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Signal sequence detection techniques for ofdm/ofdma systems
Title: Signal sequence detection techniques for ofdm/ofdma systems.Abstract: Energy in a frequency band is received at a wireless communication device and data is generated representing samples of a received time domain waveform from the received energy. Data for groups of samples of the received time domain waveform is processed to transform the data for the received time domain waveform to produce data for an intermediate domain signal that is in neither the time domain nor the frequency domain. The data representing the intermediate domain signal is analyzed to determine whether a sequence having a predetermined pattern from a set of possible sequences is present in the received energy, and ultimately to determine a sequence of the predetermined pattern whose presence is detected in the received energy. ...
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The Patent Description & Claims data below is from USPTO Patent Application , Signal sequence detection techniques for ofdm/ofdma systems.
TECHNICAL FIELD
The present disclosure relates to wireless communication devices and systems, and more particularly to detecting a signal with predetermined patterns in wireless transmissions.
BACKGROUND
In orthogonal frequency division multiplexed (OFDM) and orthogonal frequency division multiple access (OFDMA) systems, a remote client station achieves its synchronization status with respect to a base station by detecting and synchronizing to a training sequence, also known as a preamble. Usually appearing at the beginning of transmission frame, the preamble is a particular pseudo-noise (PN) sequence among a set of PN sequences known a priori to the client station.
In some OFDM-based broadband wireless technologies, preamble has some frequency or time properties that can facilitate its detection in a client station. For example, in the IEEE 802.16d/e standard, known commercially as WiMAX™, preamble occupies every third tone or subcarrier, leaving other tones unused. In the frequency domain, this property makes the representation of the preamble in the time domain have repetitive characteristics.
There is room for improving the efficiency of detecting the sequence of a wireless signal, whether a preamble sequence, ranging sequence, sounding sequence or other sequence having a predetermined pattern.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a wireless communication network in which wireless client devices are configured to detect a preamble in signals transmitted by base stations according to the techniques described herein.
FIG. 2 is a diagram showing a transmission frame and examples of sequences detected according to the techniques described herein.
FIG. 3 is a block diagram of a wireless client device configured to detect a sequence pattern using intermediate domain detection and synchronization process logic.
FIG. 4 is a flow chart generally depicting the intermediate domain detection and synchronization process logic.
FIG. 5 is a flow chart depicting process logic for transforming a time domain waveform to an intermediate domain signal as part of the intermediate domain detection and synchronization process logic.
FIG. 6 is a block diagram illustrating an architecture for Fast Fourier Transform (FFT) processing of the time domain waveform to produce the intermediate domain signal.
FIG. 7 is a flow chart depicting process logic for determining the sequence from the intermediate domain signal as part of the intermediate domain detection and synchronization process logic.
FIG. 8 is a flow chart illustrating process logic detecting presence of a predetermined pattern in the intermediate domain signal as part of the intermediate domain detection and synchronization process logic.
FIG. 9 is a flow chart illustrating in more detail process logic for determining the sequence from the intermediate domain signal as part of the intermediate domain detection and synchronization process logic.
FIG. 10 is a flow chart illustrating process logic for detecting the presence of a pattern across multiple frequency adjusted receive signals using multiple frequency offsets.
FIG. 11 is a flow chart illustrating process logic for determining the sequence across multiple frequency adjusted receive signals using multiple frequency offsets.
DESCRIPTION OF EXAMPLE EMBODIMENTS
Energy in a frequency band is received at a wireless communication device and data is generated representing samples of a received time domain waveform from the received energy. Data for groups of samples of the received time domain waveform is processed to transform the data for the received time domain waveform to produce data for an intermediate domain signal that is in neither the time domain nor the frequency domain. The data representing the intermediate domain signal is analyzed to determine whether a sequence having a predetermined pattern from a set of possible sequences is present in the received energy, and ultimately to determine a sequence of the predetermined pattern whose presence is detected in the received energy. The techniques described herein may be employed to detect the presence of a sequence having a predetermined pattern, such as a preamble sequence, a ranging signal sequence, a sounding signal sequence or any portion part of a wireless signal with structure similar to that of a preamble. Thus, while the foregoing description refers, in some cases, specifically to detecting a preamble pattern and a pseudo-noise preamble sequence, this is meant by way of example only.
Example Embodiments
Referring first to FIG. 1, a wireless communication network is shown at reference numeral 5. The network 5 is a multi-cell network comprising a plurality of wireless base stations, and in this example, three base stations 10(1), 10(2) and 10(3), each of which serves a corresponding cell or coverage area where client devices are located. For example, base station 10(1) serves Cell 1 where (in this simplified example) there are two client devices 20(1) and 20(2), base station 10(2) serves Cell 2 where there are two client devices 20(3) and 20(4) and base station 10(3) serves Cell 3 where there are two client devices 20(5) and 20(6). It should be understood that each base station may serve many more client devices in its coverage area. The base stations 10(1)-10(3) may serve as a gateway for client devices to another network, i.e., the Internet.
The base stations 10(1)-10(3) wirelessly transmit frames in their respective cells, and each frame includes a preamble portion that the client devices use to synchronize to the transmissions of the base station for a given cell. In the case of adjacent cells, such as that shown in FIG. 1, frequency reuse techniques are necessary to minimize inter-cellular interference. The preamble detection techniques described herein can be used for a variety of frequency reuse configurations, even a frequency reuse “one” (N=1) configuration.
Turning now to FIG. 2, examples of preamble sequences are shown. FIG. 2 shows an example of a transmission frame 50 comprising a preamble field 60 and a traffic field 70. This is a generalized depiction of a transmission frame. FIG. 2 shows two preamble sequences for the preamble field 60. Preamble sequence (a) is a generalized preamble sequence whereas preamble sequence (b) is a specific example of the preamble sequence used in the WiMAX standard. If multiple base stations are deployed in an area (e.g., such as that shown in FIG. 1) and they may be using the same frequency band, each base station may be configured to transmit its preamble as shown by sequence (a) in FIG. 2. For example, one base station places its preamble at the indices MM+R,M+2R, . . . ,M+kR, . . . another base station places its preamble at the indices M+1,M+R+1,M+2R+1, . . . ,M+kR+1, . . . and so on, where M is the margin. With such a structure, up to R base stations can place their preamble in the same frequency band without interfering each other. Two base stations may place their preambles on the same tones (subcarriers).
In the preamble sequence (b) shown in FIG. 2, the pseudo-noise (PN) code elements are placed every third tone, i.e., there are two tones with zero value between two consecutive placements of PN code elements. Thus, every third tone of the preamble OFDM signal is a bipolar bit of a PN sequence and the rest of the tones are unused: P(3n)=PN(n) and P(3n+1)=0, P(3n+2)=0, where P(.) is the preamble signal representation in the frequency domain whose size is the size of a Fast Fourier Transform (FFT) block. Some margins of the preamble signal might be left unused (i.e., with zero value in frequency domain) for other reasons which are not related to this topic.
The size of the FFT block used at the transmitting device, that is, the device that generates and sends the transmission with the predetermined pattern, is denoted as F. To generalize the underlying principles of the techniques described herein, the predetermined pattern (e.g., preamble pattern) is assumed to have a repetition rate of R in the frequency domain, which means there are R-1 unused tones between each two used tones in the frequency representation shown in FIG. 2. For now it is assumed that R divides F, but this is not a requirement and a more general case is explained hereinafter. The preamble signal in frequency domain is denoted P(.) and in time domain it is denoted by p(.), and the PN code by PN(.). The PN code is selected at the base station from a set of PN sequences. The client devices know the set of possible PN sequences from which the base station may choose, but they do not know the exact PN code used by the base station for any given transmission frame. The following relationship holds between p(.) and P(.) based on an FFT transform:
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Optical band gap and magnetic properties of unstrained
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Appl. Phys. Lett. 94, 212509 (Mon May 25 00:00:00 UTC 2009);
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Phase-pure, stoichiometric, unstrained, epitaxial (001)-oriented thin films have been grown on (001)
substrates by reactive molecular-beam epitaxy. Magnetization measurements show antiferromagnetic behavior with , similar to bulk . Spectroscopic ellipsometry measurements reveal that films have a direct optical band gap of .
Received Mon Feb 23 00:00:00 UTC 2009
Accepted Mon Apr 20 00:00:00 UTC 2009
Published online Thu May 28 00:00:00 UTC 2009
Acknowledgments:
We gratefully acknowledge the financial support from the National Science Foundation through Grant No. DMR-0507146 and the MRSEC Program (Grant Nos. DMR-0520404 and DMR-0820404). Work at the Advanced Photon Source is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
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