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Design of Antenna Array Used as

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Design of Antenna Array Used as

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     Design of Antenna Array Used as Smart Antenna for TD-SCDMA Systems

     Xudong Wang National,Key Laboratory of Antenna and Microwave Technology, Xidian University, Xi??an, 710071, China. College of Information Engineering, Dalian Maritime University Dalian, 116026, China. Email:xudongwang@tom.com

     Abstract-Smart antenna technology is the key technique of third generation mobile communication standard TD-SCDMA. This paper presents a scheme for designing a kind ofantenna array used as smart antenna for TD-SCDMA systems. Based on the design principle of adaptive antenna array and the concept of smart antenna, a uniform circular array consisting of some antenna elements, is employed in the scheme. The antenna elements are made by using a novel coaxial collinear printed dipole structure. An optimization design of the structure, size and performance parameters of both antenna elements and the antenna array is also performed by means of the global optimization algorithm. A large amount of simulation computation for the design is processed to instruct the manufacture of the experimental antenna array. A series of experimental tests are also performed to evaluate the performance of the developed antenna array with the experimental results given. The fact that experimental results are approximately in consistent with the simulation computation shows the effectiveness and feasibility of the design method. The design method proposed in this paper can be extended for the design of any olher array geometry. Keywords: smart antenna, adaptive antenna array, printed dipole, optimization design Feng Gao, Qizhong Liu National Key Laboratory of Antenna and Microwave Technology, Xidian University, Xi??an, 7 10071, China.

    Email:qzhliu@xidian.edu.cn

     I. INTRODUCTION

     In the standards of the IMT-2000 family, TD-SCDMA proposed by the Chinese Wireless Telecommunication Standard (CWTS) group has been considered as one of the three leading standards advocated CDMA, i.e. WCDMA, cdma2000 and TD-SCDMA. The proposal of TD-SCDMA using time division duplex (TDD) and the 2010-2025 MHz unpaired frequency band has introduced many advanced technologies (e.g. smart antenna, joint detection, synchronous code division multiple access and software radio). The advantages of TD-SCDMA can be categorized

     as

     1

     the uplink and downlink channels share the same frequency channel that is suitable for smart antenna, joint detection and CDMA techniques;

simplifying the hardware of system to lower the price

     and cost of equipments; capable of utilizing asymmetric frequency spectrum to increase the efficiency of frequency; and 1 compatible with the 2nd generation mobile cominunication systems. TD-SCDMA has shown its ability to boost traffic-carrying capacity by using the same frequency band for both uplink and downlink transmission paths. In addition, it can dynamically allocate base station resources for either one based on traffic conditions. TD-SCDMA also takes full advantage of the benefits provided by smart antennas and can simultaneously detect multiple parallel signals. Being the key technique of TD-SCDMA, smart antenna technology plays an important role in improving the performance of TD-SCDMA systems. The technology of smart antennas for mobile communications systems has received widely interest in the last couple of years [I], [2]. The principal reason for applying smart antennas is the possibility for a large increase in capacity due to controlling and reducing interferences. This can he accomplished through the use of narrow beams at the base station and as result the users separations at the space, however, base station antennas have up till now heen omni-directional or sectored. This can be regarded as a ??waste?? of power as most of it will be radiated in other directions than toward the user. In addition, other users will experience the power radiated in other directions as interference. Obviously, the base station equipped with smart antennas can cope with these problems.. Although some of smart antennas for experimental systems and commercial products currently available on the market have heen reported in the literature [3], [4], [SI, there are few reports on smart antenna used in TD-SCDMA base station. This paper focuses on a design of smart antenna used in TD-SCDMA base stations and presents a design method, also gives the simulating and experimental results. The remainder of this paper is organized as follows: in section 11, the principle of smart antenna is introduced. In section 111, a design method of the base station smart antenna used in TD-SCDMA systems is described in detail,

     1

     0-7003-8647-7/04/$20.0002004 IEEE.

     176

     in which the structure design of antenna elements and antenna array as well as theoretical analysis and experimental results are presented. In section VI, two experiments in antenna array are made to test the omni-directional radiation and beamforming performance, with the results of simulation and experiment given. Section V concludes this paper.

     11. SMART ANTENNA TECHNIQUE The theory behind smart antennas is not new, because similar techniques were already used in military radar systems. In the last couple of years the field of smart antenna

    technology is rapidly becoming one of the most promising areas of mobile communications, especially regarding the development of the first practical third generation mobile communication systems. Among those ways available to generate an adaptively adjustable antenna beam, the approach in which adopts array antenna is suggested for land-based mobile and personal communications systems. By maximizing the antenna gain in the desired direction and simultaneously placing minimal radiation pattern in the directions of the interferers, the quality of the communication link can be significantly improved [6]. A smart antenna consists of multiple radiating elements, a combining/dividing network and a control unit realized using digital signal processors (DSP), it combines antenna array with a signal processing capability to optimize reception and radiation patterns dynamically in response to the signal environment, i.e. mobile moving about the coverage area. It is obvious that a smart antenna base station system is much more complex that traditional omni-directional one because it must include very powerful numeric processors and beamforming control systems. Smart antenna systems are customarily categorized as either switched beam or adaptive array system. Switched beam antenna systems form multiple fixed beams with heightened sensitivity in particular directions. When a mobile user moves throughout the cell, the switched beam system monitors the signal strength and chooses fmm one of several predetermined, fixed beams, and switches fmm one beam to another. Switched beam systems offer many of advantages of more complex smart antenna systems at relatively low expense, however, several limitations such as incapacity for providing protection from multi-path components aniving near the desired signal and for taking advantage of path diversity by combining coherent multi-path components, as well as fluctuation of the received power level due to scalloping [l]. Adaptive arrays use sophisticated signal-processing algorithms to continuously distinguish between desired signals multi-path and interfering signals, as well as update its beam pattern. By combining adaptive digital signal processing with spatial processing techniques, adaptive array systems can achieve greater performance

     improvements than attainable using switched beam systems. Adaptive array technology represents the most advanced smart antenna approach to date. In this case, adaptive antennas are to be a type of antenna that can be adopted extensively for current and the future mobile communications systems. The array may consist of multiple antenna elements distributed in any desired pattern; however, the array is generally categorized 88 uniform linear array (ULA), uniform circular array (VCA) and uniform planar array (UPA) in terms of the means of deploying the elements. The number of elements determines the number of degrees of freedom that one has in designing array patterns, its

    typical value ranges fo four to sixteen. In general, the rm element spacing is set to half the wavelength of the carrier frequency for a ULA. When the element spacing is far apart, a large number of grating lobes appear. Presently, the ULA is the most commonly used antenna system for a sectorized cell system like the commercial cellular systems. However, in many omni-directional cell communication systems, interest in using the uniform circular array (UCA) has greatly increased. The use of a ULA as beamformer in a cellular system, especially in adaptive antenna array applications such as SDMA, is limited due to the scanning characteristics of the ULA geometry. Thus, with its much better geometry for cellular, the UCA is to a solution to this problem. In the following section, a design of a UCA used for TD-SCDMA system is to be discussed in detail.

     1 1 DESIGN SCHEME . 1. A . Antenna Element Design

     In the scheme presented in this paper, we design a novel coaxial collinear printed dipole structure with respect to the antenna elements used to compose the array antenna. Fig.1 shows the structure of the single antenna element, where the radiation element is a kind of planar dipoles. Based on the principle of sleeve dipole element, we change coaxial transmission line feed for microstrip feed, also substitute planar structure for solid form with respect to the dipole, thus, we can attain the planar dipole. The fact that the variations of antenna gain with azimuth is very small, with the width of printed circuit board far less than the wavelength of the signal is verified by means of experiments.

     feed port radist' n element /

     P-

     dielectri substrate

     2

     I '

     177.

     I-

     : :

     -, ,

     /

     Fig.1 The'struchlreof the singie antenna element

     ;BY doisittiring '&e ileiiric jfiq1g;in the pasiive region i'i' .. .. ..

     ~

     together with the boundary condition, we can derive the integral +equations of the current through the antenna element [7]. Furthermore, we can compute the current values of the coaxial collinear antenna through the use of the Method of Moment (MOM). Thereby, we can perform the optimization design in terms of the structure of antenna elements.

    The optimal objective involves the branches of element, the spacing between the branch and the coaxial feed line, the width of the coupling clearance. With practical application, antennas are frequently assembled at some high sites. It is necessary to make the beam pattern of the antenna tilt down. In the design, we make the maximal radiation direction tilt down with 7' by optimizing the spacing between the elements. Fig.2 shows the E-plane radiation patterns of the single antenna element accomplished by means of theoretical computation as well as experimental test. It can be seen in F i g 2 that experimental results are in accord with computation results, also the vertical downtilt of the radiation pattern is equal to approximately 7'.

     denoting the distance from the nth element to an arbitrary point in space, magnitude of exciting current, phase of exciting current, respectively; p is the propagation constant, equal to 2nl A ;1is the wavelength of radiated signal, r is the distance from the origin to an arbitrary observation point and x ( 0 , p ) i s the radiation pattern of the single element, it can be shown as

     where I is the length of element. The array factor of a UCA can be written as

     from Eq.s (2) and (3), the total field radiation pattern of the array is given by

     (4)

     - Theoretical computation

     270

     Fig.2 E-plane radiation pattern of single antenna element

     It is obvious that the array gain depends on the antenna element gain and the array factor. Once the antenna element gain determined, it is necessary to pay more attention to the array factor. According to the structure requirements, we optimize the array structure in terms of different N values, namely 8,lO and 16 in order to obtain the maximal gain of the array factor. Fig.3 shows the relationship of the UCA diameter and the array factor, where the optimal diameter of the UCA is 182mm with respect to N=8. The simulation results show that a diameter more than this optimal value will result in grating lobes appear. In addition, it can be verify the structure mentioned above agrees with the requirement of a fully populated array through the theoretical computation.

     B. Antenna Array Design

     A s . mentioned in section 11, in order to serve a ami-directional cell, the UCA should be a better solution. Especially, in a beamforming application, the directional patterns of a UCA can be electronically rotated throughout the azimuth without significant change in the beam shape, moreover, is less sensitive to the mutual coupling effects (compared with ULA and UPA) [XI. Therefore, we focus on the design of

    UCA. In the design, we use eight antenna elements to compose the array, which are arranged equally spaced in a horizontal circle of diameter D. According to the theory of antenna array, with respect to a one-layer UCA consisting o f N dipoles (in parallel with the Z-axis), by assuming a sinusoidal current distribution on the dipole, the radiate field can be expressed as

     a

     lfi;::

     UCA diameter (mm)

     e 4 x E 3

     2 120 140 160 180 200 220 240

     Fig.3 Relationship of UCA diameter and array factor

     where pn,I, ,{,are

     the nth element's vector potential

     C. Antenna Array Design Requirements The UCA design requirements are given by Operating frequency band 2010-2025MHz. Polarization: vertical. Omni-directional gain: 7.5dBi

     178

     ~

     I

     .

     Maximal gain: G,,.,1.>10.5dBi. Vertical downtilt: 6O-7'. Variation of gain with azimuth: < 2 IdB. Vertical beam width: 15". Number of element: N=X.

     'i RESULTS c7 z Iv. SIMULATION AND EXPERIMENT Ill, we Based on the scheme mentioned in section

     developed a UCA used for experiment. The performance of the UCA can be evaluated by the simulating and measuring experiments. All measurements have been conducted in free space. As to the simulation, we have set the simulation condition to be free space and supposed that all elements have the identical performance. Two examples are shown as follow.

     downtilt is insensitive to interaction between antenna elements. From the UCA's H-plane pattern in Fig.5, it can be seen that the variation of the UCA gain with azimuth is less than f IdB, also the omni-directional gain of the UCA exceeds 7.5dB. in both theoretical computation and experimental test. In practice, with respect to variation of gain with azimuth, the simulation result is only f 0.28dB, and the experimental result is k 0.85dB; as to omni-directional gain, 9.2dB and 8.ldB are given by the simulation and the experiment respectively. These results show that the designed UCA has a good ability of omni-directional radiation. The experimental results in Fig.4 and Fig.5 are in accord with the design requirements described in subsection C of section 111.

     A . Test of Omni-directional Radiation Performance

     Eight elements of the UCA are excited with equal amplitude and phase, on this condition, the omni-directional radiation performance should be obtained. Figs.4 and 5 present the E-plane and H-plane radiation pattern of the UCA respectively.

     ^^

     B. Test of Beamforming Performance

     Eight elements of the UCA are excited with equal amplitude and different phase shift. In this case, the beam can be controlled to point in the desired direction. We assign a number to each element of the UCA clockwise or anticlockwise so that each element has a serial number. The phase shift scheme is formed as no.1 and no.2 element with phase shift O", no.2 and 110.7 with phase shift 122", 110.3 and no.6 with phase shift -66.2", rio.4 and no.5 with phase shift 55.8". The beamforming H-plane patterns of the UCA in terms of simulation and experiment are shown in Fig.6.

     - Theorelical computation

     Experimental test

     90

     -Theoretical computation -Experimental test

     270

     Fig.4 E-plane radiation pattern of UCA

     Theoretical computation

     lxperimental test

     270

     . .

     ,

     Fig.6 Beamforming H-plane pattern of UCA The results indicate that we can steer the main beam of the UCA in the desired direction 222.5". and form four nulls in other different directions in the meantime. From Fig.6 we can see that the experimental result is approximately in consistent with the simulation computation. It shows the effectiveness and accuracy of the model made for antenna array simulation computation. It also shows the engineering feasibility of which the UCA designed in this paper has the beamfonning ability, namely, on the basis of the precision in baseband processing, the UCA can direct a main lobe toward the direction of a desired user with a side lobe or null directed toward a interferer, thus, can be used for smart antenna systems.

     01

     2 4 0 F

     Fig5 H-plane radiation pattern of UCA

     With the UCA's E-plane pattem in Fig.4, both simulation and experimental test results show the maximal gain of radiation pattern appears in the direction of 97O and 277O, which illustrates that the UCA has a vertical downtilt, equal to approximately 7'. This indicates

that the vertical

     179

     V. CONCLUSIONS

     In this paper,, a kind of smart antenna with UCA st~chq which can be used in the TD-SCDMA base station systems, is designed and implemented. Based on the theory of antenna array and smart antenna technology, we propose a UCA design scheme, also set up a simulation model with respect to the elements and m . y Combining with global optimization algorithm, we perform tho optimization design for the antenna element stmcti~re size , as well as the antenna m a y technical requirements. We develop a UCA used for smart antenna, furthermore, by a number of experimental tests, we evaluate the performance of the UCA. The performance of the developed UCA satisfies the design requirements.

     REFERENCES [I] J.H. Winters, ??Smart antennas for wireless systems??, IEEE Personal Com. Magazine, pp. 23 -27, Feb. 1998. 121 J.C. Liberti, T.S.Rappaport, Smart Anfennasf i r Wrelcss Communicafions: IS-95 and Thlrd-Generalion CDMA Applicafions,Englewood cliffs, NJ:Prentice-Hall, 1999. [3] J. H. Winters, ??Forward??Link Smart Antenna and Power. Control for IS - 136??. Fifth Stanford Workshop on Srnarf Anfennas in Mobile Wlreless Communlcations, July 23-24,

     1998. [4]?? ??F. Adachi. ??Application of Adaptive Antenna Armys to W-CDMA Mobile Radio I??, FUN, Stanford Workshop on Smarf Antennas in M b l Wfre/e.ss Communlcafions,July oie 23-24, 1998. [5] J. Strandell, M. Wennswom, A. Rydberg ,T. Oberg, 0.

     Gladh, ??Experimental Evaluation of an Adaptive Antenna iur TDMA Mobile Telephony System??, Personal Indoor and Mobile Radio Communicalions Conference 1997 (PIMRC??97),Helsinki, Sept. 1-4, 1997. [a] M. Chiyssomallis. ??Smart antennas??, IEEE Mag. on Antenna Propag.. vo1.42, pp. 129-136, Jun. 2000. [7] Akihide Sakitani. ShigeN Egashira, ??Analysis of Coaxial Collinear Antenna: Recurrence Formula of Voltages and Admittances at Connections??, IEEE %ns. on Anfenna Propag.,vol. 39, ,pp.l5-21,Jan. 1991. [SI K. Chang, RF and Micmwave Wrelus Sysfem,New York John Wiley, 2000.

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