Modulation method and radio communication system

Abstract

An input digital signal is periodically and alternately subjected to first modulation and second modulation, being thereby converted into a pair of a baseband I signal and a baseband Q signal. The first modulation and the second modulation are different from each other. The pair of the baseband I signal and the baseband Q signal are outputted. The first modulation may be at least 8-signal-point modulation while the second modulation may be phase shift keying.

1. A method for modulation, comprising the steps of:
 regularly subjecting an input digital signal to first modulation and second modulation to convert the input digital signal into a pair of a baseband I signal and a baseband Q signal, the first modulation and the second modulation being different from each other and regularly applied; and 
 outputting the pair of the baseband I signal and the baseband Q signal; 
 wherein the first modulation is at least 8-signal-point modulation, and the second modulation is phase shift keying; 
 wherein the phase shift keying provides periodically-spaced symbols which represent corresponding portions of the input digital signal in terms of differences between phases of the periodically-spaced symbols; and 
 wherein the at least 8-signal-point modulation assigns logic states of the input digital signal to respective signal points for a first symbol in response to a signal point used by a second symbol of the phase shift keying which precedes the first symbol. 
 
     
     
       2. A method as recited in  claim 1 , wherein the phase shift keying is quadrature phase shift keying. 
     
     
       3. A method as recited in  claim 2 , wherein the quadrature phase shift keying provides signal points on an I axis and a Q axis in an I-Q plane. 
     
     
       4. A method as recited in  claim 1 , wherein the at least 8-signal-point modulation is at least 8 quadrature amplitude modulation. 
     
     
       5. A method as recited in  claim 3 , wherein the at least 8-signal-point modulation is at least 8 quadrature amplitude modulation. 
     
     
       6. A method as recited in  claim 4 , wherein at least 8 quadrature amplitude modulation is 16 quadrature amplitude modulation. 
     
     
       7. A method as recited in  claim 5 , wherein the at least 8 quadrature amplitude modulation is 16 quadrature amplitude modulation. 
     
     
       8. A method as recited in  claim 4 , wherein the at least 8 quadrature amplitude modulation provides signal points which result from rotation of signal points of at least 8-value normal quadrature amplitude modulation through an angle of π/4 radian about an origin in an I-Q plane. 
     
     
       9. A method as recited in  claim 5 , wherein the at least 8 quadrature amplitude modulation provides signal points which result from rotation of signal points of at least 8-value normal quadrature amplitude modulation through an angle of π/4 radian about an origin in an I-Q plane. 
     
     
       10. A method as recited in  claim 6 , wherein the 16 quadrature amplitude modulation provides signal points which result from rotation of signal points of 16-value normal quadrature amplitude modulation through an angle of π/4 radian about an origin in an I-Q plane. 
     
     
       11. A method as recited in  claim 7 , wherein the 16 quadrature amplitude modulation provides signal points which result from rotation of signal points of 16-value normal quadrature amplitude modulation through an angle of π/4 radian about an origin in an I-Q plane. 
     
     
       12. A method as recited in  claim 1 , wherein a maximum of amplitudes corresponding to signal points of the at least 8-signal-point modulation in an I-Q plane is equal to an amplitude of a signal point of the phase shift keying in the I-Q plane. 
     
     
       13. A transmission apparatus comprising:
 first means for periodically and alternately subjecting an input digital signal to first modulation and second modulation to convert the input digital signal into a pair of a baseband I signal and a baseband Q signal, the first modulation and the second modulation being different from each other, the first modulation being at least 8-signal-point modulation, the second modulation being phase shift keying; 
 second means for outputting the pair of the baseband I signal and the baseband Q signal; 
 wherein the phase shift keying provides periodically-spaced symbols which represent corresponding portions of the input digital signal in terms of differences between phases of the periodically-spaced symbols; 
 wherein the at least 8-signal-point modulation assigns logic states of the input digital signal to respective signal points for a first symbol in response to a signal point used by a second symbol of the shift keying which precedes the first symbol; and 
 wherein said first symbol is demodulated by using said second symbol which is not a known prescribed pattern but a part of information transmitted by said transmission apparatus. 
 
     
     
       14. A transmission apparatus as recited in  claim 13 , wherein the symbols provided by the phase shift keying are used as a pilot symbol for estimating at least one of (1) a transmission path distortion and (2) a frequency offset.