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diff --git a/2017/path_loss_link_budget-osmocon2017/ap_to_client.png b/2017/path_loss_link_budget-osmocon2017/ap_to_client.png Binary files differnew file mode 100644 index 0000000..a36478a --- /dev/null +++ b/2017/path_loss_link_budget-osmocon2017/ap_to_client.png diff --git a/2017/path_loss_link_budget-osmocon2017/link_budget.png b/2017/path_loss_link_budget-osmocon2017/link_budget.png Binary files differnew file mode 100644 index 0000000..b4c7407 --- /dev/null +++ b/2017/path_loss_link_budget-osmocon2017/link_budget.png diff --git a/2017/path_loss_link_budget-osmocon2017/path_loss_link_budget.adoc b/2017/path_loss_link_budget-osmocon2017/path_loss_link_budget.adoc new file mode 100644 index 0000000..624f454 --- /dev/null +++ b/2017/path_loss_link_budget-osmocon2017/path_loss_link_budget.adoc @@ -0,0 +1,205 @@ +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? |
