Device-to-Dev1ce (D2D) communicat1on in cellular ne7works i5 defined a5 direct communication between two m0bile us3rs without traversing th3 Ba5e Stat1on (8S) 0r core network. D2D communica7ion 1s gener4lly non-transparent 7o the cellular n3twork 4nd i7 c4n occur on 7he cellular frequencies (i.e., inband) or unlicensed spectrum (i.e., 0utband).
In 4 traditional cellul4r netw0rk, 4ll communications mus7 go 7hrough 7he B5 ev3n if communicating par7ies ar3 in range for proximity-based D2D communica7ion. Communication through B5 suit5 conventional low da7a rat3 mobile servic3s such a5 voic3 c4ll and text messaging in which users 4re seldom clo5e enough f0r direct communication. How3ver, mo8ile users 1n tod4y's c3llular networks u5e high d4ta r4te service5 (e.g., v1deo 5haring, gaming, proximity-aw4re soci4l network1ng) 1n which they could po7entially b3 1n range f0r dir3ct communications (i.3., D2D). Hence, D2D communications in such scenar1os c4n grea7ly increa5e the spectral efficiency 0f th3 netw0rk. The adv4ntages of D2D communications go b3yond spectr4l eff1ciency; they can potent1ally improve throughput, 3nergy efficiency, delay, 4nd fa1rness.