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Uplink Closed Loop Power Control for LTE System Performance evaluation with the open loop error, TPC command delay and power headroom reporting

Bilal Muhammad1, Abbas Mohammed

COMSATS Institute of IT, Abbottabad Campus, Pakistan1

Blekinge Institute of Technology, Sweden

Abstract— The role of uplink power control is to suppress

interference. Power control refers to set output power

levels of transmitters, base stations in the downlink and

User Equipment (UE) in the uplink. In this paper the

performance of 3GPP Long Term Evolution (LTE) closed

loop power control combined with fractional path loss compensation factor is evaluated by simulating the effects

of open loop error, Transmit Power Control (TPC)

command delay and power headroom reporting.

Simulation results show that the closed loop power control

with fractional path loss compensation factor is

advantageous compared to closed loop power control with

full path loss compensation. The closed loop power control

with fractional path loss compensation factor is found to

improve the system performance in terms of mean bit rate

by 63%.

Keywords- LTE, power control, power headroom report, TPC

I. INTRODUCTION

Uplink transmitter power control is a key radio resource

management feature in cellular communication systems. It is

usually used to provide an adequate transmit power to the

desired signals to achieve the necessary quality, minimizing

interference to other users in the system and maximizing the

battery life of the mobile terminal. In order to achieve these

goals, uplink power control has to adapt to the radio

propagation channel conditions, including path loss,

shadowing and fast fading fluctuations, while limiting the

interference effects from other users, within the cell and from

neighboring cells.

The closed loop power control combined with fractional path

loss compensation factor sets the SINR target based on the

path loss of the users while the conventional closed loop uses

a single SINR target for all the users in a cell. The

performance evaluation of the LTE closed loop power control

combined with fractional path loss compensation factor for the

Physical Uplink Shared Channel (PUSCH) [1] is evaluated

and an optimal value of 0.8 for the path loss compensation

factor is proposed in [2]. This paper extends the work carried

out in [2] by evaluating the performance with the open loop

error, TPC command delay and power headroom reporting.

The paper is outlined as follows. In Section II power control is

discussed in general it also presents E-URTA power control

formula for the PUSCH. In Section III the modeling of

realistic scenario which includes open loop error, TPC

command delay and power headroom reporting is presented.

In Section IV the simulation assumptions are outlined. The

simulations results are described in Section V followed by

conclusions in Section VI.

II. POWER CONTROL

LTE uses single carrier frequency division multiple access

(SC-FDMA) as its radio access technology in the uplink.

Usage of an orthogonal transmission scheme eliminates intra-

cell interference; however, the system performance is still

limited by the inter-cell interference.

In order to maximize the spectral efficiency, 3GPP LTE is

designed for frequency reuse 1 [1] both for downlink and

uplink, which means that all cells in the network use the same

frequency bands. Thus with frequency reuse 1 [1], both data and control channels are sensitive to inter-cell interference.

The cell edge performance and the capacity of a cell site can

be limited by the inter-cell interference. Therefore the role of

closed loop power control becomes decisive to provide the

required SINR to maintain an acceptable level of

communication between the eNB and the UE while at the

same time controlling interference caused to neighboring cells.

Battery power is a scarce resource for portable devices such as

notebooks, ultra-portables, gaming devices and video cameras.

In the coming years these devices will operate over mobile

broadband technology such as LTE. Therefore to minimize consumption of battery power and use the available power

efficiently, power control can be helpful.

The 3GPP specifications [1] defines the setting of the UE

transmit power for PUSCH by the following equation

max 10 0min{ ,10 log ( )}PUSCH mcs iP P M P PL fα δ= ⋅ + + ⋅ + + ∆ (1) where

- Pmax is the maximum allowed transmit power.

- M is the number of physical resource blocks (PRB).

- P0 is cell/UE specific parameter. It is used to control

SNR target and is signaled by the radio resource control

(RRC). In this paper, it is assumed that P0 is cell-specific.

- α is the path loss compensation factor. It is a 3-bit cell specific parameter in the range [0-1] signaled by RRC.

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- PL is the downlink path loss estimate. It is calculated in

the UE based on the reference symbol received power

(RSRP).

- δmcs is cell/UE specific modulation and coding scheme

defined in the 3GPP specifications for LTE.

- f (∆i) is UE specific. i∆ is a closed loop correction

value and f is a function that permits to use accumulate or

absolute correction value.

In this paper, accumulate [1] correction value is considered to

correct the UE uplink power.

The accumulate correction value is calculated as

[ ]( ) ( 1) ( ) dB PUSCH PUSCHf i f i i Kδ= − + − (2)

where δPUSCH is UE specific correction value, which is also

referred as TPC command. TPC commands [-1, 0, 1, 3] dB

are used during the simulations. f (0) = 0 and

KPUSCH = 4 TTIs.

The parameter P0 is calculated [3] as

0 0 max 10 0( ) (1 )( 10 log ) [dBm]nP SNR P P Mα α= ⋅ + + − − ⋅ (3)

where,

� SNR0 is the open loop target SNR (signal-to-noise ratio)

� Pn is the noise power per PRB.

� M0 defines the number of PRBs for which the SNR target is

reached with full power. It is set to 1 for simplicity.

The LTE closed loop power control mechanism operates

around open loop point of operation. The UE adjusts its uplink

transmission power based on the TPC commands it receives

from the eNB when the uplink power setting is performed at

the UE using open loop power control.

Therefore, eq. (1) can be re-written as

maxmin{ , ( )} [dBm]PUSCH OL iP P P f= + ∆ (4)

Where POL is the uplink transmit power set by the open loop point of operation and is given by

[ ]max, 10 0min{ 10 log } dBmOLP P M P PLα= ⋅ + + ⋅ (5)

It is worthwhile to note that, if PPUSCH is set using the closed

loop power control then power limitation is neglected in

eq. (5) and is applied by eq.(4).

III. MODELLING OF REALISTIC SCENARIOS

A. Power headroom reporting Model

Power headroom (Ph) is a mechanism by which the mobile

terminal is configured to provide regular reports on its power

to the network. The power headroom report can be used by the

eNB to calculate the path loss of the users which is then used

in setting of SINR target.

Power headroom report is sent by the UE to the eNB which

indicates how much power the UE is left with to start using full power. In other words, it is the difference between the UE

transmit power and the maximum UE transmit power and is

given by:

[ ]max dBmh PUSCHP P P= −

(6)

The following triggers [4] should apply to the power

headroom reporting:

� The path loss has changed by a threshold value, since the

last power headroom report is sent. The threshold value

can be [1, 3, 6, inf] dB.

� The time elapsed from previous power headroom report is

more than [10, 20, 50, 200, 1000, inf] TTIs.

The equation for setting of SINR targets based on the path loss

presented in [2] is given by

max max

max

( 1) ( ) SINRtarge , SINRtarget [dB]

SINRtarget ,

PL PL t PL PL

PL PL

α − ⋅ − + <′ =

≥ (7)

As the power headroom reporting is taken in to account, now, in order to set the SINR target based on the path loss, the PL is

calculated [2] as:

[ ]

10 0

1 dB

{ 10 log ( )}h i

PLP M P fα

=⋅ − ⋅ − − ∆

(8)

where Ph = PPUSCH = Pmax when PL = PLmax.

B. Open loop Error Model

The open loop power control errors are usually the result of

several factors such as the accuracy of measurements of

reference symbol received power (RSRP) at the UE and

inaccuracies in the radio parts such as temperature sensitivity

and tolerances in the standard. The open loop error is

identified as a slowly varying component and varies between

manufactures of UEs. The sources of open loop power control error are illustrated in Fig. 1.

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Fig 1: Illustration of the sources of the open loop error

Since the LTE RF components are the same as that used in

WCDMA, the tolerance described in the technical

specification [5] can be used for first approximation.

Tolerance of + 9 dB is required, however a batch of UEs can

handle + 4 dB. Thus, the absolute value of + 4dB with a

uniform distribution is considered as an open loop error in

order to evaluate the effect of the closed loop power correction

using TPC commands.

By taking the absolute open loop error into account, the expression for calculating UE uplink power using closed loop

is given by:

[ ]maxmin{ , ( )} dBm PUSCH OL iP P P abserr f= + + ∆ (9)

where abserr is the absolute open loop error.

C. TPC command delay Model

The eNodeB issues the TPC command to adjust the power at

the UE. However, the adjustment takes place after some delay.

This delay is typically the propagation round trip time (RTT)

and the processing time at the UE and the eNB. The RTT

delay is due to the wave propagation, while the processing

delay at the eNodeB occurs due to measuring the received

SINR and issue of TPC command based on SINR target and

received SINR. The processing delay at the UE occurs due to

measuring the RSRP, calculating the PL, calculating the

transmit power and applying the adjustment based on the

received TPC command. The total time delay used during

simulations is KPUSCH = 4 TTIs [1].

IV. SIMULATION ASSUMPTIONS AND MODEL

Dynamic simulations have been used. The terminals having

the velocity of 3 km/h are randomly positioned in the system

area, and the radio channel between each base station and terminal antenna pair is calculated according to the

propagation and fading models. The simulator used the ray-

based 3GPP Spatial Channel Model Extension (SCME) [5] to

model the multipath fading propagation in the system. The

simulation parameters are listed in Table 1.

Table 1: Default simulation parameter

The LTE closed loop power control operates around open loop

point of operation, thus the simulator sets the open loop power

control based on the SINR target the power is corrected using

TPC commands which are issue based on the difference

between SINR target and estimated received SINR. Based on

transmit power, channel realizations, modulation scheme, and

the active interferers, an SINR is calculated for each link and

antenna including both intra- and inter-cell interference.

In contrast to full buffer traffic model a simple upload traffic

model is used. In full buffer model, neither a user leaves due

to hang up, nor does a new user arrives, since each user buffer

is filled with infinite data, and the user will not leave until and

unless it transmits all the data.

The simple upload traffic model is designed in a way such that

the users can have limited data in their buffers, thus a user

leaves when it transmits the data and new users are added in

the system. It provides the ease to define the user upload file

size and mean bearer bit rate. The mean bearer bit rate along

with the offered cell throughput defines the total number of

users in the system. Moreover, the simple upload buffer model

also allows inclusion the effect of queuing delay when calculating the user bit rate. The queuing delay reflects more

realistic results and provides a better scale for performance

comparison in choosing the optimal value of α. For different

values of α, the 5th

percentile and mean user throughput is calculated by taking the effect of queuing delay into account.

Traffic Models

User distribution Uniform

Terminal speed 3 km/h, 120 km/h

Data generation Simple upload traffic model

Radio Network Models

Distance

attenuation

L = 35.3+37.6*log(d), d = distance in meters

Shadow fading Log-normal, 8dB standard deviation

Multipath fading SCM, Suburban macro

Cell layout Hexagonal grid, 3-sector sites, 21 sectors in total

Cell radius 167m (500m inter-site distance)

System Models

Spectrum allocation

10MHz (50 resource blocks) 180kHz (1 resource block)

Max UE output power

250mW into antenna

Max antenna gain 15dBi

Modulation and

coding schemes

QPSK and 16QAM, turbo coding

Scheduling

algorithm

Round robin

Receiver MMSE [6] with 2-branch receive diversity

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V. SIMULATION RESULTS

In this section we present the simulation results using the

simple upload traffic model. Each value of α is investigated for each closed loop SINR target from a set of SINR targets. The

criterion that selects optimal value of α for a given SINR

target is optimized for cell-edge bit rate i.e. that value of α has been chosen which gives the best CELL edge performance for a given SINR target.

A. Performance analysis with the power headroom report

In this section, the behavior of the closed loop power control with the power headroom reports is analyzed. The results involve the individual effect of both the triggers applicable to the power headroom reporting namely periodicity and path loss change. The performance comparison of both the triggers is also analyzed.

a) Triggering at the periodic periodic intervals

The performance comparison in terms of user bit rate of the

closed loop power control with full compensation and α = 0.8

with or without the power headroom report is shown in Fig. 2.

It can be seen from the figure with the power headroom

reports the user bit rate is degraded for the users with good radio conditions. The reason for this degradation in the mean

bit rate is due to the fact that the SINR target setting is based

on the outdated path loss. The SINR target setting based on

the path loss aims to improve performance, in terms of bit rate,

for the users with good radio conditions. Thus, the more

outdated the path loss the more will be the degradation in the

mean bit rate, as can be seen in Fig. 2 for high periodicity

values. It is worthwhile to note that, for a longer simulation

time and setting the power headroom periodicity to infinity,

the performance of the SINR target setting based on the path

loss will be more like that of the absolute setting of the SINR

target.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

10

20

30

40

50

60

70

80

90

100

C.D

.F.

[%

]

User Bit rate [Mbps]

Performance analysis with Phr report triggering at periodic intervals

αααα : 1, Ideal

αααα : 0.8, Ideal

αααα : 0.8, periodicity: 50 ms

αααα : 0.8, periodicity: 100 ms

αααα : 0.8, periodicity: 200 ms

Fig 2: CDF plot of the user bit rate. The power headroom report

triggering after 50, 100 and 200 TTIs.

b) Triggering at change in the path loss

The simulation time is too short and mobiles speed of 3 km/h

is too slow for the UE's to experience a change in the path loss

by 3 dB. In order to analyze the behavior using the path loss

change as the power headroom trigger, the mobiles speed is

increased from 3 km/h to 120 km/hr and the path loss

threshold is set to 1 dB, while the simulation time is kept the

same.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

10

20

30

40

50

60

70

80

90

100

C.D

.F.

[%

]

Performance anaylsis of Phr report triggering at change in PL

User bit rate [Mbps]

αααα : 1, Ideal

αααα : 0.8, Ideal

αααα : 0.8, perodicity 200 ms

αααα : 0.8, plth: 1 dB

Fig 3: CDF plot of the user bit rate showing the performance of the power

headroom report. The mobile speed is 120 km/h. The simulation time is

200 ms.

The power headroom report triggering at change in the path

loss results in better performance than triggering at periodic

intervals, it can be seen in Fig. 3, the mean bit rate is more

degraded using the periodicity trigger than using the path loss threshold trigger. The reason for more degradation using

periodicity trigger is that the power headroom report triggers

at periodic intervals, if any change in the UE uplink power

occurs right after periodic interval the UE have to wait for

next interval to report the new uplink power. Thus the longer

the periodic interval the longer the UE has to wait to send

power headroom update.

On the other hand, triggering power headroom report when

path loss changes by a threshold value the uplink power is

reported immediately meaning that eNB has more updated

estimate of the UE path loss leading to better performance in terms of mean bit rate. In this specific scenario, power

headroom is reported only once using periodic intervals but on

the other hand the power headroom was sent whenever the UE

experienced path loss change of 1 dB.

c) Performance comparison of both the triggers

Fig. 4 shows the performance of the power headroom report

triggering either at the periodic intervals or when UE

experiences 1 dB change in path loss and also the combination

of both the triggers. It can be seen that the power headroom

report triggering at change in path loss alone results in the

same performance when compared to power headroom report

triggering at periodic intervals or using both of the triggers.

However, the reporting overhead will be more in case of

triggering at change in path loss than that of periodic

triggering. Thus it’s a tradeoff between reporting overhead and

mean bit rate.

5

0 1 2 3 4 5 6 7 8 9 100

10

20

30

40

50

60

70

80

90

100

C.D

.F.

[%

]

User bit rate [Mbps]

Peformance comparasion of power headroom triggers

αααα: 1

αααα: 0.8

αααα: 0.8,periodicty: 50 TTIs

αααα: 0.8,periodicty: 50 TTIs , plth: 1dB

αααα: 0.8, plth: 1dB

Fig 4: Performance comparison of the power headroom triggers.

B. Performance analysis with the open loop error and TPC

command delay

In here, the individual and combined effects of both the open

loop error and TPC command delay are investigated. Fig. 5

shows that the performance of the user bit rate is improved for

the users with good radio conditions when taking the open loop

error in to account. This is because of the increase of the uplink

power owing to the open loop error for the number of UE

which results in high received SINR. The performance in terms

of user bit rate is slightly degraded for users in the low CDF

region since the number of UE cannot satisfy the required

SINR due to open loop error. However, the performance

change in terms of bit rate due to absolute error is just the

initial phenomenon at the start of simulations (i.e., short

simulation time), and will not be visible when simulated for

longer time since the closed loop power control compensates,

for the open loop error, using the TPC commands.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

10

20

30

40

50

60

70

80

90

100

C.D

.F.

[%

]

User Bit rate [Mbps]

Performance analysis with absolute error in power setting

αααα : 0.8, Ideal

αααα : 0.8, abserr

Fig 5: Performance analysis of the user bit rate when only the absolute

error is taken into account.

The TPC delay introduces an initial delay of only 4 TTIs

before the UE starts to use the TPC command it received from

the eNB to correct its uplink power. It is worth noting that with

the round robin scheduling, it takes only 14 TTIs before all

users start to correct their uplink power using the TPC

command. Thus, the effect of TPC command delay is not

visible as can be seen in Fig. 6, where the closed loop power

control with TPC command delay shows the same performance

in terms of cell-edge and mean bit rate as that of the closed

loop without TPC command delay.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

10

20

30

40

50

60

70

80

90

100

C.D

.F.

[%

]

User Bit rate [Mbps]

Performance analysis with the TPC delay

αααα : 0.8

αααα : 0.8, TPC delay

Fig 6: CDF plot of the user bit rate showing both the closed loop power

control with or without TPC delay.

Finally, Fig. 7 shows the combined effect of both the open loop error and the TPC command delay. However, since there is no noticeable effect of the TPC command delay as discussed above, it can be concluded that the effect on the user bit rate shown is due to the open loop error only. It is also evident from the fact that the results in this figure show similar trend to those in Fig. 5.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

10

20

30

40

50

60

70

80

90

100

C.D

.F.

[%

]

User bit rate [Mbps]

Performance analysis with the abserr and TPC delay

αααα : 0.8, Ideal

αααα : 0.8, abserr and TPC delay

Fig 7: Performance analysis of the user bit rate with both the absolute

error and TPC delay are taken into account.

C. Performance analysis with power headroom report, open

loop error and TPC command delay

It is worthwhile to note that the terminal speed is increased to

120 km/h just to analyze the behavior of the power headroom

report triggering at change in path loss. In order to simulate the combined effect of the realistic scenario the mobile speed

is kept at 3 km/h while the path loss threshold of 3 dB is used.

Fig. 8 shows the performance in terms of user bit rate when

taking into account the combined effects of the absolute error,

time delay, and power headroom report triggering at periodic

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intervals of 200 ms and/or change in path loss by 3 dB. It can

be seen from this figure that the closed loop power control

using α = 0.8 shows performance gain in both mean and cell-edge bit rate. The mean bit rate is improved by 63% and at the

same time providing better cell-edge performance compared to

α = 1.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

10

20

30

40

50

60

70

80

90

100

C.D

.F.

[%

]

User bit rate [Mbps]

Performance analysis of Closed loop Power control Schemes

αααα : 1, Ideal

αααα : 0.8, Ideal

αααα : 0.8, Realisttic

Fig 8: Performance analysis in terms of the user bit rate taking in to

account the power headroom report, absolute error, and time delay.

VI. CONCLUSIONS

In this paper the performance of closed loop power control combined with fractional path loss compensation factor is evaluated by simulating the effects of open loop error, TPC command delay, and power headroom reporting.

The results shown that the performance of the closed loop

power control using α = 0.8 with the power headroom report triggering at periodic intervals and change in path loss of the UE. It was found that with the use of power headroom report the performance was degraded in the mean bit rate due to the outdated path loss used in the setting of SINR target. However, this performance in terms of the mean bit rate is still greater than the performance of the closed loop power control with the full compensation.

The TPC command delay and open loop error did not affect the

performance of closed loop power control using α = 0.8.

In the realistic case, the performance in terms of mean bit rate was improved by 63 % for a given SINR target. This shows the performance of the closed loop power control combined with the fractional path loss compensation factor is advantageous than the closed loop power control with full compensation.

REFERENCES

[1] 3GPP “E-UTRA Physical layer procedures”, TS 36.213 V8.1.0

[2] Bilal Muhammad, Abbas Mohammed. ” Performance Evaluation of Uplink Closed Loop Power Control for LTE System” in proceedings of

IEEE Vehicular Technology Conference 2009 Fall (VTC’09-Fall), 20-23 September 2009.

[3] R1-074850 “Uplink Power Control for E-UTRA – Range and

Representation of P0”.

[4] R4-081162 “LS on power headroom reporting”.

[5] D. Baum et.al. ”An Interim Channel Model for 4G Systems, Extending

the 3GPP Spatial Channel Model (SCM)” in proceedings of IEEE

Vehicular Technology Conference 2005 Spring (VTC’05-Spring), May

2005

[6] J. H. Winters, “Optimum Combining in Digital Mobile Radio with Co-channel Interference”, IEEE Journal on Selected Areas in Communications, Vol. SAC-2, No. 4, July 1984.