The invention relates to a modulator/demodulator for a 7 PSK system which transmits data at a higher speed than that of a xcfx80/4 shift QPSK system and a method of modulation/demodulation used for the same.
It is well known that a QPSK modulation system is widely used in various communication systems. In the QPSK modulation system, a single modulation signal can transmit information of 2 bits.
In the QPSK system, a single modulated carrier is selected from four modulated carriers, phase angles of which are successively shifted by xcfx80/4 in accordance with information of 2 bits supplied to a modulator.
In the QPSK system, when a phase shift of 180xc2x0 occurs, a vector representing a modulated carrier passes an origin (a xe2x80x9cOxe2x80x9d point) In case that such a phase shift occurs, since an envelope of a QPSK wave greatly changes when it is outputted from a filter, the QPSK wave is much affected by nolinearity of the power amplifier.
As a method in which an effect of nonlinearity of the power amplifier is not so much, a xcfx80/4 shift QPSK system can be cited.
According to the xcfx80/4 shift QPSK system, a single modulation signal can transmit information of 2 bits similarly to the QPSK system.
The xcfx80/4 shift QPSK system has 8 signal points phase angles of which are successively shifted by xcfx80/4 on a circle representing a phase and an amplitude of the carrier. In this system, since a variation of the envelope is small, a bandwidth of the modulated carrier is narrow when it Is outputted from the filter, and the effect of nonlinearity of the power amplifier is not so much.
As a modulation system in which a single modulation signal can transmit information of more than 2 bits, a trellis-coded 8PSK modulation system (G. Unger boeck xe2x80x9cTrellis-coded modulation with redundant signal sets, Part I and IIxe2x80x9d, IEEE Communication Magazine, Vol.25, No.2, pp.5 to 21, February, 1987) is known besides the QPSK system and the xcfx80/4 shift QPSK system.
In the aforementioned system, redundant signal sets are added to an information signal of 2 bits by the trellis-coder to output data of 3 bits, and the data thus obtained are supplied to a 8PSK modulator, in which one of 8 modulation signal points having phase angles successively shifted by xcfx80/4 is selected and a carrier having a phase angle corresponding to the selected modulation signal point is outputted.
In a PDC (Personal Digital Celluar)system, which outputs 112 symbols and information of 224 bits in a burst, a bit rate thereof is 11.2 kps in a case of full rate. As mentioned in the above, since the bit rate is quite low, an improvement of the transmission speed of the PDC system is in great demand. Since the aforementioned xcfx80/4 shift QPSK system uses a similar phase/amplitude to that used in the trellis-coded 8PSK system, if the data is transmitted through a 8PSK system, it is expected that transmission speed will be remarkably improved.
However, in the aforementioned trellis-coded 8PSK modulation system, the vector representing the modulated carrier may pass the zero xe2x80x9cOxe2x80x9d point at the time of a phase shift thereof, and the amplitude of the envelope of the modulated carrier may be zero. As mentioned in the above, if such a phase shift occurs, the envelope of the QPSK wave is sharply changed when it is outputted from the output filter, and the QPSK wave is much affected by the nonlinearity of the power amplifier.
In order to overcome the aforementioned difficulty, it is necessary that a dynamic range of the power amplifier extends from zero to a rated value, hence a cost of the system becomes extremely high.
Accordingly, it is an object of the invention to provide a modulator/demodulator which can transmit a data at a high speed without an increase in cost.
It is a further object of the invention to provide a method of modulation/demodulation used in a modulatioor/demodulator which transmits a data at a high speed without an increase in cost.
According to the first feature of the invention, a modulator/demodulator comprises:
a modulator which converts an first inputted data into a septenary data, modulates a carrier by the converted data, and radiates the modulated carrier via an antenna, and
a demodulator which demodulates a second inputted data supplied from the antenna into an octonary data, and converts the octonary data into a septenary data.
The modulator may comprise:
the first convertor for converting the inputted binary data into the septenary date in which each symbol is seven valued,
the second convertor for converting the septenary data into an octonary data in which each symbol is eight valued by adding a dummy to the each seven valued symbol,
a quadrature convertor for conducting a quadrature conversion of the octonary data to separate an output of the quadrature convertor into I and Q components, and
a quadrature modulator for modulating carries with a phase difference of xcfx80/2 by the I and Q components.
The second convertor may conduct a differential conversion in case that the septenary data is converted into the octonary data so that a vector representing a modulated carrier is prevented from passing a zero point at the time of a phase shift of the modulated carrier.
The first convertor may convert the binary data into the septenary data in terms of 5 symbols or 31 symbols.
The demodulator may comprise:
the first and second mixers for respectively mixing the second inputted data supplied from the antenna wish local carriers with a phase difference of xcfx80/2,
the first and second low pass filters for respectively removing high frequency components contained in outputs of the first and second mixers.
an inverse quadrature convertor for demodulating outputs of the first and second low pass filters to derive an octonary data functioning as a 8QAM data,
the third convertor for converting the octonary data outputted from the quadrature demodulator into a septenary data, and
the fourth convertor for converting the septenary data outputted from the third convertor into a binary data.
According to the second feature of the invention, a method of modulation/demodulation used in a modulator/demodulator comprises:
the first step comprising the steps of converting an inputted data into a septenary data, modulating a carrier by the converted data, and radiating the modulated carrier via an antenna, and
the second stop comprising the steps of demodulating a received data supplied from the antenna into a octonary data, and converting the octonary data into a septenary data.
The first step may comprise the step of converting the inputted binary data into the septenary data in which each symbol is seven valued,
converting the septenary data into an octonary data in which each symbol is eight valued by adding a dummy to the each seven valued symbol,
conducting a quadrature conversion of the octonary data to derive I and Q components, and
modulating carriers with a phase difference of xcfx80/2 by the I and Q components.
The step of converting the septenary data into an octonary data may comprise the step of conducting differential conversion in case that the septenary data is converted into the octonary data so that a vector representing a modulated carrier is prevented from passing a zero point at the time of a phase shift of the modulated carrier.
The step of converting the inputted binary data into the septenary data may comprise the step of converting the inputted binary data into the septenary data in terms of 5 symbols or 31 symbols.
The second step may comprise the steps of:
mixing the received data with local carriers with a phase difference of xcfx80/2 by means of first and second mixers,
removing high frequency components contained in outputs of the first and second mixers,
demodulating output signals of the first and second mixers from which the high frequency components are removed to derive an octonary data functioning as a 8QAM data,
converting the octonary data into a septenary data, and
converting the septenary data into a binary data.
In a modulator/demodulator and a method of modulation/demodulation used in the same, a vector representing a modulated carrier is prevented from passing a zero point by converting the first inputted data into a septenary data, modulating a carrier by the converted data, and radiating the modulated carrier via an antenna in a modulator, and by demodulating the second inputted data supplied from the antenna into an octonary data, and converting the octonary data into a septenary data in the demodulator.