SCMA Sparse Code Multiple Access

SCMA, which stands for Sparse Code Multiple Access, is a novel multiple access technique used in wireless communication systems. It is designed to improve spectral efficiency, increase the number of connected devices, and enhance overall system capacity.

To understand SCMA, let's break down its key concepts:

1. Multiple Access Technique: Multiple access techniques allow multiple users to share the same frequency band simultaneously, enabling simultaneous communication between multiple devices in a wireless network. Traditional multiple access techniques include Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and Orthogonal Frequency Division Multiple Access (OFDMA).
2. Sparse Code Multiple Access (SCMA): SCMA is a type of non-orthogonal multiple access (NOMA) technique. Unlike orthogonal multiple access techniques like FDMA and TDMA, NOMA techniques allow for overlapping signals in the frequency, time, or code domains. SCMA specifically focuses on code domain overlapping.
3. Sparse Code: Sparse code refers to a particular type of codebook design used in SCMA. The codebook consists of a set of codewords, each representing a unique combination of user assignments. The codewords are designed in such a way that only a few users are assigned to each codeword, resulting in sparsity.
4. Sparsity: Sparsity is a key property of SCMA that enables efficient resource allocation. In SCMA, the number of active users in a particular codebook is relatively small compared to the total number of users in the system. This sparsity allows for a higher number of simultaneous connections and efficient utilization of system resources.

Now let's delve into the operation of SCMA:

1. Codebook Generation: The first step in implementing SCMA is the generation of a sparse codebook. The codebook is designed in a way that ensures the codewords have a low correlation with each other. This property helps minimize interference between users sharing the same resources.
2. User Assignment: Each user in the SCMA system is assigned a unique combination of codewords from the codebook. The assignment is performed based on the available channel conditions, user priorities, or other resource allocation algorithms. The goal is to maximize the system capacity while satisfying the quality-of-service requirements of individual users.
3. Resource Mapping: After user assignment, the assigned codewords are mapped onto the available physical resources, such as subcarriers in OFDMA or time slots in TDMA. The specific mapping technique depends on the underlying multiple access scheme used in conjunction with SCMA.
4. Sparse Code Transmission: When a user wants to transmit data, it encodes its information bits into the assigned codeword. The encoded codeword is then modulated and transmitted over the shared physical channel. Due to the sparsity of user assignments, multiple users can simultaneously transmit their codewords, resulting in improved spectral efficiency and increased system capacity.
5. Receiver Processing: At the receiver side, advanced signal processing techniques are employed to detect and separate the transmitted codewords. These techniques exploit the sparsity of user assignments and leverage advanced algorithms like compressed sensing, belief propagation, or iterative detection to efficiently recover the transmitted data.

Overall, SCMA provides a promising solution for enhancing the performance of wireless communication systems by efficiently managing multiple access in the code domain. By exploiting sparsity and non-orthogonality, SCMA enables higher connectivity, increased capacity, and improved spectral efficiency compared to traditional multiple access techniques.