Technique for simultaneous communications of analog frequency-modulated and digitally modulated signals using precanceling scheme

In a system for simulcasting digitally modulated and analog FM signals over the same FM frequency band, the effect of the analog FM signal on the digitally modulated signal in the simulcast is calculated and canceled from the latter signal before its transmission. As a result, the digital transmission is free from interference from the analog FM signal. Moreover, the digital transmission is designed in such a manner that the interference caused thereby to the analog FM signal is kept at a minimal level.

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: 
Analog FM Background 
Let m(t) denote a modulating signal in FM modulation. The FM carrier 
f.sub.c after it is modulated by m(t) results in the following FM 
modulated signal x.sub.FM : 
##EQU1## 
with the assumption that 
##EQU2## 
where f.sub.d corresponds to the maximum frequency deviation. 
In the commercial FM setting, f.sub.d is typically 75 KHz, and m(t) is a 
stereo signal derived from left and right channel information represented 
by L(t) and R(t) signals, respectively. The latter are processed by 
pre-emphasis filters to form L.sub.p (t) and R.sub.p (t), respectively. 
The frequency response (H.sub.p (f)) of such filters is: 
##EQU3## 
where typically f.sub.1 =2.1 KHz, and f.sub.2 =25 KHz. 
The stereo signal, m(t), is then generated according to the following 
expression: 
EQU m(t)=a.sub.1 [L.sub.p (t)+R.sub.p (t)]+a.sub.2 cos(4.pi.f.sub.p t)[L.sub.p 
(t)-R.sub.p (t)]+a.sub.3 cos(2.pi.f.sub.p t), 
where typically 2f.sub.p =38 KHz, a.sub.1 =a.sub.2 =0.4, and a.sub.3 =0.1. 
The rightmost term, a.sub.3 cos(2.pi.f.sub.p t), 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 deriving an angle signal 
from the received version of x.sub.FM (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 [L.sub.p (t)+R.sub.p (t)]. Stereo receivers use the pilot signal to 
demodulate [L.sub.p (t)-R.sub.p (t)], which is then linearly combined with 
the estimate of [L.sub.p (t)+R.sub.p (t)] to obtain L.sub.p (t) and 
R.sub.p (t), the estimates of L.sub.p (t) and R.sub.p (t), respectively. 
These estimates are then processed by a deemphasis filter having the 
following frequency response H.sub.d (f) to obtain the estimates of the 
left and right signals at the transmitter: 
##EQU4## 
Prior Art Techniques 
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 (IBAC)" 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. 1, 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 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 
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 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. 
SUMMARY OF THE INVENTION 
In accordance with the invention, a host analog 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.

DETAILED DESCRIPTION 
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 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 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 starts at time t=0, and I.sub.n [0], . . . , 
I.sub.n [M-1] respectively represent the M symbols allocated to that 
frame, where 1.ltoreq.n.ltoreq.N. The corresponding digitally modulated 
signal to be transmitted on the n-th carrier may then be represented by 
d.sub.n (t) as follows: 
##EQU5## 
where h.sub.n (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 h.sub.n (t), the 
digital receiver would obtain the following data symbols represented by 
I.sub.n (k), assuming perfect time and carrier synchronization and an 
absence of inter-symbol interference and other impairments: 
EQU I.sub.n [k]=y(t)*h.sub.n *(-t).linevert split..sub.t=kT, 
where 0.ltoreq.k.ltoreq.M-1; y(t) represents the received digitally 
modulated signal on the FM band; and h.sub.n *(t) represents the complex 
conjugate of h.sub.n (t). However, the host analog FM signal, represented 
by x.sub.FM (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 c.sub.n [k] as follows: 
EQU c.sub.n [k]=x.sub.FM (t)*h.sub.n *(-t).linevert split..sub.t=kT. 
Thus, if 
EQU y(t)=x.sub.FM (t)+d.sub.n (t)+w(t), 
where w(t) represents noise from other sources, then 
EQU I.sub.n [k]=I.sub.n [k]+c.sub.n [k]+z.sub.n [k], 
where z.sub.n [k] is attributed to the noise w(t) and can be expressed as 
follows: 
EQU z.sub.n [k]=w(t)*h.sub.n *(-t).linevert split..sub.t=kT. 
Since the digitally modulated signal is transmitted by the transmitter 
(i.e., transmitter 300) which also transmits the host analog FM signal 
x.sub.FM (t), using the knowledge of the waveform of the FM signal, 
precanceler 307 is capable of computing c.sub.n [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 d.sub.n (t)+a.sub.n (t), where 
##EQU6## 
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 c.sub.n [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.sub.FM 
(t)+d.sub.n(t)+a.sub.n (t). Equivalently, 
EQU x(t)=x.sub.FM (t)+.differential..sub.n (t), 
where 
##EQU7## 
Thus, if y(t)=x(t)+w(t), the symbol estimates are 
EQU I.sub.n [k]=c.sub.n [k]+(I.sub.n [k]-c.sub.n [k])+z.sub.n [k]=I.sub.n 
[k]+z.sub.n [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: 
EQU x(t)=x.sub.FM (t)+d(t), 
where d(t) represents the aggregate digitally modulated signal and can be 
expressed as follows: 
##EQU8## 
and where d.sub.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 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.sub.FM 
(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 (.gamma.) used in this particular embodiment is defined as follows: 
##EQU9## 
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, .gamma..sub.max, 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 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 .gamma. 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 .gamma., 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. 
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 (l-1)-th ranked carriers, as indicated at step 603, where l=1 
initially (i.e., in the first run, the subset S 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 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 .gamma..sub.aggregate corresponding 
to the subset of carriers. At step 611, module 316 determines whether the 
value of .gamma..sub.aggregate exceeds that of .gamma..sub.max. If 
.gamma..sub.aggregate &gt;.gamma..sub.max, i.e., the aggregate degradation 
greater the maximum acceptable degradation, which is not allowed, process 
600 is prepared to exit. Specifically, the l-th ranked carrier just added 
to the subset S is eliminated therefrom, as indicated at step 613, and 
process 600 comes to an end. 
Otherwise if .gamma..sub.aggregate .ltoreq..gamma..sub.max, module 316 
determines at step 615 whether the last ranked carrier has been added to 
the subset (i.e., l=N). If l=N, process 600 again comes to an end. 
Otherwise, module 316 selects the next higher ranked carrier (i.e., l=l+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 .gamma.'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 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., 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. 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, 
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. 9C. 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 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 10B, 
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. 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 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 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. 
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 h.sub.n (t) to 
.xi..sub.n (t), where .xi..sub.n (t) represents a component spreading 
signal 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. 
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.