**REPORT ON EMWIN TESTING AT WALLOPS CDAS**

**February 22 to 24 2005**

**Prepared by**

**Peter Woolner**

**Mitretek Systems**

**SUMMARY**

Following a successful but short test performed on February 2, 2005 at
Avtec's facility at Chantilly VA, additional tests were made at the
Wallops CDA Station on February 22 to 24, 2005. These tests were
performed using the revised Avtec receive software and IF Adapter
designed for operation at 19.2 kbps user data rate. The EMWIN signal was
Offset Quadrature Phase Shift Keyed (OQPSK) modulated and used a pulse
shaping filter. The same Forward Error Correction (FEC) coding that was
used in the previous 9.6 kbps implementation was also applied to this
signal at double the data rate.

The tests were made with the Avtec transmit and receive equipment
connected back-to-back on the bench, but with the real GOES East receive
signals added to prove the receiver could tolerate that level of
interference. This was a close approximation of the level of
interference that is expected from the GOES N/P satellites.

The test results show the prototype EMWIN receive system met the
required performance criterion of one frame missing or one frame with
errors in 10,000, when the receive E~B~/N~0~ (roughly equivalent to the
signal to noise ratio) is between 3.0 and 4.5 dB. This is considered an
excellent result. It shows that any existing EMWIN user, i.e. one
receiving the current EMWIN signal at 9.6 kbps, will probably not need
to upgrade the figure of merit (G/T) of the receive system when they
change to the new EMWIN signal which is to be broadcast through the GOES
N/P series of satellites.

The new EMWIN signal format, using OQPSK and a pulse shaping filter, has
not yet been tested through its allocated transponder on a GOES N/P
series satellite. Therefore the amount of degradation caused by
non-linearities or other imperfections in that transponder is still
unknown. The tests results shown in this report indicate that satellite
degradation of up to 4.7 dB can be tolerated without requiring the
receive G/T be better than the NOAA standard that was recommended for
the existing EMWIN links on the GOES I/M series of satellites. This
writer believes it is unlikely the satellite degradation will exceed
this amount, but tests are scheduled to measure that degradation as soon
as a satellite is available.

**TEST DESCRIPTION**

All tests were performed with the equipment configured as shown figure
1.

![](media/image1.wmf)

**Figure 1. Block Diagram of Test Configuration**

All the FER tests were made using OQPSK modulation at 19.2 kbps user
data rate, but with the start and stop bits removed and the Attached
Sync Marker and frame header bits added. The FEC coding used was a rate
½ Convolutional inner code concatenated with a RS (255,223) outer code.
The level of the EMWIN signal into the receiver was set at -55 dBm. The
signal received from GOES East was combined with the test signal out of
the EMWIN modulator so that the level of the LRIT and EMWIN-I signals
from GOES East were 7 dB stronger than the EMWIN signal. The noise
generator output level was then adjusted as necessary to provide the
desired E~B~/N~0~ value.

For all tests, the frame sync parameters were set to Search Threshold =
0, Lock Threshold = 6, Check Frames = 0, and Flywheel frames = 4.

Prior to the start of the Frame Error Rate (FER) tests, the power out of
the modulator when modulated, was compared to its output power when
unmodulated. In each condition, the modulator output was fed to the
Boonton wattmeter. The power when modulated was found to be 1.0 dB
greater than when not modulated. That 1.0 dB correction has been applied
to all measurements recorded herein.

At the start of each test the C/N~0~ was measured for an unmodulated
carrier using a spectrum analyzer with internal averaging set at 100.
The offset for the N~0~ part of the measurement was chosen at the
frequency that produced the lowest noise. The C/N~0~ was also measured
at the end of each FER test and averaged with the start value to provide
the reference for that test. The E~B~/N~0~ values were then calculated
as:

E~B~/N~0~ = C/N~0~ + 1.0 -- 10\*LOG(15400) = C/N~0~ -- 40.9 dB/Hz

In each FER test, the carrier was modulated with a recorded EMWIN data
set, then the receiver was locked and allowed to run for a
pre-determined period of time. The numbers of frames transmitted and
received were recorded, plus the numbers of frames that are
uncorrectable by the RS code, Viterbi sync errors, frames missing, and
data frame sequence errors. This last item was used as a way to measure
of how many separate error events had occurred.

The count of missing frames included the uncorrectable frames, as well
as those frames that were missed because the Attached Sync Marker could
not be correctly identified. Since either event will result in a frame
that is omitted from the data passed to the user, it was the total
number of missing frames that was recorded and used to derive the FER.
The FER was calculated by dividing the total number of missing frames by
the total number of transmitted frames. Frames transmitted count was
derived from the receiver monitor values by subtracting from the
received frames count the number of fill frames (if any) then adding the
total number of missing frames minus the uncorrectable frames.

The timing of the start of each test was controlled by manually setting
the receive monitor parameters to zero, after the system had shown it
was in lock. Each test was ended by taking a screen shot of the receiver
monitor parameters at the pre-determined time. The values shown for each
parameter were used in the subsequent calculations.

**TEST RESULTS**

The following table shows the results of these tests, arranged in order
of increasing E~B~/N~0~.

  ------ ----------- -------------- -------- -------------- --------- -----------
  Test   E~B~/N~0~   Total Frames   No. of   Missing or     FER       Errors per
  Nbr    dB/Hz       Transmitted    Events   Error Frames             Event

  3      2.3         3160           31       58             1.8E-02   1.87

  1      2.6         904            48       75             8.3E-02   1.56

  9      2.8         24042          54       113            4.7E-03   2.09

  4      2.9         115504         82       645            5.6E-03   7.87

  2      3.0         10000          15       15             1.5E-03   1.00

  8      3.1         23554          2        2              8.5E-05   1.00

  5      3.95        23237          6        34             1.5E-03   5.67

  7      4.25        116023         1        79             6.8E-04   79.00

  6      4.3         24104          0        1              4.1E-05   0.00

  10     5.0         120012         0        1              8.3E-06   0.00
  ------ ----------- -------------- -------- -------------- --------- -----------

The following chart is a plot of these results for E~B~/N~0~ versus FER,
with some additional annotations that are discussed in the following
section.

> ![](media/image2.wmf){width="5.67007874015748in"
> height="5.034645669291338in"}

**Chart 1. Back-to-Back Frame Error Rate Test Results**

**DISCUSSION OF TEST RESULTS**

1.  The two points shown with red circles actually experienced no errors
    or missing frames during that test period. One error was
    artificially added to the result for each of these tests in order to
    produce the points plotted on the chart. The actual error rates for
    these points are unknown. It is probable, but not certain, that they
    lie some unknown distance below these plotted points.

2.  Test 7 (the point third from the right in the chart) was
    significantly different from all other test results. There was one
    error event in which 13 frames were uncorrectable and 66 were
    missing, giving a total of 79 frame errors in that single event.
    This is an order of magnitude worse than the next closest errors per
    event. It seems reasonable to consider this an anomaly. (The event
    occurred during an overnight test and had no witnesses.)

3.  Even if this point is eliminated, the fourth point from the right in
    the chart will remain, producing a curve that does not follow the
    normal "waterfall" shape. No explanation was found for this unusual
    effect. The only way this could be resolved was to make many more
    measurements with E~B~/N~O~ values between 3.0 and 4.0 dB/Hz and
    determine which point is supported by the additional tests. No time
    was available to perform such a check, so these results create a
    degree of extra uncertainty in the actual performance of the system.

4.  Two dashed lines were added to the chart to signify the boundaries
    within which this writer believes the true FER curve would lie if a
    sufficiently long test time had been available to collect enough
    measurement points to determine that curve with accuracy and at a
    high degree of confidence.

5.  These two dashed lines cross the FER value of 10^-4^ (the pass/fail
    criterion) at E~B~/N~0~ values of 3.0 and 4.5 dB/Hz. All the test
    results obtained are consistent with receiver performance within
    this range. However, even if the worst case value of 4.5 dB/Hz was
    the correct one, the system would be considered acceptable.

Using this worst case value, the minimum EIRP expected from the GOES N
series satellites of 44.8 dBmi, an EMWIN terminal G/T of -0.3 dB/K, and
the actual transmitted user data rate before FEC codes are added of 15.4
kbps, the worst case available E~B~/N~0~ is calculated at 11.2 dB/Hz.
(See Appendix A for the full link calculations.) Then:

> Margin = Available E~B~/N~0~ -- Back-to-back E~B~/N~0~ -- Satellite
> degradation

= 11.2 -- 4.5 -- 2.0 (If the satellite degradation is 2 dB.)

= 4.7 dB

The 4.7 dB margin is 2.7 dB over the minimum recommended operating
margin for this link, 2 dB, and is therefore available as an additional
contingency if future tests should show the actual satellite degradation
is more than the 2 dB estimated above. If the actual receiver
performance is better than the worst case as explained above, even more
satellite degradation could occur and an acceptable link performance
would still be provided.

Since the satellite transponder was not designed to carry a signal with
the modulation and using a pulse shaping root raised cosine filter as is
used on the EMWIN signal, more than usual degradation due to the
satellite transponder is expected to occur. However, as long as that
degradation does not exceed 4.7 dB, the EMWIN link as currently designed
will provide acceptable performance. It is considered unlikely that the
satellite degradation will exceed 4.7 dB, but this cannot be known for
sure until the link is tested through an actual GOES N series satellite.

**APPENDIX A. REFERENCE LINK BUDGET**

  --------------- ------------------------------------------- ------------ -----------
  **EMWIN LINK                                                             
  CALCULATIONS                                                             
  FOR 19200 bps                                                            
  NOMINAL RATE**                                                           

                  **Parameter**                               **Units**    **Value**

                  Data Rate (before framing and coding)       bps          15514

                  Modulation Type                                          OQPSK

                  FEC Coding Type                                          Conv+RS

                  FEC Coding Rate                                          0.437

  **Ground Uplink                                                           
  Transmitter**                                                            

                  Uplink Frequency                            MHz          2034.70

  Transmit EIRP                                               dBmi         76.7

  Antenna                                                     dB           0.5
  Pointing Loss                                                            

  **Earth - To -                                                            
  Space**                                                                  

                  Path Distance (for 5 deg elevation)         km           41127

  Free Space Loss                                             dB           190.9

  Atmospheric                                                 dB           0.4
  Attenuation                                                              

  **S/C Receive                                                             
  Performance**                                                            

  Polarization                                                dB           0.2
  Loss                                                                     

  U/L Incident                                                dBmi         -115.3
  Power                                                                    

                  Antenna Gain (from 5 deg elevation)         dBi          14.8

  G/T (incl cable                                             dB/K         -14.2
  losses)                                                                  

  Boltzmann                                                   dBm/Hz/K     -198.6
  constant                                                                 

  Uplink Thermal                                              dB/Hz        69.1
  C/No                                                                     

  **S/C Transmit                                                            
  Performance**                                                            

                  Downlink Frequency                          MHz          1692.70

  Transmit EIRP                                               dBmi         44.8
  (to 5 deg                                                                
  elevation)                                                               

  **Space - To -                                                            
  Earth**                                                                  

                  Path Distance (for 5 deg elevation)         km           41127

  Free Space Loss                                             dB           189.3

  Atmospheric                                                 dB           0.4
  Attenuation                                                              

  **Ground                                                                  
  Downlink                                                                 
  Receiver**                                                               

  Downlink                                                    dBmi         -144.9
  Incident Power                                                           

                  Pwr Flux Density Rqmt (EOC in 4 kHz)        dBW/m2       -154

                  Power Flux Density (EOC in 4 kHz)           dBW/m2       -159.0

  E/S G/T                                                     dB/K         -0.3

  Polarization                                                dB           0.2
  Loss                                                                     

  Boltzmann                                                   dBm/Hz/K     -198.6
  constant                                                                 

  Downlink                                                    dB/Hz        53.2
  Thermal C/No                                                             

  **Eb/No Overall                                                           
  Calculation**                                                            

  Overall                                                     dB/Hz        53.1
  Composite C/No                                                           

  Data Rate in dB                                             dB-Hz        41.9

  Eb/No                                                       dB           11.2
  Calculated                                                               
  --------------- ------------------------------------------- ------------ -----------

  ---------------- ------------------------------------------- ------------ -----------
  **Required Eb/No                                                           
  Adjustments**                                                             

                   Required BER                                             1E-06

  Theoretical                                                  dB           10.6
  Eb/No with no                                                             
  coding                                                                    

  Theoretical                                                  dB           8.0
  coding gain                                                               

  Theoretical                                                  dB           2.6
  Eb/No with FEC                                                            
  coding                                                                    

  Ground Segment                                               dB           1.9
  Implementation                                                            
  Loss                                                                      

  Satellite                                                    dB           4.7
  Segment                                                                   
  Degradations                                                              

  Required Eb/No                                               dB           9.2

  **Eb/No MARGIN**                                             **dB**       **2.0**
  ---------------- ------------------------------------------- ------------ -----------