Telecommunications – Receiver or analog modulated signal frequency converter – Local control of receiver operation
Reexamination Certificate
1998-09-24
2001-10-30
Vo, Nguyen (Department: 2743)
Telecommunications
Receiver or analog modulated signal frequency converter
Local control of receiver operation
C455S250100, C455S343200
Reexamination Certificate
active
06311048
ABSTRACT:
TECHNICAL FIELD
This invention relates to electronic communication systems, and more particularly to intelligent control of linearity in electronic receivers, such as code division multiple access (CDMA) receivers.
BACKGROUND
Certain types of radio frequency (RF) receivers that utilize a low noise amplifier (LNA) need to perform under conditions that demand very high linearity of the LNA and RF down conversion electronics. Such receivers are used in code division multiple access (CDMA) wireless communication systems. In a CDMA wireless communication system, users share all time and frequency resources simultaneously. This is accomplished by assigning each user a distinct Walsh code or user-unique digital code with special properties to achieve minimal interference between users. This code is added to the information data and modulated onto the carrier. An identical code is used in the receiver which is used to correlate the two signals. The correlation process only passes data that matches the code sequence. In this way, non-valid signals (i.e., signals from other users) are not decoded and appear as noise. Noise immunity is provided by means of the code spreading the spectrum of the information signal to a much wider bandwidth than is needed for baseband signal transmission. This ability of a CDMA system to reject unwanted signals allows for operation in high noise environments. Further information regarding CDMA systems is set forth in the well-known IS95 standard.
In one mode of operation, an IS95 compliant CDMA system permits duplex communications—that is, simultaneous transmission and reception of an RF signal. One problem of this mode is that the presence of an interference signal as the transmit (Tx) channel approaches its maximum power output generally causes cross-modulation of the Tx signal envelope. This results in “in-band” interference in the Rx channel. Such in-band interference can cause degradation in the received signal quality and subsequent call drop.
As is known in the art, the input third-order intercept point (IP
3
) of an LNA/mixer channel can be adjusted to a high value to keep cross-modulation within tolerable limits. The IP
3
is a virtual measurement of the signal strength at which the power of the 3rd-order distortion energy of a gain stage is as strong as the fundamental signal energy. Using a high IP
3
value increases the linearity of an LNA/mixer channel. However, a high IP
3
value also results in a high bias current to the LNA/mixer channel, draining battery power and thus reducing both talk and standby time for an RF telephone. Nevertheless, maintaining a high IP
3
value is the conventional approach to overcoming the cross-modulation problem noted above.
Accordingly, the inventor has determined that a better method and system is needed to control cross-modulation during duplex communications of an RF communication system that overcomes the problems noted above. The present invention provides such a method and system.
SUMMARY
The inventor has realized that, statistically, most of the operation time of an LNA-based RF communication system, such as a CDMA cellular telephone, is at low power levels of about −13 dBm. Thus, rather than maintain a high IP
3
level at all times to manage cross-modulation during the relatively infrequent occasions where such cross-modulation occurs, the invention intelligently controls the linearity of an RF receiver by selectively increasing the effective IP
3
value of an LNA/mixer channel only when needed.
In particular, a control signal is generated based on mode of operation (power consuming); received signal strength information (RSSI); transmit channel output power, as indicated by a Tx automatic gain control (AGC) signal; and the true received signal strength, as indicated by the pilot signal-to-noise ratio. The control signal is then used to selectively increase the bias current—and thus linearity—of the LNA/mixer channel, or to select one of several LNAs having differing IP
3
values to effectively increase the linearity of the LNA/mixer channel.
In one aspect, the invention includes a method and system for intelligently controlling the linearity of a receiver in a radio frequency transceiver, including determining if the transceiver is in a receive mode, and if interference is present in the receiver, and if transmit power within the transceiver is high, and if receiver power within the transceiver is low; and controlling the effective third-order intercept point (IP
3
) value of a LNA/mixer channel within the receiver in response to such determinations.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
REFERENCES:
patent: 5179724 (1993-01-01), Lindoff
patent: 5465408 (1995-11-01), Sugayama et al.
patent: 5661442 (1997-08-01), Sparks
patent: 5697081 (1997-12-01), Lyall, Jr. et al.
patent: 5722061 (1998-02-01), Hutchinson, IV et al.
patent: 5805979 (1998-09-01), Miyashita
patent: 5815821 (1998-09-01), Pettersson
patent: 5940749 (1999-08-01), Cho et al.
Nguyen Duc
Vo Nguyen
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