OFDM data demodulators synchronization
View Patent ↗Orthogonal frequency division multiplexing (OFDM) receiver embodiments of the invention provide data demodulator synchronization by finding the end of the short preamble in an IEEE-802.11a burst transmission. This method exploits the fact that there are certain symmetries in the long-preamble that can be used to determine synchronization. The long-preamble sequence is composed of a guard interval (GI) and two long-preamble symbols; the GI is the last 32 samples of the long-preamble symbol. The 32 nd sample of the long-preamble acts as a “pilot” or “anchor” sample in that the previous N and subsequent N samples are complex conjugates, or conjugate “mirror” vectors. Due to the periodicities of the long-preamble, this property repeats every 32 samples. No other samples in the long preamble exhibit this property. Coherent combining is used in one embodiment for robustness. Once this “pilot” or “anchor” sample is located, the end of the short-preamble is declared to have occurred 32 samples earlier, thus establishing a time reference.
1. A synchronization method for data demodulation in an OFDM radio receiver, comprising the steps of:
sampling and measuring an OFDM radio transmission to obtain a series of received-signal samples that represent a short preamble, a long preamble, a long preamble guard interval (GI), and a first long preamble, respectively;
computing an inner product of vectors comprised of samples x(n), x(n−1), . . . , x(n−1−N) and x(n), x(n+1), . . . , x(n+N−1) of said received-signal samples and computing the magnitude squared of a coherent and non-coherent combination determined by:
y
(
n
)
=
∑
k
=
0
N
-
1
{
x
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n
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k
)
+
x
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n
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k
+
64
)
}
{
x
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n
+
k
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+
x
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n
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k
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64
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}
+
∑
k
=
0
N
-
1
x
(
n
+
32
-
k
)
x
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32
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k
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2
assuming an index of the maximum of the result of the combination is the index of the start of the first long preamble;
subtracting a corresponding number of samples from the result of the combination to find a first received-signal sample of said long preamble guard interval (GI); and
identifying said first received-signal sample of said long preamble GI to synchronize any data demodulation of subsequent parts of said OFDM radio transmission,
wherein n is an integer identifying the n'th sample, x(n), of the vectors, N is an integer having a value determining the number of samples in the vectors, and k is an integer having a value varying from 0 to N−1.
2. A synchronization method for data demodulation in an OFDM radio receiver, comprising the steps of:
sampling and measuring an OFDM radio transmission to obtain a series of received-signal samples that represent a short preamble, a long preamble guard interval (GI), and a first long preamble, respectively;
mathematically manipulating the series of received-signal samples around x(n+32) as to achieve a coherent copy of the samples around x(n) and x(n+64) as follows:
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[
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x
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n
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32
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flipud
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conj
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n
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32
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32
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conj
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z
->
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2
assuming an index of the maximum of the result of the manipulating step is the index of the start of the first long preamble;
subtracting a corresponding number of samples from the result of the manipulating step to find a first received-signal sample of said long preamble guard interval (GI); and
identifying said first received-signal sample of said long preamble GI to synchronize any data demodulation of subsequent parts of said OFDM radio transmission,
wherein n is an integer identifying the n'th sample, x(n), of the vectors, N is an integer having a value determining the number of samples in the vectors, and k is an integer having a value varying from 0 to N−1.
3. A synchronization method for data demodulation in an OFDM receiver, comprising the steps of:
sampling and measuring OFDM radio transmission to obtain a series of received-signal samples that represent a short preamble, a long preamble guard interval (GI), and a first long preamble, respectively;
adding and collecting the vector inner products of the previous and subsequent samples starting at x(n) and x(n+32) and adding and collecting samples at x(n+64+32) as follows:
y
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n
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=
∑
k
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0
N
-
1
x
(
n
-
k
)
x
(
n
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k
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+
∑
k
=
0
N
-
1
x
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n
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32
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k
)
x
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+
32
+
k
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2
assuming an index of the maximum of the result of the adding and collecting step is the index of the start of the first long preamble;
subtracting a corresponding number of samples from the result of the adding and collegting step to find a first received-signal sample of said long preamble guard interval (GI); and
identifying said first received-signal sample of said long preamble GI to synchronize any data demodulation of subsequent parts of said OFDM radio transmission,
wherein n is an integer identifying the n'th sample, x(n), of the vectors, N is an integer having a value determining the number of samples in the vectors, and k is an integer having a value varying from 0 to N−1.