Portable terminal, communication system, and communication method

Abstract

A portable terminal communicates with a plurality of base stations by use of an OFDM signal. The portable terminal includes a communication unit sending and receiving the OFDM signal to and from the base stations, a switching timing detector, and an OFDM signal controller. The switching timing detector detects switching timing at which handover and a normal state are switched therebetween. The OFDM signal controller changes a guard time length of the OFDM signal upon detecting the timing by the switching timing detector.

1 . A portable terminal configured to communicate with a plurality of base stations by use of an orthogonal frequency division multiplexing (OFDM) signal, the portable terminal comprising:
 a communication unit configured to send and receive the OFDM signal to and from the base stations;   a switching timing detector configured to detect switching timings at which handover and a normal state are switched therebetween; and   an OFDM signal controller configured to change a guard time length of the OFDM signal upon detecting the switching timing by the switching timing detector,   wherein the OFDM signal controller is configured to make the guard time length longer than the current one when the switching timing detector detects a handover start timing at which the normal state is switched to the handover.   
   
   
       2 . The portable terminal according to  claim 1 , wherein the OFDM signal controller is configured to change a symbol length of the OFDM signal and the guard time length upon detecting the switching timing by the switching timing detector. 
   
   
       3 . The portable terminal according to  claim 1 , wherein the OFDM signal controller is configured to increase the guard time length and the symbol length when the switching timing detector detects a handover start timing at which the normal state is switched to the handover. 
   
   
       4 . The portable terminal according to  claim 1 , wherein the OFDM signal controller is configured to increase the guard time length and the symbol length by an amount of time corresponding to an increase in the guard time length when the switching timing detector detects a handover start timing at which the normal state is switched to the handover. 
   
   
       5 . The portable terminal according to  claim 1 , wherein the OFDM signal controller is configured to increase the symbol length to a value n times as great as the currently set symbol length, where n is an integer, when the switching timing detector detects a handover start timing at which the normal state is switched to the handover. 
   
   
       6 . The portable terminal according to  claim 1 , wherein
 the communication unit is configured to receive from the base stations guard time length information indicating a value of the guard time length during the handover; and   the OFDM signal controller is configured to change the guard time length to a value indicated by the guard time length information received by the communication unit when the switching timing detector detects a handover start timing at which the normal state is switched to the handover.   
   
   
       7 . The portable terminal according to  claim 1 , wherein
 the switching timing detector detects a handover completion timing at which the handover is switched to the normal state; and   the OFDM signal controller makes the guard time length of the OFDM signal shorter than the currently set guard time length when the switching timing detector detects the handover complete timing.   
   
   
       8 . The portable terminal according to  claim 1 , further comprising a spreading rate determiner configured to determine a spreading rate of the OFDM signal based on a value of the guard time length changed by the OFDM signal controller. 
   
   
       9 . The portable terminal according to  claim 8 , wherein the spreading rate determiner is configured to make the spreading rate lower than the currently set spreading rate when the switching timing detector detects a handover start timing at which the normal state is switched to the handover. 
   
   
       10 . The portable terminal according to  claim 8 , wherein the spreading rate determiner is configured to change the spreading rate to a value 1/n of the currently set spreading rate, where n is an integer, when the switching timing detector detects a handover start timing at which the normal state is switched to the handover. 
   
   
       11 . A communication system in which a plurality of base stations and a portable terminal are configured to communicate with each other by use of an OFDM signal, wherein
 the portable terminal comprises   a communication unit configured to send and receive the OFDM signal to and from the base stations;   a switching timing detector configured to detect switching timings at which handover and a normal state are switched therebetween; and   an OFDM signal controller configured to change a guard time length of the OFDM signal upon detecting the switching timing by the switching timing detector,   wherein   the OFDM signal controller is configured to make the guard time length longer than the current one when the switching timing detector detects a handover start timing at which the normal state is switched to the handover, and   the base stations each change, at the same time as the OFDM signal controller changes the guard time length, the guard time length to the same length as that changed by the OFDM signal controller.   
   
   
       12 . A method of communicating with a base station by a portable terminal, the method comprising:
 sending and receiving an OFDM signal to and from the base station;   detecting switching timings at which handover and a normal state are switched therebetween; and   changing a guard time length of the OFDM signal when the switching timing is detected,   wherein the guard time length is changed to be longer than the current one when a switching timing detector detects a handover start timing at which the normal state is switched to the handover.