Reliability vs Availability
Network reliability is similar to availability; however, instead of measuring the amount of uptime in a system, reliability is the measured likelihood of a failure occurring in a system. Reliability will track how long a network's infrastructure is functional without interruption.
Reliability - Fixed
RF link reliability is affected by three things: (1) median signal level, (2) multipath fading and (3) additional path attenuation.
First, the median signal level at the receiver has to be sufficiently higher than the receiver threshold (typically 5dB or better) to provide adequate thermal fade margin. The median signal level is found from all the gains and losses in the system including the path loss.
With a given received median signal level, the signal level at the receiver can still sometimes fall below the threshold level due to a) multipath fading, and b) additional path attenuation due to rain along the path. This dialog box gives you options for selecting how multipath fade outage probability and rain outage probability are calculated.
2. Multipath Analysis
For multipath fade outage analysis, you can select from four methods using the Fade outage analysis method drop-down list:
● Vigants-Barnett
● ITU-R 530-8 (the method in ITU-R Rec. 530-8)
● ITU-R 530-10 (the method in ITU-R Rec. 530-10)
● User Fade Outage Table (A user-defined fade outage table - see Appendix H)
● None
Depending on your selection, you must input some additional parameters in the other dialog box sections. Selecting “None” means there will be no fade outage calculation (no reduction in reliability due to fading).
3. Additional Path Attenuation Types
Exterior
You may also enter a value for the adjacent channel and external (co-channel) interference. The Other Exterior value will be used in the calculation of the flat-fade margin as described in Appendix H. An entry of -150.00 disables this option.
Dispersive Fade Margin
In this input field, you can enter a value for the dispersive-fade margin. This is applicable to digital systems only, and is incorporated in the overall fade margin as described in Appendix H. The Dispersive-fade margin is typically a specification found in manufacturer's literature for digital microwave radios. An entry of 80.0 disables this option.
Fade Occurrence Factor
This fade occurrence factor is derived from empirical data about the rate of fading. It is set using the maps found in Appendix H. A typical value for this input field is 5.0.
Rain Fade Analysis
Rain outage analysis can be done by one of the following methods:
● Crane (the Crane method)
● ITU-R Rec 530-8/9/10 (the method found in ITU-R Rec. 530-8/9/10)
● ITU-R specified rain rate (ITU-R Rec. 530 with a specified rain rate)
● User Rain Outage Table (A user-defined rain outage table - see Appendix H).
● None
Rain Rate Table Data
With EDX software you have the flexibility to use any of four rain rate tables with any of the first three rain outage calculation methods listed above. Normally you will want to use Crane data with the Crane method and ITU-R data with the ITU-R method, but you are not limited to those combinations here.
Rain Region
The rain regions available for selection in this drop-down list will depend on the rain rate table data you have selected.
Use Correlated Rain Fade Analysis
EDX software has the unique ability to take into account interfering signals which experience rain fades that are similar to the desired signal. When this option is selected, the assumption is made that the signal from the interfering transmitter is arriving along a path that is similar to that of the desired signal. In this circumstance, the rain fade along the two paths is most likely the same (correlated) because both signals experience the same rain conditions. Therefore, the C/I ratio to this interferer will not change during a fade. Recognizing this condition can be important to correctly calculating interference in high-density Point-to-Multipoint (PMP) and Consecutive Point Network (CPN) systems.
Alternate Lognormal Fading Distribution
Finally, at the bottom of the dialog box you can choose an alternate distribution method of calculating fade on the assumption that the fading is log normally-distributed (a normal or Gaussian distribution in dB). Given this assumption, there is only one other parameter to set to describe the fading distribution: the standard deviation. You can set the standard deviation of the distribution one of three ways:
● Based on the land use (clutter) type at the receiver. The actual standard deviation values for each clutter type are set on the Database>Land Use (Clutter) dialog box in the attenuation file. Click on the Edit Attenuation File button.
● Using standard deviations specified in a file as a function of path length.
● As a fixed value in dB to be used regardless of path length or land use (clutter) type. A typical value here might be 6 to 8 dB.
Regardless of the way in which the standard deviation is chosen, the program will use that value to establish a lognormal distribution to describe the fading. From that distribution, a determination can be made as to the percent of time the signal falls below the service threshold and from that, calculate link availability.
Mobile
TSB-88 Studies Setup
One of the Study Groups available from the Area Studies Details dialog box in EDX SignalPro, is the TSB-88/Monte Carlo set of studies (not available in EDX Signal). These studies follow the procedures outlined in TIA TSB-88 for calculating the percent of reliability or percent of abounded service area that meets the Channel Performance Criteria (CPC). The TIA TSB-88 document describes a method for calculating the coverage within a service area of a transmitter ("station") by calculating the C/(I+N) at many points ("tiles/Tx") within the study area. If the calculated C/(I+N) is greater than the required CPC for that system, then the point is considered served. It is difficult to guarantee 100% coverage at all points within a service area due to signal fading and incomplete knowledge of the radio environment. Therefore, licensees and operators will typically specify minimum required performance in terms of statistical signal strength reliability and/or percentage of area meeting this performance within the service area.
These studies include:
● TSB88 Monte Carlo Bounded Area Coverage
● TSB88 Radius-of-operation overlap Reliability
● Aggregate Simulcast Monte Carlo Reliability
● Simulcast delay spread (Hess)
● Hess Simulcast Monte Carlo Reliability
Along with the visual study results that are displayed in the main map, some of these studies generate a text report as well. These reports can be viewed by going to the System Reports dialog box. (Utilities>System Reports). The setup dialog box that controls the study parameters is accessed by clicking Studies>TSB-88 Studies Setup.
Service Area Boundary File
Use this to point or browse to the boundary file you want to use for the studies. This input accepts “.bna” or “.mif” boundary files.
Simulcast Delay vs Signal File
A requirement for the Hess Simulcast Monte Carlo Reliability study is a simulcast delay vs. signal performance curve file for the mobile/remote unit. This data is used to determine the criteria for successful receiver performance. This field points to that file.
Required Tile Reliability/Required Simulcast Delay/Required CPC
Required Tile Reliability (%) is the percentage of Monte Carlo sample C/(I+N) calculations in a tile that are required to be above the Required CPC in order for that tile to "pass" (i.e. provide acceptable service).
Required Simulcast Delay (us) is used for the simulcast reliability studies in this study unit (and is not needed for the interference studies).
Required CPC is a dB number based on the modulation type in use, the C/(I+N) needed for the desired DAQ level. This number is obtained from table A.1 in the annex A of the TSB-88 document that describes TSB-88 studies. The title of the table is “Projected CPC Parameters for Different DAQ’s.”
Monte Carlo
The Random Seed field sets the "random" function within a microprocessor that generates sequences of pseudo-random numbers. The "seed" is used to determine which sequence. Using the same seed for each study will ensure that the same set of Monte Carlo "random number" draws are used (this provides consistency within consecutive studies).
Standard Deviation is a dB number used by the Monte Carlo routine to set the log normal distribution used in the Monte Carlo calculations. The Samples/TX/Location field is used to enter the number of "draws" of faded signal strength for each station at each study point.
Clutter Attenuation File
The clutter attenuation file contains information that is used along with the clutter data file to determine the additional path loss to be applied at each study location. In addition, other data is included that can be used by some other study features found in the EDX software. For example, a standard deviation value can be defined for each clutter type when doing a signal reliability calculation.
This file can be created and edited using the editor within the program (Clutter File Database) or it can be created using a text editor following the format described below. A sample file named gcattn_TSB88.dat is provided with the software (in the \DATA folder) that can be used as an example of this format.
version 4 EDX land use attribute file |
numcat, numfreq |
cat(1),label(1),height(1),traf(1),stddev(1),multipath(1) |
cat(2),label(2),height(2),traf(2),stddev(2),multipath(2) |
. |
cat(numcat),label(numcat),height(numcat),traf(numcat),stddev(numcat) |
freq(1), freq(2),. . . freq(numfreq) |
cat(1), atten(1,1), atten(1,2),. . . atten(1,numfreq) |
cat(2), atten(2,1), atten(2,2),. . . atten(2,numfreq) |
. |
cat(namcat), atten(numcat,1), atten(numcat,2),. . . atten(numcat,numfreq) |
cat(1), dbkm(1,1), dbkm(1,2),. . . dbkm(1,numfreq) |
cat(2), dbkm(2,1), dbkm(2,2),. . . dbkm(2,numfreq) |
. |
cat(numcat), dbkm(numcat,1), dbkm(numcat,2),. . . dbkm(numcat,numfreq) |
where: numcat - total number of clutter category types (maximum = 99). numfreq - total number of frequencies (maximum = 20). cat - category number of clutter type (1 to 99).
lable - text label describing category (32 characters maximum). This label must be enclosed in single quotes (‘). height - clutter category height in meters. traffic - traffic load in milliErlangs (NOT USED). stddev - standard deviation in dB (reliability studies).
multipath - indicates the general radio path non-correlation condition within a clutter type
(NOT USED) freq - frequency in MHz for attenuation/dbkm values. atten - local clutter attenuation at receiver in dB. dbkm - pass-through attenuation in dB/km.
The Telecommunications Industry Association (TIA) has published a Technical Service Bulletin known as TSB-88 [8]. In this document is information on suggested communication service quality standards as well as methods for calculating/predicting these values.
TSB88 Monte Carlo Bounded Area Coverage
Following the general procedure found in the TIA document TSB-88.1-C, the mean signal levels of all transmitters are found at each tile location (EDX Study Grid point ("bin") that falls within the Service Area Boundary polygon. These signal levels are calculated using the user-selected
propagation model and databases. Next, the Monte Carlo simulation method as described in TIA-TSB-88.2-C is used to determine the probability of a "pass" for each tile. This probability is shown on the EDX map as a color.
TSB88 Radius-of-operation overlap Reliability
The TSB-88 Radius-of-operation Overlap Reliability study is identical to the Monte Carlo
Bounded Area Coverage study with two added restrictions. First, the Radius-of-operation Overlap Reliability study must have exactly one transmitter in the primary group (treated as the carrier) and exactly one transmitter in the secondary group (treated as the interferer). Second, the calculation of the Radius-of-operation Overlap Reliability study is restricted to the area that is within the approximate sector range of both transmitters. The Approximate sector range can be set by right clicking on a transmitter, selecting Site Properties, and clicking the Study sector / Sector range button.
The program requires the user to produce two polygon files that define the radius of operation area of the serving and interfering transmitters. These files are should be placed in the \ctr\ folder of the Project and are named using the Transmitter/Sector ID with a ".bna" file extension. If desired, they can be added to the Map Layers to display the service contour areas on the map.
Aggregate Simulcast Study using Monte Carlo
This examines all transmitters that have a signal at each study location. Those signals that are above the Signal level display threshold (Area Study Details dialog box parameter (Area Studies article)) are tested as possible servers to the remote unit. The strongest signal is considered the reference signal. For another signal to be considered a constructive it must have a relative time delay that is less than the Required Simulcast Delay (TSB-88 Studies Setup parameters). Otherwise, the signal is considered destructive (i.e. interference). The aggregate powers of the constructive and interfering signals are independently summed using the Monte Carlo method described in TSB-88.2. For each Monte Carlo "draw" the resulting C/(I+N) ratio is compared to a desired minimum ratio (Mobile/Remote Unit, Required C/(I+N)). The number of "passes" is displayed as a percentage of the total Monte Carlo draws (i.e. the "reliability" of the simulcast signal at that point).
Simulcast Delay Spread ("Hess")
The Hess method of simulcast RMS delay spread is used to determine the extent of time spreading at each study grid location. It is most useful for simulcast systems such as digital paging that transmit signals from the multiple locations simultaneously on the same channel.
Interference in the receiver will occur under certain conditions related to the delay time between signals arrive at a given location and their relative amplitude. The RMS delay spread is a way of characterizing the potential degradation using a single number.
The simulcast RMS delay spread is calculated as follows:
where
and
The values A_{n} are the relative amplitudes of the signals arriving from each of the N transmitters. The arrival times τ_{n} are relative to the strongest signal and can be positive or negative. The results of the calculation in μSec are multiplied by two (the Hess method) and displayed in the map view.
Hess Simulcast Monte Carlo Reliability study
This study calculates the reliability of a signal at the mobile/remote unit receiver where that signal is composed of simulcast (synchronous) transmissions from two or more transmitters. A requirement for this study is a Simulcast Delay vs. Signal performance curve file for the mobile/remote receiver. The data in this file is used to determine the criteria for successful receiver performance. This file is created using the editor found in Utilities -> Create Simulcast Curve File...
The entries in the file represent the receiver threshold under different simulcast conditions.
They are derived from manufacturer's specifications or laboratory testing of the receiver. The "dB" value is Signal to Noise Ratio ("SNR"). The "Delay" value is the simulcast signal relative delay (typically RMS delay). A point that falls in the area on or above the line in the graph represents a "pass" meaning that the receiver successfully demodulates the signal.
For each study point ("bin") in the study area the study performs the following analysis:
● Determine the mean received signal level and signal propagation transit time from each transmitter based upon the selected remote/mobile receiver. If the received signal level
is less than the Area Study details - "Signal level display threshold" (see Area Studies article) the bin is shown as a "No service" bin.
● The following group of steps are repeated based upon the number of Monte Carlo "draws" as defined by the Monte Carlo - " Samples/TX/Location" value entered in the TSB-88 System Study Setup (see figure below): o The received signal level from each transmitter is adjusted a random dB amount based on a log normal distribution using the " Standard Deviation" entry in the TSB-88 System Study Setup dialog.
o The SNR of each randomly faded signal is calculated. o Then, using all the received signals the Hess Delay at the receiver is calculated as well as the summed power received from all transmitters.
● If the summed power is less than the Mobile/Remote Unit - " Required service threshold" this Monte Carlo "draw" is a "fail" and we move on to the next draw. Otherwise, we continue....
● The SNR of the summed power and the receiver noise is calculated.
● If the SNR vs. Hess Delay values fall on or above the line in the Simulcast Curve Graph (described above) this Monte Carlo draw is considered a "pass".
● The Reliability of each bin (38.2% to 99.999%) is then the total number of "passes" divided by the total number of Monte Carlo draws.
Availability
Signal Availability and Reliability Analysis Parameters
Fade outage analysis method: Vigants-Barnett, ITU-R Rec.530-8, ITU-R 530-10, User(defined) Fade Outage Table, general lognormal, none
Vigants-Barnett C factor parameters: 0.25, 1.0, 2.0, 4.0, 6.0
Terrain type for ITU-R 530-8 method: rolling hills, mountainous, over water
ITU-R 530-8 percent of time refractivity units/km : 1 to 99%
External interference: -300 to 0.0 dBmW
Dispersive fade margin: 0.0 to 60.0 dB
Fade occurrence factor: 0.1 to 10.0
Rain outage analysis method: Crane, ITU-R Rec. 530-8/9/10, or user-defined outage table, user-specified rain rate at 0.01 % of the time, none
Rain rate region tables: A through H & LZ (local zone) for Crane, A through P for ITU-R, Crane 1996
Lognormal fading standard deviation selections: base on clutter, distance, and a user-specified fixed value in dB
Annual Link Availability
The annual multipath fade and rain outage results for a link can be converted into annual link availability using the following equation:
This annual link availability value is reported on the link profile plot and in the link analysis results file.
From an industry perspective:
Availability = Uptime ÷ (Uptime + downtime)
Percentage calculation
Availability % | Downtime per year | Downtime per month |
99.8% | 17.53 hours | 87.66 minutes |
99.9% ("three nines") | 8.77 hours | 43.83 minutes |
99.95% ("three and a half nines") | 4.38 hours | 21.92 minutes |
99.99% ("four nines") | 52.60 minutes | 4.38 minutes |
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