Nurul Huda Mahmood

Centimeter-Wave Concept for 5G Ultra-Dense Small Cells

Ultra-dense small cells are foreseen to play an essential role in the 5th generation (5G) of mobile radio access technology, which will be operating over different bands with respect to established systems. The natural step for exploring new spectrum is to look into the centimeter-wave bands as well as exploring millimeter-wave bands. This paper presents our vision on the technology components for a 5G centimeter-wave concept for ultra-dense small cells. Fundamental features such as optimized short frame structure, multi-antenna technologies, interference rejection, rank adaptation and dynamic scheduling of uplink/downlink transmission are discussed, along with the design of a novel flexible waveform and energy-saving enablers.

On the Potential of Interference Rejection Combining in B4G Networks

Beyond 4th Generation (B4G) local area networks will be characterized by the dense uncoordinated deployment of small cells. This paper shows that inter-cell interference, which is a main limiting factor in such networks, can be effectively contained using Interference Rejection Combining (IRC). By simulation we investigate two significantly different interference scenarios with dense small cell deployment. The results show that IRC brings considerable improvement in outage as well as in peak and median throughputs in both scenarios, and thus has a big potential as a capacity and coverage enhancing technique for B4G. The IRC gain mechanism depends strongly on the interference scenario and to some extent on the use of frequency reuse. These results are achieved with no coordination between cells and suggests that MIMO rank adaptation and IRC can be done independently.

Dynamic Channel Selection for Cognitive Radios with Heterogenous Primary Bands

Technological advances coupled with regulatory initiative for more efficient utilization of radio spectrum resulted in the introduction of dynamic spectrum access enabled radio called Cognitive Radio (CR). A CR network is allowed to access a frequency band owned by primary user, which can provide higher throughput and better serviceability in wireless networks. The availability of multiple primary bands requires a CR network to select the best operating band which can maximize the total system performance. The selection should be made according to heterogenous properties of primary bands which offer different maximum data rate for secondary use and generate diverse traffic pattern. In this paper, assuming such heterogenous primary bands for secondary use, simple and distributed dynamic channel selection strategies are proposed and evaluated. In addition, we introduce a measurement metric for interference experienced by primary users due to secondary network’s access in primary band. We investigate the impact of different channel selection strategies and parameters on the primary and secondary performance.

On the Potential of Full Duplex Communication in 5G Small Cell Networks

Full duplex communication promises a 100% throughput gain by doubling the number of simultaneous transmissions. In a multi-cell scenario, increasing the number of simultaneous transmissions correspondingly increases the number of interference streams observed at a particular receiver. As such, the potential throughput gain may not be 100% as promised. In this study, we evaluate the performance of full duplex communication in a dense small cell scenario as targeted by future 5th Generation (5G) radio access technology under the ideal assumptions of a full buffer, always active traffic model and perfect self interference cancellation. Advanced interference suppression/cancellation receivers are featured as well. Full duplex communication is found to provide about 30-40% mean throughput gain over half duplex transmissions for indoor scenarios, which provides an indication of the maximum throughput gains that can be achieved with full duplex communication in indoor scenarios under such idealized assumptions.

Influence of PAPR on Link Adaptation Algorithms in OFDM Systems

The impact of high power amplifier (HPA) on the performance of orthogonal frequency division multiplexing (OFDM) based wireless systems using link adaptation (LA) is analyzed in this work. LA maximizes throughput while maintaining a target bit error rate (BER) at the receiver. The non linear behaviour of the HPA introduces distortion in the OFDM signal which has high peak to average power ratio (PAPR). It is found in this work that this causes LA schemes to fail to meet the target BER. It is shown in this work that to make the LA system satisfy the BER constraint, the signal to noise ratio thresholds for changing the adaptive modulation needs to be updated.

Cognitive interference modeling with applications in power and admission control

One of the key design challenges in a cognitive radio network is controlling the interference generated at coexisting primary receivers. In order to design efficient cognitive radio systems and to minimize their unwanted consequences, it is therefore necessary to effectively control the secondary interference at the primary receivers. In this paper, a generalized framework for the interference analysis of a cognitive radio network where the different secondary transmitters may transmit with different powers and transmission probabilities, is presented and various applications of this interference model are demonstrated. The findings of the analytical performance analyses are confirmed through selected computer-based Monte-Carlo simulations.

A Distributed Taxation Based Rank Adaptation Scheme for 5G Small Cells

The further densification of small cells impose high and undesirable levels of inter-cell interference. Multiple Input Multiple Output (MIMO) systems along with advanced receiver techniques provide us with extra degrees of freedom to combat such a problem. With such tools, rank adaptation algorithms allow us to use our antenna resources to either exploit multiple spatial streams in low interference scenarios, or suppress interference in high interference scenarios. In this paper, we propose and evaluate an interference-aware distributed rank adaptation algorithm. The concept behind our approach is to discourage the choice of transmitting with multiple spatial streams in highly interfered scenarios, and to exploit and encourage the usage of multiple spatial transmission streams in low interference scenarios. We show that our proposed algorithm can be adjusted to preserve and guarantee a good outage performance, while providing the benefit of higher average throughputs, in both low and highly interfered scenarios, when compared to fixed rank configurations, and distributed selfish schemes.

On Hybrid Cooperation in Underlay Cognitive Radio Networks

Cooperative communication is a promising strategy to enhance the performance of a communication network as it helps to improve the coverage area and the outage performance. However, such enhancement comes at the expense of increased resource utilization, which is undesirable; more so in the case of opportunistic wireless systems such as cognitive radio networks. In order to balance the performance gains from cooperative communication against the possible over-utilization of resources, we propose and analyze an adaptive-cooperation technique for underlay cognitive radio networks, termed as hybrid-cooperation. Under the proposed cooperation scheme, secondary users in a cognitive radio network cooperate adaptively to enhance the spectral efficiency and the error performance of the network. The bit error rate, the spectral efficiency and the outage performance of the network under the proposed hybrid cooperation scheme with amplify-and-forward relaying are analyzed in this paper, and compared against conventional cooperation technique. Findings of the analytical performance analyses are further validated numerically through selected computer-based Monte-Carlo simulations. The proposed scheme is found to achieve significantly better performance in terms of the spectral efficiency and the bit error rate, compared to the conventional amplify-and-forward cooperation scheme.

A generalized and parameterized interference model for cognitive radio networks

For meaningful co-existence of cognitive radios with primary system, it is imperative that the cognitive radio system is aware of how much interference it generates at the primary receivers. This can be done through statistical modeling of the interference as perceived at the primary receivers. In this work, we propose a generalized model for the interference generated by a cognitive radio network, in the presence of small and large scale fading, at a primary receiver located at the origin. We then demonstrate how this model can be used to estimate the impact of cognitive radio transmission on the primary receiver in terms of different outage probabilities. Finally, our analytical findings are validated through some selected computer-based simulations.

Can Full Duplex Boost Throughput and Delay of 5G Ultra-Dense Small Cell Networks?

Given the recent advances in system and antenna design, practical implementation of full duplex (FD) communication is becoming increasingly feasible. In this paper, the potential of FD in enhancing the performance of 5th generation (5G) ultra-dense small cell networks is investigated. The goal is to understand whether FD is able to boost the system performance from a throughput and delay perspective. The impact of having symmetric and asymmetric finite buffer traffic is studied for two types of FD: when only the base station is FD capable, and when both the user equipment and base station are FD nodes. System level results indicate that there is a trade- off between multiple-input multiple-output (MIMO) spatial multiplexing and FD in achieving the optimal system performance. Moreover, results show that FD may be useful for asymmetric traffic applications where the lightly loaded link requires high level performance. In such cases, FD can provide an average improvement of up to 116% in session throughput and 77% in packet delay compared to conventional half duplex transmissions.