Frequency division multiplexing system with selectable rate
View Patent ↗An OFDM system uses a normal mode which has a symbol length T, a guard time TG and a set of N sub-carriers, which are orthogonal over the time T, and one or more fallback modes which have symbol lengths KT and guard times KTG where K is an integer greater than unity. The same set of N sub-carriers is used for the fallback modes as for the normal mode. Since the same set of sub-carriers is used, the overall bandwidth is substantially constant, so alias filtering does not need to be adaptive. The Fourier transform operations are the same as for the normal mode. Thus fallback modes are provided with little hardware cost. In the fallback modes the increased guard time provides better delay spread tolerance and the increased symbol length provides improved signal to noise performance, and thus increased range, at the cost of reduced data rate.
1. A transmitter operable to:
receive a group of bits as input and perform at least one inverse Fourier transform on said group of bits to produce orthogonal frequency division multiplex (OFDM) symbols, the inverse Fourier transform having a period of a time T;
generate a first OFDM symbol and a second OFDM symbol from the OFDM symbols, wherein:
the first OFDM symbol is expressed by superpositions of a set of sub-carriers which are orthogonal over the time T, a duration of the first OFDM symbol is T and the first OFDM symbol is associated with a first guard time having a length of T G ; and
the second OFDM symbol is expressed by superpositions of the same set of sub-carriers used to express the first OFDM symbol, the second OFDM symbol is the next OFDM symbol after or the previous OFDM symbol before the first OFDM symbol in a sequence of OFDM symbols, and the duration of the second OFDM symbol is KT and the second OFDM symbol is associated with a second guard time having a length of KT G , where K is a positive integer greater than one; and
select a first output from the generated OFDM symbols, the first output having a duration of T plus T G , the first output comprising the first OFDM symbol and a first cyclic prefix of length T G ; and
select a second output from the generated OFDM symbols, the second output having a duration of KT plus KT G , the second output comprising the second OFDM symbol and a second cyclic prefix of length KT G .
2. The transmitter as claimed in claim 1 wherein the second OFDM symbol comprises K replications of an OFDM symbol having a length of T.
3. The transmitter as claimed in claim 1 wherein the inverse Fourier transform is a fast Fourier transform.
4. The transmitter as claimed in claim 1 wherein the first guard time is prefixed to the first OFDM symbol and the second guard time is prefixed to the second OFDM symbol.
5. The transmitter as claimed in claim 1 wherein the transmitter is further operable to copy a portion of the second OFDM symbol for use in the second cyclic prefix and transmit the second cyclic prefix during the second guard time.
6. The transmitter as claimed in claim 1 wherein the transmitter is further operable to introduce forward error correction into a plurality of bits that are used to form the first and second OFDM symbols.
7. A method comprising:
receiving a group of bits as input and perform at least one inverse Fourier transform on said group of bits to produce orthogonal frequency division multiplex (OFDM) symbols, the inverse Fourier transform having a period of a time T;
generating a first OFDM symbol and a second OFDM symbol from the OFDM symbols, wherein:
the first OFDM symbol is expressed by superpositions of a set of sub-carriers which are orthogonal over the time T, the first OFDM symbol is associated with a first guard time T G , and the first OFDM symbol has a symbol duration T; and
the second OFDM symbol is expressed by superpositions of the same set of sub-carriers, the second OFDM symbol is the OFDM symbol after or before the first OFDM symbol in a sequence of OFDM symbols, the second OFDM symbol is associated with a second guard time KT G and the second OFDM symbol has a symbol duration KT, where K is a positive integer greater than 1; and
selecting a first output from the generated OFDM symbols, the first output having a duration of T plus T G , the first output comprising the first OFDM symbol and a first cyclic prefix of length T G ; and
selecting a second output from the generated OFDM symbols, the second output having a duration of KT plus KT G , the second output comprising the second OFDM symbol and a second cyclic prefix of length KT G .
8. The method as claimed in claim 7 , wherein the second OFDM symbol comprises K replications of an OFDM symbol having a length of T.
9. The method as claimed in claim 7 , wherein the inverse Fourier transform is a fast Fourier transform.
10. The method as claimed in claim 7 , wherein the first guard time is prefixed to the first OFDM symbol and the second guard time is prefixed to the second OFDM symbol.
11. The method as claimed in claim 7 , further comprising copying a portion of the second OFDM symbol for use in the second cyclic prefix and transmitting the second cyclic prefix during the second guard time.
12. The method as claimed in claim 7 , further comprising introducing forward error correction into a plurality of bits that are used to form the first and second OFDM symbols.
13. A transmitting system comprising:
an inverse Fourier transform circuit operable to receive bits as input and to perform at least one inverse Fourier transform on said bits to produce orthogonal frequency division multiplex (OFDM) symbols, the inverse Fourier transform circuit having a period of a time T; and
a prefix and window circuit coupled to the inverse Fourier transform circuit and operable to:
generate a first OFDM symbol and a second OFDM symbol from the OFDM symbols, wherein:
the first OFDM symbol is expressed by superpositions of a set of sub-carriers which are orthogonal over the time T, the first OFDM symbol is associated with a first guard time T G , and the first OFDM symbol has a symbol duration T; and
the second OFDM symbol is expressed by superpositions of the same set of sub-carriers, the second OFDM symbol is the OFDM symbol after or before the first OFDM symbol in a sequence of OFDM symbols, the second OFDM symbol is associated with a second guard time KT G and the second OFDM symbol has a symbol duration KT, where K is a positive integer greater than 1; and
select a first output of the inverse Fourier transform circuit, the first output having a duration of T plus T G , the first output comprising the first OFDM symbol and a first cyclic prefix of length T G ; and
select a second output of the inverse Fourier transform circuit, the second output having a duration of KT plus KT G , the second output comprising the second OFDM symbol and a second cyclic prefix of length KT G .
14. The transmitting system as claimed in claim 13 wherein the second OFDM symbol comprises K replications of an OFDM symbol having a length of T.
15. The transmitting system as claimed in claim 13 wherein the inverse Fourier transform is a fast Fourier transform.
16. The transmitting system as claimed in claim 13 wherein the first guard time is prefixed to the first OFDM symbol and the second guard time is prefixed to the second OFDM symbol.
17. The transmitting system as claimed in claim 13 wherein the prefix and window circuit is further operable to select a portion of the second OFDM symbol for use in the second cyclic prefix and the transmitting system is operable to transmit the second cyclic prefix during the second guard time.
18. The transmitting system as claimed in claim 13 wherein the transmitting system is further operable to introduce forward error correction into a plurality of bits that are used to form the first and second OFDM symbols.
19. The transmitting system as claimed in claim 13 wherein the transmitting system is operable to transmit the first and second OFDM symbols and first and second cyclic prefixes.