In this paper, we present two new feedback schemes, namely the network mode selection-based feedback (NMF) and the user mode selection-based feedback (UMF), that reduce the feedback load needed for opportunistic scheduling in the OFDMA downlink system. The two schemes are designed to select either the selective feedback (SF) mode which feeds back the fine-grained condition of a part of the channels or the bit-map based feedback (BF) mode which feeds back the coarse-grained condition of all the channels using 1 bit for each channel, depending on the channel conditions of the users. Specifically, the NMF scheme makes the BS determine the more desirable feedback mode by predicting and comparing the average total throughputs of the two modes for the given statistical channel characteristics of the users. The UMF scheme makes each user determine its own feedback mode based on both its channel realization and the recommendation of the network. Numerical results reveal that the proposed NMF and UMF schemes achieve significant performance gain over the existing feedback reduction schemes with similar feedback load.

In this letter, we present an opportunistic multichannel CSMA protocol for OFDMA systems in frequency-selective fading channel. In order to opportunistically exploit the frequency selectivity, an OFDM-based single-channel CSMA protocol can employ water-filling power allocation as it allows a single station to utilize the whole frequency bandwidth at each time instant. In contrast, the proposed OFDMA-based multichannel CSMA protocol achieves the multi-user diversity gain by arranging multiple stations to transmit on their favorable sub-channels simultaneously. The proposed protocol is designed as an opportunistic version of the original multi-channel CSMA protocol, which was proposed in our previous work, and adopts a channel-adaptive backoff (or transmission probability control) algorithm, enabling each station to attempt data transmission on its high-SNR sub-channel with a high probability. The opportunistic multi-channel CSMA turns out to improve the total data rate significantly over the opportunistic single-channel CSMA, and to achieve almost full multi-user diversity gain despite the distributed operation.

In this paper, we present two new feedback reduction schemes, namely the network mode selection-based feedback (NMF) and the user mode selection-based feedback (UMF), to reduce the feedback load needed for opportunistic scheduling in the OFDMA downlink system. The two schemes are designed to select either the selective feedback (SF) mode which feeds back the fine-grained condition of a part of the channels or the bit-map based feedback (BF) mode which feeds back the coarse-grained condition of all the channels using 1-bit for each channel, depending on the channel conditions of the users. Specifically, the NMF scheme makes the BS determine the more desirable feedback mode by predicting and comparing the average total throughputs of the two modes for the given statistical channel characteristics of the users. The UMF scheme makes each user determine its own feedback mode based on both its channel realization and the recommendation of the network. Numerical results reveal that the proposed NMF and UMF schemes achieve significant performance gain over the existing feedback reduction schemes with similar feedback load.

In this paper, we investigate the transmission power allocation for the support of broadcast/multicast services (BMSs) in multi-cell OFDM systems.We consider a cooperative architecture for distribution of the BMS data where the copies of the same data are transmitted by multiple base stations (BSs) in a synchronized manner and then aggregated at the receiving users. Under this architecture, we develop a power allocation scheme that optimizes the transmission power of the multiple BSs on each sub-channel to maximize the total data rate. The optimal power allocation turns out to take the form of iterative water-filling, particularly when heterogeneous modulation and coding schemes are allowed in each sub-channel. In addition, we study the two special cases where the BMS system has extended or limited capabilities: First, we consider the BMS system that supports a more sophisticated video coding technique, fine granularity scalability (FGS) video coding, which encodes the BMS data into base layer and enhancement layer. Taking the layered structure into account, we propose a power allocation scheme that maximizes the data rate of the enhancement layer while guaranteeing the minimum data rate requirement of the base layer. Second, we consider the BMS system that only allows average channel state information of users to be available at the resource manager. In this case, as it is not possible to guarantee successful transmission of every data due to instant deep fading, we propose a power allocation scheme that minimizes the probability of transmission failure. Simulation results reveal that the cooperative distribution system performs far better than the non-cooperative distribution system and the proposed power allocation schemes outperform the equal power allocation scheme significantly.

In this paper, we present a multi-channel carrier sense multiple access with collision avoidance (CSMA/CA) protocol for orthogonal frequency division multiple access (OFDMA) systems. The CSMA/CA system in conventional single-channel operation has the advantage of not requiring the signaling for bandwidth request and allocation over the scheduled access system but it sacrifices system efficiency significantly. We overcome the system efficiency limitation by exploiting the OFDMA system features that multiple stations can transmit simultaneously on different sub-channels, and the stations can obtain the channel information of all the sub-channels at each time instant. The OFDMA-based multi-channel CSMA/CA protocol is designed to enable the stations to contend with each other for channel access both in time and frequency domains through a two-dimensional backoff scheme. The protocol takes a generalized form of the conventional single-channel CSMA/CA protocol: It allows to segment the channel bandwidth into multiple narrow-band random access channels and adjust the transmission probability according to the multi-channel activity in a flexible manner. For throughput analysis, we devise a multi-channel p-persistent CSMA protocol that can emulate the multi-channel CSMA/CA protocol and then analyze its saturated throughput. The simulation and analysis results reveal that the proposed system performs far better than the single-channel CSMA/CA system while using the same total bandwidth and even close to the scheduled access system. Further, we examine how stringent time synchronization is required among the constituent stations in implementing of the OFDMA-based multi-channel CSMA/CA system.

In this paper, we present a multi-channel carrier sense multiple access with collision avoidance (CSMA/CA) protocol for orthogonal frequency division multiple access (OFDMA) systems. The protocol exploits the OFDMA system features that multiple stations can transmit simultaneously on different subchannels and the stations can obtain the channel information of all the sub-channels at each time instant. It is designed to enable the stations to contend with each other for channel access both in time and frequency domains through a two-dimensional backoff scheme. By adopting this two-dimensional backoff, the protocol takes a generalized form of the conventional single-channel CSMA/CA protocol: It allows to segment the channel bandwidth into multiple narrow-band random access channels and adjust the transmission probability according to the multichannel activity in a flexible manner. The simulation and analysis results reveal that the proposed system performs far better than the single-channel CSMA/CA system while using the same total bandwidth.

In this paper, we investigate resource allocation in MIMO interference systems where multiple MIMO links share the same frequency band and interfere with each other. For MIMO transmission scheme, we consider a spatial multiplexing scheme that uses linear precoder and decoder. We impose the SINR constraint that the received SINR of each data sub-stream should be larger than a predetermined threshold, and the QoS constraint that the data throughput of individual link should be larger than a required level. In this system, a trade-off exists between the data throughput and the power consumption due to the co-channel interference among the multiple MIMO links. So we arrange a new resource allocation algorithm such that the data throughput and the power consumption are balanced and the QoS constraint is met for each link. We model the resource allocation problem as a noncooperative game and devise a distributed resource allocation algorithm in which the constituent links find a Nash equilibrium point without coordination. The proposed algorithm turns out to perform close to the optimal point achieved by centralized optimization.

In this paper, we investigate the problem of minimizing the average transmission power of users while guaranteeing the average delay constraints in time-varying uplink channels. We design a scheduler that selects a user for transmission and determines the transmission rate of the selected user based on the channel and backlog information of the constituent users. Since it requires prohibitively high computation complexity to determine an optimal scheduler for multi-user systems, we propose a low-complexity scheduling scheme that can achieve near-optimal performance. In this scheme, we reduce the complexity by decomposing the multiuser problem into multiple individual user problems. We present the optimal solution to the user problem and then investigate the effects of the user selection algorithm on the performance for different channel models. The proposed user selection algorithm that utilizes both channel and backlog information turns out to be energy-efficient over wide range of delay requirement regardless of the channel model.

In this paper, we present a distributed resource allocation algorithm for multi-cell OFDMA systems by adopting a game theoretic approach. We first define a utility function that can represent the system performance while taking into account the co-channel interference among cells. Then we model resource allocation problem as a noncooperative game. Based on the game, we devise a iterative algorithm called DRA that requires no coordination among the base stations. We prove that there exists a Nash equilibrium for the game and the equilibrium is unique in some constrained environment. The proposed DRA algorithm turns out to converge to the equilibrium within a small number of iterations.

We investigate the problem of the lifetime maximization in wireless sensor networks under the constraint of end-to-end transmission success probability, by adopting a cross-layer strategy that considers physical layer (i.e., power control), MAC layer (i.e., retransmission control) and network layer (i.e., routing protocol) jointly. We decouple the problem into separate sub-problems of each layer and propose the optimal algorithm as well as an alternative heuristic algorithm with lower complexity for each sub-problem. We demonstrate through computer simulations that a trade-off relation exists between the network lifetime maximization and the reliability constraint, and the strategy that is designed by combining the proposed algorithm at each layer can significantly increase the network lifetime. We also investigate the effect of the retransmission control on the energy efficiency for different energy consumption models. Simulation results reveal that multiple retransmissions with low power yield little gain when the link distance is short and the power conversion efficiency of the amplifier increases with the transmission power

We investigate the problem of lifetime maximization in a wireless sensor network under the constraint of target end-to-end transmission success probability, by adopting a cross-layer strategy that considers physical layer (i.e., power control), MAC layer (i.e., ARQ control) and network layer (i.e., routing protocol) jointly. We first present a near-optimal retry limit allocation algorithm for a given routing path. This allocation determines the per-hop success probability for each link along the path in order to minimize the total energy consumption while guaranteeing the reliability constraint. Then, we develop an optimal routing and power control algorithm that maximizes the network lifetime while keeping the network stable, and an alternative heuristic algorithm that has lower and tractable complexity. Simulation results reveal that a trade-off relation exists between the network lifetime maximization and the reliability constraints and that the network lifetime can be increased significantly by employing the proposed algorithms.

In this paper, we investigate the problem of the lifetime maximization in a wireless sensor network under the constraint of the target end-to-end transmission success probability, by adopting a cross-layer strategy that considers both physical layer (i.e., power control) and network layer (i.e., routing protocol) jointly. We first present the optimal power allocation algorithm for a given routing path. This allocation determines per-hop success probability for each link along the path in order to minimize the total energy consumption while guaranteeing the reliability constraint. Then, we develop an optimal routing algorithm that maximizes the network lifetime and a heuristic algorithm that has lower and tractable complexity. Simulation results reveal that a trade-off relation exists between the network lifetime maximization and the reliability constraint, and that the proposed routing algorithm combined with the optimal power allocation algorithm increases the network lifetime significantly.

In this paper, we present a new wireless scheduling algorithm based on the cumulative distribution function (cdf) and its simple modification that limits the maximum starving time. This cdf-based scheduling (CS) algorithm selects the user for transmission based on the cdf of user rates, in such a way that the user whose rate is high enough, but least probable to become higher, is selected first. We prove that the CS algorithm is equivalent to a scheduling algorithm that regards the user rates as independent and identically distributed, and the average throughput of a user is independent of the probability distribution of other users. So, we can evaluate the exact user throughput only if we know the user's own distribution, which is a distinctive feature of this proposed algorithm. In addition, we try a modification on the CS algorithm to limit the maximum starving time, and prove that the modification does not affect the average interservice time. This CS with starving-time limitation (CS-STL) algorithm turns out to limit the maximum starving time at the cost of a negligible throughput loss.

We present a new wireless scheduling algorithm based on the cumulative distribution function (cdf) of user transmission rates and also present a simple modification of it to limit the maximum starving time. This cdf-based scheduling (CS) algorithm selects the user for transmission based on the cdf of user rates in such a way that the user whose rate is high enough but least probable to become higher is selected. It turns out that the cdf-based scheduling algorithm is equivalent to a scheduling algorithm that regards the user rates as independent identically distributed (i.i.d.) and the average throughput of a user is independent of other users' probability distribution. A distinctive feature of this proposed algorithm is that the exact user throughput can be evaluated if the user's own distribution is known. The cdf-based scheduling with starving-time limitation (CS-STL) algorithm turns out not to affect the average inter-service time but to limit the maximum starving time at the cost of a negligible throughput loss.