Interference management for self-organized femtocells towards green networks
ABSTRACT The femtocell concept is an emerging technology for deploying the next generation of the wireless networks, aiming at indoor coverage enhancement, increasing capacity and offloading the overlay macrocell traffic. One of the main challenges in short range femtocell networks is how to (re)configure the Home Node Bs (HNBs) in an autonomous manner so as to manage interference and diminish the energy consumption among nearby femtocells efficiently. In this paper, we investigate the indoor femtocell deployment making use of both the Frequency Division Duplexing (FDD) and the Time Division Duplexing (TDD) methods. In the FDD case, the HNBs share both the Uplink (UL) and Downlink (DL) channels with the macrocell without any cooperation to coordinate their access to the air interface. Conversely, in the TDD underlay case, femtocells only reuse the macrocell UL spectrum and cooperate with each other in order to minimize the interference among themselves, either with or without further coordination with the Macro User Equipment (MUE). The proposed solution is evaluated by means of system-level simulations using the Monte Carlo approach. Investigations have shown that the TDD underlay approach not only reduces the perceived interference levels, but also diminishes the outage probability.
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ABSTRACT: In recent years, the family of multicarrier code-division multiple-access (MC-CDMA) transmission technologies has drawn a lot of attention in providing broadband wireless communication services. From the literature, one can find extensive discussions in many aspects of MC-CDMA under a flat system architecture either for conventional or currently emerging cognitive radio (CR) systems. In this paper, we shift the focus from the flat architecture to the hierarchical architecture based on 2-D-spread multicarrier direct-sequenced CDMA, in which a microcell is embedded in the primitive macrocell. However, in such a hierarchical architecture, severe intercell interference can occur between the macrocell and the microcell. Via the interference avoidance code assignment strategy, this kind of intercell interference in downlink transmissions can be eliminated. To this end, we define the white, gray, and black spreading codes to facilitate the management of spreading code resources. Properly allocating the white and gray spreading codes but the black ones can achieve a higher spectrum efficiency without harming the macrocell users. For the purpose of evaluating the candidacy of the gray spreading codes, a sophisticated interference analysis is conducted to define a new multiple access interference (MAI) coefficient to quantize the amount of interference imposed on a microcell user. With the help of the new MAI coefficient, a less-interfered white or gray spreading code can be assigned to microcell users, which can bring about an extra 55% of call admissions in one of our considered cases. Moreover, the capability of the proposed hierarchical system architecture, as well as the interference management scheme for developing the femtocell system, has also been proved via simulation results with a large number of femtocells. It is believed that by cooperating with the substream deactivation technique, the proposed spreading code management associated with the hierarchical system - rchitecture can be a potential prototype of the CR-based femtocell system.IEEE Transactions on Vehicular Technology 01/2012; 61(1):166-184. · 2.06 Impact Factor
Conference Paper: Hybrid division duplex for cognitive small cell networks[Show abstract] [Hide abstract]
ABSTRACT: With the exponential increase in high rate traffic driven by a new generation of wireless devices, the volume of data traffic is expected to overwhelm cellular network capacity in the near future. To tackle this issue, small cell networks have been recently proposed as an efficient and cost-effective approach to provide unprecedented spectral efficiency and coverage. However, the dense and random deployment of small cells together with their decentralized operation bring several issues regarding interference pollution and network sustainability. Motivated by the flexible subchannel allocation capabilities of cognitive radio, we propose a cognitive hybrid division duplex (CHDD), in which a pair of frequency bands is used to perform frequency division duplex (FDD) on the macrocell while underlaid cognitive small cells simultaneously operate in time division duplex (TDD) on the same bands. We propose a spatial randomness-aware methodology on how to design optimal switching mechanism for cognitive TDD operation of small cells. Specifically, by using tools from stochastic geometry, we derive key performance measures in terms of success probability, network area spectral efficiency, and spatial average capacity for the proposed CHDD scheme for the case when the macro tier is in uplink mode.Wireless Personal Multimedia Communications (WPMC), 2012 15th International Symposium on; 01/2012
Conference Paper: Flexible duplex for cognitive femtocells in two-tier networks[Show abstract] [Hide abstract]
ABSTRACT: Small cell network architecture is considered as an effective solution to the ever growing demand for high data rate, with femtocells being a promising paradigm. The dense deployment and the uncoordinated operation of femtocells bring various challenges for interference management. Motivated by the flexible subchannel allocation capabilities of cognitive radio, we propose a cognitive hybrid division duplex (CHDD) for two-tier networks. In this scheme, macrocells operate in frequency division duplex (FDD), while the underlying cognitive femtocells employ time division duplex (TDD). The CHDD scheme has the flexibility of providing asymmetric data rates from the TDD mode, while managing inter-tier interference with FDD. Using a network model based on stochastic geometry in order to capture both interference and spatial randomness, we quantify the performance of the proposed CHDD scheme in terms of success probability, area spectral efficiency, and spatial average rate. Our analytical and numerical results show the effectiveness of introducing cognition in femtocells as a means to enhance the performance of femtocell-aided cellular networks.Global Communications Conference (GLOBECOM), 2012 IEEE; 01/2012