Method and apparatus for demodulating multi-level QAM signal

Disclosed is a demodulating apparatus for preventing a reduction in reliability of demodulated data even if a signal point of a multi-level QAM signal is detected, because of influence from fading or noises, in a position where such a signal is not normally existent. To this end, a modulated signal modulated by a multi-level QAM modulation system is orthogonally detected, and analog signals of I and Q channels placed in an orthogonal relationship with each other are outputted. By an identification device, the analog demodulated signals are digitized and then outputted as digital demodulated signals of I and Q channels. If a signal point outside a normal signal point arrangement is detected among the outputs, a digital demodulated signal within a normal signal point arrangement is substituted for the detected signal and then original code data is reproduced.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for demodulating a 
multi-level QAM (Quadrature Amplitude Modulation) signal, and more 
particularly to one for orthogonally detecting a multi-level QAM signal, 
demapping the QAM signal after its digitization and then reproducing 
original code data. 
2. Description of the Related Art 
FIG. 8 is a block diagram showing a conventional apparatus for demodulating 
a multi-level QAM signal. Referring to FIG. 8, the orthogonal detector 1 
orthogonally detects a modulated signal Sr modulated by a multi-level QAM 
modulation system and outputs analog demodulated signals of I and Q 
channels placed in an orthogonal relationship with each other. The filter 
2 performs waveform-shaping of the outputs of the orthogonally detected 
signal and then outputs analog demodulated signals Si and Sq. The decision 
device 3 digitizes the analog demodulated signals Si and Sq and outputs 
these signals as digital demodulated signals D1i and D1q. And the 
demapping circuit 5 reproduces original code data Di and Dq from the 
digital demodulated signals D1i and D1q. 
The multi-level QAM modulation system has a mapping circuit provided in a 
transmission side. For transmitting signals, the mapping circuit is 
disposed and transmits to be optimal in signal space such that a minimum 
free distance between code words can be maximum. The demapping circuit 5 
is provided in a receiving side to perform an operation contrary to that 
of the mapping circuit in the transmission side. The demapping circuit 5 
will be described more in detail later. 
Referring to FIG. 9, in which signal point arrangements are shown for 
32-level and 64-level QAM signals of a right-angle grid structure. White 
circles within an inner dotted line indicate a signal point arrangement 
for 32-level QAM signals. A signal point arrangement of 64-level QAM 
signals is indicated by black circles arrangement added to the white 
circles arrangement. The identification device 3 receives analog 
demodulated signals Si and Sq of I and Q channels arranged in such a 
signal-point manner, identifies these signals and then outputs digital 
values corresponding to respective signal points. In other words, as shown 
in FIG. 9, preset threshold levels L1, L2 and L3 are compared with each 
other concerning I and Q channels, high and low levels are identified and 
divided into 8 sections each, whereby digital demodulated signals D1i and 
D1q of bits I1, I2 and I3 and bits Q1, Q2 and Q3 are respectively 
outputted. 
In the case of 32-level QAM signals as in the case of 64-level QAM signals, 
the identification device in which levels must be identified and divided 
into 8 sections of I and Q channels respectively. When 32-level QAM 
signals are affected by fading or noises in a transmission section and, as 
a result, signal points are positioned outside the dotted line shown in 
FIG. 9 and thus shifted from a normal signal point arrangement, signals of 
signal points that are not normally existent may be identified and 
detected by the identification device. If such data regarding the signal 
outside the dotted line is detected by the identification device and 
outputted to the demapping device, the demapping circuit cannot normally 
reproduce original code data and thus outputs indefinite data. A bit error 
then occurs and, consequently, reliability of demodulated data may be 
reduced. 
The above phenomenon also occurs in the case of 128-level QAM signals. In 
FIG. 10, signal point arrangements for 128-level and 256-level QAM signals 
are shown. For convenience, only a first quadrant is shown. Also in FIG. 
10, White circles within a dotted line indicate a signal point arrangement 
for 128-level QAM signals. For I and Q channels, in the first quadrant, an 
identification device for identifying levels and dividing the levels into 
8 sections is required. Accordingly, by using the identification device, a 
signal point arrangement for 256-level QAM signals is also detected, the 
256-level QAM signals being indicated by black circles outside the signal 
point arrangement for the 128-level QAM signals. 
Thus, when a signal is shifted from a normal signal point arrangement, data 
regarding the signal outside the dotted line is also detected by the 
identification device and outputted to the demapping circuit. 
Consequently, a problem of demodulating indefinite data occurs similarly. 
A similar problem occurs for a signal point arrangement of multi-level QAM 
signals of a honeycomb structure which is disclosed in JP(A) 1-500636 
(1989). Specifically, for a signal point arrangement of a honeycomb 
structure shown in FIG. 11, since signal points exist to be detected 
outside a normal signal arrangement surrounded by a dotted line within a 
conversion input signal range of analog demodulated signals of I and Q 
channels, bit errors also occur. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an apparatus for 
demodulating a multi-level QAM signal, which can prevent a reduction in 
reliability of demodulated data even if signal points of multi-level QAM 
signals are detected, because of influence from fading or noises, in 
positions where these signal points are not normally existent. 
According to the present invention, a multi-level QAM signal demodulating 
apparatus for orthogonally detecting a multi-level QAM (Quadrature 
Amplitude Modulation) signal, demapping this QAM signal after its 
digitization and then reproducing original code data comprises 
substituting means for substituting, when a digital demodulated signal of 
a signal point shifted from a normal signal point arrangement for 
multi-level QAM signals is detected, the digital demodulated signal of a 
signal point within the normal signal point arrangement for the detected 
signal. 
Specifically, the substituting means includes a detection circuit for 
detecting the digital demodulated signal of the signal point outside the 
normal arrangement, a storage circuit for storing a substitution pattern 
beforehand, the substitution pattern being used for substituting, for the 
detected demodulated signal, a digital demodulated signal of a signal 
point within the arrangement shortest from the signal point outside the 
arrangement, and a circuit for selecting, upon receiving a detecting 
result of the detection circuit, the digital demodulated signal of the 
signal point within the arrangement based on the substitution pattern. 
Alternatively, the substituting means includes a detection circuit for 
detecting the digital demodulated signal of the signal point outside the 
arrangement, and a combinational logical circuit for producing, upon 
receiving a detecting result of the detecting circuit, the digital 
demodulated signal of the signal point within the arrangement. Otherwise, 
the substituting means includes ROM where data regarding digital 
demodulated signal after substitution is written beforehand by using the 
digital demodulated signal outputted from the digital conversion means as 
an address.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be described with reference to the accompanying 
drawings. 
FIG. 1 is a block diagram showing a first embodiment of the present 
invention, where like reference numerals denote corresponding constituting 
elements of the conventional example shown in FIG. 8. 
In FIG. 1, an orthogonal detector 1 orthogonally detects a modulated signal 
Sr modulated by a multi-level QAM modulation system and outputs analog 
demodulated signals of I and Q channels placed in an orthogonal 
relationship with each other. The filter 2 performs waveform-shaping of 
the outputs of orthogonal detection and outputs analog demodulated signals 
Si and Sq. The identification device 3 digitizes the analog demodulated 
signals Si and Sq and outputs these signals as digital demodulated signals 
D1i and D1q of I and Q channels. The substitution circuit 4 substitutes, 
when digital demodulated signals D1i and D1q of signal points outside a 
normal signal point arrangement for a multi-level QAM signal, digital 
demodulated signals D1i and D1q of signal points within the normal signal 
point arrangement for the detected demodulated signals. The demapping 
circuit 5 reproduces original code data D2i and D2q from digital 
demodulated signals outputted from the substitution circuit 4. 
A signal point arrangement for a multi-level QAM signal is uniquely decided 
by the modulation system. Accordingly, whether demodulated and detected 
signal points are within the normal signal point arrangement or not can be 
easily determined. When a signal of a signal point outside the normal 
signal point arrangement is detected, by substituting a signal within the 
normal signal point arrangement which is in the shortest distance from the 
signal point for the detected signal, outputting of indefinite data can be 
prevented during demapping. 
FIG. 2 is a block diagram showing a first embodiment of the substitution 
circuit 4. 
The out-of arrangement signal detection circuit 41 receives the digital 
demodulated signals D1i and D1q outputted from the identification device 3 
and detects a digital demodulated signal of a signal point outside the 
normal signal point arrangement. The substitution pattern storage circuit 
42 stores a substitution pattern beforehand for substituting a digital 
demodulated signal of a signal point within the normal signal point 
arrangement for the digital demodulated signal of the signal point outside 
the normal signal point arrangement. The selector 43 substitutes, when the 
out-of arrangement signal detection circuit 41 detects an out-of 
arrangement signal, a within arrangement signal for the out-of arrangement 
signal based on the substitution pattern stored beforehand in the 
substitution pattern storage circuit 42, and then outputs digital 
demodulated signals D2i and D2q. 
FIG. 3 shows an example of a substitution pattern for a 32-level QAM 
signal. Herein, normal signal points are indicated by alphabets while 
out-of arrangement signal points are indicated by bracketed alphabets. 
When out-of arrangement signal point is detected, a normal signal point 
which is in the shortest distance from the out-of arrangement signal is 
substituted for the same. For example, signal points A within the 
arrangement are substituted for out-of arrangement signal points (A1) and 
(A2), and signal points F within the arrangement are substituted for two 
signal points (F1) and (F2) outside the arrangement. 
A specific substitution method for signal points outside the normal 
arrangement detected in the receiving side will be described below. 
A cause of mistaken signal point determination during QAM demodulation may 
be attributed to fluctuation in amplitude level or a shift in carrier wave 
synchronous phase. In the former case, signal points are moved radially 
from a center point of an orthogonal coordinate. In the latter case, 
signal points are moved on places equidistant (i.e., circle) from the 
center point of the orthogonal coordinate. Since mistakenly determined 
signal points are arranged outside normal signal points, any points on the 
outer circumference of the normal signal points may have been mistaken. 
Thus, in principle, the substitution method substitutes, for a signal point 
outside the normal arrangement, a signal point which is in the shortest 
distance therefrom in a center direction of an orthogonal axis. Reference 
is now made to FIG. 3. If a distance between signal points adjacent to 
each other in parallel with I, Q and CH axial directions is d, then a 
distance between signal points (A1) and A is d, a distance between signal 
points (A1) and B is .sqroot.2.times.d and thus the distance with A is 
shortest. Therefore, the signal point A is substituted for the signal 
point (A1). 
Next, if distances are equal to a plurality of signal points A and S as in 
the case of signal points (T1) and (T2), determination is made as follows. 
For the signal point (T1), signal points A and S exist where a distance is 
shortest, and a signal point T exists in the center direction of the 
orthogonal axis. Consequently, use of only the above deciding method is 
not enough for determination. Thus, the number of errors when the signal 
point A is substituted for the signal point (T1) is considered. 
It is assumed herein that an error between signal points adjacent to each 
other in parallel with the axis is 1 bit. 
No errors when an original signal point is A. 
1 bit error when an original signal point is T. 
2 bit error when an original signal point is S. 
EQU Average error number=(0+1+2)/3=1 bit (1) 
If the signal point S is substituted for the signal point (T1), the 
following result is obtained. 
EQU Average error number=(2+1+0)/3=1 bit (2) 
If the signal point T is substituted for the signal point (T1), the 
following result is obtained. 
1 bit error when an original signal point is A. 
No errors when an original signal point is T. 
1 bit error when an original signal point is S. 
EQU Average error number=(1+0+1)/3=2/3 bit (3) 
As can be understood from the above expressions (1) to (3), the average 
error number is smallest when the signal point T is substituted for the 
signal point (T1). 
As described above, in principle, the substitution method substitutes, for 
a signal point outside the arrangement, a signal point within the 
arrangement which is in the shortest distance therefrom in the center 
direction of an orthogonal axis. If a plurality of signal points of the 
shortest distance exist, average error numbers are calculated for the 
respective signal points of the shortest distance and a signal point 
having the smallest average error number is substituted for a signal point 
outside the arrangement. FIG. 3 shows an arrangement view when all out-of 
arrangement signal points (A1) to (T2) are decided by the substitution 
method. 
Next, a substitution method for arranged signal points of a honeycomb 
structure of FIG. 11 will be described. 
FIG. 4 shows an example of a signal substitution pattern of the honeycomb 
structure. Normal signal points are indicated by alphabets while signal 
points outside the arrangement are indicated by bracketed alphabets. 
In the arranged signal points of the honeycomb structure, for example, the 
out-of arrangement signal point (e) of a first quadrant is located in a 
distance equal to those of within arrangement signal points d and e. 
Likewise, the signal point (f1) is located in a distance equal to those of 
signal points f and e, and the signal point (g) is located in a distance 
equal to those of signal points f and g. The signal point (f2) is located 
in a distance equal to those of signal points d and f. Thus, since almost 
all equidistant signal point arrangements have 2 points or more, signal 
points are substituted such that a difference in distance from the center 
point of the orthogonal axis can be minimum. As a result, for the out-of 
arrangement signal point (e), comparison of distances between the within 
arrangement signal points d and e shows that the signal point e is 
smaller. Accordingly, the within arrangement signal point e is substituted 
for the out-of arrangement signal point (e). 
For the other signal points, similar substitution is carried out by using 
the above method. As a result, as shown in FIG. 4, substitution can be 
performed for all the out-of arrangement signal points (a), (b), (e), 
(f1), (f2), (g), (i), (j1), (j2), (k), (n), (o) and (r). FIG. 5 is a view 
showing a second embodiment of the substitution circuit 4. 
In FIG. 5, the out-of arrangement signal detection circuit 44 receives 
digital demodulated signals D1i and D1q outputted from the identification 
device 3 and detects a signal outside the normal signal arrangement. The 
substitution logical circuit 45 includes a combinational logical circuit 
for receiving a detection signal from the out-of arrangement signal 
detection circuit 44 and substituting a predetermined digital demodulated 
signal within arrangement signal for a digital demodulated signal of an 
out-of arrangement signal point. 
FIG. 6 shows an example of a circuit configuration for substituting a 
digital demodulated signal within arrangement signal point A for a digital 
demodulated signal detected at an out-of arrangement signal point (A) 
shown in FIG. 9. In this case, I and Q channel digital demodulated signals 
of an out-of arrangement signal point (A1) outputted from the 
identification device 3 are D1i="010" and D1q="111" as shown in FIG. 9. 
A "010" detection section of the out-of arrangement signal detection 
circuit 44 outputs "1" when a digital demodulated signal D1i of I channel 
is "010". Its "111" detection section outputs "1" when a digital 
demodulated signal D1q of Q channel is "111". Upon receiving a detection 
signal from the out-of arrangement signal detection circuit 44, the 
substitution logical circuit 45 outputs digital demodulated signals 
D2i="010" and D2q="110" within arrangement signal point A. The out-of 
arrangement signal detection circuit 44 and the substitution logical 
circuit 45 are configured to deal with all the out-of arrangement signal 
points (A1) to (T2) shown in FIG. 3. Thus, according to the second 
embodiment, the combinational logical circuit is configured so as to 
decide out-of arrangement and within arrangement signals by using the 
substitution pattern described above with reference to FIG. 3, 
digital-demodulate a signal of each signal point, convert the signal for 
logical level reading and then realize its logical level. 
FIG. 7 shows a third embodiment of the substitution circuit 4 which is 
configured by using ROM 46. 
In ROM 46, the data of digital demodulated signals D2i and D2q after 
substitution is written beforehand by using digital demodulated signals 
D1i and D1q as addresses. For example, based on a substitution pattern 
like that shown in FIG. 3, substitution data regarding a corresponding to 
a signal point within arrangement is read when an out-of arrangement 
signal point is addressed, and the same data is read when the signal point 
within arrangement is addressed. 
In the above description, a 32-level QAM signal of the right-angle grid 
structure was taken as an example. However, the same can apply to a 
128-level QAM signal of the right-angle grid structure. In other words, 
the present invention can generally apply to 22N+1-level QAM signals (N is 
an integer of 1 or higher). Also, for a structure other than the 
right-angle grid structure, for example, in the case of a multi-level QAM 
signal of a honeycomb structure, similar substitution processing can be 
performed. As apparent from the foregoing, according to the present 
invention, even if a multi-level QAM signal is affected by fading or 
noises in a transmission section and a signal of a signal point that is 
not normally existent is detected by the identification device, the 
demapping circuit is unable to output indefinite data by substituting a 
signal of a normally existent signal point for the detected signal and 
outputting this signal to the demapping circuit. Accordingly, reliability 
of demodulated data can be increased and the occurrence of bit errors can 
be prevented. 
Although the preferred embodiment of the present invention has been 
described in detail, it should be understood that various changes, 
substitutions and alternation can be made therein without departing from 
spirit and scope of the inventions as defined by the appended claims.