INFORMATION & COMMUNICATIONS Development of Array Antenna for LTE Small Cell Base Stations Yoji Okada*, Eiji MOchida, Takashi YaMaMOTO, Yoshihiro haMada, Yoshizo Tanaka and Mitsuo SugiMOTO In recent years, mobile service providers are facing rapid increase in data traffic, and so they have much interest in picocells and femtocells which can significantly improve network capacity. Deploying picocells and femtocells, however, can potentially cause inter-cell interference in the case of co-channel operation. We have studied the interference suppression method using an array antenna system for 3GPP Long Term Evolution (LTE) uplinks based on minimum mean square error (MMSE) beamforming. This paper proposes a new method to calculate the MMSE weight in a resource block (RB) basis and shows some simulation results. Furthermore, we have successfully implemented the proposed method on a system-on-chip (SoC) and verified that it successfully suppresses interference in real time. Keywords: array antenna, interference suppression, Evolved UTRA, Long Term Evolution, femtocell, picocell 1. Introduction Meanwhile, mobile service providers are facing rapid increases in data traffic, due to the recent widespread use Mobile service providers in recent years have been fac- of smartphones and other mobile devices. LTE is antici- ing rapid increases in data traffic, so have much interest in pated as a solution. However, according to Ministry of In- small cell base stations such as picocells and femtocells. ternal Affairs and Communications statistics, cumulative However, these small cell base stations are recognized as monthly data traffic has doubled since a year ago (5). If this potential causes of inter-cell interference, since it is highly pace of traffic growth continues, data traffic will increase probable that they will be deployed using the same radio approximately 1,000 times in 10 years. Standardization frequency as used by macrocells, due to the scarcity of avail- bodies, such as 3GPP, are therefore discussing a heteroge- able radio frequencies. To address this problem, we have neous network (HetNet)(6) that serves areas combining developed an interference suppression array antenna for cells of varying sizes, such as macrocells, picocells and fem- Long Term Evolution (LTE) uplinks, to provide directivity tocells. based on a minimum mean square error (MMSE) algo- The HetNet has attracted interest in recent years as a rithm. This paper proposes a new method for calculating means of reducing loading on macrocell base stations (traf- MMSE weighting factors at the LTE resource block (RB) fic offloading) by deploying macrocell base stations to pro- level. Furthermore, we indicate that this method is excel- vide coverage for wide communication areas and by lent in that it requires neither the resource assignment in- installing (or overlaying) picocell and femtocell base sta- formation of the adjacent cells nor the scheduling tions locally in zones of greater data traffic needs. This information used by higher layers. Lastly, this paper shows arrangement enables mobile service providers to increase that the proposed method is suitable for the latest software- the communication capacity of their systems. One draw- defined radio technology, as proven through implementa- back, though, is that use of the same frequency by a macro- tion on a system-on-chip (SoC) consisting of a multicore cell and a picocell or a femtocell results in serious digital signal processor and an ARM processor. interference between them. Figure 1 shows how a mobile station in a macrocell interferes with a mobile station in a femtocell. 2. Long Term Evolution (LTE) The wideband code division multiple access (W- CDMA) standard is one of the cellular phone systems currently in use. As a successor to W-CDMA, LTE was stan- Interference dardized by the Third-Generation Partnership Project signal (3GPP). For access schemes, LTE uses orthogonal fre- quency division multiple access (OFDMA) for downlink macrocell BS macrocell MS and single carrier frequency division multiple access (SC- FDMA) for uplink. Its maximum frequency band is 20 MHz femtocell BS wide for both uplink and downlink. Maximum downlink and uplink peak rates reach 300 Mbps and 75 Mbps, re- femtocell MS spectively (1)-(4). Fig. 1. Interference in HetNet 54 · Development of Array Antenna for LTE Small Cell Base Stations 3. Interference Suppression Array Antenna freq. SC-FDMA symbols = 1subframe = 1[ms] for LTE Uplink Channels Resource block Figure 2 shows the configuration of an array antenna elements for LTE uplink channels. Resource element subcarriers =180[kHz] Reference signal Antenna #1 FFT x1 (k, l) w1 Minimum radio resource Antenna #2 assignment unit FFT x2 (k, l) w y(k, l) 2 time Antenna #P Fig. 4. Minimum radio resource assignment unit (localized type) (1) FFT xP (k, l) wP z1 (k), weight , z (k) calculator P (normally 7). Each RB has a reference signal assigned to the fourth symbol. Furthermore, the area containing two Fig. 2. Array antenna for LTE uplink channels RBs aligned on the time axis is the minimum resource as- signment unit in LTE radio frames, as shown in the figure. RBs are arranged on the time axis in two ways: localized and distributed. The former type places two RBs consecu- Each signal sent from a mobile station is received by tively on the same subcarrier frequency, while the latter antenna elements numbering P and is then divided into places them on different subcarrier frequencies. The better individual subcarriers xp(k, l ) through a fast Fourier trans- of the two types is selected according to the state of the form (FFT) algorithm where p is an antenna element num- base station radio transmission channel. ber, k is the subcarrier (frequency) number and l is the 3-2 Number of antenna elements and frequency assignment OFDM symbol (time) number. Each subcarrier is multi- Generally, a greater number of array elements trans- plied by weighting factor wp corresponding to each an- lates into more interference waves being suppressed. How- tenna and synthesized into y(k, l ). ever, fewer antenna elements are desirable, because base A weight calculator performs weighting factor wp cal- stations are subject to tight location restrictions irrespective culation so as to cancel interference waves. We designed of whether they are macrocells, picocells or femtocells. A this calculation so that maximum capacity is achieved using two-element antenna is expected to suppress only one in- the LTE communication protocol. The design considera- terference wave; accordingly, such an antenna may not tions are explained below. provide sufficient interference suppression if multiple in- 3-1 LTE frame structure terference sources are present. Figure 3 shows an LTE frequency division duplex Take, for example, communication sessions between (FDD) frame structure (1). The length of one frame is 10 a macrocell base station and four mobile stations, and a ms, which comprises 10 subframes. Each subframe consists of two slots. Macrocell base station One radio frame, Tf = 307200Ts = 10 ms One slot, Tslot = 15360Ts = 0.5 ms Macrocell Macrocell #0 #1 #2 #3 #18 #19 MS 2 MS 3 Macrocell MS 4 One subframe Interference Macrocell MS 1 Femtocell MS 1 Fig. 3. LTE FDD frame structure (1) Femtocell MS 2 Femtocell base station Frequency assignment Macrocell MS 1 Macrocell MS 2 Macrocell MS 3 Macrocell MS 4 Figure 4 shows the structure of a subframe. A resource Femtocell MS 1 Femtocell MS 2 block (RB) is an area defined by subcarriers numbering 1 2 3 4 5 6 7 8 9 10 RB RB Nsc (normally 12) and symbols (180 [kHz] along the fre- Fig. 5. Frequency assignment and interference from macrocell mobile sta- UL quency axis, 0.5 [ms] along the time axis) numbering Nsymb tions (MS) SEI TECHNICAL REVIEW · NUMBER 74 · APRIL 2012 · 55 femtocell base station and two mobile stations, as shown in nel estimates and substantially decreased weighting accu- Fig. 5. To simplify the explanation, assume that 10 RBs are racy. This drawback comes from the fact that the method present along the frequency axis and that each mobile sta- determines weighting factors so as to maximize the signal- tion is assigned frequencies by each base station, as shown to-noise power ratio (SNR) of the desired signal; in other in Fig. 5. words, the method does not take into account the effects In this situation, when the femtocell base station re- of interference waves. ceives signals from mobile station 1, three macrocell mo- On the other hand, the MMSE method determines bile stations cause interference from their respective weighting factors so as to minimize mean square errors be- different directions. Femtocell base stations generally re- tween the known desired signal and the received signal; quire four or more antenna elements. that is, it determines weighting factors while taking inter- 3-3 Convergence rate and time assignment ference wave effects into consideration. Least mean square In general, compared with small cell base stations such (LMS) and recursive least square (RLS) adaptation algo- as picocells and femtocells, macrocell base stations cover a rithms, which are widely known as MMSE solutions, require greater number of mobile stations and are subject to rapid repeated calculations until convergence, though they in- changes in the radio environment. Consequently, as shown volve little computational effort and are easy to implement. in Fig. 6, macrocell base stations are expected to assign re- If a macrocell base station adopts resource assignment sources in relatively shorter cycles than femtocells. In other change in minimum assignment units of 1 ms, the MMSE words, for femtocell base stations, it is highly likely that re- method is not a suitable calculation method for the pur- source assignment to interference sources undergoes rapid poses of the present study. temporal changes. Consequently, the convergence per- formance of array weight calculation becomes important. Table 1 compares array antenna weighting factor cal- culation methods.