CN102820923B - Transmitting light path selecting method for combined diversity reception in MIMO (Multiple-input and Multiple-Output)free space optical communication - Google Patents

Abstract

The invention discloses a transmitting light path selecting method for combined diversity reception in MIMO (Multiple-input and Multiple-Output)free space optical communication, and belongs to the technical field of MIMO free space optical communication. The transmitting light path selecting method comprises the following steps of: carrying out comparison on the light intensity gain after the gains from each laser to Nr detectors and the like are combined to obtain a serial number of the laser with maximum light intensity gain value, and then feedbacking the serial number of the laser to a light path selector at a transmitting end by a low-speed feedback channel so as to enable the transmitting end to select and activate the optimal laser for transmitting light signals. The method disclosed by the invention has the advantages that the system performances of the MIMO free space optical communication modulated on basis of OOK (On-Off Keying) are obviously improved, and compared with maximal-ratio combining reception, the complexity for realizing the method of united equal-gain combining reception, adopted by the invention, on hardware can be reduced.

Description

The transmitting optical path system of selection of associating diversity reception in MIMO free space optical communication

Technical field

The present invention relates to the transmitting optical path system of selection of associating diversity reception in a kind of MIMO free space optical communication, belong to technical field of MIMO free space optical communication.

Background technology

As everyone knows, free space optical communication due to low cost, good confidentiality, easy for installation, need not the advantage such as frequency license, in the radio communication of closely two-forty, get more and more people's extensive concerning.Especially the communication solving " last one kilometer " is used to.But, free space optical communication can be subject to the impact of rain, mist, atmospheric turbulance, there is scintillation effect in the light intensity of Received signal strength, signal strength signal intensity random fluctuation is caused to change, particularly in strong turbulence situation, light signal can be subject to severe jamming, causes the larger error rate or short time interrupted communication link.The thought (such as transmit diversity techniques has orthogonal space time-code, duplication code, and reception diversity technology has maximum-ratio combing etc.) of space diversity is introduced in free space optical communication the signal fadeout that effectively can resist atmospheric turbulance and cause, improve the reliability of communication.

Emitting antenna selecting technology is at multiple-input and multiple-output (MIMO, multiple input-multiple-output) extensively study in system, it decreases the complexity of radio frequency link on the basis maintaining the effective spatial degrees of freedom of multiple antennas, and is reduced the error rate of system by the channel condition information that transmitting terminal obtains.Because free space optical channel can regard quasistatic flat fading channel as, it is feasible that transmitting terminal obtains channel condition information by feedback channel, therefore, in Shi Yiyue, 2009 Bi Chazikasidiluowa hereby Kui (Beatriz Castillo-Vazquez) proposes transmitting terminal at " electronics bulletin " (Electronics Letters) and utilizes channel condition information to select the emission diversity method of light beam transmission path (being called for short " optical path "), and research shows that its performance is better than traditional orthogonal space time-code and duplication code transmit diversity techniques under strong turbulence condition.

But, the method of Bi Chazikasidiluowa hereby Kui is mainly in Nt laser of transmitting terminal, utilize channel condition information to select best optical path to carry out utilizing emitted light signal, receiving terminal is only furnished with a detector, does not make full use of the reception diversity technology in space diversity.

Summary of the invention

For the defect of prior art, the invention provides the transmitting optical path system of selection of associating diversity reception in a kind of MIMO free space optical communication.This method is the transmitting optical path system of selection of combining equal gain diversity reception in a kind of strong turbulence MIMO free space optical communication.

Method of the present invention is as follows:

To Nt × Nr MIMO free-space optical communication system, wherein Nt is number of lasers, Nr is photodetector number, when supposing without alignment error mistake, adopt the on-off keying OOK modulation system of intensity modulated/direct-detection, here the OOK signal format considered is if Bi Chazikasidiluowa is hereby described in Kui Wenzhang: have Gaussian pulse shape and low duty ratio, and its average transmit power is the Euclidean distance in P, OOK planisphere between two equiprobability signaling points wherein T _bbit period, as employing kT _bduring the Gaussian pulse in cycle

1≤i≤Nt, 1≤j≤Nr, i-th laser is to the light intensity gain I of a jth detector _ijprobability density obey complex exponential distribution, namely

$f_{I_{\mathrm{ij}}}\left(I_{\mathrm{ij}}\right)=e^{-I_{\mathrm{ij}}},I_{\mathrm{ij}}>0---\left(1\right)$

For MIMO free space optical communication environment, I _ijbe a slow fading process and meet independent same distribution between each component;

At Nt × Nr MIMO free-space optical communication system, the mathematic(al) representation of the transmitting optical path system of selection that the associating equal gain diversity that the present invention proposes receives is as follows:

$I_{\max }=\underset{1\≤i\≤N_t}{\max }\{B_i=\underset{j=1}{\overset{N_r}{\mathrm{\Σ}}}{I}_{\mathrm{ij}}\}$ $K=\underset{1\≤i\≤N_t}{\arg \max }\{B_i=\underset{j=1}{\overset{N_r}{\mathrm{\Σ}}}{I}_{\mathrm{ij}}\}---\left(2\right)$

Wherein, B _irepresenting i-th laser to the light intensity gain of Nr detector after equal gain combining, therefore combining transmitting optical path system of selection that equal gain diversity receives namely by comparing B ₁~ B _ntlight intensity gain after this Nt equal gain combining, obtains maximum I _max, then by feedback channel, the sequence number K of laser corresponding to this maximum is passed to transmitting terminal, thus transmitting terminal is selected and the laser activating this optimum carrys out utilizing emitted light signal.

The mathematic(al) representation of the Bi Chazikasidiluowa hereby method of Kui is

$I_m=\underset{1\≤i\≤N_t,j=0}{\max }{I}_{\mathrm{ij}}---\left(3\right)$

By (2) formula compared with (3) formula, known I _max>I _m, the conditional bit error rates P under above-mentioned two kinds of methods _b(e I _max), P _b(e I _m) be

$P_b(e\backslash I_{\max })=Q\left(\sqrt{2\mathrm{\&gamma;}{I_{\max }}^2}\right)<P_b(e\backslash I_m)=Q\left(\sqrt{2\mathrm{\&gamma;}{{\mathrm{\&xi;I}}_m}^2}\right)---\left(4\right)$

Wherein, Q refers to Q function, average signal-to-noise ratio γ=P ²t _b/ N ₀, N ₀the one-sided power spectrum density of white Gaussian noise.

And then, the bit error rate under above-mentioned two kinds of methods for

$P_{b,I_{\max }}={\&Integral;}_0^{\&infin;}{P}_b(e\backslash I_{\max })f\left(I_{\max }\right){\mathrm{dI}}_{\max }<P_{b,I_m}={\&Integral;}_0^{\&infin;}{P}_b(e\backslash I_m)f\left(I_m\right)d{I}_m---\left(5\right)$

Wherein, bit error rate is the integration of the probability density function product of conditional bit error rates and light intensity gain, f (I _max), f (I _m) be I respectively _max, I _mprobability density function.

(5) formula shows, the transmitting optical path system of selection that the associating equal gain diversity that the present invention proposes receives is lower than the method bit error rate of Bi Chazikasidiluowa hereby Kui, namely illustrates and systematic function is significantly improved.

The method that the method that the present invention proposes compares Bi Chazikasidiluowa hereby Kui is significantly improved on system performance, and adopts associating equal gain combining method of reseptance can reduce its complexity than maximum-ratio combing method of reseptance on hardware implementing.

Accompanying drawing explanation

Fig. 1 is the free-space optical communication system block diagram based on method of the present invention.

Wherein, 1, OOK modulator, 2, optical path selector, 3, the 1st lasers, 4, the 2nd lasers, 5, the 1st detectors, 6, the 2nd detectors, 7, a Nr detector, 8, equal gain combining receiver, 9, decision device,

1. be feedback channel.

Fig. 2 is the comparison diagram of method of the present invention and Bi Chazikasidiluowa hereby method bit error rate when adopting that Gaussian pulse OOK modulates, number of lasers is 2 of Kui.

Fig. 3 is software flow pattern of the present invention.

Embodiment

Embodiment:

With transmitting terminal number of lasers for 2, receiving terminal number of detectors is the MIMO free-space optical communication system of Nr=2 or 3 is that example is to introduce the present invention.System block diagram is as shown in Figure 1: input data sequence carries out through OOK modulator gaussian pulse is modulated, and then by optical path selector selected optimum optical path from two lasers, and is launched by the laser at this optimum optical path place.

The acquisition of optimum optical path is as follows:

Receiving terminal known channel state information, has namely known light intensity gain I _ij, suppose that receiving terminal photoelectric conversion efficiency is that 1, i-th laser is to Nr the light intensity gain of (Nr=2 or 3) detector after equal gain combining if B ₁< B ₂, according to (2) formula, the optical path of known 2nd laser is optimum, therefore the sequence number of this laser is fed back to the optical path selector of transmitting terminal by the feedback channel of a low speed, and the 2nd laser is selected and activates to launch thus, otherwise, B ₁>B ₂, selected 1st laser is launched.

The present invention is using bit error rate as weighing the index of systematic function, and the method for the transmitting optical path system of selection that associating equal gain diversity the present invention proposed receives and Bi Chazikasidiluowa hereby Kui compares.As shown in Figure 2, software emulation flow chart as shown in Figure 3 for comparative result.

In Fig. 2, method of the present invention, when receiving terminal number of detectors is 2 or 3, is obviously found out, method of the present invention is lower than the method bit error rate of Bi Chazikasidiluowa hereby Kui.

Claims (1)

1. a transmitting optical path system of selection for associating diversity reception in MIMO free space optical communication, is characterized in that,

To Nt × Nr MIMO free-space optical communication system, wherein Nt is number of lasers, Nr is photodetector number, when supposing without alignment error mistake, adopt the on-off keying OOK modulation system of intensity modulated and/or direct-detection, here the OOK signal format considered is: have Gaussian pulse shape and low duty ratio, and its average transmit power is the Euclidean distance in P, OOK planisphere between two equiprobability signaling points wherein T _bbit period, as employing kT _bduring the Gaussian pulse in cycle $\mathrm{\&xi;}=4/k\sqrt{\mathrm{\&pi;}},0<k\&le;1;$

1≤i≤Nt, 1≤j≤Nr, i-th laser is to the light intensity gain I of a jth detector _ijprobability density obey complex exponential distribution, namely

$f_{I_{\mathrm{ij}}}\left(I_{\mathrm{ij}}\right)=e^{-I_{\mathrm{ij}}},I_{\mathrm{ij}}>0---\left(1\right)$

For MIMO free space optical communication environment, I _ijbe a slow fading process and meet independent same distribution between each component;

At Nt × Nr MIMO free-space optical communication system, the mathematic(al) representation of the transmitting optical path system of selection that associating equal gain diversity receives is as follows:

$\begin{array}{cc}{I}_{\max }=\underset{1\&le;i\&le;N_t}{\max }\{B_i=\underset{j=1}{\overset{N_r}{\mathrm{\&Sigma;}}}{I}_{\mathrm{ij}}\}& K=\underset{1\&le;i\&le;N_t}{\arg \max }\{B_i=\underset{j=1}{\overset{N_r}{\mathrm{\&Sigma;}}}{I}_{\mathrm{ij}}\}\end{array}---\left(2\right)$

Wherein, B _irepresenting i-th laser to the light intensity gain of Nr detector after equal gain combining, therefore combining transmitting optical path system of selection that equal gain diversity receives namely by comparing B ₁~ B _ntlight intensity gain after this Nt equal gain combining, obtains maximum I _max, then by feedback channel, the sequence number K of laser corresponding to this maximum is passed to transmitting terminal, thus transmitting terminal is selected and the laser activating this optimum carrys out utilizing emitted light signal.