doc: Improved README

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

docs/README.md

@@ -6,31 +6,26 @@ This is a non realistic channel model.
 It models NxN independent paths each with a configurable attenuation
 This attenuation can also be configured to change over time.
 
-The default atennuation for all paths is set with the command line options
+The default atennuation for all paths is set with the command line option
 `-at=<attenuation>`.
 
 A file can be used to set the attenuation for some/all of the individual paths.
-This file is specified with the command line option `-file=<attenuations_file>`
+This file is specified with the command line option `-file=<att_matrix_file>`
 
 This file shall have one separate line per path like:<br>
-    `x y : {value|"<attenuation_file>"}`<br>
-e.g.:
-```
-	  1 2 : 65
-	  2 1 : 65
-	  1 3 : "/myfolder/att_file.txt"
-	  3 1 : "/myfolder/att_file.txt"
-```
+    `x y : {value|"<timed_attenuation_file>"}`<br>
 
 Where:
 
 * x is the transmitter number, x = 0..N-1
 * y is the receiver number     y != x
 * N being the number of devices in the simulation
 * value is a floating point value in dB
-* `<attenuation_file>` : is the path to a `<attenuation_file>` as described
-  below which applies to that path
+* `<timed_attenuation_file>` : is the path to a `<timed_attenuation_file>` as
+  described below which applies to that path.
   Note that this file name shall be provided in between `""`
+* Not all paths need to be provided. Those omited will default to the attenuation
+  provided from the command line.
 
 The file names can be either absolute or relative to `bin/`
 
@@ -46,10 +41,10 @@ Therefore the channel model won't be called to reevaluate conditions during a
 packet unless the devices which are transmitting change.
 
 
-### The `<attenuation_file>`:
+### The `<timed_attenuation_file>`:
 
 This file contains 2 columns, the first column represents time in
-microseconds, and the second column the attenuation at that given time.
+microseconds (an integer value), and the second column the attenuation at that given time.
 The times shall be in ascending order.<br>
 If the simulation time is found in the file, the channel will use the
 corresponding attenuation value.<br>
@@ -59,3 +54,38 @@ If the simulation time is before the first time value in the file, the
 channel will use the first atttenuation provided in the file.<br>
 If the simulation time is after the last time value in the file, the channel
 will use the last attenuation provided in the file.<br>
+
+### The `-atextra=<extra_attenuation>` command line option:
+
+You can provide an extra attenuation to be added to all paths, independently of
+how their attenuation was originally calculated, with the `-atextra` command line
+option.
+This `extra_attenuation` value will just be added to all paths.
+
+### An example:
+
+`./bs_2G4_phy_v1 -D=3 -s=Hola -channel=multiatt -argschannel -at=40 -file=paths_att_file.txt`
+
+`paths_att_file.txt`:
+```
+0 1 : 65
+1 0 : 65
+0 2 : "/myfolder/att_file.txt"
+2 0 : "/myfolder/att_file.txt"
+#Note that the paths from 1<->2 are not specified. They will default to the -at command line parameter
+```
+
+`att_file.txt`:
+```
+1000000    60
+11000000  120
+11000001   60
+```
+
+With this configuration, the radio traffic between device:
+
+* device 0 and device 1 will have an attenuation of 65dB
+* device 0 and device 2 will have an attenuation of
+    * 60dB up to the first second
+    * An increasing attenuation between 60->120 dB, between the first second and the 11th. At a rate of 6dB per second
+    * A sudden reduction in the attenuation to 60 dB right after, which is maintained thereafter.