CN102395061A - PON (passive optical network) transmission system based on asymmetrical cut-off light OFDM (orthogonal frequency division multiplexing) - Google Patents

Abstract

The invention provides a PON (passive optical network) transmission system based on asymmetrical cut-off light OFDM (orthogonal frequency division multiplexing), belonging to the field of fiber optic communication. In the PON ascending transmission system provided by the invention, a modulation mode of the asymmetrical cut-off light OFDM is used, and has the advantages of not needing to transmit direct current optical carrier wave and being energy-saving, as well as realizing multiple access of time division multiplexing. In the asymmetrical cut-off light OFDM, the part of the OFDM signal, which is smaller than 0, is set to be 0, the part of the OFDM signal, which is larger than 0, is reserved, and the method can be realized by a direct modulation laser offset at a threshold value point. If the OFDM signal before cut-off only has odd number frequency component, the cut-off does not interference the odd number of frequency subcarrier waves. The invention has the advantages that: an optical user unit is colorless, the modulation mode is energy-saving, and the direct modulation laser is used, so that the cost is lowered; and due to the application of the high-stage modulation mode, the transmission rate is greatly improved.

Description

Based on the asymmetric passive light network transmission system that blocks light positive friendship frequency division multiplexing

Technical field

The invention belongs to fiber optic communication field, proposed a kind of uplink implementation of EPON.

Background technology

In recent years, along with professional development such as internet and multimedia, the user is exponential increase to the demand of access network bandwidth.Use the access way of conventional transmission medium such as twisted-pair feeder, coaxial cable to be difficult to satisfy the user's bandwidth demand.Advantages such as capacity is big because optical fiber communication has, long transmission distance, as the capacity and the span that can significantly improve Access Network in the Access Network of transmission medium, Fiber to the home (FTTH) is regarded as the final solution of Access Network with optical fiber.Simultaneously, Access Network has characteristics such as cost is quick, the operation maintenance expense is big.(Passive Optical Network PON) can be for tens of even up to a hundred users provide access service to EPON simultaneously, and network center uses passive power division device, and is as shown in Figure 1.PON has advantages such as cost is low, the network operation maintenance cost is cheap, has competitiveness.The PON network is made up of three parts:

Optical line terminal (Optical Line Terminal, OLT): be the local side apparatus in the PON network, for user's access service be provided simultaneously.

2. (Optical Network Unit, ONU): the user side equipment in the PON network, ONU provides broadband access for the user to optical network unit.

3. (Optical Distribution Network, ODN): couple together OLT and ONU, be generally the star structure, the center of network is passive power division device to Optical Distribution Network.

At present, standardization and extensive commercial PON technology comprise EPON and GPON, even 10G-EPON etc.They can provide up, the descending access bandwidth of 1Gb/s and even 10Gb/s.On transmission means, EPON and GPON use simple NRZ modulation and time-division multiplex mode.In downlink transfer (OLT is to the transmission of ONU), OLT sends continuous bit stream to all ONU broadcasting, and which data each ONU can judge to one's name and store according to procotol, and which to one's name and with it does not abandon.In uplink (ONU is to OLT), ONU sends light signal at specific time slot, can not be overlapping when the light signal that requires each ONU to send arrives OLT.Because wide range of commercial is used, time-multiplexed optical transport technology is ripe, and the time division multiplexing PON of 10Gb/s is up and downlink transfer is technological can realize.

Because the continuous growth of access bandwidth demand and to the competition of low-cost technologies, academia and industrial quarters are all in that actively to inquire into the PON of new generation that seeks to have high power capacity more technological.A new generation's PON technology is providing the jumbo while, and the characteristics that need to satisfy comprise: compatible existing Optical Distribution Network, promptly only use the ODN of power division; ONU is colourless, and promptly each ONU is identical on hardware, as not needing the laser of specific wavelength.

Continue to improve the access rate of PON, time division multiplexing and NRZ modulation format have got into bottleneck.In downlink system, improve baud rate and require the receiving terminal circuit bandwidth to increase, increased the realization difficulty and the cost of receiver.The chromatic dispersion of optical fiber link will limit PON system span.In the uplink, the OLT end, the above burst receiver of 10Gb/s also is difficult to realize.

In order further to improve the access rate of PON; We have proposed a kind of based on asymmetric light positive friendship frequency division multiplexing (Asymmetric Clipping Optical Orthogonal Frequency Division Multiplexing, passive optical network uplink transmission system ACO-OFDM) of blocking.

Summary of the invention

In order further to improve the access rate of PON; We have proposed a kind of based on asymmetric light positive friendship frequency division multiplexing (Asymmetric Clipping Optical Orthogonal Frequency Division Multiplexing, passive optical network uplink transmission system ACO-OFDM) of blocking.Its feature ONU utilizes time division multiplexing to carry out up access, and advantage is that ONU is colourless and energy-conservation.In this system, each ONU sends the Frame that one or more OFDM symbols are formed in distributed time slot.The moment of Frame arrival OLT can not be overlapping.As shown in Figure 2.

Description of drawings

Fig. 1: EPON sketch map;

Fig. 2: based on the asymmetric passive optical network uplink transmission system sketch map that blocks light positive friendship frequency division multiplexing;

Fig. 3: common orthogonal frequency-division multiplex singal and the asymmetric contrast of blocking light positive friendship frequency-division multiplex singal;

Fig. 4: the asymmetric light positive that blocks is handed over frequency division multiplexing principles of modulation and demodulation figure;

Embodiment

In the PON of the present invention system, each ONU uses the ACO-OFDM modulation system.Being about to that ofdm signal blocks less than 0 part is 0; Part less than 0 is constant, keeps original value.Fig. 3 is the contrast of common ofdm signal and ACO-OFDM signal.The asymmetric operation meeting of blocking is disturbed the ofdm signal precocity.But can know according to theory analysis,, through the signal after asymmetric the blocking odd subcarriers not disturbed so if original ofdm signal has only odd subcarriers.Therefore, ONU is modulation signal on odd subcarriers, and even subcarriers zero setting re-uses the ACO-OFDM modulation, and OLT just can normally accept this signal.

ONU can use direct modulated laser to realize the ACO-OFDM modulation, and directly the input/output relation of modulated laser is approximately:

$P=\left\{\begin{array}{cc}s(i-i_{\mathrm{th}})& i\&GreaterEqual;i_{\mathrm{th}}\\ 0& i<i_{\mathrm{th}}\end{array}\right.$

Wherein, i _ThThreshold current for laser.

If laser bias is at the threshold current place, the electric current of input is:

i＝d(t)+i _th

Laser output power is:

$P=\left\{\begin{array}{cc}\mathrm{sd}\left(t\right)& d\left(t\right)\&GreaterEqual;0\\ 0& d\left(t\right)<0\end{array}\right.$

The principles of modulation and demodulation figure of ACO-OFDM is as shown in Figure 4.Light subscriber unit (ONU) 410 produces the ACO-OFDM signal, and the transmission of process optical fiber 401 is received by optical line terminal (OLT) 420.

In light subscriber unit 410, at first bit 411 is produced qam symbol through QAM mapping 412.

Inversefouriertransform (IFFT) the 414th, the fast algorithm of Inverse Discrete Fourier Transform (IDFT), the computing formula of IDFT and IFFT is:

$x_n=\frac{1}{N}\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}{X}_k\exp \left(\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HSA00000605527200011.png,HSA00000605527200012.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;\&omega;}\frac{k}{N}n\right)$

Wherein, X _kBe the input of IFFT, in ofdm system, be called subcarrier.K is the X of odd number _kBe called odd subcarriers, k is the X of even number _kBe called even subcarriers.For the time-domain signal that makes IFFT output is a real number, X _kNeed satisfy the conjugation symmetry, that is:

X _N-k＝X _k ^*，1＜k＜N/2

Wherein, * representes conjugation.

412 qam symbols that produce, as sequence number less than the N/2 odd subcarriers.Simultaneously, these qam symbols are got conjugation 413, as the odd subcarriers of sequence number greater than N/2, need satisfy formula X _N-k=X _k ^*Odd subcarriers all is set to 0.

To subcarrier X _kCarry out after the inversefouriertransform 414; Produce time-domain signal, the signal insertion Cyclic Prefix 415 to 414 outputs passes through parallel serial conversion 416 and digital-to-analogue conversion (DAC) 417 modules again; Produce electric territory ofdm signal, and the modulation signal of the direct modulated laser 418 of conduct.Directly modulated laser 418 bias points are made as threshold point, and so direct modulated laser can produce the ACO-OFDM signal.

The demodulation method of ACO-OFDM signal is consistent with common ofdm signal demodulation method.Hold at OLT; Photodiode 421 converts light signal into the signal of telecommunication; Analog to digital converter (ADC) 422 is analog signal conversion digital signal, carries out serial to parallel conversion 423 again, behind the deletion Cyclic Prefix 424; Carry out fast Fourier transform (FFT) 425 operations, take out the subcarrier that carries data and carry out the demodulation that the ACO-OFDM signal has just been accomplished in QAM demodulation 426.

Narration is based on the implementation method of the time division multiplexing PON of ACO-OFDM below.In no input signal, ONU does not send light signal, can realize that therefore time division multiplexing inserts.In this system, each ONU sends the Frame that one or more OFDM symbols are formed in distributed time slot.The moment of Frame arrival OLT can not be overlapping.As shown in Figure 2.So just realized the time-division multiplexing multiple access access.

The major technique advantage

The present invention proposes and a kind ofly block the passive optical network uplink transmission system that light positive is handed over frequency division multiplexing based on asymmetric, its advantage is:

1. colorless ONU: use time-division multiplexing multiple access to insert, can realize colorless ONU;

2. energy-conservation modulation system: as an ONU not signal the time, he does not send any light signal, does not need the direct current optical biasing.

3. cost is low: the present invention uses direct modulated laser, has cost advantage

Transmission rate high, be easy to realize: can improve transmission rate and reduce character rate and make the receiver of OLT end realize easily with the method that improves order of modulation.

Claims (4)

1. based on the asymmetric passive light network transmission system that blocks light positive friendship frequency division multiplexing, it is characterized in that:

(1) modulation format that uses is that the asymmetric light positive that blocks is handed over frequency division multiplexing;

(2) be applied in the uplink of EPON.

2. the asymmetric light positive that blocks according to claim 1 is handed over frequency-division multiplex singal, it is characterized in that, can use the direct modulated laser that is biased in threshold point to realize blocking of pair of orthogonal frequency-division multiplex singal.

3. the asymmetric light positive that blocks in the uplink that is applied in EPON according to claim 1 is handed over frequency division multiplexing, it is characterized in that, can realize that time-division multiplexing multiple access inserts.

4. time-division multiplexing multiple access according to claim 3 inserts; It is characterized in that; Each light subscriber unit sends one or more asymmetric Frames that light positive hands over frequency division multiplexing symbol to form that block at specific time slot in the EPON, and the moment of Frame arrival optical line terminal is not overlapping.

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