5G RSMA vs SCMA: Understanding the Differences

This page compares 5G’s RSMA and SCMA technologies, highlighting the differences between them.

RSMA stands for Resource Spread Multiple Access, while SCMA stands for Sparse Code Multiple Access. Both are multiple access techniques used in 5G wireless technology.

Figure 1 illustrates the resource allocation of both RSMA and SCMA.

As shown, both techniques spread the user signal across time and/or frequency resources.

RSMA | Resource Spread Multiple Access

There are two main types of RSMA: single-carrier RSMA and multi-carrier RSMA.

SCMA | Sparse Code Multiple Access

SCMA is based on Low-Density Signature (LDS) CDMA. It offers low-density spreading, similar to LDS CDMA, but utilizes multi-dimensional constellations, unlike LDS-CDMA.

Figure 3 depicts the block diagram of the LDS-CDMA and SCMA transmitter parts. Each user has a unique codebook that maps each of M codewords to a length N constellation. The length N constellation is extended to length L by inserting (L-N) zeros. It requires an iterative multi-user joint detection technique at the receiver.

Figure 4 shows the SCMA transmit part. Input bits to be transmitted are directly mapped to codewords and spread over multiple subcarriers. Codewords can be assigned to the same UE or different UEs.

SCMA features the following functionalities:

• Non-orthogonal: Multiplexes code layers.
• Over-Loading: Increases overall rate and connectivity.
• Sparsity: Limits receiver complexity for detection.
• Spreading: Provides robust link adaptation and coverage.
• Multi-dimensional: Uses codewords with shaping gain.

Advantages and Disadvantages of SCMA

Advantages of SCMA:

• When there are fewer users, it allows for low-complexity iterative message-passing multi-user detection.
• Users occupy the same resource blocks in a low-density way.
• Affordable low multi-user joint detection complexity.
• Less collision even for a large number of concurrent users.
• Better coverage due to spreading gain.

Disadvantages of SCMA:

• It offers a higher PAPR (Peak-to-Average Power Ratio) compared to SC-RSMA.
• It requires synchronous multiplexing.
• It offers a lack of scalability/flexibility to users needing different spreading factors.
• This technique does not exploit full diversity.