Baseband versus subcarrier:

Baseband

In our experiments, we have used "basedband" modulation for the conveying of voice modulation atop a modulated light beam and with it, we have managed to break a number of distance records.  In a nutshell, this system may be described thusly:

• A "standing" current is established - typically near the maximum "safe" continuous current of the LED being used.

• Audio is imposed on the LED's operating current and its waveform goes from 0% (zero current - the LED is off) to 100% (twice the "standing" current.)  During the excursions to "100%" the LED's maximum current may be briefly exceeded, but this has not been found to be a cause for concern due to its brief nature and the rather conservative ratings assigned to good-quality LEDs - particularly those that are adequately heat-sinked.

• At the detector, audio is AC-coupled and applied to an audio amplifier and speakers/headphones.
  

The above system is very simple, requiring no exotic equipment other than the easily-built modulator and detector.  Being "baseband", its frequency response need only range from 200 Hz or so to 2500-3000 Hz to convey speech and this spectral range is low enough that the inevitable degradation due to the capacitances of the detector's active devices (e.g. photodiode, input amplifier, etc.) and the operating impedance is reduced by the maximum degree practical.  Because of the nature of the system, its ultimate sensitivity is unmatched by any other for voice-grade communications.


Subcarriers - FM

Subcarrier operation implies that the voice information is conveyed at a frequency higher than that encountered in baseband.  In past years, many "optical communicator" circuits have been described that utilize a frequency-modulated (FM) subcarrier at a fairly high ultrasonic frequency - typically in the 30-80 kHz range.  Because it is FM, its occupied bandwidth is, when modulated by speech, in the 10-15 kHz region, hence the need for the fairly high frequencies.  FM has the obvious advantage in that it is, by definition, immune to amplitude variations in the carrier level, the intelligence being conveyed solely by the variation in frequency, and as such it may be minimally affected by amplitude fluctuations due to scintillation, insects, rain and interference from lighting - provided that the minimum signal/noise level is above the threshold of the FM detector!

When using an FM subcarrier system there are several things that can contribute to its lesser ability to be used under weak signal conditions than straight baseband operation:

• Detection bandwidth is necessarily higher than for baseband.  Being FM, its occupied bandwidth can be calculated using "Carson's Rule" which states that the signal will require (2*deviation) + (2*highest modulated frequency) - which means that if 2.5 kHz deviation is used and the highest audio frequency being modulated is 3 kHz (from the human voice), this turns out to be:

(2 * 2.5 kHz deviation) + (2 * 3 kHz modulated frequency) = 11 kHz.

If this is compared with that used by baseband - which is simply that of the highest audio frequency (e.g. 3 kHz) one can see that nearly 4 times the bandwidth is required - and communications theory tells us that the wider the bandwidth in front of the "demodulator", the more "diluted" the desired signal becomes from the various noise sources.  In other words, we may get more noise in our 11 kHz FM detector bandwidth than we do in our 3 kHz baseband bandwidth!

In the simpler circuits, this detection bandwidth is often ignored:  Many FM "optical communicator" schemes consist of a simple voice-modulated oscillator on the transmit end and a PLL-type detector on the receive end - but there is often NO attempt made to precede the detector with a bandpass filter and more often than not, the output from the optical receiver is simply dumped into the detector unfiltered - noise and all!  To be sure, a PLL-type of detector does, in fact, have a somewhat limited bandwidth intrinsically, but even off-frequency noise can degrade its performance owing to the necessity of such a detector to have limiting action.

The "better" FM subcarrier systems will, in fact, have a proper bandpass filter preceding the FM demodulator - either an op-amp type filter at the ultrasonic frequency of the subcarrier itself (say, +/- 5.5 kHz from the center of the subcarrier) or that ultrasonic frequency may actually be up-converted to a higher frequency and applied to an already-existing FM communications receiver:  Past articles have appeared that convert the ultrasonic frequencies to HF or even VHF to allow the FM mode of a radio (or handie-talkie) to be used - proper filtering and all!  In addition to bandpass filtering, a narrowband FM scheme will also employ pre-emphasis on transmit and de-emphasis on receiver - a clever (but simple) trick to further-enhance the performance of an FM system under weak-signal conditions.

Provided that the detection bandwidth is taken care of, there's still the issue of the FM threshold.  While FM is generally immune to variations in amplitude causing effects on the demodulated signal, that signal must maintain a minimum signal level above the noise in order for degradation due to that noise to be avoided.  While it varies with the FM demodulator method and bandwidth, this threshold is generally in the 8-10dB range for a fairly "noise-free" signal and below this, the recovered signal rapidly gets lost in the noise!

In baseband operation, the skilled listener can easily make out speech that has just a 6 dB of signal-noise ratio - and this is not only 2-4 dB lower than that detectable by an FM-type circuit, but because the bandwidth of the baseband is less than 1/3rd, it's also less-affected by the noise as well!

Comment:

• On a baseband system, one would practically set the receive bandwidth by the application of a low pass filter of 2.5-3 kHz.  Because of the nature of the human auditory system, we are remarkably good at mentally filtering out the higher-frequency components that contain little or no voice information so in many cases the addition of a low-pass filter will likely offer little advantage to even a semi-skilled listener.  For an FM system, since we impose a demodulator between the signal and our ears, we lose the direct advantage of our brain's ability to do such filtering before the original audio signal is recovered by the demodulator.

Finally, there's also another issue that arises in a practical subcarrier scheme using common photodiode detectors:  The loss of ultimate sensitivity as the frequency is increased.

As noted above, the capacitance of the photodiode combined with the high impedance of a very sensitive detector conspires to form a low-pass filter.  For practical photodiodes (e.g. 1mm² to 10mm² in area) and typical circuits this "knee" frequency (where the frequency rolloff starts to become apparent) typically occurs right in the voice range of 200 to 2000 Hz, depending on the particular circuit and photodiode used.

Above this "knee" frequency, the audio response of the circuit drops off at approximately 6dB/octave while the noise floor itself generally stays constant (once one gets above 1/F noise - which we will ignore for the purposes of this discussion) which means that the signal-noise ratio (and ultimate sensitivity) of the detector as a whole decreases with increasing frequency.  Knowing this, one quickly realizes that for purposes of ultimate system sensitivity, it is best to use as low a frequency as possible!

If we take a 50 kHz FM subcarrier as an example using a receiver with its "knee" at 1 kHz, we can quickly calculate that at a rate of 6dB/octave, we may lose 30-40dB of ultimate sensitivity - this, atop the degradation caused by our necessarily-wider bandwidth and the lower threshold of our FM detector!  Practically speaking, it's been observed that a well-designed receive system can, to a degree, mitigate this degradation, but even the best implementation will suffer somewhat!

Why use FM then?  The main attraction of FM is, again, the fact that it's insensitive to amplitude variations - and provided that the minimum signal level is maintained, it can be essentially noise-free:  If the path is fairly short, the air is clear and the transmit/receive optics are reasonably well-designed, this scheme can work very well.

The avoidance of man-made optical interference:

An advantage of subcarrier schemes not yet mentioned is that the necessarily-higher frequencies involved move it away from the "buzz" and "hum" of manmade urban lighting which contains strong components of twice the mains frequency and its harmonics (e.g. 100, 200, 300, 400 etc. Hz for 50 Hz mains and 120, 240, 360, 480 Hz for 60 Hz mains.)  If the optical path involves passing over/near areas in which such lighting is used, the baseband frequency range can become heavily contaminated with the mains-related harmonics, possibly making communications difficult.

Comment:

These mains-related harmonics (buzz, hum) may be filtered out with the proper equipment:  For more information about how this may be done, read the page "A Comb Filter to combat mains-induced hum from urban lighting" at this web site.

Fortunately, the amplitude of these harmonics tends to drop off fairly rapidly with increasing frequency and by the time one gets to 10-20 kHz, they are typically reduced to the point of either being inaudible or minimally intrusive and in this way interference from mains-powered lighting may be avoided.

Comments:

• With the increasing prevalence of solid-state (e.g. LED) urban lighting, the scenario described above is shifting.  These lights are typically driven with high-frequency (30-300 kHz) switching regulators - sometimes run in a "discontinuous" mode - that is, the regulation is simply switched on/off to maintain an average LED current rather than adjusting the duty cycle.  Because of often-minimal mains filtering and operating in this discontinuous mode - and the ability of the LED itself to respond at ultrasonic frequencies - these light sources can produce "noise" over a very wide bandwidth from baseband well into ultrasonic - including the frequency of the switching regulator itself.  At the time of writing (mid 2012) LED outdoor lighting is small - but growing - percentage of urban lighting.  The good news is that such lighting is often far more efficiently directed onto the area being illuminated and "spillover" is significantly reduced.

• Electronic advertising and similar signs can produce a lot of "racket" at all frequencies from baseband, well into the ultrasonic range, so it's best to pick optical paths to avoid them altogether!

The use of SSB subcarrier schemes:

More recently (2010 and later) the UK optical group has been a proponent of the use of single sideband (SSB) on optical system - and for some valid reasons:

• The use of a modest "carrier" frequency (22-25 kHz) is high enough to avoid the majority of the buzz and hum of urban lighting, yet it's not so high that sensitivity is as severely affected by the high-frequency degradation of the detectors..

• An "HF to VLF" transverter is implemented to converter the input/output of an amateur HF rig between the 22-25 kHz frequency and (typically) the 80 meter amateur band.  Because many of the optical enthusiasts already use an all-mode transceiver in conjunction with their VHF, UHF and microwave transverters, they already have this "IF" radio on-hand.

• The use of SSB is somewhat more power-efficient that the "AM" used for baseband communications.  For example, the LED is not illuminated unless there is speech present - this, contrasted to baseband AM where the LED is running at half-current all of the time.  This has the potential of significantly reducing the power consumed by the optical transmitter - particularly if very high-power LEDs are used - but this may be partially offset by the power required by the IF radio.  (Some optical SSB users will set their LED's idle current at 10%-25% of maximum to provide a continuous visual reference to the party at the receiving end.)

• SSB is also a more efficient than "AM" in the conveying of modulation - a fact that partially offsets the effects of the receiver sensitivity degradation encountered when using subcarrier frequencies.
  

• For full-duplex operation, it's practical for each end to slightly offset the transmit frequency to reduce the amount of crosstalk.
  

To be sure, the use of SSB is a bit more complex than simple baseband - and from a purely theoretical standpoint its weak-signal performance is going to be demonstrably inferior, but based on practicalities and the environment experienced by the UK folks it does make some sense:

• The population density of the UK makes it extremely difficult to find any reasonably long path that is devoid of urban lighting.  Given this fact, some means of mitigation of mains-induced interference will usually be necessary, whether it's a subcarrier scheme or baseband filtering.  By comparison, in the western U.S. or Australia it's perfectly reasonable to find both short and long optical paths that have no significant urban lighting!
  

• The lay of the land in the UK effectively precludes the longest possible optical paths and as of the time of writing, the longest path utilized in the UK is on the order of 120km.  With these relatively shorter paths there is less need for the ultimate in optical detector performance and thus the degradation incurred through the use of subcarriers may well be an acceptable tradeoff!  For casual and contest operation distances in the 5km-50km range are more likely and given clear air and well-implemented optics, these distance are well within the reach such a system!
  

• Attraction for those already interested in VHF/UHF/Microwave operation.  The construction of a subcarrier-based optical communication system involves, first, a baseband-type system onto which is overlaid the circuitry to permit subcarrier use.  While it might seem a bit redundant to add complexity to an already-useful system, it's worth noting that once built, the encumbrance of the extra circuit is of little importance to the VHF/UHF/Microwave operator that already has the SSB "IF" transceiver available.  Practically speaking, such an enthusiast could consider the subcarrier-based optical box to be yet another self-contained transceiver for operating on yet another "band."  If the preponderance of operation is over modest (5-50km) ranges, such equipment will perform perfectly adequately!
  

In the future, I hope to add more links, but in the meantime one may join and peruse the UKNanowave Yahoo group for more information.

If you have questions or comments concerning the contents of this page, if have information about other suppliers, feel free to contact me using the information at this URL.
Keywords:  Lightbeam communications, light beam, lightbeam, laser beam, modulated light, optical communications, through-the-air optical communications, FSO communications, Free-Space Optical communications, LED communications, laser communications, LED, laser, light-emitting diode, lens, fresnel, fresnel lens, photodiode, photomultiplier, PMT, phototransistor, laser tube, laser diode, high power LED, luxeon, cree, phlatlight, lumileds, modulator, detector