Subject: [NEXTEL-1] Advanced P25 Phase II Linear Simulcast ModulationTM analysis

From: [email-masked]
Date: Tue, 16 Dec 2003 20:19:10 -0500

Advanced P25 Phase II Linear Simulcast ModulationTM analysis

Within the P25 standard, the modulation formats for over the
air transmission of digital data have been harmonized to allow
for different, but compatible modulation formats to be utilized
depending on application and bandwidth requirements. This
application note will deal with the utilization of CQPSK
modulation used in new P25 systems that employ Linear
Simulcast ModulationTM technology and in P25 Phase II systems.

Understanding P25 Modulation Formats

P25 systems utilize digital modulation techniques to transmit the
digital ones and zero’s across the air interface. These formats are
defined in the TIA/EIA-102 standards and apply to Phase I P25
systems and Phase II systems. Each of the modulation formats
accomplishes the task of transmitting digital information over the
air in a bandwidth efficient manner. Two modulation formats are
called out in the P25 standard, C4FM modulation in a 12.5 kHz
bandwidth which is used for current P25 Phase I systems and
CQPSK or Compatible Quadrature Phase-Shift Keying modulation
which is intended to be utilized for Phase II P25 systems operating
in a 6.25 kHz bandwidth. (See note).

Phase 1 P25 systems utilize a form of digital modulation called
C4FM. C4FM is a 4-carrier modulation format where the carrier
is shifted in frequency at a particular rate (time) to a particular
location around a center frequency. This allows for each of the 4
"states" to represent a binary number. Each state is a "DiBit" or
"Symbol" which contains two bits of information. Figure 1.0
shows how C4FM is used to impart the digital information over the
air.

Figure 1.0 - C4FM Modulation
As you can see, each of the carriers depicts a particular symbol.
This is a simplified representation of C4FM since, in reality, the
carriers would not be seen as individual, stand alone carriers
shifted off center from the fundamental frequency. Although each
carrier has a fixed offset (-1800 Hz, -600 Hz, +600 Hz, +1800
Hz), the carrier never returns to center frequency. Each transition
of the carrier is encoded to start from where it was last positioned.
Figure 2.0 shows how a C4FM signal would look on a frequency
plane or a vector signal analyzer like the Agilent 89440A/89431A
after the decoding process.

Note: P25 Phase II Systems are in development and there are at least two
different
Phase II approaches that are either specified or in development. They include
potentially a TDMA system that provides similar bandwidth efficiency. This
application
note deals with the Phase II P25 systems utilizing 6.25 kHz bandwidths
and CQPSK modulation.

Figure 2.0 C4FM in a Frequency Plane with Decision Points
As can be seen, there are 4 decision points (indicated by the red
dots) that represent the frequency states of the C4FM signal at a
particular time. Each are valid decision points that tell us what the
value of the symbol will be (11, 10, 00 or01). Each of the decision
points has to fall within a valid range to be recognized as a
symbol. If it falls outside the "window" (shown as the blue circle),
it becomes an invalid symbol and the demodulator in the radio will
decode it as an erroneous symbol and will therefore see a higher
"BER" or Bit Error Rate.

As we begin to look at the modulation format for P25 and Linear
Simulcast ModulationTM, we need to understand that the CQPSK
signal can more easily be depicted in an I/Q plane. I/Q stands for
In-Phase and Quadrature-Phase and it is simply a plot of phase
and amplitude. Every sinusoid can be expressed as the sum of a
sine function (phase zero) and a cosine function (phase p/2).
Taking the "sine", or In-phase component, and combining it with
the "phase-Quadrature" derives an I/Q plot. The
"phase-Quadrature" is shifted 90 degrees. The resulting In-phase
and Quadrature phase signal can be depicted in an I/Q Plot
shown in figure 3.0.

Figure 3.0 An I/Q plot showing phase and amplitude
differentials.

Since C4FM is a constant amplitude modulation, the I/Q plot of
the modulation looks like a circle, with the 4 decision points
referenced by the amount of phase offset from zero. Figure 4.0
shows how C4FM would look on an I/Q plot.

\
Figure 4.0 C4FM Modulation depicted in an I/Q plot.
Although we do not see an amplitude shift, we do see a phase
shift which is attributed to the frequency shift of the carrier. This
is the key to compatibility with CQPSK modulation in that the
symbols "appear" in the same position in the I/Q plane regardless
of modulation formats. (Note: The IFR 2975 does not perform
C4FM plot analysis in the I/Q domain.)

Since the data rate, bandwidth and bits per symbol are identical,
the main difference between the two modulations is that C4FM,
using a frequency shift, provides a fixed amplitude signal. CQPSK,
since it requires a phase shift to depict a symbol, imparts an
amplitude component to the signal.

The modulation format for LSM is the same digital modulation as
P25 Phase II, or CQPSK. Figure 5.0 shows CQPSK modulation in
the I/Q plane. Here you can clearly see the amplitude component
of the CQPSK modulation, and the decision points, which would
be the same as C4FM.

Figure 5.0 CQPSK Modulation

The primary difference between P25 Phase II and LSM is the
Alpha, or shaping of the filter. Where the P25 standard calls for a
more aggressive Alpha of .2 to keep within the 6.25 kHz channel,
LSM uses an Alpha of .35, which provides a more gradual roll-off
of the side skirts of the occupied bandwidth, and is used in a 12.5
kHz channel. Figure 6.0 shows the comparison between P25
Phase II and LSM filter shaping.
Figure 6.0 Alpha Depiction of .35 and .2

Why LSM is Utilized

Linear Simulcast ModulationTM is utilized mostly in dense
metropolitan areas. The amplitude component of the PSK
modulation allows for better in-building penetration of the signal.
In addition, due to the use of highly linear amplifiers, the range of
the signal is significantly extended, sometimes up to twice the
distance.

For Control Channels, this equates to better range of the repeater
station that controls mobile access to the network. For Voice
Channel use, the range of the users’ listening capacity is
extended.

How do you Measure Frequency Error of a CQPSK

Signal?

With digital modulation, seemingly easy measurements like
frequency error become difficult. This is due to the fact that with
the phase shifts occurring on the RF carrier, a stable frequency
measurement is difficult obtain. To determine the actual
frequency of the signal, a mathematical rendering of the RF signal
must be calculated as a separate computation and then
compared against a mathematically derived "ideal" signal. Only
then can we determine accurate frequency error measurements.
The 2975 with the LSM Advanced Option performs these
calculations for fast and accurate frequency error measurements
of the LSM CQPSK signal.

What is measured with CQPSK modulation?

In addition to frequency error, there are various parameters that
we can derive from the I/Q plot of the phase-shifted symbols.
The primary measurements for this digital modulation format
include EVM, Phase Error and Magnitude Error. Figure 7.0 shows
the various parameters.

Figure 7.0 EVM, Phase Error and Magnitude Error
Understanding the impact of this is important in testing digital
modulation. The ability of the modulator to produce as close as
possible to an ideal waveform is critical to the receiving radio's
ability to decode the digital data being sent over the air.
As you can see, we start with the ideal symbol point indicated by
the (+). This is the point where the symbol "should" be. With
digital modulation, these symbol points come close, but rarely
land where they should. Here, the actual landing point is
depicted by the black dot. The resulting error vector - the red
arrow - shows how far the actual landing point is from the ideal
point.

ErrorVector Magnitude is the ratio of the length of the error vector
over the normalized ideal or "reference" signal and is expressed
as a percentage. For example, if the magnitude of the error
vector is .03 compared to a normalized (to 1.0) ideal signal, the
ErrorVector Magnitude (EVM) would be 3%.
In addition, more information about the nature of the error is
derived by looking at the magnitude error of the actual signal
compared to the magnitude of the desired signal. The Magnitude
Error is the excess (or shortfall) amplitude of the signal compared
to what the magnitude should be. This is also called I/Q
magnitude error.

Phase error is another test to verify the accuracy of the modulator.
If the "symbol" is shifted in phase from the ideal signal, we have
a phase (f) shift between the ideal signal and the actual signal.
This is expressed as a phase error, in degrees, from the ideal
signal path.

Understanding Origin Offset

Origin Offset, also known as carrier bleed-through or DC offset of
the I/Q base-band, shows where the digital signal is offset at the
beginning of the I/Q plot. It is defined as an imbalance in the I/Q
modulator.

Using the IFR 2975 to analyze CQPSK signals
With Option 11, the 2975 is set up to generate and receive P25
Phase II and LSM signals. With Option 13, the 2975 can further
analyze CQPSK signals with a high degree of accuracy. This
option allows users of Linear Simulcast Modulation systems to
quickly ascertain the modulation accuracy of LSM systems.
Setting Up The IFR 2975 for Analysis of CQPSK Signals
To enable CQPSK analysis on the 2975, we first need to be in
either the duplex or receiver mode. Aeroflex's CQPSKanalysis has
two primary modes of operation: the actual constellation display
of the signal received, and the corresponding analysis of the PSK
parameters. To observe the constellation display from the Duplex
Mode <Mode 3>, we select the "I-Q Plot" option.
(Note: Options are numbered based on how many options are
activated on the unit at the time and may vary between differently
configured units.)

To enable the CQPSK data tile, we select the "EVM Data" option.
This option displays the actual parametric parameters of the
CQPSK signal including EVM, Peak EVM, magnitude error, peak
magnitude error, phase error, peak phase error, origin offset and
frequency error.

See figure 8.0 on how to select the I-Q Plot and EVM Data display.
Figure 8.0 Selecting the IFR 2975's CQPSK Constellation
Display

After both tiles are enabled, we can see both the constellation
and the corresponding data fields for analysis of the CQPSKsignal.
Figure 9.0 shows how the two tiles together are displayed.

Figure 9.0 Display of both the I-Q Plot and EVM Data tiles
Once the constellation and data tiles are enabled, the user can
further analyze the CQPSK signal. By turning off the Trajectory
(signal path) of the constellation display, the user can clearly view
how closely the actual decision points are landing compared to
the ideal landing points. To turn off the trajectory, simply toggle
the trajectory button. This analysis provides a very clear display of
how accurately the modulator is performing.

The persistence mode of the 2975 CQPSK constellation allows for
long-term analysis of how accurately the decision points are
landing in the desired "zone" over a long period of time. By simply
enabling the persistence mode, the 2975 shuts off the trajectory
and then shows the actual symbol points as they accumulate over
time. See figure 10.0 which displays how the 2975 accumulates
data points.

Figure 10.0 Persistence Mode enabled
To further analyze the signal received, we can turn off the
constellation display. This allows the user to see the envelope of
the modulation and determine problems within the modulator.

Conclusion

Using the IFR 2975 to perform analysis of the CQPSK signals used
in Linear Simulcast ModulationTM systems provides a cost effective
alternative to expensive stand alone spectrum analyzers or vector
network analyzers for determining modulation accuracy. The
accuracy of the 2975 for P25 CQPSK signals comes very close to
more expensive solutions and provides the added benefit of not
having to carry an additional piece of equipment.

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From: "SueNY" <[email-masked]>
Date: Tue, 16 Dec 2003 23:35:01 -0500

Hi...
  Do you realize your last two posts were 461KB and *926*KB? That's over a
MEG combined! Can you please have mercy on us poor folks who are stuck on
dialup and just add a link instead? Thanks and Happy Holidays! :-)

SueNY

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From: [email-masked]
Date: Tue, 16 Dec 2003 23:41:55 -0500

You are right Sue......

Sorry but these documents are important to us "techies" . I hate links they
expire so quick when I go back to reference them they are always DEAD! I will
try to keep it down.

Happy Hoilidays! How are things in the confines of the 122PCT or is that 123?

SueNY wrote:

 

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From: "SueNY" <[email-masked]>
Date: Tue, 16 Dec 2003 23:47:17 -0500

That would be the 122. :-) Things here are really good. Do you have your
Christmas shopping done yet? I'm hoping Santa will bring me the cable I need
to connect my i95 to my PDA. Happy Holidays to you too!! :-)

Sue
----- Original Message -----
From: <[email-masked]>
To: <[email-masked]>
Sent: Tuesday, December 16, 2003 11:41 PM
Subject: Re: [NEXTEL-1] Please Have Mercy!

 

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From: [email-masked]
Date: Tue, 16 Dec 2003 23:56:55 -0500

Sorry 122 Pct........Ah Nice and Quiet!

No I have not brought anything I have been real busy..... Looks like I going to
have to "Delegate" a shopper. I thought you  got that cable. I'll email Santa
and see what he can do.

 

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From: [email-masked]
Date: Wed, 17 Dec 2003 13:01:55 -0000

Sue, (and others),
You can read these posts on line rather than by email.
Go to;
groups.yahoo.com/group/NEXTEL-1

 

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