$4\mathrm{.}4$ Suppose that the multiplexing parameter r and diversity parameter d approximately satisfy the diversity$/$multiplexing tradeoff d$($r$)=($N$\_($t$)-$r$)($N$\_($r$)-$r$)$ at any large finite SNR$,$ where N$\_($t$)$ and N$\_($r$)$ denote the numbers of transmit and receive antennas, respectively. For a $8\backslash$times $8$ MIMO system with an SNR of $15$dB$,$ if we require a data rate per unit Hertz of R$=30$bps$,$ what is the maximum diversity gain the system can provide?