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 receiver operable to:
receive a first orthogonal frequency division multiplex (OFDM) symbol, the first OFDM symbol being expressed by superpositions of a set of sub-carriers which are orthogonal over a time T, wherein the 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
receive a second OFDM symbol, the second OFDM symbol expressed by superpositions of the same set of sub-carriers used to express the first OFDM symbol, the second OFDM symbol being the next OFDM symbol after or the previous OFDM symbol before the first OFDM symbol in a sequence of OFDM symbols, wherein 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.
2. The receiver as claimed in claim 1 wherein the first guard time is interposed between successive OFDM symbols.
3. The receiver 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.
4. The receiver as claimed in claim 1 including a fast Fourier transform (FFT) block to perform a Fourier transform on a received signal.
5. The receiver as claimed in claim 4 wherein a period of the Fourier transform is the time T.
6. The receiver as claimed in claim 5 further including a filter operably coupled to an input of the FFT block.
7. The receiver as claimed in claim 6 wherein the filter averages an input signal over K intervals, each interval having a duration T.
8. The receiver as claimed in claim 7 wherein the filter includes a memory having a delay line established to receive M samples, where M corresponds to the number of samples in each block of data operated upon by the FFT block.
9. The receiver as claimed in claim 4 further including a filter operably coupled to an output of the FFT block.
10. The receiver as claimed in claim 9 wherein the filter averages K outputs from the FFT block.
11. The receiver as claimed in claim 4 further including an antenna for receiving the first OFDM symbol and the second OFDM symbol.
12. The receiver as claimed in claim 11 further including an RF down-converter.
13. The receiver as claimed in claim 1 , wherein the second OFDM symbol consists of K copies of a third OFDM symbol, the third OFDM symbol being expressed by superpositions of the same set of sub-carriers used to express the first OFDM symbol, the third OFDM symbol having a symbol duration T.
14. A method comprising:
receiving a first orthogonal frequency division multiplex (OFDM) symbol, the first OFDM symbol being expressed by superpositions of a set of sub-carriers which are orthogonal over a time T, wherein the 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
receiving a second OFDM symbol, the second OFDM symbol expressed by superpositions of the same set of sub-carriers used to express the first OFDM symbol, the second OFDM symbol being the next OFDM symbol after or the previous OFDM symbol before the first OFDM symbol in a sequence of OFDM symbols, wherein 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.
15. The method of claim 14 , wherein the first guard time is interposed between successive OFDM symbols.
16. The method of claim 14 , 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 method of claim 14 further comprising:
performing a Fourier transform on the first OFDM signal, where a period of the Fourier transform is the time T.
18. The method of claim 14 further comprising:
determining an average of K successively received OFDM signals.
19. The method as claimed in claim 14 , wherein the second OFDM symbol consists of K copies of a third OFDM symbol, the third OFDM symbol being expressed by superpositions of the same set of sub-carriers used to express the first OFDM symbol, the third OFDM symbol having a symbol duration T.
20. A receiving system comprising:
an orthogonal frequency division multiplex (OFDM) receiver comprising:
an analog to digital converter (ADC); and
a fast Fourier transform (FFT) block, operably coupled to the ADC, configured to perform a Fourier transform on an output of the ADC;
the receiver being configured to:
receive a first OFDM symbol, the first OFDM symbol being expressed by superpositions of a set of sub-carriers which are orthogonal over a time T, wherein the first OFDM symbol is associated with a first guard time T G and the first OFDM symbol has a symbol duration T; and
receive a second OFDM symbol, the second OFDM symbol expressed by superpositions of the same set of sub-carriers used to express the first OFDM symbol, the second OFDM symbol being the next OFDM symbol after or the previous OFDM symbol before the first OFDM symbol in a sequence of OFDM symbols, wherein 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.
21. The receiving system as claimed in claim 20 further comprising a receiver front-end that converts a waveform at an RF carrier frequency into a baseband waveform.
22. The receiving system as claimed in claim 21 further comprising an antenna for receiving the waveform at the RF carrier frequency.
23. The receiving system as claimed in claim 20 wherein a period of the Fourier transform is the first OFDM symbol duration T.
24. The receiving system as claimed in claim 20 further comprising a filter coupled to the FFT block.
25. The receiving system as claimed in claim 24 further including a symbol timing circuit, operably coupled to the FFT block and the filter, that provides a timing signal to the FFT block and the filter such that the timing signal indicates the start of a new FFT interval to the FFT block and the filter.
26. The receiving system as claimed in claim 20 wherein K=2 such that the second guard time associated with the second OFDM symbol is twice the length of the first guard time associated with the first OFDM symbol.
27. The receiving system as claimed in claim 20 wherein the first guard time T G is interposed between successive OFDM symbols.
28. The receiving system as claimed in claim 20 wherein the first guard time T G is prefixed to the first OFDM symbol.
29. The receiving system as claimed in claim 20 , wherein the second OFDM symbol consists of K copies of a third OFDM symbol, the third OFDM symbol being expressed by superpositions of the same set of sub-carriers used to express the first OFDM symbol, the third OFDM symbol having a symbol duration T.
30. A transmitter operable to:
transmit a first orthogonal frequency division multiplex (OFDM) symbol, the first OFDM symbol being expressed by superpositions of a set of sub-carriers which are orthogonal over a time T, wherein the 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
transmit a second OFDM symbol, the second OFDM symbol expressed by superpositions of the same set of sub-carriers used to express the first OFDM symbol, the second OFDM symbol being the next OFDM symbol after or the previous OFDM symbol before the first OFDM symbol in a sequence of OFDM symbols, wherein 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.
31. The transmitter as claimed in claim 30 wherein the first guard time is interposed between successive OFDM symbols.
32. The transmitter as claimed in claim 30 wherein the first guard time is prefixed to the first OFDM symbol and the second guard time is prefixed to the second OFDM symbol.
33. The transmitter as claimed in claim 30 , wherein the second OFDM symbol consists of K copies of a third OFDM symbol, the third OFDM symbol being expressed by superpositions of the same set of sub-carriers used to express the first OFDM symbol, the third OFDM symbol having a symbol duration T.
34. A method comprising:
transmitting a first orthogonal frequency division multiplex (OFDM) symbol, the first OFDM symbol being expressed by superpositions of a set of sub-carriers which are orthogonal over a time T, wherein the 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
transmitting a second OFDM symbol, the second OFDM symbol expressed by superpositions of the same set of sub-carriers used to express the first OFDM symbol, the second OFDM symbol being the next OFDM symbol after or the previous OFDM symbol before the first OFDM symbol in a sequence of OFDM symbols, wherein 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.
35. The transmitter as claimed in claim 34 wherein the first guard time is interposed between successive OFDM symbols.
36. The transmitter as claimed in claim 34 wherein the first guard time is prefixed to the first OFDM symbol and the second guard time is prefixed to the second OFDM symbol.
37. The method as claimed in claim 34 , wherein the second OFDM symbol consists of K copies of a third OFDM symbol, the third OFDM symbol being expressed by superpositions of the same set of sub-carriers used to express the first OFDM symbol, the third OFDM symbol having a symbol duration T.
38. A transmitting system comprising:
an orthogonal frequency division multiplex (OFDM) transmitter comprising:
a coding circuit that forms symbols from groups of bits; and
an inverse Fourier transform circuit, operably coupled to the coding circuit, that performs an inverse Fourier transform on said symbols to produce OFDM symbols; and
wherein the OFDM transmitter is configured to:
transmit a first OFDM symbol, the first OFDM symbol being expressed by superpositions of a set of sub-carriers which are orthogonal over a time T, wherein the first OFDM symbol is associated with a first guard time T G and the first OFDM symbol has a symbol duration T; and
transmit a second OFDM symbol, the second OFDM symbol expressed by superpositions of the same set of sub-carriers used to express the first OFDM symbol, the second OFDM symbol being the next OFDM symbol after or the previous OFDM symbol before the first OFDM symbol in a sequence of OFDM symbols, wherein 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.
39. The transmitting system as claimed in claim 38 , wherein the second OFDM symbol consists of K copies of a third OFDM symbol, the third OFDM symbol being expressed by superpositions of the same set of sub-carriers used to express the first OFDM symbol, the third OFDM symbol having a symbol duration T.