Sandip Karar

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.

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.

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.

An Overlay Cognitive Radio Model Exploiting the Polarization Diversity and Relay Cooperation

Interference mitigation is one of the significant challenges in overlay cognitive radio (CR). This paper proposes a scheme to avoid the interference in an overlay CR scenario by exploiting the polarization diversity and relay cooperation. In this scheme, a dual-polarized antenna is incorporated at the secondary transmitter, one of which acts as a relay to the primary user’s signal and cancels out the interference via space–time coding. Analytical results about the error performance and the outage probability for the scheme have been calculated. The performance degradation in the absence of perfect CSI has also been shown.

Interference Mitigation in Overlay Cognitive Radio Using Orthogonal Polarization

Interference mitigation is one of the significant challenges in overlay cognitive radio. This paper proposes two schemes to avoid the interference in an overlay cognitive radio scenario by exploiting the polarization diversity. In the first scheme both the primary and the secondary user use dual polarized antenna at the receiver end which combines the received signals in a proper way to cancel out the interference. In the second scheme a dual polarized antenna is incorporated at the secondary transmitter one of which acts as a relay of the primary user’s signal and cancels out interference via space–time coding. Analytical results about the error performance and the outage probability for the two schemes are shown.

Overcoming the SNR-wall for energy detection using the autocorrelation of cyclic prefix based OFDM signal

Energy detection is one of the basic schemes of spectrum sensing. In case of energy detection because of the uncertainty in estimating the noise variance there exists a fundamental limit of minimum SNR, called the SNR-wall, below which the signal could not be reliably detected. In this paper, we propose a method to overcome this SNR-wall for OFDM signal by exploiting the autocorrelation property of the cyclic prefix both in AWGN and multipath fading channel. Required SNR for signal detection versus noise uncertainty for a given false alarm probability (0.01) and detection probability (0.99) has been found out in case of both energy detector and the autocorrelation based detector for OFDM signal. Our result shows that there is about 23 dB advantage with our proposed method for AWGN channel and about 12 dB advantage for multipath fading channel over conventional energy detector for noise uncertainty parameter U = 5 with ∼ 5,000 signal samples.