Multiplex communications – Trasmultiplexers
Reexamination Certificate
1999-06-25
2003-07-08
Urban, Edward F. (Department: 2685)
Multiplex communications
Trasmultiplexers
C370S343000, C370S344000, C370S347000, C370S481000
Reexamination Certificate
active
06590871
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a multi-carrier modulation apparatus and more particularly, to a transmitter for use in mobile communication.
A transmultiplexer (TMUX) is available as an apparatus which can process a multiplex signal in its multiplexed state, that is, without performing an operation of lowering the sampling frequency to restore the multiplex signal to individual signals. The TMUX is an apparatus adapted to carry out mutual conversion between a time-division-multiplex (TDM) signal and a frequency-division-multiplex (FDM) signal. By applying the TMUX, a plurality of filter banks having the same pass-band characteristic but having different center frequencies can be constructed to permit a collective processing of a plurality of filtering and modulation operations of the multi-carrier modulation apparatus.
TMUX is detailed in, for example, “Modulation Form Conversion Apparatus” in JP-A-1-117437 and “TDM-FDM Transmultiplexer: Digital Polyphase and FFT” by Maurice G. Bellanger et al, IEEE Trans. on Communications, vol. COM-22, No. 9, September 1974. It is also described in “Applications of Digital Signal Processing” edited by The Institute of Electronic Information and Communication Engineers of Japan, page 126, FIG. 5.27, May 20, 1981.
At first, a prior art multi-carrier modulation method utilizing a TMUX will be described below.
A TMUX and filter bank will first be outlined with reference to
FIGS. 3 and 4
.
FIG. 3
is a spectrum diagram for explaining the operation principle of the TMUX and filter bank along signal processing steps, where abscissa represents frequency and ordinate represents signal level. In the figure, f
S
designates a sampling frequency of polyphase filters
43
0
to
43
M−1
, f
k
designates a shift frequency, f
B
designates a pass-bandwidth, k−1 designates a spectrumof (k−1)-th filter, k designates a spectrum of k-th filter and k+1 designates a spectrum of (k+1)-th filter where k is an integer as defined by M≧k≧1.
FIG. 4
is a block diagram showing an example of the construction of the TMUX for realization of TDM-FDM conversion similar to that in the TDM-FDM converter explained in the aforementioned IEEE Trans. or illustrated in FIG. 5.27 of the aforementioned “Applications of Digital Signal Processing”. The TMUX has a TDM input
40
, a demultiplexing switch
41
, an M-point inverse discrete Fourier transform (IDFT) unit
42
, polyphase filters
43
0
to
43
M−1
, phase shifters
44
0
to
44
M−1
delay circuits
45
0
to
45
M−1
, an adder
46
and a FDM output terminal
47
. Here, suffixes 0 to M−1 of the reference numerals mean that there are M constituent components of 0-th to (M−1)-th components. The TDM input terminal
40
is connected to the demultiplexing switch
41
which in turn is connected to the respective input terminals of the M-point IDFT unit
42
. Output terminals of the M-point IDFT unit
42
are connected to the polyphase filters
43
0
to
43
M−1
, respectively. Especially, the polyphase filter
43
0
is called an original filter. The polyphase filters
43
0
to
43
M−1
are connected to the delay circuits
45
0
to
45
M−1
, respectively, through the corresponding phase shifters
44
0
to
44
M−1
and the respective delay circuits
45
0
to
45
M−1
are connected to the adder
46
which in turn is connected to the FDM output terminal.
In
FIGS. 3 and 4
, it is presupposed that all signals are complex signals in order to handle quadrature modulation waves. A frequency shift f
K
for conversion of the TDM signal into the FDM signal is so selected as to satisfy such a condition that the sampling frequency f
S
of the polyphase filters is related to the shift frequency f
K
of each channel in a relation of f
k
=(k+1/2) f
s
/M (see (a) in FIG.
3
). When a TDM signal inputted to the TDM input terminal
40
is demultiplexed by the demuliplexing switch circuit
41
, the sampling frequency f
S
is reduced to f
S
/M so as to take a spectrum form as shown at solid line at (b) in FIG.
3
. The signal having this spectrum is again sampled at the aforementioned sampling frequency f
S
with the result that aliasing components as shown at dotted line at (b) in
FIG. 3
develop. By extracting components of a necessary frequency band from those components by means of the filter bank, the conversion can be completed.
The filter bank is constructed of a group of band-pass filters having the same pass-band width f
B
and having center frequencies which are separated by f
B
from each other. Accordingly, as shown at (c) in
FIG. 3
, the k-th filter H
k
(Z) is obtained by frequency-shifting the original filter H
0
(Z) having the same frequency characteristic by (k+1/2)f
B
and is equal to substitution of equation (1) in which f of delay operator Z=exp(j2&pgr;f/f
S
)in the original filter H
0
(Z) is replaced with f−(k+1/2)f
B
.
exp
(
j
2
π
(
f
-
k
f
B
-
f
B
/
2
)
f
s
)
=
z
exp
(
-
j
2
π
(
k
+
1
/
2
)
f
B
/
f
s
)
=
z
exp
(
-
j
2
π
(
k
+
1
/
2
)
/
M
)
=
z
W
k
exp
(
-
j
π
/
M
)
(
1
)
where, W=exp(−j2&pgr;/M)
Namely, H
k
(Z) is given by equation (2),
H
k
(
z
)=
H
0
(
zW
k
exp(−
j&pgr;/M
)) (2)
Incidentally, an arbitrary filter can be expressed by a polyphase filter in which the sampling frequency is set to 1/M, as indicated by equation (3)
H
(
z
)=&Sgr;
H
(
z
M
)
i
z
−i
(3)
Thus, by decomposing the k-th filter
43
K
into polyphase and applying kf
B
frequency shift, equation (3) can be reduced to
H
z
(
z
)
=
∑
H
k
(
z
M
)
i
z
-
i
=
∑
H
0
(
-
z
M
)
i
z
-
i
W
-
ik
exp
(
j
π
i
/
M
)
(
4
)
where
W
−ik
=(
W
)
−ik
=exp(
j
2
&pgr;ik/M
) (5)
By adding all of M outputs from respective k-th filters
43
k
, the FDM signal can be obtained.
As will be seen from equation (4), the filter bank of FDM scheme can be realized by the multiplication of matrix W
−ik
pursuant to equation (5) (the M-point IDFT unit
42
of FIG.
4
), the M polyphase filters
43
0
to
43
M−1
pursuant to equation (3), the phase rotation exp(j&pgr;i/M) by means of the M phase shifters
44
0
to
44
M−1
of FIG.
4
and the delay z by means of the M delay circuits
45
0
to
45
M−1
of FIG.
4
. The multiplication of matrix W
ik
is the butterfly operation used in the fast Fourier transform (FFT) and therefore its speedup can be achieved by the same algorithm. Since the M polyphase filters
43
0
to
43
M−1
which are respectively developed from the M filter banks are all common to each other, the hardware and software process can be reduced in scale. In this manner, the TDM-FDM conversion can be realized with the TMUX.
SUMMARY OF THE INVENTION
An example of the multi-carrier modulation method applied with the TMUX which is employed to convert four complex base-band input signals into a FDM signal will be described with reference to
FIGS. 5 and 6
. It is to be noted that this modulation method is contrived by the present inventors in the course of achieving the present invention.
FIG. 6
is a spectrum diagram for explaining the operation procedure of multi-carrier modulation of 4-channel multiplexing (CH
1
, CH
2
, CH
3
and CH
4
), in which a TMUX having a 8-point IDFT is used. In the figure, abscissa represents frequency, ordinate represents signal level, f
S
represents sampling frequency for signal processing, f
sym
represents sampling frequency for input complex base-band signal, and f
B
represents filter pass-band width. Note that the scale of the abscissas shown in
FIG. 6
is not drawn to coincide with that of
FIG. 3
due to drafting, a
Hitachi Denshi Kabushiki Kaisha
Urban Edward F.
Zewdu Meless
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