$6.$ The cellular network shown in Figure $3$ has $10$ omni$-$directional cells and a total bandwidth of $1\mathrm{.}68$ MHz $.$ All cells are designed to support the same number of voice users with data rate of $20$ Kbps per user.

Figure $3$

$($a$)$ If the above cellular network implements a frequency division multiple access $($FDMA$)$ system with a per$-$channel bandwidth of $20$ KHz and a $3-$cell re$-$use pattern in the uplink, compute the total uplink capacity of the FDMA system. You may assume that all re$-$use cells are sufficiently separated and hence do not interfere with each other.

$(5$ Marks$)$