Multiple antennas for physical layer secrecy

Due to the broadcast nature of the wireless channel, information transmitted over a wireless network can be accessed by unauthorized users, also referred to as eavedroppers. Ensuring that only the legitimate receiver will be able to decipher the transmitted information has been traditionally achieved by cryptographic approaches operating at various layers of the network protocol stack, e.g., WEP (LAN, 1990), at the link layer, SET (SET, 1997), at the application layer, IPSec (IPSec, 2000), at the network layer, TLS/SSL (SSL, 1996), and WTLS (WAP, 2002) at the transport layer. However, cryptographic protocols, mainly developed for wireline networks, impose significant challenges to wireless devices, which typically have limited resources. They require significant storage overhead (Liu et al., 2006) and computational power, thus impacting battery life (Potlapally et al., 2006). Further, they rely on secret keys, and key distribution and management over wireless networks is difficult and expensive (NIST Report, 2009). It should be noted that a cryptographic approach does not prevent an unauthorized user from receiving the transmitted packet; it just makes it computationally difficult for the unauthorized user to decrypt the message contained in the packets.