ABSTRACT A graph‐based framework is proposed in this paper to implement dynamic fractional frequency reuse (D‐FFR) in a multicell Orthogonal Frequency Division Multiple Access (OFDMA) network. FFR is a promising resource‐allocation technique that can effectively mitigate intercell interference (ICI) in OFDMA networks. The proposed D‐FFR scheme enhances the conventional FFR by enabling adaptive spectral sharing as per cell‐load conditions. Such adaptation has significant benefits in practical systems where traffic loads in different cells are usually unequal and time‐varying. The dynamic adaptation is accomplished via a graph framework in which the resource‐allocation problem is solved in two phases: (1) constructing an interference graph that matches the specific realization of FFR and the network topology and (2) coloring the graph by use of a heuristic algorithm. Various realizations of FFR can easily be incorporated in the framework by manipulating the first phase. The performance improvement enabled by the proposed D‐FFR scheme is demonstrated by computer simulation for a 19‐cell network with equal and unequal cell loads. In the unequal‐load scenario, the proposed D‐FFR scheme offers significant performance improvement in terms of cell throughput and service rate as compared to conventional FFR and previous interference management schemes. Copyright © 2011 John Wiley & Sons, Ltd.

Block transmission with cyclic prefix is a promising technique to realize high-speed data rates in frequency-selective fading channels. Many popular linear precoding schemes, including orthogonal frequency-division multiplexing (OFDM), single-carrier (SC) block transmission, and time-reversal (TR), can be interpreted as such a block transmission. This paper presents a unified performance analysis that shows how the optimal precoding strategy depends on the optimization criterion such as capacity, mean-square error, and secrecy. We analyze three variants of TR methods (based on maximum-ratio combining, equal-gain combining and selective combining) and two-types of pre-equalization methods (zero-forcing and minimum mean-square error). As one application of our framework, we derive optimal precoding (i.e., OFDM with optimal power and phase control) in the presence of interference limitation for distributed antenna systems; we find that without power/phase control, OFDM does not have any capacity advantage over SC transmissions. When comparing SC and TR, we verify that for single-antenna systems in the high SNR regimes, SC has a capacity advantage; however, TR performs better in the low SNR regime. For distributed multiple-antenna systems, TR always provides higher capacity, and the capacity of TR can approach that of optimal precoders with a large number of distributed antennas. Furthermore, we make an analysis of secrecy capacity which shows how high-rate messages can be transmitted towards an intended user without being decoded by the other users from the viewpoint of information-theoretic security. We demonstrate that TR precoding can be the best candidate among the non-optimal precoders for achieving high secrecy capacity, while the optimal precoder offers a significant gain over those non-optimal precoders.

As mobile stations (MSs) in the next generation cellular wireless systems will more frequently operate on multiple applications, such as web browsing, VoIP, online video etc., energy saving becomes more critical and face new challenges from the quality of service (QoS) requirements. A special operation state, called sleep mode, is designed for energy saving of MS, in which MS operates on continuous sleep cycles, where every sleep cycle is the sum of a listening window and a sleep window. This paper proposes an energy saving method that adaptively determines sleep cycles and shifts listening window. When MS is in sleep mode, the sleep cycles are extended by probabilistic decisions related to the traffic statistic attributes. Different from conventional methods, our method is also able to deal with mixed traffic pattern by shifting listening window. Extensive simulation results validate the advantages of our method both in terms of energy saving and shorter response time.

Fractional frequency reuse (FFR), using different frequency reuse factors for cell center and edge regions, is able to effectively improve spectrum efficiency in multi-cell OFDMA networks. However, optimal performance is hard to achieve in practice as the efficiency of resource allocation could drop drastically due to the constraint from the frequency partitions formed by FFR. Since the radio resource is pre-partitioned for cell edge and center, fair resource allocation in a cell is also difficult to implement. Conventional frequency partition adjustment either has high complexity due to global optimization or suffers from heavy performance degradation due to absence of effective control on inter-cell interference (ICI). To solve this issue, we create models for analyzing geographical distribution of interference in multi-cell networks. Based on the observed non-uniform distributed ICI, we redefine the zones for fractional reuse and propose clustering based FFR, which offers resource allocation higher flexibility and better fairness with additional spatial dimension. Extensive simulation has been performed to validate practicality and effectiveness of our proposed scheme.

The performance unfairness problem in a single cell IEEE 802.11 wireless local area network (WLAN) is considered. While existing research is based on the assumption that all nodes have the same transmission success probability and per-node throughput, this fairness exists only if all nodes within range of the access point can sense each other. Recent measurements suggest that this is not necessarily true and terminals can be hidden from each other. In this paper, the impact of hidden terminals on the performance unfairness among individual nodes is investigated via analysis, simulation and experimental measurements in a real network. In the presence of hidden terminals, it is observed that the widely accepted conclusion of equal performance among nodes does not hold any more. Instead, nodes far from the access point (AP) see more hidden terminals than those close to the AP, so they get more packet losses and lower throughput. This phenomenon is not due to inter-cell interference, or channel disparities among nodes, and is significant even when the RTS/CTS mechanism designed to mitigate the impact of hidden terminals is turned on. The simulation results show that for a 16-node WLAN with a fixed data rate of 6 Mbps, the throughput of a node close to the AP is more than twice that of an edge node, due to hidden terminals.

A novel concept of portable charging station networks to serve Electric Vehicles is described. An optimum charging station deployment method is explained, and its performance has been simulated for single highway, two intersecting highways, and Manhattan-like grid traffic models. Outage probability and service waiting delay performances are evaluated. Impact of the number PCSs and the ratio of EVs to PCSs in the service area on outage probability and waiting delay are studied. The gained insights will be used in extending this pioneer step to a stochastic framework with more realistic traffic models.

To mitigate inter-cell interference (ICI) and achieve higher spectrum efficiency, fractional frequency reuse (FFR) has been widely adopted by the next generation wireless systems, wherein different frequency reuse factors are applied to cell center and cell edge zones. In such conventional FFR, a contiguous radio spectrum is partitioned in a fixed fashion across all cells for edge and cell center zones. This approach evidently lacks the flexibility of inter-cell and/or intra-cell resource allocation adjustment and the capability of dealing with traffic load fluctuation and quality of service (QoS) requirements variations. This paper models the implementation of FFR in a multi-cell network environment and proposes a scheme called D-FFR that can adaptively partition radio spectrum in a distributed manner to achieve different FFR configuration among different cells. Resource demands and various inter-cell/intracell allocation constraints are accounted in D-FFR to enable differentiable inter-cell and/or intra-cell throughput and deliver higher spectrum efficiency. Analytical and extensive simulation results are provided in the paper to validate the effectiveness of our proposed scheme.

Abstract The IEEE 802.16/WiMAX standard has fully embraced multi‐antenna technology and can, thus, deliver robust performance and high transmission rates. Nevertheless, due to its inherent cost concerns, a WiMAX mobile station (MS) should preferably contain fewer radio frequency (RF) chains than antenna elements. This is because the former is often substantially more expensive than the latter. Thus, antenna selection, wherein a subset of antennas is dynamically selected to connect to the limited number of RF chains for transceiving, is a highly appealing performance enhancement technique for multi‐antenna WiMAX terminals. In this paper, a novel protocol for antenna selection in space division multiple access (SDMA) transmission is developed for the next‐generation IEEE 802.16 mobile stations. Both locally and globally optimal selection rules are considered together with various linear precoding designs at the base station (BS). The proposed protocol can readily accommodate various channel situations (e.g., reciprocal and non‐reciprocal channels). As demonstrated by analysis and simulation, the proposed protocol delivers considerable performance improvement over conventional IEEE 802.16 terminals that lack antenna selection capability. Moreover, the proposed protocol leverages the existing signaling method defined in IEEE 802.16, thereby incurring a negligible signaling overhead and requiring only minimal modifications of the standard. To the best of our knowledge, this paper represents the first effort to support antenna selection capability for SDMA transmissions in IEEE 802.16 mobile stations. Copyright © 2009 John Wiley & Sons, Ltd.

We address the problem of collaborative sensing in cognitive radios. In a cognitive radio network, all the nodes may sense the spectrum simultaneously. They should then exchange their sensing results in order to improve the reliability of the detection. This exchange of information has to be done efficiently to improve on the bandwidth efficiency of the network. We propose a medium access control (MAC) signaling protocol and study its performance behavior. For the case of a single-band channel, we present a thorough analysis of the proposed protocol and use the results to pick the protocol parameters that minimizes the signaling time for a given probability of detection. Analysis of the proposed protocol for multiband channels is solved by introducing a matrix formulation of the proposed protocol that allows its evaluation numerically.

Next generation wireless system such as advanced WiMAX (i.e., IEEE802.16m) and LTE advanced will fully embrace multi-hop relay architecture. The conventional Automatic Repeat reQuest (ARQ) and the more recent Hybrid ARQ (HARQ) are two simple yet highly effective error control techniques designed for single hop system. Nevertheless, extending them in a synergistic manner to support multi-hop relay networks is by no means a trivial undertaking. This paper explores a variety of multi-hop error control techniques such as hop-by-hop ARQ, 2-link ARQ and end-to-end ARQ, and various possible combinations with HARQ. We further establish an analytical framework for each of these key techniques and evaluate the performance. Based on the analysis and comparison, we propose a low complexity error control mechanism tailored for the multihop transmission features. Extensive simulation results compare the performance and validate our analytical framework.

A graph-based framework for dynamic fractional frequency reuse (FFR) in multi-cell OFDMA networks is proposed in this work. FFR is a promising resource allocation technique that can effectively mitigate inter-cell interference (ICI) in OFDMA networks. The proposed scheme enhances the conventional FFR by enabling adaptive spectral sharing per cell load conditions. Such adaptation has significant benefits in a practical environment where traffic load in different cells may be asymmetric and time-varying. The dynamic feature is accomplished via a graph approach in which the resource allocation problem is translated to a graph coloring problem. Specifically, in order to incorporate various versions of FFR in our framework, we construct a graph that matches the specific version of FFR and then color the graph using the corresponding graph algorithm. The performance improvement enabled by the proposed dynamic FFR scheme is further demonstrated by computer simulation for a 19-cell network with asymmetric cell load. For instance, the proposed dynamic FFR scheme can achieve a 12% and 33% gain in cell throughput and service rate over conventional FFR, and render a 70% and 107% gain in cell throughput and service rate with respect to the reuse-3 system.

The IEEE 802.16/WiMAX standards has fully embraced multi-antenna technology and can, thus, deliver robust performance and high transmission rates. Nevertheless, due to its inherent cost concerns, a WiMAX mobile station (MS) should preferably contain fewer radio frequency (RF) chains than antenna elements. Thus, antenna selection, wherein a subset of antennas is dynamically selected to connect to the limited number of RF chains for transceiving, is a highly appealing performance enhancement technique for multiantenna WiMAX terminals. In this paper, a novel protocol for antenna selection in space division multiple access (SDMA) transmission is developed for the next-generation IEEE 802.16 mobile stations. Both locally and globally optimal selection rules are considered at the base station (BS). The proposed protocol can readily accommodate various channel situations (e.g., reciprocal and non-reciprocal channels). As demonstrated by analysis and simulation, the proposed protocol delivers considerable performance improvement over conventional IEEE 802.16 terminals that lack antenna selection capability. Moreover, the proposed protocol leverages the existing signaling method defined in IEEE 802.16, thereby incurring a negligible signaling overhead and requiring only minimal modifications of the standard.

A novel, practical and low-complexity multi-cell OFDMA downlink channel assignment method using a graph- based approach is proposed in this work. The inter-cell interference (ICI) information is obtained through inference from the diversity set of mobile stations (MSs) and presented in the form of an interference graph. The proposed downlink channel assignment method consists of two phases. The task of ICI reduction is mapped to the MAX fe-CUT problem in graph theory and solved in the first phase. Then, channel assignment is conducted by taking into account instantaneous channel conditions in the second phase. State-of-the-art ICI management techniques such as ICI coordination (ICIC) and base station cooperation (BSC) are incorporated in our framework. Heuristic algorithms are proposed to solve both phases of the problem efficiently. Simulation is conducted to demonstrate the effectiveness of the proposed solution, where the SINR improvement can be as high as 4.5 dB. The proposed solution can be used in next generation cellular systems such as 3 GPP long term evolution (LTE) and IEEE 802.16 m.

The IEEE 802.16/WiMAX standard has fully embraced multi-antenna technology and can, thus, deliver robust and high transmission rates and higher system capacity. Nevertheless, due to its inherent form-factor constraints and cost concerns, a WiMAX mobile station (MS) should preferably contain fewer radio frequency (RF) chains than antenna elements. This is because RF chains are often substantially more expensive than antenna elements. Thus, antenna selection, wherein a subset of antennas is dynamically selected to connect to the limited RF chains for transceiving, is a highly appealing performance enhancement technique for multi-antenna WiMAX terminals. In this paper, a novel antenna selection protocol tailored for next-generation IEEE 802.16 mobile stations is proposed. As demonstrated by the extensive OPNET simulations, the proposed protocol delivers a significant performance improvement over conventional 802.16 terminals that lack the antenna selection capability. Moreover, the new protocol leverages the existing signaling methods defined in 802.16, thereby incurring a negligible signaling overhead and requiring only diminutive modifications of the standard. To the best of our knowledge, this paper represents the first effort to support antenna selection capability in IEEE 802.16 mobile stations.

As the two key technologies that have the potential to reshape the landscape of next-generation wireless network, cooperative communications and directional antenna system so far have been developed in parallel, if not in isolation from each other. In order to establish a thorough comparison between the relative system performance of cooperative diversity and directional transmission in an adhoc environment, we design and quantitatively evaluate three medium access control protocols, namely O-CoopMAC, D-NoopMAC and D-CoopMAC. The study yields the unexpected yet crucial observation that cooperative forwarding significantly limits the spatial reuse created by transmission directionality, and therefore can appreciably degrade the performance of a directional system with a sufficiently narrow antenna beam. To the best knowledge of the authors, this is the first paper to systematically compare the performance of these two technologies in an adhoc environment and reveal the key fact that cooperation and directionality can be rather foes than friends!

Cooperative communications is a promising technology that tends to change the conventional access and transmission schemes in wireless networks. By enabling additional collaboration from nodes that otherwise will not directly participate in the transmission, it enables spatial diversity and dramatically improves the performance of the network. In this paper we propose a cross-layer cooperative protocol based on a MAC protocol called CoopMAC (Liu et al., 2005; Korakis et al., 2007; and Liu et al., 2006) for ad-hoc wireless networks in order to leverage cooperation in both MAC and PHY layer. Exploiting physical layer combining at the receiver, this simple yet efficient scheme illustrates a new paradigm for realistic cross-layer cooperative protocol design for next generation wireless ad-hoc networks. We have evaluated the performance of the proposed protocol by extensive simulations in a large scale wireless ad-hoc network. Simulation results show that the new protocol significantly improves the network performance in terms of throughput and delay.

Hybrid automatic repeat-request (HARQ) is critical to an IEEE 802.16e OFDMA network, as it can significantly improve the reliability of wireless link. However, as revealed by our simulation, the maximum number of parallel HARQ channels defined in the current system will render a potential bottleneck and preponderantly limit the overall system capacity of next generation WiMAX network, wherein the wireless link is anticipated to support much higher data rate. Moreover, the current IEEE 802.16 standard fails to tap into the considerable synergy inherent in HARQ and MAC layer fragmentation, as these two schemes have been designed separately. In order to enhance the throughput performance and improve the preparedness of multichannel HARQ protocol for adoption by future 802.16J and 802.16m system, we then propose two remedies, namely expansion of ACID field and an adaptive fragmentation scheme1. The performance evaluation results further confirm that the ACID expansion readily doubles the link level throughput, while the adaptive fragmentation can effectively stave off the dismal degradation that plagues the IEEE 802.16e HARQ protocol in high bit error rate regime2. Equally importantly, the two proposed schemes substantially reuse the existing mechanisms in the current 802.16 standard and only incur diminutive additional complexity, thereby speeding up the possible acceptance and implementation of the technology.

Frame structure is critical to an IEEE 802.16e OFDMA network, as it governs the fundamental channel access in both time and frequency domain. The frame structure design is even more complicated in the new mobile multi-hop relay-based (MMR) network architecture, as numerous dimensions of design constraints and challenges have been introduced therein. In this paper, we propose a simple yet flexible framework based upon the current 802.16e OFDMA frame structure design, which enables multihop operation while still maintaining the backward compatibility with the legacy mobile stations. Further performance evaluation not only demonstrates the capacity improvement an MMR network can achieve based upon the proposed frame structure, but also establishes a more profound understanding on the range extension aspect of a relay network.

Due to the broadcast nature of wireless signals, a wireless transmission intended for a particular destination station can be overheard by other neighboring stations. A focus of recent research activities in cooperative communications is to achieve spatial diversity gains by requiring these neighboring stations to retransmit the overheard information to the final destination. In this paper we demonstrate that such cooperation among stations in a wireless LAN (WLAN) can achieve both higher throughput and lower interference. We present the design for a medium access control protocol called CoopMAC, in which high data rate stations assist low data rate stations in their transmission by forwarding their traffic. In our proposed protocol, using the overheard transmissions, each low data rate node maintains a table, called a CoopTable, of potential helper nodes that can assist in its transmissions. During transmission, each low data rate node selects either direct transmission or transmission through a helper node in order to minimize the total transmission time. Using analysis, simulation and testbed experimentation, we quantify the increase in the total network throughput, and the reduction in delay, if such cooperative transmissions are utilized. The CoopMAC protocol is simple and backward compatible with the legacy 802.11 system. In this paper, we also demonstrate a reduction in the signal-to-interference ratio in a dense deployment of 802.11 access points, which in some cases is a more important consequence of cooperation

IEEE 802.15.4-2003 (2003) was introduced to address the market need for connecting low-rate devices in a wireless personal area network (WPAN). A slotted CSMA/CA medium access control (MAC) protocol is defined in the standard to coordinate the channel access of a large number of wireless devices. In this paper, we propose a novel Markov chain for IEEE 802.15.4 MAC, which faithfully captures all the essential features of the protocol, and thus can provide valuable insight into the strengths and weaknesses of this multiple access scheme. The evaluation reveals that the double carrier sensing mechanism specified in 802.15.4 MAC is not an optimal design, and a slight modification in the protocol can result in further performance improvement in terms of throughput, delay and energy efficiency

In this letter, we propose a three-dimensional Markov chain model for the 802.11e enhanced distributed channel access (EDCA) mode. This model can be used to compute the maximum sustainable throughput and service delay distribution for each priority class when under saturation load. The new framework models the performance impact of major quality-of-service (QoS)-specific features (e.g., CWMin, CWMax, AIFS, internal collision resolution) of the 802.11e EDCA mode, and hence can provide an analytical approach to pick the parameter values associated with EDCA to meet the QoS requirements of each priority.

In this paper, a novel idea of user cooperation in wireless networks has been exploited to improve the performance of the IEEE 802.11 medium access control (MAC) protocol. The new MAC protocol leverages the multi-rate capability of IEEE 802.11b and allows the mobile stations (STA) far away from the access point (AP) to transmit at a higher rate by using an intermediate station as a relay. Two specific variations of the new MAC protocol, namely CoopMAC I and CoopMAC II, are introduced in the paper. Both are able to increase the throughput of the whole network and reduce the average packet delay. Moreover, CoopMAC II also maintains backward compatibility with the legacy 802.11 protocol. The performance improvement is further evaluated by analysis and extensive simulations.

The IEEE 802.11e protocol is designed to enhance the QoS capability of wireless local area networks (WLAN). In this paper, we propose a three dimensional Markov chain model for the 802.11e enhanced distributed coordination function (EDCF) mode and compute the maximum sustainable throughput and service delay distribution for each priority class when under heavy load. This provides an analytical approach to pick the parameter values associated with EDCF to meet the QoS requirements of each priority. This is accomplished by modeling the performance impact of all the major QoS-specific features (i.e. CWMin, CWMax, AIFS, internal collision resolution) of the 802.11e EDCF mode.

The IEEE 802.11e protocol is designed to enhance the QoS capability of wireless local area networks (WLAN). In this paper, we propose a multidimensional Markov model for the 802.11e enhanced distributed coordination function (EDCF) mode and compute the maximum sustainable throughput and service delay distribution for each priority class when under heavy load. Since the QoS mechanisms and their associated parameters in IEEE 802.11e interact with each other in a complex way, it is important to model the aggregate effect of all the mechanisms. The approach we present does so and therefore provides an analytical approach to pick the parameter values associated with EDCF to meet the QoS requirements of each priority.