Sandeep Narayanan

Distributed space shift keying for the uplink of relay-aided cellular networks

Space Shift Keying (SSK) is a novel low-complexity digital modulation technique for multiple antenna wireless systems. In SSK, blocks of data are encoded in the index of a specific transmit antenna of the antenna array, which is switched on for data transmission while all the other antennas radiate no power. Recent results have shown that SSK can outperform, with low computational complexity, conventional modulation schemes when many antennas at the transmitter are available. Unfortunately, this makes SSK a useful candidate for transmission technology only in the downlink of cellular networks. In this paper, by exploiting the concept of virtual antenna arrays, we propose a novel relay-aided scheme to make SSK useful in the uplink of cellular networks. The specific contributions of this paper are as follows: i) the optimal decoder which takes into account decoding errors at the relays is developed; and ii) the performance in terms of Average-Bit-Error-Probability (ABEP), and the energy gain in terms of Relative-Average-Energy-Reduction (RAER) per bit is studied. Via Monte-Carlo simulations, it is shown that the proposed optimal decoder outperforms the conventional decoder for SSK, which does not account for decoding errors at the relays.

Multi-user spatial modulation MIMO

Spatial Modulation (SM) is a recently proposed single-RF multiple-input-multiple-output (MIMO) technique, which is capable of outperforming many conventional MIMO transmission schemes with low implementation and computational complexity. Recently, there have been some attempts in understanding the performance of SM in multi-user environments. However, most of the work has been oriented towards uplink multi-access scenarios. Also, conventional downlink/broadcast MIMO precoding techniques such as Zero Forcing (ZF) or Minimum Mean Square Error (MMSE) cannot be used in Multi-User SM (MU-SM), as part of the data in SM is also encoded into the Channel Impulse Responses (CIRs). In this paper, a novel precoding scheme for single-cell downlink MU-SM systems is proposed with a two-fold objective: i) the precoder needs to be able to completely eliminate the Multi-User Interference (MUI) by taking advantage of the Channel State Information (CSI) at the transmitter and ii) it needs to allow the users to use a single-user Maximum Likelihood (ML) optimum detector while achieving the same performance as interference-free point-to-point SM transmission. Finally, we also develop an interference-aware multi-user detection scheme, which does not require any CSI at the transmitter, and compare its performance with that of single-user detection schemes based on precoding.

Distributed Spatial Modulation: A Cooperative Diversity Protocol for Half-Duplex Relay-Aided Wireless Networks

In this paper, distributed spatial modulation (DSM) is introduced. DSM is a cooperative diversity protocol for multirelay wireless networks, which is based on the concept of spatial modulation (SM). The distinguishable feature of DSM lies in improving the reliability of the source via distributed diversity and by increasing the aggregate throughput of the network since new data is transmitted during each transmission phase. This is achieved by encoding the data transmitted from the source into the spatial positions of the available relays and by exploiting the signal domain to transmit the data of the relays. At the destination, a diversity combiner that is robust to demodulation errors at the relays is proposed, and its end-to-end error probability and achievable diversity are studied. It is mathematically proved that DSM allows the source to achieve second-order diversity. With the aid of Monte Carlo simulations, DSM is compared against state-of-the-art cooperative protocols, and it is shown to provide a better error probability.

On the Energy Consumption of the Decision-Fusion Rules in Cognitive Radio Networks

Cognitive radio has been proved to be an efficient solution for spectrum shortage and underutilization problems. However, energy consumption during spectrum sensing is a significant drawback of cognitive radio especially in energy-limited systems. An important stage in spectrum sensing playing a role in increasing the energy consumption is the decision fusion at the fusion center. Many decision-fusion rules have been proposed such as Likelihood Ratio rule (LR), Maximum Ratio Combining rule (MRC) and Equal Gain Combining rule (EGC). A comparison among these rules is presented in this paper. The comparison is in terms of the consumed energy and the achievable detection probability at a given false alarm probability threshold, under a limited time assumption. Simulation results show that in critical conditions, such as short sensing time, low SNR, and large number of users, EGC has a better performance in both energy saving and detection probability.

Energy-Efficient Partial-Cooperative Spectrum Sensing in Cognitive Radio over Fading Channels

Energy efficiency in cooperative spectrum sensing in cognitive radio is investigated in this paper, where a novel approach is proposed for reducing the energy consumed in spectrum sensing and improving the resultant energy efficiency of the cognitive transmission. The proposed approach is based on limiting the number of users that participate in the spectrum sensing task. The participation decision of each user is taken individually by the user itself, where each user estimates the expected amount of consumed energy based on its distance from the base station, and compares it to a predefined threshold. The user will participate only if the estimated energy is less than the threshold. Besides reducing energy consumption, our proposal increases the amount of successfully transmitted data as well. Moreover, an optimization of the threshold is carried out through simulation in order to optimize the energy efficiency. Our results show a considerable amount of reduction in energy consumption (up to 80%) compared to the conventional approach.

Distributed Spatially-Modulated Space-Time-Block-Codes

In this paper, a novel energy-efficient protocol, intended for wireless networks with large number of relay nodes, is proposed. The main distinguishable feature of the proposed protocol is that it offers throughput enhancement, by having the same diversity-gain and having to activate the same number of relay nodes at any given time-instance, as the the conventional distributed space-time-block-codes (D-STBC). The proposed protocol applies the recently introduced idea of Spatially-Modulated Space-Time-Block-Codes (SM-STBC) devised for multiple-input-multiple-output (MIMO) systems to cooperative relay networks. The specific contributions of this paper are: i) an error-aware Maximum-Likelihood (ML) demodulator, which is robust to demodulation errors at the relays is developed; ii) a low-complexity error-aware demodulator is also developed; and iii) it is shown, with the help of Monte Carlo simulations, that the proposed protocol outperforms state-of-the-art relaying protocols.

On the Achievable Performance-Complexity Tradeoffs of Relay-Aided Space Shift Keying

Future cellular networks require transmission technologies and protocols that are energy-efficient, and that can meet the growing demands of mobile data traffic with the best performance versus complexity tradeoff. The recently proposed singleRF space shift keying (SSK-) multiple-input-multiple-output (MIMO) transmission scheme is one of such physical layer technology that satisfies these criteria. In general, SSK requires a large number of antenna elements (with only one of them active) at the transmitter for high data rate transmission. This makes SSK particularly useful for the downlink of cellular systems. In this paper, we exploit the virtual MIMO concept to design SSK transmission schemes for the uplink of cellular networks. The idea is to take advantage of nearby nodes to the mobile terminal as a “virtual spatial-constellation diagram,” where the information can be encoded and transmitted to the final destination. By taking into account the virtual nature of the spatial-constellation diagram, we develop advanced demodulation techniques, perform a comprehensive mathematical analysis, as well as analyze the system complexity and energy consumption. It is also shown that the proposed scheme is capable of outperforming certain state-of-the-art protocols.

Simultaneous Uplink/Downlink Transmission Using Full-Duplex Single-RF MIMO

In this letter, we introduce a full-duplex protocol for simultaneous transmission between the uplink and the downlink of cellular networks. The protocol takes advantage of the inactive antenna(s) in multiple-input-multiple-output (MIMO) systems with a single active radio frequency (RF) front-end. More precisely, for the downlink transmissions, we make use of spatial modulation (SM), and for the uplink, we make use of the coordinate-interleaved orthogonal design (CIOD)-based space-time block code (STBC). We provide accurate mathematical expressions for evaluating the error-performances and the achievable diversity order at the base station (BS) and at the mobile terminal (MT) in the presence of self-interference. Our results demonstrate clearly the potential of SM and CIOD for full-duplex operation.

A base station switching on-off algorithm using traditional MIMO and spatial modulation

In this paper, we propose a novel rate adaptive Base Station (BS) switch on-off algorithm that employs Spatial Modulation (SM) and traditional Multiple-Input Multiple-Output (MIMO). We consider a cluster of BSs and based on the required average data rate, our algorithm selects the most energy efficient scheme between SM and MIMO with only Channel State Information at the Receiver (MIMO-CSIR). In order to further increase the Energy Efficiency (EE) (in bits/J), we activate or deactivate BSs in conjunction to switching between different transmission modes (SM or MIMO-CSIR). In both cases, we ensure that the User Terminals (UTs) do not face a reduction in Quality-of-Service (QoS) in the sense of ergodic capacity. Using the fundamental limit of Shannon capacity and a BS power model, we show that the use of SM, in such an algorithm, offers significant EE improvements. Finally, we demonstrate simulation results which show that, depending on the required average data rate, our algorithm achieves the same or better EE compared to the employed benchmark systems.

A virtual full duplex distributed spatial modulation technique for relay networks

Spatial modulation, a multiple-input multiple-output (MIMO) technology which uses the antenna index to transmit part of the incoming data, is an attractive way to reduce the energy cost and transceiver complexity in future wireless networks. In particular, the recently proposed technique of distributed spatial modulation (DSM) for relay networks can lead to better spectral efficiency, as it allows the relays to transmit their own data while simultaneously relaying the data of the source. A new distributed spatial modulation protocol is introduced in this paper which achieves virtual full duplex (VFD) communication. In this protocol, the source and relays transmit their own data in every time slot; thus, the spectral efficiency is significantly improved compared to conventional DSM. Simulation results indicate that at high signal-to-noise ratio (SNR), the proposed protocol has similar bit error rate (BER) performance versus SNR-per-bit compared to the standard full duplex relaying protocol of successive relaying; however, in contrast to successive relaying, the relays are simultaneously transmitting their own data, which is received at the destination with an error rate similar to that of the sources data.