Communication system

Abstract

At the transmitter side, carrier waves are modulated according to an input signal for producing relevant signal points in a signal space diagram. The input signal is divided into, two, first and second, data streams. The signal points are divided into signal point groups to which data of the first data stream are assigned. Also, data of the second data stream are assigned to the signal points of each signal point group. A difference in the transmission error rate between first and second data streams is developed by shifting the signal points to other positions in the space diagram expressed at least in the polar coordinate system. At the receiver side, the first and/or second data streams can be reconstructed from a received signal. In TV broadcast service, a TV signal is divided by a transmitter into, low and high, frequency band components which are designated as a first and a second data streams respectively. Upon receiving the TV signal, a receiver can reproduce only the low frequency band component or both the low and high frequency band components, depending on its capability. Furthermore, a communication system based on an OFDM system is utilized for data transmission of a plurality of subchannels, wherein the subchannels are differentiated by changing the length of a guard time slot or a carrier wave interval of a symbol transmission time slot, or changing the transmission electric power of the carrier.

1. A receiving apparatus comprising:
 a channel selector connected to a variable oscillator for changing a frequency of an oscillation signal of said variable oscillator;  
 a mixer connected to said variable oscillator for converting a received signal by using said oscillation signal, into a an M-value VSB modulation modulated signal, where M is an integer;  
 a demodulator for demodulating said M-value VSB modulation modulated signal to reproduce an encoded signal; and  
 a trellis decoder for trellis decoding said encoded signal to obtain a trellis-decoded signal, wherein  
 said received signal is a time division multiplexed signal transmitted by a time division multiplexing, and is demodulated as an 8-value VSB modulation modulated signal in a first mode and is demodulated as an N-value VSB modulation modulated signal in a second mode, where N is an integer less than 8.  
 
     
     
       2. The receiving apparatus in accordance with  claim 1 , wherein said trellis-decoded signal is an encoded video signal, and wherein said apparatus further comprises a video decoder for decoding said trellis-decoded signal to obtain a video signal. 
     
     
       3. The receiving apparatus in accordance with  claim 2 , wherein the video decoder is an MPEG decoder. 
     
     
       4. A receiving apparatus comprising:
   a channel selector operable to change a frequency of an oscillation signal of a variable oscillator;        a mixer operable to convert a received signal by using the oscillation signal into a VSB modulated signal;        a demodulator operable to demodulate the VSB modulated signal into an N - level demodulated signal and an  8   - level demodulated signal, where N is an integer less than  8 ; and        a trellis decoder operable to trellis decode the  8   - level demodulated signal to obtain a trellis - decoded signal, wherein        the received signal is a time division multiplexed signal transmitted by a time division multiplexing.     
     
     
       5. The receiving apparatus in accordance with  claim 4 , wherein the trellis- decoded signal is an encoded video signal, and wherein said apparatus further comprises a video decoder operable to decode the trellis - decoded signal to obtain a video signal.   
     
     
       6. The receiving apparatus in accordance with  claim 5 , wherein the video decoder is an MPEG decoder.