Transmitter and method of transmitting and receiver and method of detecting ofdm signals

Abstract

A transmitter transmits payload data using Orthogonal Frequency Division Multiplexed (OFDM) symbols. The first OFDM symbol is a first type having a number of sub-carriers which is less than or equal to the number of sub-carriers of the one or more second OFDM symbols of a second type and a guard interval for the first OFDM symbol is selected in dependence upon the longest possible guard interval of the second OFDM symbol. Accordingly an OFDM communications system can be formed in which data is transmitted using a frame structure in which a guard interval is adapted to allow a mix of different types of OFDM symbols.

1 . A transmitter for transmitting payload data using Orthogonal Frequency Division Multiplexed (OFDM) symbols, the transmitter comprising:
 a frame builder configured to receive the payload data to be transmitted and to receive signalling data for use in detecting and recovering the payload data at a receiver, and to form the payload data with the signalling data into frames for transmission,   a modulator configured to modulate a first OFDM symbol with the signalling data forming a part of each of the frames and to modulate one or more second OFDM symbols with the payload data to form one or more of the frames, and   a transmission unit configured to transmit the first and the one or more second OFDM symbols, wherein the first OFDM symbol is of a first type having a number of sub-carriers which is less than or equal to a number of sub-carriers of the one or more second OFDM symbols of a second type and a guard interval for the first OFDM symbol is selected in dependence upon a longest possible guard interval of the one or more second OFDM symbols, wherein said longest possible guard interval is in a finite set of possible guard intervals.   
     
     
         2 . The transmitter as claimed in  claim 1 , wherein the number of sub-carriers of the first OFDM symbol is selected in dependence upon a duration of the selected guard interval. 
     
     
         3 . The transmitter as claimed in  claim 1 , wherein the number of sub-carriers of the first OFDM symbol is selected to increase a likelihood of a receiver being able to detect and recover the signalling data from the first OFDM symbol and the number of sub-carriers of the one or more second OFDM symbols is selected to maximize spectral efficiency. 
     
     
         4 . The transmitter as claimed in  claim 1 , wherein the number of sub-carriers in the one or more second OFDM symbols is substantially thirty two thousand, sixteen thousand or eight thousand, and the number of sub-carriers of the first OFDM symbol is substantially eight thousand. 
     
     
         5 . The transmitter as claimed in  claim 4 , wherein the guard interval is 57/128 of a duration of the first OFDM symbol. 
     
     
         6 . The transmitter as claimed in  claim 4 , wherein a duration of said guard interval is the same as the longest duration guard interval for the one or more second OFDM symbols. 
     
     
         7 . The transmitter as claimed in  claim 1 , wherein the signalling data is encoded with a first error correction code and the payload data is encoded with at least one other error correction code, an encoding rate of the first error correction code being lower than an encoding rate of the at least one other error correction code. 
     
     
         8 . The transmitter as claimed in  claim 7 , wherein the signalling data is encoded with an error correction code, an encoding rate of the error correction code being lower than rate one quarter. 
     
     
         9 . A method of transmitting payload data using Orthogonal Frequency Division Multiplexed (OFDM) symbols, the method comprising:
 receiving the payload data to be transmitted;   receiving signalling data for use in detecting and recovering the payload data to be transmitted at a receiver;   forming the payload data with the signalling data into frames for transmission;   modulating a first OFDM symbol with the signalling data forming a part of each of the frames and modulating one or more second OFDM symbols with the payload data to form one or more of the frames; and   transmitting by circuitry the first OFDM symbol and the one or more second OFDM symbols, wherein the first OFDM symbol is of a first type having a number of sub-carriers which is less than or equal to a number of sub-carriers of the one or more second OFDM symbols of a second type, wherein the transmitting step includes   selecting, from a finite set of available guard intervals, a guard interval for the first OFDM symbol depending upon the maximum allowable duration of the guard interval useable for the one or more second OFDM symbols.   
     
     
         10 . The method as claimed in  claim 9 , wherein the step of selecting the guard interval for the first OFDM symbol includes selecting the number of sub-carriers of the first OFDM symbol in dependence upon the size of the selected guard interval. 
     
     
         11 . The method as claimed in  claim 9 , wherein the step of selecting the guard interval for the first OFDM symbol includes selecting the number of sub-carriers of the first OFDM symbol to increase a likelihood of a receiver being able to detect and recover the signalling data from the first OFDM symbol, wherein the method further comprises selecting the number of sub-carriers of the one or more second OFDM symbols to maximize spectral efficiency. 
     
     
         12 . The method as claimed in  claim 9 , wherein the step of selecting the number of sub-carriers of the one or more second OFDM symbols includes selecting the number of sub-carriers in the one or more second OFDM symbols to be substantially thirty two thousand, sixteen thousand, or eight thousand, and the step of selecting the number of sub-carriers of the first OFDM symbol comprises selecting the number of sub-carriers of the first OFDM symbol to be substantially eight thousand. 
     
     
         13 . The method as claimed in  claim 12 , wherein the step of selecting the guard interval comprises selecting the guard interval to be 57/128 of the duration of the first OFDM symbol. 
     
     
         14 . The method as claimed in  claim 12 , further comprising selecting the duration of said guard interval to be the same as the longest duration guard interval for the one or more second OFDM symbols. 
     
     
         15 . The method as claimed in  claim 9 , further comprising:
 encoding the signalling data with a first error correction code; and   encoding the payload data with at least one other error correction code, an encoding rate of the first error correction code being lower than an encoding rate of the at least one other error correction code.   
     
     
         16 . The method as claimed in  claim 15 , further comprising:
 encoding the signalling data with an error correction code, an encoding rate of the error correction code being lower than rate one quarter.   
     
     
         17 . A receiver for detecting and recovering payload data from a received signal, the receiver comprising:
 a detector configured to detect the received signal, the received signal comprising the payload data and signalling data for use in detecting and recovering the payload data, the signalling data and the payload data forming frames in the received signal, the signalling data in each frame being carried by a first Orthogonal Frequency Division Multiplexed (OFDM) symbol, and the payload data being carried by one or more second OFDM symbols, wherein the first OFDM symbol has a number of sub-carriers which is less than or equal to a number of sub-carriers of the one or more second OFDM symbols and a guard interval for the first OFDM symbol having been arranged in dependence upon the maximum allowable duration of the guard interval useable for the one or more second OFDM symbols from a finite set of guard intervals; and   a demodulator configured to detect the first OFDM symbol and the second one or more OFDM symbols and to recover the signalling data from the first OFDM symbol in the presence of the guard interval using a forward Fourier transform with respect to the number of sub-carriers of the first OFDM symbol and using the signalling data to recover the payload data from the second one or more OFDM symbols.   
     
     
         18 . The receiver as claimed in  claim 17 , wherein the number of sub-carriers of the first OFDM symbol has been determined in dependence upon the size of the arranged guard interval. 
     
     
         19 . The receiver as claimed in  claim 17 , wherein the number of sub-carriers in the one or more second OFDM symbols is substantially thirty two thousand, sixteen thousand, or eight thousand, and the number of sub-carriers of the first OFDM symbol is substantially eight thousand. 
     
     
         20 . The receiver as claimed in  claim 19 , wherein the guard interval 57/128 of a duration of the first OFDM symbol. 
     
     
         21 . The receiver as claimed in  claim 20 , wherein the duration of said guard interval is the same as the longest duration guard interval available for the one or more second OFDM symbols. 
     
     
         22 . A method of detecting and recovering payload data from a received signal, the method comprising:
 detecting the received signal, the received signal comprising the payload data and signalling data for use in detecting and recovering the payload data, the signalling data and the payload data forming frames in the received signal, the signalling data in each frame being carried by a first Orthogonal Frequency Division Multiplexed (OFDM) symbol, and the payload data being carried by one or more second OFDM symbols and a guard interval for the first OFDM symbol having been arranged in dependence upon the maximum allowable duration of the guard interval useable for the one or more second OFDM symbols from a finite set of guard intervals;   demodulating by circuitry the first OFDM symbol and the second one or more OFDM symbols to recover the signalling data from the first OFDM symbol in the presence of the guard interval using a forward Fourier transform with respect to the number of sub-carriers of the first OFDM symbol; and   using the signalling data to recover the payload data from the one or more second OFDM symbols.