Pulse or digital communications – Systems using alternating or pulsating current – Antinoise or distortion
Reexamination Certificate
2000-04-07
2004-03-23
Bocure, Tesfaldet (Department: 2731)
Pulse or digital communications
Systems using alternating or pulsating current
Antinoise or distortion
C375S296000, C375S346000
Reexamination Certificate
active
06711214
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to transmitter methods and apparatus and, more particularly, to a transmitter method and apparatus wherein energy in a portion of a spectrum on a first side of a predetermined frequency is transferred to a second side of the predetermined frequency in such a manner that at least some of the transferred energy results in a reduction of transmitted bandwidth and receivers responsive to the transmitter respond to the transferred energy as if it were on the first side of the predetermined frequency.
BACKGROUND ART
The problem of frequency spectrum crowding has become so severe that government allocated spectra for different types of transmission results, in some cases, in overlapping spectra for different transmitters. For example, the United States Federal Communications Commission (FCC) in establishing a table of channel allocations for the Advanced Television Standards Committee (ATSC), eight level vestigial sideband (8VSB) digital television (DTV) transmitters has created severe technical problems for television channel
14
(470-476 MHz). There are terrestrial mobile radio transmitters which the FCC has licensed at 469.975 MHz, i.e., at a frequency allocation very close to the lower band edge of channel
14
. It is extremely difficult to protect the low power 469.975 MHz radio emission from the high power DTV signal having energy at 470 MHz, only 0.025 MHz away from the 469.975 MHz emission. This is particularly the case since the FCC has required the 470 MHz band edge to be down only 36 dB from the mid-band amplitude. If prior techniques were used, the channel
14
allocation would be useless in most cases because it would overpower the 469.975 MHz emission, a situation the FCC will not permit.
Another situation that is problematic is the so-called “N+1” allocation. In the “N+1” allocation, a National Television Systems Committee (NTSC) licensee is assigned a DTV channel that is the next channel up from the licensee's NTSC channel. In this situation, the NTSC aural carrier has significant sidebands that extend to within about 75 kHz of the NTSC channel edge, and therefore to within about 75 kHz of the DTV signal. This creates problems for stations that wish to use frequency selective combiners with separate NTSC and DTV transmitters. Yet another problem is the radio telescope allocation at channel
37
in the UHF DTV band (608-614 MHz). DTV stations on channel
36
or
38
may interfere with radio telescope operations on channel
37
.
A typical prior art digital television transmitter adapted to transmit signals containing information indicative of digitally encoded video and aural signals for deriving an ATSC A/53 standard signal is illustrated in
FIG. 1
as including multi-bit digital baseband television signal source
10
which drives the cascaded combination of data randomizer
11
, Reed-Solomon encoder
12
, data interleaver
14
, trellis encoder
16
and multiplexer
18
. In the 8VSB system, the signals derived from source
10
, randomizer
11
, encoders
12
and
16
, as well as interleaver
14
and multiplexer
18
include three parallel bits having a symbol rate (i.e., sampling frequency) of
1.539
×
10
9
143
,
i.e., the encoded television signal that source
10
derives is sampled 10,762,237.76 times per second. Because each symbol includes two or three bits, the bit rate is substantially higher than the symbol rate. The three parallel bits represent
8
amplitude levels of the encoded television signal.
Multiplexer
18
, in addition to being responsive to the output of trellis encoder
18
, responds to segment synchronizing source
20
and field synchronizing source
22
to derive an output having the same number of bits as applied to the multiplexer by encoder
16
. Multiplexer
14
supplies a multi-bit output signal to pilot inserter
24
which inserts a constant bit pattern representing a DC amplitude equal to 0.625 of the weight of a single bit in the incoming 3 bit pattern. Pilot inserter
24
derives a multi-bit output signal which drives pre-equalizer filter
26
.
Pilot inserter
24
derives a multi-bit output signal which drives pre-equalizer filter
26
. Pre-equalizer filter
26
supplies a multi-bit intermediate frequency (IF) signal to vestigial sideband modulator or generator
28
. Generator
28
feeds a multi-bit digital IF signal to digital to analog converter
30
, which supplies an analog IF signal to frequency up converter
32
. Converter
32
is a frequency synthesizer that heterodynes the IF output frequency of converter
30
to a radio frequency (RF) carrier frequency. Up converter
32
also inverts the IF spectrum digital to analog converter
30
derives so up converter
32
converts (1) the lowest frequencies in the IF spectrum into the highest frequencies in the RF spectrum and (2) the highest frequencies in the IF spectrum to the lowest frequencies in the RF spectrum. RF up converter
32
applies the modulated carrier frequency signal to antenna
34
via power amplifier
36
.
The output signal of digital to analog converter
30
includes orthogonal I and Q channels or components. At predetermined time intervals, after the receiver's root raised cosine filtering, the I channel has one of multiple levels, corresponding to the number of amplitude levels in the 3 bit signal source
10
derives. The Q channel contains no independent information, but causes part of the unwanted lower sideband appearing at the output of up converter
32
to be reduced substantially to zero amplitude. The unwanted lower sideband is removed by circuitry included in vestigial sideband generator
28
and up converter
32
does not reintroduce it. Because up converter
32
“flips” (i.e., inverts) the IF spectrum that digital to analog converter
30
derives, the upper sideband RF output of converter
30
is reduced substantially to zero.
To enable digital to analog converter
30
to produce the desired vestigial sideband signal, vestigial sideband modulator or generator
28
derives the baseband vestigial sideband spectrum illustrated in FIG.
2
. The spectrum of
FIG. 2
has a 6 MHz bandwidth and includes the 309.44056 kHz pilot carrier frequency that pilot inserter
24
derives, as well as a vestigial sideband that extends 309.4405594 kHz below the pilot carrier frequency, to the left of the carrier as illustrated in FIG.
2
. The response curve of
FIG. 2
has a flat portion
38
that extends throughout the vast majority of the transmitter 6.0 MHz bandwidth. The response curve has monotonic root raised cosine (RRC) transitions
40
and
42
at its upper and lower band edges. Transitions
40
and
42
are symmetrical, each having a frequency extent of about 619 kHz (i.e., twice the pilot carrier frequency) between the edges of the transmitter bandwidth and the corner frequencies where the transitions end and flat portion
38
begins. Receivers responsive to transmitters of the type illustrated in
FIG. 1
include a filter with the same response as FIG.
2
.
The amplitude response of each of transitions
40
and
42
at any frequency (f) from the mid-point frequency (f
t
) of each transition is:
Rrc
(
f
)
=
sqrt
(
1
2
[
1
+
cos
π
(
f
-
f
t
2
f
t
)
)
)
(
1
)
Vestigial sideband modulator
28
for deriving the ATSC A/53 standard has generally used a Hilbert filter or phasing method. My co-pending commonly assigned application, Ser. No. 09/239,668, filed Jan. 29, 1999, entitled “Vestigial Sideband Generator Particularly for Digital Television,” discloses a modified Weaver modulator as the device for generating the vestigial sideband spectrum that modulator
28
derives.
The ATSC standard 8VSB transmission system, like many other digital transmission systems, includes a certain amount of “excess bandwidth.” Because the 8VSB transmitter transmits symbols at a rate of 10.76223776 . . . million per second, the transmitter requires a minimum theoretical bandwidth of 5.381118881 . . . MHz (half the symbol rate). Although this is a theor
ADC Broadband Wireless Group, Inc.
Bocure Tesfaldet
Fogg & Associates LLC
Ryan Laura A.
LandOfFree
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