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Spatial modulation

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Spatial modulation is a technique that enables modulation over space, across different antennas (radio) at a transmitter.[1][2] Unlike multiple-input and multiple-output (MIMO) wireless (all the transmitting antennas are active and transmitting digital modulated symbols such as Phase-shift keying and Quadrature amplitude modulation), in spatial modulation technique, only a single antenna among all transmitting antennas is active and transmitting symbol, while all other remaining transmitting antennas are sitting idle. The duty of the receiver (information theory) is two fold: antenna index estimation of the active antenna at the transmitter and decoding the symbol which has been sent by the active transmitting antenna.

Both processes carries message bit. Since only one transmitting antenna is active at a particular instant of time, one single RF chain for the active antenna is required, unlike conventional 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 than MIMO wireless systems. Conventional MIMO systems suffer from problems such as inter antenna interference and transmit antenna synchronization issues[3] due to all transmitting antennas being active. The above mentioned problems are non-existent for SM, since a single antenna at the transmitter is active and remaining transmitting antennas are sitting idle.

Spatial Modulation Procedure

In spatial modulation procedure, from a series of information bits incoming to the SM transmitter, the transmitter will divide 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 antenna index also carries information of k bits, hence in total k+l bits will be decoded at the receiver[4].

Illustrative Example

SM procedure can be illustrated with an example of SM transmitter having NT=2 antennas and employing 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]. For incoming message bits 10, from the look-up table 1, it is the third row. In bit numbering, the most significant bit (MSB) is 1 and the least significant bit (LSB) is 0. MSB will decide transmitting antenna index whereas LSB will decide which BPSK symbol to transmit. If MSB=0 first antenna will transmit the symbol whereas, MSB=1 then second antenna will transmit the symbol. For LSB=0, BPSK symbol 1 will be transmitted whereas for LSB =1, BPSK symbol -1 will be transmitted. Hence, when the incoming bits are 10, second antenna will transmit BPSK symbol 1. In this case, k=l=1, in practice, only 1 message bit is transmitted from the second antenna, but receiver decodes both the message bit transmitted from the second antenna as well active antenna index of the transmitter, hence, effectively 2 message bits are decoded at the receiver. Therefore, the spectral efficiency of the SM transmitter in this case is 2 bit/s/Hz.

Table 1: SM mapping table for NT=2 and M=2 (BPSK)
Incoming bits Antenna index BPSK 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 which 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 advanced SM methods, more than one transmitting antenna is active at a time at the transmitter in order to improve spectral efficiency of the traditional SM scheme. Besides, SM and its advanced versions are also used in Free-space optical communication termed as Optical spatial modulation[13] and Advanced Optical Spatial Modulation [14] respectively.

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. S2CID 2747250.
  2. ^ Mesleh, Raed; Abdelhamid, Alhassi (May 2018). Space Modulation Techniques. John Wiley & Sons Inc. doi:10.1002/9781119375692. 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. doi:10.1109/VTCSpring.2014.7022789.
  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. S2CID 3180691.
  7. ^ Bhowal, Anirban; Kshetrimayum, Rakhesh Singh (Dec 2020). Advanced Spatial Modulation Systems. Singapore: Springer Nature. p. 229. doi:10.1007/978-981-15-9960-6. 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. S2CID 7269587.
  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. doi:10.1109/EuCAP.2012.6206031.
  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. doi:10.1109/ACSSC.2010.5757786.
  11. ^ Khandani, A.K. "Media-based modulation: A new approach to wireless transmission". 2013 IEEE International Symposium on Information Theory. Istanbul, Turkey: IEEE. doi:10.1109/ISIT.2013.6620786.
  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. S2CID 2947859.
  13. ^ Mesleh, R.Y.; Elgala, H.; Haas, H. (3 March 2011). "Optical Spatial Modulation". IEEE/OSA Journal of Optical Communications and Networking. 3 (3): 234–244. doi:10.1364/JOCN.3.000234.
  14. ^ Bhowal, A.; Kshetrimayum, R. S. (3 November 2020). "Advanced Optical Spatial Modulation Techniques for FSO Communication". IEEE Transactions on Communications. 69 (2): 1163–1174. doi:10.1109/TCOMM.2020.3035400.
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