I'm working on visible light communication where I fabricate my own organic LEDs. One of the ways to characterize them is "small signal analysis." I understand we are applying this analysis to see the bandwidth of the OLED, but I do not understand how this bandwidth is related to the communication side.
Some people directly apply a bitstream to the LED while some modulate the bitstream with AC and apply. What is the difference?
how this bandwidth is related to communication side?
Since this deals with optical telecommunication, I think it is relatively easy to see why the frequency response would be important. If you have an LED that can modulate quickly, then you can send bits at a higher rate.
Also some people directly applies bitstream to LED while some modulating bitstream with AC and apply. What is the difference?
The most straight-forward approach for sending data through light, is using a Non-Return-to-Zero scheme (NRZ) where you just turn on the light for a '1' and turn it off for a '0'. There are encodings that guarantee enough edges to keep finding the signal even when there is background light but you might still modulate it to distinguish it more easily. Nevertheless, there are many more modulation schemes that can (if the noise is low enough) allow you to encode more information than an NRZ scheme. For example, you could encode information by frequency modulation (FM) or amplitude modulation (AM), or you can encode information in both phase shifts and amplitude (QAM), etc. You can even have multiple frequency bands working in parallel.
Bandwidth of something is generally defined to be the frequency range where the AC waveform amplitude drops by 3dB. That's the half power point.
The larger bandwidth you have the faster you can turn the LED on and off (or modulate it) to send data. For example send square wave at 100 kHz, it will still look like 100 kHz square wave as the LED can fully turn on and off at the rate it can. But because the LED has some finite speed to turn on and off, at 100 MHz it may not be able to fully turn on and off any more. So it has limited bandwidth.
Sometimes the LED can be directly driven with data stream, but sometimes it may need to be modulated for some reason, as it would not othwerwise pass through the system.
For example, if you use 1 Mbps data rate and make a receiver for it, the receiver circuitry might work if there is enough transitions in the data, like if it ideally is 50% high and low, but if the detector has automatic gain control to adjust itself to varying ambient light conditions and varying data signal powers due to distance or whatever, too long light pulse or too long time between light pulses may cause the receiver to disregard a too long pulse as ambient light or consider too long space between pulses to adjust it to original ambient light.
Therefore the transmitted data is modulated or encoded for it to pass the system and not be affected much by the data pattern.
You asked why to use modulation.
Modulation changes the frequencies of the signal so it "fits" better with the frequency response of the system.
For example, radio antennas are (more-or-less) band-pass filters, and you're only allowed to use a certain band anyway. When you transmit a signal by radio you have to make it fit within that band.
With Visible Light Communication you should have some more freedom to choose your own frequency, but your system still might not like certain frequencies.
Ambient light consists of very low frequencies <2Hz including 0Hz. So if your system relies on 0Hz for communication, it's likely to be confused by ambient light. If light off means 0 and light on means 1, then your system will get confused when I turn on the light in the room. So you design it to avoid very low frequencies. If you make it so a 100Hz flash means 0 and a 200Hz flash means 1 (that's FSK), it's not going to be confused by ambient light.
You might discover additional constraints when you construct your system and measure its frequency response. Most likely, the LED and/or the photodetector won't be able to respond to high frequencies. When you measure it, you'll find a "window" of frequencies that can get from the signal source, through the transmit amplifier (if you have one), LED, optical path, detector, and receive amplifier (if you have one), and to the signal receiver.
This "window" is your band, and the width of the band is the bandwidth (who woulda thunk?). It won't be a sharp transition, though. It won't be like 990kHz works and 991kHz doesn't. Rather, frequencies will get transmitted worse and worse as you move closer to the edge of the band. So you have to cut it off at some point and say "past this frequency the transmission is too poor". Traditionally, we find where the frequency response is 3dB weaker than the middle, and we call that the edge - though that rule of thumb does assume the edge is nicely shaped.
Then, to send data as quickly as possible, you want to work out a modulation scheme that packs your data into the band as efficiently as possible. That's a whole separate topic and out of scope here. The Shannon-Hartley formula gives a theoretical maximum amount of data that you can send in a certain bandwidth with a certain signal power and noise power.
Of course, you don't have to design an optimal modulation scheme - if you're not too concerned about data rate you can use something simple, like FSK. Many systems do.
One gotcha when thinking about modulation for beginners: you need to realize that every signal has frequency content. It's tempting to think that AM or ASK has only one frequency, since it modulates the amplitude, not the frequency. But when you take your 1MHz carrier frequency, and modulate it with a 0.2MHz signal, and then measure the frequencies in your modulated signal, you find that it does actually use the whole band 0.8MHz-1.2MHz. If you try to send that through a communications link whose frequency response only works for the band 0.95MHz-1.05MHz, the other side will get a mangled signal.
Another gotcha: if you have, say, a 200kbps digital signal, as a wave it actually contains frequencies up to 2MHz and higher, not just 200kHz. To limit the signal to 200kHz you have to "smear out" the edges, e.g. with a low-pass filter. When you measure the frequencies in a square wave you see that it contains not just the square wave frequency, but also odd-numbered multiples of it. If you send it through a low-pass filter to remove those, the wave looks a lot more sine-y.
Also some people directly applies bitstream to LED while some modulating bitstream with AC and apply. What is the difference?
You don't say what sort of communication you're doing.
If it's across open space in a lit area, then there are many sources of "noise" that can confuse the receiver. Outdoors, the flickering of sunlight through trees is pretty random. Indoors, fluorescent or LED lamps will have a flicker frequency.
If you transmit only your raw datastream, this makes it very hard for the receiver to recognise what's data and what's noise.
If you modulate the data onto a fixed frequency carrier, the receiver can use a bandpass filter tuned to the same frequency to reject most of the unwanted background light.
Small signal analysis is used to analyze the AC (or frequency) effects of nonlinear circuit elements. You can linearize an element such as a diode to turn it into a linear element and it makes the analysis and math easier.
Since communication with LED's involves turning them on and off very fast, it is important to understand how fast you can do that and also how to optimize the current source that will drive the LED. The bandwidth will also be determined by the diode and parasitic elements such as inductance and capacitance that arise from materials in the LED and construction.
Also some people directly applies bitstream to LED while some modulating bitstream with AC and apply. What is the difference?
A bitstream is nonlinear, usually to analyze frequency we do an (AC) frequency sweep and to help determine bandwidth and also for linear analyses
