Nowadays, a variety of powerful smart devices are designed to handle a wide range of traffic, such as VoIP, video streaming, mobile gaming, and so on. In order to satisfy the growing traffic demand for wireless communications, the Third Generation Partner Project (3GPP) Long Term Evolution (LTE) has been developed to support higher transmission rate by adopting some novel technologies, such as Multi-input Multi-output (MIMO) and Orthogonal Frequency-Division Multiple Access (OFDMA). However, the high complexity of these new technologies may introduce large power consumption.
Discontinuous reception (DRX) vs. QoS
While the LTE transmission rate is many times faster than the 3G rate, the battery, the power source of mobile devices, has not any sizeable advancement. Thus, power saving is still one of the important issues for mobile devices. To save the energy of mobile devices, 3GPP LTE has defined the discontinuous reception (DRX) scheme to allow devices to turn off their radio interface and go for a sleep state for a length of time, while staying connected to the network, thereby reducing the power consumption when there is no data transmission. Nevertheless, if packets arrive at sleep state, these packets would pose unexpected delay, which may affect their Quality of Service (QoS) requirements. Thus, how to determine DRX parameters to minimize the power consumption and guarantee QoS simultaneously is still an open issue.
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When Channel Condition is Considered
Further, channel condition should be considered when the DRX parameters are determined. The adaptive modulation and coding (AMC) in LTE offers a link adaptation method that can dynamically choose the modulation and coding scheme (MCS) according to current channel condition for each user, known as UE (user equipment). The UE uses channel quality indicator (CQI) [1] to report channel condition for base station, also called evolved Node B (eNB), to decide the MCS level.
A higher MCS level (i.e., with 64 Quadrature Amplitude Modulation (64QAM) modulation) has higher transmission rate but is more prone to errors due to interference and noise. A lower MCS level (i.e., QPSK modulation) has lower rate but can tolerate a higher level of interference. When channel condition is good, AMC assign a higher MCS, which means that the UE can transmit data with a higher transmission rate during a short period. Thus, the UE can turn off its radio interface for a long period for power saving. In contrast, when channel condition is bad, AMC assign a lower MCS, which has a lower transmission rate. Thus, the UE needs to extend the turn-on duration in order to meet the QoS requirements. Therefore, CQI information should be considered when determining optimal DRX parameters.
Related Work
Previous studies have investigated how to dynamically adjust the DRX parameters based on channel condition [5, 6]. A multi-threshold adaptive DRX (M-ADRX) mechanism [5] is proposed, where UEs are divided into several states according to their channel condition. UEs with better channel condition will be configured with lower power consumption parameters, and vice versa. In [6], DRX parameters are adjusted depending on the system load and the channel variation to
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improve power saving efficiency. For UEs with slow-varying channel, power saving parameter is used. In order to obtain an accurate channel condition, i.e., up-to-date channel condition information, for packet scheduling and DRX parameter setting, an update period of channel condition before the actual turn-on time is proposed by [7].
However, this work does not consider the QoS features, such as packet delay, packet loss rate, and required data rates. A DRX-aware scheduling scheme [8] is proposed to reduce the packet loss rate due to DRX sleep, but it increases power consumption. In [11], they study the use of DRX for VoIP traffic under different scheduling strategies.
Instead of proposing new adaptive DRX scheme, they only use several specific parameter settings to observe the power savings and QoS impact. In [15], the authors determine DRX parameters by considering the QoS requirements for Internet of Thing (IoT) applications, but it does not dynamically adjust the DRX parameters based on channel condition. In addition, they propose DRX aware scheduling algorithm, but it increases power consumption too.
Dynamic Scheduling with Extensible Allocation and Dispersed Offsets (DXD)
In this study, a novel DRX scheme, called Dynamic Scheduling with Extensible Allocation and Dispersed Offsets (DXD) scheme, is proposed to meet QoS requirements, minimize power consumption, and increase system capacity in the DRX mode. In order to maximize of power savings, the UE should turn off the radio interface as long as possible under the QoS constraints. The proposed scheme dynamically determines DRX parameters depending on QoS. On the other hand, when the UE turns on the radio interface, the buffered packets should be forwarded as soon as possible. Thus, we disperse turn-on time of UEs so that each UE can fully utilize the bandwidth for resource efficiency and fast transmission. In addition, the
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proposed method includes a DRX-aware scheduler. It can determine whether or not to extend the turn-on time for QoS satisfaction. If system loading is high, extension may not improve QoS but only increases power consumption.
The rest of this thesis is organized as follows. In Chapter 2, we firstly describe the operations of DRX in LTE systems, and review relevant literature to justify the issues. The problem statement and its notations are defined in Chapter 3. In Chapter 4, we describe the proposed DXD algorithm and illustrate the detailed operations with an example. In Chapter 5, the simulation results are presented. Finally, we conclude this work in Chapter 6.
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