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Furthermore, there are additional advantages in using DRX, such as over-the-air resource saving on both the uplink and downlink to increase overall system capacity. One of the enhancements over 3G wireless systems is that in LTE DRX mode can be enabled even when the user equipment is registered with the evolved node-B. However, there is a need to optimize the DRX parameters, so as to maximize power saving without incurring network re-entry and packet delay. In particular, care should be exercised for real-time services.
In this article the power saving methods in both network attached and network idle modes as outlined in LTE are explained. The optimum criteria to select the DRX mode are defined for different applications. These high data rates over the access part of the network are achieved through the use of higher order modulation, such as 64quadrature amplitude modulation QAM , advanced coding techniques, convolutional turbo codes combined with advanced antenna techniques, such as multiple-input multiple-output MIMO , space-division multiple access SDMA , and so on.
Furthermore, DRX offers significant improvement with respect to resource utilization, particularly for applications characterized by extended OFF periods. Based on the application type, the DRX parameters are selected such that the energy and resource savings are maximized.
This may include network re-entry in some cases. Furthermore, UE has to perform scanning of the neighboring eNB in the event of detecting signal quality degradation with respect to the serving eNB [6, 7]. The rest of the article is organized as follows. A detailed description of the UE and network functionalities during different DRX modes is given in the next section. Examples of network re-entry times are presented. Finally, some concluding remarks with pointers to future evolution are presented.
In this state the UE can be paged for DL traffic. Restrictions apply. Network architecture. As shown in Fig. Only the discontinuous data exchange is on the air interface. The rest of the network is unaware of the DRX operation. More details on these modes are covered in subsequent sections. In this mode the UE is still registered with eNB i. When UE is not listening to the DL transmission, most of its circuitry is turned off.
DRX parameters in this mode are provided by the eNB during the radio bearer setup. Since various applications have varying delay sensitivity, RRC chooses DRX parameters based on the quality of service for each application.
This procedure is repeated cyclically. When multiple data bearers are established, DRX is enabled only when all the data bearers met their corresponding DRX inactivity timer condition. The shortest DRX cycles among all the data bearers are followed. UE resets the DRX mode and returns to the active mode as soon as a packet arrival is detected.
However, as shown in Fig. The delay depends on the length of the DRX cycle. In the UL the additional delay is a result of the bandwidth grant from the eNB. For each radio bearer, the DRX parameters are defined during the bearer setup procedure.
This is to reduce the UE wake up time in case of unexpected data arrival immediately after the DRX cycle is enabled. The provisioning of a short DRX cycle is mostly dependent on the characteristics of the application packet arrival.
The DRX parameters associated with each data bearer are as follows :? DRX inactivity timer T1 indicating the time in number of consecutive subframes without the scheduled traffic to wait before enabling DRX. This timer is reset to zero and enabled immediately after successful reception of PDCCH resource grant or allocation. Figure 2. These are reasonable values suggested by the authors. The defined DRX cycles shall be cyclic with respect to 10, subframes. The allowed TON values in number of subframes or milliseconds are 1, 2, 3, 4, 5, 6, 8, 10, 20, 30, 40, 50, 60, 80, , and Retransmission timer TR indicates the maximum number of subframes the UE should wait before turning off the circuits if a retransmission of data is expected from the eNB.
That is, when retransmissions are expected, TON is extended. In the ensuing sections a mathematical formulation is derived to give insight into the delay performance in the DL. Furthermore, we assume that the energy spent per frame is Esleep and Eawake, respectively, during the sleep and normal modes. The ratio of Eawake and Esleep is directly related to the number of circuits powered down during the DRX mode. The packet delay 95th percentile , as derived in the previous section, is plotted as a function of the percentage UE energy savings in Fig.
Here we assume that the packets are of fixed size, and the eNB allocates enough resources to transmit that packet within one subframe. This assumption is to make the analysis independent of the quality of service allocated to the user as well as the type of application. Throughout this article, 75 percent energy is assumed to be saved during the OFF time. The results show that the packet delay increases exponentially with the UE energy savings.
Various DRX cycles indicated on the plot show that the packet delay increases rapidly when the DRX cycle is greater than 80 subframes. This result is true for various ON duration timer settings. Percentage of energy saving vs. DRX and thus no power savings. VoIP is characterized by the periodic arrival of fixed length packets for the duration of talk spurt. One way of enabling the DRX is to exploit this characteristic. Assuming that the VoIP packets are arriving at 20 ms and the power saving neglecting the retransmissions, etc.
Video Streaming — Video streaming is characterized by fixed video frame rate e. The interpacket delay may vary based on the video coder delay. The received packets are buffered and passed on to decoder at the receiver end. Simulations are performed based on video streaming model proposed in . The energy savings are measured across multiple video streaming sessions and plotted against 95 percent packet delay.
For video traffic the guaranteed packet data rate affects the packet delay. If the data rate is too low, the time for DRX reduces as shown in Fig. Figure 4b shows the packet delay at 95 percent as a function of data rate for different DRX cycle settings. At higher data rates, the short DRX cycles does not affect the packet delay performance because the DRX opportunity is increased by sending the data too fast. The short DRX cycle can be used efficiently as a tool to shape the packet delay distribution.
For UL, the additional delay is as a result of the bandwidth grant from the eNB. The paging DRX cycle has to be optimized to reduce this delay, T4. Energy savings as a function of packet data rate. Furthermore, eNB removes the UEs context from the database.
During the idle mode, the mobility is fully controlled by UE, since the network is not aware of the UE existence continuously. UE should perform the signal quality measurements with respect to the serving and neighboring eNBs according to measurement thresholds recommended by the serving eNB.
When the system information advertised by the new serving eNB does not include its tracking area, UE will perform a tracking area update to indicate its presence so that the network knows where to page the UE in case of DL data transfer. UE may be paged by the network when there is data addressed to that particular UE.
In Mode-0 the paging message is not scheduled on all the radio frames. Mode-1 allows configuration of the paging message on any radio frame. Furthermore, in Mode-1 the paging message can be distributed across the subframes within the radio frame.
Furthermore, we explored the possibility of page messages repeated over multiple subframes within the radio frame to increase the probability of reception at the UE. If successfully received, eNB responds by sending a random access response granting enough bandwidth to the UE to send the RRC connection request.
Over-the-air encryption is enabled by sending the security mode command by the eNB. Call flow for idle mode exit for DL data transfer. The detailed call flow is shown in Fig. The MME retransmits the page request a preconfigured number of times if a response is not received from the UE. The timer for retransmission should be carefully configured by measuring the expected delay between the transmission of a Page request message from the MME and the reception of an NAS request from the UE.
T rtx is the retransmission timer for the paging message from the MME. Ppage is the probability of UE being paged unsuccessfully. Idle mode exit time for different paging cycles.
These results are generated assuming that over-the-air paging messages are successfully received by the UE with a probability of 0. The maximum page retransmissions are limited to 4. The RACH preamble detection error rate is assumed be 3 percent, and the maximum number of preamble retransmissions is set to 5.
RACH transmission backoff time is assumed to be 6 ms. The advantage of sending the page message multiple times over the air is also shown. Here the assumption is that the page messages are independently decoded. Multiple pages per radio frame improve the reentry time significantly.
In particular, for applications characterized by extended OFF periods, the power savings and resource utilization are maximized.
LTE-A Network through DRX Configuration and Development of power saving device