Multiplex communications – Duplex
Reexamination Certificate
1999-09-28
2004-05-04
Pham, Chi (Department: 2663)
Multiplex communications
Duplex
C370S295000, C370S320000
Reexamination Certificate
active
06731611
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for duplex data transmission with quadrature amplitude modulation (QAM) via two conductors using the time division multiplex and frequency division multiplex method of two oppositely transporting channels. The transmission is in a first direction via an “upper” channel with a higher frequency band f
o
±B
o
/2 and, in a second direction, via a “lower” channel with a lower frequency band f
u
±B
u
/2, which does not overlap the higher frequency band. Each of the channels transports two digital signals, namely a normal and a quadrature data signal, at a respective symbol frequency f
T
, which may be identical or different for the two channels. The system further performs at both ends modulation of a carrier signal with the two digital data signals for the purpose of transmission, and sampling of the received modulated carrier signal at a sampling frequency f
Ao
in the case of the upper channel and f
Au
in the case of the lower channel, respectively. The pulse train resulting from the sampling is subsequently demodulated, which is followed by low-pass filtering and sampling at the symbol frequency of the data signal for the purpose of recovering the two digital data signals used for modulation at the other end. The following frequencies are defined:
f
o
=center frequency of the two upper channels;
B
o
=bandwidth of the upper channels;
f
u
=center frequency of the lower channels;
B
u
=bandwidth of the lower channels;
f
T
=symbol frequency of the digital data signal;
f
Ao
=sampling frequency of the receiver-end sampling of the upper channel prior to demodulation;
f
Au
=sampling frequency of the receiver-end sampling of the lower channel prior to demodulation;
and the following inequalities hold true:
f
o
>f
u
; f
Ao
>f
T
; and
f
Au
>f
T
.
The invention furthermore relates to a demodulator for use in a preferred embodiment of the method according to the invention, having a respective demodulation path for the normal signal and the quadrature signal. The demodulator has a first sampler for sampling at a sampling frequency, a multiplier for multiplication by a cyclic signal, a low-pass filter, and a second sampler for sampling at the symbol frequency.
In the case of duplex data transmission via two-wire lines, the transmission is interfered with to an increasing extent by near-end crosstalk as the speed increases. This is due to the fact that near-end crosstalk attenuation decreases with increasing frequency. In the case of the time division multiplex method with echo compensation, which utilizes the channel bandwidth optimally, from a certain speed the range is limited by the near-end crosstalk. The range can then be increased only by compensating for the near-end crosstalk, which requires considerable technical sophistication that cannot yet be realized. If the time division multiplex method is combined with the frequency division multiplex method, in which the transmission in the two directions takes place in different frequency bands, then although the required bandwidth is increased, the near-end cross-talk can be eliminated by suitable selective filtering. A precondition in this case is that all the signals transmitted in the same direction are transmitted in a multi-pair cable in the same frequency band. From speeds of a few Mbits and above, a greater range can be achieved with the frequency division multiplex method than with the time division multiplex method with echo compensation.
As the transmission method, the quadrature amplitude modulation method is taken as a basis. This enables a plurality of bits to be transmitted per symbol by means, for example, of quaternary or even higher encryption (cf. the textbook by K. D. Kammeyer “Nachrichtenubertragung” [Tele-communications], Stuttgart 1996, p. 372, FIGS.
11
.
1
.
2
). According to the frequency division multiplex method, transmission in one direction takes place in a different frequency band from transmission in the other direction, in which case, in order to be able to eliminate the near-end crosstalk by selective filtering, the carrier frequencies for the two transmission directions are chosen in such a way that the two bands do not overlap.
At the receiver end, the received modulated signal has to be filtered out of the composite signal on the line and sampled at a suitable sampling frequency. The choice of sampling frequency has a very great influence on the realization complexity. The lower it can be chosen to be, the lower the complexity is with regard to processing speed and number of coefficients in the case of the digital filtering. The sampling frequency must be a multiple of the symbol rate. In addition, the known sampling theorem states that the sampling frequency must be at least twice as high as the maximum frequency of the sampled signal.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a duplex data transmission method with QAM and a corresponding demodulator, which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which has a further reduced complexity in the case of demodulation and which achieves a reduction in the sampling frequency.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for duplex data transmission with quadrature amplitude modulation via two conductors using time division multiplex and frequency division multiplex methods of two oppositely transporting channels, which comprises:
modulating a carrier signal with two digital data signals;
transmitting the modulated signal in a first direction in an upper channel having a relatively higher frequency band f
o
±B
o
/2 and in a second direction in a lower channel having a relatively lower frequency band f
u
±B
u
/2, whereby the lower frequency band does not overlap the higher frequency band, and whereby each of the channels transports two digital signals, including a normal data signal and a quadrature data signal, at a respective symbol frequency f
T
;
sampling the modulated carrier signal received via the upper channel at a sampling frequency f
Ao
and sampling the modulated carrier signal received via the lower channel at a sampling frequency f
Au
;
subsequently demodulating a pulse train resulting from the sampling step; and
subsequently low-pass filtering and sampling at the symbol frequency of the data signal for recovering the two digital data signals used for modulation at the respectively other end;
wherein the following frequencies are defined:
f
o
=center frequency of the upper channel;
B
o
=bandwidth of the upper channel;
f
u
=center frequency of the lower channel;
B
u
=bandwidth of the lower channel;
f
T
=symbol frequency of the digital data signal;
f
Ao
=sampling frequency of a receiver-end sampling of the upper channel prior to demodulation;
f
Au
=sampling frequency of the receiver-end sampling of the lower channel prior to demodulation;
and wherein:
f
o
>f
u
, f
Ao
>f
T
, f
Au
>f
T
;
f
o
/f
u
is a rational number
K
; and
f
Ao
=2
f
o
.
These relationships may be partly combined to require the ratio f
o
/f
u
to be a rational number K>1.
By definition a rational number is a number which can be expressed as the quotient of two integers with a non-zero denominator.
In accordance with an added feature of the invention, a delay is introduced at the receiver end in the demodulation path of a respective one of the two data signals of at least one of the channels for phase-shifting the respective data signal with respect to a cyclic multiplier used for demodulation.
In accordance with an additional feature of the invention, the normal data signal and the quadrature data signal are brought into phase after demodulation by low-pass filtering with a do low-pass filter having a signal propagation time longer than a signal propagation time of
Do Nhat
Greenberg Laurence A.
Locher Ralph E.
Pham Chi
Siemens Aktiengesellshaft
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