Frequency division multiplexing system with selectable rate
First Claim
1. A receiver for receiving an orthogonal frequency division multiplex (OFDM) communications signal, the ODFM signal being transmitted using a set of subcarriers which are orthogonal over a time T, the receiver comprising:
 an analogtodigital converter; and
a fast Fourier transform (FFT) block operably coupled to the analogtodigital converter;
the receiver being configured to operate in one of a plurality of signaling modes in each of which the duration of a received symbol is KT where K is a positive integer and ones of the plurality of signaling modes have different values of K, but the same set of subcarriers;
wherein a guard time is associated with the received signal, the length of the guard time being greater for signaling modes having a greater value of K.
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Abstract
An OFDM system uses a normal mode which has a symbol length T, a guard time TG and a set of N subcarriers, 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 subcarriers is used for the fallback modes as for the normal mode. Since the same set of subcarriers 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.
76 Citations
42 Claims

1. A receiver for receiving an orthogonal frequency division multiplex (OFDM) communications signal, the ODFM signal being transmitted using a set of subcarriers which are orthogonal over a time T, the receiver comprising:

an analogtodigital converter; and
a fast Fourier transform (FFT) block operably coupled to the analogtodigital converter;
the receiver being configured to operate in one of a plurality of signaling modes in each of which the duration of a received symbol is KT where K is a positive integer and ones of the plurality of signaling modes have different values of K, but the same set of subcarriers;
wherein a guard time is associated with the received signal, the length of the guard time being greater for signaling modes having a greater value of K.  View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)


15. A receiver for receiving an orthogonal frequency division multiplex (OFDM) communications signal, the ODFM signal being transmitted using a set of subcarriers which are orthogonal over a time T, the receiver comprising:

a filter operative to average an input signal over K intervals, each interval having a duration T; and
a fast Fourier transform (FFT) block operably coupled to the filter;
the receiver being configured to operate in one of a plurality of signaling modes in each of which the duration of a received symbol is KT where K is a positive integer and ones of the plurality of signaling modes have different values of K;
wherein the FFT block is arranged in a signal path of the receiver ahead of the filter such that the filter averages K outputs from the FFT block.  View Dependent Claims (16)


17. A method for receiving an orthogonal frequency division multiplexed (OFDM) signal, the OFDM signal being transmitted using a set of subcarriers which are orthogonal over a time T, the method comprising the steps of:

determining an average of at least part of the OFDM signal over K intervals, each interval having a duration T; and
performing a fast Fourier transform (FFT) on the determined average;
wherein the OFDM signal is characterized by a plurality of signaling modes in each of which the duration of each OFDM symbol is KT where K is a positive integer and ones of the plurality of signaling modes have different values of K, but the same set of subcarriers, and further wherein a guard time is associated with the OFDM symbol and the length of the guard time is greater for signaling modes having a greater value of K.  View Dependent Claims (18, 19, 20, 21, 22, 23, 24, 25)


26. A receiver for receiving an orthogonal frequency division multiplex (OFDM) communications signal, the ODFM signal being transmitted using a set of subcarriers which are orthogonal over a time T, the receiver comprising:

a filter operative to average an input signal over K intervals, each interval having a duration T; and
a fast Fourier transform (FFT) block operably coupled to the filter;
the receiver being configured to operate in one of a plurality of signaling modes in each of which the duration of a received symbol is KT where K is a positive integer and ones of the plurality of signaling modes have different values of K;
wherein each of the received symbols is associated with a guard time, the length of the guard time being greater for signaling modes with a greater value of K.  View Dependent Claims (27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41)


42. A receiver for receiving an orthogonal frequency division multiplex (OFDM) communications signal, the ODFM signal being transmitted using a set of subcarriers which are orthogonal over a time T, the receiver comprising:

a filter operative to average an input signal over K intervals, each interval having a duration T; and
a fast Fourier transform (FFT) block operably coupled to an output of the filter;
the receiver being configured to operate in one of a plurality of signaling modes in each of which the duration of a received symbol is KT where K is a positive integer and ones of the plurality of signaling modes have different values of K wherein the filter averages across K parts of the received symbol, each of the K parts having a duration of T, to generate an average result, and wherein the FFT block determines a fast Fourier transform based upon the filter average result.

1 Specification