The Enhanced UMTS Radio Access Network Extensions (EURANE) provided UMTS and HSDPA support in the Network Simulator (NS-2). The main components of EU-RANE include the Radio Link Control (RLC) layer with the Acknowledged and Unacknowledged Modes, MAC layer for the main UMTS and HSDPA transport channels and a physical-layer simulation tool for simulating the fast channel variations in HSDPA. Although it has added most of the UMTS protocol stack to NS-2, EURANE does not support multiple cells, user mobility or handover which are necessary to study the system-level performance of RLC layer, handover and Iub flow control. In this paper, we describe new extensions and a redesign of EURANE to overcome these limitations and provide additional functionality. The new simulator supports user mobility using Random Waypoint mobility models and handover. In addition, the simulator provides an enhanced signaling protocol for Iub flow control that allow adding new flow control algorithms to EURANE whenever needed.

The primary challenge in heterogeneous cellular networks is mitigating inter-cell interference especially in dense deployments of residential femtocells. Traditionally, interference in wide-area cellular networks has been studied from a multicell resource allocation perspective, where radio resources, e.g. power, bandwidth, in each cell are centrally managed to reduce interference. This approach generally assumes a fully loaded network (i.e. many simultaneous active users in a cell) where the system performance is insensitive to the activity of a single user. This assumption is not suitable for femtocells which are designed to serve very few users and thus lack the presumed traffic aggregation. In this paper, we quantify the impact of the user activity factor by comparing the performance of centralized resource allocation schemes represented by a highly idealized power control protocol to that of a simple random access protocol. Our analysis shows that, with a high probability, a simple unoptimized random access protocol would perform much better than a fair, optimized power-controlled network in most of the unsaturated traffic scenarios. If the network is saturated, then a fixed orthogonal partitioning of the channel bandwidth among femtocells would be the best strategy to handle the interference. This result highlights the importance of modeling the random user activities and traffic patterns in the design and analysis of femtocell interference mitigation solutions.Index Terms-OFDM multiple access interface, random access protocols, OFDMA femtocells, 4G mobile communication. 0733-8716 (c)

Abstract-OFDMA is the basis of future broadband access, due to its many inherent advantages such as scalability and fine granularity for multi-user access. OFDMA-Aloha combines the flexibility of OFDMA with basic Aloha's collision resolution mechanism over sub-carriers, in an attempt to reduce packet collisions and achieve faster retransmission. However, this comes at the expense of a larger slot size, due to lower channel rates per subcarrier. The above gives rise to a fundamental question: whether to use a single wide-band Aloha channel and retransmit via random back-off in next K time slots, or to retransmit immediately in one of K narrow-band sub-channels which are each 1/K slower (OFDMA-Aloha)? We answer this question, by analyzing the two protocols: Aloha and OFDMA-Aloha under the same total bandwidth and load conditions. We first derive the exact distribution of the packet access delay of OFDMAAloha in the saturated case. Then, we extend the analysis to the unsaturated case and derive the mean queue length and packet delay by decomposing the system of interfering queues into multiple independent queues utilizing the symmetry in our system. Our results show that if the network is already saturated, channelization does not bring substantial reduction in the collision rate to the point where it outweighs the effect of expanded slot size. In this case the single channel Aloha performs better than OFDMA-Aloha especially when the gap between the number of channels and the number users is large. On other hand, when the network is lightly loaded, OFDMA-Aloha enjoys smaller packet delays, but not for long as it saturates faster than the single channel Aloha.

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