VSB RECEIVER

Abstract: A method and device for improving the reception of a vestigial sideband signal by reducing the loss of information at the band edges in sever multipath. This is achieved by using a complex equalizer in conjunction with a real equalizer.

Description: VSB receiver. BACKGROUND OF THE INVENTION The digital terrestrial television transmission standard for the U.S. is a trellis coded 8-VSB system as described in detail in ATSC Digital Television Standard, Doc. A/53, http:// www.atscorg/stan& s.html, Sep. 1995. Briefly, three trellis-coded bits are encoded into one of eight symbols +/-1, +1-3, +1-5, +1-1. This real symbol stream at 10.76 Msyms/s has a double-sided spectrum in the digital frequency domain. One of the sidebands is filtered with a square-root-raised-cosine(SQRC) filter to give a vestigial-sideband (VSB) signal which is then up-converted to RF and transmitted. At the receiver, the conventional method of demodulation consists of first recovering the carrier frequency and phase, matched filtering, and then recreating the double- sideband real signal prior to equalization with a real equalizer operating at the symbol rate of 10.76Msyms/s. In fact, one of the so called "advantages" of VSB is the fact that, unlike QAM, complex equalization is not necessary. However, the above receiver structure with a real equalizer is optimal only in the case of an ideal channel with perfect carrier and timing phase recovery. When the signal suffers multipath distortion, processing only the real part of the signal at the receiver causes information about the channel at the band edges to be lost. The situation is analogous to QAM where it has been shown that fractionally-spaced equalization can compensate for band-edge distortions in the channel that symbol-spaced equalization cannot. In VSB, taking the real part of the signal causes aliasing of the band-edges in the same way that sampling a QAM signal at the symbol rate does. SUMMARY OF THE INVENTION Accordingly, it is an object of the invention to provide a receiver structure for VSB which reduces the loss of information at the band edges in severe multipath by using a blind complex equalizer in conjunction with a real equalizer. This object is achieved because the complex VSB signal at the symbol rate (i.e. 10.76 Msyms/s) is not aliased and any band- edge distortions can be equalized with a complex equalizer. It is another object of the invention to equalize the band edges of the complex baseband signal prior to recreating the double sideband signal. It is yet a further object of the invention to simplify the complex equalizer by using a complex equalizer with a small number of taps to just equalize the band edges and then recover the field synchronization signal from the output of this complex equalizer for training a longer equalizer that equalizes the rest of the band. It is even a further object of the invention to use a composite equalizer which has a feedback section having real taps and a forward section having an inner section with complex taps to compensate for the band edges and an outer section with real taps. The invention accordingly comprises the methods and features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the independent claims. Advantageous embodiments are defined in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention reference is made to the following drawings: Figure la shows the frequency spectrum of a SQRC filter, p(t); Figure lb shows the frequency spectrum of the transmitted signal, s(t); Figure lc shows the low-pass equivalent of the multipath channel; Figure Id shows the frequency spectrum of the recovered VSB signal showing aliasing of the band-edge; Figure 2 shows a receiver in accordance with the invention; and Figure 3 shows a receiver in accordance with the invention which includes a forward equalizer with embedded real and complex taps. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figures la- Id show the basic idea behind the invention. Let a n be the 8 level, real symbol stream with pilot tone at the symbol rate of 10.76 Msyms/s. This symbol stream has a double-sided spectrum of bandwidth 10.76 MHZ. The VSB transmitted signal at RF, s(t), is formed by first filtering this symbol stream to extract one side-band and a "vestige" of the other side-band, and then up-converting to the transmission frequency. The transmitted signal at RF, s(t), can then be written as: where p(t) is the SQRC pulse shape with 11.5% roll-off of which the frequency characteristic F{p (t) } is shown in Figure la. Note that p(t) is a complex filter in the time domain and combines the operations of vestigial filtering and minimum intersymbol interference filtering when the same filter is used as a matched filter in the receiver. The spectrum F { s(t) } of s(t) is shown in Figure lb. T is the symbol interval of 1/10.76 Msyms/s =92.94 ns and et c is the carrier frequency. Let h(t) be the bandpass multipath channel that the RF signal s(t) encounters. This bandpass channel has a low-pass equivalent II L O of which the frequency characteristic is shown in Figure lc. This low-pass equivalent h L (t) is related to h(t) as follows: h(t)= Re {h L (t) e Jϋ "'} (2) The received signal r(t) at RF is the convolution of s(t) and h(t) and can be written as: r(t) = Re \ ∑ a n d(t - nT) e Jω A (3) where d(t) =p(t) .\ h L (t) and .\ denotes convolution. Conventional demodulation of r(t) involves down-conversion to baseband, either in the analog or digital domain, matched filtering and then recreating the original double-sided transmitted spectrum by taking the real F {x (t) }part as shown in Figure Id. This baseband double-sideband signal x(t) can be written as: x(t) = Re [{ r(t) e - Jω< '}* p(t)] (4) where i.e. c(t) is the composite baseband pulse-shape at the receiver. From Figure Id it is clear that the process of taking the real part of the down-converted signal creates aliasing at the lower band-edge. Furthermore, sampling of x(t) at the symbol rate of 10.76 MHZ will create aliasing of the other band-edge. This will lead to a performance loss in severe multipath channels. Instead, this invention proposes the general idea of equalizing the band edges of the complex baseband signal y(t) prior to recreating the double sideband signal. In the 8-VSB ATSC standard, equalization is usually carried out after the field synch segment that contains the training sequence is detected. However, if one wants to do complex equalization, it is preferable to use a blind algorithm since in severe multipath channels the signal may be too distorted for reliable synch extraction. Assuming perfect carrier recovery, y(t) can be written as: y (t) = {r (t) e- jω< ' }* P (t) ( 6 ) Complex equalization of y(t) could be performed by sampling y(t) at the symbol rate of 10.76 MHZ to form the complex sequence y(k) and then equalizing y(k) with a complex equalizer with coefficients f(k). Since the final goal is the transmitted, real, VSB symbol sequence, only the real part of the complex equalizer output is required for further processing. This reduces the number of multiplications required per tap to 2 from 4. This is still twice the number of multiplications required by a real equalizer. In general, f(k) could be chosen to equalize the entire band. However this would require a large number of taps, each with twice the number of multipliers than a real equalizer. There are 2 ways to simplify the equalizer, one is to place a complex equalizer by itself with a smaller number of taps to just equalize the band-edges and then recover the field synchronization signal from the output of this complex equalizer for training a longer, real equalizer that equalizes the rest of the band. The complex equalizer then needs to be adapted blindly, at least for the first data field, since the segment and field synchronization signals have not yet been recovered. This architecture is shown in Figure 2. The recovery of segment synch after the complex equalizer makes synch recovery extremely robust even in severe multipath channels with band-edge distortions that can cause conventional synch detection on the real, aliased signal to fail. Referring to Figure 2, the real output, z(k) of the complex equalizer with taps [f(0),...f(Lr 1)] is related to the complex input y(k)=y R (k) +jyτ(k) as follows: where di is the delay through the complex equalizer. From the above, it is clear that for each tap 2 multiplications are required as opposed to 4 for a full complex multiplication as for QAM and 1 for a real equalizer. The second way of accomplishing this is to have a composite equalizer which has both a forward and feedback section as shown in Figure 3. The feedback section has all real taps since the symbols are real. The forward section has an inner and outer section. The inner section has complex taps to compensate for the band-edges while the outer taps are real. Again, such an equalizer can either start up in the blind mode and then switch to a trained mode of adaptation. The advantage of this structure is that the total number of multipliers required would be less than the previous approach. Let [g(0),...g( Li -1), g( Lι),...g( Li +L 2 - l),g( Lι+L 2 ),...g(L g -l)] be the taps of the composite forward equalizer of length L g . The first Li taps are real, the next L 2 taps are complex and the remaining taps again are real. The real output, z(k) of the composite forward equalizer is related to the complex input y(k) = y R (k)+jyι(k) and the taps of the composite equalizer g(k) as follows: z(k) = ∑(g(n)yR(k + d, - n) n=0 L1+L2+I + Re [ ∑ (gR(n) + jgI((n))yR(k + d l - n) + jyl(k + d l - n)) n=Lι Lg-1 (8) + ∑ g(n)yR(k + d, - n) n=L,+L2 rl = g(n)yR(k + d, - n) n=0 L1+L2+I + £ [gR(n)yR(k + dj - n) - gI(n)yI(k + d - n)] n "==LLιι LLgg--11 (9) + ∑ g(n)yR(k + d, - n) n+Lι+L2 Here, di is the delay through the forward equalizer (i.e. center tap) and should be chosen such that ( L]-l)<d ! <( L Ϊ +L Z ). The real output of the forward equalizer is then added to the real output of the feedback equalizer to form the final equalizer output. The blind algorithm that could be used to adapt either of the above structures could be the Godard algorithm, D.N. Godard, "Self-recovering equalization and carrier tracking in two dimensional data communications systems," IEEE Trans. Commun., vol. COM-28, no. 11, pp. 1867-1975, Nov. 1980, the Sato algorithm, Y.Sato, "A method of self recovering equalization for multilevel amplitude-modulation systems," LEEE Trans. Commun., vol. COM-23, pp. 679-682, June 1975, or the sign-error algorithm, M.Ghosh, "A sign-error algorithm for blind equalization of real signals," ICASSP, May 1998. Of the three, the last one has the least complexity. The general method and device to improve a receiver structure for VSB reception has been described above. The specific circuitry is now described. Figure 2, as stated above, shows a short complex equalizer 15 followed by a longer real equalizer 17. The up converted VSB signal s(t) is provided at the input of tuner 11. It is then A/D converted at N/D converter 12. The carrier is recovered at the carrier recovery circuit 13 and the signal and pilot are complex square-root-raised cosine filtered and the pilot is removed at the processing circuit 14 and the signal is sampled at the symbol rate of 10.76 MHZ to form the complex sequence y(k). The complex equalizer 15 having taps f(k) equalizes the complex sequence y(k). In a preferred embodiment only the band edges of y(k) are equalized meaning f(k) is relatively of a small number. The field synchronization signal is then recovered by the sync recovery unit 16 from the real output z(k) of the complex equalizer 15. The recovered sync signal sy is then used to further update the taps f(k) of the complex equalizer 15. The real equalizer 17 then equalizes the real output z(k). The complex equalizer 15 further receives the blind adaption information BA. In an alternate embodiment 20 shown in Figure 3, a composite forward equalizer 25 with embedded real and complex taps is shown. The up converted VSB signal s(t) is provided at the input of tuner 11, it is A/D converted at A/D converter 12, the carrier is recovered at the carrier recovery circuit 13. The signal and the pilot are complex-square-root-raised cosine filtered and the pilot is removed at the processing circuit 14 and the signal is sampled at the symbol rate of 10.76 MHZ to form the complex sequence y(k). Composite forward equalizer 25 and the composite feedback equalizer 29 form a composite equalizer. The feedback section 29 contains all real taps since after the slicer 28 all symbols are real. The forward equalizer 25 includes an inner section with complex taps to compensate for the band edges and an outer section with real taps to equalize the remainder of the VSB signal. The real output z(k) of the composite forward filter 25 is summed with the sum circuit 26 with the real output of the feedback equalizer 29. This summed uotput signal o is supplied to a back en (not shown). From this summed output signal o the sync signal sy is recovered by the sync recovery unit 27. This sync signal sy is used to train the taps of the composite forward filter 25 once it is reliably detected. It will thus be seen that the objects set forth above, and those made apparent from the preceding description are efficiently obtained and, since certain changes may be made in the above construction without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are intended to cover all the generic and specific features of the invention herein described, and all statements of the scope of the invention, which, as a matter of language, might be said to fall therebetween.

Claims: CLAIMS: 1. A VSB receiver, comprising: a device which receives a VSB signal which includes a field synch segment; a complex equalizer (15) which complex equalizes the VSB signal and provides an output; a sync recovery device (16) which recovers the field synch segment from the output: and a real equalizer (17) for real equalizing the output. 2. A VSB receiver comprising: a device which receives a VSB signal which includes a field synch segment and band edges; a complex equalizer (15) which equalizes the band edges and provides an output; a synch recovery device (16) which recovers the field sync segment from the output of the complex equalizer; and a real equalizer (17) for equalizing the remaining portion of the VSB signal. 3. The device as claimed in claim 2, wherein the recovered field synch trains the complex equalizer. 4. The device as claimed in claims 2 and 3, further including a blind adaptation device which blindly adapts the complex equalizer at least before the recovery of the synch segment by the synch recovery device. 5. A VSB receiver, comprising: a device which receives a VSB signal including band edges; a composite equalizer including 1) a forward section (25) having an inner section including complex taps for equalizing band edges and an outer section including real taps for equalizing a remainder of the VSB signal, and 2) a feedback section (29) having real taps for equalizing the output of the composite equalizer. 6. A VSB receiver, comprising: a complex equalizer (15) which receives a VSB signal and provides a real output; a sync recovery unit (16) which recovers a field synch segment from the real output; and a real equalizer (17) which equalizes the real output. 7. A VSB receiver comprising: an input which receives an up converted VSB signal; an A/D converter (12) which A/D converts the up converted VSB signal and provides a digital output; a square-root-cosine filter (14) for filtering the digital output; a complex equalizer (15) for equalizing band edges of the square root cosine filter digital output and for providing a real output; a sync recovery unit (16) for recovery of a field synch segment from the real output; and a real equalizer (17) for equalizing the real output. 8. A VSB receiver, comprising: an input which receives an up converted VSB signal; an A/D converter (12) which A/D converts the up converted VSB signal and provides a digital output; a square-root-cosine filter (14) for filtering the digital output; a composite forward equalizer (25) having real and complex taps, the complex taps for complex equalizing band edges of the square root cosine filter digital output, the real taps for filtering the remaining portion of the square root cosine filter digital output and for providing a real output; a slicer (28) for estimating the real output to the nearest symbol; a feedback equalizer (29) for equalizing the output of the slicer and for providing an equalized output; an adder (26) for adding the equalizing output and the real output to provide a finalized equalizer output; and a sync recovery unit (27) for recovering a field sync segment from the finalized equalized output for adapting the taps of the composite forward equalizer. 9. A method of equalizing a VSB signal, comprising the steps of: receiving the VSB signal which includes a field synch segment and band edges; complex equalizing the band edges and providing an output; recovering the field synch segment from the output; and real equalizing the output. 10. A method of equalizing a VSB signal comprising the steps of: receiving the VSB signal which includes a field sync segment and band edges; complex equalizing the band edges to provide a real output; recovering the field sync segment from the real output; real equalizing the real output; and adapting the step of complex equalizing using the recovered field synch segment.

Inventors: GHOSH MONISHA, GHOSH, MONISHA
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1999
Main classification: H04N5
IPC codes: H04B3, H04B7, H04L27, H04N5