A Comparative Analysis of Various Spatial Modulation Techniques

Abstract

Spatial modulation (SM) is an emerging digital modulation technology that fulfills the requirement of higher Spectra efficiency and high energy efficiency and has a very wide range of applications. The concept used in Spatial Modulation can further be extended to other domains such as frequency, time, angle or even across multiple domains. The basic idea behind SM is to convey extra information with the ON/OFF states of transmitting antennas. In single RF Spatial Modulation, only one antenna is activated at a time. Therefore, it requires only one RF chain which reduces the cost as compared to MIMO [1] system. In other variants of Spatial Modulation, more than one antenna may be activated at the same time. Different forms of Spatial Modulation have been described in this paper. Single RF spatial modulation, generalized spatial modulation, and differential spatial modulation. A comprehensive comparative analysis has been presented with plots of spectral efficiency against bit error rate. It has been concluded that spectral efficiency increases up to 15 times in differential spatial modulation as compared to single RF spatial modulation. Whereas, the energy efficiency increases up to 4 times in generalized spatial modulation as compared to single RF spatial modulation. Spatial modulation has low diversity gain because only one antenna is active at a time and it also requires channel state information at the receiver side for demodulation of the received signal which makes SM costly so a new technique Space-Time Block Code (STBC) is introduced in spatial modulation which increases diversity gain and reduces overall system cost because this technique does not require channel state information at receiver side for demodulation of the signal.

Introduction

I. INTRODUCTION

The demand of high data rate and high bandwidth increases with the increase in population so researchers mainly focus on other technology having ultra-high capacity, ultra-low latency, low power consumption, and massive connectivity over scarce wireless resources [2,3] to full fill requirements of people. Due to very high mobile data traffic researchers are motivated to develop new transmission technology having maximum achievable throughput and minimizing the development cost so in the beginning of 21st-century spatial modulation has been developed to full fill requirements but in the early 21st century prelim SM [4] has developed which do not gain much attention but after 2008 due to development of GSM and DSM [5], SM becomes prominent technology for wireless communication.

SM often transmits additional information bits through the index of one active transmit antenna. The active antenna index varies based on the antenna-switching mechanism in accordance with the bits of spatial information. So SM is termed as a new technique in different modulation technique which conveys additional information through the activation state of transmission antennas. Particular, only one RF chain is needed at the SM transmitter to activate one of many transmit antennas for a constellation symbol transmission. So SM considerably decreases downlink communications energy consumption and uplink communications' hardware costs at the user interface.

For high-mobility wireless communication systems, where the channel correlation is weaker but the inter-channel interference is free, SM can be a better candidate technology than Vertical Bell Laboratories Layered Space-Time (VBLAST). In GSM k antenna out of total Nt antenna are chosen for transmission of signal using spatial modulation technique .because k antenna active at a time so k RF chain are required which increase diversity gain and spectral efficiency. In DSM all Nt antenna are activated at a time and differential encoded space-time shift keying (STSK) uses Cayley Unitary Transform and conveys information by activation state of Space-Time Dispersion Matrix.

II. SINGLE RF SPATIAL MODULATION

Single RF Spatial modulation is a special case of MIMO system in which only one antenna is activated to save energy and the active antenna index is used to convey additional information based on the antenna switching mechanism[6] for example if we consider BPSK and the number of transmitter antenna 4 to convey 3 bit of information we have to transfer only 2 bit only and information of which antenna is used to transfer particular bit is to be transferred to receiver side for demodulation on signal at receiver side .antenna  

Table1 Single RF spatial modulation having 4 transmitting antenna

In single RF spatial modulation, we require information of modulation index such as one phase shift keying (PSK) / quadrature amplitude modulation (QAM) symbol and we require the index of one active antenna for each Chanel use during transmission. Spectral efficiency depends on two things modulation index and the number of transmit antenna so

 Spectral efficiency is given as.

Conclusion

In MIMO System all antenna are activated at a time so it requires large amount of power and it also require RF chain equals to number of transmitter antenna which makes system costly and complicate. SM space domain to convey activated antenna index information which is extra information without requirement of any additional bits to be transferred through channel so spectral efficiency and energy efficiency both increases in SM as compare to MIMO but there is one drawback that activated antenna must be in power of 2 eg 2,4,8,16 . In GSM we chose k antenna out of total Nt antenna to activate at a time which increases energy efficiency and in GSM there is no constrain that activated antenna must be in power of 2 . In DSM all antenna are activated at a time and differential encoded Space-Time Shift Keying (STSK) uses Cayley Unitary Transform and convey information by activation sate of space time dispersion matrix in DSM spectral efficiency is increase up to 15 time as compare to SM. Spatial modulation requires channel state information at receiver side to retrieve signal which make system costly and complicate so space time block code technique is use which don’t require channel state information to retrieve signal at receiver side. Graph in figure 4 represent increase in spectral efficency with increase in nummer of transmitting antenna. Graph is ploted using modulation index M= 4. In GSM k= NT-2 means we activate two antenna less than total transmitting antennas. eg we activate 2 antenna out of 4 and 6 antenna out of 8 transmitting antenna are activated. GSM has low spectral efficency as compare to SM but GSM has heigh energy efficency as compare to SM . In DSM spectral efficency is increase to 15 times as compare to SM.

References

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Copyright

Copyright © 2022 Rohit Singh, Dr. Ashutosh Singh, Dr. A. K. Shankhwar . This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.