JPH0519342B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is widely used in communication networks.
Concerning a branching circuit for FDM signals.

[Conventional technology]

Digital transformer multiplexer (hereinafter referred to as
(abbreviated as TMUX) is FDM-
Widely used for TDM interconversion. Its basic function is separation of FDM signals by digital signal processing.

A conventional TMUX type FDM signal branching circuit is shown in FIG. 1 is a local oscillator with the center frequency of the input IF signal, 2 is a π/2 phase shifter, 3 and 4 are mixers,
5 and 6 are LPFs (low pass filters), 7 and 8 are A/D converters, 9 is a multiplexed clock (NΔf) oscillator, 10 is an N frequency divider, 11 is a time/space division conversion switch, 12-1 to 12-N is a delay device, 13-
1 to 13-N are digital sub-filters, and 14 is an N-point FFT.

FIG. 8 is a basic block diagram showing the basic operation of TMUX. 21 is a BPF (band pass filter) tuned to ω _K , 32 is a mixer, 33 is a local signal generator, 34 is an N frequency divider, and 35 is a sampler.

FIG. 9 shows the configuration of a digital filter (Figure a) and the sub-filter division configuration (Figure b). 61
is a shift register, 62-1 to 62-L are weighters, and 63 and 64 are adders.

The basic operation of conventional TMUX is as shown in Figure 8, ω _k =k・Δω=k・2πΔf (1) (k=0, ±1, ..., ±(N/2-1)) The signal of the k-th channel is frequency-selected by the BPF 31 whose center frequency is ω _k . It is further converted to 0Hz by complex multiplication exp{−jω _k t}.
The frequency is converted to the baseband centering on , and the output is sampled using a channel clock Δf.

Therefore, as shown in FIG. 6b, the frequency spectrum of the output signal repeats the same spectrum at Δf periods. Therefore, in order to avoid signal distortion caused by aliasing, that is, overlapping of the spectrum of harmonic frequencies, as shown in Figure 6a,
The characteristics of the basic filter of TMUX must be completely limited within Δf.

[Problem that the invention seeks to solve]

Due to this inherent limitation, traditional TMUX
It is basically impossible to realize a filter group having completely flat frequency characteristics within the frequency band Δf. Furthermore, in order to realize frequency characteristics that are as flat as possible within the band, extremely steep filter characteristics are required, which increases the circuit scale of the digital filter. Of course, it is obvious that it is impossible to process signals with a bandwidth wider than Δf.

The present invention overcomes the above-mentioned drawbacks of conventional TMUX,
A variable bandwidth FDM signal branching circuit that can process signals with a bandwidth wider than Δf using a multiple sampling type transformer multiplexer (hereinafter abbreviated as MS-TMUX) that can realize basic filter characteristics wider than Δf. The purpose is to realize the following.

[Means for solving problems]

The variable bandwidth FDM branching circuit according to the present invention inputs an FDM signal consisting of a channel having a channel frequency interval Δf or an integer multiple thereof, and outputs the N-branched outputs from the N-branched outputs having a basic filter whose bandwidth is wider than the Δf. m times (m is an integer greater than or equal to 2, usually 2)
MS-TMUX outputs at a sampling rate of
Among the N output signals of the MS-TMUX, those whose signal bandwidths are within the Δf are output as they are, and for channels with a bandwidth greater than Δf, the signals corresponding to the occupied frequencies of the channels are output as is. MS-TMUX output port (corresponding channel number k+i; i=0, 1, 2,
..., M-1) and the timing of the output signal of the output port (an interpolated clock of m times the Δf), it operates at a sampling frequency of at least (M+1) Δf or more. a digital signal interpolation circuit that performs signal interpolation using a digital filter; i=0, 1, 2,
. . . M-1) It has M frequency shift circuits for performing the frequency shift and an adder for summing the outputs of the frequency shift circuits, and the output of the adder has M.
It is characterized by obtaining a channel signal with a bandwidth of Δf.

〔Example〕

The first variable bandwidth FDM signal branching circuit of the present invention
As shown in the figure. 21 is an MS-TMUX, 22 is a switch matrix, 23-1 to 23-K are signal interpolation circuits, 24-1 to 24-5 are frequency shift circuits using complex multipliers, and 25-1 and 25-2 are frequency shift circuits using complex multipliers. Adders 26-1 and 26-2 are post filters.

FIG. 2 shows an interpolation type digital sub-filter that plays an essential role in the MS-TMUX used in the present invention. Note that Figure a shows the case of 0lN/2-1, and Figure b shows the case of N/2l<N-1. 7
1, 72, 73 are N-1-l and N/2-1- respectively
l, 3/2N-1-l (however, l=0, 1, 2,...
..., N-1)-th digital sub-filter. 74 is an N/2 sample advancer (with multiplexed clock), 75 is an N/2 sample delayer, and 76 is an adder.

FIG. 3 shows the input/output timing of the interpolation type (m=2) sub-filter.

FIG. 4 shows a signal interpolation circuit used in the present invention.
41 is a resettable counter, 42 is a ROM,
43R, 43I are shift registers, 44R-1~
44R-3, 44I-1 to 44I-3 are multipliers,
45R and 45I are adders.

FIG. 5 shows a frequency shift circuit used in the present invention. 51 is a counter, and 52R and 52I are cos {(i-(M-1)/2)Δωt} and sin{(i-(
M
-1)/2)Δωt}. Here, the time t is designated by address control by the counter 51. 53R, 53I are D-F/F (flip-flop), 54R-1, 54R-2, 54
I-1 and 54I-2 are mixers, and 55R and 55I are adders.

FIG. 6 shows the relationship between the output signal of a conventional TMUX, the fundamental filter of the MS-TMUX according to the present invention, and the spectrum of the output signal, as well as the spectral operation of the FDM branching circuit of the present invention.

The operation of the present invention will be explained with reference to FIGS. 1 to 6. Here, the case of m=2 interpolation sampling will be explained. The output spectrum of MS-TMUX has a spectrum that repeats at a frequency of 2Δ as shown in Figure 6d, so the basic filter characteristics of MS-TMUX are as shown in Figure 6c if it is narrower than 2Δ. , Δ does not cause aliasing distortion even at a bandwidth wider than Δ. Therefore, as shown by the solid line in Figure 6c, even if the bandwidth is the same as that of the conventional TMUX, the steepness of the filter characteristic is not only considerably gentler, but also as shown by the broken line in Figure 6c. , it is possible to realize a completely flat basic filter characteristic within Δ.

The output of the MS-TMUX 21 is connected by a switch matrix 22 to interpolation circuits 23-1 to 23-5 for signals having a bandwidth wider than Δ. The other channels are connected to interpolation circuits 23-6 to 23-K. As shown in FIG. 4, each interpolation circuit is
The weight of each tap is stored in ROM42 using FIR type LPF.
This can be executed by reading the data at high speed and performing sum-of-products calculations. As a result of the interpolation, the spectrum is as shown in FIG. 6e.

Now, a case will be described in which a signal with a bandwidth twice Δ is reproduced. Frequency shift circuit 24-1, 24-
2, the k+1st channel is +Δω/2,
When the frequency of the k-th channel is shifted by -Δω/2, the spectra shown in FIG. 6f and g are obtained. When these are added, the output spectra of the adder 25-1 become as shown in FIG. 6h and FIG. 6i. That is, in the case of a solid line, it can be output as is, but in the case of a broken line, the frequency characteristics near 0 Hz are raised as shown in FIG. 6i, so the frequency characteristics shown in FIG. It has a completely flat frequency characteristic at 2Δ as shown in Figure 6k.
TMUX can be realized. In a similar way Δ
3 times or any integer multiple of the signal.
FDM demultiplexing is possible.

〔Effect of the invention〕

The present invention can achieve the following effects.

(1) It is possible to perform FDM demultiplexing even on signals that have completely flat frequency characteristics within the basic bandwidth Δ and have a bandwidth that is an integral multiple of Δ.

(2) Therefore, it is possible to set up a completely transparent transmission path that includes this FDM branching circuit.

(3) As a result, the modulation method and data rate of the signals that can be processed are free, and it can be easily introduced into existing systems.

(4) Since the bandwidth of the filter group is variable, it can be used in commercial digital communication systems where various data rates are used.

(5) Digital signal processing makes it possible to realize a large number of filter groups in a compact and lightweight manner, so it is expected to have a wide range of applications in communication satellites, central stations of small station systems, etc.

[Brief explanation of the drawing]

FIG. 1 shows a variable bandwidth FDM branching circuit according to the present invention, FIG. 2 shows an interpolation type digital filter used in the MS-TMUX of FIG. 1, and FIG. 3 shows an interpolation type subfilter of FIG. 2. 4 shows the interpolation circuit in FIG. 1, FIG. 5 shows the frequency shift circuit in FIG. 1, and FIG. 6 shows the conventional example (Fig. a, Figures showing filter characteristics and output signal characteristics for comparison between Figure b) and the present invention (Figures c to k), Figure 7 shows a conventional digital transformer multiplexer FDM branching circuit, and Figure 8 shows a conventional digital transformer multiplexer FDM branching circuit. This is a diagram for explaining the operating principle of a transformer multiplexer, and FIG. 9 shows the configuration of a digital filter. In the figure, 21 is a multiple sampling type transformer multiplexer, 22 is a switch matrix, 23 to
1 to 23-K are signal interpolation circuits, 24-1 to 24-
5 is a frequency shift circuit.

Claims (1)

[Claims] 1. An FDM signal consisting of a channel with a channel frequency interval Δf or a bandwidth of an integer multiple thereof is input, and the bandwidth of the basic filter is wider than the Δf, and the N-branched output is m times the Δf (m is an integer greater than or equal to 2, and is normally referred to as a multiple sampling type transformer multiplexer (MS) that outputs at a sampling rate of 2).
-TMUX) and the N of the MS-TMUX
Among the output signals, those whose signal bandwidth is within the above Δf are output as they are, and channels with a bandwidth larger than the above Δf are output as they are.
the MS- corresponding to the occupied frequency of the channel;
TMUX output port (corresponding channel number is k+i; i=0, 1, 2, ..., M-1)
At least (M+1) based on the switch matrices respectively connected to
a digital signal interpolation circuit that performs signal interpolation using a digital filter operating at a sampling frequency of Δf or more; /2) 2πΔft, (i=0, 1, 2,
. . . M-1) It has M frequency shift circuits for performing the frequency shift and an adder for summing the outputs of the frequency shift circuits, and the output of the adder has M.
A variable bandwidth FDM signal branching circuit characterized by obtaining a channel signal with a bandwidth of Δf.