One possible solution is to modify the current LTE air interface so that mobile users can sense the spectrum and use existing schemes such as listen-before-talk and request-to-send/clear-to-send to access the network [7]. However, changing the LTE air interface is not easy, so it is more desirable to use the existing LTE air-interface for either licensed or unlicensed spectrum [6, 8] to reduce the operating cost and system complexity. Therefore, this article focuses on backward-compatible network access schemes for LTE-CR.