Telecommunications – Transmitter – Carrier amplitude control
Reexamination Certificate
1999-02-05
2002-01-22
Urban, Edward F. (Department: 2683)
Telecommunications
Transmitter
Carrier amplitude control
C455S127500, C330S279000, C370S342000
Reexamination Certificate
active
06341219
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of wireless communications and, more particularly to a single-source frequency diverse power control method and apparatus for CDMA wireless cellular handsets.
2. Description of the Related Art
Code Division Multiple Access (CDMA) transmission schemes have become increasingly popular due to the recent growth of the cellular industry. CDMA is a spread spectrum technique whereby data signals are modulated by a pseudo-random signal, known as a spreading code, before transmission. The modulation of the data signals spreads the spectrum of the signals and makes them appear like noise to an ordinary receiver. When the same pseudo-random signal is used to demodulate (despread) the transmitted data signal at the CDMA receiver, the data signal can be easily recovered.
Currently, there exists an industry standard requiring CDMA handset transmitters to achieve a minimum of 73 dB of output power control. As is known in the art, the terms “gain control” and “power control” may be used synonymously since gain can be translated into power at any point during the transmit chain. The maximum required gain minus the minimum required gain is often referred to as gain range. In practice, the circuit components employed to achieve this output power control must also achieve additional power control to compensate for device, frequency, mode and temperature variations. These variations increase the minimum output power control of the components used in the CDMA transmitter to slightly over 100 dB of gain range.
FIG. 1
illustrates a typical transmit chain
100
used in a CDMA handset transmitter. The transmit chain
100
includes a digital-to-analog converter (DAC)
102
, modulator
104
, an intermediate frequency (IF) stage
116
and a radio frequency (RF) stage
118
. The IF stage
116
includes an IF amplifier
106
and an IF filter
108
. The RF stage
118
includes a RF upconverter
110
, oscillator
112
and RF amplifier
114
.
The DAC
102
receives digital baseband data from a remaining portion of the CDMA handset. Typically, the baseband data is received from a microcontroller or processor, such as a digital signal processor, responsible for controlling the operation of the handset. The baseband data is comprised of two signals known in the art as the in-phase I and quadrature Q signals. The DAC
102
converts the digital baseband data into analog I and Q signals and outputs the analog I and Q signals to the modulator
104
.
The modulator
104
inputs the analog I and Q signals, combines and modulates the signals into one IF signal which output to the IF amplifier
106
. The IF amplifier
106
has a gain controlled by a control signal. The IF amplifier
106
amplifies the IF signal and outputs it to the IF filter
108
. The IF filter
108
filters out any noise from the amplified IF signal and outputs the filtered IF signal to the RF upconverter
110
. As known in the art, the upconverter
110
converts the amplified IF signal into a RF signal. This conversion is controlled in part by the oscillator
112
connected to the upconverter
110
. The RF signal is output by the upconverter
110
to the RF amplifier
114
. The RF amplifier
114
has a gain controlled by a control signal. The RF amplifier
114
amplifies the RF signal such that the transmission power of the RF signal has the desired output power. This signal would then be supplied to an antenna where it is radiated to a CDMA base station.
Allocation of both gain and gain range through the transmit chain
100
results from tradeoffs based on the noise performance, linearity, current consumption and isolation issues of the chain
100
. Noise performance and linearity are known to have the biggest impact on system performance.
A CDMA system or handset is a full duplex system, that is, both the transmitter and receiver are operating simultaneously. In a CDMA handset, the transmit chain
100
must be designed to eliminate noise appearing at the receiver frequency band. This noise would interfere with a RF received signal. Therefore, the design of the transmitter must be such that its thermal noise is much lower than the thermal noise generated in the receiver. It is this constraint which drives the noise figure requirements, the IF gain allocation and is a critical factor in determining the electrical characteristics of the IF filter
108
. The high gain range is the constraint which forces the output power control to be performed across the two stages
116
(i.e., a two-stage power or gain control) ,
118
as opposed to being performed in either the RF or IF stage (i.e., a one stage power or gain control).
Today, almost all power control is performed via a two-stage power control having a variable gain. Typically, these methods utilize separate signals to control the gain of the two stages. It is desirable, however, to control the two frequency-diverse variable-gain stages (i.e., the IF and RF stages) with a single control signal. The use of one control signal would greatly enhance the overall handset design and circuitry by simplifying the interface between the transmit chain and the handset micro-controller. This would improve the cost associated with manufacturing the handset as well as its performance.
FIG. 2
is a block diagram illustrating an exemplary automatic and adjustable power control (APC) circuit
120
for controlling the two frequency-diverse variable-gain stages (i.e., the IF and RF stages
116
,
118
) of the transmit chain with a single APC control signal VapcMaster. The APC circuit
120
is implemented either in analog or digital circuitry. As shown in
FIG. 2
, the APC circuit
120
utilizes the VapcMaster signal to generate a first signal Vapc
IF
to control the gain of the IF amplifier
106
and a second signal Vapc
RF
to control the gain of the RF amplifier
114
. The VapcMaster signal is an analog voltage level whose amplitude is output by the handset micro-controller. As described below, this signal is used by the APC circuit
120
to generate the Vapc
IF
and Vapc
RF
control signals which are then respectively applied to the IF and RF amplifiers
106
,
114
. In most wireless handset applications, the VapcMaster signal will be approximately 2.0 volts at maximum.
One method of controlling the gains of the IF and RF stages
116
,
118
is by a sequential control method. This sequential control method varies the gain of one of the stages over the stages entire gain range prior to “handing off” the power control to the other stage. For example, the method would begin by varying the gain of the RF stage over the entire RF stage gain range. When this is complete, the method would continue by varying the gain of the IF stage over the entire IF stage gain range. During this method, the total gain G
Tot
, which is the addition of the gains of the IF and RF stages, must remain within the required gain range. The gain control of the sequential method is performed by the APC circuit as follows:
G
Tot
=G
IF
+G
RF
=Vapc
IF
*GS
IF
+Vapc
RF
*GS
RF
, where
Vapc
IF
=VapcMaster, Vapc
RF
=Vapc
RF−min
when VapcMaster
Min
<VapcMaster<APC Handoff Level,
Vapc
IF
=Vapc
IF−max
, Vapc
RF
=VapcMaster when APC Handoff Level<VapcMaster<VapcMaster
Max
,
GS
IF
=the gain slope of the IF stage in dB/V=(total gain range of IF stage)/(control range of the IF stage in Volts) and
GS
RF
=the gain slope of the RF stage in dB/V=(total gain range of RF stage)/(control range of the RF stage in Volts).
It must be noted that the APC Handoff Level is a voltage level of the APC control signal VapcMaster at which the RF gain range has been completely exercised. Once the VapcMaster reaches the APC Handoff Level, the sequential method begins to exercise the gain of the IF stage over its entire gain range.
A second method of controlling the gains of the IF and RF stages
116
,
118
is by a simultaneous control method. In the simultaneous control method the con
Poirier John R.
Strobel Christopher J.
Agere Systems Guardian Corp.
Dickstein , Shapiro, Morin & Oshinsky, LLP
Urban Edward F.
LandOfFree
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