Growth And Integration Of Various Wireless Technologies

Published Date: 02 Nov 2017

Abstract

Power consumption has been the prime issue with the growth and integration of various wireless technologies in a User Equipment (UE). In the latest Fourth Generation Long Term Evolution (LTE), a huge increase in data throughput and decrease in latency during data transmission is observed. All these come with a reduction in the UE’s size and complex receiver design for the UE [1]. In such a scenario, efficient utilization of power resources by the UE becomes a necessity to maintain constant communication with the Base Station (BS). LTE utilizes a Single Carrier-Frequency Division Multiple Access (SC-FDMA) scheme in the uplink and Orthogonal Frequency Division multiplexing (OFDM) in the downlink. One solution to reduce power consumption is to make the UE sleep during the inactive time periods i.e. the periods during which there is no active data transmission/reception at the UE. Thus, LTE has shown a significant reduction in battery consumption on the UE side by implementing the enhanced Discontinuous Reception (DRX) power saving mechanism. By doing this the UE circuitry is shut down when not in use. Thus, the resources are freed up when not in use and thus the uplink and downlink capacity can be significantly increased [1]. The following paper discusses the DRX concept, its implementation in LTE for RRC_Connected and RRC_Idle States and its Challenges.

I. Introduction

With the enhancements in the data rates and integration of various technologies like Bluetooth, Wireless Local Area Network (WLAN), Global Positioning System (GPS) and Cellular data all in one device, the UE is not just a calling device today. It uses different applications like texting, online gaming, web browsing and cellular data, which consume a lot of battery from the mobile handset. The notion of "Always stay connected" has resulted in tremendous utilization of the UE constantly throughout the day; thus leading to less standby time of the mobile [7]. Charging the mobile every 5 to 6 hours is not only time consuming but agitating for the end user who expects high Quality of Service (QoS) constantly by the technology the mobile supports.

The Third Generation Partnership Project (3GPP) has evolved with different generations of wireless technologies starting from 1st generation (1G) Analog Mobile Phone Service (AMPS) devices to the current Fourth Generation (4G) Long Term Evolution (LTE). These technologies evolved for providing higher data rates, less latency, higher QoS and proper utilization of the available resources than its previous generations. Thus, techniques majorly focused on adoption of higher modulation schemes, different coding rates, different multiple access techniques, multi-antenna concepts and many more for efficient bandwidth (B.W) utilization. But while doing all this, there was lot of concern regarding the amount of power consumption the device would undergo given its small form factor. Thus, there became a need for the devices to implement efficient and power saving techniques that could help the evolved node-B (eNB) provide QoS to its serving customers at all times.

Different power saving techniques like Power Saving Multi Poll (PSMP) or Spatial Multiplexing Power Save (SMPS) in Wireless Local Area Network (WLAN) and idle/sleep modes in Wireless Interoperability for Microwave Access (WiMAX) have been implemented [1]. Similarly, enhanced Discontinuous Reception (DRX) power saving mechanism has been implemented in LTE for efficient battery utilization [2]. It relies on the fact that the UE should be put in sleep mode when no data is being transmitted by it or scheduled for it. DRX in LTE can occur when the UE is connected to the Radio Resource Control (RRC) and doing data transmission/reception (RRC_Connected state) or when there is no data transfer between UE and eNodeB (RRC_Idle state) [1].

II. Need for DRX in LTE

According to the specifications, LTE aims at achieving a data rate of 300Mbps in the downlink and 75Mbps in the uplink for a 20MHz Channel Bandwidth [15]. The standard also requires the eNB in LTE to support up to 200 active users at a time for a 5 MHz channel [16]. LTE uses OFDM in the downlink, which provides spectrum efficiency and higher throughput values. OFDM utilizes the concept of multicarrier transmission where several orthogonal narrowband overlapping frequency subcarriers are placed in tight frequency spacing and each of the subcarrier represents a frequency flat channel. Hence, it gives protection against deep fades that occur for non Line of Sight (non-LOS) users in practical scenarios. However, the disadvantage of OFDM is high PAPR as compared to SCS. Due to this high PAPR, it is advisable to use OFDM in linear systems only where large back off is not required for the power amplifiers. LTE also implements a Multiple Input Multiple Output (MIMO) antenna scheme that uses multiple antennas at the transmitter and receiver to increase the data throughput [1][2]. For the uplink, SC-FDMA is used which is less power consuming than OFDM but higher than SCS. Hence, in order to increase the efficiency of the data transmission and reception, power saving mechanism like DRX is used by the UE.

III. DRX parameters

To understand DRX mechanism in LTE, listed below are few of its basic parameters. The following parameters are summarized from [1] – [4], [7] and [13]. All these parameters are set and updated every 10ms i.e. for each and every LTE sub frame. These are set by the RRC on a per user basis depending on the UE’s QoS requirements, Bandwidth allocation and resource availability at the eNB.

DRX Cycle: The period between the active state and the end of DRX mode i.e. sleep state is termed as a DRX Cycle.

DRX inactivity timer: The UE listens to the Physical Downlink Control Channel (PDCCH) or the Physical Downlink Shared Channel (PDSCH) for the reception of control or data packets respectively sent to it by the eNB. Hence, after successful reception of the control or data packet, the UE starts the DRX inactivity timer. This timer indicates the extra amount of time for which the UE needs to stay in the active state assuming more data arrival before entering the DRX mode. Once the UE receives data, it resets the value of this timer and listens to the PDSCH or PDCCH and continues to remain active, else it follows the Short DRX Cycle.

Short DRX Cycle: The number of sub frames for which the UE initially goes into the DRX mode is denoted by the Short DRX Cycle. These are short sleep cycles in which the UE wakes up after every 2^n (where n is an integer and 1ms to 9ms) or 5x (2^n) (where n is an integer and n=1 to 6 ms) shorter intervals to check if there is any data available from the eNB side (similar to beacon interval frame in WLAN) [1].

DRX Short Cycle timer: The number of short DRX Cycles for which the UE sleeps before entering into the Long DRX Cycle is termed as DRX Short Cycle Timer. Hence, the UE can comparatively come back to active state with less amount of delay as compared to the Long DRX Cycle if it has data packets intended for it from the eNB.

Long DRX Cycle: This denotes the DRX Cycle, which is followed right after the Short DRX Cycle. The efficiency of the DRX mechanism depends highly on this timer value. Higher the Long DRX Cycle, higher is the power saving done by the UE. However, this comes at the cost of increased latency in case the UE needs to come out of the DRX mode and send or receive data from the eNB. Hence, depending on the certain application’s QoS and latency requirement, an intelligent decision should be made for this particular long DRX Cycle value.(have to write more here)

ON Duration: Once there is data scheduled for the UE when it is in DRX mode, it needs to come out of the DRX Mode and receive the PDCCH control information. Hence, the time during which the UE stays in active mode to receive this data is termed as ON Duration.

ON Duration Timer: There can be data sent for the UE from the eNB while the UE is in DRX mode. The UE needs to exit from the DRX mode to receive the incoming control packets via the PDCCH. Hence, the number downlink sub frames for which the UE remains ON after exiting from the DRX mode is termed as ON Duration Timer.

Retransmission Timer: There maybe chances when few data packets received at the UE are erroneous. In such cases, UE requests for a retransmission. The time for which the UE extends its ON Duration in case of retransmission is termed as Retransmission Timer. During this the UE waits for the eNB to retransmit the erroneous packets and hence indirectly, increases the efficiency of the DRX mechanism by not going in the sleep mode.

IV. DRX in RRC_Connected and RRC_Idle State

DRX in RRC_Connected State: The UE in RC_Connected State is in the process of reception and/or transmission of data to the eNB. In this mode, the UE can be put to sleep for the small durations in between the transmission and reception when the UE is waiting to receive/send packets or once it receives the DRX Command MAC Control element from the eNB [1]. When there are no more packets left in its sending buffer for transmission or no more packets left to receive from the eNB, it starts the DRX power saving mechanism. Thus, DRX mode is enabled in RRC_Connected State by either the eNB or the UE.

The following Figure 1 depicts the DRX mechanism in the RRC_Connected State for the scenario when the UE switches between active mode, power saving mode and back to active mode.

Untitled1.pngFigure 1: DRX Mechanism in RRC_Connected State [1] [2] [3] [4] [6] [7] [11] [13] [14]

As shown in Figure 1, during the active state, the UE receives the control information for the time specified by ON Duration. Once the last successful PDCCH sub frame packet is received, the UE starts the Inactivity timer. Once the Inactivity timer expires and there is no more pending data to be received by the UE, the short DRX Cycle is initiated by the UE. In between the short DRX Cycle, the UE periodically listens to the PDCCH to see if there is any data intended for it with the help of ON duration timer. After ‘N’ no. of short DRX Cycles, if still there are no data packets for the UE, it enters into the long DRX Cycle. Even during this long DRX cycle, the UE periodically keeps listening to PDCCH for the time specified by the ON Duration timer. Once there is downlink data packet intended for the UE from the eNB or if the UE wants to send data to the eNB in the uplink, the long DRX Cycle is exited and the UE returns back to the active state. However, there is some delay, which occurs between this, switching from DRX state to active state as shown in the Figure 1. This delay is due to the fact that UE takes time for correct synchronization with the eNB. Also in DL, the eNB needs time to allocate the proper bandwidth according to its total available resources and scheduled users. Hence, proper tradeoff between the long DRX Cycle and the time to enter back into the active state from the long DRX Cycle should be decided by the RRC for different delay sensitive applications. Once the UE is back in active state, it again waits for the ON duration looking for control segment information through PDCCH and then remains inactive for the rest of the interval and enters the short DRX mode as explained before.

DRX in RRC_Idle State: In RRC_Idle State, the UE does not have any active control or data transmission for an extra amount of time as compared to the RRC_Connected state and hence eNB releases the UE connection [1] [2]. The MME and SGW also keep only the control plane related UE contents [1].

Figure 2 explains the flow for the DRX mechanism in RRC_Idle State.

11.pngFigure 2: DRX Mechanism in RRC_Idle State [1] [2] [3] [4] [6] [7] [11] [13]

As seen in Figure 2, similar to that of the RRC_Connected State, the UE in the RRC_Idle State which is initially in the DRX mode keeps on listening to the PDCCH periodically with the help of ON Duration timer and then enters the Long DRX Cycle once the Short DRX Cycle timer expires. The only difference here is that the UE takes additional time for network re-entry once it wants to send any uplink data or receive a page intended for it from the eNB. The network re-entry starts with the UE sending the RRC_Connection Request to the eNB until it receives the downlink page from the SGW via the eNB [1][6]. Also, in RRC_Idle mode, the network has no exact information about the UE’s location constantly and the UE also has no dedicated channel for reporting the Channel Quality Indicator (CQI) to the eNB[ref]. The UE has to constantly do the uplink channel measurements for any handoff, if needed. Once a new eNB with higher signal strength as compared to the serving eNB is detected, the UE initiates a handover by sending the request to the serving eNB. If the new eNB does not broadcast the name of the UE in its system information broadcast message, the UE updates the tracking area update and informs the new eNB about its location [1]. Then, a successful handoff can take place.

V. DTX mechanism at the eNB in LTE

The figure below shows the power consumption model per cell from [17].

Capture.PNG

Figure 3: Illustration of the power consumption model used in the implementation [17]

The power consumption at the eNB is due to the high power amplifiers which it uses for data transmission and reception. Hence, as shown in the Figure 2, when the power consumption level reduces to a lower value ‘D’ i.e. when the eNB has less load conditions, the cell can be put into the DTX mode. In LTE, since there are different synchronization and signaling signals to be sent by an eNB, it cannot be completely shutdown [ref].

One OFDM frame structure which is used to downlink of LTE is as shown below:Untitled3.png

Figure 4: One OFDM Downlink Frame in LTE [15]

As seen in Figure 4, one OFDM frame consists of two slots with seven OFDM symbols each. The cell specific reference pilot signals are present in the first and the fourth symbol of every slot [15]. Also, there are symbols 2, 3, 4, 6 and 7 which carry only user data and no reference signals. Hence, we can shut down the transmitter at the eNB during these symbols to save the unnecessary power transmissions. Also, Multicast Broadcast Single Frequency Network (MBSFN) sub frames which are transmitted by a group of eNBs in tight time synchronization help in power saving at the eNB since they carry less reference signals and either multicast data to a group of users or broadcast data to all users [ref?]. Hence, less number of data frames would have to be sent by the eNB and hence it can save power. However the drawback with the many MBSFN frame transmissions is reduction in cell capacity and throughput as less data frames are transmitted by the eNBs.

VI. Challenges of DRX mechanism in LTE

The power reduction at the UE side is possible due to the two main factors, the Short DRX Cycle and the Long DRX Cycle. The long DRX Cycle results into the highest power saving for the UE. However, it takes the maximum delay to come out the DRX mode and enter the active mode when paged or for UL transmission. Hence, there is a tradeoff between latency and power saving by selecting the appropriate parameters for these cycles. This has to be very precise in case of delay sensitive applications like Voice over Internet Protocol (VoIP) and video calling. Also in case of multiple users, the DRX mode is enabled only once all the UEs have no data scheduled for them from the eNB. The DRX Cycle in this case is enabled for that UE which has the least value of the inactivity timer. The eNB also needs to schedule the users correctly and set its resources efficiently to avoid quality of performance of the network without the DRX mode. Also, there could be issues in handovers when the UE has to wakeup and scan and register to a new eNB which could shorten the DRX cycle in spite of no data transmission. In case of Inter-Radio Access Technology (IRAT) handovers, the power saving for parameters for different technologies may vary causing inconsistencies in power saving.

VII. Conclusion

The DRX mechanism which is implemented at the UE for the power saving is better than the DRX mechanism for UMTS since the latter only has one DRX Mode as opposed to Short and Long DRX Mode in UE. However, if the traffic model does not follow a continuous traffic pattern with short periods of data transmission and long waiting periods, then it is not a good idea to implement the DRX mode in the UE for LTE. If the DRX mode is still implemented, then it would result in more poor battery life for the mobile. Thus, the flexibility of the different DRX parameters should be correctly optimized and set according to the traffic pattern observed during the data transmission. Also, the delay due to the network reentry after the Long DRX Cycle should be kept as low as possible.