Not Applicable
1. Technical Field
This invention relates in general to digital communications and, more particularly, to a GMSK/EDGE modulator with transition smoothing.
2. Description of the Related Art
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
The amplitudes of the signal correspond to √{square root over (I2+Q2)}, where I and Q are the In-phase and Quadrature components of the modulation in the IQ-plane shown in
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.
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.
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.
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.
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.
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:
a illustrates a modulation circuit including a smoothing circuit;
b illustrates a flow chart describing operation of the modulation circuit of
a through 4c illustrates examples of the smoothing operation on transitions between 8PSK and GMSK modulation types;
a through 5c illustrates examples of the smoothing operation on transitions between GMSK and 8PSK modulation types;
a illustrates an 8PSK to GMSK transition without smoothing;
b illustrates a graph of amplitude corresponding to the transition of
a illustrates an 8PSK to GMSK transition with smoothing;
b illustrates a graph of amplitude corresponding to the transition of
The present invention is best understood in relation to
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.
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.
It will be assumed that the guard bits for both GMSK and 8PSK are selected to be all 1s.
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.
As shown in
Referring to
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<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'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).
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
The operation of the modulation circuit is described in greater detail in connection with
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
Examples of the smoothing operation for switching from 8PSK to GMSK are shown in
In
In
Examples of the smoothing operation for switching from GMSK to 8PSK are shown in
In
In
a and 6b illustrate a transition from 8PSK to GMSK without the smoothing operation.
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.
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.
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.
Number | Date | Country | Kind |
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05292792 | Dec 2005 | EP | regional |
This application claims the benefit of the filing date of 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.
Number | Name | Date | Kind |
---|---|---|---|
20050215206 | Granstrom et al. | Sep 2005 | A1 |
20050281353 | Huang et al. | Dec 2005 | A1 |
20050286653 | Lai et al. | Dec 2005 | A1 |
20080310543 | Helfenstein et al. | Dec 2008 | A1 |
Number | Date | Country | |
---|---|---|---|
20070164834 A1 | Jul 2007 | US |
Number | Date | Country | |
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60867705 | Nov 2006 | US |