2. 2
Outline
Base station antenna
specification and
meanings
Antenna types and
trends
Antenna Type And
Developments
Other Elements
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4. 4
Antennas
Categories
Omnidirectional antennas
radiation patterns is constant in the
horizontal plain
useful in flat rural areas
Directional antennas
concentrate main energy into certain
direction
larger communication range
useful in cities, urban areas, sectorised
sites
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6. 6
Antennas - Antenna Gain
Measures the antenna´s capability to transmit/extract energy
to/from the propagation medium (air)
dB over isotropic antenna (dBi)
dB over dipole (dBd)
Antenna gain depends on
mechanical size: A
effective antenna aperture area: w
frequency band
Antenna Gain:
G A w=
4
2
π
λ
Pt
Gain
(Dbi)
Isotropic radiated Power
Equivalent isotropic
radiated power:
EIRP = Pt+Gain(Dbi)
radiated
power
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11. 11
1 dipole (received power) 1mW
Multiple dipole matrix
Received power 4 mW
GAIN = 10log(4mW/1mW) = 6dBd
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12. 12
Gain = 10log(8mW/1mW) = 9dBi
“Sector antenna”
Received power 8mW
“Omnidirectional array”
Received power 1mW
(Overlook
Antenna
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13. 13
GSM 900 : 890-960MHz
GSM 1800 : 1710-1880MHz
GSM dual band : 890-960MHz &
1710-1880MHz
eg.824-960MHz 1710-1900MHz
CDMA2000 1x
Frequency Range
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31. 31
Ratio of maximum
mainlobe to maximum
sidelobe
F/B = 10 log(FP/BP) typically 25dB
Back power Front power
Front to back ratio
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36. 36
IMD@2×43dBm
f1, f2, 2f1-f2, 2f2-f1
913MHz,936MHz,959MHz,982MHz
Third Order Intermodulation
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37. 37
Intermodulation
Intermodulasi
Terjadi akibat penguatan
sistem yang non linier
Hanya orde ke-3 dan
kadang-kadang orde ke-5
yang signifikan
Sinyal dengan amplituda
yang sama menghasilkan
level IM yang sama pada
frek tinggi dan rendah
Sinyal dengan amplituda
berbeda memberikan
level IM yang berbeda
pula
Untuk mencegah
intermodulasi,penguat
dioperasikan pada
penguatan bukan-
maksimum
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38. 38
Intermodulation
Intermodulasi
Komp. Orde 1 :
diharapkan linier
Komp. Orde 2 : frek 2ω
diredam oleh filter
Komp. Orde 3 : frek 3ω
diredam dengan
filter
Penguat
Non-linier
( )
( )tB
tAv
B
Ai
ω
ω
cos
cos
+
=
++
+=
3
2
i
iio
cv
bvavv
Yang bermasalah :
Komponen yang lain
amplituda kecil
( ) ( )ABBA ωωωω −− 2,2
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40. 40
10 Simple Guidelines for RF Safety
All personnel should have EME awareness training
All personnel entering the site must be authorized
Obey all posted signs
Assume all antennas are active
Before working on antennas, notify owners and disable appropriate
transmitters
Maintain minimum 3 feet clearance from all antennas
Do not step in front of antennas
Use personal RF monitors while working near antennas
Never operate transmitters without shields during normal operation
Do not operate base station antennas inside equipment rooms
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45. 45
Antenna Types
By frequency band: GSM900, GSM1800, GSM900/1800
By polarization: Vertical, Horizontal, ±45º linear
polarization, circle polarization
By pattern: Omni-directional, directional
By down-tilt: Non, mechanical, electronic
adjustment, remote control
By function: Transmission, receiving, transceiving
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50. 50
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 the
environment is infinitely large and
flat
Diversity
Coverage Improvement?
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51. 51
Network Elements
MHA
MastHead Amplifier
(Low Noise
Amplifier)
RX signal amplified
near the antenna in the
top of the mast
Offers better coverage
Eliminates the antenna
cable loss
Increased receiver
sensitivity of the BTS
and cell size
Increased network
quality
Noise Figure £ 2.0 dB (typical)
RX Gain: Up to 12 dB
Dimensions : 266 x 130 x 123 mm
Weight : 5.6 kg (duplexed)
Volume : 4.2 l
IP 65 Enclosure Protection
Power Feeding Through Antenna Coax
Alarms handled in BTS
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52. 52
Booster
• TX signal amplified
• Nokia Booster Configuration
• Booster (PA) Unit (TBU)
• Booster Filtering Unit (AFH)
• Masthead Preamplifier equipment (MHA)
• Output power before combining can be up to 49 dBm
Isolator + combiner + filter (AFH) give roughly 2.5 dB losses
Booster BTS is suitable for all the environments where
enhanced coverage or high output power is needed
Theoretically, cell radius is enhanced up to 60% and the
coverage area is roughly the triple
Network Elements
Booster
TRXTBUAFH
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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.
10
11
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 17): 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. The most used methods in cellular network planning are space and polarisation diversity, as far as base station antennas are concerned. 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. 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. 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.)
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.