Transform:

$$W_R(\omega) = M\cdot \hbox{asinc}_M(\omega) \isdef \frac{\sin\left(M\frac{\omega}{2}\right)}{\sin\left(\frac{\omega}{2}\right)}$$

The DTFT of a rectangular window is shown in Fig.3.1.

Figure 3.1: Rectangular window discrete-time Fourier transform.