Pulse or digital communications – Receivers – Automatic gain control
Patent
1995-06-26
1998-04-28
Bocure, Tesfaldet
Pulse or digital communications
Receivers
Automatic gain control
4552391, H04L 2708
Patent
active
057455317
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a gain control apparatus of a receiver for mobile communication systems, and more particularly to an automatic gain control apparatus and method of a receiver for mobile communications using CDMA systems which perform multiple access using a spread-spectrum.
BACKGROUND ART
The propagation characteristic of terrestrial mobile communications is represented by three factors: variations involved in the distance variations between a base station and a mobile station (distance variations); moderate variations over an interval of about several tens of meters (median variations); and rapid variations in an interval of several tens of meters (instantaneous variations). The variations in a received signal involved in the distance variations is the greatest among these variations. For example, the received signal will attenuate more than 70 dB at an edge of a cell separated far from the base station. Accordingly, a dynamic range of more than 70 dB is required of a mobile communication receiver.
Thus, since the received signal level fluctuates more than 70 dB in a mobile communication system, and other users are carrying on telephone conversations through other channels in adjacent frequency bands, it is necessary to amplify the received signal to a level sufficient for detection after having removed unnecessary signals of adjacent channels. Analog signal transmission systems or systems using only phase shift information such as a .pi./4 shift QPSK modulation which will be employed in digital systems introduced at the next term, regulate level variations of an input signal of a receiver by amplifying the signal more than 70 dB using a limiter amplifier after band limiting.
However, when a linear modulation is performed, or when information is carried by amplitude components, a linear reception is required. Since a conventional limiting amplifier will corrupt amplitude information, an automatic gain control amplifier which performs linear amplification is necessary in this case. Recently, the digital signal processing has become the main current, owing to the development of the digital signal processing, in which a received signal is A/D converted before processing. In the mobile communications, however, quantization error of the A/D conversion increases when a received level drops, because variations in the received level due to fading are very large of about 60 dB. To handle this problem, an automatic gain control circuit is essential which compensates for the received level variations.
FIG. 2 is a block diagram showing a conventional feedback type automatic gain control circuit. The conventional feedback type automatic gain control circuit includes a variable gain control amplifier 11 whose gain is varied in accordance with a bias voltage, and an instantaneous envelope level detector 12 which detects an envelope level of the output signal. The instantaneous envelope level from the detector 12 is supplied to the variable gain control amplifier 11 as the bias voltage. Thus, the gain of the variable gain control amplifier 11 fluctuates in accordance with the variations in the instantaneous envelope level, thereby keeping constant the output signal level of the variable gain control amplifier 11 using the closed loop.
In the feedback type automatic gain control circuit, it will be sufficient for the instantaneous envelope level detector 12 to have an input dynamic range on the order of a few tens of dB at most because the output signal of the variable gain control amplifier 11 is inputted thereto. Correction using the closed loop, however, takes a rather long time for convergence of the level. This makes it difficult to track instantaneous variations in the signal level, and presents a problem in that it is difficult for the closed loop control to follow instantaneous variations such as rising edges of burst signals.
On the other hand, a feedforward type automatic gain control circuit has an open loop configuration, and an input signal to a variable gain control ampli
REFERENCES:
patent: 3605018 (1971-09-01), Coviello et al.
patent: 4984287 (1991-01-01), Kaufman et al.
patent: 5065410 (1991-11-01), Yoshida et al.
patent: 5452332 (1995-09-01), Otani et al.
Adachi Fumiyuki
Sawahashi Mamoru
Bocure Tesfaldet
Hoare, Jr. George P.
NTT Mobile Communications Network Inc.
Webster Bryan
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