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 for transmitting an orthogonal frequency division multiplex (OFDM) communication signal, the transmitter comprising:
apparatus that transmits an OFDM signal using a set of sub-carriers which are orthogonal over a time T, symbols being expressed by superpositions of the sub-carriers, the apparatus configured to selectively operate in one of a plurality of signaling modes in each of which duration of each of the symbols is KT, where K is a positive integer, and different ones of the plurality of signaling modes have different values of K and the same set of sub-carriers, there being guard times associated with the symbols, guard time lengths being greater for signaling modes having a greater value of K.
2. The transmitter as claimed in claim 1 including a circuit to receive the superpositions of the sub-carriers expressing the symbols and to derive a K-fold repetition in time of each said superposition.
3. The transmitter as claimed in claim 1 further including a cyclic prefixer block that copies a last portion of the symbol, the copy of a last portion of the symbol being transmitted during the guard time.
4. The transmitter as claimed in claim 1 including a coding circuit that receives a stream of bits and partitions the stream into groups of bits.
5. The transmitter as claimed in claim 4 including a forward error correction circuit that introduces forward error correction coding into the groups of bits.
6. The transmitter as claimed in claim 1 including an inverse Fourier transform circuit that performs an inverse Fourier transform of the superpositions of the sub-carriers.
7. The transmitter as claimed in claim 6 wherein a period of the inverse Fourier transform is the time T.
8. The transmitter as claimed in claim 7 wherein the symbol is formed from K repetitions of results of the inverse Fourier transform.
9. The transmitter as claimed in claim 8 wherein K=2.
10. The transmitter as claimed in claim 9 including a transmitter front-end operably coupled to the digital to analog converter, the transmitter front end converting the baseband waveform to a waveform at an RF carrier frequency to form the OFDM signal for transmission by the transmitter.
11. The transmitter as claimed in claim 6 including a digital to analog converter operably coupled to the inverse Fourier transform circuit to provide a baseband waveform comprising the superpositions of sub-carriers.
12. The transmitter as claimed in claim 11 further including an antenna for transmitting the OFDM signal.
13. The transmitter as claimed in claim 1 wherein one of the plurality of signaling modes is a normal mode and another of the plurality of signaling modes is a fallback mode.
14. The transmitter as claimed in claim 1 wherein the length of said guard time is KT G where T G is the same for all of the said signaling modes.
15. The transmitter as claimed in claim 1 wherein the guard time is interposed between successive symbols.
16. The transmitter as claimed in claim 1 wherein the guard time is prefixed to the symbols.
17. A method of transmitting an orthogonal frequency division multiplex (OFDM) communication signal, the method comprising:
transmitting an OFDM signal using a set of sub-carriers which are orthogonal over a time T, symbols being expressed by superpositions of the sub-carriers;
selectively operating in one of a plurality of signaling modes in each of which duration of each of the symbols is KT where K is a positive integer and ones of the plurality of signaling modes have different values of K and the same set of sub-carriers; and
providing guard times in association with the symbols, guard time lengths being greater for signaling modes having a greater value of K.
18. The method of transmitting as claimed in claim 17 including:
receiving the superpositions of the sub-carriers expressing the symbols; and
deriving a K-fold repetition in time of each superposition.
19. The method of transmitting as claimed in claim 17 further including:
copying a last portion of the symbol; and
transmitting the copy of a last portion of the symbol during the guard time.
20. The method of transmitting as claimed in claim 17 further including:
partitioning a stream of bits into groups of bits, each group of bits forming a symbol.
21. The method of transmitting as claimed in claim 20 further including:
introducing forward error correction coding into the groups of bits.
22. The method of transmitting as claimed in claim 17 further including:
performing an inverse Fourier transform on the superpositions of sub-carriers.
23. The method of transmitting as claimed in claim 22 wherein a period of the inverse Fourier transform is the time T.
24. The method of transmitting as claimed in claim 23 wherein the symbol is formed from K repetitions of results of the inverse Fourier transform.
25. The method of transmitting as claimed in claim 24 wherein K=2.
26. The method of transmitting as claimed in claim 22 further including:
performing a digital to analog conversion to produce a baseband waveform signal.
27. The method of transmitting as claimed in claim 26 further including:
converting the baseband waveform to a waveform at an RF carrier frequency to form the OFDM signal; and
transmitting the OFDM signal over an antenna.
28. The method of transmitting as claimed in claim 17 wherein one of the plurality of signaling modes is a normal mode and another of the plurality of signaling modes is a fallback mode.
29. The method of transmitting as claimed in claim 17 wherein the length of the guard time is KT G where T G is the same for all of the signaling modes.
30. The method of transmitting as claimed in claim 17 wherein the guard time is interposed between successive symbols.
31. The method of transmitting as claimed in claim 17 wherein the guard time is prefixed to the symbols.
32. A transmitting system comprising:
an orthogonal frequency division multiplex (OFDM) transmitter selectively operable in a plurality of modes, each of the plurality of modes employing the same set of sub-carriers, which are orthogonal over a time T, the transmitter comprising:
a coding circuit that forms symbols from groups of bits;
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
another circuit, operably coupled to the inverse Fourier transform circuit, capable of switching between a first mode having a guard time T G and an OFDM symbol duration T, and a second mode having a guard time KT G and an OFDM symbol duration KT, where K is a positive integer greater than 1.
33. The transmitting system as claimed in claim 32 , wherein the coding circuit maps N symbols onto N orthogonal sub-carriers.
34. The transmitting system as claimed in claim 33 wherein the coding circuit introduces forward error correction coding into the groups of bits.
35. The transmitting system as claimed in claim 32 , wherein a period of the inverse Fourier transform is the OFDM symbol duration T.
36. The transmitting system as claimed in claim 32 including a digital to analog converter operably coupled to the output of the another circuit.
37. The transmitting system as claimed in claim 36 including a transmitter front-end that converts a baseband waveform to a waveform at an RF carrier frequency for transmission by the transmitter.
38. The transmitting system as claimed in claim 37 further including an antenna for transmitting the waveform at the RF carrier frequency.
39. The transmitting system as claimed in claim 32 further including a cyclic prefixer block that copies a last portion of the symbol, the copy of a last portion of the symbol being transmitted during the guard time.
40. The transmitting system as claimed in claim 32 further including control circuitry responsive to results of monitoring signal quality.
41. The transmitting system as claimed in claim 32 , wherein the another circuit is capable of deriving a K-fold repetition of each OFDM symbol of duration T to form an OFDM symbol of duration KT.
42. The transmitting system as claimed in claim 32 , wherein the another circuit is a prefix and window circuit.
43. The transmitting system as claimed in claim 42 , wherein the prefix and window circuit performs windowing by applying a gradual roll-off pattern.
44. The transmitting system as claimed in claim 32 , wherein, in the second mode, the another circuit provides K−1 copies of the OFDM symbol of length T in order to form an OFDM symbol of length KT.
45. The transmitting system as claimed in claim 32 wherein K=2 in the second mode such that the guard time in the second mode is twice the length of the guard time in the first mode.
46. The transmitting system as claimed in claim 32 wherein the guard time is interposed between successive OFDM symbols.
47. The transmitting system as claimed in claim 32 wherein the guard time is prefixed to the OFDM symbols.
48. A method of transmitting an orthogonal frequency division multiplex (OFDM) communication signal, the method being selectively operable in a plurality of modes, the method comprising:
forming symbols from groups of bits;
performing an inverse Fourier transform on said symbols to produce OFDM symbols; and
selectively switching between a first mode having a guard time T G and an OFDM symbol duration T, and a second mode having a guard time KT G and an OFDM symbol duration KT, where K is a positive integer greater than 1, the first and second modes employing the same set of sub-carriers, which are orthogonal over a time T.
49. The method of transmitting as claimed in claim 48 , further including:
providing N orthogonal sub-carriers; and
mapping N symbols onto the N orthogonal sub-carriers.
50. The method of transmitting as claimed in claim 49 further including:
introducing forward error correction coding into the groups of bits.
51. The method of transmitting as claimed in claim 50 wherein a period of the inverse Fourier transform is the OFDM symbol duration T.
52. The method of transmitting as claimed in claim 48 further including:
performing a digital to analog conversion of the OFDM symbols.
53. The method of transmitting as claimed in claim 52 further including:
transmitting the OFDM symbols at an RF carrier frequency over an antenna.
54. The method of transmitting as claimed in claim 48 further including:
copying a last portion of the OFDM symbol; and
transmitting the copy of the a last portion of the OFDM symbol during the guard time.
55. The method of transmitting as claimed in claim 48 further including:
windowing the OFDM symbol by applying a gradual roll-off pattern.
56. The method of transmitting as claimed in claim 55 further including:
monitoring a quality of an OFDM signal and controlling the windowing responsive to the results of the monitoring.
57. The method of transmitting as claimed in claim 48 further including:
receiving superpositions of the sub-carriers expressing the OFDM symbols; and
deriving a K-fold repetition of each superposition to form another OFDM symbol.
58. The method of transmitting as claimed in claim 48 , wherein, in the second mode, the function provides K−1 copies of an OFDM symbol of length T in order to form an OFDM symbol of length KT.
59. The method of transmitting as claimed in claim 48 , wherein, in the second mode, each OFDM symbol is prefixed with K−1 copies of the OFDM symbol.
60. The method of transmitting as claimed in claim 48 wherein K=2 in the second mode such that the guard time in the second mode is twice the length of the guard time in the first mode.
61. The method of transmitting as claimed in claim 48 wherein the guard time T G is interposed between successive OFDM symbols.
62. The method of transmitting as claimed in claim 48 wherein the guard time T G is prefixed to the OFDM symbols.
63. An orthogonal frequency division multiplex (OFDM) transmitter comprising:
means for transmitting an OFDM signal using a set of sub-carriers which are orthogonal over a time T, symbols being expressed by superpositions of the sub-carriers;
means for selectively operating in one of a plurality of signaling modes in each of which duration of each of the symbols is KT where K is a positive integer and ones of the plurality of signaling modes have different values of K and the same set of sub-carriers; and
means for providing guard times in association with the symbols, guard time lengths being greater for signaling modes having a greater value of K.
64. An orthogonal frequency division multiplex (OFDM) transmitter comprising:
means for forming symbols from groups of bits;
means for performing an inverse Fourier transform on said symbols to produce OFDM symbols; and
means for switching between a first mode having a guard time T G and an OFDM symbol duration T, and a second mode having a guard time KT G and an OFDM symbol duration KT, where K is a positive integer greater than 1, the first and second modes employing the same set of sub-carriers, which are orthogonal over a time T.