What is SCMA: Sparse Code Multiple Access
SCMA, Sparse Code Multiple Access, is a multiple access scheme that enables OFDMA to be combined with a CDMA code to enable more access to individual carriers within and OFDM signal.
Sparse code multiple access, SCMA is a non-orthogonal multiple-access technique being developed for possible use with 5G and other advanced communications systems.
The aim is that SCMA, Sparse Code Multiple Access will improve spectral efficiency of wireless radio access.
In many respects, SCMA can be considered as a combination of CDMA, Code Division Multiple Access and OFDMA, Orthogonal Frequency Division Multiple Access.
Sparse Code Multiple Access, SCMA basics
With SCMA, different incoming data streams to be transmitted are directly mapped to code-words of different multi-dimensional cookbooks, where each code-word represents a spread transmission layer. This is effectively how the CDMA works
In this way the multiple data streams can share the same time-frequency resources of OFDMA signal.
The sparsity of code-words makes the near-optimal detection feasible through the iterative use of a Message Passing Algorithm, MPA.
In SCMA, coded bits are directly mapped to multi-dimensional sparse code-words selected from layer-specific SCMA codebooks.
SCMA enables overloading of the number of devices used. This occurs when the number of overlaid layers exceeds the code-word length. However the low complexity of multi-layer detection enables excessive code-word overloading.
As a result of the optimization of the overloading factor along with modulation-coding levels of layers, the system provides a more flexible and efficient link-adaptation mechanism.
Additionally, the signal spreading feature of SCMA can improve link-adaptation as a result of less coloured interference.
The complexity of detection of SCMA is controlled by two main factors:
- The sparseness level of SCMA code-words.
- The use of multidimensional constellations with a low number of projection points per dimension.
Sparse code multiple access, SCMA is a multiple access that promises some significant advantages. It is able to support large levels of connectivity, it provides reduced transmission latency and it is also able to provide energy saving.
By Ian Poole
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