FI105973B - A method for determining the frequency bandwidth of modems employing a multicarrier system and a system employing the method - Google Patents

Description

, 105973

A method for determining the frequency bandwidth of modems employing a multicarrier system, and a system employing the method. The present invention relates to a method according to the preamble of claim 1 for determining the location and the widths of the frequency bands used for data transmission of modems employing a multicarrier system.

The invention also relates to a rigid system for optimizing the frequency band assignment of modems employing a multicarrier system.

10

The invention thus relates to a method for protecting a modem connection from radio frequency interference whose frequencies cannot be considered as known in advance. The invention also relates to an apparatus for protecting e.m. from radio frequency interference. The method is applicable to modems that generally employ 15 known linear modulation methods: QAM (Quadrature Amplitude Modulation) and CAP (Carrierless Amplitude / Phase Modulation).

Increasing speeds will be transmitted along the copper cable. The ETSI (European Telecommunication Standards Institute) working group is working on the definition of a VDSL modem 20 (Very High Speed Digital Subcriber Line). The highest transfer rates are tens of megabits per second, and the frequency range at which the transfer occurs is between 300 kHz and • 30 MHz.

When operating in this frequency band using bandwidths of several megahertz, 25 different types of radio frequency interference will become a problem, especially in countries where overhead cables are used as a subscriber cable (eg in Finland in rural areas). There is ample AM radio stations and also amateur radio activities in this band. The radio waves may interfere with the receiver of the modem connection by connecting to the overhead cable and partially changing the cable. transverse interference that can propagate along the cable to the receiver 30. On the other hand, the modem connection itself may interfere with the radio because the modulated signal in the cable contains power at radio frequencies and this power is in part:. emits into the environment.

2,105,973

In terms of modem design, radio interference can be divided into two categories. For interferences with a known frequency and for interferences with a frequency not known in advance. Radio interference of known frequency is due to amateur radio interference, since amateur interference occurs in certain internationally standardized frequency bands. On the other hand, broadcasts from a large number of AM stations are subject to interference, the frequencies of which cannot be considered to be known at the design stage of the modem, since the transmission frequencies vary considerably between countries.

A particular directional connection (subscriber to center, US (Up Stream) or downstream to DS (Down Stream)) may be implemented using one continuous frequency band (1) or multiple frequency bands (2) as shown in FIG. When multiple frequency bands are used, there may be an area between the bands for which virtually no guard band (3) is applied, or the bands may be partially overlapped as shown in Figure 2. In the case of overlapping bands, the situation is as if there were one single band, but from the point of view of modulation technology, talking about multiple bands makes sense.

A principle diagram of a rigid system using more than one frequency band per transmission direction is shown in Figure 3 for one transmission direction. The data to be transmitted (D) 20 is divided into parallel branches (D1 ... Dn) which are up-modulated (ml ... mn) each in its own band. . Partial signals S1 ... Sn are formed, the sum of which is the payload signal (S) passing through the channel. In reception, certain functions (h) (e.g. part of the filtering) are applied to the incoming signal (S). Subsequently, the corresponding portions (sl ... sn) of each band are extracted and down-modulated (m'1 ... m'n) from the signal and subjected to other necessary operations on the receiver such as channel offset, detection, timing and necessary filtering (k). The received data stream (d) is formed by combining parallel data streams d1 to .dn. The received data stream (d) is the same as the transmitted data stream (D) unless there are transmission errors. In this text, the frequency bands corresponding to each sub-signal are hereinafter referred to as sub-bands. The subbands carrying the signal in a particular direction (US or DS) do not have to be adjacent to one another in the frequency band, but the different directional subbands may be interleaved in different ways. The direction of transmission of a single 3 105973 subband is irrelevant for future review. The center frequencies of the subbands are called carriers, and a system with only one subband per transmission direction is called a single carrier system, and a system with multiple subbands per transmission direction is called a multicarrier system.

5

The bandwidth used in a single carrier system is several megahertz at VDSL speeds, typically between 1 and 12 MHz. Amplitude and phase distortion of the channel must be corrected by adaptive receiver equalizers. The magnitude of the amplitude and phase distortion is proportional to the width of the frequency band and thus to the transmission rate 10 (symbol frequency). It follows that increasing the transmission rate extends the required length of the receiver equalizers. Increasing the length of the equalizers requires increasing the number of tap coefficient equalizers. The increase in the number of loss coefficients, in turn, contributes to the number of calculation operations required and thus to the power consumption of the integrated circuits, which is a critical quantity for the performance of the apparatus. The advantage of a single carrier system is the simplicity of the fixed transmitter and receiver filters over the multi carrier systems.

The subbands of the multicarrier system may be in the frequency band with each other of the same width or with different widths and may be located in the frequency band either unevenly or unevenly. In conventional DMT (discrete multi tone) modulation, the subchannels are of the same width and are equally spaced. The DMT is formed by a filter bank that implements an inverse discrete Fourier transform (EDFT) at the transmission end and a discrete Fourier transform (DFT) at the receiving end, a filter bank that implements flat-plate and even-distributed filter banks. Similarly, a filter bank that does not satisfy the condition of equivalence and / or distribution is called a non-uniform filter bank. For example, source [Cox] describes the formation of both equal and unequal filter banks. Such filter banks 30 allow for the implementation of both uniform and non-distributed multicarrier systems. The multicarrier system may also be considered: consisting of a plurality of parallel logically, but not necessarily implementationally, 4 105973 independent single-carrier systems. In Figure 3, each branch (e.g., ml-channel-h-m'1-k) may be, for example, a conventional system employing QAM modulation. Description of QAM modulation is e.g. at [Lee & Messerschmitt].

5 The advantage of multi-carrier systems over single-carrier systems is that in multi-carrier systems, data transmission can be allocated to those frequency bands that have the best signal-to-noise ratio, while avoiding those bands that are not allowed to be interfered by the modem. A disadvantage of multi-carrier systems is the complexity of the transmitter and receiver filter structures 10 (filter bank) and the high peak-to-peak ratio of the signal compared to the single-carrier system. Complexity increases as the number of subbands increases, but a large number of subbands is advantageous in terms of overall transmission capacity because the frequency bands with the best signal to noise ratio can be best utilized. As the number of subbands increases and 15 total transmission capacity is constant, either all or at least some of the subbands are reduced. The impulse interference tolerance improves as the subband narrows, as the symbol's time duration becomes longer.

Further narrowing the subband allows the duration of the berths to be shortened because as the 20 bands narrow, the channel distortion decreases. This reduces the total number of landing operations required by all subband correctors per unit time. The increase in the number of subbands is compensated for by the decrease in the symbol frequency of all or part of the subbands, and thus also the computational frequency of the equalizers.

Next, it is considered how the above-described prior art methods can address the problem of radio interference in a receiver.

In the VDSL Single Carrier System, the band of a single radio interference is substantially narrower than a signal transmission band. In this case, a single radio interference in the 30 frequency bands may be considered a point interference. On the other hand, in a VDSL single-carrier system, the signal transmission band is so wide that radio interference in the transmission band is generally • unavoidable. Such point interference can be eliminated by band-pass filters.

5, 105973

If the frequencies of the radio interferences are known in advance, fixed band-pass filters designed for specific frequency ranges may be used, which may be analogue, digital or both. If the frequencies of the radio interferences are not known in advance, an adaptive filter must be used which, on a case-by-case basis, provides the necessary interference bands for the frequencies of radio interference.

Fixed band-pass filters cause additional distortion, which must be compensated by increasing the length of the receiver anchors and thereby increasing the number of pin coefficients. This will increase the amount of computational work. In addition, setting blocking bands on the 10-signal transmission band sometimes significantly reduces the signal-to-noise ratio [Salz]. In practice, the use of an adaptive filter means that the linear receiver receiver (FFE) length is increased in time so that it is able to handle its share of channel distortion echoing and to provide the necessary barrier bars. The DFE of the feedback receiver must also be increased in time in order to be able to perform its role of correcting channel distortion and also compensate for the distortion effect of the gain bands formed in the linear portion. The same applies to the loss of signal-to-noise ratio as for fixed band-pass filters.

In multicarrier systems, two different well-known methods of protecting against radio interference can be used. 1) The affected frequency band may • be left unused for signal transmission, or 2) appropriate bandwidth filtering may be used to eliminate radio interference in the same manner as in a single-carrier system.

25

In the case of radio interferences whose frequencies are known in advance, the application of the first - # method in practice means that the sub-bands used for data transmission are allocated to the frequency bands in which k.o. no radio interference.

30 For radio interferences whose frequencies are not known in advance, the former method is suitable for the case of a large number of carriers; tens or hundreds. The principle is illustrated in Figure 4. Based on a suitable error criterion, it is determined that 105973 o when a subband (2) is subject to radio interference (4), and when the interference is so large that k.o. a subband capable of transmitting a payload signal of sufficient quality terminates data transmission in that subband. Not using a few (1-10) subbands does not disturb the transmission capacity [bits / s], because one subband represents a very small proportion of the total transmission capacity of the waste system 5. The data transmission is thus allocated on a case-by-case basis to those frequency bands where no radio interference occurs. The disadvantage of a system with tens or hundreds of carriers is the complexity of the transmitter and receiver structures and the high peak-to-peak ratio of the signal.

10 In a multicarrier system where there are so few subbands that interrupting data transmission in any of the subbands would cause a disturbing reduction in transmission capacity, radio interference at unknown frequencies must in principle be attenuated by adaptive filters as well as single-carrier interference. Preparation for this increases the number of computations required for signal processing 15 per unit of time and thereby the power consumption of the integrated circuits. In the future, the upper limit of the number of calculations required per unit of time is called computational capacity.

If the interference is sufficiently weak, the situation can be addressed by reducing the 20 bit bits of the subband in question, whereby the interference tolerance increases as the number of detector levels decreases. This method is also better suited to a situation with a large number of subbands (tens to hundreds), since reducing the bit rate of a few subbands does not disturb the overall transmission capacity.

It is an object of the present invention to overcome the drawbacks of the above described technique and to provide a completely new type of method employing a multicarrier array. to determine the subband of modems and to optimally allocate the computational capacity needed for signal processing between different subbands.

The subband assignment according to the invention is implemented by dividing the transmission band into narrower subbands than other subbands in the frequency ranges where statistically * · the most unobserved interference frequencies occur.

More specifically, the method according to the invention is characterized in what is stated in the characterizing part of claim 1.

% 5 In turn, the icing system according to the invention is characterized in that the narrowest subbands are located in the frequency ranges which statistically have the highest number of previously unknown interfering frequencies.

The allocation of computing capacity according to the invention between different subbands is implemented by allocating a greater proportion of the available computing capacity than would be achieved by conventional design to those subbands located in the frequency bands where statistically most unexpected interference frequencies occur.

In one preferred embodiment of the invention, the apparatus is followed by the possibility that the allocation of available computing capacity between different subbands may be changed, either automatically or manually, as the case may be.

The icing system according to the invention is characterized in that the allocation of the computing capacity between the different subbands is implemented in such a way that a larger proportion of the available computing capacity 20 is allocated than the conventional design would have for those subbands located in those frequency ranges which are statistically noise frequencies.

More specifically, the system of the invention is characterized by what is set forth in the characterizing part of claim 8.

The invention provides significant advantages.

The subband according to the invention provides the following benefits:

The adverse effects of radio interference on a single subband can be reduced as the number of radio interferences on the subband decreases (talking about the number of radio interferences 30 8 105973 makes sense since the individual interference can be considered as a point in the frequency band).

The complexity of the waste system is not unnecessarily increased, as the subbands are only narrowed where it is most useful, so that the increase in the number of subbands is small.

The distribution of computing capacity according to the invention between different subbands achieves the following benefits: 10 The available computing capacity at the design stage of the device can be allocated where it is statistically most useful.

The available computing capacity may be assigned, either manually or automatically, during use and / or operation of the device to where it is most useful in each situation.

The invention will now be further explored by way of exemplifying embodiments of the accompanying drawings.

20

Figure 1 is a graph showing prior art signal transmission over a single continuous frequency band.

FIG. 2 is a graph showing prior art signal transmission over a plurality of non-overlapping frequency bands.

· m.

FIG. 3 is a graph showing prior art signal transmission over a plurality of overlapping frequency bands.

Fig. 4 shows a block diagram of a single transmission direction (US or DS) of the prior art multicarrier system.

«105973 9

FIG. 5 is a graph showing one transmission band according to the invention divided into multiple subchannels in a modem apparatus.

The following abbreviations are used throughout this document: * 5 AM Amplitude Modulated CAP Carrierless Amplitude / Phase Modulation DFE Decision Feedback Equalizer DFT Discrete Fourier Transform 10 DMT Discrete Multitone DS Downstream (Downstream to Subscriber) ETSI European Telecommunication Standards Institute FFE Feed Forward Equalizer IDFT Inverse Discrete Fourier Transform 15 QAM Quadrature Amplitude Modulation US Up stream (subscriber to center)

BACKGROUND OF THE INVENTION The invention relates to a method and apparatus for protecting against radio interference at unknown frequencies. ; up to tens of percent. In the VDSL-25 application, such a multi-carrier power system has a sub-bandwidth of the order of 0.3 to 4 MHz, whereby radio interference can be considered as point-to-point interference along the frequency axis as in the "single-carrier radio system.

Conventionally, in multicarrier systems, subbands are located in the frequency ranges where the signal-to-noise ratio is most advantageous and, on the other hand, the frequency ranges for which interference power is prohibited are not used. The appropriate placement of the subbands to circumvent radio interference at known frequencies is obvious to one skilled in the art. In contrast, the other 10 105973 radio interferences cannot be circumvented by appropriate placement of the subbands at the design stage because k.o. the frequency of radio interference can vary considerably from country to country and even from local to local. In the future, radio interference will be understood to mean only radio interference of unknown frequency from the point of view of modem design.

The present invention consists of three parts, the first of which is a novel method for determining subband configuration and an apparatus for applying it.

The first part of the invention is based on the fact that the harmful effects of radio interference are proportional to the number of radio interferences that hit the subband. The number is a quantifiable quantity because the interferences in the frequency range can be considered as points.

What is new is that the multicarrier radio system of the present invention is prepared for radio frequencies of unknown frequency by selecting narrower subbands in the frequency bands where radio frequency is most common and where radio stations occur most frequently on the frequency axis, such frequency is 90.6 MHz. The direction of transmission (US or DS) of each subband is irrelevant to the invention.

20

The principle of sub-band placement is shown in Figure 5. The narrowest sub-bands (2) are located in the region (5), where statistically the highest number of previously unknown interference frequencies occur. The subbands may be overlapping or there may be a guard-band between them. What is still irrelevant is the direction of transmission (US or DS) of each subband.

e - * - Of course, a single carrier system can also be considered as a starting point, the transmission band of which is subdivided into subbands, which are implemented by the same method as the original single carrier system. The sub-band division is implemented such that the narrowest 30 sub-bands are generated where the radio operation is most common and the radio stations occur most frequently on the frequency axis. After the subband, of course, the transmission direction of each subband can be freely selected.

II

105973

The second part of the invention is based on the following known issues:

The number of calculations required to implement the device per unit of time is limited due to the power consumption / warm-up, size and price of the integrated circuits. In the future, this limitation will be referred to as: computing capacity.

The required time lengths of the coefficients associated with the different subbands and / or the adaptive filters for attenuating the radio interference increase as the number of radio interferences in the subband 10 increases.

Increasing the temporal length of the coefficients and / or other adaptive filters increases the number of pin coefficients of the coefficients and / or other adaptive filters and thereby the number of calculations required per unit time.

15

What is new is that the multicarrier band system according to the invention is prepared for radio frequencies of unknown frequency by selecting a plurality of pin coefficients for measuring the longest beacons and / or adaptive filters for radio frequency suppression only for those subbands within the frequency bands 90 kHz - 3.6 MHz.

Conventional dimensioning of berths based solely on channel distortion, for example in the situation of Figure 5, would not lead to the same result. 25 Channel distortion design would result in a smaller number of pin coefficients per subband, the narrower the subband. In the dimensioning of the present invention, the opposite is true. The number of pin coefficients per subband is highest in the narrowest subbands. Selecting the number of subband specific pin coefficients involves dividing the available 30 computing capacity among the different subbands.

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The third part of the invention is a novel method for selecting the lengths of adaptive filters for attenuation of different co-bands and / or discrete radio interferences when using a modem and the apparatus implementing it. Parts 1 and 2 of the invention are based a priori on the frequency ranges most likely to occur. The method described in Part 2 of the invention provides a statistically advantageous distribution of available computing capacity between different subbands. In the individual case, however, it is possible that the allocation of computational capacity based on the above statistical thinking is not optimal.

10

What is new is that the multicarrier array system of the present invention is prepared for radio frequency interference of unknown frequency by adjusting the number of pin coefficients of adaptive filters for attenuating different subbands and / or discrete radio interferences so that the available computation can be varied.

What is new is that the rigid system is provided with a mechanism capable of modifying the allocation of available computing capacity between different subbands during modem operation by modifying pin numbers to optimize the quality and / or speed of data transmission. The optimization criterion may be, for example, the ratio of the detection error to the distance between adjacent detection levels.

References: 25 [Cox] R.V.Cox, The design of uniform and nonuniform spaced pseudoquadrature * mirror filters, IEEE Trans. ASSP, Vol 34, Oct. 1986, p. 1090-1096.

[Lee & Messerschmitt] E. A. Lee and D.G. Messerschmitt, Digital Communication, 30 Kluwer Academic Publishers 1994.

105973 13 [Salz] J. Salz, Optimum Mean-Square Decision Feedback Equalization, The Bell System Technical Journal, Vol. 52, no. 8, Oct. 1973, p. 1341-1373.

Claims (14)

A method for determining the frequency band distribution of modems applying the multi-path system, according to which the transmission band is divided into at least three sub-bands (2), considering the two transmission directions, characterized in that the width of the sub-band (2) in the frequency ranges (5) at which most probable previously unknown interference frequencies (4) occur are selected at narrower than the width of the other subbands (2) and the number of said narrower subbands is selected at least two, when the two transmission directions are taken into account when the number is determined. Method according to Claim 1, characterized in that the width of the subband (2) in the frequency range 90 kHz - 3.6 MHz (5) is selected for at least 30% narrower than the width of the other subband and the number of said narrowband the subbands are selected for at least two, when both transmission directions are taken into account when the number is determined. Method according to Claim 1, characterized in that the method protects itself against radio interference at a previously unknown frequency by selecting the adaptive filters for sub-bands (2) with the number of tapping coefficients in the longest equalizers and / or only for attenuation of radio interference. the frequency ranges (5) on which radio activity is most common and on which radio stations occur most closely on the frequency axis. 25 Method according to claim 3, characterized in that the number of tapping coefficients relating to the frequency band 90 kHz - 3.6 MHz (5) ', · the appropriate subband (2) equalizers and / or adaptive filters are intended only for attenuation of radio interference, tili is selected for greater than the corresponding number of tapping coefficients, which relate to the other subband of the frequency band. Method according to claim 3 or 4, characterized in that the distribution of the available computing capacity used between different sub-bands is organized to be changed from case to case so that the number of tap coefficients connected to the sub-bands can be changed. 5 Method according to Claim 3 or 4, characterized in that the system is provided with a mechanism which, while the modem is running, changes the distribution of the available standing counting capacity between different sub-bands by changing the number of coefficients of the aforementioned sub-bands in order to optimize the quality and / or speed of the data transfer. Method according to claim 6, characterized in that for the optimization criterion, the relation between the demodulator error and the distance between adjacent demodulator levels is selected. 15 8. Multi-carrier system, whose transmission band is divided into at least three sub-bands, taking into account both transmission directions, characterized in that the width of the sub-band (2) in the frequency ranges (5) at which it is most likely to occur at previously unknown noise frequencies (4) the width of the other sub-bands (2) and that the number of previously mentioned narrower sub-bands is at least two, when the two transmission directions are taken into account when the number is determined. System according to claim 8, characterized in that the width of the sub-band (2) in the frequency range 90 kHz - 3.6 MHz (5) is at least 30% narrower than the width of the other sub-bands and that there are at least two of the aforementioned sub-bands. narrower subbands, when both transmission directions are taken into account when the number is determined. System according to Claim 8, characterized in that the system has hedged against previously unknown radio interference by selecting the adaptive filters for the sub-bands (2) measured with the number of tapping coefficients and the only adaptive filters intended for attenuation of radio interference. ) located in the frequency bands (mm) (5) on which radio activity is most common and on which radio stations are closest to the frequency axis. 11. A system according to claim 10, characterized in that the number of tap coefficients relating to the equalizers and / or adaptive filters intended for the frequency band 90 kHz - 3.6 MHz (5) only intended for attenuation of radio interference, is greater than the corresponding number of tapping coefficients, which relate to subband located elsewhere in the frequency range. System according to claim 10 or 11, characterized in that the system comprises means for distributing the available counting capacity between different sub-bands from case to case, so that the number of tap coefficients connected to the sub-bands can be adjusted. System according to claim 10 or 11, characterized in that the system is provided with a mechanism which, while the modem is running, changes the distribution of the available standing capacity between the various sub-bands by changing the previously mentioned pin numbers. to optimize the quality and / or speed of data transfer. 20 The system according to claim 13, characterized in that the optimization criterion consists of e.g. the relationship between the demodulator error and the distance between the adjacent demodulator levels. =>. «« «