Radio-relay system

A radio-relay system is a system of Transmission of signals (today mainly numerical) between two fixed points. It uses as support the radio waves, with carrier frequencies 1 GHz with 40 GHz (field of the Micro-ondes), very strongly concentrated using antenna S directives.

These waves are mainly sensitive to maskings (relief, vegetation, buildings…), to precipitations, to the conditions of refractivity of the atmosphere and have a rather strong sensitivity to the phenomena of reflection.

Transmitted signal

For each microwave link, one defines two frequencies corresponding to the two directions of transmission. For reasons of distance and visibility, the hertzian way between the transmitter and the receiver is often cut out in several sections, called jumps, connected by relay stations.

The support Radioélectrique used is common to everyone. The Wavebands thus represent a rare resource and their use is regulated by national and international official organizations. In the case of a network made up of several jumps or close connections geographically, problems of interferences can appear, affecting the quality of the transmissions. The definition of a good plan of attribution of frequencies (and polarizations) must make it possible to minimize the disturbances while optimizing the use of the spectral resource.

The signal to be transmitted is transposed in frequency by Modulation. The operation of modulation transforms the signal, at the origin in Baseband, signal with narrow band, whose spectrum is located inside the band-width of the channel. The modulations used are:

to 4 or 16 states (QPSK, 4 QAM, 16QAM…) for the signals PDH
to 64 or 128 states (64 QAM, 128 QAM…) for the signals SDH

Increase in the number of state reduced for a flow given the Band-width necessary of a factor 2. In against part, the least good tolerance with the noise of the modulated signals supposes a reduction of the effective range of the connections.

The following table summarizes the bandwidths necessary according to the flows met in the hertzian one and the type of modulation used:

Factors influencing the propagation

One of the methods of forecast most worked out for the design of microwave links in direct sight is given by the recommendation UIT-R P.530-8, which makes it possible to envisage the most important parameters of propagation.

During its propagation, the Hertzian wave undergoes mainly three types of attenuations:

That corresponding to its radiation in free space, which is fixed, and always presents (about 140 dB in general), and moreover sometimes accentuated by the presence of obstacles.
That coming then from the random variations of the climatological conditions: guidance, and precipitations (possible losses of about thirty dB).
Those of the phenomena of Interference S, consequences of the principal reflection, or multi-ways (possible losses of about thirty dB).

Propagation in free space, release

The transmitting station radiates. The electromagnetic Ondes convey an energy per unit of area which decrease like the square of the distance.

Moreover, on the whole of the way traversed by the wave, it is imperative to take care of the release of the connection. Relief, vegetation, building intercepting the beam involve losses of which account should be held.

The essence of energy is concentrated in the zone which one calls “first ellipsoid of Fresnel”. The extent of this zone (a few meters with several tens of meters) varies proportionally with the wavelength and the length of the connection. One thus takes care of the release of this volume.

Atmospheric refraction

This volume however is not fixed. As one notices it on the following diagram, it is necessary to hold account for the definition of this zone of the conditions of the atmosphere along the way of the wave. Indeed, the rays are not propagated in straight line, but preferentially follow the zones of strong electromagnetic index, that is to say the densest layers of the atmosphere.

According to the climatic parameters, the provision of these layers changes (one speaks about Réfraction). The hertzian rays thus are more or less curved towards the ground (super refraction), or on the contrary, “point” towards the sky (refraction will infra). In this last case, the release of the connection is made more difficult.

It is thus necessary to undertake statistical studies to quantify the duration during which these phenomena will be able to harm the quality of the connection, and with which intensity.

It is noticed that for the whole of calculations, that amounts giving an average curve to the ray. A convenient representation, used in the following figure, is to make as if the rays always travelled in straight line, and to curve the profile of the connections consequently. That facilitates in particular the description of the geometry of the considered rays.

One thus introduces a “apparent terrestrial ray”, taking account of the virtual deformation of the ground with respect to the propagated waves. He is deduced from the real ray by a parameter, called “factor K”, translating the vertical gradient of coïndice of refraction. Its median value in Europe east from approximately 4/3.

Release/diffraction

The ellipsoid of Fresnel is sometimes partially blocked by an obstacle. One distinguishes usually three types of obstacle:

blade, for “thin” obstacles,
, for a pair of obstacles of the type spherical “blade”
, for obstacles blocking the beam at an important distance.

For each one, methods of calculating make it possible to envisage the additional attenuation to take into account in the assessments.

If the obstacle blocks on a too important portion the ray, the connection can always be established, but this time by diffraction (specific method of calculating).

Guidance and precipitations

Certain characteristics of the propagating medium are thus “random”. For those, one has recourse to climatological statistics (e.g. average steam concentration). It is advisable to consider mainly two phenomena:

Phenomena of guidance

During a certain time, the atmospheric conditions can involve a guidance of the beam, generally in super refraction. The result is then similar to a dépointage of antenna. The probability of occurrence, over the unspecified month, of these “nonselective faindings” is given by a statistical parameter called factor PL (from 2% to 30% in France).

This phenomenon of guidance is dimensioning in the engineering of the connections whose band frequency is lower than 15GHz. It will reduce the possible length of the jump for requirements for availability given.

Attenuations due to the hydrometeors

For the FH of frequency higher than 7GHz, precipitations involve also considerable losses, more especially as the rate of precipitation (in mm/h) and the frequency are high. The intensity of rain varies from 22 with 60mm/h 0,01% of the average year.

This phenomenon of precipitation is dimensioning in the engineering of the connections whose band frequency is higher than 8GHz. It will reduce the possible length of the jump for requirements for availability given.

The wave is partially dispersed on cross polarization (phenomenon of transpolarisation). Attenuation and transpolarisation are marked for a signal in polarization H.

Reflection, ways multiple

The received signal is the sum of the principal signal, and all the considered signals (on the ground, the vegetation, and especially the stretches of water). The interferences generated between all these signals involve on-fields and under-fields sometimes extremely important but also of the distortions (selective fadings).

The principal reflection is the phenomenon of multi-way dominating. There exists however of other case of importance.

multiple reflections in a layer of guidance, the atmospheric conduit playing a part similar to a guide of wave: the wave “rebounds” on the “edges” of the conduit.
scintillation: at the time of the overflight of a forest for example, part of the wave is propagated through the trees, undergoing strong transpolarisations, and dephasings. The resulting field of interference is very unstable.

Assessment of connection

The typical synoptic diagram of a radio-relay system is given below:

Calculation of the assessment of connection

The characteristics of the equipment of end to be taken into account for the calculation of the energy assessment are:

Power of emission : It is the power of the signal which the hertzian equipment can deliver. It usually lies between 20 and 30dBm.

Thresholds of reception : Defined compared to a binary error rate given (TEB=10-3 or 10-6), they translate the capacity for the receiver to treat the signal weakened after propagation (with respect to the thermal Bruit). Depending on the waveband, flow and type of modulation, they generally lie between -70 and -95dBm

Losses of connection (Guide of wave, connector industry…) : For the equipment not presenting antennas integrated, it is necessary to connect by a Coaxial cable or a guide of wave the transmitter/receiving to the antenna. These offsets induce linear losses of 1 to several dB, to which the losses due to the connectors and other junctions are added.

Profit of the antenna : The antennas, mainly parabolic, bring a profit of power (about 25 with 45dB) all the more large as their diameter is important. The directivity of the beam increases with the waveband and the diameters of the antenna.

Obtaining the assessment of connection rests on the simple report: the distant station must receive a signal such that it can retranscribe it with an acceptable error rate, taking into consideration requirement for quality of the connection. The assessment of connection, summation of the emitted power and all the profits and the losses met to the receiver, must thus be such as the level of received signal is higher than the threshold of reception.

However, if the characteristics of emission/reception of the FH to the antenna can be known with precision, it is on the other hand impossible to know the characteristics of the medium at any moment crossed by the waves.

Definition of the margins

The criteria of performance of a connection define the percentages of times allocated during which the signal must be received with a sufficient quality and an availability. Being given the fluctuating conditions of propagation which can even degrade to stop the connection occasionally, one defines in reception the margins of operation allowing to fill these criteria.

the margin with the threshold : To compensate for the majority of the occasional losses of power (nonselective faindings) which undergoes the signal, reception is made with a margin called uniform margin or margin with the threshold. It is the power which one will be able to lose by degradation of the conditions of propagation without losing for as much the quality of the connection.

the selective margin : As one saw, the signal does not undergo only one weakening during the propagation. It also undergoes distortions. This still complicates the task of reception. To translate the capacity of equipment to correctly translate a signal sullied with distortion, a margin known as selective is introduced, which rises from the characteristic of signature of the receiver.

The presence of a disturber (for example another connection emitting on a too close frequency) can also bring a degradation of the effective threshold of the receiver, and reduces consequently these margins.

Devices of countermeasure

Devices make it possible to improve the availability and the quality of the connections, as well with respect to the risks of propagation as of the reliability of the equipment. It is for example possible to double the connection but there exist less heavy and less expensive means.

Protection Hot stand-by

It is possible to choose a configuration of equipment known as of “active day before” ( Hot-stand-by ), in order to mitigate the possible failures of materials. One can also add a " diversité" : it is about a second distinct channel to the connection.

With the emission, in the event of failure of the transmitter, one rocks automatically on a second transmitter, help. This one is thus inactive the major part of time.

In reception, the two receivers receive. The equipment automatically chooses the way by which the signal is the best. In the event of breakdown, one of the two ways remains always available, and allows the breakdown service without interruption of the connection.

Diversity of space and frequency

By introducing a diversity one can benefit from the phenomena of interference evoked earlier.

Diversity of space : One of the main issues already mentioned relates to the presence of a ray reflected in addition to the direct ray which involves the formation of interferences in the vertical plan of the reception antennas. The measurable power thus presents peaks of on-field and hollows of under-field following a vertical axis. The idea is to place a second distant reception antenna of the first of a half interference ring, or of an odd multiple of those, so that the fields the main thing and of diversity are correlated in opposition. The combined field thus makes it possible to be freed very largely from instabilities of the field due to the reflections or the multiple ways.

Diversity of frequency : the idea is similar to that of diversity of space. It is also a question of combining two fields whose dephasings are complementary. One exploits this time the differences in properties of propagation waves of close frequencies. One thus emits in a redundant way on a second couple of frequencies, preferentially on a cross polarization.

Diversity mixed and hybrids : it is possible also to propose configurations mixing the two preceding types of diversity. One can thus emit at two frequencies different out of the two antennas from diversity of space (one speaks then about quadruple diversity). It is also possible to place only one cross antenna on a side, and to benefit from the diversity of space in dissymmetrical reception of way (triple diversity).

Profit on the assessment

According to the connections considered, these techniques make it possible to maintain a power received stable with a few dB whereas in their absence, faindings of field could reach until - 40 dB.

the profits obtained by these methods are measured in terms of increased availability, although the margins uniform and selective remain identical.
They are of true interest only for the situations where the reflections are prevalent (connection with strong overflight of wide very reflective: water, plains desert) and probability of occurrence of multiple ways high (long connections or in zones with strong factor PL).

Choice of diversity

The diversity of frequency has the advantage of requiring only one antenna. The efforts on the bearing structures are thus less; their size can also be less. On the other hand, once given the heights of antenna, the optimal variation in frequency is fixed. This requirement is not always compatible with the plans of frequency imposed in addition. It also presents a weak spectral output

The diversity of space requires two antennas (is there the place on the pylon corresponding to desired spacing?) but their size is often less. In addition, the method has the advantage of a much greater flexibility, and generally higher performances. It is of more sparing in frequencies, resource O how much rare.

Sources

1. " Telecommunications and infrastructures (microwave, space and optical Links) " by Gerard Barué, editions Ellipses.

2. Recommendations of the ITU-R

External bonds

ITU official website
ITU-R official website
Excels site perso on the old FH from France Telecom

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