Pulse or digital communications – Systems using alternating or pulsating current
Reexamination Certificate
1998-06-12
2001-04-10
Vo, Don N. (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
C375S295000, C375S316000, C375S353000
Reexamination Certificate
active
06215826
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to methods of coding a signal for digital optical communication for use in electric home appliances and information equipment capable of infrared communication, and more particularly, to a method of coding a signal for digital optical communication, the spectrum of which attains a zero level at a prescribed frequency within a main lobe band, a baseband transmitter or an ASK transmitter using the method, and a baseband receiver or an ASK receiver decoding a signal coded by the method.
2. Description of the Related Art
Conventional digital optical coding methods are roughly divided into coding method using a subcarrier and coding methods without using a subcarrier. Herein, the subcarrier refers to a carrier wave artificially created by turning on/off light at a certain cycle. A rectangular wave of light simply turning on/off is often substituted for the subcarrier. A method of deforming the waveform of a signal for communication data by a prescribed rule for transmission without using the subcarrier is called a baseband coding method. Meanwhile, a method of transmitting communication data by changing any of a amplitude, phase and frequency of a subcarrier based on the data is called a carrier band coding method.
Example of Conventional Baseband Coding Method
As examples of the baseband coding method, NRZ (Non Return to Zero), RZ (Return to Zero), PPM (Pulse Position Modulation) and Manchester coding methods are known. Coding waveforms for these coding methods are set forth in FIG.
1
.
Spectrum According to Conventional Baseband Coding Methods
A spectrum according to a baseband coding method generally has a main lobe having its highest power spectrum density in the low frequency region, and a side lobe having its power spectrun density reduced in higher frequency regions. The width of a side band of the main lobe in this case is usually the reciprocal of the minimum pulse width used in that coding method. For example, since the minimum pulse width of an NRZ signal is equal to 1-bit time (time necessary for transmitting 1 bit), the band width of the main lobe is equal to the bit rate. If the bit rate is 100 Kbps, for example, the band width of the main lobe is 100 kHz.
According to the ½ RZ coding method, 4-value PPM coding method or Manchester coding method, since the minimum pulse width is equal to half 1-bit time, and therefore the band width of the main lobe is twice as large as the bit rate. If the bit rate is 100 Kbps, for example, the band width of the main lobe is 200 kHz. The minimum pulse width is usually set at a value equal to or lower than 1-bit time.
Stated differently, in a normal communication method, the main lobe has a band width equal to or larger than the bit rate. Spectra according to the above coding methods are shown in
FIGS. 2
to
5
. As can be clearly seen from these graphs, the zero level of each spectrum does not lie within the main lobe band according to the conventional coding methods, and the spectrum is present in the entire main lobe frequency band.
As a special conventional example, Japanese Patent Laying-Open No. 7-107125 discloses a method of communicating by repeating the same symbol at least twice. In the communication method, the zero level of a spectrum appears within the main lobe band. For example, transmission waveforms and spectra when each symbol according to the 4-value PPM method is repeated twice for transmission are shown in
FIGS. 6 and 7
. According to the coding method, the pulse width is reduced as compared to the bit rate, and the main lobe width of the spectrum increases, which generally makes it difficult to design a receiver.
Examples of Conventional Carrier Band Coding Methods
Meanwhile, among various carrier band coding methods, the simplest is a method of changing the amplitude, which is called “ASK (Amplitude Shift Keying)” method. Among ASK methods, the simplest method uses two kinds of amplitudes, in other words a prescribed amplitude and amplitude zero, which is called “OOK (On Off Keying)”. Coding methods by superposing on an output signal according to each of the conventional baseband coding methods a subcarrier for transmission may be considered, which are also ASK coding methods in a broad sense. The coding waveforms according to these methods are set forth in FIG.
8
. Methods of coding by changing the phase or frequency of the subcarrier are called PSK (Phase Shift Keying) method, and FSK (Frequency Shift Keying) method, respectively (see PSK and FSK coding waveforms in FIG.
9
).
Spectrum According to Conventional Carrier Band Coding Methods
A spectrum according to a carrier band coding method has a main lobe in a frequency band around a subcarrier frequency. In a method which uses a plurality of subcarrier frequencies as the FSK method, there are a plurality of main lobes in frequency bands around the subcarrier frequencies. The width of a sideband of a main lobe according to a carrier band coding method is normally the reciprocal of the minimum “subcarrier non-changing time” used in the coding method.
Spectrum According to Conventional Carrier Band Coding Method, Particularly ASK Coding Method
A spectrum according to an ASK coding method in which a subcarrier is superposed on an output signal according to a baseband coding method is produced by shifting a spectrum according to the original baseband coding method to a frequency band around the subcarrier. In this case, however, the spectrum according to the baseband coding method does not entirely shift to the high frequency band, and a certain amount of the spectrum according to the baseband coding method remains as unnecessary radiation in the low frequency band. Spectra according to ASK coding methods produced by superposing subcarriers on the output signals accordingly to the baseband coding methods are set forth in
FIGS. 10
to
13
.
Spectrum According to Conventional Carrier Band Coding Method, Particularly PSK and FSK Coding Methods
Meanwhile, in a PSK or FSK method, no spectrum appears in the low frequency band unlike the ASK methods. These methods, however, generally consume more electric power than the ASK methods, and require complex receiving circuit configurations, and therefore the ASK methods or baseband methods are preferred in the field of optical communication.
As can be seen from the foregoing, in the baseband communication methods and ASK communication methods, a spectrum having a main lobe of a band width equal to or larger than the bit rate appears in the low frequency band. Therefore, if communication devices using a plurality of communication methods are used, mutual interference between them is caused.
If, however, remote controlls for TV sets usually employ an ASK communication method at a bit rate of about 1 Kbps, using a subcarrier at around 40 kHz, the spectrum of which has a main lobe of about 2 kHz on one side around the vicinity of 40 kHz. Assuming that another method is newly employed for communication at about 75 Kbps, and a baseband coding method or an ASK communication method is employed, a signal spectrum appears in the entire low frequency band up to about 0 Hz-75 kHz area, which interferes with the optical communication of the remote controll. More specifically, according to a conventional baseband coding method or an ASK coding method, interference between communication at 75 Kbps and remote communication at 40 kHz may be hardly avoided.
If a PSK or FSK coding method is used rather than an ASK coding method, no spectrum appears in the low frequency band, and the interference may be avoided. In this case, however, the configuration on the receiver side is more complicated than the case of using an ASK coding method, which increases the cost.
If a coding method of creating a gap in a spectrum by transmitting a symbol at least twice, in order to avoid such interference, is used, the pulse width is narrowed relative to the bit rate, and the main lobe expands, which makes it difficult to design a receiver.
SUMMARY OF THE I
Ikeda Yutaka
Ohtani Yoshihiro
Uno Hiroshi
Conlin David G.
Dike Bronstein, Roberts & Cushman LLP
Sharp Kabushiki Kaisha
Vo Don N.
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