Telecommunications Industry Association (TIA)
Document Number: TR45.WGMS-161130-366
DOCUMENT SUBMITTED TO:
TR45 Working Group on Microwave Systems
SOURCE: Lockard and White
CONTACT: Steve Hill, [email protected]
TITLE: Proposed revisions to Chapter 3
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TR45 WORKING GROUP ON MICROWAVE SYSTEMS
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Proposed text edits
1
Chapter 3
Digital Receiver Interference
Fixed point-to-point microwave radio systems use transmitters and receivers deployed
miles apart to transport high speed digital signals. The reliability of the transmission is
directly related to the path fade margin (the difference between the normal received
signal power and the lowest received signal power which still supports receiver
operation). In the absence of external interference, the lowest operational received
power (receiver threshold) is determined by the receiver’s front end (Gaussian) noise.
External interference can cause the receiver threshold to occur at a larger (stronger)
received power, thereby reducing the effective path fade margin (and path reliability).
In many situations (especially in bands where different radio services are used),
receiver fade margins are either not known or are different for different radio users (or
services) in the same frequency band. In these situations it is desirable to determine
interference objectives which limit receiver degradation to a defined level regardless of
user fade margin. Interference is defined in such a way that it only decreases receiver
performance a specified amount. See the section “Interference Estimation” for details
regarding C/I and T/I, two commonly used methods of managing external interference in
digital receivers. In North America, T/I is used exclusively.
Threshold to Interference (T/I) Criterion
Threshold to Interference (T/I) curves are used to estimate interference caused by an
interfering signal into a victim digital receiver. They represent the maximum interfering
signal level at with the victim receiver’s 10-6 bit error ratio (BER) threshold has been
degraded 1 dB.
The interference objective defined by T/I is given by the following:
Icoord
= coordinated interference objective (dBm)
(3-1)
= RSLmin (dBm) – T/I (dB)
RSLmin
= received signal level at radio 10-6 BER threshold (dBm)
(3-2)
= receiver threshold specification (dBm)
= RSLnorm (dBm) – FM (dB)
RSLnorm
= normal received signal level (dBm)
(3-3)
FM
= radio fade margin (dB) = RSLnorm - RSLmin
(3-4)
2
If both the desired (“victim”) and interfering spectrums are similar, we say they are “like
modulation.” The following are typical like modulation QAM T/I curves:
40
30
QAM
512
256
128
64
32
16
4
T/I (dB)
20
10
0
-10
-20
-30
0.0
0.5
1.0
1.5
Normalized Frequency
Fig. 3-1: Typical T/I Curves
|Normalized Frequency| = Absolute Value [ (Interfering Signal Center Frequency (3-5)
– Desired Signal Center Frequency) | / Desired Signal Channel Bandwidth ]
T/I is a function of receiver bandwidth, modulation format and interfering signal spectral
power and frequency. For like modulation, the modulation types of Interferer and victim
are the same (e.g. both are QAM), though the modulation levels may be different (e.g.
4QAM vs 256QAM, etc.).
The use of T/I simplifies analysis of the effect of interference into a receiver. Typically
T/I is specified by the manufacturer for similar signal interference which is co-channel,
adjacent channel or next to adjacent channel. Generally the desired and interfering
signals are similar (“like”) bandwidth QAM signals. However there is an occasional
need for estimating T/I data for cases where exact data is not available.
Notice that the implied assumption for the following material is that the radios involved
are operating using QAM (or TCM) technology. While the general methodology can be
applied to any form of radio, the specific values given in this document are assuming
QAM technology.
3
Co-Channel T/I
To begin an analysis, the co-channel similar interfering signal T/I value is fundamental
to frequency planning analysis. The following average values are based upon a major
coordinator’s T/I data base.
Modulation
4096 QAM
2048 QAM
1024 QAM
512 QAM
256 QAM
128 QAM
64 QAM
32 QAM
16 QAM
QPSK
Average T/I
6 GHz
Average T/I
11 GHz
Average T/I
40.1
37.4
33.5
31.6
29.7
25.6
22.8
16.0
40.3
39.0
34.3
31.1
28.5
25.3
22.7
15.0
40.2
38.2
33.9
31.4
29.1
25.5
22.8
15.5
Default Value
47.0 dB
44.0 dB
41.0 dB
38.0 dB
35.0 dB
32.0 dB
29.0 dB
26.0 dB
23.0 dB
17.0 dB
Table 3-1: Typical Co-Channel Like Interference T/I Values
It is proposed that the above default values be used if none are provided by the
manufacturer.
CW T/I Curves
C/W T/I curves represent receiver sensitivity to unmodulated carrier (carrier wave or
CW) interference. They are typically used to coordinate analog FM transmitters which
can be characterized as a carrier with insignificant sideband modulation.
CW T/I curves are simply a scaled version of the digital receiver’s pass band power
response. Individual receiver curves vary slightly depending upon the designer’s
partitioning of the raised cosign and X/Sin(X) filtering. The following graph is based
upon actual receiver measured data.
4
Normalized Amplitude Response (dB)
0
Normalized
Measured
Receiver
CW T/Is
-20
-40
Default Curve
-60
-80
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
|Normalized Frequency Offset|
Figure 3-2: Normalized Receiver CW T/I Curves
We will use a “default” curve for normalized radio amplitude versus frequency response
(receiver bandpass characteristic) when actual values are not available. From this we
may infer the receiver CW T/I curve.
CW T/I (dB) = 2 dB + receiver similar modulation co-channel T/I value
+ typical normalized CW T/I value (at frequency of interest)
(3-6)
The 2 dB value compensates for the fact that the interfering unmodulated signal has
different statistical characteristics (effective peak to average power ratio) than does a
digitally modulated interfering signal. This 2 dB value is based upon lab tests and has
been shown to be accurate across a wide range of QAM and TCM based radios.
Similar Interference T/I Curves
Similar interference T/I curves, if normalized to the co-channel (zero frequency offset)
value, are quite similar.
5
10
0
Normalized T/I (dB)
QAM
512
256
128
64
32
16
4
-10
-20
-30
-40
-50
-60
0.0
0.5
1.0
1.5
Normalized Frequency
Figure 3-3: Typical T/I Curves
It can also be observed that in general the coordination interference limit derived from
Icoord = RSLmin (dBm) – T/I (dB) is similar for all receivers.
-30
Interference Limits (dBm)
-40
-50
-60
-70
-80
-90
-100
-110
0.0
QAM
4
16
32
QAM64
4 128
16 256
32 512
64
128
256
512
0.5
1.0
1.5
Normalized Frequency
Figure 3-4: Typical Interference Limits
In general one curve is adequate to represent all curves – except for low order QAM
near the normalized frequency of one. All digital radio receivers have a limited analog
to digital conversion range. Typically the converters have 15 to 20 dB head room
(maximum to typical input signal range). That means that at the input to the converter,
6
the most negative T/I can be measured is -15 to -20 dB. More negative values are
simply not possible (overdriving the converter will create significant intermodulation
noise). Interference with significant frequency offset relative to channel center
frequency will be heavily filtered by the receiver internal analog filter which precedes the
analog to digital converter. Interference with significant frequency offset will not
experience the converter head room limitation (with the possible exception of very low
order QAM). However, for interference with about adjacent channel offset, receiver
filtering may not be enough to keep the analog to digital converter head room from
being a limitation. For large QAM radios, adjacent channel T/I is modest. However, for
low order QAM (such as 4 and 16 QAM), the converter head room becomes a
determining factor in achievable T/I. The adjacent channel T/I value of low order
modulation radios will typically have slightly worse (more positive) T/I values than those
shown above because of limitations in analog to digital converter headroom. We need
to keep this potential limitation in mind when we consider T/I curves for a wide range of
receivers.
A radio interference limit (receiver threshold – T/I) is typically constant for all large to
moderate QAM radios of similar bandwidth. However, it tends to become smaller (more
negative) for low order QAM radios because of reasons just mentioned. For adaptive
modulation radios using multiple QAM modes, the natural question is “What interference
limit should be used to frequency coordinate the radio?” If a wide range of receiver
QAM states (“profiles”) are being used on a path, the most highly used state (typically
the largest QAM) should be used to determine the path interference limit. If Adaptive
Coding and Modulation (ACM) mode is specified, the mode with the most transmission
capacity (least error correction) should be used for interference estimations.
All
interference limits will be the same dBm value except for low order QAMs which will be
the most briefly used states.
Requirements for
Measurements
Transmitter
Spectrum,
Receiver
Selectivity
and
T/I
It is proposed manufacturers provide transmitter spectrums, receiver selectivity curves
and similar interference T/I curves for their receivers. It is suggested that the curves
have the following characteristics:
7
Figure 3-5: Sampling Rates for Transmitter, Receiver and T/I Curves
Frequency symmetric T/I curves are preferred. If the values are not symmetric relative
to channel center frequency, the frequency values should correctly represent the
relative offset of the interfering frequency (at the transmitter or receiver RF input)
relative to channel center frequency.
Irrespective of whether the victim receiver is equipped with a channel specific RF filter
or not, it is safe to assume that interference indicated by using the methods described in
this document can be dismissed, provided that the following conditions are met
simultaneously:
a) The interfering signal falls in the 2nd adjacent channel of the victim receiver (or
farther out).
b) Both the interfering and victim radios are specified by their respective manufacturer
to be able to receive a similar modulation co-polarized signal in the 2nd adjacent
channel, without degradation of receiver performance.
c) The interfering level at the victim receiver’s input is not stronger than the desired
signal power at the same point.
Estimating Dissimilar Interference T/I Curves
Sometimes the interfering signal is not similar bandwidth to the desired signal. No
manufacturer can be expected to provide T/I curves for all possible interference cases.
In these cases, the T/I values must be theoretically derived. By the Convolution
Theorem, if two devices are cascaded (multiplicative) in the time domain, the frequency
domain representation of the output is the convolution of the frequency domain
representation of the two devices. We apply this theorem to derive the frequency
domain representation of a digital signal passing through a linear receiver.
A normalized T/I curve T/INormalized may be estimated using the following equation:
T / INormalized  10log {[

 
s(  f)c()d] / [


s(  f0 )c( )d]}
(3-7)
The term c(f) is the normalized power transfer function (bandpass characteristic) of the
victim receiver. C(f) is a normalized CW T/I curve with power expressed in dB.
Therefore c(f) = 10C(f)/10. The term s(f) is the normalized spectral power density of the
interfering signal being applied to the input of the receiver. For the interfering spectral
density function S(f) with power expressed in dB, s(f) = 10S(f)/10. The convolution
integral in the denominator is referenced to the receiver center frequency f 0. That
integral’s function is to normalize the result of the convolution integral in the numerator.
Sometimes the transmitter and/or receiver characteristics are not known. We will use
the following “default” curves when actual values are not available.
8
10
Normalized Spectral Density (dB)
0
-10
Normalized
Default
Transmitter
Spectrum
-20
-30
-40
-50
-60
-70
-80
-90
-100
0
1
2
3
4
|Normalized Frequency|
Normalized Amplitude Response (dB)
Figure 3-6: Default Transmitter Spectrum
0
Normalized
Default
Receiver
Bandpass
Characteristic
-50
-100
-150
-200
0
1
2
3
4
|Normalized Frequency|
Figure 3-7: Default Receiver Bandpass Characteristic
The default values are listed in the tabs “Transmitter” and “Receiver” of the Excel file
<TR45.WGMS-160527-245-Task I - Estimated TI Curves RevB.xls>.
9
We further define the following relationships:
Bandwidth Ratio (PR) = Interfering Spectrum Bandwidth / Desired Spectrum Bandwidth
(3-8)
Bandwidth Factor (dB) = 10 Log10 [ Bandwidth Ratio (PR) ] if Bandwidth Ratio (PR) > 1
= 0 if Bandwidth Ratio (PR)  1
(3-9)
|Normalized Frequency| = Absolute Value [ (Interfering Signal Center Frequency
– Desired Signal Center Frequency) / Desired Signal Channel Bandwidth ] (3-10)
T/I (dB) = Normalized T/I (dB) + Receiver Co-Channel T/I (dB) - Bandwidth Factor (dB)
(3-11)
Using the above default curves, normalized T/I values were calculated for dissimilar
interference bandwidths. The results are provided in tab “Widerange TI Curves” of the
Excel file <TR45.WGMS-160527-245-Task I - Estimated TI Curves RevB.xls>.
Normalized T/I (dB)
The calculated results are graphed below:
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
-120
-130
-140
-150
Calculated
Normalized
T/I Curves
0
1
2
3
|Normalized Interfering Spectrum Offset|
Figure 3-8: Small Bandwidth Ratio Interference T/I Curves
10
4
Normalized T/I (dB)
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
-120
-130
-140
-150
Calculated
Normalized
T/I Curves
5
1
10
15
20
25
30
0
0
50
100
150
200
|Normalized Interfering Spectrum Offset|
Figure 3-9: Large Bandwidth Ratio Interference T/I Curves
It can be observed that for bandwidth ratio  0.05, the normalized T/I is equal to the
normalized receiver bandpass characteristic within 1 dB. For bandwidth ratio ≥ 30, the
normalized T/I is equal to the normalized transmitter spectral characteristic with
(normalized frequency) replaced with (normalized frequency x bandwidth ratio).
As noted previously, receiver T/I curves are typically measured without an external RF
filter. Some receivers are operated with external narrow bandwidth RF filters and some
are not. The manufacturer is requested to indicate if narrow bandwidth RF filtering is
used with the receiver in normal operation. If this filter is present at the receiver, the T/I
is limited to not less than -80 dB. If the RF filter is not present or is unknown, the
estimated T/I is limited to not less than -65 dB.
Determining the Interference and Receiver Bandwidth
A critical factor is determining T/I is the receiver Bandwidth Ratio (Interfering Spectrum
Bandwidth / Desired Spectrum Bandwidth). That, of course, requires us to know the
bandwidth of the interference and the receiver. The receiver bandwidth is inferred from
the transmission bandwidth of the normal transmit signal. This, in turn, can be
determined from the transmitter emission designator.
11
The transmitter emission designator is a coded word defining the type of signal
modulation and its bandwidth. The FCC and ITU-R format for the emission designator
is three numerals and a capital letter to express necessary bandwidth followed by three
capital letters describing the form of modulation. The necessary bandwidth uses the
letter to indicate the magnitude and the decimal location.
Bandwidths between 0.001 and 999 Hz shall be expressed in Hz (letter H), between
1.00 and 999 kHz shall be expressed in kHz (letter K), between 1.00 and 999 MHz shall
be expressed in MHz (letter M) and between 1.00 and 999 GHz shall be expressed in
GHz (letter G).
Examples:
0.002 Hz
0.1 Hz
25.3 Hz
400 Hz
2.4 kHz
6 kHz
12.5 kHz
180.4 kHz
=>
=>
=>
=>
=>
=>
=>
=>
H002
H100
25H3
400H
2K40
6K00
12K5
180K
180.5 kHz
180.7 kHz
1.25 MHz
2 MHz
10 MHz
202 MHz
5.65 GHz
=>
=>
=>
=>
=>
=>
=>
181K
181K
1M25
2M00
10M0
202M
5G65
The bandwidths of the interfering and victim radios determine the interference
objectives according to the default method. The radio bandwidths as expressed in the
emission designators are expected to be assigned as follows.
The authorized bandwidth is the maximum bandwidth authorized to be used by a station
as specified in the station license. Since the emission designator is the only data
element on the license of a FCC Part 101 microwave station that indicates the
bandwidth to be used, the bandwidth indicated in the emission designator is the
authorized bandwidth. The emission mask requirements (Section 101.111) are based
on the authorized bandwidth and the payload capacity requirements (Section 101.141)
are based on emission bandwidth which is equivalent.
Emissions are designated according to their classification and their necessary
bandwidth. The minimum value of necessary bandwidth that should be used is the
greater of: (1) the occupied (99% power) bandwidth, or (2) the minimum bandwidth for
which the actual transmitter emissions meet the Section 101.111 mask requirements.
The maximum value of necessary bandwidth that may be used is the channel
bandwidth of the plan the radio system is intended to occupy (see Section 101.147).
Radio vendors should specify the emission bandwidth within this range, consistent with
meeting the required payload capacity. Assigning a narrower bandwidth is preferred to
promote more efficient use of spectrum.
12
The choice of emission bandwidth for a radio affects the objectives found using the
default method. In particular, for frequency separation near adjacent-channel a
narrower bandwidth produces more relaxed objectives, while a wider bandwidth
produces more stringent objectives. Vendors should recognize these impacts when
assigning the radio emission designator.
Finally, changes of modulation profiles for adaptive radios are intended to be nonimpactful in terms of potential interference with other systems. Following this principle,
radios should be designed to maintain consistent emissions and bandwidth across the
modulation profiles that are implemented. Furthermore, to simplify frequency
coordination analysis and licensing, vendors should assign the same necessary
bandwidth in the emission designators for the various modulation profiles.
Estimating T/I for Unlike Interference
Manufacturers will provide T/I curves for like interference. It is impractical for them to
provide curves for every case of unlike interference. For unlike interference the above
default calculations could be used to estimate T/I. However, those calculations are
rather pessimistic. For cases where the interference and receiver bandwidths are unlike
but close, using the like interference T/I curves will get more accurate results than the
default calculations.
For the unlike interference case two estimates should be made.
estimated using the default calculations described previously.
First, the T/I is
Next, T/I is estimated using the manufacturer’s like interference T/I data with the
following modifications. If the interference bandwidth is less than the victim receiver,
determine T/I assuming the interference is the same bandwidth as the receiver. If the
interference bandwidth is greater than the victim receiver, determine T/I assuming the
interfering signal has reduced separation f R.
13
Figure 3-10: Modified Transmitter Spectrums
fR
fS
fS
BW I
BW V
= reduced frequency separation
(3-12)
= fS - fS
= actual frequency separation between interfering signal and victim receiver center
frequencies
= shift in a hypothetical like interference signal center frequency if the
hypothetical like interfering signal is shifted as close to the victim
receiver frequency as possible while remaining within the spectrum of the
actual interfering signal
= (BW I – BW V)/2
= interfering signal bandwidth
= victim receiver bandwidth
The best estimate of unlike interference T/I is the lesser of the two above estimates.
This process is formalized in the following flow chart:
14
Three Dimensional Interpolation
15
Three dimensional interpolation can be used to derive values between those provided in
the above referenced tables.
Figure 3-11: Rectangular Three Dimensional Interpolation
It is assumed that all data is organized as a set of related X, Y and Z values. X and Y
values are assumed equally spaced. The task is to estimate the Z value associated
with and X and Y pair not contained in the original data. Using two dimensional
interpolation (below), interpolate a set of Z values for the desired X value and multiple Y
values (above figure, left side). Next, again using two dimensional interpolation,
interpolate the desired Z value for the desired X and Y values (above figure, right side).
Cubic interpolation is suggested for values which vary non-linearly.
Two Dimensional Cubic Interpolation
Sometimes it is necessary to interpolate between tabular data points. The following
method uses a cubic (Y = A + BX + CX2 + DX3) polynomial. If the data has a known
nonlinear relationship to X, (such as squared or logarithmic components), first transform
the input X values. For example, make the substitution Z = X2 or Z = log(X)
respectively. Replace X below with Z. The results will be the appropriate polynomials
(e.g., Y = A + BZ + CZ2 + DZ3 which equivalent to Y = A + B(X2) + C(X4) + D(X6) or Y =
A + B log(X) + B log2(X) + B log3(X).). Of course, the X values must be inversely
transformed to complete the process.
Figure 3-12: Two Dimensional Interpolation
16
The following LaGrangian interpolation method uses a limited number of data points to
produce a polynomial that exactly reproduces the input data:
Given a set of four X,Y values, (X1,Y1), (X2,Y2), (X3,Y3) and (X4,Y4) with X1 < X2 < X3 <
X4, interpolate a value of Y for X such that X1 < X < X4 (The X values do not need to be
equally spaced. Also, the data samples do not need to be centered on the region of
interest but should encompass the region of interest with at least one sample on one
side or the other of the X or Y data.):
C21 = X2 – X1
C31 = X3 – X1
C32 = X3 – X2
C41 = X4 – X1
C42 = X4 – X2
C43 = X4 – X3
K1 = C21 * C31 * C41
K2 = C21 * C32 * C42
K3 = C31 * C32 * C43
K4 = C41 * C42 * C43
CA1 = (X2 * X3 * X4) / K1
CA2 = – (X1 * X3 * X4) / K2
CA3 = (X1 * X2 * X4) / K3
CA4 = – (X1 * X2 * X3) / K4
CB1 = – (X2 * X3 + X2 * X4 + X3 * X4) / K1
CB2 = (X1 * X3 + X1 * X4 + X3 * X4) / K2
CB3 = – (X1 * X2 + X1 * X4 + X2 * X4) / K3
CB4 = (X1 * X2 + X1 * X3 + X2 * X3) / K4
CC1 = (X2 + X3 + X4) / K1
CC2 = – (X1 + X3 + X4) / K2
CC3 = (X1 + X2 + X4) / K3
CC4 = – (X1 + X2 + X3) / K4
CD1 = – 1 / K1
CD2 = 1 / K2
CD3 = – 1 / K3
CD4 = 1 / K4
A = CA1 * Y1 + CA2 * Y2 + CA3 * Y3 + CA4 * Y4
B = CB1 * Y1 + CB2 * Y2 + CB3 * Y3 + CB4 * Y4
C = CC1 * Y1 + CC2 * Y2 + CC3 * Y3 + CC4 * Y4
D = CD1 * Y1 + CD2 * Y2 + CD3 * Y3 + CD4 * Y4
Y = A + B * X + C * X2 + D * X3
See Appendix 1 of Digital Communication Systems [2] for more details.
References:
17
[1] Committee, ETSI EN 301 126-1 V1.1.2, Fixed Radio Systems; Conformance testing;
Part 1: Point-to-Point equipment - Definitions, general requirements and test
procedures. Valbonne: European Telecommunications Standards Institute, pp. 24 – 29,
September 1999.
[2] Kizer, G., Digital Microwave Communication. Hoboken: Wiley John Wiley and Sons,
pp. 702 – 708, 2013.
[3] TIA Fixed Point-to-Point Microwave Section Committee 14-11, “Interference Criteria
for Microwave Systems,” TIA/EIA Telecommunications Systems Bulletin 10-F.
Washington DC: Telecommunications Industry Association, pp. B-1 – B-8, June 1994.
18