Pulse or digital communications – Systems using alternating or pulsating current
Reexamination Certificate
2001-02-21
2003-09-30
Pham, Chi (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Reexamination Certificate
active
06628724
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the transfer of information and a suitable system therefor.
2. Description of Related Art
In many sectors of technology waves are used for the transfer of information. These may be electromagnetic or acoustic waves, for example, which are disseminated either in a special conductor or freely in a given transfer medium, and in this way pass from the transmitter or transmission unit to the receiver or reception unit. With analog information transfer, the values which are to be transferred are formed into a stepless continuous spectrum of physical states. This occurs typically in the form of an amplitude, frequency, and/or phase modulation of the carrier waves. This enables very large volumes of information to be transferred in a given interval of time. With digital information transfer, by contrast, there is a restriction to specific discrete states. With regard to the transfer rate, however, if electromagnetic waves are being used, there have still been no restrictions encountered in practice hitherto, since the frequencies of the carrier waves concerned are very high, and different digital states can be realised in extremely short spaces of time.
In some transfer media, however, such as water for example, information transfer by electromagnetic waves is only possible to a limited degree, since these have only a short range. Accordingly, in this situation the use of sound waves for the transfer of information is a possibility, which can often be propagated over substantially greater distances. These sound waves, however, are mechanical pressure waves, which, apart from the substantially lower frequency, which naturally has an effect on the transferable information rate, also differ in respect of general propagation. Their propagation speed, for example, depends very much on the particular ambient conditions.
The wide range of problems which can arise with acoustic information transfer, can be illustrated briefly by the example of the transfer of sound signals under water. With the propagation in space of the sound waves emanating from a transmitter, a part of the waves may be reflected from the water surface and/or from the bed of the body of water, depending on the depth, from various objects, particles in suspension, and even from layered inhomogeneities in the water, or bent by them. The various different components of sound waves will then arrive at the receiver with differing amplitude and phase relationship, depending on the length of run, angle relationships, and acoustic properties of the relevant limit surfaces or media. As a consequence of the interference, the actual signal at the reception point may be amplified, weakened, distorted, or even totally deleted, in an unforeseeable manner, or reception may also be distorted by what is referred to as reverberation.
To explain the problems in greater detail, the simple situation will first be considered in which only a very short signal of a specific frequency, referred to as a CWP (Continuous Wave Pulse) is transmitted. In this situation (so-called Multipath Propagation), a receiver can obtain not only an individual signal, but a whole group of temporally-displaced individual pulses of different strengths. This effect is referred to as “channel response”. While in this case it is still possible for the individual pulses to be distinguished on the receiver side, and, for example, the most suitable pulse to be selected as the “actual signal” (whereupon the other pulses can, as a consequence, be regarded as “interference signals” and treated accordingly), a separation of this nature in the transmission of a longer wave package cannot normally be effected any longer, since the receiver receives only a summary or composed signal, which may indeed still have the same frequency as the initial signal, but in which the actual signal and the interference signals, with their different amplitudes and phase positions, are overlaid in such a way that unforeseeable fluctuations in the amplitude and also in the phase location may arise. This undesirable effect, which renders the evaluation of the signal difficult or can even, under certain circumstances, make this impossible, is referred to as “Intersymbol Interaction” (ISI). If transmitter and receiver move relative to one another, an additional problem may arise in the form of frequency shifts as a result of Doppler effects.
This wealth of problems makes underwater communications very difficult, such as by means of ultrasonics between divers and/or underwater vehicles, as well as the remote control of underwater equipment. Hitherto, analog information transfer in particular has only been practicable to a very limited degree. It was and is, however, still frequently used for the transfer of speech, whereby use is made of the fact that human beings can identify known words and sense associations even in cases of reception subject to very heavy noise interference. By appropriate practice and agreement on a restricted vocabulary, the identification rate can be somewhat improved. This process is not suitable, however, for transferring, for example, computer data or other information by mechanical means. Accordingly, in the acoustic information transfer sector too, suitable digital processes are being sought.
Today's technical digital systems, especially for underwater use, are based mostly on the sequential transfer of sound signals of consistent height, which are located in a more or less narrow frequency band.
A further development represents broad band procedures (see e.g. U.S. Pat. No. 5,124,955) using a plurality (100) of parallel frequency channels. For reducing the influences of multipath propagation, these procedures use a stepwise switching between the frequency channels. Certain channels are provided for submitting a binary 1, while other channels are provided for submitting a binary 0. Five channels carry the same information, wherein the power portions of the redundant channel groups are added in the receiver and compared for reducing fading effects. Accordingly, the natural redundancy caused by the multipath propagation is reduced by the introduction of an additional synthetic redundancy (10 frequency channels are used for each bit). This common procedure is relatively stable. However, it does not allow modulations with an increased graduation.
Irrespective of whether the transmission takes place in a narrow or broad frequency band, encoding by means of serial “clicks” only allows for a limited information transfer rate. With a shortening of the pulses, the band broadening increases. Furthermore, Doppler effects may be compensated in a restricted manner only.
Another common multichannel system (see WO 99/19058) uses the so-called Orthogonal Frequency Division Multiplexing (OFDM) also for channels with constant frequencies in combination with a Forward Error Correction (FEC). This is in particular provided for a reduction of errors caused by the superposition of multipath components. This procedure is described as allowing a Differential Quadrature Phase Shift Key (DQPSK) modulation with bit rates up to 3000 bps (OF 31 carriers and FNR =10 dB) and up to 9600 bps (with 100 carriers). Unmodulated pilot signals with constant frequencies are transmitted above and below the frequency band used for information transmission for compensating Doppler effects. The frequencies of the pilot signals are permanently monitored with two separate PLL's which submit corrections to a Discrete Fourier Transformer (DFT) unit. This procedure represents a complicate method which requires a complex technical equipment. Furthermore, this procedure uses the transmission physics in a restricted manner only.
The prior art development of transmission techniques is directed on complex post-transmission processing with complicate equalizers, PLL and correction algorithms which are implemented with the DSP technique. A further improvement has been obtained with the so-called beam forming (see
Bannasch Rudolf
Kebkal Konstantin
Evologics GmbH
Piper Rudnick LLP
Tran Khanh Cong
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