The airwaves are a shared medium to improve performance, you need to be spectrum efficient 802.11ac is all about optimization to do that. 

此句有两处错误，重新检查原句，提供出处。

1. 逗号使用错误

2. you need to be spectrum efficient 802.11ac is ... 这个句子，要么主语从句缺少 that，要么后面的 is 有问题。