Discontinuous transmission for energy efficiency in 5G

1. Introduction


Energy efficiency is one of the main goals for the next mobile network generation. As we mentioned in previous posts, theeNodeBs are the elements in the network with highest energy requirements. In addition to the techniques applied at the base stations that we mention in our Telecoms.com publicationdiscontinuous transmission (DTX) techniques are promising solutions to save energy in the network without compromising the rest of the parameters, i.e., the QoS. In this post we are going to define the concept of discontinuous transmission in order to be able to focus in one subcategory named “fast sleep mode” which is gaining popularity among the traditional “long sleep modes”, for offering a greater flexibility and accuracy in the instants where the base stations need to be switch on or off. However, the accuracy trade-off involves a faster capability of the Power Amplifier (PA) to be able to switch at those speeds, therefore higher requirements for these PAs are needed.

DTX has been widely use in user terminals to enhance the battery life and the basic principle consists in transmitting only when it’s needed, turning the transmitted power down in periods of time when the traffic load is lower. When this technique is applied in the eNodeB, it’s named “cell DTX“, and this is the concept that we will study in this post, because as mentioned, the base station is the element of the network that more energy consumes. Let’s stablish the main differences between thetwo types of DTX before we focus and explain in more detail the fast sleep mode, also called fast DTX.

Fast cell DTX

The fast DTX technique acts on the slot level. There are two subtypes: the micro DTX, or micro sleep, it’s used when there is not user data to transmit, and the DTX transmission is performed on the Cell-specific Reference Signal (CRS). The second subtype is the MBSFN-based DTX where the Multicast-Broadcast Single-Frequency Network (MBSFN) are transmitted to mkae room for longer sleep periods and the CRS are not transmitted.

Long cell DTX

This is the traditional sleep mode where the periods in the off state are longer and the power consumption is also considerably reduced. It has been shown that this mode is specially useful for densely deployed networks such as dense urban scenarios. Under these conditions, the macro-cells can be in sleep mode during low traffic periods.

2. Fast DTX

Let’s recall how the LTE sub-frame is formed: as you can observe in Figure 1 [1], the first part, the downlink control region, begins with 1 to 3 OFDM symbols. The Primary and Secondary Synchronization Signals (PSSS and SSSS, respectively) are transmitted in the sub-frames number 0 and 5 and the Physical Broadcast Channel (PBCH) is transmitted in the sub-frame number 0. Figure 1 also shows a zoom (inside the circle) of one Resource Block (RB) formed by 14×12 resource elements (OFDM symbols x subcarrier), assuming normal cyclic prefix. The resource elements in blue are used to transmit the Cell-specific Reference Signal (CRS), which are related to the antenna port.
The  MBSFN sub-frames begin with a small control region, same as normal (unicast) sub-frames, while the rest may be empty and there are 6 out of 10 sub-frames that can be configured to be MBSFN (1, 2, 3, 6, 7, and 8). Therefore, when the traffic load is low, we can set up to 6 free MBSFN sub-frames to save energy. Figure 1 shows the minimum amount of signals needed when there is no traffic at all in a cell. When transmitting these 6 MBSFN sub-frames and assuming the PA is able to commute in 30 µs, the PA can transmit 74% of the time in low power mode when there is no traffic in a cell. This implies that the energy savings when using DTX are efficient.
Figure 2 [1] shows the cell average transmitted power for 4 different MDSFN configurations in comparison with the case where no MDSFN are transmitted.

3. Achievements and final notes

Ericsson has published results where cell DTX is able to save 47-63%  power in low traffic hours and 38-60% power in peak traffic hours (see Figure 3 [3] for traffic loads variation). When using fast DTX in the 50% of the cells in a network, the daily energy consumption is reduced up to 37% in comparison with a network that doesn’t use fast DTX. If fast DTX and sleep mode are combined, the daily energy consumption can up to 73% lower than networks with no energy saving policies. Therefore, a dynamic combination of both techniques is a potential option for energy efficiency.
load variation_vs_time
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[1] Frenger P. et al, “Reducing Energy Consumption in LTE with Cell DTX”, IEEE, 2011.
[2] Rapone D., et al, “Energy efficiency Solutions for the Mobile Network Evolution Towards 5G: an Operator Perspective”, 2015.
[3] Hiltunen K., “Utilizing eNodeB Sleep Mode to Improve the Energy-Efficiency of Dense LTE Networks”, IEEE, 2013.

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