Frequency Planning

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Interference

 

 

Summary

Use of Frequencies in P-P links

Channel arrangements, ITU-R Recs.

Interference classification

Internal Interference sources

Degradation due to Interference

 

 

 

2001-2016, Apus Cloud Project e Luigi Moreno

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Summary

 

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.

 

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Use of frequencies in P-P links

 

"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.

 

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Channel arrangements, ITU-R Recs.

 

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, e ach 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.

 

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Interference classification

 

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.

 

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Internal Interference sources

 

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 f requency 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).Fur ther 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.

 

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Degradation due to 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.

 

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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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2001-2016, Apus Cloud Project e Luigi Moreno