A communication link with capability to support connections in both transmit and receive directions at the same time and over the entire frequency band is called full-duplex, or two-way. In contrast, a link that can support connection in only one direction at a time (over a given frequency band) is called one-way or half-duplex. Current wireless systems are one-way and rely on either separate time slots (time division duplex) or separate frequency bands (frequency division duplex) to transmit and to receive. These alternatives have their pros and cons, but both suffer from lack of ability to transmit and to receive concurrently over entire frequency band. Even in the context of Orthogonal Frequency Division Multiple Access (OFDMA), where different frequency tones are used to simultaneously service multiple users, there is no method known to use the OFDMA tones in opposite directions. A similar shortcoming exists in the context of Code Division Multiple Access (CDMA). Although full-duplex wireless is theoretically possible, its implementation is difficult due to an excessive amount of self-interference, i.e., the interference each transmitter generates to its own receiver(s).
Full-duplex communication is currently used in many applications, e.g., wired telephones, digital subscriber line, wireless with directional antennas, and free-space optics. The impact of full-duplex links in these earlier applications is limited to doubling the rate by providing two symmetrical pipes of data flowing in opposite directions. In contrast, in multi-user wireless systems, due to the broadcast nature of transmission (everyone hears everyone else), full-duplex capability has the potential to do more than merely doubling the rate. A summary of some of the benefits of full-duplex are as follow.
Full-duplex facilitates wireless networking. In particular, the ability to handle asynchronous users enables superimposing a half-duplex, low bit rate, low power, easy to detect network for control signaling superimposed (physical overlay, rather than logical) on top of the network of primary full-duplex data links. The superimposed links are separated from the primary full-duplex data links in the code domain, and use time multiplexing plus Carrier Sense Multiple Access (CSMA) among themselves. However, the conventional problems of CSMA are avoided as control links operate in parallel with primary full-duplex data links. The physical layer of control links is designed such that full-duplex data links can detect and cancel the interference caused by the superimposed control links.
Full-duplex enhances security through desirable jamming.
Full-duplex facilitates multi-node distributed & collaborative signaling, including realization of Network Information Theoretic setups, and cognitive wireless.
Full-duplex, through exploiting feedback, improves point-to-point throughput, and enables ultra-low power transmission.
Full-duplex doubles the point-to-point throughput.