A Novel Transmit Array Structure for Optical Spatial Modulation
The performance of multiple input single output (MISO) and multiple input multiple output (MIMO) systems is limited by spatial channel correlation. This limitation becomes particularly severe in light lidelity (LiFi) systems, which use intensity modulation/direct detection (IM/DD) for data transmission, because light emitting diodes (LEDs) can only emit incoherent light. This paper proposes a novel transmitter structure that provides spatially separated channels and enables optical MISO/MIMO spatial modulation (SM) transmission without the need for a power allocation algorithm or a transmit precoding technique. A single LED array, which consists of multiple LEDs with different characteristics, is considered in an indoor environment. Different numbers of transmit LEDs are chosen for SM transmission based on the relation between channel correlation and bit error probability. It is shown that the proposed structure provides reliable SM transmission when the modulation order for M-ary pulse amplitude modulation (PAM) is considered as 2 and 4. Computer simulations also show that higher modulation order, e.g., 8-PAM, can be supported but at a slightly degraded bit error ratio (BER) performance.