Sparse Code Multiple Access - SCMA

Sparse code multiple access (SCMA) is another waveform configuration of the flexible new air interface. This non-orthogonal waveform facilitates a new multiple access scheme in which sparse codewords of multiple layers of devices are overlaid in code and power domains and carried over shared time-frequency resources. Typically, the multiplexing of multiple devices may become overloaded if the number of overlaid layers is more than the length of the multiplexed codewords.

However, with SCMA, overloading is tolerable with moderate complexity of detection thanks to the reduced size of the SCMA multi-dimensional constellation and the sparseness of SCMA codewords. In SCMA, coded bits are directly mapped to multi-dimensional sparse codewords selected from layer-specific SCMA codebooks. The complexity of detection is controlled through two major factors. One is the sparseness level of codewords, and the second is the use of multidimensional constellations with a low number of projection points per dimension [3]. An example of device multiplexing with a low projection codebook and the resulting constellation mapping is shown Figure   A device’s encoded bits are first mapped to a codeword from a codebook. In the example, a codeword of length 4 is used. The low projection codebook has a reduced constellation (from 4 points to 3 points). Furthermore, each point (e.g. “00”) has non-zero component only in one tone. A codebook with one non-zero component is a zero-PAPR codebook.

                                        SCMA multiplexing and low projection codebook constellation

Furthermore, a blind multi-device reception technique can be applied to detect device activities and the information carried by them simultaneously. With such blind detection capability, grant-free multiple access can be supported. Grant-free multiple access is a mechanism that eliminates the dynamic request and grant signaling overhead. It is an attractive solution for small packets transmission. SCMA is an enabler for grant-free multiple access. Due to these benefits,SCMA can support massive connectivity, reduce transmission latency and provide energy saving.