Fast Power and Channel Adaptation for Mobile Users in OFDMA Multi-Cell Scenarios

Methods for interference mitigation and adaptive multi-user resource allocation are among the most promising technological breakthroughs that should improve capacity of the last generation broadband wireless systems. In multi-cell scenarios characterized by radio technologies based on OFDMA (Orthogonal Frequency DivisionMultiple Access), co-channel interference is the performance limiting factor and techniques that reduce transmission power, act indirectly on the co-channel interference levels. In this work we investigate multi-cell scenarios with mobile users in which power and channel adaptation act as positive factors on the interference reduction. We are interested particularly on the role of closed loop power control and of simple techniques for fast channel assignment in multi-cell configurations with low reuse factors (possibly between 1 and 3). From the numerical findings presented in this chapter, it turns out that spectral efficiency can be improved by algorithms that are simple and fast enough to be exploited for mobile users, at least at pedestrian velocities. The ambitious capacity requirements for future broadband wireless networks expose system designers to the challenging compromise between the scarcity of spectral resources and the impairments introduced by radio propagation randomness. At the heart of this challenge there is the ability to exploit radio resources as efficiently as possible in all available dimensions (space, time, frequency or channel, power, modulation and coding). Since mitigation of interference and resource allocation strategies are usually studied separately, in this work we aim to investigate more deeply the relations between the application of simple techniques for allocating power and channels to mobile users and their impact on interference and, consequently, on multi-cell overall performance. Adaptive allocation techniques have already demonstrated their ability to mitigate the effect of deep fading by encouraging channel access to users temporarily experiencing better propagation conditions, taking advantage by the so-called ’multiuser diversity gain’ (Knop & Humblet (1995)). In our previous works (Galati et al. (2010); Reggiani et al. (2007); Galati et al. (2008)) we have shown how resource allocation algorithms in multi-cell environments have a non-trivial impact on downlink mutual interference among cells when the simple mechanism of power adaptation is applied to the system sub-channels: when the power, after channel assignment, is reduced to the minimum level required for the selectedmodulation and coding profile, all the network acquires an advantage from the interference reduction, giving rise to a ’virtuous loop’ in which performance parameters as spectral efficiency or coverage turn out Fast Power and Channel Adaptation for Mobile Users in OFDMA Multi-Cell Scenarios 8