TCIL 17 Microwave Link Design

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Micrwave Link Design 1 MICROWAVE LINK DESIGN 04th September 2009

What is Microwave Communication : 

Micrwave Link Design 2 What is Microwave Communication A communication system that utilizes the radio frequency band spanning 2 to 60 GHz. As per IEEE, electromagnetic waves between 30 and 300 GHz are called millimeter waves (MMW) instead of microwaves as their wavelengths are about 1 to 10mm.

What is Microwave Communication : 

Micrwave Link Design 3 What is Microwave Communication Small capacity systems generally employ the frequencies less than 3 GHz while medium and large capacity systems utilize frequencies ranging from 3 to 15 GHz. Frequencies > 15 GHz are essentially used for short-haul transmission.

Advantages of Microwave Radio : 

Micrwave Link Design 4 Advantages of Microwave Radio Less affected by natural calamities Less prone to accidental damage Links across mountains and rivers are more economically feasible Single point installation and maintenance Single point security They are quickly deployed

Line-of-Sight Considerations : 

Micrwave Link Design 5 Line-of-Sight Considerations Microwave radio communication requires a clear line-of-sight (LOS) condition Under normal atmospheric conditions, the radio horizon is around 30 percent beyond the optical horizon Radio LOS takes into account the concept of Fresnel ellipsoids and their clearance criteria The Fresnel Zone must be clear of all obstructions.

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Micrwave Link Design 6 Radius of the first Fresnel zone R=17.32(x(d-x)/fd)1/2 where d = distance between antennas (in Km) R= first Fresnel zone radius in meters f= frequency in GHz x y d=x+y R

Line-of-Sight Considerations : 

Micrwave Link Design 7 Line-of-Sight Considerations Typically the first Fresnel zone (N=1) is used to determine obstruction loss The direct path between the transmitter and the receiver needs a clearance above ground of at least 60% of the radius of the first Fresnel zone to achieve free space propagation conditions Earth-radius factor k compensates the refraction in the atmosphere Clearance is described as any criterion to ensure sufficient antenna heights so that, in the worst case of refraction (for which k is minimum) the receiver antenna is not placed in the diffraction region

Effective Earth’s Radius = k * True Earth’s Radius True Earth’s radius= 6371 Km k=4/3=1.33, standard atmosphere with normally refracted path (this value should be used whenever local value is not provided) : 

Micrwave Link Design 8 Effective Earth’s Radius = k * True Earth’s Radius True Earth’s radius= 6371 Km k=4/3=1.33, standard atmosphere with normally refracted path (this value should be used whenever local value is not provided) Variations of the ray curvature as a function of k K= True Earth’s curvature = 6,371 Km K=1 K=0.5 K=0.33

Line-of-Sight Considerations : 

Micrwave Link Design 9 Line-of-Sight Considerations Clearance criteria to be satisfied under normal propagation conditions - Clearance of 60% or greater at the minimum k suggested for the certain path - Clearance of 100% or greater at k=4/3 - In case of space diversity, the antenna can have a 60% clearance at k=4/3 plus allowance for tree growth, buildings (usually 3 meter)

Microwave Link Design : 

Micrwave Link Design 10 Microwave Link Design Microwave Link Design is a methodical, systematic and sometimes lengthy process that includes Loss/attenuation Calculations Fading and fade margins calculations Frequency planning and interference calculations Quality and availability calculations

Microwave Link Design Process : 

Micrwave Link Design 11 Microwave Link Design Process The whole process is iterative and may go through many redesign phases before the required quality and availability are achieved Frequency Planning Link Budget Quality and Availability Calculations Fading Predictions Interference analysis Propagation losses Branching losses Other Losses Rain attenuation Diffraction- refraction losses Multipath propagation

Loss / Attenuation Calculations : 

Micrwave Link Design 12 Loss / Attenuation Calculations The loss/attenuation calculations are composed of three main contributions Propagation losses (Due to Earth’s atmosphere and terrain) Branching losses (comes from the hardware used to deliver the transmitter/receiver output to/from the antenna)

Loss / Attenuation Calculations : 

Micrwave Link Design 13 Loss / Attenuation Calculations Miscellaneous (other) losses (unpredictable and sporadic in character like fog, moving objects crossing the path, poor equipment installation and less than perfect antenna alignment etc) This contribution is not calculated but is considered in the planning process as an additional loss

Propagation Losses : 

Micrwave Link Design 14 Propagation Losses Free-space loss - when the transmitter and receiver have a clear, unobstructed line-of-sight Lfsl=92.45+20log(f)+20log(d) [dB] where f = frequency (GHz) d = LOS range between antennas (km) Vegetation attenuation (provision should be taken for 5 years of vegetation growth) L=0.2f 0.3R0.6(dB) f=frequency (MHz) R=depth of vegetation in meter’s (for R<400m)

Propagation Losses : 

Micrwave Link Design 15 Propagation Losses Obstacle Loss –also called Diffraction Loss or Diffraction Attenuation. One method of calculation is based on knife edge approximation. Having an obstacle free 60% of the Fresnel zone gives 0 dB loss 0 dB 20dB 16dB 6dB 0 dB First Fresnel Zone

Link Budget : 

Microwave Link Design 16 Link Budget The link budget is a calculation involving the gain and loss factors associated with the antennas, transmitters, transmission lines and propagation environment, to determine the maximum distance at which a transmitter and receiver can successfully operate

Link Budget : 

Micrwave Link Design 17 Link Budget Receiver sensitivity threshold is the signal level at which the radio runs continuous errors at a specified bit rate System gain depends on the modulation used (2PSK, 4PSK, 8PSK, 16QAM, 32QAM, 64QAM,128QAM,256QAM) and on the design of the radio

Link Budget : 

Micrwave Link Design 18 Link Budget The gains from the antenna at each end are added to the system gain (larger antennas provide a higher gain). The free space loss of the radio signal is subtracted. The longer the link the higher the loss These calculations give the fade margin In most cases since the same duplex radio setup is applied to both stations the calculation of the received signal level is independent of direction

Link Budget : 

Micrwave Link Design 19 Link Budget Receive Signal Level (RSL) RSL = Po – Lctx + Gatx – Lcrx + Gatx – FSL Link feasibility formula RSL  Rx (receiver sensitivity threshold) Po = output power of the transmitter (dBm) Lctx, Lcrx = Loss (cable,connectors, branching unit) between transmitter/receiver and antenna(dB) Gatx = gain of transmitter/receiver antenna (dBi) FSL = free space loss (dB)

Link Budget : 

Micrwave Link Design 20 Link Budget The fade margin is calculated with respect to the receiver threshold level for a given bit-error rate (BER).The radio can handle anything that affects the radio signal within the fade margin but if it is exceeded, then the link could go down and therefore become unavailable

Link Budget : 

Micrwave Link Design 21 Link Budget The threshold level for BER=10-6 for microwave equipment used to be about 3dB higher than for BER=10-3. Consequently the fade margin was 3 dB larger for BER=10-6 than BER=10-3. In new generation microwave radios with power forward error correction schemes this difference is 0.5 to 1.5 dB

Radio path link budget : 

Micrwave Link Design 22 Radio path link budget Transmitter 1 Receiver 1 Splitter Splitter Transmitter 2 Receiver 2 Output Power (Tx) Branching Losses waveguide Propagation Losses Antenna Gain Antenna Gain Branching Losses Received Power (Rx) Receiver threshold Value Fade Margin

Fading and Fade margins : 

Micrwave Link Design 23 Fading and Fade margins Fading is defined as the variation of the strength of a received radio carrier signal due to atmospheric changes and/or ground and water reflections in the propagation path.Four fading types are considered while planning links.They are all dependent on path length and are estimated as the probability of exceeding a given (calculated) fade margin

Fading and Fade margins : 

Micrwave Link Design 24 Fading and Fade margins Multipath fading - Flat fading - Frequency-selective fading Rain fading Refraction-diffraction fading (k-type fading)

Fading and Fade margins : 

Micrwave Link Design 25 Fading and Fade margins Multipath Fading is the dominant fading mechanism for frequencies lower than 10GHz. A reflected wave causes a multipath, i.e.when a reflected wave reaches the receiver as the direct wave that travels in a straight line from the transmitter If the two signals reach in phase then the signal amplifies. This is called upfade

Fading and Fade margins : 

Micrwave Link Design 26 Fading and Fade margins Upfademax=10 log d – 0.03d (dB) d is path length in Km If the two waves reach the receiver out of phase they weaken the overall signal.A location where a signal is canceled out by multipath is called null or downfade As a thumb rule, multipath fading, for radio links having bandwidths less than 40MHz and path lengths less than 30Km is described as flat instead of frequency selective

Fading and Fade margins : 

Micrwave Link Design 27 Fading and Fade margins Flat fading A fade where all frequencies in the channel are equally affected.There is barely noticeable variation of the amplitude of the signal across the channel bandwidth If necessary flat fade margin of a link can be improved by using larger antennas, a higher-power microwave transmitter, lower –loss feed line and splitting a longer path into two shorter hops On water paths at frequencies above 3 GHz, it is advantageous to choose vertical polarization

Fading and Fade margins : 

Micrwave Link Design 28 Fading and Fade margins Rain Fading Rain attenuates the signal caused by the scattering and absorption of electromagnetic waves by rain drops It is significant for long paths (>10Km) It starts increasing at about 10GHz and for frequencies above 15 GHz, rain fading is the dominant fading mechanism Rain outage increases dramatically with frequency and then with path length

Frequency planning : 

Micrwave Link Design 29 Frequency planning The objective of frequency planning is to assign frequencies to a network using as few frequencies as possible and in a manner such that the quality and availability of the radio link path is minimally affected by interference. The following aspects are the basic considerations involved in the assignment of radio frequencies

Frequency planning : 

Micrwave Link Design 30 Frequency planning Determining a frequency band that is suitable for the specific link (path length, site location, terrain topography and atmospheric effects) Prevention of mutual interference such as interference among radio frequency channels in the actual path, interference to and from other radio paths, interference to and from satellite communication systems Correct selection of a frequency band allows the required transmission capacity while efficiently utilizing the available radio frequency spectrum

Frequency planning : 

Micrwave Link Design 31 Frequency planning Frequency channel arrangements The available frequency band is subdivided into two halves, a lower (go) and an upper (return) duplex half. The duplex spacing is always sufficiently large so that the radio equipment can operate interference free under duplex operation. The width of each channel depends on the capacity of the radio link and the type of modulation used

Frequency planning : 

Micrwave Link Design 32 Frequency planning The most important goal 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 because of radio interference.

Frequency planning : 

Micrwave Link Design 33 Frequency planning Frequency planning of a few paths can be carried out manually but, for larger networks, it is highly recommended to employ a software transmission design tool. One such vendor independent tool is Pathloss 4.0. This tool is probably one of the best tools for complex microwave design. It includes North American and ITU standards, different diversity schemes, diffraction and reflection (multipath) analysis, rain effects, interference analysis etc.

Frequency planning for different network topologies : 

Micrwave Link Design 34 Frequency planning for different network topologies Chain/cascade configuration L U U f1 HP f1 VP f1 HP

Ring configuration : 

Micrwave Link Design 35 Ring configuration If the ring consisted of an odd number of sites there would be a conflict of duplex halves and changing the frequency band would be a reliable alternative U L U L L U f1 HP f1 VP f1 VP f1 VP f1 HP f1 VP

Star configuration : 

Micrwave Link Design 36 Star configuration The link carrying the traffic out of the hub should use a frequency band other than the one employed inside the cluster L U U U U U f1 HP f2 VP f1 HP f1 HP f2 VP

Interference fade margin : 

Micrwave Link Design 37 Interference fade margin In normal unfaded conditions the digital signal can tolerate high levels of interference but in deep fades it is critical to control interference. Adjacent-channel interference fade margin (AIFM) (in decibels) accounts for receiver threshold degradation due to interference from adjacent channel transmitters

Interference fade margin : 

Micrwave Link Design 38 Interference fade margin Microwave Link Multipath Outage Models A major concern for microwave system users is how often and for how long a system might be out of service. An outage in a digital microwave link occurs with a loss of Digital Signal frame sync for more than 10 sec. Digital signal frame loss typically occurs when the BER increases beyond 1 x 10-3.

Quality and Availability : 

Micrwave Link Design 39 Quality and Availability The main purpose of the quality and availability calculations is to set up reasonable quality and availability objectives for the microwave path.The ITU-T recommendations G.801, G.821 and G.826 define error performance and availability objectives. The objectives of digital links are divided into separate grades: high, medium and local grade. The medium grade has four quality classifications.

Quality and Availability : 

Micrwave Link Design 40 Quality and Availability The following grades are usually used in wireless networks:- Medium grade Class 3 for the access network High grade for the backbone network

Improving the Microwave System : 

Micrwave Link Design 41 Improving the Microwave System Hardware Redundancy Hot standby protection Diversity Improvement Space Diversity Frequency Diversity Media Diversity Repeaters Active repeaters Passive repeaters

Basic Recommendations : 

Micrwave Link Design 42 Basic Recommendations Use higher frequency bands for shorter hops and lower frequency bands for longer hops Avoid lower frequency bands in urban areas Use star and hub configurations for smaller networks and ring configuration for larger networks In areas with heavy precipitation , if possible, use frequency bands below 10 GHz. Use protected systems (1+1) for all important and/or high-capacity links Leave enough spare capacity for future expansion of the system

Slide 43: 

Micrwave Link Design 43 Space diversity is a very expensive way of improving the performance of the microwave link and it should be used carefully and as a last resort The activities of microwave path planning and frequency planning preferably should be performed in parallel with line of sight activities and other network design activities for best efficiency. Use updated maps that are not more than a year old. The terrain itself can change drastically in a very short time period.Make sure everyone on the project is using the same maps, datums and coordinate systems.

Difficult Areas for Microwave Links : 

Micrwave Link Design 44 Difficult Areas for Microwave Links In areas with lots of rain, use the lowest frequency band allowed for the project. Microwave hops over or in the vicinity of the large water surfaces and flat land areas can cause severe multipath fading.Reflections may be avoided by selecting sites that are shielded from the reflected rays. Hot and humid coastal areas

Slide 45: 

Micrwave Link Design 45 Thank you