This application claims the benefit, under 35 U.S.C. §365 of International Application PCT/CN2007/000850, filed Mar. 16, 2007, which was published in accordance with PCT Article 21(2) on Sep. 25, 2008 in English.
The present invention generally relates to communications systems and, more particularly, to wireless systems, e.g., terrestrial broadcast, cellular, Wireless-Fidelity (Wi-Fi), satellite, etc.
Today, the number of communication signals being broadcast is on the rise. In addition, these broadcast communication signals may use different types of modulation. One form of receiver that supports multiple modulation types is represented by the currently proposed Chinese Digital Television System (GB) 20600-2006 that specifies a receiver support a single carrier (SC) modulation mode and a orthogonal frequency division multiplexing (OFDM) modulation mode. This receiver determines the type of modulation in the received signal by setting itself to each type of modulation until the receiver correctly recovers data in the received signal. For example, the receiver may first configure itself to receive an OFDM signal, and then test for the presence of predefined data in the received signal. If this test succeeds, the receiver assumes that the received signal is an OFDM signal. However, if this test should fail, then the receiver configures itself to receive a single carrier signal and then, again, tests for the presence of the predefined data in the received signal. Unfortunately, the presence of multipath effects may make it difficult for the receiver to locate the predefined data—whatever modulation type the receiver is set to. As a result, the receiver may take a long time to correctly determine the modulation type since the receiver will continue to switch back and forth between modulation types searching for the predefined data.
In a communications environment that supports different types of modulation, it would be beneficial if a receiver could adapt to any received signal to correctly recover the information conveyed therein whatever the modulation type without having to test for the presence of predefined data—even in a multi-path environment. Therefore, and in accordance with the principles of the invention, a receiver determines a fluctuation range (MFR) as a function of at least a fourth-order cumulant of a received signal; and classifies a modulation type of the received signal as a function of the determined fluctuation range.
In an embodiment of the invention, a receiver supports a single carrier (SC) form of modulation and a multi-carrier form of modulation such as orthogonal frequency division multiplexing (OFDM). Upon receiving a broadcast signal, the receiver downconverts the received broadcast signal to a received base-band signal. The receiver then determines a fourth-order cumulant and a second-order cumulant of the received base-band signal for use in calculating a normalized fourth-order cumulant of the received base-band signal. The receiver then measures a maximum fluctuation range (MFR) of the normalized fourth-order cumulant of the received base-band signal and classifies a modulation type of the received base-band signal as either SC or OFDM as a function of the measured maximum fluctuation range. After determining the modulation type of the received signal, the receiver switches to the classified modulation type, i.e., that modulation mode, to recover data from the received signal.
In view of the above, and as will be apparent from reading the detailed description, other embodiments and features are also possible and fall within the principles of the invention.
Other than the inventive concept, the elements shown in the figures are well known and will not be described in detail. For example, other than the inventive concept, familiarity with Discrete Multitone (DMT) transmission (also referred to as Orthogonal Frequency Division Multiplexing (OFDM) or Coded Orthogonal Frequency Division Multiplexing (COFDM)) is assumed and not described herein. Also, familiarity with television broadcasting, receivers and video encoding is assumed and is not described in detail herein. For example, other than the inventive concept, familiarity with current and proposed recommendations for TV standards such as NTSC (National Television Systems Committee), PAL (Phase Alternation Lines), SECAM (SEquential Couleur Avec Memoire) and ATSC (Advanced Television Systems Committee) (ATSC) and Chinese Digital Television System (GB) 20600-2006 is assumed. Likewise, other than the inventive concept, other transmission concepts such as eight-level vestigial sideband (8-VSB), Quadrature Amplitude Modulation (QAM), and receiver components such as a radio-frequency (RF) front-end, or receiver section, such as a low noise block, tuners, down converters and demodulators, correlators, leak integrators and squarers is assumed. Further, other than the inventive concept, familiarity with statistical processing of signals, such as forming cumulants, is assumed and not described herein. Similarly, other than the inventive concept, formatting and encoding methods (such as Moving Picture Expert Group (MPEG)-2 Systems Standard (ISO/IEC 13818-1)) for generating transport bit streams are well-known and not described herein. It should also be noted that the inventive concept may be implemented using conventional programming techniques, which, as such, will not be described herein. In this regard, the embodiments described herein may be implemented in the analog or digital domains. Further, those skilled in the art would recognize that some of the processing may involve complex signal paths as necessary. Finally, like-numbers on the figures represent similar elements.
Referring now to
Turning now to
Antenna 105 of
Before describing the inventive concept in detail, attention should now be directed to
This is also referred to herein as the normalized fourth-order cumulant. Signal 216 is then applied to element 220, which provides a signal 221, which is representative of the magnitude of the normalized fourth-order cumulant,
Loop filter 225 filters the normalized fourth-order cumulant to remove high frequency components and provides a filtered signal 226 to MFR element 230.
Turning briefly to
(signal 221) and filtered signal 226. Difference signal 276 is applied to filter 280, which filters difference signal 276 by
to provide signal 281 to combiner 285. The latter adds signal 281 to filtered signal 226. In the loop filter, performance is decided mainly by the value for n. Although the value for n is determined experientially, some illustrative values may be n=8, or n=10.
Returning to
Turning now in more detail to the inventive concept, for the purposes of this example received signal 111 is assumed to be a baseband signal received in a multipath environment. In this context, received signal 111, also referred to herein as r(n), is:
where hl(n) is the path complex gain for a particular path, l; τl is the path delay; L is the total number of paths; w(n) is additive which Gaussian noise (AWGN); and s(n) depends on the modulation type. In terms of s(n), the following signal models are used:
where P is the power of signal for all three equations (2), (3) and (4). With respect to equations (2) and (3), these represent SC modulations and M is the level of SC modulation. In this example, equation (2) represents phase-shift keying (PSK) and equation (3) represents quadrature amplitude modulation (QAM). With respect to equation (4), this represents OFDM modulation, where H is the number of carriers (or subcarriers) in the OFDM signal, and ch is the symbol sequence, which is assumed to be centered, independent and identically distributed (I.I.D.).
As described above, and in accordance with the principles of the invention, a receiver performs modulation classification as a function of at least a fourth-order cumulant. In particular, the inventive concept takes advantage of the fact that in applying the Central-Limit Theorem it is known that OFDM probability converges on the Gaussian distribution and that SC modulations are known to be non-Gaussian distributions. As such, the fourth-order cumulants of Gaussian signals are zeros theoretically, which does not happen to non-Gaussian signals. Although the fourth-order cumulant itself could be used, multi-path effects may cause scale problems in data. As such, the fourth-order cumulant is normalized, e.g., with the square of the second-order cumulant, in order to alleviate any scaling problems. The normalized fourth-order cumulant is also referred to herein as |{tilde over (C)}40|, where:
and Cum4 is the equation for a fourth-order cumulant. Therefore, equation (6b) can be rewritten as:
However, since, as noted above, the fourth-order cumulant of a Gaussian process is zero, equation (6c) can be further rewritten as:
It should be noted that since C21 is the average power, i.e., C21=E[|r(n)|2], C212 is a finite value. Similarly, the path gain hl(n) is also a finite value.
Now, the following observations are made. If the received signal is an OFDM signal, which is presumed to follow the Gaussian distribution, then,
Cum4(s(n−τl))→0. (7)
However, since each path, hl(n), is a finite value, it can be deduced that:
Cum4(hl(n)s(n−τl))→0. (8)
Therefore,
Based on the above analysis, if the received signal is an OFDM signal then:
|{tilde over (C)}40(rOFDM)|→0. (10)
In contrast to an OFDM signal, if the received signal is a SC signal, which is non-Gaussian, then Cum4(s(n−τl)) will be non-zero. As a result, |{tilde over (C)}40(r(n))| for an SC signal, i.e., |{tilde over (C)}40(rSC)|, is non-zero and shows the channel properties. In other words, when a SC signal is transmitted in a multipath environment, each channel will have different amplitude properties, such that |{tilde over (C)}40(rSC)| will fluctuate over a large range. It should be noted that the obtained fourth-order cumulant may have many high frequency components, which will affect the estimation of the MFR. As such, it is advantageous, though not required, to use the earlier-described first-order loop filter to filter out these high frequency components and obtain an envelope of the fourth-order cumulant to improve the estimation performance.
As a result of the above analysis, and in accordance with the principles of the invention, |{tilde over (C)}40| of the received signal is calculated and analyzed to determine if the received signal uses an OFDM modulation or an SC modulation. Reference should now be made to
MFR≦threshold. (11a)
Otherwise, the receiver decides the received signal is a SC type of modulation, i.e.
MFR>threshold (11b)
Illustratively, a value for threshold is derived from the above-described MFR measurements, e.g., illustrated in
The receiver can estimate the SNR to determine particular MFROFDM and MFRSC values to use from
Referring now to
Illustrative performance results are shown in
As described above, and in accordance with the principles of the invention, a receiver performs modulation classification in multipath environments and, as such, is able to adapt to the received signal by setting the modulation type. The inventive concept is also representative of a blind modulation classification method and apparatus since the receiver determines, or estimates, the modulation type without looking for predefined data in the received signal. It should be noted that although the inventive concept was illustrated in the context of a DTV broadcast signal, the inventive concept is not so limited and is applicable to other types of receivers that perform adaptive reception, such as a software defined radio receiver, etc.
In view of the above, the foregoing merely illustrates the principles of the invention and it will thus be appreciated that those skilled in the art will be able to devise numerous alternative arrangements which, although not explicitly described herein, embody the principles of the invention and are within its spirit and scope. For example, although illustrated in the context of separate functional elements, these functional elements may be embodied in one, or more, integrated circuits (ICs). Similarly, although shown as separate elements, any or all of the elements may be implemented in a stored-program-controlled processor, e.g., a digital signal processor, which executes associated software, e.g., corresponding to one, or more, of the steps shown in, e.g.,
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/CN2007/000850 | 3/16/2007 | WO | 00 | 9/16/2009 |
Publishing Document | Publishing Date | Country | Kind |
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WO2008/113202 | 9/25/2008 | WO | A |
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