1. Field
The present invention relates to electronic devices and communications, and more particularly, to a method and apparatus for processing a communication signal in a receiver.
2. Background
Noise figure (NF) and linearity are characteristics of receivers, and can be used to specify the performance of receivers. NF is a measure of the degradation of a signal to noise ratio (SNR), caused by components in a communication signal chain. More specifically, NF is the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard noise temperature T0 (e.g., 290° K.). In other words, NF is the ratio of actual output noise to that which would remain if the device itself did not introduce noise. It is a number by which the performance of a radio receiver can be specified.
The linearity of a receiver, on the other hand, can be characterized by the input-referred third-order intercept point (IIP3). Typically, an output communication signal (e.g., RF signal) and third-order intermodulation products are plotted versus the input communication signal (e.g., RF signal). As the input communication signal is increased, the IIP3 is a theoretical point where the desired output communication signal and the third-order products become equal in amplitude. The IIP3 is an extrapolated value since the active device typically goes into compression before the IIP3 point is reached.
In cellular applications, it is common to have more than one communication system operating within the same geographic coverage area. Furthermore, these systems can operate at or near the same frequency band. When this occurs, the transmission from one system can cause degradation in the received signal of another system. For example, CDMA is a spread spectrum communication system which spreads the transmit power to each user over the entire 1.2288 MHz signal bandwidth. The spectral response of an FM-based transmission can be more concentrated at the center frequency. Therefore, FM-based transmission can cause jamming to appear within the allocated CDMA band and very close to the received communication signal. Furthermore, the amplitude of the jamming can be many times greater than that of the communication signal. This jamming can cause third-order intermodulation products, which can degrade the performance of the CDMA system.
To minimize degradation due to intermodulation products caused by jamming, the receiver can be designed to have high IIP3. However, design of a high IIP3 receiver typically requires the active devices within the receiver to be biased with high DC current, thereby consuming large amounts of power. This design approach is undesirable for cellular application wherein the receiver is a portable unit powered by a battery and power is limited.
Thus, a receiver with improved linearity and noise figure, and reduced power consumption is desired.