206 lines
6.6 KiB
Plaintext
206 lines
6.6 KiB
Plaintext
Path Loss and Link Budget
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=========================
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:author: Harald Welte <hwelte@sysmocom.de>
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:copyright: 2017 by Harald Welte (License: CC-BY-SA)
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:backend: slidy
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:max-width: 45em
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[[rf-path-loss]]
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== Path Loss
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A fundamental concept in planning any type of radio communications link
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is the concept of 'Path Loss'. Path Loss describes the amount of
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signal loss (attenuation) between a receive and a transmitter.
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As GSM operates in frequency duplex on uplink and downlink, there is
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correspondingly an 'Uplink Path Loss' from MS to BTS, and a 'Downlink
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Path Loss' from BTS to MS. Both need to be considered.
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It is possible to compute the path loss in a theoretical ideal
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situation, where transmitter and receiver are in empty space, with no
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surfaces anywhere nearby causing reflections, and with no objects or
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materials in between them. This is generally called the 'Free Space
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Path Loss'.
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[[rf-path-loss]]
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== Path Loss
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Estimating the path loss within a given real-world terrain/geography is
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a hard problem, and there are no easy solutions. It is impacted, among
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other things, by
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* the height of the transmitter and receiver antennas
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* whether there is line-of-sight (LOS) or non-line-of-sight (NLOS)
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* the geography/terrain in terms of hills, mountains, etc.
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* the vegetation in terms of attenuation by foliage
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* any type of construction, and if so, the type of materials used in
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that construction, the height of the buildings, their distance, etc.
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* the frequency (band) used. Lower frequencies generally expose better
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NLOS characteristics than higher frequencies.
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The above factors determine on the one hand side the actual attenuation
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of the radio wave between transmitter and receiver. On the other
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hand, they also determine how many reflections there are on this path,
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causing so-called 'Multipath Fading' of the signal.
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== Radio Propagation Models
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Over decades, many different radio propagation models have been designed
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by scientists and engineers. They might be based on empirical studies
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condensed down into relatively simple models, or they might be based on
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ray-tracing in a 3D model of the terrain.
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Several companies have developed (expensive, proprietary) simulation
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software that can help with this process in detail. However, the
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results of such simulation also depend significantly on the availability
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of precise 3D models of the geography/terrain as well as the building
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structure in the coverage area.
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In absence of such simulation software and/or precise models, there are
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several models that can help, depending on the general terrain:
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== Common Path Loss Models
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[[path-loss-models]]
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.List of common path loss models
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[options="header",cols="20%,20%,20%,40%"]
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|===
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|Type|Sub-Type|Bands|Name
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|Terrain|-|850, 900, 1800, 1900|ITU terrain model
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|Rural|Foliage|850, 900, 1800, 1900|One woodland terminal model
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|City|Urban|850, 900|Okumura-Hata Model for Urban Areas
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|City|Suburban|850, 900|Okumura-Hata Model for Suburban Areas
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|City|Open|850, 900|Okumura-Hata Model for Open Areas
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|City|Urban|1800, 1900|COST-231 Hata Model
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|Indoor|-|900, 1800, 1900|ITU model for indoor attenuation
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|===
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In <<path-loss-models>> you can see a list of commonly-used path loss
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models. They are typically quite simple equations which only require
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certain parameters like the distance of transmitter and receiver as well
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as the antenna height, etc. No detailed 3D models of the terrain are
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required.
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== RF Power in a Wireless Link
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image::link_budget.png[width="90%"]
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[[rf-link-budget]]
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== Link Budget
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The link budget consists of the total budget of all elements in the
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telecommunication system between BTS and MS (and vice-versa).
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This includes
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* antenna gains on both sides
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* coaxial cabling between antenna and receiver/transmitter
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* losses in duplexers, splitters, connectors, etc
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* gain of any amplifiers (PA, LNA)
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* path loss of the radio link between the two antennas
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== Simplified Link Budget Equation
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The simplified link budget equations looks like this:
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Rx Power (dBm) = Tx Power (dBm) + Gains (dB) − Losses (dB)
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Gains is the sum of all gains, including
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* Gain of the transmitter antenna
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* Gain of the receiver antenna
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* Gain of any PA (transmitter) or LNA (receiver)
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Losses is the sum of all losses, including
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* Loss of any cabling and/or connectors on either side
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* Loss of any passive components like duplexers/splitters on either side
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* Path Loss of the radio link
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== Link Budget Equation vs. Path Loss
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* Using the Link Budget equation and resolving it for the path loss will
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give you an idea of how much path loss on the radio link you can afford
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while still having a reliable radio link.
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* Resolving the path loss into a physical distance based on your path
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loss model will then give you an idea about the coverage area that
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you can expect.
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NOTE:: The Rx Power substituted in the Link budget equation is
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determined by the receiver sensitivity. It is customary to add some
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some safety margin to cover for fading.
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== RF Link
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image::ap_to_client.png[width="90%"]
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== Uplink Link Budget
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[graphviz]
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----
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digraph G {
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rankdir = LR;
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MS -> MSAnt -> Path -> BTSAnt -> Cabling -> Duplexer -> Cable -> BTS;
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MSAnt [label="MS Antenna"];
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BTSAnt [label="BTS Antenna"];
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}
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----
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The transmit power of a MS depends on various factors, such as the MS
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Power Class, the frequency band and the modulation scheme used.
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[options="header"]
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.Typical MS transmit power levels
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|===
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|Power Class|Band|Modulation|Power
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|4|850 / 900|GMSK|33 dBm (2 W)
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|1|1800 / 1900|GMSK|30 dBm (1 W)
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|E2|850 / 900|8PSK|27 dBm (0.5 W)
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|E2|1800 / 1900|8PSK|26 dBm (0.4 W)
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|===
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The minimum reference sensitivity level of a normal GSM BTS is specified
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in 3GPP TS 05.05 and required to be at least -104 dBm. Most modern BTSs
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outperform this significantly.
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FIXME: Example calculation (spreadsheet screenshot?)
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== Downlink Link Budget
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[graphviz]
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----
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digraph G {
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rankdir = LR;
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BTS -> Cable -> Duplexer -> Cabling -> BTSAnt -> Path -> MSAnt -> MS;
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MSAnt [label="MS Antenna"];
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BTSAnt [label="BTS Antenna"];
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}
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----
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The transmit power of the BTS depends on your BTS model and any possible
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external power amplifiers used.
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The minimum reference sensitivity level of a GSM MS is specified in 3GPP
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TS 05.05 and can typically be assumed to be about -102 dB.
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FIXME: Example calculation (spreadsheet screenshot?)
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== Optimization of the Link Budget
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If the coverage area determined by the above procedure is insufficient,
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you can try to change some of the parameters, such as
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* increasing transmit power by adding a bigger PA
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* increasing antenna gain by using a higher gain antenna
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* reducing cable losses by using better / shorter coaxial cables
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* increasing the height of your antenna
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include::rf.adoc[]
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== The End
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Questions?
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