the meaning of management of bandwidth and energy resources

Question

I have read in this paper, Single-Carrier Frequency Domain Equalization, second page, first paragraph, that Single Carriers (SC) cannot offer the same flexibility as OFDM in the management of bandwidth and energy resources.

I could not understand what is the exact meaning of "management of bandwidth and energy resources"? I mean how the OFDM offers management of bandwidth and energy resources however the SCFDE almost uses the same technique to equalize the channel at the receiving end.

In brief, my question is to understand this paragraph:

On the other hand, FDE does not represent an optimal solution to signal detection over ISI channels and SC systems cannot certainly offer the same flexibility as OFDM in the management of bandwidth and energy resources, both in single user and in multiuser communications.

Answer 1

I mean how the OFDM offers management of bandwidth and energy resources however the SCFDE almost uses the same technique to equalize the channel at the receiving end.

So, the fact that OFDM offers these is because it's a multi-carrier system; SCFDE explicitly is not, so it does not have these advantages. It's similar, but different in the aspects important here.

In OFDM, you create an OFDM symbol with $N$ subcarriers in it, that will all carry separate, independent values ("subcarrier symbols"). So, each OFDM symbol contains $N$ independent-from-each-other values. (The independence is what the "O" in OFDM means, the fact that they're sent at the same time is the "M" and the fact that these are subcarriers in frequency is the "FD".)

So, you can tell your one receiver that

hey, there's subcarrier 1, 3, 7, 29 and 71 in the first OFDM symbol, subcarriers 2, 6, 8, 12, 120 and 122 in the second; these are for you, dear receiver A, and they contain QPSK. I'll multiply these with a factor $a_A$ so that they have the right power at your antenna.

and tell

hey, there's subcarrier 2, 4, 8, 30, 31, 32, 34, 35, 36, 36 and 72 in the first OFDM symbol, subcarriers 1, 3, 4, 5, 7, 9, 10, 11, 13, 14, 15, 90, 92 and 121 in the second; these are for you, dear receiver B, and they contain 64-QAM. I'll multiply these with a factor $a_B$ so that they have the right power at your antenna.

and send the two OFDM symbols. Note that you did not create two separate transmissions – it's just that you split up the OFDM symbols into groups of subcarriers carrying symbols for different users, and that you had the freedom to scale the values as you saw necessary. In this example, clearly, user A achieves a much lower data rate (maybe they need less data, so it's advantageous to send something that doesn't need much SNR, i.e., BPSK, at a low power) than user B.

The ability to just pick a subset of carriers to send to each user, and to use constellations and scalings as desired, is the flexibility the paragraph refers to.

Since SCFDM doesn't have the independent subcarriers, you can't do it with that. (If your question now is "How does OFDM achieve this orthogonality of subcarriers, I will have to refer you to textbooks on OFDM, or to other questions and answers on this site, as it exceeds the scope of your question :) )