WCDMA Outdoor Antenna Selection for Dense Urban Areas

IEEE defines "antenna" as "a part of the transmission or receiving system, designed for transmitting or receiving of
electro-magnetic waves", and a key component in the communications system. The antenna plays a very important role in improving the capacity and performance of mobile networks.

  The basic function of the antenna is the coupling of electro-magnetic energy between the free space and conductive equipment[1]. The Communications Antenna Industry Standard of China prescribes three categories of base station antennas in mobile communications system: omni antennas, directional monopole antennas, and directional 45° dual polarized antennas[2]. The main question for a network-planning engineer is how to choose electrical and engineering parameters of an antenna to achieve optimal network performance and to provide the best service for the users?

1 Antenna Selection
Interference is an important factor that inhibits system performance and capacity of a cell[3]. A network-planning engineer can control the interference by selecting the antenna electrical parameters and modifying antenna-related engineering parameters. For example, he can select the gain and radiation pattern, and modify parameters like height, azimuth and downtilt to directly influence the field intensity, distribution of radio signals and effectively and reasonably distribute the electro-magnetic energy. The difference in environments is embodied in the path loss, which is then embodied in the difference of transmission models. The 3GPP Hata model defines four radio environments: the densely populated urban area, ordinary urban area, suburban area and rural area. Here we only discuss antenna selection in the densely populated urban area, with the simulation platform of ZTE’s WCDMA planning system[4].

  (1) Selection of Antenna Horizontal Lobe Angle
  In the radio access system, cell sectorization is used to improve the utilization ratio and capacity of radio resources. Cell sectorization means using a directional antenna to cover only a part of a cell. A WCDMA system chooses the directional antenna to reduce co-channel interference and increase system capacity[5].

  Ideally, a sectorized cell can receive only 1/3 of original signals of a cell by using the 120° antennas, thus reducing 2/3 of the interference and doubling the capacity. In reality, the radiation of the antennas overlaps somewhat, so the interference can hardly be reduced by 2/3. During network planning, we can choose either omni antennas with the horizontal lobe width of 360° or directional antennas with the horizontal lobe 3 dB width of 20°, 30°, 65°, 90°, 105°, 120° and 180° and the gain ranging from 0 dBi to more than 20 dBi[2]. Then we move on to how to choose an antenna with an appropriate horizontal lobe angle to achive maximal RF channel capacity?

  As shown in Figure 1 and Figure 2, the simulation done by ZTE shows that regardless of downlink transmission power, or uplink noise rise, the antenna with 65°, 6.5° and 18 dB is the best. Therefore, it is recommended that an antenna with 65°, 6.5° and 18 dB be used in the densely-populated urban area, which also tallies with our engineering practices.

  (2) Selection of Antenna Downtilt
  Antenna downtilt refers to a certain angle implemented by declining an antenna to slope across its main lobe. It is used to reduce the power level arriving at the adjacent station, i.e. to reduce the interference. It is a main approach to control the cell coverage after an antenna’s electrical parameters are set. In practice, the antenna downtilt is directly related to the antenna height, coverage radius, vertical lobe and electrical downtilt. Considering the coverage radius is fixed, the higher an antenna, the larger the downtilt. However, if we consider antenna height as fixed, the smaller the coverage radius, the larger the antenna downtilt.

  In antenna installation, the fasteners of an antenna can support a mechanical downtilt of about 14°. In practice, when the mechanical downtilt exceeds 8°, the main lobe of the antenna will distort；when it exceeds 10°, the distortion is rather severe. Therefore, it is better for the mechanical downtilt to be less than 10°. In a densely-populated urban area with heavy traffic, the Node Bs are at a distance of around 400-800 m apart, and we have to choose antennas with adjustable electrical downtilt or antennas whose fixed electrical downtilt is more than 6°. The horizontal half-power width won’t change abnormally when the main lobe downtilt is within 10°-20°, so it can meet the requirement of Node B radius control in the densely populated urban area with heavy traffic.

  The simulation model designed by ZTE helps analyze the relation between the cell capacity and antenna downtilt (shown in Figure 3 and Figure 4), and the simulation results can tell us how the downtilt affects the cell capacity, and further help set the antenna downtilt according to special requirements in network planning.

  The above two figures show the system capacity and pilot coverage in a cell with a radius of around 700 m in a densely-populated urban area when an antenna with 65°, 6.5° and 18 dB is used. We can see the performance is best when the downtilt is from 5° to 8°. If the downtilt is not set optimally, the capacity loss can be as high as 40%, and the pilot coverage loss can be as high as around 30%. For the system as a whole, if the downtilt is adjusted too much, blind areas are easy to emerge. However, when the adjustment is too small, there will be pilot pollution. Hence the adjustment of the antenna lobe angle given in this essay is a good and practical reference.
Our engineering practice has proved that the simulation results tally with the antenna downtilt adjustment in the actual network.

  (3) Diversity Receiving
  In mobile communications, multipath transmission causes the fast fading of a signal, and the fading level can reach 30 dB[3]. The technology of antenna diversity can greatly decrease the fading of a received signal, and increase the link quality and system gain. The diversity modes include spatial diversity and polarized diversity. Their common principle is to ensure the uncorrelated or quasi-uncorrelated fading of the different branches of an antenna. The diversity antenna first separates irrelevant multipath signals. The signal independence is measured with the correlation coefficient of the branch signals, and the correlation coefficient of received signals should be smaller than 0.7. Then the separated signals are combined by the combining technology to achieve the maximal Signal/Noise ratio[5]. The often-used combination methods include selective combination, handover combination, maximal ratio combination, equal gain combination, etc.

  (4) Selection of Monopole Antenna Diversity Distance 
  The Node B requires a horizontal spatial diversity distance of 20 ?姿 and vertical spatial diversity distance of around 15 ?姿. When the distances between the Node B antennas are constant, increasing the antenna height may reduce the correlation between the received signals of different antennas. The diversity gain of horizontal spatial diversity is around 3-5 dB, and the gain of vertical spatial diversity is around 2-4 dB. The performance of horizontal spatial diversity is better than that of vertical spatial diversity. Table 1 shows the minimal and recommended distances of both horizontal spatial diversity and vertical spatial diversity in practical engineering. In practice, the diversity distance of two monopole antennas in the same sector is no less than 10 ?姿[3,5].
 

  (5) Dual-Polarized Antenna Diversity
  In the urban area, there is not much free space for the antenna, and it is difficult to install an antenna according to the required spatial diversity distance. Therefore, polarized diversity becomes an important choice.

  There are two kinds of dual-polarization antennas: vertical/horizontal polarized antennas and ±45° polarized antennas. When the signal with horizontal polarized transmission is near to the ground, it will generate polarized current on the ground surface, and the signal fades rapidly because of the energy loss. However, the vertical polarization hardly generates the polarized current. The ±45° polarized antenna performs better than the vertical/ horizontal polarized antenna. Currently, we only use the ±45° polarized antenna. Such a dual-polarized antenna consists of one +45° and one -45° antennas that work in the duplex mode at the same time, which greatly reduces the number of antennas used in each cell. Besides, the orthogonal polarization ensures a good effect of diversity reception. Compared with the spatial diversity antenna, the dual-polarization antenna has some performance loss in the downlink, but in the uplink, it has better correlation statistics and equivalent or even higher diversity gain, when the mobile phone antenna is inclined.

2 Conclusions
In an urban network, the ±45° dual-polarized antenna and spatial diversity antenna can provide the same gain, and occupy smaller space, when the mobile phone antenna is transmitting with 45° inclination.

  In an urban network, when the Node B distance is definite, there is an optimal downtilt from 5° to 8°. The antenna downtilt has almost the same influence on the number of uplink and downlink subscribers, which is shown in Figure 3. It should be noted that the capacity limit will be different after the "inflexion".

  When the Node B distance is relatively small or the antenna is relatively high, the downtilt must be large enough to reduce adjacent cell interference, and to reduce the soft handover rate. If the downtilt is too large, the antenna with the electrical downtilt can be chosen.

References
[1] YD26-89 Microwave Station Anti-lightening and Grounding Design Specification[S].
[2] YD/T1059-2000 Technical Conditions of Base Station Antennas in Mobile Communications System[S].
[3] Lee Jhong Sam, Miller Leonard E. CDMA System Engineering Manual[M]. Translated by Xu Xibin. Beijing: People’s Posts and Telecommunications Press, 2001.
[4] 3GPP TR25.942 RF System Scenarios[S].
[5] Kim Kyong II. CDMA System Design and Optimization[M]. Translated by Liu Xiaoyu. Beijing: People’s Posts and Telecommunications Press, 2000.

Manuscript received: 2004-12-10