DFT Leakage

The DFT frequency index $k$ represents the pointer to the real-world frequency of the signal. This has been established through the relation

$$k = N \frac{f}{f_s}$$

where $N$ is the DFT size and $f_s$ is the sample rate. Since $f$ is a continuous variable (e.g., 1.5 GHz, 2.4 GHz), the frequency index $k$ can be an integer or a fractional number.