Patent Publication Number: US-2003231887-A1

Title: Method and apparatus for receiving/transmitting an optical signal through the air with a variable fading margin

Description:
[0001] The present invention relates to the field of wireless transmissions and in particular to the field of optical wireless transmissions. Still more in particular, the present invention relates to a method and apparatus for receiving/transmitting an optical signal through the air with a variable fading margin.  
       [0002] It is widely recognized that the wireless optical telecommunication systems could become an extremely advantageous alternative to both standard optical fiber and microwave radio telecommunication systems mainly for economic and practical reasons but also for the high capacity in stable weather conditions. Very often, in fact, lying optical fibers results in being time consuming, highly expensive or even impossible (just consider, for instance, historic places in old cities and towns). Free space optics systems are also convenient for overcoming emergency situations. A further advantageous use of wireless optical systems is for mobile telecommunication providers that are highly in trouble when they have to find right locations for their infrastructures (the so called problem of site availability).  
       [0003] A typical optical wireless telecommunication system comprises a transmitting apparatus and a receiving one. The transmitting apparatus is based on one or more laser sources whose beam/s, after being duly modulated and filtered by a proper optical system, is directed towards a corresponding remote receiver.  
       [0004] The receiving side comprises one or more apertures focusing the light of the laser beam in a photo-detector or in a multimode fiber linked to the detector.  
       [0005] In other words, the known wireless optical systems use air as transmitting medium (differently from optical fiber systems where optical fibers are used). The behaviour of an optical fiber is generally highly predictable while the behaviour of air as transmission medium is not. In fact, as it is known, air is characterized by high changes according to altitude, latitude, season characteristics, . . . It results in that the properties of an optical communication channel also depend on meteorological parameters, namely reduced visibility, rainfalls, winds, temperature, as well as system parameters, for instance wavelength, divergence of the laser beam, receiver aperture, distance between receiver and transmitter, etc.  
       [0006] The main natural agents influencing the laser propagation into the troposphere are two, namely:  
       [0007] time and space changes of the refractive index n;  
       [0008] visibility reductions (resulting in a strong attenuation) due to fog, mist, snow and rainfalls.  
       [0009] The time and space variations of refractive index cause misalignments (due to variations of radius of curvature of the electromagnetic beam) and fast fluctuations of the received signal (due to the combination of field contributions received with random phase). Furthermore, the visibility reductions cause a long and very strong decreasing.  
       [0010] In any case, such effects reduce the optical power that is received at the receiving side for time periods ranging from a few milliseconds to some hours in the worst case.  
       [0011] The known method for overcoming such a reduction of the received signal is to design the whole optical system in such a way that it has a proper power/fading margin. If P TX  is the optical power (in dBm) at the focus of the transmitting optics, L OPT  are the power losses (in dB) caused by the transmitting and receiving optics to the detector, namely the optical-electrical transducer, L GEOM  are the power losses (in dB) deriving from the ratio between the actual receiving area and the beam section at the receiver, and S RX  the sensitivity (in dBm) at the receiver, namely the signal threshold under which the signal is not detected by the transducer, the power/fading margin FM (in dB) for a given distance, which is available for the attenuations caused by atmospherical events, is calculated as follows: 
         FM=P   TX   −L   OPT   −L   GEOM   −S   RX   (1) 
       [0012] For a given detector technology (including the receiving area) and for a given Bit Error Rate, sensitivity depends on the transmission bandwidth. In principle, a low sensitivity is appreciated but in this case bandwidth is low. On the contrary, because in principle the object of any transmission system is to have a large bandwidth, the sensitivity will be high (this means that a signal just below a certain high value will not be detected).  
       [0013] As because of certain atmospheric phenomena, the signal level can easily decrease and become close to the threshold, resulting in that data could be lost and the optical link interrupted.  
       [0014] In view of the above, the outstanding problem to solve is to have more fading margin available in an optical wireless system in order to counteract atmospherical phenomena causing the fading of the received power during communication over air by means of laser carriers.  
       [0015] In addition to the above solution consisting in oversizing the optical link in terms of available fading margin, this being expensive and unpractical (as during most of the time the oversized fading margin is not useful), it is known to provide a telecommunication system comprising a laser-on-air system and a standard microwave wireless system. In such an arrangement, when the laser system fails due to meteorological phenomena or other reasons, the high capacity microwave wireless link is operated. This solution works efficiently but has the disadvantage to be highly expensive as the additional radio link should be established (possibly the radio licenses should be reserved). Furthermore, for regulated radio frequences, bandwidth is drammatically reduced.  
       [0016] The main object of the present invention is to overcome the above problem. This and further objects are obtained by a method and apparatus according to the present invention having the features indicated in independent claims 1 and 8, respectively. Further advantageous characteristics are indicated in the respective dependent claims.  
       [0017] The basic idea of the present invention is to provide a selective method for increasing the fading margin only when it is necessary by slowing down the transmitting and the receiving bit rate, i.e. the laser signal bandwidth. In other words, when the signal level becomes low, the laser signal bandwidth is reduced and the fading margin is correspondingly increased. 
     
    
    
     [0018] The invention will become clear in view of the following detailed description, given by way of example only, to be read making reference to the attached figures, wherein:  
     [0019]FIG. 1 shows a Free Space Optics system;  
     [0020]FIG. 2 is a schematic representation of a first embodiment of free space optics transceiver according to the present invention; and  
     [0021]FIG. 3 is a schematic representation of a second embodiment of free space optics transceiver according to the present invention. 
    
    
     [0022] Reference should be made to equation (1) set forth above. For a certain geographic area, the statistics of attenuative events is available. Thus, having fixed the parameters of equation (1), the annual optical link unavailability percentage P% can be calculated. It is clearly desiderable to have such an unavailability percentage as low as possible.  
     [0023] A possible solution to the problem of reducing said percentage P% is to decrease the nominal value of the sensitivity S RX  at the receiver by acting on the signal bandwidth of the wave incident on the detector. It could be demonstrated that the sensitivity S RX  is proportional to such a bandwidth.  
     [0024] Sensitivity of an optical transmitter could be defined as the minimum value of optical power (P 0 ) providing a certain signal-to-noise ratio (S/N) determijned by a particular application. In case of transmission through OOK (On-Off Key) modulation and based on the use of an APD (Avalanche Photodiode) receiver, we could say that 
       P   0   =hf   0   R   b (18 F+ 6(2 TKT   L   /R   L ) 1/2   /M   q ) 
     [0025] where:  
     [0026] f 0 : signal frequency  
     [0027] R b : Bit-rate  
     [0028] F: noise digit at the receiver  
     [0029] K: Boltzmann constant  
     [0030] T L : temperature of the load resistor  
     [0031] R L : load resistor  
     [0032] M q : average gain  
     [0033] It is thus realized that sensitivity is proportional to signal bit rate.  
     [0034] Just making some practical examples,  
     [0035] sensitivity for APD at 2,5 Gbit/s: −34 dBm BER 10 −9    
     [0036] sensitivity for APD at 622 Mbit/s: −42 dBm BER 10 −9    
     [0037] sensitivity for APD at 155 Mbit/s: −48 dBm BER 10 −9    
     [0038] In view of the above consideration, the basic idea of the present invention is to provide a dynamic transmitting/receiving system operating according to both the field and meteorological information that is received and able to correspondingly reduce the bandwidth of the signal that is transmitted and received on the same wavelength. This is done by a proper algorithm.  
     [0039] Necessarily, some traffic should be lost and for this reason a decision should be taken in advance about traffic priority. The algorithm takes into account such a priority and, in case the bandwidth should be reduced, only the most privileged information will be transmitted. Low priority traffic will be lost. In any case, the bandwidth reduction results in an increased power margin PM.  
     [0040] For instance, in case of a transmission with a regular bit-rate of 2,5 Gbit/s, such a bit-rate could be reduced up to 155 Mbit/s profitably results in a power margin PM that is increased by about 14 dB.  
     [0041] Again, the advantage consists in having a non-interrupted link even if the amount of the transmitted information is reduced, in principle up to the system capacity limit. Clearly, the solution according to the present invention provides a further advantage over the prior art arrangements as an exceeding power margin is not allocated for most of the time, thus reducing the design costs.  
     [0042] A further advantage of the present invention is that the complexity of the system is unchanged, differently from backed-up free space optics systems (for instance the above mentioned FSO systems with radio back-up).  
     [0043]FIG. 1 shows a Free Space Optics (FSO) system. It simply comprises two FSO apparatus, namely a first FSO transceiver and a second FSO transceiver placed at a certain distance one to each other. Each FSO transceiver comprises a Tx side (with a laser transmitter) and an Rx side (with a laser receiver). The laser transmitter transmits laser signals through the air, the laser signals being received at the laser receiver of the corresponding FSO transceiver.  
     [0044] In FIG. 2 a first embodiment of the present invention is shown. Clearly, FIG. 2 and FIG. 3 show a FSO transceiver very schematically with the purpose of illustrating the main features of the present invention. Details not directly related to the present invention will be not shown.  
     [0045] The receiving side RX of the embodiment of FIG. 2 comprises an optical interface OI for receiving optical signals ROS transmitted through the air, a laser detector LD for detecting and focusing the laser signal ROS, an electrical interface EI for converting the signal into electrical, a transimpedance amplifier TA for transducing a current into a signal voltage, a RX switch which is fed by the output of the transimpedence amplifier TA, a number of output filters FLT 1 , FLT 2  for producing signals at different bit-rates (Bit-Rate  1 , Bit-Rate  2 ). The signals output from the filters FLT are sent to demodulators DEMOD (not shown) and will be managed as usual.  
     [0046] The transmitting side of the embodiment of FIG. 2 comprises a laser transmitter LT which is fed by a signal at a certain bit-rate of a number of bit-rates, the bit-rate by which the laser transmitter is fed being selected by a TX Switch.  
     [0047] Both the RX and TX switches are driven through a switching algorithm block SAB. In turn, the switching algorithm mainly operates according to the power level PL of the optical signal that is received and to the information from auto-tracking systems. Power level information PL are derived by the transimpedance amplifier TA and fed to the switching algorithm block SAB for performing a power monitoring. Advantageously, the switching algorithm block receives further information INFO. For instance, such further information INFO relates to link visibility and meteorological conditions possibly decreasing the power level of the signal at the receiving side.  
     [0048] In normal conditions of good visibility and acceptable weather conditions, the transmission bit-rate is, for instance, Bit-Rate  1 . If the detected power level becomes lower for any cause (generally not for mis-alignment problems) or the meteorological conditions become bad, the bit-rate is changed to a lower bit-rate, for instance Bit-Rate  2 . Preferably, the same bit-rate switching command SC from the switching algorithm block is sent to the RX Switch and to the TX Switch at the very same time because it is assumed that the transmission medium (air) has mutual characteristics in transmission and in reception. This switch operation results in a transmission at a lower bit-rate and in the receiver that is arranged for receiving at such a lower bit-rate.  
     [0049] Naturally, as soon as better weather conditions and/or higher power levels are detected, the bit-rate will be changed back to Bit-Rate  1 . It is thus realized that the present invention could be termed as a “co-lambda back-up technique” as the bit-rate switch is operated at the same wavelength.  
     [0050] When the transmission bit-rate is switched to a lower bit-rate (for instance Bit-Rate  2 ), the output from the transimpedence amplifier TA is sent to the second corresponding filter FLT 2  and to common demodulation blocks DEMOD (not shown). Analogously, the laser transmitter LT will receive the signal to be air-transmitted at a lower bit-rate (for instance Bit-Rate  2 ) from the TX Switch, driven by the switching command SC from the switching algorithm block SAB.  
     [0051] The above switching command SC is also provided out of the transceiver for selecting the information to be transmitted in case of lower bit-rate. In other words, as mentioned above, in case the selected transmission bit rate is lower than the one used in normal conditions, some information could not be transmitted. For this reason, it is requested to decide in advance a certain priority of the information to be transmitted. For instance, there will be high priority information to be transmitted also at the lowest bit-rate; medium priority information to be transmitted at an intermediate bit-rate (together with the high priority information); low priority information to be transmitted at the higher bit-rate (together with both the high and medium priority information). In the example of FIG. 2, as there are provided only two bit-rates, the traffic will be organized in only two priorities: low priority traffic will be sent only when the first bit-rate is selected.  
     [0052]FIG. 3 shows a further embodiment of the transceiver TX/RX according to the present invention. The main difference is in the transmission side and lies in using an optical switch ORX-Switch and a number of different receiving lines that are optimized for a corresponding bit-rate. Thus, the receiving side RX of the embodiment shown in FIG. 1 comprises: an optical interface OI, an optical switch ORX-Switch, a number n of laser detectors LD, a number n of electrical interfaces EI, a number n of transimpedence amplifiers TA and a number n of filters FLT. Conventionally, the output from filters FLT is sent to demodulation blocks (not shown). As in the example of FIG. 2, only two bit-rates are provided and thus n is equal to 2 in the example of FIG. 3.  
     [0053] The transmission side is the same as the one of FIG. 2. In this further embodiment, the switching algorithm block is fed by the same information as in the first embodiment (power level PL of the received signal from any of the transimpedence amplifiers, visibility and meteorological information INFO). The main difference is that it will drive the optical switch ORX-Switch by a switching command SC. Also in this case, the switching command SC is also provided out of the transceiver for selecting the information to be transmitted in case of lower bit-rate.  
     [0054] Herebelow a possible switch criteria will be described in detail. If RL RX  is the received level (in dBm) and TE RX  is the threshold errors in reception (in dBm), EWT, namely the early warning threshold (in dBm), will be EWT=TE RX +EWM where EWM, the early warning margin, could be 3 dB, for instance. If RL is the received level in dBm, the possible switch criteria could be: “issuing a switching command and perform a bit-rate switch when RL&lt;EW for a time longer than a certain period”. The persistence period of a certain situation before operating the switch is provided in order to avoid useless and frequent changes. An acceptable exemplifying period could be 3 minutes. The period is calculated by a counter that is started as soon as it is detected that RL&lt;EW: the start time is set at present time (the time when it is detected the RL&lt;EW condition). If present time−start time is equal to x, then switch is true (the bit-rate is switched), otherwise switch is false (the bit-rate is not switched).  
     [0055] There have thus been shown and described a novel method and a novel free space optics apparatus which fulfills all the objects and advantages sought therefor. Many changes, modifications, variations and other uses and applications of the subject invention will, however, become apparent to those skilled in the art after considering the specification and the accompanying drawings which disclose preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention which is limited only by the claims which follow.