Base Station Schedules Research Articles

Adaptive downlink and uplink channel split ratio determination for TCP-based best effort traffic in TDD-based WiMAX networks

In this paper, we study the determination of downlink (DL) and uplink (UL) channel split ratio for Time Division Duplex (TDD)-based IEEE 802.16 (WiMAX) wireless networks. In a TDD system, uplink and downlink transmissions share the same frequency at different time intervals. The TDD framing in WiMAX is adaptive in the sense that the downlink to uplink bandwidth ratio may vary with time. In this work, we focus on TCP based traffic and explore the impact of improper bandwidth allocation to DL and UL channels on the performance of TCP. We then propose an adaptive split ratio (ASR) scheme which adjusts the bandwidth ratio of DL to UL adaptively according to the current traffic profile, wireless interference, and transport layer parameters, so as to maximize the aggregate throughput of TCP based traffic. Our scheme can also cooperate with the base station (BS) scheduler to throttle the TCP source when acknowledgements (ACKs) are transmitted infrequently. The performance of the proposed ASR scheme is validated via ns <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> simulations. The results show that our scheme outperforms static allocation (such as the default value specified in the WiMAX standard and other possible settings in existing access networks) in terms of higher aggregate throughput and better adaptivity to network dynamics.

On the performance anomaly in WiMAX networks

AbstractThe WiMAX system carries a wide range of services in urban and rural environments supporting quality of service. A key element of the QoS framework is the scheduling algorithm adopted by the Base Station (BS). In this paper, we analyze the saturation throughput perceived by Mobile Stations in the cases of two BS scheduling algorithms: Deficit Round Robin (DRR) and time‐based DRR. We demonstrate that the WiFi issue of ‘Performance Anomaly’ also occurs in WiMAX networks: when the BS uses scheduling approaches aimed at achieving throughput‐fairness, like DRR. Performance Anomaly means that when some Mobile Stations (MSs) use a very low bit rate, the throughput of MSs with a high bit rate is dramatically degraded. We propose time‐based DRR as a viable solution to remove the Performance Anomaly. Time‐based DRR is a simple modification of the DRR algorithm that achieves time‐fairness. Its implementation is feasible in WiMAX. The analysis is carried out by means of analytical models supported by NS2 simulations. Two scenarios are considered: the first is suitably set up to highlight and understand the phenomenon of Performance Anomaly; the second examines the impact of Performance Anomaly on a system level focusing on a rural environment. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Performance evaluation of the MAC protocol in IEEE 802.16 systems with data and VoiP traffic scheduling

AbstractIn the last few years, the metropolitan area networks (MAN) have increased their popularity and attracted the interest of the most important research groups all over the world. Among several standards, IEEE 802.16 has taken a relevant role providing high data rate in a big covering range with low implementation costs and multi‐traffic communications. The IEEE 802.16 networks can have a pre‐defined structure, with a central base station (BS) covering a cell in which a variable number of subscriber stations (SSs) can work. This paper deals with the proposal of a quality of service (QoS) driven scheduling algorithm to be used in an IEEE 802.16 network where different traffic types coexist. In particular, the paper mainly focuses on best effort data and VoIP communications, by proposing a scheduling technique that allows an efficient resource management of both traffic types by considering their specific QoS flavor. The performance evaluation has been carried out by considering both the phases of contention and packet scheduling, by means of a theoretical approach and computer simulations. Numerical results show the performance of the proposed algorithm by focusing on a scenario where the BS schedules the best effort and VoIP traffics of several SSs. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Low complexity MIMO scheduling with channel decomposition using capacity upperbound

In multiuser MIMO systems, the base station schedules transmissions to a group of users simultaneously. Since the data transmitted to each user are different, in order to avoid the inter-user interference, a transmit preprocessing technique which decomposes the multiuser MIMO downlink channel into multiple parallel independent single-user MIMO channels can be used. When the number of users is larger than the maximum that the system can support simultaneously, the base station selects a subset of users who have the best instantaneous channel quality to maximize the system throughput. Since the exhaustive search for the optimal user set is computationally prohibitive, a low complexity scheduling algorithm which aims to maximize the capacity upper bound is proposed. Simulation results show that the proposed scheduling algorithm achieves comparable total throughput as the optimal algorithm with much lower complexity.

The Design and Evaluation of Unified Cellular and Ad-Hoc Networks

In third-generation (3G) wireless data networks, providing service to low data-rate users is required for maintaining fairness, but at the cost of reducing the cell's aggregate throughput. In this paper, we propose the unified cellular and ad hoc network (UCAN) architecture for enhancing cell throughput while maintaining fairness. In UCAN, a mobile client has both 3G interface and IEEE 802.11 -based peer-to-peer links. The 3G base station forwards packets for destination clients with poor channel quality to proxy clients with better channel quality. The proxy clients then use an ad hoc network composed of other mobile clients and IEEE 802.11 wireless links to forward the packets to the appropriate destinations, thereby improving cell throughput. We refine the 3G base station scheduling algorithm so that the throughput gains are distributed in proportion to users' average channel rates, thereby maintaining fairness. With the UCAN architecture in place, we propose novel greedy and on-demand protocols for proxy discovery and ad hoc routing that explicitly leverage the existence of the 3G infrastructure to reduce complexity and improve reliability. We further propose secure crediting mechanisms to motivate users that are not actively receiving to participate in relaying packets for others. Through both analysis and extensive simulations with HDR and IEEE 802.11b, we show that the UCAN architecture can increase individual user's throughput by more than 100 percent and the aggregate throughput of the HDR downlink by up to 50 percent.

Joint packet scheduling and dynamic base station assignment for CDMA data networks

AbstractIn current code division multiple access (CDMA) based wireless systems, a base station (BS) schedules packets independently of its neighbours, which may lead to resource wastage and the degradation of the system's performance. In wireless networks, in order to achieve an efficient packet scheduling, there are two conflicting performance metrics that have to be optimized: throughput and fairness. Their maximization is a key goal, particularly in next‐generation wireless networks. This paper proposes joint packet scheduling and BS assignment schemes for a cluster of interdependent neighbouring BSs in CDMA‐based wireless networks, in order to enhance the system performance through dynamic load balancing. The proposed schemes are based on sector subdivision in terms of average required resource per mobile station and utility function approach. The fairness is achieved by minimizing the variance of the delay for the remaining head‐of‐queue packets. Inter‐cell and intra‐cell interferences from scheduled packets are also minimized in order to increase the system capacity and performance. The simulation results show that our proposed schemes perform better than existing schemes available in the open literature. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Experiments on HSDPA Throughput Performance in W-CDMA Systems

In this paper, we present laboratory and field experimental results using High Speed Downlink Packet Access (HSDPA) test-beds in order to reveal the actual HSDPA performance based on key technologies such as base station (BS) scheduling, adaptive modulation and coding, hybrid automatic repeat request, and advanced receiver design. First, this paper evaluates the effects of advanced user equipment capabilities such as the maximum number of multi-codes, transmit diversity, receive diversity, and a chip equalizer. Increases in throughput of 60% and 85% due to using 10 and 15 codes were observed compared to 5 codes, respectively. The gain of 22% was obtained by applying closed-loop transmit diversity to the HSDPA network. Receive diversity improves the throughput in the region from low to high signal-to-interference ratio, and the gain of 45% was obtained by applying receive diversity to the conventional RAKE receiver. A throughput gain of approximately 17% due to the use of the chip equalizer was obtained and it was observed mainly in the high Ior/Ioc region and under multi-path conditions. Second, field experiments are conducted to elucidate the effects of multi-user diversity using a BS scheduling algorithm, and reveal that proportional fairness scheduling provides both the increase in sector throughput of 18% and a sufficient degree of fairness among users. The transmit control protocol (TCP)-level throughput performance is also investigated in order to reveal the actual end-user throughput. The results show that the throughput rate of approximately 90% of the throughput of the MAC-hs layer is achieved in the TCP layer in the laboratory experiments and in the field experiments.

On asymptotically fair transmission scheduling over fading channels with measurement delay

We examine the effect of measurement delays on the throughput of certain downlink data packet systems operating over Rayleigh fading channels. In these systems the base station (BS) schedules a single user at a time and transmits at a rate which is based on measurements of the BS's received power performed by the user. The same subject was also discussed in a recent paper, titled Asymptotically fair transmission scheduling over fading by Berggren and Jantti (2004). They studied the effects of measurement delays on the operation of the scheduler. As a consequence of the delay, it is possible that by the time the BS actually transmits data, the propagation channels might have changed, such that the scheduler's choice no longer conforms to the desired scheduling policy, resulting in loss of system throughput and multi-user gain. Berggren and Jantti assume that perfect link adaptation is nevertheless always assured, i.e., that the BS transmits at a rate that matches precisely the current state of the selected user's channel. However, measurement delays lead also to mismatch between the data rate the BS transmits to the chosen user, and the current state of the propagation channel to that user. We believe that this mismatch must also be taken into account. In this letter we propose to use a backoff factor and analyze its mitigating effect on losses introduced by measurements delay in a Rayleigh fading environment. We then show the resulting gain in system throughput for optimal choices of the backoff factor

Joint Base Station Scheduling

Consider a scenario where radio base stations need to send data to users with wireless devices. Time is discrete and slotted into synchronous rounds. Transmitting a data item from a base station to a user takes one round. A user can receive the data item from any of the base stations. The positions of the base stations and users are modeled as points in Euclidean space. If base station b transmits to user u in a certain round, no other user within distance at most kb􀀀uk2 from b can receive data in the same round due to interference phenomena. The goal is to minimize, given the positions of the base stations and users, the number of rounds until all users have received their data.We call this problem the Joint Base Station Scheduling Problem (JBS) and consider it on the line (1D-JBS) and in the plane (2D-JBS). For 1D-JBS, we give an efcient 2-approximation algorithm and polynomial time optimal algorithms for special cases. We model transmissions from base stations to users as arrows (intervals with a distinguished endpoint) and show that their conict graphs, which we call arrow graphs, are a subclass of the class of perfect graphs. For 2D-JBS, we prove NP-hardness and show that some natural greedy heuristics do not achieve approximation ratio better than O(log n), where n is the number of users.

무선 ATM망에서 rt-VBR 서비스를 위한 동적 슬롯 할당 기법

본 논문에서는 무선 ATM망에서 rt-VBR 서비스를 위한 동적 슬롯 할당 기법을 제안한다. 제안된 방식에서는 슬롯 할당을 요구하기 위한 예약 요청 패킷을 경쟁 기반으로 전송하고, 예약 요청이 성공하면 단말기에서는 비 경쟁 방식으로 동적 매개변수를 전달한다. 기지국 스케줄러는 예약이 성공한 단말기들이 잔여수명과 요청하는 슬롯 수와 같은 동적 매개변수를 전송하기 위한 DPS 미니슬롯을 할당한다. 한편 기지국 스케줄러는 단말기들이 전송한 잔여수명을 기반으로 잔여수명이 가장 짧은 단말기에게 상향 데이터 슬롯을 우선적으로 할당한다. 본 논문에서는 제안한 방식의 성능을 시뮬레이션을 통하여 분석하였다. 시뮬레이션의 결과, 제안한 동적 슬롯 할당 기법은 셀 손실률을 낮게 유지하면서 rt_VBR 서비스가 요구하는 전송 지연을 보장하고 있음을 알 수 있다. This paper proposes the dynamic slot allocation method for real-time VBR (rt-VBR) services in wireless ATM networks. The proposed method is characterized by a contention-based mechanism of the reservation request, a contention-free polling scheme for transferring the dynamic parameters. The base station scheduler allocates a dynamic parameter minislot to the wireless terminal for transferring the residual lifetime and the number of requesting slots as the dynamic parameters. The scheduling algorithm uses a priority scheme based on the maximum cell transfer delay parameter. Based on the received dynamic parameters, the scheduler allocates the uplink slots to the wireless terminal with the most stringent delay requirement. The simulation results show that the proposed method guarantee the delay constraint of rt-VBR services along with its cell loss rate significantly reduced.

An uplink CDMA system architecture with diverse QoS guarantees for heterogeneous traffic

The uplink access control problems for cellular code-division multiple-access (CDMA) systems that service heterogeneous traffic with various types of quality-of-service (QoS) and use multicode CDMA to support variable bit rates are addressed. Considering its distinct QoS requirements, class-I real-time traffic (e.g., voice and video) is differentiated from class-II non-real-time traffic (e.g., data). Connection-oriented transmission is achieved by assigning mobile-oriented code channels for class-I traffic, where each corresponding mobile needs to pass an admission test. Class-II traffic is transmitted in a best-effort manner through a transmission-rate request access scheme which utilizes the bandwidth left unused by class-I traffic. Whenever a mobile has class-II messages to transmit, the mobile requests code channels via a base station-oriented transmission-request code channel, then, according to the base station scheduling, the transmission is scheduled and permitted. Addressed are the admission test for class-I connections, transmission power allocation, and how to maximize the aggregate throughput for class-II traffic. The admission region of voice and video connections and the optimum target signal-to-interference ratio of class-II traffic are derived numerically. The performance of class-II traffic transmissions in terms of average delay is also evaluated and discussed.