Spatial modulation
signal transmission technique
In signal processing, spatial modulation is a technique that enables modulation over space, across different antennas at a transmitter. Unlike multiple-input and multiple-output (MIMO) wireless (where all the transmitting antennas are active and transmitting digital modulated symbols such as phase-shift keying and quadrature amplitude modulation), in spatial modulation, only a single antenna among all transmitting antennas is active and transmitting, while all other remaining transmitting antennas sit idle. The duty of the receiver is: to estimate the active antenna index at the transmitter and to decode the symbol sent by the transmitting antenna.
Both processes carry a message bit. Since only one transmitting antenna is active at a particular instant, one single RF chain for the active antenna is required, unlike MIMO systems in which NT (number of transmitting antennas) antennas are active and correspondingly NT number of RF chains are required. RF chains are costly, which makes spatial modulation (SM) much cheaper to implement. Conventional MIMO systems suffer from problems such as inter-antenna interference and transmit antenna synchronization issues because all transmitting antennas are active.
Procedure
In SM, a series of information bits come to the transmitter. The transmitter divides the incoming bits in a chunk of k+l bits, where k is an exponent of two used for deciding the antenna index from which the l bits will be transmitted after applying an M-ary transmission or modulation scheme. In fact, only l bits are transmitted practically, since the antenna index also carries information of k bits, hence in total k+l bits will be decoded at the receiver.
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