Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Analysis of complex waves
Reexamination Certificate
2001-11-26
2003-07-22
Le, N. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Analysis of complex waves
C324S076190, C324S076410, C324S076570
Reexamination Certificate
active
06597161
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to signal detectors and methods, for use for example in optical or electrical systems, and methods and apparatus for spectrum analysis.
BACKGROUND OF THE INVENTION
In wavelength-division multiplexed (WDM) optical systems it is useful to detect channel power of channels of an optical signal as it propagates through a communications network. Channel power of individual channels of the WDM optical signal can be measured by de-multiplexing the WDM optical signal and then making a direct measurement but such a technique is expensive. To avoid this, in another approach [G. R. Hill, et al., “A Transport Network Layer Based on Optical Network Elements”, Journal of Lightwave Technology, Vol.11, no. 5/6, pp.667-679, May/June 1993] each channel is modulated with one or more respective dither signal(s) resulting in each channel having a unique tone within its power spectral density, the remaining spectrum being that of the data carrying signal. The channel power for each channel is determined by identifying the respective dither signals and measuring the power of the respective dither signals. Detection of the channel power of individual channels becomes difficult when there are large variances in channel power between channels of the WDM optical signal. More specifically, the power spectrum associated with individual channels of a WDM optical signal may vary over a dynamic range up to 30 dB. Such a large dynamic range is due to, for example, channel add/drop throughout a communications network in which the WDM optical signal propagates with or without wavelength dependent attenuation along an optical fiber or wave-guide. In cases where the power spectral density of a WDM optical signal varies over a large dynamic range, the data spectrum density of more powerful channels may act as noise in the detection of less powerful channels. As such, optical systems using modulation techniques to detect channel power require very powerful DSPs (digital signal processors). These DSPs collect data for long periods of time up to (for example 100 s) for each channel to correctly identify channel power and this results in a long detection latency. The collection of data for such a long period of time requires extensive computations and large memories. The long detection latency effectively results in non-real-time detection of channel power, large memory requirements and a requirement for expensive DSPs. This solution is clearly impractical.
SUMMARY OF THE INVENTION
Provided are a spectrum analyzer, a signal detector and methods for spectrum analysis and for measuring power of one or more channels of an electrical or optical signal. Each channel may carry a unique modulation tone. The spectrum analyzer performs a DFT (discrete Fourier transform) on the signal. Only frequency bands of interest which contain a tone that need to be detected are processed. Higher layers of coherent integrations are performed on the frequency bands of interest which contain a modulation tone with a SNR (signal-to-noise ratio) which does not exceed a minimum threshold suitable for power measurement and thereby require finer resolution. The higher layer coherent integrations are performed by collecting additional data and performing a coherent integration. Further higher layers of coherent integrations are performed until all tones have been detected with a SNR exceeding the minimum threshold or a maximum detection latency has been reached. Processing only frequency bands of interest and performing higher layers of coherent integrations on only those bands of interest requiring a finer resolution provides a variable detection latency and efficient use of memory and computations thus allowing power measurements to be performed in real-time.
In accordance with a first broad aspect, the invention provides a method of performing a spectrum analysis. DFTs are performed upon a sequence of time domain measurements. The DFTs produce frequency domain samples associated with respective frequency bands. At least one higher layer of coherent integrations is then performed for at least one frequency sub-band of at least one of the respective frequency bands.
In some embodiments, the DFTs may be evaluated using a FFT (fast Fourier transform) algorithm. In such embodiments, of the respective frequency bands, only frequency bands of interest which carry a respective tone that requires detection may be monitored.
In some embodiments, frequency domain samples may be produced only for frequency bands of interest, of the respective frequency bands, which carry a respective tone that requires detection. A higher layer of coherent integrations may be performed within a layer j wherein j≧2. Within layer j a number R
j
of frequency domain samples within a previous layer j−1 having identical center frequencies, f
cj−1,s
, may be coherently integrated. The frequency domain samples within the previous layer j−1 may be frequency domain samples of a frequency band or sub-band, s, of frequency bandwidth, &Dgr;f
j−1
, within layer j−1. The frequency domain samples within the previous layer j−1 may be coherently integrated to produce frequency domain samples, within layer j, each having an associated frequency sub-band, t, of frequency bandwidth, &Dgr;f
j
=&Dgr;f
j−1
/R
j
. In some embodiments, at least one of the frequency domain samples within the previous layer j−1 may be obtained from at least one additional sequence of time domain measurements. Furthermore, the at least one additional sequence of time domain measurements may be collected at a particular time interval. This time interval may allow the frequency domain samples within the previous layer j−1 to be coherently integrated without having to apply a global phase shift to synchronize the frequency domain samples within said previous layer j−1. In some embodiments, when being coherently integrated within the layer j, the frequency domain samples within the previous layer j−1 may be synchronized using a twiddle factor, W
&phgr;
gj
(r)
=e
−j&phgr;(r)
, wherein &phgr;
gj
(r) is a global phase shift. Furthermore, the global phase shift may satisfy &phgr;
gj
(r)=2&pgr;f
cj−1.s
&Dgr;t
r
wherein &Dgr;t
r
may be a time interval between sampling of respective sequences, i and i+r, of time domain measurements associated with the frequency domain samples within the previous layer j−1. The respective sequences, i and i+r, of time domain measurements may be sampled in a manner that the time interval, &Dgr;t
r
, may be an integral multiple of rN/f
s
wherein N may be a number of time domain measurements within each one of the sequences, i and i+r, of time domain measurements. f
s
may be a sampling frequency of the time domain measurements.
In some embodiments a local phase shift may be applied to the frequency domain samples within the previous layer j−1. This may be done to allow the frequency domain samples within the previous layer j−1 to be coherently integrated at center frequencies, f
cj,t
, different from the center frequencies, f
cj−1,s
. In such embodiments, the center frequencies, f
cj,t
, may be center frequencies of the respective frequency sub-bands, t. As such the respective frequency sub-bands, t, which may be within said layer j, may be monitored. Furthermore, within the layer j, only frequency sub-bands within a subset of the respective frequency sub-bands, t, may be monitored.
The frequency domain samples within said previous layer j−1, may be coherently integrated using a twiddle factor, W
&phgr;
tj
(t)
=e
−j&phgr;
tj
(t)
wherein &phgr;
tj
(t) may be a local phase shift. Incorporation of the twiddle factor may allow the respective frequency sub-bands, t, which may be within the layer, j, to be monitored. Furthermore, within the layer j, the local phase shift, &phgr;
lj
(t), may satisfy &phgr;
lj
(t)=2&pgr;t&Dgr;f
j
.
In some embodiments, for each one of the frequency
Chimfwembe Patrick Chilufya
Jin Dongxing
Remedios Derrick
Wan Ping Wai
Donnelly Victoria
Le N.
Teresinski John
Tropic Networks Inc.
LandOfFree
Method and apparatus for spectrum analysis with variable... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for spectrum analysis with variable..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for spectrum analysis with variable... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3099099