求教：sum-rate 是什么，如何计算的，谁能给我点拨一下

wishing兄弟

In a 4 antenna (Nt  = 4) system operating at 10 dB with Tfb  = 100 bits, for example, it is near-optimal to have 5 users (arbitrarily chosen from a larger user set) feed back B = 20 bits each. This provides a sum rate of 9.9 bps/Hz, whereas 10 users with B = 10 with and 25 users with B = 4(i.e., operating with less accurate CSI ) provide sum rates of only 8.5 and 4.6, respectively

这里的sum-rate of 9.9bps/Hz是怎么计算出来的，有没有明白的，给我讲讲，不胜感激……