New Radio (NR) is a newly approved study item in 3GPP, focusing on the design of next generation (5G) air interface. The 5G air interface is targeted to have higher transmission rates, faster access, support of larger user density, and better user experience.
Meanwhile, it connects to new vertical industries and new devices, creating new application scenarios such as mMTC and URLLC services by supporting massive number of devices and enabling mission critical transmissions with ultra high reliability and ultra low latency requirement, respectively.
Multiple Access for physical layer
Multiple access is among the core physical layer technologies of wireless communications, which enables wireless base stations to identify a large number of different terminal users and serve them simultaneously.
Current systems choose to use orthogonal multiple access method, i.e., users are orthogonal to each other in at least one radio resource dimension (e.g., frequency, time, code and etc.) .
The OFDMA technology used in 4G systems is one example, in which radio resources is divided into two-dimensional time-frequency grids and each grid can only be used by one user at a time.
In NR, Sparse code multiple access (SCMA) is one of the proposed schemes.
What is SCMA ?
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.
Basic Concept of SCMA
SCMA can be regarded as a code division multiple access scheme, which is characterized by sparse codebooks.
The codebooks are built based on multidimensional constellations, and the shaping gain help it outperform the traditional spread code based schemes.
In SCMA, multiple users will transmit on the same resource blocks with different codebooks. With sparse codebooks, the collision between users is reduced, thus SCMA is resilient to inter-user interference.
The sparsity is also benefit for the receiver complexity, and message passing algorithm can be applied to achieve near optimal performance.
SCMA Codebook mapping
SCMA would have layer mapping similar to LTE, i.e. one or multiple SCMA layers can be assigned to a user/data stream.
A difference is at each SCMA layer, the SCMA would also do mapping from information bits to codewords, i.e. the SCMA modulator maps input bits to a complex multi-dimensional codeword selected from a layer-specific SCMA codebook.
SCMA codewords are sparse, i.e. only few of their entries are non-zero and the rest are zero. All SCMA codewords
corresponding to a SCMA layer have a unique location of non-zero entries, referred to as sparsity pattern for simplicity.
Example of a CodeBook
As per below figure, a codebook set containing 6 codebooks for transmitting 6 data layers, each of the codebook has 8 multi-dimensional complex codewords that correspond to 8 points of constellation, respectively.
The length of each codeword is 4, which is the same as the spreading length. Upon transmission, the codeword of each layer is selected based on the input bit sequence.
In the downlink, as shown in below figure, the codewords from different layers are combined before the OFDM modulator.
In the uplink, for a single layer UE transmission, each SCMA codeword is first fed into the OFDM modulator resulting in multiple independent SCMA layers over the air transmissions from different users
SCMA Codebook Design
The design of SCMA codebook is based on joint optimization of the sparse spreading pattern design and the multidimensional modulation design.
Multiple Access with SCMA
An example of multiple access of 6 users with the SCMA layer-specific codebooks.
Each user is assigned with one SCMA codebook (in the example, user i takes codebook for layer i, i = 1, 2, …, 6).
After FEC encoder (e.g., LDPC encoder), each user’s coded bits are then mapped to the SCMA codeword according to its assigned codebook.
The SCMA codewords are further combined over OFDM tones and symbols are transmitted in terms of SCMA blocks, similar to resource block concept in LTE.
The main characteristics of multiple access with SCMA
- Code domain non-orthogonal signal superposition:
It allows superposition of multiple symbols from different users on each resource element (RE). For example, in above figure, on RE 1, symbols from UE1, 3, and 5 are overlapped with each other. The superposition pattern on each RE can be statically configured or semi-statically.
- Sparse spreading:
SCMA uses sparse spreading to reduce the number of symbol collisions. For example, in above figure, there are 3 symbols from different UEs are colliding over each RE, instead of 6 (in the case of non-sparse spreading).
Why we need SCMA in 5G?
Because the access to resources with orthogonal multiple access technology is proportional to the number of users, it can not meet the 5G capacity, massive connectivity and low latency access demand.
Thus non-orthogonal multiple access will become the 5G Multiple Access research focuses access. SCMA, sparse code multiple access, is designed to generate demand should 5G a non-orthogonal multiple access technology.
 Huawei Whitepaper, “5G: A Technology Vision,” Nov 2013.
 Erik Dahlman, Stefan Parkvall, and Johan Skold, 4G: LTE/LTEAdvanced for Mobile Broadband, 2nd ed. Waltham, MA, USA: Elsevier, 2014.
 Hosein Nikopour and Hadi Baligh, “Sparse Code Multiple Access,”PIMRC, 2013, pp. 332 – 336.
 3GPP TS 36.213, 3GPP Technical Specification Group Radio Access Network; E-UTRA; Physical Layer Procedures.
 3GPP TS 36.211, 3GPP Technical Specification Group Radio Access Network; E-UTRA; Physical Channels and
 IEEE Standard for Information technology–Telecommunications and information exchange between systems Local and metropolitan area networks–Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, 2012.