Note: Descriptions are shown in the official language in which they were submitted.
CA 02321941 2000-10-24
1
TECHNIQUE FOR SIMULTANEOUS COMMUNICATIONS
OF ANALOG FREQUENCY-MODULATED AND
DIGITALLY MODULATED SIGNALS USING PRECANCELING SCHEME
This is a division of co-pending Canadian Patent
Application Serial No. 2,208,830 filed June 25, 1997.
Field of the Invention
The invention relates to systems and methods for
communications using analog and digitally modulated
signals, and more particularly to systems and methods for
simulcasting digitally modulated and analog frequency-
modulated (FM) signals over an FM frequency band.
Background of the Invention
The explosive growth of the digital
communications technology has resulted in an ever-
increasing demand for bandwidth for communicating digital
data. Because of the scarcity of available bandwidth for
accommodating additional digital communications, the
industry recently turned its focus on the idea of utilizing
the preexisting analog FM band more efficiently to help
make such accommodation. However, it is required that any
adjustment to the FM band utilization do not significantly
affect the performance of the analog FM communications.
A licensing authority grants FM broadcast
stations licenses to broadcast on different carrier
frequencies. The separation of these carrier frequencies is
200 KHz and are reused geographically. However, in order to
accommodate for the fairly gradual power reduction at the
tails of the spectrum of an analog FM signal, closely
located stations are licensed to use frequency bands
separated by typically at least 800 KHz. The following
provides background information on FM communications:
CA 02321941 2000-10-24
2
~nalocr FM Backaro,~,nr~
Let m(t) denote a modulating signal in FM
modulation. The FM carrier f~ after it is modulated by
m(t) results in the following FM modulated signal x~:
x~(t)- cos(2nf~t . 2nfd~~ r~(t)dz),
J
with the assumption that
wax (m(t) ( . 1,
c
where fd corresponds to the maximum frequency deviation.
In the commercial FM setting, fd is typically 75
KHz, and m(t) is a stereo signal derived from lef t and
right channel information represented by L(t) and R(t)
signals, respectively. The latter are processed by pre
emphasis filters to form Ln(t) and R~(t), respectively.
The frequency response (Hp(f)) of such filters is:
1~j (f/fl)
p(f)~ 1~j(f/f2)
where typically fl = 2.1 KHz,~and fa = 25 KHz.
The stereo signal, m(t), is then generated
according to the following expression:
~a(t). al[Lp(t).Rp(t)].aZCOS(4r1 pt) [Lp(t)-Rp(t)].a3cos(2tI pt),
where typically 2fp = 38 KHz, ai = az = 0.4, and a3 = 0.1.
The rightmost term, a3cos(2rtfpt), in the above expression
is used by FM receivers to coherently demodulate the
passband term involving the difference of the left and
right signal, and is generally referred to as the "Pilot
Signal."
A conventional FM receiver includes a device for
CA 02321941 2000-10-24
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deriving an angle signal from the received version of
x~(t). A mathematical derivative operation of this angle
signal provides m~(t), an estimate of m(t). For
monophonic receivers, a lowpass filter is used to obtain
an estimate of the [Lp(t) + 1~(t)]. Stereo receivers use
the pilot signal to demodulate [LD(t) - F~(t)], which is
then linearly combined with the estimate of [Lp(t) +
Rp(t)] to obtain L~p(t) and R~p(t), the estimates of LD(t)
and Rp(t), respectively. These estimates are then
processed by a deemphasis filter having the following
frequency response Hd(f) to obtain the estimates of the
left and right signals at the transmitter:
x . 1
l.j (fl fl)
A number of techniques have been proposed to
achieve the aforementioned goal of simulcasting digital
data and analog FM signals using a preexisting FM band.
One such technique referred to as an "In Band Adjacent
Channel (IHAC)" scheme involves use of an adjacent band to
transmit the digital data. Fig. 1 illustrates the
relative location of the IBAC for digital broadcast in
accordance with this scheme to the power spectrum of a
host analog FM signal in the frequency domain. As shown
in Fig. l, the center frequencies of the IBAC and the host
signal are, for example, 400 KHz apart. However, the
implementation of the IBAC scheme requires a new license
from the licensing authority. In addition, in a crowded
market like a large populous city in the United States,
the transmission power level using the IBAC scheme needs
CA 02321941 2000-10-24
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to be kept low to have minimal interference with other
channels. As a result, the IBAC scheme may not afford
broad geographic coverage of the digitally modulated
signal. However, digital transmission is more robust than
analog FM transmission, thus leading to broader coverage
with digital transmission if the power levels of the two
transmissions are equal. The actual coverage depends on
the location of the transmitter and interference
environment.
When the IBAC scheme is utilized with removal of
existing analog FM transmitters, an in-band reserved
channel (IBRC) scheme emerges. In accordance with the -
IBRC scheme, the power level of digital transmission is
comparable to that of analog FM transmission, resulting in
at least as broad a digital coverage as the FM coverage.
By successively replacing analog FM transmitters with
IBAC/IBRC transmitting facilities, a migration from a 100$
analog to a 100 transmission of audio information over
the FM band is realized.
Another prior art technique is referred to as an
"In Band on Channel (IBOC)" scheme. Referring to Fig. 2,
in accordance with this scheme, digital data is
transmitted in bands adjacent to and on either side of
the power spectrum of the host analog FM signal, with the
transmission power level of the digitally modulated signal
significantly lower than that of the FM signal. As shown
in Fig. 2, the relative power of the digitally modulated
signal on the IBOC to the host signal is typically 25 dB
lower. Unlike the IBAC scheme, the current FM license is
applicable to implementing the IBOC scheme, provided that
the transmission power level of the digitally modulated
signal satisfy the license requirements. Because of the
requirement of the low power transmission level of the
CA 02321941 2000-10-24
digitally modulated signal, the IBOC scheme may also be
deficient in providing broad geographic coverage of same,
more so than the IBAC scheme. As discussed hereinbelow,
broad coverage of transmission pursuant to the IBOC scheme
5 without an analog host is achievable using a relatively
high transmission power level. As such, a migration from
a 100% analog to a 100% digital transmission of audio
information over the FM band is again realizable.
Other prior art techniques include one that
involves use of a frequency slide scheme where the center
frequency of digital modulation is continuously adjusted
to follow the instantaneous frequency of a host FM
waveform. According to this technique, while the spectra
of the analog and digital waveforms overlap, the signals
generated never occupy the same instantaneous frequency,
thereby avoiding interference of the digitally modulated
signal with the host analog FM signal. For details on
such a technique, one may be referred to: "FM-2 System
Description", USA Digital Radio, 1990-1995. However, the
cost of a system implementing the technique is undesirably
high as its design is complicated, and the system is
required to be of extremely high-speed in order to react
to the constantly changing instantaneous frequency of the
host FM waveform.
Accordingly, it is desirable to have an
inexpensive system whereby digitally modulated signals can
be simulcast with host analog FM signals, with broad
coverage of the digitally modulated signals and virtually
no interference between the digitally modulated signals
and the FM signals.
of the Iaventioa .
In accordance with the invention, a host analog
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FM signal representing analog data and a digitally
modulated signal representing digital data are communicated
over an allocated FM frequency band. The analog FM signal
and a modified version of the digitally modulated signal
are simultaneously transmitted over the FM band. The
digitally modulated signal is modified to account for the
effect of the FM signal on the modified signal when they
are simultaneously transmitted. This effect is canceled
from the digitally modulated signal before the
transmission. As a result, the digital transmission is free
from interference from the analog transmission and affords
a broad coverage. In addition, the rate and power level of
digital transmission are selected in such a manner that the
interference caused by the digital transmission to the
analog transmission is kept at an acceptably low level.
In accordance with one aspect of the present
invention there is provided a method for use in a
communications receiver comprising: receiving a data symbol
corresponding to one of predetermined data symbols in a
signal constellation; extending lines from an origin of
said signal constellation through the predetermined data
symbols therein, respectively; translating the received
data symbol onto a selected one of the extended lines, the
selected line being the closest to said received data
symbol of all of the extended lines; and determining the
predetermined data symbol corresponding to said translated
received symbol.
In accordance with another aspect of the present
invention there is provided a communications apparatus
comprising: a receiver for receiving a data symbol
corresponding to one of predetermined data symbols in a
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6a
signal constellation; means for extending lines from an
origin of said signal constellation through the
predetermined data symbols therein, respectively; means for
translating the received data symbol onto a selected one of
the extended lines, the selected line being the closest to
said received data symbol of all of the extended lines; and
means for determining the predetermined data symbol
corresponding to said translated received symbol.
Brief Description of the Drawings
The present invention taken in conjunction with
the invention described in co-pending Canadian Patent
Application Serial No. 2,208,830 filed June 25, 1997, will
be described in detail hereinbelow with the aid of the
accompanying drawings, in which:
Fig. 1 illustrates the relative power and
location of an in band adjacent channel (IBAC) scheme to an
analog FM carrier in the frequency domain in prior art;
Fig. 2 illustrates the relative power and
locations of in band on channel (IBOC) scheme to a host
analog FM carrier in the frequency domain in prior art;
Fig. 3 is a block diagram of a transmitter for
transmitting digitally modulated and analog FM signals in
accordance with the invention;
Fig. 4 illustrates a composite power spectrum of
the digitally modulated and analog FM signals transmitted
by the transmitter of Fig. 3 during a given time frame;
Figs. 5 and 6 are flow charts depicting the steps
of selecting carriers for digital transmission by
transmitter of Fig. 3;
CA 02321941 2000-10-24
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Fig. 7 illustratively charts the carriers
selected for digital transmission during each transmission
interval;
Fig. 8 is a block diagram of a receiver for
receiving the digitally modulated and analog FM signals
from the transmitter of Fig. 3;
Figs. 9A-9C respectively depict three possible
scenarios where the precancelation scheme in accordance
with the invention may or may not be needed;
Figs. l0A and lOB respectively depict two
possible scenarios where an improved precancelation scheme
in accordance with the invention is applicable;
Fig. 11 illustrates a composite power spectrum
of a host analog FM signal and a multiple sequence spread
spectrum signal in a first direct~sequence code division
multiple access (DSCDMA) system in accordance with the
invention; and
Fig. 12 illustrates a composite power spectrum
of a host analog FM signal and two multiple sequence
spread spectrum signals in a second DSCDMA system in
accordance with the invention.
Detailed D~scri~~tion
Fig. 3 illustrates transmitter 300 for
simulcasting digitally modulated signals and analog FM
signals in accordance with the invention. FM modulator
301, which may reside in a FM radio station, in a standard
way generates a stereo FM signal in response to an analog
input signal. The FM signal is to be transmitted over a
frequency band, which in this instance is 200 KHz wide,
allocated to the FM broadcast. Transmitter 300 is also
used to transmit digital data in accordance with, an
inventive scheme to be described which is an improvement
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over the prior art IBOC scheme. Like the latter, the
inventive scheme may be used to transmit digital data
outside the host FM signal band. However, in a
significant departure from the prior art scheme, the
inventive scheme may also be used to transmit over the
same FM band both digitally modulated and host analog FM
signals.
One of the objectives of the invention is to
allow an FM receiver to process the host analog FM signals
in a conventional manner and provide virtually
undeteriorated FM quality, despite the fact that the FM
signals sharing the same frequency band with the digitally
modulated signals. To that end, digitally modulated
signals are inserted in the host FM band at low enough
power levels to avoid causing significant co-channel
interference at the FM receiver.
Coverage of digitally modulated signals
transmitted at a low power level is normally limited.
However, the inventive scheme improves such coverage. In
addition, the inventive scheme includes a precanceling
scheme whereby the interference which would otherwise be
caused by the host analog FM signal at a digital data
receiver is precanceled.
In accordance with the precanceling scheme,
cancellation or elimination of a calculated response of
the analog FM signal from the digitally modulated signal
is performed at transmitter 300. Since the waveform of
the FM signal is a priori known at the transmitter, the
precancelation is achievable by eliminating from the
digitally modulated signal, before its transmission, the
effect of the FM signal with which the digitally modulated
signal is to be simulcast. Thus, with the precanceling
scheme, the digital data transmission, though sharing the
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same band with the analog FM transmission, is devoid of
interference from the analog FM signal at the digital data
receiver and subject only to the background noise.
In transmitter 300, digital data is transmitted
pursuant to an adaptive orthogonal frequency division
multiplexed scheme. To that end, digital data is input at
multicarrier (or multitone) modem 303, which provides
multiple carrier frequencies or tones for digital data
transmission. The input digital data are channel coded
and interleaved in a conventional manner to become more
immune to channel noise.
The digital data transmission by multicarrier
modem 303 is achieved using N pulse shaping tones or
carriers, each occupying a subband having a bandwidth of
200/N KHz, where N is a predetermined integer having a
value greater than 1. Accordingly, modem 303 includes N
pulse shaping filters, denoted 305-1 through 305-N, each
associated with a different carrier.
The digital data to be transmitted is
represented by data symbols. In accordance with the
invention, modem 303 transmits the data symbols on a
frame-by-frame basis, with each frame containing M
symbols, where M is a predetermined integer having a value
greater than 0.
Within each frame only a subset of carriers of
modem 303 are used for digital data transmission. Fig. 4
shows such a subset populating the FM band during a
particular frame. The frequencies and number of carriers
in the subset vary from frame to frame, and are selected
to minimize the interference caused by the digital data
transmission to the host analog FM signal.
Without loss of generality, let's assume that
only the n-th carrier is used in the current frame, which
y
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starts at time t = 0, and In[0],..., In[M-1] respectively
represent the M symbols allocated to that frame, where 1 s
n s N. The corresponding digitally modulated signal to be
transmitted on the n-th carrier may then be represented by
5 c~, ( t ) as follows
do ( t ) '~ In [ k ] h n ( t-k T) r
x.o
where hn(t) represents the impulse response of pulse
shaping filter 305-n associated with the n-th carrier. If
this were the only signal transmitted in the signal space .
direction defined by hn(t), the digital receiver would
10 obtain the following data symbols represented by 3n(k),
assuming perfect time and carrier synchronization and an
absence of inter-symbol interference and other
impairments:
In[k]'Y(t)' hn(-t) ~~~kT ~
where 0 s k s M-1; y(t) represents the received digitally
modulated signal on the FM band; and h'n(t) represents the
complex conjugate of hn(t). However, the host analog FM
signal, represented by x~(t), is also transmitted on the
same band. As such, the analog signal would make a non-
zero contribution to the received symbol. Such a
contribution is represented by cn[k] as follows:
Cn[k]~XFK(t). hn(-t) ~t.kT .
Thus, if
Y(t) - x~(t) + dn(t) + w(t) ,
where w(t) represents noise from other sources, then
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rn[k]~In[k]'Cn[k]~zn[k]r
zn[k]-w(t)~ hn(-t) ~t-kT .
where zn[k] is attributed to the noise w(t) and can be
expressed as follows:
Since the digitally modulated signal is transmitted by the
transmitter (i.e., transmitter 300) which also transmits
the host analog FM signal x~(t), using the knowledge of
the waveform of the FM signal, precanceler 307 is capable
of computing cn[k]'s at the cost of a short delay. Using -
the computed results, precanceler 307 then precancels the
effect that the FM signal would otherwise have on the
digitally modulated signal when the two signals are
simulcast over the same band. The precanceled digitally
modulated signal at the output of precanceler 307 can be
represented by dn(t) + an(t), where
an(t)~~-Cn[k]hn(t-kT) .
k-0
The precanceled digitally modulated signal is
applied to adder 309 where the precanceled signal is added
to a delayed version of the host FM analog signal. The
latter comes from the output of delay element 311 which
injects into the analog FM signal a delay as long as that
incurred by precanceler 307 in computing cn[k]'s.
Similarly, other delays may be introduced into various
components of circuit 300 to better synchronize their
operations, and should be apparent to a person skilled in
the art in implementing the invention as disclosed.
The output of adder 309 can be expressed as x(t)
- x~(t) + dn(t) + an(t). Equivalently,
y
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X(t)'X~y(t)'dn(t) ~
where
dn(t)'~, (In~k)-cn~kl)l7~(t-kT). (1)
k-0
Thus, if y(t) - x(t) + w(t), the symbol estimates are
rn~k~'cn~k~'(rn~k~-en~k))~Zn(k~-r~~k~'Zrt~k~
In general, a subset S of the N carriers in
multicarrier modem 303 is selected. In that case the
output of adder 309 (x(t)) can be generically represented
as follows:
x(t) - x~(t) +. d(t) ,
where d(t) represents the aggregate digitally modulated
signal and can be expressed as follows:
d(t)-~dn(t)
neS
and where d-n(t) is given by expression (1) above for each
value of n.
The output of adder 309 is applied to linear
power amplifier 313 of conventional design. The latter
transmits an amplified version of the composite signal
x(t) over the allocated FM frequency band.
The manner in which the subset S of the N
carriers in modem 303 is selected for digital data
transmission will now be described. The precanceling
scheme described above guarantees that the digital data is
transmitted without interference from the host analog FM
signal. However, the host analog FM signal may be
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significantly affected by the digitally modulated signal
using such a scheme. Thus, one of the objectives of the
invention is to select as large a subset (S) of the
carriers as possible while the total degradation incurred
to the host analog FM signal is kept at an acceptable
level.
One way to evaluate this degradation is by
simulating an analog FM receiver. Let L(t) and R(t)
respectively denote the left and right channel estimates
of the analog FM receiver subject to an input x(t) -
x~(t)+ d(t). Given the values of L(t) and R(t) which are
available at transmitter 300, L~(t) and R~(t) can be
predetermined whether they are of acceptable quality. By
way of example, but not limitation, the figure of merit
(Y) used in this particular embodiment is defined as
follows:
f [L(t)- L(t) ]Zdt . ~ [R(t)- R (t) ]Zdt
time-frame time-frame
LZ(t)dt ~ f RZ(t)dt
time-frame time-frame
The subset (S) of carriers are selected by
carrier insertion module 316 on a time-frame by time-frame
basis. Module 316 runs an insertion algorithm to turn on
as many carriers as possible during each frame, subject to
a preselected constraint, y"~X, representing the maximum
acceptable degradation to the host analog FM signal. The
precancelation effect of each selected carrier on the FM
signal is taken into consideration in the insertion
algorithm.
The insertion algorithm for. each time frame
comprises carrier pre-ranking process 500 and carrier
selection process 600, which are depicted in Figs. 5 and
P
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14
6, respectively. Turning to Fig. 5, in pre-ranking
process 500, each n-th carrier, for n = 1, 2, ...., N, in
modem 303 takes turn in emulating its transmission with
the host analog FM signal, as indicated at step 503 where
n = 1 initially. At step 505, an interference analysis of
the emulated transmission of the current carrier together
with the FM signal is performed by carrier insertion
module 316. In this particular embodiment, the carrier
contains random digital data in the emulated transmission.
However, in an alternative embodiment, the carrier
contains the actual digital data to be transmitted in the
emulation. In that embodiment, although the emulation
would be more realistic, the bookkeeping of each carrier
for the associated data used in the emulation is
necessary. The above interference analysis also takes
into account the precancelation effect of the current
carrier on the FM signal. Based on the interference
analysis, the value of y corresponding to the carrier in
the time frame under consideration is computed at step
507. The current carrier is then ranked among the
previously ranked carriers in the order of increasing
value of y, as indicated at step 509. At step 511,
module 316 determines whether the last carrier (i.e., n =
N) has gone through the pre-ranking process. If the last
carrier has been ranked, process 500 then comes to an end.
Otherwise, module 316 selects the next carrier (i.e., n =
n + 1) at step 513, and returns to step 503 previously
described.
w Referring now to Fig. 6, in carrier insertion
process 600, the 1-th ranked carrier from process 500 is
added to the subset S of carriers consisting of 1 through
(1-1)-th ranked carriers, as indicated at step 603, where
1 = 1 initially (i.e., in the first run, the subset S
CA 02321941 2000-10-24
consists of the first ranked carrier only). Transmission
of the carriers in the subset S together with the host
analog FM signal is emulated at step 604. At step 605,
module 316 performs an interference analysis of the
5 emulated transmission, taking into account the
precancelation effect of the subset of carriers on the FM
signal. Based on the interference analysis, module 316 at
step 607 computes the value of YagQregace corresponding to
the subset of carriers. At step 611, module 316
10 determines whether the value of Y$qgregace exceeds that of
Y,~' If Yaqgregate ' Y",~~ i . e. , the aggregate degradation
greater the maximum acceptable degradation, which is not
allowed, process 600 is prepared to exit. Specifically,
the 1-th ranked carrier just added to the subset S is
15 eliminated therefrom, as indicated at step 613, and
process 600 comes to an end.
Otherwise if YaQQregae. s Y",~,~, module 316
determines at step 615 whether the last ranked carrier has
been added to the subset (i.e., 1 = N). If 1 = N, process
600 again comes to an end. Otherwise, module 316 selects
the next higher ranked carrier (i.e., 1 = 1 t 1) at step
617, and returns to step 603 previously described.
Since, in practice, processes 500 and 600 take
certain time to run, for synchronization purposes, the
corresponding delay is introduced to the analog signal
transmission using delay element 311 described above.
However, this delay can be significantly shortened if
parallel processing is applied. For example, by using
parallel processing, module 316 can compute the respective
Y's in process 500 in parallel.
Fig. 7 illustratively charts the results of a
simulation where the above insertion algorithm was
applied. Each column in Fig. 7 is associated with a
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16
transmission interval T. That is, the first column is
associated with the first transmission interval; the
second column is associated with the second transmission
interval; and so on and so forth. Each box in a column
represents the status of a carrier in modem 303 requiring
a subband of 200/N KHz during a given frame. A selected
carrier is indicated by a shaded box. As shown in Fig. 7,
during each transmission interval, only a subset of the
carriers are selected. In addition, the carriers in the
subset vary adaptively with time.
It should be pointed out at this juncture that
since the carriers selected by carrier insertion module
316 vary from frame to frame, a control channel is
required to convey information about the selected carriers
to the receiver, which is described hereinbelow.
Specifically, the receiver needs to be informed of which
particular carriers are on or off during each frame. For
conveying such information, control channel 401 in Fig. 4
is reserved outside the analog signal spectrum. In
addition, control channel processor 319 is employed to
generate one-bit information per carrier per. frame (i.e.,
N bits per transmission interval) to be transmitted over
control channel 401.
As an alternative to the above control channel
arrangement, it will be appreciated that a person skilled
in the art may use a limited control channel arrangement
where when certain carriers are always on or off, no
control information is transmitted for those carriers, or
when carriers are turned on or off as a group, only one
bit per frame is transmitted for that group of carriers.
Other possibilities include use of an adaptive control
channel arrangement where a different control channel is
used depending on the type of the data communicated (e. g.,
CA 02321941 2000-10-24
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a conversation, a pause, music, etc.).
Fig. 8 illustrates receiver 800 for receiving
from the FM frequency band a composite signal x'(t)
corresponding to x(t) and the control channel information
generated at transmitter 300. Because of the
precancelation performed at the transmitter in accordance
with the invention, the design of receiver 800 is
advantageously simple. As mentioned before, FM receiver
803 in receiver 800 is of conventional design and, in a
standard way recovers the original analog signal.
Synchronization control decoder 805 decodes the control
channel information in x'(t) to identify the selected
carriers used for digital transmission in each
transmission interval. The identities of the carriers are
conveyed to demodulator 807. With the knowledge of the
selected carriers, demodulator 807 performs the inverse
function to modulator 303 on x'(t) to recover therefrom
the digital data, albeit channel-coded and interleaved.
The foregoing merely illustrates the principles
of the invention. It will thus be appreciated that those
skilled in the art will be able to devise numerous other
schemes which embody the principles of the invention and
are thus within its spirit and scope.
For example, it will be appreciated that a
person skilled in the art will apply the inventive
precanceling scheme with a variety of standard digital
modulation techniques including, for example, MPSK and
MQAM techniques.
Moreover, the precanceling scheme described
above may be selectively applied. Under certain
situations, precancelation may not be necessary. One such
situation is demonstrated here where a well-known QPSK
constellation is used for generating data symbols. Figs.
CA 02321941 2000-10-24
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9A through 9C respectively show three possible scenarios
where we assume that the symbol transmitted was at 1 + j.
In the scenario of Fig. 9A, without
precancelation, the received symbol in the absence of
noise is indicated by "x" inside the square whose corners
are marked by the four possible symbols. Since the
received symbol is closer to the decision boundaries than
1 + j which is the intended symbol (indicated by a
circle), the effective SNR of this received symbol has
been lowered. Precancelation in this case effectively
moves the symbol in the direction of the dashed arrow to
the position 1 + j to regain the desired SNR.
In the scenario of Fig. 9B, however, the
effective SNR of the received symbol without precanceling
is higher than that of 1 + j. Since precancelation would
reduce the SNR of the received symbol, and possibly
introduce additional distortion to the host FM signal, we
may want to refrain from applying precancelation in this
case.
In the scenario of Fig. 9C, even though
precancelation is necessary in this case, the
precancelation described above moves the received symbol
in the direction of the dashed arrow to the position of 1
+ j. However, such precancelation is inferior to the one
that, for example, moves the received symbol in the
direction of the solid arrow shown in Fig. 4. The
precancelation represented by the solid arrow further
improves the SNR of the symbol, and possibly the host FM
signal distortion.
Based on the above observation and the
disclosure heretofore, it will be appreciated that a
person skilled in the art will devise other precanceling
schemes which may be more immune to carrier recovery
CA 02321941 2000-10-24
19
errors than the present scheme. For example, an improved
precanceling scheme is depicted here in Figs. 10A and 10B
where the scheme is applied to the scenarios of Figs. 9B
and 9C, respectively. As.shown in Figs. 10A and lOB, the
improved precancelation moves the received symbol "x" in
the direction of the solid arrow perpendicularly to a
solid line denoted L. Line L is an extension of the
dashed line emanating from the origin of the
constellation, and extends outwardly from the point 1 + j.
Lines involving other symbols in the constellation can be
formed in a similar manner. However, the received symbol
is translated onto the closest line, which is L in this
instance, with the minimum Euclidean distance (i.e.,
perpendicularly to the line). To minimize intersymbol
interference in case of incorrect sampling instants, we
may limit the amplitude of the translated symbol by
limiting the length of line L. It should be noted that
this improved precanceling scheme is applicable to digital
transmission not only involving QPSK, but also other
constellations, such as MPSK, MQAM, PAM, and
multidimensional constellations.. In the case of MPSK, the
improved precanceling scheme can be applied to all signal
points therein, while in the case of MQAM, the improved
precanceling scheme should be selectively applied to the
outer signal points therein.
In addition, the disclosed precanceling scheme
can be applied to digital signaling based on direct
sequence code division multiple access (DSCDMA) sequences,
which are of the type commonly used in cellular mobile
radio downlink (base-to-mobile) transmission. In
accordance with the DSCDMA scheme, a direct sequence
spread spectrum signal is obtained by multiplying a slowly
varying data signal and a fast varying spreading sequence.
CA 02321941 2000-10-24
The sequence is a pseudo-noise code known to the receiver.
For example, by using the so-called "Walsh" functions,
orthogonal spread spectrum signals are generated on the
same carrier. Fig. 11 shows an IBOC scheme where digital
5 spectrum signals are generated on the host carrier. Since
all sequences are originated from the same site,
coordination by means of Walsh functions is feasible.
Fig. 12 shows another example where Walsh
functions are applied to two subcarriers individually to
10 generate two groups of spread spectrum signals. These two
groups of signals are frequency orthogonal to each other.
As shown in Fig. 12, the spectra of the two groups of
signals partially overlap the spectrum of the host analog
FM signal.
15 The disclosed precanceling scheme for the
multicarrier system needs only to be slightly modified
when it is applied to a direct sequence spread spectrum
system. The modification involves the change of hn(t) to
~n(t), where ~n(t) represents a component spreading signal
20 based on the standard spreading code and Walsh functions.
The insertion algorithm for the multicarrier system is
also applicable to the direct sequence spread spectrum
system.
One advantage of the multicarrier system over the DSCDMA
system is that the former can populate close to the edges
of the 200 KHz band most of the time, especially when the
analog message rate is low, resulting in a temporarily
small frequency deviation.
It will be appreciated that based on the above
disclosure that the inventive precanceling scheme is
applicable to a DSCDMA system, a person skilled in the art
will similarly apply the inventive technique to orthogonal
frequency hopping (FH) systems.
CA 02321941 2000-10-24
21
In addition, although in the disclosed
embodiment, a particular digitally modulated signal which
is linearly modulated is simulcast with an analog FM
signal which is non-linearly modulated, the invention
broadly applies to a simulcast of any linearly modulated
signals with any non-linearly modulated signals.
Finally, the disclosed precanceling scheme is
also applicable to the prior art IBOC scheme of Fig. 2.
In an IBOC system, precancelation of the analog FM signal
spectral tail provides at least two benefits to the
digital receiver. The performance of the digital receiver
improves since any interference from the analog signal has
been eliminated. As a result, for given digital reception
quality, a lower transmitting power for digitally
modulated signals may be used. In addition, the
performance of the digital receiver can be readily
determined since it is independent of the host analog FM
signal. More importantly, the digital data rate in such
an IBOC system can be increased, as the digital carriers
can be inserted closer to the analog host carrier.
