SCMA Uplink Decoding With Codebook Collision

Sparse code multiple access (SCMA) is a non-orthogonal multiplexing technique proposed to the 5G mobile communication system, which can improve the system performance in massive connectivity scenarios. However, during the random access phase of the SCMA system, the codebook used by a user is unknown to the base station. It is very likely some of the users will choose the same codebook in the same time-frequency resource. Therefore all the data in this resource block will be corrupted and can not be recovered. This paper proposes a new method to deal with collision situation by dividing the decoding process into two parts. First, we use a short pilot to detect the codebook used by each user in an SCMA system. Then, with the knowledge of codebook usage, the decoding of actual user data can be simplified. Using the proposed method, codebook collision can be detected. Although the conflicted users can't be decoded by the receiver, other users using the same resource block can still be recovered with relatively low performance penalty. The random access protocol we propose here is called SCMA-Aloha. Its peak throughput can be 20 percent higher than traditional slotted-Aloha.