Aug 18, 2021
ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (OFDM) – Understanding Network and Security for Far-Edge Computing
This is a scheme in which multiple closely spaced orthogonal subcarrier signals with overlapping spectra are transmitted to carry data in parallel.
In traditional Frequency Division Multiplexing (FDM), the subcarriers (also known as channels) are kept apart using a little bit of space between them called a guard band:
Figure 3.18 – FDM using guard bands
This is done to prevent crosstalk, noise, or interference between the channels. It also makes it easier for the demodulators to single out the channels when demuxing them.
OFDM deliberately overlaps the channels in a specific way – this is where the orthogonal part comes in. Orthogonal means “at right angles,” but in this context, it refers to a precise mathematical relationship between how the channels are spaced across the frequency band. This technique can save as much as 50% of the bandwidth, which can now be used to carry additional channels:
Figure 3.19 – OFDM
OFDM uses digital signal processing techniques to perform coherent demodulation on these overlapping channels 7. The mathematics are beyond the scope of this book. At its core, this is simply another example of how we can exploit the fact that light always travels at the same speed. OFDM techniques can, and often are, used in combination with MIMO.
7 Fourier transforms can be performed to convert the time domain of a digital signal’s square waves into frequency domains corresponding to the channels.
Shannon-Hartley theorem (signal-to-noise ratio)
Developed in the 1940s, the Shannon-Hartley theorem describes the maximum rate at which information can be transmitted over a communications channel of a specified bandwidth in the presence of noise:
Here, we have the following:
C is the channel capacity in bits per second
B is the bandwidth of the channel in hertz
S is the average received signal power in watts
N is the average power of noise/interference in watts
Let’s zero in on the two most important terms to remember:
Signal (S): Average power of the received signal in watts
Noise (N): Average power of noise (that is, interference) in watts
These two terms are grouped into a single expression known as the Signal-to-Noise Ratio (SNR). An SNR of 2:1 means there is twice as much signal as there is noise. An SNR of 1:1 means there is the same amount of noise as there is a signal.
Another way to put this is to say a signal that is suffering throughput loss from degradation due to interference can be improved by increasing the signal’s power 8.
8 Keep in mind that increasing the power of your signal can create interference for others. This is why there are often laws limiting how powerful a given device’s transmitter is allowed to be.
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