© 2001-2014, Luigi Moreno, Torino, Italy
In this Session we introduce the basic configuration of a Point-to-Point Radio Link. The fundamental parameters useful to describe radio site installations are presented, including antenna, radio equipment and ancillary subsystems. Hops with passive repeaters are finally discussed.
A Point-to-Point radio-relay link enables communication between two fixed points, by means of radiowave transmission and reception. The link between two terminal radio sites may include a number of intermediate radio sites.
The direct connection between two (terminal or intermediate) radio sites is usually referred as a "Radio Hop". In some cases, a radio hop may include a passive repeater.
A multi-hop radio-relay link, connecting A to B,
divided in two Radio Sections
A multi-hop radio-relay link can be divided in a number of "Radio Sections", each of them being made of one or more radio hops. Transmission performance are usually summarized on a radio section basis.
General criteria for radio network planning and design are not discussed here; just a very brief summary is given below. The overall process can be usually divided in two steps :
1) Preliminary network or link planning. A partial list of activities carried on at this stage is :
· Consideration of Regulatory environment;
· Identification of Terminal radio sites;
· Service and Capacity requirements;
· Performance objectives;
· Frequency band selection;
· Identification of suitable Radio equipment and Antennas;
· Sample design of typical hops, estimate of maximum hop length.
2) Route and Intermediate Site Selection. A number of factors have influence on this choice; among others :
· Maximum hop length;
· Nature of Terrain and Environment;
· Site-to-Site terrain profile; visibility and reflections:
· Angular offset from one hop and adjacent hops (to avoid critical interference);
· Need for passive repeaters.
· Availability of existing structures (buildings, towers);
· New structures requirements;
· Access roads (impact on installation and maintenance operations);
· Availability of Electric Power sources;
· Weather conditions (wind, temperature range, snow, ice, etc.);
· Local restrictions from regulatory bodies (authorization for new buildings, air traffic, RF emission in populated areas, etc.);
When a tentative selection of intermediate sites is available, the final design goes through an iterative process :
· Hop configuration and detailed hop design;
· Prediction of Hop and Section performance;
· Identification of critical hops;
· Revision of Route and Site selection;
· Revision of Hop configuration and of detailed hop design.
In the following Sections we focus on this design process, going through site and hop configuration and leading to performance predictions. As a first topic, we discuss the parameters useful to describe the site and hop configuration.
A radio hop is described in terms of :
a) Topographical data and terrain description :
· Radio site position: geographical coordinates or other mapping information; elevation above sea level (a.s.l.);
· Path length and orientation (azimuth: note that in plane geometry the azimuths computed at the two extremes of a line segment differ by 180 deg, while this is not true in spherical geometry; so, two path azimuths, referred to each radio site, are usually indicated);
· Path profile as derived from paper or digital maps: note that accuracy requirements are widely different throughout a radio path, since the elevation of possible obstructions should be accurately estimated, while significant profile portions (where no obstruction or reflection is expected) could be almost ignored.
b) Radio equipment, antennas and ancillary sub-systems installed at each radio site; in the following sections, the main parameters useful to describe the radio site installation will be discussed.
c) Specific aspects on equipment installation and operation :
· Antenna positioning: installation height and pointing; space diversity option, antenna spacing;
· Frequency used: (average) working frequency (usually referred in hop computations and link budget); detailed frequency plan (go and return RF channels at each radio site, required for interference analysis);
· RF protection systems (use of 1+1 or n+1 frequency diversity, hot stand-by, etc.)
· Use of passive repeaters: flat reflector or back-to-back antenna system, repeater site parameters, reflector or antenna positioning and pointing.
d) Climatic and environmental parameters: they are usually required by propagation models (atmospheric refraction, rain, etc.), so they will be discussed while presenting such models.
Finally, let us consider several attenuation or degrading factors, such as :
· Atmospheric absorption loss;
· Obstruction loss;
· Any other systematic loss throughout the radio path (additional losses);
· Rx threshold degradation due to ground reflections;
· Rx threshold degradation due to interference.
The above impairments will be discussed in the following sessions, where suitable models to estimate their impact on hop performance are considered.
However, it may happen that the inputs required to apply such models are not fully available or that other reasons suggest not to go through a specific analysis.
In that case, we can include among hop parameters also a rough estimate (or a worst case assumption) of losses or degradations caused by the impairments listed above.
A simplified block diagram of a sample radio site installation is shown below.
An example of radio equipment block diagram,
in the case of multiple RF channel operation, using a single antenna
for both transmission and reception.
Even if this example shows a specific configuration, it is useful as a reference in the following presentation. Other configurations of particular interest are:
· Single RF channel installations, where no branching system is needed;
· Outdoor installations, where radio equipment is directly connected to the antenna, without feeder line.
From the viewpoint of a single radio hop design, we can limit information about Radio Equipment to the very basic parameters :
· Range of operating frequencies;
· Transmitted power PT;
· Receiver threshold PRTH (minimum received power required to guarantee a given performance level);
Note that :
1) Both the transmitted power and the receiver threshold are usually referred at the equipment input / output flanges, not including branching filter losses.
2) When the transmitter is equipped with an Automatic Transmitted Power Control (ATPC) device, the Tx power to be considered in hop design is the maximum power level (which should be applied every time the received signal quality is deeply affected by propagation impairments);
3) The receiver threshold is the minimum received power required to achieve a given performance level; in digital systems, the reference performance is usually set at Bit Error Rate (BER) = 10-3, while other reference levels may be adopted if needed.
Performance objectives in digital radio links will be discussed in the final Session of this course.
Additional parameters can be useful for a more complete understating of the equipment operation, even if they are not directly involved in the hop design :
· Equipment user capacity; for digital systems, bit-per-second or number of standardized signals, like STM-1 or DS1 signals; for analog systems, number of telephone or television channels;
· Bit rate (R) of the modulated (emitted) signal (this may differ from the user capacity, mentioned above, since the transmission equipment may include additional bits for service and monitoring channels, channel coding, etc.);
· Modulation technique;
· Symbol rate of the modulated (emitted) signal; in analog systems, an equivalent parameter is the baseband (modulating) signal bandwidth;
· Emitted spectrum and modulated signal bandwidth.
The Symbol rate SR depends on the emitted signal bit rate R and on the modulation technique :
where L is the number of bits coded in a single modulated waveform (L = 2 in QPSK modulation, L = 6 in 64QAM modulation).
For advanced tasks in Radio Hop design, more detailed data on radio equipment are required. This includes :
· Rx noise bandwidth BN and Rx noise figure NF;
· Signal-to-Noise (S/N) ratio at the Rx threshold;
· Co-channel Carrier-to-Interference ratio at receiver input, producing the threshold BER, in the absence of thermal noise (high Rx level);
· Typical spacing between adjacent RF channel;
· Net Filter Discrimination (NFD) at the above spacing;
· Results of signature measurement;
· Possible use of Automatic Transmitted Power Control (ATPC) and related parameters;
· Possible use of Cross-Polar Interference Canceller (XPIC) and related parameters.
The Signal-to-Noise (S/N) ratio can be expressed in terms of received power PR, receiver noise bandwidth BN, and receiver noise figure NF :
where PR is expressed in dBm and BN in MHz; the expression in square brackets gives the receiver thermal noise power.
Digital Equipment Signature
The equipment signature gives a measure of the sensitivity of radio systems to channel (amplitude and group delay) distortions as produced during multipath propagation events. More specifically, it is used for digital radio systems with signal bandwidth larger than about 10-12 MHz (on this type of signals, significant frequency selective distortion is not produced if the bandwidth is narrower; other signals may be sensitive to frequency selective multipath even with a narrower bandwidth).
Measurement Set-up - The Tx signal is modulated by a test sequence and is transmitted through a simulated multipath channel, modeled as a two-path channel (direct plus echo branches).
Signature measurement setup.
As shown in the above figure, the power level and the phase of the delayed signal can be adjusted by means of a variable attenuator and a variable phase shifter.
Assuming a normalized signal amplitude equal to 1 in the direct branch and b (< 1) in the delayed branch, then the Two-Path Channel Transfer Function is:
t = Echo delay, assumed as constant ( = 6.3 ns in the original Bell Labs / Rummler model );
fo = j / 2 p t = Notch Frequency (corresponding to the minimum amplitude of the transfer function);
B = Notch depth (in dB) = - 20 Log10 (1 - b).
Two-path channel transfer function,
with definition of Notch Frequency and Notch Depth.
The above definition refers to a Minimum-Phase Transfer Function. Otherwise, if the signal amplitude is b ( < 1) in the direct branch and 1 in the delayed branch, then similar definitions apply, but a Non-Minimum-Phase Transfer Function is obtained.
Measurement Procedure - As shown by the above definitions, the notch frequency is controlled by varying the echo phase f; while the notch depth depends on the echo amplitude b.
The first step in the measurement procedure is to select a given Notch Frequency fo, with echo amplitude close to zero. Then, the echo amplitude is increased, making the transmission channel more and more distorting. Consequently, the Bit Error Rate (BER) will increase.
The notch is made deeper, up to the "Critical Depth BC", when BER = 10-3 (or any other desired threshold). The point [BC, fo] is a signature point.
in the Notch Depth / Notch Frequency plane.
In that plane each point corresponds to a pair of notch parameters, so it is representative of a particular channel state. The points below the signature show the channel states for which BER > Threshold. Therefore, the area below the signature gives a measure of the receiver sensitivity to multipath distortions. For an unequalized signal, typical signature width may be of the order of 1.5 times the symbol rate, while using equalization it is halved at least.
To predict multipath outage, it is often required that the equipment signature be defined by only two parameters (signature width and depth). In most cases the shape of actual equipment signatures allow for a "square brick" approximation.
Equipment parameters used in Interference analysis
Net Filter Discrimination (NFD) - It is used to characterize the radio system ability to limit the interference coming from an adjacent radio channel.
NFD gives the improvement in the Signal-to-Interference ratio passing through the Rx selectivity chain (RF, Intermediate, baseband stages) :
where (C/I)RF is defined at the RF input stage and (S/I)DEC at the decision circuit stage.
Signal spectra at the receiver input ant output
and Rx selectivity.
As shown by the figure above, the NFD depends on :
Interfering signal spectrum (Tx filtering);
Overall Rx selectivity in the Useful Channel.
The NFD can be measured or evaluated for interference between identical signals (adjacent channel interference in a homogeneous channel arrangement) and also when the interfering signal is different (in capacity and/or modulation format) from the useful one (interference in a mixed signal network).
So, for any pair of useful and interfering signals and for each value of the channel spacing, a NFD value can be evaluated.
Threshold Carrier-to-Interference ratio - In some applications the received signal may be interfered by a co-channel signal, with identical capacity and modulation format (for example in co-channel frequency arrangements, with use of both orthogonal polarizations).
The receiver sensitivity to co-channel interference is estimated by a Bit Error Rate (BER) vs. C/I curve, as shown in the figure below.
Bit Error Rate (BER) vs. Carrier-to-Interference ratio (C/I),
with indication of C/I threshold for BER = 10-3.
The measurement is made in absence of any significant thermal noise contribution (high Rx power level).
From this measurement, it is possible to know the Carrier-to-Interference ratio corresponding to the threshold error rate (for example BER = 10-3).
Cross-Polar Interference Canceller (XPIC) Gain - The Interference Canceller is used to reduce the interference coming from a signal transmitted on the same frequency with orthogonal polarizations (usually the useful and interfering signals have identical capacity and modulation format).
We assume that the signal-to-interference ratio at the receiver RF input is (C/I)RF.
The interference canceller works in such a way that the signal-to-interference ratio appears to be improved to a higher value (C/I) APP defined as
where XPICGain is defined as the gain produced by the cross-polar canceller. The interference impairment is computed by assuming (C/I) APP to be the actual signal-to-interference ratio.
Let us consider a radio transmitter with power pT coupled to an Isotropic Antenna (an ideal source of EM Radiation, that radiates uniformly in all directions). At the distance L from the antenna, the emitted power will be uniformly distributed on. the surface area of a sphere of radius L, so that the Power Density rI is :
EM power emission from an Isotropic Antenna (left)
and from a Directive Antenna (right)
Then we substitute the Isotropic Antenna with a Directive Antenna, while the transmitted power is again PT. We imagine to measure the Power Density where the antenna axis intercepts the sphere surface, with result rD
The antenna gain gives a measure of how much the emitted power is focused in the measurement direction, compared with the isotropic case. As a result of the "experiment" described above, the antenna gain is defined as :
This definition leads to g = 1 for the isotropic antenna.
where D is the reflector diameter, h is called "antenna efficiency" (typically in the range 0.55 - 0.65), A is the reflector area and AE = h A is the Antenna Effective Area.(or Aperture).
In logarithmic (decibel) units :
where the ±0.5 dB term depends again on antenna efficiency; it is assumed to express the diameter D in meters [m] and the frequency F in GigaHertz [GHz].
Note that, for given dimension, the antenna gain increases with frequency (6 dB higher if the frequency is doubled). Similarly, at a given frequency, the gain increases 6 dB if the antenna diameter is doubled.
Below, some examples of antenna gain vs. diameter and frequency are given.
Antenna gain vs. diameter and frequency; the double (red, black) line gives a range of possible gains, depending on antenna efficiency.
The 3-dB beamwidth BW (see graphical definition below) is related to antenna gain; as the gain increases, the EM energy is focused in a narrower beam.
Definition of the antenna 3-dB Beamwidth BW.
For reflector antennas, some simple "rules of thumb" are useful in relating antenna diameter D [m], working frequency F [GHz], gain G [dB], and the 3-dB beamwidth BW [deg] :
Note that these are approximate relations, fitting with "real" values within some margin; in particular cases this margin may be even large.
An additional concept in antenna operation is the Far Field Region. It is the region sufficiently distant from the antenna, where the electromagnetic (EM) field can be well approximated as a plane wave and the antenna diagram is stabilized. Closer to the antenna, the Near Field Region and the Fresnel (transition) Region are defined, where the antenna radiation diagram is not easily predicted. The boundary between the Fresnel and the Far Field Region is approximately at the distance :
Antenna Parameters for hop design
Point-to-point radio hops usually make use of high-gain directive antennas, which offer several advantages :
· both Transmission and Reception : the antenna gain is maximized in the desired direction.
· Transmission : the emitted radio energy is focused toward the receiver, thus reducing the emission of interfering radio energy in other directions;
· Reception : the receiver sensitivity to interfering signals coming from other directions is reduced.
However, in special cases, also antennas with sectorial or even omnidirectional coverage may be used (this is true mainly for point-to-multipoint applications).
In most cases, directive antennas are parabolic antennas or other reflector antennas (like Horn or Cassegrain antennas). The directivity patterns can be measured both in the vertical (elevation) and in the horizontal (azimuth) planes; however, we can often adopt the simplifying assumption that one diagram is applicable both to the vertical and to the horizontal planes. In that case, also the 3-dB antenna beamwidth is assumed to be the same in the two planes.
As far as interference problems are not considered in a single radio hop design, we can limit information about the antennas to the very basic parameters :
· Range of operating frequencies;
· Single or Double Polarization operation;
· Antenna gain;
· 3dB beamwidth in the vertical plane (this may be useful to analyze reflection paths).
An example of the antenna connection to radio equipment is given in the Block diagram shown above. Note that the antenna gain (as well as other antenna parameters) is referred to the antenna I/O flange.
Additional parameters can be useful for a more complete description of antenna operation :
· Antenna type (Parabolic, Horn, Cassegrain, etc.);;
· Coverage type (omnidirectional, sectorial, directive);
· 3dB beamwidth in the horizontal plane (for sectorial antennas);
· Diameter (or more generally, physical dimensions);
· Voltage standing wave ratio (VSWR);
Antenna radiation diagram (mask) for Co-polar e X-polar operation
(a different horizontal scale is used in the 0 - 20 deg range and in the 20 - 180 deg range).
More on the Antenna radiation diagram
Some additional comments on antenna diagrams :
The result of the antenna directivity measurement usually exhibits multiple lobes and nulls. A sidelobe envelope is estimated, giving a "mask diagram", useful to characterize the antenna directivity.
In interference analysis the need arises to estimate the antenna gain in any direction and the antenna mask gives a conservative result.
The pattern of co-pol and cross-pol antenna diagrams, close to the pointing direction, are significantly different, as shown in the figure below.
Example of Co-pol. and Cross-Pol. antenna diagrams,
close to the antenna pointing direction
While the co-pol pattern is rather flat, in the range of some tens of degree around pointing direction (maximum gain), the cross-pol pattern has a very narrow minimum in the same direction.
In some cases it is convenient to point the antenna by searching for the minimum cross-pol signal level, instead of searching for the maximum co-pol signal. By this way, it is assured that, not only the maximum gain, but also the maximum cross-pol discrimination are obtained.
The antenna directivity diagram is usually measured in a controlled environment, in order to characterize the "true" antenna response, without influence or errors produced by any external element.
In actual operation, the antenna response may by significantly altered by the surrounding environment. For example, an obstacle close to the main antenna lobe may produce a signal reflection, about 180° from the antenna pointing direction. This apparently reduces the antenna front-to-back decoupling, both in the co-pol and cross-pol diagrams.
The correct antenna positioning is a key factor in order to get antenna performance in real operating conditions as close as possible to measured parameters.
A number of additional equipment and subsystems are working in a radio site. In the present context, we consider only what is strictly related to the design of a radio hop (so, we do not discuss power lines and back-ups, air conditioning, grounding, and other subsystems, even if they are of significant importance in the overall site operation).
As shown in the block diagram above, a branching filter is required in radio transceivers for multiple RF channel operation.
In transmission, the function of the branching system is to multiplex RF channels on a single wide-band RF signal, suitable to be transmitted on a single antenna. Similarly, in reception, the branching system splits the multi-channel signal coming from the antenna into multiple RF channels, each addressed to the corresponding receiver.
The branching loss is different for the various RF channels (in Tx and Rx), depending on the number of filter ports and circulators to be passed through by the signal. However, in hop design, it is advisable to take account of highest loss, resulting from Tx and Rx branching.
In a branching configuration with a common Tx/Rx antenna (see block diagram), the branching loss include the loss of the circulator used to separate the Tx and the Rx branches.
Tx / Rx Attenuators
Power attenuators may be added in the transmitter or in the receiver chain, mainly to avoid an excessive power level at the receiver input (which may saturate the Rx front-end stage) and/or to avoid unnecessary power emission in short hops (interference reduction).
Note that many radio equipments now include power setting options or ATPC (Automatic Transmitted Power Control) devices, so that in most cases the use of external attenuators is no longer required.
In the context of radio hop design, the only parameter to be associated with Tx and Rx attenuators is the attenuation level itself.
A feeder line is required to connect the antenna I/O flange to the radio equipment I/O port (or to the branching system I/O port). The exception is the outdoor configuration, with direct equipment-to-antenna connection.
The basic feeder parameters for radio link design are :
· Range of operating frequencies;
· Specific loss (expressed in dB per unit length).
Additional parameters, giving more details on feeder description :
· Feeder type (cable, rectangular waveguide, etc.);
· Weight (expressed in kg per unit length).
Passive Repeaters are used mainly in hops over irregular terrain, to by-pass an obstruction along the path profile.
Three Passive Repeater configurations are described below, while the corresponding Link Budget equations are presented in the next Session.
Single plane reflector - it is implemented as a metal surface, which is close to a 100% reflection efficiency. The surface flatness must be more and more accurate for increasing frequency (smaller signal wavelength).
Passive repeater implemented as a single plane reflector
Each path from a radio site to the repeater is called a "leg". So a radio hop with a single reflector is made of two legs.
Note that the useful or "effective" area AE of the plane reflector is given by :
where AREAL is the real reflector area and j is the angle between the two rays.
It is estimated that for j > 120° (corresponding to b < 60°) the effective area is so reduced that it is not practical the use of a single reflector, since very large panels should be installed.
Double plane reflector - It is used when the change in signal direction (b) is lower than 60° or when it is not possible to find a suitable position for a single reflector, where visibility with both hop terminals is assured.
Passive repeater implemented as a double plane reflector
A radio hop with a double reflector is made of three legs.
With a double reflector arrangement it is possible to operate even if the angle b is close to 0°.
The reflector effective area is given by the same formula used for the single reflector, so that the angle between the two rays, at both reflectors, should be as low as possible.
Passive repeater implemented
as a two-antenna back-to-back arrangement
From a geometrical point of view, the back-to-back antenna system has a wider and more flexible application field, compared with a single reflector system. From a given repeater position, any change in signal direction (b) can be obtained.
However, single or double reflectors may be implemented, if needed, with surfaces much wider than the usual antenna size. Moreover, the reflector efficiency is close to 100%, compared to some 55% antenna efficiency.
So, when the power budget is limited, the back-to-back antenna system may be a poor solution.
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End of Session #1
© 2001-2014, Luigi Moreno, Torino, Italy