The most effective method to prevent co-channel interference (CCI) between floors in a multi-story building is to implement a three-dimensional (3D) channel plan. By assigning different non-overlapping channels (e.g., 1, 6, and 11 in the 2.4 GHz band) to adjacent floors, you ensure that an access point (AP) on one floor does not interfere with an AP directly above or below it on the next floor. This spatial separation of channels is a fundamental principle in RF design to mitigate CCI and is a primary consideration within the RUCKUS Wi-Fi Planner tool for multi-floor deployments.

A. Lower transmit power on lower floors: While lowering transmit power is a valid technique to reduce cell size and overall interference, a proper channel plan is the more direct and effective primary strategy for preventing CCI.

C. Increase the number of APs per floor: Increasing the number of APs without a corresponding channel and power optimization plan would almost certainly increase, not prevent, co-channel interference.

D. Enable SmartMesh across floors: SmartMesh is a wireless backhaul technology used for AP connectivity where Ethernet is unavailable. It is not a feature designed to mitigate client-facing co-channel interference.

1. RUCKUS Wi-Fi Best Practices for High-Density Wi-Fi Network Design Guide: In the "Channel and Power Plan" section, the guide emphasizes the need for a 3D channel reuse plan for multi-story buildings. It explicitly advises against placing APs on the same channel directly above or below each other on adjacent floors to minimize inter-floor interference. This directly supports the strategy of using different channels per floor. (Typically found in sections discussing RF planning and deployment best practices).

2. RUCKUS Certified Wireless Administrator (RCWA) 100 Study Guide: The official study materials for the RCWA certification cover RF fundamentals and network planning. The "Channel Planning" module details the importance of creating a channel reuse pattern in three dimensions for multi-floor deployments to avoid co-channel interference, reinforcing that staggering channels (e.g., Floor 1: Ch 1, Floor 2: Ch 6, Floor 3: Ch 11) is a critical design step.