Non-Orthogonal Multiple Access (NOMA) is an advanced multiple access technique used in wireless communication systems to improve spectral efficiency and increase the number of users that can be supported simultaneously in a given bandwidth. NOMA is considered a promising technology to address the ever-growing demand for high data rates and massive connectivity in the era of 5G and beyond.
In contrast to Orthogonal Multiple Access (OMA) techniques such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), and Code Division Multiple Access (CDMA), which allocate separate frequency, time, or code resources to different users, NOMA allows multiple users to share the same resources by exploiting the power domain and/or code domain.
There are two main types of NOMA:
- Power-domain NOMA: This type of NOMA uses superposition coding at the transmitter and successive interference cancellation (SIC) at the receiver. Superposition coding combines multiple users' signals into a single signal, which is then transmitted over the channel. The receivers decode the combined signal using SIC, which involves decoding the strongest user's signal first, then subtracting it from the combined signal and decoding the next strongest user's signal, and so on. This process continues until all user signals are decoded. Power-domain NOMA can achieve high spectral efficiency by allocating different power levels to users based on their channel conditions.
- Code-domain NOMA: This type of NOMA uses non-orthogonal spreading codes to multiplex users' signals in the code domain. Unlike CDMA, which uses orthogonal codes to separate users, code-domain NOMA allows users to share the same spreading codes with different code rates. Code-domain NOMA techniques include Sparse Code Multiple Access (SCMA), Pattern Division Multiple Access (PDMA), and Lattice Partition Multiple Access (LPMA). These techniques can support a large number of users with low complexity and high spectral efficiency.
NOMA provides several benefits, including:
- Improved spectral efficiency: By allowing multiple users to share the same resources, NOMA can provide higher spectral efficiency compared to OMA techniques, which is crucial for meeting the requirements of 5G and beyond.
- Massive connectivity: NOMA can support a large number of users simultaneously, making it suitable for applications like the Internet of Things (IoT), where a vast number of devices need to be connected.
- Enhanced fairness: NOMA can provide better fairness among users by allocating power or code resources according to users' channel conditions and quality of service requirements.
However, there are also some challenges associated with NOMA, such as increased complexity at the receiver, inter-user interference, and the need for accurate channel state information. Despite these challenges, NOMA has been widely investigated as a key technology to meet the requirements of future wireless communication systems.