WO2010120340A1

WO2010120340A1 - Method and apparatus for spectrum sensing of atsc and ntsc television signals

Info

Publication number: WO2010120340A1
Application number: PCT/US2010/000961
Authority: WO
Inventors: Hou-Shin Chen; Wen Gao
Original assignee: Thomson Licensing
Priority date: 2009-04-01
Filing date: 2010-03-31
Publication date: 2010-10-21

Abstract

Methods and apparatus for spectrum sensing for the presence of ATSC digital television signals and NTSC analog television signals in the VHF/UHF TV bands are provided. Similar time-domain signal structures for the ATSC and NTSC signals are used to perform spectrum sensing. For ATSC digital television signals, a Segment-Sync based spectrum sensing method is provided, based upon the correlation of Segment Sync symbols inserted in the beginning of each data segment. Additionally, a general adjacent channel interference alleviation scheme by using a narrower receiver filter is also provided. For NTSC analog TV signals, a horizontal-sync based spectrum sensing method is provided which is based upon the correlation of horizontal sync signals present at the beginning of each line of an NTSC analog TV signal. The proposed spectrum sensors can reliably detect the target signals when a strong adjacent channel interference exists and the signal power is as low as dBm as set in FCC test models.

Description

METHOD AND APPARATUS FOR SPECTRUM SENSING OF ATSC AND NTSC

TELEVISION SIGNALS

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Serial No. 61/211599 , filed April 1 , 2009, which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present principles relate to spectrum sensing for ATSC and NTSC television signals and systems.

BACKGROUND OF THE INVENTION Recently, the Federal Communications Commission (FCC) has approved the operation of unlicensed radio transmitters in the broadcast television spectrum at locations where that spectrum is not being used by licensed services (this unused TV spectrum is often termed "white spaces") under certain rules. A major regulation is that the white space devices will be required to sense, at levels as low as - 1 14 dBm, TV signals (digital and analog), wireless microphone (WM) signals, and signals of other services that operate in the TV bands on intermittent basis. The noise power in a 6 MHz TV channel under normal temperature is about -96 dBm assuming that the noise figure of a sensing device is 1O dB. Thus, the sensing requirement set by FCC is about -18 dB in terms of signal-to-noise power ratio (SNR). Power detection, or energy detection, was widely used to determine the presence of signals without prior knowledge of signals. However, power detectors do not function well when SNR is low [9]. Under low SNR conditions, accurate noise power levels and large number of data samples are needed to achieve good sensing performance. However, an accurate noise power level is hard to reach because it can be affected by several factors, e.g., temperature and system calibration. The lack of knowledge about the noise power is called noise uncertainty. The amount of noise uncertainty can be as large as ± 1 dB. When the noise uncertainty is equal to 1 dB, a power detector fails if the SNR is below -3.3 dB even with a very long sensing time [10]. TV signals (analog and digital) and wireless microphone signals are primary and secondary licensed signals in the TV broadcast bands.

TV stations in United States have now converted from analog to digital transmissions. During the transition to digital transmissions, each full service TV station that was authorized before 1997 was required to broadcast on two channels, one for digital and one for analog. Thus, there were NTSC analog TV signals [5] and ATSC Digital TV (DTV) signals [4] on air in North America. A variety of sensing algorithms for ATSC DTV signals have either been reported in the prior art, or proposed to IEEE 802.22 Group. Some of these claim to be able to achieve the sensing requirement set by the FCC. In addition, several white space prototypes developed by different companies have been tested by the FCC. The FCC test report (FCC, "Evaluation of the Performance of Prototype TV-Band White Space Devices Phase II," OET 08-TR-1005, October 2008) gives indications that all these white space prototypes (spectrum sensors) do not function well in Two-Signal (Adjacent Channel Interference such as an adjacent TV signal) model especially when a strong DTV signal exists in the lower adjacent channel. It is stated in the FCC test report that "although different algorithms are utilized among the prototype devices to detect the presence of a DTV signal, they all appear to share a similarity in that they all sample the spectrum in proximity of an anticipated DTV pilot signal." Because the DTV pilot tone is only 310 kHz away from the edge of the lower adjacent channel as shown in Fig. 3, when there is a strong DTV signal in the lower channel, this pilot tone signal is severely interfered by the adjacent channel interference as shown in Fig. 4. As a result, the white space devices, which utilize the pilot tone signal, fail to determine the availability of channels when strong TV signals exist in the adjacent channels. Spectrum sensing of ATSC DTV signals draws more attention than that of NTSC analog TV signals and wireless microphone signals. Although full service NTSC TV stations are expected to be converted to ATSC TV stations, the digital transition is not required for low power NTSC stations or translators. In addition, wireless microphones have been popularly used in studios and during sports or news gathering events. There are about approximately 35,000 to 70,000 licensed wireless microphone devices operated in United States. Thus, spectrum sensing of the NTSC analog TV signals and wireless microphones is important as well. The FCC report also reveals that the white space devices fail to distinguish the presence of wireless microphone or NTSC signals from interference signals in the Two-Signal model. Spectrum sensing algorithms for ATSC DTV and NTSC analog TV signals are provided by the methods and apparatus described herein.

SUMMARY OF THE INVENTION

These and other drawbacks and disadvantages of the prior art are addressed by the present principles, which are directed to a method and apparatus for spectrum sensing.

According to an aspect of the present principles, there is provided a method for spectrum sensing. The method includes a step of correlating synchronization segment data and accumulatingcorrelation functions. The method further includes a step of generating a decision statistic by finding a maximum value of the correlation functions. The method also includes a step of detecting occupied spectrum space by using the decision statistic.

According to another aspect of the present principles, there is provided another method of performing spectrum sensing. The method includes a step of finding the conjugate product of accumulated segment synchronization correlation functions. The method also includes a step of coherently combining the aforementioned conjugate products for different correlation delays. The method also includes a step of generating a decision statistic by finding the maximum of the sum of weighted conjugate products over a sensing interval. The method also includes a step of detecting occupied spectrum space by using the aforementioned decision statistic. According to another aspect of the present principles, there is provided another method for spectrum sensing. The method includes a step of correlating horizontal synchronization data and accumulating correlation functions. The method further includes a step of generating a decision statistic by finding a maximum value of the correlation functions. The method also includes a step of detecting occupied spectrum space by using the decision statistic.

According to another aspect of the present principles, there is provided another method of performing spectrum sensing. The method includes a step of finding the conjugate product of accumulated horizontal synchronization correlation functions. The method also includes a step of coherently combining the aforementioned conjugate products for different correlation delays. The method also includes a step of generating a decision statistic by finding the maximum of the sum of weighted conjugate products over a sensing interval. The method also includes a step of detecting occupied spectrum space by using the aforementioned decision statistic.

According to another aspect of the present principles, there is provided an apparatus. The apparatus includes a correlator for correlation of two synchronization functions. The apparatus also includes an accumulator for accumulating the correlation functions over a sensing interval and a processor for generating a decision statistic.

The apparatus also includes a detection unit that uses the decision statistic to determine if spectrum space is occupied.

According to another aspect of the present principles, there is provided a method for alleviating interference in an ATSC receiver. The method includes a step of receiving a digital television receiver. The method further includes a step of filtering the received digital television receiver using a receiver filter that is narrower than the channel bandwidth to reduce adjacent channel interference.

These and other aspects, features and advantages of the present principles will become apparent from the following detailed description of exemplary embodiments, which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an ATSC DTV signal data segment. Figure 2 shows the magnitude response of a pure ATSC DTV.

Figure 3 shows the magnitude response of a pure ATSC DTV signal with lower adjacent channel interference.

Figure 4 shows the magnitude response of lower adjacent channel interference. Figure 5 shows the magnitude response of a segment sync signal. Figure 6 shows a burst blanking sequence in M/NTSC systems.

Figure 7 shows the levels in the composite NTSC signal and details of horizontal line synchronization signals.

Figure 8 shows an embodiment of a method for spectrum sensing under the present principles.

Figure 9 shows another embodiment of a method for spectrum sensing under the present principles

Figure 10 shows an apparatus for spectrum sensing under the present principles.

Figure 11 shows a method for alleviating interference in an ATSC receiver.

DETAILED DESCRIPTION OF THE INVENTION An approach for spectrum sensing of both NTSC analog and ATSC digital TV signals is described herein. A Segment-Sync-Based spectrum sensing algorithm for ATSC DTV signal is first described. The method is based on the correlation between Segment Sync symbols inserted in the beginning of each data segment. A general adjacent channel interference alleviation scheme using a narrower receiver filter is also described. The interference alleviation scheme can be applied for spectrum sensing of different signals e.g., ATSC DTV, NTSC analog TV, OFDM signals, provided that the feature used for spectrum is a wide band signal. For example, the Segment Sync is a time-domain feature of the ATSC signal and the energy of the Segment-Sync signal is spread over the whole 6 MHz TV channel. Thus, if we use a narrower receiver filter instead of the original 6 MHz receiver filter, although the Segment-Sync signal is distorted a little bit, the interference power is reduced significantly. And although the filter is narrow compared to the original 6 MHz receiver filter, it is still a wide band filter which does not result in large complexity. By using the Segment-Sync-Based method and a narrower receiver filter, the spectrum sensing methods described herein are robust against the adjacent channel interference and can achieve the sensing requirement specified by the FCC.

Herein we describe an unified spectrum sensing approach for both NTSC analog and ATSC digital TV signals.

Segment-Sync-Based Spectrum Sensing First, we briefly describe the structure of ATSC DTV signals [4]. ATSC DTV signals consist of consecutive data segments as shown in Fig. 1. A complete data segment has 832 symbols: four symbols for data segment SYNC, and 828 data symbols. The two-level data segment SYNC employs a 1001 pattern and the data symbols are eight-level PAM (8-PAM) symbols. Vestigial Sideband (VSB) modulation is applied to improve bandwidth efficiency. An 8-PAM with VSB modulation is also called an 8-VSB modulation. Note that before VSB modulation, a constant of 1.25 is added to each symbol for the purpose of creating a small pilot carrier. As mentioned before, this pilot carrier is widely used to perform spectrum sensing in variety of approaches. However, methods utilizing this pilot carrier severely suffer from adjacent channel interference. Thus, instead of utilizing the pilot carrier, we use data segment SYNC to perform spectrum sensing.

There are two reasons to utilize data segment SYNC for spectrum sensing. First, as shown in Fig. 5, the signal power of the data segment SYNC spreads over the whole 6 MHz TV channel so that unlike the pilot carrier, the data segment SYNC signal will not be fully shaded by the adjacent channel interference. Second, because the time difference between any two data segment SYNC for a given sensing time is at most tens of milli-seconds, it is reasonable to assume that they encounter the same channel effects including timing offset, frequency offset, and multi-path fading effect. Thus, the correlation of two data segment SYNC generates a constant term. The interference and noise signals do not exhibit this property. As a result, the correlation of two data segment SYNC elements is used as a basic approach to perform spectrum sensing for ATSC DTV signals. Let y[n] be the received complex baseband signal, and define the accumulated SYNC correlation function to be

C(i,s) = -±- f_j∑y[i + k + n-L)y^t[i + k + (n + s) -L)] (1)

where / is the starting sample timing and s is correlation delay in terms of data segments. The parameter K is number of samples within a data Segment Sync and L is number of samples of a data segment. For ATSC signals, K=4 and L=832. In addition, the parameter N_s is number of correlations of the two data Segment Syncs with delay of s data segments for a given sensing time, e.g., for a sensing time of 10 data segments, Λ/i = 9, N₂ = 8, etc. Note that although symbol timing information is lacking, the absolute value of C(i,s) is maximum when the starting sample timing is the first sample of the Segment Sync. Consequently, the decision statistic for the Segment-Sync-Correlation-Based spectrum sensing approach using the accumulated Segment-Sync correlation function is given by

Note that s must be a non-zero positive integer and s is fixed to 1 in our example. An alternative decision statistic

which is the Maximum-to-Average Amplitude Ratio of C(i,s) over a window of L samples, can be used to perform spectrum sensing as well. Using decision statistic Rc(s) has an advantage that because it is a relative value, the threshold need not be adjusted according to signal strength or processing gain. For a given sensing time, the Segment-Sync correlation function C(i,s) at different delay of s data segments are available. In another embodiment of this invention, various accumulated Segment-Sync correlation functions C(i,s) for different values of s are ccombined to form decision statistics. However, the C{i,s) corresponding to different s suffers different phase rotation caused by carrier frequency offset and thus linearly (coherently) combining cannot be applied. An easy way is to use non-coherently combining which is to combine the absolute values of C(i,s) for different values of s and the decision statistic is then given by

where a_s is the combining ratios. Similarly, the decision statistic based on Maximum-to-Average Amplitude Ratio can be formed as

In order to coherently combine accumulated Segment-Sync correlation functions C(i,s) for different values of s, let

Q(i,s,s + d) = C(i,s)C'(i,s + d) (6) which is the conjugate product of two accumulated Segment-Sync correlation functions. Now, the phase term embedded in Q(i,s,s+d) becomes a function of d , and hence, we can coherently combine Q(i,s,s+d) for different s. The decision statistic for this coherent combining is given by

where b_s is a combining ratio. In general, d is set to 1 , but other values can be used. Again, the decision statistic based on Maximum-to-Average Amplitude Ratio can be formed as

In the FCC report [3], none of the white space devices submitted by different companies can determine the availability of a channel in the Two-Signal (Adjacent Channel Interference) model especially when a strong DTV signal exists in the lower adjacent channel. Fig. 3 and Fig. 4 show the frequency response of an ATSC DTV signal and an ATSC DTV signal with a lower adjacent channel interference. The center frequency of the signals in these two figures is 5.38 MHz. In Fig. 3, the signal power is at a level of -114 dBm. The amplitude of the pilot tone is about 13 dB higher and it can be used to perform spectrum sensing when SNR is as low as -20 dB. This is the reason that the pilot tone signal is widely used in different approaches to perform spectrum sensing. It is stated in the FCC report [3] that "although different algorithms are utilized among the prototype devices to detect the presence of a DTV signal, they all appear to share a similarity in that they all sample the spectrum in proximity of an anticipated DTV pilot signal."

In the Two-Signal model, the signal power of the adjacent channel can be as high as -28 dBm. In Fig. 4, the signal power and adjacent signal power are -114 dBm and -28 dBm, respectively. It is obviously that the pilot tone signal is completely shaded by the interference. It is very difficult for a pilot-tone-based spectrum sensor to tell if there is a pilot tone of ATSC signals when such a strong interference exists. This explains why the white space devices which sample the spectrum in proximity of an anticipated DTV pilot signal to perform spectrum sensing do not function well when there is a strong adjacent channel interference.

Figure 5 plots the frequency response of the ATSC data segment SYNC signal. We can see that the data segment SYNC signal spreads mainly from 3 MHz to 8 MHz. Thus, in order to alleviate the effect of interference, instead of using a 6 MHz receiver filter, we can use a narrower filter, e.g., 5 MHz. Note that the narrower filter can be implemented in analog or digital domain. By using a narrower filter, the data segment SYNC signal is mainly preserved and the interference signal is suppressed. Then, the SYNC-Correlation-Based spectrum sensing method described in previous Sections can reliably determine the availability of a channel even with a strong adjacent channel interference. The proposed interference alleviation scheme can be applied not only to Sync-Correlation-Based spectrum sensing algorithms for ATSC and NTSC TV signals, but also to spectrum sensing algorithms of other types of signals e.g., OFDM signals, provided that the feature used for spectrum sensing is a wide band signal.

Horizontal-Sync-Based Spectrum Sensing Algorithms The NTSC TV signal is an analog signal. The TV signal consists of frames (figures) at a rate of 30/1.001 = 29.97 frames/s. Each frame consists of a total of 525 scan lines (M system used in North American). The NTSC TV signal uses interlaced scan so that each frame is scanned in two fields and each field contains half the number of lines in a frame. As shown in Fig. 6, the first 19 or 20 lines of each field are vertical blanking interval which is used to perform equalization/synchronizations and are not displayed. The remaining 486 scan lines have a Horizontal Sync signal in the beginning of a line as shown in Fig. 6 and Fig. 7. Actually, it can be found that the last 11 lines of vertical blanking interval have a Horizontal Sync signal, too. The signal interval of the Horizontal Sync signal is about 4.7 micro second and the interval of a line is 63.55 micro second. Thus the density of the Horizontal Sync signal is 4.7/63.55 == 7.4%. On the other hand, the density of the data segment SYNC of ATSC signals is 4/832 « 0.48% . Thus, the Horizontal Sync contained in NTSC signals is a much stronger feature than the data segment SYNC contained in ATSC signals. It is reasonable to assume that for a small given sensing time, the transmitted signal encounter the same channel effects including frequency offset, timing offset, and multi-path fading effect. Thus, the correlation of any two Horizontal Syncs in a small given sensing time will generate a non-zero constant term. Since the noise signal does not employ this property, the correlation of two Horizontal Syncs can be used as a basic approach to perform spectrum sensing for the NTSC analog TV signals. Let y[n] be the sampled received signal and define the accumulated Horizontal-Sync correlation function to be

C(i,s) = -J-_r ∑∑^! _dy[i + k + n -L)y [i + k + (n + syL)] (9)

where / is the starting sample timing and s is correlation delay in terms of scan lines. The parameter K is number of samples within a data Segment Sync and L is number of samples of a data segment. The sampling rate should be carefully chosen such that each line consists of an integer number of samples. For example, K - 148 and L = 2002 correspond to a sampling rate of 31.5 MHz. In addition, the parameter N₅ is number of correlations of the two Horizontal Syncs with delay of s lines for a given sensing time, e.g., for a sensing time of 20 lines, /V₁ = 19, N₂ = 18, etc. Note that because the first 9 lines of vertical blanking interval do not have the Horizontal Sync signal, the sensing time should not be less than 9 lines. Although symbol timing information is lacking, the absolute value of C(i,s) is maximum when the starting sample timing is the first sample of the Horizontal Sync. Consequently, the decision statistic for the Horizontal-Sync-Correlation-Based spectrum sensing algorithm using the accumulated Horizontal-Sync correlation function is given by r_t (j) = max |C(ι,5)| . (10) Note that s must be a non-zero positive integer. An alternative decision statistic

which is the Maximum-to-Average Amplitude Ratio of C{i,s) over a window of L samples, can be used to perform spectrum sensing as well. Using decision statistic R_c(s) has an advantage that because it is a relative value, the threshold need not be adjusted according to signal strength or processing gain. For a given sensing time, the Horizontal-Sync correlation function C(i,s) at different delay of s lines are available. In another embodiment of this invention, various accumulated Horizontal-Sync correlation functions C(i,s) for different values of s are ccombined to form decision statistics. However, the C(i,s) corresponding to different s suffers different phase rotation caused by carrier frequency offset and thus linearly (coherently) combining cannot be applied. An easy way is to use non-coherent combining which is to combine the absolute values of C(i,s) for different values of s and the decision statistic is then given by

where a_s is the combining ratios. For example, the combining ratio a_s = N_s is the Maximal-Ratio combining. Similarly, the decision statistic based on Maximum-to-Average Amplitude Ratio can be formed as

In another embodiment of this invention, in order to coherently combine accumulated Segment-Sync correlation functions C(i,s) for different values of s, let

which is the conjugate product of two accumulated Horizontal-Sync correlation functions. Now, the phase term embedded in Q{i,s,s+d) becomes a function of d , and hence, we can coherently combine Q(i,s,s+cf) for different s. The decision statistic for this coherent combining is given by

T_n = s,s + d)\ (15)

Note that because the Horizontal Sync signal is a wide band signal, the adjacent channel interference alleviation scheme described in the previous pattern application can be used with the Horizontal-Sync-Correlation-Based spectrum sensing algorithms presented in this invention.

One embodiment of the present principles is illustrated in Figure 8, which shows a method for spectrum sensing. Correlation of synchronization data and accumulation of correlation functions is performed in step 810. Generation of a decision statistic according to the present principles is performed in step 820. The decision statistic is then used in step 830 to determine whether spectrum space is occupied.

Another embodiment of the present principles is illustrated in Figure 9, which shows a method for spectrum sensing. The conjugate products of synchronization data is performed in step 910. The conjugate products are combined in step 920. Following the combining step, a decision statistic is generated according to one of the principles of the present invention in step 930. The decision statistic is then used in step 840 to determine whether spectrum space is occupied.

Another embodiment of the present principles is illustrated in Figure 10, which shows an apparatus for spectrum sensing. A correlator 1010 receives a signal and performs correlation on synchronization data. The output of correlator 1010 is input to accumulator 1020 which performs accumulation of the correlation functions according to the present principles. Accumulator 1020 output is input to processor 1030 which generates a decision statistic as an output. The output of accumulator 1020 is input to detector 1040 which determines whether spectrum space is occupied.

A further embodiment of the present principles is illustrated in Figure 11 , which shows a method of alleviating interference in an ATSC receiver. A digital television signal is received in step 1110. The received signal is then filtered using the principles of the present invention in step 1120. The present description illustrates the present principles. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the present principles and are included within its spirit and scope. All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the present principles and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions.

Moreover, all statements herein reciting principles, aspects, and embodiments of the present principles, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. Thus, for example, it will be appreciated by those skilled in the art that the block diagrams presented herein represent conceptual views of illustrative circuitry embodying the present principles. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudocode, and the like represent various processes which may be substantially represented in computer readable media and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.

The functions of the various elements shown in the figures may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term "processor" or "controller" should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor ("DSP") hardware, read-only memory ("ROM") for storing software, random access memory ("RAM"), and non-volatile storage.

Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context.

In the claims hereof, any element expressed as a means for performing a specified function is intended to encompass any way of performing that function including, for example, a) a combination of circuit elements that performs that function or b) software in any form, including, therefore, firmware, microcode or the like, combined with appropriate circuitry for executing that software to perform the function. The present principles as defined by such claims reside in the fact that the functionalities provided by the various recited means are combined and brought together in the manner which the claims call for. It is thus regarded that any means that can provide those functionalities are equivalent to those shown herein. Reference in the specification to "one embodiment" or "an embodiment" of the present principles, as well as other variations thereof, means that a particular feature, structure, characteristic, and so forth described in connection with the embodiment is included in at least one embodiment of the present principles. Thus, the appearances of the phrase "in one embodiment" or "in an embodiment", as well any other variations, appearing in various places throughout the specification are not necessarily all referring to the same embodiment.

Claims

CLAIMS:

1. A method of spectrum sensing, comprising: generating an accumulated correlation function of received complex baseband signals containing data segment SYNC signals wherein said correlation is performed with a correlation delay over a sensing interval; generating a decision statistic by finding the maximum of said correlation functions for a correlation delay over a sensing interval; and using said decision statistic value to detect occupied spectrum space.

2. The method of Claim 1 , wherein the correlation delay is equal to one.

3. The method of Claim 1 , wherein the decision statistic is formed by further dividing said maximum of the sum of weighted conjugate products over a sensing interval by an average amplitude of said accumulated SYNC correlation functions.

4. The method of Claim 1 , wherein the correlations are weighted and summed for different values of correlation delay before finding a maximum value over a sensing interval.

5. The method of Claim 4, wherein said maximum of correlation functions for different values of correlation delay over a sensing interval is divided by an average amplitude of said accumulated SYNC correlation functions to generate said decision statistic.

6. A method of spectrum sensing, comprising: finding the conjugate product of two accumulated segment SYNC correlation functions; coherently combining said conjugate products for different correlation delays; finding the maximum of the sum of weighted conjugate products over a sensing interval to generate a decision statistic; and using said decision statistic value to detect occupied spectrum space.

7. The method of Claim 6, wherein the decision statistic is formed by further dividing said maximum of the sum of weighted conjugate products over a sensing interval by an average amplitude of said accumulated SYNC correlation functions.

8. A method of spectrum sensing, comprising: accumulating correlation functions of received horizontal synchronization signals wherein said correlation is performed with a correlation delay over a sensing interval; generating a decision statistic by finding the maximum of said correlation functions for a correlation delay over a sensing interval; and using said decision statistic value to detect occupied spectrum space.

9. The method of Claim 8, wherein the decision statistic is formed by further dividing said maximum of the sum of weighted correlation functions over a sensing interval by an average amplitude of said accumulated horizontal synchronization correlation functions.

10. The method of Claim 8, wherein the correlations are weighted and summed for different values of correlation delay before finding a maximum value over a sensing interval.

11. The method of Claim 10, wherein said maximum of correlation functions for different values of correlation delay over a sensing interval is divided by an average amplitude of said accumulated horizontal synchronization correlation functions to generate said decision statistic.

12. A method of spectrum sensing, comprising: finding the conjugate product of two accumulated horizontal synchronization correlation functions; coherently combining said conjugate products for different correlation delays; finding the maximum of the sum of weighted conjugate products over a sensing interval to generate a decision statistic; and using said decision statistic value to detect occupied spectrum space.

13. The method of Claim 12, wherein the decision statistic is formed by further dividing said maximum of the sum of weighted conjugate products over a sensing interval by an average amplitude of said accumulated horizontal synchronization correlation functions.

14. An apparatus for spectrum sensing, comprising: a correlator for correlation of two synchronization functions with correlation delay; an accumulator for accumulating said correlation functions over a sensing interval; a processor for generating a decision statistic wherein said decision statistic is comprised of the maximum value of said correlation functions; a detection unit that uses said decision statistic to determine if spectrum space is occupied.

15. The apparatus of Claim 14, wherein said processor forms the decision statistic by further dividing said maximum of the sum of weighted correlation functions over a sensing interval by an average amplitude of said accumulated synchronization correlation functions.

16. The apparatus of Claim 14, wherein said accumulator applies weights to the correlations and sums them for different values of correlation delay before finding a maximum value over a sensing interval.

17. The apparatus of Claim 14, wherein said maximum of correlation functions for different values of correlation delay over a sensing interval is divided by an average amplitude of said accumulated synchronization correlation functions to generate said decision statistic.

18. The apparatus of Claim 14, wherein said correlator further finds the conjugate product of two accumulated segment synchronization correlation functions; said accumulator coherently combines said conjugate products for different correlation delays; said processor finds the maximum of the sum of weighted conjugate products over a sensing interval to generate a decision statistic; and said detection unit uses said decision statistic value to detect occupied spectrum space.

19. The apparatus of Claim 18, wherein the decision statistic is formed by further dividing said maximum of the sum of weighted conjugate products over a sensing interval by an average amplitude of said accumulated horizontal synchronization correlation functions.

20. A method of alleviating interference in an ATSC receiver, comprising: receiving a digital television signal; filtering said received digital television signal using a receiver filter that is narrower than the channel bandwidth to reduce the adjacent channel interference.