Analysis of RF requirements for Active Antenna System

Transcription

1 212 7th International ICST Conference on Communications and Networking in China (CHINACOM) Analysis of RF requirements for Active Antenna System Rong Zhou Department of Wireless Research Huawei Technology Beijing, China Abstract Active Antenna System (AAS) is a Base Station equipped with an antenna array system, the radiation pattern of which may be dynamically adjustable. Since the interactions between the transmitters and receivers within the AAS might be different from the convention BS and the convention antenna system, the impacts of the transmitted or received radio signals on the transmitter and receivers shall be studied. In this article, we present an overview on some typical new characteristics of the AAS, such as spatial ACLR, in-band blocking. Then quantitative study of in-band blocking is conducted according system level simulations. Keywords - AAS; RF;In-band blocking; spatial ACLR; I. INTRODUCTION Presently, 3GPP (3 rd Generation Partnership Project) RAN Working Group 4 is studying on the RF and EMC aspects of Active Antenna System (AAS). By integrating the transceivers and the radiating elements in one package and applying beam forming vectors to the radiating elements digitally which is different from legacy Base Station (BS), AAS has the capability of applying additional beamforming (BF) on vertical direction in addition to horizontal-only BF. This capability of 3-Dimension (3D) spatial processing has the potential to improve the system performance. Furthermore AAS could eliminate the cable loss and reduce site engineering complexities costs. Since the interactions between the antenna array system and the transmitters and receivers within the AAS might be different from the legacy BS and the convention antenna system, the impacts of the transmitted or received radio signals on the transmitter and receivers shall be investigated. The methodologies employed to determine AAS radio transmission and reception characteristics could follow the similar methodologies for convention BS, i.e. coexistence studying of AAS with other system is required with taking into account of system scenarios and implementation issues to ensure that the coexisting systems will not significantly degrade the performance of each other. In this article we give a detailed analysis of new characteristics of AAS, focusing on Adjacent Channel Leakage power Ratio (ACLR) and In-band blocking requirement, which might be different from the legacy BS. Then we provided some preliminary simulation results for AAS in-band blocking level, followed by a discussion of simulation assumptions, including Wu Rong Zhang, Ph.D., IEEE member Department of Wireless Research Huawei Technology Beijing, China ronnie.zhang@huawei.com 3D antenna modeling. In final, the suggestion for defining AAS Radio Frequency (RF) requirement was provided according to the analysis. II. NEW CHARACTORISICS FOR AAS A. Receive RF requirements For AAS receive requirements, let s start the discussions from the in-band blocking requirement, and other receiver requirements such as in-channel selectivity, dynamic range, and the receiver inter-modulation could follow the similar discussion. The in-band blocking interference power represents the total received power at the antenna connector (via the connected antenna) from all the UEs in the adjacent system within the same operating band but based on the uncoordinated deployment. The methodology of defining BS in-band blocking requirements for UTRA and E-UTRA is described in TR and TR36.942, i.e. system simulations were performed to evaluate the CDF distribution of the received power from UEs within the systems at the adjacent channel. The in-band blocking requirement shall be the power level that the BS may receive with very low possibility, for example,.1% for Macro BS. As shown in Fig.1, the signals collected from the antenna elements are combined before being fed into the front end of the receiver for traditional BS. While for AAS BS, each receiver is connected with one or multiple antenna elements. Since the antenna pattern connected with the receiver is different between traditional BS and AAS, the received blocking level could be different as well /12/$ IEEE

2 Figure 1. Scenarios of received blocking signals at traditional BS and AAS. B. Transmit RF requirement For transmit requirements, we would like to compare the ACLR of traditional BS with AAS at the first step. For traditional BS, both the wanted signal and unwanted (Out-of-Band and Spurious) emissions are transmitted from a single RF transmit chain, and the weighting vectors are applied to the identical (wanted / unwanted) signal at each antenna element and the weighting process is implemented as phase shift network inside the antenna. Thus the composite antenna generates the similar beam in the wanted signal domain and the unwanted emission domain, as shown in Fig. 2. Figure 3. Spatial selectivity of the transmitter signals. According to the simulation, the main beam pattern (un-normalized) for uncorrelated signals is usually wider than that of wanted signal, shown in Figure 4. Gain / Y direction Gain / X direction Figure 4. Antenna pattern of uncorrelated distortion and noise Figure 2. Traditional BTS signal emissions. However, this process is different for AAS. For the i th transmitter in the AAS BS, the output signal P i can be denoted as, P i = S + F + F i + N i (1) where S is the wanted signal to be transmitted. The wanted signals are identical for all the transmitters. F is the correlated distortions due to pre-distortion, crest factor reduction, and etc, and its radiation pattern is identical as the one for wanted signal. F i is the uncorrelated distortions due to transmitter chain inconsistence, such as the discrepancies of the RF modules in the transmitter chains, which can t be controlled by the weighting vectors. N i is the thermal noise in the transmitter chain which should be uncorrelated between the transmitters and can t be controlled by the weighting vectors. The phenomenon in Fig.3 and Fig.4 leads to the spatial selectivity of the relative differences between the wanted signal and the distortions plus noise, or the ACLR. The ACLR selectivity means the interference leakage to the adjacent band are not uniformly distributed in the spatial domain, i.e. some of the areas covered by the adjacent systems may suffer more interference from AAS BS while some others may suffer less. The ACLR selectivity is determined by the correlation level between the transmitters. The spatial ACLR shall be properly modeled and the impacts on the coexistence between adjacent systems shall be evaluated. III. SYSTEM SIMULATION To study AAS in-band blocking requirement quantitatively, system level simulation is performed. In this section we first describe the simulation assumptions and parameters used in the study. Then some initial simulation results are present to show the blocking level received at AAS. A. Simulation Case The most typical application of the AAS BS is the macro cell coverage with 3-sectors at each site. Correspondingly, Macro legacy system to Macro AAS system is considered at 68

3 the first, meanwhile Macro legacy system to Macro legacy system is also simulated for comparison, as shown in Table 1. where θ3db = 65 degrees is the vertical 3dB bandwidth, and SLAV = 25 db is the front-back ratio on vertical domain. TABLE 1 SIMULATION CASES FOR IN-BAND BLOCKING EVALUATION Aggressor Victim 1-a 1-b 1-c 1 z Case -1 1 B. Simulation Assumptions -1 1) 3D antenna model To evaluate AAS blocking requirement, we could first establish the radiating element pattern, and then, on db domain, make superposition of the element pattern with the array pattern which is determined by the vertical transmit weighting factor [1]. The antenna modeling is based on the preliminary geometry as shown in Fig.5. 1 y 3 2 x Figure 6. Radiating element pattern b) Composite antenna pattern The proposed composite pattern is a superposition of the element pattern and the vertical array pattern which is given by, N G (θ, ϕ ) = AE (θ, ϕ ) + 1 log1 wn u n n=1 2 (5) where un is the signal arrived at the n th radiating element with different phase shift. wn is the complex transmit weights applied to the vertical antenna array, assumed to be : wn = (6) where βetilt is electrical down tilt of BS, and dv is the element vertical spacing. Figure 5. Antenna array geometry. a) Radiating element pattern It is assumed that the radiating element pattern model is in a similar form to the 3GPP model in TR as below, and antenna pattern is shown in Fig z AE(φ,θ) = Gmax min{ [AE,H(φ)+AE,H(θ)], Am} d 1 exp i 2π (n 1) v sin β etilt λ N (2) -5 where -1 φ and θ is the given azimuth and elevation angle which is defined in Fig Gmax is the maximum directional gain of the radiating element in db y Figure 7. Composite antenna pattern 2) Cell layout For uncoordinated network simulations, identical cell layouts for each network shall be applied with worst case shift between sites. The second network s sites are located at the first network s cell edge. Inter site distance (ISD) of 75 meter is applied. (3) where φ3db = 65 degrees is the horizontal 3dB bandwidth and Am = 25dB is the front-back ratio. AE,H is the vertical pattern: AE,V (θ) = min[12(θ/θ3db)2,slav] 1 x AE,H is the horizontal pattern: AE,H (φ) = min[12(φ/φ3db)2,am] (4) 681

4 For 3D model, UEs are distributed on the flat ground, with the uniform height of 1.5 meter, and the height of the macro site are uniformly 3 meter. 3) Propagation Modeling For Macro cell deployed in urban or suburban area, the path loss model defined in 3GPP TR is applied, assuming a carrier frequency of 2GHz. L(R) = Log 1 (R), R in kilometer; Path_Loss_a = max {L(R), Free_Space_Loss}+ LogF; Path_Loss_macro= max {Path_Loss_a, Free_Space_Loss} G_Tx G_Rx; 4) Simulation parameters Other simulation assumptions and parameters are listed in Table 2. TABLE 2 GENERAL SIMULATION ASSUMPTIONS Parameters Cellular layout Duplex UE distribution Carrier frequency System bandwidth Inter Site Distance (ISD) Minimum distance UE<->BS Log normal shadowing Shadow correlation coefficient Scheduling algorithm RB number per active UEs number of active UEs UE max Tx power UE min Tx power BS max Tx power Power control parameters Antenna configuration at MS Down tile angle of BS Cable loss of legacy BS Cable loss of AAS The height of BS The height of MS Radiation Element Size (Row * Column) Vertical radiating element spacing Output statistics (Interferer levels) Values Hexagonal, 3 sectors/cell (19 cells wrap-around), uncoordinated FDD Average 1 UEs per cell (sector). UEs on flat ground 2GHz 1MHz 75m 35m Standard Deviation of 1 db.5 (inter site) / 1. (intra site) Round Robin, full buffered UL: 16RBs (total: 48 RBs) UL: 3 UEs 23 dbm -4 dbm 46dBm TR Section : PC Set 1 (alpha=1; P=-11dBm) Omni-directional 1 degree electrical down tile 1 db db 3 m 1.5 m 1*1.9λ CDF of the received interference power in dbm from an aggressor (Enlarged to show 99.99% point) C. Simulation Result We now present some preliminary simulation results for in-band blocking level received at AAS individual receiver and the receiver of the traditional BS under different Macro deployment scenarios, by using the simulation methodology and assumptions described above. CDF Blocking level at BS receiver (dbm) Figure 8. Received in-blocking level. Case 1-a Case 1-b Case 1-c The 99.99% CDF power levels reading from Fig.8 are summarized in Table 3 below. TABLE 1 BLOCKING LEVELS AT 99.99% CDF Case Aggressor Victim 1-a 1-b 1-c 99.99% CDF (dbm) Some conclusions could be obtained based on the above simulation results: In the real network, the blocking interference signals presented at the individual radiating element of the AAS are almost the same with that of BS equipped with traditional antenna. The reason is that comparing with traditional antenna, the element antenna shows larger gain than the composited antenna pattern in a wide direction. This implies that the interference received by element antenna would not be 1log 1 (N) lower than that of the traditional antenna in tradition BS. Therefore it is suggested that the in-band blocking requirement for tradition BS could applied for each receiver of AAS. In the existing testing specifications for BS using antenna array[2], it defines that for receiver testing, the test signals applied to the receiver antenna connectors shall be such that the sum of the powers of the signals applied equals the power of the test signal(s) specified in the test, and this requirements apply to each test, as shown below. 682

5 Test input port P s Splitting network Rx antenna interface P i Base Station REFERENCES [1] Xue-Song Yang, Hao Qian, Bing-Zhong Wang and Shaoqiu Xiao, Radiation Pattern Computation of Pyramidal Conformal Antenna Array with Active-Element Pattern Technique, Antennas and Propagation Magazine, IEEE, Volume 53, Feb.211, pp [2] TS , Release 1, UTRA Base Station conformance testing P s = sum(p i ), where P s is the required input power specified Figure 9. Receiver test set-up. If we use this testing approach for AAS, the wanted signal and blocking interference are equally allocated to each antenna port, and the blocking capability of each receiver is tested as: Blocking interference: Legacy BS in-band blocking level 1Log 1 (N); Wanted signal: Legacy BS wanted signal level 1Log 1 (N); where N is number of antenna ports in the AAS. However, according to our simulation result, there is not a 1log 1 (N) db relations between the blocking interference power level at individual element in AAS and the legacy BS as implied by the array testing setup using power splitter. Defining and testing the in-band blocking performance of each receiver in AAS at 1Log 1 (N) db scaled to legacy BS requirement would expose the AAS to big interference risks which eventually impact the stability of the network. The similar issues can be found for other receiver tests such as Dynamic Range, In-band Selectivity, and Inter-modulation. IV. CONCLUSION AAS is an emerging technology which is an integration of multiple transceivers and the antenna array in one package and offers significant benefit on site engineering and system performance gain. Since the interactions between the transmitters and receivers within the AAS might be different from the legacy BS and the legacy antenna system, the impacts of the transmitted or received radio signals on the transmitter and receivers could be different as well. This implies that new requirements shall be defined for the AAS with taking into account of its new characteristics. 683