Automatic gain control method and apparatus for wireless communication systems

A system and method are provided for automatic gain control (AGC) using an adaptive reference and a threshold. An adaptive reference allows for adjusting the target power based on variations in operating parameters and conditions, such as a change to a modulation schemes which requires a different signal to noise ratio (SNR) to maintain performance. A gain adjustment threshold allows for rapid AGC response while reducing the potential effects of hysteresis. AGC levels may be changed during symbol prefixes or suffixes, and avoided during a data portion of a signal. Embodiments allow for symbol-by symbol AGC changes in OFDM, OFDMA, and TDD systems.

RELATED APPLICATIONS

This application is related to and claims priority to Chinese Application No. 200610160846.7 filed Nov. 30, 2006 entitled “AUTOMATIC GAIN CONTROL METHOD AND APPARATUS FOR WIRELESS COMMUNICATION SYSTEMS”, the disclosure of which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to wireless communication, and more particularly to automatic gain control (AGC) for wireless systems.

BACKGROUND

Fast fading channels can present dynamic range challenges for symbol based wireless systems, such as orthogonal frequency division multiplexing (OFDM), orthogonal frequency division multiple access (OFDMA), and time division duplex (TDD) systems with frame-based AGC. Problems exist because signal strength may fluctuate significantly during a frame which contains multiple symbols, resulting in differing signal to noise ratios (SNRs) among symbols. Further compounding the challenges are differing SNR requirements for various common modulation schemes.

For example, 16 quadrature amplitude modulation (16 QAM) will have poorer bit error rate (BER) performance than quadrature phase shift keying (QPSK) for a given SNR. To compensate for this difference, the SNR requirement for 16 QAM is higher than it would be for QPSK. For a system that uses multiple modulation schemes, meeting a minimum performance level may require significant dynamic range in order to accommodate the differing SNR requirements over fading channels.

In order to maintain an acceptable SNR within a system's dynamic range, receivers often employ AGC. Traditionally, AGC circuits control the gain of a variable gain amplifier to maintain an amplified signal within a target power range. By comparing the amplified signal power to a reference, a determination can be made as to whether the amplifier should be adjusted to provide more gain or less gain. A control signal then causes the gain adjustment. Since the feedback loop has a time lag, if power adjustments are made too quickly, the feedback loop may become an oscillator which could introduce a ping-pong effect in the power level. To combat this, TDD-OFDM systems often adjust gain on a relatively slow, frame-by-frame basis, presenting the above-mentioned problem of differing symbol SNRs.

SUMMARY

An adaptive AGC reference allows for adjusting signal power based on system needs when operating on a certain type of signal. For example, when a higher signal power is required, possibly due to changing the modulation scheme to one requiring a higher SNR, the adaptive reference may be increased. Conversely, the adaptive reference may be lowered when system performance will allow, such as when using a more robust modulation scheme.

A gain adjustment threshold, which prevents small changes in amplifier gain, introduces a non-linearity in the AGC feedback loop. This reduces the likelihood of feedback-induced oscillations or a ping-pong effect, allowing symbol-based AGC, which provides for more consistent symbol SNRs. By adjusting the gain only during a symbol prefix or suffix, the gain level is not altered over the data, resulting in more consistent demodulation. Embodiments of the invention provide for symbol-based AGC by determining a power level, comparing the power level with an adaptive reference to determine a target gain change, comparing the target gain change with a threshold, and only adjusting signal gain if the target gain change exceeds the threshold.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows prior art AGC circuit10using a traditional implementation. AGC10controls the gain applied to signals arriving at input101in order to maintain a target power level at output102. Analog to digital converter (ADC)104digitizes the amplified signal, and provides the digital signal to both output102and power measurement unit105. The measured power is then compared to a target power level by target power comparison subsystem106. The result of this comparison is then fed back to variable amplifier103as a gain adjustment signal. Typically, gain may be adjusted on a frame-by-frame basis.

FIG. 2shows AGC circuit20, according to an embodiment of the invention. AGC20controls the gain applied to signals arriving at input101in order to maintain a target power level at output102. Specifically, the gain of variable amplifier103is adjusted based on feedback from adaptive target power comparison subsystem201. Optional analog to digital converter (ADC)104digitizes the amplified signal, and provides the signal to both output102and power measurement unit105. The function of power measurement unit105may be provided by any known method for determining power level, including, for example, estimation. Adaptive reference202may change based the modulation scheme being used, or it may change based on an operating parameter or condition that affects system performance, such as noise levels. For example, a change from QPSK to 16 QAM could require the increase of adaptive reference202by 4 to 6 dB. The result of the comparison, which is target gain change, is then compared to threshold204, in threshold trigger203. If the target gain change identified in comparison unit201exceeds the threshold, then the gain of variable amplifier103is adjusted.

Threshold204may provide separate values for increasing and decreasing gain, and may be defined in terms of ratios or absolute differences. A threshold is a condition that must be met order for an event to occur. Threshold204is chosen to allow changes in gain levels that are determined to be significant enough to warrant change, while insignificant gain level changes that fall below threshold204may be prevented from occurring. The setting of threshold204may be made based on engineering decisions or historical performance data. Since feedback systems often introduce a lag between identifying a need for a change, and the change taking effect, they risk producing undesirable oscillations with frequencies related to the lag period. In general, threshold trigger203is a non-linear device, which may reduce oscillations in output power. Threshold204may itself be adjusted based on whether hysterisis is detectable in the output signal. For example, if power oscillations due to the AGC lag time are identifiable, the values in threshold204may be adjusted either up or down in order to perturb the feedback cycle. Additionally, threshold204may be adjusted based on conditions of the input signal, such as how rapidly or significantly it varies.

AGC20may adjust gain on a symbol-by-symbol basis, which provides a more consistent symbol power level as compared with frame-by-frame adjustment. With a fast fading channel and frame-by-frame adjustment, a frame, which contains multiple symbols, may have some symbols with higher signal power than others, resulting in differing SNRs for the symbols as well as a potentially wide dynamic range. However, adjusting gain too often, such as during the symbol, may cause ambiguities during demodulation. Thus, AGC20may limit the time-frame for gain adjustment to a symbol prefix or a cyclic prefix (CP), such that gain is constant during the data portion of the signal. In an OFDM system, for example, a CP may last 10 microseconds (μs) and precedes the data portion, which may last 100 μs. For OFDMA, the CP follows the data portion of the signal.

FIG. 3shows method30which is one embodiment for providing AGC according to an embodiment of the invention. An input signal is received during process301, and its power is determined in process302. The power level may be determined by either measurement or estimation. The signal power level is then compared with an adaptive reference by process303. The adaptive reference may change in response to identified or expected system performance needs or parameter changes, such as variations in the modulation scheme, estimated interference level, or any other need for adjusting SNR.

Process304compares the result of process303with a threshold. The threshold itself may be varied, based on whether gain may be safely adjusted more often, or whether a ping-pong effect is occurring in the output signal. A determination is made in process305as to whether the threshold is exceeded. If it is not, process306does nothing. If the threshold is exceeded, then process307adjusts gain. Gain may be adjusted on a symbol-by symbol basis during a symbol prefix or a CP, but held constant during the data portion of the signal.

Embodiments of the invention thus provide symbol-based AGC with an adaptive reference. The adaptive reference may be based on the modulation scheme in use, such that when the modulation changes to a one that needs higher signal power to maintain a particular performance level, such as a minimum BER, a higher power reference is used. Conversely, the power reference may be lowered when the modulation changes to one that does not require as much signal power to maintain a particular performance level. Symbol-based AGC may provide more consistent SNR levels than frame-based AGC for fast fading channels, due to the more rapid response in gain adjustment.

Embodiments of the invention also provide for an AGC adjustment threshold, such that the AGC level is not adjusted when the amount of gain change is below the threshold. Minor power fluctuations may occur rapidly, possibly producing a ping-pong effects, or power-level oscillations, if the AGC is allowed to vary too rapidly. The threshold limits prevents AGC level adjustment during minor fluctuations, limiting AGC level adjustment to the more serious power fluctuations. Adjustment of the gain may be accomplished during a symbol prefix or suffix, so that the gain level is established and is not altered for the data. Embodiments of the invention may simultaneously provide symbol-based AGC with an adaptive reference and a threshold.