UE-specific Interference-Aware Open-loop Power Control in 3GPP LTE-A Uplink HetNet Woon Kim, Zeeshan Kaleem and KyungHi Chang* Department of Electronic Engineering Inha University Incheon, South Korea Email:
[email protected],
[email protected], and
[email protected]*
Abstract— Heterogeneous networks (HetNet), that is the
latest evolution of cellular networks, has been well acknowledged to meet the increasing demand of data traffic. However, unplanned small cell deployments along with the aggressive frequency reuse scheme lead to severe inter-cell interference, bringing the system performance degradation. In this paper, we propose UE-specific Interference-Aware Open-loop Power Control (IA-OPC) scheme which considers the outgoing and incoming interference in the cell in order to fully take the complicated interference situation of HetNet into account. By using the proposed scheme, the system interference existing in HetNet environment can be evidently reduced; therefore the user throughput performance is increased. Keywords— Uplink Power Control, Interference, Heterogeneous Network, 3GPP LTE-A.
I.
INTRODUCTION
Although 3GPP LTE-A uplink uses the Single CarrierFrequency Division Multiple Access (SC-FDMA) that removes the intra-cell interference by providing orthogonality [1], the frequency reuse factor of 1 for efficient use of limited frequency leaves the inter-cell interference problem to be solved. In the conventional homogeneous network that is the macro cell only based network, the fractional power control (FPC) is used to deal with the interference between the cells [12]. HetNet has been a promising technique for future cellular networks. However, dense small cell deployments lead to worse performance in inter-cell interference compared to the homogeneous network. The FPC scheme applied in homogeneous networks is shown not effective enough in HetNets, therefore the cell-specific uplink power control parameters for macro cell and a femto cell respectively were set and evaluated in [3]. In [4], the user equipment (UE) transmit power is controlled by varying uplink signal to interference plus noise ration (SINR) target by exchanging the closed-loop (CL) commands based on the interference generated to neighbor cells under the HetNet environment. However, most of the literature has only used cell-specific open-loop power control (OL-PC) parameter, which means all the UEs in a cell will have same OL-PC parameter. According to [5], the UE-specific OL-PC parameter is also defined. In this
*Address correspondence to Prof. KyungHi Chang.
paper, UE-specific Interference-Aware Open-loop Uplink Power Control (IA-OPC) scheme is proposed which considers the outgoing and incoming interference in the cell to control the UE-specific OL-PC parameter in addition to the cellspecific parameter. By using the UE-specific offset, the uplink transmit power can be set more specific to the UE interference condition. By considering outgoing and incoming interference at the same time, the system interference can be fully taken into account while setting the transmit power. II.
UPLINK POWER CONTROL SCHEME IN 3GPP LTE-A SYSTEM
In 3GPP LTE-A standard [5], transmission power P (dBm) for Uplink data transmission is set as: ìï üï Ptx,j = min í Pmax , 10log10 ( M j ) + P0 + a × PL j + D mcs + f (i ) ý 14243 ïî closed -loop ï þ
(1)
where Pmax is the maximum allowable transmit power defined as 23dBm. Mj is the number of physical resource block (PRB) allocated to the UE j determined by using scheduling algorithm such as Round Robin (RR) or Proportional Fair (PF). P0 is the target SINR control parameter or the target received power broadcasted by the higher layer through the System Information Block 2 (SIB2). P0 can be expressed as: P0 = P0 _ CELL + P0 _ UE
(2)
where P0_CELL and P0_UE is cell-specific and UE-specific value respectively. α is the pathloss compensation factor provided by the higher layer, and the PLj is the pathloss between the UE j and its base station. The P0 + α∙PL denotes the power spectral density (PSD) and forms the fractional OL-PC. ΔMCS and f(i) are the CL power control (CL-PC) parameter that represents the transport format or the modulation and coding scheme (MCS) dependent parameter and the transmit power control (TPC) command respectively. This paper discusses OL-PC only, resulting ΔMCS and f(i) set as 0. III.
PROPOSED UE-SPECIFIC INTERFERENCE-AWARE OPEN-LOOP UPLINK POWER CONTROL
In the proposed UE-specific IA-OPC procedure, the OLPC parameter P0 is tuned by giving UE-specific offset P0_UE to the cell-specific P0_CELL which is set fixed in this paper.
A. System Model The system considers one-tier HetNet environment with 7 eNB sites with the simple 5x5 grid HeNB urban deployment model [6] for each sector. In this paper, both eNBs and HeNBs will use the same frequency band with the frequency reuse factor of one in order to utilize the spectrum efficiently. B. Proposed P0_UE Decision Procedure based on the Incoming Interference In the proposed IA-OPC scheme, the UE-specific offset is determined based on the incoming interference at the cell. The P0_UE decision mechanism in IA-OPC scheme is as follows: a) Measure the average interference level at the serving base station The base station i will measure cell specific IoTBS,i which is the interference level averaged over the total PRB Mi allocated to the base station i. Interference over thermal (IoT) is the uplink constraint that needs to be considered when designing the system [7]. The IoT is calculated as : Mi
IoTBS ,i =
å m I BSm ,i + N
P0 _ UE
IoTUE , j IoTTH
ì æ P0 _ UE _ max , ö ï ç ÷ æ P0 _ UE _ min , ö÷, ïmin ç =í çç max çç (1 - b ) × ( PL - PL ) ÷÷ ÷÷ max j øø ï è è ïP , 0 _ UE _ min î
PL j
