Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-01-14
2003-05-20
Negash, Kinfe-Michael (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06567199
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to digital optical communication used for electric home appliances or information equipment having an infrared communication function, and more particularly, to a digital optical transmission apparatus and a method for performing ASK modulation to alleviate mutual interferences.
2. Description of the Background Art
In optical communication in general, the amplitude, phase or frequency of a subcarrier is usually changed for transmission depending upon data desired to be communicated. This is called carrier bank modulation as compared to modulation not utilizing a subcarrier (base band modulation). The subcarrier is created artificially by turning on/off light in a certain cycle. The subcarrier is often replaced with a square wave of light simply turned on/off.
Among carrier band modulation techniques, the simplest technique changes the amplitude. The technique is called ASK (amplitude-shift keying). Among various ASK techniques, the simplest one utilizes two kinds of amplitudes, a prescribed amplitude and an amplitude zero. The technique is more particularly called OOK (on/off keying).
A signal modulated according to base band modulation techniques such as RZ (Return to Zero) modulation, PPM (Pulse Position Modulation) and Manchester modulation may be superposed by a subcarrier for transmission. These modulation techniques are also ASK modulation techniques in a board sense. The waveforms according to these modulation techniques are given in FIG.
1
. The modulation technique which changes the phase and frequency of a subcarrier are called PSK (Phase Shift Keying) modulation and FSK (Frequency Shift Keying) modulation, respectively. A waveform produced by PSK modulation and a waveform produced by FSK modulation are shown in FIG.
2
.
Spectrum According to Conventional Carrier Band Modulation
A spectrum according to carrier band modulation has a main lobe in a frequency band around the frequency of the subcarrier. According to a technique using a plurality of subcarriers such as FSK, there are a plurality of main lobes in frequency bands around the frequencies of the subcarriers. One of the side bands of a main lobe according to a carrier band modulation technique is the inverse of the minimum “subcarrier unchanged time” normally used according to the modulation technique.
Spectrum According to Conventional Carrier Band Modulation Technique, Particularly ASK Modulation Technique
A spectrum according to an ASK modulation technique produced by superposing a waveform according to a base band modulation technique by a subcarrier corresponds to a spectrum produced by shifting the original spectrum according to a base band modulation technique to a frequency band around the frequency of the subcarrier. Note, however, the entire spectrum according to the base band modulation is not shifted to a high frequency band, and a part of the spectrum according to the base band modulation remains in the low frequency band as unwanted radiation. A spectrum according to an ASK modulation technique produced by superposing the waveform of an NRZ (Non Return to Zero) modulation technique by a subcarrier is given in FIG.
3
.
Spectra According to Conventional Carrier Band Modulation Technique, Particularly According to PSK and FSK Modulation Techniques
According to PSK and FSK modulation, unlike the ASK modulation technique, no spectrum appears in a low frequency band. According to these techniques, however, more power is generally consumed than the ASK modulation and the circuit configuration of a receiver for the modulation technique is more complicated. As a result, ASK modulation techniques or base band modulation techniques are more often used in the optical communication industry.
As described above, according to the base band and ASK modulation techniques, a spectrum having a main lobe of a bandwidth of at least a bit rate appears in a low frequency band. Therefore, if there are transmitter/receivers according to a plurality of communication techniques, disadvantageous mutual interferences are caused among these transmitter/receivers.
A remote control used for a television, for example, employs an ASK communication technique at a bit rate of 1 Kbps using a subcarrier in the vicinity of 40 KHz. The spectrum has a main lobe of about 2 KHz around the vicinity of 40 KHz on one side. Herein, if a communication at about 75 Kbps is newly performed according to another communication method, a spectrum appears in a low frequency band about in the range from 0 Hz to 75 KHz whether the base band modulation or ASK modulation technique is employed. Therefore, optical communication using the remote control will be interfered with. Thus, while having a communication at about 75 Kbps according to the conventional base band modulation or ASK modulation technique, the interference between that communication and the remote control using a subcarrier at about 40 KHz has been hardly eliminated.
Spectrum According to IrBUS Method as Related Art
In order to solve the above problem, a 16 PSM (Pulse Sequence Modulation) coding method has been proposed in the IrBUS method that the United States Infrared Data Association (IrDA) is presently trying to standardize as a coding method to replace conventional base band modulation techniques. In the proposed coding method, as shown in Table 1, 4-bit data is allocated to 16 symbols represented by 8 slots.
TABLE 1
Transmission Data
Parallel Symbol Pattern
D3
D2
D1
D0
P7
P6
P5
P4
P3
P2
P1
P0
0
0
0
0
1
0
1
0
0
0
0
0
0
0
0
1
0
1
0
1
0
0
0
0
0
0
1
0
0
0
1
0
1
0
0
0
0
0
1
1
0
0
0
1
0
1
0
0
0
1
0
0
0
0
0
0
1
0
1
0
0
1
0
1
0
0
0
0
0
1
0
1
0
1
1
0
1
0
0
0
0
0
1
0
0
1
1
1
0
1
0
0
0
0
0
1
1
0
0
0
1
1
1
1
0
0
0
0
1
0
0
1
0
1
1
1
1
0
0
0
1
0
1
0
0
0
1
1
1
1
0
0
1
0
1
1
0
0
0
1
1
1
1
0
1
1
0
0
0
0
0
0
1
1
1
1
1
1
0
1
1
0
0
0
0
1
1
1
1
1
1
0
1
1
0
0
0
0
1
1
In the IrBUS method, this 16 PSM base band signal is superposed by a subcarrier at 1.5 MHz to produce a power spectrum valley in the range 36 KHz to 40 KHz, a peak power spectrum band for a subcarrier normally used by a remote control for electric home appliances, such that mutual interference is alleviated. The use of this method may restrain mutual interferences to about ½ the level of the conventional base band or ASK modulation method.
FIGS. 4A
to
4
C show an optical signal produced by superposing a 16 PSM base band signal by a subcarrier, its subcarrier component and its base band component (details of which will be described later), respectively.
FIGS. 5A and 5B
show a conventional LED driving circuit. The LED driving circuit shown in
FIGS. 5A and 5B
includes an LED current limiting resistor
105
, a transistor
107
having its output controlled by an IrTx electrical signal
800
, and an LED
106
which emits light when transistor
107
is turned on and outputs an IrTx optical signal
801
. Note that the LED driving circuit shown
FIG. 5A
has LED current limiting resistor
105
between a power supply
104
and LED
106
, and transistor
107
has its collector and emitter connected to LED
106
and ground, respectively. The LED driving circuit shown in
FIG. 5B
has LED
106
between power supply
104
and LED current limiting resistor
105
, and transistor
107
has its collector and emitter connected to LED current limiting resistor
105
and ground, respectively.
When IrTx electrical signal
800
attains a high level, transistor
108
is turned on, which allows a current to be passed to LED
106
through LED current limiting resistor
105
, and LED
106
emits light which is output as IrTx optical signal
801
. When IrTx electrical signal
800
attains a low level, transistor
107
is turned off, so that no current is passed to LED
106
, which therefore does not emit light.
FIGS. 6A and 6B
show a power spectrum in the range from 0 to 2 MHz and a power spectrum in the range from 0 to 50 KHz, respectively when a 16 PSM base band signal superposed by a subcarrier is transmitted.
FIGS. 7A and 7B
show power spectra shown in
FI
Nakaoka Hiroyuki
Ohtani Yoshihiro
Uno Hiroshi
Conlin David G.
Dike Bronstein, Roberts & Cushman IP Group, Edwards & Angell, LL
Negash Kinfe-Michael
Sharp Kabushiki Kaisha
Tucker David A.
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