Hiding your signal in a noise like modulation type such as CDMA. The RF bandwidth however will be much wider than your signal bandwidth, thats part of the trick. Also you can use (pseudo random) high speed frequency hopping, which is an older method (from WW2) that might also work well for you. What is "the best" depends on your boundary conditions.

hello mohamad,

The   security concern in the physical layer in wireless communications is important and effective because it can omit upper-layer secure protocols in ad hoc or multi-hop relay transmissions, or it can enhance security together with upper-layer protocols.So, i think the modulation techniques like QPSK and QAM will be the best choice.

Dear Mohamed, 

Code division multiplexing is the modulation type that may be cosidered the most secure among the other types of digital modulation where the signal is multiplied by a pseudo random number rendering the signal noise like. This modulation technique is based on the spread spectrum used in the military communication to combat the interception of the signal. The security increases as the spreading factor increases. However the signal bandwidth increases largely after spreading  reducing the bandwidth efficiency. The spreading is considered to be more secure against jamming.

The data security of the spread spectrum is relatively weak and the spreading sequence can be discovered.

To secure the data one has to encrypt them using one of standard encryption algorithms like AES  advanced encryption algorithm or any other private nonstandard algorithm.

Best wishes