Asynchronous Massive Access and Neighbor Discovery Using OFDMA

The fundamental communication problem in the wireless Internet-of-Things (IoT) is to discover a massive number of devices and to provide them with reliable access to shared channels. Oftentimes these devices transmit short messages randomly and sporadically. This paper proposes a novel signaling scheme for grant-free massive access, where each device encodes its identity and/or information in a sparse set of tones. Such transmissions are implemented in the form of orthogonal frequency-division multiple access (OFDMA). Under some mild conditions and assuming device delays to be bounded unknown multiples of sampling intervals, sparse OFDMA is proved to enable arbitrarily reliable asynchronous device identification and message decoding with a codelength that is O(K(logK + log S + logN)), where N denotes the device population, K denotes the actual number of active devices, and log S is essentially equal to the number of information bits each device can send. The computational complexity for discovery and decoding can be made to be O(K(logK)(logK + logS + logN) + K2 logK). As a proof of concept, a specific design is proposed to identify up to 200 active devices out of N = 296 possible devices with up to 20 samples of delay, moderate signalto-noise ratios, and fading. If the device population is N = 248 instead, each active device can also transmit 48 bits to the access point at the same time. The codelength compares much more favorably with those of standard slotted ALOHA and carrier-sensing multiple access (CSMA) schemes.