1.1.1 Antenna Types

Many different types and mechanical forms of antennas exist. Each is specifically designed for special needs.

In mobile communications the two main categories to consider are:

omnidirectional antennas: radiate with same intensity to all directions (in azimuth)

directional antennas: main radiation energy is concentrated to certain directions

Omnidirectional antennas are useful in rural areas, while directional beam antennas are preferable in urban areas. They provide a more controllable signal distribution and energy concentration.

The most common antenna types are:

Dipoles: the basic antenna type. Simple design, low gain, omnidirectional radiation pattern.

Arrays: combination of many elementary arrays. High achievable gains, special radiation pattern can be engineered. Active arrays use many actively fed dipole elements. Passive arrays merely use the reflecting properties of array elements.

Yagi antenna: Very popular passive array antenna. Widespread use as TV-reception antenna. Very high gain and good directional effects.

Parabolic antenna: Used for microwave links, optical antennas and satellite links. Very high gains and extremely narrow beamwidth. Most commonly used for line-of-sight propagation paths. (satellites, microwave links)

1.1.2 Antenna Characteristics

Antennas can be characterised with a number of attributes:

Radiation pattern: the main characteristic of antennas is the radiation pattern. The horizontal pattern (“H-plane”) describes azimuth distribution of radiated energy. The vertical pattern (“E-plane”) describes the energy distribution in elevation angle.

Figure 1. Horizontal and vertical antenna radiation patterns

Antenna gain is a measure for the antenna’s efficiency. Reference antenna configuration to compare with is by convention the isotropic antenna. Gain is measured usually in “decibel above isotropic” (dBi) or in “decibel above Hertz dipole” (dBd). Hertz dipole has a gain of 2.2 dB compared to the isotropic antenna, therefore dBd + 2.2 = dBi. Antenna gain depends on the mechanical size of the antenna, the effective aperture area, the frequency band and the antenna configuration. Antennas for GSM1800 can achieve some 5...6 dB more antenna gain than antennas for GSM900 while maintaining the same mechanical size. Antenna gain can be estimated by the formula:

G A w4

2

where A is the mechanical size and w the effective antenna aperture area.

Note

Catalogues usually show dBi values, since they are higher numerical values and therefore look more impressive...

Antenna lobes: main lobe, side-lobes; ratio of main lobe to max. side lobe is a measure for quality of radiation pattern

Half-power beamwidth: 3-dB beamwidth; the angle (in both azimuth and elevation plane), at which the radiated power has decreased by 3 dB with respect to the main lobe. Narrow angles mean good focusing of radiated power (= larger communication distances possible)

Antenna downtilt (mechanical or electrical): directional antennas may be tilted either mechanically or electrically in order to lower the main radiation lobe.

By downtilting the antenna radiation pattern, field strength levels from this antenna at larger distances can be reduced substantially. Therefore antenna downtilting reduces interference to neighbouring cells while improving spot coverage also. Two types of downtilting exist:

Mechanical downtilting means that the antenna is pointed towards the ground in the main beam direction. At the same time the back lobe is uptilted.

Electrical downtilting has the advantage that the antenna pattern is shaped so that the main beam and the back lobe are downtilted. In order to be able to control the interference situation it is better to use electrical down tilting.

With omnidirectional antennas, mechanical downtilting is not applicable, but only electrical. Electrical downtilting is performed by internal slight phase shifts in the feeder signals to the elementary dipoles of the antenna system.

Figure 2. Radiation pattern of an antenna with electrical downtilt

5..8 deg

Figure 3. Mechanical downtilting

Polarisation: polarisation plane is the propagation plane of the electrical field vector (by definition). Antennas are usually vertically polarised. Cross-polarised antennas achieve some dB gain in signal quality in environments where the radio wave is subjected to polarisation shifts, e.g. by multipath propagation and reflection on dielectric materials.

Antenna bandwidth: defined as the bandwidth, within which the VSWR (Voltage Standing Wave ratio) is less than 1:2. Typical values for antenna bandwidths are approx. 10% of the operating frequency.

Antenna impedance: maximum power coupling into antennas can be achieved when the antenna impedance matches the cable’s impedance. Antenna impedance depends on the design used. Impedance can be trimmed to practically any value by micro strip stubs, coils and capacitors. This is done by the antenna supplier and not relevant to the network planner. Typical value is 50 Ohm.

Mechanical size: mechanical size is related to achievable antenna gain. Large antennas provide higher gains, but also need more care in deployment (optical impact!) and apply higher torque to the antenna mast (static). Wind load and icing of antennas in winter may cause static problems to the mast. Usual values for wind velocities are assumed at 150 km/h or 200 km/h.

1.1.3 Coupling Between Antennas

Antenna radiation pattern will become superimposed when distance between antennas becomes too small. This means the other antenna will mutually influence the individual antenna patterns.

As a rule of thumb, 5 ..10horizontal separation provides sufficient decoupling of antenna patterns. The exact distance needed depends on the individual radiation patterns.

As vertical radiation patterns often have very much narrower half-power beamwidth, the vertical distance needed for decoupling is also much smaller. As the rule of thumb, 1 vertical separation is sufficient in very most cases.

main lobe

5 .. 10

1

Figure 4. Horizontal and vertical separation

1.1.4 Installation Examples

Antenna installation configurations depend on the operator’s preferences, if any. It is important to keep sufficient decoupling distances between antennas. If TX and RX direction use separated antennas, it is advisable to keep a horizontal separation between the antennas in order to reduce the TX signal power at the RX input stages.

• Recommended decouplingTX - TX: ~20dBTX - RX: ~40dB

• Horizontal decoupling distance depends on

antenna gain

horizontal rad. pattern

• Omnidirectional antennasRX + TX with vertical separation (“Bajonett”)RX, RX div. , TX with vertical separation (“fork”)

Vertical decoupling is much more effective

0,2m

omnidirectional.: 5 .. 20mdirectional : 1 ... 3m

Figure 5. Antenna coupling

Figure 6. Antenna installation examples

1.1.5 Nearby Obstacles Requirement

Nearby obstacles are those reflecting or shadowing materials that can obstruct the radio beam both in horizontal and vertical planes. When mounting the antenna system on a roof top, the dominating obstacle in the vertical plane is the roof edge itself and in the horizontal plane, obstacles further away, e.g. surrounding buildings, can act as reflecting or shadowing material.

It is possible that the antenna beam will be distorted if the antenna is too close to the roof. In other words, the antenna must be mounted at a minimum height above the rooftop or other obstacles. As a practical planning / installation rule, the first Fresnel zone (vertical plane) must be kept clear. The clearance is between the bottom of the antenna and the most dominant obstacles. As a rule of thumb, in the horizontal plane the 3dB beamwidth must be clear within 150m.

Figure 7. Required height clearance from the antenna to the edge of the rooftop

Figure 8. Antenna tilting near an edge of the rooftop

Antenna downtilt affects previous results. The following graph shows how the clearance requirement changes when antenna downtilt varies from 0 to 6 degree.

Figure 9. Height clearance versus antenna tilt

If antennas are wall mounted, a safety margin of 15 between the reflecting surface and the 3-dB lobe should be guaranteed, see Figure 10.

Figure 10. Horizontal clearance

1.2 Diversity Techniques

Diversity techniques are based on the fact that receiving multiple uncorrelated copies of the same signal, at the same or delayed time, can reduce fast fading dips. When two received signals are combined, the achieved signal quality is better than either of the partial signals separately.

There are different diversity reception schemes (see Figure 11): both the base station and the mobile station implement time diversity already by interleaving. Frequency diversity can be achieved with frequency hopping: since fast fading is frequency dependent, many frequencies are quickly and cyclically hopped so that if one frequency is in a fading dip, it is just for a very brief time. Traditionally two base station receiver antennas have been separated horizontally (usually) or vertically (seldom) to create space diversity. In urban environment, the same diversity gain can be achieved by using polarisation diversity: signals are received using two orthogonal polarisations at the reception end.

In the mobile radio channel multipath propagation is present. The delayed and attenuated signal copies can be combined in a proper way to increase the level of the received signal (multipath diversity). In GSM it is performed by an equaliser, while in W-CDMA (Wideband-CDMA) a so called "rake receiver" is utilized.

• Time diversity

• Frequency diversity

• Space diversity

• Polarisation diversity

• Multipath diversity

Transmit the same signal at leasttwice (with time delay t)

Transmit the same signal on at leasttwo different frequency bands

multiple antennas

crosspolar antennas

equaliser, rake receiver

t

f

Figure 11. Diversity techniques

The most used methods in cellular network planning are space and polarisation diversity, as far as base station antennas are concerned.

1.2.1 Space Diversity

Space diversity is a traditional diversity method, especially used in macrocells. Spatial antenna array separation causes different multipath lengths between a mobile station and a base station. Partial signals arrive at the receiving end in different phases. The two antenna arrays must be separated horizontally in order to achieve uncorrelated signals. Space diversity performs very well with macrocells in all environments, giving diversity gain of about 4-5 dB.

In microcells, the large antenna configurations are not often possible due to site acquisition and environmental reasons. Antennas must be small and easily hidden. The amount of physical antenna equipment must be minimised. Antennas are often placed on lampposts or other existing structures, in which spatial separation is not possible. On the other hand, arranging the antenna arrays within one physical antenna doesn’t provide big enough separation between the arrays. Therefore other means of providing diversity is required in urban microcellular environment.

1.2.2 Polarisation Diversity

Uncorrelated signals can be provided without physical separation by applying different orthogonal linear polarisation at the receiving end. Signals can be received using for example horizontal and vertical or 45 slanted polarisation in cross-polarised antennas. The performance of polarisation diversity technique depends on the environment and the reflections between mobile station and base station. The more the partial signals reflect and diffract along the route, the more uncorrelated the signals are at the receiver, and the more gain can be achieved.

The polarisation diversity gain can be measured as improved bit error rate (BER) or frame erasure rate (FER) at the receiver. In very dense urban areas, where narrow streets and high buildings surround the site, more than 5 dB diversity gain – equal to that of space diversity – has been measured. On the other hand, in the open areas and LOS situations, signal does not reflect enough on the way and cross-polarisation would not give any additional gain. This must be taken into account as slightly decreased signal quality with low field strength levels. Since cross-polarised antennas are small and suitable for urban areas, cross-polarisation diversity is the preferred diversity method for microcells.

1.2.3 Combining

Two main combining methods are used to take advantage of the signals in space or polarisation diversity:

Selection combining: every antenna signal branch is demodulated, C/I and bit error rates (BER) are calculated and then all signal branches are sampled at regular time intervals, always the best signal branch is selected for further processing. This method passes only a single branch and rejects all other signals.

Maximal ratio combining: antenna signals are individually amplified at the same amplitudes, the signal phasing is assessed. Signal samples are added (vector addition) with correct phase adjustments. Then the combined signal is demodulated and further processed. This diversity method achieves a C/I improvement due to the fact that the wanted information (carrier signal) from different antenna branches are strongly correlated, while the additive noise components are uncorrelated (assuming white Gaussian noise process). In the superposition of both signals the wanted components will constructively add, while the noise components eliminate each other. (Note: If antennas are not sufficiently separated from each other, also the noise processes of both antennas will be correlated and the C/I improvement therefore decreases to zero.)

1.2.4 Coverage Improvement by Diversity?

In link budget calculations, antenna diversity brings a signal improvement of ~ 5 dB. Note that this is not a physical improvement, i.e. a signal that is stronger by 5 dB (physically impossible), but rather an equivalent gain. The improvement in signal quality, i.e. in bit error rate, is the same as could be expected by a signal stronger by 5 dB. It is an “indirect gain”. This higher equivalent gain allows for a higher tolerable path loss, i.e. a larger communication range.

One supplier company claims that by 3 dB more allowable path loss they could provide 20% more coverage range, i.e. 40% more coverage area per cell. Conclusion was, that therefore they need 40% less base stations to cover the same area size. This cunningly simple calculation is also stunningly wrong. It would be in theory true if the environment were infinitely large and flat, if there were exactly zero overlap between cells and the cells were placed exactly regularly and there were absolutely no obstacles within the entire area. This obviously is not the case in real life.

• Diversity gain depends on environment

• Is there coverage improvement by diversity ?antenna diversity

equivalent to 5dB more signal strength

more path loss acceptable in link budget

higher coverage range

R

R(div) ~ 1,3 R

A 1,7 A ??70% more coverage per cell ??needs less cells in total ??

True only (in theory)if environment is infinitely large and flat

Figure 12. Diversity gain is equivalent gain

1.3 Antenna Cables

Coaxial cables of different diameters are usually used to transport the RF signal from the RX/TX units of the BTS to the antenna itself. Distances are typically in the range 10..50 m. Thin coax feeder cables are easier to install (bending radii!), but also cause higher losses per distance unit. Connectors, material ageing, jumper cables etc. cause additional losses to the most valuable RF signal. Typical values are 10 dB/100m for thin cables and 4 dB/100m for thick coax cables.

Typical values for cable losses between BTS and antenna are 3..5 dB. This means that some 50...70% of total signal energy is lost even before it arrives at the transmitting antenna or the receiver unit! Antenna cables shall therefore be kept as short as possible.

• Cable typescoaxial cables : 1/2”, 7/8”, 1 5/8”losses approx. 10 .. 4 dB/ 100m

==> power dissipation is exponential withcable length ! !

• Connector losses approx. 1 dB per connection(jumper cables etc..)

• Thick antenna cables lower losses per length

large bending radiimuch more expensive

jumper(2 m)

40 .

. 70

m

jumper(2 m)

Keep antenna cables short