Two-tier networks, comprising a conventional cellular network overlaid with shorter range hotspots (e.g. femtocells, distributed antennas, or wired relays), offer an economically viable way to improve cellular system capacity. The capacity-limiting factor in such networks is interference. The cross-tier interference between macrocells and femtocells can suffocate the capacity due to the near-far problem, so in practice hotspots should use a different frequency channel than the potentially nearby high-power macrocell users. Centralized or coordinated frequency planning, which is difficult and inefficient even in conventional cellular networks, is all but impossible in a two-tier network. This paper proposes and analyzes an optimum decentralized spectrum allocation policy for two-tier networks that employ frequency division multiple access (including OFDMA). The proposed allocation is optimal in terms of Area Spectral Efficiency (ASE), and is subjected to a sensible Quality of Service (QoS) requirement, which guarantees that both macrocell and femtocell users attain at least a prescribed data rate. Results show the dependence of this allocation on the QoS requirement, hotspot density and the co-channel interference from the macrocell and surrounding femtocells. Design interpretations of this result are provided.
The problem considered in this paper is related to Yeung and Nanda @cite_10 , who propose frequency partitioning in a microcell macrocell system based on mobile speeds and the loading of users in each cell. Similar dynamic channel allocation schemes have been proposed in @cite_27 and @cite_12 . Their frequency partitioning is derived based on choosing handoff velocity thresholds and maximizing the overall system capacity, subject to per-tier blocking probability constraints that ignore co-channel interference (CCI). In contrast, our work determines the spectrum allocation which maximizes the system-wide ASE considering interference from neighboring BSs, path loss and prevailing channel conditions.
@cite_27 @cite_10 @cite_12
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Multitier networks that provide coverage to the same areas by several cells of different sizes are useful to accommodate high traffic density while keeping high quality of service. The author gives an overview of a number of contributions on this subject. Two main issues are considered: spectrum sharing between different layers with a focus on F TDMA systems and teletraffic performance of multitier networks given different handover policies. In this paper, we study spectrum management in a two-tier microcell macrocell cellular system. Two issues are studied: micro-macro cell selection and frequency spectrum partitioning between microcells and macrocells. To keep the handoff rate in a two-tier cellular system at an acceptable level, low mobility users (with speed spl upsi V sub 0 ) should undergo handoffs at macrocell boundaries. The mobile determines user mobility from microcell sojourn times and uses it for channel assignment at call origination and handoff. The probability of erroneous assignment of a mobile to a microcell or macrocell is shown to be significantly lower than previous approaches. We investigate the optimal velocity threshold, V sub 0 , and propose that it may be dynamically adjusted according to traffic load. Finally, we propose a systematic way for finding an optimal partition of frequency spectrum between microcells and macrocells. This partitioning is based on the traffic load and velocity distribution of mobiles in the system. The umbrella cell system, where the same radio system is used for microcells and overlaying macrocells, is the most promising strategy for deploying microcell service to cope with increased portable radio subscribers. A practical approach to implementing a microcell system overlaid with an existing macrocell system is proposed. There is no need to design the channel assignment and transmit power management for the microcell system by introducing the channel segregation, a self-organized dynamic channel assignment and the automatic transmit power control. The system channels are reused automatically between the macrocells and microcells. The interference from the macrocell to the microcell is compensated by slight increase of transmit power for the microcell system. It is shown by computer simulation that locally increased traffic is accommodated with the microcells laid under macrocells without any effort for channel management. >