For the most part, the light consists
of the signal transmitted by the FSO system at the other end of the link. However, the
collected light also consists of all the background light that exists within the FoV. The
background light thus collected acts as noise that, when sufficient, can degrade the
performance of the optical wireless link. Therefore, a system with a larger FoV collects
more background noise than a system with a smaller FoV, though it may collect the
same amount of signal, thus reducing the overall signal-to-noise ratio.
The amount of background noise can also be reduced significantly by optically filtering
the received signal. Narrowband optical filters are routinely used in optical wireless
products to knockout unwanted background light from the receiver. However, the ratio
of background light received by receivers with different FoVs remains the same. For
example, regardless of the amount of filtering used, an FSO system collects four times
as much background light as a similar system with half the FoV. Additionally, doing
optical filtering poses its own limitations. For example, using too narrow a filter, which
is often costly, may also knock off signals from wider spectrum sources such as LED.
Pages:
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595