Femtocell Coverage Research Articles

Interference reduction through femto-relays

Femtocell, also known as home base station is one of the promising cost-effective technologies that ensure a steady increase in system capacity, improvement in service quality and a better data rate for users mainly in the indoor environment. However massive rollout of femtocells cause severe interference to the existing macrocell network and drive users into outage. Moreover, though femtocells can provide improved home coverage and capacity for indoor users, many a times, these femtocells are under-utilised when they do not serve any voice or data request. In this paper a novel idea to extend the coverage of femtocells beyond the home environment through multiple hops is proposed. The idea is to exploit the under-utilised femtocells to serve the high data rate requests of many outdoor user equipments and reduce the probability of their outage. This will not only extend the coverage of femtocell and relax the burden on the macrocell base station, but also helps in reducing the interference to the co-existing macrocell network, because of reduced transmission power and path loss, achieved through relaying.

LTE 기반 펨토셀 네트워크에서 간섭 완화를 위한 적응적 전력 제어 기법

가중되는 기지국의 트래픽 부하를 줄이면서 실내 음영 지역 문제를 해결하기 위해 저비용, 고성능의 펨토셀(femtocell)에 대한 연구가 활발히 진행되고 있다. 그러나 사무실 빌딩 환경 등 다수의 펨토셀이 설치되는 밀집된 펨토셀 환경에서는 동일 주파수간 간섭 현상으로 인해 전송률이 저하되고 전송품질이 저하되는 등 많은 문제가 발생하게 된다. 본 논문에서는 3GPP LTE 기반 밀집된 펨토셀 환경에서 간섭 완화를 위한 전력 제어 기법을 제안한다. 특히, 기존 매크로셀(eNB)로 부터 서비스 받는 mUE(macro User Equipment)가 펨토셀 주변에 매우 가까이 존재할 경우, 밀집된 펨토셀로부터 mUE에게 영향을 주는 펨토셀의 간섭을 최소화하고 전송 성능을 향상시킬 수 있는 최적의 전력제어 기법을 제안한다. 제안하는 전력 제어 기법은 펨토셀의 밀도와 펨토셀의 전송 성능을 보장하면서 mUE의 성능을 향상시킬 수 있도록 설계하였다. 단말의 Outage Probability와 매크로셀과 펨토셀의 신호대 잡음비(SINR)를 주요 성능 지표로 분석한 결과 본 논문에서 제안하는 전력제어 기법은 간섭 제어를 통해 펨토셀의 성능을 유지하면서 mUE의 성능을 30% 이상 개선할 수 있는 것으로 나타났다. The low-power, low-cost femtocell network has been proposed not only to alleviate traffic load to the macro base station (eNB) but also to cover the indoor coverage hole problem. However, in the dense femtocell environment where many femtocells are deployed to cover the whole large office building, performance of such femtocell environment can be deteriorated due to severe co-channel interference problem between the eNB and femtocells and among neighboring femtocells. In particular, a macro UE(mUE) located within femtocell coverage may experience severe co-channel interference from surrounding femtocells. Therefore, In this paper, we propose a novel power control schemes to mitigate interference to a mUE under such dense LTE femtocell environment. With proposed femtocell power control schemes, performance of the mUE can be greatly improved in terms of the outage probability and the SINR while maintaining satisfying femtocell performance. Simulation based performance study shows that the proposed power control scheme is able to enhance mUE performance more than 30% than the conventional dense femtocell in terms of the two performance metrics.

Femtocells: Smart Approach Analysis to Macrocells

are intelligent cellular access points that support any mobile device using standard cellular air interfaces, such as UMTS, CDMA2000 and LTE. They contain cellular radios that are tightly integrated with the existing macrocell radio network and thus create a seamless experience for mobile users as they move in and out of femtocell coverage, whether on an active call or in standby. Femtocells learn their radio frequency (RF) surroundings and use this information to self-optimize their operation. In particular, femtocells use their RF awareness to control interference between themselves and between femtocells and macrocells. The surest way to increase the system capacity of a wireless link is by getting the transmitter and receiver closer to each other, which creates the dual benefits of higher quality links and more spatial reuse. In a network with nomadic users, this inevitably involves deploying more infrastructure, typically in the form of microcells, hotspots, distributed antennas, or relays In this paper contains the technical view for femtocells, and describe the state-of-the-art on each front. Also describe the technical challenges facing femtocell networks, and give some preliminary ideas for how to overcome them. Keywords-Advanced, 3GPP, Relay deployment, backhaul, amplify and forward relaying.

셀룰러 펨토 시스템에서 부하 분산을 통한 분산적 부채널 ON/OFF 스케쥴링 기법

셀룰러 펨토 시스템에서 펨토 기지국(f-BS: femto base station)은 좁은 영역에 중복 설치 및 과밀 설치될 수 있다. 이러한 불필요한 설치는 채널을 공유하는 인접 f-BS간 간섭문제를 야기하여서 시스템의 용량과 커버리지에 영향을 미칠 수 있다. 본 논문은 셀룰러 펨토 시스템에서 발생할 수 있는 이러한 성능하락 문제를 해결하기 위하여 강제 핸드오버를 이용한 부하분산과 확률적 자원 이용방법을 제시한다. 제안하는 기법은 중앙 컨트롤러의 조정이 아닌 이웃 f-BS간의 통신을 통한 분산적인 방법이며, f-BS가 주변 정보를 수집하여 스스로 과밀지역에 분포하였음을 인식하고 부하 및 자원 이용의 조절하는 방법을 포함한다. 평균 셀 수율, 사용자당 평균 수율을 바탕으로 제안하는 기법의 성능 향상을 모의실험을 통해 검증하였다. In cellular femto systems, femto Base stations(f-BSs) can be installed unnecessarily and overcrowded in small areas. This will cause an interference problem and it can impact on the capacity, blocking probability, and coverage of femtocells in the shared channel systems. In this paper, we propose a load distribution scheme by using forced handover and probabilistic subchannel scheduling policy to resolve the problem. The proposed scheme operates in distributed manner though communication with neighboring f-BSs, and includes self-detection of overcrowded area and radio resource management based on measurements. We evaluate the performance of the proposed scheme in terms of average cell throughput and average throughput per users.

Optimisation approach for femtocell networks using coordinated multipoint transmission technique

Proposed is a new coordinated multipoint transmission technique to optimise femtocell coverage as well as to eliminate interference between them, The performance of the proposed algorithm is analysed and evaluated.

On Two-Tier Femtocell over Fading Environments

In this paper the system performance of an MC-CDMA (multi-carrier coded-division multiple-access) system operating over single-cell with two-tier femtocell environment is analyzed. Consider two-tier scenario is deployed with a macrocell site in which is being surrounded several femtocells, which are designed to serve a group of subscribers locate in a small coverage area such as small office, home office or a house. The coverage area is typically up to 100 meters in radius. Mostly, the femtocell is applied to serve indoor subscribers, thus, the Rayleigh fading is adopted to characterize the propagation channel. The technique of TH-MC- CDMA (time-hopped multi-carrier coded-division multiple-access) technique is supposed to transmit each symbol alternatively with fair time (slotting)/frequency (coding) for each user in the hotspots (the area around 0th femtocell). The intensity of signals estimated at a mobile unit located in the second tie, i.e., the femtocell coverage area, is an important issue. The contribution of the paper is mainly to evaluate the system performance with both the BER (bit error rate) and OP (outage probability) according to the most important parameters, for example, the activating user number, the hopping number provided by TH-MC-CDMA system and the subcarrier number. Furthermore, the discussion of interference avoidance is also discussed with non-analytically.

Self-Optimized Coverage Coordination in Femtocell Networks

This paper proposes a self-optimized coverage coordination scheme for two-tier femtocell networks, in which a femtocell base station adjusts the transmit power based on the statistics of the signal and the interference power that is measured at a femtocell downlink. Furthermore, an analytic expression is derived for the coverage leakage probability that a femtocell coverage area leaks into an outdoor macrocell. The coverage analysis is verified by simulation, which shows that the proposed scheme provides sufficient indoor femtocell coverage and that the femtocell coverage does not leak into an outdoor macrocell.

Interference Avoidance and Resource Allocation for OFDMA Downlink of Femtocells with Distributed Power Control

OFDMA femtocells in the macrocellular network of which frequency reuse factor is 1 cause serious interference to macrocell users, while the femtocells improve the performance of indoor users. In this letter, a novel downlink resource allocation algorithm for OFDMA femtocell networks is proposed to reduce interference between the macrocells and the femtocells. This algorithm allocates femtocell subchannels to avoid interference to macrocell users in the femtocell coverage, and minimizes the total transmission power of the femtocell to reduce the negative effect on the performance of the macrocell. Simulation results are provided to present the performance of the proposed algorithm.

Comparing coverage quality for femtocell and macrocell broadband data services

This article presents the results of a series of field tests to characterize femtocell coverage quality in typical residential houses with respect to high-speed data services. The study proves femtocell users can expect to observe an improvement of more than five times in average airlink data speeds relative to the existing macrocell network. The article presents the methodology used in the tests, and offers summary results for both 1xEV-DO and HSDPA technologies. The results presented here show that the case for using femtocells for broadband data services is even stronger than for voice-based services.

Self-Optimization of femtocell coverage to minimize the increase in core network mobility signalling

Femtocells are low-cost, low-power cellular base stations that are deployed by the end user to supplement macrocellular coverage and provide high data rates in the customer's premises. In femtocell deployments, leakage of the pilot signal to the outside of a house can result in a highly increased signalling load to the core network, as a result of the higher number of mobility events caused by passing users. In this paper, the impact of these mobility events on the core network signalling load is investigated for urban areas. It is shown that without further optimization, the resulting increase in signalling is unacceptably high and as a result prevents the large-scale deployment of femtocells. Different self-optimization methods for residential femtocell coverage are investigated that can significantly reduce the resulting mobility events. First, mobility event based pilot power self-optimization is proposed for femtocells with a single antenna that matches the coverage to the size of the house and thereby can provide both good indoor coverage and a reduced level of mobility events. It is shown that the proposed coverage optimization can significantly outperform simpler methods that aim to achieve a constant cell radius. Then, this concept is further refined by using a low-cost switched pattern multi-element antenna solution. Antenna gain pattern measurements of a prototype with two patch and two inverted F antennas are presented and a corresponding feeder network is discussed. Self-optimization methods are proposed that jointly select an appropriate antenna pattern and optimize the pilot power. Finally, a second multi-element antenna solution is investigated where lobes in different directions, generated by several patch antennas, can be individually attenuated to shape the coverage. Both multi-element antenna solutions allow for a better match of femtocell coverage to the shape of each individual house, and result in a further improvement of both indoor coverage and core network signalling resulting from mobility events.

Coverage in multi-antenna two-tier networks

In two-tier networks comprising a conventional cellular network overlaid with shorter range hotspots (e.g. femtocells, distributed antennas, or wired relays) with universal frequency reuse, the near-far effect from cross-tier interference creates dead spots where reliable coverage cannot be guaranteed to users in either tier. Equipping the macrocell and femtocells with multiple antennas enhances robustness against the near-far problem. This work derives the maximum number of simultaneously transmitting multiple antenna femtocells meeting a per-tier outage probability constraint. Coverage dead zones are presented wherein cross-tier interference bottlenecks cellular and femtocell coverage. Two operating regimes are shown namely 1) a cellular-limited regime in which femtocell users experience unacceptable cross-tier interference and 2) a hotspot-limited regime wherein both femtocell users and cellular users are limited by hotspot interference. Our analysis accounts for the per-tier transmit powers, the number of transmit antennas (single antenna transmission being a special case) and terrestrial propagation such as the Rayleigh fading and the path loss exponents. Single-user (SU) multiple antenna transmission at each tier is shown to provide significantly superior coverage and spatial reuse relative to multiuser (MU) transmission. We propose a decentralized carrier-sensing approach to regulate femtocell transmission powers based on their location. Considering a worst-case cell-edge location, simulations using typical path loss scenarios show that our interference management strategy provides reliable cellular coverage with about 60 femtocells per cell-site.