The word “orthogonal” in OFDM isn’t just a mathematical curiosity – it’s literally what keeps the carriers from stepping on each other’s toes.
If you plot the sinc‑shaped spectra of two adjacent carriers, the null of one coincides exactly with the peak of the other. That precise alignment is why the receiver can separate them with a single FFT, even though the carriers overlap in frequency.
| Aspect | What it means | Why it matters | |--------|----------------|----------------| | Orthogonal sub‑carriers | A wideband channel is split into dozens or even thousands of narrowband sub‑carriers that are mathematically orthogonal (i.e., their spectra do not interfere). | Allows the transmitter to pack many sub‑carriers tightly together, achieving high spectral efficiency without inter‑carrier interference. | | Robustness to multipath fading | Because each sub‑carrier’s symbol duration is long (often several micro‑seconds), the relative delay spread caused by reflections becomes a small fraction of that duration. | The system tolerates severe multipath without the need for complex equalizers; a simple cyclic prefix eliminates inter‑symbol interference (ISI). | | Simple equalization | In the frequency domain, each sub‑carrier experiences a flat (i.e., constant) gain, so a single complex gain per sub‑carrier suffices. | Equalization reduces to a per‑tone complex multiplication, which is computationally cheap (FFT‑based). | | Scalable bandwidth | Adding or removing sub‑carriers adjusts the total occupied bandwidth in steps of the sub‑carrier spacing (e.g., 15 kHz in LTE). | Operators can tailor channel width to the available spectrum (e.g., 5 MHz, 10 MHz, 20 MHz). | | Flexible resource allocation | Sub‑carriers can be grouped into resource blocks and assigned independently to different users, services, or QoS classes. | Enables multi‑user OFDMA (as in LTE/5G) and dynamic scheduling, maximizing overall system throughput. | | Support for multiple modulation & coding schemes (MCS) | Each sub‑carrier (or block of them) can carry QPSK, 16‑QAM, 64‑QAM, or even 256‑QAM, with appropriate forward error correction (FEC). | Adaptive modulation adapts to instantaneous channel quality, delivering the highest possible data rate while preserving reliability. | ilija stojanovic osnovi telekomunikacija pdf 14
To give you a flavor of what "ilija stojanovic osnovi telekomunikacija pdf 14" would contain, here is a paraphrased summary typical of his Chapter 14 on baseband digital transmission: The word “orthogonal” in OFDM isn’t just a
“After quantization, the PCM signal is a discrete-amplitude, discrete-time sequence. This signal must be encoded into a line code that matches the transmission medium. The choice of line code determines the presence of a DC component, clock recovery capability, and bandwidth efficiency. In this chapter, we analyze the power spectral density of NRZ, RZ, and Manchester codes. Then, we introduce the Nyquist criterion for zero ISI, leading to the raised cosine spectrum. Finally, we derive the probability of error for a binary baseband system with a matched filter: ( P_e = Q\left(\sqrt\frac2E_bN_0\right) ).” | Aspect | What it means | Why
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