Method for the spectral configuration of signals modulated by means of orthogonal frequency division multiplexing (ofdm) for an electrical network

- 28036 Madrid

ES 2 220 208 A1

DESCRIPTION

Spectral configurability procedure for signals modulated by orthogonal frequency division multiplexing (OFDM) for electrical network.

Object of the invention

The present invention, as expressed in the wording of this specification refers to a spectral configurability procedure of signals modulated by orthogonal frequency division multiplexing (OFDM), which is applicable to communication systems that use the electrical network. as a means of transmission. With the method of the invention, the signal is adapted to the characteristics of the channel and to the regulations that the law in force in different countries establishes for the transmission of signals through the electrical network.

Background of the invention

In most telecommunication systems it is advantageous to have means to configure the spectral characteristics of the signal, to adapt to future regulations or in force in different countries and to the characteristics of the channel.

The method object of the invention is specially designed for its use with signals modulated by orthogonal frequency division multiplexing (OFDM) that are known in the state of the art.

There is prior art in the state of the art on scalable modulation procedures for OFDM signals such as that described in patent US 6175550 "Orthogonal frequency division multiplexing system with dynamically scalable operating parameters and method thereof" in which an OFDM system that varies time is described. of the OFDM symbol or equivalently the bandwidth of the signal and the cyclic prefix dynamically from measurements made by the receiver. The problem is that to vary the symbol time, the signal sampling frequency in the converters is varied, which forces the design of the analog components that are conventionally located after the digital-analog converter (DAC) of the transmitters to vary. and before the analog-digital converter (ADC) of the receivers. This is because the spectral positions of the signal and its replicas depend on the sampling frequency, so the necessary filtering characteristics are different as said frequency varies. The method object of the invention solves the problem and achieves that the variation of the duration of the OFDM symbol is carried out without affecting the design of the analog components, since the frequency of the converters is not varied.

Description of the invention

To achieve the objectives and avoid the drawbacks indicated in previous sections, the invention consists of a spectral configurability method of signals modulated by orthogonal frequency division multiplexing (OFDM) for the electrical network, which includes sending and receiving signals by OFDM modulation. through the electrical network, and is characterized in that it varies independently, for the transmission and reception path, the bandwidth and spectral position of the OFDM signal, the shape and level of the spectrum of the OFDM signal being transmitted, or some combination of the above. All these parameters are changed without varying the sampling frequency of the digital-analog converters of the transmitter and the analog-digital converters of the receiver. In this way, the communications system can be adapted to the regulations of each country both in terms of frequency range and injected or radiated power, and the type of channel.

The variation of the bandwidth of the signal in transmission is carried out by means of an interpolation with a configurable interpolation factor that is a function of the required variation, and in reception it is carried out by decimating with an equally configurable factor.

To place the spectrum of the signal in the desired position, the method performs a digital band translation with a variable frequency, the value of which is a function of the desired position.

The method of the invention provides for the use of a power mask to select the signal level of each carrier (carrier to carrier), which allows precompensating the channel attenuation, and eliminating carriers, thereby narrowing the bandwidth. signal, and gaps are created in the signal spectrum; achieving the desired shape and level for the signal spectrum, according to the requirements established in each country.

To attenuate the secondary lobes of the signal and achieve more abrupt spectral gaps, the OFDM symbols in the time domain are multiplied by a raised cosine window.

The method of the invention makes it possible to obtain a communication system over the electrical network in which communication is carried out with independent spectra of the OFDM signal for transmission and reception in terms of bandwidth, spectral location, shape and signal level.

It is also possible that several independent systems that use different bandwidths, spectral location, shape and signal level, communicate with the equipment that follows the same communication system simultaneously, sharing the channel through frequency division multiplexing.

Finally, it is also possible to vary the duration of the cyclic prefix that conventionally precedes each symbol of the OFDM signal, so that the system can adapt to the type of channel.

With all this, it is possible to adapt communication systems through the electrical network to the standards and regulations of the different countries and to the different types and topologies of the channel, without variations in the physical implementation of the system.

Below, to facilitate a better understanding of this specification and forming an integral part thereof, some figures are attached in which the object of the invention has been represented by way of illustration and not limitation.

Brief description of the figures

Figure 1.- Represents a block diagram of a transmitter that works according to the procedure of the invention.

Figure 2.- Represents a block diagram of a receiver that works according to the procedure of the invention.

Figure 3.- Represents the power spectral density (PSD) of the transmitted signal.

Figure 4.- Represents the raised cosine windows used in two consecutive symbols.

Figure 5.- Represents two sets of nodes that use frequency division multiplexing with

ES 2 220 208 A1 different parameters in transmission and reception.

Description of an embodiment of the invention

A description of an example of the invention is made below, referring to the numbering adopted in the figures.

When designing a communications system through the electrical network, we are faced with the lack of common regulations on the frequency ranges that can be achieved. use for each service and the power spectral density that can be injected into the line. These regulations change from one country to another, and there are even cases in which they are not yet defined. In other cases, it is necessary to design a system that adapts to different use scenarios, such as access, local area network or transport or trunk network, in which the needs are also different. To solve all these problems, the procedure of the invention proposes a method to be able to adapt the characteristics of the signal of a transmission system through the electrical network to each specific case, varying only digital configuration parameters, that is, with the same physical embodiment. of the system.

The present method allows to vary the bandwidth and the position of the OFDM signal independently for transmission and reception, as well as the shape and level of the signal spectrum and the cyclic prefix of the OFDM signal in transmission.

To carry out this process in transmission, a block diagram such as the one shown in Figure 1 is used, in which a block (18) is observed that applies the power mask on the signal, after which a block (1) appears that performs the Inverse Fourier Transform (IDFT) that is conventionally used to perform OFDM modulation, and a block (19) that inserts the cyclic prefix and multiplies the signal by a raised cosine window. At the output of this block, an interpolation (2) of the signal is carried out with a variable factor (5) to vary the bandwidth. As the procedure of the invention does not affect the sampling frequency (7) of the digital-analog converter (DAC) (4), that is, it remains fixed, the higher the interpolation factor (5), the greater the number of samples per symbol, with so the symbol time is longer and the bandwidth occupied by the signal is smaller, and conversely, with a lower interpolation factor, a greater bandwidth is obtained. To carry out this, interpolation by a variable factor, it is possible to have several interpolators of different factors that can also be combined with each other to obtain other factors. Once the signal with the desired bandwidth has been obtained, it is necessary to place it in the appropriate spectral position. For this, a digital band translation is performed by means of the block (3) that has a variable translation frequency (6) to perform the required translation. Subsequently, the digital signal is converted to an analog signal by means of a fixed frequency converter (4) (7).

At reception the process is carried out in reverse. The OFDM analog signal is received and converted into a digital signal by means of an analog-digital converter (ADC) (8) which, in this embodiment, samples the signal at the same fixed frequency (7) used in the transmitter. Afterwards, the band transfer is carried out to pass the received signal in band pass to base band by means of the block (3) with a translation frequency that in this embodiment coincides with the translation frequency (6) used in transmission. Subsequently, the signal is decimated (10) by a variable factor, which in this embodiment coincides with the interpolation factor used in transmission, (5), whose value is selected so that the block (11) that performs the transformation of Fourier (DFT) always get the same number of samples for each symbol.

Throughout this process, it is not necessary for the transmission and reception parameters in the same node to be the same.

To configure the power level or power spectral density of the signal to be transmitted, a power mask per carrier is used, which in figure 1 is applied in block (18), that is, before passing the signal to the time domain with block (1). That is, when the OFDM symbol is generated in frequency, each carrier is multiplied by a value that determines the power of said carrier. As said power is relative to the other carriers, since the analog components will determine the final power of the signal, this value is between zero and one, where zero corresponds to eliminating the carrier and one corresponds to the maximum power, these values ​​or any other intermediate value can be used.

In some cases it may be interesting to eliminate certain carriers to generate gaps in the spectrum. These gaps may be necessary so as not to interfere with other communication services that use the same frequencies, such as radio amateurs. It should also be noted that the frequencies of these gaps vary in different countries. Carriers can also be removed at the ends of the bands to reduce the bandwidth of the signal. The intermediate values ​​of the power mask can be used to compensate for channel effects. Most channels attenuate high frequencies to a greater extent than low frequencies. The power mask can be used to power the high carriers so that all carriers arrive with similar power to the receiver. A spectrum generated with an arbitrary shape is illustrated in Figure 3, in which the different levels of injected power are observed for each frequency.

The elimination of carriers within the band does not imply that the power injected at those frequencies is zero, since there is power due to the secondary lobes of the adjacent carriers. These secondary lobes appear due to the transitions between the symbols of the OFDM signal, and to reduce them, each OFDM symbol can be multiplied in the time domain by a raised cosine window, which in figure 1 is performed by block (19) , which inserts the cyclic prefix in a configurable way and multiplies the signal to be transmitted through a raised cosine window. Figure 4 shows the shape of said window for two consecutive symbols. The effective duration of a symbol is a time (12) and it is observed that there is some overlap (13) between symbols. In this way, by applying the raised cosine window, it is necessary to eliminate a smaller number of carriers to achieve a certain depth of gap in the spectrum.

The method of the invention makes it possible to have a node that transmits using certain frequencies and receives on others. Obviously, any node that wants to communicate with it will have to work with

ES 2 220 208 A1 the transmission and reception frequencies inverted with respect to the first. This is advantageous in a tree-like channel topology like the one shown in Figure 5, which is common in the low voltage power grid, where all nodes hanging from a branch only communicate with the base of that branch. If it is required to provide service in two branches of the tree, this can be done by placing a node (16) at the base of each of the branches. These nodes transmit in one frequency range and receive in another, so that they cannot communicate with each other, but neither do they interfere with each other. In each of the branches hang other nodes (17) that communicate with their master (16). The transmission of a slave (17) will be received by its master and will be received by the other master to which it will arrive more attenuated, as it is at a greater distance in terms of electrical cable length. If this attenuation is sufficient, the transmission of the slave node will not affect the reception of the other master, so that the two groups of nodes can share the channel without interference between them.

Another possible configuration is that several nodes communicate using certain frequencies and another group of nodes uses different frequencies, so that there is no communication or interference between the two groups of nodes.

In most OFDM systems the.

use of a cyclic prefix, which consists of repeating the last samples of the symbol at the beginning of it. This is done to avoid inter-symbol interference (ISI) due to channel delay-spread, for which the length of the cyclic prefix must be greater than said spread. Obviously, each channel will have its own spread, so a large enough cyclic prefix is ​​normally used for most channels. Depending on the application of the system, different types of channel can be found, such as, for example, medium or low voltage channels, access channels or inside a house (inhome), which statistically present different dispersions.

Typically, in the electrical network, the dispersion is less in the medium voltage lines and in the connections within the same house than in the access sections. For all this, it is advantageous to have a system in which the duration of the cyclic prefix can be varied and thus be able to take better advantage of the characteristics of each channel, for this reason the method of the invention provides that this modification can be carried out using, in this example of embodiment of the invention, the block (19) to insert the cyclic prefix in a configurable way and multiply the signal by a raised cosine window.