Telecommunications – Transmitter – Power control – power supply – or bias voltage supply
Reexamination Certificate
2001-12-18
2004-05-11
Vuong, Quochien B. (Department: 2685)
Telecommunications
Transmitter
Power control, power supply, or bias voltage supply
C455S115100, C455S126000, C455S522000
Reexamination Certificate
active
06735420
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to wireless communications, and more particularly to an efficient multiple data rate, closed-loop transmit power scheme for wireless packet communications.
DESCRIPTION OF RELATED ART
The data rate of wireless data communications, including Wireless Local Area Networks (WLANs) and associated devices, continue to increase. Methods of increasing data rate includes different coding schemes and higher capacity modulation schemes. Exemplary modulation schemes include Binary Phase Shift Keying (BPSK), Quadrature PSK (QPSK), 16 Quadrature Amplitude Modulation (QAM), and 64 QAM employing corresponding constellation types with 2, 4, 16 or 64 constellation points, respectively, for modulating 1, 2, 4 or 6 bits, respectively. BPSK uses a relatively low capacity constellation type whereas 64 QAM uses a relatively high capacity constellation type. The Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.11a standard employs the BPSK, QPSK, 16 QAM and 64 QAM modulation schemes for WLAN operation in the 5 Gigahertz (GHz) frequency band to achieve data rates of 6, 9, 12, 18, 24, 36, 48 and 54 megabits per second (Mbps) employing orthogonal frequency division multiplexing (OFDM).
OFDM is a multi-carrier modulation technique in which data is carried on a plurality of “tones” or “sub-carriers” associated with a multi-carrier signal. In the OFDM embodiment, communication is established using packets of information including one or more synchronization data fields followed by a plurality of OFDM symbols. In an OFDM configuration, 52 sub-carrier signals are incorporated within each OFDM symbol, including 48 data tones and 4 pilot tones as known to those skilled in the art. Data is incorporated on each data tone using a selected modulation schemes, such as BPSK, QPSK, 16 QAM, and 64 QAM. Each of the modulation schemes employs a corresponding constellation type with 2, 4, 16 or 64 constellation points, respectively, for modulating 1, 2, 4 or 6 bits, respectively. The data rate range is determined by the decoding scheme and by the modulation scheme and corresponding constellation typeused. For example, BPSK is used for 6 or 9 Mbps, QPSK is used for 12 or 18 Mbps, 16 QAM is used for 24 or 36 Mbps, and 64 QAM is used for 48 or 54 Mbps. A constellation point is selected for each bit group according to the selected constellation and data rate, and each tone is modulated with an amplitude and phase according to the selected constellation point. A different data rate may be achieved for each constellation type using a different encoding technique employing different proportions of redundant information. Although the present invention is illustrated using 5 GHz OFDM, it is understood that different RF bands and modulation schemes may be used.
Increases in required transmit power is a necessary consequence of increased path loss in the 5 GHz band. With increased path loss, coverage (communications distance) is at a premium. In order to maximize distance, it is desired to push the transmit power amplifier to a maximum power limit for optimal performance. A given wireless network may be operated at multiple data rates. An Access Point (AP), for example, may communicate with several devices in a wireless area, where each device operates at a different data rate. In fact, an IEEE 802.11a OFDM-based WLAN requires transmission at multiple data rates. Maximum transmit power, however, is a function of the particular data rate of transmission, which is further related to the encoding scheme and constellation type. As the data rate increases, distortion tolerance decreases. For example, the simpler constellations (e.g. BPSK) used in the lower data rates may be transmitted with greater power, albeit higher distortion. As the constellation type becomes more complex, the distance between constellation points decreases (and the corresponding transmit waveform becomes more complex). Distortion tolerance is consequently reduced. Also, a higher data rate for a given constellation type using less redundancy information is more sensitive to distortion and therefore less robust. Because lower data rates can sustain more distortion, they can be transmitted with greater total power.
In this manner, to increase output power (range), it is desired to transmit with as much distortion as possible. On the other hand, in order to prevent serious distortions and spectral leakage into adjacent channels, the power amplifier output should be reduced to limit the effect of waveform peak saturation. As a result of these competing considerations, the target power level is different for each data rate and/or constellation type employed. The problem arises as to how to efficiently maintain closed-loop power control in a radio (such as an AP or the like) that transmits multiple data rate packets at multiple output power levels. Two primary means of automatic level control (ALC) control are available, including an open loop method and a closed loop method. In the open loop method, a control device, such as a media access control (MAC) device or the like, simply writes a default controlling set point value to a register. The controlling set point value may be determined at time of manufacture using a calibration procedure. The use of default values carry the penalty of being less than optimal since conservative assumptions must be made. Differences from channel to channel are less likely to be a problem and can be minimized by interpolation. A more severe concern is that the radio may drift over time causing significant distortion and errors. Without closed loop control, unconstrained power drifts may cause spectral violations and bleed-over into adjacent channels thereby interfering with other users.
In the closed loop method, the control device initially sets a nominal set point value and closes the loop by sampling the output transmit power and driving it to the desired set point. The power amplifier, however, is a non-linear device. The efficiency, distortion, nonlinearity and interference issues dictate a different set point for each data rate or modulation scheme used. In this manner, a single control loop is insufficient to achieve the optimal power set point for each constellation type or data rate and separate loops would otherwise be required. Separate control loops contribute to the complexity and cost of radio devices. Furthermore, certain data rates may only be used infrequently so that significant drift may lead to significant error over time. In other words, infrequently used loops exhibit significant errors and inefficient operation.
SUMMARY OF THE INVENTION
A power controller according to an embodiment of the present invention is used for a wireless transmitter that operates using multiple constellation types to achieve several different data rates, where the transmitter includes a variable output power amplifier having a gain input for controlling output power. The power controller includes an output power detector, an error circuit and a power control circuit. The output power detector provides an output power level value indicative of output power. The error circuit compares the output power level value with a target power value and asserts an error value indicative thereof. The power control circuit asserts a power control signal based on a power control value for controlling the gain input of the output power amplifier, where the power control circuit regulates the power control value based on the error value. In order to maximize or otherwise optimize operation for each of the multiple data rates, the power controller further includes an input normalization circuit that selectively offsets the output power level value using one or more input scale values selected by a data rate select signal. Also, the power controller includes an output normalization circuit that selectively offsets the power control value using one or more output scale values selected by the data rate select signal.
It is appreciated that typical power amplifier used for wireless communicati
GlobespanVirata Inc.
Stanford Gary R.
Vuong Quochien B.
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