Abhirup Das Barman

Mitigation of chromatic dispersion electronically in a coherent optical communication system

High speed optical fiber transmission performance is severely degraded due to chromatic dispersion in the fiber. Contrary to conventional dispersion compensation techniques by dispersion compensated fiber, in this work electronic compensation techniques are investigated to mitigate chromatic dispersion in a phase modulated optical coherent detection system. Such electronic compensation techniques add flexibility to the dynamically routed optical networks. Performances of electronic mitigation techniques of chromatic dispersion are compared and their applicability limits are evaluated.

Opportunistic Sub-channel and Transmit Power Allocation in an OFDMA Based Cognitive Femtocell Network

The paper proposes a scheme to minimize the co-tier and cross-tier interference by properly allocating the sub-channels and power among the femtocell users (FUs) in an OFDMA based cognitive femtocell network. An efficient graph coloring based sub-channel allocation scheme is adapted in which the sub-channels in the uplink band of the macrocell are shared by the femtocells dynamically in an opportunistic way through spectrum sensing. A price-based power allocation scheme using game theory is proposed to assign the transmission power among the FUs, whereby the macrocell base-station (MBS) controls the transmission power of the FUs by pricing their resulted interference power levels at the MBS with the constraint that the total interference created at the MBS in each sub-channel is kept below a tolerable threshold. The game theoretic approach ensures that no FUs can improve its utility by changing its own transmission power selfishly. The allocation of sub-channels among the FUs is centrally controlled whereas the power allocation is handled in a distributed way which makes the process efficient. Finally, numerical examples are presented to analyze the proposed scheme. The results show that substantial number of FUs with good quality of services can be accommodated in the macrocell reusing the uplink frequency sub-channels and thereby enhancing the throughput and coverage of the network.

Circuit model of UTC-PD with high power and enhanced bandwidth technique

An electrical equivalent circuit model of InGaAs/InP uni travelling carrier photodiode is presented. The model is suitable to be built on any electrical circuit simulator to perform design and optimize the device parameters. We have shown a novel technique of increasing bandwidth of the device by inserting a small shunt inductance in series with the load without sacrificing the device output photocurrent and linearity to a large extent.

Q-Learning Based Co-Operative Spectrum Mobility in Cognitive Radio Networks

In cognitive radio systems, fast and efficient spectrum selection is a vital task to minimize the overhead of spectrum scanning, and hence to improve the response time of the system. So, the choice of channel sensing sequence plays an important role for better performance of the system. This paper proposes a co-operative Q-learning based spectrum sensing technique for the secondary users of an ad hoc network to access the primary channels. By the proposed technique, every secondary user (SU) maintains a dynamic priority list of channels based on Q-learning from its own action-observation history, as well as from spatial channel information exchange among its local neighbors. Whenever there is a demand an SU scans the spectrum according to the order in the priority list until there is a success. Simulation studies show that with significantly less computing and scanning overhead, our proposed Q-learning based approach improves the response time and call block / drop rate to offer better performance compared to other contemporary reinforcement learning based approaches.

Adaptive pre-compensation of LEDs for improved decoding of N-CSK in visible light communication

We propose an adaptive pre-compensation using second order memory less polynomial to mitigate dynamic nonlinearity in LEDs. The scheme will improve the performance of IEEE 802.15.7 standard N-level colcur shift keying (N-CSK) modulation for indoor visible-light optical wireless communication system. The N-CSK system is implemented and Voronoi segmentation is employed to decode N-CSK constellation. Decoding performance is simulated by Monte Carlo method and verified experimentally. Symbol error rate (SER) and error vector magnitude (EVM) at the receiver shows improved decoding performance of CSK through pre-distortion compensation of LEDs.

Promoting femtocell cooperation through incentive for improving data rate of indoor users in underlay heterogeneous network

This study proposes an incentive-based femtocell cooperation scheme for indoor macrocell users (MUs) in an underlay two-tier heterogeneous network which helps the indoor MUs to achieve higher data rate through time sharing access from the femtocell. Due to underlay spectrum sharing nature of the femtocell, the network suffers from severe cross-tier interference. To manage this cross-tier interference, a price-based power control scheme is also proposed for the femtocell access point (FAP) whereby the macrocell base station (MBS) controls the transmit power of the FAP by pricing their resulted cross-tier interference power level subject to a maximum tolerable interference limit. A Stackelberg game has been formulated and analysed for joint pricing-based power control and incentive-based cooperation game. Numerical results show the performance of the proposed scheme in terms of achievable rate of indoor users and utilities of both the MBS and the FAP for different locations of the femtocells.

Photoresponse characteristics from computationally efficient dynamic model of uni-traveling carrier photodiode

A time domain model of bulk InGaAs/InP uni-traveling carrier photodiode (UTC-PD) is developed in terms of integral carrier density rate equation. The wavelength dependent responsivity at different absorption width is derived from the model which shows good agreement with the experimental results. The bandwidth of the device is estimated from time dependent photocurrent response.

Equivalent circuit model of UTC photodiode

In this work we have examined different characteristics of uni travelling carrier (UTC) photodiode (PD) by mathematical modeling of the device. Based on the basic circuit elements contain in the mathematical model, an equivalent circuit model of UTC-PD is developed. The frequency response and pulse photocurrent response agrees well with reported data. To the authors believe such kind of circuit model of UTC photodiode has not been reported elsewhere. The influences of different parameters and device mounting parasitic effects on frequency response have been investigated. Such simplified circuit model will be helpful in predicting different characteristics of UTC photodiode before fabrication.

Indoor visible light communication with smart lighting technology

An indoor visible-light communication performance is investigated utilizing energy efficient white light by 2D LED arrays. Enabled by recent advances in LED technology, IEEE 802.15.7 standardizes high-data-rate visible light communication and advocates for colour shift keying (CSK) modulation to overcome flicker and to support dimming. Voronoi segmentation is employed for decoding N-CSK constellation which has superior performance compared to other existing decoding methods. The two chief performance degrading effects of inter-symbol interference and LED nonlinearity is jointly mitigated using LMS post equalization at the receiver which improves the symbol error rate performance and increases field of view of the receiver. It is found that LMS post equalization symbol at 250MHz offers 7dB SNR improvement at SER10-6

Opportunistic spectrum access for cooperative unlicensed femtocells in two-tier heterogeneous networks utilizing HARQ feedback

Abstract The co-channel deployment of femtocells can lead to severe cross-tier interference to macrocell users which is resolved through the use of cognitive radio technology in which the femtocells, acting as unlicensed users, opportunistically access the spectrum of the primary macrocell in a cognitive way. This research proposes a cooperative scheme to improve the spectrum access opportunity for unlicensed femtocell in a two-tier heterogeneous network by exploiting the hybrid-automatic repeat request (HARQ) feedback information of macrocell transmission assuming the macrocell spectrum is considerably underutilized. This HARQ based scheme is very useful to get rid of the ‘hidden node problem’ and ‘exposed node problem’ encountered in conventional ‘listen-before-talk’ strategy used by the cognitive users. The outage performance of macrocell user is also improved due to cooperation from the femtocell. Performance analysis of the scheme is presented in Nakagami fading environment for a single femtocell scenario and compared with the conventional non-cooperative spectrum access via numerical examples. Appreciable improvements in terms of macrocell outage, macrocell and femtocell throughput over the conventional non-cooperative spectrum access have been found by considering a fixed macrocell traffic arrival rate. Moreover the dependence of femtocell location on the performance of the proposed scheme is further explored.