Abstract:
To minimize abrupt changes in modulated signal amplitude when switching between modulation types in a multi-modulation system, a “smoothing” circuit is used. When the modulation type is changed in a multi-slot modulation, the last symbol of the previous slot is overwritten and modulation is controlled by a smoothing circuit during the time of the final symbol of the previous slot to effect the trajectory smoothing. The IQ-plane is divided into two (or more) zones, each with an associated safe point. After the next-to-last symbol is modulated, modulation is made to jump to the safe point associated with the zone and modulation continues to a starting point. From the starting point, the next slot can be modulated using the new modulation type.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of the filing date of copending provisional application U.S. Ser. No. 60/867,705, filed Nov. 29, 2006, entitled “GMSK/EDGE MODULATOR WITH SWITCHING TRANSITION SMOOTHING”, which is incorporated by reference herein, and claims priority under 35 U.S.C. 119(b) to EP 05292792.8, filed Dec. 23, 2005. 
     
    
     STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Technical Field  
         [0004]     This invention relates in general to digital communications and, more particularly, to a GMSK/EDGE modulator with transition smoothing.  
         [0005]     2. Description of the Related Art  
         [0006]     EGPRS (Enhanced General Packet Radio Service) standard allows mixing consecutive GMSK and 8PSK modulations slot-by-slot within a single TDMA time-frame. To switch between modulation types, the modulator is reset. As shown in  FIG. 1 , if the modulator is reset in GMSK mode, the I-vector is set to its maximum value, while the Q-vector is set to 0, i.e., the modulator is set to zero phase. If the modulator is reset in 8PSK mode, the phase is initialized close to 0 degrees at a slightly smaller amplitude.  
         [0007]     The amplitudes of the signal correspond to √{square root over (I 2 +Q 2 )}, where I and Q are the In-phase and Quadrature components of the modulation in the IQ-plane shown in  FIG. 1 . In the modulator trajectories shown below the 8PSK peak amplitude is the same as the GMSK peak (and rms) amplitude. Therefore the 8PSK rms amplitude will be less than that of GMSK in this case. This is also true when burst data bits are all 1&#39;s in both modulations, which is the case during power ramping between bursts where guard bits (user defined as all 1&#39;s) are modulated.  
         [0008]     At the instant where the modulation type changes, the transition may cause a zero-crossing through the origin of the IQ-plane, because the (I,Q) vector at the end of the previous slot is switched instantaneously to the (I,Q) reset vector of the modulation type in the next slot. In practice, zero crossing in the IQ-plane can cause power amplifier nonlinearity or desaturation. It can deteriorate RF/PA (radio frequency power amplifier) performance and should be avoided.  
         [0009]     In the prior art, solutions to this problem rely on reducing the power level of the power amplifier circuit to mask or attenuate such transitions without addressing the problem of the transitions themselves.  
         [0010]     Therefore, a need has arisen for a modulator that avoids zero crossings in the IQ-plane when switching between modulation types, for example, between GMSK to 8PSK or between 8PSK to GMSK.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     In the present invention, data is selectively modulating in a first modulation type or a second modulation-type, where modulation-types can be changed between slots. Smoothing changes between slots of different modulation types by determining a zone associated with a current position in an IQ plane at a predetermined point of modulation, jumping to a safe point associated with the zone and modulating from the safe point to a starting point prior to modulating data in a next slot.  
         [0012]     The present invention eliminates the possibility of passing through a zero crossing during a switch between modulation types, thereby eliminating the problems associated with power amplifier circuits during a switch.  
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0013]     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0014]      FIG. 1  illustrates trajectories for GMSK and 8PSK modulation types;  
         [0015]      FIG. 2  illustrates safe points used for smoothing transitions in an multi-slot modulator;  
         [0016]      FIG. 3   a  illustrates a modulation circuit including a smoothing circuit;  
         [0017]      FIG. 3   b  illustrates a flow chart describing operation of the modulation circuit of  FIG. 3   a;    
         [0018]      FIGS. 4   a  through  4   c  illustrates examples of the smoothing operation on transitions between 8PSK and GMSK modulation types;  
         [0019]      FIGS. 5   a  through  5   c  illustrates examples of the smoothing operation on transitions between GMSK and 8PSK modulation types;  
         [0020]      FIG. 6   a  illustrates an 8PSK to GMSK transition without smoothing;  
         [0021]      FIG. 6   b  illustrates a graph of amplitude corresponding to the transition of  FIG. 6   a;    
         [0022]      FIG. 7   a  illustrates an 8PSK to GMSK transition with smoothing;  
         [0023]      FIG. 7   b  illustrates a graph of amplitude corresponding to the transition of  FIG. 7   a.    
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     The present invention is best understood in relation to  FIGS. 1-7   a - b  of the drawings, like numerals being used for like elements of the various drawings.  
         [0025]     The present invention provides a dual-mode modulator with transition smoothing circuitry that avoids zero-crossing in the IQ-plane during transition between modulation types.  
         [0026]     In the GSM05.02 specification (Section 5.2), each slot contains 156.25 symbols (156.25 bits for GMSK and 468.75 bits for 8PSK). For GMSK, this means that each slots holds 142 bits of data, 6 tail-bits and 8.25 guard bits. For 8PSK, each slot holds 426 bits of data, 18 tail bits and 24.75 guard bits. Table 1 specifies the tail bits and guard bits for GMSK and 8PSK.  
                                     TABLE 1                           Tail bits and Guard Bits for GMSK and 8PSK                GMSK   Bit   8PSK   Bit       Modulation   Bits   Value   Bits   Value               Tail bits (1)   B0 to B2   {0,0,0}   B0-B8   {1,1,1,1,1,1,1,1,1}       Tail bits (2)   B145 to   {0,0,0}   B435-B443   {1,1,1,1,1,1,1,1,1}           B147       Guard bits   B148 to   User   B444-B468   User defined           B156   defined                  
 
         [0027]     It will be assumed that the guard bits for both GMSK and 8PSK are selected to be all 1s.  
         [0028]     To provide smoothing, the following method is used. When the modulation type is changed in a multi-slot modulation, the modulation of the last symbol of the previous slot will be replaced modulation that will be controlled by a smoothing circuit during the time of the final symbol of the previous slot to effect the trajectory smoothing. For GMSK, each symbol is one bit, so the last guard bit of the slot is replaced. For 8PSK, each symbol is three bits, so the last three bits are replaced.  
         [0029]     As shown in  FIG. 2 , for GMSK modulation, the I-Q diagram trajectory is on a circle; for 8PSK modulation, when 1&#39;s are modulated, the I-Q diagram trajectory is on another circle with smaller amplitude. The slot starts and finishes with guard bits that are user defined. If guard bits are 1&#39;s, then the transition between two modulations is the pass from one circle to the other.  
         [0030]     Referring to  FIG. 2 , the smoothing circuit effects the following events. If the modulation is in the positive half plane just prior to modulating the last symbol of the slot (i.e., Q&gt;0), the last symbol of the slot is replaced with the smoothing data such that the modulation is made to jump to point P at [0,1] on the circle (where the maximum amplitude of the modulation-types are normalized to 1). Then, by modulating 1&#39;s using GMSK modulation, the quarter circle from point P to point S is traversed. From point S, data in the new slot is modulated using the new modulation type (GMSK or 8PSK).  
         [0031]     On the other hand, if the modulation is in the negative half plane just prior to modulating the last symbol of the slot (i.e., Q&lt;0), the last symbol of the slot is replaced with a smoothing data such that the modulation progresses to point N a [0,−1] on the circle. Then, by modulating 1&#39;s using GMSK modulation, the quarter circle from point N to point S is traversed. From point S, data in the new slot is modulated using the new modulation type (GMSK or 8PSK).  
         [0032]      FIG. 3   a  illustrates a block diagram of a modulation circuit  10  supporting multi-slot operation, such as described in the GPRS specification. The modulator is capable of modulating in either of two modulation-types, shown as an 8PSK modulator  12  and a GMSK modulator  14  (although other modulation-types such as QPSK could be used as well). A smoothing circuit performs the operations to modulate to a safe spot (P or N in  FIG. 2 ) during the last symbol of a slot, where the modulation-type is scheduled to switch on the next slot.  
         [0033]     The operation of the modulation circuit is described in greater detail in connection with  FIG. 3   b . In steps  20  and  22 , if the modulation is not in multi-slot mode or there is no modulation-type change on the next slot, then modulation proceeds as normal in block  24 . Similarly, modulation proceeds as normal if the time for the last symbol of the current slot has not been reached in block  26 .  
         [0034]     If, however, the modulation is in multi-slot mode and the modulation-type is to be changed with the next slot and the time of the final symbol has been reached in block  26 , then the smoothing circuit makes a jump to a safe point depending upon the current zone of the modulator in the IQ plane (steps  28  through  32 ). In the preferred embodiment, the zone is determined by whether Q for the last symbol to be modulated (i.e., the next-to-last symbol of the current slot) is positive or negative. In  FIG. 2 , safe point P is at (0, 1) and safe point N is at (0, −1). The safe points could be at different locations so long as the path from any point in the associated zone to the safe point does not come near the zero crossing. From the safe point, modulation using 1&#39;s traverses the quarter circle back to the GMSK starting point. In step  34 , data from the new slot is modulated in using the new modulation type.  
         [0035]     Examples of the smoothing operation for switching from 8PSK to GMSK are shown in  FIGS. 4   a - c . In  FIG. 4   a , the modulation of the next to last symbol in an 8PSK modulated slot ends at the arrow point. Since Q is negative at this point in the modulation, the safe point is N. The modulation jumps to point N (it should be noted that this jump cannot come near the origin from any point in the zone). Then, using GMSK modulation of 1&#39;s, the quarter circle is traversed to the starting point S. Modulation of data from the new slot can take place from this point using GMSK modulation.  
         [0036]     In  FIG. 4   b , the modulation of the next to last symbol in an 8PSK modulated slot has a positive Q; thus, the safe point is P. The modulation jumps to point P and, using GMSK modulation of 1&#39;s, the quarter circle is traversed to the starting point S. Modulation of data from the new slot can take place from this point using GMSK modulation.  
         [0037]     In  FIG. 4   c , the modulation of the next to last symbol in an 8PSK modulated slot has a negative Q; thus, the safe point is N. As compared to  FIG. 4   a , this point has a positive I. The modulation jumps to point N and, using GMSK modulation of 1&#39;s, the quarter circle is traversed to the starting point S. Modulation of data from the new slot can take place from this point using GMSK modulation.  
         [0038]     Examples of the smoothing operation for switching from GMSK to 8PSK are shown in  FIGS. 5   a - c . In  FIG. 5   a , the modulation of the next to last symbol in a GMSK modulated slot ends at the arrow point. Since Q is negative at this point in the modulation, the safe point is N. The modulation jumps to point N. Then, using GMSK modulation of 1&#39;s, the quarter circle is traversed to the starting point S. Modulation of data from the new slot can take place from this point using 8PSK modulation.  
         [0039]     In  FIG. 5   b , the modulation of the next to last symbol in a GMSK modulated slot has a positive Q; thus, the safe point is P. The modulation jumps to point P and, using GMSK modulation of 1&#39;s, the quarter circle is traversed to the starting point S. Modulation of data from the new slot can take place from this point using 8PSK modulation.  
         [0040]     In  FIG. 5   c , the modulation of the next to last symbol in a GMSK modulated slot has a negative Q; thus, the safe point is N. The modulation jumps to point N and, using GMSK modulation of 1&#39;s, the quarter circle is traversed to the starting point S. Modulation of data from the new slot can take place from this point using 8PSK modulation.  
         [0041]      FIGS. 6   a  and  6   b  illustrate a transition from 8PSK to GMSK without the smoothing operation.  FIG. 6   a  illustrates an eye diagram of a transition between an 8PSK modulation with a slightly positive Q to a GMSK modulation. As can be seen, the transition crosses very close to the origin of the IQ plane. As can be seen in  FIG. 6   b , the amplitude at the point of transition spikes downward to almost zero. By contrast, the same transition is shown in  FIGS. 7   a  and  7   b , with the smoothing operation. In this case, there is only a slight amount of noise at the transition.  
         [0042]     The present invention provides significant improvement over the prior art. Rather than adjusting the power amplifier to compensate for problems caused by modulation-type transitions, the present invention provides smooth transitions during the time of the last symbol for a slot by jumping to a safe point, where the jump cannot incur a zero crossing. From the safe point, the quarter circle is traversed to the starting point.  
         [0043]     While the present invention is described in relation to 8PSK and GMSK modulation-types, other modulation types, such as QPSK could also be used as one of the modulation-types.  
         [0044]     Although the Detailed Description of the invention has been directed to certain exemplary embodiments, various modifications of these embodiments, as well as alternative embodiments, will be suggested to those skilled in the art. The invention encompasses any modifications or alternative embodiments that fall within the scope of the Claims.