Question about graded algebras

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I have the following question. Let us suppose that we have a $\mathbb{Z}$- graded algebra $R$ with generators $a_{1},...,a_{r}$ in degrees $d_{1},...,d_{r}$ respectively. What kind of conditions we need to impose on $R$ in order to verify: $$R_{i}\otimes R_{j}\simeq R_{i+j}$$ for all $i$ and $j$?