Adrish Banerjee

Energy Harvesting Cognitive Radio With Channel-Aware Sensing Strategy

An energy harvesting cognitive radio scenario is considered where a secondary user (SU) with finite battery capacity opportunistically accesses the primary user (PU) channels. The objective is to maximize the throughput of SU under energy neutrality constraint and fading channel conditions in a single-user multi-channel setting. A channel selection criterion based on the probabilistic availability of energy with SU, channel conditions, and primary networks belief state is proposed, which chooses the best subset of channels for sensing, yielding higher throughput. We construct channel-aware optimal and myopic sensing strategies in a Partially Observable Markov Decision Process framework based on the proposed channel selection criterion. The effects of sensing errors and collisions between PU and SU on the throughput of the latter are studied. It is shown that there exists a trade-off between the transmission duration and the energy lost in collisions.

Secure Communication via a Wireless Energy Harvesting Untrusted Relay

The broadcast nature of the wireless medium allows unintended users to eavesdrop on confidential information transmission. In this regard, we investigate the problem of secure communication between a source and a destination via a wireless energy harvesting untrusted node that acts as a helper to relay the information; however, the source and destination nodes wish to keep the information confidential from the relay node. To realize the positive secrecy rate, we use destination-assisted jamming. Being an energy-starved node, the untrusted relay harvests energy from the received radio-frequency (RF) signals, which include the sources information signal and the destinations jamming signal. Thus, we utilize the jamming signal efficiently by leveraging it as a useful energy source. At the relay, to enable energy harvesting and information processing, we adopt power splitting (PS) and time switching (TS) policies. To evaluate the secrecy performance of this proposed scenario, we derive analytical expressions for two important metrics, viz., the secrecy outage probability and the ergodic secrecy rate. The numerical analysis reveals design insights into the effects of different system parameters such as PS ratio, energy harvesting time, target secrecy rate, transmit signal-to-noise ratio (SNR), relay location, and energy conversion efficiency factor, on secrecy performance. Specifically, the PS policy achieves better optimal secrecy outage probability and optimal ergodic secrecy rate than that of the TS policy at higher target secrecy rate and transmit SNR, respectively.

Nonsystematic turbo codes

In this paper, we introduce the concept of nonsystematic turbo codes and compare them with classical systematic turbo codes. Nonsystematic turbo codes can achieve lower error floors than systematic turbo codes because of their superior effective free distance properties. Moreover, they can achieve comparable performance in the waterfall region if the nonsystematic constituent encoder has a low-weight feedforward inverse. A uniform interleaver analysis is used to show that rate R=1/3 turbo codes using nonsystematic constituent encoders have larger effective free distances than when systematic constituent encoders are used. Also, mutual information-based transfer characteristics and extrinsic information transfer charts are used to show that rate R=1/3 turbo codes with nonsystematic constituent encoders having low-weight feedforward inverses achieve convergence thresholds comparable to those achieved with systematic constituent encoders. Catastrophic encoders, which do not possess a feedforward inverse, are shown to be capable of achieving low convergence thresholds by doping the code with a small fraction of systematic bits. Finally, we give tables of good nonsystematic turbo codes and present simulation results comparing the performance of systematic and nonsystematic turbo codes.

Performance of hybrid ARQ schemes using turbo trellis coded modulation for wireless channels

In this paper, bandwidth efficient Type-I and Type-II hybrid-ARQ (HARQ) schemes using turbo trellis coded modulation (TTCM) are proposed. These schemes combine the power efficiency of turbo codes with the bandwidth efficiency of trellis coded modulation (TCM) to create an effective hybrid FEC/ARQ system. Several packet combining schemes are presented for use in conjunction with iterative turbo decoding over wireless time-varying Rayleigh fading channels. The packet combining schemes provide improved throughput and reliability compared to a standard Type I hybrid ARQ system without combining with only a small increase in transmitter and receiver complexity. Simulation results show that, for high throughput values, HARQ schemes based on TTCM give substantial improvement over conventional TCM schemes with the same throughput over wireless channels.

Malicious user suppression for cooperative spectrum sensing in cognitive radio networks using Dixon's outlier detection method

Cooperation among multiple secondary users improves the cognitive radio sensing system performance, but the presence of malicious secondary users may severely degrade the same. In this paper, we study the detection and elimination of such malicious users in a cooperative sensing system using Dixons outlier test and compare its performance with Grubbs test and boxplot test. We have shown using receiver operating characteristics curves that Dixons test outperforms Grubbs test and boxplot test for the case of a single malicious user. We also illustrate the limitations of Dixons test for several malicious users using an example of two malicious users in a cooperative spectrum sensing setting for cognitive radio.

Resource Allocation and Fairness in Wireless Powered Cooperative Cognitive Radio Networks

We integrate a wireless powered communication network with a cooperative cognitive radio network, where multiple secondary users (SUs) powered wirelessly by a hybrid access point (HAP) help a primary user relay the data. As a reward for the cooperation, the secondary network gains the spectrum access where SUs transmit to HAP using time division multiple access. To maximize the sum throughput of SUs, we present a secondary sum-throughput optimal resource allocation (STORA) scheme. Under the constraint of meeting target primary rate, the STORA scheme chooses the optimal set of relaying SUs and jointly performs the time and energy allocation for SUs. In particular, by exploiting the structure of the optimal solution, we find the order in which SUs are prioritized to relay primary data. Since the STORA scheme focuses on the sum throughput, it becomes inconsiderate toward individual SU throughput, resulting in low fairness. To enhance fairness, we investigate three resource allocation schemes, which are: 1) equal time allocation; 2) minimum throughput maximization; and 3) proportional time allocation. Simulation results reveal the tradeoff between sum throughput and fairness. The minimum throughput maximization scheme is the fairest one as each SU gets the same throughput, but yields the least SU sum throughput.

Sensing-Throughput Tradeoff in Cognitive Radio With Random Arrivals and Departures of Multiple Primary Users

This letter analyzes the sensing-throughput tradeoff for a secondary user (SU) under random arrivals and departures of multiple primary users (PUs). We first study the case where PUs change their status only during SUs sensing period. We then generalize to a case where PUs change status anytime during SU frame, and compare the latter case with the former in terms of the optimal sensing time and SU throughput. We also investigate the effects of PU traffic parameters and the number of PUs on the sensing-throughput tradeoff for SU. Results show that, though the increase in the number of PUs reduces the optimal sensing time for SU, the opportunity to find a vacant PU channel reduces simultaneously, in turn, reducing SU throughput. Finally, we validate the analysis by Monte Carlo simulations.

A comparison of low complexity turbo-like codes

In the past few years, several approaches to low complexity turbo-like code designs have appeared. All of these code designs are based on very simple graph structures or 2-state trellises that result in low decoder complexity. One such approach is based on multiple turbo codes. The low complexity multiple turbo code designs are compared to 8-state turbo codes used in third generation mobile systems on the basis of error performance and computational complexity.

‘n-ratio’ logic based cooperative spectrum sensing using double threshold energy detection

In this paper we describe a new cooperating sensing method using double threshold energy detection technique for cognitive radio. Each secondary cognitive user takes a local decision on spectrum occupancy based on two threshold energy detection and uses 1 bit information to convey its decision to the fusion center that collects decisions from all cooperating users who are able to detect presence or absence of signal. Fusion center takes a final decision using what we called ‘n-ratio’ logic. We will show that OR logic proposed in the literature is a special case of the ‘n-ratio’ logic decision. We derive the expression for probability of detection and probability of false alarm for our ‘n-ratio’ logic cooperating sensing method for Additive White Gaussian Noise (AWGN) and flat fading Rayleigh channels. Finally we present some results that shows improvement in spectrum sensing of the proposed method in comparison to OR logic, while keeping network overhead low.

Interference-Assisted Wireless Energy Harvesting in Cognitive Relay Network with Multiple Primary Transceivers

We consider a spectrum sharing scenario, where a secondary network coexists with a primary network of multiple transceivers. The secondary network consists of an energy-constrained decode-and- forward secondary relay which assists the communication between a secondary transmitter and a destination in the presence of the interference from multiple primary transmitters. The secondary relay harvests energy from the received radio- frequency signals, which include the information signal from the secondary transmitter and the primary interference. The harvested energy is then used to decode the secondary information and forward it to the secondary destination. At the relay, we adopt a time switching policy due to its simplicity that switches between the energy harvesting and information decoding over time. Specifically, we derive a closed-form expression for the secondary outage probability under the primary outage constraint and the peak power constraint at both secondary transmitter and relay. In addition, we investigate the effect of the number of primary transceivers on the optimal energy harvesting duration that minimizes the secondary outage probability. By utilizing the primary interference as a useful energy source in the energy harvesting phase, the secondary network achieves a better outage performance.