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The demand for ultra-reliable low-latency communications (URLLC) has led to the adoption of grant-free (GF) access techniques by the 5G NR, with the goal of reducing uplink access time. When GF access is employed, the base station (BS) preallocates multiple transmission opportunities (TOs) that can be utilized by the user equipment (UE) as needed. However, this approach results in inefficient resource utilization as unused TOs are wasted. To overcome this inefficiency, the 5G NR allows the assignment of configured grants (CG) to a group of UEs instead of a single one. This development has led research into group-based CG (GCG) schemes, whose reliance on shared resources can result in collisions. The collisions can be prevented by the use of stop-and-wait IR-HARQ schemes. Nevertheless, the delay caused by feedback latency is also undesirable as it severely affects latency performance. This work proposes two new IR-HARQ GCG schemes to efficiently handle feedback latency. The first one is able to eliminate the feedback latency overhead and is proven to simultaneously achieve the latency of a one-shot transmission and the energy efficiency of IR-HARQ, even in the presence of non-instantaneous feedback signaling. The second one features both a feedback latency protection mechanism, similar to the first scheme, and a mechanism specifically designed to further reduce latency. The performance of the proposed schemes is compared with scenarios where each UE uses either an individual one-shot or an IR-HARQ scheme. These comparisons encompass scenarios with either power or energy constraints. The results have shown that the second scheme always outperforms the IR-HARQ scheme and that it is able to outperform the one-shot scheme on a wide interval of feedback latency values, achieving a lower latency both for power and energy constrained cases.

The ultra-reliable low-latency communications (URLLC) tight latency requirements paired with transmission of small payload packets motivates the development of techniques that reduce or eliminate the need for dynamic scheduling. This justifies the study of grant free (GF) leveraged techniques in order to reduce both the latency and control signaling overhead. Previous works considered preallocating resources not only for the first transmission, but also for all possible IR-HARQ transmissions, effectively reducing the scheduling latency and control signaling overhead. However, this has several drawbacks, as it translates into wasted resources. To address these issues, we propose a group-based preallocation method combined with IR-HARQ. Initially, a pool of preallocated resources is assigned to a group of users, which then cooperatively use IR-HARQ feedback signals to distribute, on the fly, the resources amongst them without collisions. The proposed method has two phases: a preallocation phase that takes place once at the group formation stage and a transmission phase which happens at each uplink transmission. The transmission parameters for all possible transmission scenarios are selected at the preallocation stage, with the goal of reducing the latency under reliability and energy constraints. The transmission parameters are obtained through a constrained latency optimization procedure, which considers the stochastic nature of the underlying process. We prove that, asymptotically, the proposed scheme is able to reduce the latency, at least, down to the average latency of any single user (SU) HARQ. The numerical results show that the latency and resources wastage is significantly reduced comparatively to single user IR-HARQ with preallocated resources.

Passive technologies, including intelligent reflecting surfaces (IRS), are gaining traction thanks to their ability to enhance communication systems while maintaining minimal cost and low complexity. They can assist a wireless sensor network (WSN) by achieving low power requirements for sensors and aid communication needs in many applications, for instance, environmental monitoring. In this paper, we propose an IRS-equipped WSN which describes sensors equipped with IRSs instead of active radio frequency (RF) electronics. The IRS sensor node (ISN) intercepts a dedicated signal from a power source such as a base station (BS) and modulates the transmission of that signal to an intended recipient. In order to enable multiple sensors to transmit to the receiver, we study opportunistic scheduling (OS) utilizing multi-sensor diversity while considering blind IRS operation, and compare it with round-robin (RR), proportional fairness (PF), and a theoretical upper bound. We study the effect of the choice of the number of IRS elements N and number of ISNs L on the average throughput of the system under OS. Finally, we provide pertinent comparisons for the different scheduling schemes and IRS configurations under relevant system performance metrics, highlighting different scenarios in which each scheme performs better. [ABSTRACT FROM AUTHOR]

Passive technologies, including intelligent reflecting surfaces (IRS), are gaining traction thanks to their ability to enhance communication systems while maintaining minimal cost and low complexity. They can assist a wireless sensor network (WSN) by achieving low power requirements for sensors and aid communication needs in many applications, for instance, environmental monitoring. In this paper, we propose an IRS-equipped WSN which describes sensors equipped with IRSs instead of active radio frequency (RF) electronics. The IRS sensor node (ISN) intercepts a dedicated signal from a power source such as a base station (BS) and modulates the transmission of that signal to an intended recipient. In order to enable multiple sensors to transmit to the receiver, we study opportunistic scheduling (OS) utilizing multi-sensor diversity while considering blind IRS operation, and compare it with round-robin (RR), proportional fairness (PF), and a theoretical upper bound. We study the effect of the choice of the number of IRS elements N and number of ISNs L on the average throughput of the system under OS. Finally, we provide pertinent comparisons for the different scheduling schemes and IRS configurations under relevant system performance metrics, highlighting different scenarios in which each scheme performs better.

This letter presents a detailed investigation on the performance of reconfigurable intelligent surface (RIS)-assisted communication system with user scheduling. Depending on the availability of channel state information (CSI) at the RIS, two separate scenarios are considered, namely without CSI and with CSI. Closed-form expressions are derived for the ergodic capacity of the system in both scenarios. It is found that CSI has a significant impact on the performance of the system. Without CSI, the RIS provides an array gain of N, where N is the number of reflecting elements, and user scheduling provides an multi-user gain of $\log \log \text {M}$ , where M is the number of users. With CSI, the RIS provides an array gain of $\text {N}^{2}$ , while no multi-user diversity gain can be obtained. [ABSTRACT FROM AUTHOR]

For the efficient user scheduling (US) and interference mitigation, channel state information (CSI) is mandatory at the base station (BS), particularly for the frequency division duplexing (FDD) systems where the users need to provide some CSI back to the BS for performing the scheduling process. As the feedback overhead increases drastically with increase of antennas as well as users, so, US with limited/reduced feedback is essential. Deploying massive multiple-input multiple-output frequency division duplexing (mMIMO FDD) systems remains an open area for research with major difficulties of associated uplink CSI feedback load as the downlink and uplink channels are not reciprocal. Therefore, smart balancing is required between feedback data and achievable throughput while scheduling the users. In order to handle the above mentioned issue, we employed a selective 4-bit quantized CSI feedback based user-antenna paired/joint scheduling scheme for the multi user (MU) FDD mMIMO systems. The key idea involves in this process is that, only a limited set of users qualifying a predefined selection threshold feed back the 4-bit quantized signal-to-interference-plus-noise ratio (SINR) values based on angle-of-departure (AoD) along with the antenna indices for the scheduling process. The BS schedules the users with respective best antenna. Furthermore, using this scheme the multi user diversity gain is also achieved. This dynamic adjustment of antenna/user selection and collision free antenna sharing between the users majorly helps in improving the throughput of the system with this selective 4-bit quantized limited feedback. The mathematical sum rate performance analysis has been demonstrated along with simulation results for different system parameters. [ABSTRACT FROM AUTHOR]

Millimeter wave (mmWave) beamforming (BF) system can increase the spatial capacity by being installed in hotspot areas where a number of users exist. However, it is hard to obtain the channel state information of a large array system at transmitter. Even with the codebook-based BF scheme, the digital–analog hybrid BF structure may require a two-step feedback procedure. In order to reduce the feedback and feedforward overhead required for joint user scheduling and hybrid BF, we propose a codebook-based radio frequency (RF) only BF scheme with one-step feedback procedure. We show that the achievable sum rate of the RF-only BF scheme double-logarithmically scales with the number of users in mmWave channel model, which coincides with the results for the digital BF schemes in rich scattering channel model. We also show that the RF-only BF scheme can outperform the hybrid BF scheme with limited feedback. In addition, we examine the error performance in extremely sparse channel, and find the number of transmitted beams to achieve the desired rate per user in a small number of users regime. [ABSTRACT FROM AUTHOR]

Multiple Input Multiple Output (MIMO) based Spatial Time Division Multiple Access (STDMA) Wireless Mesh Networks (WMNs) have attracted extensive research attention. However, there are two problems in existing studies: (1) the employed MIMO link rate models are not suitable for a MIMO link of a practical STDMA WMN, and (2) the designed scheduling algorithms usually cannot take advantage of the multi-user diversity in a WMN. In this paper, we develop an analytical model for determining the MIMO link rate of an STDMA WMN. Based on a node-based slot assignment and scheduling algorithm (Chen and Lea in IEEE Trans Veh Technol 62(1):272–283, 2013), we propose a static/opportunistic hybrid scheduling framework that can exploit multi-user diversity and channel fading. The performance evaluation shows that the proposed framework has 33–46 % throughput gain over the prior joint routing and time slot assignment schemes for MIMO WMNs. [ABSTRACT FROM AUTHOR]

Underwater acoustic (UWA) communication suffers from large propagation delays, limited bandwidth, frequency dependent attenuation and severe multipath fading. Low power consumption, acceptable bit error rate and minimal complexity are three critical factors for UWA sensor network design. Orthogonal frequency division multiple access (OFDMA) is an attractive option because of its robustness against frequency selective underwater channel. But, OFDMA suffers from high peak-to-average power ratio (PAPR) and high sensitivity to frequency offsets. Single carrier FDMA (SC-FDMA) has not been well investigated over UWA channel in literature. Two variants of SC-FDMA namely interleaved FDMA (IFDMA) and localized FDMA (LFDMA) are considered here. Firstly, through simulation it has been observed that IFDMA performs significantly better than OFDMA, in terms of SER and PAPR. However, IFDMA outperforms LFDMA in terms of PAPR, but SER performance of IFDMA is poor as compared to that of LFDMA. In case of power limited uplink UWA communication, IFDMA serves as a feasible solution. But for downlink, where lower SER is an important performance parameter, LFDMA is an efficient multiple access scheme. The use of carrier interferometry (CI) codes with SC-FDMA further enhances the SER and PAPR performance over highly frequency selective UWA transmission. We have also evaluated the effect of multi-user diversity for CI/SC-FDMA over 5-tap frequency selective UWA channel by applying scheduling algorithm. The algorithm selects the sensor mote with best channel condition for transmission. [ABSTRACT FROM AUTHOR]

In spatial time division multiple access wireless mesh networks, not all links can be activated simultaneously, as links scheduled for transmission must satisfy the specified SINR requirements. Previously, slot assignment has been done on a link basis, where a set of links is selected for transmission in a given slot. However, if selected links are in deep fade or have no traffic to transmit, the slot is wasted. Thus, a node-based scheme was proposed, where a set of nodes is selected for transmission. Which link to be used by a node depends on the links’ instantaneous traffic load. Although this allows us to exploit multi-user diversity, it creates a planning discrepancy: slot assignment is designed based on long-term channel statistics, but scheduling on short-term channel fading conditions. Consequently, the performance gain of the node-based scheme is not consistent: it is marginal under certain scenarios. To avoid the design discrepancy, we develop a new slot-assignment and routing framework in this paper. The new approach incorporates short-term channel fading statistics to optimize the long term slot assignment, routing and scheduling simultaneously. Hence, multi-user diversity can be exploited more efficiently. Not only is the performance gain of the resulting system significant (can be as much as 64% higher throughput than the scheme introduced by Chen and Lea), it is also less topology dependent compared with the one by Chen and Lea. [ABSTRACT FROM PUBLISHER]

In this paper, we consider joint realtime (RT) and nonrealtime (NRT) flows packet scheduling and resource block (RB) allocation in orthogonal frequency division multiple access (OFDMA) wireless networks. Radio RBs in the OFDMA plane are to be distributed among RT and NRT flows. In the conventional approach, RT and NRT flows are served sequentially. This sequential approach is inefficient because an RT flow may presumably have enough time until its delay deadline while its channel is in deep fade. In this situation, the transmission of NRT flows with higher level of efficiency can be performed. Intuitively speaking, the conventional sequential approach is too conservative, which can be reengineered. We propose a novel joint RT and NRT flows disutility-based packet scheduling and RB allocation in a common pool of RBs. The proposed joint approach enlarges the effective capacity of the associated wireless system when compared with the separated pool of RBs. The joint approach is particularly relevant for improving voice over LTE (VoLTE). We use mean bit-rate, mean queue-length, and instantaneous queuing delay information, in addition to channel information, to match the demand and supply. Furthermore, we develop a novel model for input–output bit-rate behavior of the mixture of RT and NRT flows. This model sheds light on the identification of different load regions and understanding of the system in an intuitive manner. Our approach and methodology can be extended for broader quality-of-service (QoS) requirements and for the utility of future applications. Simulation results show that the proposed framework is able to unify the serving mechanism of the RT and NRT flows and is able to achieve higher admissible bit-rate when handling mixed RT and NRT flows, compared with various baselines. [ABSTRACT FROM AUTHOR]

In this work, we propose a unified approach to evaluating the CDF and PDF of indefinite quadratic forms in Gaussian random variables. Such a quantity appears in many applications in communications, signal processing, information theory, and adaptive filtering. For example, this quantity appears in the mean-square-error (MSE) analysis of the normalized least-mean-square (NLMS) adaptive algorithm, and SINR associated with each beam in beam forming applications. The trick of the proposed approach is to replace inequalities that appear in the CDF calculation with unit step functions and to use complex integral representation of the the unit step function. Complex integration allows us then to evaluate the CDF in closed form for the zero mean case and as a single dimensional integral for the non-zero mean case. Utilizing the saddle point technique allows us to closely approximate such integrals in non zero mean case. We demonstrate how our approach can be extended to other scenarios such as the joint distribution of quadratic forms and ratios of such forms, and to characterize quadratic forms in isotropic distributed random variables. We also evaluate the outage probability in multiuser beamforming using our approach to provide an application of indefinite forms in communications. [ABSTRACT FROM AUTHOR]

In this paper, distributed user scheduling schemes are proposed for the multi-user MIMO-Y channel, where three NT-antenna users ( NT=2N,3N) are selected from three clusters to exchange information via an NR-antenna amplify-and-forward (AF) relay ( NR=3N), and N\geq 1 represents the number of data stream(s) of each unicast transmission within the MIMO-Y channel. The proposed schemes effectively harvest multi-user diversity (MuD) without the need of global channel state information (CSI) or centralized computations. In particular, a novel reference signal space (RSS) is proposed to enable the distributed scheduling for both cluster-wise (CS) and group-wise (GS) patterns. The minimum user-antenna (Min-UA) transmission with N_{T}=2N is first considered. Next, we consider an equal number of relay and user antenna (ER-UA) transmission with N_{T}=3N$, with the aim of reducing CSI overhead as compared to Min-UA. For ER-UA transmission, the achievable MuD orders of the proposed distributed scheduling schemes are analytically derived, which proves the superiority and optimality of the proposed RSS-based distributed scheduling. These results reveal some fundamental behaviors of MuD and the performance-complexity tradeoff of user scheduling schemes in the MIMO-Y channel. [ABSTRACT FROM PUBLISHER]

This work proposes a suboptimal multi-user scheduling scheme for Gaussian broadcast channels which improves upon the classical single user selection, while considerably reducing complexity as compared to the optimal superposition coding with successful interference cancellation. The proposed scheme combines the two users with the maximum weighted instantaneous rate using superposition coding. The instantaneous rate and power allocation are derived in closed-form, while the long term rate of each user is derived in integral form for all channel distributions. Numerical results are then provided to characterize the prospected gains of the proposed scheme. [ABSTRACT FROM AUTHOR]

A single primary user cognitive radio system with multiuser diversity at the secondary users (SUs) is considered, where there is an interference constraint between SUs and the primary receiver. The SU with the highest instantaneous signal-to-noise ratio is selected for communication from a set of active users, which also satisfies the interference constraint. The active number of SUs is shown to be binomial, negative binomial, or Poisson-binomial distributed depending on various modes of operation. The outage probability in the slow-fading scenario is also studied. This is then followed by a derivation of the scaling law of the ergodic capacity and bit error rate (BER) averaged across the fading and the user distribution for a large mean number of users. The ergodic capacity and average BER under the binomial user distribution are shown to outperform the negative binomial case with the same mean number of users. Moreover, the Poisson distribution is used to approximate the user distribution under the non-independent and identically distributed interference scenario and compared with binomial and negative binomial distributions in a stochastic ordering sense. Monte Carlo simulations are used to supplement our analytical results and compare the performances under different user distributions. [ABSTRACT FROM PUBLISHER]

We consider a wireless system where multiple users are served in Downlink (DL) by one Base Station (BS) and one Relay Station (RS). Previous research has shown that, combining two users' messages via Superposition Coding (SC) could enhance their achievable rate and fairness, in both two-user and multi-user systems. In this work, we propose two novel SC schemes referred as Relayed/Direct (RD-SC) and Relayed/Relayed SC (RR-SC) schemes, that combine the messages to a pair of Relayed/Direct users or Relayed/Relayed users into three or four SC layers, where the number of SC layers is equal to the number of available links. For each scheme, power allocation under sum-rate maximization is analyzed, enabling to derive the optimal power ratios to each SC layer numerically. Then, we design the non-orthogonal Max Rate-Two User-SC (MR-TU-SC) Scheduler and Proportional Fair-Two User-SC (PF-TU-SC) Scheduler where a pair of selected users are allocated simultaneously based on RD-SC and RR-SC schemes. The simulation results show that the proposed schedulers significantly outperform the conventional orthogonal schedulers where a single user is allocated on a unit resource block, in terms of system throughput, fairness and outage, while approaching the upper bound performance. [ABSTRACT FROM PUBLISHER]

Channel sensing order setting is crucial for efficient channel exploration and exploitation in cognitive radio (CR) networks. This paper investigates the sensing order setting problem in multi-channel multi-user CR networks for both distributed scenario and centralized scenario. As the optimal solution is too complicated, two suboptimal greedy search algorithms with much less computational complexities are proposed. The channel availability, channel achievable rate, multi-user diversity and collisions among CR users are considered comprehensively in our proposed methods. For the distributed scenario, a novel potential function is proposed to represent the relative advantage of a channel used by a user among multi channels and multi users, based on which each user can get its own sensing order. For the centralized scenario, a sensing matrix is obtained by a coordinator for all the users. It is shown that, CR users' average throughput increases and collision probability decreases with the number of channels due to increased transmission opportunities. The total network throughput increases with the number of user pairs due to multi-user diversity. Simulation results validate the efficacy of the proposed schemes in elevating CR users' throughput and decreasing the probability of collision, and show the performance improvement of the proposed schemes by comparisons with existing works. [ABSTRACT FROM AUTHOR]

In this paper, we propose a limited feedback based underlay spectrum sharing scheme where a downlink secondary network shares the spectrum with a pair of primary users. The primary receiver and N secondary receivers each sends 1-bit feedback of channel quality information to their corresponding transmitters. By overhearing the primary feedback and receiving the secondary feedback, one of the secondary receivers is scheduled at the secondary transmitter in each fading block. The optimal channel quantization thresholds and power allocation are jointly determined by maximizing the average throughput of the secondary user under the average secondary power constraint and the average primary rate loss constraint. The average throughput of the secondary user grows as log log N. [ABSTRACT FROM PUBLISHER]

In LTE systems with heterogeneous user mobilities, low-mobility users favor frequency selective scheduling while high-mobility users benefit from frequency diversity scheduling. To benefit both low- and high-mobility users simultaneously, scheduling exploiting frequency selectivity and diversity is desired. To enable the scheduling, low-complexity user mobility classification to distinguish these two types of users is required. In this paper, we first propose a user mobility classification algorithm, which is robust to different channel delay profiles (CDPs), for single-transmit-antenna systems. Then, we extend it to multiple-input multiple-output (MIMO) systems. A low-complexity scheduling algorithm, exploiting both frequency-selectivity and diversity for low- and high-mobility users simultaneously, is also developed. As demonstrated by the simulation results, the proposed user classification algorithm is robust to different CDPs and the proposed scheduling algorithm is effective. [ABSTRACT FROM PUBLISHER]

In multi-rate 802.11 networks, stations with better uplink conditions should access the shared channel more frequently than the ones with worse uplink conditions to enhance the aggregated throughput, which is referred to as multi-user diversity. In this letter, we propose Adaptive Basic Rate Set (ABRS) that uses the time gap between DIFS and EIFS for a prioritized channel access while minimizing starvation. ABRS requires the change in the firmware only at access points with no modification to the standard. Simulation results reveal that ABRS effectively prioritizes stations with higher bit rates; the aggregate uplink throughput is improved up to about 30-70% even in mobility scenarios. [ABSTRACT FROM PUBLISHER]

In this letter, we consider the uplink wiretap network which consists of a base station, N legitimate users, and several eavesdroppers. We propose a novel user scheduling algorithm based on a threshold, which achieves the optimal multi-user diversity gain, i.e., log log N. To the best of our knowledge, there has been no such result in uplink wiretap networks. In order to obtain good throughput performance in the network, the threshold value needs to be carefully chosen. Through extensive simulations, we observe that the proposed user scheduling outperforms the conventional scheduling algorithms and it approaches the throughput performance of the optimal user scheduling algorithm in various scenarios. [ABSTRACT FROM PUBLISHER]

We propose joint user-and-hop scheduling over dual-hop block-fading broadcast channels in order to exploit multi-user diversity gains and multi-hop diversity gains all together. To achieve this objective, the first and second hops are scheduled opportunistically based on the channel state information. The joint scheduling problem is formulated as maximizing the weighted sum of the long term achievable rates of the users under a stability constraint, which means that in the long term the rate received by the relay should equal the rate transmitted by it, in addition to power constraints. We show that this problem is equivalent to a single-hop broadcast channel by treating the source as a virtual user with an optimal weight that maintains the stability constraint. We show how to obtain the source weight either off-line based on channel statistics or on real-time based on channel measurements. Furthermore, we consider special cases including the maximum sum-rate scheduler and the proportional fair scheduler. We also show how to extend the scheme into one that allows multiple user scheduling via superposition coding with successive decoding. Numerical results demonstrate that our proposed joint scheduling scheme enlarges the rate region as compared to scheduling schemes that exploit the diversity gains partially. [ABSTRACT FROM PUBLISHER]

We investigate achievable degrees-of-freedom (DoF) of an (N,K)-user interference channel where only K user (transmitter-receiver) pairs among N user pairs are allowed to simultaneously communicate in a dense network (N >> K). Each node is assumed to have M antennas and to be randomly located. We propose a distributed scheduling protocol to achieve the maximum DoF (i.e., MK), which sequentially and opportunistically selects a user pair causing/receiving interference lower than a pre-determined threshold to/from already selected user pairs in each step. It is proven that the proposed protocol achieves the maximum DoF, MK, in the (N,K)-user interference channel with less stringent network size N, compared with the conventional centralized protocol which has been known as the best. With zero-forcing detector at receiver, we prove that it is sufficient that the network size N scales at least as ω(SNR^M^2K(K-1)) to achieve the maximum number of DoF MK, where SNR denotes the received signal-to-noise ratio. We also investigate the required feedback overheads of the proposed protocol and show that it is quite small when the network is strongly interference-limited because only a small number of users are required to transmit their signaling. Our numerical results show that our proposed scheme controls interference more effectively than the centralized protocol. [ABSTRACT FROM PUBLISHER]

Scheduling can obtain multi-user diversity if channel state information (CSI) is known, such as for low-mobility users and can exploit frequency diversity if CSI is not available at the transmitter, such as for high-mobility users. In this paper, we investigate resource allocation exploiting frequency and multi-user diversity for LTE downlink systems with users of different mobilities. To facilitate resource allocation, we first develop a user classification algorithm to identify high- and low-mobility users. Based on user mobility classification, we then propose a scheduling algorithm to simultaneously obtain multi-user diversity for those low-mobility users and frequency diversity for those high-mobility users. It is demonstrated by computer simulation that the performance of the proposed scheduling algorithm provides 6% and 23% gain of overall cell throughput, and 5.6% and 18% gain of 10th percentile throughput over proportional fairness based frequency-selective and frequency-diversity scheduling algorithms, respectively. Furthermore, the proposed scheduling algorithm has the same order of computational complexity as the frequency-selective scheduling algorithm. [ABSTRACT FROM PUBLISHER]

In general, it has been demonstrated that the performance of conventional random beamforming (RBF) approaches that of ideal eigen beamforming when the number of users is large in a cell by exploiting multi-user diversity. However, if the number of users decreases, such as femto or pico cell in the 3GPP Long Term Evolution-Advanced standard, the performance degradation occurs in the RBF scheme due to the lack of multi-user diversity. In this paper, we present a novel precoder based on singular value decomposition (SVD) using feedback of weight values in an RBF environment. For achieving performance improvement in the femto cell environment, we generate a precoding matrix appropriate to user channel by controlling the feedback values with an equivalent feedback quantity. In the simulation results, we verify the outstanding performance of the proposed scheme for a small number of users. [ABSTRACT FROM AUTHOR]

This paper considers coordinated user scheduling in a multi-user two-hop multi-input multi-output relay system with limited feedback. The proposed scheme utilizes a quantized transmit correlation and channel quality information, enabling to achieve both interference mitigation and multi-user diversity (MUD) gain. To this end, we first investigate the effect of quantization error on the statistical characteristics of co-channel interference (CCI) cased by the relay. Then, the coordinated user-scheduling strategy is designed with the use of eigen-beamforming to maximize the desired signal power in an instantaneous manner while minimizing the CCI in an average sense. Analytic and numerical results show that the proposed scheme can maximize the achievable sum-rate by handling a tradeoff between interference mitigation and MUD gain according to the number of quantization bits, providing a large sum-rate performance in the presence of quantization error. [ABSTRACT FROM AUTHOR]

We introduce a distributed opportunistic scheduling (DOS) strategy, based on two pre-determined thresholds, for uplink K-cell networks with time-invariant channel coefficients. Each base station (BS) opportunistically selects a mobile station (MS) who has a large signal strength of the desired channel link among a set of MSs generating a sufficiently small interference to other BSs. Then, performance on the achievable throughput scaling law is analyzed. As our main result, it is shown that the achievable sum-rate scales as K\log(SNR\log N) in a high signal-to-noise ratio (SNR) regime, if the total number of users in a cell, N, scales faster than SNR^\fracK-11-ε for a constant ε∈(0,1). This result indicates that the proposed scheme achieves the multiuser diversity gain as well as the degrees-of-freedom gain even under multi-cell environments. Simulation results show that the DOS provides a better sum-rate throughput over conventional schemes. [ABSTRACT FROM AUTHOR]

In a multiple transmit antenna, single antenna per receiver downlink channel with limited channel state feedback, we consider the following question: given a constraint on the total system-wide feedback load, is it preferable to get low-rate/coarse channel feedback from a large number of receivers or high-rate/high-quality feedback from a smaller number of receivers? Acquiring feedback from many receivers allows multi-user diversity to be exploited, while high-rate feedback allows for very precise selection of beamforming directions. We show that there is a strong preference for obtaining high-quality feedback, and that obtaining near-perfect channel information from as many receivers as possible provides a significantly larger sum rate than collecting a few feedback bits from a large number of users. In terms of system design, this corresponds to a preference for acquiring high-quality feedback from a few users on each time-frequency resource block, as opposed to coarse feedback from many users on each block. [ABSTRACT FROM PUBLISHER]

We consider a flat fading multiantenna downlink system with a large number of users where the objective is to deliver equal rates to nonoutage users with a low complexity. We show that in the limit of a large number of users, a zero-forcing beamforming strategy combined with a low complexity user grouping algorithm based on a semi-orthogonal user selection achieves asymptotically optimal performance, with respect to an upper bound that can be achieved when no interference is present among users. [ABSTRACT FROM AUTHOR]

In wireless packet networks, the performance of multiuser diversity schedulers has been analyzed with the saturation assumption; however, the finite queue length effect should be taken into account. We propose a new scheduling scheme exploiting multiuser diversity that considers not only channel information but queue information as well. Considering the finite queue length effect and adaptive modulation and coding scheme (AMCS), we analyze the performance of the proposed scheduler. [ABSTRACT FROM PUBLISHER]

The heterogeneous cooperative relaying technique can be utilized to complete the hierarchical convergence for the multi-radio access networks, where the single heterogeneous cooperative relay is selected and the maximal-ratio combining (MRC) scheme is utilized to achieve the cooperative diversity gain. In order to evaluate performances of the hierarchical convergence mechanism, this paper theoretically investigates the key factors of the multi-user diversity (MUD) gain, the heterogeneous cooperative diversity gain and the large scale fading of the first and second links. The tight closed-form expressions in terms of the outage probability and the average symbol error rate are derived for evaluating how and with what factors impact on the system performance. The analytical and simulation results show that the number of heterogeneous cooperative relay nodes (HCRNs) M and the number of destination stations (DSs) K have great impacts on performances, and the order of outage probability is ( M + 1) K. The large scale fading ratio of the first hop to the second hop also has a big impact on performances. Thus in the real network, we can utilize advanced radio resource management schemes to achieve a high multi-user diversity, instead of configuring too many HCRNs for the heterogeneous cooperative diversity gain. Furthermore, we can guarantee the transmission quality between the BS and HCRNs via the network planning to optimize the overall network performance. [ABSTRACT FROM AUTHOR]

In this paper, we present a hierarchical convergence scheme for multi-radio access networks via heterogeneous cooperative relaying technique, where heterogeneous cooperative relay node (HCRN) acts as the convergence gateway and provides cooperative diversity and antenna array gains. The multiple antennas are configured on base station (BS), HCRN, and destination node (DS) to achieve the antenna array gain. Considering the power consumption and implementing complexity at HCRN, only a single antenna is configured at HCRN to transmit decoded packages to the destination node (DS). A joint heterogeneous cooperative relay selection and maximal-ratio combining scheme is proposed to maximize both cooperative and multi-user diversity (MUD) gains. Tight closed-form expressions for the outage probability and average symbol error rate are derived to evaluate performances, which are related strictly to the cooperative relay selection scheme, multiple antenna configurations, and fading channels. The analytical and simulation results show that the numbers of HCRNs and DSs play identical roles in the performance improvement, while the antenna number of DS provides a more significant diversity gain. Thus, in a practical application, we should aim to achieve a high MUD gain, instead of approaching the cooperative diversity gain via deploying too many HCRNs. In addition, the antenna array gain via configuring multiple antennas in DSs is preferred because it is bigger than the MUD gain or the cooperative diversity gain. Copyright © 2009 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

It is well known that high multi-user and multi-carrier diversity gains can be achieved via the use of DCA (Dynamic Channel Allocation). However, there is no framework for analyzing the system performance of DCA algorithms over multi-cell environments. This paper presents a numerical analysis for measuring the performance gain for various DCA algorithms. Based on the analysis, an LQE (Location-based QoS Enhanced) DCA algorithm is proposed, in which a higher priority is assigned to users in the outer region for fairness by exploiting multi-carrier diversity. In the inner region, each carrier is allocated to a user with the best channel gain for higher throughput by exploiting multi-user diversity. In the simulation, we demonstrate that the proposed DCA algorithm is very effective from the perspective of fairness and throughput. [ABSTRACT FROM AUTHOR]

In this paper, we discuss different algorithms that can be used to encode channel state information (CSI) in realistic multi-user multiple-input multiple-output (MIMO) systems where there are only few users experiencing similar propagation conditions and the mobile user receivers do not necessarily have the same number of receive antennas. We divide systems with CSI encoding into four classes: time-division multiplexing (TDM) with and without linear pre-coding, and multiple user scheduling with and without linear pre-coding. The practical aspects such as system’s complexity and approaches for transmitting the CSI feedback and rate information from the mobile receivers to the base station are discussed and compared for different bit rates in the feedback link. We show that significant increases of the mean throughput of the multi-user scheduling systems demand much higher feedback link bit rates than TDM solutions. We also demonstrate that, while optimum, the non-linear pre-coding systems may introduce unacceptable degree of complexity into the base station design while linear pre-coding offers a very good trade-off between performance and complexity. [ABSTRACT FROM AUTHOR]

It has been shown that random beamforming using partial channel state information (CSI) achieves the same throughput scaling as obtained from dirty paper coding for a broadcast (downlink) channel with M transmit antennas and K users where K is large [1]. In this paper, we apply this scheme to wideband MIMO broadcast channels. By using OFDM, an L-tap wideband channel can be decomposed to N parallel narrowb- and channels (subcarriers), where N > L. Neighboring subcarriers are highly correlated. Therefore, we consider neighboring subcarriers as a cluster and find the closed form solution for the joint characteristic function of SINR values at two subcarriers in a cluster. We show numerically how the knowledge of the quality of the center subcarrier sheds light about the quality of other subcarriers in the same cluster, and address the issue of cluster size. In addition, through complex and asymptotic analysis, we show that for cluster size of order N/L√logR (for large K), users need only feedback the best SINR at the center subcarrier of each cluster in order for the transmitter to perform opportunistic beamforming and maintain the same throughput scaling as when full CSI is available. Using simulation results, we verify our analytical result and show that even fewer feedback can be tolerated, and larger clusters (N/2L) can be implemented for a small throughput hit. [ABSTRACT FROM AUTHOR]

Abstract: Wireless multi-carrier communication systems that use packet schedulers based on channel knowledge have been proved their performance. Proportional fairness scheduling (PFS), if used in these systems, promises an attractive trade-off between fairness among users and system throughput. In this paper, an analytical solution is presented for the PFS throughput. Then, a simple scheme is presented to reduce the feedback signaling of the PFS without a significant loss in performance. This reduced complexity PFS attempts to meet a trade-off between multi-user diversity gain, fairness among users and low rate feedback signaling. As shown by simulations, for a large number of users compared to the number of sub-channels, this scheme kept fairness among users while minimizing the feedback signaling. [Copyright &y& Elsevier]

This paper considers the effect of spatial correlation between transmit antennas on the sum-rate capacity of the MIMO Gaussian broadcast channel (i.e., downlink of a cellular system). Specifically, for a system with a large number of users n, we analyze the scaling laws of the sum-rate for the dirty paper coding and for different types of beamforming transmission schemes. When the channel is i.i.d., it has been shown that for large n, the sum rate is equal to M log log n + M log P/M + o(1) where M is the number of transmit antennas, P is the average signal to noise ratio, and o(1) refers to terms that go to zero as n → ∞. When the channel exhibits some spatial correlation with a covariance matrix R (non-singular with tr(R) = M), we prove that the sum rate of dirty paper coding is M log log n + M log P/M + log det(R) + o(1). We further show that the sum-rate of various beamforming schemes achieves M log log n +M log P/M +M log c + o(1) where c ≤ 1 depends on the type of beamforming. We can in fact compute c for random beamforming proposed in [1] and more generally, for random beamforming with precoding in which beams are pre-multiplied by a fixed matrix. Simulation results are presented at the end of the paper. [ABSTRACT FROM AUTHOR]