CN110677197A - CDMA coding system based on time lens - Google Patents

CDMA coding system based on time lens Download PDF

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CN110677197A
CN110677197A CN201910910814.1A CN201910910814A CN110677197A CN 110677197 A CN110677197 A CN 110677197A CN 201910910814 A CN201910910814 A CN 201910910814A CN 110677197 A CN110677197 A CN 110677197A
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time
pulse sequence
lens
pulse
subsystem
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CN110677197B (en
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郭淑琴
林雅妮
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Zhejiang University of Technology ZJUT
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation
    • H04B10/524—Pulse modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J13/00—Code division multiplex systems

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  • Computer Networks & Wireless Communication (AREA)
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  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

Abstract

A CDMA coding system based on a time lens comprises an optical fiber delay line coding subsystem and a pulse sequence compression subsystem based on the time lens, wherein the output end of the optical fiber delay line coding subsystem is connected with the input end of the pulse sequence compression subsystem based on the time lens; the optical fiber delay line coding subsystem divides the input narrow light pulse into w parts, carries out different time delays according to address codes distributed to users, and obtains a pulse sequence synthesized by the narrow light pulses with different delays at the output end of the optical fiber delay line coding subsystem; the time lens based pulse sequence compression subsystem compresses a pulse sequence having a width exceeding one information bit period into one information bit period to obtain a final output encoded signal. The invention can reduce the requirement of CDMA coding on pulse width and pulse accurate delay, and make the system bear more users.

Description

CDMA coding system based on time lens
Technical Field
The invention relates to a CDMA coding system based on a time lens.
Background
Code division multiplexing, also known as code division multiple access multiplexing, abbreviated as CDMA, is a rapidly developing signal processing method in cellular mobile communications. In CDMA, which allows all stations to transmit simultaneously in the entire frequency band, i.e. each user can communicate using the same frequency band at the same time, a method of dividing channels according to chip sequences (i.e. address codes) is used to distinguish data of each user: each user is assigned a unique m-bit chip sequence, and if a user wants to transmit bit 1, it is actually the m-bit chip sequence of itself, and if a user wants to transmit bit 0, it is actually the binary complement of the m-bit chip sequence of itself. CDMA coding refers to a process of modulating between user information and an address code, in a time-domain CDMA coder, one bit information period is divided into m (m is the length of the address code) time slots, and different time slots correspond to different time delays. At the transmitting end of CDMA coder, a pulse representing information "1" is divided into w (w is address code weight) optical pulse forming sequence according to specific time delay information and transmitted. Time-domain CDMA coding suffers from code length problems: in order to provide the largest number of users, the code length of the address code must be long enough, which results in that the pulse width of the optical pulse needs to be very short, the optical pulse with very narrow pulse width will interfere with itself when passing through the beam splitter and delay and recombining, which brings interference noise, and the shorter the pulse width of the optical pulse, the greater the difficulty of accurately delaying the optical pulse.
The time lens is an optical device capable of generating a secondary time phase shift on an optical signal, and has four main implementation modes: a time lens implemented based on EOPM, a time lens implemented based on XPM, a time lens implemented based on sum and difference frequency effects, and a time lens implemented based on FWM effects. In recent years, many researchers mainly study a time lens imaging system based on a four-wave mixing effect, and the time lens imaging system has the advantages that the phase matching with a higher angle can be realized, the resolution is higher, the system can be built by low-power pumping, the system can be built in many materials, and the like.
Amplification or compression of the input signal is achieved with an imaging system and the envelope of the output signal is kept consistent with the envelope of the input signal, a function called time-domain imaging, which is also one of the main functions of a time-lens imaging system. The time lens imaging system consists of three parts, namely an input end dispersion part, a time lens and an output end dispersion part. Beta is as、βiSecond order dispersion coefficients, beta, of the input and output fibers, respectivelypIs the second order dispersion coefficient of the pump light transmission fiber; l iss、LiLength of input and output optical fibres, L, respectivelypIs the length of the fiber that the pump light undergoes dispersion broadening; the input end dispersion accumulation and the output end dispersion accumulation are respectively phis＝βsLs，φ″i＝βiLi，φ″p＝βpLpFor pumping pulses through LpThe latter dispersion accumulation amount. When phis、φ″iPhipSatisfies the imaging condition
Figure BDA0002214638930000021
Then, the amplification or compression of the input optical signal can be realized, wherein the amplification factor M ═ phi ″ "i/φ″sWherein when 0 < M < 1, compression of the input optical signal can be achieved.
Disclosure of Invention
In order to overcome the defect of the code length problem of the existing time domain CDMA coding, the invention provides a time lens-based CDMA coding system, which can reduce the requirements of the CDMA coding on the pulse width and the pulse accurate delay and enable the system to bear more users.
The technical scheme adopted by the invention is as follows:
a CDMA coding system based on a time lens comprises an optical fiber delay line coding subsystem and a pulse sequence compression subsystem based on the time lens, wherein the output end of the optical fiber delay line coding subsystem is connected with the input end of the pulse sequence compression subsystem based on the time lens;
the optical fiber delay line coding subsystem is used for dividing the input narrow optical pulse into w parts of same sub-pulses according to an address code allocated to a user, wherein w is the address code weight, then the w parts of sub-pulses are delayed differently according to the address code, the different pulse delay sequences reflect address code information, user distinguishing is realized, and finally a pulse sequence synthesized by the sub-pulses with different delays is obtained at the output end of the optical fiber delay line coding subsystem;
the time lens based pulse sequence compression subsystem is used for compressing a pulse sequence with the width exceeding one information bit period into one information bit period to obtain a final output coded signal.
Furthermore, the pulse sequence compression subsystem based on the time lens is composed of an input section optical fiber, the time lens and an output section optical fiber, and the second-order dispersion of the output section optical fiber is measured by phi ″iThe second-order dispersion of the input section of the optical fiber is represented by phi ″)sRepresents; the magnification M ═ phi ″' of the time lens imaging subsystemi/φ″sM is more than 0 and less than 1; during the duration of the pump light pulse of the pulse sequence compression subsystem based on the time lens, the pulse sequence can be covered at the same time, and the compression of the pulse sequence is realized by 0 < M < 1.
Still further, in the pulse sequence compression subsystem based on the time lens, the FWM is generated between the signal light and the pump light in the high-nonlinearity fiber to realize the time lens effect.
Or the following steps: in the pulse sequence compression subsystem based on the time lens, the FWM of the signal light and the pump light occurs in a high nonlinear medium to realize the time lens effect.
In the pulse sequence compression subsystem based on the time lens, the width of the pump light pulse is controlled, so that one pump light pulse width can cover the pulse sequence width at the same time.
Preferably, the delay is realized by adopting an optical fiber delay line technology.
The invention has the beneficial effects that: after a pulse sequence compression subsystem based on a time lens is added, the optical pulse used for encoding can adopt a low-cost broadband optical pulse, a wider optical pulse passes through an optical fiber delay line encoding subsystem to obtain a pulse sequence with the width exceeding one information bit period, and the pulse sequence compression subsystem can compress the pulse sequence into one information bit period. The invention can reduce the requirement of the time domain CDMA coding system on the ultrashort optical pulse and improve the number of users which can be borne by the time domain CDMA coding system.
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FIG. 1 is a system diagram of the present invention, including a fiber delay line encoding subsystem and a time lens based pulse train compression subsystem, wherein (a) is user information and (b) is a system schematic of an embodiment of the present invention.
FIG. 2 is a schematic diagram of time lens compression, which performs a compression function on an input light pulse when the magnification is 0 < M < 1.
FIG. 3 is a schematic diagram of a pair of optical pulses compressed after passing through a time lens based pulse train compression subsystem, wherein (a) is an input signal to the pulse train compression subsystem; and (b) is the output signal of the pulse train compression subsystem.
Detailed Description
The invention will be further explained by means of embodiments in conjunction with the attached drawings, without limiting the scope of the invention thereto.
Referring to fig. 1-3, a time lens based CDMA encoding system includes a fiber delay line encoding subsystem and a time lens based pulse train compression subsystem. The optical fiber delay line coding subsystem divides the input narrow light pulse into several parts, carries out different time delays according to the address code allocated to the user, obtains a pulse sequence synthesized by the narrow light pulses with different delays at the output end of the optical fiber delay line coding subsystem, and the optical fiber delay line coding is a very classical method in the current CDMA coding method, so the implementation process is not repeated here; the pulse sequence compression subsystem based on the time lens comprises an input section optical fiber, a time lens and an output section optical fiber, wherein the second-order dispersion quantity phi of the output section optical fiber is formediThe second-order dispersion of the input section of the optical fiber is represented by phi ″)sRepresents; the magnification M ═ phi ″' of the time lens imaging subsystemi/φ″sM is more than 0 and less than 1; controlling the width of the pump light pulse to make the duration of the pump light pulse cover the pulse sequence, and compressing the pulse sequence with the width exceeding one information bit period into one information bit period by 0 < M < 1 to obtain the final output coding signal.
In the pulse sequence compression subsystem based on the time lens, the FWM of the signal light and the pump light occurs in a high-nonlinearity fiber to realize the time lens effect. Or the following steps: the time lens effect is realized by FWM of signal light and pump light in a highly nonlinear medium.
Referring to FIG. 2, to satisfyThe parameters of both temporal lens imaging subsystems are selected as: l iss＝1km，βi＝20ps2/km，Li＝1km，βp＝-80ps2/km，L p1 km. At this time, M is 1/2.
FIG. 3 illustrates a schematic diagram of a pair of optical pulses compressed after passing through a time lens based pulse train compression subsystem, wherein (a) is an input signal to the pulse train compression subsystem; (b) is the output signal of the pulse train compression subsystem.
As described above with reference to fig. 1 to 3, the optical pulse passes through the CDMA coding system based on the time lens to obtain an output coded signal required for CDMA communication. The invention can reduce the requirement of the time domain CDMA coding system on the ultrashort optical pulse and improve the number of users which can be borne by the time domain CDMA coding system.

Claims (6)

1. A CDMA coding system based on a time lens is characterized by comprising an optical fiber delay line coding subsystem and a pulse sequence compression subsystem based on the time lens, wherein the output end of the optical fiber delay line coding subsystem is connected with the input end of the pulse sequence compression subsystem based on the time lens;
the optical fiber delay line coding subsystem is used for dividing the input narrow optical pulse into w parts of same sub-pulses according to an address code allocated to a user, wherein w is the address code weight, then the w parts of sub-pulses are delayed differently according to the address code, the different pulse delay sequences reflect address code information, user distinguishing is realized, and finally a pulse sequence synthesized by the sub-pulses with different delays is obtained at the output end of the optical fiber delay line coding subsystem;
the time lens based pulse sequence compression subsystem is used for compressing a pulse sequence with the width exceeding one information bit period into one information bit period to obtain a final output coded signal.
2. A time-lens based CDMA coding system as claimed in claim 1, wherein: the pulse sequence compression subsystem based on the time lens is composed of an input section optical fiber, the time lens and an output section optical fiber, and the second-order dispersion quantity of the output section optical fiber is phii"indicates that the second-order dispersion of the input section fiber is represented by phis"represents; the magnification factor M phi of the time lens imaging subsystemi″/φsM is more than 0 and less than 1; during the duration of the pump light pulse of the pulse sequence compression subsystem based on the time lens, the pulse sequence can be covered at the same time, and the compression of the pulse sequence is realized by 0 < M < 1.
3. A time-lens based CDMA coding system as claimed in claim 1 or 2, characterized in that: in the pulse sequence compression subsystem based on the time lens, the FWM of the signal light and the pump light occurs in a high-nonlinearity fiber to realize the time lens effect.
4. A time-lens based CDMA coding system as claimed in claim 1 or 2, characterized in that: in the pulse sequence compression subsystem based on the time lens, the FWM of the signal light and the pump light occurs in a high nonlinear medium to realize the time lens effect.
5. A time-lens based CDMA coding system as claimed in claim 1 or 2, characterized in that: in the pulse sequence compression subsystem based on the time lens, the width of the pump light pulse is controlled, so that one pump light pulse width can cover the pulse sequence width at the same time.
6. A time-lens based CDMA coding system as claimed in claim 1 or 2, characterized in that: the delay is realized by adopting an optical fiber delay line technology.
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