The growth of mobile broadband services is phenomenal in recent years, with mobile traffic doubling almost every year. Currently, the industry, research community, and government agencies are working together to push the next generation wireless communication system to be up to 1000 times faster. For instance, the 5G system is expected to offer 1000 times data rate and much smaller latency, compared with the 4G system. Such an ambitious goal relies on significant advances of the underlying technologies, among which efficient spectrum sharing plays a key role. To enable efficient spectrum sharing, an effective approach is to provide incentives to licensed users to compensate the interference from unlicensed users. With sufficient incentives, licensed users are willingly cooperative in spectrum sharing. This can be done at either network layer or physical layer.
The network layer approach is to utilize the network coding technique to piggyback packets from unlicensed users onto the licensed user packets to achieve simultaneous transmissions. Specifically, unlicensed users serve as relays between licensed users that are not connected or connected with weak links, to increase licensed user’s throughput and reduce their packet delay. Furthermore, an unlicensed user can utilize network coding to encode one or multiple packets from unlicensed users with one licensed user’s packet, and send out the coded packet through one transmission. The physical layer approach is to split the transmission power of unlicensed user to accomplish spectrum co-access between licensed users and unlicensed users. Specifically, an unlicensed user’s transmitter splits its transmission power into two portions, with one portion to transmit its own packet, and the other portion to transmit the licensed user’s packet. As a result, the signal power at the licensed user receiver is increased, and the achievable data rate of the licensed user can be maintained at the same level or even higher level compared with the case without the simultaneous transmission from the unlicensed user. Meanwhile, the unlicensed user can utilize dirty paper coding to cancel the interference from the licensed user’s transmission. With new radio and network technologies, we envision that new spectrum co-access approaches can be designed accordingly to exploit the progresses in these fields.