Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2614—Peak power aspects
  • H04L27/2623—Reduction thereof by clipping
  • H04L27/2624—Reduction thereof by clipping by soft clipping
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
  • H04L27/20—Modulator circuits; Transmitter circuits
  • H04L27/2096—Arrangements for directly or externally modulating an optical carrier
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
  • H04L27/22—Demodulator circuits; Receiver circuits
  • H04L27/223—Demodulation in the optical domain
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
  • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
  • H04L27/36—Modulator circuits; Transmitter circuits
  • H04L27/366—Arrangements for compensating undesirable properties of the transmission path between the modulator and the demodulator

Definitions

• the present invention relates to the encoding / decoding of the amplitude of the data signal, generated with radio frequencies or digitally, using symmetry and folding in axes or limits determined by the physical characteristics of the optical modulator.
• the technique consists in indicating the upper and lower limits of the amplitude of the signal in which the characteristics of the modulators and amplifiers are sufficiently linear that will serve as symmetry axes on which the signal will be folded as many times as necessary until obtaining a signal whose values are within the defined limits. Together with the resulting signal, auxiliary information labels will be added to indicate the number of times the signal that will be used for its correct reception has been folded.
• the transmission of multi-level and multi-carrier signals includes advantages such as increasing the amount of information transmitted.
• modulation in multiple RF carriers offers advantages such as greater tolerance to dispersion and multi-path fading of the signals. It also simplifies the equalization in the frequency domain and selectively select the carriers to be used, as well as the bits to be transmitted in each one, allowing greater granularity of the total bandwidth.
• One type of multi-carrier modulation is orthogonal frequency division modulation (OFDM) that has gained popularity among operators and has been chosen for several applications including copper-based user access networks (DSL) and wireless local area networks (WiFi, WiMAX). Recently, the application of OFDM in optical access networks has gained a lot of interest from different research groups.
• PAPR maximum power and average signal power
• optical access networks based on multi-carrier modulation face another challenge when switching from optical to electrical domain and vice versa.
• the most cost-efficient systems for optical access networks are based on intensity modulation and direct detection (IMDD) due to their relatively simple implementation.
• IMDD intensity modulation and direct detection
• Optical transmission systems based on intensity modulation require that the signal to be sent be real and unipolar [2].
• the first condition can be satisfied using hermitian symmetry in the subcarriers.
• a DC voltage is usually added to the signal to raise its level.
• this produces a high average power in the optical carrier, in addition to limiting the range of the optical modulator to correctly detect the signal.
• the information of the phase of the optical signal is lost due to the quadratic relation of the photodetector when transforming the photons into electrons. Maintaining this information would increase the performance of systems based on direct detection.
• the present invention aims to reduce the impact of PAPR on data signals with multiple carriers, and also, for that matter, increase the performance of IMDD systems by recovering phase information by modifying the signal to be transmitted and adding auxiliary information. .
• the present invention provides a method to reduce the ratio of maximum and average signal power and thus improve the performance of transmission systems based on amplitude modulation.
• an encoder will determine a series of limits that will serve as axes over which the signal components will be folded thus maintaining the amplitude values within an acceptable operating range (for example, within the corresponding range, subsequently , to the linear zone of the first period of the transfer function of the optical modulator Mach-Zehnder -MZM).
• Folding is understood as doubling the signal at an inflection point by changing the increasing or decreasing characteristic of the signal in its opposite and maintaining continuity. Within the same term "fold" and in order to avoid nonlinearities and discontinuities, it is interpreted that the slope of the folded signal Can be modified.
• the amplitude folding function consists in replacing the signal levels that exceed the equivalent values located between the defined limits.
• the relationship between the original and modified levels can be represented by a sawtooth function, characterized by the alternation of positive and negative slope periods (or vice versa), with absolute slope values that can be different in each period.
• the encoded signal will be confined or "folded" between the limits of the sawtooth signal (Fig. 1)
• the information of the number of folds will be included as an auxiliary label which will be detected and used by the receiver to recover the original data signal .
• the object of the invention is an optical or electrical communication system that transmits data modulated in amplitude by optical fiber or some other means (Fig. 2) and which consists of: a) A transmitter based on an amplitude modulator for signals OFDM type or other.
• the transmitter also includes a non-linear preprocessing block that determines the axes of symmetry and folds the input / output function and identifies the number of folds for each frame of the signal.
• This block can be replaced by a modulator whose transfer function allows the natural folding of the function (such as an MZM optical modulator, fig. 2c).
• a transmission link c) A receiver that will include a photodetector and dispersion compensator (when required) in the optical case, a decoder that will determine the number of folds and frames and a non-linear block that performs the proper operation with the signal and its auxiliary information to recover the original signal. It will also include an OFDM or other electric demodulator. that allow you to control the maximum levels of the signal by keeping it in a range within which the devices have linear characteristics or recover the information that is lost by the quadratic characteristic of the photodetector (detection of photons to electrons), which helps to improve the performance of the system.
• the direct detection receiver is the main objective of the present invention.
• this type of receiver is popular for its low cost.
• its quadratic photon to electron detection ratio loses the phase information of the optical signal.
• the transmitter components in particular electrical amplifiers and optical modulators have relationships that are not linear in the entire signal range. This is most evident when the signal has many levels and there are values that stand out from the average, causing non-linear effects that reduce the overall performance of the system.
• the proposed invention allows both limitations to be reduced depending on the ranges chosen and can be extrapolated to the electrical case. This will have a pre-encoder in the transmitter that will do the following:
• the receiver will have a decoder that will perform the following functions:
• y is the decoded signal
• r is the received signal
• z and z indicate the number and amplitude value of the limit or axis on which the signal is folded.
• Fig. 1 illustrates the "fold" of a multilevel signal (input signal) into a signal
• Fig. 2 represents a scheme of a communication link
• Fig. 2b shows the block diagram of an optical communication system.
• the data signal is encoded (1 b01) and transformed to the analog domain (1 b02).
• Fig. 2c shows a period and a quarter of the periodic transfer signal of an MZM type optical modulator.
• Fig. 3 illustrates the limit in case you want to retrieve the phase information in the received signal.
• the dotted signal will be the result of the encoding of the generated signal.
• Fig. 4 exemplifies the defined limits that will serve as folding axes for a multilevel signal.
• the dotted signal will be the result of the coding in the generated signal
• Fig. 5a shows the transmitted signal of duration T, the first section contains the modified data and the second the auxiliary information, sign vector, required for its correct detection.
• Fig. 5b shows the transmitted signal of duration T, the first section contains the modified data and the second the auxiliary information, frame vector, required for its correct detection.
• Fig. 6 illustrates the operation of the decoder after detecting the section of the data signal and auxiliary information for the case in which the signal has been folded once or several times and reducing the maximum values.
• This decoder is an example of digital type.
• Fig. 7 presents the operation of the decoder after having detected the section of the data signal and auxiliary information in the case of recovering the phase information.
• the auxiliary signal values indicate the sign of the sample.
• This decoder is an example of analog type
• Fig. 8 shows a block diagram of the encoder in the transmitter.
• the generated signal (701) is divided into two.
• the signal (701) suffers a delay caused by (702) and is encoded in (704).
• the copy of the generated signal (701) passes through a comparator (703) that will indicate the area defined by the limits established from the level of the signal (d [n]).
• This value goes through the block (706) that checks in a reference table (707) the index of the zone (c [n]) as well as the limit value (c v [n]) that has been exceeded.
• These values are sent to the encoder (704) and the Zone index is passed to buffer (708) to generate the auxiliary signal.
• the delayed signal is encoded in (704) by the operations defined in fig. 8b and then stored in the buffer (705).
• a switch will alternate the sending of the encoded data signals and the auxiliary signal.
• Fib.8b shows the encoder block diagram (704) of fig8: the value of the limit c v [n] (7b05) obtained from the module (706) is subtracted from the delayed data signal x [n] (7c01). ) of fig. 8. The samples of the resulting signal are multiplied by the value obtained from the operator (7c04) that raises the constant "- ⁇ to the zone index, c [n] (7b04).
• Fig. 8c presents the flow chart of block (706) of fig. 8 and which generates the auxiliary signal and the levels required to modify the data signal.
• the maximum or minimum limit is defined, depending on whether the value is positive or negative, which has exceeded the value of the current sample of the signal (7b02).
• the obtained value will be sent to the encoder (704) as the signal c v [n] (7b05). It will also be used to check the corresponding zone in which the value of the signal sample is located (7b03) in a reference table (707 of fig. 8).
• the value obtained from the reference table, c [n] (7b04) is sent to both the encoder (704) and the buffer (708) represented in fig. 8.
• Fig. 9 illustrates a block diagram of the decoder in the receiver.
• the received signal (801) is divided by a switch in one part with the information signal and in the auxiliary signal.
• the information signal, and r [n] is stored in a buffer (802) while the auxiliary signal is sent to a comparator (803) to determine the zones, s [n], in which each of them were located. the samples of the frame received before being encoded.
• the signals y r [n] and s [n] are sent to the decoder (804) that uses a reference table (805) to retrieve the signal and whose operation is explained in fig. 9b.
• Fig. 9b presents the block diagram of the decoder in the receiver.
• the maximum value of the received signal is determined, and r [n] in block (8b01). From this maximum value and the number of levels defined in memory (8b03) the values of the limits, c v [n], are obtained. The indexes of the zones, c [n], are also identified from the auxiliary signal s [n]. Both tasks are carried out in the block (8b02).
• Fig. 10 represents a block diagram of the proposed method, with a coding (C) and decoding (D) function of a signal that is transmitted by means (M).
• the preferred implementation consists of an encoder in the transmitter and a decoder in the receiver that allow to modify in an intelligent and predetermined way the signal in a part of data and to add a section of auxiliary information to recover the original signal. It is not necessary to make modifications to the fiber optic distribution plant or some means.
• the encoder can be digital or analog and will precede the signal modulator (Fig. 8). Its operation is as follows:
• the modified signal will be modulated and sent by the chosen transmission medium.
• the receiver on the other hand, must include a decoder (Fig. 9) that allows it to recover the original signal.
• This device can be digital or analog and will work as follows:

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Cited By (3)

Citations (2)

• 2012
  • 2012-09-14 ES ES201231426A patent/ES2455116B1/es active Active
• 2013
  • 2013-09-13 WO PCT/ES2013/070634 patent/WO2014041227A1/fr not_active Ceased

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