Pulse or digital communications – Receivers
Reexamination Certificate
1998-09-08
2001-11-06
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
C375S230000, C375S345000
Reexamination Certificate
active
06314144
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a digital wireless receiving apparatus, and in particular to a wireless apparatus which is used for digital mobile communication.
In recent years, with a stringent condition of frequency resources in wireless communication, a high-efficiency transmission system by digitalization has been generally used. On the other hand, for a channel occupation bandwidth, a narrow one has been demanded.
This demand has been achieved, on a transmitter side, by reducing a roll-off factor and by adopting a linearization art while on a receiver side, an analog filter which is superior in an attenuation characteristic has been developed in order to reduce an interfering wave between adjacent channels. In addition, for the improvement of performance a removal of the interfering wave has been recently attempted with a digital filter.
2. Description of the Related Art
FIG. 22
shows a prior art digital wireless receiving apparatus which uses an analog filter and a digital filter as mentioned above. In this prior art, after being amplified at an RF (Radio Frequency) amplifier
32
, an RF reception signal from an antenna (ANT)
31
is transformed into an intermediate frequency (IF) signal (a base band signal) at a mixer
33
with a local oscillation signal from a local oscillator
34
. Then the intermediate frequency signal is amplified at an IF amplifier
35
and sent to an IF filter
101
. The above components
31
-
35
form a high frequency portion
100
.
The IF filter
101
secures a pass band characteristic in the base band signal, removes an interfering wave and sends the base band signal to a quadrature demodulator
102
. In the quadrature demodulator
102
, by using a regenerated carrier wave
90
, I/Q quadrature components are separated from the base band signal at a quadrature detector
9
and are transformed into digital signals at A/D converters
11
-
1
,
11
-
2
. Then, the digital signals secure the pass band characteristic for demodulation at digital filters (FIR)
12
-
1
,
12
-
2
, and are taken over by an identification portion
103
for the identification operation.
In such a prior art apparatus, the IF filter
101
uses an analog filter with a high selectivity (a high attenuation characteristic) to satisfy a selection characteristic between adjacent channels where the interfering wave exists. Accordingly, the analog filter becomes large in shape and expensive. Also, it is technically difficult to realize an analog filter with a narrow band and a high attenuation characteristic, and is not practical to mount such an analog filter on a base station, especially on a mobile station (a portable device or a car-mounted device).
To avoid such a defect, the IF filter
101
has to use a cheep and small-sized analog filter with a normal pass band and attenuation characteristic. On the contrary, if a demodulator (a base band portion) uses digital filters
12
-
1
,
12
-
2
which are composed of a DSP or the like to satisfy the above mentioned characteristic (a narrow band and a high attenuation characteristic), a tap number of the digital filter has to be increased, so that and a delay quantity for the demodulation can not be neglected, which is not practical either.
In addition, in a system which provides an adjacent channel selection characteristic with the digital filter there is a tendency that a dynamic range for securing the characteristic becomes small, compared with a system which achieves the same with the prior art analog filter.
To avoid such a defect, after the high frequency portion removes the interfering wave to some extent with a small-sized analog filter, the demodulator has to use the digital filter with a small delay quantity so that the whole receiving system may satisfy the adjacent channel selection characteristic.
In this case, as an attenuation amount of an interfering wave between adjacent channels is greatly influenced by a linear characteristic of a pre-stage circuit of the digital filter, by providing an AGC circuit in the RF amplifier or the like of the pre-stage circuit of the digital filter the linear characteristic has to be secured and the adjacent channel selection characteristic has to be made advantageous.
According to an arrangement shown in
FIG. 23
, a case will now be considered where an AGC control is executed from the IF filter
101
to the RF amplifier
32
through a level detector
41
and an AGC controller
42
.
If a desired electric field wave is inputted from the antenna
31
, an output electric field of the IF filter
101
shows two cases where the AGC control is executed and where no AGC control is executed, as shown in
FIGS. 24A
,
24
B respectively.
Assuming that there is an interfering wave which is larger by 60 dB than the desired wave in the adjacent channel and that the removal performance for the adjacent channel of the IF filter
101
is 20 dB, the detected electric fields in
FIGS. 24A
,
24
B become those as shown in
FIGS. 25A
,
25
B respectively by the influence of the interfering wave. That is, if the AGC control system is ruled by the interfering wave and the interfering wave increases in magnitude, the execution of the AGC control will lead desensitization restraining an original gain of the desired wave. Namely, it is impossible to execute an appropriate AGC control in an arrangement shown in FIG.
23
.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a digital wireless receiving apparatus which can perform a suitable AGC control in addition to avoiding an increase in scale and cost of an analog filter as well as a process quantity (a delay quantity) of a digital filter as much as possible.
[1] To achieve the above-mentioned object, a digital wireless receiving apparatus according to the present invention is arranged to perform an AGC control by using a filter exclusively used for detecting a desired wave which is different from the filter in a demodulation system of
FIG. 23
to accurately detect an input level of the desired wave.
Namely, as shown in a schematic arrangement (
1
) of
FIG. 1
, in order to remove an interfering wave existing in a frequency band except the desired wave, the IF filter
101
and the first digital filters
12
-
1
,
12
-
2
have only to possess a pass band characteristic for a
15
demodulation operation different from the prior art shown in FIG.
23
. The input level of the desired wave is detected by a second digital filter
12
-
3
and a level detector
104
which have a high selectivity (a high attenuation characteristic). From the detected result, an AGC controller
105
performs a gain control for an RF amplifier
32
in a high frequency (RF) portion
100
.
The attenuation characteristic of the digital filters
12
-
1
,
12
-
2
is not relatively good, while a tap number is small such that it can be neglected upon the demodulation. Also, the digital filter
12
-
3
has a high selectivity.
In the above-mentioned arrangement, removal performances of the interfering wave in the IF filter
101
, the digital filters
12
-
1
,
12
-
2
and the digital filter
12
-
3
are assumed to be 20 dB, 40 dB and 60 dB, respectively.
When the desired wave and the interfering wave the input level of which is the desired wave input level +60 dB are inputted, an input electric field into the level detector
104
reveals a characteristic shown in FIG.
26
A and only the input level of the desired wave can be detected, so that the AGC control can be performed by the AGC controller
105
as shown in FIG.
26
B. At this time, because the digital filter
12
-
3
has nothing to do with the demodulation, the high attenuation characteristic can be realized by e.g. making a narrow band without increasing the tap number.
[2] Also, as the above-mentioned invention [1], this invention may use both of the analog filter, and the first and the second digital filters, wherein the tap number or the series stage number of the first digital filter is arranged to be chang
Matsumoto Shigeaki
Moriyama Yukihiro
Al-Beshrawi Tony
Chin Stephen
Fujitsu Limited
Helfgott & Karas, PC
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