Spatial modulation is a technique that enables

modulation In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...

over space, across different

antennas (radio) In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an ...

at a

transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna (radio), antenna. The transmitter itself generates a radio frequency alternating current, which i ...

. Unlike

multiple-input and multiple-output In radio, multiple-input and multiple-output, or MIMO (), is a method for multiplying the capacity of a radio link using multiple transmission and receiving antennas to exploit multipath propagation. MIMO has become an essential element of wir ...

(MIMO) wireless (where all the transmitting antennas are active and transmitting digital modulated symbols such as

phase-shift keying Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs at a ...

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 (information theory) is: to estimate the active antenna index at the transmitter and to

decode Decoding or decode may refer to: is the process of converting code into plain text or any format that is useful for subsequent processes. Science and technology * Decoding, the reverse of encoding * Parsing, in computer science * Digital-to-analog ...

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 An RF chain is a cascade of electronic components and sub-units which may include amplifiers, filters, mixers, attenuators and detectors.Steer M., "Microwave and RF Design", Scitech Publ., Inc., N.C., 2010, also from Yes Dee Publ., India, 2016 It ...

for the active antenna is required, unlike MIMO systems in which ''N''_T (number of transmitting antennas) antennas are active and correspondingly ''N''_T 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 Information is an abstract concept that refers to that which has the power to inform. At the most fundamental level information pertains to the interpretation of that which may be sensed. Any natural process that is not completely random ...

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 An ''M''-ary transmission is a type of digital modulation where instead of transmitting one bit at a time, two or more bits are transmitted simultaneously. This type of transmission results in reduced channel bandwidth. However, sometimes, two o ...

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.

Example

An SM transmitter with ''N''_T=2 antennas uses a binary

(BPSK) modulator. In that case, the 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 carries an additional bit of information as illustrated in Table 1. An incoming message bit string 10 matches the third row in the lookup table. In

bit numbering In computing, bit numbering is the convention used to identify the bit positions in a binary number. Bit significance and indexing In computing, the least significant bit (LSB) is the bit position in a binary integer representing the binary 1 ...

, the most significant bit (MSB) is 1 and the least significant bit (LSB) is 0. The MSB indicates the transmitting antenna index while LSB indicates which BPSK symbol to transmit. If MSB=0 the first antenna will transmit the symbol. If MSB=1 then the second antenna will transmit. For LSB=0, BPSK symbol 1 will be transmitted whereas for LSB =1, BPSK symbol -1 will be transmitted. In this case, , so only one message bit is transmitted from the second antenna. The receiver decodes both the message bit as well as the active antenna index, effectively two message bits are decoded. Therefore, the

spectral efficiency Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth in a specific communication system. It is a measure of how efficiently a limited frequency spectrum is ut ...

of the SM transmitter in this case is 2 bit/s/Hz. The receiver must estimate the antenna index, as well as decode the symbol.

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 In some of the above advanced SM methods, more than one transmitting antenna is active at a time at the transmitter in order to improve spectral efficiency. SM and its advanced variants are used in

free-space optical communication Free-space optical communication (FSO) is an optical communication technology that uses light propagating in free space to wirelessly transmit data for telecommunications or computer networking. "Free space" means air, outer space, vacuum, or ...

termed as Optical spatial modulation and Advanced Optical Spatial Modulation, respectively.

References

{{reflist Radio resource management Radio technology Optical communications