Apparatus and method for detecting preambles according to IEEE 802.11A wireless LAN standard
A novel method and apparatus for short preamble detection within the framework of the 802.11a standard is described. The present invention can be easily implemented in both the radio part (preferred embodiment) and the baseband part (general case) of the receiver to provide short preamble detection. The inventive method and apparatus utilize either a notch comb filter to remove the rake of subcarriers composing the short preamble or a battery of bandpass filters to enhance them. A decision on the presence of the short preamble is made by comparing the magnitude associated to certain qualities of the input and output sequences to a preset threshold. The proposed innovation offers a significant advantage over conventional approaches in terms of gate count and power consumption.
1 . A OFDM preamble detector for detecting a 802.11a preamble out from a received analog input signal, comprising:
means for converting the analog input signal into a n-bit digital input signal,
filtering means for either canceling or enhancing the subcarriers included in the digital input signal and for providing an output signal,
evaluating means for evaluating certain qualities/parameters of the digital input and/or output signals, and
decision means for making a decision based on a comparison of the qualities/parameters of the digital input and/or output signals to a preset threshold, and for generating a detection signal if the presence of a 802.11a preamble is detected.
2 . The preamble detector of claim 1 , characterized in that it is implemented in the RF part of a receiver.
3 . The preamble detector of claim 1 , characterized in that it is implemented in the BBP part of a receiver.
4 . The preamble detector of claim 1 , characterized in that the means for converting the analog input signal into a digital signal is a comparator/single bit ADC.
5 . The preamble detector of claim 1 , characterized in that the means for converting the analog input signal into a digital signal is a n-bit ADC.
6 . The preamble detector of claim 1 , characterized in that the filtering means for canceling the subcarriers has either one of the following filter characteristics:
H 40 DCO =1+2 z −1 + . . . +2 z −31 +z −32 ,
H 20 DCO =1+2z −1 + . . . +2z −15 +z −16 , when a DC offset is to be coped with,
Or otherwise,
H 40 =1− z −2 +z −32 +z −34 ,
H 20 =1− z −2 +z −16 +z −18 .
7 . The preamble detector of claim 1 , characterized in that the decision means are arranged for making a decision on the presence of a short preamble in the input signal by comparing the maximum number of consecutives zeros found in the output sequence to a preset threshold.
8 . The preamble detector of claim 1 , characterized in that the filtering means for enhancing the subcarriers has either one of the following filter characteristics:
H
40
=
1
1
+
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-
2
+
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4
+
z
-
6
+
z
-
8
+
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10
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+
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H
20
=
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1
+
z
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14
.
9 . The preamble detector of claim 1 , characterized in that the decision means are arranged for making a decision on the presence of the short preamble by comparing the energy of the input sequence to that of the output sequence multiplied by a preset threshold.
10 . The preamble detector of claim 1 , characterized in that the magnitude of a complex sample is calculated as follows:
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+
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.
11 . A method for OFDM preamble detection according to 802.11a standard from a received analog input signal, comprising the steps of:
converting the analog input signal into a n-bit digital input signal,
filtering the digital input signal by either canceling or enhancing the subcarriers included in the digital input signal and providing an output signal,
evaluating certain qualities/parameters of the digital input and/or output signals,
making a decision based on a comparison of the qualities/parameters of the digital input and/or output signals to a preset threshold, and generating a detection signal if the presence of a 802.11a preamble is detected.
12 . The method of claim 12 , characterized in that the digital input signal is a sequence of 1-bit complex samples (I & Q) delivered by a pair of comparators/single bit ADCs.
13 . The method of claim 12 , characterized in that the input signal to be processed is a sequence of n-bit complex samples (I & Q) delivered by a pair of n-bit ADCs.
14 . The method of claim 12 , characterized in that the decision on the presence of a short preamble in the input signal is made by comparing the maximum number of consecutives zeros found in the output sequence to a preset threshold.
15 . The method of claim 12 , characterized in that the decision on the presence of the short preamble is made by comparing the energy of the input sequence to that of the output sequence multiplied by a preset threshold.
16 . The preamble detector of claim 2 , characterized in that the means for converting the analog input signal into a digital signal is a comparator/single bit ADC.
17 . The preamble detector of claim 3 , characterized in that the means for converting the analog input signal into a digital signal is a comparator/single bit ADC.
18 . The preamble detector of claim 2 , characterized in that the means for converting the analog input signal into a digital signal is a n-bit ADC.
19 . The preamble detector of claim 3 , characterized in that the means for converting the analog input signal into a digital signal is a n-bit ADC.
20 . The preamble detector of claim 2 , characterized in that the filtering means for canceling the subcarriers has either one of the following filter characteristics:
H 40 DCO =1+2 z −1 + . . . +2 z −31 +z −32 ,
H 20 DOC =1+2z −1 + . . . 2z −15 +z −16 , when a DC offset is to be coped with,
Or otherwise,
H 40 =1− z −2 +z −32 +z −34 ,
H 20 =1 −z −2 +z −16 z −18 .