BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
FIG. 1 is a block diagram of a conventional LINC architecture;
FIGS. 2A and 2B are respectively phasor diagrams of a signal and components thereof before and after amplification;
FIG. 3 is a block diagram of a multilevel LINC transmitter according to an embodiment of the invention;
FIGS. 4A and 4B are respectively phasor diagrams showing out-phasing angles of single-level and multilevel scaling techniques;
FIGS. 5A and 5B are respectively a detailed phasor diagram and a generalized phasor diagram showing out-phasing angles of multilevel scaling techniques;
FIG. 6 is a schematic diagram showing signal envelope distribution in WCDMA;
FIG. 7 is a block diagram of a multilevel scaler 313 in FIG. 3;
FIG. 8 is a block diagram of the envelope modulator 340 in FIG. 3;
FIG. 9A is a schematic diagram of VDD-to-PM distortion which degrades linearity;
FIG. 9B is a schematic diagram showing characteristics of the distortion compensator 350 in FIG. 3; and
FIG. 9C is a schematic diagram showing constant phase of the output signal of the multilevel LINC transmitter with different PA supply votlages.
DETAILED DESCRIPTION OF THE INVENTION
The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.
FIG. 3 is a block diagram of a multilevel LINC transmitter according to an embodiment of the invention. The multilevel LINC transmitter 300 comprises a multilevel signal component separator 310, a phase modulator block 320, and an RF block 330. The multilevel signal component separator 310 comprises a polar converter 311, a multilevel scaler 313 coupled to the polar converter 311, an inverse cosine module 315 coupled to the multilevel scaler 313, and a phase calculator 317 coupled to the polar converter 311 and the inverse cosine module 315. The polar converter 311 receives and converts the input signal S(t) to polar form. Then, an envelope signal A(t) is scaled by a multilevel scaler 313 and the inverse cosine module 315 generates an out-phasing angle θ′(t). Thereafter, the phase calculator 317 generates phase signals φ(t)+θ′(t) and φ(t)−θ′(t). In other words, the multilevel signal component separator 310 converts the input signal S(t) into phase signals φ(t)+θ′(t) and φ(t)−θ′(t). The phase modulator block 320 comprises two phase modulators 321 coupled to the multilevel signal component separator 310. The RF block 330 comprises a plurality power amplifiers 331 coupled to the phase modulator block 320 and the multilevel scaler 313 and a power combiner 333 coupled to the power amplifiers 331.
In an embodiment of the invention, a Wilkinson power combiner is adopted in a LINC transmitter, however, scope of the invention is not limited thereto. Other hybrid couplers, lossless Wilkinson power combiner, Chireix-outphasing combiner, or the like are also applicable to the invention. For a Wilkinson power combiner, efficiency η(t) thereof is defined as,
η(t)=cos2θ(t)
It is noted that η(t) is high when θ(t) is low. When the out-phasing angle θ(t) is substituted by the formula disclosed previously, the efficiency η(t) is expressed as,
As a result, to utilize high power efficiency of a Wilkinson power combiner, the value of r0 must be close to and not less than the maximum of A(t).
Rather than the conventional scaling technique using single-level r0, the multilevel scaler 313 in FIG. 3 reduces θ(t) such that high Wilkinson power combiner efficiency is achieved. A 2-level design example is illustrated in FIG. 4B. When A(t) is much smaller than r0, the multilevel scaler adapts scale factor from r0 to r1., and out-phasing angle. θ′(t) in FIG. 4B is much smaller than the conventional out-phasing angle θ(t) in FIG. 4A. Thus, the multilevel scaling technique enhances Wilkinson combiner efficiency. The multilevel scaling technique can be generalized to N levels in FIG. 5A, and RN is a general expression for multilevel scaling as shown in FIG. 5B, where RN=rk, for rk+1<A(t)≦rkk=0,1, . . . , N−1, where rN=0, r0=max(A(t)). The definition of out-phasing angle θ′(t) in multilevel scaling technique is modified as
To maximize the Wilkinson power combiner efficiency, optimal scale factors of each level rk need to be determined in advance. Since multilevel scale factors are used in the LINC transmitter, Wilkinson power combiner efficiency formula is modified as
FIG. 6 shows envelope distribution of WCDMA where A(t) is a probability function. To acquire an expected value of η(t), the envelope A(t) is divided into several regions, illustrated in FIG. 5. Then, the expectation value of each region is summed to derive E(η)(t)),
wherein p(A(t)) is a probability density function of A(t), rk is a value of a kth level scale factor, N is a number of a scale factor level, and max(A(t)) is a maximum input signal envelope. To maximize the Wilkinson power combiner efficiency, E(η(t)) is differentiated such that
whrerein k=0, 1, . . . , N. As a result, an optimal set of RN is obtained. With the optimal set of RN, the multilevel scaler dynamically adapts RN close to and no lower than the envelope A(t).
FIG. 7 is a block diagram of a multilevel scaler 313 in FIG. 3. The multilevel scaler 313 comprises a slicer 510 and a ROM 530 coupled to the slicer 510. The slicer 510 is used to select and output a specific rk to the inverse cosine module 315. Preferably, the slicer 510 comprises a comparator. The comparator determines in which range the envelope A(t) is and which rk should be selected according thereto. The ROM 530 stores the optimal set of RN.
Moreover, the multilevel LINC transmitter according to an embodiment of the invention further comprises an envelope modulator 340 coupled to the multilevel scaler 313 and the power amplifiers 331, as shown in FIG. 3. FIG. 8 is a block diagram of the envelope modulator 340 in FIG. 3. The envelope modulator 340 comprises a digital to analog converter (DAC) 341 coupled to the multilevel scaler 313, a low pass filter (LPF) 343 coupled between the DAC 341, and a low low drop-out (LDO) regulator 345 coupled between the LPF 343 and the power amplifier 331. An input signal of the envelope modulator 340 is a digital control signal from the multilevel scaler 313. The digital-to-analog converter (DAC) 341 converts the control signal to an analog signal. Then the analog control signal passes through the low pass filter (LPF) 343. Finally, the highly power efficient low drop out (LDO) regulator 345 ensures a robust power supply voltage to the PA 331. Due to RC delay, the control signal path group delay and the phase path delay are different. An additional delay compensator is inserted in a phase path, between the phase modulator block and the RF block, to partially overcome distortion due to RC delay.
Additionally, the multilevel LINC transmitter according to an embodiment of the invention further comprises a distortion compensator 350 coupled between the multilevel signal component separator 310 and the envelope modulator 340. Since adjustment of the supply voltage of two RF power amplifiers 331 introduces another distortion, VDD-to-PM distortion, a distortion compensator 350 is incorporated in the multilevel LINC transmitter to compensate VDD-to-PM distortion. FIG. 9A is a schematic diagram of VDD-to-PM distortion which degrades linearity. To correct VDD-to-PM distortion, a digital distortion compensator 350 with characteristics shown in FIG. 9B is incorporated in the multilevel LINC transmitter. Thus, phase of the output signal remains constant even with different PA supply voltages, as shown in FIG. 9C.
Moreover, the multilevel LINC transmitter according to an embodiment of the invention further comprises a temperature sensor 360. Since temperature variation may result in different VDD-to-PM distortion, a temperature sensor 360 is incorporated in the the multilevel LINC transmitter such that VDD-to-PM distortion is compensated.
The invention provides a multilevel LINC transmitter with a multilevel scaler in a multilevel signal component separator thereof. The multilevel scaler dynamically adapts a scale factor according to the input signal and therefore the out-phasing angle is adjustable. As a result, high power efficiency and linearity are achieved.
While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
Patent Application
20080019459
