Amplifiers – With pilot frequency control means
Reexamination Certificate
2000-12-13
2002-12-03
Pascal, Robert (Department: 2817)
Amplifiers
With pilot frequency control means
C330S151000
Reexamination Certificate
active
06489844
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a feed-forward amplifier, which contributes to low-distortion amplification of electric signals, and a controller of the same.
The present invention is applicable to low-distortion amplification of multi-carrier signals or spread spectrum modulated signals. Expressed in more general terms, the present invention contributes to improvement in the quality of transmission signals in the fields of radio communications and cable communications. The feed-forward amplifier relating to the present invention is usable in base stations and repeaters for cellular telephones, broadcasting stations and relay stations for terrestrial wave digital television broadcasting systems, and various other systems requiring low-distortion amplification.
2. Description of the Related Art
Cellular telephone systems, in particular such systems using terrestrial waves, have a common configuration in which a large number of base stations are geographically dispersed from each other while users carry mobile stations. Furthermore, there are instances where a repeater is provided between the base station and the mobile station. Radio frequency amplifiers are provided at the base stations and at the repeaters to power-amplify the transmitting radio frequency signals to the mobile stations, to which strict low-distortion characteristics are required due to the reasons given in the following.
First, the input versus output characteristics of the amplifier inevitably exhibit some non-linearity of varying degrees. Distortion components generated by this non-linearity are of many types including harmonics, cross-modulation distortion, and intermodulation distortion. It is necessary to reject or suppress these distortion components that cause degradation of the signal quality. Some distortion components, such as harmonics, which often appear at frequencies considerably separated from the frequency band occupied by the input signal to the amplifier can be rejected by a filter provided at a stage subsequent to the amplifier. However, the remaining distortion components, such as cross-modulation distortion and intermodulation distortion having a frequency that is identical or extremely close to the frequency of the input signal to the amplifier are difficult or impossible to reject by such a filter. In particular, when the amplifier amplifies a plurality of carriers having frequencies extremely close to each other, neither the cross-modulation nor intermodulation distortion component can be rejected with a filter.
Cellular telephone systems are implemented according to mutually different standards in various countries in the world. Among these standards, the PDC (Personal Digital Cellular) standard cellular telephone system implemented in Japan, the GSM (Global System for Mobile communication) standard cellular telephone system implemented in many countries, such as in Europe, the IS-54/IS-136 standard cellular telephone system implemented in the US, and the EDGE (Enhanced Data-rates for GSM Evolution) standard cellular telephone system and the GPRS (General Packet Radio System) standard cellular telephone system, which are both called 2.5 generation cellular telephone systems, use many carriers having frequencies extremely close to each other. The CDMA (Code Division Multiple Access) standard cellular telephone system, which is currently being popularized or developed in various countries, transmits spread spectrum modulated signals. This type of system has been implemented in the U.S., Japan, and South Korea under the designation of cdmaOne or IS-95, and there are plans for its implementation under the designation of W-CDMA (Wideband CDMA), IMT-2000 (International Mobile Telecommunication-2000) or cdma2000.
As was clearly summarized in the above, a common characteristic of current and future cellular telephone systems is that the signals to be transmitted between a base station or repeater and a mobile station, or between a base station and a repeater include a plurality of components having frequencies extremely close to each other. More precisely, a signal including a plurality of frequency components having frequencies extremely close to each other is input to a radio frequency power amplifier (multi-carrier amplifier) in base stations or repeaters for the PDC, GSM, IS-54/IS-136, EDGE, and GPRS standards, or spread spectrum modulated signal is input to a radio frequency power amplifier in base stations or repeaters for the CDMA standard. Thus, the frequency components in the input signal are susceptible to cross-modulation or intermodulation. Furthermore, since the input signal includes many carriers or is spread spectrum modulated, distortion components are liable to appear due to the non-linearity of the amplifier. Namely, the radio frequency-power amplifier of base stations and repeaters for cellular telephone systems requires a scheme to suppress the generation of distortion components represented by the intermodulation distortion components, or an improvement to lower or eliminate distortion.
The radio frequency power amplifier of base stations and relay stations for terrestrial wave digital television broadcasting systems also requires a similar scheme. For example, since the terrestrial wave digital television broadcasting system planned for implementation in Japan transmits signals in which many carriers are multiplexed in accordance with the OFDM (Orthogonal Frequency Division Multiplex) standard, its amplifiers require an improvement to reduce or eliminate distortion.
However, it is impossible to realize an amplifier having ideal low-distortion characteristics, and it is often difficult to realize even an amplifier having near-ideal low-distortion characteristics due to constraints in terms of cost and circuit size. One approach for solving this problem is to add to the amplifier a circuit to reject or suppress the distortion components that are generated in the amplifier. This approach has been implemented heretofore in a form where a circuit is provided to detect the distortion components included in the output signal of the amplifier, and in accordance with the result thereof, to perform automatic control so as to minimize the distortion components included in the output signal of the amplifier. This type of amplifier having an additional circuit is called a distortion compensation amplifier.
One known type of distortion compensation amplification system of the prior art is the feed-forward system. Generally, the feed-forward system is adopted with an object to enable the maximum suppression or rejection, among the residual distortion components in the amplifier output, of the distortion components that are difficult to reject at a filter stage or the like, to enable the maintenance of desirable distortion component rejection and suppression performance even in the event of temperature variations or deterioration due to aging, and once this is achieved, to maintain and improve the quality of the transmission signals by obtaining a low-distortion amplified output. The distortion compensation amplifier adopting the feed-forward system is called a feed-forward amplifier.
The feed-forward amplifier comprises a main amplifier for amplifying signals, a distortion detection loop, which is a feed-forward loop for detecting the distortion components generated at the main amplifier, a distortion rejection loop, which is a feed-forward loop for rejecting or suppressing these distortion components from the output signal, and a controller for automatically controlling the operation of the distortion detection loop and the distortion rejection loop. Hitherto, various improvements and modifications have been proposed for the feed-forward amplifier. Relevant references include patents issued in Japan such as Japanese Patent Publication No. Hei 7-77330, Patent Nos. 2711413, 2711414, 2799911, 2804195, 2948279, 2945451, and 2945447, or corresponding laid-open publications including the original disclosure by one of the applications i
Ito Akira
Ito Keijiro
Kawasumi Norihisa
Sakamoto Hironori
Yamada Akira
Japan Radio Co. Ltd.
Nguyen Khanh Van
Samuels , Gauthier & Stevens, LLP
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