This application note covers the following path loss information:

• Introduction
• Packet Format
• Example Path Loss Server
• ACE Project Configuration
• Advanced Parameters
• Troubleshooting and Checklist

In ACE Studio, simulated radios communicate when they are in-tune and in-range. Path loss effects are used to simulate radio degradation due to weather effects, terrain, time-of-day, etc. The radio receiver finds in-tune radios and then sends out a PDU request to the server for each in-tune radio. The server returns a response packet and the radio applies the amount of path loss to the radio calculation.

The path loss interface feature is a separately purchased software option. If your Target has the Path Loss Interface Software Package it is displayed on the RMS "Options File" page as the "Prop. Loss I/F" option.

The general operation of the path loss interface is as follows:

1. The receiver finds any in-tune radios.
2. The receiver sends out one request for each in-tune radio to the server.
3. The server returns a response packet to the radio.
4. The radio applies the amount of loss in the packet to its radio calculation.

The packet is in a DIS-like format. The packet is formatted as follows:

  RXpathPDU = record
     PDUheader     	: record
     			ProtocolVer   	: byte;
    	 		ExerciseID    	: byte;
     			PDUtype       	: byte;
     			PDUfamily     	: byte;
     			TimeStamp     	: Unsigned_32bit;
    			Length        	: Unsigned_16bit;
     			Pad1          	: Unsigned_16bit;
   		end;
    RXEntityID   	: record
     			Site   			: Unsigned_16bit;
     			Application/Host   	: Unsigned_16bit;
     			Entity     		: Unsigned_16bit;
       		end;
    RXDeviceID   	: Unsigned_16bit;
    TXEntityID   	: record
     			Site   			: Unsigned_16bit;
     			Application/Host   	: Unsigned_16bit;
     			Entity     		: Unsigned_16bit;
       		end;
    TXDeviceID   	: Unsigned_16bit;
    PathFactor   	: IEEE_float_32bit;
    LocationRX   	: record
     			X : IEEE_float_64bit;
     			Y : IEEE_float_64bit;
     			Z : IEEE_float_64bit;
       		end;
    LocationTX   	: record
     			X : IEEE_float_64bit;
     			Y : IEEE_float_64bit;
     			Z : IEEE_float_64bit;
       		end;
    FrequencyMSW : Unsigned_32bit;
    FrequencyLSW : Unsigned_32bit;
  end;

All PDUs are delivered in network byte order (Big Endian). Path factor loss is of the range 0.0 - 1.0 with 1.0 indicating no loss at all (0dB) and 0.0 indicating complete loss (-300dB).

If the path loss interface shares the same Ethernet interface (for example eth0) as the host interface, it is highly recommended that the port numbers are set to different numbers.

Packets are transmitted to the host address and port number specified in the DIS Gateway. Upon reception of a packet containing path loss PDUs, the host computer must perform the following operations:

1. Increment PDU type by 1 changing PDU type from request to reply (30 for request and 31 for reply).
2. Calculate the PathFactor value between the locations indicated by LocationRX and LocationTX and insert this value into PathFactor. Note that LocationRX and LocationTX are given by geocentric coordinates. Path factor is a value of range 0.0 - 1.0 with 1.0 representing no loss (0.0dB loss) and 0.0 representing complete loss of signal (-300.0dB loss).
3. Retransmit the entire PDU back to the sourceIP/sourceUDP port.

It is important that the reply packet contains all of the ID numbers in the original request packet. The host should not modify any field except as described above.

As a simple example, if line-of-sight occulting was being used:

• PathFactor = 1.0 would be clear line of sight, while
• PathFactor = 0.0 would be occulted by path loss.

If the host has a more sophisticated model, which takes into account frequency, the frequency of the transmitter/receiver pair is provided in the PDU.

1. The Target must have the "Prop. Loss I/F" option enabled, see the RMS Options File page.
2. The project is setup with DIS radios that are on, in-tune and within range.
3. A path loss server is sitting on the same network as the Target's path loss interface and is listening on a known port.

There is not a global path loss enable for a radio. Instead, each of the radio’s mode/waveform/rxgain combinations must be configured to ensure the radio issues the path loss requests in those modes.

1. Open the project and double-click the Commplan icon to open the Commplan Editor.
2. Click the 'Receivergain' folder to expand it and select the 'rxgain' file.
3. In the Path Loss Request column turn on each of the rxgains that correspond with the mode/waveform that needs to issue the path loss requests.
4. Click 'Apply' and 'OK'.

1. In the Project Manager, expand the 'servers' folder on the left side.
2. Right-click on the canvas and add a DIS gateway file.
3. Fill in the path interface, port and path according to your installation’s setup.
4. Click 'OK'.

1. In the Project Manager, open the Telestra Editor by right-clicking on the Target icon and selecting 'Edit'.
2. Click the 'Sim Server' tab.
3. Select the 'DIS Gateway' that was previously created.
4. Click 'Update'.

The path loss interface also has a set of parameters that control when a radio should send a path loss request. These parameters are set in the Parameters.ini file located inside the Domain. These settings help control timeouts, default values, and allow the radio to send path loss requests even if it is located at the center of the earth. The default file is included below:

The Path Periodic Timeout occurs once a radio has already received a path loss response and is stationary. This is similar to the normal timeout used by radios. The default is 60,000 milliseconds.

path_periodic_timeout = 60000

The Path Moving Timeout occurs if either the transmitter or receiver are moving. It is similar to the moving timeout used by radios. If the path moving timeout occurs, a new request is sent by the receiver. The default is 2000 milliseconds.

path_moving_timeout = 2000

If the transmitter or the receiver move more than the Path Moving Threshold before the Path Moving Timeout occurs, a new path loss request is sent out. The default is 500 meters.

path_moving_threshold = 500

The Path Request Timeout is used whenever a request is sent. It is the amount of time the receiver will wait before sending another request. If any of the above timeouts or thresholds are triggered and a path loss request is sent, the receiver will always use this timeout until hearing back from the server. The default is 2500 milliseconds.

path_request_timeout = 2500

The Path Timeout Count is the number of Path Request Timeouts that must occur before using the Path Timeout Factor.

path_timeout_count = 2

The Timeout Path Factor is the number the radio will use for path loss after reaching the Path Timeout Count. The default is 1.0 which is the same as if no path factor or additional attenuation were present.

path_timeout_factor = 1.0

The Initial Path Factor is the number used by the radio before a response is received from the server. The default value is 0, which is the same as a completely blocked path.

path_initial_factor = 0.0

The Force Path Request option allows radios that are in tune with a transmitter located at the center of the earth to still send out path loss requests. The default value is 0. To enable, set the value to 1.

path_force_request = 0

1. Do you have a valid DIS radio? See, how do I configure my network radios to communicate on the DIS network?
2. Is your radio set to send path loss requests? Check in Transceiver > RCUParameters > Rcvrgain and set in CommPlan > Receivergain.
3. Is your radio in-tune with another DIS radio?
4. Are the radio world positions at least 500 meters apart?
5. Is your Target sending out path loss request packets? Check the RMS Health System in Radio Networking > DIS > Statistics > Path Sent. Also in the Target command line type:

   tcpdump -i <interface> udp port <port number>
   

   For example:

   tcpdump -i eth0 udp port 55001
   

6. Is your Target receiving path loss responses? See the RMS Health system in Radio Networking > DIS > Statistics > Path Received and the 'tcp dump' command above.
7. Is your radio using the path loss value? Use the Radio Monitor to verify, see below for details.

1. In the top left menu bar select Applications > ASTi > Radio Monitor.
2. In the Radio Monitor, select your Target from the pull-down menu on the upper right.
3. Select 'Receivers' in the bottom-left pull-down menu.
4. Double-click the radio to open a detailed radio page.
   
   At the bottom of the radio details page you should see a valid range and a path factor between 0-1. Note that this value will not be directly from the server and will include any additional propagation that is applied by ACE.