This paper considers a two-tier femto-macro overlay network architecture, where groups of femtocells are overlaid by macrocells. The capacity of the femtocells is limited, thus redirection of blocked femto calls to the overlaid macrocell is used as a QoS assurance strategy. Similar overflow approaches have been used in telephony; however, extension of this approach to capacity-constrained femto-macro overlay networks poses significant challenges due to the wide variety of bandwidth requirement of Internet services, the large number of femtocells overlaid by a macrocell, and the need for fast computations imposed by practical requirements of dynamic real-time resource allocation. We propose a new computationally efficient numerical method for the loss probability approximation in such two-tier femto-macro overlay networks. The accuracy of the method is verified by exact Markov-chain solutions for small systems and by simulations for larger systems.

Performance modeling is important for the purpose of developing efficient dimensioning tools for large complicated networks. But it is difficult to achieve in heterogeneous wireless networks, where different networks have different statistical characteristics in service and traffic models. Multiservice loss analysis based on multi-dimensional Markov chain becomes intractable in these networks due to intensive computations required. This paper focuses on performance modeling for heterogeneous wireless networks based on a hierarchical overlay infrastructure. A method based on decomposition of the correlated traffic is used to achieve an approximate performance modeling for multiservice in hierarchical heterogeneous wireless networks with overflow traffic. The accuracy of the approximate performance obtained by our proposed modeling is verified by simulations.

The emerging IEEE 802.16j mobile multi-hop relay (MMR) network is currently being developed to increase the user throughput and extend the service coverage as an enhancement of existing 802.16e standard. In 802.16j, the intermediate relay stations (RSs) help the base station (BS) communicate with those mobile stations (MSs) that are either too far away from the BS or placed in an area where direct communication with BS experiences unsatisfactory level of service. In this paper, we investigate the uplink Erlang capacity of a two-hop 802.16j relay system supporting both voice and data traffics with adaptive modulation and coding (AMC) scheme applied in the physical layer. We first develop analytical models to calculate the blocking probability in the access zone and the outage probability in the relay zone, respectively. Then a joint algorithm is proposed to determine the bandwidth distribution between the access zone and the relay zone, and to derive the Erlang capacity region of the system. The numerical examples show that some capacity gains can be obtained with a relay-enhanced 802.16j system compared to the conventional single-hop 802.16e system.

Erlang capacity is traditionally defined as the maximum value of offered traffic among different service classes that the system can support when the blocking probabilities at the call admission control (CAC) level do not exceed certain thresholds. That is valid when a fixed amount of bandwidth is allocated to each user in each frame during the whole service time. However, with the introduction of adaptive modulation and coding (AMC) scheme employed at the physical layer, outage might occur due to the fact that the allocation of bandwidth is dynamic based on the time-varying wireless channel conditions. In this paper, we evaluate the Erlang capacity of a TDMA system with AMC supporting voice and data traffics, by taking both the blocking and the outage probabilities into account. The analytical models for calculating the blocking and the outage probabilities are developed separately, and a joint algorithm for determining the Erlang capacity of the system is proposed with some numerical examples.

To solve the performance degradation issue in current WLAN caused by the crowded unlicensed spectrum, we propose a cognitive radio (CR) media access protocol, C-CSMA/CA. The basic idea is that with cognitive radio techniques the WLAN devices can not only access the legacy WLAN unlicensed spectrum but opportunistically access any other under-utilized licensed spectrum without a license. The application scenario of C-CSMA/CA is infrastructure BSS (Basic Service Set) WLAN. C-CSMA/CA efficiently exploits the inherent characteristics of CSMA/CA to design distributed cooperative outband sensing to explore spectrum hole; moreover, it designs dual inband sensing scheme to detect primary user appearance. Additionally, C-CSMA/CA has the advantage to effectively solve the cognitive radio self-coexistence issues in the overlapping CR BSSs scenario. It also realizes station-based dynamic resource selection and utilization. It is compatible with any legacy WLAN (BSS) system. We develop and implement the simulation of C-CSMA/CA by OPNET. The simulation results show that C-CSMA/CA highly enhances throughput and reduces the queuing delay and media access delay.

IEEE 802.16, known as WiMAX, has received much attention for its capability to support multiple types of applications with diverse QoS requirements. Beyond what the standard has defined, radio resource management (RRM) still remains an open issue. In this paper, we propose a hierarchical downlink resource management framework for OFDMA based WiMAX systems. Our framework consists of a dynamic resource allocation (DRA) module and a connection admission control (CAC) module. DRA emphasizes on how to share the limited radio resources in term of subchannels and time slots among WiMAX subscribers belonging to different service classes with the objective of increasing the spectral efficiency while satisfying the diverse QoS requirements in each service class. CAC highlights how to limit the number of ongoing connections preventing the system capacity from being overused. Through system-level simulation, it is shown that the proposed framework can work adaptively and efficiently to improve the system performance in terms of high spectral efficiency and low outage probability.

In this paper we propose the one4all cooperative access strategy to introduce a more efficient media access strategy for wireless networks. The one4all scheme is designed for the infrastructure based distributed wireless network architecture. The basic idea is that mobile devices can form a cooperative cluster using their short-range air interface and one device contends the channel for all the devices within the cluster. This strategy reduces the number of mobile devices involved in the collision process for the wireless medium resulting in larger throughput, smaller access delay, and less energy consumption. Based on an analytical model, the proposed strategy is compared with the two existing strategies RTS/CTS (request to send/ clear to send) and packet aggregation. The results show that the proposed cooperative scheme has similar throughput performance as packet aggregation and it has much higher throughput than the conventional RTS/CTS scheme. Furthermore, the newly introduced cooperative scheme outperforms packet aggregation in terms of channel access delay and energy consumption.

Packet reservation multiple access (PRMA) protocol is an implicit reservation MAC protocol. It is initially designed for voice packets in the cellular networks, but it is currently also used for data packets in OFDM based fixed wireless access networks. When it is applied for data packets, the system throughput depends on the size of packets and the number of consecutive packets. From the statistics of existent wireless data networks using PRMA protocol, it shows that the system throughput is quite low because of the inconsecutive small packets. In order to improve the throughput, packet aggregation scheme is considered to be applied in PRMA. Before designing packet aggregation algorithm, it is worth investigating the effect of packet aggregation scheme on the performance of throughput and delay. In this paper we develop a generic Markov chain model for PRMA with packet aggregation. Based on this model the throughput and delay are derived and analyzed. A numerical example is calculated, which illustrates the effect of packet aggregation on the throughput and delay with varying packet arrival rate. The results of the paper are valuable inputs for designing optimal packet aggregation algorithm, considering the tradeoff between throughput and delay.

This paper studies loss calculation in hierarchical networks with multiservice overflows which have different call arrival rates, mean holding times, bandwidth requirements and share a common link. The loss calculation involves two challenging problems: 1) the computation of the two moment characterizations of multiservice overflow traffic over the shared link, 2) the calculation of the loss probabilities for multiservice non-Poisson overflow traffic in hierarchical systems. An efficient approximation method, known as multiservice overflow approximation (MOA), is proposed to enable multiservice networks designs with hierarchical architecture. Two contributions are included in the MOA method. First, an approximation based on blocking probabilities matching is proposed to compute the variances of multiservice overflows over the shared link. Second, a modified Fredericks & Hayward's approximation is used to calculate the loss probabilities of multiservice non-Poisson over flow traffic. The performance of the MOA method is evaluated in a two-tier hierarchical cellular network and compared with an existing approximation method based on multi-dimensional Markov-modulated Poisson process (MMPP). Verified by simulations, the MOA method achieves better accuracy in the general heterogeneous cases at lower computational cost than the MMPP method.

Reliable multicast applications such as software distribution, data distribution and replication and mailing list delivery, etc. [1] are getting more and more interests from network and service providers. The conventional error/loss recovery schemes are not efficient when they are applied to multicast scenarios in wireless networks. The reason lies in the unreliable wireless channel, the limited wireless bandwidth and resource, the battery powered wireless devices, and others. To have an effective error/loss recovery scheme for reliable multicast in wireless networks, we advocate a new communication architecture. It is referred to as cellular controlled peer-to- peer network, where the mobile devices communicate directly with each other to perform cooperative retransmissions using their short-range communication capabilities in addition to their cellular links. Based on the cooperative architecture a novel retransmission scheme is proposed exploiting the short-range retransmission in this paper. The state of the art, the non-cooperative error recovery schemes (e.g., ARQ, Layered FEC and Integrated FEC II) and the proposed scheme are compared with each other in terms of energy consumption to show the benefit of the newly introduced scheme.

Packet reservation multiple access (PRMA) protocol is an implicit reservation MAC protocol. It is initially designed for voice packets in the cellular networks [2, 3] but it is currently also used for data packets in OFDM based fixed wireless access networks [8, 9], When it is applied for data packets, the system throughput depends on the size of packets and the number of consecutive packets. From the statistics of existent wireless data networks using PRMA protocol, it shows that the system throughput is quite low because of the inconsecutive small packets. In order to improve the throughput, packet aggregation scheme is considered to be applied in PRMA. Before designing packet aggregation algorithm, it is worth investigating the effect of packet aggregation scheme on the performance of throughput and delay. In this paper we develop a generic Markov chain model for PRMA with packet aggregation. Based on this model the throughput and delay are derived and analyzed. A numerical example is calculated, which illustrates the effect of packet aggregation on the throughput and delay with varying packet arrival rate. The results of the paper are valuable inputs for designing optimal packet aggregation algorithm, considering the tradeoff between throughput and delay.

Fast recovery from software and hardware failures is very essential to communication systems, especially, when it is used for mission-critical applications such as public safety systems. A failure in the network infrastructure can affect a large number of users and may result in loss of lives. The infrastructure software applications that provide services to the mobile stations according to some defined communication protocols play a key role for system availability. The real-time peer-to-peer nature of these communication protocols poses a real challenge in developing a recovery mechanism that can work in such environments. In this paper, we introduce a new recovery method that takes into account the layered architecture of the communication protocols and their peer-to-peer communication pattern. The method is based on communicating extended finite state machine and does not assume transient and fail-stop failures. Furthermore, an experimental testbed has been implemented to evaluate our new approach. The experimental results have shown that the infrastructure applications can reliably recover and quickly restore the servicing level that the system was performing immediately prior to the failure. Moreover, the failure-free overhead caused by this approach is relatively low, and is experimentally found to be less than 5%.