Amplifiers – With control of power supply or bias voltage – With control of input electrode or gain control electrode bias
Reexamination Certificate
2001-10-22
2004-03-23
Tokar, Michael (Department: 2819)
Amplifiers
With control of power supply or bias voltage
With control of input electrode or gain control electrode bias
C330S20700P, C455S069000, C455S070000, C455S522000
Reexamination Certificate
active
06710651
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to wireless communication and more particularly to systems and methods for controlling the output power in a wireless communication device.
2. Background
There are several factors that impact the transmit power level in the transmitter of a wireless communication device. Two factors that limit the transmit power level, for example, are: 1) Specific Absorption Rate (SAR) requirements; and 2) Adjacent Channel Power Ratio (ACPR) requirements. SAR is a metric used to evaluate compliance of portable devices with the maximum permissible exposure limits as defined in the FCC guidelines on human exposure to Radio Frequency (RF) emissions. Effectively, the FCC guidelines place a limit on the maximum transmit power of a communication device in order to prevent exposure by users of such devices to excessive levels of RF energy.
ACPR is generally defined as the ratio of the average power in the adjacent frequency channel to the average power in the transmitted frequency channel. In other words, a wireless communication device is configured to transmit over a specific frequency channel at any given time. But due to inherent linearity and other limitations of the components that comprise a communication device transmitter, it is very difficult to prevent the energy transmitted by the device from spreading over into adjacent channels. If too much energy resides in the adjacent channels, then it can interfere with devices operating on those channels. Therefore, many wireless communication standards define limits for ACPR, and even when the applicable standard does not define a limit, ACPR is still a practical limitation.
In order to maintain acceptable SAR and ACPR limits, conventional communication device transmitters typically comprise a power detector, to detect the transmit power level, and an isolator to isolate the transmitter from reflected energy generated at the interface between the transmitter and the device's antenna. For example, in a Frequency Modulation (FM) transmitter, SAR is the limiting issue. Therefore, a power detector can be used to ensure that the output power of the transmitter does not exceed the FCC specified limits. In a transmitter that is implementing a complex modulation scheme, such as Code Division Multiple Access (CDMA) or Time Division Multiple Access (TDMA), on the other hand, there are much more stringent linearity requirements. Thus, ACPR is the limiting issue, although SAR still applies. If the transmitter attempts to produce too much or excessive power is reflected back from the antenna into the transmitter, the linearity and, therefore, the ACPR can be substantially degraded. Accordingly, conventional devices often insert an isolator to block the reflected power and have some means to limit the maximum RF output power if there is a danger of exceeding the transmitter rating before reaching the SAR threshold
While the conventional detector/isolator approach has certain advantages, it also has certain limitations that can substantially impact the performance of a wireless communication device. For example, the impedance of the transmission line that conveys the transmitted power to the antenna is designed to match the impedance of the antenna in order to reduce the amount of reflected energy and increase transmission efficiency. But when the communication device is placed next to the human head, for example, the impedance of the antenna changes due to the proximity with the head. As a result, more power is reflected back toward the transmitter. When this reflected energy reaches the isolator it is dissipated as heat. Therefore, the resulting radiated transmit power is much lower than it otherwise could be, even taking into account the SAR limitation.
Additionally, the isolator introduces extra loss into the transmission path that is typically on the order of 0.5 dB. Therefore, the transmitter must supply an extra 0.5 dB of power in order to compensate for the extra loss. Increasing the power, however, also increases the ACPR, i.e., increases the amount of energy in the adjacent channels. Because ACPR is predominantly a 3rd order product, the resulting increase in ACPR is approximately 3 times the increase in transmit power, or 1.5 dB, which can lead to noncompliance with the ACPR requirements. Thus, as can be seen, the conventional detector/isolator approach can have a substantial negative impact on the performance of a wireless communication device.
FIG. 1
illustrates an exemplary wireless communication transceiver
100
. Such a transceiver can be included in a wireless communication device, thus enabling the device to communicate over a wireless communication channel
124
in a wireless communication system. Transceiver
100
actually comprises a receive path
106
and a transmit path
110
. Preferably, both paths are interfaced with antenna
102
via a duplexer
108
. Duplexer
108
essentially acts as a filter that is configured to shunt incoming RF signals received by antenna
102
to receive path
106
. Duplexer
108
is further configured to send outgoing RF signals from transmit path
110
to antenna
102
, while providing isolation between paths
106
and
110
so that the incoming and outgoing signals do not interfere with each other.
The received RF signals are then demodulated and processed so as to extract a baseband information signal in the receive portion of transceiver
100
(not shown). Preferably, the baseband information signal is then decoded and processed in a baseband processor (not shown), such as a Mobile Station Modem (MSM). The MSM, or equivalent, is also preferably responsible for generating and encoding baseband information signals that are to be transmitted over communication channel
124
. The baseband information signals generated by the MSM (not shown) are then modulated with a RF carrier in the transmit portion of transceiver
100
, which generates a RF transmit signal to be transmitted via antenna
102
.
The transmit portion of transceiver
100
is also preferably configured to set the power level of the RF transmit signal. In general, Power Amplifier (PA)
120
in conjunction with Variable Gain Amplifier (VGA)
122
generate the required power level as demanded by the MSM. PAs are typically key components in any high frequency RF transmitter design. This is because RF transmitters typically require high output power to compensate for path losses in communication channel
124
and to ensure an adequate signal strength at the base station associated with channel
124
. Since the base station can be miles away, it can be difficult to achieve adequate receive power at the base station. At the same time, if the signal power at the base station is too high, then it may interfere with reception by the base station of transmit signals from other devices within the communication system. Transmitting at higher power levels also reduces battery operating time. Therefore, while it is important to ensure an adequate transmit power level, it is also important to ensure that the transmit power level is not too high. Thus, power control in a wireless communication device is an important aspect of wireless communication.
In conventional wireless communication systems, power control is often performed in the wireless communication device. For example, the base station can be configured to measure the power level of a received transmit signal and determine if it is too high or too low. The base station can then be configured to transmit commands to the wireless communication device instructing the device to turn its power up or down. CDMA communication systems, for example, use such a power control loop. In a CDMA system, the goal of the base stations is to receive signals from each of the devices with which it is communicating at the same receive power level. In fact, such power equalization at the base station for each of the devices in communication with the base station is a critical aspect of CDMA operation. Thus, power control is a critical
Kyocera Wireless Corp.
Nguyen Khai
Tokar Michael
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