In recent years, with the widespread use of information processing technology and the rapid progression of the so-called IT (Information Technology) introduced society, the expansion and demand has been remarkable in information communications. For the communication infrastructure, introduction of wireless and high-speed communications is desired naturally to connect among the society, and further connect between a person and the society. Such increasing demand for mobile communications will exhaust abundant frequency resources.
Currently, a main subject under study to enhance the spectral efficiency is improvements in technology relating to radio propagation as typified by MIMO (Multi Input Multi Output). However, there are various difficulties in reserving a desired radio communication path in free space, especially space in outdoor environments. Particularly, more difficulties arise under circumstances where terminals move at high speed. Multiplexing is further difficult.
In view of the foregoing, it is considered that reliable improvement should be first established in baseband. With respect to improvements in baseband, pioneers have been developing successively new schemes such as ASK, PSK, QAM, CDMA and OFDM. Thus, as an essential resolving method, improvements in modulation efficiency in baseband are desired earnestly.
First, considering a signal density in doubling a signal rate is as shown in FIG. 1B. In addition, FIG. 1A shows a Nyquist signal waveform on one axis, and one wave of Nyquist signal is configured every symbol duration T. FIG. 1B shows the case where two waves of Nyquist signal are accommodated within the symbol duration T, and the transmission rate is doubled. However, when two waves of Nyquist signal are accommodated simply within the symbol duration T as shown in FIG. 1B, a frequency band is broadened by two times as compared with the case as shown in FIG. 1A, thus being not preferable.
Conventionally, it is known that a SSB (Single Side Band) scheme is resistant to variations in propagation environments except that some device is required for carrier reproduction in a reception system. The technique to improve the bit error rate characteristic by applying the SSB scheme is described in U.S. Pat. No. 6,091,781 (JP H01-239189), for example.
FIGS. 2A, 2B and 2C illustrate the principle described in the above document. By applying the SSB scheme to a basic I-axis signal and Q-axis signal as shown in FIG. 2A, an I-axis signal and Q-axis signal provided with the SSB scheme as shown in FIG. 2B are obtained and combined in order to form a SSB-QPSK signal as shown in FIG. 2C.
This processing is specifically implemented by a circuit configuration as shown in FIG. 3. First, interpolators 1 and 2 interpolate zero respectively to an in-phase data signal X(n) and quadrature data signal Y(n). An output of interpolator 1 is output to signal combiner 7 via delay circuit 3, while being output to signal combiner 8 after undergoing Hilbert transform by Hilbert filter 4. An output of interpolator 2 undergoes Hilbert transform by Hilbert filter 5 and then is output to signal combiner 7, while being output to signal combiner 8 via delay circuit 6. An output of signal combiner 7 is provided to mixer 11 via pulse shaping filter 9, and an output of signal combiner 8 is provided to mixer 12 via pulse shaping filter 10. Mixer 11 modulates cosine carrier cos(ωct) with the output signal of pulse shaping mixer 9, and mixer 12 modulates sine carrier sin(ωct) with the output signal of pulse shaping mixer 10. An I-channel RF signal and Q-channel RF signal respectively from mixers 11 and 12 are combined in signal combiner 13, and SSB-QPSK signal Z(t) is thereby obtained. Thus, according to the configuration as described in the above-mentioned document, to apply the SSB scheme, respective Hilbert transform components of an I-axis signal and Q-axis signal are generated and subjected to quadrature modulation.
By this means, according to the above-mentioned document, the SSB scheme resolves the conventional defect that the I-axis signal and Q-axis signal are uniquely undergone cosine multiplication and sine multiplication, and enables improvements in transmission characteristics. It is thus described in the above-mentioned document that SSB-QPSK theoretically has the spectral efficiency (for example, 2 bps/Hz) equal to that in QPSK and SSB, and is more resistant to equaling inadequacy on a Rayleigh fading path than QPSK and SSB, and that changes in envelop in SSB-QPSK is less than that in QPSK by 6 dB.
However, the technique as described in the above-mentioned document is to improve the bit error rate characteristic by applying the SSB scheme, and is fundamentally not to enable greatly increased signal transmissions in a limited frequency band as compared with the conventional technique.