United Kingdom

Testing of ITU-R P.1812 with respect to long-term measurement data

# Introduction

The recently-adopted point-area propagation model, ITU-R P.1812, has
been developed to a large extent, from algorithms already embodied in
ITU-R P.542. The earlier Recommendation has been extensively tested
against empirical data, and has gained wide acceptance as a reliable
model.

The new Recommendation, however, is intended to cover a frequency range
with a lower limit of 30 MHz, compared with a lower limit of \~700 MHz
in P.452. A previous 3K contribution \[1\] examined the frequency
dependence of the algorithms in P.452 and concluded that they should,
generally, be applicable over the wider frequency range, with the caveat
that the model be limited to use at greater than 1%-time below 700 MHz.
Furthermore, it was noted that the troposcatter and ducting/layer
reflection models might require further optimisation.

Workers in the UK have now produced six independent implementation of
P.1812 that provide results agreeing to better than 0.001dB for a set of
agreed test paths. These results give confidence that the algorithms
have been correctly implemented, but offer no indication of the
performance of the Recommendation.

It is likely that, to inform the current discussions within the CG
regarding the choice of diffraction model, the bulk of the initial
testing will focus on relatively short paths, for which diffraction is
the dominant mechanism. This contribution, however, concerns the testing
of the overall Recommendation against empirical data gathered from
long-term measurements on paths of, typically, 100km-400km. This note is
intended only to give an initial insight into the performance of the
overall Recommendation, and further contributions will present more
detailed analysis of the behaviour of individual elements of the model.

# Long-term measurement data

A substantial body of empirical data has been gathered over some 60
years, relating to the variability of transmission loss over long paths.
Unfortunately, the bulk of this data relates to North West Europe and
North America, with little coverage of other latitudes.

An attempt is currently being made to assemble a reliable database of
such measurement results, for the purpose of testing P.1812 and other
ITU-R models. The main source for this work is the set of data collated
by R.S.Sandell (RSS) of the UK some 15 years ago, and currently archives
on the SG3 website at
<http://www.itu.int/ITU-R/study-groups/software/dbsg5/index.html> under
the heading "*Databanks of VHF/UHF measurements relating to terrestrial
broadcasting*". Part of this data follows a format similar to, but not
identical with, that given in Table VI-I of P.311 for signal variation
with time, and includes some 3,300 records.

The amount of directly usable data is, however, considerably less than
this figure might suggest. Almost half of the 3,300 records relate to
the statistics for monthly subsets of longer measurements. In a further
300 records, information is missing on terminal position or aerial
height. Finally, 1300 records are for durations significantly shorter
than a year. Excluding these records leaves a database of 328 records,
spanning frequencies between 45 MHz and 3 GHz, as indicated in Figure 1.

![](media/image1.wmf){width="6.692361111111111in"
height="4.295138888888889in"}

***Figure 1: Long-term measurement database statistics***[^1]

The vast majority of the paths represented in the original and filtered
database are in North West Europe, with others in North America, Japan
and the Gulf. The geographical extent of these links is indicated in
Figure 2, with details (where colour indicates frequency band) in
Figures 3 and 4.

![](media/image2.png){width="6.584722222222222in" height="3.29375in"}

***Figure 2: Geographical extent of filtered data***

![](media/image3.png){width="3.1902777777777778in"
height="2.814583333333333in"}
![](media/image4.png){width="2.9993055555555554in"
height="2.8208333333333333in"}

***Figure 3: American links Figure 4: European links***

In addition to the RSS data, some long-term measurements are also
included in the data already converted to 3K1 format by Mr Fryderyk
Lewicki; the set 'IRTs' describes 11 paths at Band II in Southern
Germany, while 'YLEs' relates to paths in Finland that are also included
in the filtered data set. The data set 'ABU' makes a very useful
geographical extension to the existing data, but currently requires some
further validation.

The 'sanitising' of the available measurement database is still
continuing, and it is hoped to add further data as missing path
information becomes available from original documentation. Conversely,
some existing data will need to be excluded as measurement errors are
traced (examples already detected include paths with undocumented local
clutter and measurements at the receiver noise floor).

Some of the records in the original database include details of the path
profile. In all cases, the new, filtered, database includes terrain data
taken from the best source, which is of 50m horizontal resolution around
the UK, from SRTM data at latitudes below 60, and from GLOBE data
elsewhere. Regardless of underlying terrain resolution, these profiles
have been generated with a point-spacing of 1km (the impact of
point-spacing is considered in 3.2 below)

It is intended that a collated and filtered long-term database will be
made available to the group (in XML and 3K/1 formats) by the end of
January.

# Testing of P.1812 using long-term data

One of the UK implementations of P.1812 has been exercised using the
filtered database described above. No corrections[^2] for clutter were
made as it is believed that all terminal locations in the database
described above were intended to be free of local clutter.

The initial results are presented below on a band-by band basis.

![](media/image5.wmf){width="6.588888888888889in"
height="3.157638888888889in"}

***Figure 5: Band 1 results***

Examination of the detailed model output shows that at the smallest
%-times, the ducting model is underestimating the loss, while at 50%
both the troposcatter and diffraction models show (a smaller) tendency
to underestimate the loss.

In common with results for other bands, the ducting model shows lower
mean errors for longer paths, while the performance at 50% is largely
independent of path length.

![](media/image6.wmf){width="6.588888888888889in"
height="3.157638888888889in"}

***Figure 6: Band 1I results***

Much the same trends are evident as for the Band I results. The outlier
at 50% time represents an 860km path for which it is suspected that the
signal was below the noise floor of the receiver.

![](media/image7.wmf){width="6.588888888888889in"
height="3.157638888888889in"}

***Figure 7: Band 1II results***

Note that the Y-axis in Figure 7 has been extended to accommodate the
outliers. These points relate to a single path of 930km in the Gulf on
which path losses approached the free space value for a significant
portion of the time, and remained within 40dB of free-space at 50% time.
The statistics given in the tables below exclude this path.

![](media/image8.wmf){width="6.588888888888889in"
height="3.157638888888889in"}

***Figure 8: Band 1V results***

At Band IV frequencies (400-600 MHz) there is some evidence that the
ducting model is starting to perform somewhat better, as might be
expected from its origin in P.452.

![](media/image9.wmf){width="6.588888888888889in"
height="3.157638888888889in"}

***Figure 9: Band V results***

The pairs of outliers between 1% and 50% relate to two paths in the Gulf
. The statistics given in the tables below exclude these paths.

![](media/image10.wmf){width="6.588888888888889in"
height="3.157638888888889in"}

***Figure 10: Band VI results***

The results for frequencies between 1 and 3 GHz appear quite good, but
the small number of measurements in the database allows little
confidence.

The positive outliers at 10% and 50% relate to a path in Germany, the
negative outlier at 10% to a path in Florida. No explanation is apparent
in either case.

  ------------ ---------- ---------- ---------- ---------- ---------- ----------
  **%-time**   **Band I** **Band     **Band     **Band     **Band V** **Band
                          II**       III**      IV**                  VI**

  **0.01**     -19.2      -19.1      -13.9      -6.4       -8.1       -7.9

  **0.1**      -16.5      -18.5      -11.9      -5.7       -9.2       -8.1

  **0.3**      -9.0       -13.6      -8.8       -5.9       -8.2       -3.9

  **0.5**      -7.6       -13.0      -9.7       -3.3       -8.3       -4.0

  **1.0**      -10.3      -15.7      -8.5       -7.7       -11.2      -2.6

  **3.0**      -4.4       -7.3       -6.5       -6.0       -7.2       -6.0

  **5.0**      -5.9       -8.7       -2.4       -5.5       -4.6       -1.2

  **10**       -6.1       -4.9       -3.4       -5.4       -8.0       -3.2

  **50**       -7.3       -3.5       -3.4       -3.5       -6.0       -2.8
  ------------ ---------- ---------- ---------- ---------- ---------- ----------

***Table 1: Average errors***

  ------------ ---------- ---------- ---------- ---------- ---------- ----------
  **%-time**   **Band I** **Band     **Band     **Band     **Band V** **Band
                          II**       III**      IV**                  VI**

  **0.01**     9.4        12.6       8.6        13.8       11.5       8.1

  **0.1**      10.7       12.9       9.4        14.2       10.4       7.3

  **0.3**      5.0        17.5       10.9       13.9       9.9        2.1

  **0.5**      5.0        18.3       11.7       11.5       10.2       1.6

  **1.0**      10.3       13.6       11.0       12.7       10.1       9.3

  **3.0**      2.7        17.4       14.7       11.9       9.2        7.2

  **5.0**      6.4        11.3       12.1       10.7       8.5        1.1

  **10**       5.4        7.4        7.5        12.6       10.5       13.8

  **50**       4.6        8.6        6.7        12.8       9.4        13.6
  ------------ ---------- ---------- ---------- ---------- ---------- ----------

***Table 2: Standard deviation of errors***

## Impact of profile point spacing

A brief check was made of the sensitivity of the model to the choice of
profile point spacing. This issue will be of greater relevance for
shorter paths, and it was expected that differences would be slight for
the database tested here, and confined to the higher time-percentages.

  ----------------- ----------------- ----------------- -----------------
  **%-time**        **0.5 km**        **1 km**          **2 km**

  **0.01**          -7.9              -8.1              -8.2

  **0.1**           -8.9              -9.2              -9.4

  **0.3**           -7.9              -8.2              -8.4

  **0.5**           -8.5              -8.3              -8.9

  **1.0**           -11.4             -11.2             -11.6

  **3.0**           -7.1              -7.2              -7.3

  **5.0**           -4.5              -4.6              -4.7

  **10**            -7.8              -8.0              -8.5

  **50**            -5.6              -6.0              -6.6
  ----------------- ----------------- ----------------- -----------------

***Table 3: Average errors versus point-spacing***

  ----------------- ----------------- ----------------- -----------------
  **%-time**        **0.5 km**        **1 km**          **2 km**

  **0.01**          11.5              11.5              11.7

  **0.1**           10.2              10.4              10.5

  **0.3**           9.8               9.9               10.1

  **0.5**           10.0              10.2              10.2

  **1.0**           10.2              10.1              10.2

  **3.0**           9.2               9.2               9.1

  **5.0**           8.5               8.5               8.4

  **10**            10.6              10.5              10.6

  **50**            9.6               9.4               9.5
  ----------------- ----------------- ----------------- -----------------

***Table 4: Standard deviation of errors versus point-spacing***

As shown in tables 3 and 4, these expectations were met, with only the
statistics for average error at 10% and 50% showing a significant
improvement as the profile resolution is increased. No reduction in the
standard deviation of error is evident.

# Conclusions

It appears that the model given in the new Recommendation, P.1812
performs reasonably well for long paths, at times down to 1% and
frequencies down to 40 MHz.

There is a general tendency to underestimate both diffraction and
troposcatter loss by 4-6dB, and there are some indications that the
ducting model works better at frequencies above 500 MHz.

The model fails to reproduce the (few available) measured statistics of
path loss experienced in the extreme radio-climactic conditions found in
the Gulf.

Further work will examine the performance of the component algorithms in
more detail.

# References

\[1\] 3K/132 Information document on using Rec. ITU-R P.452 in the
frequency range 30 MHz to 3000 MHz: a candidate for a deterministic
Point-To-Area model, UK

*RFR / 21 Jan 08*

\_\_\_\_\_\_\_\_\_\_\_\_\_\_

[^1]: Band I: 41-72 MHz, Band II: 88-108 MHz, Band III: 160-216 MHz,
    Band IV: 400-600 MHz, Band V: 600-950 MHz, Band VI:1297-3000 MHz.

[^2]: either by additions to along-path terrain heights or in the form
    of terminal corrections