Mohamed Rihan

Iterative interference alignment in macrocell-femtocell networks: A cognitive radio approach

The deployment of femtocells in current and future communication systems promises an effective solution for limited indoor coverage problem and a possible gateway for mobile data offloading. In this paper, we cast a cognitive interference alignment approach (IA) suitable for heterogeneous cellular networks with a mixed macrocell and femtocell deployment Specifically, in our approach a restricted waterfilling (RWF) algorithm is used to maximize the downlink data rate, while reserving some eigenmodes for giving the femtocell basestations the opportunity to do their transmissions even at high signal-to-noise power ratio (SNR) for the macrocell basestation. Additionally, both the cross-tier and co-tier interference is to be aligned at each femtocell users receiver using an Iterative Reweighted Least Squares(IRLS) algorithm. The simulation results show that the proposed IA approach provides an improved sum rate for the femtocell users, compared to the conventional IA techniques, like, the leakage minimization approach and the nuclear norm based rank constraint rank minimization approach.

On ZigBee coexistence in the ISM band: Measurements and simulations

Wireless Sensor Network (WSN) technology enables design and implementation of novel applications that can be used to address numerous industrial, environmental, social and economical challenges. The IEEE 802.15.4 standard specifies WSN as a personal unlicensed network that operates in the Industrial, Scientific and Medical (ISM) unlicensed band. Unfortunately, this band is also available for many other wireless technologies like Bluetooth/ IEEE 802.15.1 and Wireless Local Area Networks (WLANs)/IEEE 802.11b/g. The blind coexistence between wireless technologies in the ISM band can lead to severe performance degradation due to undesired interference. This paper provides an overview on the performance of ZigBee WSNs/IEEE 802.15.4 coexisting with IEEE 802.11b/g WLANs. Through extensive measurements for packet error rate (PER), link quality indication (LQI) and received signal strength indication (RSSI) performance metrics, we were able to clarify the performance degradation due to the blind coexistence between IEEE 802.15.4 and IEEE 802.11b/g networks. We support the measurement results by extensive simulations using OPNET V.16 for large scale ZigBee WSNs coexisting with WLANs. We show that for robust operation in unlicensed bands, WSNs should be equipped with cognitive radio capability.

Interference Alignment with Limited Feedback for Macrocell-Femtocell Heterogeneous Networks

Interference alignment (IA) emerged on the communication scene as a solution to the interference problem in all interference-limited networks, including heterogeneous cellular systems. However, the performance of IA is greatly related to the accuracy of the channel state information at transmitters (CSIT), namely the number of feedback bits. Accordingly, in order to improve the performance of IA, it would be useful to analyze the number of feedback bits with respect to the sum rate loss. Motivated by that, this paper studies a limited feedback-based IA scheme suitable for two tier macrocell-femtocell heterogeneous networks. First, an approximate analytical expression for the upper bound on the total sum rate loss due to limited feedback in the studied IA system, is derived. Then, a simulation based evaluation of the sum-rate loss due to the implementation of limited feedback IA in heterogeneous networks is obtained. Simulation results confirmed the severe effect of quantization of CSI on the interference alignment performance.