one antenna working as receiver and transmitter
If you want frequency proportional to range of wall, consider using an FM continuous wave (FMCW) or linear FM homodyne (LFMH) radar. LFMH and FMCW are almost synonyms. Transmit and receive occur (or can occur) simultaneously. The transmit waveform consists of a linear frequency ramp. The TX and RX are mixed, and the difference frequency is selected using a low-pass filter. When there is a single dominant radar target (such as a wall) the difference frequency is basically a sine wave with frequency proportional to range.

This is one of the simplest types of radar.

http://en.wikipedia.org/wiki/Continuous-wave_radar#Modulated_continuous-wave
http://demonstrations.wolfram.com/FrequencyModulatedContinuousWaveFMCWRadar/