SUS (semi-orthogonal user selection)

SUS, which stands for Semi-Orthogonal User Selection, is a user selection technique used in multi-user multiple-input multiple-output (MU-MIMO) systems. MU-MIMO allows simultaneous transmission to multiple users by utilizing multiple antennas at the transmitter. SUS is a method that optimizes user selection in MU-MIMO systems to maximize system capacity and improve overall performance.

Here is a detailed explanation of Semi-Orthogonal User Selection (SUS):

1. MU-MIMO Systems: In MU-MIMO systems, a transmitter equipped with multiple antennas can serve multiple users simultaneously by transmitting independent data streams to different users. This technology improves system capacity, spectral efficiency, and overall performance by exploiting spatial multiplexing and interference management techniques.
2. User Selection in MU-MIMO: User selection plays a crucial role in MU-MIMO systems as it determines the set of users to be served simultaneously. The selection process impacts system performance, capacity, and fairness. The goal is to select a subset of users that optimizes the system's spatial multiplexing gains while minimizing interference between users.
3. Orthogonal User Selection: In MU-MIMO systems, orthogonal user selection refers to the selection of users whose channels are nearly orthogonal to each other. Orthogonal users have minimal or no interference between their channels, enabling efficient spatial multiplexing without significant performance degradation. However, finding a completely orthogonal user set may be challenging in practical scenarios due to channel conditions and system constraints.
4. Semi-Orthogonal User Selection: SUS is a technique that relaxes the requirement for orthogonal user selection by allowing a limited amount of interference between users' channels. Instead of seeking completely orthogonal user sets, SUS aims to select users with semi-orthogonal channel conditions, where interference levels are manageable. This allows for a compromise between system capacity and complexity.
5. Channel Characteristics: In SUS, user selection is based on the channel characteristics between the transmitter and potential users. The channels' properties, such as channel gains, channel correlation, and interference levels, are taken into account to determine the semi-orthogonal user set. The channels are typically estimated using pilot signals transmitted by users.
6. Selection Metrics: Various metrics can be used in SUS to evaluate the suitability of users for selection. These metrics consider the trade-off between spatial multiplexing gains and interference levels. Commonly used metrics include signal-to-interference-plus-noise ratio (SINR), signal-to-interference ratio (SIR), channel capacity, or a combination of these metrics. The specific metric used depends on the system design and optimization objectives.
7. Optimization Algorithms: SUS involves optimization algorithms to select the semi-orthogonal user set that maximizes the system performance. The algorithms consider the channel characteristics, selection metrics, and system constraints to make optimal or near-optimal user selection decisions. These algorithms can be based on techniques such as greedy algorithms, exhaustive search, beamforming, or convex optimization.
8. Practical Considerations: In practical MU-MIMO systems, SUS must consider various factors, including system overhead, computational complexity, feedback limitations, and real-time implementation. The selection algorithm should strike a balance between performance improvement and implementation complexity to ensure practicality in real-world scenarios.
9. System Performance: By selecting users with semi-orthogonal channels, SUS can improve system capacity, spectral efficiency, and user throughput in MU-MIMO systems. It reduces interference between users, enabling more efficient spatial multiplexing and enhancing overall system performance.

In summary, Semi-Orthogonal User Selection (SUS) is a user selection technique used in MU-MIMO systems. SUS aims to select a subset of users with semi-orthogonal channel conditions, balancing spatial multiplexing gains and interference levels. It optimizes system performance, capacity, and throughput by considering channel characteristics, selection metrics, and system constraints. SUS strikes a compromise between full orthogonality and practical implementation considerations, enabling efficient multi-user transmission in MU-MIMO systems.