Telecommunications – Transmitter and receiver at separate stations – With control signal
Reexamination Certificate
1998-06-16
2004-05-04
Nguyen, Lee (Department: 2682)
Telecommunications
Transmitter and receiver at separate stations
With control signal
C455S226200, C455S245100, C455S522000, C370S318000, C370S321000, C375S345000
Reexamination Certificate
active
06731910
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a data transmission system as a communication system for a mobile communication and a mobile satellite communication, and a data transmitter and a data receiver which are used in the data transmission system.
2. Discussion of the Prior Art
In a mobile communication, a receiving signal level is greatly varied (in several tens dB or greater) since when propagating, it is subjected to various interferences, e.g., shadowing, fading and the like. One of the measures possibly taken for this problem is to correct the receiving signal level by the utilization of the AGC (automatic gain control).
FIG. 8
shows in block form an overall wireless communication system.
FIGS. 9 and 10
show also in block form a prior transmitter and a prior receiver in the mobile wireless communication system, which are prescribed in “TIA/EIA/IS-139.1-A” (TIA/EIA INTERIM STANDARD: TDMA Cellular/PCS-Radio Interface-Mobile Station-Base Station-Compatibility-Digital Control Channel) or “RCR STD-32”.
In
FIG. 8
showing the communication system, reference numeral
1
is a transmitter for converting a transmission data signal S
1
into a radio signal which is to be transmitted in the form of a transmission signal S
2
;
2
is a channel for transmitting the wireless signal; and
3
is a receiver for extracting reception data S
4
from a reception signal S
3
. In
FIG. 9
showing the transmitter, numeral
11
is a burst generator;
12
is a preamble adder;
13
is a modulator; and
14
is an antenna. The preamble adder
12
generates a transmission burst signal S
12
by use of the transmission data S
1
, an information signal S
11
output from the preamble adder
12
, and the like. A pattern of the preamble is a repetition pattern, e.g., ALL
0
. The modulator
13
modulates the transmission burst signal S
12
and produces a modulated signal S
13
. The antenna
14
emits a radio wave containing the modulated signal S
13
. In
FIG. 10
showing the receiver, numeral
31
is an antenna;
32
is a low noise amplifier (LNA);
33
is an RF/IF portion for frequency converting a signal S
31
output from the low noise amplifier
32
;
34
is an AGC portion for automatically controlling a low-frequency signal S
32
output from the RF/IF portion
33
so that its output signal level has a fixed value;
35
is a demodulator for demodulating an AGC output signal S
33
; and
36
is a reception controller for extracting the reception data S
4
from a demodulated signal S
34
output from the demodulator
35
. The wireless communication system thus arranged employs a repetition pattern, e.g., ALL
0
, for the preamble pattern.
Description will be given about the operation of the thus arranged mobile wireless communication system when it sends a signal from a mobile station MS to a base station BMI. The wireless communication system used here is prescribed in “TIA/EIA/IS-139.1-A”. In the mobile station MS, the burst generator
11
constructs a transmission burst by adding the bits of a ramp (R), sync words (SYNC=synchronization, SYNC+=additional synchronization), and an AGC preamble (PREAM=preamble), to data (DATA, already error-correction coded) to be transmitted. A format of the transmission burst is shown in FIG.
11
. In the “TIA/EIA/IS-139.1-A”, the preamble consisting of eight symbols of the &pgr;/4 shift modulated as a repetition of “1” and “0” is added as the preamble pattern to the transmission data.
Those symbols of the preamble are phased as shown in FIG.
12
. As shown, the preamble portion takes a fixed envelope level. The output signal S
12
of the burst generator
11
is modulated by the modulator
13
and amplified, and then the thus processed signal is radiated from the antenna
14
. When propagating through the channel
2
, the radio signal is greatly affected by fadings (e.g., Rayleigh fading and frequency-selective fading), and its waveform is greatly distorted. Further, the signal level of the radio signal largely varies depending on the distance between the base station BMI and the mobile station MS, shadowing, and fadings. In the base station BMI, the radio signal S
3
thus distorted in waveform and varied in amplitude is received by the antenna
31
; it is amplified by the LNA
32
; and it is converted into a low frequency signal by the RF/IF portion
33
. An A/D converter and the like are provided at the input of the demodulator
35
. Therefore, the input signal level must fall within a predetermined range of levels. An output signal of the RF/IF portion
33
has a great level variation. The level variation of the output signal must be removed before it is input to the demodulator
35
. To remove the level variation, the AGC portion
34
is used. An output signal S
34
of the AGC portion
34
is demodulated by the demodulator
35
, and applied to the reception controller
36
. The reception controller
36
extracts the data portion from the burst signal. The demodulated signal S
34
is processed for its error removal, for example, and finally is output as a reception data signal S
4
.
The operation of the AGC portion
34
for effecting the level correction will be described.
FIG. 13
shows a basic arrangement of the conventional AGC portion
34
. In the figure, reference numeral
41
is an AGC amplifier;
42
is a level detector; and
43
is a low-pass filter (LPF). A low-frequency signal S
32
input to the AGC portion
34
contains a great level variation, as mentioned above. The AGC amplifier
41
of the AGC portion
34
amplifies or attenuate the low-frequency signal S
32
, and produces an output signal S
33
. The AGC output signal S
33
is input to the level detector
42
. The level detector compares a signal level of the AGC output S
33
with a reference signal level, and produces a signal representative of a difference between those signal levels. The difference signal is input to the LPF
43
. The LPF removes a minute variation of the difference signal and outputs an AGC amplifier control voltage signal or an RSSI (received signal strength indicator) signal S
40
. The RSSI signal S
40
determines a gain of the AGC amplifier
41
. The AGC amplifier
41
is thus controlled, and through its control, the output signal strength level of the amplifier
41
is approximate to the reference signal level. The operation of the AGC portion
34
is as described above. A given time is taken till a close loop consisting of the AGC amplifier
41
, level detector
42
and LPF
43
settles down in operation. In the “TIA/EIA/IS-139.1-A”, to secure a satisfactory demodulation, as shown in
FIGS. 14A and 14B
, the AGC output signal (input to the demodulator) is settled down at the preamble (PREAM) portion (located in the head portion of the reception burst format) of the reception burst, so that the AGC output levels of the SYNC and DATA portions (follows the PREMA portion in the reception burst format) are put within a desired range of signal levels.
In a case where the fixed repetition pattern is used for the AGC preamble as mentioned, the radio signal having undergone a frequency selective fading channel improperly operates the AGC portion or circuit. This problem is remarkably revealed in particular in a case where a symbol rate is faster than a fading variation rate and a state of fading little varies within the preamble portion. The frequency selective fading will be described. The frequency selective fading occurs where a delay quantity of a delayed wave, reflected by a distant obstacle, e.g., mountain and building, is not negligible when comparing with the symbol period (FIG.
15
).
A fading phenomenon producing a delay quantity of the delayed wave which is at least {fraction (1/10)} as large as the symbol period sometimes is categorized into the frequency selective fading. If no measure is taken for this fading, the preceding wave interferes with the delayed wave, thereby causing a misjudgment of the received signal (FIG.
16
). A composite wave of the preceding wave and the delayed wave is depicted
Ishizu Fumio
Murakami Keishi
Taira Akinori
Mitsubishi Denki & Kabushiki Kaisha
Nguyen Lee
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