diff options
Diffstat (limited to '2016/path_loss_link_budget/rf.adoc')
| -rw-r--r-- | 2016/path_loss_link_budget/rf.adoc | 530 |
1 files changed, 530 insertions, 0 deletions
diff --git a/2016/path_loss_link_budget/rf.adoc b/2016/path_loss_link_budget/rf.adoc new file mode 100644 index 0000000..c86e7a7 --- /dev/null +++ b/2016/path_loss_link_budget/rf.adoc @@ -0,0 +1,530 @@ +// 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 +|=== + + |