531 lines
20 KiB
Plaintext
531 lines
20 KiB
Plaintext
// original version part of osmo-gsm-manuals.git
|
|
|
|
== Introduction into RF Electronics
|
|
|
|
Setup and Operation of a GSM network is not only about the configuration
|
|
and system administration on the network elements and protocol stack,
|
|
but also includes the physical radio transmission part.
|
|
|
|
Basic understanding about RF (Radio Frequency) Electronics is key to
|
|
achieving good performance of the GSM network.
|
|
|
|
[[rf-coaxial-cabling]]
|
|
=== Coaxial Cabling
|
|
|
|
Coaxial cables come in many different shapes, diameters, physical
|
|
construction, dielectric materials, and last but not least brands and
|
|
types.
|
|
|
|
There are many parameters that might be relevant to your particular
|
|
installation, starting from mechanical/environmental properties such as
|
|
temperature range, UV resilience, water/weatherproofness, flammability,
|
|
etc.
|
|
|
|
For the subject of this manual, we will not look at those mechanical
|
|
properties, but look at the electrical properties instead.
|
|
|
|
The prime electrical parameters of a coaxial cable are:
|
|
|
|
* its attenuation over frequency and length
|
|
* its maximum current/power handling capability
|
|
* its propagation velocity (ignored here)
|
|
* its screening efficiency (ignored here)
|
|
|
|
==== Coaxial Cable Attenuation
|
|
|
|
The attenuation of a coaxial cable is given in dB per length, commonly
|
|
in 'dB per 100m'. This value changes significantly depending on the
|
|
frequency of the signal transmitted via the cable. Cable manufacturers
|
|
typically either provide tables with discrete frequency values, or
|
|
graphs plotting the attenuation per 100m (x axis) over the frequency (y
|
|
axis).
|
|
|
|
FIXME: Example.
|
|
|
|
So in order to estimate the loss of a coaxial cable, you need to
|
|
|
|
. determine the frequency at which you will use the cable, as determined
|
|
by the GSM frequency band of your BTS. Make sure you use the highest
|
|
frequency that might occur, which is typically the upper end of the
|
|
transmit band, see <<gsm-bands>>
|
|
. determine the attenuation of your cable per 100m at the given
|
|
frequency (check the cable data sheet)
|
|
. scale that value by the actual length of the cable
|
|
|
|
A real cable always has connectors attached to it, please add some
|
|
additional losses for the connectors that are attached. 0.05 dB per
|
|
connector is a general rule of thumb for the frequencies used in GSM.
|
|
|
|
FIXME: Example computation
|
|
|
|
As you can see very easily, the losses incurred in coaxial cables
|
|
between your antenna and the BTS can very quickly become significant
|
|
factors in your overall link budget (and thus cell coverage). This is
|
|
particularly relevant for the uplink power budget. Every dB you loose
|
|
in the antenna cable between antenna and the BTS receiver translates
|
|
into reduced uplink coverage.
|
|
|
|
Using the shortest possible coaxial cabling (e.g. by mounting the BTS
|
|
high up on the antenna tower) and using the highest-quality cabling are
|
|
the best strategies to optimize
|
|
|
|
WARNING: If you plan to assemble the coaxial connectors yourself, please
|
|
make sure you ensure to have the right skills for this. Properly
|
|
assembling coaxial connectors (whether solder-type or crimp-type)
|
|
requires precision tools and strict process as described by the
|
|
manufacturer. Any mechanical imprecision of connector assembly will
|
|
cause significant extra signal attenuation.
|
|
|
|
==== Checking coaxial cables
|
|
|
|
If you would like to check the proper operation of a coaxial cable,
|
|
there are several possible methods available:
|
|
|
|
* The more expensive method would be to use a 'RF Network Analyzer' to
|
|
measure the S11/S12 parameters or the VSWR of the cable.
|
|
* Another option is to use a TDR (time domain reflectometer) to
|
|
determine the VSWR. The TDR method has the added advantage that you
|
|
can localize any damage to the cable, as such damage would cause
|
|
reflections that can be converted into meters cable length from the
|
|
port at which you are testing the cable. Mobile, battery-powered TDR
|
|
for field-use in GSM Site installation are available from various
|
|
vendors. One commonly used series is the 'Anritsu Site Master'.
|
|
|
|
|
|
[[rf-coaxial-connectors]]
|
|
=== Coaxial Connectors
|
|
|
|
A coaxial connector is a connector specifically designed for mounting to
|
|
coaxial cable. It facilitates the removable / detachable connection of
|
|
a coaxial cable to a RF device.
|
|
|
|
There are many different types of coaxial connectors on the market.
|
|
|
|
The most important types of coaxial connectors in the context of GSM
|
|
BTSs are:
|
|
|
|
* The 'N type' connector
|
|
* The 'SMA type' connector.
|
|
* The '7/16' type connector
|
|
|
|
FIXME: Images
|
|
|
|
The above connectors are tightened by a screw-on shell. Each connector
|
|
type has a specific designated nominal torque by which the connector
|
|
shall be tightened. In case of uncertainty, please ask your connector
|
|
supplier for the nominal torque.
|
|
|
|
NOTE: Always ensure the proper mechanical condition of your RF
|
|
connectors. Don't use RF connectors that are contaminated by dust or
|
|
dirt, or which show significant oxidization, bent contacts or the like.
|
|
Using such connectors poses significant danger of unwanted signal loss,
|
|
and can in some cases even lead to equipment damage (e.g. in case of RF
|
|
power at PA output being reflected back into the PA).
|
|
|
|
|
|
[[rf-duplexers]]
|
|
=== Duplexers
|
|
|
|
A GSM BTS (or GSM TRX inside a BTS) typically exposes separate ports for
|
|
Rx (Receive) and Tx (Transmit). This is intentionally the case, as
|
|
this allows the users to add e.g. additional power amplifiers, filters
|
|
or other external components into the signal path. Those components
|
|
typically operate on either the receive or the transmit path.
|
|
|
|
You could now connect two separate antennas to the two ports (one for
|
|
Rx, one for Tx). This is commonly done in indoor installations with
|
|
small rubber-type antennas directly attached to the BTS connectors.
|
|
|
|
In outdoor installations, you typically (want to) use a single Antenna
|
|
for Rx and Tx. This single antenna needs to be connected to the BTS
|
|
via a device that is called 'Duplexer'.
|
|
|
|
The 'Duplexer' is actually a frequency splitter/combiner, which is
|
|
specifically tuned to the uplink and downlink frequencies of the GSM
|
|
band in which you operate the BTS. As such, it has one port that passes
|
|
only uplink frequencies between the antenna and that port, as well as
|
|
another port that passes only downlink frequencies between antenna and
|
|
that port.
|
|
|
|
.Illustration of the Duplexer functionality
|
|
[graphviz]
|
|
----
|
|
digraph G {
|
|
rankdir = LR;
|
|
|
|
BTS -> Duplexer [label="Tx band"];
|
|
Duplexer [shape=box];
|
|
Duplexer -> BTS [label="Rx band"];
|
|
Duplexer -> Antenna [label ="All frequencies",dir=both];
|
|
Antenna [shape=cds];
|
|
}
|
|
----
|
|
|
|
WARNING: *The ports of a duplexer are not interchangeable*. Always make
|
|
sure that you use the Rx port of the duplexer with the Rx port of the
|
|
BTS, and vice-versa for Tx.
|
|
|
|
|
|
[[rf-pa]]
|
|
=== RF Power Amplifiers
|
|
|
|
A RF Power Amplifier (PA) is a device that boosts the transmit power of
|
|
your RF signal, the BTS in your case.
|
|
|
|
RF power amplifiers come in many different characteristics. Some of the
|
|
key characteristics are:
|
|
|
|
Frequency range::
|
|
A PA is typically designed for a specific frequency range. Only
|
|
signals inside that range will be properly amplified
|
|
Gain in dB::
|
|
This tells you how many dB the power amplifier will increase your
|
|
signal. `Pout = Pin + Gain`
|
|
Maximum Output Power::
|
|
This indicates the maximum absolute output power. For example, if the
|
|
maximum output power is 40 dBm, and the gain is 10dBm, then an input
|
|
signal of 30dBm will render the maximum output power. An input signal
|
|
of 20dBm would subsequently generate only 30dBm of output power.
|
|
Efficiency::
|
|
The efficiency determines how much electrical power is consumed for
|
|
the given output power. Often expressed as Power Added Efficiency
|
|
(PAE).
|
|
|
|
WARNING: If you add external power amplifiers to a GSM BTS or any other
|
|
transmitter, this will invalidate the regulatory approval of the BTS.
|
|
It is your responsibility to ensure that the combination of BTS and PA
|
|
still fulfills all regulatory requirements, for example in terms of
|
|
out-of-band emissions, spectrum envelope, phase error, linearity, etc!
|
|
|
|
[graphviz]
|
|
.Addition of a RF Power Amplifier to a GSM BTS Setup
|
|
----
|
|
digraph G {
|
|
rankdir = LR;
|
|
BTS;
|
|
PA [label="PA 14dB gain"];
|
|
Duplexer [shape=box];
|
|
|
|
BTS -> PA [label="Tx 23 dBm"];
|
|
PA -> Duplexer [label="Tx 37dBm"];
|
|
Duplexer -> BTS [label="Rx band"];
|
|
Duplexer -> Antenna [dir=both];
|
|
Antenna [shape=cds];
|
|
}
|
|
----
|
|
|
|
|
|
=== Antennas
|
|
|
|
The Antenna is responsible for converting the electromagnetic waves
|
|
between the coaxial cable and the so-called 'air interface' and
|
|
vice-versa. The properties of an antenna are always symmetric for both
|
|
transmission and reception.
|
|
|
|
Antennas come in many different types and shapes. Key characteristics
|
|
distinguishing antennas are:
|
|
|
|
Antenna Gain::
|
|
Expresses how much more efficient the antenna converts between cable
|
|
and air interface. Can be expressed in dB compared to a theoretical
|
|
isotropic radiator (dBi) or compared to a dipole antenna (dBd). Gain
|
|
usually implies directivity.
|
|
|
|
Frequency Band(s)::
|
|
Antennas typically have only a relatively narrow band (or multiple
|
|
narrow bands at which they radiate efficiently. In general, the
|
|
higher the antenna gain, the lower the usable frequency band of the
|
|
antenna.
|
|
|
|
Directivity::
|
|
Antennas radiate the energy in all three dimensions.
|
|
|
|
Mechanical Size::
|
|
Mechanical Size is an important factor depending on how and where the
|
|
antenna is mounted. Size also relates to weight and wind-load.
|
|
|
|
Wind Load::
|
|
Expresses how much mechanical load the antenna will put on its
|
|
support structure (antenna mast).
|
|
|
|
Connector Type::
|
|
Your cabling will have to use a compatible connector for the antenna.
|
|
Outdoor antennas typically use the 7/16 type connector or an N type
|
|
connector. Indoor antennas either N type or SMA type.
|
|
|
|
Environmental Rating::
|
|
Indoor antennas cannot be used outdoor, as they do not offer the level
|
|
of protection against dust and particularly water / humidity /
|
|
corrosion.
|
|
|
|
Down-tilt Capability::
|
|
Particularly sector antennas are typically installed with a fixed or
|
|
(mechanically / electrically) variable down-tilt in order to limit the
|
|
radius/horizon of the antenna footprint and avoid excess interference
|
|
with surrounding cells.
|
|
|
|
VSWR::
|
|
The Voltage Standing Wave Ratio indicates how well the antenna is
|
|
matched to the coaxial cable, and how much of the to-be-transmitted
|
|
radio signal is actually converted to radio waves versus reflected
|
|
back on the RF cable towards the transmitter. An ideal antenna has a
|
|
VSWR of 1 (sometimes written 1:1). Real antennas are typically in the
|
|
range of 1.2 to 2.
|
|
|
|
Side Lobes::
|
|
A directional antenna never radiates only in one direction but always
|
|
has certain side lobes pointing outside of the main direction of the
|
|
antenna. The number and strength of side lobes differ from antenna
|
|
to antenna model.
|
|
|
|
NOTE: Whenever installing antennas it is important to understand that
|
|
any metallic or otherwise conductive object in their vicinity will
|
|
inevitably alter the antenna performance. This can affect the radiation
|
|
pattern, but also de-tune the antenna and shift its frequency band
|
|
outside the nominal usable frequency band. It is thus best to mount
|
|
antennas as far as practically possible from conductive elements within
|
|
their radiation pattern
|
|
|
|
|
|
==== Omni-directional Antennas
|
|
|
|
Omni-directional antennas are typically thin long dipole antennas covered
|
|
with fiberglass. They radiate with equal strength in all directions and
|
|
thus result in a more or less circular cell footprint (assuming flat
|
|
terrain). The shape of the radiation pattern is a torus (donut) with
|
|
the antenna located in the center of that torus.
|
|
|
|
Omni-directional antennas come with a variety of gains, typically from 0
|
|
dBd to 3 dBd, 6 dBd and sometimes 9 dBd. This gain is achieved by
|
|
compressing the radiation torus in the vertical plane.
|
|
|
|
Sometimes, Omni-directional antennas can be obtained with a fixed
|
|
down-tilt to limit the cell radius.
|
|
|
|
|
|
==== Sector Antennas
|
|
|
|
Sector antennas are used in sectorized cell setups. Sector antennas can
|
|
have significantly higher gain than omni-directional antennas.
|
|
|
|
Instead of mounting a single BTS with an omni-directional antenna to a
|
|
given antenna pole, multiple BTSs with each one sector antenna are
|
|
mounted to the same pole. This results in an overall larger radius due
|
|
to the higher gain of the sector antennas, and also in an overall
|
|
capacity increase, as each sector has the same capacity as a single
|
|
omni-directional cell. And all that benefit still requires only a
|
|
single physical site with antenna pole, power supply, back-haul cabling,
|
|
etc.
|
|
|
|
Experimentation and simulation has shown that typically the use of three
|
|
sectors with antennas of an opening angle of 65 degrees results in the
|
|
most optimal combination for GSM networks. If more sectors are being
|
|
deployed, there is a lot of overlap between the sectors, and the amount
|
|
of hand-overs between the BTSs is increased.
|
|
|
|
|
|
|
|
[[rf-lna]]
|
|
=== RF Low Noise Amplifier (LNA)
|
|
|
|
A RF Low Noise Amplifier (LNA) is a device that amplifies the weak
|
|
received signal. In general, LNAs are combined with band filters, to
|
|
ensure that only those frequencies within the receive band are
|
|
amplified, and out-of-band interferers are filtered out. A duplexer
|
|
can already be a sufficient band-filter, depending on its
|
|
characteristics.
|
|
|
|
The use of a LNA typically only makes sense if you
|
|
. have very long and/or lossy coaxial cables from your antenna to the
|
|
BTS, and
|
|
. can mount the duplexer + LNA close to the antenna, so that the
|
|
amplification happens before the long/lossy coaxial line to the BTS
|
|
|
|
Key characteristics of a LNA are:
|
|
|
|
Frequency range::
|
|
A LNA is typically designed for a specific frequency range. Only
|
|
signals inside that range will be properly amplified
|
|
Gain in dB::
|
|
This tells you how many dB the low noise amplifier will increase your
|
|
signal. `Pout = Pin + Gain`
|
|
Maximum Input Power::
|
|
This indicates the maximum RF power at the PA input before saturation.
|
|
Noise Figure::
|
|
This indicates how much noise this LNA will add to the signal. This
|
|
noise will add to the interference as seen by the receiver.
|
|
|
|
[graphviz]
|
|
.Addition of a RF Low Noise Amplifier to the GSM BTS Setup
|
|
----
|
|
digraph G {
|
|
rankdir = LR;
|
|
|
|
BTS -> LNA [label="Rx",dir=back];
|
|
LNA -> Duplexer [label="Rx",dir=back];
|
|
BTS -> Duplexer [label="Tx"];
|
|
Duplexer -> Antenna [dir=both];
|
|
|
|
Duplexer [shape=box];
|
|
Antenna [shape=cds];
|
|
}
|
|
----
|
|
|
|
[graphviz]
|
|
.Addition of a RF LNA + RF PA to the GSM BTS Setup
|
|
----
|
|
digraph G {
|
|
rankdir = LR;
|
|
|
|
subgraph {
|
|
rank = same;
|
|
PA;
|
|
LNA;
|
|
}
|
|
|
|
BTS -> LNA [label="Rx",dir=back];
|
|
BTS -> PA [label="Tx 23 dBm"];
|
|
LNA -> Duplexer [label="Rx",dir=back];
|
|
PA -> Duplexer [label="Tx 37 dBm"];
|
|
Duplexer -> Antenna [dir=both];
|
|
|
|
PA [label="PA 14dB gain"];
|
|
Duplexer [shape=box];
|
|
Antenna [shape=cds];
|
|
}
|
|
----
|
|
|
|
As any LNA will add noise to the signal, it is generally discouraged to
|
|
add them to the system. Instead, we recommend you to mount the entire
|
|
BTS closer to the antenna, thereby removing the losses created by
|
|
lengthy coaxial wire. The power supply lines and Ethernet connection to
|
|
the BTS are far less critical when it comes to cable length.
|
|
|
|
|
|
== Introduction into GSM Radio Planning
|
|
|
|
The main focus of the manual you are reading is to document the
|
|
specifics of the Osmocom GSM implementation in terms of configuration,
|
|
system administration and monitoring. That's basically all on the
|
|
software part.
|
|
|
|
However, successful deployment and operation of GSM networks depends to
|
|
a large extent on the proper design on the radio frequency (RF) side,
|
|
including the right cabling, duplexers, antennas, etc.
|
|
|
|
Planning and implementing GSM deployment is a science (or art) in
|
|
itself, and in most cases it is best to consult with somebody who has
|
|
existing experience in the field.
|
|
|
|
There are three parts to this:
|
|
|
|
GSM Radio Network Planning::
|
|
This includes an analysis of the coverage area, its terrain/geography,
|
|
the selection of the right sites for your BTSs, the antenna height, a
|
|
path loss estimate. As a result of that process, it will be clear
|
|
what amount of transmit power, antenna gain, cable length/type, etc.
|
|
you should use to obtain the intended coverage.
|
|
GSM Site Installation::
|
|
This is the execution of what has been determined in the previous
|
|
step. The required skills are quite different, as this is about
|
|
properly assembling RF cables and connections, duplexers, power
|
|
amplifiers, antennas, etc.
|
|
Coverage testing::
|
|
This is typically done by driving or walking in the newly-deployed GSM
|
|
site, and checking of the coverage is as it was expected.
|
|
|
|
NOTE: This chapter can only give you the briefest overview about the
|
|
process used, and cannot replace the experience and skill of somebody
|
|
with GSM RF planning and site deployment.
|
|
|
|
[[rf-radio-net-plan]]
|
|
=== GSM Radio Network Planning
|
|
|
|
In GSM Radio Network Planning, the number and location of sites as well
|
|
as type of required equipment is determined based on the coverage
|
|
requirements.
|
|
|
|
For the coverage of a single BTS, this is a process that takes into
|
|
consideration:
|
|
|
|
* the terrain that needs to be covered
|
|
* the type of mobile stations to be supported, and particularly the
|
|
speed of their movement (residential, pedestrians, trains, highways)
|
|
* the possible locations for cell sites, where BTSs and Antennas can be
|
|
placed, as well as the possible antenna mounting height
|
|
* the equipment choices available, including
|
|
** type and capabilities of BTS. The key criteria here is
|
|
the downlink transmit power in dBm, and the uplink receive
|
|
sensitivity.
|
|
** antenna models, including gain, radiation pattern, etc.
|
|
** RF cabling, including the key aspect of attenuation per length
|
|
** RF duplexers, splitting the transmit and receive path
|
|
** power amplifiers (PAs), increasing the transmit power
|
|
** low noise amplifiers (LNAs), amplifying the received signal
|
|
|
|
For coverage of an actual cellular network consisting of neighboring
|
|
cells, this process also must take into consideration aspects of
|
|
'frequency planning', which is the allocation of frequencies (ARFCNs) to
|
|
the individual cells within the network. As part of that, interference
|
|
generated by frequency re-use of other (distant) cells must be taken
|
|
into consideration. The details of this would go beyond this very
|
|
introductory text. There is plenty of literature on this subject
|
|
available.
|
|
|
|
[[rf-db]]
|
|
=== The Decibel (dB) and Decibel-Milliwatt (dBm)
|
|
|
|
RF engineering heavily depends on the Decibel (dB) as a unit to express
|
|
attenuation (losses) or amplification (gain) impacted on radio signals.
|
|
|
|
The dB is a logarithmic unit, it is used to express the ratio of two
|
|
values of physical quantity. You can thus not express an absolute value
|
|
in dB, only relative.
|
|
|
|
NOTE: *Relative loss* (cable, connector, duplexer, splitter) *or gain*
|
|
(amplifiers) are power *is expressed in dB*.
|
|
|
|
In order to express an absolute value, you need to use a unit like
|
|
'dBm', which is referencing a power of 1 mW (milli-Watt).
|
|
|
|
NOTE: *Absolute power* like transmitter output power or receiver input
|
|
power *is expressed in dBm*.
|
|
|
|
[options="header",cols="15%,15%,70%"]
|
|
.Example table of dBm values and their corresponding RF Power
|
|
|===
|
|
|dBm|RF Power|Comment
|
|
|0|1 mW|
|
|
|1|1.26 mW|transmit power of sysmoBTS 1002 when used with `max_power_red 22`
|
|
|3|2 mW|
|
|
|6|4 mW|
|
|
|12|16 mW|
|
|
|12|16 mW|
|
|
|20|100 mW|
|
|
|23|199 mW|Maximum transmit power of indoor sysmoBTS 1002
|
|
|26|398 mW|
|
|
|30|1 W|Maximum transmit power of a MS in 1800/1900 MHz band
|
|
|33|2 W|Maximum transmit power of a MS in 850/900 MHz band
|
|
|37|5 W|Maximum transmit power of 1 TRX in sysmoBTS 2050
|
|
|40|10 W|Maximum transmit power of sysmoBTS 1100
|
|
|===
|
|
|
|
[[rf-gsm-bands]]
|
|
=== GSM Frequency Bands
|
|
|
|
GSM can operate in a variety of frequency bands. However,
|
|
internationally only the following four bands have been deployed in
|
|
actual networks:
|
|
|
|
[options="header"]
|
|
.Table of GSM Frequency Bands
|
|
|===
|
|
|Name|Uplink Band|Downlink Band|ARFCN Range
|
|
|GSM 850|824 MHz .. 849 MHz|869 MHz .. 894 MHz|128 .. 251
|
|
|E-GSM 900|880 MHz .. 915 MHz|925 MHz .. 960 MHz|0 .. 124, 975 .. 1023
|
|
|DCS 1800|1710 MHz .. 1785 MHz|1805 MHz .. 1880 MHz|512 .. 885
|
|
|PCS 1900|1850 MHz .. 1910 MHz|1930 MHz .. 1990 MHz|512 .. 810
|
|
|===
|
|
|
|
|