- Open Access
Energy-saving centric uplink scheduling scheme for broadband wireless access networks
© Chen et al.; licensee Springer. 2014
Received: 16 June 2013
Accepted: 2 April 2014
Published: 1 May 2014
This study proposes an energy-saving centric uplink scheduling (ESC-US) scheme to support efficient energy usage and satisfy the quality of service (QoS) requirements of Worldwide Interoperability for Microwave Access (WiMax) networks. The uplink resource allocation is different from that of the downlink resource allocation scheme because the uplink traffic is queued at the mobile station (MS) and the base station (BS) has no information regarding it without using a polling procedure. The considered resource scheduling schemes maximize the sleep efficiency and consider the QoS requirements of individual MSs. The proposed scheduling scheme in this study considers the delay budget of MSs with real-time connections and the required minimum reserved traffic rate (MRTR) of MSs with non-real time connections when maximizing sleep efficiency. Both scheduling schemes for the traffic of real-time polling services (rtPS) and non-real-time polling services (nrtPS) apply the ‘just enough QoS’ and ‘sleep before transmission’ (SbT) concepts to achieve this energy-saving centric objective. Exhaustive simulations were conducted to examine the performance of the proposed schemes. The simulation results show that both schemes guarantee the desired QoS and achieve superior energy-savings efficiencies compared to the conventional scheme.
The rapid progress of broadband wireless access technologies, such as Worldwide Interoperability for Microwave Access (WiMax)  and long-term evolution (LTE) , and power mobile devices stimulates the flourishing deployment of mobile internet services. However, the slow improvement in battery technologies  has led to an exponentially increasing gap between available and required battery capacities . Therefore, conserving energy is a crucial factor for mobile devices in practical applications. Because wireless internet is a shared medium, device energy efficiency is affected not only by the layers that compose the point-to-point communication link but also the interaction between the links in the entire network . Therefore, the efficient conservation of energy to achieve longer mobile station (MS) operation times is vital to the success of mobile internet services. Furthermore, most wireless access networks provide the quality of service (QoS) mechanism to ensure that users receive the desired service qualities of various services, such as data, voice, video, and multimedia, through appropriate radio resource allocation and scheduling at the base station (BS). WiMax defines three types of power-saving classes (PSC) for various types of traffic for the purposes of energy saving. Type I of PSC is recommended for the connection of best effort (BE) and non-real-time variable rate (NRT-VR) services. Type II of PSC is recommended for the connection of unsolicited grant service (UGS) and real-time variable rate (RT-VR) services. Type III of PSC is recommended for multicast connections and management operations. PSC I and PSC II operate alternatively between an awake and sleep mode. The sleep mode includes two operational windows: sleep window and listen window. PSC III does not contain a listen window, and its sleep cycle consists of only one predefined sleeping interval. The MS can issue a sleep request (MOB_SLP-REQ) message to its serving BS for the approval of entering sleep mode. The BS transmits the sleep response (MOB_SLP-RSP) message to indicate whether the sleep request is accepted or rejected. Sleep parameters are provided if the request is accepted. The sleep parameters include the starting sleep time, initial sleep window size, final sleep window size, and the listen window size, which are adopted by an MS in its sleep mode. PSC I doubles its sleep window size if the BS does not inform it of the presence of data packets at its listen interval, but it is constrained by the final sleep window size. The sleep window of PSC II is consistently the same size because of the real-time requirement. The sleep management procedure defined in WiMax is generally an operational procedure that does not consider the energy-saving efficiency in real applications, especially for uplink traffic. The network side generally does not know whether mobile devices require the uplink radio resource; thus, the BS schedules the uplink radio resource based on the bandwidth request issued by mobile devices. In WiMax, an MS awakes and issues a bandwidth request (BR) message when it has traffic to send. It remains awake and waits for the uplink radio resource allocated by the BS. This situation may cause the MS to halt the sleeping mode and switch to the awake mode even when no sufficient radio resources exist for its uplink transmission. The BS is responsible for radio resource allocation and the MS may wake without transmitting any information if the BS does not allocate resources for it because of insufficient radio resources. The MS wastes its limited energy in this scenario. From an energy-saving viewpoint, an inefficient uplink scheduling scheme may cause the MS to halt the sleeping mode and switch to the awake mode even when insufficient radio resources exist for its uplink transmission. Conversely, an efficient sleep mechanism provides a longer sleep interval to reduce energy consumption and maintains QoS requirements. This study proposes the energy-saving centric uplink scheduling (ESC-US) scheme, which properly manages sleep periods to conserve the energy of MSs under the QoS constraint. In this approach, the BS schedules the traffic for a better sleeping arrangement when receiving the BR from the MS to achieve better sleep efficiency and reduce switching between the sleep and awake modes. Both real-time and non-real-time connections were considered in the proposed scheduling schemes to ensure the desired QoS of each user. The proposed ESC-US scheme applies the ‘sleep before transmission’ (SbT) concept using hybrid sleep arrangement policies for connections with different services to achieve better radio resource usage in a heuristic manner.
The remainder of this study is arranged as follows: Section 2 presents reviews of related studies for this topic, Section 3 presents the proposed system model, Section 4 shows this study's approach and details the proposed ESC-US scheme, Section 5 provides the simulation results of the proposed schemes with discussions, and Section 6 offers a summary of the conclusions of this study.
2 Related works
Several recent studies have investigated the issue of energy consumption with various sleep mode operations. Xiao  proposed an analytical model for the PSC I to evaluate the effects of sleep parameters and traffic load by solely considering the downlink transmission. Moreover, both the downlink and uplink transmissions were considered and proposed analytical model to evaluate the energy management in the IEEE 802.16e . In , the authors examined the performance in terms of the dropping probability and mean waiting time of packets in the queue of the BS by using an M/GI/1/N queuing model. In , the authors presented a semi-Markov chain queuing model to derive the performance and discuss the selection of proper sleep parameters. The performance of PSC II for VoIP traffic was examined in  for the given network delay model by using a simulation method that additionally provided a guideline to determine the sleep interval and discover the most energy-efficient sleep interval length which satisfied the given delay constraints from the results. The authors of  proposed a maximum unavailability interval (MUI) scheme for the given sleep parameters of PSC II to determine the start frame number for each of the connections with various classes by applying the Chinese reminder theorem. Conversely, in , the authors evaluated and compared the performance of PSC I and II. It proposed a sleep mode switching scheme based on analytical results to achieve optimal energy performance according to various traffic conditions.
Conversely, a number of studies are currently considering not only energy savings but also the QoS issues as well [13–17]. In , the authors proposed an energy-saving centric scheme for downlink scheduling. The authors presented a longest virtual burst first (LVBF) scheduling algorithm to improve the energy efficiency of MSs and meet the requirement of minimal data rates for the connection with PSC I . A minimum wakeup time (MWT) scheduling scheme for PSC II was proposed in  to determine the maximal sleeping time for a single MS with constant bit rate (CBR) traffic and analyzed the upper bound for multiple MS power-saving scheduling with a MWT. Tsao and Chen  proposed two power-saving scheduling algorithms. The first algorithm is a periodic on-off scheme based on the idea of allowing an MS to sleep for a fixed interval and, subsequently, to listen for another fixed interval in a round-robin manner to determine feasible solutions for minimal energy consumption under the delay constraint. The aperiodic on-off scheme was proposed to solve the energy-wasting problem of no traffic transmission during a fixed listen period, which adjusts the length of sleep and listen periods according to the traffic status. The energy-efficient architecture for two-level scheduling was proposed in . The proposed architecture sets a priority order for the first-level scheduling by assuming that a mobile and fixed station coexist in the network and that mobile stations have a higher priority than fixed stations from an energy-saving viewpoint. Their study proposed a two-timer mechanism (i.e., empty timer and overloading timer) to dynamically adjust the state transition timing to achieve optimal energy efficiency. Most previous studies considered either sleeping period management for one type of connection or sleep synchronization of multiple connections, but resource scheduling for uplink traffic from an energy saving viewpoint was seldom considered. However, a BS must serve various types of connections for multiple MSs in a practical environment. Hence, the purpose of this study is to schedule sleeping and resources in a tradeoff manner so that an MS can conserve as much energy as possible and sufficiently provision its desired QoS.
3 The proposed ESC-US system model
The definitions of important parameters of the proposed scheme
The predicted number of frames that the traffic generated prior to the BR issue
The m th bandwidth request issued at the n th frame for a specific MS i
Number of frames that BS will request the MS to sleep for the BR m,n issued by rtPS MS
The bandwidth that needs to be allocated in the newly generated frames
The average minimum traffic rate of each frame
The MRTR of the MS i, M i indicates that the BS can calculate the average bandwidth that must be allocated in each frame F i
BS can estimate the farthest frame of this allocation based on the desired bandwidth specified in BR m,n
The frame duration
BS can arrange the delay of the nrtPS traffic because of the sleep period
Number of frames that BS will request the MS to sleep for the BR m,n issued by nrtPS MS
Number of bits of preallocated nrtPS bandwidth from the x th frame
Number of bits that BS can allocate for MS in each frame
4 The proposed ESC-US scheme
The proposed scheme shall decide the farthest frame that can be allocated for the issued BR without violating the desired QoS. The real-time polling service (rtPS) and non-real-time polling service (nrtPS) have different QoS requirements. The delay tolerance is critical for rtPS connections, but the MRTR is a major parameter for nrtPS connections when fair resource usage is concerned. Therefore, the acceptable farthest frames of the rtPS connection and nrtPS connection are decided in a different manner.
where |BRm,n| and |BRm-1,x| denote the required bandwidth specified in the BR m,n and BRm-1,x messages, respectively. The function of BWD(x,y) allocates the bandwidth from the farthest frame x backward to the nearest frame for the required bandwidth y and returns the number of the nearest frame after allocation. It can be calculated by decreasing the bandwidth that can be allocated in prescheduled frames from the newly generated bandwidth, which is the difference between the bandwidths specified in the BRm- 1,x and BR m,n messages. The function Av i (a, b) in (3) calculates the available bandwidth from the a th frame to the b th frame of MS i. The newly generated required bandwidth may be fully allocated in the prescheduled frames. Thereafter, B m is zero. In this scenario, BWD(n + D i - α,0) equals n + D i - α, and it makes the sleep period krt m equal to (n + D i - α) - 2 - (n + 1 + f) = D i - α - 3 - f in (2), as shown in Figure 4.
For the nrtPS traffic, there is no delay constraint. Thus, the BS is not required to predict when the uplink was generated. The larger value of β is the shorter sleep period and the transmission delay the MS acquires. A negative β value indicates that the MS can acquire a much longer sleep period, but the MRTR requirement may not be satisfied if the MS has data to transmit.
5 Performance simulation
System simulation parameters
BW per frame
The real-time VoIP traffic model parameters
Voice over IP (VoIP)
AMR (12.2 kbps)
Talk spurt length
Exponential, mean = 1,026 ms
Exponential, mean = 1,171 ms
RTP/UDP/IP header compression
where N p is number of sent packets, P s and N f denote the packet size and the number of used frames, respectively.
The average sleep efficiency is defined as the average percentage of the number of sleep frames to the number of total frames per connection during the simulations.
5.1 Fixed number of nrtPS MS
5.2 Effect of rtPS on/off ratio
5.3 Sleep threshold effect for MS
The sleep efficiency of rtPS Class I with respect to the sleep threshold
The sleep efficiency of rtPS Class II with respect to the sleep threshold
The uplink scheduling scheme is proposed from an energy-saving viewpoint in this study. Contrary to the traditional concept, the novel SbT concept is introduced in the proposed ESC-US scheme. The proposed scheme analyzes the sleep efficiency with the QoS constraints to determine whether the MS is suitable to enter sleep mode and calculates the proper sleep period in a systematic manner if the MS can enter sleep mode. Both rtPS and nrtPS connections are considered in the proposed scheme. The main achievement of this study was improving the energy-saving efficiency under the desired QoS constraint. The proposed scheme arranged the radio resources to satisfy its QoS in a just enough manner to maximize the sleep efficiency. Several performance indices, such as the average delay, sleep efficiency, sufficient rate, and MRTR, were examined through exhaustive simulations. The performance of the proposed scheme is compared to the traditional uplink scheduling scheme. The simulation results demonstrate that the proposed SbT concept is meaningful and the energy-saving performance is superior to the traditional scheme. The simulation results clearly show the tradeoff between energy savings and performance. The proposed SbT with the just enough QoS concept provides an alternative method for considering the tradeoff issue. Additionally, the approach can be adopted for LTE network by extending the proposed concept to be applied for the relative power-saving parameters such as the decision of DRX cycles. Although the ESC-based uplink scheduling algorithm is proposed in this study and downlink scheduling is proposed in , the integration of the downlink and uplink energy-saving centric scheduling is more complex. Further study for the harmonic operation between these two independent algorithms is required to maximize the overall system performance.
This research work was supported in part by the grants from the National Science Council (NSC) (grant numbers: NSC 98-2221-E-008-063, NSC 99-2218-E-159-001, and NSC 100-2221-E-008-097).
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