Chapter 8
Summary and Future Directions
8.1 Summary
Space modulation techniques (SMTs) were presented, analyzed, and thoroughly discussed in this book. The book presented a comprehensive overview of different SMTs and analyzes their performance. SMTs are new and unique multiple‐input multiple‐output (MIMO) wireless communication systems. Most of the existing analysis in literature cannot be directly applied to SMTs. It is shown in this book that SMTs need different system designs for both transmitter and receiver. Also, the performance analysis for these systems requires different studies, which go beyond existing theory. In SMTs, new constellation diagram is defined and used to convey information. In previous theory of MIMO systems, multipath propagation channel is used to increase the channel capacity of wireless systems. Uncorrelated channel paths from different transmit antennas are utilized to transmit independent and cochannel data streams to increase the data rate. Such schemes generally achieve spatial multiplexing gain and they are called spatial multiplexing (SMX) MIMO systems. SMX systems suffer from inter‐channel interference (ICI), which degrades their error performance and complicates the receiver design. Also, each transmit antenna must be driven by a complete radio frequency (RF) chain including IQ modulators, filters, power amplifier (PA), and others. These components are generally very expensive and produce good amount of heat, which require proper cooling. Also, transmit antennas must be synchronized to start the transmission simultaneously.
In SMTs, the MIMO channel is utilized in a different manner, which relaxes all previous SMX limitations. SMTs consider the multipath propagation channel as a spatial constellation diagram and use the uncorrelated channel paths from each transmit antenna to all receive antennas as constellation points in the spatial constellation diagram. As such, SMX gain is achieved, which depends on the number of available symbols in the spatial domain. Thus, in SMTs, data bits can be transmitted through two domains, spatial and signal domains. Some SMTs as space shift keying (SSK) and generalized space shift keying (GSSK) use only the spatial domain. Other SMTs as spatial modulation (SM) and quadrature spatial modulation (QSM) use both signal and constellation domains. However, in all SMTs, single‐RF carrier signal is transmitted, and the transmitted carrier is either modulated or unmodulated. Hence, a maximum of one RF‐chain is needed to implement the transmitter of any SMT. It was demonstrated in this book that SMTs transmitting unmodulated carrier signal, using only the spatial domain, require no RF‐chain and can be implemented through RF switches and some other basic components. Thereby, a simple and low‐cost implementation of SMTs is very feasible through simple RF components such as RF switches, RF couplers, RF splitters, and RF combiners. The book evaluated the performance of different SMTs in terms of energy efficiency, implementation cost, receiver computational complexity, and average bit error ratio (ABER) and compared it to SMX. It was reported that SMTs can be designed to achieve much better performance in all these metrics as compared to SMX.
Considering the working mechanism of SMTs, analyzing their performance in different environments and over variant channel conditions is not trivial and witnessed tremendous studies in literature. The book presented a detailed performance analyses for SMTs over Rayleigh fading channel with perfect and imperfect channel knowledge at the receiver. Also, a general analytical framework for the ABER performance of SMTs was developed and shown to be accurate. Noncoherent SMTs ABER performance analyses were also discussed and developed.
In addition, the book presented the capacity analysis of SMTs and discussed the conditions under which capacity can be achieved. Again and similar to bit error analysis, SMTs are unique wireless communication systems and their capacity cannot be derived by applying existing theory. It was demonstrated in this book that the derivation of SMTs capacity should consider both spatial and signal constellation diagrams. It was revealed that the capacity can be achieved if the multiplication of the signal symbol by the spatial symbol follows a complex Gaussian distribution. This means that for a given channel distribution, the signal symbols should be shaped such that their multiplication with the spatial symbols is complex Gaussian distributed. These novel results were validated through different analysis, and it is a major contribution of this book.
The adoption of SMTs in two emerging technologies, millimeter‐wave (mmWave) and cooperative communications, was presented as well in the book. mmWave is an auspicious technology for future wireless systems. Applying SMTs in mmWave systems promises significant enhancements as reported in the book. Detailed performance analysis and results were presented and discussed. Also, cooperative communications were a key‐enabling technology for fourth‐generation (4G) wireless standard, and it will play a significant role in fifth‐generation (5G) and beyond systems. SMTs promise significant enhancements when considered in cooperative communication. Detailed system design and performance analysis for different cooperative topologies were presented and discoursed.
8.2 Future Directions
SMTs attracted significant research interests in different areas. SMTs were developed almost 10 years ago, and many emerging technologies are considering the adoption of SMTs at the moment. Hereinafter, we will summarize some possible future directions and interesting applications of SMTs.
8.2.1 SMTs with Reconfigurable Antennas (RAs)
An antenna is reconfigurable if it is possible to change its frequency, polarization or radiation characteristics. This is attained through variant techniques, which redistribute the antenna current to modify the electromagnetic fields of its effective aperture. Reconfigurable antennas (RAs) witnessed significant research interest very recently to accommodate sophisticated system requirements by modifying their geometry and electrical behavior. Such modifications are generally performed to adapt to anticipated changes in environmental conditions or system requirements [287]. RA promises several advantages including reduced number of hardware components, complexity, and cost.
The use of RA to implement SMTs was first proposed in [62] with the aim to enhance the spectral efficiency and to reduce the implementation complexity. As discussed in this book, the implementation of SMTs can be easily facilitated through RF switches, which can be easily deployed using RA. In [62], RA are considered to provide a low complexity implementation of SM system. This is achieved by modifying the radiation patterns of RA based on the sequence of incoming data bits. Each incoming data bits sequence modulates a certain polarization parameters that produces unique radiation pattern. The transmitted radiation pattern is detected at the receiver side and used to estimate the transmitted data bits. The spectral efficiency of such scheme depends on the number of possible radiation patterns, and the probability of error is determined by the uniqueness of such patterns. It is important to note that different radiation patterns will be received as a unique signal due to interaction with the available scatterers in the propagation medium [61]. Therefore, RA can be used to create a new constellation dimension and can be combined with other signal and spatial dimensions to enhance the overall spectral efficiency.
Recently, an implementation of a communication system utilizing RA for SM is proposed in [288]. The RA are based on a meander line radiating element surrounded by two L‐shaped wire resonators connected to a metallic ground plane with two PIN diodes. The design system allows for a generation of four radiation patterns through switching the PIN diodes. It is reported that the cross correlation among different patterns is in the range of 11–80%. Other antenna geometries are proposed in order to increase the number of radiation patterns that can be generated while still preserving good correlation properties for practical communication applications.
The world first testbed that uses RA to realize SM is proposed in [289]. Novel RA designs are considered to encode the information bit stream and their different energy patterns are visualized with the aid of an innovative radio wave display that is capable of measuring the received power. The display clearly shows that distinct received energy patterns are obtained when different radiation patterns of RA are activated at the transmitter. Finally, in [290], RA are used to enhance the performance of SSK system over Rician fading channels.
8.2.2 Practical Implementation of SMTs
So far, intensive theoretical analysis of SMTs have been conducted in literature. However, a major asset for SMTs is the promise to simplify hardware designs, which promise significant enhancements in energy consumption, computational power, and hardware cost. Yet, hardware designs and practical implementation of SMTs are not yet addressed in literature. Some attempts are conducted utilizing existing hardware components, which are not tailored to the specific working mechanism of SMTs. In [39], a hardware testbed for a 
SM MIMO system is developed using National Instruments (NI) PXI MIMO platform. Recently, Mesleh et al. [40] proposed transmitter designs for different SMTs with minimum number of RF chains. It is, also, reported in this book that SMTs based on this optimum design can achieve significant enhancements as compared to SMX MIMO systems. However, the impact of several hardware blocks on the performance of SMTs are yet to be studied. Some recent attempts in this direction were reported where the impact of IQ imbalance on the performance of QSM system is analyzed in [77]. Furthermore, the impact of antenna switching time on the performance of SM system is studied in [156, 291].
Therefore, the investigation of the potential of SMTs via practical implementation testbeds and under real‐time channel conditions is very much needed to demonstrate their true potential. This is a significant research direction for future studies.
8.2.3 Index Modulation and SMTs
Index modulation (IM) techniques are developed based on the concept of SMTs. They have witnessed significant research interest in literature. However, their full potential is not yet explored. Noncoherent techniques for IM are to be developed. Also, IM techniques can be deployed with single transmit antennas by modulating the orthogonal frequency division multiplexing (OFDM) subcarriers. Hence, combining them with different SMTs might lead to significant enhancements in terms of spectral and energy efficiencies. The impact of different imperfections such as frequency offset, peak to average power ratio (PARP), frequency, and timing errors are to be investigated and analyzed.
8.2.4 SMTs for Optical Wireless Communications
Optical wireless communications (OWC) is another promising technology for future wireless systems [107–110]. It utilizes a huge portion of an unlicensed spectrum that promises an immense increase in the data rate. Indoor systems are generally called visible light communications whereas outdoor applications are named free space optics. Transmitting wireless signals over optical carriers is not trivial, and specific modulation techniques can only be considered. In optical systems, the intensity of the light is modulated, which must be real and unipolar and quadrature modulation is not possible. For instant, SSK system can be directly applied to OWC. Other SMTs can be applied in conjunction with multicarrier communication such as OFDM [99–104]. However, the major challenge in applying SMTs in OWC is the high spatial correlation among different channel paths from different transmit light sources to multiple receivers. Uncorrelated channel paths can be created by properly spacing transmitters and receivers, such that the spacing is larger than the fading correlation length. In particular, for OWC, the total length of the transmitters must not exceed the capture zone of the receiver. However, designing such an OWC setup may not be feasible in practical systems, since the available spacing among different transmitters may not be adequate for these requirements. A solution that attracted significant interest in literature to decorrelate the optical wireless (OW) MIMO channel paths is the imaging receiver. Imaging receivers are shown to be efficient in eliminating the effects of ambient light noise, cochannel interference, and multipath distortion. Such advantages are attained through allowing signals arriving from certain angles to be passed and all other signals are discarded.
Designing an OW system utilizing all advantages of SMTs and achieving good performance is still an open research question that needs to be studied and is a very interesting topic for future research.