Source: http://www.google.com/patents/US8159932?dq=%22robert+sheehan%22
Timestamp: 2014-09-30 14:31:50
Document Index: 178640469

Matched Legal Cases: ['art 11', 'art 11', 'art 11', 'art 11', 'art 1', 'art 2']

Patent US8159932 - Initial timing estimation in a wireless network receiver - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign in<nobr>Advanced Patent Search</nobr>PatentsAn apparatus for and a method of wirelessly receiving a signal, and detecting a start of packet (SOP) from the received signal using at least one SOP detection criterion. In the case an SOP is detected, determining a plurality of metrics from the received signal, and using at least two of the plurality...http://www.google.com/patents/US8159932?utm_source=gb-gplus-sharePatent US8159932 - Initial timing estimation in a wireless network receiverAdvanced Patent SearchPublication numberUS8159932 B1Publication typeGrantApplication numberUS 12/346,637Publication dateApr 17, 2012Filing dateDec 30, 2008Priority dateOct 31, 2003Also published asUS7480234Publication number12346637, 346637, US 8159932 B1, US 8159932B1, US-B1-8159932, US8159932 B1, US8159932B1InventorsBrian Hart, Milind D. Paranjpe, John D. O'SullivanOriginal AssigneeCisco Technology, Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (37), Non-Patent Citations (27), Classifications (7) External Links: USPTO, USPTO Assignment, EspacenetInitial timing estimation in a wireless network receiverUS 8159932 B1Abstract An apparatus for and a method of wirelessly receiving a signal, and detecting a start of packet (SOP) from the received signal using at least one SOP detection criterion. In the case an SOP is detected, determining a plurality of metrics from the received signal, and using at least two of the plurality of metrics to determine an initial timing for a received packet. Different versions combine the metrics in different ways to determine the initial timing. The apparatus includes a processing circuit coupled to a radio receiver to determine a plurality of metrics from a wirelessly received signal, and an initial time determining circuit coupled to the SOP detector and the processing circuit. In the case an SOP is detected, the initial time determining circuit uses at least two of the plurality of metrics to determine an initial timing for a received packet.
wherein a packet according the wireless networking standard includes a preamble, and wherein detecting the SOP includes:
determining a logical function of at least two of a set of logical indicators, the set of logical indicators including:
that a threshold was exceeded by a weighted sum of the measure of the average received signal power and the measure of the correlation of the input signal with the known part of the preamble.
2. A method as recited in claim 1, wherein the measure of the correlation quality is a measure of the correlation power normalized by the power of the received signal.
3. A method as recited in claim 1, wherein the wireless networking standard is one of a set of one or more wireless networking standards, wherein a packet according each of the wireless networking standards of the set of wireless networking standards includes a corresponding preamble, and wherein detecting the SOP includes:
for each wireless networking standard of the set of wireless networking standards, determining a corresponding logical function of at least one of the set of logical indicators, and
determining the OR of the one or more the corresponding logical functions.
4. A method as recited in claim 1, wherein the standard is one of the IEEE 802.11 standards.
5. A method as recited in claim 1, wherein the measure of the correlation quality is a comparison measure of the instantaneous correlation power with the average correlation power in the recent past.
6. A method as recited in claim 1, wherein a packet according to the wireless networking standard includes a preamble that has a first part that includes a series of periodic symbols and a second part, further comprising:
determining a measure of the correlation of the input signal with at least one of the periodic symbols to provide a measure of the correlation of the input signal with a known part of the preamble.
7. A method as recited in claim 6, wherein the wireless networking standard is an OFDM standard according to which the first part of the preamble includes a periodic series of short symbols and the second part includes long symbols and a guard interval.
8. A method as recited in claim 1, wherein determining that a threshold was exceeded by the average power rise of the received signal includes computing the ratio of the received power during the presence of a packet and the received signal power before the packet arrived.
9. A method as recited in claim 1, wherein a packet according to the wireless networking standard includes a preamble that has a first part that includes a series of periodic symbols and a second part, and wherein determining the plurality of metrics includes determining at least two of the set of metrics that comprises:
a measure of the carrier to noise ratio (CNR) of the received signal;
a measure of the correlation of the input signal with at least one of the periodic symbols; and
whether or not a measure of the carrier to noise ratio (CNR) of the received signal is within a CNR range;
that a threshold was exceeded by a measure of the correlation of the input signal with at least one of the periodic symbols; and
the time a measure of the correlation of the input signal with at least one of the periodic symbols peaks to indicate an SOP time; and
the time a measure of the correlation of the input signal with at least one of the periodic symbols changes to indicate the time of the end of the series of periodic symbols; and
the time a measure of the correlation of the input signal with the start of a second part of the preamble peaks to indicate the time of the start of the second part.
a radio receiver configured to receive a signal and output a received signal;
a start of packet (SOP) detector coupled to the radio receiver and configured to detect an SOP from a received signal using at least one SOP detection criterion for a packet that conforms to a wireless networking standard, wherein the SOP detector is configured to determine a measure of the average power rise of the received signal;
an initial time determining circuit coupled to the SOP detector and the processing circuit, the initial time determining circuit using at least two of the plurality of metrics and configured to determine an initial timing for a received packet in the case an SOP is detected,
wherein a packet according the wireless networking standard includes a preamble, and wherein the SOP detector detects by one of the set of SOP methods that comprises:
detecting a rise in the average received signal power;
detecting a rise in the average power rise of the received signal;
detecting a rise in a measure of the quality of the correlation of the input signal with a known part of the preamble; and
11. An apparatus as recited in claim 10, wherein the SOP detector detects an SOP using different sets of at least one SOP criterion for each of a set of at least one wireless networking standard that an arriving packet may conform to,
wherein a packet according each of the at least one wireless networking standard includes a preamble, and wherein the SOP detector detects a packet by detecting whether any logical function corresponding to any of the at least one standard is true:
each corresponding function being of at least one of a set of logical indicators for each standard, each set of logical indicators comprising:
that a threshold was exceeded by the average power rise of the received signal; and
that a threshold was exceeded by a measure of the quality of the correlation of the input signal with a known part of the preamble.
12. An apparatus as recited in claim 10, wherein the SOP detector detects an SOP using different sets of at least one SOP criterion for each of a set of at least one wireless networking standard that an arriving packet may conform to,
13. An apparatus as recited in claim 10, wherein a packet according to the wireless networking standard includes a preamble that has a first part that includes a series of periodic symbols and a second part, and wherein the processing circuit includes at least two of the set that comprises:
a circuit configured to determine a measure of the carrier to noise ratio (CNR) of the received signal;
a circuit configured to determine a measure of a rise in the received signal power;
an autocorrelation circuit configured to determine a measure of the autocorrelation of the input signal at the period of the symbols;
a symbol correlation circuit configured to determine a measure the correlation of the input signal with at least one of the periodic symbols; and
a second correlation circuit configured to determine a measure of the correlation of the input signal with the start of the second part of the preamble,
and wherein the initial timing determining circuit determines the initial timing from at least one of:
the time a measure of the auto correlation of the input signal at the period of the symbols changes to indicate the time of the end of the series of periodic symbols;
14. An apparatus as recited in claim 13, wherein the wireless networking standard is an OFDM standard according to which the first part of the preamble includes a periodic series of short symbols and the second part includes long symbols and a guard interval.
means for detecting a start of packet (SOP) from the received signal using at least one SOP detection criterion for a packet that conforms to a wireless networking standard;
means for determining an initial timing for a received packet in the case an SOP is detected by the means for detecting the SOP, the means for determining an initial timing using at least two of the plurality of metrics to determine,
wherein a packet according the wireless networking standard includes a preamble, and wherein the means for detecting the SOP determines a logical function of at least two of a set of logical indicators, the set of logical indicators including:
16. An apparatus as recited in claim 15, wherein the measure of the correlation quality is a measure of the correlation power normalized by the power of the received signal.
17. An apparatus as recited in claim 15, wherein the measure of the correlation quality is a comparison measure of the instantaneous correlation power with the average correlation power in the recent past.
18. An apparatus as recited in claim 15, wherein a packet according to the wireless networking standard includes a preamble that has a first part that includes a series of periodic symbols and a second part, the apparatus further comprising:
means for determining a measure of the correlation of the input signal with at least one of the periodic symbols to provide a measure of the correlation of the input signal with a known part of the preamble.
19. An apparatus as recited in claim 15, wherein in the case the two of the set of logical indicators includes that a threshold was exceeded by the average power rise of the received signal, the means for determining the SOP computes the ratio of the received power during the presence of a packet and the received signal power before the packet arrives to determine whether a threshold was exceeded by the average power rise of the received signal.
20. A non-transitory computer readable storage medium containing computer executable code that when executed by at least one processor causes carrying out of a method comprising, for a wirelessly received signal:
that a threshold was exceeded by a weighted sum of the measure of the average received signal power and the measure of the correlation of the input signal with the known part of the preamble. Description
RELATED APPLICATIONS This invention is a continuation of U.S. patent application Ser. No. 10/698,703 to Hart et al., filed Oct. 31, 2003, now U.S. Pat. No. 7,480,234. The contents of such U.S. application are incorporated herein by reference.
In one embodiment, determining of the initial timing includes detecting whether or not a measure of the CNR is in a CNR range wherein a first metric of the set of metrics is expected to be effective, and using the first metric for the initial timing determining only if it is detected that the measure of the CNR is in the CNR range. For OFDM variants of the IEEE 802.11 standard, the first metric is the measure of autocorrelation, and determining the initial timing uses that a change was detected in the auto correlation, e.g., by detecting that a range being reached by the autocorrelation measure only if it is detected that the measure of the CNR is in the CNR range.
The invention will be described herein in terms of a WLAN station that operated according to OFDM variants of the IEEE 802.11 standard and proposed amendments. One receiver embodiment supports the IEEE 802.11g and 11a variants operating in the 2.4 GHz and 5 GHz frequency ranges, respectively. The invention is also applicable to a radio that operates under other wireless standard for which accurate timing determination is important, including other variants of the IEEE 802.11 standard.
The preamble 201 is 16 μs long and has two 8 μs parts: a first part (�short preamble part�) consisting of set of 10 short symbols 202, and a second part (�long preamble part�) consisting of two long symbols 207 and 209, and a cyclic extension part (guard interval) 205. In a typical system, the short preamble part provides for the SOP detection, AGC, diversity selection when diversity is used, coarse frequency offset estimation and timing synchronization, while the long preamble part then provides for channel estimation and fine frequency offset estimation.
FIG. 3 shows the modem 111 of FIG. 1 in more detail. The modem 111 is implemented as a single chip and includes a controller 324 that controls the different states of the receiver (receive controller 321) the operation of the automatic gain control circuit (AGC controller 323), and receive/transmit control (Rx/Tx controller 320) for controlling the function of the modem under control of the off-chip MAC controller and that provides status signals to the off-chip MAC controller 119.
The 11a average power rise is constructed by delaying the 11a average power by its averaging length plus 200 ns, i.e. 1000 ns or 1800 ns if use�2sym_for_corr�11a is false or use�2sym_for_corr�11a is true, respectively, using a FIFO 517. A subtractor 519 subtracts the delayed 11a average power from the 11a average power to generate the average power rise rel_rssi_iq�11a_hdb in units of 0.5 dB
To generate the 11a correlation quality signal, the downconverted signals are input to a FIFO 521 that is coupled to a correlator 523 includes a pair of one-symbol correlators. The correlator 523 correlates the input signal at 40 MHz over one short symbol if use�2sym_for_corr�11a is false or two short symbols if use�2sym_for_corr�11a is true. In the latter case the outputs of the two- and one-symbol correlators are added by adder 525 to form the correlation signal. A magnitude circuit 527 computes the correlation magnitude and a logarithmic converter 529 converts the magnitude to a correlation power in a dB scale with 0.5 dB resolution.
In one version, the measure of the correlation quality is a comparison measure of the instantaneous correlation power with the average correlation power in the recent past. In another version, the measure of the correlation quality is a measure of the correlation power normalized by the power of the received signal. FIG. 5 uses the latter. A subtractor 533 calculates the correlation quality corr�11a_hdb as the correlation power normalized by the 11a average power in units of 0.5 dB.
At the same time as the SOP signals are generated, sop_rssi_ic_quiescent_hdb is captured and stored in a register buffer as part of the register set of the modem 111. sop_rssi_iq_quiescent_hdb is the power average delayed by 1000 or 1800 ns obtained from the FIFO 517. In this way, the average signal power is measured well before the packet is detected and nominally immediately before the packet arrives, so sop_rssi_iq_quiescent_hdb should reflect the power of the noise on the medium. sop_rssi_iq_quiescent_hdb is updated whenever the SOP circuit detects an apparent packet.
The initial timing determining method includes determining a plurality of metrics from the received signal, and using at least two of the plurality of metrics to determine an initial timing for a received packet. Different versions combine the metrics in different ways to determine the initial timing.
The RSSI jump at the start of packet. The first peak in the correlation of the received signal with the short symbol. A drop in the correlation peaks of the received signal correlated with the short symbol. A drop in the autocorrelation of the received signal in the short symbol interval as detected, e.g., by an inverse measure of the autocorrelation exceeding a threshold. Detect the transition between the short and long symbol part of the preamble by performing a correlation of the received signal with the first guard interval and a threshold to detect the correlation peak. The first peak in the correlation with the long symbol. Rise in the autocorrelation of the signal in the long symbol part of the preamble, as detected, e.g., by a measure of the autocorrelation exceeding a threshold. A change in the short-term power spectral density from short to long symbols, as indicated by appearance in energy in the subcarriers not used in the short symbols but used in the long symbols; The time shift in the channel impulse response determined during the long symbols that produces minimum interference between OFDM symbols. Each of these is now discussed.
Above-mentioned incorporated by reference U.S. patent application Ser. No. 10/095,668 describes one method of how to use the relatively sudden rise in RSSI at the start of packet as the start of packet trigger, and also how such a measure can be refined.
For a received packet conforming to the IEEE 802-11b standard, the time for the packet energy to rise from 10-to-90% may be as wide as 2 μs. We have found that this is too wide for accurate initial timing determination.
For a received packet conforming to one of the OFDM variants of the IEEE 802-11 standard, e.g., the IEEE 802.11a or 0.11g, there is no parameter specified for the time for the packet energy to rise, but presumably it should be under 2 μs. The inventors decided not to rely on this to be short enough. Using our lab equipment (e.g. a multi-purpose vector signal generator (SMIQ), manufactured by Rhode and Schwarz), at relatively high carrier-to-noise ratio (CNR), the inventors found a pre-packet �pedestal�, where the RSSI level jumps up 1-2 μs before the packet actually begins. They then observed the RSSI jumping up again when the packet truly begins. Note that one embodiment of the AGC of the modem shown in FIG. 3 is described in above referenced incorporated-by-reference U.S. patent application Ser. No. 10/622,175. One version of that AGC method includes the AGC process starting as soon as a rise in RSSI is detected. The inventors decided that there is a very limited window to accurately determine when the RSSI jumped most quickly. The inventors also were reluctant to change the AGC settings to help out initial timing estimation. We therefore decided to use the RSSI rise or a combination of RSSI and correlation-based signals to generate an initial SOP trigger, but also to seek a method of make the accurate initial timing determination that is independent of the RSSI jump at the start of packet.
FIG. 8 shows one embodiment of a circuit that calculates the autocorrelation metric and that determines the autocorrelation indicator and timing. The I,Q input signal after downconverting and subsampling to 20 MHz is used. We found that 4 bits of phase for the phase estimation is adequate, so a measure of the phase is determined with an 16PSK detector 803. An 800 ns (16 sample) delay is used to delay the measure of phase and a comparison circuit 807 is used to obtain a measure of the phase difference with the phase from a symbol period time (16 samples) earlier. Now because the phase change of the PSK detected signal varies differently�albeit monotonically�than does the autocorrelation, in one embodiment the error signal is squared by a squarer 809 to account somewhat for the difference in variation. Alternate embodiments can use another monotonic function. A filter 811 is used to average the noise. The filter has an impulse response summing to zero. The filter 811 shown has a finite impulse response 1, 1, . . . , 1, −1, −1, . . . , −1 (i.e. 16 1's followed by 16 −1's), and is implemented as an infinite impulse response filter using the two delays 812, 813, the coefficient summer 815 and the integrator using summer 817 and one unit delay 818. The filter output starts to rise at the short-to-long symbol transition, peaks 16 samples later, then tends to taper away again.
FIG. 9 shows one embodiment of a circuit that determines the Corr_GI metric and the Corr_GI indicator and timing. The input is the set of complex (I,Q) baseband signals after downconversion and subsampling to 20 MHz. The Corr_GI correlator 905 correlates the input with the known sample values of 1600 ns, the guard interval. In one implementation, correlator 905 is implemented as a 32-tap FIR filter with the coefficients chosen as the time-reversed, complex conjugated 32 samples of the guard interval GI2 quantized to {�1,0}+j{�1,0}, i.e. to values of �1. The correlator 905 includes a scaler (not shown) that scales the output by a settable scale factor. The result is clipped in correlator 905 to a 12-bit value.
Another method of detecting the time of transition from the short symbol preamble to the long symbol preamble is to detect a change in the short-term power spectral density. In the short symbol preamble, only 12 subcarriers are whereas the long symbols exercise 52 subcarriers. Therefore, detecting when energy appears at the 40 new subcarriers provides an indication of short-to-long symbol transition. Determining the subcarriers not in the short symbols includes taking a DFT (using an FFT operation) of the input signal, selecting the 40 new subcarriers that are known to not be in the short symbols, e.g., filtering those that are in the short symbols, and summing the energy in the subcarriers. Alternatively, the filtering/selecting can be eliminated. The method detects when the short term spectral power spectral density exceeds a threshold. How to calculate a measure of the short term spectral power density using the FFT would be is known to those in the art.
A further embodiment includes a pair of FIR or IIR filters, one with notches at the short symbol tone frequencies, and one with multiple passbands at the short symbol tone frequencies. Timing is determined from the rise in power on the first filter and the fall in power on the second filter.
In one embodiment, the peak detection logic for the results of filtering distinguishes peaks due to the GI2, LS1 and LS2.
One embodiment uses simulations. We examined the probability distribution function (PDF), obtained by simulation, of when the maximum value of the metric occurs for the Corr_GI, RSSI change, CorrDrop, and Autocorr methods. The PDFs were examined for the following range of CNRs: 1, 2.5, 5, 10, 20, 30 dB and for the channels that has the following range delay spreads: 0, 50, 100, 150, 250, 600 ns. Since we were more interested in both tails of the PDF rather than the PDFs themselves, we plotted the cumulative distribution functions (CDF) and 1 minus the plotted the cumulative distribution functions (1−CDF) on a logarithmic scale. For a good metric, we would expect the CDF to go from a low to a high value rapidly around a zero timing error, and (1−CDF) to change from a high to a low value rapidly around a zero timing error.
We plotted the CDF and 1−CDF during the false alarm period and during the positive detect period, which for correlation methods is the expected peak period, to check that a threshold can be selected that prevents false alarms without causing too many misses.
RSSI-IQ=ceil(10(log 10(rssi_average)+6)/0.5) for non-zero rssi_averageRSSI-IQ=0 when rssi_average is zero.
As described above, the noise power sop11a_rssi_iq_quiescent is a signal that is recorded by the SOP detector.
While embodiments has been described for operation with a wireless network receiver that operates according to the OFDM variants, e.g., the 802.11a and 802.11g variants of the IEEE 802.11 standard, the invention may be embodied in receivers and transceivers operating in other standards than the IEEE 802.11 OFDM standards, for example other WLAN standards and other wireless standards where receivers that determine the initial timing determination would be beneficial. Applications that can be accommodated include IEEE 802.11 wireless LANs and links, wireless Ethernet, HIPERLAN 2, European Technical Standards Institute (ETSI) broadband radio access network (BRAN), and multimedia mobile access communication (MMAC) systems, wireless local area networks, local multipoint distribution service (LMDS) IF strips, wireless digital video, wireless USB links, wireless IEEE 1394 links, TDMA packet radios, low-cost point-to-point links, voice-over-IP portable �cell phones� (wireless Internet telephones), etc.
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