I have started working on TDMA algorithms for SDR utilizing narrow band waveforms, and I have started reading about narrowband waveforms. But I am confused about the concepts of channel bandwidth and occupying frequencies of narrow band signals. Narrow band channel bandwidth is 25 kHz and occupying frequency range is 30 - 512 MHz. But I have read so far that the bandwidth is the difference between maximum frequency, and minimum frequency and 25 kHz is not the difference of 512 MHz - 30 MHz = 482 MHz. Please elaborate the whole concept and difference?
What that marketing brochure (a terrible way to start learning about a technical concept, by the way) means is that it uses narrowband channels from somewhere within that 30 to 512 MHz range.
That just means it allows you to pick one (or a few) of 25kHz pieces of spectrum to operate on.
"Narrowband" is not defined to be 25 kHz, by the way. The definition of narrowband depends on who you ask: For someone normally dealing with e.g. DVB-T signals, a 2 MHz channel might be considered narrow. For someone dealing with morse operation in HF, 150 Hz might be narrow. For someone doing modeling of channels, you'd typically expect them to say that a channel is narrow if it occupies significantly less than the coherency bandwidth – and that is something that will wildly vary between 30 MHz and 500 MHz. So, anyway you put it, calling 25 kHz "narrowband" is really just a marketing term here.