zabetia@tiger.UUCP (03/12/87)
So how would I go about sending the data over the laser, or the LED's? Should I just pulse them on for a 1 and keep them off for a 0? Should I constantly send a square wave(on-off-on-off) at about ten times the frequency of my data and then keep it on for a 1 or off for a 0? I mean: data: 1 ------ ------------ ------ levels: ? ------ (no data) ------ -------- 0 ------ ------ ------ no bit bit bit two bits bit none bit bit no bit Emitter: on - - - - ------ ------------ - - - ------ - - - - off - - - - ------ ------ - - ------- - - - - - Anybody have better ideas? How do they do it for fiber optics? Thank you again for your help. -- Mahboud Zabetian allegra! --\ zabetia@tiger.princeton.edu 232 Pyne Hall mhuxi! -----\ (609) 452-2285 Princeton University seismo! -----\ (609) 734-0246 Princeton, NJ 08544 attunix! ------ princeton!zabetia
larry@kitty.UUCP (03/14/87)
In article <177@tiger.Princeton.EDU>, zabetia@tiger.Princeton.EDU (Mahboud Zabetian) writes: > So how would I go about sending the data over the laser, or the LED's? > Should I just pulse them on for a 1 and keep them off for a 0? Since we are talking about a free-air wireless link (not fiber optic), a simple encoding scheme like the above will never work in any reliable fashion due external light influence. Furthermore, in the trivial case, any apparatus failure or optical path interruption would be interpreted as a logical 0. We need a method to ascertain the integrity of the communication apparatus and propagation path. [read on...] > Should I constantly send a square wave(on-off-on-off) at about ten times the > frequency of my data and then keep it on for a 1 or off for a 0? I mean: This is a step in the right direction (sort of like 1/2 FSK :-) ), but pretty risky data communication. The problem, as in the first example, is no validation that a detected logical 0 is a _real_ logical 0, or is the result of optical link failure. The simplest modulation method is FSK (frequency shift keying). For example, to send a 1200 bps signal a 5 kHz carrier frequency might be a logical 0, and 5.8 kHz might be a logical 1, with the amount of the frequency shift being 800 Hz. The receiver must always detect either of the above frequencies on an exclusive-OR basis, otherwise the communication link is presumed to have failed. Both the 5.0 and 5.8 kHz frequencies are individually detected; examples of detection methods are: (1) band-pass filters (switched-capacitor filters work nice) coupled to energy detectors; (2) phase-locked loop frequency detectors; (3) off-the-shelf FSK modem IC's. For full-duplex operation, each direction should use carrier frequencies and frequency shifts selected to minimize interference from intermodulation products resulting from detecting the signal from the other direction (there is usually some problem of local receiver-transmitter optical coupling, often caused by reflection off objects close to one local end). I have designed optical links using PPM (pulse-position modulation) with good results. PPM has the advantage of requiring only one constant carrier frequency (for each direction), one bandpass filter, and one energy detector. PPM circuits are a bit trickier to design, but in my opinion, PPM is more reliable than FSK and other techniques (like phase-shift modulation) for low-speed (< 10 kbps) free-air optical data links. > How do they do it for fiber optics? Fiber optics solve one major problem which plagues free-air optical links: fiber optic transmission effectively eliminates all extraneous light other than the transmitted signal, since fiber optic cables all have an opaque sheath. As a result of their being no extraneous light, fiber optic systems generally use direct data pulses (i.e., pulse rate = bit rate) rather than any carrier frequency type of modulation scheme. While simple mark-space data encoding may be used for fiber optic links, such a simple technique no longer becomes feasible when data rates exceed 50 kbs; at this point modulation codes are necessary. While fiber optic signals are directly detected as 0's and 1's, these bits are encoded using a modulation code not unlike that used in magnetic media recording. Modulation codes used in fiber optics include: RZ, NRZ, NRZI, biphase-mark, biphase-space, Manchester, MFM, and a delay modulation scheme called Miller. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"