Since an antenna is passive, the only way to obtain gain in any direction is to increase the directivity by concentrating the radiation in the wanted direction. For a loss free antenna the directivity can be given with the same number as the gain if the latter is given with respect to an isotropic antenna. Hence, in this chapter the distinction between gain and directivity is not always strictly maintained. The directivity can be increased by reflectors or by stacking dipoles on the same vertical line. The latter method can be used because a number of coherent radiation sources interfere constructively (in directions where they radiate in phase) and destructively (in directions where they are in “anti phase” and more or less cancel each other out). Each doubling of the number of dipole elements (corresponding to a doubling in length) increases the gain in the main direction by 3 dB. Figure 1 shows some different antenna arrays. The gain is different in different directions. However, when the antenna gain is quoted it is usually given for the direction of maximum radiation.
Figure 1 Antenna Arrays
Since the concentration of radiation is inversely proportional to the solid angle of the beam, the gain can be estimated if the beamwidths are known:
G = 10 x log 31000/(V3 x H3)
G = Antenna gain relative isotropic antenna (dBi)
V3 = Vertical beamwidth relative -3 dB points (degree centigrades)
H3 = Horizontal beamwidth relative -3 dB points (degree centigrades)
BEAMWIDTH
Vertical Beamwidth
Since the concentration of radiation is proportional to L/l, the vertical beamwidth decreases as the gain increases. The vertical beamwidth can be estimated if the length of the antenna is known:
The vertical beam of an antenna is normally directed towards the horizon, assuming the antenna is correctly mounted. Lowering the beam below the horizon is known as “down tilt” (Figure 2). Consequently, if the beam is directed above the horizon, “up tilt” is achieved. Below is a description of the methods used to achieve down tilt and a discussion on how down tilt can improve the performance of a system. Up tilt will not be discussed further.
Figure 2 Antenna Down Tilt (Basic Geometry)
ELECTRICAL TILT
Electrical down tilt requires an antenna with a number of vertically stacked dipoles. (Here, the word “dipole” represents other radiating elements as well.) The individual dipoles can be oriented vertically, which is the most common orientation in cellular systems. They can also be oriented horizontally or at a slant (±45°) position. If all dipoles are fed with the same phase, the main beam of the vertical pattern will be perpendicular to the mechanical axis of the antenna (towards the horizon). A phase difference between the dipoles will result in a beam that deviates from the horizontal. Different tilt angles are available, depending on the antenna manufacturer. Typical values are 2° and 6°. An advantage of using electrical tilt is that the antenna is always mounted in a vertical position irrespective of tilt. A disadvantage is that the antennas must be ordered with a certain tilt angle. (Antennas with adjustable electrical tilt are available on the market to avoid the disadvantage of fixed tilt values. The antennas have a limited gain and are expensive.)
MECHANICAL TILT
Mechanical tilt is achieved by changing the mechanical alignment of the antenna. All antenna manufacturers have adjustable brackets designed for this purpose. It is possible to combine the electrical and mechanical methods.
CELL PLANNING ASPECTS ON DOWN TILT
Down tilt can be used to overcome coverage and/or interference problems. To be able to discuss down tilt from a general point of view, some special applications must be excluded, i.e. antennas on extreme hill tops, the “Manhattan syndrome”, etc. In these cases, tilt can always be motivated. As a general rule, to reduce co-channel interference, three criteria must be fulfilled:
1. Short site-to-site distances (small cells)
2. High mounted antennas
3. High gain antennas (narrow vertical beam)
Figure 3 Schematic of a Regular Network With Site to Site Distance of 1 km α=1° β=2°
Let us start with a case based on medium values (Figure 3). Site-to-site distance: 1 km; antenna height: 25 m; and an antenna with 14° vertical beamwidth (approximately at the -3 dB point). As a starting point, let us reduce the signals from the interfering site (Alpha) towards the interfered site (Bravo) by 7 dB. The diagram in Figure 3-4 shows that a tilt of 10° is needed to achieve a reduction of 7 dB towards the horizon. However, to reduce the signal by 7 dB at the cell border, a tilt of 11° is needed (10 + 1°) since the angle ( a) towards the cell border is 1°. Note that the gain reduction at the cell border for no tilt is almost zero.
We started with site Alpha which is a potential interferer to site Bravo. As we down tilted Alpha by 11°, the interference situation in site Bravo is improved by 7 dB. But if the network is regular (in a reasonable sense) site Bravo is also an interfering site to site Cairo. Now we have to down tilt Bravo as well with the same values as Alpha and the result in a regular network is that almost all sites must be down tilted The next step is to see what happens in the own site area when the antenna is down tilted. The angle (b ) between the horizontal and a mobile on street level on the cell border is 2° (Figure 3). It is obvious that the mean vertical beam is pointing somewhere inside the cell border. 11° corresponds to a distance of 129 m. From the same antenna diagram, it can be seen that the signals at the cell border are reduced by 5 dB, found in the diagram at 11 - 2° = 9°. Note that the gain reduction at the cell border for no tilt is almost zero.
The net result regarding C/I increase is only 2 dB -- at the expense of 5 dB coverage loss!
Figure 4 Typical gain reductions as a function of tilt angle for three different antennas (beamwidths are 7, 14, and 28 degrees)
Note that the figures are not drawn to scale (i.e. that the horizontal scale is different from the vertical scale). It is common to make figures in this way but it can be misleading. It is obvious that the calculations so far are based on a network in open terrain as no obstacles can be seen between the base station and the mobile. A more realistic case with respect to co-channel interference problems is in urban or suburban areas with buildings in-between (Figure 5).
Figure 5 This figure illustrates the fact that there is seldom line-of-sight between two antennas in an urban environment
It is unlikely that the radio signals follow the direct line between the base station antenna and the mobile, passing all the buildings in-between. It is more realistic to see the signals coming from (by reflection and diffraction) the roof tops down to the street. The angle to the cell border can then be calculated from the base station antenna height above roof tops (e.g. 5 m). Assuming a site-to-site distance of 1 km, it is an angle of 0.4° to the cell border. The conclusion is: Signals from the interfering site and the interfered site arrive at the cell border with a very small difference in the vertical angle - regardless of how much down tilt is applied. However, down tilting means that less radiation is transmitted across the roof tops and the coverage might decrease.
Returning to the three requirements in this section:
1. Short site-to-site distances (small cells)
2. High mounted antennas
3. High gain antennas (narrow vertical beam)
It can be seen that the first requirement (small cells) gives the possibility to achieve a difference in the two vertical angles towards the roof tops on the cell border and towards the roof tops on the interfered site. The second requirement helps to increase that difference. Finally, with a narrow vertical beam, a C/I increase by 2-3 dB is possible if 1° angle difference can be achieved (e.g. by mounting the antennas 20 m above the roof tops) and that not more than 5 dB coverage reduction is acceptable. For example, if a 7° antenna is tilted 5°, the gain reduction towards the roof tops for the interfered site is 3 dB (found at 5° - 2° = 3° in Figure 4); whereas for the interference, it is 5 dB (found at 5° - 1° = 4°) i.e. a 2 dB increase in C/I.
NULL FILL-IN
As previously mentioned, antenna gain is different in different directions (Figure 6). This means that areas at a certain distance (depending on the antenna height) from the antenna will be radiated by the first null rather than the main direction. Hence, the signal level will not decrease monotonically as the distance between the transmitting antenna and the receivers increases, but more as it is illustrated in Figure 7 and Figure 8. For parallel-fed collinear arrays, it is possible to reduce the gain reduction in the direction of the first null by simply adjusting the power fed to the different antenna elements slightly. This gives a small reduction in gain in the main direction but this is compensated for by much more predictable signal strengths in areas closer to the transmitting antenna.
Figure 6 Gain Reduction as a Function of Vertical Angle
Figure 7 High gain antenna at 25 m height
Figure 8 High gain antenna at 75 m height
DIVERSITY
There is a need for receiver diversity in cellular systems to improve the uplink. Space diversity is the conventional method used where the two RX antennas are separated by a certain distance. Based on experience from measurements and simulations (and because of installation advantages) polarization diversity is used in standard configurations. The signals from the two RX antennas are later combined in the base station. The result is an increase in signal strength of three to six dB. (The exact value depends on the similarity between the signals received from the two antennas where the two receiving antennas are separated by 90 degrees in the polarization plane.)
SPACE DIVERSITY
Figure 9 shows a traditional configuration with space diversity. The horizontal space needed for the antennas is dependent on the required diversity separation.
Figure 9 Antenna configuration with space diversity
POLARIZATION DIVERSITY
A dual-polarized antenna is an antenna device with two arrays within the same physical unit. The two arrays can be designed and oriented in different ways as long as the two polarization planes have equal performance with respect to gain and radiation patterns.
Figure 10 Dual polarized antennas
The two most common types are vertical/horizontal arrays and arrays in +/-45 degree slant orientation (Figure 10). The two arrays are connected to the respective RX branches in the BTS.
The two arrays can be used as combined TX/RX antennas (Figure 11) and then the number of antenna units is reduced compared with space diversity. The use of a duplex filter reduces the number of antenna units to only one per cell depending on configuration.
Figure 11 Antenna configuration with polarization diversity
The diversity gain obtained from polarization diversity is slightly less then the gain from space diversity. In the most critical environments (such as indoors and inside a car) the gain is, however, almost as good as if space diversity were used. A dual polarized antenna offers very low correlation between the two received signals, but the power reception of each branch is slightly better with space diversity. This implies a small benefit for space diversity in noise-limited environments. For most applications, the difference is negligible. In interference limited environments on the other hand, the low correlation obtained by polarization diversity is advantageous. Due to slightly different propagation characteristics for different kinds of polarization, the downlink from a +/-45 degree dual polarized antenna suffers from about 1.5 dB extra loss compared to a vertically polarized antenna. This loss only affects the downlink.
The isolation between the two polarization planes needs to be 30 dB. The size of the antenna must remain small, as the intention with polarization diversity is to reduce the outlook of the antenna installation.
INTERMODULATION (IM)
When two signals of a different frequency mix in a non-linear device, the result is InterModulation (IM). The non-linear devices can be, e.g. antennas, combiners, connectors, and duplex filters. IM can be a problem at any site that has two or more transmitters. The IM problems can be caused by a transmitter in the same system or by a transmitter in another system that is cosited or has a site in the neighborhood. Finding the intermodulation source can be time-consuming since the problem is often intermittent.
Second order products are given by the formula f1 +/- f2 . Both these frequencies are outside the receiver passband. In fact, all the even-order products will be well outside the receiver passband. Third order products are given by the formulae 2 x f1 - f2 and 2 x f2 - f1. These frequencies fall inside the band. All oddorder products can cause problems (Figure 12). However, higher order products (usually 7th order and higher) decrease rapidly in power and therefore do not cause any problems.
Figure 12 Intermodulation in 900 MHz cellular system
The allocated frequency band and the duplex distance are what determines if the IM will cause problems (Table 1 and Table 2). IM3 products are strong enough to degrade the receiver sensitivity even though there is no combining; just backwards coupling from one antenna to the other. IM5 is a problem if the frequencies are combined before entering the duplex filter.
Table 1 The maximum band (B) to avoid intermodulation. Based on worst case scenario, which is IM from the lowest and the highest frequencies in the allocated band
Table 2 Worst case relations for IM in the RX band. D= Duplex distance (MHz), B = allocated band (MHz)
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