142 results about "Optical ofdm" patented technology

A transmitter for an optical transmission system transmits an optical sub carrier multiplexed signal comprising number of sub-carriers, onto an optical transmission path, and provides spectral shaping by different magnitudes of the sub-carriers, or different modulation formats for different sub carriers. This spectral shaping can reduce performance degradation by Kerr effect optical non linearities. This can mean higher input powers may be launched. The magnitudes can provide a signal spectrum which is lower near a centre of a band of sub carriers than near an edge of the band. Such spectral shaping can be provided in the receiver either to undo the pre emphasis in the transmitter, or to reduce non linearities from components at the receiving side.

A method of transmitting digital information over a dispersive optical channel includes encoding the digital information into a series of data blocks, wherein each block comprises a plurality of substantially orthogonal frequency domain subcarriers. Each data block is then divided into at least two frequency bands, each band comprising a plurality of contiguous subcarriers. A frequency/time transformation is then performed, in order to form a corresponding plurality of transformed bands, each transformed band comprising a sequence of time domain data samples. A time delay is applied to at least one of the transformed bands relative to at least one other of the transformed bands. The bands are then combined to produce an electrical signal waveform embodying the digital information. Finally, an optical source is modulated using the electrical signal waveform, to produce a corresponding optical signal for transmission over the dispersive optical channel. The invention enables a reduction in the duration of guard intervals that may need to be inserted into the transmitted data blocks, in order to avoid received signal degradation due to inter-symbol interference caused by dispersion in the optical channel. Transmission overheads may thereby be reduced, resulting in an improvement in transmission efficiency.

The invention discloses a sensing system based on optical OFDM (Orthogonal Frequency Division Multiplexing) and an FBG (Fiber Bragg Grating) monitoring method thereof, relating to the field of optical fiber sensing. The sensing system comprises an adjustable optical source, an electrooptical modulator, an optical circulator, a weak reflective FBG array, a wavelength tuning control circuit, a signal generator, an optical detector and a digital decoding and control unit, wherein the adjustable optical source is used as a signal optical source; the electrooptical modulator is used for modulating data onto signal optical waves; the optical circulator is used for separating optical signals which are in an incident direction from optical signals which are in a reflective direction; the weak reflective FBG array is used as a sensing element for reflecting the optical signals to the optical circulator; the wavelength tuning control circuit is used for tuning the emitting wavelength of the adjustable optical source; the signal generator is used for generating electric signals which are encoded based on OFDM; the optical detector is used for converting the reflective optical signals into the electric signals; and the digital decoding and control unit is used for positioning an FBG sensor and acquiring variables to be detected. In the invention, by combining the FBG monitoring method based on the optical OFDM, the arrangement density and the sensing distance of FBG sensors are improved, the scanning frequency and the spatial revolution of the system are improved, and large-capacity measurement requirements are satisfied.

The invention relates to a power-adjustable partially-superimposed training sequence-based time synchronization system and a power-adjustable partially-superimposed training sequence-based time synchronization method for an optical orthogonal frequency division multiplexing (OFDM) system, and relates to the technical field of optical communication. A novel superimposed training sequence-based time synchronization method aims to overcome the shortcomings of low synchronization performance of the conventional optical OFDM system and great interference with transmission data, and cyclic prefix information is fully used for acquiring synchronization information. A training sequence is linearly superimposed onto the latter L sampling points of an OFDM symbol and a cyclic prefix under the condition that the length L of the cyclic prefix is determined, so that the shortcoming of interference with the data during the superimposition of a long sequence is overcome. The method is high in time synchronization accuracy, the interference to OFDM data is small, and a training sequence superimposition power allocation factor can be selected more flexibly.

To reduce PAPR in an optical OFDM communication system, light is phase-modulated by a base-band OFDM signal which is the output of a transmission signal processing unit 100 provided in an inner section of an optical receiver. When optical communication is implemented with the light as signal light, PAPR of low level of almost 0 dB can be achieved at places in an inner section of an optical fiber wherein the optical power is large. The signal light is propagated along an optical fiber which is a transmission path, the light is passed through a delay interferometer, and converted to electrical signals by two optical receivers. Each of the two electrical signals is converted to a subcarrier by FFT signal processing and an AD converter disposed in an inner section of the optical receivers, then, subcarriers having the same frequency are subtracted and decoded, and data is regenerated.

One aspect provides an optical communication system. The system includes an optical-to-digital converter, a frequency estimator and a symbol synchronizer. The optical-to-digital converter is configured to receive an optical OFDM bit stream including an OFDM symbol bearing payload data and a symbol header preceding the OFDM payload data. The frequency estimator is configured to determine a carrier frequency offset of the payload data from the symbol header. The symbol synchronizer is configured to determine a starting location of the payload data within the bit stream by cross-correlating a synchronization pattern within the symbol header with a model synchronization pattern stored by the symbol synchronizer.

The invention provides methods, devices and a system for recovering the corrupted subcarrier at the local oscillator (LO) frequency in coherent optical OFDM transmission. The method includes performing advanced coding on a data signal to obtain an encoded signal; performing high order modulation on the encoded signal to obtain a high-order-modulated signal; performing OFDM modulation on the high-order-modulated signal to obtain an electrical OFDM signal; and performing up-conversion on the electrical OFDM signal to obtain an optical OFDM signal to be output. The inventive technique of employing advanced coding with low rate combining with higher order modulation can be used to reduce the decoding bit error ratio (bit error rate) level, so that the LO subcarrier can be fully recovered while the bandwidth of the transmitted signal may be substantially the same as the existing optical OFDM system, and there is no need to add any feedback control module or feedback loop support or the like to the existing optical OFDM system, so that the complexity of the receiving side can be reduced.

A self-polarization diversity technique to combat PMD in a direct-detection optical OFDM system. This technique does not require any dynamic polarization control, and can simultaneous compensate PMD in a WDM system with one device. Simulation results show that this technique virtually completely eliminates the PMD impairments in direct-detection optical OFDM systems.

The invention provides a WDM-PON-RoF wired/wireless selectable access system and method based on SSB-OFDM. In the WDM-PON wired/wireless selectable access system, an OLT enables N ways of 4QAM-OFDM radio frequency signal single side bands to be modulated on N different optical carriers, and the N ways of 4QAM-OFDM radio frequency signal single side bands are reused as down link signals. RN distributes one way of optical OFDM signals and two ways of optical carriers with different radio frequencies for each ONU. According to the different user demands, each ONU conducts beat frequency on the optical OFDM signals and one way of optical carriers with small/large frequency spaces with the optical OFDM signals, therefore, wired/wireless access is achieved, and the other way of optical carriers are used as a modulating light source of up-link signals. The WDM-PON wired/wireless selectable access system and method are used, selectable access of wired signals or 60 GHz frequency band wireless signals can be achieved, the purpose of being free of light sources of the ONUs is achieved, and the cost and the complexity of the ONUs are lowered. An SSB and OFDM modulating technology is used in the WDM-PON wired/wireless selectable access system, and the spectrum efficiency is improved.

From an real valued OFDM signal (S₀(t)) is a baseband signal (S_B(t)) derived and converted into a complex single sideband modulation signal (n(t)). This is modulated onto an optical carrier (f_OC) to generate a SSB transmission signal (S_OT) having a small bandwidth an carrying the information in the envelope or in the power of the envelope. According to the modulation direct detection is possible. Only a small bandwidth is necessary for the transmission.

A method for transmitting digital information having a predetermined bit-rate (B) via a nonlinear optical channel includes determining a combination of transmission parameters comprising a number (N) of subcarriers, an information symbol rate (S) for each subcarrier, and a number (b) of bits per symbol on each subcarrier, such that B=NSb. An optical signal is generated comprising the plurality (N) of orthogonal optical subcarriers. Each subcarrier is modulated to carry a proportion (Sb/B) of the total bit-rate (B) of the digital information. The optical signal is then transmitted to a corresponding optical receiver via the nonlinear optical channel. The received signal has a measurable signal quality (Q) which is a function of the transmission parameters (N, S, b). Determining the combination of transmission parameters comprises selecting the parameters such that the associated signal quality is greater than a corresponding quality for a single-carrier transmission, and greater than a corresponding quality employing a larger number (M>N) of subcarriers.

In accordance with some embodiments of the present disclosure a method for receiving and processing an optical orthogonal frequency-division multiplexed signal containing a plurality of traffics comprises receiving the optical orthogonal frequency-division multiplexed signal. The method further comprises down-converting the optical orthogonal frequency-division multiplexed signal into the electrical domain to obtain an electrical signal; filtering the electrical signal to obtain a first portion of the electrical signal containing a first of the plurality of traffics and preprocessing the first portion of the electrical signal in a first parallel preprocessor; filtering the electrical signal to obtain a second portion of the electrical signal containing a second of the plurality of traffics and preprocessing the second portion of the electrical signal in a second parallel preprocessor; and combining the preprocessed first and second portions of the electrical signal to yield a combined electrical signal and demodulating the combined electrical signal.

A method includes producing interference between a received optical OFDM signal and an optical carrier extracted from the received optical OFDM signal to provide optical coherent detection of the received optical OFDM signal. Preferably, producing the interference includes optically splitting the received optical OFDM signal into a first part that is filtered to extract the carrier from the received optical OFDM signal and a second part similar to the received optical OFDM signal.

The invention discloses a polarization multiplexing band interpolation based OFDMA-PON (orthogonal frequency division multiple access-passive optical network) system which is characterized in that: two paths of OFDM (orthogonal frequency division multiplexing) signals are respectively produced at an optical line terminal, and sub-bands of the two paths of OFDM signals are mutually interleaved; and through respectively carrying out optical modulation and polarization control on the OFDM signals, two paths of optical OFDM signals in the x and y polarization directions are respectively obtained, and then combined into one path so as to form optical OFDM signals without protection bands, in such a way, the bandwidth of the system is fully used. An optical distribution network divides the received optical OFDM signals into optical OFDM signals in the x and y polarization directions, then an optical beam splitter divides the OFDM signals into multiple paths, and an optical filter extracts the optical OFDM signals divided by the optical beam splitter, selects required different sub-band optical OFDM signals or different numbers of sub-band optical OFDM signals, and sends the selected signals to respective optical network units, thereby achieving the variable-speed access of the optical network units.

The invention discloses a cyclic prefix (CP) and virtual carrier based blind frequency offset estimation method in an optical OFDM (Orthogonal Frequency Division Multiplexing) system, which can estimate the decimal part and the integral part of the frequency offset at the same time. Without the assist of training sequence and other known data, the cyclic prefix (CP) and virtual carrier based blind frequency offset estimation method improves the effective speed and frequency spectrum efficiency of the system. By adopting a large amount of data of a receiving terminal to carry out blind frequency offset estimation, the method not only can reduce the influence of noise, laser line width and other factors, but also can a trace frequency offset change situation and improve the accuracy of frequency offset estimation and system performance. Moreover, the cyclic prefix (CP) and virtual carrier based blind frequency offset estimation method is applicable to an optical transmission system which adopts the OFDM technology and can be used together with other frequency offset estimation technologies. The invention has the advantages of high practicability, high frequency offset estimation accuracy, simple structure, low cost, low requirement on signal processing and the like.

The invention discloses a time synchronization method in a system of an optical orthogonal frequency division multiplexing-based passive optical network (OOFDM-PON). The scheme comprises an optical line terminal (OLT) and an optical network unit (ONU), wherein the OLT comprises a laser, a modulator, an OFDM signal generating module and an optical multiplexer. The ONU comprises an optical demultiplexer, a time delay compensator, an optical detector and an OFDM receiving module. At the transmitting end of the OLT, optical signals for modulating optical OFDM signals and timing information are divided into two branches. The optical signals are divided into two branches after transmitted by optical fibers, wherein a timing signal is extracted from the branch for carrying the timing information by the optical detector after time-delay compensation, and the OFDM signals are accurately received at the receiving end of the ONU. The invention has the characteristics of simple design principle, easy realization, low cost and the like.

Methods and systems for secure communication include assigning data streams to respective orthogonal sub-carriers according to random number information at a transmitter. The data streams are modulated onto the respective orthogonal sub-carriers before the orthogonal sub-carriers are transmitted. The random number information is transmitted on a second medium, separate from the transmission medium. The orthogonal sub-carriers are received from the transmission medium at a receiver, while the random number information is received from the second medium at the receiver. The orthogonal sub-carriers are associated with data streams using the random number information and demodulated to produce data for the associated data stream.

The invention provides a parameter measuring device of a distributed optical fiber sensing system. The parameter measuring device comprises the distributed optical fiber sensing system based on the BOTDA (Brillouin optical time domain analysis) technology, an optical OFDM (orthogonal frequency division multiplexing) signal generating module, on OFDM detecting module and a signal processing module. The distributed optical fiber sensing system comprises a laser, a coupler, a first electro-optical modulator, a second electro-optical modulator, a pulse/random sequence generator, a circulator and a single mode fiber serving as a sensing fiber. By the parameter measuring device, the problems the measuring precision is low, measuring range is limited and measuring results are singular in the prior art can be solved.

The invention provides a reversed polarity multiple pulse position modulation optical OFDM system combining with dimming control. The system combines MPPM and RPO-OFDM technologies efficiently, and improves the data transmission rate under the precondition of keeping a bandwidth unchanged by utilizing an extra-bit carried by an MPPM mode while supporting a flexible dimming function. Compared withthe traditional RPO-OFDM, the method can realize higher spectrum efficiency under the condition that calculation complexity and energy consumption are similar to those in the prior art.

The invention discloses a method for restraining PAPR of optical OFDM system based on FHT and improved Mu law companding transform to firstly improve the traditional Hadamard transform and Mu law companding transform and then perform the cascade connection. The computation complexity of fast Hadamard transform is much lower than Hadamard transform and the improved Mu law companding transform may undertake joint optimization for peak-to-average ratio performance and error rate performance, control the single BER performance while reducing single PAPR and eliminate the band spectrum regeneration resulting from signal distortion in the course of repetition. After the fast Hadamard transform for single is completed, the PAPR of single reduces a little. While the companding transform is performed for the single, the predistortion of single will not be too serious. Thus, when undertaking the inverse transformation in the receiving end, the single will be recovered better. The PAPR performance after using the provided method is better than the performance after separately adopting one of methods and the comprehensive performance is superior to traditional SLM and PTS.