Microwave Engineering Considerations

Free Space & Atmospheric Attenuation:


Free space & atmospheric attenuation is defined by the loss the signal undergoes traveling through the atmosphere.

Changes in air density and absorption by atmospheric particles.


Reflections:

Reflections can occur as the microwave signal traverses a body of water or fog bank; cause multipath conditions.

Diffraction:

Diffraction is the result of variations in the terrain the signal crosses.

Rain Attenuation:

Raindrop absorption or scattering of the microwave signal can cause signal loss in transmissions.

Skin Affect:

Skin Affect is the concept that high frequency energy travels only on the outside skin of a conductor and does not penetrate into it any great distance.

Skin Affect determines the properties of microwave signals.

Microwave Impairments

Equipment, antenna, and waveguide failures.

Fading and distortion from multipath reflections.

Absorption from rain, fog, and other atmospheric conditions.

Interference from other frequencies.

XPIC

Cross Polarization Interference Cancellation (XPIC).

How does Microwave having XPIC capabilities effectively double the potential capacity of a Microwave Path.

It allows the assignment of the same frequency to both the vertical & horizontal Polarization on a Path.

Where available frequencies are limited then it is possible to assign the same frequency twice on the same path using both Polarizations.

Using standard Microwave equipment from any of the major manufacturers, if a full block of eight frequencies were available for a 6 GHz Lower band path then eight frequencies could be assigned in each direction on the path, four per polarization.

Using equipment with XPIC capability, sixteen frequencies may be assigned each way on the same path (eight per polarization).

Types of Microwave Links

Frequency Diversity:

Frequency Diversity uses separate frequencies (dual transmit and receive systems).

It monitors primary for fail over and switches to standby.

Interference usually affects only one range of frequencies.

Not allowed in non-carrier applications because of spectrum scarcity.

Space Diversity:

Space Diversity protects against multi-path fading by automatic switch over to another antenna place below the primary antenna.

This is done at the BER failure point or signal strength attenuation point to the secondary antenna that is receiving the transmitted signal at a stronger power rating.

Line of Sight (LoS)

Fresnel Zone Clearance:

Fresnel Zone Clearance is the minimum clearance over obstacles that the signal needs to be sent over.

Reflection or path bending will occur if the clearance is not sufficient.

Repeaters

A microwave repeater with antenna isolation comprises a first receiving antenna for receiving an input signal from a distant source.

An amplifier section for amplifying and modulating the input signal to produce an output signal.

A transmitting antenna coupled to the amplifier section and an isolation control means including a second receiving antenna for isolating the first receiving antenna from the transmitting antenna.

In one preferred embodiment, the second receiving antenna is aligned so as to match the gain and phase from the transmitting antenna to the first receiving antenna.

The isolation control means comprises a phase shifter coupled to the second receiving antenna; and a power combiner having inputs coupled to the first receiving antenna and the phase shifter and an output coupled to the amplifier section.

The power combiner receives the input signal from the first receiving antenna and a 1 SO phased shifted signal from the phase shifter.

SDH

Synchronous Digital Hierarchy:

Management is very inflexible in PDH, So SDH was developed.

Synchronous Digital Hierarchy (SDH) originates from Synchronous Optical Network (SONET) in the US.

It includes capabilities for bandwidth on demand and is also made up of multiples of E1.

STM-1 (155Mb/s) is 63 x E1, STM-4 (622Mb/s) is 4 x STM-1 and STM-16 (2.5Gb/s) is 4 x STM-4.

The benefits of SDH are:

1) Different interfaces or different bandwidths can connect (G708, G781).

2) Network topologies are more flexible.

3) There is flexibility for growth.

4) The optical interface is standard (G957).

5) Network Management is easier to perform (G774 and G784).

PDH

Plesiochronous Digital Hierarchy:

The Plesiochronous Digital Hierarchy (PDH) is a technology used in telecommunication networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems.

The term plesiochronous is derived from Greek plesio, meaning near, and chronos, time, and refers to the fact that PDH networks run in a state where different parts of the network are nearly, but not quite perfectly, synchronised.

PDH is typically being replaced by Synchronous Digital Hierarchy (SDH) or Synchronous optical networking (SONET) equipment in most telecommunications networks.

PDH allows transmission of data streams that are nominally running at the same rate, but allowing some variation on the speed around a nominal rate.

By analogy, any two watches are nominally running at the same rate, clocking up 60 seconds every minute.

However, there is no link between watches to guarantee they run at exactly the same rate, and it is highly likely that one is running slightly faster than the other.

E1

An E1 link operates over two separate sets of wires, usually twisted pair cable. A nominal 3 Volt peak signal is encoded with pulses using a method that avoids long periods without polarity changes. The line data rate is 2.048 Mbit/s (full duplex, i.e. 2.048 Mbit/s downstream and 2.048 Mbit/s upstream) which is split into 32 timeslots, each being allocated 8 bits in turn. Thus each timeslot sends and receives an 8-bit sample 8000 times per second (8 x 8000 x 32 = 2,048,000). This is ideal for voice telephone calls where the voice is sampled into an 8 bit number at that data rate and reconstructed at the other end. The timeslots are numbered from 0 to 31.

One timeslot (TS0) is reserved for framing purposes, and alternately transmits a fixed pattern. This allows the receiver to lock onto the start of each frame and match up each channel in turn. The standards allow for a full Cyclic Redundancy Check to be performed across all bits transmitted in each frame, to detect if the circuit is losing bits (information), but this is not always used.

One timeslot (TS16) is often reserved for signalling purposes, to control call setup and teardown according to one of several standard telecommunications protocols. This includes Channel Associated Signaling (CAS) where a set of bits is used to replicate opening and closing the circuit (as if picking up the telephone receiver and pulsing digits on a rotary phone), or using tone signalling which is passed through on the voice circuits themselves. More recent systems used Common Channel Signaling (CCS) such as ISDN or Signalling System 7 (SS7) which send short encoded messages with more information about the call including caller ID, type of transmission required etc. ISDN is often used between the local telephone exchange and business premises, whilst SS7 is almost exclusively used between exchanges and operators. SS7 can handle up to 4096 circuits per signalling channel[citation needed], thus allowing slightly more efficient use of the overall transmission bandwidth (for example: uses 31 voice channels on an E1).

Unlike the earlier T-carrier systems developed in North America, all 8 bits of each sample are available for each call. This allows the E1 systems to be used equally well for circuit switch data calls, without risking the loss of any information.

While the original CEPT standard G.703 specifies several options for the physical transmission, almost exclusively HDB3 format is used.

E-carrier

In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations, worldwide standards have been created and deployed.

The European Conference of Postal and Telecommunications Administrations (CEPT) originally standardized the E-carrier system, which revised and improved the earlier American T-carrier technology, and this has now been adopted by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T).

This is now widely used in almost all countries outside the USA, Canada and Japan.

The E-carrier standards form part of the Plesiochronous Digital Hierarchy (PDH) where groups of E1 circuits may be bundled onto higher capacity E3 links between telephone exchanges or countries.

This allows a network operator to provide a private end-to-end E1 circuit between customers in different countries that share single high capacity links in between.

In practice, only E1 (30 circuit) and E3 (480 circuit) versions are used. Physically E1 is transmitted as 32 timeslots and E3 512 timeslots, but one is used for framing and typically one allocated for signalling call setup and tear down.

Unlike Internet data services, E-carrier systems permanently allocate capacity for a voice call for its entire duration.

This ensures high call quality because the transmission arrives with the same short delay (Latency) and capacity at all times.

E1 circuits are very common in most telephone exchanges and are used to connect to medium and large companies, to remote exchanges and in many cases between exchanges.

E3 lines are used between exchanges, operators and/or countries, and have a transmission speed of 34.368 Mbit/s.

Alignment

Simplifying the Process:

We have laid the groundwork for establishing the Path Align-R™ test set as a viable and effective solution for thepath alignment process. We will now describe how this process of aligning the microwave link is greatlysimplified.

1. Review the Engineering Profile to determine the expected RSL (received signal level, or path loss)for the link under test (this should include the free space path loss and the gain of the antennas). Notethat the loss exhibited by the installed cable or waveguide transmission lines are not included in thisvalue because the Path Align-R™ test sets are connected directly to the antennas with a short coax cablewith minimal insertion loss to impact the test.

2. Both tower climbers (technicians) should go over their check lists, prior to climbing the tower, toassure that, included with the Path Align-R™, they have:

• A charged battery installed (possibly a spare if you are planning to be up for over 4–5 hours);
• The proper waveguide-to-coax adapter for the antenna;
• A coax cable (SMA to coax connector on the coax-to-waveguide adapter);
• The supplied headset; and,
• A carabiner and a nylon runner, strap, or lanyard of appropriate length to attach the back-pack's'D' ring to the tower.

3. Set one Path Align-R™ unit to ‘Master’ and the other Path Align-R™ unit to ‘Slave’ (it doesn’t matter whichis which, as long as they’re different).

4. Using the front panel thumbwheel switches, select the proper link frequency (Note: both units mustbe set to the same frequency, e.g. 6.2 GHz).

5. After reaching the antenna, check that the antenna polarization is properly setup.

6. Locate each Align-R™ unit near the back of the antenna, with its weather-resistant instrument backpack’s‘D’ ring attached to the tower structure using a carabiner and a nylon runner, strap, or lanyard.

7. Attach the coax-to-waveguide adapter at the back of the antenna’s waveguide flange.

8. Connect the coax cable from the appropriate output connector on the Path Align-R™ to the adapter.

9. Connect the headset to the Path Align-R™.

10. Turn the Power Switch ON. An LED will indicate which output connector is active (make sure thecable is connected to that connector) and begin talking. Typically, the antennas can be off alignmentby as much as several beamwidths and the voice channel will still operate.

11. While one technician simply observes the reading of the path loss meter at his/her site, the other sitecommences Azimuth (horizontal) adjustment. As the voice channel is full duplex (FM), eachtechnician can communicate with the other during the alignment process. Check for both side lobes aswell as the main lobe response to ensure that the antenna is being aligned to the main lobe and notone of the side lobes. The Path Align-R™ test sets have enough sensitivity (-100 dB) and update speed(300 ms) to quickly check for side lobe and main lobe response.

12. Once Azimuth has been optimized, the Elevation adjustment is performed.

13. When steps 11 & 12 have been completed at the first site, the technicians switch roles and steps 11 &12 are repeated by the other technician at the second site.

14. Once step 13 is completed, for both Azimuth and Elevation, the link is aligned. As a record, note thefinal path loss value displayed on the meter.

Summary:

The difficulties of scheduling an antenna alignment test with the radios can cause significant delays, asdescribed earlier. Furthermore, traditional test methods can produce significant costs in mobilization,deployment and testing of the antenna system. If the transmission lines (waveguides, etc.) connecting theantenna to the radio were to have a problem, the test is in jeopardy until they are corrected. If the path undertest is questionable (due to site choice, obstructions, etc.), the Path Align-R™ can step right in with minimaleffort to verify if there are any problems. Using the Path Align-R™ test set, the overhead associated with offeringpath alignment services (cost of equipment, level of expertise required, and training time) has been greatlyreduced. Offering path alignment services with the Path Align-R™ translates to providing more revenue opportunities for your company.

Alignment

Alternative Methods:

Traditionally, the radios that will be placed at each site are used to complete the task of optimizing the path.However, there are several reasons for not utilizing the radios to complete the process. The radios may not beavailable at the time the test has been scheduled or their reliability may be questionable, thus alternatemethods are needed. Another possible reason the radios might not be able to be used is that the FCC permitshave not been granted, but the contractor needs to complete the path test on time to meet the customer’srequirements, or ahead of expected turbulent weather. In addition, if the anticipated path is questionable, aquick, cost effective, reliable method is needed to test the link, prior to the significant investment ofconstructing towers, purchasing radios and other expensive equipment and hardware, etc.

Alternative test instrumentation must be utilized, in lieu of the radios, whenever the previously mentionedcircumstances arise. Scheduling of the path alignment test and installation of the associated hardware (cables,waveguide, antenna, etc.) can be facilitated to reduce excess mobilization costs. Some of the most widelyused equipment for this application are signal generators (used as the transmitter) and spectrum analyzers(used as the receiver). The signal generator should be a broadband, synthesized device (phase locked to areference clock) with accurate output power, equal to or greater than 0 dBm. The spectrum analyzer should betunable, and have at least –100 dBm of sensitivity at the frequency band of interest. Signal acquisition speedof the receiver is essential to quickly spot subtle changes in antenna response or to support investigation ofproblem paths. Most importantly, ease of use is vital in order to reduce the training costs of test technicians.Much of the test equipment used in this alternate method of path alignment tends to be expensive and complicated.

Application Specific Solution:

Recent test instrumentation has brought on feature rich, expensive systems that shoot for the stars in offeringthe latest and greatest in technology. However, these systems are mostly not fit, nor are they practical, forspecific applications such as antenna path alignment. The most successful new instrumentation in the industryhas been equipment that is application specific, portable, battery powered, and easy to use, withoutcompromising performance. The Path Align-R™, is a new addition to this practical, application specific category.It is specifically designed for use as a microwave path alignment test set. Among its many features andfunctions, eight specific traits fit those identified as important to microwave antenna path alignment.

1. Portable - Each of the two units in the Path Align-R™ Test Set weighs only 7 pounds, fully loaded, andis 3.5” x 8.375” x 13.1” (HxWxD) in size. This significantly reduces the shipping costs and logisticsof transporting test equipment to and from the site.

2. Battery Powered – A rechargeable lead acid battery is fitted to provide four to five hours of operation.Not only is this ideal for field use but each unit can easily be taken up the tower, in its weatherresistantinstrument back-pack, and attached directly behind the antenna using a carabiner and a nylonrunner, strap, or lanyard.

3. Ease of Use – A pair of units (sold as a ‘set’), one at each site, reads out the path loss, identically, atboth ends of the link. Simply turn them on, connect the output to the antenna using the supplied cable,tune the frequency, connect the headset, and you are ready to test. Rarely has an instrument been soeasy to use without compromising on performance.

4. Performance – A Path Align-R™ test set provides 100 dB of effective dynamic range. This is the resultof the 100 dBm sensitivity of the receiver and 0 dBm output power of the transmitter. Four bands(1.8–2.5 GHz, 5.8–6.6 GHz, 11–12 GHz and 18.1–19.4 GHz) are provided in the standard Test Set.The specific operating frequency can be tuned by the operator within these band limits, using thethumbwheel switches, to a resolution of 1 MHz. An antenna system whose link frequency is designed outside the frequency band edge of the Path Align-R™ (e.g. 6.8 GHz) can still have its path alignmentcorrectly adjusted, as long as the antenna system (antenna, waveguide, etc.) can operate at both thelink frequency (6.8 GHz) and a nearby frequency covered by the Path Align-R™ (e.g. 6.6 GHz).

5. Speed - An LCD readout of direct path loss, with a 0.1 dB resolution, automatically updates itsreading every 300 milliseconds, and can quickly find subtle changes to the antenna response, thusallowing for small adjustments to antenna azimuth or elevation for optimum signal transfer.

6. Voice Channel – Communication between sites, and from radio room to tower top, is significantlyimproved, with a full duplex voice channel. Each tower technician can speak to the other through theincluded headset, without having to key a radio. Voice communication is enabled immediately aftersetup and activation. The antennas do not even have to be fully aligned for the voice channel to work.

7. Accurate – The Path Align-R™’s output is a synthesized source that provides a very accurate and stablesignal. When compared to a synthesized source (HP 8360) and a fully featured spectrum analyzer (HP8594E), the Path Align-R™ test set measures path loss within 1.0 dB of each other.

8. Cost Effective – The Path Align-R™ test set is priced far below that of a synthesized source and spectrumanalyzer combination. However, price is not the only savings achieved with the use of the test set.The cost of communicating between sites can become quite expensive when using cellular technology(about $300 per event when considering long distance, roaming, and calling time charges that aretypically the case in a remote microwave test). The use of two technicians operating the radios, andtwo tower climbers working behind the antennas, can potentially be reduced to three; with thesupervisor level on the ground floor and two tower climbers. The transportation costs of shipping thetest sets to job sites from the central office can significantly reduce company expenses.

It is apparent that the Path Align-R™ test set is well featured to offer impressive results in accomplishing the goalof an optimum solution for the microwave path alignment test process.

Alignment

Simplifying the Path Alignment of Microwave Communication Systems:

Wherever microwave links exist, the path between antennas has always required accurateantenna alignment. This process requires highly trained tower crews to physically align theantennas as well as ground technicians and sophisticated, expensive, and complex testequipment to monitor the results. The process of optimizing the transmission path of microwavecommunication systems is about to undergo a significant development in process, simplification and costbenefit without compromising performance or accuracy. The process can now be accomplished with the useof the Path Align-R™ , Models 2200–2241, µwave Antenna Path Alignment Test Sets from PendulumInstruments, formally XL Microwave. Tower installation crews can now perform the entire alignment processthemselves, up the tower, at the antenna, without the need of additional ground technicians, equipment, orindeed, even the waveguides or radios installed.

The Traditional Process:

The traditional process, more typically described as “microwave path alignment,” requires the use of atransmitter and a receiver located at each end of the microwave link. The transmitter generates the signal thatpasses through the transmission line to the antenna, which radiates the signal over the free space link. Thesignal propagates towards the other end of the path and is received by the antenna, forwarding the signalthrough the transmission line to the receiver, connecting the two sites. The receiver processes the signal,producing information on its value relative to the amount that was originally generated at the transmittingend. If the antennas are optimally pointed to each other (aligned), the largest concentration of signal (mainbeam) is emitted and received, reducing the free space attenuation of the signal. Provided the transmissionlines do not lose too much of the signal between the antennas and the radios, maximum signal transfer isachieved. If the antennas are not optimally aligned, then signal transfer is degraded and received dynamicrange is lost.

There are several steps involved in the traditional preparation and process of aligning the antennas of amicrowave communication system. These steps may include making sure the cable or waveguidetransmission line was properly installed, with minimal RF reflection of the microwave signal; that eachantenna polarization is properly setup; and that the transmitter output power is calibrated. A voicecommunication link between the personnel inside the radio room of each site and the tower technicians,located at each antenna, needs to be established using two way mobile radios or cellular phones. Some spreadspectrum radios have an order wire over which to communicate, however, communication to each of thetower technicians will still need to be completed. The engineering profile is reviewed to determine theexpected RSL (receive signal level) for the path under test and any adjustments for output power are applied.Once this setup is complete, the tower technicians are instructed to commence the adjustment of the azimuthalignment (bearing) of the antennas, one at a time. The antennas are panned over their azimuth profile andreadings of the receiver signal output power are taken. Careful observation of the output power reading isnecessary to distinguish the antenna side-lobe to main-lobe response. Once the maximum signal is achieved,the antennas are aligned for elevation optimization. It is evident that the communication between site to siteand tower technician to receiver technician needs to be continuous and clear to ensure that the antennasoptimum alignment setting is achieved.

Microwave Transmission System

Microwave Spectrum:

Range is approximately 1 GHz to 40 GHz
Total of all usable frequencies under 1 GHz gives a reference on the capacity of in the microwave range.

Components of a Microwave System:

1) Digital Modem.
2) Radio Frequency (RF) Unit.
3) Antenna.


Digital Modem:

The digital modem modulates the information signal.
(intermediate frequency or IF)

RF Unit:

IF is fed to the RF unit which is mounted as close physically to the antenna as possible.
(direct connect is optimal)

Waveguides:

Waveguides are hollow channels of low-loss material used to direct the signal from the RF unit to the antenna.