Communications: electrical – External condition vehicle-mounted indicator or alarm
Reexamination Certificate
2000-03-03
2001-01-30
Pope, Daryl (Department: 2736)
Communications: electrical
External condition vehicle-mounted indicator or alarm
C340S933000, C340S905000, C340S539230
Reexamination Certificate
active
06181259
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electronic toll collection (ETC) system for an intelligent transport system (ITS).
2. Description of the Related Art
As shown in
FIG. 1
of the accompanying drawings, an ETC system is a road-to-vehicle communication system for collecting expressway tolls from running vehicles via radio communications between a vehicle-mounted device
50
on a vehicle and a road radio unit
100
installed in a toll gate. A conventional radio communication process which is carried out by the vehicle-mounted device
50
on the vehicle that has entered an expressway, for radio communication with the road radio unit
100
in the ETC system will be described below.
As shown in
FIG. 1
, the road radio unit
100
in the ETC system normally uses two frequencies, i.e., frequencies F
1
, F
2
. These two frequencies F
1
, F
2
are used to avoid radio wave interference between plural lanes at entrance and exit toll gates. The frequencies F
1
, F
2
are assigned respectively to adjacent ones of the lanes, and the same frequency is not assigned to successive ones of the lanes. The road radio unit
100
has a narrow communication range in order to avoid radio wave interference between the lanes.
In order for the vehicle-mounted device
50
to effect normal radio communications with the road radio unit
100
at a toll gate or lane of an expressway, the vehicle-mounted device
50
needs to perform bi-directional data communications with the road radio unit
100
within a short period of time of several 100 msecs. in which the vehicle-mounted device
50
runs into and out of the narrow communication range of the road radio unit
100
. Therefore, the vehicle-mounted device
50
has to detect, at a high speed, that it enters the communication range of the road radio unit
100
. However, since the vehicle-mounted device
50
cannot recognize, in advance, the lane along which the vehicle enters a toll gate, the vehicle-mounted device
50
is unable to know, in advance, which of the frequencies F
1
, F
2
the carrier transmitted from the road radio unit
100
is using. Consequently, the vehicle-mounted device
50
carries out a frequency search process for switching between the frequencies F
1
, F
2
at a high speed so that the vehicle-mounted device
50
can detect the carrier of either one of the frequencies F
1
, F
2
transmitted from the road radio unit
100
whenever the vehicle-mounted device
50
may enter the communication range of the road radio unit
100
. The vehicle-mounted device
50
carries out the frequency search process at all times even if it is outside of the communication range of the road radio unit
100
.
An arrangement of the conventional vehicle-mounted device
50
will be described below with reference to
FIG. 2
of the accompanying drawings. As shown in
FIG. 2
, the conventional vehicle-mounted device
50
comprises a controller
41
, a receiver
43
, and an RF (Radio Frequency) module
4
.
The controller
41
, which comprises a programmed CPU, exchanges data with the receiver
43
and the RF module
4
via a CPU bus line
5
. The controller
41
also controls the RF module
4
to perform a frequency search process to switch between the received frequencies F
1
, F
2
at a constant switching period T
1
. When the controller
41
detects the reception by the RF module
4
of a carrier sent from the road radio unit
100
in response to a level-detected signal
10
from the RF module
4
, the controller
41
stops the frequency search process, and performs road-to-vehicle communications with the road radio unit
100
at a fixed frequency.
The receiver
43
detects a unique word from demodulated data
8
from the RF module
4
and checks the demodulated data
8
from the RF module
4
with a CRC (Cyclic Redundancy Check) code, and sends the detected result via the CPU bus line
5
to the controller
41
.
The RF module
4
has a local oscillator with a frequency synthesizer. The RF module
4
receives the carrier sent from the road radio unit
100
at a frequency indicated from the controller
41
via the CPU bus line
5
, and demodulates the received carrier. The RF module
4
has a level detector which outputs a level-detected signal
10
to the controller
41
when it detects the carrier sent from the road radio unit
100
.
Operation of the conventional vehicle-mounted device
50
will be described below with reference to FIG.
2
. When the controller
41
is turned on, the controller
41
performs the frequency search process to switch between the received frequencies F
1
, F
2
at the constant switching period T
1
(see
FIG. 3
of the accompanying drawings). In the RF module
4
, the frequency of the local oscillator is controlled so as to be able to receive the frequencies F
1
, F
2
. The switching period T
1
is generated by an internal counter of the controller
41
, and is set, in advance, to a suitable value according to the frame period of the carrier, which is of a time-division frame structure, sent from the road radio unit
100
.
Operation of the conventional vehicle-mounted device
50
as the vehicle enters the communication range of the road radio unit
100
will be described below. It is assumed, for example, that the vehicle enters the communication range of the road radio unit
100
which is sending the carrier at the frequency F
1
.
When the vehicle with the vehicle-mounted device
50
enters the communication range of the road radio unit
100
, the reception frequency of the vehicle-mounted device
50
is switching alternately between frequencies F
1
, F
2
at the constant switching period T
1
according to the frequency search process. When the reception frequency of the vehicle-mounted device
50
is tuned with the carrier frequency F
1
sent from the road radio unit
100
as a result of the frequency search process and the level detector in the RF module
4
detects the carrier from the road radio unit
100
, the RF module
4
sends a level-detected signal
10
to the controller
41
. The controller
41
stops the frequency search process, and fixes the reception frequency of the RF module
4
to F
1
.
Subsequently, the carrier, which is of a time-division frame structure, sent from the road radio unit
100
is demodulated by the RF module
4
of the vehicle-mounted device
50
. The receiver
43
detects a unique word from demodulated data
8
from the RF module
4
and checks the demodulated data
8
from the RF module
4
with a CRC code. The detected result is supplied to a reception management area in the receiver
43
. The controller
41
monitors the reception management area via the CPU bus line
5
. When the carrier is received well in as many successive frames as a back guard count (N
1
), the controller
41
judges the reception condition as the establishment of frame synchronization. After the establishment of frame synchronization, road-to-vehicle bi-directional data communications are carried out according to a time-division multiplex communication process.
When the vehicle-mounted device
50
finishes the bi-directional data communications and the vehicle runs out of the communication range of the road radio unit
100
, the controller
41
attempts to detect a reception failure in the reception management area in the receiver
41
. When the carrier is not received well in as many successive frames as a forward guard count (N
2
), the controller
41
judges the reception condition as the lack of frame synchronization. The controller
41
then restarts the frequency search process and enters a steady state. The above communication procedure is applicable in the communication range of the road radio unit
100
both at a toll gate and in a lane.
The vehicle-mounted device
50
of the conventional road-to-vehicle communication system repeats the frequency search process in order to detect the carrier sent from the road radio unit
100
at all times even outside of the communication range of the road radio unit
100
. This is because the vehicle-mounted device
50
shou
NEC Corporation
Pope Daryl
Sughrue Mion Zinn Macpeak & Seas, PLLC
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