Self-interference cancellation

Transmitting and receiving on exactly the same frequency at exactly the same time has multiple purposes. In-band full duplex can potentially double spectral efficiency. It permits true full duplex operation where only a single frequency is available. And it enables “listen while talking” operation (see cognitive radio, below).

Though most small cells are expected to be fed using fiber optic cable, running fiber isn't always practical. Reuse of the frequencies used by a small cell to communicate with users (“access”) for communication between the small cell and the network (“backhaul”) will be part of the 3GPP's 5G standards. When implemented using SIC, the local backhaul radio's transmit signal is cancelled out at the small cell's receiver, and the small cell's transmit signal is cancelled out at the local backhaul radio's receiver. No changes are required to the users’ devices or the remote backhaul radio. The use of SIC in this applications has been successfully field-tested by Telecom Italia Mobile and Deutsche Telekom.[5][6]

SIC enables satellite repeaters to extend coverage to indoor, urban canyon, and other locations by reusing the same frequencies. This type of repeater is essentially two radios connected back-to-back. One radio faces the satellite, while the other radio faces the area not in direct coverage. The two radios relay the signals (rather than store-and-forward data bits) and must be isolated from each other to prevent feedback. The satellite-facing radio listens to the satellite and must be isolated from the transmitter repeating the signal. Likewise, the indoor-facing radio listens for indoor users and must be isolated from the transmitter repeating their signals to the satellite. SIC may be used to cancel out each radio's transmit signal at the other radio's receiver.

Cable networks have traditionally allocated most of their capacity to downstream transmissions. The recent growth in user-generated content calls for more upstream capacity. Cable Labs developed the Full Duplex DOCSIS 3.1 standard to enable symmetrical service at speeds up to 10 Gbit/s in each direction. In DOCSIS 3.1, different frequencies are allocated for upstream and downstream transmissions, separated by a guard band. Full Duplex DOCSIS establishes a new band allowing a mix of upstream and downstream channels on adjacent channels. The headend must support simultaneous transmission and reception across the full duplex band, which requires SIC technology. The cable modems are not required to transmit and receive on the same channels simultaneously, but they are required to use different combinations of upstream and downstream channels as instructed by the headend.[7]

Mesh networks are used to extend coverage (to cover entire homes) and for ad-hoc networking (emergency communication). Wireless mesh networks use a mesh topology to provide the desired coverage. The data travels from one node to another until it reaches its destination. In mesh networks using a single frequency, the data is typically store-and-forwarded, with each hop adding a delay. SIC can enable wireless mesh nodes to reuse frequencies so that the data is retransmitted (relayed) as it is received. In mesh networks using multiple frequencies, such as whole-home Wi-Fi networks using “tri-band” routers, SIC can enable greater flexibility in channel selection. Tri-band routers have one 2.4 GHz and one 5 GHz radio to communicate with client devices, and a second 5 GHz radio that is used exclusively for internode communication. Most tri-band routers use the same pair of 80 MHz channels (at opposite ends of the 5 GHz band) to minimize interference. SIC can allow tri-band routers to use any of the six 80-MHz channels in the 5 GHz band for coordination both within networks and between neighboring networks.

The military frequently requires multiple, high power radios on the same air, land, or sea platform for tactical communication. These radios must be reliable even in the face of interference and enemy jamming. SIC enables multiple radios to operate on the same platform at the same time. SIC also has potential applications in military and vehicular radar, allowing radar systems to transmit and receive continuously rather than constantly switching between transmit and receive, yielding higher resolution. These new capabilities have been recognized as a potential 'superpower' for armed forces that may bring about a paradigm shift in tactical communications and electronic warfare.[8][9]

National regulatory agencies, such as the Federal Communications Commission in the U.S., often address the need for more spectrum resources by permitting sharing of underutilized spectrum. For instance, billions of Wi-Fi and Bluetooth devices compete for access to the ISM bands. Smartphones, Wi-Fi routers, and smart home hubs frequently support Wi-Fi, Bluetooth, and other wireless technologies in the same device. SIC technology enables these devices to operate two radios in the same band at the same time. Spectrum sharing is a topic of great interest to the mobile phone industry as it begins to deploy 5G systems.

Radios that dynamically select idle channels to make more efficient use of finite spectrum resources are the subject of considerable research. Traditional spectrum sharing schemes rely on Listen-before-talk protocols. However, when two or more radios choose to transmit on the same channel at the same time there is a collision. Collisions take time to detect and resolve. SIC enables listen-while-talking, ensuring immediate detection and faster resolution of collisions.[10]