Wireless Modulation

Wireless
1

Hello, everyone.

The ENCOR OCG goes pretty deep into Wireless and this is the final L1 topic that I have to cover :smiley:

In contrast, wireless LANs must carry data at high bit rates, requiring more bandwidth
for modulation. The end result is that the data being sent is spread out across a range of frequencies.

This is known as spread spectrum. At the physical layer, modern wireless
LANs can be broken down into the following two common spread-spectrum categories:
ā–  Direct sequence spread spectrum (DSSS): Used in the 2.4 GHz band, where a
small number of fixed, wide channels support complex phase modulation schemes and
somewhat scalable data rates. Typically, the channels are wide enough to augment the
data by spreading it out and making it more resilient to disruption.
ā–  Orthogonal Frequency Division Multiplexing (OFDM): Used in the 2.4, 5, and
6 GHz bands, where a single 20 MHz channel contains data that is sent in parallel
over multiple frequencies. Each channel is divided into many subcarriers (also called
subchannels or tones); both phase and amplitude are modulated with quadrature
amplitude modulation (QAM) to move the most data efficiently

Unfortunately, I donā€™t think I understand anything mentioned here..

We have a whole frequency spectrum that ranges from things that we can see all the way to xRay and such. So what exactly then is this spread spectrum, then?

The spectrum of frequencies that WiFi uses, or? The DSSS and OFDM explanations are pretty complex too to be honest and then they also talk aboutā€¦ QAM without any further explanations.

Thank you.
David

2

Hello David

Spread spectrum as a technique is a method of increasing the resilience of a wireless signal. Such a technique spreads data signals across a wider range of frequencies (channel width) than strictly necessary. This approach makes the signal less susceptible to interference making it more resilient.

The concept itself is about how data is ā€œspreadā€ over a channelā€™s bandwidth, not the entire electromagnetic spectrum. So if you have a 40 MHz channel width, a spread spectrum technique is used to place the data on the wireless signal in a robust manner.

Now itā€™s interesting that the OCG states that OFDM is a spread spectrum technology, because strictly speaking it isnā€™t, although it does spread data across multiple frequencies.

I talk more about OFDM in this post. But as a summary, let me just say that OFDM splits the data into many low-rate streams and transmits them in parallel over many closely spaced subcarriers. The subcarriers are orthogonal to each other (no interference between them), which makes OFDM very spectrally efficient.

Real spread spectrum, in contrast, such as DSSS, intentionally spreads a signal over a much wider bandwidth than needed, often using a pseudo-random code (like in DSSS) or hopping between frequencies (like in FHSS, which is another such method).

Even so, I still suggest that for the purposes of the exam, stick with what the OCG says. I just hope my explanation has helped to decipher the information a bit more successfully.

Now QAM is a modulation technique. This is the method with which data is actually encoded onto each subcarrier, and this is used specifically with OFDM. DSSS also uses modulation techniques, but not QAM. It typically uses simpler techniques. I think at this point youā€™re probably beyond the information you need for certification, but it is interesting to further explore these concepts.

I hope this has been helpful!

Laz