Pulse or digital communications – Receivers – Automatic gain control
Reexamination Certificate
1999-04-05
2004-08-24
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Automatic gain control
C455S250100
Reexamination Certificate
active
06782061
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an AGC circuit for calculating the average received level per slot, obtaining the difference between the calculated level and a reference value, and feeding back the difference to an AGC amplifier, thereby correcting any variations in the received level.
2. Description of the Prior Art
In mobile communications, the received signal strength greatly varies (a maximum of 80 dB is expected) owing to the influences of the distance between a base station and a terminal, fading due to the movement of a terminal, shadowing due to obstacles such as buildings, and the like. To stably receive and demodulate/decode such signals, the received levels must be corrected by using an AGC (Automatic Gain Control) circuit so as to level the received baseband signals.
This AGC circuit has been generally used in TV receivers and radios. According to a portable telephone based on the GSM or IS-95 CDMA scheme, a signal is transmitted from a base station in units of slots, and the signal is received and demodulated in units of slots on the terminal side. In this scheme, AGC must be performed to make the gain of the AGC amplifier constant within one slot, i.e., to make the relative received signal strength constant within one slot. Such an AGC circuit cannot be realized by a conventional AGC circuit designed to perform only analog signal processing. To realize this circuit, a method including digital signal processing in
FIG. 1
is required.
FIG. 1
shows a conventional AGC circuit having such an arrangement.
FIG. 1
is a block diagram showing the schematic arrangement of the AGC circuit.
Since this block diagram of
FIG. 1
aims at explaining only AGC operation, other portions that are not directly associated with AGC are omitted.
In this circuit, a received signal is a QPSK-modulated wave and is demodulated into two baseband signals, i.e., an I (In-phase component) and Q (Quadrature component) signals.
The received signal is converted into an IF signal by a receiver
1
. The IF signal is amplified or attenuated by an AGC amplifier
2
and demodulated into I and Q baseband signals by a quadrature demodulator
3
. The I and Q signals are respectively converted into digital signals by 8-bit A/D converters
4
and
5
(not limited to 8 bits).
Assume that a CDMA scheme with a chip rate of 4.096 MHz is used as a modulation scheme, and each A/D converter samples at 16.384 MHz, which is a conversion rate four times the chip rate. Assume also that the slot length is 625 &mgr;sec (i.e., 2,560 chips). In this case, therefore, each of the numbers of I and Q signal samples obtained amounts to 2560×4=10240 in a 1-slot interval.
A calculator
6
of received level calculates the average received level in the 1-slot interval from the above digital I and Q signals, and outputs the calculation result as an 8-bit straight binary code.
The calculator
6
of received level calculates the average received level in the following manner (for example).
Since each of the I and Q signals is an 8-bit signal from a positive peak to negative peak, the absolute value of each signal is obtained first. The maximum absolute value is obtained when each signal is completely saturated to the positive and negative peaks to have a rectangular waveform. This value of each of the I and Q signals is “01111111” in binary notation (“127” in decimal notation).
A received amplitude A should be calculated by:
A={square root over (I
2
+Q
2
)}
(1)
However, since it is difficult to implement this calculation by hardware, an approximate value A′ given by:
A
′
=
Max
(
|
I
|
,
|
Q
|
)
+
Min
(
|
I
|
,
|
Q
|
)
2
(
2
)
is used.
FIG. 2
shows how the amplitudes A and A′ differ from each other.
The circle in
FIG. 2
(sequence 3) is the result obtained by plotting a vector (I, Q) when the amplitude A is normalized with 1. A plot of the amplitude A′ on each vector yields the graphic pattern of sequence 1.
As can be seen from
FIG. 2
, the value A′ is always slightly larger than the value A. The value A′ is larger than the value A by an average of about 1.087 times, i.e., about 0.723 dB. Such a difference poses no problem in the AGC circuit. When the average received level of many samples corresponding to one slot (4×2560 samples=10240 samples) or more is to be obtained, we can assume that the obtained amplitude is always larger than the true amplitude by 0.723 dB.
A method of calculating the average received level in a 1-slot interval will be described next.
The average received level is obtained by dividing the sum of the values A′ corresponding to one slot by the number of samples. Although the number of samples corresponding to one slot is 4×2560=10240, since it is difficult to divide by using this number, 2
13
=8192, which is the nearest power of 2, is used.
That is, the sum is shifted by 13 bit positions to the right. In other words, the output from the calculator
6
of received level is the result obtained by adding up the approximate values A′ corresponding to one slot (10,240 samples) and shifting the sum by 13 bit positions to the right.
Since the maximum value of each of the I and Q signals is 127, a maximum value Amax obtained by the above calculation is expressed by:
A
max
=
(
127
+
127
2
)
×
10240
÷
8192
≈
237
(
3
)
This value can be expressed by an 8-bit straight binary code.
As described above, the average received level is calculated by calculating an approximate value of the average of amplitudes.
To be exact, the received level must be determined by obtaining the average of received powers. It is, however, difficult to calculate an average received power, because no suitable approximation method is available. Under the circumstances, an amplitude average approximate value is unwillingly used. The difference between the received level values respectively obtained by using power averages and amplitude averages will be examined.
A received signal S from a base station has in-phase and quadrature components written as:
S=I
(
t
)·cos(2
&pgr;f
c
t
)−
Q
(
t
)·sin(2
&pgr;f
c
t
) (4)
In CDMA, since I(t) and Q(t) represent the sums of many independent speech channels, interference waves, and noise, the central limiting theorem holds. It therefore follows from this that I(t) and Q(t) each exhibit a Guassian distribution. If quadrature components respectively have independent Guassian distributions in this manner, the amplitude distribution of the synthetic signal exhibits a Rayleigh distribution. If amplitudes R have a Rayleigh distribution, the probability density distribution is given by:
P
(
R
)
=
R
b
0
·
exp
(
-
R
2
2
·
b
0
)
(
5
)
In equation (5), b
0
is a positive constant. Omitting a description of intermediate calculation, the power average is expressed by:
R
2
_
=
∫
0
∞
R
2
·
R
b
0
·
exp
(
-
R
2
2
·
b
0
)
·
ⅆ
R
=
2
b
0
(
6
)
Omitting a description of intermediate calculation, the amplitude average is be expressed by:
R
_
=
∫
0
∞
R
·
R
b
0
·
exp
(
-
R
2
2
·
b
0
)
·
ⅆ
R
=
b
0
·
π
2
(
7
)
The dB difference between the received levels obtained by using the power and amplitude averages is given by:
d
=
20
·
log
(
R
2
_
R
_
)
=
(
2
·
b
0
b
0
·
π
2
)
=
20
·
log
(
4
π
)
≈
1.05
dB
(
8
)
That is, the received level calculated by using the power averages is equivalent to the value obtained by adding 1.05 dB to the received level calculated by using amplitude averages.
A method of determining reference level as an AGC target will be described next.
For example, the peak factor of a single code of a CDMA received signal is about 6 dB. When many users, e.g., 32 users, use this device, the peak factor increases b
Kim Kevin
NEC Corporation
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