Telecommunications – Receiver or analog modulated signal frequency converter – Local control of receiver operation
Reexamination Certificate
1999-11-24
2001-09-04
Maung, Nay (Department: 2681)
Telecommunications
Receiver or analog modulated signal frequency converter
Local control of receiver operation
C455S235100, C455S245100, C330S278000, C330S282000, C330S291000
Reexamination Certificate
active
06285863
ABSTRACT:
BACKGROUND
1. Technical Field
The present invention relates generally to wide-band code division multiple access (WCDMA) receivers and other receivers for wireless communications. More particularly, the present invention relates to system and method for providing an automatic gain control (AGC) system with high dynamic range (over 100 dB), whereby the AGC can operate over a broad range of varying signal level while maintaining a good signal-to-noise (SN) ratio performance over the entire signal varying range and maintaining inter-modulation components at low levels.
2. Description of Related Art
In general, wireless communication systems employing wide-band receivers (e.g., WCDMA receivers) typically share one carrier channel among a plurality of users. As a result, the signal strength may vary over a very wide range depending on factors such as the distance between a mobile station and a service station and the number of neighboring subscribers sharing the same RF carrier. Indeed, the signal received by a service center or by a mobile station may vary over a 90 dB range and a received signal may even be as high as −25 dBm.
A conventional wide-band receiver typically comprises a RF-IF (radio frequency—intermediate frequency) section that inlcudes an AGC circuit. In general, the function of the AGC circuit is to process an input IF signal to generate an IF signal with a fixed power level at the output of the AGC (regardless of the level of the input IF signal level). This fixed signal level is required for further IF processing, both within the RF-IF section and in a base-band processing unit. A conventional AGC circuit is typically designed as a closed loop AGC circuit comprising a gain variable amplifier (GVA) and some control hardware/software to control the gain of the GVA.
FIG. 1
is a block diagram of a conventional receiver comprising a single, closed loop AGC circuit according to the prior art. The receiver comprises an antenna
100
for receiving a RF signal. A LNA (low noise amplifier)
101
amplifies the RF signal and the amplified RF signal is filtered by a band-pass filter
102
to generate a RF signal (e.g., 2120 MHz to 2180 MHz). A down-converter
103
processes the bandpass-filtered RF signal to generate an IF signal. The IF signal comprises a plurality of channels (e.g., 12 channels with a channel bandwidth of 5 MHz in the range 2120 MHz to 2180 MHz). A SAW (surface acoustic wave) filter
104
filters the IF signal to select a desired channel within the IF signal. Each of these components and their respective functions are well-known the art.
The IF signal (i.e., selected channel) is fed to a closed loop AGC circuit
105
comprising a GVA
106
and an IF sensor and control signal generator
107
. The GVA
106
amplifies the IF signal to predetermined power level. The IF signal is further processed by other IF processing units
108
(e.g. filters, amplifiers, etc). A power splitter
109
divides the IF power received from the IF processing units
108
. A demodulator
110
receives an IF signal from one output from the power splitter
109
and extracts a baseband signal from the IF signal. Another output of the power splitter
109
is connected to the IF sensor and signal generator
107
to provide a feedback control of the gain of the GVA
106
. In particular, the IF sensor and signal generator
107
comprises a logarithmic amplifier which converts the IF Sample signal to a voltage signal (based on the signal level of the IF Sample). The voltage level is compared to a Desired level setting signal to generate an appropriate control signal for the GVA
106
based on the comparison. The gain of the GVA
106
will be automatically adjusted to reach a balance under which the output signal of the GVA
106
will be fixed at the predetermined level.
One problem associated with the conventional AGC architecture based on the single, closed-loop design is that it does not provide high dynamic range with respect to the range of RF power levels that may be input to the receiver. A detailed analysis of the system of
FIG. 1
with respect to dynamic range will now be provided. For purposes of illustration, it is assumed that the system of
FIG. 1
is a wide-band receiver that operates in a frequency range of 2120 MHz to 2180 MHz. It is further assumed that the channel bandwidth is 5 MHz per channel and that the RF signal level input to the receiver varies between −25 dBm/channel and −115 dBm/channel. In addition, the following system requirements are assumed. First, the required signal-to-noise (S/N) ratio of the demodulated baseband signal should not be less than 0 dB (the baseband processing unit of the receiver has a processing gain, so the demodulated baseband data is not required to have a high margin of S/N ratio). Second, the signal level at the input of any device in the receiver chain should not be higher than −18 dBc of the IIP
3
value of that device (where the IIP
3
parameter denotes the input third inter-modulation cross-point in dBm) so as to ensure that the IM
3
of the receiver chain will be at least −36 dBc (where the IM
3
parameter is the third inter-modulation component in dBc). It is to be understood that the above system requirements are typical for WCDMA systems or CDMA- 2000 systems.
In addition, for illustrative purposes, the GVA
106
is assumed to have the performance parameters as set forth in Table 1 below. It is to be understood that these performance parameters correspond to parameters of a GVA of current technology such as the RF2607 GVA by RF Micro Devices. Another GVA device that may be considered is the Q5500 by QUALCOMM, which has similar performance as that of RF2607. As shown in Table 1, the noise figure (NF) and IIP
3
parameters of the illustrative GVA (e.g., RF2607) vary with the change in gain of the GVA.
TABLE 1
Gain
45
30
20
10
0
−10
−20
−30
−45
(dB)
NF (dB)
5
6
10
18
25
34
42
48
60
IIP
3
−46
−34
−23
−18
−14
−12
−7
−4
−3
(dBm)
In addition, for illustrative purposes, the signal level (SL), noise figure (NF), GVA gain (GA), and S/N ratio S/N at different locations in the receiver chain of
FIG. 1
using a single, closed loop AGC topology are listed below in Table 2.
TABLE 2
SL at the input of
−115
−105
−95
−85
−75
−65
−55
−45
−35
−25
the receiver (dBm)
SL at the input of
−111
−101
−91
−81
−71
−61
−51
−41
−31
−21
the GVA (dBm)
GA of the GVA
45
35
25
15
5
−5
−15
−25
−35
−45
(dB)
SL at the output of
−66
−66
−66
−66
−66
−66
−66
−66
−66
−66
the GVA (dBm)
NF of the GVA
5.0
5.5
8.0
14
21
29
38
45
52
60
(dB)
NF at the output
6.9
7.0
7.9
11.3
17.3
25.1
34.0
41.0
48.0
56.0
of the GVA (dB)
Noise floor at the
−100
−100
−99.1
−95.7
−89.7
−81.9
−73.0
−66.0
−59.0
−55.0
output of the GVA
(dBm)
S/N at the exit of
34
34
33
30
24
16
7
0
−7
−11
the GVA (dB)
The performance of the conventional single, closed loop AGC with respect to dynamic range will now be discussed in to detail with respect to the illustrative system parameters and values set forth in Tables 1 and 2 above. As noted above, the SL at the input of the receiver (i.e., antenna
100
) is assumed to vary in the range from a low level of −115 dBm/channel to a high level of −25 dBm/channel (as shown in Table 2 varying in the range in increments of 10 dBm). The SL at the output of the GVA
106
is maintained at a fixed level in accordance with the closed loop requirement. As shown in Table 2, the fixed level at the output of the GVA
106
is assumed to be −66 dBm. This fixed level is based on factors such as (1) the assumed maximum available gain GA (e.g., 45 dBm) of the GVA for the lowest SL at the input of the receiver (e.g., −115 dBm) and (2) an assumed total gain of 4 dBm for the “rec
F. Chau & Associates,LLP
Lucent Technologies - Inc.
Maung Nay
Vuong Quochien B.
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