Path & Frequency Planning

I

PATH AND FREQUENCY PLANNING

This chapter covers some of the issues that may arise concerning path profiles, line-of-sight requirements, input signals and their variation, diversity, reflections and frequency planning.

In addition, surveying possible radio-link paths and site requirements are discussed.

TABLE OF CONTENTS

1 Objective and scope 1 2 Basic definitions and advices 1 2.1 Radio path 1 2.2 Microwave system 2 2.2.1 Point-to-point radio-relays 2 2.2.2 Point-to-multipoint radio-relays 3 2.3 Equivalent and true earth radii 3 2.4 Fresnel zones 4 2.5 Line of sight / clearance 6 2.6 Path profiles 7 2.7 Outage 9 3 Mobile transmission network 9 3.1 Transport network 9 3.1.1 Core/backbone network 10 3.1.2 Access network 11 3.2 Physical and logical networks 12 3.3 The SDH transport capacity 13 3.3.1 High capacity 13 3.3.2 Medium capacity 13 3.3.3 Low capacity 13 3.4 Capacity planning 13 3.5 Network topologies 14 3.5.1 Chain/Tandem 14 3.5.2 Star network � Case A 15 3.5.3 Star network � Case B 16 3.5.4 Ring (loop) 17 3.5.5 Mesh 18 3.6 Evolution in a mobile access transport network 18 3.6.1 Capacity growth and new sites 19 3.6.2 Improving protection and extending coverage 19 3.6.3 Integrating nodes and network manager 20 3.7 Network hierarchy 21 4 Path and site survey 22 5 Microwave design 23 5.1 Initial planning 24 5.2 Loss/attenuation block 25 5.2.1 Link budget 25 5.2.2 Fade Margin 26 5.2.3 Losses and attenuations 26 5.3 Fading Block 30

II

5.3.1 Rain 31 5.3.2 Multipath - flat 31 5.3.3 Multipath � frequency selective 31 5.3.4 Refraction-diffraction 32 5.4 Frequency Planning Block 32 5.4.1 Introduction 32 5.4.2 Assignment of radio-relay frequencies 32 5.5 Chanel arrangements 33 5.5.1 Far interference in frequency planning 33 5.5.2 Near interference in frequency planning 34 5.5.3 How may interference be avoided? 36 5.5.4 Interference tolerance for digital channels 36 5.5.5 Simplified step-by-step procedure 36 5.5.6 Additional scenarios in frequency planning 40 5.6 Quality and availability block 42 6 Improving microwave paths 45 6.1 Hardware failure 45 6.2 Diversity 45 6.2.1 Space diversity 46 6.2.2 Frequency diversity 46 6.2.3 Improvement factor 46 6.3 Other techniques 47 6.3.1 Passive repeaters 47 6.3.2 One-plane reflector 47 6.3.3 Two-plane reflectors 47 6.3.4 Back-to-back antennas 48


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1 Objective and scope In the process of designing of microwave networks, a number of tasks must be performed before path calculations are carried out. These tasks are described herein and deal, for the most part, with terrain, climate, equipment data and site configuration. Frequency planning is considered as one of the more important tasks in the planning of a network.

This section will cover some of the issues that may arise concerning path profiles, requirements regarding line-of-sight, input signal and their variation, diversity, reflections and frequency planning. A section is included which deals with the survey of possible radio-link paths and site requirements.

2 Basic definitions and advices

2.1 Radio path A radio path (or path) is the physical (geographical) distance between a transmitter and its corresponding receiver in which electromagnetic signals are propagated. It is also known as radio hops (or hop). One or more radio paths connected in tandem form a radio-relay circuit or radio-relay link. The radio stations (radio-relay stations) between two terminal stations are called repeater stations (active or passive); see Figure 1.

Terminalstation

Terminalstation

Repeaterstation

Repeaterstation

Repeaterstation

Figure 1: Radio link between two terminal stations showing repeater station.

Radio-relay transmission is a very attractive transmission alternative for applications ranging from the coverage of the rural, sparsely populated areas of developing countries having ineffective or minimal infrastructures to the well-developed industrial countries that require rapid expansion of their telecommunications networks. Most of the commercial radio links currently in use are in the frequency range 2�50 GHz.

RADIO TRANSMISSION NETWORK AND FREQUENCY PLANNING

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Low capacity radio-relay links are encountered in the access portions of a transmission network (access is the part of the network between the subscribers and the exchanges) where PDH radio-relay links are frequently used. Particularly, radio-relay links are used in the access portion to connect RBS sites to the MSC or possibly BSC.

Radio-relay links may also be found in the backbone portions of a transmission network (backbone is the part of the network collecting the traffic from the access portion and leading it into the nearest service platform). In the backbone portion of a network, radio-relay links are typically used to interconnect the RNCs/MSCs.

Today, most of the manufacturers are currently providing equipment with capacity in the range of 2 Mbit/s to 155 Mbit/s.

Advices:

1) Radio-relay links are recommended in inaccessible terrain and difficult environments.

2) Radio-relay links are recommended when quick coverage of large areas by new operators is demanded.

3) Radio-relay links are recommended when higher security is required (equipment can be physically concentrated).

4) Be aware that the term �radio relay� is also intended for radio equipment.

2.2 Microwave system A microwave system can be build of point-to-point (PP) or point-to-multipoint (PMP) radio relays, normally an optimal combination of both categories.

2.2.1 Point-to-point radio-relays A point-to-point microwave system is composed of microwave radio units, transmission lines (waveguides or coaxial cables), and parabolic antennas on both ends. Generally, they present two different and broad alternatives: standard and split configuration.

Standard configuration consists of a complete indoor radio unity and a waveguide connecting the transmitter to the outdoor parabolic dish antenna. This configuration requires a shelter and at least one full rack of space mounting.

The split configuration is a microwave terminal with outdoor radio units and an indoor access module.


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2.2.2 Point-to-multipoint radio-relays In dense urban areas where the site density is high, microwave point-to-multipoint systems might be optimal with regard to capital cost, operational cost as well as speed of installation. The PMP technology is a complement to point-to-point (PP) microwave application where a combination of the PP and the PMP technologies might give the most cost and spectrum efficient microwave solution for the entire network.

2.3 Equivalent and true earth radii Due to the refraction in the atmosphere, a radio beam may be bent upward or downward. The refraction in the atmosphere is described by its refraction index, which is dependent on the humidity, temperature and pressure of the atmosphere, all together function of the height. In order to facilitate path profiling at the time computers were not currently available, radio transmission engineers introduced the earth-radius factor k that compensates for the refraction in the atmosphere. Applying appropriate k-values to the true-earth radius, an equivalent-earth radius is geometrically obtained and consequently straight rays can be drawn; see Figure 2.

Curved ray

True-earth radius

Straight Ray

Equivalent-earth radius

Figure 2: Equivalent and true earth radii.

Different values of the earth-radius factor yield different equivalent-earth curvatures; see Figure 3.


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k=1

k= ∞

k=0.5

k=0.33

k= -1

Earth Surface

Figure 3: Equivalent earth with different ray beam curvatures for different values of the earth-radius factor.

Advices:

1) The earth-radius factor k can be obtained in the following way: A) select the appropriate value of the refractivity gradient (dN/dh) in the charts given Chapter 15.2. Use the chart that gives the highest monthly value. B) The k-value is given by k = 157/(157 + dN/dh).

2) The refractivity gradient values presented in the charts Chapter 15.2 are negative.

3) N = -39 N-units/km (normal atmosphere) yields k = 4/3 = 1.33. This value should be used whenever local value is not provided.

4) Low k-values lower the LOS (demand higher antenna heights) but offer better protection against interference from other stations. Higher k-values give higher LOS (demand lower antenna heights) but expose the link to interference from other stations.

2.4 Fresnel zones Fresnel zones are specified employing an ordinal number that corresponds to the number of half-wavelength multiples that represents the difference in radio wave propagation path from the direct path. The first Fresnel zone is therefore an ellipsoid whose surface corresponds to one half-wavelength path difference and represents the smallest volume of all the other Fresnel zones; see Figure 4.

Even if one finds that a path exhibits proper line-of-sight characteristics, path obstacles may have attenuating effects on the signal if they are situated sufficiently close to the path.


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dA= Distance from antenna A to point M, km

dB= Distance from antenna B to point M, km

R = Radius of the Fresnel zone at point M, m

dA + dB = d = Distance between antennas A and B, km

MA BR

dBdA

MA BR

dBdA

f = Frequency, MHz

( )dfdddR AA

⋅−⋅⋅= 547 ( )

dfdddR AA

⋅−⋅⋅= 3217.

GHz

Figure 4: Fresnel zone between two stations located on an equivalent earth surface (the ray beam is straight).

Provided that there is no obstacle within the first Fresnel zone, obstacle attenuation can be ignored, and clearance demands are in most cases satisfied. If one has, for example, a backbone network operating at a lower frequency than for example 7 GHz, the path length may require more clearance than that required by the first Fresnel zone. One may be required to keep the first Fresnel zone free from obstacles at a smaller effective earth-radius than for k=4/3. For example, the requirement may entail a free first Fresnel zone for k=0.5.

On the other hand, at frequencies less than about 2 GHz, one may be able to tolerate some obstacle attenuation. The need for clearance for these frequency bands must be calculated for each individual path.

Some examples of how the radius of the Fresnel zone varies with path length for different frequency bands are shown in Table 1. The table shows the Fresnel zone�s mid-path, which provides an indication of the clearance requirements that are demanded.


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Frequency (GHz) Path length (km) 0.45 7 15 23 26 38

5 29 7.3 5.0 4.0 3.8 3.1 15 50 12.7 8.7 7.0 6.6 - 40 82 20.7 - - - -

Table 1: The radius (m) of the first Fresnel zone (mid-path) for some frequencies. The symbol (-) is not practically applicable for the correspondent frequency.

Advices:

1) The higher the frequency the smaller the Fresnel zone and consequently more vulnerable to non-LOS effects (object attenuation).


2.5 Line of sight / clearance The refractive properties of the atmosphere are not constant. The variations of the refraction index in the atmosphere (expressed by the earth-radius factor k) may force terrain irregularities to totally or partially intercept the Fresnel zone. Clearance can be described as any criterion to insure that the antenna heights are sufficient so that in the worst case of refraction (for which k is minimum!) the receiver antenna is not placed in the diffraction region.

hchc = LOS-clearance

Figure 5: The LOS-clearance.

The direct path between the transmitter and the receiver needs a clearance above the ground or any obstruction of at least 60% of the radius of the Fresnel zone in order to insure free-space propagation. Clearance values have to fit the local climate conditions.


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Frequencies above 7 GHz require free line-of-sight between the transmitting and receiving antennas. Obstructions that penetrate into and above the line-of-sight cause signal attenuation that may cause the path to be unusable. Such obstructions may be composed of terrain, forests, buildings, chimneys, etc. If one uses maps to investigate free line-of-sight conditions, one should be especially observant as to obstructions close to the sites (in the vicinity of 100-200 meters) that may not be indicated due to inaccuracies in the map due to insufficient resolution. Maps are not the best tool to judge the height of buildings and other man-made obstructions. A line-of-sight investigation should always be performed on site before finally selecting station sites.

Clearance is normally considered by applying �clearance criteria� that are climate dependent or by properly handling diffraction-diffraction fading (k-type fading).

Advices:


2) The most common discrepancy arises when the radius of the first Fresnel zone is not compensated for its vertical projection. The more inclined the path is the more correction is required.

2.6 Path profiles The intention of the path profile is to provide material for the decision as to whether a free line-of-sight exists between the selected sites for the stations and whether sufficient clearance exists to avoid obstacle attenuation. The path profile is also useful when calculating variations in received signals (fading).

The path profile is essentially a plot of the Earth�s elevation as a function of distance along the path between the transmitting and receiving sites. Data is derived by locating the two terminals on an elevation contour map, drawing a straight line between the two points, and then reading the elevation contours at suitable distance intervals.

Topographical information, used in the construction of path profiles, may also be derived from topographical databases. Such databases are required to include both altitude data and land-use data.


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A path profile is plotted in a so-called path-profile chart where the height of the Earth bulge ∆h at a given point along the path expressed in meters is obtained as a function of the Earth radius factor; see Figure 6. Path profile charts are constructed by computing the height of the Earth bulge, ∆h, along the entire path.

d2d1

∆h

Earth

ele

vaiti

on

Distance, km

Elev

atio

n, m Rk

ddh

⋅⋅⋅

=2

∆ 21

Figure 6: The Earth bulge.

A radio ray beam may be shown as a straight line in a path profile that is constructed having an earth radius factor that corresponds to the conditions defined by a normal atmosphere for the particular geographical locations at which the sites are located.

The parameter (dN/dh) that may be used for the calculation of the particular k-value for different parts of the world is discussed in section 2.2.

The path profile chart may now be completed. Antenna height and line-of-sight information are added to the chart. Adding the first Fresnel radius to the chart will allow the determination of free line-of-sight and whether or not sufficient clearance exists along the path. The path profile is to clearly indicate any forest areas, buildings and other man-made obstructions, see Figure 7.


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Figure 7: Path profile.

2.7 Outage Outage is generally defined as the probability that a pre-defined bit-error ratio is exceeded during a certain measured period. The concept of outage comprises quality (error performance) and availability events that are referred to a given bit-error ratio.

3 Mobile transmission network

3.1 Transport network The transport network is generally divided into two main portions: core (trunk) and access. Two words that are commonly employed when discussing transport networks can be somewhat misleading: 1) �transport� is often used to refer to the core portion, 2) �backbone� (core) is in many applications the same as �core� but having a more physical signification.

A mobile transmission network is normally composed of three general capacity ranges as illustrated by Figure 9.


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HUB

Low Capacity RadioAccess Network (LRAN)

HUB HUB

BSC/RNCHigh Capacity RadioAccess Network (HRAN)

Core Network

PDH/SDH links

PDH links

SDH links

Transmissionhub site (medium capacity)

Ring/star

Star/chain

Ring/mesh

Switch site (high capacity)

MSC/MG

MSC/MG

(LRAN)

Figure 8: A mobile transmission network showing the connection between different sites and capacity levels. MSC=Mobile Switching Center, MG=Media Gateway, BSC=Base Station Controller, and RNC=Radio Node Controller.

3.1.1 Core/backbone network The core/backbone transport network is the portion of the network assembling the traffic from smaller access areas and then carrying it into the nearest service platform. The backbone microwave presents high reliability, normally characterized by its high capacity (SDH links) at LOS sites having sufficient power supply and of easy accessibility (close to roads and reliable infra-structures).

Since a high degree of availability and easy maintenance is required, the backbone portion is commonly build as a ring having nodes (hubs) assembling the subscribers from the access portion in order to minimize the capacity and number of connections to higher network layers. The concentration of traffic on core/backbone nodes demands tall, steady, and rugged towers located at distinct LOS sites. In Figure 9, high capacity backbone links (34 Mbit/s) are converging to an RNC-node. The portion of the network comprises a number of clusters forming star configuration, interconnected by a backbone chain/tandem of radio-relay links.


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34 Mb/s

34 Mb/s

34 Mb/s

34 Mb/s

34 Mb/s

8 Mb/s

2 Mb/s2 Mb/s

8 Mb/s

8 Mb/s2 Mb/s

RNC

Figure 9: Typical access network containing a number of clusters forming star configuration interconnected by core/backbone chain/tandem of radio-relay links.

3.1.2 Access network The access network (also known as subscriber network, local loop and local network) is the part of the network between the subscriber and the local exchange. It is considered the lowest layer in a microwave network and built forming star/tree or chain structures. Similarly to core/backbone nodes, access nodes aim at concentrate the traffic from the subscriber in order to minimize the number and capacity of access connections before �delivering� it into higher network layers. Access nodes also demand tall, steady, and rugged towers located at distinct LOS sites. The low capacity links illustrated in Figure 9 are typical examples in the access portions of a network.

Until the middle of the 80s operators were typically governmental (monopoles) entities that invested in copper lines between subscribes and their local exchange. The rate of expansion of access networks was therefore relatively low. However, with the advent of competitive mobile networks in the beginning of the 90s, investments on access networks increased dramatically and now accounts for approximately 50% of the investments in telecommunication networks.

Three wide subscriber categories are now part of the access network: fixed, mobile and cordless applications (terminal mobility in a fixed network, for instance DECT) as used in the office by several medium and large companies.


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3.2 Physical and logical networks The physical sites in a network normally form a huge net of hundreds or thousands of interconnections where the sites are the main building blocks. This is the physical network. However, the structure of a mobile transmission network can also be considered from the point of view of logical building blocks instead of its physical sites. This is the logical network. The logical blocks are somewhat arbitrary and general, in some way attempting to describe the functionalities present in the network. The logical part of a network consists of paths, which as far as their extensions are concerned, can be controlled from operations support systems. In this sense, a microwave network can be implemented as a combination of end (smallest building block) and aggregation sites.

Considering the physical and logical aspects, it is safe to say that the access network is the only portion of a network showing both physical and logical characteristics.

The end site supports transmission in only one direction (for instance, carrying traffic into the access portion or out of it). It can either be a site providing the user access to a radio base station or an end customer, or it can be a site interfacing the mobile switches and the core network. For an end site providing user access, the capacity normally ranges from 2 up to 34 Mbit/s. Redundancy is normally not required.

For a site interfacing switches and the core network, see Figure 8, the capacity normally ranges from 34 Mbit/s up to 155 Mbit/s. Redundancy is normally required. Both point-to-point and point-to-multipoint end sites are foreseen. The end site should be flexible in order to support traffic interfaces ranging from multiple E1s, E2, E3, and STM-1.

Medium-capacity aggregation sites, see Figure 8, are nodes/hubs (with or without redundancy) where low/medium capacity traffic is concentrated and carried through high-capacity aggregation sites into the core/backbone portion where redundant 155 Mbit/s microwave links can be used. Optical fibers are commonly carrying the traffic in ring or redundant chains.


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3.3 The SDH transport capacity The transport network has to be properly dimensioned to avoid traffic delay and blocking due to lack of capacity. In addition, if a fault occurs in one certain direction, an alternative path is immediately taken into operation (route diversity). When it comes to microwave networks, SDH links are the main responsible for transporting high capacity in the HCRAN and core portion of the network. Some characteristics of SDH links in three main capacity ranges are briefly described in the following sections.

3.3.1 High capacity SDH links in the long-haul/core network typically operate at STM-1, n×STM-1 or STM-n transmission rates. They are typically deployed in the lower (2-12 GHz) frequency bands.

SDH links in short-haul or access network typically operate at the STM-1 or n×STM-1 transmission rates. They are usually deployed in frequency bands higher than 12 GHz.

3.3.2 Medium capacity There might be applications when traffic requirements are below those of an STM-1 signal and effectively utilize the available radio bandwidth. In this case, radio-relay systems typically operate at the STM-RR transmission rate with a VC-3 traffic payload. In addition, there are also PDH links (E3=34 Mbit/s).

3.3.3 Low capacity SDH links with payload transport capacity below VC-3, e.g. n×VC-12, are still under study. In addition, there are also PDH links (E1=2 Mbit/s, E2=8 Mbit/s).

3.4 Capacity planning Clear rules for capacity planning are generally not applicable for the entire transport network because:

1) Capacity planning is strongly dependent on the application: fixed, mobile, data communication and enterprise network.

2) Capacity expansion in transport networks takes place distinctly; currently the �lifetime� is normally 5-10 years before expansion is demanded.


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3) Upgrading of existing transport networks is network portion dependent; for instance, the upgrading of the core and access portions is normally not �synchronized�.

3.5 Network topologies There are three main aspects that directly affect the network topology: cost, reliability and speed of deployment. For instance, improved reliability and system performance can be attained if adequate topology is employed, although it may lead to more expensive solution. The best strategy is therefore to search for an acceptable balance between overall system performance and costs. A number of possible alternatives are included below, including common network configurations in which a number of RBS sites are to be connected to a mobile telephone exchange (MSC/RNC); see Figure 10.

MSC

Figure 10: RBS sites that are to be connected to an MSC.

3.5.1 Chain/Tandem This type of configuration consists of linking RBS sites in a chain such that the previous RBS sites in the chain act as active repeaters for the last one; see Figure 11. This often provides minimum length per link and normally a cost-effective solution.

Figure 11 illustrates two chains converging to a common RNC. In this particular case, the configuration can also be considered as a �tree�.

A common application of chain is the connection of RBS sites along roads. Parts of a chain can also be used in mixed topologies.


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MSC

Figure 11: Chain/tandem configuration.

This configuration has two main disadvantages: 1) since the links are connected in sequence it is expected poorer hardware availability caused by hardware faults, 2) capacity requirement increases along the chain toward the RNCs. Drop insert or DCC/DXC (Digital Cross Connect) may help to minimize capacity requirements.

At the very bottom of the network hierarchy, where capacity is relatively low (2 Mbit/s), the paths are normally unprotected. Closer to the RNC where the capacity is accumulated, it is strongly recommended to have some degree of protection.

�Cascade� configuration is similar to chain/tandem as previously described in this section, but the traffic may be concentrated at some RBS sites in the chain (hubs). Protection is strongly recommended in the �feeder� link.

3.5.2 Star network � Case A Figure 12 illustrates a common pattern, in which all RBS sites are directly connected to the RNC forming a star network. In principle, this configuration is simple and offers the following advantages:

• The RBS sites may be established to expand capacity requirements in a particular area separately from capacity requirements in other parts of the network.

• The network may be gradually taken into service in accordance with the establishment of new sites.

This configuration presents some disadvantages:

• It involves a large number of incoming RNC routes and their corresponding antennas. This may cause space and strength problems for antenna support structures. Robust structures are generally more expensive.

• The high number of incoming routes may lead to problems in finding sufficient number of available channels.


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• Some sites may be situated too far from the RNC, thus increasing fading probabilities.

MSC

Figure 12: Star network, case A.

3.5.3 Star network � Case B Figure 13 illustrates another option of the star configuration. In this specific case, the connection is made in two stages. The farthest sites are connected first to a common node, which is connected to the RNC. The common node might be a hub or a PoC (Point of Concentration). The link from the common node to the RNC will generally have higher capacity than the individual RBS site connections. In order to handle longer distance, it may be necessary to assign a lower frequency band to the link between the common node and the RNC. Higher frequency bands are therefore reserved for the connection of the individual RBS sites.

The main drawback with star configurations is generally the vulnerability for hardware failure in the common node.

MSC

Figure 13: Star network, case B.


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3.5.4 Ring (loop) In Figure 14, all RBS sites are connected in a ring (loop). The main advantage of this configuration is to improve the availability of network. In the event of a failure in one link, the traffic can be sent in the other direction of the ring. If the ring has sufficient capacity to carry all the traffic from every site in both directions, then complete redundancy has been achieved. The capacity requirement is the total sum of the individual capacity requirements. Likewise chain configuration, drop insert or DCC/DXC helps to minimize capacity requirements.

MSC

Figure 14: RBS sites connected as ring (loop).

Unavailable time caused by hardware failure is reduced without the necessity of doubling the radio equipment. On the other hand, if network capacity is not increased, the ability to handle traffic decreases.

Figure 15 illustrates the access portion of a network. In this specific case, the links carry the same traffic capacity and the access network is found in the two lowest levels of the network (PDH 34 Mbit/s and STM1). The backbone network is found in the highest level (STM1/4).

Node B Node B

Node B

Node B

Node BNode B

RNC

RNC

RNC RNC

PDH 34 Mbit/s

STM1

STM1/4

Figure 15: Access network designed as a ring.


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3.5.5 Mesh The mesh configuration is a mixture of the previous described configurations and currently employed to improve the availability to the network, see Figure 16.

RNC

Figure 16: Mesh configuration.

This configuration is commonly a non cost-effective solution and therefore it is somewhat rare. Further, the traffic distribution presents more complexity in the physical layer. Other configurations normally exhibit equivalent reliability for less cost.

3.6 Evolution in a mobile access transport network Figure 17 shows a mobile transport network composed of 23 sites connected by 22 paths forming an access microwave network. With the advent of new technologies and strategies, the network will need to be upgraded in several phases to face new business opportunities. In what follows, different phases of the evolution of an access transport network will be illustrated.

As pointed in the previous sections, most mobile access networks for 2G-transmission are today built with PDH microwave radio, providing capacities from 2 to 34 Mbit/s. The networks are designed with combinations of star, tree, ring and cluster topologies.

The current expansion of 2G and 2.5 G mobile networks and the introduction of 3G mobile system set new requirements on the mobile access network in terms of transmission. The capacity growth and network expansion have clear impacts on the transport network.


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BSC/RNC

Figure 17: Mobile access transport network.

3.6.1 Capacity growth and new sites The number of subscribers has been geographically increased, and then forcing the increase of geographical coverage, see Figure 18. New sites and new links are therefore added to extend coverage, and the existing links are being upgraded to higher capacities. For instance, the capacity can be rapidly changed within the range of 4 to 36 Mbit/s. The capacity growth is indicated in Figure 18 with thicker lines and new paths with dashed lines.

BSC/RNC

Figure 18: The capacity growth (thicker lines) and new sites and paths (dashed lines).

3.6.2 Improving protection and extending coverage In a later phase of the network evolution, higher protection might be demanded along with high capacity. The direct connection from a node to the BSC/RNC is removed and a high capacity ring of SDH links is introduced into the network; see Figure 19.


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A possible option to cover several sites simultaneously and improve the traffic is the introduction of point-to-multipoint (PMP) microwave links.

BSC/RNC

Figure 19: SDH-rings improve the network protection and point-to-multipoint (PMP) microwave links can be a reasonable alternative to face coverage increase.

3.6.3 Integrating nodes and network manager A common strategy in dense access networks is to apply concentration (node) in the network. Concentration reduces transmission costs in the upstream direction and also is more effective when connected subscribes lines are poorly utilized.

As the network density is increasing, the number of complex node-sites will certainly increase. New aspects as the integration of microwave equipment and traffic routing, handling PDH and SDH technologies as well as increased ring protection, will force the introduction of more �intelligence� at the nodes, in Figure 20 indicated as shady rings.

In order to simplify the whole network, a network manager (in Figure 20 indicated as a PC-icon) is introduced. A single network manager brings several advantages as reducing time to operation, minimize maintenance cost and maximize the network availability.


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BSC/RNC

Figure 20: More intelligence at the nodes and a network manager are introduced when the transport network becomes more dense and complex.

3.7 Network hierarchy Network hierarchy is a natural consequence of network evolution. Subscribers are connected to their respective exchanges and to provide extended coverage the exchanges are connected. It is easy to understand that a large network that is built connecting hundreds or even thousands of local exchanges will rapidly become a clumsy, rigid, static and ineffective network. This was the approach in the early days of telephone networks. Modern transport networks, as transport networks supporting mobile phone systems, are built using another strategy based on hierarchy levels. New nodes are strategically introduced in the network while other nodes are rearranged with focus on functionality and purposes such that not all nodes are needed to be connected. The complicated mesh configuration is therefore substituted by a more dynamic star configuration.

The ITU-T defines six levels of hierarchy: International Center, Quaternary Center, Tertiary Center, Secondary Center, Primary Center and Local Exchange. Such a structure is, however, most applicable for very large countries.

Hierarchies easily adjustable to most of countries and operators normally exclude some of the levels recommended by the ITU-T. The number of levels varies from one operator to another, but it is normally kept to about four levels: international Exchange, National Transit Exchange, Regional Transit Exchange and Local Exchange.


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Few-levels networks (flat networks) are surely more attractive because of its relatively simple structure and management, although the complexity in more �intelligent� nodes can make the network more vulnerable if proper protection is not introduced.

Figure 21 illustrates possible hierarchical strategy in a SDH transport network.

National

Regional

Local

Access

DXC ADMSTM-16 (2.5 Gbit/s)

STM-4 (622 Mbit/s)

STM-4 (155 Mbit/s)

Core

Access

Figure 21: Hierarchical strategy for a SDH transport network.

4 Path and site survey Site and path surveys are often necessary in conjunction with the planning of proposed radio-links. This process may be subdivided into two activities. The first of which is the actual inspection of the path itself and the nature and positions of any obstacles along the path, the study of the reflective attributes of the path, the actual positions of obstacles that are expected to attenuate reflections, etc. The second relates to the inspection of the station site, including activities such as the physical inspection of the antenna masts, their height, their structural properties and their ability to carry the required antenna equipment, whether or not sufficient space has been allotted for the mounting of antennas, the availability of secure and sufficient electrical power, etc.

The checklist below includes a number of the essential points that should be investigated.

Find/verify:


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• Geographical position of the site.

• Antenna carrier height above ground level.

• Antenna carrier type, strength and torsional strength.

• Ground level above mean sea level.

• Possibility to mount antennas at necessary heights.

• Obstacles in path directions, height and width.

• Potential reflecting surfaces.

• Radio environment, other radio equipment in the vicinity or potential sources of signal interference.

• Distance between indoor and outdoor equipment.

• Floor/wall space for mounting indoor equipment. Power.

• Battery backup.

• Possibility of mounting antenna feeder or multi-cable between the indoor and outdoor equipment, considering space, wall entrance, bend radius, etc.

• New sites - proximity to roads and power transformer stations.

5 Microwave design Microwave design is necessary when a completely new or currently existing network is build. Some general aspects involved in the design of a microwave network are its purpose, technical requirements, flexibility, and future network expansion and implementation time. For upgrade or expansion of existing network a detailed analysis of the current network status is very important.

Microwave design is a process and as such it is composed of a start-up phase and conclusion phase. The process chart illustrated in Figure 22 is intentionally simplified. Attempts to make detailed process charts for general application normally fail because a complete microwave design involves too many uncertain and unpredictable input such as organizational structure, customer requirements, cost prediction, logistic aspects, environment effect, mixed technologies, network integration, etc.


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InitialDesign/planning

SiteSelection/survey

DetailedDesign/planning

Star

t-up

Conc

lusi

on

INPUT

Figure 22: Simplified microwave design process.

5.1 Initial planning Before starting the actual planning of a radio link path, one should acquire an overview of the construction of the entire network (of which the path in question is to be a part of), and of the network functionality that the proposed path is to provide. This background knowledge will enable decision as to the quality and availability standards that should be conformed to when dimensioning the path.

Network planning is generally based upon the network�s operational requirements. These can be expressed in terms of:

• Quality

• Availability/unavailability

• Traffic requirements and capacity

The manner in which one goes about determining the requirements pertaining to the dimensioning of individual radio-link paths is a function of the configuration and the dimensioning of the local network. In some cases surrounding networks may also be involved. Every network component path is to exhibit a level of availability and quality such that the entire connection, subscriber-to-subscriber, maintains the overall standards that were selected.

The International Telecommunication Union, ITU, publishes recommendations that provide guidance as to the dimensioning of networks intended for international connection. Practical examples of this type of network is found in the transmission to/from the radio base stations of a mobile telephone network or in the internal company networks that are connected to public communications networks.


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The Prediction Cycle of the multi-task process of planning a microwave network is described in Chapter 2. As it was described therein, the whole process is highly iterative and may pass through many re-design phases before final �convergence� is attained. Four main blocks in the Prediction Cycle are outlined: Loss/Attenuation (Link Budget), Fading, Frequency Planning and finally Quality and Availability.

In what follows, each block will be summarized and some general planning advices are furnished.

5.2 Loss/attenuation block

5.2.1 Link budget A link budget is established to enable calculations involving signal reception under fade-free conditions. The budget contains a summation of all losses and amplifications of the signal as it propagates from the transmitter to the receiver. This is illustrated in Figure 23 where the power received by the radio-link terminal and the path fade margin is shown.

The main purpose of the link-budget is to determine the fade margin of each path and �delivery� its value to the Fading Block and Quality and Availability Block.

AntennaSplitter

Transmitter /Receiver


Wave guide

Wave guide

WaveguideAntenna Splitter



Wave guide

Wave guide

Waveguide

OutputPower

Branching Losses Antenn

a Gain

Propagation Losses

Antenn

a Gain

Branching Losses

Receiver Thresh.Value

Received Power

UnfadedFade

Margin

Tran

smitt

ed P

ower

Figure 23: Losses and gains along a microwave path.

Advices

1) If power splitters are employed in redundant systems, an additional loss should be introduced in the link budget.


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5.2.2 Fade Margin The incoming signal that is calculated with the help of the link budget applies to fade-free time. Actual incoming signals to the radio-link receiver vary over time due to fading. To allow for a sufficient power range in connection with incoming signal variations, paths are dimensioned so that a given margin is attained between fade-free incoming signal levels and the receiver threshold value. This is referred to as the fade margin. The fade margin is to be sufficiently large so that the probability of it being exceeded due to fading is sufficiently small in order to meet with the functional demands that are placed on the path. The requirements placed on fade-margin size are indirectly set as a result of the norm used when dimensioning the path.

Advices

1) The fade margin is calculated with respect to the receiver threshold level for a given bit-error ratio (BER). The threshold level for BER=10-

6 for MINI-LINK equipment is about 4 dB higher than the threshold level for BER= 10-3. Consequently, the fade margin is 4 dB larger for BER=10-3 than for BER=10-3. For other equipment than MINI-LINK, it is generally common to have one dB of threshold level for each decade of BER.

2) Fade margins lying in the range 25 to 40 dB are most common

3) The fade margin is NOT an input parameter for the design of a microwave network.

5.2.3 Losses and attenuations Two kinds of loss/attenuation are ALWAYS present and predictable:

• Free-space, Chapter 4, section 7.1

• Gas, Chapter 4, section 7.2

Three kinds of loss/attenuation are predictable IF present:

• Reflection, Chapter 4, section 7.3

• Rain, Chapter 4, section 7.4.

• Obstacle, Chapter 4, section 7.5

5.2.3.1 Free-space attenuation Advices


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1) If the distance is doubled while maintaining constant frequency, the free-space loss is increased by 20·log 2= 6 dB. The same applies to a doubling of the frequency while maintaining a constant distance. In other words, an additional attenuation of 6 dB will be caused for every doubling of either the distance or the frequency

2) Comparing to other kinds of loss/attenuation, free-space loss gives the major contribution. Expressed in the GHz range, the free-space loss has a minimum of approximately 92 dB. If it is expressed in the MHz range the minimum is 92 dB � 60 dB = 32 dB (1 GHz = 1000 MHz → 20·log 1000 = 60 dB).

3) This relatively small increase of free-space attenuation by only 6 dB with increased distance might give the impression that long paths can easily be obtained by simply increasing the transmitter output power, or the receiver sensitivity or the antenna gain. This is not so easy to accomplish because the total path attenuation is also determined by other negative contributions, for example gas attenuation.

4) Cell-planners commonly refer to half-wave dipole antenna gains. Comparing to the above presentation for which the gain of an �ideal� isotropic antenna is 1 (0 dBi), the gain of a half-wave dipole antenna is 1.64 (2.15 dBi). Considering both stations of a radio link, the difference between free-space loss comparison using isotropic and half-wave dipole antennas is about 4.30 dB.

5.2.3.2 Gas attenuation Advices

1) If gas absorption is calculated as a function of relative humidity and temperature, be aware both parameters are reciprocally consistent.

2) If local values of temperature are available, select the average summer temperature.

3) For tropic climate nearby large bodies of water, annual temperature charts can be employed for the selection of temperature.

4) Frequency bands located close to the strong features of water vapor (23 GHz) and oxygen (50-60 GHz) are strongly affected by attenuation. Depending on the values of temperature and humidity, the specific attenuation can be as much as 1 or 2 dB/km. This gives a large negative contribution to the fade margin, then making the quality and availability objectives harder to be accomplished. On the other hand, such high values of specific attenuation are also of benefit because it provides valuable shielding to co-channel interference.


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5) Considering the �atmospheric isolation� described above as a positive contribution, the use of high frequency systems will improve the efficiency of spectrum utilization through the enhanced opportunity for multiple frequency re-use for short-distance communication systems operating within the same part of the frequency band.

5.2.3.3 Reflection loss Advices

1) The grazing angle (φ) of radio-relay paths is normally very small, currently lower than 1 degree.

2) It is strongly recommended to avoid ground reflection. This can be achieved by �shielding� the path against the indirect ray; see Figure 24. If possible do not select paths that cross-large bodies of water. If, however, one is forced to select paths that are likely to cause reflections, one should attempt to select antenna heights and a wave propagation path such that reflected waves are, as far as is possible, attenuated by obstructions that are situated along the path of reflection. The risk of interference is as a result considerably reduced through planned attenuation.

A B

φ

Figure 24: Obstacle attenuation in a reflected wave.

3) Vertical polarization gives less loss. For large grazing angles the difference between vertical and horizontal polarization is substantial.

4) Changing the antenna heights can move the location of the reflection point. This approach, usually known as the �Hi-Lo technique�, force the reflection point to move closer to the lowest antenna by affecting the height of the higher antenna. The grazing angle increases and the path becomes less sensitive to k-value variations.


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5) Space diversity normally provides good protection against reflection. It is currently applied for paths over open water surfaces. The distance between the primary and the diversity antennas should be adapted to the path�s geometry so that one always achieves the best possible signal in one of the antennas.

6) In addition to the reflection problems caused when radio waves propagate across large bodies of water, these regions also cause other transmission difficulties due to the propagation-impairing atmospheric conditions that often prevail in these areas. These factors sufficiently motivate the use of space-diversity when constructing such paths, even in the event that one feels fairly certain that the effects of reflections have been reduced through planned obstacle attenuation.

7) The contribution due to �reflection loss� is NOT automatically included in the link budget, but in the case reflection cannot be avoided the fade margin may be adjusted by including this contribution as �additional loss� in the link budget.

5.2.3.4 Rain attenuation Advices

1) The contribution due to rain attenuation is NOT included in the link budget. It is only used in the calculation of rain fading.

2) Horizontal polarization gives more rain attenuation.

3) Rain attenuation increases exponentially with rain intensity (mm/h).

4) Rain attenuation increases with frequency. From about 10 GHz, rain attenuation might be dimensioning for long path lengths. For higher frequencies, say above 18 GHz, rain attenuation is directly dimensioning, irrespectively the path length.

5) The more intensive rain, the shorter it will last and the more limited the rain cells will be.

6) Only short-time integration or instantaneous rain intensity data gives reasonable rain attenuation values

7) �Meteorological� rain intensity data are normally based on long-integration rain measurements and therefore not applicable.

5.2.3.5 Obstacle attenuation Advices


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1) If knife-edge approximation is considered, the values given in Figure 25 are reasonable good approximations.

0 dB 0 dB 6 dB 16 dB 20 dB

Figure 25: Rough values for obstacle loss if knife-edge approximation is considered.

2) If 60% of the first Fresnel zone is free from obstacle then the obstacle loss is 0 dB.

3) If the LOS is about to strike the obstacle, then the obstacle loss is 6 dB.

4) If smooth earth approximation is considered, the values given in Figure 26 are reasonable good approximations.

10 dB

20 dB

40 dB

Figure 26: Rough values for obstacle loss if smooth earth approximation is considered.

5) Different k-values result in different obstruction loss values. Small k-values give more attenuation while large k-values give less attenuation.

5.3 Fading Block Three fading mechanisms are considered:

• Rain, Chapter 4, section 8.6

• Multipath - Flat, Chapter 4, section 8.8 and 8.7

• Multipath � Frequency Selective, Chapter 4, section 8.11

• Refraction-diffraction, Chapter 4, section 8.12

Advices


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1) Climate, terrain and path length are factors that affect the degree to which a radio-link path is sensitive to fading.

5.3.1 Rain Advices

1) Rain fading starts increasing noticeable at about 10 GHz. For frequencies above 15 GHz, rain fading is normally the dominating fading mechanism and consequently the dimensioning factor.

2) Other forms of precipitation (snow, hail, fog and haze) do not affect radio-relay links as much as rain events and can therefore be considered as negligible.

3) Snow covering antennas and radomes, so-called ice coating, can result in two problems: increased attenuation and deformation of the antenna�s diagram.

5.3.2 Multipath - flat Advices

1) The method used for �unknown terrain� is commonly employed for fading estimations during initial planning and/or tender activities.

2) Multipath fading (flat or frequency selective) is the dominating fading mechanism for frequency lower than approximately 10 GHz.

3) Increasing path inclination reduces the effects of flat fading.

4) Reducing path clearance will reduce the effect of flat fading because risk for multipath propagation is decreased. However, this technique may increase the risk for refraction-diffraction fading.

5) Multipath flat fading is more likely on path across flat ground than on paths over rough terrain.

6) Multipath flat fading is normally more active over bodies of water (lakes, sea, etc) than over land.

7) Multipath flat fading is normally most active during early and later summer (late spring and early autumn).

8) Calm weather favors atmospheric stratification and that gives multipath fading

5.3.3 Multipath � frequency selective


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Advices

1) A rule of thumb is that multipath fading, for radio links having bandwidths less than 40 MHz and path lengths less than approximately 30 km, is described as being flat instead of frequency selective.

2) Increasing the output power in order to reduce the outage time for selective fading doesn�t give any improvement. It only increases the flat fading or reduces the thermal noise power received without having any influence on the effects (amplitude and group delay distortions across the channel) of selective fading.

3) The advices 5, 6, 7, and 8 are also applicable for multipath frequency selective fading.

5.3.4 Refraction-diffraction Advices

1) Diffraction-refraction fading is a path clearance procedure in which antenna heights can by optimized for local climatic conditions.

2) The advices 5, 6, 7, and 8 are also applicable for refraction-diffraction fading.

5.4 Frequency Planning Block

5.4.1 Introduction The objective of frequency planning is to assign frequencies to a network using as few frequencies as possible and in such a manner, that the quality and availability of the radio-link path is minimally affected by interference.

Frequency planning is often based on the �3 dB threshold degradation approach�. In this method, threshold degradation at the receiver, higher than 3 dB, is not accepted. In practice, this requires that the fade margin is 3 dB higher than the demands made due to wave propagation and hardware. This method may lead to over dimensioning or under dimensioning of the network. Frequency planning shall therefore rely on the concept of quality and availability objectives.

5.4.2 Assignment of radio-relay frequencies The following aspects are the basic considerations involved in the assignment of radio frequencies:


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• Prevention of mutual interference such as the interference between the radio frequency channels in the actual path, interference to/from other radio paths, interference to/from satellite communication system, etc.

• Aim at frequency economy of the available radio frequency spectrum.

• Proper and reasonable selection of frequency band that conforms to the required transmission capacity.

• Frequency band suitable to both path characteristics (path length, site location, terrain topography) and atmospheric effects.

5.5 Chanel arrangements The ITU-R recommends that the preferred radio-frequency channel arrangements should be developed following the homogeneous patterns alternated, co-channel band re-use, and interleaved band re-use. This is discussed in Chapter 8, section 5.4.

5.5.1 Far interference in frequency planning Figure 27 illustrates a typical far interference scenario with two �real� paths (AB and CD) and two �virtual� paths (AD and BC). The �virtual� paths are the �interfering� paths.

Tx= f1Rx = f2

Tx= f2Rx = f1

Tx= f1Rx = f2 Tx= f2

Rx = f1

Real path

Real path

Virtual paths

Virtual paths

A

B

C

D

Figure 27: Far interference scenario with two radio-link paths and four interfering paths.

Far interference is often the primary factor that limits the number of paths that can be set up within a given geographical area. Planning a network that is �free� from the effects of far interference requires the following issues:


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• Knowledge of the geographic locations of the sites, the layout, and dimensioning of the radio-link paths.

• Equipment data

• Existing network frequency assignment

• Reasonable radio-wave propagation models

In addition, far interference also affects the possibility of realizing a variety of network solutions, for example, the number of possible paths within a node located in a star network.

5.5.2 Near interference in frequency planning Near interference refers to receiver disturbances that are generated by transmitters that are grouped at the same site, i.e. co-located stations. These disturbances may be caused both by in-house and neighbor equipment, either individually or as a result of their interaction.

By allocating the same duplex band (lower/upper) to all the transmitters on the same site, all receivers on the site will automatically operate in the other part of the duplex bands (upper/lower) and near interference can in many cases be neglected.

Disturbances may appear in the form of intermodulation effects, i.e., the mixture of two or more transmitter frequencies that may arise close to a particular receiver frequency; see Figure 28.

f2f1

frx ftx

3f2-2f12f2-f1

Figure 28: Near interference due to intermodulation.

Disturbances arising from intermodulation effects in cases where waveguide-bound frequency bands are used (i.e., ≥ 6 GHz) are negligible. Of course wave-guide-bound systems may contribute to intermodulation effects in systems working at lower frequency bands and therefore using coaxial cables as antenna feeders. It should be noted that this effect is due to the fact waveguides are more frequency discriminating (hi-pass filter characteristic) then antenna feeder cables


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Degraded performance can also be the result of transmitter frequencies that are located too close to a receiver�s frequency thereby directly degrading reception quality.

An additional cause of degraded reception quality is blocking. Blocking can arise even if the disturbing transmitter frequency lies well separated from the receiver frequency if the emitted field-strength by the transmitter is sufficiently strong. Examples of situations in which blocking can arise is when the radio-links are co-located with high-power transmitters such as radar stations and radio broadcasting stations.

Another important characteristic, that should be considered when calculating the effect of near interference, is the coupling loss between two antennas located at the same site; see Figure 29.

P1

A = Coupling loss

P2

A

P P A2 1= −

Figure 29: Coupling loss between two antennas.

The following isolation values may be used when performing rough estimations (the values are, in fact, dependent on the distance and angles between the two antennas):

• Approximately 40 dB between two antennas made up of dipoles

• Approximately 80 dB between two parabolic antennas.

The selection of proper duplex-bands for transmitter and receiver equipment, during the frequency-allocation process, is essential if one is to control the risk of disturbances that arise as a result of insufficient transmitter-receiver frequency separation. Allocating all the transmitters to one of the duple-bands and all the receivers to the other always attains maximum site frequency economy for a specific radio-frequency channel arrangement. This often results in also satisfying the requirement of maintaining the necessary frequency separation between transmitter and receiver frequencies.


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Making use of one of the ITU�s standardized radio-frequency channel arrangement generally facilitates the planning process. These standards are internationally and widely accepted by numerous government frequency regulating bodies.

5.5.3 How may interference be avoided? The following conditions shall be met:

• Sufficiently weak interference signals.

• No frequency overlap (receiver frequencies are sufficiently separated from interfering signals).

The first condition may be very difficult to meet, often as the result of frequent occurrence of co-located radio systems (occasionally forced co-location) � while the second condition may be attained but requires careful frequency planning.

5.5.4 Interference tolerance for digital channels The tolerance of digital channels to interference depends on the modulation scheme. In particular, modulation scheme that requires low C/I for a certain bit-error ratio is more tolerant to interference. Robust modulation schemes are 2PSK and 4PSK, while more complex modulation schemes, as 128QAM require much larger C/I-values.

5.5.5 Simplified step-by-step procedure The most important target of frequency planning is to allocate available channels to the different links in the network without exceeding the quality and availability objectives of the individual links due to radio interference. Frequency planning of a few paths can be carried out manually. For many paths, portions of a network, or the entire network, it is highly recommended to employ a transmission tool.

The following simplified step-by-step procedure is recommended:

STEP 1: Outline the overall structure of the network by pointing out the location of RBS sites, RNCs, Hubs, PoCs and MGWs.

STEP 2: Allocate the appropriate quality and availability objectives for every portion of the network (no frequencies are involved in this step). Perform quality and availability calculations and ensure that the objectives are reasonably accomplished for every portion of the network (no over/under dimensioning!).


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STEP 3: Estimate the traffic requirements and capacity. The highest concentration of the links is generally around the RNCs, where the links having the highest capacity are located. It is a good practice to start frequency planning with highest capacity links in the most concentrated node. This will normally decide the number of frequencies needed in the network. Other links should normally be enabled to reuse the same frequencies. In some cases, it may be necessary to use channels from more than one frequency band due to the limited number of available channels in the first selected band.

STEP 4: Start assigning a duplex half (lower/upper) for the transmitter in the sites of the network. Generally, two alternatives are possible:

A) In a chain of sites there will be alternating lower/upper sites, that is, the transmitter in site 1 is L (lower), site 2 is U (upper), site 3 is L, and so on.

B) In a ring with odd number of sites, the transmitter of the first site will be assigned the same duplex half as the receiver of the last site (which is the first site in a closed ring). This would probably cause serious interference. For instance, assigning L to the transmitter in the first site of a ring composed of five sites would give the following result: site 1 (L), site 2 (H), site 3 (L), site 4(H) and site 5 (L).

Site 1 will have one more transmitter sending in H, which is exactly the same, duplex half used by the receiver in the connection between site 1 and site 2. This problem is best resolved by using channels from another suitable band.

Transmitters using upper and lower duplex halves are sometimes forced to mix on the same site. In this case, near interference has to be evaluated. The main parameters in the evaluation of near interference are the isolation (antenna separation) between transmitters and receivers on the same site, the frequency separation and the receiver�s capability of blocking (filtering) unwanted signals.

STEP 5: Consider antenna discrimination aspects in the early stages of frequency planning. For instance, in a common site (e.g., a node or hub) the links having sufficient separation angle may use the same channels. In addition to angle separation, distance separation (coupling loss) between two antennas may also give a certain degree of discrimination.

STEP 6: Re-use frequencies and polarization as often as possible. That is, if one channel is assigned to a node, always try to assign the same channel for the other paths on the same node. Note that the required number of frequencies may depend on the order of allocation. Therefore, try other combinations to minimize the required number of frequencies.


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STEP 7: Proceed with the next node. It should normally be possible to plan this with the same frequencies as used in the former node. If necessary, change frequency for a link that is aligned to a link using the same frequency and connected to the former node.

STEP 8: Perform a new quality and availability calculation (after the frequency allocation) and identify the links that do not meet the quality and availability objectives. Far interference calculation is carried out and the receivers having relatively high threshold degradation values are probably a part of the links not meeting the quality and availability objectives. Make the appropriate changes (polarization, channel, frequency band, antenna size, etc) and ensure that a new interference calculation gives lower threshold degradation values. There might be some situations in which higher output power of a transmitter may improve the quality and availability figures without considerable interference contributions to the network. These favorable situations are however not so common.

STEP 9: Repeat step 8 until the quality and availability objectives of all portions of the network are accomplished.

STEP 10: The total analysis is finished and the network will have the final parameter values (channel, polarization, antenna size, frequency band, etc) as given by the last iteration circle.

Advices

1) Frequency planning is not carried out with the purpose of avoiding interference, rather to accomplish the quality and availability objectives of each portion of the network!

2) Reuse frequencies, i.e., repeat frequencies as often as is possible! Good frequency economy is always encouraged!

3) Use antennas having high front-to-back ratios and large side-lobe suppression. These result in both good frequency economy and, in the final analysis, good overall network-economy. High performance antennas may be a suitable alternative.

4) Do not use higher radio-link output power than necessary! Start always frequency planning with the lowest available output power.

5) If the choice is between higher transmitter output power and larger antennas, choose (if possible) a larger antenna. Power will be concentrated along the intended path, i.e., towards the receiver and a certain degree of interference may be avoided.


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6) Changing polarization is very effective measure against interference for paths having sharp angles (see the antenna diagram) while very ineffective for paths forming wide angles.

7) When assigning specific channels to the individual links in a network it is strongly recommended to start with high capacity links (the links demanding wider bandwidth).

8) At radio-relay frequencies, the antenna discrimination increases rapidly with the angle separation and is an extremely efficient factor in suppressing interference. Thus, if the two links are not closely aligned to a common line and with the (upper or lower) transmitters transmitting in the same direction, it is normally possible to reuse frequencies between two such links.

9) Generally, interference problems in radio-relay networks are greatest for links connected to a common node. For other links, interference can normally be avoided.

10) Interfering signals are not always in line of sight! Low k-values decrease the line of sight (demand higher antenna heights) but offer better protection against interference from other stations.

11) Never mix transmitters (or receivers) using upper and lower duplex halves on the same site. All transmitters (or receivers) on one site should have the same duplex half!

12) It is not economically feasible to achieve totally interference-free networks.

13) The threshold degradation (given in dB) is a reasonable good indicator of the amount of interference from all transmitters at one particular receiver. It is, however, not recommended as a criterion for deciding on quality aspects.

14) The main purpose of an interference analysis is to obtain the effective fade margin that is used in the re-calculation of the quality and availability parameters.

15) A rule of thumb is that the requirement on signal-to-interference ratio in the case of co-channel interference should be C/I ≅ 20 dB.

16) A rule of thumb gives that C/I = -15 dB is sufficient to avoid adjacent-channel interference. This value is, however, equipment dependent and varies as a function of frequency separation between the disturbing and the disturbed signal. The bandwidth of the interfering signal in relation to the bandwidth of the disturbed receiver also affects the demand placed on the C/I ratio.


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17) Far interference is often the primary factor that limits the number of paths that can be set up within a given geographical area.

5.5.6 Additional scenarios in frequency planning Interference aspects may severely limit the number of links in a network if appropriate caution is not taken into account in the earlier stages of frequency planning. In what follows, some general aspects based on former sections are illustrated.

5.5.6.1 Chain/cascade configuration Despite the differences in traffic capacity in chain and cascade configuration, the frequency planning is similar in both configurations.

Since paths of a chain have very sharp angles, using the same channels by changing polarization (H/V) may be a good alternative to alternate two channels in the chain. Figure 30 shows channel 1 used alternately with horizontal (H) and vertical (V) polarization. Upper (U) and lower (L) duplex halves for the transmitters are illustrated in each site.

f1/V f1/H f1/VToward RNCU L U L

Figure 30: Alternating polarization with the same channel along the chain.

5.5.6.2 Tree configuration For sharp angle, the polarization discrimination ensures the possibility of using the same channel with different polarization (H/V). Both transmitters on the common node of Figure 31 have the same duplex half (U).

Toward RNCf1/V

f1/H

U

L

L

Figure 31: Alternating polarization with the same channel for sharp angle separation in a tree configuration.


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5.5.6.3 Ring configuration The same channel with the same polarization is employed in the perpendicular paths but with different polarization in the parallel paths. The transmitters are alternately labeled Upper (U) and lower (L) duplex halves; see Figure 32.

U L

UL

Toward RNC

f1/V

f1/V

f1/H

f1/H

Figure 32: Same polarization for perpendicular paths and different polarization for parallel paths, although the same channel for the entire ring.

If the ring is composed of odd number of sites there would be a conflict of duplex halves and the changing the frequency band would be a reliable alternative.

5.5.6.4 Star configuration As pointed out earlier, all transmitters on the common node should have the same duplex half (L); see Figure 33. The configuration displays a difficult frequency-planning scenario and is very sensible to the geometry (mutual angles). If the node is a concentration point for high capacity links wide bandwidth is required, thus making the allocation of smaller channels in other portions of the network quite complicated. The link caring the traffic toward the RNC is supposed to use another frequency band than the one employed inside the cluster.

UU

L

U

U

UU

Toward RNCf1/H f2/V

f1/V

f2/V

f1/Vf2/H

Figure 33: The transmitters on the common node of a star configuration have the same duplex half. Channel allocation is very dependent on the geometry.


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5.5.6.5 Mesh configuration Meshes, see Figure 34, present a complicated frequency-planning scenario due to several conflicts of duplex halves. In addition, it normally requires more channels than other configurations.

Toward RNC

U

Figure 34: The frequency planning of meshes is considerably difficult.

5.6 Quality and availability block The quality and availability block is described in Chapter 6.

The main purpose of the quality and availability block is to set up reasonable quality and availability objectives for the radio-relay path. The entire procedure can be structured in five general steps; see Figure 35.

Selection of a transmission networkmodel (the reference path!)

Calculation of quality andavailability parameters

1 2

34

Comparison between allocated andcalculated objectives

Allocation of quality andavailability objectives

If the objectives are not accomplishedchange network parameters and performa new quality and availability calculation

5

Figure 35: Procedures involved in the quality and availability block.


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Appropriate network model is selected in step 1 along with the selection of quality and availability objectives for the corresponding portions and sections of the network model given in step 2. In step 3, the quality and availability parameters are calculated and compared to the objectives in step 4. If the objectives are not met (step 5), appropriate network parameters (antenna size, antenna height, output power, channel arrangements, polarization, etc) are changed and the quality and availability parameters are re-calculation as illustrated in step 3. The procedure is continued in step 4 as an iterative process.

Advices

1) Unavailability is more suitable to radio-link planning than availability. The main reason for employing unavailability is that unavailability contributions can be added together when calculating the total unavailability of a system.

2) Since parts of a network can be extended in the future, a �spare fraction� (used for design according to Rec. ITU-T G.826 and G.828) should be allowed along with the allocation of the objectives presented in the objective tables. The appropriate value for the �spare fraction� has to be considered after careful analysis of the network. The value 0% means that the objectives presented in the objective tables are fully allocated and nothing is left for future expansion of the network.

3) The quality and unavailability objectives for the all portions in the HRX and HRP are to be accomplished concurrently.

4) The objectives shall account for effects caused by fading, interference, and other sources of performance degradation.

5) Block allocation is to be used in the access and short haul portions, while in the long haul portion it is a combination of block allocation and length-related allocation.

6) Quality objectives for SDH paths using equipment designed before March 2000 shall follow the ITU-R objectives based on Rec. ITU-T G.826.

7) The objectives are applicable to each direction of transmission independently

8) Rain fading and diffraction-refraction fading (k-type fading) give unavailability while multipath (flat and frequency selective) fading gives ESR, SESR and BBER.


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9) Unavailability is the dominating and dimensioning factor for frequencies higher than 15 GHz while quality is the dominating and dimensioning factor for frequencies lower than 15 GHz. There is, however, a frequency range between 10 and 15 GHz where both quality and availability might be comparable; see Figure 36.

Sign

ifica

nce

of fa

ding

mec

hani

sms

10 20Frequency (GHz)

Availability (rain andrefraction-diffraction fading)

Quality (flat and frequencyselective fading)

Figure 36: The dimensioning pattern showing the relative importance of fading mechanisms as a function of frequency.

10) Distance-based allocation in the access and short haul is not supported by any ITU-R recommendation based on Rec. ITU-T G.821, G.826, G.827, and G.828 (portions of the network model are not length-defined).

11) Distance-based allocation distributes the objectives equally along a radio link (chain of paths). This is not always effective since �difficult paths� normally require more objective apportionment than �easy paths�.

12) Unavailability due to hardware failure is not length-related while unavailability due to radiowave propagation (rain and refraction-diffraction fading) can be strongly length dependent.

13) If a portion of a network (for instance the national portion) is made length-dependent for allocation purposes, incomplete sections may leave unused objectives.

14) Allocation of quality and availability per path can be a reasonable and rapid approach when large networks are considered.

15) Networks that are better than good are a waste of money!


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6 Improving microwave paths

6.1 Hardware failure Equipment failure normally results in longer interruptions (unavailability). Hardware redundancy can in many cases be the only option of improving the total unavailability figure of a radio-link system.

The simplest form of hardware redundancy is that of a standby system operating in parallel with the active system. This standby system takes over whenever the on-line (active) system fails. Thus, a failure on the on-line system constitutes a total system failure only if the standby system is unavailable. A switching device connects the standby and the active system.

Monitored hot standby is preferred and means that the functions of the standby component should work properly and optimally whenever required. Monitored implies electronic control/supervision.

The most important parameters in the calculation of hardware failure (unavailability) are the Mean Time Between Failure (MTBF) and the Mean Time To Repair (MTTR).

Advices

1) Hardware redundancy does not affect the unavailability due to propagation effects. It only improves the unavailability due to hardware failure.

2) In the definition of MTTR, it is included the time necessary to detect, report, locate and repair the failure. Then, considering that the MTTR of non-redundant systems can be somewhat large in some applications, redundancy is recommended if high availability (low unavailability) is required and short MTTR cannot be accomplished.

3) Hardware redundancy demands more equipment and consequently it is more expensive than non-redundancy.

6.2 Diversity Diversity is a well-established method to overcome the effects of multipath fading. The most common forms of diversity in LOS links are space and frequency diversity. Frequency and space diversity can also be combined giving better improvement than sole frequency or space diversity. Other diversity forms (for instance, polarization, angle diversity, and �hybrid� diversity) are known, although not commonly employed.


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6.2.1 Space diversity Space diversity is the simultaneous transmission of the same signal over a radio channel by using two or more antennas for reception and/or transmission. The antennas are therefore physically separated in a tower or mast and the distance between the antennas (antenna separation) at the receiver or transmitter is such that the individual signals are assumed to be uncorrelated.

Space diversity is the most used diversity option against multipath fading and it does not have any impact on rain fading.

6.2.2 Frequency diversity Frequency diversity is the simultaneous transmission of the same signal over two or more radio frequency channels located at the same frequency band. Another form of frequency diversity is cross-band diversity in which the RF bearers are situated in different frequency bands.

One great drawback of frequency diversity is the inefficient use of the available frequency spectrum.

6.2.3 Improvement factor Improvements that are achieved as the result of diversity are expressed as a factor, referred to as the improvement factor, which affects the calculated probability of multipath fading, see Figure 37. The improvement varies for different fade depths. It is greatest for deep fading where improvements of up to 100 times can be achieved. The improvement factor is calculated using a number of algorithms, depending on the selected diversity method. The factor is affected, for the most part, by antenna separation in the case of space diversity and by frequency difference in the case of frequency diversity.

Probability of exceeding the fading depth, %

Fadi

ng d

epth

, dB

Without diversity

With diversity

improvement

gain

A

B

C

10-2 10-3 10-4 10-5 10-6 10-7

-10

-20

-30

-40

-50

0

Figure 37: Diversity improvement.


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6.3 Other techniques

6.3.1 Passive repeaters Passive repeaters are used to change the direction of the radio-relay signal in order to overcome obstacles in an otherwise direct line of sight between two radio-relay stations. They are also employed when the cost with an �active repeater� is comparatively too high. The use of passive repeater is not only less expensive than an active repeater, but the operation cost is also substantially reduced as neither power nor access roads are required.

Two types of passive repeaters are used:

• Plane reflectors - reflect the radio-relay signal in the same way as mirrors reflect light.

• Back-to-back antennas - work like ordinary active repeaters, but without generating radio frequency signals.

6.3.2 One-plane reflector It consists in a flat reflexive surface that changes the ray direction avoiding by this way the obstacle. The performance of this set-up is given by the reflector surface (height and width) and the angle α, formed between the incident and reflected ray. Also the reflection coefficient has to be taken into account. This solution is used for fixed service; see Figure 38.

Reflector

Site A Site B

Path A Path B

Obstacle

Figure 38: One-plane reflector.

6.3.3 Two-plane reflectors It is a variation of one-plane reflector solution by changing the direction of the radio-relay signal two times; see Figure 39. Its application is limited due to costs and difficulty of aligning the radio-link.


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Reflector 1

Site A

Path A

Path B

Site B

Path C

Reflector 2

Figure 39: Two-plane reflector.

6.3.4 Back-to-back antennas It consists of two antennas connected by a wave-guide; see Figure 40. It is the most useful passive repeater due to the cost and the availability of its components. The total path length, the two additional antenna gains and the loss introduced by the wave-guide are included in the link budget. Back-to-back antennas introduce considerable losses.

waveguide

Site A Site B

Path

A

Path B

Back-to-BackAntennas

Figure 40: Back-to-Back antennas.

Advices

1) The optimal location of a passive repeater is as close as possible to the transmitter or receiver. Never half way between the transmitter and the receiver!

2) Back-to-back antennas are primarily used in cities where line-of-sight transmission is not possible due to buildings and other obstructions.

3) Plane reflectors are normally employed in rural areas, often on mountain peaks.