Evaluating Communications Links
STK allows you to determine the times one object can "access," or see, another object. Also, you can impose constraints on accesses between objects to define what is a valid access. These constraints are defined as properties of the objects whose accesses are being calculated. This capability is used in STK Communications to define communications links.
You can model two types of communications links: basic and multi-hop. To create a basic communications link, simply define an access between a transmitter and a receiver. By changing the receiver constraints, you can define the limiting characteristics of the link you wish to analyze.
Communications links can only be defined between receivers and transmitters. Although you can calculate an access between a receiver or transmitter and other objects in STK, the communication parameters are not used in determining whether an access exists and the communications link data is unavailable.
To create a multi-hop communications link, define a group of communication objects consisting of a transmitter, receiver, re-transmitter, and another receiver. You specify the group of objects by creating a chain object using a chain, where the transmitters and receivers are listed in the chain in the order that you wish the signal to pass through. You can analyze more complex multi-hop links by creating constellations of receivers and transmitters for use when creating the chain.
When no access visibility is available between the objects in a chain, the data provider will report that there is no data available for reporting.
Link Budget Report
A specialized Access Report for basic link budget analysis is available using the Link Budget button in the Reports frame of the Access window.
The Link Budget report includes all the link parameters associated with the selected receiver or transmitter, as explained in the following table. The fields summarized in the table are available for communications links. Link budget reports take light speed delay into account. The units mentioned are shown as an example and, in some cases, are the default report units. The report data elements have units you can select and can be formatted in the report styles.
Link budget reports take computed refraction into account if enabled on the transmitter or receiver objects. TThe model settings used to compute refraction for the transmit side of the link will be taken from the transmitter object. The model settings for the receive side will be taken from the receiver object. As the refraction models compute the refraction differently, it is highly recommended to use the same model on both objects.
Depending on the model types of the objects involved, some parameters may be blank when the report is generated.
Fields in the basic Link Budget report
| Parameter | Description |
|---|---|
| Time | Scenario time. |
| EIRP | The effective isotropic radiated power in the link direction. This value is the power of the transmitter (in dB), plus the antenna gain (in dB), plus any filter and post-transmit gains/losses (in dB). |
| Rcvd Iso Pwr | Received Isotropic Power at the receiver antenna. The value is defined in the bandwidth of interest, so it includes a bandwidth overlap factor (see "Computing Bandwidth Overlap Factor"). This value does not include any pre-receive gains or losses. |
| Flux Density | Power flux density at the receiver antenna. This value does not include any pre-receive gains or losses. The units for this are dBW/m2. |
| g/T | Receiver gain over the equivalent noise temperature. Units are dB/K. |
| C/No | Carrier-to-Noise density at the receiver input. This value includes any pre-receive gains or losses. |
| Bandwidth | The receiver's RF bandwidth will be equal to your specified bandwidth if the autoscaling option is disabled for the receiver. Otherwise it will show the bandwidth that the receiver is using to match the transmitted signal. If spread-spectrum/CDMA is enabled, the bandwidth will be the spread bandwidth. |
| C/N | Carrier-to-Noise ratio at the receiver input. This value includes any pre-receive gains or losses. |
| Eb/No | Signal-to-Noise ratio at the receiver. This value includes any pre-demod gains or losses. |
| BER | Bit Error Rate. This value includes any pre-demod gains or losses. |
Generating Link Budget Data for the STK MATLAB Interface
To generate link budget data between a transmitter and a receiver that is suitable for the STK MATLAB interface, you will need to create a new report style from the default Link Budget Report style. To create a new style:
- Open the Report & Graph Manager for the Access object.
- Copy the Link Budget report style and paste it under My Styles. The pasted style is also named Link Budget.
- Right-click Link Budget and select Properties.
- Under Report Contents, select Link Information-Rcvd Iso. Power and click Options....
- Under Data Format, select Scientific (e) in the Notation field, enter 12 in the Number of Decimal Digits field, and click OK.
- With Link Information-Rcvd Iso. Power still selected, click Units....
- Clear the Use Defaults field and select Watts (W) and click OK.
- Generate the Link Budget report in STK MATLAB using the stkAccReport command. (When STK receives the report call from MATLAB, it searches the My Styles folder first and the Install folder last to return the correct report to MATLAB.)
Creating custom report styles from default styles enables you to easily change the units and the data formats as required. AGI recommends that you use the Scientific (e) notation format when you expect very small values.
Multi-Hop Links
Multi-hop links have some new parameters besides those available in the basic link performance report. The fields below are available for multi-hop links. The fields from the basic link performance report are modified to include an extra numeric identifier attached on the end. The identifier indicates for which individual hop in the overall multi-hop link the parameter in question is being calculated. For a simple transmitter to receiver to re-transmitter to receiver link two hops exist, one for the uplink (transmitter to receiver) and one for the downlink (re-transmitter to receiver). The fields will then be identified using a 1, 2, 3, 4, etc. (corresponding to link number in the multi-hop link) at the end. Additional fields available for Multi-Hop Links are listed in the table below. These fields provide accumulated totals for link performance parameters.
Additional fields available for Multi-Hop Links
| Parameter | Description |
|---|---|
| IBO | Input backoff in dB. Available only for re-transmitters. |
| OBO | Output backoff in dB. Available only for re-transmitters. |
| C/No Tot. | Total Carrier-to-Noise density at the receiver input as carried through the whole link. Includes uplink noise contributions and any pre-receive gains or losses. |
| C/N Tot. | Total Carrier-to-Noise ratio at the receiver input as carried through the whole link. Includes uplink noise contributions and any pre-receive gains or losses. |
| Eb/No Tot. | Total Signal-to-Noise ratio as carried through the whole link. Includes uplink noise contributions. |
| BER Tot. | Total Bit Error Rate as carried through the whole link. Includes uplink noise contributions and any pre-demod gains or losses. |
More complicated multi-hop links using constellations of communication objects may result in multiple successful paths through the chain. Following the nomenclature used in connection with the Chain object, these successful paths are called strands. Since each strand represents a potential link, multi-hop link reports will contain a subsection for each strand in the chain.
You may generate new report styles by opening the STK Report & Graph Manager for a chain, selecting a report style, and clicking Properties. In the Report Style Properties window, you may assign any link parameter from the link information and parameter list. For example, to generate a report for a two- or three-hop chain, you can select the Bent Pipe Comm Link report and click Make Copy in the Report & Graph Manager, override the new report name with "TwoHopBentPipe" and click Change to add the name to the list. Then, you can click Properties and select Link Information from the Elements list and transfer the new element to the Report Contents list. You may reformat this style by adding new lines or sections. Once you are finished, you can click OK and click Create in the Report & Graph Manager to generate the new report.
Receiver and Transmitter Basic Properties Reports
The basic properties of a receiver or transmitter can be generated by highlighting the object of interest in the Object Browser, selecting Report & Graph Manager from the Analysis menu and selecting the Basic Properties style.
Computing Bandwidth Overlap Factor
Bandwidth overlap factor is computed using one of the following ways:
- Transmitter Power Spectral Density (PSD) and Transmit and Receive Filters Disabled: The transmitted spectrum is modeled as a flat spectrum with unity magnitude across the transmitter’s bandwidth. The receiver’s frequency response is also modeled as a flat response across the receiver’s bandwidth. Therefore, the bandwidth ratio is computed as just a simple ratio of the receiver’s bandwidth to the transmitter’s bandwidth. If the transmitter’s bandwidth is totally contained within the receiver’s bandwidth, the ratio will be 1.0. For a receiver that has an autoscaled bandwidth and an autotracked frequency, this value will be 1.0, as well. Otherwise, this value may be less than 1.0 if the receiver center frequency and the transmitter frequency are not the same or the receiver’s bandwidth is totally contained within the transmitter’s bandwidth.
- Any combination of Transmitter PSD, Transmitter Filter, or Receiver Filter Enabled: In this case the bandwidth overlap factor is computed by integrating the composite received PSD over the receiver’s bandwidth and dividing by the total power in the original transmitter PSD. For example, if the transmitter PSD and filter is enabled and receiver filter is enabled, the composite PSD will be the multiplication (in frequency) of the transmitter PSD (based on the selected modulation type), the transmitter frequency response, and the receiver frequency response. If any of the frequency responses are disabled, they are modeled by a flat spectrum with unity magnitude over the appropriate bandwidth, and therefore are not a factor in the computation.