DAS, Uncoordinated Femto and Joint Scheduling Systems for In-building Wireless Solutions Abstract—Small cells are popular deployment options for coverage holes and capacity hotspot areas. Due to the poor outdoor-to-indoor propagation property of in-building environment, a dedicated wireless system installed inside the building is often preferred for providing indoor users high- data-rate services. Distributed antenna systems and Femto cells are cost-efficient techniques for this application. In this paper, their performance is evaluated in an LTE downlink context along with a proposed joint scheduling system, which maximizes the supported number of users under a QoS constraint. The selection of the enterprise building model includes a general office building model described in the WINNER II project and a site-specific office building with large scale path-loss values retrieved from measurements. Results show superior performance of the Femto system compared to DAS in providing high-data-rate services in most cases with a quality-guaranteed scheduler, while the centralized joint scheduling system gives the best performance. The centralized scheme can also help improve the system robustness in obtaining high performance even in the situation where access points are placed non-optimally. I. INTRODUCTION Nowadays, the rapid growth of high data rate applications has stressed the current macro cellular network infrastructure and facilitated the deployments of small cells. Therefore, Pico and Micro base stations (BS) are deployed in large scale networks at coverage holes and hotspot areas. By decreasing cell size, higher capacity is expected benefiting from massive geometrical reuse of available resources. On the other hand, the majority amount of the mobile traffic is originated from indoor. According to [1], in 2007, more than 50% and 70% of voice calls and data traffic, respectively, are generated by indoor users. However, providing indoor coverage by outdoor sites leads to poor indoor radio quality due to high wall penetration loss. The dedicated in- building wireless system with radio sources emitting inside building is regarded a more efficient approach for providing indoor services. Current in-building wireless solutions include indoor distributed antenna system (DAS) and the envisioned Femto cell systems. DAS is composed of many remote antenna ports distributed over a large area and connected to a single BS by fiber, coax cable or microwave links. Without advanced signal processing at the central BS, the same downlink signal is broadcasted on all of its antennas, or so called simulcast. Studies show that simulcasting is an effective means to combat shadowing in noise limited environment thanks to transmitter macrodiversity [3]. DAS is first introduced for indoor usage by Saleh [2]. Indoor DAS can help increase coverage and signal-to-noise ratio (SNR) at the same transmission power or achieve power saving and low inter-cell interference. DAS is not implemented in large scale networks, i.e. macro networks, due to high cost, but because of shortened cabling distance and high indoor capacity, literatures argue that DAS is cost-efficient to be used indoor comparing to microcells [3]. Femto is an alternative cost-efficient way to extend indoor coverage and capacity. Femtos are portable, low cost and low range BSs first designated for home usage, where they are deployed by end users and connected to the operator network by residential digital subscriber line (DSL) or cable broadband. With a bit higher Radio Frequency (RF) power and higher supported user number, Femto can be well suited for enterprise offices or campus buildings. In such an environment, due to its low cost, Femto BSs could potentially be deployed with high density, i.e., number of Femto as many as remote antenna ports in DAS. The problem of close-by Femto deployment is the severe inter-cell interference: in user random deployment cases, without any interference management, Femto system performance is significantly degraded [4]. To solve this problem, many studies look for solutions by using soft frequency reuse or flexible spectrum reuse, such as F-ALOHA [5] and ACCS [6]. F- ALOHA is a decentralized interference avoidance algorithm proposes that each Femto cell accesses a random subset of the candidate frequency sub-channels, whereas ACCS takes advantage of exchanged cell load information and measured interference level to dynamically allocate frequency chunks to each cell. In this study, we regard to Femto as uncoordinated Femto with no inter-cell information exchange or interference management. Besides the DAS and uncoordinated Femto systems, we propose a centralized coordinated scheduling scheme which assumes all cells are connected to a central node where cell information is gathered and coordinated resource management could be achieved. Centralized algorithms are also developed addressing the inter-cell interference issue in other studies [7][8], but often with increased implementation complexity and are hardly realized in practice. The algorithm we use is of low complexity, which is expected implementable in real products. It facilitates resource sharing subject to maximizing individual user effective spectral efficiency. In this paper, we investigate the performance of these systems in an enterprise office building environment in LTE downlink context. Results are generated with WINNER II [9] office building model and channel model, and further verified with a site-specific building model with large scale path-loss values retrieved from real-life measurement [10]. The comparison of different systems is only provided from the radio performance point of view. The purpose is to compare solutions with increasing but practical level of complexity and relevance, based on a fair comparison of radio equipment such as equal number of access points, equal output power per access point, etc. However, other important features such as economical cost analysis and practical implementation considerations have to be included to provide an integrated future enterprise wireless solution vision, which will be the target of our future work. II. ENTERPRISE SOLUTIONS In this study, we will compare the radio performance of DAS, un-coordinated Femto and joint scheduling system when serving an enterprise building. (a) (b) Figure 1. Multi-AP scenarios Zhen Liu (1) , Troels Sørensen (1) , Jeroen Wigard (2) , Preben Mogensen (1,2) (1) Aalborg University, (2) Nokia Siemens Networks, Aalborg, Denmark e-mail: zl@es.aau.dk 978-1-4244-8331-0/11/$26.00 ©2011 IEEE