PPT ON MICROWAVE LINK DESIGN

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MICROWAVE LINK DESIGN  Presentation Transcript

1.MICROWAVE LINK DESIGN

2.What is Microwave Communication

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.

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

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

6.Line-of-Sight Considerations Fresnel Zone - Areas of constructive and destructive interference created when electromagnetic wave propagation in free space is reflected (multipath) or diffracted as the wave intersects obstacles. Fresnel zones are specified employing ordinal numbers that correspond to the number of half wavelength multiples that represent the difference in radio wave propagation path from the direct path The Fresnel Zone must be clear of all obstructions.

7.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

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)

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 2-3 meter)

10.Microwave Link Design Microwave Link Design is a methodical, systematic and sometimes lengthy process that includes

11.Microwave Link Design Process

12.Loss / Attenuation Calculations

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

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 = total distance 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)

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

16.Gas absorption Primarily due to the water vapor and oxygen in the atmosphere in the radio relay region.The absorption peaks are located around 23GHz for water molecules and 50 to 70 GHz for oxygen molecules.The specific attenuation (dB/Km)is strongly dependent on frequency, temperature and the absolute or relative humidity of the atmosphere.

17.Gas attenuation versus frequency

18.Propagation Losses

19.The specific attenuation of rain is dependent on many parameters such as the form and size of distribution of the raindrops, polarization, rain intensity and frequency Horizontal polarization gives more rain attenuation than vertical polarization Rain attenuation increases with frequency and becomes a major contributor in the frequency bands above 10 GHz The contribution due to rain attenuation is not included in the link budget and is used only in the calculation of rain fading

20.Ground Reflection Reflection on the Earth’s surface may give rise to multipath propagation The direct ray at the receiver may interfered with by the ground-reflected ray and the reflection loss can be significant Since the refraction properties of the atmosphere are constantly changing the reflection loss varies.

21.The loss due to reflection on the ground is dependent on the total reflection coefficient of the ground and the phase shift The highest value of signal strength is obtained for a phase angle of 0o and the lowest value is for a phase angle of 180o The reflection coefficient is dependent on the frequency, grazing angle (angle between the ray beam and the horizontal plane), polarization and ground properties

22.The grazing angle of radio-relay paths is very small – usually less than 1o It is recommended to avoid ground reflection by shielding the path against the indirect ray The contribution resulting from reflection loss is not automatically included in the link budget.When reflection cannot be avoided, the fade margin may be adjusted by including this contribution as “additional loss” in the link budget

23.Signal strength versus reflection coefficient

24.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

25.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

26.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

27.Link Budget

28.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

29.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

30.Fading and Fade margins

31.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

32.Frequency-selective fading There are amplitude and group delay distortions across the channel bandwidth It affects medium and high capacity radio links (>32 Mbps) The sensitivity of digital radio equipment to frequency-selective fading can be described by the signature curve of the equipment This curve can be used to calculate the Dispersive Fade Margin (DFM)

33.Fading and Fade margins

34.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 than with path length

35.Microwave path lengths must be reduced in areas where rain outages are severe The available rainfall data is usually in the form of a statistical description of the amount of rain that falls at a given measurement point over a period of time.The total annual rainfall in an area has little relation to the rain attenuation for the area Hence a margin is included to compensate for the effects of rain at a given level of availability. Increased fade margin (margins as high as 45 to 60dB) is of some help in rainfall attenuation fading.

36.Reducing the Effects of Rain Multipath fading is at its minimum during periods of heavy rainfall with well aligned dishes, so entire path fade margin is available to combat the rain attenuation (wet-radome loss effects are minimum with shrouded antennas) When permitted, crossband diversity is very effective Route diversity with paths separated by more than about 8 Km can be used successfully

37.Radios with Automatic Transmitter Power Control have been used in some highly vulnerable links Vertical polarization is far less susceptible to rainfall attenuation (40 to 60%) than are horizontal polarisation frequencies.

38.Fading and Fade Margins

39.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

40.Frequency planning Assignment of a radio frequency or radio frequency channel is the authorization given by an administration for a radio station to use a radio frequency or radio frequency channel under specified conditions. It is created in accordance with the Series F recommendations given by the ITU-R. In India the authority is WPC (Wireless Planning & Coordination Wing)

41.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

42.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.

43.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.

44.Frequency planning for different network topologies

45.Ring configuration

46.Star configuration

47.Interference fade margin

48.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 (IFM) is the depth of fade to the point at which RF interference degrades the BER to 1x 10-3 . The actual IFM value used in a path calculation depends on the method of frequency coordination being used.

49.There are two widely used methods. The C/I (carrier to interference) and T/I (threshold to interference) methods. C/I method is the older method developed to analyse interference cases into analog radios. In the new T/I method, threshold-to-interference (T/I) curves are used to define a curve of maximum interfering power levels for various frequency separations between interfering transmitter and victim receivers as follows

50.Interference fade margin

51.For each interfering transmitter, the receive power level in dBm is compared to the maximum power level to determine whether the interference is acceptable. The T/I curves are based on the actual lab measurements of the radio.   Composite Fade Margin (CFM) is the fade margin applied to multipath fade outage equations for a digital microwave radio

52.Interference fade margin

53.Interference fade margin

54.Outage (Unavailability) (%) = (SES/t) x 100 where t = time period (expressed in seconds) SES = severely errored second   Availability is expressed as a percentage as : - A = 100 - Outage (Unavailability) A digital link is unavailable for service or performance prediction/verification after a ten consecutive BER> 1 x 10-3 SES outage period.

55.Quality and Availability

56.The following grades are usually used in wireless networks:- Medium grade Class 3 for the access network High grade for the backbone network

57.Improving the Microwave System

58.Antireflective Systems Repeaters Active repeaters Passive repeaters

59.Basic Recommendations

60.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.

61.Perform detailed path surveys on ALL microwave hops.Maps are used only for initial planning, as a first approximation. Below 10 GHz , multipath outage increases rapidly with path length.It also increases with frequency , climatic factors and average annual temperature.Multipath effect can be reduced with higher fade margin. If the path has excessive path outage the performance can be improved by using one of the diversity methods.

62.Difficult Areas for Microwave Links