ElectricRob

So you want to send data wirelessly? Starlink does it. 5G (cellular) does it. Wi-Fi does it. What do they all have in common? QAM modulation. Data is just 1s and 0s. Eight bits make a byte. A byte can represent something as simple as a character or part of a pixel in a digital photograph. Put enough bytes together and you have sentences, images, video, and just about everything else we send digitally. But how does all of this travel "through the air"? Via radio signals, of course. More specifically, through a technique called Quadrature Amplitude Modulation (QAM). Here's how it works. The transceiver generates a radio wave whose amplitude and phase can be precisely controlled. Each unique combination represents a symbol. The receiver reads that symbol and translates it back into bits. This isn't unique to Starlink. 5G and Wi-Fi use the same basic technique, often employing higher-order modulation schemes such as 256-QAM or even 1024-QAM, depending on the technology and signal conditions. The key is signal quality. The higher the signal-to-noise ratio (SNR), the more reliably a receiver can distinguish between closely spaced symbols. That allows the system to use higher-order QAM, packing more bits into each symbol and increasing data throughput. Starlink does the same thing. It uses adaptive modulation, dynamically shifting between QAM schemes based on the quality of the connection. When SNR is high, it can use higher-order QAM and transmit more bits per symbol. When conditions deteriorate, it shifts to a more robust, lower-order scheme. Let's look specifically at 64-QAM. The "64" refers to the number of unique symbols, or states, that can be transmitted. QAM combines two carrier signals that are 90 degrees apart in phase (hence "quadrature"). By varying their amplitudes, the system creates 64 unique combinations. Since 64 = 2⁶, each symbol represents 6 bits of data. For example, the binary sequence 110011 maps to one specific point in the QAM constellation. A constellation diagram illustrates this nicely. It plots the 64 possible symbols in a two-dimensional space. Each point represents a unique combination of the two carrier signals, and therefore a unique 6-bit value. Now let's put some numbers around this. At a symbol rate of 90 million symbols per second, with 64-QAM carrying 6 bits per symbol, the theoretical data rate is: 90 million × 6 = 540 Mbps. That's before accounting for coding overhead and other protocol details. But there's another interesting aspect: the bandwidth is shared between transmitting and receiving data. In the example we're looking at, approximately 74 milliseconds of each second is allocated to uploading data from the subscriber terminal, while the remaining 926 milliseconds is allocated to downloading data from the satellite. In other words, the link is asymmetric, with considerably more capacity allocated to downloads than uploads.