FR2927207A1 - OPTICAL MIMO SYSTEM - Google Patents

Abstract

The present invention relates to a method of transmitting information symbols belonging to an OOK 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 on-off 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.

Description

OPTICAL MIMO SYSTEM

TECHNICAL FIELD 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, also called FSO (Free-Space Optical) using emission intensity modulation and direct detection are currently used for very short-distance transmissions, as in the IrDA (Infrared Data Association) systems. They are also envisaged to replace the local loop, ie to provide subscriber coverage (last 20 mile problem), particularly in densely populated urban environments, when fiber optic 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 so-called atmospheric scintillation phenomenon, that is to say index fluctuations due to atmospheric turbulence creating variations in luminous intensity at light. reception. To make optical communications more resistant to scintillation phenomena, it has been proposed in the article by M.K. Simon et al. titled 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 spatio-temporal 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 above-mentioned article applies to an optical system equipped with two photoemitters and encodes a block of two information symbols belonging to an O0K (On-Off Keying) or 2-PPM modulation alphabet ( Pulse Position Modulation). It is recalled that the O0K modulation alphabet consists of two symbols 0.1 corresponding to an on / off modulation. The 2-PPM modulation alphabet consists of two symbols respectively corresponding to a first and a second time positions. The proposed spatio-temporal code can be written in the following form: (s1 s2 \ s2 si

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; s1, s2 are two information symbols to be transmitted and s ~, s2 are their respective complementary symbols, defined by: for symbols O0K, if s; = 0 (Off), = 1 (On) and vice versa, and for 2-PPM symbols, if s; corresponds to first (respectively second) position PPM, corresponds to second (respectively first) position PPM. C = (1) 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 four-emitter system 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, Vol. 11, No. 5, May 2007, pages 390-392. In the system described, a block of four information symbols s1, s2, s3, s4 belonging to a modulation alphabet O0K or 2-PPM, is encoded by the following space-time code:

/ S1 S2 S3 S4 c = S2 S1 S4 S3 (2) S3 where S4 S1 S2 S4 S3 S2 S1 where, as before, the lines represent the times of use, the columns the photoemitters and is the complementary symbol of s ;. 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 is correlatively translated to the reception by respective decoding delays of 2 and 4 time-symbol. 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.

DISCLOSURE OF THE INVENTION The present invention is defined by a method of transmitting information symbols belonging to a modulation alphabet 00K or M-PPM with M> 2 by an optical MIM0 system comprising a plurality P of photoemitters, according to which, for each information symbol 00K or M-PPM to be transmitted during a symbol time, a signal is modulated by means of said information symbol, respectively by an on-off modulation and an M-PPM modulation, the signal modulated by a same information symbol being provided during the same symbol time to P photoemitters to be transmitted in 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 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. 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 a modulation alphabet M-PPM, 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 a modulation alphabet M-PPM, and the receiver comprises a plurality of photodetectors, each photodetector being followed by a plurality M 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 the M modulation positions being compared 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 a modulation alphabet 00K, and said photodetector is followed by an integrator adapted to supply an energy value of the signal received 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 00K, 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 characteristics and advantages of the invention will appear on reading a preferred embodiment of the invention with reference to the appended figures in which: FIG. 1 schematically represents an optical MIMO system according to a first embodiment of the invention; Fig. 2 schematically shows an optical MIMO system 5 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 DESCRIPTION OF PARTICULAR EMBODIMENTS An optical MIMO system comprising a plurality of light emitters and at least one photodetector is again considered. More specifically, it will be assumed in the following that the optical MIMO system comprises P> 2 photoemitters, for example laser diodes or LEDs, and Q> 1 photodetectors, for example photodiodes. The channel between a photoemitter p = 1,..., P and a photodetector q = 1,..., Q can be modeled by an attenuation coefficient, it is a 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 00K or M-PPM with M> 1, each of the P photoemitters simultaneously transmits a signal modulated this symbol. We will first consider the case of an M-PPM modulation.

If we denote by the symbol M -PPM transmitted simultaneously by the P photoemitters, each photoreceptor q receives the optical power:

P rq = Ehgps + nq (3) p = 1 where s = (s1, s2, ..., sM) 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; nq = (ng1, ..., ngM) is a random vector whose components are centric iid Gaussian random variables representing the reception noise on the photoreceptor q for the different positions PPM and rq = (1 1,1 2, ..., 1gM) 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 hgp are random variables iid with values in [0,1] and whose probability density follows an exponential law, that is to say: P (h) = hexp ùh (4) where h is the value average 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 E between the modulation positions. On reception, the detection of the symbol is performed in an incoherent manner, that is to say without prior knowledge of the attenuation coefficients he. More precisely, the symbol s is detected as the vector s = (s (mi), m = 1,..., M, where b is the Dirac symbol and the PPM position of the s symbol is estimated in the maximum likelihood sense. (ML) is obtained by:

(Q = arg max L rqm (5) m = 1, ..., M \ q = 1 / In practice the values rqm represent energies, the optical power received by a photodetector being integrated over the symbol duration. (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 comprises only one photodetector, the operation of This detection, which is particularly robust, can be carried out at each symbol time, without any processing delay, and it applies whatever the values of M> 2, P> 2 and Q> 1 and thus allows a 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 transmitting device 101 and a receiving device 102. The transmitting device comprises a PPM modulator 110 common to the P photoemitters 120 and receiving the information symbols belonging to the alphabet M-PPM. For each PPM symbol s = (s (mμmP) m = 1,..., M, the modulator 110 modulates an elementary signal in position, for example a pulse of waveform cp (t): s (t) = ( p (t- (6) where E is the time difference separating 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 into the integrators 140 for each of the M modulation positions, after possible filtering adapted to the waveform (not shown), to provide the components Yqm, m = 1, .. The components Yqm are then summed by the M summers 150 to provide the decision variables Ergm, m = 1, .., M The q = 1 M decision variables are compared in the comparator 160 for provide the position mp according to (5) and therefore the estimated symbol Î.It should be noted that if the system comprises a single photod the summators 150 are omitted.

We will now consider the case of an O0K modulation. If we denote a symbol O0K with s = 0 (Off) or 1 (On), the signal received by a photodetector q can be written as:

PY q = E hgys + nq (7) p = 1 where the coefficients lie have the same meaning as before, nq is a centered Gaussian random variable representing the noise, Yq represents the energy received by the photodetector, by integration of the power optical received on a symbol time. The detection of the symbols O0K is coherent, that is to say that it supposes the prior knowledge of the attenuation coefficients hqy, for example by virtue of the transmission of a sequence of pilot symbols. To do this, the symbol s = 1 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.

The maximum likelihood detection can be performed by comparing the sum of the energies Q with a predetermined threshold T q = 1 QQ rq <T = O and rqT = 1 (8) q = 1 q = 1 where the threshold T is determined , according to the coefficients hqp: (QP hqp AND = 1 q = 1 p = 1 2 (9) where E is the average energy of the emitted symbols.

Fig. 2 schematically shows an optical MIMO system 15 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 20 provides an elementary signal, for example a pulse of waveform cf (t), in particular a rectangular pulse, to all the photoemitters and, conversely, when s = 0, no signal n is transmitted, either simply: 25 s (t) = s.cp (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 the Q photodetectors 230. The signal received by a photodetector q is temporally integrated into an integrator 240, after optional matched filtering (not shown) to the pulse waveform, over a symbol duration to provide the value Yq. 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: (\ i ~ QP kn LL qp 0 = 119 = 1 where the Q (.) Function is the tail Gaussian (Gaussian Q function), k is a modulation-dependent constant used and the signal-to-noise ratio (SNR) In contrast, the bit error probability of optical MIMO systems of the state of technology cited in FIG. the introductory part is given by: Pe = Q 15 Peold = QQP - \ 2 hl hqp q = 1 p = 1 (12) the Schwartz inequality Since hqp> _ 0, p = 1, .., P, q = 1, .., Q, we have, according to (QP (QP LLhgp LLhe and so 0 = 119 = 1, \ q = 1 p = 1, Pe <polo, in other words the optical MIMO systems according to the invention a lower BER than those known from the state of the art, at the same SNR level.

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 (1xl). Curves 320 and 330 correspond to a MIMO system with two photoemitters and a photodetector (2x1 system), using 2-PPM information symbols, 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 (9)

A method of transmitting information symbols belonging to a modulation alphabet 00K or M -PPM with M> 2 by an optical MIM0 system comprising a plurality P of light emitters, characterized in that for each information symbol 00K ( s) or M -PPM (s) to be transmitted during a symbol time, a signal ((p (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 00K or M -PPM with M> 2, the modulator being adapted to provide, during the same symbol time, the signal 25 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 light emitters 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 a modulation alphabet M-PPM, and that said photodetector (130) is followed by a plurality M of integrators (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 of highest energy. corresponding to the value A MIMO system according to claim 5, characterized in that the information symbols belong to a modulation alphabet M-PPM, and the receiver (102) comprises a plurality of photodetectors (140), each photodetector being followed by a plurality M 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 a summator (150), and the energy values thus summed for the M modulation positions being compared in a comparator (160) to provide the modulation position (mÊ) corresponding to the highest summed energy value. 8. MIMO system according to claim 5, characterized in that the information symbols belong to a modulation alphabet O0K, 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 O0K modulation alphabet, and the receiver (202) comprises a plurality of photodetectors (230), each photodetector being followed by an integrator ( 240) adapted to provide an energy value of the received signal over a symbol time, said energy values being summed by an adder (250) and the summed value being compared in a comparator (260) to a predetermined threshold to provide a estimation of the information symbol received during the symbol time.