Which option describes a practical implementation of "null space projection" in antenna arrays for SEW?

• A. It concentrates energy in the direction of the jammer
• B. It uses time-division multiplexing to avoid jam
• C. It uses MIMO/array processing to place nulls along jammer directions while maintaining service in other directions
• D. It encrypts signals to prevent interception

Answer: (D)

Null space projection in antenna arrays leverages the array’s degrees of freedom to suppress interference by shaping the beam pattern so the response toward jammer directions is driven to near zero, while still providing strong response toward legitimate directions. In practice, you identify the jammer directions with their steering vectors, construct a projection that places nulls in those directions (the null space of the interference channel), and apply this to the array weights. With MIMO/array processing, you can create multiple nulls to cover the jammer directions and still maintain service elsewhere by optimizing the remaining gain toward desired users.

Why the other ideas don’t fit: directing energy toward the jammer would worsen interference, not suppress it; using time-division multiplexing avoids jamming in time but doesn’t implement spatial nulls; encryption protects confidentiality, not the spatial suppression of interference.