Abstract:
A method that allows an optical wireless communication link between transmitting and receiving stations to be established and used reliably without the need for position sensing capabilities. A small random nudge of the center of the acquisition spiral is used to prevent a link from being established before proper alignment is attained or to correct the alignment of an established link which does not have sufficient alignment to maintain a high bandwidth link.

Description:
RELATED APPLICATIONS 
   This patent application is related to U.S. patent application Ser. No. 09/940,763, entitled Method And Apparatus For Aligning Optical Wireless Link, filed on Aug. 27, 2001 by Heminger et al., claiming benefit of Provisional Application Ser. No. 60/285,461, filed on Apr. 20, 2001; and also U.S. patent application Ser. No. 09/942,265, entitled Method And Apparatus For Re-Acquisition After Link Disruption In An Optical Wireless Link, filed on Aug. 27, 2001 by Oettinger et al., claiming benefit of Provisional Application Ser. No. 60/285,460, filed on Apr. 20, 2001. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to wireless communications systems, and more particularly, to a method of controlling alignment of an optical wireless communication link between an optical transmitting station (transceiver) and an optical receiving station (transceiver) which are not capable of sensing positional information of the remote station&#39;s beam relative to the local station&#39;s detector. 
   2. Description of the Prior Art 
   An optical wireless link system comprises at least two stations, each of which contains an optical transmitter (or transceiver) and an optical receiver (or transceiver). The transmitter is able to change the direction of its transmitted beam by known amounts of angular displacement. The receiver generally sees this motion and sends position correction information back to the transmitter. 
   In some instances however, the receiver is not equipped with any position sensing capability to detect the positioning accuracy of the incident beam relative to the receiving station detector. In such systems, active tracking between stations is not typically employed; rather, the stations will switch to a “hold” mode, where the direction of the beam is held constant once a link is established. In order to establish the link (a procedure termed “acquisition”), such architectures must rely solely on binary feedback in which a sample was received or it was not received (either the beam is aligned with the receiving station data detector or it is not so aligned). This is problematic since some feedback may be detected by the receiving station as soon as the beam is close to the receiving station&#39;s data detector (i.e. locking onto the “halo” or edge of the laser). Ideally, the beam should be centered on the detector since the center is brighter (higher SNR), and the system will be more immune to slight vibrations. Once the foregoing feedback is detected, some method must be used to determine if further adjustment is necessary, or if the alignment is sufficient for data communication. In one method, the acquisition spiral may be continued until the alignment is good enough to meet some predefined criteria (e.g. 100 samples in a row). If the alignment is slightly off center, however, it will likely be incapable of transferring data at a high rate due to insufficient alignment. The poor data rate may trigger the need for a “re-acquisition”. Such techniques may then switch back and forth between the acquisition and hold modes, without ever establishing a good data link. 
   In view of the foregoing, it would be desirable and advantageous in the optical wireless communication art to provide a technique that allows an optical wireless communication link to be used reliably without the need for position sensing capabilities. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a method of controlling alignment of an optical wireless communication link between two optical transceivers. The method allows an optical wireless communication link between the stations to be used reliably without the need for position sensing capabilities simply by modifying acquisition and re-acquisition algorithms such as those defined in U.S. patent application Ser. No. 09/940,763 and U.S. patent application Ser. No. 09/942,265 referenced herein before. The &#39;763 and &#39;265 patent applications are assigned to Texas Instruments Incorporated, the assignee of the present application, and are incorporated by reference in their entirety herein. Randomly modifying the center of the acquisition spiral that is used for a scan pattern by a small amount (for example, a fraction of the beam divergence) has the effect of moving away from locations where the alignment is sufficient to feedback some positional data, but not enough to provide a high bandwidth link. “Sensorless acquisition” has been successfully demonstrated by the present inventor at distances ranging from 1 to 50 meters using this random nudge technique. This random nudge technique can be used at different times during the wireless communication process. 
   In one aspect of the invention, a method of controlling alignment of an optical wireless communication link between an optical transmitting station and an optical receiving station is implemented by randomly adjusting the center of the spiral used for a scan pattern during the final signal acquisition cycle. Using this aspect, the initial alignment between the stations can be performed at a much faster rate. For example, by applying the nudge if more than a couple samples in a row are missed, the algorithm will not dwell in locations which have poor error rates. 
   In another aspect of the invention, a method of controlling alignment of an optical wireless communication link between an optical transmitting station and an optical receiving station is implemented by randomly adjusting the starting point of the center of the spiral used for a scan pattern during a signal re-acquisition cycle. Using this aspect, will prevent the stations from the endless cycle between “hold” and “acquisition”. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects, features and advantages of the present invention will be readily appreciated, as the invention becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing figures wherein: 
       FIG. 1  is a flowchart depicting a method of controlling alignment of an optical wireless communication link between an optical transmitting station and an optical receiving station without using position sensing and feedback information by randomly adjusting the center of the acquisition spiral used for a scan pattern “during a signal re-acquisition process” according to one embodiment of the present invention; and 
       FIG. 2  is a flowchart depicting a method of controlling alignment of an optical wireless communication link between an optical transmitting station and an optical receiving station without using position sensing and feedback information by randomly adjusting the center of the acquisition spiral used for a scan pattern “during a signal acquisition process” according to one embodiment of the present invention. 
   

   While the above-identified drawing figures set forth particular embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments are best explained by first considering the case in which the transmitting and receiving units have first completed a coarse acquisition process. During the coarse acquisition process, signals are monitored to determine if any new feedback information is received. This feedback information may be monitored using an “ICUC” (“I see, you see”) acquisition spiral which spirals in and out, transmitting both its local position as it goes, along with the most recent value of the remote that it has seen. The spiral is therefore transmitting what “I see” and the remote is transmitting what “you see”. Upon completion of the coarse acquisition, a determination must then be made as to when the receiving stations should drop into a “hold” mode. Since positioning accuracy cannot be monitored, some other criteria must be used to make this determination. Continued use of the ICUC spiral acquisition method until a predetermined number of consecutive sightings count is reached can be used to program the units into their “hold” modes. 
   The approach described herein above is problematic since the laser can be aligned well enough to get the required “consecutive samples”, but not well enough to reliably transmit high bandwidth data. For example, if the error rate is 1/100, it is likely to perform a lock with little problem; but the data throughput would be poor. 
   In the “hold” mode, the units assume that the lock has been lost after some number of missed samples; and the units attempt to re-acquisition. This re-acquisition would proceed, starting in the last known good location. The foregoing method may easily end up in a cycle where a solid lock with a good data rate is never achieved. 
   The present method of controlling alignment of an optical wireless communication link between an optical transmitting station and an optical receiving station however, does not use the last known good location as a starting point for the scan in order to acquire or re-acquire a “lock”. In stead, a small random nudge of the center of the acquisition spiral used for a scan pattern is added to the last “good” position; and then the scan begins from that point. “Sensorless acquisition” has been successfully demonstrated by the present inventor at distances ranging from 1 to 50 meters using this random nudge technique. This random nudge technique can be used at different times during the wireless communication process. The initial alignment between the transmitting and receiving stations, for example, can be improved at a much faster rate; and the endless cycle between “hold” and “acquisition” modes can be avoided simply by using this technique during a final acquisition at which time a random adjustment of the center of the scan pattern can be made, for example, if more than a couple samples in a row are missed. 
   An alternative approach might be to restart the scan from the center, but this approach will be even slower, especially if the initial alignment is poor. Further, if the receiving station is locking onto the “halo”, it will consistently find the “halo” instead of the beam, since the “halo” precedes the beam in its motion. 
   Looking now at  FIG. 1 , a flow chart shows a method  100  of controlling alignment of an optical wireless communication link between an optical transmitting station and an optical receiving station without using position sensing and feedback information, but instead randomly adjusts the center of the acquisition spiral used for a scan pattern during a signal re-acquisition process according to one embodiment of the present invention. This embodiment is useful immediately following any known course acquisition process such as the “ICUC” acquisition technique discussed herein before. 
   When the transmitting and receiving stations have completed a coarse signal acquisition, for example, some criteria other than that related to positioning accuracy is monitored to determine whether the stations should drop into a “hold” mode, since positioning accuracy information is no longer available. 
   With continued reference to  FIG. 1 , the process can be seen to begin upon detection of a link disruption  102 ; at which time the process starts from the last known good location, with a small radius as shown in block  104 . A small nudge of the starting point is then implemented to remove any lock-up conditions as depicted in block  106 . The small nudge is most preferably implemented right on the border of close enough to lock, but far enough away that it keeps going through re-acquire. The re-acquisition process then continues by commencing the ICUC acquisition spiral at the new center as shown in block  108 . 
     FIG. 2  is a flowchart depicting a method  200  of controlling alignment of an optical wireless communication link between an optical transmitting station and an optical receiving station without using position sensing and feedback information by randomly adjusting the center of the acquisition spiral used for a scan pattern “during a signal acquisition process” according to one embodiment of the present invention. The process can be seen to begin by periodically sampling a signal stream to make a determination of whether there is new ICUC feedback information in the packet as seen in blocks  202  and  204 . If new feedback information is present, the spiral size is collapsed and a determination is made as to whether the position is close to the final position or still away from the final position. If the position is still far away from the final position, the center of the spiral is set to the last seen position and the signal acquisition continues from that point by resetting the radius to spiral from its new center as seen in block  206 . This process continues until a predetermined acquisition criterion is met (i.e. a desired number of contiguous samples is received) as depicted in block  208 , at which time the acquisition is finished, and the link is switched into a “hold” state such shown in block  210 . If after periodic sampling however, a determination is instead made that no new ICUC feedback information has been received, a further determination is made as to whether just a few samples (i.e. 5 samples) have been received without feedback as shown in block  212 . If a determination has been made that such a few samples have been received without any new ICUC feedback information, the center of the ICUC spiral is nudged in a random direction, and the acquisition continues from that point as depicted in block  214 . If a further determination is then made regarding yet a different acquisition criteria, i.e. whether a greater number of samples (i.e. 100) have been received without any new ICUC feedback information using the new center, then the maximum radius of the ICUC acquisition spiral is increased by a predetermined amount, i.e. 10%, and a “No Feedback Counter” is reset to zero as depicted in blocks  216  and  218 . Subsequent to increasing the maximum radius and resetting the “No Feedback Counter”, the acquisition continues and transmitted ICUC data is intended for the remote station as shown in block  220 . A final determination is then made as to whether a predetermined acquisition criterion is met as shown in block  208 . If the predetermined acquisition criteria are in fact met, then the acquisition process is finished and the communication link is put into a “hold” state as depicted in block  210 . If the predetermined acquisition criteria have not been met, the acquisition process loops back and starts from the beginning as depicted in blocks  202  and  208 . Looking again at block  216 , if a determination has been made that the different acquisition criteria has not been met, i.e. whether a greater number (i.e. 100) of samples, have not been received without any new ICUC feedback information using the new center, then the acquisition process continues without either increasing the maximum ICUC spiral radius or resetting the “no Feedback Counter”; and transmitted ICUC data is again intended for the remote station as shown in block  220 . A final determination is then similarly made as to whether a predetermined acquisition criterion is met as shown in block  208 . If the predetermined acquisition criteria is in fact met, then the acquisition process is similarly finished and the communication link is put into a “hold” state as seen in block  210 . If the predetermined acquisition criteria have not been met, the acquisition process loops back and starts from the beginning as depicted in blocks  202  and  208 . 
   In summary explanation, a method allows an optical wireless communication link between the transmitting and receiving stations to be used reliably without the need for position sensing capabilities. A small random nudge of the center of the acquisition spiral that is used for a scan pattern is added to the last “good” position; and then the scan begins from that point. The method can be used at different times during a communication link, including but not limited to, during final acquisition and/or re-acquire modes. 
   In view of the above, it can be seen the present invention presents a significant advancement in the art of optical wireless link control techniques. Further, this invention has been described in considerable detail in order to provide those skilled in the optical wireless communication art with the information needed to apply the novel principles and to construct and use such specialized components as are required. In view of the foregoing descriptions, it should be apparent that the present invention represents a significant departure from the prior art in construction and operation. However, while particular embodiments of the present invention have been described herein in detail, it is to be understood that various alterations, modifications and substitutions can be made therein without departing in any way from the spirit and scope of the present invention, as defined in the claims which follow.