SC-FDMA

 Yesterday and today I have been reading (and watching videos) about SC-FDMA, or Single Carrier Frequency Division Multiple Access. It's quite fascinating. 

One thing that I have been curious about as it pertains to Cellular, especially since I was deep in Wi-Fi for over ten years, was why do UEs (clients) have such a disparity in upload and download speeds? Wi-Fi was never perfectly symmetrical, but what's up with LTE showing about 100DL / 14UL on a perfect signal? That's where SC-FDMA steps in. The issue comes down the the technologies utilized (OFDMA on DM and SC-FDMA on UL) and the power consideratioons. Think of it like this - cellular base stations are large and have a lot of power and muscle at their disposal. Big high-gain antennas, low-loss RF cables and beefy radios in the hut below. Power is conditioned with batteries and UPSes. OFDMA is subject to large variations in power, and thus these aforementioned things are needed for a reliable signal to be transmitted at such great distances. Now what of the UEs? They are small, relatively inexpensive and survive via a battery. "So what"?, one may think. Think about how RF works, it utilizes the power it has to "normalize" it's waveforms. There is no perfect radiator and physics apply, so the signals coming from the amps must be smoothed out and allow for OFDMA to work properly and to be decoded by the UEs sometimes miles away from the tower. This is where SC-FDMA comes to light.

SC-FDMA utilizes an addition FFT (fast fourier transform - see more here) that will allow, with a lower power hit and variation, the ability to decode data from a UE in this case, to the base station. What happens under the hood is rather than relying on a one-to-one mapping of symbols to sub carriers, the symbols are mixed and then map them to a subcarrier(s) and this allows for less power variation and creates a more stable signal. Since the UE's tx power is so much lower than the base station, this can grant the UE some insurance that a stronger signal will be received at the base station and increase the likelihood that it can be decoded successfully.

This post is certainly short, but I hope helpful nonetheless. If I've posted anything incorrect in this post, PLEASE let me know in the comments. I strive for accuracy and clarity.

Other points of learning here:
What is SC-FDMA? And why is it used for the Uplink of 4G/5G Mobile?
SC - FDMA vs. OFDMA
Difference between SC-FDMA and OFDMA