osmo-gsm-manuals/common/chapters/rf.adoc

== 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 <<rf-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
|===

[[rf-path-loss]]
=== Path Loss

A fundamental concept in planning any type of radio communications link
is the concept of 'Path Loss'.  Path Loss describes the amount of
signal loss (attenuation) between a receive and a transmitter.

As GSM operates in frequency duplex on uplink and downlink, there is
correspondingly an 'Uplink Path Loss' from MS to BTS, and a 'Downlink
Path Loss' from BTS to MS.  Both need to be considered.

It is possible to compute the path loss in a theoretical ideal
situation, where transmitter and receiver are in empty space, with no
surfaces anywhere nearby causing reflections, and with no objects or
materials in between them.  This is generally called the 'Free Space
Path Loss'.

Estimating the path loss within a given real-world terrain/geography is
a hard problem, and there are no easy solutions.   It is impacted, among
other things, by

 * the height of the transmitter and receiver antennas
 * whether there is line-of-sight (LOS) or non-line-of-sight (NLOS)
 * the geography/terrain in terms of hills, mountains, etc.
 * the vegetation in terms of attenuation by foliage
 * any type of construction, and if so, the type of materials used in
   that construction, the height of the buildings, their distance, etc.
 * the frequency (band) used.  Lower frequencies generally expose better
   NLOS characteristics than higher frequencies.

The above factors determine on the one hand side the actual attenuation
of the radio wave between transmitter and receiver.  On the other
hand, they also determine how many reflections there are on this path,
causing so-called 'Multipath Fading' of the signal.

Over decades, many different radio propagation models have been designed
by scientists and engineers.  They might be based on empirical studies
condensed down into relatively simple models, or they might be based on
ray-tracing in a 3D model of the terrain.

Several companies have developed (expensive, proprietary) simulation
software that can help with this process in detail.  However, the
results of such simulation also depend significantly on the availability
of precise 3D models of the geography/terrain as well as the building
structure in the coverage area.

In absence of such simulation software and/or precise models, there are
several models that can help, depending on the general terrain:

[[path-loss-models]]
.List of common path loss models
[options="header",cols="10%,10%,20%,60%"]
|===
|Type|Sub-Type|Bands|Name
|Terrain|-|850, 900, 1800, 1900|ITU terrain model
|Rural|Foliage|850, 900, 1800, 1900|One woodland terminal model
|City|Urban|850, 900|Okumura-Hata Model for Urban Areas
|City|Suburban|850, 900|Okumura-Hata Model for Suburban Areas
|City|Open|850, 900|Okumura-Hata Model for Open Areas
|City|Urban|1800, 1900|COST-231 Hata Model
|Indoor|-|900, 1800, 1900|ITU model for indoor attenuation
|===

In <<path-loss-models>> you can see a list of commonly-used path loss
models.  They are typically quite simple equations which only require
certain parameters like the distance of transmitter and receiver as well
as the antenna height, etc.  No detailed 3D models of the terrain are
required.

FIXME: Example calculations

[[rf-link-budget]]
=== Link Budget

The link budget consists of the total budget of all elements in the
telecommunication system between BTS and MS (and vice-versa).

This includes

* antenna gains on both sides
* coaxial cabling between antenna and receiver/transmitter
* losses in duplexers, splitters, connectors, etc
* gain of any amplifiers (PA, LNA)
* path loss of the radio link between the two antennas

The simplified link budget equations looks like this:

 Rx Power (dBm) = Tx Power (dBm) + Gains (dB) − Losses (dB)

Gains is the sum of all gains, including

* Gain of the transmitter antenna
* Gain of the receiver antenna
* Gain of any PA (transmitter) or LNA (receiver)

Losses is the sum of all losses, including

* Loss of any cabling and/or connectors on either side
* Loss of any passive components like duplexers/splitters on either side
* Path Loss of the radio link

Using the Link Budget equation and resolving it for the path loss will
give you an idea of how much path loss on the radio link you can afford
while still having a reliable radio link.  Resolving the path loss into
a physical distance based on your path loss model will then give you an
idea about the coverage area that you can expect.

The Rx Power substituted in the Link budget equation is determined by
the receiver sensitivity.  It is customary to add some some safety
margin to cover for fading.

==== Uplink Link Budget

[graphviz]
----
digraph G {
	rankdir = LR;
	MS -> MSAnt -> Path -> BTSAnt -> Cabling -> Duplexer -> Cable -> BTS;
	MSAnt [label="MS Antenna"];
	BTSAnt [label="BTS Antenna"];
}
----

The transmit power of a MS depends on various factors, such as the MS
Power Class, the frequency band and the modulation scheme used.

[options="header"]
.Typical MS transmit power levels
|===
|Power Class|Band|Modulation|Power
|4|850 / 900|GMSK|33 dBm (2 W)
|1|1800 / 1900|GMSK|30 dBm (1 W)
|E2|850 / 900|8PSK|27 dBm (0.5 W)
|E2|1800 / 1900|8PSK|26 dBm (0.4 W)
|===

The minimum reference sensitivity level of a normal GSM BTS is specified
in 3GPP TS 05.05 and required to be at least -104 dBm.  Most modern BTSs
outperform this significantly.

FIXME: Example calculation (spreadsheet screenshot?)

==== Downlink Link Budget

[graphviz]
----
digraph G {
	rankdir = LR;
	BTS -> Cable -> Duplexer -> Cabling -> BTSAnt -> Path -> MSAnt -> MS;
	MSAnt [label="MS Antenna"];
	BTSAnt [label="BTS Antenna"];
}
----

The transmit power of the BTS depends on your BTS model and any possible
external power amplifiers used.

The minimum reference sensitivity level of a GSM MS is specified in 3GPP
TS 05.05 and can typically be assumed to be about -102 dB.

FIXME: Example calculation (spreadsheet screenshot?)


==== Optimization of the Link Budget

If the coverage area determined by the above procedure is insufficient,
you can try to change some of the parameters, such as

* increasing transmit power by adding a bigger PA
* increasing antenna gain by using a higher gain antenna
* reducing cable losses by using better / shorter coaxial cables
* increasing the height of your antenna