Method and System for Optimizing Power Consumption in LTE Radio Base Stations.
Technical area
Optimization of Radio Base Station Power Consumption, Self-Organizing Networks (SON), Operational Expenditure (OPEX) Reduction, Dynamic Bandwidth Adjustment, Adaptive RBS MIMO Support Switching, and Flexible eNB Cell Site Sharing within RBS.
2. Technical problem to be solved by this Energy Saving Method
Currently LTE radio network design & dimensioning is performed considering the different types of traffic carried by LTE networks, subscriber density, over subscription ratio to meet the busy hour traffic requirements, radio coverage confidence, etc.,
The LTE channel bandwidth of 20 MHz or lesser is decided based on the number of active data clients support with minimum achievable DL & UL throughput requirements for the desired market or region.
In reality, the entire cell sites in a given market will not be fully loaded as designed all the time i.e. the entire 24 hrs. in a day. There will always be no load or less than 25 % load for most of the cell sites for at least 25% of the time in a day. The 25% of the time amounts to 6hrs in a day which is close to the average human sleeping time/ no activity period of 7 ~ 9 hrs per day.
This means the cell sites do not require 20 MHz all the time and it needs less than that for at least 25% of the time in a day.
Electrical power accounts for approximately 30% of OPEX, of which 50% is for power amplifiers.
The power consumption of a 20 MHz LTE system will always be higher than the power consumption of a 5 MHz or a 1.4 MHz LTE system even under no load conditions.
The reason behind the higher power consumption is the fact; the power amplifier working on a larger channel bandwidth needs more energy to drive based on the operating channel bandwidth.
Another reason being, the number of reference signal resource element which is transmitted all the time irrespective of the data traffic is four times higher for a 20 MHz system in comparison to a 5 MHz system and is approximately 10 times higher for 20 MHz system in comparison to a 1.4MHz LTE system.
In general, more traffic will also increase the power consumption.
However, even with zero traffic, the Radio Base Station consumes power, since it is transmitting control channels in order to provide coverage, allowing subscriber mobility.
Generally, the more service the RBS provides to the subscribers connected to it, the higher the required output power from the radios belonging to the RBS since the PRB Utilization is higher.
So in order to conserve energy, we need to dynamically adapt to a lean channel bandwidth possible to maintain the same radio coverage with reduced power consumption with no compromise of live traffic capacity.
It is possible to maintain the same radio coverage if we maintain the same energy per resource element of the reference signals irrespective of the channel bandwidth.
To use only as much channel bandwidth required and as many capacity as it needs, so that nothing is wasted meaning we do not operate power amplifiers in higher channel bandwidth than is required to operate at that interval to conserve energy to reuse those saved energy to reduce OPEX and also to reduce carbon footprints.
Low power modes reduce electric grid consumption, mitigate demand for costly diesel generation in remote locations and reduce carbon footprints.
The power consumption of site equipment like Air Conditioner adds approximately 20 % ~ 35% to the power consumption of Indoor RBS Site Power. This number varies typically between 20% and 60%, depending upon how much power is required for cooling purposes.
Note that the total site power consumption may differ depending on RBS type. For an instance, the entire RBS is placed in an indoors environment, which often requires site level cooling, whereas only the RBS Main Unit is placed in an indoors environment, since the RRUS-01 is placed outdoors.
The novel method helps the SON Energy Saving function to optimize energy consumption [reduction of energy consumption] by enabling scaling of channel bandwidth of individual Radio Base Station (RBS) cell site dynamically based on the instantaneous load. The function also incorporates dynamic tuning of MIMO configuration i.e., dynamic switching of MIMO to SISO & SISO to MIMO based on the User load & the current operating QoS Class Index of the serviced subscribers.
Energy Saving Reference Calculations
General base stations are used for wide area coverage, most of the radio network is built from such base stations. Micro and Pico base stations provide spot coverage.
A base station is located at what is called a site. The site provides power and environmental protection to the base station.
All RANs have complex algorithms to allocate sufficient, but not too much, output power to each user traffic channel and also to common channels to achieve the requested services. Sufficient output power is achieved when the signal to noise ratio for each service is achieved with a target error rate.
Scaled Channel Bandwidth means less Reference Signal Resource Element which means less output power is needed for reduced channel bandwidth.
3. Background
3.1 Technical Background / Existing Technology
Energy saving function is currently realized in a deployment where capacity boosters (additional carriers) can be distinguished from cells providing basic coverage, to optimize energy consumption enabling the possibility for a cell providing additional capacity, to be switched off when its capacity is no longer needed and to be re-activated on a need basis.
The decision is typically based on cell load information, consistently with configured information. The switch-off decision may also be taken by O&M.
3.2 Problems with Existing Solutions
The existing solution helps to save energy consumption of additional carriers if any based on the cell loading. This helps save energy in case of lower channel bandwidth and it becomes less efficient when the operating bandwidth is large.
In the current energy saving method there is no option to save energy of the primary cell either by turning off MIMO, reducing the channel bandwidth, reducing the power amplifier output power or by sharing the same power amplifier with other sectors in the same RBS by power tapping and sharing with other sectors in no load or reduced load conditions.
4. Brief summary of the Energy Saving Method
The Novel Method helps the SON system to make a decision to scale the current operating channel bandwidth which helps in energy efficient utilization.
The aim of this approach is to significantly reduce the operational expenses (OPEX) through energy savings.
The function allows, for example in any deployment model to identify the little/no user connected time period per cell and reduce/scale down their operating channel bandwidth to the least possible channel bandwidth which is sufficient to handle the need at that moment.
These identified cells provide basic radio coverage which is provided by a larger channel bandwidth with reduced radio throughput capacity, to optimize energy consumption of the power amplifier by scaling down the power amplifier operating bandwidth to a smaller bandwidth when its capacity is no longer needed and to be re-activated on a need basis.
By adopting this method, every cell site switch its mode between coverage and capacity based on the instantaneous demand at that moment.
5. Detailed description of the Energy Saving Approach
Assumption:
- Cell 1 Operating in 20 MHz Channel Bandwidth.
- No users active in Cell 1 for a past 15 minutes and is going to continue for next 6 hours and it is a routine every day based on the analyzed statistics history.
- Cell 1 consumes X Watts of power for 6 hours when its operating channel bandwidth is 20 MHz with 2T 2R Configurations and Y Watts of power for 6 hours when its operating channel bandwidth is 1.4 MHz respectively under no load condition. [ X > Y and Energy saving in Watts = X-Y]
- Cell 1 consumes X/2 Watts of power for 6 hours when OBW = 20MHz with 1T 1R configuration when it is operating in 20 MHz and Y/2 Watts of power for 6 hours when OBW = 1.4MHz with 1T 1R configuration.
Criteria Check No.1:
Check for OBW greater than 1.4MHz for cell1. If OBW > 1.4 MHz then check for number of active UE’s in the cell site for a predefined interval. If the number of active UE’s is zero then scale down the OBW to 1.4MHz and turn off MIMO if enabled in the cell site.
For an instance: when pmActiveUeDlSum & pmPrbUsedUlDtch= 0 for a predefined interval say 15 minutes.
pmActiveUeDlSum
Number of UEs considered active in the downlink direction.
Condition: The counter aggregates for each TTI the number of UEs with DRB data to send.
Counter type: ACC
Sampling rate: Per TTI
Counter is reset after measurement period: Yes
pmActiveUeUlSum:
Number of UEs considered active in the uplink direction.
Condition: The counter aggregates for each TTI the number of UEs with buffer status reports indicating DRB data to be sent in the uplink direction.
Counter type: ACC
Sampling rate: Per TTI
Counter is reset after measurement period: Yes
Calculation on Power saving:
1. By adapting bandwidth scaling achievable power savings from Reference Signal Resource Element (RSRE).
Cell site operating in 20 MHz channel bandwidth has 100 Physical Resource Blocks (PRBs).
Each PRB contains 4 Reference Signal Resource Element in 0.5ms time slot per antenna port. Every Cell site can operate in 4T 4R / 2T 2R MIMO Configuration based on the hardware and license support.
Let us do energy consumption calculation for 2T 2R MIMO configuration RBS cell site with the following assumptions:
a. 20Watts Tx power per antenna port.
b. Operating Bandwidth = 20 MHz
c. MIMO Configuration = ON, Config Type: 2Tx2Rx
d. No Active / Connected Users
e. Excluding Control channel energy consumption computation
LTE System with 20 MHz channel bandwidth contains 1200 subcarriers inclusive of pilot and data subcarriers excluding one DC/Null subcarrier and guard subcarriers on the left and right side of the channel bandwidth extremes.
Power per Subcarrier per Resource Element = 20Watts / 1200 = 0.01667 approximated to 0.0167Watts per RE.
The Power requirement for RSRE in a 20 MHz system for one Antenna Port is as follows:
No of PRBs in 20MHz LTE system (A) = 100
No of RSRE in 0.5ms time slot per PRB (B) = 4
Power per RSRE with 20Watts Output Power per Antenna Port (C) = 0.0167Watts
Power consumption of RSREs in 20 MHz LTE system every 0.5ms timeslot (D) =
A * B * C = 100 * 4 * 0.0167 = 6.68 Watts per 0.5 ms per Antenna Port
Power Consumption per Hour in Watt-hour (E) = D * 2 * 1000 * 3600
= 6.68 Watts * 2 * 1000 * 3600 = 13.33 kilowatt-hour or 47988 kJ (Energy per hour)
RSRE Power Requirement per Hour for 2TxRx for 20MHz system (F) = E * 2
= 26.67 kilowatt-hour or 95976 kJ
LTE System with 1.4 MHz channel bandwidth contains 72 subcarriers inclusive of pilot and data subcarriers excluding one DC/Null subcarrier and guard subcarriers on the left and right side of the channel bandwidth extremes.
Power per Subcarrier per Resource Element = Power Amplifier Output Power / No of modulation/useful Subcarriers.
Let us do the reverse calculation to arrive at the Power Amplifier Output power of 1.4MHz LTE system, keeping the RSRE power constant i.e 0.0167Watts to meet the same radio coverage requirement of 20 MHz LTE system of 20Watt Output Power.
Power Amplifier Output Power of 1.4 MHz = 0.0167 Watts (RSRE Power) * No of useful subcarriers in 1.4MHz
= 0.0167 * 72 = 1.2024 Watts
So the required output power of the power amplifier is 1.2Watts for 1.4 MHz system.
But for the 20 MHz to meet the same radio coverage in no load condition, the output power of the power amplifier is 20 Watts.
Note: There is a significant amount of energy savings when power amplifier operates in 1.4MHz in comparison to 20 MHz system under no load as well as low load conditions.
Power Amplifier Output Power of 20MHz and 1.4MHz are 20 Watts & 1.2 Watts respectively for the same radio coverage under no load / reduced load conditions (when < 10 % loaded).
Under reduced load conditions we can scale to the better channel bandwidth based on the load level.
No of PRBs in 1.4MHz LTE system (a) = 6
No of RSRE in 0.5ms time slot per PRB (b) = 4
Power per RSRE with 1.2 Watts Output Power per Antenna Port (c) = 0.0167Watts
Power consumption of RSREs in 1.4 MHz LTE system every 0.5ms timeslot (d) =
A * B * C = 6 * 4 * 0.0167 = 0.4008 Watts per 0.5 ms per Antenna Port
Power Consumption per Hour in Watt-hour (e) = d * 2 * 1000 * 3600
= 0.4008 Watts * 2 * 1000 * 3600 = 0.8 kilowatt-hour or 2880 kJ (Energy per hour)
RSRE Power Requirement per Hour for 2TxRx for 20MHz system (f) = e * 2
= 1.6 kilowatt-hour or 5760 kJ
SON Energy Savings can be achieved in adapting the following methods either as individual or in combinations of the following:
1. Channel Bandwidth Scaling
Condition a: No Active Users in the Cell
For an instance: Switching to 1.4 MHz channel bandwidth from the current operating bandwidth under no load conditions.
Note: Every Cell Site can probably be idle up to 6hours per day. (Considering the average sleeping time of the users which is 7.5 hours per day as per survey statistics)
Condition b: Low Load (Based on the data demand of the connected users)
For an instance: Switching to the least possible channel bandwidth which could meet the data demand requirements (From 20MHz to 5 MHz channel Bandwidth based on the PRB requirement say 16 PRBs to meet the instantaneous data demand).
2. MIMO Switching:
a. When there are no connected users in the cell site, we can dynamically turn off MIMO and operate in SIMO mode so that we can turn down the redundant power amplifiers used and conserve energy out of it.
b. When none of the connected users in the cell site are operating in spatial multiplexing mode and also during no MU-MIMO users, we can dynamically turn off MIMO and operate in SIMO mode so that we can turn down the redundant power amplifiers used and conserve energy out of it if transmit diversity gain is not significant.
When using SIMO/SISO it is sufficient with one Radio Unit (RU) with one power amplifier operational per sector. The SIMO power consumption is then equal to 0.5 * [the corresponding MIMO value] if 2Tx 2 Rx and 0.25 * [the corresponding MIMO value] if 4 Tx 4 Rx .
This formula accounts for removing one/three RU per sector but still having one DUL-20.
This method gives a total SIMO output power per sector of one half the total MIMO output power, eg 2x20W becomes 1x20W or one fourth the total MIMO output power in case of 4 Tx 4 Rx.
3. Cell Site PA Sharing within RBS:
When there are no connected users in the RBS for a predefined (User defined) interval say for 30 minutes along with historical statistics from OSS-RC data,
SON function can direct the RBS to use one Power Amplifier for all 3 cell sites (sectors) by tapping the antenna port to all cell sites within RBS under no load conditions.[Same PCI per RBS]
2. Criteria Check No.2:
Check for Cell site load less than user defined threshold for a predefined interval.
For an instance: pmPrbUsedDlDtch & pmPrbUsedUlDtch < 25 % of the available PRB for a predefined interval say 15 minutes.
pmPrbUsedDlDtch:
The total number of Physical Resource Block (PRB) pairs used for Data Radio Bearers (DRB) in the downlink.
Condition: This measurement is applicable to the Dedicated Traffic Channel (DTCH) on the Physical Downlink Shared Channel (PDSCH).
pmPrbUsedUlDtch:
The total number of Physical Resource Block (PRB) pairs used for Data Radio Bearers (DRB) in the uplink.
Condition: This measurement is applicable to the Dedicated Traffic Channel (DTCH) on the Physical Uplink Shared Channel (PUSCH).
In case the criterion is met, scale the operating channel bandwidth to the next lowest possible bandwidth (Say from a. OBW = 20 MHz to OBW = 10 MHz, b. OBW = 10 MHz to OBW = 5 MHz, c. OBW = 5 MHz to OBW = 3 MHz & 1.4 MHz respectively) and continue step 1 if the OBW is greater than 1.4 MHz.
6. Novel Approach Flow Diagram
7. Abbreviations
UE: User Equipment
ANR: Automatic Neighbor relation
TA: Timing Advance
SIB: System Information Block
PCI: Physical Cell ID
OSS –RC : Operation Sub System – Radio and Core
RTD: Round Trip Delay
RTT: Round Trip Time
RACH: Random Access Channel
ECGI- Extended Cell Global Identity
RBS: Radio Base Station
RU: Radio Unit
RRU: Remote Radio Unit
BW: Bandwidth
MIMO: Multiple Input Multiple Output
SISO: Single Input Single Output
DUL-20: Digital Unit LTE – 20 MHz
OBW: Operating Channel Bandwidth
PRB: Physical Resource Block
RE: Resource Element
CBW: Channel Bandwidth
