Handoff Management in Macro-Femto Cellular Networks

Introduction

Over the last few years, there is a tremendous increase in the ratio of smart phones and tablets etc. As a result, the number of users in the network are increasingly aggressively and their requirements of bandwidth and throughput are increasing exponentially in recent times. Cisco research group has shown a 39 fold increase in the data traffic and throughput demand from year 2009 to 2014 (Reardon 2010). Due to this rapid increase in the data traffic and users demand of higher throughput for their multimedia application like video streaming, video conferencing and online gaming etc. Should we keep small cells or femto cells? Conventional wireless cellular network was becoming short of resources which was severely degrading the QoS of the users. To cope up with these limitations of the network, concept of heterogeneous network was evolved which integrates the small cells like femto, micro and pico cell in the conventional network and forming multi-tier communication. The next generation wireless communication networks are promising in terms of providing the larger bandwidth and higher throughput while assuring the quality of service (QoS) to the users for both voice and different data and multimedia services. Femtocell networks provides good quality of services with enhanced throughput to the users in the indoor environment with lower cost. Transmitter and receiver deployed closer to the user increase the capacity of wireless link as well as it creates dual benefits of better quality of links and more spatial reuse of spectrum resources (Liu et al. 2010). So, femtocell is one of the best approaches for the heterogeneous convergence networks for coverage area extension and capacity maximization.

A femtocell is a small base station with shorter communication range and low transmit power also referred to as home-enhanced NB (H(e)NB). This base station supports fewer users as compared to macro base station and employs the frequency reuse concept to enhance the system capacity. Femto cells are normally installed indoor to provide communication coverage to the users affected penetration losses and low SINR of macro cell signals (Chandrasekhar, Andrews, and Gatherer 2008a). One of the most important feature of a cellular communication system is the freedom of mobility but in this system, this service will be provided by supporting handoff process also known as handover from one cell to another without any interreption. Handoff is the process of changing the frequency, time slot, channel, spreading code, or combination of them from one base station to another while communication session is in progress. It is often initiated when any user crosses the cellular region or degradation of QoS in current channel, Different types of handoff algorithms are investigated by research community which are based on received signal strength (RSS), user’s velocity, dwell time, SINR value and users association with predefined threshold have been studied in (Halgamuge et al. 2005). The threshold sets a minimum handoff criteria for handoff initiation but hysteresis model adds a margin H(hysteresis threshold) to the threshold of RSS or dwell time from the serving BS.

The integration of femtocell with conventional macrocell networks is not very simple and easy because it leads several types of problems i-e, radius size of femtocell is very small which means, we required a large number of femto cells up to several hundred within a macrocell. If a UE attempts handoff between macrocell and femtocell then it will experience more severe SINR degradation than UE moving between macrocell at the speed. In otherwords, we can say that interference also increases by deploying the femotocells within a macrocell. Interferrence also increase as we increase the femtocells within a macrocell.