It is known that the achievable data rate per user can be increased when relays are deployed in wireless networks. However, the drawback of this solution is that some of the network's resources should be allocated to the relays. In this paper, we consider a two-tier network in which all users send or receive data in two hops. By applying vector quantization, we compute the relays' locations to improve network's average transmission rate. These locations are also computed analytically when the number of relays is less than six. Having determined the relays' locations, the network's average transmission rate is evaluated. Subsequently, we define the "neutrality-surface" such that the performance of any relay network operating below this surface is inferior to that of the same network without relays. Finally, we study the relative relaying gain for different network configurations.

Channel Inversion, and its Minimum Mean Square Error (MMSE) variation, are low complexity methods for Space Division Multiple Access (SDMA) in Multiple Input Multiple Output Broadcast Channel (MIMO-BC). As the channel matrix deviates from orthogonal, these methods result in a waste of transmit power. This paper proposes a trellis precoding method (across time and space) to improve the power efficiency. Adopting a 4-state trellis shaping method from [1], the complexity of the proposed method, which is entirely at the transmitter side, is equivalent to the search in a trellis with 4(N) states where N is the number of transmit antennas. Numerical results are presented showing that the achievable gains, which depend on the channel realization, can be significantly higher than the traditional shaping gain which is limited to 1.53dB.

With the co-existence of different wireless networks, which exhibit largely different bandwidth and coverage characteristics, much interest has been involved in integrating these networks to support smooth and efficient multimedia services. In this paper, we present an analytical framework for variable-bit-rate (VBR) video streaming in a two-tier wireless network with VBR channels. We derive the expected number of jitters and average buffering delay during video playback as measures of system performance. Our objective is to discover heterogeneous networking attributes that may influence the streaming performance, in terms of the tradeoff between jitter frequency and buffering delay. Through experimenting with a wide range of fixed, separate, and jointly optimal jitter-recovery buffering schemes, based on buffering delay, buffered data, and buffered playback duration, we quantify the benefit of incorporating user location information in streaming over heterogeneous wireless networks.

In this paper, we consider an interworking architecture of wireless mesh backbone and propose an effective vertical handoff scheme between 802.11 and 802.16 wireless access networks. The proposed vertical handoff scheme aims at reducing handoff signaling overhead on the wireless backbone and providing a lower handoff delay to mobile nodes. The handoff signaling procedure in different scenarios is discussed. Together with call admission control, the vertical handoff scheme directs a new call request in the 802.11 network to the 802.16 network, if the admission of the new call in the 802.11 network can degrade quality-of-service (QoS) of the existing real-time traffic flows. Simulation results demonstrate the performance of the handoff scheme with respect to signaling cost, handoff delay, and QoS support.

It is known that the achievable data rate per user can be increased when relays are deployed in wireless networks. However, the drawback with this solution is that some of the network resources should be allocated to the relays. In this paper, we consider a two-tier network where all users should send/receive data in two hops (via a relay). Applying vector quantization, we approximately find the location of the relays. These approximate relays' locations are also computed analytically when the number of relays is less than six. Having the relays' locations, the network average transmission rate is evaluated in terms of a set of network parameters. Then, in the multi-dimensional space of these network parameters, we introduce the concept of neutrality-surface. The neutrality-surface is defined such that the performance of any relay network operating below this surface is inferior to that of a simple no-relay network with the same parameters. Finally, we study the relative and differential relaying gain for different network configurations.

In this paper, we propose a novel node clustering algorithm with effective tax-based subcarrier allocation tailored for wireless mesh networks with quality-of-service support. With effective frequency reuse, our proposed approach is shown to achieve a higher system throughput than a conflict-graph approach and a baseline approach. Also, our approach is demonstrated promising in performance tradeoff between packet delay and end-to-end transmission rate.

The future-generation wireless systems will combine heterogeneous wireless access technologies to provide mobile users with seamless access to a diverse set of applications and services. The heterogeneity in this inter-technology roaming paradigm magnifies the mobility impact on system performance and user perceived service quality, necessitating novel mobility modeling and analysis approaches for performance evaluation. In this paper, we present and compare three mobility models in two-tier integrated heterogeneous wireless systems, the independence model as a naive extension of the traditional cell residence time modeling techniques for homogeneous cellular networks, the basic Coxian model which takes into consideration the correlation between the residence time within different access technologies, and the extended-Coxian model for further improved estimation accuracy. We propose a general stochastic performance analysis framework based on application session models derived from these mobility models, applying it to a 3G-WLAN integrated system as an example. Our numerical and simulation results demonstrate the general superiority of Coxian-based mobility modeling over the independence model. Furthermore, using the proposed modeling and analysis methods, we investigate the impact of different parameters on system performance metrics such as network utilization time, handoff rates, and forced termination probability, for a wide range of user applications.

The integration of different wireless access technologies is propelled by the need to support new services and better resource utilization in next-generation wireless networks. This integration complicates the system design due to the interaction of different factors including network-oriented, application-oriented, and user-oriented system parameters. In this work, we present an analytical framework to estimate different session-level performance metrics in two-tier systems, using the 3G-WLAN integrated network as an example. We investigate the impact of the amount of coverage overlap and the topology of the underlay technology on different session performance metrics as well as the total session cost. The obtained results show that clustering can significantly reduce the vertical-handoff signaling load and the forced termination probability of different applications in comparison with a random topology. Additionally, the proposed cost analysis provides design guidelines for developing economical WLAN management mechanisms to maintain reduced session cost with extended WLAN coverage.

The integration of wireless local area network (WLAN) hotspot and the 3G cellular networks is imminently the future mode of public access networks. One of the key elements for the successful integration is vertical handoff between the two heterogeneous networks. Service disruption may occur during the vertical handoff because of the IP layer handoff activities, such as registration, binding update, routing table update, etc. In this paper, the network interface switching and registration process are proposed for the integrated WLAN/cellular network. Two types of fast vertical handoff protocols based on bicasting and non-bicasting supporting real-time traffic, such as voice over IP, are modeled. The performance of a bicasting based handoff scheme is analyzed and compared with that of fast handoff without bicasting. Numerical results and the simulation are given to show that packet loss rate can be reduced by the bicasting during handoff scheme without increasing bandwidth on both wireless interfaces. Copyright © 2006 John Wiley & Sons, Ltd.

In this paper, an agent based WLAN/cellular network integrated service model and relevant authentication scheme is proposed. The service model, or solution, does not require cumbersome peer-to-peer roaming agreements to provide seamless user roaming between WLAN hotspots and cellular networks, which are operated by independent wireless network service providers. Security analysis and overhead evaluation are given to demonstrate that the proposed service model and the supporting schemes are secure and effective.

The Third Generation (3G) cellular networks provide ubiquitous connectivity but low data rates, whereas Wireless Local Area Networks (WLANs) can offer much higher data rates but only cover smaller geographic areas. Their complementary characteristics make the integration of the two networks a promising trend for next-generation wireless networks. With combined strengths, the integrated networks will provide both wide-area coverage and high-rate data services in hot spots. There are many aspects involved in their interworking, such as mobility, security and Quality of Service (QoS) provisioning. In this paper, we present a survey of most recent interworking mechanisms proposed in the literature, and outline some important open issues to achieve seamless integration.

In this paper, an agent-based integrated service model for wireless local area network (WLAN)/cellular networks and the relevant authentication and event tracking for billing support schemes are proposed. The service model does not require inefficient peer-to-peer roaming agreements to provide seamless user roaming between the WLAN hotspots and the cellular networks, which are operated by independent wireless network service providers. The proposed authentication and event-tracking schemes take the anonymity and intractability of mobile users into consideration and operate independently so that the integrated billing service can be applied to the cellular network, even if it still uses a traditional authentication scheme. Security analysis and overhead evaluation are given to demonstrate that the proposed service model and the supporting schemes are secure and efficient.

In this paper, a ticket ID system is proposed for service agent based WLAN/cellular network integrated service architecture. The proposed system accelerates the authentication process for the mobile terminal in the visited network, which effectively compensates the additional cost introduced by the service agent. The design of the ticket ID system also considers the user anonymity feature of the integrated service architecture. The performance evaluation demonstrates the ticket ID system effectively reduces the overall overhead in service agent based integrated service architecture.

Joint source-channel coding is an effective approach for the design of bandwidth efficient and error resilient communication systems with manageable complexity. An interesting research direction within this framework is the design of source decoders that exploit the residual redundancy for effective signal reconstruction at the receiver. Such source decoders are expected to replace the traditionally heuristic error concealment units that are elements of most multimedia communication systems. In this paper, we consider the reconstruction of signals encoded with a multistage vector quantizer (MSVQ) and transmitted over a noisy communications channel. The MSVQ maintains a moderate complexity and, due to its successive refinement feature, is a suitable choice for the design of layered (progressive) source codes. An approximate minimum mean squared error source decoder for MSVQ is presented, and its application to the reconstruction of the linear predictive coefficient (LPC) parameters in mixed excitation linear prediction (MELP) speech codec is analyzed. MELP is a low-rate standard speech codec suitable for bandwidth-limited communications and wireless applications. Numerical results demonstrate the effectiveness of the proposed schemes

Vector precoding is arguably the best form of precoding for the multi-user multiple input multiple output (MIMO) broadcast channel. However, conventional vector precoding schemes are designed to minimize the transmit energy, which is suboptimal in terms of the received signals mean square error (MSE). This paper proposes modifications to vector precoding to overcome this shortcoming. Improvements of about 2 dB are realizable in a fast fading environment. Also, conventional vector precoding schemes usually result in unbalanced levels of interference, resulting in a poor performance for some users. Noting the above, another improvement is proposed by directly minimizing the bit error rate rather than the MSE. This improvement adds another 1 dB of gain, resulting in an overall gain of 3 dB in a quasi static fading environment. These improvements are also applied to Tomlinson-Harashima precoding with similar results.

When transmitting over the multi-user MIMO broadcast channel, the transmitter has both knowledge of, and control over, the interference between users. Methods based on interference cancellation focus only on the transmitter's knowledge of the interference. This paper proposes a method which takes advantage of the transmitter's control over the interference among users. This is done by considering all users simultaneously. First, redundancy is introduced into the transmitted data similarly to the trellis shaping method, then the mean square error (MSE) is minimized by exploiting this redundancy. The proposed method is unique in that it considers all users simultaneously, and that the MSE is minimized rather than the transmit power. This method can achieve a BER of 10 -5 , only 1 dB above the minimum signal to noise ratio predicted by the capacity of the channel. Depending on the channel parameters, this demonstrates a 1-3 dB improvement over the best reported results. The improvement in the performance is achieved at the price of an increase in the complexity (equivalent to the Viterbi decoding of a trellis with 2 2N states for N transmit antennas).

Call admission control plays an important role in quality of service (QoS) provisioning in the interworking between the cellular network and wireless local area network (WLAN). Within the WLAN coverage, a service request can be admitted into the cellular network or the WLAN. Due to the heterogeneous underlying QoS support of the cellular network and WLANs, the admission of traffic in the WLAN coverage has a significant impact on QoS satisfaction and overall resource utilization, especially when multiple services are considered. A popular admission strategy (referred to as WLAN-first scheme) is to admit the incoming service requests into the WLAN whenever it is available so as to take advantage of the low cost and large bandwidth of the WLAN. In this paper, we investigate the performance of the WLAN-first scheme. It is observed that the overall resource utilization can be maximized when the admission regions for voice and data services in a cell and a WLAN are properly configured.

In this paper, we study the voice and data service provisioning in an integrated system of cellular and wireless local area networks (WLANs). To maximize the overall resource utilization of the integrated system, complementary quality of service (QoS) support capabilities of the two networks are exploited to serve voice and data traffic. As an essential resource allocation aspect, admission control can be used to properly admit voice and data calls to the overlaying cellular cells and WLANs. In this study, a generalized admission scheme is analyzed to investigate the dependence of resource utilization on admission parameters, which vary with user mobility and traffic variability. By applying an effective QoS evaluation approach, the admission parameters can be determined using a search algorithm.

A sub-optimum a-posteriori probability (APP) detector is proposed for iterative joint detection/decoding in a multiple-input multiple-output (MIMO) wireless communication system employing an outer code. The proposed detector searches inside a given sphere in a parallel manner to simultaneously find a list of m-best points based on an additive metric. The metric is formed by combining the channel output and the a-priori information. The parallel structure of the proposed method is suitable for hardware parallelization. The radius of the sphere and the value of m are selected according to the channel condition to reduce the complexity. Numerical results are provided showing a significant reduction in the average complexity (for a similar performance and peak complexity) as compared to the best earlier known method. The proposed scheme is applied for the decoding of the rate 2, 4 times 2 MIMO code employed in the 802.16e standard