Joint Spectrum Allocation and Relay Selection in Cellular Cognitive Radio Networks

Cognitive Radio Network (CRN) has been proposed in recent years to solve the spectrum scarcity problem by exploiting the existence of spectrum holes. One of the important issues in the cellular CRNs is how to efficiently allocate primary user (PU) spectrum inside a CRN cell without causing harmful interference to PUs. In this paper, we present a cross-layer framework which jointly considers spectrum allocation and relay selection with the objective of maximizing the minimum traffic demand of secondary users (SUs) in a CRN cell. Specifically, we consider (1) CRN tries to utilize PU spectrum even when the CRN cell is not completely outside the protection region of the PU cell, and (2) cooperative relay is used in cellular CRNs to improve the utilization of PU spectrum. We formulate this cross-layer design problem as a Mixed Integer Linear Programming (MILP) and propose a low complexity heuristic algorithm to solve it. Compared to a simple channel allocation scheme, the numerical results show a significant improvement by using our proposed method and the performance is close to the optimal solution. We further consider the spectrum allocation among several CRN cells with the objective of maximizing the overall minimum throughput of all cells while ensuring each individual cell’s minimum throughput requirement. A low complexity algorithm is proposed to achieve the objective with satisfactory performance.     

Fairness Analysis of Inter-cell Relay in Downlink OFDMA Cellular Networks

Wireless Personal Communications - In the OFDMA cellular network, in order to improve the throughput of the cell system and the overall resource utilization of the system as well as the quality of...     

Outage Probability in Full Spectrum Reuse Cellular Systems with Discontinuous Transmission

Co-channel interference has a very strong impact on the performance of cellular mobile radio systems; a performance measure to evaluate its effect is the outage probability. This work presents an analytical general formula for the outage probability evaluation in full spectrum reuse cellular systems with discontinuous transmission. In particular, both Nakagami and Rician fading along with log-normal shadowing have been considered in the signal propagation model. In addition, the discontinuous transmission strategy is considered. The results are applied to the throughput evaluation of a packet cellular radio network based on the slotted Aloha protocol.     

Bandwidth Partitioning and SINR Threshold Analysis of Fractional Frequency Reuse in OFDMA Cellular Networks for Real Time and Best Effort Traffic

To achieve high capacity in cellular networks, frequency reuse factor of unity is used. However, it suffers from heavy co-channel interference at cell edge regions. This leads to poor Signal to Interference plus Noise Ratio (SINR) and hence poor performance. fractional frequency reuse (FFR) is one of the methods being considered to improve cell edge performance. In this work we present the impact of SINR threshold and bandwidth partitioning on the successful deployment of FFR scheme. We have considered both real time (RT) and best effort (BE) traffic. In FFR, the total frequency resource is divided logically into cell center and cell edge user bands. However, while frequency resource is partitioned into two segments, the amount of bandwidth required by cell center and cell edge users is quite different and influences the system performance. We developed the semi analytical approach to evaluate the average bandwidth required by cell center and edge band users. From the analysis and simulation results it is seen that while grade of service (GoS) fair based method of choosing bandwidth partitioning ratio is best for RT traffic, while the only feasible method for BE traffic is dividing the bandwidth based on probability of a user to be in a given band which we termed as ‘Probability’ method.     

A Framework to Optimize the OFDM-Based Multiuser Dynamic Spectrum Access Networks with Frequency Reuse

This paper presents a novel framework to optimally solve the spectrum access problem at the central spectrum moderator (CSM) for multiple radio pairs. In the system model considered, the available spectrum is partitioned into N subcarriers, and the proposed CSM assigns subcarriers to multiple radio pairs with the objective to maximize the total achievable bit rate in the network. Each transceiver adopts the OFDM frontend, and transmits only on those subcarriers assigned to it. The proposed algorithm jointly considers the effect of interference from multiple co-channel radios, while at the same time supports all transmitters at their minimum guaranteed bit rate, under individual total transmit power constraint. Our proposed method converts the original NP-hard mixed integer non-linear programming problem into a mixed binary linear programming problem, which can be solved optimally using standard algorithms.     

Joint Relay Selection and Power Allocation for Cooperative Cellular Networks

The principal purpose of this paper is to develop a joint relay selection and power allocation algorithm in cooperative cellular networks with multiple users assisted by multiple relays. By solving a rate optimization problem, we can obtain the optimal solutions and the distributed implementation based on the primal-dual decomposition. An admission control algorithm is also presented when there exists the minimal rate requirement for each user. Simulation results indicate that the algorithm with joint design is superior to the existing ones with isolated design in large total transmission rates and small deviations. Moreover, with the admission control algorithm, the rate requirements for users participating in cooperation are also satisfied.     

OFDMA中继网络基于效用的资源分配

研究半双工正交频分复用（OFDMA）中继网络的资源分配。解决动态子信道分配,自适应功率分配,传输策略选择和中继选择的联合优化问题。为了公平利用中继用户的资源,把中继用户所能帮助的最大子信道数和每个子信道的功率设为常量。在此基础上,提出贪婪算法以最大化总效用。当算法每次分配资源时,把资源分配给潜在效用增长最大的用户。仿真结果表明所提算法与固定的资源分配策略相比,能更高效地利用资源,并且改善了用户间的公平性。     

OFDMA中继网络动态节能子载波、比特和功率联合分配策略(英文)

To reduce energy consumption while maintaining users’ Quality of Service (QoS) in Orthogonal Frequency Division Multiplex Access (OFDMA) relay-enhanced networks, an adaptive energy saving subcarrier, bit and power allocation scheme is presented. The optimal subcarrier, bit and power allocation problems based on discrete adaptive modulation and coding scheme have been previously formulated for relay-enhanced networks, and have been reformulated into and solved by integer programming in optimization theory. If the system still has a surplus of subcarriers after resource allocation, we carry out Band- width Exchange (BE) to enable more subcarriers to participate in transmission to save energy. In addition, as the relay selection scheme is closely linked with resource allocation, a heuristic energy saving relay selection scheme is proposed. Simulation results indicate that the proposed algorithm consumes less energy when transmitting the same number of bits than greedy energy saving schemes, although its spectrum efficiency is worse.     

Interference Coordination in Cellular OFDMA Networks

Orthogonal frequency division multiple access is the basis for several emerging mobile communication systems. Prominent examples are the 3GPP Long Term Evolution as the successor of UMTS high-speed packet access and the IEEE 802.16 system, advanced by the WiMAX forum. On a system level, OFDMA is basically a combination of time and frequency division multiple access. In cellular TDM/FDM systems, inter-cell interference is a major issue that traditionally has been solved by avoiding the use of the same frequency bands in adjacent cells. However, this solution incurs a waste of precious frequency resources. An attractive alternative is the use of beamforming antennas in combination with interference coordination mechanisms, where the transmission of adjacent base stations is coordinated to minimize inter-cell interference. Interference coordination is an important aspect of the system level, which influences many other issues, such as network planning or scheduling mechanisms. In this article, we give an overview of interference coordination as it would apply, for example, to IEEE 802.16e and review the relevant literature. We also discuss and compare interference coordination algorithms, which can be based either on global system knowledge or purely on local system knowledge.     

Spatial Spectrum Reuse for Opportunistic Spectrum Access in Infrastructure-Based Systems

Opportunistic spectrum access (OSA) receives a constantly growing interest due to its potential to mitigate spectrum scarcity and meet the increasing communication needs of mobile users. OSA refers to identifying and exploiting spatiotemporal unused portions of licensed spectrum to allow communication among unlicensed–secondary users (SUs) without adverse impact to the licensees (primary users—PUs). Key parameters in OSA are the spectrum opportunities detection method used by the SUs, and the interference level perceived by the PUs. A spatial spectrum reuse framework is proposed, where broadcast messages of an infrastructure-based primary system are exploited and combined with location-aware methods to detect spectrum opportunities and establish interference-free secondary links. The study of secondary link establishment probabilities revealed a spectrum reuse of up to 25% for omni-directional and up to 90% for directional antennas. Moreover, increased throughput is achieved in both cases, with directional antennas attaining significantly better performance.     

Cross-layer QoS Analysis of Opportunistic OFDM-TDMA and OFDMA Networks

Performance analysis of multiuser orthogonal frequency division multiplexing (OFDM-TDMA) and orthogonal frequency division multiple access (OFDMA) networks in support of multimedia transmission is conducted in this work. We take a cross-layer approach and analyze several quality-of-service (QoS) measures that incilude the bit rate and the bit error rate (BER) in the physical layer, and packet average throughput/delay and packet maximum delay in the link layer. The authors adopt a cross-layer QoS framework similar to that in IEEE 802.16, where service classification, flow control and opportunistic scheduling with different subcarrier/bit allocation schemes are implemented. In the analysis, the Rayleigh fading channel in the link layer is modeled by a finite-state Markov chain, and the channel state information (CSI) is assumed to be available at the base station. With the M/G/1 queueing model and flow control results, the analysis provides important insights into the performance difference of these two multiaccess systems. The derived analytical results are verified by extensive computer simulation. It is demonstrated by analysis and simulation that OFDMA outperforms OFDM-TDMA in QoS metrics of interest. Thus, OFDMA has higher potential than OFDM-TDMA in supporting multimedia services.     

Spectrum Sharing between Cellular and Mobile Ad Hoc Networks: Transmission-Capacity Trade-Off

Spectrum sharing between wireless networks improves the efficiency of spectrum usage, and thereby alleviates spectrum scarcity due to growing demands for wireless broadband access. To improve the usual underutilization of the cellular uplink spectrum, this paper addresses spectrum sharing between a cellular uplink and a mobile ad hoc networks. These networks access either all frequency subchannels or their disjoint subsets, called spectrum underlay and spectrum overlay, respectively. Given these spectrum sharing methods, the capacity trade-off between the coexisting networks is analyzed based on the transmission capacity of a network with Poisson distributed transmitters. This metric is defined as the maximum density of transmitters subject to an outage constraint for a given signal-to-interference ratio (SIR). Using tools from stochastic geometry, the transmissioncapacity trade-off between the coexisting networks is analyzed, where both spectrum overlay and underlay as well as successive interference cancelation (SIC) are considered. In particular, for small target outage probability, the transmission capacities of the coexisting networks are proved to satisfy a linear equation, whose coefficients depend on the spectrum sharing method and whether SIC is applied. This linear equation shows that spectrum overlay is more efficient than spectrum underlay. Furthermore, this result also provides insight into the effects of network parameters on transmission capacities, including link diversity gains, transmission distances, and the base station density. In particular, SIC is shown to increase the transmission capacities of both coexisting networks by a linear factor, which depends on the interference-power threshold for qualifying canceled interferers.     

Dynamic Channel Assignment for Multihop Cellular Networks with Any Reuse Factor

A clustered multihop cellular network (cMCN) architecture is recently proposed and studied using fixed channel assignment (FCA) with a reuse factor, N_r=7. In this Letter, we propose and develop a multihop dynamic channel assignment (mDCA) scheme applicable with any reuse factor. The mDCA assigns channels with the knowledge of the information about interference in surrounding cells. Simulations show that the capacity improvements at call blocking probability of 1% for mDCA over the conventional FCA are 96% and 210% for N_r=4 and N_r=7, respectively.     

OFDM中继网络中的资源复用策略

OFDM与中继技术是下一代无线通信系统的重要候选方案。针对OFDM蜂窝中继网络,为进一步提高系统吞吐量与资源利用率,提出了一种集中式的小区内资源复用方案。基于对网络中直传用户与中继用户的信干噪比分析,通过设定一个有效的滞后门限以及各节点的有限反馈,基站能预估直传资源被复用后系统总容量的变化趋势。仿真表明,该方法能提升后5%用户的平均信干噪比,且能增强系统的总容量,是一种良好的资源复用方案。     

协同DF-OFDM系统中基于MRC的机会中继研究

针对机会中继译码转发协同系统中未考虑直传链路的问题,提出一种由目的节点根据系统中断概率进行中继选择的方案,研究了将机会中继信号与直传信号在目的节点进行最大比合并时的系统性能,推导了采用最大比合并时的系统中断概率。研究结果表明,当直传链路存在时,通过目的节点的中继选择,可使系统性能得到进一步提高,其性能优于不考虑直传链路时的机会中继系统。     

一种动态寻路中的机会转发模型

动态寻路（Dynamic Routing）是一项用以解决城市交通拥堵的智能交通技术.通过让一些车辆产生和转发路况拥堵消息,另一些车辆能够避开拥堵路段,从而缓解交通状况.以“存储-携带-转发”为通信模式的机会网络被广泛运用于动态寻路研究.文中提出了一种新型路况信息的机会转发算法,算法综合考虑车辆的位置和行驶方向.仿真实验表明,算法在保持信息散布效果的前提下,显著降低了网络负载,提升了效率.     

Optimal and Approximate Mobility-Assisted Opportunistic Scheduling in Cellular Networks

This paper considers the problem of scheduling multiple users in the downlink of a time-slotted cellular data network. For such a network, opportunistic scheduling algorithms improve system performance by exploiting time variations of the radio channel. We present novel optimal and approximate opportunistic scheduling algorithms that combine channel fluctuation and user mobility information in their decision rules. The algorithms modify the opportunistic scheduling framework of Liu et al., (1993) with dynamic constraints for fairness. These fairness constraints adapt according to the user mobility. The adaptation of constraints in the proposed algorithms implicitly results in giving priority to the users that are in the most favorable locations. The optimal algorithm is an offline algorithm that precomputes constraint values according to a known mobility model. The approximate algorithm is an online algorithm that relies on the future prediction of the user mobility locations in time. We show that the use of mobility information in opportunistic scheduling increases channel capacity. We also provide analytical bounds on the performance of the approximate algorithm using the fundamental inequality of Dyer et al., (1986) for linear programs. Simulation results on high data rate (HDR) illustrate the usefulness of the proposed schemes for elastic traffic and macrocell structures     

Performance Analysis of the Primary User in the Secondary User Relay Assisted Spectrum Sharing Networks

In this paper, inaccurate spectrum detecting by the secondary user (SU) is taken into account. The impact of the interference caused by the SUs due to miss detection on the primary user (PU) in a spectrum sharing network is analyzed, and those SU nodes of correct detection are assumed to act as potential relays to assist the PU transmission process based on two proposed cooperative transmission schemes, referred to as, the distance based and the signal-to-noise-ratio (SNR) based schemes. We utilize stochastic geometry to analyze the impact of the secondary network parameters and cooperative transmission schemes on a typical primary source–destination (S–D) pair for the SU relay assisted spectrum sharing networks in Rayleigh fading environment. Using this approach, closed-form expressions for the primary system success probabilities with those cooperative transmission schemes as well as the PU direct re-transmission scheme are derived respectively. Simulations confirm our analytical derivations and results demonstrate that significant improvement on the PU success probability by using SU cooperative transmission schemes, and the SNR based scheme is superior to the distance based scheme.     

非理想信道状态信息对频谱共享认知中继网络性能的影响

在频谱共享认知中继网络中,非理想信道状态信息可能导致次级用户的干扰超过主用户所能容忍的最大干扰值,从而影响到主用户的正常通信。为了衡量次级用户的传输对主用户性能的影响,该文提出了干扰概率这一性能指标。在非理想的信道状态信息条件下,推导了次级用户采用机会中继与选择协作协议时的干扰概率闭式解析式。理论分析表明,从干扰概率角度看,选择协作协议优于机会中继协议,但二者的极限干扰概率相同。此外,在非理想信道状态信息条件下,无论是采用机会中继还是选择协作协议,增加中继数量都会对主用户造成更大的干扰。最后,仿真结果验证了理论分析的正确性。     

Dynamic Spectrum Sharing for OFDMA-Based Multi-hop Cognitive Radio Networks with Priority

In this paper, we propose a resource assignment scheme of a secondary user (SU) for a multi-hop cognitive radio network. In multi-hop networks, since each link has different SNR because of their different distance between stations and multipath fading, the link of the smallest SNR is the bottleneck. For overcoming this problem, it is proposed to give a priority to each link based on the link SNR and to assign the resource blocks (RBs) in ordering of instantaneous SNR on the link. This link priority information is shared among nodes through a control channel. Then, we assign the selected SU RB to each SU node according to the SNR ordering with distributed manner. We confirm the effectiveness of the proposed SU RB assignment by using computer simulation.