The communications subsystem is the part of the CubeSat mission that facilitates data downlink (the process of sending information down to the ground station) and command uplink (the process of sending information up to the CubeSat from our ground station). This part of a CubeSat system is critical to actually giving the system value. A satellite that launches into space successfully but can't talk to us on the ground isn't much more useful than a floating rock in orbit.
Often, downlink data consists of payload information (such as images from an onboard camera), and telemetry information (such as GPS data and information about the health of electrical systems on board). Uplink data, meanwhile, is usually commands that tell the satellite what manoeuvres it should perform to carry out particular payload objectives (such as which way to orient a camera).
The main components that compose the communications subsystem include the transceiver and antenna onboard both the CubeSat and on the Ground Station.
Before we get into learning about the deeper technical details that govern a communications system, it's important to learn about some key ideas that will be useful no matter which part of comms you decide to get involved in.
Although most people have a rough idea of what a decibel is and have heard the term in passing, the exact definition and usage of the decibel are often not well understood.
At its core, the decibel is really a unitless value that measures the logarithmic ratio between two numbers. Most often, decibels are used to measure power, such as sound intensity or the strength of an electromagnetic wave. Although technically unitless, the "dB" symbol is often used to clarify that the given value is based on a logarithmic scale.
To be specific, power (whether it be for a sound wave or an EM wave) can be converted to decibels using the following formula:
$$ dB = 10log_{10}(\frac{P}{P_{ref}}) $$
where $P$ is the power you are trying to write in dB, and $P_{ref}$ is the reference power. This reference power can be a static value, like 1W, or a function of distance.
A measure of power in decibels relative to 1W is abbreviated as a dBW (decibel relative to a watt of power). When measured relative to 1mW, the dBm abbreviation is given instead. Another common unit is dBi: used commonly in antenna theory, this represents the power at a given distance relative to a perfectly "isotropic" antenna, meaning one that perfectly radiates a signal into a sphere. Similarly, a dBiC is relative to a perfect circularly polarized antenna, dBd is relative to an ideal dipole antenna, and dB(monopole) is relative to a perfect monopole antenna. Don't worry if you don't understand what each of those terms mean yet, that's all covered in the Antenna Theory Hub section.
Since the decibel scale is logarithmic, it's nice to remember common values: -3dB is half the reference power, 3dB is double the reference power, and 6dB means it is 4 times larger. Note that decibel values can obviously be negative if the ratio is less than 1, and can also be 0 if the ratio is exactly 1 (for example, 1W=0dBW).
https://www.youtube.com/watch?v=wGE4tjATecY
Bandwidth is another term commonly used in everyday language that isn't always well defined. It simply refers to the range of frequencies which a given signal can lie within.
When a government agency assigns a spacecraft a particular section of the radio spectrum that they can operate in, those limits are referred to as the bandwidth within which the spacecraft can transmit and receive data. A range is required because, as transmitters aren't always perfect, it's hard to very precisely maintain all the power of a transmit signal within a singular frequency, and therefore it's more common to see a distribution of power within an allocated band, as seen in the image below.
For an antenna, bandwidth refers to the range of frequencies that it can receive.
Also note that for certain radio communication methods like Frequency Modulation (FM), it is necessary for there to be a sizable bandwidth so that receivers can easily detect changes in the frequency and interpret them as either 1s or 0s.