WO2009095428A1

WO2009095428A1 - Optical mimo system

Info

Publication number: WO2009095428A1
Application number: PCT/EP2009/050981
Authority: WO
Inventors: Chadi Abou Rjeily
Original assignee: Commissariat A L'energie Atomique
Priority date: 2008-01-31
Filing date: 2009-01-29
Publication date: 2009-08-06
Also published as: FR2927207A1

Abstract

The present invention relates to a method of transmitting information symbols belonging to a 0OK or M-PPM modulation alphabet with M≥ 2 by an optical MIMO system comprising a plurality P of photoemitters, in which, for each information symbol OOK (s) or M-PPM (s) to be transmitted during a symbol time, a signal (φ (t)) is modulated by means of said information symbol, respectively by an all or nothing modulation and an M-PPM modulation, the signal modulated by one and the same information symbol (s(t)) being provided during one and the same symbol time to the P photoemitters so as to be transmitted in free space.

Description

OPTICAL MIMO SYSTEM

DESCRIPTION

TECHNICAL AREA

The present invention generally relates to the field of optical communications in free space and more particularly that of optical Multiple Input Multiple Output (MIMO) systems.

STATE OF THE PRIOR ART

Open-space optical communications, still called FSO (Free-Space Optical) using transmit intensity modulation and direct detection, are currently used for very short-range transmissions, as in IrDA (Infrared Data Association) systems. ). They are also envisaged to replace the local loop, ie to ensure the subscribers' service (last mile problem), especially in densely populated urban environments, when optical fiber links can not be deployed or if their deployment would be prohibitively expensive.

Optical communications in free space, however, suffer from severe limitations, mainly due to a phenomenon called atmospheric scintillation, that is to say, index fluctuations due to atmospheric turbulence creating variations in light intensity at the reception. To make optical communications more resistant to the phenomenon of scintillation, it has been proposed in the article by M. K Simon et al. entitled "Alamouti-type space-time coding for free space optical communication with direct detection" published in IEEE Trans. on wireless comm., vol. 4, No. 1, Jan. 2005, pages 35-39, to use a multi-antenna system with spatiotemporal coding.

Wireless telecommunication systems of the multi-antenna type are well known in the state of the art. These systems use a plurality of transmit antennas and one or more antenna (s) on reception and are called, according to the type of configuration adopted, MIMO (Multiple Input Multiple Output) or MISO (Multiple Input Single Output) . Subsequently we will use the same term MIMO to cover the MIMO and MISO variants mentioned above. In its optical variant, a MIMO system comprises a plurality of light sources, typically light-emitting diodes or laser diodes, and at least one photodetector, typically a photodiode.

In a space-time coded MIMO system and more precisely space-time block coding (STBC), a block of information symbols to be transmitted is encoded into a matrix of transmission symbols. one dimension of the matrix corresponding to the number of antennas and the other corresponding to the number of consecutive instants of transmission. The spatio-temporal coding proposed in the aforementioned article applies to an optical system equipped with two photoemitters and codes a block of two information symbols belonging to a modulation alphabet 0OK (On - Off Keying) or 2 - PPM (Puise Position Modulation). It is recalled that the modulation alphabet 0OK consists of two symbols 0.1 corresponding to an all or nothing modulation (On / Off). The 2-PPM modulation alphabet consists of two symbols respectively corresponding to a first and a second time positions. The proposed space-time code can be written in the following form:

C =: D

where the first and second lines of C correspond respectively to the first and second transmission times also known as PCUs (Per Channel Use); the first and second columns of C correspond respectively to the first and second photoemitters;

S ₁₉ S ₂ are two information symbols to be transmitted and Sj ₉ S ₂ are their respective complementary symbols, defined by: for symbols 0OK, if S ₁ = O (Off), Sj = I (On) and vice versa, and for 2-PPM symbols, if S ₁ corresponds to first (respectively second) position PPM, sj corresponds to second (respectively first) position PPM. Like the Alamouti code from which it is derived, the spatio-temporal code defined by (1) is at maximum diversity.

This spatio-temporal code has been extended to a system with four photoemitters in the article by A. Garcia-

Zambrana entitled "Error rate performance for STBC in free-space optical communications through strong atmospheric turbulence" published in IEEE Com. Letters,

Flight. 11, No. 5, May 2007, pages 390-392. In the system described, a block of four information symbols

S ₁₉ S ₂₉ S ₃₉ S ₄ belonging to an OOK modulation alphabet or

2-PPM, is encoded by the following space-time code:

S ₁ S ₂ S ₃ S ₄ s, C = ² % (2)

S ₄ S ₁ S ₂

S ₄ S ₃ S ₂ S ₁

where, as before, the lines represent the times of use, the columns the photoemitters and sj is the complementary symbol of S ₁ . This code is also at maximum diversity. The optical MIMO systems mentioned above, however, suffer from a number of constraints. First of all, they only work with a very limited number of photoemitters (2 or 4) and a single PPM modulation (2-PPM). In addition, the spatio-temporal codes defined (1) and (2) respectively require 2 and 4 times of use to transmit a block of symbols, which translates correlatively to the reception by respective decoding delays of 2 and 4 symbol times.

The object of the present invention is to provide an optical MIMO system overcoming the disadvantages set out above, in particular to propose an optical MIMO system that can operate with any number of photoemitters and a PPM modulation alphabet having any number of positions. modulation.

STATEMENT OF THE INVENTION

The present invention is defined by a method of transmitting information symbols belonging to an OOK or M-PPM modulation alphabet with Af> 2 by an optical MIMO system comprising a plurality P of photoemitters, according to which, for each symbol of 0OK or Af-PPM information to be transmitted during a symbol time, a signal is modulated by means of said information symbol, respectively by an all-or-nothing modulation and an Af-PPM modulation, the signal modulated by the same information symbol being supplied during the same symbol time to the P photoemitters to be transmitted in the free space.

The present invention also relates to a transmission device for optical MIMO system, comprising a plurality P of photoemitters, further comprising a modulator connected to P photoemitters, adapted to modulate a signal by means of information symbols belonging to a modulation alphabet 00K or Af-PPM with Af> 2, the modulator being adapted to provide, during the same symbol time, the signal modulated by the same information symbol, P photoemitters to be transmitted in free space.

The photoemitters may be laser diodes or light emitting diodes.

The invention also relates to an optical MIMO system for communication in free space, comprising a transmission device as defined above and a reception device comprising at least one photodetector.

According to a first embodiment, the information symbols belong to an Af-PPM modulation alphabet, and said photodetector is followed by a plurality M of integrators, each integrator being adapted to provide for each symbol time, a value of the energy of the signal received by said photodetector for a modulation position, said energy values then being compared with each other in a comparator to provide the modulation position corresponding to the highest energy value.

According to a variant of the first embodiment, the information symbols belong to an Af-PPM modulation alphabet, and the receiver comprises a plurality of photodetectors, each photodetector being followed by a plurality Af of integrators, each integrator being adapted to provide for each symbol time, an energy value of the signal received by said photodetector for a modulation position, the energy values relating to the same modulation position being summed by an adder, and the energy values thus summed for Af positions in a comparator to provide the modulation position corresponding to the highest summed energy value.

According to a second embodiment, the information symbols belong to an OOK modulation alphabet, and said photodetector is followed by an integrator adapted to supply an energy value of the received signal over a symbol time, said energy value being compared to a predetermined threshold in a comparator to provide an estimate of the received information symbol during the symbol time.

According to a variant of the second embodiment, the information symbols belong to a modulation alphabet 0OK, and the receiver comprises a plurality of photodetectors, each photodetector being followed by an integrator adapted to supply an energy value of the signal received on a symbol time, said energy values being summed by an adder and the thus summed value being compared in a comparator to a predetermined threshold to provide an estimate of the received information symbol during the symbol time.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will appear on reading a preferred embodiment of the invention with reference to the attached figures among which: Fig. 1 schematically represents an optical MIMO system according to a first embodiment of the invention;

Fig. 2 schematically represents an optical MIMO system according to a second embodiment of the invention;

Fig. 3 represents the probability of error for two optical MIMO systems according to the invention and for two optical MIMO systems according to the state of the art.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

An optical MIMO system is again considered comprising a plurality of light emitters and at least one photodetector. More precisely, it will be assumed in the following that the optical MIMO system comprises P≥2 photoemitters, for example laser diodes or LEDs, and β≥1 photodetectors, for example photodiodes. The channel between a photoemitter p = \, ..., P and a photodetector q = \, .., Q can be modeled by an attenuation coefficient, h ^ real number and less than 1, reflecting the optical attenuation on the direct line path (LOS) between the photoemitter p and the photodetector q. The idea underlying the present invention is to transmit the same symbol by the different photoemitters, in other words to eliminate the temporal dimension of the space-time code. More precisely for a symbol to be transmitted, belonging to a modulation alphabet 0OK or M-PPM with M> \, each P photoemitters simultaneously transmit a signal modulated this symbol.

We will first consider the case of an M-PPM modulation. If we denote by s the symbol Af-PPM transmitted simultaneously by the P photoemitters, each photoreceptor q receives the optical power:

where s = (s _ι , s ₂ , ..., s _M ) is a vector of dimension M whose components are all zero except for one, equal to 1, corresponding to the modulation position of the symbol ; n _q = yn _ql , ..., n _qM j is a random vector whose components are centered Gaussian iid random variables representing the reception noise on the photoreceptor q for the different positions PPM and r _q = (r _ql , r _q2 , ..., r _qM ) is a vector of dimension M associated with the photodetector q and whose components give the respective powers received for the different PPM modulation positions; and h ^ are random variables iid with values in [0,1] and whose probability density follows an exponential law, that is to say:

where h is the average value of h. It will be assumed in the following that the differences in propagation time between couples of light emitters and photodetector (s) are substantially less than the time difference ε separating the modulation positions.

At reception, the detection of the symbol is performed in an incoherent manner, that is to say without prior knowledge of the attenuation coefficients h. Specifically, the s symbol is detected as

the vector s = (δ (m-m ^)) where δ is the symbol of

Dirac and m _e the PPM position of the symbol s estimated in the sense of maximum likelihood (ML) is obtained by:

)

In practice the values r _qm represent energies, the optical power received by a photodetector being integrated over the symbol duration.

It is understood from (5) that the detection consists in summing the energies received by the different photodetectors for each of the PPM positions and in selecting the position which collects the highest energy. If the system has only one photodetector, the summing operation is of course omitted. This detection, particularly robust, can be performed at each symbol time, without processing time. It also applies regardless of the values of M≥2, and O _P≥2. ≥ 1 and thus allows great flexibility in the realization of the system.

Fig. 1 represents an optical MIMO system according to a first embodiment of the invention.

The system comprises a transmission device 101 and a reception device 102. The transmission device comprises a PPM modulator 110 common to the P photoemitters 120 and receiving the information symbols belonging to the M-PPM alphabet. For each PPM symbol s = (δ (m-m ^), the modulator 110 modulates in position an elementary signal, for example a waveform pulse φ (t):

s (t) = φ (t-mfi) (6)

where ε is the time difference between two consecutive modulation positions.

Each of the photoemitters 120 transmits the signal s (t) in the form of a light wave, which is received by each of the Q photodetectors 130. The signal received by each photodetector q is temporally integrated in the integrators 140 for each of the M positions of modulation, after optional filtering adapted to the waveform (not shown), to provide the components r _qm , m = \, .., M of the vector r _q . The components r _qm are then summed by the M summers 150 for

Q provide the decision variables Σr _qm , m = l, .., M. The M decision variables are compared in the comparator 160 to provide the position m _e according to (5) and therefore the estimated symbol s. Note that if the system comprises a single photodetector, the summers 150 are omitted.

We will now consider the case of an OOK modulation. If we write s a symbol 0OK with s = 0 (Off) or 1 (On), the signal received by a photodetector q can be written:

where the coefficients h ^ have the same meaning as before, n _q is a centered Gaussian random variable representing the noise, r _q represents the energy received by the photodetector, by integrating the received optical power into a symbol time.

The detection of 0OK symbols is coherent, that is to say that it assumes prior knowledge of the attenuation coefficients h ^, for example by virtue of the transmission of a sequence of pilot symbols. To do this, the symbol s = \ can be successively transmitted by each of the photoemitters, the other photoemitters being passive (that is to say s = 0). It will be noted that the transmission of the pilot sequence derogates from the principle of simultaneous transmission of the same symbol by the different photodetectors. Maximum likelihood detection can be performed by comparing the sum of the energies

Q

Vr with a predetermined threshold T:

Q Q

Σr _q <T → s = O and Σr _q ≥ T => s = l (8) q = \ q = \

where the threshold T is determined, as a function of the coefficients h ^:

, 9)

where E is the average energy of the emitted symbols.

Fig. 2 schematically represents an optical MIMO system according to a second embodiment of the invention.

The system includes an On-Off modulator 210, common to P photoemitters 220, and receiving the information symbols s. When s = 1, the modulator supplies an elementary signal, for example a waveform pulse φ (t), in particular a rectangular pulse, to all the photoemitters and, conversely, when s = 0, no signal n ' is transmitted, either simply:

s {t) = s.φ {t) (10) Each of the photoemitters 220 transmits the signal s (t) in the form of a light wave, which is received by each of Q photodetectors 230. The signal received by a photodetector q is temporally integrated into an integrator 240, after any adapted filtering

(not shown) at the pulse waveform, over a symbol duration to provide the value r _q . These values are summed in the summator 250 and the sum obtained is compared with the above-mentioned detection threshold T in the comparator 260. The result of this comparison gives the estimate s of the transmitted symbol.

Note that if the system comprises a single photodetector, the summator 250 is omitted.

It can be shown that the bit error probability (BER) of an optical MIMO system according to the present invention is given by:

where the function (? (.) is the Gaussian tail (Gaussian

Q function), k is a constant depending on the modulation used and η the signal-to-noise ratio (SNR).

In contrast, the bit error probability of optical MIMO systems of the state of technology cited in the introductory part is given by:

Since h ≥ 0, p = \, .., P, q = \, .., Q, we have, according to

the inequality of Schwartz I and so

P _e ≤ P °, in other words the optical MIMO systems according to the invention have a lower BER than those known from the state of the art, at the same level of SNR.

Fig. 3 represents the bit error rate for different optical MIMO systems.

The curve 310 corresponds to the SISO system (Single In Single Out) of reference, with a single photoemitter and a single photodetector (1x1).

Curves 320 and 330 correspond to a MIMO system with two photoemitters and a photodetector

(2x1 system), using information symbols 2-

PPM, respectively with the spatio-temporal coding described in the article by M. K. Simon et al. and with the coding according to the first embodiment of the invention.

Similarly, the curves 340 and 350 correspond to a MIMO system with four photoemitters and a photodetector (4x1 system), using 2-PPM information symbols, respectively with the spatio-temporal coding described in the article by A. Garcia-Zambrana and with the coding according to the first embodiment of the invention. As noted above, it can be seen that the bit error rate obtained with an optical MIMO system according to the invention is actually lower than with a system of the state of the art.

Claims

1. Method of transmitting information symbols belonging to a modulation alphabet

OOK or M-PPM with M≥2 by an optical MIMO system comprising a plurality P of light emitters, characterized in that, for each information symbol 0OK (s) or M-PPM (s) to be transmitted during a symbol time, a signal (φ (t)) is modulated by means of said information symbol, respectively by an all - or - nothing modulation and an M - PPM modulation, the signal modulated by the same information symbol

(s (t)) being supplied during the same symbol time to the P photoemitters to be transmitted in the free space.

2. Transmission device for optical MIMO system, comprising a plurality P of light emitters (120, 220), characterized in that it further comprises a modulator (110, 210) connected to the P light emitters, adapted to modulate a signal by means of symbols of information belonging to a modulation alphabet 0OK or M-PPM with M> 2, the modulator being adapted to provide, during the same symbol time, the signal modulated by the same information symbol, P photoemitters to be transmitted in the 'free space.

Transmitting device according to Claim 2, characterized in that the photoemitters are laser diodes.

4. Transmission device according to claim 2, characterized in that the light emitters are light emitting diodes.

5. optical MIMO system for communication in free space, characterized in that it comprises a transmitting device (101,201) according to one of claims 2 to 4, and a receiving device (102,202) comprising at least one photodetector.

A MIMO system according to claim 5, characterized in that the information symbols belong to an Af-PPM modulation alphabet, and said photodetector (130) is followed by a plurality of integrators M (140), each integrator being adapted to provide for each symbol time an energy value of the signal received by said photodetector for a modulation position, said energy values then being compared with each other in a comparator (160) to provide the modulation position ( rh _f ) corresponding to the highest energy value.

A MIMO system according to claim 5, characterized in that the information symbols belong to an Af-PPM modulation alphabet, and the receiver (102) comprises a plurality of photodetectors (140), each photodetector being followed by a plurality Af of integrators, each integrator being adapted to provide for each symbol time, an energy value of the signal received by said photodetector for a modulation position, the energy values relating to the same modulation position being summed by an adder (150), and the energy values thus summed for the M modulation positions being compared in a comparator (160) to provide the modulation position (^) corresponding to the highest summed energy value.

8. MIMO system according to claim 5, characterized in that the information symbols belong to an OOK modulation alphabet, and that said photodetector (230) is followed by an integrator (240) adapted to provide an energy value of signal received on a symbol time, said energy value being compared to a predetermined threshold in a comparator (260) to provide an estimate of the information symbol (s) received during the symbol time.

9. MIMO system according to claim 5, characterized in that the information symbols belong to an OOK modulation alphabet, and the receiver (202) comprises a plurality of photodetectors (230), each photodetector being followed by an integrator ( 240) adapted to supply an energy value of the received signal over a symbol time, said energy values being summed by an adder (250) and the thus summed value being compared in a comparator (260) to a threshold predetermined to provide an estimate of the received information symbol (s) during the symbol time.