Use of Frequencies in P-P links
Channel arrangements, ITU-R Recs.
Degradation due to
Interference
© 2001-2014, Luigi Moreno, Torino, Italy
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In this
Session the use of different frequency bands for Point-to-Point radio systems
is first considered and the ITU-R approach for RF channel arrangements is
presented. Then, the various types of
interference arising in P-P systems is examined, together with classification
criteria. This allows to list the main
interference sources and to give brief notes about each of them. Finally, the
interference effects are discussed.
"Radio Regulations" are the international
agreements issued (and updated from time to time) by the International
Telecommunications Union (ITU), as a result of WARC (World Administrative Radio
Conference) meetings.
"Radio Regulations" specify which
radio systems are allowed to use the various frequency bands, in the
Radiofrequency Spectrum. In particular,
point-to-point radio links are mentioned as "Fixed radio service" in
frequency bands from VHF up to tens of GHz.
In the following, we briefly review the
main criteria in the use of frequency bands in the range 1-60 GHz, for P-P
applications.
Frequency Bands
The Table below reports the main
applications of P-P radio links operating in different frequency ranges. The
typical hop lengths and the most relevant propagation problems are indicated.
Frequency Band |
Typical Hop Length |
Propagation Problems |
Typical Applications |
< 5 GHz |
50 - 60 km; long hops > 100 km |
Multipath (rain not significant). |
Long-haul
networks; Over-the-sea
hops; hops with reduced clearance. |
5-11 GHz |
40 - 50 km |
Multipath, rain
in some regions. |
Long-haul
networks. |
12-15 GHz |
20 - 40 km |
Multipath
and rain. |
Short-haul
networks; metropolitan
links. |
17-20 GHz |
10 - 20 km |
Rain. |
Metropolitan
links. |
> 20 GHz |
< 10 km |
Rain,
atmospheric absorption around 23 and 60 GHz. |
Access
networks; feeder links to BTS; P-MP; WLL (*). |
(*) BTS = Base Transceiver Station in cellular
networks;
P-MP
= Point-to-Multipoint systems; WLL =
Wireless Local Loop.
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Note that, at frequencies above 15
GHz, the hop length limitation due to
rain attenuation makes multipath outage almost negligible, even if multipath
propagation should be a significant problem on longer hops.
We now consider frequency planning techniques,
as implemented for P-P applications, in the context of different network models
and with reference to frequency plans recommended by ITU-R..
Go - Return Frequency plans
Typically, P-P radio links operate for
bi-directional communications. To this
end, the most common technique is to divide a frequency band, assigned to P-P
radio systems, in two sub-bands (usually with the same bandwidth). One or more radio channels in one sub-band
are used for transmission in one direction, while the corresponding radio
channel(s) in the other sub-band transmit(s) in the opposite direction.
Sub-division of the assigned bandwidth in two
sub-bands.
This explains why the two sub-bands are
often labeled as "GO" and "RETURN" sub-bands, respectively.
In a long-haul network model the above
technique is implemented as shown in the figure below.
Use of sub-bands in a long-haul network
(red arrows for lower sub-band, blue arrows for
upper sub-band).
A given sub-band is used in a radio site for
transmission in both directions. The other sub-band is used for reception
only. Clearly, the condition is
reversed at the two nearest sites.
So, the same frequency is never used in a
radio site for both transmission and reception, in any direction. This avoids complex problems in decoupling
receivers and transmitters located at the same site.
In a radio node (or star network model) the
"Go / Return" technique is implemented as shown in the figure below.
Use of sub-bands in a star network.
The radio node transmits in a given
sub-band and receives in the other one.
All the surrounding sites work in the opposite condition.
Interleaved
and co-channel frequency arrangements
In a Go-Return frequency plan, each sub-band is divided in a number of radio
channels. The way radio channels are positioned in each sub-band is called an
"RF channel arrangement".
A number of ITU-R Recommendations deal with
frequency arrangements in various frequency bands.
In an Interleaved Frequency
Arrangement the adjacent RF channels are allocated on alternate polarizations,
as shown in the figure below.
Interleaved frequency arrangement.
The frequency arrangement is
defined by three parameters:
· X = channel spacing between co-polar channels (the channel spacing between cross-polar channels is X/2);
· Y = central guard band (key parameter to decouple Tx and Rx signals at a radio site);
· Z = edge guard band (to avoid interference from / to other radio systems in adjacent frequency bands).
On the other hand, in a Co-channel
Frequency Arrangement, as shown in the figure below, the adjacent RF channels
are allocated on both the orthogonal polarizations (H / V).
Co-channel frequency arrangement.
As in the
case of the interleaved plan, three parameters (X, Y, Z) define the frequency
arrangement. However, in the co-channel
case, X is the channel spacing between co-polar and
cross-polar channels.
Comment
Analog radio systems were mainly developed in frequency bands below 12 GHz, using the interleaved frequency arrangement, since analog signals are not suitable to accept a co-channel interference on the same radio hop.
Subsequently, the development of digital radio systems, mainly in frequency bands above 12 GHz, suggested the adoption of co-channel frequency plans, in order to get a higher efficiency in radio spectrum utilization (more radio channel packed in a given frequency band).
Presently, the co-channel frequency arrangement is recommended for use with digital systems (as an alternative to the interleaved plan) also in several frequency bands below 12 GHz.
The need arises of identifying various types of interfering signals and classifying them on the basis of different criteria. This allows the designer of a radio system to apply standard procedures to deal with each class of interfering signals.
Two aspects in the interference mechanisms can be considered : the source of the interfering signal and the impact of propagation conditions.
Source of Interference
A general classification of Interference sources is :
· Internal interference, when the interfering signal is emitted by a transmitter which is part of the same radio system of the interfered (victim) receiver.
· External interference, in the opposite case (the interfering signal is emitted by a transmitter which is part of a different radio system).
Usually, internal Interference in a radio network can be well estimated, since all the system parameters are under the control of the network designer.
On the other hand, external interference is more difficult to predict in detail, since not all the technical data about the interfering system (power levels, antenna pointing and diagrams, etc.) may be available at the designer of the interfered (victim) system. So, in most cases, external interference is taken into account with some approximation and including some conservative margin.
Coordination procedures are recommended in some cases by ITU-R to avoid interference between different radio systems, sharing a common frequency band.
A more specific classification of interference sources refers to the transmitter / hop / radio system emitting the interfering signal:
· Co-site Interference (internal or external) : Produced by transmitters located at the same radio site where the interfered (victim) receiver is located.
· Same Hop Interference (internal only) : Produced by transmitters working on the same hop at the same frequency (co-channel, cross-pol. interference) or at adjacent frequencies (co-pol. or cross-pol. interference) with reference to the interfered (victim) receiver.
· Interference from other P-P Hops (internal or external): Produced by transmitters working on a different radio hop, at the same frequency (co-channel interference) or at adjacent frequencies with reference to the interfered (victim) receiver.
· Interference from other radio systems (external only): Produced by transmitters in radio systems other than P-P systems, sharing the same frequency band with P-P systems (e.g. satellite systems).
Propagation conditions
Another criterion to classify interference is related to the propagation conditions suffered by the interfering signal, compared with the propagation conditions which affect the useful (interfered) signal. We consider :
· Correlated Interference, when the interfering signal suffers the same propagation impairment as the useful signal. Specifically, in the case of rain events, this happens when the useful and the interfering paths are identical or so close that they are both affected by a raincell in the same way.
· Uncorrelated Interference, when the above conditions are not established, so that we can assume that additional attenuation (caused by multipath or rain) affects in a different measure the useful and the interfering signals. As a worst case assumption, we consider that the useful signal is received at the threshold level, while the interfering signal may be received with no additional attenuation (nominal power level).
Correlated (1) and uncorrelated (2) interference
paths
when the useful path is affected by rain.
In some cases, the term "partially correlated" will be used, in particular when more precise models are available (like in the case of co-channel, cross-polarized same-hop interference, with rain or multipath fading).
The correlated / uncorrelated interference model appears as a rather approximated one (also the term "correlated" is not fully correct, as used in this context). However, even a rough model is useful to analyze the interference scenario in a simple way and worst case assumptions are often required to evaluate the most critical interference effects.
An example of a possible implementation of the rain correlation model is given in the figure below.
Interfering Tx in the yellow region produces a
correlated interference;
in the blue and brown regions,
clauses a) and b) below are not satisfied,
respectively;
(CD = Correlation Distance).
In this model, interference is assumed to be correlated if:
a) Separation from useful transmitter (Tu) to interfering path is below a given "Correlation Distance" CD;
b) Interfering path length is at least equal to the useful path length.
The above requirements guarantee that the interfering signal travels through the same raincell as the useful signal, along a path not shorter than the useful one.
Typical values of "Correlation Distance" are in the range 0.5-1.0 km (this is a fraction of the expected raincell size). However, a suitable choice of correlation distance allows to scale the model to local rain conditions. More specifically, zero correlation distance forces the model to assume as correlated only the interfering signals emitted at the same radio site as the useful signal; this may be an extremely conservative assumption.
In this section
we list a number of interference sources which may be present as internal
interference in P-P radio networks.
For each
interfering signal, information is given about frequency
spacing and polarization, useful-to-interfering signal decoupling, and about
the effect of propagation conditions (rain, multipath) on interference
correlation or uncorrelation.
Co-site
Interference
· Frequency spacing and polarization : central guard-band (minimum spacing); usually cross-pol. channels at the minimum spacing.
· Useful-to-Interfering signal decoupling : Tx & Rx signal filtering (NFD). Further decoupling depending on Tx/Rx implementation: if Tx & RX channels on the same antenna, then decoupling is produced by the branching system; if Tx & RX channels on the different antennas, then decoupling is given by the side-to-side antenna decoupling (see comments on antenna field performance vs. laboratory measurements).
· Effect of propagation: uncorrelated interference in any case (rain, multipath).
Same Hop
- Co-channel,
cross-polarized signal
· Useful-to-Interfering signal decoupling : only from antenna XPD (cross-polarization discrimination), zero frequency spacing, no filtering effect.
· Rain effects : even if the useful and the interfering signals travel along the same path, so that attenuation is correlated, the reduction in cross-polar discrimination due to rain makes the interference partially uncorrelated. The rain XPD model described in another session gives a practical tool to predict the overall effect.
· Multipath effects : partially uncorrelated Interference, due to XPD degradation under multipath propagation. The multipath prediction model gives a tool to estimate the overall effect of multipath attenuation and XPD degradation.
Same
Hop - Adjacent channel, co-polarized signal
· Useful-to-Interfering signal decoupling : Tx & Rx signal filtering (NFD), depending on the RF channel spacing.
· Rain effects : correlated Interference;
· Multipath effects : partially uncorrelated interference (the ITU-R multipath models do not cover this type of interference).
Same
Hop - Adjacent channel, cross-polarized signal
· Useful-to-Interfering signal decoupling : only from antenna XPD (cross-polarization discrimination), zero frequency spacing, no filtering effect.
· Rain effects : same as for co-channel, cross-polarized signal;
· Multipath effects : same as for co-channel, cross-polarized signal.
Long-haul Networks - Backward Interference
· Frequency spacing and polarization : (usually) co-channel, cross-polar.
· Useful-to-Interfering signal decoupling : from Tx antenna front-to-back decoupling (see comments on antenna field performance vs. laboratory measurements).
· Rain effects : correlated interference (same path for useful and interfering signals).
· Multipath effects : uncorrelated Interference (useful and interfering transmitters are co-located, but signals are emitted by different antennas; equivalent to a Tx diversity system).
Long-haul Networks - Forward Interference
· Frequency spacing and polarization : (usually) co-channel, cross-polar.
· Useful-to-Interfering signal decoupling : from Rx antenna front-to-back decoupling (see comments on antenna field performance vs. laboratory measurements).
· Rain effects : uncorrelated interference (different paths for useful and interfering signals).
· Multipath effects : uncorrelated Interference
Long-haul Networks - Over-reach Interference
· Frequency spacing and polarization : co-channel, co-polar.
· Useful-to-Interfering signal decoupling : Tx and Rx Antenna angular discrimination (if hops are not aligned). Additional Free Space Loss (interfering path length)
· Rain effects : correlated interference in the critical case of almost aligned hops.
· Multipath effects : uncorrelated interference.
Star Networks - Up-link Interference
· Frequency spacing and polarization : co-channel, co-polar (worst case).
· Useful-to-Interfering signal decoupling : Rx (node) antenna angular discrimination. Tx & Rx signal filtering (NFD) if not co-channel.
· Rain effects : uncorrelated Interference.
· Multipath effects : uncorrelated Interference.
Star Networks - Down-link Interference
· Frequency spacing and polarization : co-channel, co-polar (worst case).
· Useful-to-Interfering signal decoupling : Tx (node) antenna angular discrimination. Tx & Rx signal filtering (NFD) if not co-channel.
· Rain effects : correlated interference.
· Multipath effects : uncorrelated interference.
Performance
degradation caused by interference can be evaluated following a two-step
process:
· To estimate the power level of the interfering signal at the (useful)
receiver input. The interfering power
is evaluated under two alternative assumptions: (1) useful signal received at
nominal power level; (2) useful signal
received at threshold level.
· To estimate the effect of a given interference power on the interfered
receiver. This depends on a number of
system parameters, including the receiver threshold, the modulation format and
interference sensitivity.
Let us
consider four interference classes:
· Same hop interference:
Degradation caused by co-channel or adjacent-channel interference in the
same radio hop is usually included in outage prediction models. This has been discussed in previous
sessions, in connection with multipath
propagation and rain
attenuation.
· Co-site interference (internal interference): this is usually considered as part of the radio system design;
equipment manufacturer gives specifications about the required decoupling
between Tx and Rx radio channels, for the suggested system configurations (Tx
and Rx channels on the same antenna or on separate antennas).
· Co-site interference (external interference): in this case, coexistence is required of different radio systems
and a general analysis is not possible. High level interfering signals (even at
a quite different frequency) may be responsible of anomalous receiver response,
related to Rx saturation and non-linearity,
intermodulation, spurious emissions, etc.
This point will not be considered in the following.
· Interference coming from other radio hops: this case is discussed below.
Interference
power estimate
The
figure defines the geometrical parameters in the interference scenario.
Interference from site Ti to useful receiver Ru:
definition of geometrical parameters.
As a
first approach, the Basic
Radio Link equation (used to predict Rx power in the useful hop) gives an
estimate of interference power IR at the useful receiver input:
where: PIR = output power (dBm) at the
interfering Tx;
GT(a) = Tx antenna gain (dB) in the direction of
the interfered (victim) receiver;
GR(b) = Rx antenna gain (dB) in the direction of
the interfering transmitter;
FSL = Free Space Loss (dB) over the TI to RU
path.
The Net Filer Discrimination (NFD)
gives the measure of the interfering signal attenuation, as a result of the
useful receiver selectivity. If the
interfering signal spectrum is within the Rx filter passband, then NFD=0 dB.
The
signal-to-interference ratio, under the assumption of no additional attenuation
of the useful signal, is defined as "Unfaded S/I" (S/I)U
and is computed as:
where PR
is the nominal useful power at the receiver input and IR is given
above.
Similarly,
the signal-to-interference ratio, under the assumption that the useful signal
is at the threshold level, is defined as "Faded S/I" (S/I)F. For uncorrelated interference (no attenuation suffered by the
interfering signal) it is computed as:
where: PTH = useful receiver threshold;
FM = PR - PTH = Fade
Margin in the useful hop.
On the
other hand, for correlated
interference (same attenuation on the useful and interfering signals), we
have:
Up to
now, we have assumed that no obstruction exists between the interfering Tx and
the useful (victim) Rx. If the
interfering path is not perfectly clear, a clearance analysis should be
performed.
A more
general approach to path loss prediction for interfering signals is given by
ITU-R Rec. P.452 ("Prediction procedure for the evaluation of microwave interference between
stations on the surface of the Earth at frequencies above about 0.7 GHz").
In that
recommendation, all the propagation mechanisms which can contribute to
interference power reception at the useful (victim) receiver, are considered:
· line-of-sight ;
· diffraction;
· tropospheric scatter;
· surface and elevated ducting;
· hydrometeor scatter.
This
allows a quite detailed analysis of interference levels, which cannot be
summarized in these notes.
Effect
of Interference
The interference
effect can be estimated by assuming that the interference power is equivalent
to an additional noise power at the receiver.
This
assumption allows to predict the receiver performance with satisfactory
approximation, in particular for adjacent-channel interference and when we have
multiple interference. In most cases it
is only slightly pessimistic.
Alternatively, for co-channel interference, it may be advisable to refer
to the measured Rx
performance.
The chart
below gives a graphical interpretation of threshold degradation caused by the
combined impairment of noise and interference.
Increase of Rx threshold power due to the combined
disturbance
of noise and interference power.
The
overall result of an interfering signal on system performance is to shift the
BER vs. Rx power curve to the right, as in the figure below.
BER vs. Rx power without (A) and with (B) the
presence
of interference; D = Rx threshold
degradation.
The two
curves allow to estimate the performance degradation for any BER value. Note that the figure above refers to an
interfering signal, with given C/I ratio, modulation format and frequency
spacing.
Further Readings
Lehpamer H., Transmission systems design handbook for wireless networks, Artech House Inc., 2002.
Smith W.E. et al., "Recent advances in microwave interference prediction", IEEE Int. Conf. Communications, Seattle 1987.
Barber S., "Co-frequency cross-polarized operation of a 91 Mb/s digital radio", IEEE Int. Conf. Communications, Denver 1981.
Vogel k., "Frequency re-use with 7bit/s/Hz for 140 Mb/s system with orthogonal co-channel arrangement", European Conf. Radio-Relay, Munich 1986.
Segal B., "Spatial correlation of intense precipitation with reference to the design of terrestrial microwave networks", IEE Int. Conf. on Antennas and Propagation (ICAP), Norwich 1983.
Moreno L., "Spectrum utilization in a digital radio-relay network", IEEE Tr. Electromagnetic Compatibility, vol. 24, n. 1, February 1982, pp. 40-45.
Pagones M.J. and Prabhu V.K., "Effect of interference from geostationary satellites on the terrestrial radio network", GlobeCom, New Orleans 1985.
End
of Session #7
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© 2001-2014,
Luigi Moreno, Torino, Italy