Integrated sensing and communication (ISAC) has been widely recognized as a key technology in future sixth-generation (6G) wireless networks, especially for emerging applications and scenarios demanding both high-performance sensing and communication functionalities. The ISAC technique has the potential of enabling sensing and communication functionalities in a single hardware platform, even sharing the same waveform, thus, achieving the reduced cost of hardware implementation and the efficient use of spectrum resources. In this paper, we commence with the discussion on sensing and communication at the current stage from the view of motivations, applications, and challenges. Then, we provide a comprehensive survey of the state-of-the-art approaches to the ISAC technique from the waveform design perspective. To be specific, we classify the waveform design methods into three categories, namely, communication-centric waveform design, sensing-centric waveform design, and joint waveform optimization and design. In addition, potential research directions and challenges for future ISAC waveform design are outlined.

Integrated Sensing and Communication Waveform Design: A Survey

This paper develops three low-complexity detection schemes for the spatial modulation (SM) Multiple Input Multiple Output (MIMO) systems. We first propose a reduced complexity maximum likelihood (RC-ML) detection scheme. For the practical values of the modulation index, the proposed RC-ML method is simpler than the existing hard-limiting-based ML schemes. The two other proposals are based on a suboptimal Maximum Ratio Combining (MRC) detection method. We present a new analysis of the MRC detector in the uncorrelated Rayleigh fading channel and identify the system operational scenarios for which the MRC performance is nearly identical to the ML performance. Our proposals — a hybrid MRC-ML method and an MRC scheme with rank reduction — are inspired by this analysis. The analytical and simulation results show that the proposed MRC-based schemes can attain a near-optimal performance while reducing the complexity of the ML-based SM receiver.

Low Complexity Optimal and Suboptimal Detection at Spatial Modulation MIMO Receivers

In this paper, a measurement-based three-dimensional (3-D) spatial modulation (SM) system is first proposed in the existing literature. The major contributions of this work include: 1) a 3-D SM measurement system with a <inline-formula><tex-math notation="LaTeX">$\boldsymbol{16 \times 16}$</tex-math></inline-formula> virtual uniform rectangle array (URA) at the transmitter and a virtual uniform line array (ULA) with 16 elements at the receiver, 2) a comprehensive measurement activity indoors taking both line-of-sight (LOS) and none-LOS (NLOS) scenarios into account, and 3) the first measurement-based average bit error probability (ABEP) evaluation for 3-D SM system. Numerical results reveal that the widely used independent and identically distributed (i.i.d.) Rayleigh channel model significantly overestimates the performance of the SM system. Moreover, we find that the ABEP performance of 3-D SM is worse than that of the 2-D SM under the same spacing among the antenna array, while 3-D SM outperforms 2-D SM under the same size of the antenna array. 

On the Performance of 3-D Spatial Modulation Over Measured Indoor Channels

We consider quadrature spatial modulation (QSM) schemes, which achieve high spectral efficiency (SE) via the dispersion of a relatively small number P of M-ary modulated symbols over a large number of combinations of nT transmit antennas and T transmit instances. In particular, we design a new space-time block code (STBC)-based scalable QSM scheme combining high SE with maximum diversity and optimum coding gains. Deriving a closed-form expression for the optimum SE, we show that scaling the size T with nT not only is required to achieve SE optimality, but also results in further gains in bit error rate (BER) performance. Building on the latter optimal parameterization, a fully optimized scalable QSM (OS-QSM) transmitter design is then obtained by introducing a new dispersion matrix index selection algorithm that ensures even utilization of spatial-temporal resources. Finally, a new greedy boxed iterative shrinkage thresholding algorithm (GB-ISTA) QSM receiver is proposed, which exploits the inherent sparsity of QSM signals and while detecting spatially and digitally modulated bits in a greedy fashion. The resulting low complexity of the new receiver, which is linear on nT , enables the utilization of OS-QSM in systems of previously prohibitive dimensions.

Scalable Quadrature Spatial Modulation

We consider quadrature spatial modulation (QSM) schemes, which achieve high spectral efficiency (SE) via the dispersion of a relatively small number <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P$ </tex-math></inline-formula> of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M$ </tex-math></inline-formula> -ary modulated symbols over a large number of combinations of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n_{T}$ </tex-math></inline-formula> transmit antennas and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T$ </tex-math></inline-formula> transmit instances. In particular, we design a new space-time block code (STBC)-based scalable QSM scheme combining high SE with maximum diversity and optimum coding gains. Deriving a closed-form expression for the optimum SE, we show that scaling the size <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T$ </tex-math></inline-formula> with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n_{T}$ </tex-math></inline-formula> not only is required to achieve SE optimality, but also results in further gains in bit error rate (BER) performance. Building on the latter optimal parameterization, a fully optimized scalable QSM (OS-QSM) transmitter design is then obtained by introducing a new dispersion matrix index selection algorithm that ensures even utilization of spatial-temporal resources. Finally, a new greedy boxed iterative shrinkage thresholding algorithm (GB-ISTA) QSM receiver is proposed, which exploits the inherent sparsity of QSM signals and while detecting spatially and digitally modulated bits in a greedy fashion. The resulting low complexity of the new receiver, which is linear on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n_{T}$ </tex-math></inline-formula> , enables the utilization of OS-QSM in systems of previously prohibitive dimensions. 

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Many variants of spatial modulation (SM), aiming to enhance data rate and bit error rate (BER) performance, have been proposed, however, their performance has not been compared for large-scale multiple-input multiple-output (MIMO) systems. This paper looks at various such schemes, namely spatial modulation (SM), generalized spatial modulation (GSM), quadrature spatial modulation (QSM) and enhanced spatial modulation (ESM) to study their performance for large-scale MIMO systems under generalized fading conditions. Our results indicate that for the same spectral efficiency and transmit power, QSM and ESM perform better than GSM and SM schemes. For lower order modulation, QSM outperforms all other schemes under various fading conditions, whereas ESM takes over for higher order modulation. The BERs for QSM and ESM under various transmit configurations have been evaluated via extensive Monte Carlo simulations. The results reveal that in QSM, a higher number of total transmit antennas, N t , with a lower order modulation scheme provides a much better BER than a lower number of N t with a higher order modulation scheme, for the same data rate. This performance difference, however, becomes smaller in ESM, which can provide comparable performance using a higher order modulation and half the transmit antennas.

On the Performance of Spatial Modulation Schemes in Large-Scale MIMO under Correlated Nakagami Fading

Devices with full-duplex (FD) radios are able to transmit and receive at the same time without requiring orthogonal resources, thereby creating strong self-interference (SI) that results from their own transmissions. Spatial modulation (SM), on the other hand, is a multi-antenna scheme that activates only one transmit antenna to send a data symbol, where the index of the activated antenna depends on the input bits. In this manner, additional data can be conveyed implicitly via the selection of the active transmit antenna. Quadrature spatial modulation (QSM) is a variant of SM, in which the quadrature components of the constellation symbol are separately modulated on the indices of the transmit antenna, leading to an increase in the data rate by implicitly encoding on the spatial domain. This article provides an overview of the research activity on FD-SM and proposes a novel FD-QSM scheme that exploits multiple antennas to achieve antenna cancellation at the receiving side to mitigate the SI signal. Assuming active cancellation mechanisms are also in place, the performance of FD-QSM is studied in the presence of residual SI (RSI). The results reveal that FD-QSM can provide more than 40 percent capacity gain over its half-duplex (HD) counterpart in the presence of strong RSI and roughly the same gain over HD spatial multiplexing (SMX)-based multiple-input-multiple-output (MIMO) systems with moderate RSI. When applied to the downlink of a cellular network, FD-QSM provides 2dB gain over FD-SM and 5dB gain over FD-MIMO, operating at the same spectral efficiency, while huge gains are observed when FD-QSM is applied to non-orthogonal multiple access (NOMA)-aided FD relay network.

Full-Duplex Quadrature Spatial Modulation for Multi-Antenna Systems

In this paper, we propose a channel estimation scheme for spatial modulation (SM) systems. In general, SM systems require each transmit antenna to separately send pilot symbols for channel estimation. This is a lengthy process, which may incur significant channel estimation errors in a time varying channel. Thus, we propose correlation-based channel estimation (CBCE) scheme, which exploits the correlation between transmit antennas to estimate channels of inactive antennas using the pilot-based estimate of the active antenna. The change in the active channel from the last pilot-based estimate is calculated, and a time-proportionate amount of that change is scaled according to the channel correlation coefficients to estimate the channel state of the inactive channels. With pilot slots designed to spread out in a data frame, the estimation process for every channel is carried out $N_t$ times in a data frame with $N_t$ denoting the number of transmit antennas. We observe that in a high signal-to-noise ratio (SNR) regime, our proposed scheme provides about 2 dB and 5 dB gains compared to conventional channel estimation (CCE) method for moderately correlated and highly correlated antennas, respectively. Through Monte Carlo simulations with different correlation $\rho$ and user speeds, we validate our analysis and show that the proposed scheme outperforms CCE when $\rho \geq 0.3$ , while it provides comparable performance for small $\rho$ .

Ayesha Bint Saleem

Papers

On the Performance of Spatial Modulation Schemes in Large-Scale MIMO under Correlated Nakagami Fading

Full-Duplex Quadrature Spatial Modulation for Multi-Antenna Systems

Channel Estimation for Spatial Modulation Schemes in Spatially Correlated Time Varying Channels