution bandwidth (ResBW) of 1 kHz. For instance in  FIG. 1  the frequencies around 10.2 MHz to 11 MHz should have a transmit PSD of −40 dBm. Due to the side suppression of notch filters at 9.5 MHz and 11.7 MHz the communication spectrum around 10.2 MHz to 11 MHz is attenuated by 10 dB or more. 
     With a modem  200  as depicted in  FIG. 2  OFDM carriers at carrier frequencies around 10 MHz to 11 MHz can be amplified, boosted or pre-distorted by at least approximately the same amount as the weak slopes of the notch filters attenuate them. In this embodiment all transmitted carriers would have identical power spectral density. This raises the signal-to-noise ratio (SNR) at a receiver and enables at an adaptive OFDM system the use of higher constellations of a carrier involved. 
     In  FIG. 2  an OFDM part of a modem  200  is schematically depicted, where amplitudes of carriers could be adjusted individually. Payload data is transmitted to a Feed Forward Error Correction (FEC) Unit  202 . Afterwards, a plurality n of corresponding Quadrature Amplitude Modulators (QAM)  204 _ 1 ,  204 _ 2 ,  204 _ 3 , . . . ,  204   —   n  maps information of the Individual bits to its complex constellation points. Each of the Quadrature Amplitude Modulators (QAM)  204 _ 1 ,  204 _ 2 ,  204 _ 3 , . . . ,  204   —   n  is connected to one of a plurality n of amplifiers  206 _ 1 ,  206 _ 2 ,  206 _ 3 , . . . ,  206   —   n  and a corresponding carrier of a plurality n of carriers is multiplied by an amplitude factor of said amplifier. Carriers, which are known to be notched, may already become suppressed at this stage and are not injected into summation. All active carriers are summed in a transformation unit  208 , e.g. an Inverse Fast Fourier Transformation (IFFT)-unit, and a transmission signal in time domain is generated. Due the weak side loop suppression of OFDM systems this signal will be filtered additionally by a flexible notch filter  210  and afterwards digital-to-analog converted by an digital-to-analog converter (DAC)  212  and transmitted to a corresponding receiver (not depicted). 
     The flexible notch filter  210  can also attenuate frequencies adjacent to the notched frequency band. This unwanted side-attenuation can be compensated by a pre-amplification of the affected carrier. 
     The depicted embodiment of the OFDM system is able to set an amplitude of each carrier individually. This may be achieved by multiplying each affected carrier after the corresponding QAM mapper  204 _ 1 ,  204 _ 2 ,  204 _ 3 , . . . ,  204   —   n  with its amplitude factor. This multiplication boosts or amplifies the carrier by an amount identical or at least approximately identical to the amount of attenuation of this carrier resulting from the flexible notch filter  210 . The value of attenuation for each carrier resulting from the flexible notch filter  210  can be derived from the frequency response of the flexible notch filter  210 . 
     This embodiment ensures that the carriers are boosted to their original (not attenuated) level. Therefore, an amplification of the PSD beyond regulatory limits can be avoided. 
     A notch control unit  214  is provided, which is connected to the flexible notch filter  210  and the amplifiers  206 _ 1 ,  206 _ 2 ,  206 _ 3 , . . . ,  206   —   n , the notch control unit  214  being configured to set the filter coefficients of the flexible notch filter  210 , to calculate the frequency response of the filter  210  and to boost or amplify the attenuated carriers by setting a corresponding amplitude factor. The notch control unit  214  may be connected to an antenna  216  and a corresponding first measuring unit  218  in order to determine frequency bands of external transmissions like e.g. radio amateur frequency bands or radio transmissions from radio stations, e.g. on short wave channels. Another embodiment which is depicted as well in  FIG. 2  comprises a further measuring unit  220 , which is configured to snoop noise ingress on mains  222  in order to determine said frequency bands. The modem  200  may comprise the antenna  216  and the first measuring unit  218  and/or the further measuring unit  218  alternatively or in combination, which is depicted schematically by the use of broken lines. 
     These measured frequency bands can be used to correspondingly determine said filter coefficients of said flexible notch filter  210 , so that corresponding parts of the transmission signals are notched and the corresponding amplitude factors are accordingly adjusted. 
     In  FIG. 3  a schematically block diagram for the method of transmitting data is depicted. In a first step S 1  the noise ingress is measured and the frequency bands which should be notched are detected. In a second step S 2  filter coefficients for the notch filter  210  are calculated and the corresponding frequency response of the notch filter  210  is determined. In a third step S 3  an attenuation of used adjacent carriers due to this frequency response is evaluated and in a fourth step S 4  the amplifiers  206 _ 1 ,  206 _ 2 ,  206 _ 3 , . . . ,  206   —   n  are programmed to boost the carriers. 
     REFERENCE SYMBOLS 
     
         
           200  Modem 
           202  Forward Error Correction Unit 
           204 _ 1 ,  204 _ 2 , Quadrature Amplitude Modulator QAM 
           204 _ 3 , . . . ,  204   —   n    
           206 _ 1 ,  206 _ 2 , Amplifier 
           206 _ 3 , . . . ,  206   —   n    
           208  Inverse Fast Fourier Transformation Unit 
           210  Flexible Notch Filter 
           212  Digital to Analog Converter 
           214  Notch Control Unit 
           216  Antenna 
           218  Measuring Unit 
           220  Further Measuring Unit 
           222  Mains 
         S 1  First Step 
         S 2  Second Step 
         S 3  Third Step 
         S 4  Fourth Step