Data-driven methods for learning near-optimal control policies through approximate dynamic programming (ADP) have garnered widespread attention. In this paper, we investigate how data-driven control methods can be leveraged to imbue near-optimal performance in a core component in modern factory systems: the electric motor drive. We apply policy iteration-based ADP to an induction motor model in order to construct a state feedback control policy for a given cost functional. Approximate error convergence properties of policy iteration methods imply that the learned control policy is near-optimal. We demonstrate that carefully selecting a cost functional and initial control policy yields a near-optimal control policy that outperforms both a baseline nonlinear control policy based on backstepping, as well as the initial control policy.

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.

In this paper, we develop a general reduced-rate orthogonal frequency division multiplexing (OFDM) transmission scheme for inter-subchannel interference (ICI) self-cancellation over high-mobility fading channels. Via transmit and receive processing, we transform the original OFDM system into an equivalent one with fewer subcarriers. By reducing transmission rate, we are able to design a transmitted signal structure with inherent ICI self-cancellation capability without requiring the instantaneous channel state information. We develop a general structure of transmit and receive processing matrices so that all equivalent subchannels in the transformed OFDM system have the same average signal-to-interference ratio (SIR). For the developed structure, we further optimize the transmit and the receive processing coefficients to maximize the SIR based on channel statistics. Numerical and simulation results demonstrate that the developed reduced-rate OFDM transmission achieves an SIR gain of around 5 dB over the existing ICI self-cancellation schemes and significantly reduces the error floor at the receiver.

In this paper, a multi-antenna two-way relaying protocol is proposed whereby two source nodes wishing to exchange information via a relay node additionally send private messages intended solely for the relay. In particular, we investigate the performance of this bidirectional relaying protocol over fading channels when the relay and source nodes are equipped with multiple antennas thus enabling them to leverage diversity and/or multiplexing gains. Specifically, provided enough antennas are available at the relay, the latter may opt for a demodulate-and-forward approach whereby it demodulates all incoming streams before broadcasting solely the messages to be exchanged between the source nodes or it may opt for a generalized analog network coding approach whereby it only demodulates the private messages destined for the relay's own sake while treating the messages to be exchanged between the source nodes as colored noise, subtracting the demodulated information from the overall received signal and then broadcasting the remaining part of the received signal. We compare how the two approaches fare when coupled with different multiple-input multiple-output (MIMO) detection techniques thus striking suitable tradeoffs between performance and implementation complexity for the generalized multi-antenna two-way relaying channel under consideration.

In this paper, we develop a general reduced-rate orthogonal frequency division multiplexing (OFDM) transmission scheme for inter-subchannel interference (ICI) mitigation in a high-mobility environment. By transmit and receive preprocessing, we transform the original N-subcarrier OFDM system into an equivalent K-subcarrier one with significantly reduced ICI. In particular, we develop a general structure of transmit and receive preprocessing matrices so that the K subchannels in the equivalent OFDM system share a common average signal-to-interference ratio (SIR). Without requiring the instantaneous channel state information, we optimize the preprocessing coefficients to maximize the SIR based on channel statistics. Numerical and simulation results demonstrate that the developed reduced-rate OFDM transmission achieves significant performance improvements over the existing ICI self-cancellation schemes.

In Dedicated Short Range Communications (DSRC) based vehicular networking, each vehicle periodically broadcasts control updates (that contain location and speed information etc.) to its neighbors, as a key component of traffic management and safety applications. The effectiveness of such a broadcast feature can be measured by two metrics: (1) the efficiency, or the average rate (number of nodes per sec) to which a source can deliver its broadcast packets, and (2) the reliability, or the average number of nodes that receive a specific transmission successfully. We demonstrate theoretical limits to and achievable tradeoffs between efficiency and reliability for a linear network under Rayleigh fading links. We then provide power control and congestion control strategies that maximize broadcast efficiency. A strategy that achieves near-optimal broadcast efficiency when the network nodes have high mobility is also described. Ns-2 simulations are used to validate our analytical results.

In this paper, we consider a two-hop multi-input-multi-output (MIMO) amplify-and-forward (AF) relay system consisting of a source node (SN), a destination node (DN), and a relay node (RN) that simply amplifies and forwards its received signal to the DN without any further processing. In this system, the overall channel from the SN to the DN is a cascade of the backward relay channel over the SN-RN hop, the amplifying matrix at the RN, and the forward relay channel over the RN-DN hop. We investigate the estimation of the two cascaded relay channels at the DN based on the predefined amplifying matrix applied at the RN and the corresponding overall channel obtained through the conventional channel estimation algorithms with the help of pilots transmitted by the SN. In particular, we find necessary and sufficient conditions on the pilot amplifying matrix sequence at the RN to ensure feasible relay channel estimation at the DN. Based on these conditions, we present rules to design diagonal or quasi-diagonal pilot amplifying matrices so that the cascaded relay channels can be estimated with minimum complexity at the RN. In the presence of imperfect overall channel state information at the DN, we further develop the approximate linear least-square estimation of the relay channels based on the designed pilot matrix sequence and demonstrate its performance by simulation results.

Statistics-based inter-subchannel interference (ICI) mitigation schemes are developed for orthogonal frequency division multiplexing (OFDM) transmission over high-mobility channels with line-of-sight components. By utilizing the channel statistics, we develop the Wiener filtering in the downlink and the transmit preprocessing in the uplink for ICI mitigation. Numerical and simulation results demonstrate that the proposed schemes effectively mitigate ICI and lower the error floor, especially in the presence of a strong line-of-sight path.

This paper presents a novel technique for multi-hop localization by using mobility (MLM) in mixed line-of-sight/non-line-of-sight (LOS/NLOS) environments. In MLM, nodes equipped with ultra-wideband (UWB) radio first move randomly for collecting time-of-arrival (TOA) measurements and then apply a modified biased Kalman filter (MBKF) designed for mitigating NLOS errors. NLOS bias in the measurements is further mitigated by using the shortest path distance (SPD) selection. Node positions are initialized by using multidimensional scaling (MDS) and then estimated by using iterative trilateration accompanied by an error accumulation management. Our simulation results demonstrate that the positioning accuracy of MLM outperforms previous methods in obstructed building environments with a limited anchor heard (AH).

In this paper, we study the impact of uncertain channel state information (CSI) on the performance of demodulate-and-forward relaying protocols with higher order modulation formats such as pulse-amplitude modulation (PAM) and rectangular quadrature-amplitude modulation (QAM). Assuming a single source and a single destination node assisted by $N$ relay nodes, we study the average bit error probability (BEP) performance of $M$-ary PAM and rectangular QAM constellations with Gray code mapping and imperfect CSI at the relay nodes as well as the destination. The main contributions of this paper are the derivation of closed-form expressions for $a)$ the cumulative distribution functions of the demodulator test statistics, $b)$ the transition probability of error at a given relay, and $c)$ the average BEP for independent and not necessarily identically distributed Rayleigh fading channels with imperfect receiver CSI.

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.

Two of the major challenges facing OFDM/OFDMA systems are their sensitivity to frequency selective fading and ICI due to CFO or Doppler shift, especially when the subcarrier spacing becomes smaller. We propose an OFDM transceiver design that employs frequency redundant subcarrier mapping to mitigate frequency selective fading and subcarrier spreading to achieve ICI self cancellation. Both our theoretical analysis and simulation show that such a code-spread-interleaved-redundant OFDM system design offers significant (over 10 dB) improvement in CIR and robust BER performance in different channel conditions.

Reliable and timely multi-hop propagation of messages among vehicles is essential for a safer and greener transportation system. Various broadcast-based forwarding strategies are envisioned for infrastructure-less vehicle-to-vehicle (v2v) communications. This paper proposes a prioritized broadcast contention control (PBCC) module/layer that provides reliable and low latency multi-hop connection. The PBCC forwarding algorithm optimizes the back-off distribution to improve the probability of successful broadcast and prioritizes forwarders based on location information. This module can be implemented in WAVE devices with minimum system modification. We integrate simple vehicular mobility models into ns-2 and implement a WAVE/802.11p communication protocol stack. Extensive simulations demonstrate PBCC's superiority in multi-hop delay.

In a vehicular network, every vehicle broadcasts update messages that contain location and speed information periodically to its one hop neighbors. The broadcast efficiency measures the average rate at which a vehicle receives these packets from any of its neighbors. As the node density increases, heightened interference lowers broadcast efficiency if congestion control mechanism is not used. In this paper, we analyze the broadcast efficiency under Rayleigh fading channel, and provide congestion control and power control strategies that maximize the efficiency. A worst-case guaranteed strategy achieving at least 95% of the optimal is also provided for cases when the network nodes have high mobility. Ns-2 simulations show that our analytical results accurately predict the system dynamic.

IEEE 802.15.4 specification for MAC and PHY offers a standard for general purpose wireless sensor networks. The TG4e of IEEE 802.15 is currently engaged in defining a specification particularly suitable for industrial and commercial applications, which impose severe constraints of low latency and high reliability. In this work, we present a simple MAC scheme to address these requirements of emergency response sensing applications for wireless sensor networks. We evaluate the proposed scheme for varying channel and traffic load conditions using simulations. Our results show that the latency and packet loss rate performance of emergency response traffic, consisting of guaranteed time slot (GTS) frames, significantly improves in comparison with the performance of original IEEE 802.15.4 MAC. The proposed MAC was also evaluated for potential adverse impact on the non-critical traffic, which consists of frames that use contention access period (CAP) of the superframe. It is shown that under realistic load conditions, the impact on the CAP traffic is minimal.

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 propose an intercell downlink orthogonal frequency division multiple access (OFDMA) scheduling technique in a sectorized cellular network. Adjacent sectors from neighboring cells form a cluster and each OFDMA resource block is allocated to the rate-maximizing sector. Compared to a cellular network that uses conventional fractional frequency reuse (FFR) technique, our proposed system requires only slightly more backhaul traffic while providing an appreciable performance gain. Intercell scheduling, which grants a resource exclusively to the rate-maximizing cell within a cluster, is a simple and powerful base station cooperation technique that balances non-cooperation and full cooperation. We find out that a tri-sectored network is particularly well-suited to applying inter-sector scheduling as each cluster is relatively isolated from other clusters. We provide an option to adjust the load on backhaul traffic by adjusting the granularity of the OFDMA resource under contention. We also provide an option to optimally swap resources. Analysis on performance gain for a few configurations are given. Simulations are provided to verify and illustrate the claimed performance gain.

Adoption of relay stations has been commonly accepted as a key technique for future IMT-advanced 4G systems to improve the link performance. While study in relay systems is often carried out for individual users independently, in this paper, we present a design termed as relay-based multi-user cooperative communication specifically for the uplink of 4G systems. In this system, data streams from multiple users are coded at the relay with an invertible matrix in finite field. These coded data, with information of one user being spread into other data streams, are forwarded by the RS to the base station (BS). The relayed data received at the BS will work together with those received through the direct links between mobile stations (MS) and BS to implement a cooperative process. This is carried out using a turbo decoding process that alternates the decoding between a multi-user decoder and a number of single-user decoders. It is shown through the simulations that the decoding converges within two or three iterations, yet indeed provides significant gains compared with a reference relay system where each user is processed individually.

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.

In this paper, we are concerned with cross-talk interference from the transmit to the receive antenna of a wireless channel-reuse-relay-station (CRRS) that forwards signals over the same channel as it receives signals. By estimating the coupling channel from the transmit to the receive antenna, the proposed scheme performs cross-talk reconstruction and cancellation at the RS. Different from the conventional coupling channel estimation schemes that require the RS to transmit dedicated pilots, the proposed scheme utilizes the random forwarded signals of the RS as pilots for coupling channel estimation, thus avoiding changing the structure of the RS's transmitted signal. For a general RS with any relay mechanism, we propose a least square coupling channel estimator; for an RS with the decode-and-forward mechanism, we further propose a minimum mean-square error coupling channel estimator. Also, we investigate the performance of the proposed cross-talk cancellation scheme with both analytical and numerical results.

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.

In this paper, we are concerned with a two-hop multi-input-multi-output (MIMO) amplify-and-forward (AF) relay system consisting of a source node (SN), a relay node (RN), and a destination node (DN). Since the simple RN in this system is unaware of the structure of its received signal and incapable of performing complicated signal processing, the interim channels over the SN-RN and the RN-DN hops can not be estimated directly. Therefore, we develop a novel interim channel estimation approach in this paper. Furthermore, we find necessary and sufficient conditions for the pilot amplifying matrix sequence at the RN to ensure successful interim channel estimation at the DN, and present rules to design low-complexity pilot amplifying matrices meeting these conditions.

Relays play an important role for increasing rate and reducing energy consumption of wireless networks. In this paper we consider a three-node network (source, relay, destination) in which we want to minimize total energy consumption for a given transmission rate. We analyze and optimize the Split-Combine-Relaying (SCR) protocol that for many typical parameter settings performs better than traditional decode-and- forward. SCR splits a data packet into two fragments, which are then transmitted in two phases. In the first phase of transmission, the source sends the first fragment to the relay. In the second phase of the transmission, the source sends the second fragment to the destination, while, at the same time, the relay forwards the first fragment to the destination. We provide an optimization framework to decide the amount of data in each fragment, the amount of time spent in each of the phases, and the corresponding transmission powers. We also show that an extension of SCR that employs Slepian-Wolf coding of the fragments leads to further reduction of energy consumption.

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.

In this paper, we propose a static power allocation algorithm for a two-hop multi-input-multi-output (MIMO) amplify-and-forward (AF) relay system in which the interim channel state information over the first and the second hops is unavailable. Based on the path losses over the first and the second hops, this algorithm performs static power allocation between the source and relay nodes to maximize the equivalent received SNR of the system. We further investigate the optimal location of the relay node when the conventional fixed and the proposed optimal static power allocation schemes are applied. Our comparison between direct transmission and relay-based two-hop transmission indicates that whether the latter outperforms the former depends on a tradeoff between the received SNR gain and the multiplexing loss in the relay-based two-hop transmission scheme.

Wireless sensor networks are emerging as an important area for communications. They enable a wealth of new applications including surveillance, building control, factory automation, and in-vehicle sensing. The sensor nodes have to operate under severe constraints on energy consumption and form factor, and provide the ability for precise self-location of the nodes. These requirements can be fulfilled very well by various forms of ultra-wide-band (UWB) transmission technology. We discuss various techniques and tradeoffs in UWB systems and indicate that time-hopping and frequency-hopping impulse radio physical layers combined with simple multiple-access techniques like ALOHA are suitable designs. We also describe the IEEE 802.15.4a standard, an important system that adopts UWB impulse radio to ensure robust data communications and precision ranging. In order to accommodate heterogeneous networks, it uses specific modulation, coding, and ranging waveforms that can be detected well by both coherent and noncoherent receivers.

Contention-based multiple access is a crucial component of many wireless systems. Multiple-packet reception (MPR) schemes that use interference cancellation techniques to receive and decode multiple packets that arrive simultaneously are known to be very efficient. However, the MPR schemes proposed in the literature require complex receivers capable of performing advanced signal processing over significant amounts of soft undecodable information received over multiple contention steps. In this paper, we show that local channel knowledge and elementary received signal strength measurements, which are available to many receivers today, can actively facilitate multi-packet reception and even simplify the interference canceling receiver's design. We introduce two variants of a simple algorithm called dual power multiple access (DPMA) that use local channel knowledge to limit the receive power levels to two values that facilitate successive interference cancellation. The resulting receiver structure is markedly simpler, as it needs to process only the immediate received signal without having to store and process signals received previously. Remarkably, using a set of three feedback messages, the first variant, DPMA-Lite, achieves a stable throughput of 0.6865 packets per slot. Using four possible feedback messages, the second variant, turbo-DPMA, achieves a stable throughput of 0.793 packets per slot, which is better than all contention algorithms known to date.

Industrial sensing, monitoring and automation offer a lucrative application domain for networking and communications. Wired sensor networks have traditionally been used for these applications because such networks adequately meet two vital requirements, i.e., low latency and high reliability, needed for an industrial deployment. Wired sensor networks, however, are not very cost effective due to higher components' cost. These networks also lack the flexibility needed for subsequent topological changes. Wireless sensor networks (WSN), on the other hand, are less expensive and offer high degree of flexibility. Wireless networks, therefore, can offer an attractive and viable solution for industrial sensing and automation. IEEE 802.15.4 standard defines a specification for MAC and PHY layers for short- range, low bit-rate, and low-cost wireless networks. However, the specified system is inefficient in terms of latency and reliability and fails to meet the stringent operational requirements for industrial applications. In this paper, we propose a set of new MAC superframes with an aim to enhance both performance metrics. We then use simulation to compare the performance of our proposed systems with that of the one specified in IEEE 802.15.4 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.

In this paper, we propose a novel hybrid QRD- MCMC MIMO detector that combines the features of a QRD-M detector and a Markov chain Monte Carlo (MCMC) detector. The QRD-M algorithm is applied first to obtain initial estimates of the transmitted signal vector. Subsequently, the QRD-M estimate is used to initialize one of the Gibbs samplers for MCMC detection. The MCMC detection reduces the M parameter required by the QRD-M detector, while the QRD-M initialization effectively alleviates the well-known high-SNR problem in existing MCMC detectors. Performance of the QRD-M/MCMC detector is examined under both an idealized MIMO channel with perfect channel side information (CSI) and a practical IEEE 802.16e MIMO- OFDMA system with imperfect CSI. Numerical results show that, compared to the stand-alone QRD-M or MCMC detectors, the QRD-MCMC detector achieves superior performance at a reduced complexity.

We develop a game theoretic framework for effective and adaptive inter-cell interference management in the OFDMA-based WiMAX/3GPP-LTE systems. Such approach requires no fixed spectrum planning beforehand and does not affect the spectrum usage for users that are not subject to inter-cell interference. Based on the handoff mechanisms in WiMAX systems, we also examine means of categorizing users who are, or are not, subject to potential interference. Different optimization opportunities for BSs are identified and analyzed. The interactions between BSs are modeled by a Stackelberg game in which BSs can make intelligent and rational decisions to reduce interference with minimum optimization cost. The algorithm to attain stable Nash Equilibrium is investigated and the behaviors of BSs with different preferences in the game are examined.

A game theoretic framework is developed in this paper to facilitate inter-cell interference management through cognitive sensing distributively performed by mobile stations (MSs). Using stochastic geometry, we reveal the relationship between the effectiveness of interference management and MS's "willingness" to perform cognitive sensing. Such cognitive sensing performed by MS is motivated by the associated beneficial results as well as by the rewards from base station(BS) that encourage sensing. Different tradeoffs for BS and MSs exist in their interactions, which are modeled as a Stackelberg game in this paper. While both BS and MS seek to manage interference at its own minimum cost, we design algorithm to achieve Nash equilibrium in such a game and investigate the optimal strategies taken by the players (BS and MSs).

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.

In this paper, we design a novel energy-detection based receiver architecture to detect unknown UWB signals in a strong narrowband interference (NBI) environment. Designed receiver is capable of suppressing NBI at low cost without any need for searching its frequency location. This is made possible by preprocessing the received signal using a cascaded nonlinear energy operator followed by a high-pass filter before regular energy detection.

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!

In this paper, we investigate the low-complexity synchronization design for multi-band orthogonal-frequency-division-multiplexing (MB-OFDM) ultra-wideband (UWB) systems. We propose a unified synchronizer design based on auto-correlation-function. The key component in the proposed synchronizer is a parallel signal detector structure in which multiple auto-correlation units are instantiated and their outputs are shared by other functional units in the synchronizer, including time-frequency pattern detection, symbol timing, carrier frequency offset estimation and correction and frame synchronization. We show that, via analysis and simulations, such a design achieves not only a low computation cost which makes it attractive in implementation, but also equal or better performance compared to the cross-correlation based designs.

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.

Contention-based multiple access is a crucial component of many wireless systems. It is known that using interference cancellation techniques to receive and decode multiple packets that arrive simultaneously can improve the efficiency of multiple access. However, such multi-packet reception (MPR) schemes proposed in the literature require complex receivers capable of performing advanced signal processing over significant amounts of soft undecodable information received over multiple contention steps. In this paper, we show that local channel knowledge and elementary received signal strength measurements, which are made by many receivers today, can actively facilitate multi-packet reception and even simplify the interference canceling receiver's design. We introduce a simple algorithm called turbo-dual power multiple access (turbo-DPMA) that uses local channel knowledge to limit the receive power levels to two discrete values that are carefully chosen to facilitate successive interference cancellation. As we shall see, limiting the receive power in such a manner not only facilitates the simultaneous reception of up to two packets, but it also enables the receiver to derive additional useful information about the contending users from its received signal strength indicator. The resulting receiver structure is markedly simpler, as it needs to process only the immediate received signal, without having to store and process signals received previously. Even more remarkably, the turbo-DPMA is stable for packet arrival rates as high as 0.793 packets/slot, which is significantly better than all the contention algorithms known to date.

This paper considers a multi-hop network in which relay nodes cooperate to minimize the total energy consumed in transmitting a (unicast) packet from a source to a destination. We propose the Progressive Accumulative Routing (PAR) algorithm, which progressively performs relay discovery, relay ordering and relay power allocation in a distributed manner, such that each relay node only needs local information. We assume Destination Energy Accumulation, in which the destination accumulates the energy of multiple received copies of a packet, each of which is too weak to be reliably decoded by itself, while the lower complexity relay nodes use a decode-and-forward approach. We also provide a closed-form analysis of the energy-savings achieved by the PAR when a relay node is added to an already existing DEA route. Simulations verify that the algorithm considerably reduces the total energy consumption, and can be implemented efficiently.

We derive very tight closed-form lower bounds of symbol/bit error probability for maximum-likelihood sequence detection (MLSD) on clipped OFDM channels with frequency- selective quasi-static Rayleigh fading. Contrary to common knowledge, we show that as a result of both clipping and channel frequency-selectivity, clipping at the transmitter leads to achievable frequency-diversity and performance gain over unclipped systems with MLSD. We derive closed-form expressions of the diversity and the performance gain over the no-clipping case, as well as the optimal clipping ratio for a given system. We consider both Nyquist-rate and oversampled OFDM systems with rectangular QAM signaling.

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 preamble design for mixed-mode transmission of 802.11n, carrier-frequency offset (CFO) estimation errors (or residual CFO) may cause significant errors in estimating the high-throughput (HT) MIMO channel because of the time-varying phase error as a result of residual CFO. In this paper, we propose a joint residual CFO and channel estimation technique using HT preambles. We show that the proposed technique is robust against residual CFO. We also present a simple technique for performing residual CFO estimation utilizing the 802.11n cyclic structure of pilot patterns. We demonstrate that with the proposed techniques, the performance loss due to residual CFO can be essentially recovered.

Low power consumption is a critical issue in many UWB systems. This paper investigates the application of dynamic voltage scaling (DVS) and other low power design techniques to a multiband-OFDM UWB transceiver baseband circuit design in order to reduce average power consumption of the chip. The results show significant power savings over the conventional approach.

We propose a link adaptation technique for IEEE 802.11n MIMO-OFDM systems with convolutional coding. The technique dynamically selects the best modulation and coding scheme (MCS) based on channel conditions and on the medium access control (MAC) layer performance (coded BER or PER) requested by applications. We show that the coded performance at decoder output can be modeled as a function of decoder-input uncoded BER and the code rate. For MIMO-OFDM systems with different modulation types for different spatial streams at each subcarrier, we propose a simple and accurate method for computing the BER for each spatial stream mixed from different subcarriers. We also discuss approaches for searching and selecting the MCS.

In the paper, a novel timing synchronization method is proposed for MIMO-OFDM based wireless local area networks (WLAN) systems. For coarse timing synchronization, a sliding window differentiator is used after a conventional autocorrelation device so that the timing ambiguity caused by the auto-correlation metric plateau can be removed. For fine timing synchronization, a metric called signal-to-interference ratio (SIR) is devised. Based on this metric, a fine timing synchronization method is developed to improve the timing precision. Compared with the conventional cross-correlation synchronization methods, this method is more robust to multipaths and pseudo multipaths caused by the cyclic delay diversity (CDD) scheme, which may be adopted in the next generation WLAN IEEE 802.11n system. At last, the timing is refined to precisely estimate the time of arrival for location applications by searching the cross-correlation peak in a small window around the fine timing estimation.

In this paper, a non-unitary differential space-time coding scheme is proposed based on the super orthogonal space-time trellis coding scheme (SOSTTC). First, we extend the SOSTTC scheme to support quadrature amplitude modulation (QAM) constellations. Then we present a differential space-time coding scheme based on these power-efficient SOSTTCs. Several low-complexity suboptimal differential decoders are developed based on the per-survivor processing technique (PSP). The performance analysis shows that the proposed PSP decoding schemes only have a 0.4dB performance loss compared with the coherent scheme under slow fading channels. Simulation results demonstrate that the proposed scheme can achieve much higher coding gain compared with the known unitary space time codes in the high data rate region.

We propose a modified layered structure for multiple-input multiple-output systems, where the layer detection order is fixed and the data rate for each layer is allocated based on the detection order and channel statistics. Using a Gaussian approximation of the layer capacities, we derive an asymptotic optimum data-rate-allocation approach. For optimum data-rate allocation, the amount of backoff from the mean layer capacity is proportional to the standard deviation of the layer capacity. With statistical data-rate allocation, only limited channel feedback is needed to update channel statistics at the transmitter. Simulation results show significant performance improvement with the proposed algorithm. We also find that the performance gap between the layered structure and the channel capacity diminishes with increasing ergodicity within each codeword. Numerical results show a singal-to-noise ratio improvement of 6.3 and 3.6 dB for TGn channels B and D, respectively, for 1% outage probability and 9 b/s/Hz spectral efficiency.

This paper considers signaling schemes for heterogeneous ultrawideband communications networks that contain both coherent (rake) and transmitted-reference (TR) receivers. While coherent receivers are capable of receiving TR signals, they do so with a 3 dB penalty, because they cannot make use of the energy invested into the reference pulse. We propose a new signaling scheme that avoids this drawback, by encoding redundant information on the reference pulse. The resulting scheme does not affect the operation of a TR receiver, while recovering the 3 dB penalty and furthermore providing an additional 1.7 dB coding gain to a coherent uncoded binary scheme. This can be explained by interpreting the scheme as a trellis-coded modulation. We also provide an alternative implementation that can be viewed as a recursive systematic convolutional encoder. Combining this version further with a simple forward error correction encoder results in a concatenated code that can be decoded iteratively, providing a bit-error rate of 10-3at 2.8 dB signal-to-noise ratio in additive white Gaussian noise. The convergence behavior of this iterative code is analyzed by using extrinsic information transfer charts.

Ultra Wide Band technology has attracted a lot of attention recently as a viable solution for high data rate, low power, short-range wireless link. The growing multi-media home networking is demanding more bandwidth and wireless throughput has become a bottleneck for high quality multi-media services. With the maximum data rate above 100 Mbps, UWB is a perfect solution for such applications. This paper describe such a system that takes advantage of the high data rate offered by UWB. We first implemented the complete MB-OFDM PHY layer using multi-FPGA hardware and discrete RF design. Out implementation is fully compliant to the WiMedia/MBOA PHY specifications and the FCC power regulations. The system achieved 110Mbps maximum data rate with a BER better than 10-6 over a range of 4 meters. Subsequently, we developed a testbed that demonstrates simultaneous transmission of multiple High Definition video streams over the MB-OFDM link. The use-case scenario is a Multimedia Client-Server application where we have one Media Server (transmitter) and several Media

We consider a MIMO system where error-prone feedback from the receiver is used by the transmitter to select a single optimum antenna to transmit data. Such error-prone feedback is common in the bandwidth-limited real systems, and is in marked contrast with the idealizations assumed in the selection literature. We show how the signaling assignment, which maps the antenna indices to the codewords that are fed back to indicate the index of the best transmit antenna, affects the performance of transmit antenna selection. The impact is intimately coupled with the receiver design. We derive approximate closed-form expressions for the average symbol error probability of QPSK modulated data in a spatially correlated channel, and then systematically find the optimal signaling assignment. Performance improvements are demonstrated for different antenna topologies without introducing any additional redundancy.

We investigate a cooperative communications scheme withNparallel relays, where both the transmissions from the source to the relays and from the relays to the destination use fountain codes. Receivers for fountain codes can accumulate mutual information, while traditional energy collection methods, such as repetition or cooperative space-time codes, only accumulate energy. As a consequence, using fountain codes can reduce the total energy required for transmitting data from the source to the destination. We first analyze the scenario where the source stops transmitting and the relay nodes start transmitting afterLrelay nodes have successfully decoded the source data. We optimizeL, and also give closed-form equations for the energy savings that can be achieved by the use of mutual-information-collection at the receiver instead of traditional energy-collection methods. We then analyze an alternate scenario where each relay node starts its transmission to the destination as soon as it has decoded the source data, and helps the other relay nodes that are still in reception mode. Doing so further reduces the total transmission time and energy consumption.

This paper considers a sensor network where relay nodes cooperate in order to minimize the total energy consumption for the unicast transmission of a message from a single source to a single destination. We assume Destination Energy Accumulation, i.e., the destination can accumulate the energy of multiple copies of the message, each of which is too weak to be reliably decoded by itself, while the relay nodes use a decode-and-forward approach. We propose the Progressive Accumulative Routing (PAR) algorithm, which performs relay discovery, relay ordering and power allocation in a distributed manner so that each relay node only needs information about its neighboring nodes. Simulations verify that the algorithm considerably reduces the total energy consumption, and can be implemented efficiently. Furthermore, it performs close to the optimal DEA route with high probability.

IEEE 802.11 wireless local area networks (WLANs) legacy standards can provide up to 54 Mbps, and the next generation WLAN will offer a physical-layer data rate at least an-order-of-magnitude higher than the existing standards. However, when the current medium access control (MAC) is applied directly to the higher-data-rate WLAN, it leads to high protocol overhead and significant throughput degradation because of its design for lower data rates. Designing efficient MAC schemes becomes critical and crucial. In this paper, a novel reverse direction (RD) protocol is proposed to transfer data in both directions, to overcome the fundamental overhead and to improve performance. The aim is to render the MAC efficient and robust not only for current IEEE 802.11a/b/g standards, but also for the next generation WLAN with higher data rate and higher throughput, i.e, upcoming IEEE 802.11n. Simulations with OPNET are performed and the comprehensive simulation results show that the novel RD protocol improves performance significantly.

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

This paper studies efficient and simple data broadcast in IEEE 802.15.4-based ad hoc networks (e.g., ZigBee). Since finding the minimum number of rebroadcast nodes in general ad hoc networks is NP-hard, current broadcast protocols either employ heuristic algorithms or assume extra knowledge such as position or two-hop neighbor table. However, the ZigBee network is characterized as low data rate and low cost. It cannot provide position or two-hop neighbor information, but it still requires an efficient broadcast algorithm that can reduce the number of rebroadcast nodes with limited computation complexity and storage space. To this end, this paper proposes self-pruning and forward node selection algorithms that exploit the hierarchical address space in ZigBee networks. Only one-hop neighbor information is needed; a partial list of two-hop neighbors is derived without exchanging messages between neighboring nodes. The ZigBee forward node selection algorithm finds the minimum rebroadcast nodes set with polynomial computation time and memory space. Using the proposed localized algorithms, it is proven that the entire network is covered. Simulations are conducted to evaluate the performance improvement in terms of the number of rebroadcast nodes, number of duplicated receivings, coverage time, and communication overhead.

Ultra-wideband (UWB) communication is a potential technique for future high-speed networks. In this paper, we investigate channel estimation and interference suppression for OFDM based UWB systems. In particular, we modify an existing channel estimation approach for UWB systems and develop an exponential window based approach to estimate correlation of the receive signals for interference suppression. Computer simulation results show that these approaches can be effectively used in OFDM based UWB systems.

Current standardization activities for wireless local area networks (WLAN) have focused the effort on providing high throughput and supporting applications with QoS requirements. In order to achieve such goals, the MAC mechanisms of legacy IEEE 802.11 and .11e protocols need to be improved. Multipolling mechanisms are often employed to improve the protocol efficiency, however, current available multipolling mechanisms suffer from hidden terminal and/or inefficient channel usage problems. In this paper, we propose a multi-polling mechanism that utilizes novel metric for channel access and thus is capable to utilize channel resource efficiently and overcome the hidden terminal problem. Analysis shows that the proposed mechanism improves the throughput from 5% to 40% compared to legacy 802.11e.

A two-dimensional post normalization scheme is proposed to improve the performance of conventional min-sum (MS) and normalized MS decoding of irregular low density parity check codes. An iterative procedure based on parallel differential optimization algorithm is presented to obtain the optimal two-dimensional normalization factors. Both density evolution analysis and specific code simulation show that the proposed method provides a comparable performance as belief propagation decoding while requiring less complexity. Interestingly, the new method exhibits a lower error floor than that of belief propagation decoding in the high SNR region. With respect to standard MS and one-dimensional normalized MS decodings, the two-dimensional normalized MS offers a considerably better performance.

We propose a modified layered structure for multiple-input multiple-output (MIMO) systems, where the layer detection order is fixed and the data rate for each layer is allocated based on the detection order and channel statistics. With Gaussian approximation of layer capacities, we derive the optimum data rate allocation and the amount of backoff from mean layer capacity is proportional to the standard deviation of the layer capacity. The minimum overall outage probability of a layered system is uniquely determined by the normalized capacity margin. We then investigate how to select the total information rate to maximize effective throughput. Simulation results show significant performance improvement with the proposed algorithm, and the performance gap between layered structure and the channel capacity diminishes with increasing ergodicity within each codeword.

Basic service set (BSS) is the fundamental building block of an IEEE 802.11 wireless local area network (WLAN). The overlapping BSS problem refers to situations that two or more systems, unrelated to each other, are in close enough proximity to hear each other physically. Overlapping BSS problem may degrade the network performance severely. In this paper, a two-level carrier sensing mechanism is proposed to solve the overlapping BSS problem. In the proposed solution, a new network allocation vector is introduced to solve the overlapping BSS problem with minimal hardware requirement. Moreover, the proposed mechanism introduces very little complexity - when there is no overlapping BSS problem exists, the proposed mechanism works the same as legacy 802.11 MAC mechanism.

Due to scarce resources, such as transmission power, storage space and communication bandwidth, current broadcast approaches for general ad hoc networks can not be applied to IEEE 802.15.4 based ad hoc networks (e.g., ZigBee networks). This paper proposes a forward node selection algorithm that significantly reduces broadcast redundancy. The algorithm exploits the hierarchical address space in ZigBee networks. Only one-hop neighbor information is needed: a partial list of two-hop neighbors is derived at a node without exchanging messages between neighboring nodes. The complexity of the proposed algorithm is polynomial in terms of both computation time and memory space. The localized algorithm provides an optimal and feasible solution of selecting the minimum number of rebroadcast nodes in ZigBee networks, which is an NP-hard problem for general ad hoc networks. The proposed algorithm is extended to deal with packet loss during data transmission. A ZigBee rebroadcast algorithm is also proposed to further reduce the number of rebroadcast nodes and cover the whole network faster by assigning a non-random rebroadcast timer determined by the number of neighbors to be covered, distance and link quality. Simulations are conducted to evaluate the broadcast redundancy, coverage time, and coverage ratio.

With fast deployment of wireless local area networks (WLANs), the ability of WLANs to support real time services with stringent quality of service (QoS) requirements has come to the fore. We evaluate the capability of QoS support in the IEEE 802.11e standard, which provides medium access control (MAC) enhancements for QoS support in 802.11. Both the enhanced distributed channel access (EDCA) and the polling based channel access modes are evaluated, and their performance under real time audio and video traffic is shown through simulations. We find that EDCA provides satisfactory service differentiation among its four access categories. However, in the presence of heavy load traffic, such as a high definition television (HDTV) signal transmission, it is more desirable to place such load under the HCF (hybrid coordination function) polling mode to avoid the adverse impact of other traffic on this class of traffic. With a hybrid polling and EDCA protocol, network capacity is effectively increased for better support of real-time audio and video transmissions in future home networks.

An adaptive space time transmit diversity scheme with simple feedback is proposed for the next generation wireless communication systems. By combining orthogonal space time block codes with adaptive sub-group antenna encoding, this new diversity scheme can effectively exploit the diversity potential provided by multiple antenna arrays without introducing interference among the signals transmitted at different antennas. In order to reduce the amount of feedback information as well as the computational complexity, a new quadrant phase constraining method is introduced for the computation of the feedback information. With simple operations at both the transmitter and the receiver, the new adaptive diversity scheme outperforms not only open loop space time block encoding techniques, but also some closed loop transmit diversity techniques with the same amount of feedback.

The next-generation wireless technologies, e.g., 802.11n and 802.15.3a, offer a physical-layer speed at least an-order-of-magnitude higher than the current standards. However, direct application of current MACs leads to high protocol overhead and significant throughput degradation. We propose ADCA, a high-performance MAC that works with high-capacity physical layer. ADCA exploits two ideas of adaptive batch transmission and opportunistic selection of high-rate hosts to simultaneously reduce the overhead and improve the aggregate throughput. It opportunistically favors high-rate hosts by providing higher access probability and more access time, while ensuring each low-rate host certain minimum amount of channel access time. Simulations show that the ADCA design increases the throughput by 112% and reduces the average delay by 55% compared with the legacy DCF. It delivers more than 100 Mbps MAC-layer throughput as compared with 35 Mbps offered by the legacy MAC.

We investigate the effect of symbol spreading on the performance of ultrawideband systems based on multiband OFDM. Ultrawideband channels offer the possibility of very high frequency diversity, as the used bandwidth is much larger than the coherence bandwidth of the channel. In multiband-OFDM, frequency diversity stems from (i) forward error correction coding, which distributes the symbols associated with one bit among different tones, and (ii) interleaving, which distributes the symbols among different employed frequency bands. We show that for weak (rate 3/4) error correcting codes, the achieved diversity order is insufficient. However, the use of Walsh-Hadamard spreading gives additional diversity within each frequency band and greatly improves the performance. Furthermore, we propose a novel scheme for grouping OFDM tones before the spreading is performed. This grouping gives additional flexibility in the system design.

An adaptive transmit diversity scheme with quadrant phase constraining feedback is proposed in this paper. With simple linear operations at both transmitter and receiver, the proposed algorithm can achieve better system performances with only 2M-2 bits of feedback information for systems with M transmit antennas. Theoretical performance bounds of the proposed transmit diversity scheme are derived. Simulation examples and theoretical analyses show that the proposed transmit diversity scheme outperforms not only the conventional open-loop transmit diversity techniques, but also some closed-loop transmit diversity techniques with more information transmitted in the feedback channel.

In this paper, reverse link capacity of a signal-to-interference ratio (SIR) based power-controlled direct-sequence code-division multiple-access (DS-CDMA) system, with the use of an antenna array and a Rake receiver in a multiple cell environment, is investigated. Both transmit and receive beamforming in the reverse link are considered. Instead of using tedious iterative methods, reverse link user capacity represented by a simple closed-form expression is derived, which relates to the number of antennas, the number of Rake receiver fingers, a target SIR, and the processing gain. The most efficient distribution of antenna elements between the base stations and the mobile stations to maximize the user capacity is observed through the numerical results, which also show significant capacity improvement by increasing the number of the antennas and Rake receiver fingers.

UWB systems based on impulse radio have the potential to provide very high data rates over short distances. In this paper, a new pulse shape is presented that satisfies the FCC spectral mask. Using this pulse, the link budget is calculated to quantify the relationship between data rate and distance. It is shown that UWB can be a good candidate for reliably transmitting 100 Mbps over distances at about 10 meters.

The upcoming IEEE 802.11e standard supports the applications with QoS requirements by using differentiated medium access mechanism for different traffic categories. In order to protect the high priority data flows and improve network performance in a heavy-loaded IEEE 802.11e network, a new measurement-based distributed call admission control method is introduced in this paper. The proposed method is based on the measurement of the existing traffic load over IEEE 802.11e network. Depending on the amount of the existing traffic load, the admission controller decides whether or not to allow a data unit to have the right to access the wireless medium. The simulation results show that the proposed mechanism works well.

In this paper we evaluate the average throughput of two types of W-CDMA systems with Multiple-input-multiple-output and adaptive modulation and coding by numerical simulation and theoretic analysis. One system employs space-time transmit diversity, another implements per antenna rate control. In both systems the modulation levels and turbo coding rates are dynamically adjusted depending on the channel conditions. The channels are assumed to be Rayleigh fading channels. Closed-form expressions are derived for the average throughput of both systems. The simulation results prove the accuracy of the analysis.

We address the antenna subset selection problem in spatially correlated MIMO channels. To reduce the severe performance degradation of the traditional antenna selection scheme in correlated channels, we propose to embed DFT operations in the RF chains. The resulting system shows a significant advantage both for diversity schemes and for the capacity of spatial multiplexing, while requiring only a minor hardware overhead.

In this paper, we investigate the design of pulse shaping UK filters for impulse radio ultrawideband (UWB) communications systems. The goal of the shaping is to meet an arbitrary spectrum mask, e.g., the mask mandated by the FCC for UWB emissions. Compared with classical FIR filter designs, the current problem introduces three new challenges: (1) it is minimax with quadratic constraints, (2) a single-sided distortion function is used, (3) delay positions are treated as tuning parameters. We first approach this problem by constructing a least-squares approximation to the minimax problem where the optimization over delays can be easily solved. With the LS solution serving as an initialization, nonlinear optimization techniques are employed to fine tune the solutions.

Transmit diversity is one of the key contributions to the 3G (third generation) WCDMA (wide-band code division multiple access) systems to accommodate high data-rate applications, where spatial diversity is introduced into the signal by transmitting through multiple antennas. While turbo codes are well understood to be able to provide powerful error correction ability in a SISO (single input single output) system, therefore achieve improved system performance, space-time block coded (STBC) MIMO (multiple input multiple output) transmission is receiving more and more interests as an effective method to combat adverse effects of fading, improve the performance and increase the capacity. This paper addresses the issue of introducing space-time coding to turbo coded HSDPA (high-speed downlink packet access) systems in order to obtain both diversity gain and coding gain. This novel scheme extends STBC, originally proposed for perfectly known frequency nonselective fading channels to frequency selective fading channels. Channel equalization is utilized to mitigate the intersymbol interference (ISI) caused by multipath propagation. Numerical results are presented to reveal the performance merits of the proposed system gained by using turbo space-time coding.

In this paper, a trellis based blind channel estimation and equalization technique is presented. First, blind channel estimation is accomplished by incorporating a list parallel Viterbi algorithm with the least mean square (LMS) updating approach. In this operation, multiple trellis mappings are preserved and ranked in terms of path metrics. Equivalently, multiple channel estimates are maintained and updated once a single symbol is received. Second, at a certain point, which is determined by the evolution of path metrics and the linear constraint possessed in the trellis mapping, only the best channel estimate is maintained. Third, this channel estimate is adopted to construct the whole trellis that is used by a conventional adaptive Viterbi algorithm. Signal detection and further channel updating are then conducted alternatively. To alleviate the noise impact, a small delay is introduced before the feedback of detected symbols to further update the channel estimate. Simulation has shown the overall good performance of the proposed scheme in terms of mean square error (MSE) convergence of the channel estimation, stability to the initial channel guess, and computational complexity etc.