Spatial Modulation (SM)

Information is mapped onto two distinct constellations simultaneously. The first is the classical signal constellation (e.g., QPSK, 16-QAM), which determines the symbol transmitted. The second is the spatial constellation diagram, where each point corresponds to a specific physical transmit antenna index. This allows the system to transmit extra bits without increasing bandwidth or power.

Unlike conventional MIMO systems that require a dedicated Radio Frequency (RF) chain for every active antenna, SM activates only one antenna per symbol period. This drastically reduces hardware cost, complexity, and power consumption. A single RF chain can be switched between multiple antennas via a low-cost switch, making SM ideal for massive MIMO uplinks and low-power IoT devices.

Because only one antenna radiates at a time, there is zero inter-channel interference at the receiver. This eliminates the need for complex matrix inversion or successive interference cancellation algorithms. Detection complexity scales linearly with the number of antennas rather than exponentially, enabling low-complexity optimal Maximum Likelihood (ML) detection.

SM prioritizes Energy Efficiency (EE) over raw Spectral Efficiency (SE). By encoding information in the antenna index, SM achieves a higher data rate than Single-Input Single-Output (SISO) systems while consuming significantly less power than spatial multiplexing. The spectral efficiency grows logarithmically with the number of transmit antennas, making it a green communication enabler.

An extension of basic SM where a subset of antennas is activated simultaneously to transmit the same or different symbols. This breaks the single-antenna constraint to increase spectral efficiency while retaining partial ICI avoidance. GSM offers a flexible trade-off between the high data rates of spatial multiplexing and the energy efficiency of pure SM.

SM performance is highly sensitive to Channel State Information (CSI) accuracy. Since the spatial bit depends on distinguishing between channel paths, deep fading or spatial correlation between antennas can cause antenna index detection errors. Advanced receivers often use joint detection algorithms to estimate both the active antenna index and the transmitted symbol simultaneously.

A MIMO technique that partitions a high-rate data stream into multiple independent lower-rate streams transmitted simultaneously over different spatial paths. Unlike SM, which uses antenna indices to carry information, spatial multiplexing focuses purely on maximizing spectral efficiency by sending parallel data streams. The capacity scales linearly with the minimum number of transmit and receive antennas.

A transmit diversity technique that sends multiple copies of a data stream across different antennas and time slots. The primary goal is diversity gain—improving link reliability against fading—rather than increasing data rate. The Alamouti scheme is the classic orthogonal STBC for two transmit antennas, providing full diversity without requiring channel state information at the transmitter.

An extension of SM where a subset of multiple antennas is activated simultaneously in each transmission interval. This increases spectral efficiency by transmitting more bits through antenna index combinations while retaining SM's energy efficiency advantages. GSM bridges the gap between pure SM and full spatial multiplexing.

The optimal detection method for SM receivers that jointly estimates both the active antenna index and the transmitted symbol. MLD performs an exhaustive search over all possible antenna-symbol combinations to minimize the Euclidean distance to the received signal. While optimal, its complexity grows exponentially with the number of antennas and constellation size.

The critical preprocessing step that characterizes the propagation channel's impulse response using known pilot symbols. Accurate channel estimation is essential for SM receivers to correctly identify which antenna is active. Errors in channel state information directly degrade antenna index detection probability and overall bit error rate performance.

A beamforming technique that weights signals across multiple antennas to maximize power at the intended receiver. In SM systems, transmit precoding can be applied to shape the spatial constellation and improve the distinguishability of different antenna indices at the receiver, enhancing detection reliability in correlated channels.