Patent Publication Number: US-7719376-B2

Title: Avoidance of discontinuities when switching between modulation schemes

Description:
The invention relates to a modulator system comprising a first modulator for modulating an input signal according to a first modulation scheme and a second modulator for modulating the input signal according to a second modulation scheme, to a transmitter comprising such a modulator system, to a modulator, to a method and to a processor program product. 
     Examples of such a transmitter are mobile radio terminals and base stations and network nodes operating in a Global System for Mobile telecommunication (GSM) supporting an Enhanced General Packet Radio Service (EGPRS) or a Universal Mobile Telecommunication System (UMTS). 
     A prior art modulator system is known from the article “Combined GMSK and 8PSK Modulator for GSM and EDGE, by Peter Bode and Alexander Lampe, Philips Semiconductors Nuremberg, Germany, and Markus Helfenstein, Philips Semiconductor Zürich, Switserland, which discloses in its  FIG. 5  a first modulator (modulation scheme: eight Phase Shift Keying or 8PSK) and in its  FIG. 6  a second modulator (modulation scheme: Gaussian Minimum Shift Keying or GMSK). During a modulation scheme change, discontinuities in the output signal of the modulator system may arise, which increase the adjacent channel interference. To avoid this adjacent channel interference, a power amplifier for amplifying this output signal is ramped down during the modulation scheme change. This ramping down is realized by creating a zero output signal of the modulator system through signal shaping during the modulation scheme change. 
     The known modulator system is disadvantageous, inter alia, due to causing the power amplifier to be ramped down. This limits the possible power amplifiers to be used. Some power amplifiers may not be ramped down and require an always non-zero signal at their input. 
     It is an object of the invention, inter alia, to provide a modulator system having relatively little discontinuities in its output signal during a modulation scheme change, without this output signal being ramped down. 
     Furthers objects of the invention are, inter alia, to provide a transmitter comprising such a modulator system having relatively little discontinuities in its output signal during a modulation scheme change, without this output signal being ramped down, and a modulator, a method and a processor program product all having relatively little discontinuities in their output signal during a modulation scheme change, without this output signal being ramped down. 
     The modulator system according to the invention comprises a first modulator for modulating an input signal according to a first modulation scheme and a second modulator for modulating the input signal according to a second modulation scheme, which modulator system comprises a compensator for combining at least one modulator signal with at least one waveform for compensating at least one signal parameter of an output signal for discontinuities resulting from a modulation scheme change. 
     By providing the modulator system with the compensator for compensating one or more signal parameters of the output signal of the modulator system for discontinuities, these discontinuities resulting from modulation scheme changes are reduced to a relatively large extent. The combining like for example multiplying and/or adding of a for example modulated signal with a waveform allows at least one modulated signal to be adapted in such a way that a discontinuity between a first modulation signal modulated according to a first modulation scheme and a second modulation signal modulated according to a second modulation scheme is smoothened to a relatively large extent. Usually this will be done during a so-called guard interval located between some data symbols and (three) tail symbols respectively on one side and (three) tail symbols and some data symbols respectively on the other side. Alternative guard intervals can be located between some data symbols on one side and some data symbols on the other side. 
     A first embodiment of the modulator system according to the invention is defined by further comprising at least one pulse shaper, with the compensator being located after the pulse shaper. By locating the compensator after the one or more pulse shapers of the modulators, these modulators themselves do not need to be amended. 
     A second embodiment of the modulator system according to the invention is defined by the compensator comprising a multiplier for multiplying the modulator signal in the form of at least one pulse shaped modulated signal with the waveform in the form of a complex valued waveform, with the at least one signal parameter comprising an amplitude and a phase. By providing the compensator when located after the one or more pulse shapers with the multiplier, discontinuities in the amplitude as well as in the phase of at least one pulse shaped modulated signal are smoothened via one multiplier receiving the complex valued waveform. 
     A third embodiment of the modulator system according to the invention is defined by further comprising at least one pulse shaper, with the compensator being located before the pulse shaper. By locating the compensator before the one or more pulse shapers of the modulators, the compensator can be integrated into the modulators. Usually, a first part of the compensator will be integrated into the first modulator, and a second part of the compensator will be integrated into the second modulator. 
     A fourth embodiment of the modulator system according to the invention is defined by the compensator comprising at least one multiplier for multiplying the modulator signal in the form of at least one modulated signal with the waveform, with the at least one signal parameter comprising an amplitude. By providing the compensator when located before the one or more pulse shapers with at least one but usually two multipliers, discontinuities in the amplitude of at least one modulated signal are smoothened via the one or two multipliers receiving the waveform. This waveform may be in the form of a complex valued waveform, which possibly but not exclusively comprises a real valued waveform only. 
     A fifth embodiment of the modulator system according to the invention is defined by each modulator comprising at least one multiplier for multiplying a mapped input signal with a complex valued signal, with the compensator comprising at least one multiplier for multiplying the modulator signal in the form of the complex valued signal with the waveform in the form of a complex valued phase offset, with the at least one signal parameter comprising a phase. By providing the compensator when located before the one or more pulse shapers with the at least one but usually three additional multipliers, one additional multiplier for one prior art multiplier in the first modulator and two multipliers for two prior art multipliers in the second modulator, thereby suggesting that the second modulator is based on a two branch modulation scheme, discontinuities in the phase of at least one modulated signal are smoothened. Thereto, per modulation scheme, usually a different phase offset will need to be multiplied to the complex valued signal. 
     A sixth embodiment of the modulator system according to the invention is defined by the first modulation scheme being a Phase Shift Keying modulation scheme and the second modulation scheme being a Gaussian Minimum Shift Keying modulation scheme. 
     The transmitter according to the invention comprises a modulator system comprising a first modulator for modulating an input signal according to a first modulation scheme and a second modulator for modulating the input signal according to a second modulation scheme, which modulator system comprises a compensator for combining at least one modulator signal with at least one waveform for compensating at least one signal parameter of an output signal for discontinuities resulting from a modulation scheme change, which transmitter further comprises a power amplifier for amplifying the output signal. Of course, this output signal may be the output signal originating from the modulator system or a derived version thereof, like for example a digitized version thereof. 
     The modulator according to the invention for modulating an input signal according to a modulation scheme comprises a compensator for combining at least one modulator signal with at least one waveform for compensating at least one signal parameter of an output signal for discontinuities resulting from a modulation scheme change. This modulator either has the ability to adapt its modulation scheme, or is used in combination with an other modulator. 
     The method according to the invention for modulating an input signal according to a first modulation scheme and for modulating the input signal according to a second modulation scheme comprises a step of combining at least one modulator signal with at least one waveform for compensating at least one signal parameter of an output signal for discontinuities resulting from a modulation scheme change. 
     The processor program product according to the invention for modulating an input signal according to a first modulation scheme and for modulating the input signal according to a second modulation scheme comprises a function of combining at least one modulator signal with at least one waveform for compensating at least one signal parameter of an output signal for discontinuities resulting from a modulation scheme change. 
     Embodiments of the transmitter according to the invention and of the modulator according to the invention and of the method according to the invention and of the processor program product according to the invention correspond with the embodiments of the modulator system according to the invention. 
     The invention is based upon an insight, inter alia, that the ramping down of an output signal of the modulator system is to be avoided, and is based upon a basic idea, inter alia, that discontinuities in amplitudes and/or phases of a signal can be smoothened by adapting at least the last part of this signal before the discontinuity and/or at least the first part of this signal after the discontinuity. 
     The invention solves the problem, inter alia, to provide a modulator system having relatively little discontinuities in its output signal during a modulation scheme change, without this output signal being ramped down, and is advantageous, inter alia, in that this modulator system can be combined even with a power amplifier which may not be ramped down and which requires an always non-zero signal at its input. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments(s) described hereinafter. 
    
    
     
       In the drawings: 
         FIG. 1  shows in block diagram form a modulator system according to the invention comprising a compensator of a first type; 
         FIG. 2  shows in block diagram form a modulator system according to the invention comprising a compensator of a second and a third type; 
         FIG. 3  shows in an upper graph (prior art) an absolute value of an output signal created without compensation and in a lower graph (invention) an absolute value of an output signal created through compensation before pulse shaping; and 
         FIG. 4  shows in block diagram form a transmitter according to the invention. 
     
    
    
     The modulator system  1  according to the invention as shown in  FIG. 1  comprises a first eight Phase Shift Keying (8PSK) modulator  2  and a second Gaussian Minimum Shift Keying (GMSK) modulator  3 ,  4  having a first modulating part  3  (first branch) and a second modulating part  4  (second branch). An input signal A is supplied to an input of a first multiplexer  5  and via a first output of the first multiplexer  5  supplied to an input of the modulator  2  and via a second output of the first multiplexer  5  supplied to inputs of the modulating parts  3 ,  4 . In modulator  2 , the input signal A is supplied to a serial-to-parallel-converter  6  and then mapped by a mapper  7 , after which the mapped signal is multiplied through a multiplier  8  with a signal B and a resulting modulated signal is supplied to a first input of a second multiplexer  9 . An output signal of the second multiplexer  9  is supplied to an upsampler  10  and then pulse shaped by a pulse shaper  11  (a Finite Impulse Response (FIR) Filter), after which a first pulse shaped signal is added via an adder  12  to a second pulse shaped signal coming from the modulating part  4 . 
     In modulating part  3 , the input signal A is supplied to a mapper  14 , after which the mapped signal is multiplied through a multiplier  15  with a signal C and a resulting modulated signal is supplied to a second input of the second multiplexer  9 . In modulating part  4 , the input signal A is supplied to a Finite State Machine (FSM)  16  and then mapped by a mapper  17 , after which the mapped signal is multiplied through a multiplier  18  with a signal D and a resulting modulated signal is supplied to a first input of a third multiplexer  19 . An output signal of the third multiplexer  19  is supplied to an upsampler  20  and then pulse shaped by a pulse shaper  21  (a Finite Impulse Response (FIR) Filter), after which this second pulse shaped signal is supplied to the adder  12  discussed before. The added first and second pulse shaped signals are combined with a complex valued waveform E for compensating at least one signal parameter like an amplitude or a phase of the output signal F for discontinuities resulting from a modulation scheme change, as discussed below. 
     The first modulator  2  modulates the input signal A according to a first modulation scheme 8PSK. Thereto, the first multiplexer  5  is controlled in such a way that the input signal is supplied to the modulator  2  and not to the modulators  3 ,  4 . The input signal A is serial-to-parallel converted and then mapped and then multiplied with the signal B, with the signal B for example being equal to exp j(3πk/8). The second multiplexer  9  is controlled in such a way that the resulting modulated signal is upsampled and pulse shaped. In a prior art situation, with the multiplier  13  not being present, the upsampled and pulse shaped modulated signal forms the output signal F of the modulator system  1  in case of this modulator system  1  being in a 8PSK mode. Thereto, the third multiplexer  19  is controlled in such a way that a second input of this third multiplexer  19  receiving a zero signal is coupled to the third multiplexer&#39;s output. 
     The second modulator  3 ,  4  modulates the input signal A according to a second modulation scheme GMSK. Thereto, the first multiplexer  5  is controlled in such a way that the input signal is supplied to the modulator  3 ,  4  and not to the modulator  2 . In the modulating part  3 , the input signal A is mapped and then multiplied with the signal C, with the signal C for example being equal to exp j(πk/2). In the modulating part  4 , the input signal A is processed by the FSM  16  and is mapped and then multiplied with the signal D, with the signal D for example being equal to exp j(π[k−1]/2). The second multiplexer  9  and the third multiplexer  19  are controlled in such a way that the resulting modulated signals can be upsampled and pulse shaped. In a prior art situation, with the multiplier  13  not being present, the upsampled and pulse shaped modulated signals form the output signal F of the modulator system  1 . 
     So, by controlling the multiplexers  5 ,  9  and  19 , the modulation scheme can be selected. In case of a modulation scheme change, discontinuities in the output signal F of the modulator system  1  may arise, which increase the adjacent channel interference. To avoid increased adjacent channel interference, these discontinuities can be avoided by introducing multiplier  13  (a compensator of a first type). For example, in case of four GMSK symbols arriving from the adder  12  and being fed into the multiplier  13  and then four 8PSK symbols arriving from the adder  12  and being fed into the multiplier  13  during a guard interval comprising eight symbols, the amplitudes for example equal 1, 1, 1, 1, 0.7, 0.7, 0.7 and 0.7. To avoid amplitude discontinuities, the amplitudes of eight samples of the complex valued waveform E could be chosen as 1, 0.96, 0.91, 0.86, 1.14, 1.09, 1.03 and 1. The amplitudes of the output signal of the multiplier  13  then equal 1, 0.96, 0.91, 0.86, 0.8, 0.76, 0.72 and 0.7. As a result, the amplitude smoothly decreases from 1 to 0.7, and any amplitude discontinuities in the output signal F have been avoided. The same way, any phase discontinuities in the output signal F can be avoided by supplying the eight samples of the complex valued waveform having appropriately chosen phases. 
     The modulator system  1  according to the invention as shown in  FIG. 2  corresponds with the modulator system  1  according to the invention as shown in  FIG. 1 , apart from the following. Instead of multiplier  13 , now multipliers  25 ,  26  respectively have been introduced (a compensator of a second type) located between multiplexers  9 ,  19  and upsamplers  10 ,  20  respectively for multiplying the output signals of the multiplexers  9 ,  19  with waveforms S, T respectively, and multipliers  22 ,  23 ,  24  respectively have been introduced (a compensator of a third type), coupled to multipliers  8 ,  15 ,  18  respectively for multiplying the signals B, C, D respectively with the complex valued phase offsets X, Y, Z respectively. 
     A possible setting of the multiplexers  9  (mux  9 ) and  19  (mux  19 ) during a transition from GMSK to 8PSK is as follows: 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
             
            
               
                 Mux 9 
                 . . . G G G 
                 G G G 
                 G G G G P P P P 
                 P P P 
                 P P P . . . 
               
               
                 Mux 19 
                 . . . G G G 
                 G G G 
                 G G G G 0 0 0 0 
                 0 0 0 
                 0 0 0 . . . 
               
               
                   
               
            
           
         
       
     
     The first column with symbols shows the last three data symbols, the second column shows three tail symbols, the third column shows eight guard symbols, the fourth column shows three tail symbols, and the fifth column shows the first three data symbols. 
     The complex valued phase offset X for example equals exp j[φ 8PSK ]=exp j[arg(g[k])+πk/2+Φ GMSK −arg(p[k+1])−3π(k+1)/8+ΔΦ], thereby suggesting that the last GMSK symbols and the first 8PSK symbols are fed into the multiplexers  9 ,  19  at symbol interval k and k+1 respectively, and that φ GMSK  denotes the phase offset of a preceding GMSK modulated burst and that A, denotes a phase angle defining the phase difference between the last GMSK symbol and the first 8PSK symbol. In case all symbols G[k], P[k] equal “1” in the guard interval, an appropriate choice for Δφ is for example 3π/8. With this choice, the GMSK symbol fed into the multiplexer in symbol interval k looks like a preceding 8PSK “1” for the 8PSK symbol passed to the multiplexer in symbol interval k+1. The phase offset φ8PSK is updated when switching from GMSK to 8PSK and remains constant during an 8PSK modulated burst. 
     A possible setting of the multiplexers  9  (mux  9 ) and  19  (mux  19 ) during a transition from 8PSK to GMSK is as follows: 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
             
            
               
                 Mux 9 
                 . . . P P P 
                 P P P 
                 P P P P G G G G 
                 G G G 
                 G G G . . . 
               
               
                 Mux 19 
                 . . . 0 0 0 
                 0 0 0 
                 0 0 0 0 G G G G 
                 G G G 
                 G G G . . . 
               
               
                   
               
            
           
         
       
     
     The first column with symbols shows the last three data symbols, the second column shows three tail symbols, the third column shows eight guard symbols, the fourth column shows three tail symbols, and the fifth column shows the first three data symbols. 
     The complex valued phase offset Y, Z for example equals exp j[φ GMSK ]=exp j[arg(p[k])+3πk/8+φ 8PSK −arg(g[k+1])−π(k+1)/2+Δφ], thereby suggesting that the last 8PSK symbols and the first GMSK symbols are fed into the multiplexers  9 ,  19  at symbol interval k and k+1 respectively, and that φ 8PSK  denotes the phase offset of a preceding 8PSK modulated burst and that Δφ denotes a phase angle defining the phase difference between the last 8PSK symbol and the first GMSK symbol. In case all symbols P[k], G[k] equal “1” in the guard interval, an appropriate choice for Δφ is for example π/2. With this choice, the 8PSK symbol fed into the multiplexer in symbol interval k looks like a preceding GMSK “1” for the GMSK symbol passed to the multiplexer in symbol interval k+1. The phase offset φ GMSK  is updated when switching from 8PSK to GMSK and remains constant during a GMSK modulated burst. 
       FIG. 3  shows in an upper graph (prior art) an absolute value of an output signal F after digital-to-analog-conversion created without compensation and in a lower graph (invention) an absolute value of an output signal F after digital-to-analog-conversion created through compensation by multiplying the modulator signal in the form of the complex valued signal B, C, D with the waveform X, Y, Z in the form of a complex valued phase offset X, Y, Z, before pulse shaping. The modulation scheme change from 8PSK to GMSK takes place between symbol interval  56  and  57 , with all symbols P(k), G(k) chosen to have the value “1” in the guard interval ( 53 - 60 ). 
     When demanding equal peak values for GMSK and 8PSK, the samples of the waveforms S, T should be “1” for 8PSK and about “1.5” for GMSK, when demanding equal root mean square values these samples should all be “1”. 
     The modulator system  1  as shown in  FIG. 2  calculates a phase offset Y, Z and rotates GMSK pulse shaping filter input symbols therewith, and calculates a phase offset X and rotates 8PSK pulse shaping filter input symbols therewith, and calculates an amplitude waveform S, T and multiplies GMSK/8PSK pulse shaping filter input symbols therewith. 
     The modulator system  1  as shown in  FIGS. 1 and 2  minimizes adjacent channel interference, improves the stability of phase and/or amplitude loops controlled by the output signal F, does not require ramping and ramping calculations, is simple, allows a fast transition between modulation schemes, and offers amplitude smoothing when changing between equal root mean square values and peak values for GMSK and 8PSK for free. 
     The transmitter  30  as shown in  FIG. 4  comprises an input stage  31  for generating the input signal A for the modulator system  1 , and comprises for example a digital-to-analog-converter  32  for converting the output signal F into an analog signal, and comprises for example a power amplifier  33  for amplifying the analog output signal. 
     Alternative modulation schemes like for example 4PSK or 16PSK instead of 8PSK and alternative modulator system constructions are possible. Instead of multiplying exp j(a) and exp j(b), the adding of a+b could be performed, and vice versa: exp j(a)*exp j(b)=exp j(a+b). 
     A complex valued waveform comprises a real valued waveform, an imaginary valued waveform, or a combination of both. 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb “to comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.