In Spatial modulation, modulation is done over space, across different antennas at the transmitter . It is a MIMO wireless technique in which information is generally transmitted from a single antenna at the transmitter and that antenna index also carries information. This MIMO wireless technique is simple to implement, it requires only one Radio frequency (RF) chain unlike the conventional MIMO systems. Hence it is much cheaper to implement. It has also overcome the problems of traditional MIMO systems such as inter antenna interference and transmit antenna synchronization. In fact the above problems are non-existent for spatial modulation (SM) since a single antenna at the transmitter is active and remaining antennas sit idle. Let us consider a series of information bits are incoming to the SM transmitter. The transmitter will divide the incoming bits in a chunk of k+l bits. k which is an exponent of 2 is used for deciding the antenna index from which the l bits will will be transmitted after applying an M-ary modulation scheme. In fact, only l bits are transmitted practically, since antenna index also carries information of k bits, hence in total k+l bits will be decoded at the receiver.
It can be easily illustrated with a simple example. Assuming series of bits are incoming to the transmitter. Assume that the transmitter has 2 antennas and Binary phase shift keying (BPSK) modulator. In that case, transmitter can transmit a BPSK symbol by performing BPSK modulation, which will carry a message bit, the antenna index from which the BPSK symbol is transmitted will also carry an additional bit of information as illustrated in Table 1 . Consider the third row of table 1, in this case the input bit is 10, the most significant bit (MSB) 1 will indicate that the least significant bit (LSB) 0 will be transmitted from the antenna 2 and the LSB will be modulated using BPSK whose corresponding symbol is 1.
|Incoming bits||Index for antenna||Symbol transmitted|
The duty of the receiver is two-fold: estimate the antenna index from the symbol has been sent from the transmitter as well as decode the symbol which has been transmitted from the transmitter  .
Advanced Spatial Modulation
In order to improve the spectral efficiency, SM has been modified to various advanced SM schemes.:
- Quadrature Spatial modulation
- Improved Spatial modulation
- Generalized Spatial modulation
- Spatial media Based modulation
- Enhanced Spatial Modulation, etc.
In some of the above methods, more than one antennas are active at time so that spectral efficiency of the SM schemes can be improved.
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