Spatial modulation

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In Spatial modulation, modulation is done over space, across different antennas at the transmitter [1][2]. 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[3]. 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[4].

Illustration

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 [5]. 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.

Table 1: SM mapping table for NT=2 and M=2 (BPSK)
Incoming bits Index for antenna Symbol transmitted
00 1 1
01 1 -1
10 2 1
11 2 -1

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 [6] .

Advanced Spatial Modulation

In order to improve the spectral efficiency, SM has been modified to various advanced SM schemes[7].:

• Quadrature Spatial modulation[8]
• Improved Spatial modulation[9]
• Generalized Spatial modulation[10]
• Spatial media Based modulation[11]
• Enhanced Spatial Modulation[12], 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.

References

1. Mesleh, R.Y.; Haas, H.; Sinanovic, S; Ahn, C.W.; Yun, S (15 July 2008). "Spatial Modulation". IEEE Transactions on Vehicular Technology. 57 (4): 2228–2241. doi:10.1109/TVT.2007.912136.
2. Mesleh, Raed; Abdelhamid, Alhassi (May 2018). Space Modulation Techniques. John Wiley & Sons Inc. ISBN 9781119375654.
3. Kumbhani, Brijesh; Kshetrimayum, Rakhesh Singh (June 2017). "Spatial Modulation". MIMO Wireless Communications Over Generalized Fading Channels. Boca Raton, Florida: CRC Press. p. 267. ISBN 9781138033009.
4. Kshetrimayum, Rakhesh Singh (July 2017). "Antenna Selection and Spatial Modulation". Fundamentals of MIMO Wireless Communications. Cambridge, UK: Cambridge University Press. p. 348. ISBN 9781108415699.
5. Fu, Yu; Wang, Cheng-Xiang; Mesleh, Raed; Cheng, Xiang; Haas, Harald; He, Yejun. "A Performance Study of Spatial Modulation Systems Under Vehicle-to-Vehicle Channel Models". 2014 IEEE 79th Vehicular Technology Conference (VTC Spring). Seoul, South Korea: IEEE.
6. Renzo, M. D.; Haas, H.; Ghrayeb, A.; Sugiura, S.; Hanjo, L. (17 December 2013). "Spatial Modulation for Generalized MIMO: Challenges, Opportunities, and Implementation". Proceedings of the IEEE. 102 (1): 56–103. doi:10.1109/JPROC.2013.2287851.
7. Bhowal, Anirban; Kshetrimayum, Rakhesh Singh (Dec 2020). Advanced Spatial Modulation Systems. Singapore: Springer Nature. p. 229. ISBN 978-981-15-9959-0.
8. Mesleh, R.Y. (30 July 2014). "Quadrature Spatial Modulation". IEEE Transactions on Vehicular Technology. 64 (6): 2738–2742. doi:10.1109/TVT.2014.2344036.
9. Luna-Rivera, J.M.; Gonzalez-Perez, M.G. "An improved spatial modulation scheme for MIMO channels". 2012 European Conference on Antennas and Propagation (EUCAP). Prague, Czech Republic: IEEE.
10. A., Younis; Serafimovski, N.; Mesleh, R.; Haas, H. "Generalised spatial modulation". 2010 Conference Record of the Forty Fourth Asilomar Conference on Signals, Systems and Computers. Pacific Grove, CA, USA: IEEE.
11. Khandani, A.K. "Media-based modulation: A new approach to wireless transmission". 2013 IEEE International Symposium on Information Theory. Istanbul, Turkey: IEEE.
12. Cheng, C.-C.; Sari, H.; Sezginer, S.; Su, Y.T. (13 April 2015). "Enhanced Spatial Modulation With Multiple Signal Constellations". IEEE Transactions on Communications. 63 (3): 2237–2248. doi:10.1109/TCOMM.2015.2422306.