- Open Access
Power headroom report-based uplink power control in 3GPP LTE-A HetNet
© Kim et al. 2015
- Received: 1 June 2015
- Accepted: 16 October 2015
- Published: 26 October 2015
In a 3rd Generation Partnership Project Long Term Evolution-Advanced (3GPP LTE-A) uplink, user equipment (UE) has a maximum transmission power limit defined by the UE power class. Generally, the cell edge UE has a higher probability to be constrained by the maximum transmission power level owing to the compensation of the large pathloss. When the UE transmission power is constrained by the maximum level, allocating a higher number of physical resource blocks (PRBs) than the UE power capability can afford will reduce the transmission power to be allocated per PRB, resulting in inefficient use of power resources. To avoid this power inefficiency, the uplink transmission power can be controlled according to the number of PRBs allocated using the power headroom report-based power efficient resource allocation (PHR-PERA) scheme proposed in this paper. Furthermore, adaptive open-loop power control (OL-PC) based on the signal-to-interference-plus-noise ratio (SINR) and the uplink interference is used to improve the cell capacity. By the uplink power control employing the proposed PHR-PERA scheme, the macro and femto UE throughputs were increased by 49.9 and 5 %, respectively, compared to the case of conventional fractional power control (FPC). Additional gains of 21.9 and 4.8 % for macro and femto UE throughputs, respectively, were achieved by adaptive OL-PC. The performance of fast closed-loop power control (CL-PC) based on the received SINR is also evaluated in this paper. The simulation results demonstrate that CL-PC supports OL-PC by compensating the fading effect for the UE uplink SINR to meet the target SINR.
- Uplink power control
- Power headroom report
- Heterogeneous networks
- 3GPP LTE-A
In a 3rd Generation Partnership Project Long Term Evolution-Advanced (3GPP LTE-A) uplink, the orthogonality provided by single carrier-frequency division multiple access (SC-FDMA) removes intra-cell interference—i.e., the interference between users in the same cell . However, the inter-cell interference problem remains to be solved because the band allocated to a user in a cell can be used by another user in any of the neighboring cells. In a conventional homogeneous network—i.e., a network based on macro cells only—fractional power control (FPC) is used to cope with inter-cell interference [1, 2]. The impact of the FPC scheme on the signal-to-interference-plus-noise ratio (SINR) was evaluated in [3, 4] in detail. The FPC scheme partially compensates for the pathloss such that users with high pathloss will operate at a low SINR requirement, thus reducing interference caused to the neighboring cells. In the overload indicator (OI)-based uplink power control proposed in , the base station measures the uplink interference and sends the OI to the neighboring base stations to broadcast its interference situation. Based on the number of OIs received, the target’s received power is dynamically adjusted to control the uplink transmission power and avoid system interference.
The latest evolution of cellular networks—i.e., heterogeneous networks (HetNet)—has been well acknowledged to meet the increasing demand for data traffic. In HetNet, there is a possibility of deploying the picocells or femtocells with macrocells as one of the candidate for small cells. The picocells’ deployment is the same as the macrocells’ deployment, that is, they are deployed by telecom operators after doing proper planning in order to reduce the inter-cell interference. However, unplanned user deployed femtocell deployments lead to severe inter-cell interference in the aggressive frequency reuse scheme and result in system performance degradation. Allocating different frequency bands to the macro and femtocell by using fractional frequency reuse schemes  can be one of the solutions to prevent the severe inter-cell interference; however, the goal of using the aggressive frequency reuse scheme is to provide the spectral efficiency under the condition of bandwidth limited situation. Similarly, the inter-cell cooperation scheme such as muting the base station requires the exchange of the control information that gives the overhead resulting in spectral inefficiency . Therefore, in this paper, all the cells are using the same frequency band to provide spectral efficiency; the uplink power control is used to mitigate the severe interference situation of HetNet. In , the cell-specific uplink power control scheme was proposed considering the HetNet environment. It is verified that by using a separate set of uplink power control parameters such as target received power for macro- and femtocells, it is possible to increase the average femtocell capacity and coverage without jeopardizing the performance of macro user equipment (MUE). The aggregated resource usage of femto user equipments (FUEs) was used to control the FUE transmission power in . As the aggregated resource usage increases, FUE transmission power is suppressed to maintain the uplink throughput of macro users. In , the target received power is controlled based on the interference generated to neighboring cells by exchanging the closed-loop commands under the HetNet environment. The exchange of the interference state between the cells allows the base station to send a power control command to the user equipment (UE).
However, less literature has considered the bandwidth allocated to the user for controlling the uplink transmission power. The allocated bandwidth can be represented as the number of PRBs allocated to the user, and the more PRBs are allocated, the more UE transmission power is required. Because the UE has the transmission power constraint , allocating more number of PRBs than the UE power capability can afford will reduce the transmission power allocated per PRB, which is also referred to as spectral density (PSD). This causes inefficient use of power resources. The UE transmitting with maximal power can also cause severe inter-cell interference with neighboring cells. In the 3GPP LTE-A system, the UE can inform the base station of its transmission power state by using the parameter called power headroom (PH). In this paper, the power headroom report-based power-efficient resource allocation (PHR-PERA) scheme is proposed in order for the base station to consider the UE transmission power state while allocating PRBs. Eventually, the UE transmission power can be controlled by the number of PRBs allocated by using the proposed PHR-PERA scheme. By employing the proposed scheme, reduction in the PSD can be avoided by allocating the UE with the number of PRBs that the UE power capability can afford. Furthermore, adaptive open-loop power control (OL-PC) based on the SINR and the uplink interference averaged over a certain period is used to optimize cell capacity.
The rest of the paper is organized as follows: “System model” section describes the system model that includes the network setup and the channel model. In “General power control mechanism in 3GPP LTE-A uplink” section, the general 3GPP LTE-A uplink power control mechanism is described. “Proposed uplink power control procedure for 3GPP LTE-A system” section explains the proposed uplink power control procedure, including resource allocation, OL-PC, and closed-loop power control (CL-PC). In “Performance evaluation” section, the performance of the proposed uplink power control scheme is evaluated using system-level simulations. Finally, conclusions are drawn in “Conclusions” section.
2.1 Network setup
For the data transmission in the 3GPP LTE-A uplink, each user is allocated a certain number of PRBs. One PRB, which is the smallest radio resource unit, has a size of 180 kHz in the frequency domain and 0.5 ms in the time domain, allowing 50 PRBs to be utilized in a 10-MHz system bandwidth .
In this paper, a one-tier HetNet environment with seven eNode B (eNB) sites is considered. The center cell is the region of interest and consists of eNBs, Home eNode B (HeNBs), MUEs, and FUEs. A 5 × 5 grid model that is one of the valid HeNB urban deployment models is considered in each sector. It is composed of 25 adjacent apartments that are 10 × 10 m in size. The deployment of the HeNBs is random in each cell. Thus, there is a possibility of deployment of HeNBs at the cell-edge. This deployment results in the severe interference from FUEs to the uplink service of MUEs, especially at the cell-edge.
2.2 Channel model
The channel gain represents the propagation loss that occurs when the signal travels from the transmitter to the receiver. It can be calculated as the antenna gain minus the losses, which include pathloss, shadowing, and fading.
Shadowing is caused when the obstacles are in the paths between the UEs and the base station. It is modeled by using lognormal distribution with a mean of 0 dB and standard deviation of 4 dB for the link between the HeNB and the UE . For other interference links, the standard deviation is 10 dB, and the inter-site correlation value is 0.5 .
Fast fading is responsible for the short-term signal variations that can occur owing to the mobility of the UEs or other reflectors . In this paper, fast fading is generated according to the 3GPP LTE-A Ped B channel model.
5.1 Simulation parameters
Number of PRBs
48 + 2 (control channel)
Cellular layout of macrocells
Hexagonal grid, 3 sector cells/eNB
Number of sites
7 cells (21 sectors)
Region of interest (ROI)
Center cell (3 sectors)
HeNB deployment model
5 × 5 grid model
Number of HeNBs/macrocell
Number of MUEs/eNBs
Number of FUEs/HeNBs
UE maximum transmission power (P max)
30 drops, 500 TTIs
5.2 Evaluation of the proposed PHR-based uplink power control procedure
5.2.1 Initial parameter setting in conventional open-loop power control
As shown in Fig. 4a, the throughput increases as P 0 increases. The initial values of −62 dBm and 0.8 are selected for the MUE P 0 and α, respectively, and −60 dBm and 0.7 were selected for the FUE P 0 and α, respectively, which improved the MUE and FUE throughput performance at the same time. In Fig. 4b, as P 0 of the MUE and FUE increase, the transmission powers of the MUE and FUE also increase. The initial value of P 0 for the MUE is also used for the macro only case. In Fig. 4a, the throughput of the macro-only case is a bit higher compared to the MUE throughput of the HetNet case with the same MUE parameters because no femto users are causing interference to the macrocell. For the without-power-control case, as reference, all the MUEs and FUEs transmission power are set as 23 dBm as shown in Fig. 4b. As the transmission power is set as maximum, the high value of cumulative distribution function (CDF) region, which is the cell interior user throughputs, is higher than the case of with power control. However, the CDF of lower region, which is the cell mean, and the edge user throughputs are damaged significantly compared to the with power control case. Therefore, the necessity of the power control is also proven in this section. This trend will be similar in the case of the performance evaluation for the proposed PHR-PERA schemes discussed in the following sections.
5.2.2 Conventional fractional power control with the proposed PHR-PERA algorithm
Although improvement in throughput can be achieved by increasing P 0 of the MUE, the transmission powers of 20 % of the MUEs that are in the CDF range of 0.8 to 1 are constrained by P max , which causes a reduction in the power per PRB, resulting in power inefficiency. Therefore, by using the proposed PHR-PERA resource allocation scheme, the throughput can be maximized even further through efficient use of UE transmission power.
Sum throughput using the proposed PHR-PERA scheme
Conventional FPC with PHR-PERA
MUE sum throughput (Mbps)
FUE sum throughput (Mbps)
5.2.3 Adaptive open-loop power control with the proposed PHR-PERA scheme
Sum throughput using adaptive OL-PC
Conventional FPC with PHR-PERA
Adaptive OL-PC with PHR-PERA
MUE sum throughput (Mbps)
FUE sum throughput (Mbps)
5.2.4 Closed-loop power control with adaptive open-loop power control and the PHR-PERA scheme
In this paper, the uplink power control procedure for the 3GPP LTE-A system is proposed under the HetNet environment. The proposed PHR-PERA efficiently utilizes the limited bandwidth and power resources by allocating the PRBs to the UE considering the UE power capability. Additionally, adaptive OL-PC improves the capacity of both macro- and femtocells by setting the open-loop parameter based on the average received SINR and the uplink interference caused by neighboring cells. The proposed PHR-based uplink power control scheme is verified by system-level simulation. The simulation results for the proposed PHR-PERA scheme with conventional OL-PC shows a remarkable increase of approximately 49.9 % in macrocell capacity and 5 % in femtocell capacity. By employing the proposed PHR-PERA with adaptive OL-PC, the macrocell and the femtocell capacity has been increased by 21.9 and 4.8 %, respectively, compared to the case of employing conventional OL-PC alone. Hence, the proposed PHR-based uplink power control scheme shows remarkable performance improvement in the HetNet environment, and the limited spectral and power resources in the 3GPP LTE-A uplink can be more efficiently utilized. Furthermore, the evaluation result of OL-PC with CL-PC clearly demonstrates that CL-PC supports OL-PC for the UE uplink SINR to meet the target SINR determined by the base station.
This research was funded by the MSIP (Ministry of Science, ICT & Future Planning), Korea, in the ICT R&D Program 2014, and was supported by the Industrial Strategic Technology Development Program of MSIP/IITP, Republic of Korea. (R0101-15-0057, Development of small cell base station supporting IMT-Advanced TDD radio technology for evolution of TDD network). This research was also supported by the MSIP (Ministry of Science, ICT and Future Planning), Korea, under the ITRC (Information Technology Research Center) support program (IITP-2015-H8501-15-1019) supervised by the IITP (Institute for Information & communications Technology Promotion).
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