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What is the intuitive way of understanding concept of constellations in digital communication? Understanding a constellation is easy if you understand how a signal is represented as I/Q data. For a good introduction, I'd recommend What Is I/Q Data - National Instruments. I'll summarise here. Consider what modulation is. We have some carrier, and then we ...


8

OFDM isn't a modulation technique. It stands for "Orthogonal frequency-division multiplexing" and is a way of splitting the spectrum into discrete channels for sending data. Each of those channels, which is just a single frequency carrier in its own right, requires the data to be modulated onto it. In this example BPSK is used.


7

LoRa is a chirp-based spread-spectrum modulation. A symbol is a chirp. To generate symbols/chirps, the modem modulates the phase of an oscillator. The number of times per second that the modem adjusts the phase is called the chip rate and defines the modulation bandwidth. Chip rate is a direct subdivision of the quartz frequency (32 MHz). Example for 125 ...


5

According to the data sheet, the N1996a spectrum analyser has a 'zero span' mode, where you can use it as a fixed tuned receiver, and the trace sweeps in time rather than frequency, just like an oscilloscope. See page 101 of the manual N1996-90028.pdf Set the resolution bandwidth to be wide enough to follow your signal. If you tune it to the centre of your ...


5

NxN-QAM means Quadrature Amplitude Modulation and it is a modulation scheme where the transmitted signal is the "mix" of two quadrature carriers whose amplitude is digitally modulated independently so as to give N different possible amplitude levels per each carrier. Therefore 64-QAM is 8x8-QAM, for example. The total bandwidth of such a signal is ...


5

AFAIK it doesn't have a fixed name, apart from 'the WS2812 protocol'. As described, it is a fixed-cell pulse-width encoding. FYI: in practice, it isn't fixed-cell at all: provided that the pauses (T1L, T0L) are shorter than Treset, all that matters is the pulse width (T0H, T1H). This makes it a plain pulse-wdith encoding, and it makes bit-banging waaaaay ...


4

My understanding of constellations is that they are just a way to represent the sinusoidal signal that is used to send the data. For instance, let's say you wanted to encode a single bit in a sinusoidal signal. You have 2 choices for the bit value and you would want the signal for each choice to look as different from each other as possible. This is an ...


4

The word 'width' in Pulse Width Modulation is a bit misleading. PWM actually encodes an analog value as the ratio of High or On time ('pulse width') to the total period of a rectangular waveform. VPW is another name for Pulse Length or Pulse Duration Modulation (PDM). It encodes an analog value in the absolute width of the pulse. The period between pulses ...


4

[The term "digital signal" is only a convenient shorthand. There is digital information imposed on an analog signal.] The analog current will generate a voltage drop in the common wire too, and it will affect the digital line. However, the drop in the common wire would have to be larger than a logic threshold in order to affect a digital signal. That's ...


4

Definitions So, what is a bit, symbol, chip, and chirp, and what do these mean? Bit Bit is the smallest unit of information. Most of the time, we try to send these bits from the sender (TX) to the receiver (RX). In order to send these bits to RX, they have to go through some kind of medium to reach their destination. It can be any metal, air, water, ...


4

PSK is very much alive and used all over the place! Let me correct a misunderstanding first: Or when PSK is talked about, is bandlimited PSK implied which amounts to QAM with a circular constellation? Every communication signal is bandlimited. The constellation diagram has nothing to do with bandlimiting – that's job of the pulse shaping filter (this is ...


4

I seek a device that can be programmed to output physical voltages based on a set of datapoints. This kind of device is called a digital-to-analog converter or DAC. The device should also support multi-channel outputs (and inputs), Up to 8 output channels per chip is pretty common. You can probably find 16 channels if you look, but you might have to ...


4

Yes, it is possible. The approach that I would probably take would be to use a small-to-medium size FPGA to do the DSP (basically 100 DDS generators), a high-speed (250 MHz) DAC, and a reasonably good LC bandpass filter at the output of the DAC. This could be interesting: I did some digging, and it is now possible to do general-purpose programming on the ...


3

In most cases, a digital signal is much faster than an analog signal, meaning that it contains a higher frequency content. As you probably know by now, traces that are close to each other can have parasitic capacitance. As frequency increases, the capacitance between two traces begins to look like a short circuit and can result in something commonly known ...


3

QAM really has nothing to do with PSK at all. It's just two channels of ASK that are combined together using two subcarriers that have a fixed phase shift between them. To prevent ISI in an ASK signal, you need to be able to transmit at a minimum the main "lobe" of the power spectrum with linear phase shift. The lobe is defined by the first nulls in the PSD,...


3

The reason why OFDM is confusing is mainly because it is never fully presented. There are always gaps and holes in the details. For old-school OFDM, you see details of multiple orthogonal carriers (eg. a fundamental sinusoid and harmonics of it are all orthogonal), where each orthogonal carrier amplitude may be altered (eg. either make the amplitude equal to ...


3

QAM does not require any subcarriers at all, it modulates the carrier signal at the carrier frequency.Both the in-phase (I) and the quadrature (Q) components in QAM are modulated at the carrier frequency, the only difference is that the carrier waves used for modulation are \$90^0\$ out of phase i.e the I-component is modulated with \$\cos(2\pi f_ct)\$ ...


3

This is a complement to Lorenzo Donati rather than an answer The bandwidth is the range of frequency beyond which a sinusoid is considered to be attenuated (conventionally, by -3dB or a factor of $1/\sqrt{2}$). However, digital signals are not sinusoids. The Fourier transform shows that any digital signal (any signal for that matter) is made of a defined ...


3

Honestly, if it isn't immediately apparent to you why orthogonality of subcarriers eliminates IC interference, you should review what these terms mean. Orthogonality literally means that these are independent - non-divisible - frequencies. This in turn makes it so that any interference caused by interaction between these frequencies is not within the data ...


3

There may be special modulation we don't know about, but you wont likely notice any significant change in modulated spectrum After reading about the Receiver technology , your Pulse position servo information will be digitized and sent as DATA , therefore there is no change in your spectrum due your misleading video. It is wideband DSSS !!! not narrow ...


3

Take a look at Figure 49-7 in clause 49: Idles are type of control code. When sending control codes in a block, you must start the block with a "block type field" to indicate the mix of control and data octets in the block. The value of the block-type field is nonzero. Therefore the scrambler will never see a stream of all-zero's, even if idles are sent ...


3

What you are referring to does exist, and is called amplitude-shift keying.


3

As others have stated amplitude modulation is indeed used for digital modulation. However, the biggest issue with AM modulation is that you are using a natural phenomenon, the envelope variation, to carry your signal. That means your signal is highly susceptible to interference by outside causes, eg. neighboring frequency transmitters, variability in local ...


3

Why amplitude modulation is not used for sending digital data? It is used. In fact, this is all there was in early radio. The carrier was simply switched on and off according to the dits and dahs of Morse code. This was all these early transmitters could do. Only later did people get clever and encoded a analog signal in the amplitude of the carrier. ...


3

My question is, what compels us to transmit the frequency-shifted version of the negative frequency part? Indeed, nothing compels us to do so. Transmitting the same (baseband) signal twice is inefficient, it unnecessarily doubles the use of frequency space. why isn't this done for digital modulation methods like PSK, QAM or APSK? What makes you think it ...


3

One of the most common uses of PSK is carried in (almost) everybody's pocket, the 2G GSM system. Although many phones use 3G and 4G modulation for traffic, 2G is still required for a common 'lowest level' that all support for call setup, and to revert to should 3G or 4G not be served in the area. No commercial communication system uses raw 'PSK', as the ...


3

Companding is a non-linear time-domain operation that alters the signal in way that reduces the influence of perceived noise. Small amplitude signal variations are increased above the noise floor so that when these are decreased on the receiving end the noise floor is lowered. As any non-linear operation if transmitter and receiver are not exactly matched ...


3

An "arbitrary waveform generator" (look it up) in its simplest form is an oscillator that drives a counter that drives a memory that drives a DAC: simulate this circuit – Schematic created using CircuitLab The number of address bits you need is determined by the number of steps in your waveform. The number of data bits is determined by how much ...


2

I assume you are in north america, so, for a 15km radio link at the 900MHz band, you will need A modem and antenna that will give you a link budget of ~110dB, this assumes you use the maximum allowed by the FCC (1W). For a USB port that can deliver 0.5W at best, you will need another power source, or limit your transmissions to very short bursts. You might ...


2

There are two roads: one that lets you understand what's happening, and another that is fast. Let's start with the firts. please note: I'm assuming 1Mbitps = \$2^{20}\$bps and not \$10^6\$bps. Your transmitter would divide the bitrate across 32 modulators: each modulator would see a reduced bitrate, i.e. 1Mbps/32 = 32768bps = 32kbps. Since you have two ...


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