path loss / link budget basics

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+Path Loss and Link Budget
+=========================
+:author:	Harald Welte <hwelte@sysmocom.de>
+:copyright:	2017 by Harald Welte (License: CC-BY-SA)
+:backend:	slidy
+:max-width:	45em
+
+[[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'.
+
+[[rf-path-loss]]
+== 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.
+
+== Radio Propagation Models
+
+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:
+
+== Common Path Loss Models
+
+[[path-loss-models]]
+.List of common path loss models
+[options="header",cols="20%,20%,20%,40%"]
+|===
+|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.
+
+== RF Power in a Wireless Link
+
+image::link_budget.png[width="90%"]
+
+[[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
+
+== Simplified Link Budget Equation
+
+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
+
+== Link Budget Equation vs. Path Loss
+
+* 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.
+
+NOTE:: 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.
+
+== RF Link
+
+image::ap_to_client.png[width="90%"]
+
+
+== 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
+
+include::rf.adoc[]
+
+== The End
+
+Questions?