Title: Method and apparatus for reducing dynamic range of a power amplifier
Abstract: An output-power threshold is selected such that one or more signal requirements is not outside a pre-determined range when output power of a polar transmitter is less than the output-power threshold. A determination is made whether the output power is less than the threshold. In response to a determination that the output power is less than the threshold, amplitude modulation of a polar signal transmitted by the polar transmitter is disabled. When the output power meets or exceeds the output-power threshold, the polar signal transmitted by the polar transmitter is both amplitude and phase modulated.
Patent Number: 6,892,057 Issued on 05/10/2005 to Nilsson
| Inventors:
|
Nilsson; Magnus (Lund, SE)
|
| Assignee:
|
Telefonaktiebolaget LM Ericsson (Publ) (Stockholm, SE)
|
| Appl. No.:
|
215127 |
| Filed:
|
August 8, 2002 |
| Current U.S. Class: |
455/126; 330/10; 330/129; 455/108; 455/114.2; 455/127.1 |
| Intern'l Class: |
H04B 001/04; H04G003/20 |
| Field of Search: |
455/126,1271-1274,102,108,110,114.2,114.3,115.1,115.3
330/127,129-136,10,149,75,96
|
References Cited [Referenced By]
U.S. Patent Documents
| 4600892 | Jul., 1986 | Wagner et al.
| |
| 5124665 | Jun., 1992 | McGann.
| |
| 5369789 | Nov., 1994 | Kosugi et al.
| |
| 5633893 | May., 1997 | Lampe et al.
| |
| 5784412 | Jul., 1998 | Ichihara.
| |
| 5834987 | Nov., 1998 | Dent.
| |
| 5886573 | Mar., 1999 | Kolanek.
| |
| 6046630 | Apr., 2000 | Kim.
| |
| 6101224 | Aug., 2000 | Lindoff et al.
| |
| 6147553 | Nov., 2000 | Kolanek.
| |
| 6160855 | Dec., 2000 | Nakamura et al.
| |
| 6236267 | May., 2001 | Anzil.
| |
| 6373345 | Apr., 2002 | Kimppa et al.
| |
| 6633199 | Oct., 2003 | Nielsen et al.
| |
| 6734724 | May., 2004 | Schell et al.
| |
| Foreign Patent Documents |
| 0584534 | Mar., 1994 | EP.
| |
| 1 164 694 | Dec., 2001 | EP.
| |
| WO 9907066 | Feb., 1999 | WO.
| |
| WO 0035080 | Jun., 2000 | WO.
| |
Other References
EP Standard Search Report completed Jul. 7, 2003.
Hajmiri, Ali et al., "Jitter and Phase Noise in Ring Oscillators", IEEE Journal
of Solid-State Circuits, vol. 34, No. 6, Jun. 1999 (pp. 790-804).
"TS 25.101 v3.0.0", 3rd Generation Partnership Project (3 GPP), Oct.
1999 (pp. 1-66).
|
Primary Examiner: Nguyen; Duc M.
Attorney, Agent or Firm: Jenkens & Gilchrist, P.C.
Claims
1. A method of operating a polar transmitter, the method comprising:
transmitting first information at a first output power greater than an output-power
threshold, wherein the first information is amplitude and phase modulated;
transmitting second information at a second output power not exceeding the output-power
threshold, wherein the second information is only phase modulated; and
wherein the output-power threshold is selected such that at least one signal
requirement is not outside a pre-determined range when the output power does not
exceed the output power threshold.
2. The method of claim 1, wherein the at least one signal requirement comprises
an error vector magnitude value.
3. The method of claim 2, wherein the pre-determined range is 0-17.5%.
4. The method of claim 1, wherein the at least one signal requirement comprises
an adjacent channel leakage ratio.
5. The method of claim 1, wherein the steps transmitting are performed in the
order listed.
6. The method of claim 1, wherein the at least one signal requirement comprises
a measure of information quality.
7. The method of claim 1, further comprising, in response to the output power
not exceeding the pre-determined output-power threshold, running a power amplifier
in a saturated mode and controlling the output power by varying a bias level of
the power amplifier.
8. The method of claim 1, further comprising, in response to the output power
not exceeding the pre-determined output-power threshold, running a power amplifier
in a linear mode and controlling the output power by a variable gain amplifier.
9. The method of claim 1, further comprising:
decreasing the output power toward the output-power threshold;
decreasing an effective modulation depth linearly in relation to the decrease
in the output power;
responsive to the output power not exceeding the output-power threshold, performing
the step of transmitting the second information.
10. A method of transmitting a polar signal, the method comprising:
selecting an output-power threshold, the output-power threshold being selected
such that at least one signal requirement is not outside a pre-determined range
when output power is less than the output-power threshold;
determining whether the output power is less than the threshold; and
in response to a determination that the output power is less than the threshold,
disabling amplitude modulation of the polar signal.
11. The method of claim 10, wherein the at least one signal requirement comprises
an error vector magnitude value.
12. The method of claim 11, wherein the pre-determined range is 0-17.5%.
13. The method of claim 10, wherein the at least one signal requirement comprises
an adjacent channel leakage ratio.
14. The method of claim 10, further comprising, in response to a determination
that the output power is not less than the threshold, enabling amplitude modulation
of the polar signal.
15. The method of claim 10, wherein the at least one signal requirement comprises
a measure of information quality.
16. The method of claim 10, further comprising, in response to the determination
that the output power is less than the pre-determined output-power threshold, running
a power amplifier in a saturated mode and controlling the output power by varying
a bias level of the power amplifier.
17. The method of claim 10, further comprising, in response to the determination
that the output power is less than the pre-determined output-power threshold, running
a power amplifier in a linear mode and controlling the output power by a variable
gain amplifier.
18. The method of claim 10, further comprising:
decreasing the output power toward the output-power threshold; and
decreasing an effective modulation depth linearly in relation to the decrease
in the output power.
19. An apparatus comprising:
a modulator, wherein the modulator is adapted to perform phase modulation of
an input signal, thereby yielding a phase-modulated input signal;
a power amplifier adapted to perform amplitude modulation of the phase-modulated
input signal in response to output power of the apparatus exceeding a pre-determined
output-power threshold; and
wherein the output-power threshold is selected such that at least one signal
requirement is not outside a pre-determined range when the output-power threshold
is not exceeded.
20. The apparatus of claim 19, wherein the at least one signal requirement comprises
an error vector magnitude value.
21. The apparatus of claim 20, wherein the pre-determined range is 0-17.5%.
22. The apparatus of claim 19, wherein the at least one signal requirement comprises
an adjacent channel leakage ratio.
23. The apparatus of claim 19, wherein the at least one signal requirement comprises
a measure of information quality.
24. The apparatus of claim 19, wherein, when the output power does not exceed
the pre-determined output-power threshold, the power amplifier is run in a saturated
mode and the output power is controlled by varying a bias level of the power amplifier.
25. The apparatus of claim 19, wherein, when the output power does not exceed
the pre-determined output-power threshold, the power amplifier is run in a linear
mode and the output power is controlled by a variable gain amplifier.
26. The apparatus of claim 19, wherein:
when the output power is within a pre-determined range of a lower bound of a
dynamic range of the apparatus and is decreased toward the output-power threshold,
an effective modulation depth is decreased linearly in relation to the decrease
in the output power; and
upon the output-power threshold being reached, the power amplifier no longer
performs the amplitude modulation.
Description
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention generally relates to reducing a dynamic range of a power
amplifier. In particular, the present invention relates to reducing a dynamic range
of a power amplifier used as part of a polar transmitter while still meeting all
other transmitter signal requirements.
2. Description of Related Art
Many communication-system standards such as, for example, the Global System
for Mobile Communications (GSM), use constant-envelope modulation. In constant-envelope
modulation, all modulating information is contained in a phase part of a transmitted
signal. As a result, all blocks of a modulator used as part of a GSM transmitter
can be run in saturation.
Many newer communication-system standards require linear modulation. Linear
modulation is more spectrally-efficient than constant-envelope modulation. Systems
based on, for example, Enhanced Data for GSM Evolution (EDGE), Universal Mobile
Telecommunications System (UMTS), Wideband Code Division Multiple Access (WCDMA),
and Code Division Multiple Access 2000 (CDMA2000) utilize linear modulation. In
linear modulation, modulating information is contained in both an amplitude part
and a phase part of a transmitted signal. Therefore, all blocks of a transmitter,
and especially a power amplifier block of the transmitter, must be run in a linear
mode in order to avoid distorting the transmitted signal. Running the blocks in
the linear mode causes the power efficiency of the transmitter to drop and results
in more noise being generated.
Dividing a modulating signal into the phase part and the amplitude part
permits the phase part to be introduced in a phase locked loop (PLL) or an in-phase
quadrature (IQ) modulator and the amplitude part to be introduced in a power amplifier
positioned after the PLL or IQ modulator. In this way, switching blocks can be
used, which results in better power efficiency. If the blocks are run in switching
mode, noise is amplified only at instants of switching, which results in less noise.
FIG. 1 shows an exemplary polar transmitter
100 that permits the modulating
signal to be divided as discussed above. The polar transmitter
100 includes
a PLL modulator
102 that serves as a two-point frequency modulator, a power
amplifier (PA)
104, and an antenna
106. A polar signal is divided
into a phase part (f
inst(s))and an amplitude part (A(s)) and is input
to the transmitter
100 as described in detail below. FIG. 1 represents one
of several polar-modulation schemes described in U.S. Pat. No. 5,834,987 to Dent,
which is incorporated herein by reference.
The phase part of the polar signal is input to a Sigma-Delta modulator
108.
The Sigma-Delta modulator
108 outputs control signals, which are fed to
a prescalar division block
110. An output of the prescalar division block
110 is fed to a phase frequency detector
112. A PLL reference signal
θ
ref(s) is also input to the phase frequency detector
112.
Output of the phase frequency detector
112 is fed to a PLL loop filter (H
LP(s))
114. The PLL loop filter provides an integration function. Output of the
PLL loop filter
114 and a scaled version of the phase part (f
inst(s)/K'vco)
are summed at a sum block
116. Output of the sum block
116 is input
to a voltage-controlled-oscillator (VCO) block
118. The voltage-controlled-oscillator
block
118 functions as a direct modulation injector. Output of the voltage-controlled
oscillator block
118 is fed back to an input of the prescalar division block
110 and also to the power amplifier
104. The amplitude part of the
polar signal is input to the power amplifier
104 as well. Output of the
power amplifier
104 is fed to the antenna
106 for transmission.
A transmitter such as, for example, the transmitter
100, must fulfill
certain
standards-based signal requirements in order to achieve a good radio link and also
to avoid interfering with other users. For example, UMTS transmitter signal requirements
are specified in 3G TS 25.101, 3rd Generation Partnership Project—Technical
Specification Group Radio Access Networks: UE radio transmission and reception
(FDD) (Release 1999), which has been promulgated by the Third Generation Partnership
Project (3GPP). While polar transmitters such as the transmitter
100 can
serve as very power-efficient linear modulators, standards-based signal requirements
on the transmitter blocks can become quite stringent. For example, the UMTS power-amplifier
dynamic-range requirements are especially difficult to meet. Therefore, an apparatus
and method for reducing the dynamic range of a power amplifier that eliminates
the drawbacks mentioned above and other drawbacks is needed.
SUMMARY OF THE INVENTION
These and other drawbacks are overcome by embodiments of the present invention,
which provides an apparatus and method for reducing the dynamic range of a power
amplifier. In an embodiment of the present invention, an apparatus includes a modulator
and a power amplifier. The modulator is adapted to perform phase modulation of
an input signal. The phase modulation yields a phase-modulated input signal. The
power amplifier is adapted to perform amplitude modulation of the phase-modulated
input signal in response to output power of the apparatus exceeding a pre-determined
output-power threshold. The output-power threshold is selected such that at least
one signal requirement is not outside a pre-determined range when the output-power
threshold is not exceeded.
In another embodiment of the present invention, a method of operating a polar
transmitter includes transmitting first information at a first output power greater
than an output-power threshold and transmitting second information at a second
output power not exceeding the output-power threshold. The first information is
amplitude and phase modulated. The second information is only phase modulated.
The output-power threshold is selected such that at least one signal requirement
is not outside a pre-determined range when the output power does not exceed the
output power threshold.
In another embodiment of the present invention, a method of transmitting a polar
signal includes selecting an output-power threshold. The output-power threshold
is selected such that at least one signal requirement is not outside a pre-determined
range when output power is less than the output-power threshold. The method also
includes determining whether the output power is less than the threshold and, in
response to a determination that the output power is less than the threshold, disabling
amplitude modulation of the polar signal.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of exemplary embodiments of the present invention
can be achieved by reference to the following Detailed Description of Exemplary
Embodiments of the Invention when taken in conjunction with the accompanying Drawings, wherein:
FIG. 1, previously described in part, shows an exemplary polar transmitter 100
that permits a modulating signal to be divided into an amplitude part and a phase
part; and
FIG. 2 is a flow chart that illustrates operation of an exemplary embodiment
of the present invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION
In UMTS, the dynamic range of a power amplifier is defined as a range in dB in
which all required UMTS specification points (i.e., signal requirements) are met.
For example, for a UMTS user equipment (UE) of power class 4, the output power
range must be from -50 dBm to 24 dBm (i.e., 74 dB). A UMTS-required modulation
depth must then also be added to the output power range to obtain the minimal acceptable
dynamic range of the power amplifier.
The modulation depth is defined as a ratio of the maximal and minimal envelope
of the transmitted signal. A phase-modulated signal includes no amplitude information;
therefore, the modulation depth is 0 dB. An amplitude-modulated (AM) signal has
all information stored in the envelope; therefore, the modulation depth is greater
than 0 dB and is infinite in the case of a 100% AM signal. The modulation depth
is ideally infinite. However, UMTS spectrum-emission-mask signal requirements are
the limiting factor in determining a minimal acceptable modulation depth. Simulations
have revealed that only an additional 15-20 dB of dynamic range is required in
order to achieve sufficient modulation depth to meet the UMTS spectrum-emission-mask
signal requirements. To avoid distortion, the power amplifier
104 must pass
the modulation depth at all power levels; therefore, the power amplifier
104
dynamic range must be greater than the modulation depth plus the output power range.
Thus, the power amplifier
104 must support a dynamic range of at least approximately
90 dB in order to fulfill the UMTS signal requirements.
The 90 dB requirement is very hard to fulfill mainly due to leakage of the phase
part through the power amplifier
104. The power amplifier
104 typically
has limited isolation from input to output. At low output levels, a signal input
to the power amplifier
104 is greater than a signal output by the power
amplifier
104 and the minimum output power by the power amplifier
104
is limited by leakage. Because the output power of the power amplifier
104
is a combination of signal leakage from input to output and an amplified signal
through the power amplifier
104, if the amplification by the power amplifier
104 is small, the leaking signal tends to dominate the output signal. The
power amplifier
104 actually works as an attenuator at very low output power
levels. As a lower limit of the dynamic range of the power amplifier
104
is approached, the power amplifier
104 reduces the modulation depth and
thereby adds distortion. The dynamic range of the power amplifier
104 is
limited at the upper end of the dynamic range by the maximal output power, which
in turn is set by the impedance level and supply voltage used.
The UMTS signal requirements include absolute requirements on adjacent-channel
leakage ratio (ACLR). ACLR is the ratio of transmitted power to power measured
after a receiver filter in adjacent channel(s). Analysis of the absolute requirements
of the ACLR reveals that 15-20 dB modulation depth is required only for output-power
levels greater than approximately -30 dBm. Therefore, dynamic-range requirements
of the power amplifier
104 can be reduced substantially.
Under appropriate conditions, sufficient information can be transmitted in
the phase part only of the transmitted signal. In UMTS, the measure of information
quality is the signal requirement of error vector magnitude (EVM), which must be
less than 17.5%. CDMA systems are inherently immune to error vector magnitude degradation
due to the receiver gain when the transmitted signal is despread. If a low error
vector magnitude is achieved, a high-quality radio link can be set up.
In simulations of a UMTS system performed by the inventor, a signal was divided
into the amplitude part and the phase part, the dynamic range of the amplitude
signal was limited, and the amplitude part and the phase part were re-combined.
Various simulations are then performed on the re-combined signal. These simulations
have shown that detection of the phase part of the transmitted signal by a receiver
results in an error vector magnitude of only 11%, which is well below the maximal
acceptable error vector magnitude of 17.5%. Therefore, information to be transmitted
can be included in only the phase part of the transmitted signal, so long as the
output spectrum meets all UMTS signal requirements. For UMTS systems, simulations
have demonstrated that, when only the phase part is used, the output spectrum is
sufficient and all signal requirements are met for output power levels up to -30
dBm. Thus, for output power levels up to -30 dBm, an amplitude path of the power
amplifier
104 can be turned off (i.e., disabled), such that A(s)=0. Since
the amplitude path typically includes a digital-to-analog converter (DAC), filters,
and other circuitry, substantial power savings can accrue from turning off or disabling
the amplitude path.
UMTS signal requirements also mandate that peak2avg be supported at peak output
power. For UMTS, peak2avg is approximately 3.4 dB. UMTS defines peak2avg as the
ratio of the peak value to the average value of the envelope of the transmitted
signal. Thus, when peak2avg support requirements are considered, the dynamic range
of the power amplifier
104 is reduced from approximately 90 dB to approximately
57.4 dB (i.e., 54 dB+3.4 dB=57.4 dB).
Even if a full modulation depth of 15-20 dBm is inserted in the amplitude part
that is input to the power amplifier
104, at low output-power levels, the
effective modulation depth will be lessened due to leakage through the power amplifier
104. For example, if the dynamic range of the power amplifier
104
is 57.4 dB and the power amplifier
104 is transmitting at an output power
of -14.0 dBm, 16.0 dB of effective modulation depth remains (i.e., -14.0 dBm-(-30.0
dBm)=16.0 dB).
As the output power of the power amplifier
104 is decreased further below
-14.0 dBm, the effective modulation depth is reduced linearly along with the output
power. When the output power has been reduced to -30 dBm, the effective modulation
depth becomes 0 dB. At this point, the amplitude path of the transmitter
100
can be turned off (i.e., disabled) and swapped to a DC level corresponding to a
desired output-power level. As noted above, since the amplitude path includes filters
and signal processing (pre-distortion), power savings results therefrom.
Output power control when the amplitude path has been turned off (i.e., disabled)
can be attained in at least two ways. First, the power amplifier
104 can
be run in a saturated mode and the output power controlled by a voltage for amplitude
power control (Vapc). Amplitude power control is achieved by varying the power
amplifier
104 bias level, as in a constant-envelope case. Second, the power
amplifier
104 can be run in linear mode and a variable gain amplifier (VGA)
prior to the power amplifier
104 used to control the output power. In the
second case, the power amplifier
104 is biased to avoid saturation and has
a constant gain.
FIG. 2 is a flow chart that illustrates operation of an exemplary embodiment
of the present invention. A flow
200 begins at step
202, at which
step an output-power threshold is selected such that at least one signal requirement
is not outside a pre-determined range when output power of a polar transmitter,
such as, for example, the polar transmitter
100, is less than the output-power
threshold. The signal requirements can be, for example, error vector magnitude
or adjacent channel leakage ratio. From step
202, execution proceeds to
step
204. At step
204, a determination is made whether the output
power is less than the output-power threshold. If it is determined at step
204
that the output power is less than the output-power threshold, execution proceeds
to step
206.
At step
206, amplitude modulation of the polar signal by the polar transmitter
is disabled. As noted above, disabling amplitude modulation by the polar transmitter
results in substantial power savings. If, at step
204, it is not determined
that the output power is less than the output-power threshold, execution proceeds
to step
208. At step
208, amplitude modulation by the polar transmitter
is enabled. From both steps
206 and
208, execution proceeds to step
204.
As noted above, the signal requirements will often times include a measure of
information quality, such as the error vector magnitude discussed above. In addition,
in connection with step
206, a power amplifier of the polar transmitter
can be run in a saturated mode and the output power controlled by bearing a biased
level of the power amplifier. In the alternative, in connection with step
206,
the power amplifier can be run in a linear mode and the output power controlled
by a variable gain amplifier.
Application of principles of the present invention yields a substantial
reduction in the dynamic power range of the power amplifier
104, lessens
problems associated with the phase part leaking through the power amplifier
104
at low power levels, and results in power savings at low output-power levels. While
UMTS and the polar transmitter
100 have been used for the exemplary embodiments
described herein, the principles of the present invention are not limited to UMTS
applications, standards-based signal requirements, or to any particular polar transmitter design.
Although embodiment(s) of the present invention have been illustrated in
the accompanying Drawing and described in the foregoing Detailed Description, it
will be understood that the present invention is not limited to the embodiment(s)
disclosed, but is capable of numerous rearrangements, modifications, and substitutions
without departing from the invention defined by the following claims.
*
