ES2414529B1 - IMPULSIVE NOISE CANCELLATION METHOD IN POWER LINE COMMUNICATIONS (PLC) SYSTEMS THROUGH PROCESSING OF FREE TRANSMISSION CARRIERS - Google Patents

Abstract

Communication systems through power supply lines, called Power Line Communications (PLC) often leave certain carriers or portions of the spectrum without power transmission to respect possible interference with other radio communication services, such as amateur radio. This method of impulse noise cancellation for PLC systems consists in processing these free carriers in the receiver, to find out the amplitude and phase of the impulse noise pulses when they appear in the transmission lines and thus be able to proceed with their cancellation.

Description

Impulse noise cancellation method in Power Line Communications (PLC) systems through transmission-free carrier processing

Technical sector

This method finds application in the telecommunications sector. More specifically in relation to the systems of transmission and reception of signals per line. Among these, in those with digital modulation and transmission system. And especially, those with multitone type modulation such as Orthogonal Frequency Division Multiplexing (OFDM). The main application is for power supply line communication systems, called Power Line Cornmunications (PLC) that are regulated by international standards such as IEEE P 190 1 and similar ones.

State of the art

Most of the proposed methods to try to minimize the effects of impulse noise (IR) are intended for digital radio transmissions. Some patents such as W02004002101, EP1309095 and EP1180851 base their operation on the location of the RI peaks from the information signal itself received; when they believe they have recognized an RI peak they eliminate the corresponding signal portion. Other publications of interest are US Patents 20050213692 and EP1361720 that use complex IR compensation algorithms. Also the authors of the present patent have previously published two procedures applicable to radio systems: P200402455 that monitors the IR in an orthogonal polarization to that in which the transmission occurs and P200402706 that monitors the IR in another channel at a different frequency.

It would therefore be desirable to have a method specifically designed against RI in PLC systems that not only detects or eliminates signal samples that are affected but is capable of eliminating the contribution of the RI leaving the original PLC signal unaltered.

For this purpose, the present invention takes advantage of the fact that PLC systems leave certain carriers or portions of the spectrum without power transmission in order to respect possible interference with other communication services. This method of RI cancellation for PLC systems consists in processing these free carriers in the receiver, to find out the amplitude and phase of the RI pulses when they appear in the transmission lines and thus be able to proceed with their cancellation. Here, the verb cancel means that the RI pulses are excised from the signal without affecting the latter.

Explanation of the procedure

The term Power Line Communications (PLC), which can be translated as Power Line Communication, is used to identify technologies, equipment, applications and services designed to provide users of communication systems whose means of transmission are the cables they carry electricity.

Impulsive noise (RI) is called large amplitude pulses that arise unexpectedly in a communication channel. The RI can be classified according to its origin as natural noise (not from human activity and produced mainly by lightning in storms) and noise of human origin that includes: Transients in transformers and induction motors, sparks produced in the motor collector universal electric and direct current, transients by electrical switching, voltaic arcs, etc. The combined effect of pulse amplitude, rate of appearance and bandwidth of the receiver can greatly impair the operation of digital communication systems, even if the signal to noise ratio is high.

The IR especially harms the transmission of communications in the power supply lines. Its effect becomes more evident in those using digital modulation and transmission systems, and with special virulence in those with OFDM modulation, such as the PLC systems referred to in this patent.

RI is considered to be a broadband phenomenon, that is, when an RI pulse reaches the receiver, spectral components must be expected to extend throughout the bandwidth of the communication channel.

The IEEE P 190 1 standard has been created by the Institute of Electrical and Electronics Engineers (IEEE) to regulate PLC systems. This rule, among others, specifies that those carriers within an OFDM symbol that correspond to the amateur radio bands (which can be seen in Figure 1) must not transmit power so as not to interfere with these services; therefore, when demodulating an OFDM symbol on these carriers that we will call free, only background noise and a certain level of interference should be found. When an RI pulse reaches an OFDM receiver, its power is distributed among all the carriers of the symbol due to its high bandwidth properties. This RI is also introduced in the free carriers and will have a much greater power than its background noise, this is the basic principle on which the method presented is based: Use the information of the RI detected in the free carriers to cancel it. This cancellation of the RI leaving the original PLC signal unaltered implies greater refinement with respect to other existing techniques that limit the elimination of the RI pulse together with the corresponding sample of the PLC signal.

Description of the drawings

Figure 1 describes an OFDM symbol in the frequency domain with a carrier distribution according to IEEE P1901. The symbol covers 50 MHz and includes 2048 carriers, of which only 917 (spaced hatch shading) are effectively modulated by. üFDM information. In the 237. carriers. corresponding to the bands of the amateur radio service (dense striped shading) no power is transmitted; however, the RI (dotted shadow) is present in all carriers to a greater or lesser extent. The carriers of the beginning and the end (without shading) remain unused, those of the beginning correspond to the bands of the medium-wave broadcasting service by AM and the rest of the lower bands, and those of the end are left unmodified although they could Be used in the future.

Figure 2 is a block diagram describing the operation of the RI cancellation method presented. Block (1) calculates a threshold of detection of RI peaks and block (2) detects those peaks according to the previous threshold. Block (3) calculates a fast Fourier Transform (Fast Fourier Transform) and block (4) extracts the 237 free carriers corresponding to the bands of

radio amateur described in Figure 1. Finally, block (5) is responsible for generating the RI canceling sequence.

Detailed statement of the procedure

In the block diagram of Figure 2, the basic method, object of this patent, is represented.

The arrival of the signal s [n] is indicated in the upper left, which is the OFDM symbol in the time domain. This starting signal s [n] is complex discrete. To obtain it, several usual procedures must be performed, such as amplification, filtering, sampling, synchronization, channel estimation and other processes, which are independent of what is stated in this patent. The signal s [n] contains the information that you want to receive, and added to it, the disturbance of RI you want to eliminate. The basic method consists of four stages.

Stage A: The signal s [n] passes through the block (1) of Figure 2. Here the threshold of amplitude level Th is calculated which will be used to discriminate which samples of s [n] will be studied to determine if are affected or not by RI. This study is carried out in block (2), where all samples of s [n] are examined and those that have an amplitude that exceeds the Th threshold are marked. In block (2) of Figure 2 it is assumed as For example, that M samples exceed the threshold of amplitude Th: These samples are called s [no], s [n1], ..., s [nM-t1, and with them it is found in which position of s [n] Find the marked samples. With these positions a delta train called t [n] is created.

Stage B: At the same time, the original signal s [n] enters block (3) of Figure 2, where its complex frequency carriers or components are extracted by Fast Fourier Transform (FFT), obtaining its version in the discrete domain of the frequency S [k]. In block (4) of figure 2, only those carriers that must be free of transmission according to IEEE P1901 standard and which match the shaded ones with dense grating in figure 1 are extracted from S [k]. These samples where no power has been transmitted shall be called X [k] and the values found therein shall be deemed to be due exclusively to RI.

Stage C: The block (5) of Figure 2 arrives on one side t [n], found in Stage A and that consists of the positions of those samples of s [n] that exceeded the Th threshold. And on the other hand X [k], found in Stage B and containing the frequency information of the complex carriers in which no power was transmitted. From all this information, block (5) of Figure 2 calculates d [n], which is an estimate of the complex values corresponding to the RI of the pulses located at the positions determined by t [n]. Those samples of d [n] that are zero or near zero will not be affected by RI. If, on the contrary, any sample of d [n] has a significant complex value, it will be because it contains RI.

Stage D: It can be considered that d [n] is entirely the disturbance of pure RI. Therefore, to eliminate from s [n] the RI will suffice by subtracting d [n], obtaining the final clean sequence c [n] that is indicated in the upper right part of Figure 2.

The basic method described can have several variants:

Variant1: Due to imperfections in the PLC modulation equipment it is possible to find small power contributions in the carriers prohibited by the standard. This may be due to intermodulation products or the lack of sufficient attenuation of the filters at these frequencies. The method should monitor the possible presence of these carriers, only partially free of transmission and exclude them from the calculation process, or they may also be included if their amplitude is not excessive.

Variant 2: The method presented does not have to be restricted only to the free transmission carriers represented in Figure 1. There are more frequencies outside the spectrum occupied by the PLC system and which could be used to calculate the sequence of RI.

Variant 3: In the same way, it is not necessary to use all of the carriers represented in Figure 1 to determine the IR. The calculation can also be done using only part of them. In this way precision would be lost, but with the advantage that the computational load would be lower, the overall process capacity of the receiver increased.

Variant 4: There is a possibility that narrow band interference will appear continuously within the prohibited carriers. For these cases, and similar to what is stated in variant 1, the presence of interfered carriers must be monitored and excluded from the calculation process.

Variant 5: The described method could be used without including Stage D, only to detect IR and not to cancel it.

Variant 6: On the basic method, an iterative procedure could be implemented that would feed the calculation result at the entrance of the process, in order to refine the method of canceling the IR, at the expense of greater computational complexity.

Variant 7: Although initially it has been assumed that the signal s [n] is already sampled, synchronized and with the channel estimate made, it is also possible to apply the method just after sampling, before the synchronization and channel estimation stages .

Variant 8: Similar to what is stated in variant 7, the method could be applied not just after sampling, nor at the end of all the synchronization and channel estimation stages, but interleaved between them.

Variant 9: The method object of this patent is likely to be combined with other known methods, such as the previous clipping or clipping of the signal. Such combinations do not mean a real variation of the nature of the method that is the subject of this patent.

Variant 10: The base method is described according to the IEEE P1901 standard, which is the one shown in Figure 1 ..

Variant 11: The base method is described considering OFDM as a signal modulation system s [n]. However, there are other modulation schemes for which the method remains equally valid, such as Windowed-OFDM, OFDM-Wavelet or other variants.

Variant 12: It is also not strictly necessary that an OFDM modulation variant be used for the correct operation of the method. The method that is patented does not depend on the modulation system used.

Variant 13: In this document the term PLC has been constantly used to refer to the line communication systems that are affected by IR. Other line communication systems affected by RI can benefit from the exposed method even if they are not specifically called PLC, as long as they share the same characteristic that consists in preventing the transmission of signal in certain frequency carriers, which are those that can be used to calculate the RI as described in the basic method.

Application of the procedure to the industry

The RI in power lines. from. Food is a serious problem for communication systems on this physical environment. The cancellation procedure presented can be added to the firmware of any receiver of a PLC system to improve its behavior against the RI. It constitutes an improvement with respect to other existing methods that are only limited to eliminating samples suspected of containing impulsive noise, and with respect to other methods that use complicated algorithms that consume a large amount of processing time.

Claims (6)

one.

Method of impulse noise cancellation (RI) in a Power Line Communications (PLC) system comprising at least the following stages: a) On a processed electrical signal s [n] that has undergone usual signal processing procedures, all samples of s [n] and those that have an amplitude that exceeds a defined threshold are marked. b) On the same signal s [n] the transform in the frequency domain is calculated. The values of those carriers that must be free or theoretically free of power according to the standard used by the transmission system are then extracted. c) On the information obtained in stages a) and b) an estimate of the complex values corresponding to the IR in the time domain of the pulses marked according to stage a) is calculated. d) Cancellation of the RI by subtracting the reconstructed sequence in step c) from the signal s [n].

2.

Impulse noise cancellation (RI) method in a Power Line Communications (PLC) system comprising at least the following stages: a) On an electrical signal processed s [n], which has undergone usual signal processing procedures, are examined all samples of s [n] and those that have an amplitude that exceeds a defined threshold are marked. b) On the same signal s [n] the transform in the frequency domain is calculated. The values of those carriers that must be free or theoretically free of power in the chosen working range are then extracted, regardless of what the standard used for the transmission sets. c) On the information obtained in stages a) and b) an estimate of the complex values corresponding to the IR in the time domain of the pulses marked according to stage a) is calculated. d) Cancellation of the RI by subtracting the reconstructed sequence in step c) from the signal s [n].

3.

Method of impulse noise cancellation (RI) in a Power Line Communications (PLC) system, according to claims 1-2 characterized by not using in step b) all of the carriers, but using only a subset of them.

Four.

Impulse noise cancellation (RI) method in a Power Line Communications (PLC) system, according to claims 1-3, characterized by excluding from the calculation in step b) those carriers in which power is detected continuously

5 due to narrow band interference. 5. Impulse noise cancellation (RI) method in a Power Line Communications (PLC) system, according to claims 1-4 characterized by using the information resulting from steps c) and / or d) to refine the calculation process by means of a 10 feedback process. 6. Impulse noise cancellation (RI) method in a Power Line Communications (PLC) system, according to claims 1-5 characterized in that it is based on the IEEE P1901 standard for determining free carriers in step b). s [n] c :;) c [n] s [n] . +,. Sequence I,. + clean d [n] Extract free carriers Calculate

-

""

sequence T 1 ~ 11111111111k X [k] canceling ~ F ~) S [k] (4) (5) Detect samples that exceed M-1 8 [no] s [n1] --- s [nM-1] "t [n] = r l) [n-nq] t t t q = O ... -.-one._____ Th .._-- ~ 'f .--- i-. , 1 LJ 1l, LuLllL (one ) nM-1 n s [n] (2) FIGURE 2