Network Engineering
The Application of Hybrid Diversity to Digital Microwave Radio-Relay Links Scope Hybrid (Space+Frequency) Diversity is the most effective configuration available to greatly improve microwave radio link performance in a multipath fading atmosphere. This technical brief defines hybrid diversity, its features and its economic benefits.
Overview Hybrid Diversity is even more effective than Quadruple Diversity, a more costly, complex scheme requiring four receivers and dual a ntennas at each site and thus no longer recommended for today’s high-performance errorlessly switched 1:1 digital microwave radiorelay links even in long over-water paths traversing adverse geoclimatic regions. Quadruple diversity is, however, very effective when deployed in MegaStar 155 1:N multi-line digital microwave links with IF combiners. Three- and four-antenna Hybrid Diversity protection schemes meet ITU-R performance objectives with less complexity, complexity, providin g higher MTBF’s and MTBO’s at much lower cost. cost. The following sidebar provides a summary of some of Hybrid Diversity’s main characteristics and features:
Characteristics of Hybrid Diversity
Hybrid Diversity is an enhancement of Space Diversity by adding Frequency Diversity to errorlessly-switched radio-rela y links. Hybrid Diversity is the most effective of all of the diversity arrangements, and is preferred in difficult propagation areas. Three-dish Hybrid Diversity is used in space-limited mountain-top, urban area, and in other links restricted to a single antenna at one end. Four-dish Hybrid Diversity pro vides antenna feeder system protection at both sites and (in many 1:N and other links) impro ved performance. Hybrid Diversity is as effective (same or better performance) as Quadruple Diversity, but is not nearly as complex: Hybrid Diversity requires only dual receivers compared to four receivers with IF combining for Quadruple Diversity. Hybrid Diversity uses three (or four, for antenna feeder system protection) single-polarized antennas per path. Quadruple Diversity typically requires four dual polarized antennas antennas per path.
Three-Antenna Hybrid Diversity Arrangement
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The sketch below shows a three-antenna ((TTRR-TR/TR) hybrid dive rsity protection arrangement providing diversity improvement factors higher than Space Diversity alone, and equal to or greater than Quadruple Diversity. The single antenna, shown at the left in the sketch, is best located at the higher elevation site with the dual vertically spaced antennas assigned to the lower elevation site.
DATA IN
DATA IN
DATA OUT
DATA OUT
The radio terminal in this three-antenna Hybrid Diversity arrangement located at the single (usually the higher elevation) antenna site is configured fo r standard frequency diversity (TTRR) protection. This single antenna is easily located on the side of a microwave building or stub tower, which is a great advantage since this higher elevation site is often mountainous and thus more exposed to the elements with more dif ficult access and tower constraints.
Fresnel Patterns and Antenna Spacings The Fresnel interference patterns generated by a specular reflection at the low- and high-sites in a 3-dish Hybrid Diversity link a re shown in the following sketch. SIGNAL STRENGTH VARIATION ALONG TOWER (HIGH END)
SIGNAL STRENGTH VARIATION ALONG TOWER (LOW END)
TTRR
TR (A)
TR (A) TR (B)
TTRR k = 4/3
a) At low end, with high end fixed (Recommended)
k = 4/3
b) At high end, with low end fixed (Not recommended)
Interference Patterns Caused By Reflections. Hybrid Diversity Rules: Always assign the highest T/R frequencies to the upper antenna at the two-antenna end.
Always assign the vertically-separated SD dishes to the low (usually the more secure and accessible) end of a three-dish hybrid diversity microwave link.
Radios at the dual antenna (preferably the lower) end of the path are in a TR/TR arrangement Network Engineering with each TR connected to one of the separated antennas. Two-way diversity improvements (reductions in outage) are provided primarily by the spaced antennas. Very close T-T/R-R frequency separations can be assigned with little reduction in hybrid diversity’s improvement factors. The Fresnel pattern resulting from the addition of the direct (wanted) and reflected (interference) rays forms closely spaced nulls at the low end of the path and a wide null pattern at the high end of the path. The optimum vertical antenna spacing is small at this low-end site, and space antennas at this end provides an anti-correlative fade characteristic between the main and diversity receivers in both directions of the path. Very wide spacings are necessary if deploying dual antennas at the high-end site, assignments neither recommended nor required in Hybrid Diversity links. The lower site is best configured with dual antennas optimally spaced for anti-correlative fad ing between the diversity receivers. Separate TRs are assigned to the upper and lower antennas that provide Space Diversity protection against multipath fade activity in both directions of the path. Since the specular reflection point on a high-low path is always much closer to this lower site, a very close diversity antenna spacing of perhaps only 3-8m(10-25ft) provides the required anticorrelative protection fro m outage caused by specular ground or water reflections. If the paths were configured with Quadruple Diversity (not recommended e xcept in MegaStar 155 1:N links), dual antennas with much greater spacing (10-20m) would be required at the higher elevation site. And it is at these higher elevation, often mountainous, sites that the construction of tall towers to accommodate specular reflection multipath fade activity is difficult and costly.
Four-Antenna Hybrid Diversity Arrangement Dual vertically spaced antennas are assigned to both microwave sites in a four-antenna TR/TRTR/TR Hybrid Diversity arrangement. The main advantage of a four-antenna Hybrid Diversity arrangement vs. a three-antenna scheme is that all antenna feeder systems are now fully protected from failure, misalignment, and contamination by the errorless switching of the receivers’ data streams. In a four-antenna arrangement, the radios are then arranged in a split TR/TR configuration at both sites, with each TR now connected to an upper or lower single polarized antenna. The vertical antenna separation in a four-antenna Hybrid Diversity link must be reeva luated to ensure anti-correlative receiver fading caused by an exposed specular reflection, of course. The addition of a fourth antenna to the Hybrid Diversity link will provide only a minimal improvement in the path’s error performance that is ignored in the path calculations. Since this addition of a 4th antenna, two at each end of the path, doubles the separation of the maindiversity rays over a mid-path specular reflections point, geometry calculations must ensure that the main-diversity ray Fresnel clearances are not now correlative, i.e. both even or both odd Fresnel zone clearances. The TR assignments are optimized in a high-low/low-high arrangement rather than high-high and low-low antenna couplings.
Hybrid Diversity Improvement Factor (Ihd) Calculation The improvement (reduction in link outage time) with Hybrid Diversity is Ihd = Isd + Ifd, where Isd and Ifd are the computed Space and Frequency Diversity Improvement Factors respectively. This Hybrid Diversity Improvement Factor closely approximates the improvement factor computed for a similar Quadruple Diversity link.
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In neither arrangement (Hybrid or Quadruple Diversity) are the Space and Frequency Diversity Improvement Factors multiplied – i.e. Iqd or Ihd ≠ Isd x Ifd.
Canary Islands 69.7km High-Low 7 GHz Overwater Path Example As an example, errorlessly-switched MicroStar M/H 7 GHz radios configured for Hybrid Diversity protection against multipath fade activity are optimally utilized in the Canary Islands’ Alajero – Cemisore 69.7km high-low 34 Mbit/s E3 digital microwave radio-relay link . With the 836m Alajero and 20m Cemisore site elevations and this 69.7km path almost entirely over exposed seawater, three-antenna Hybrid Diversity was the optimum solution for meeting the ITU-R performance objectives for this link. A single 2.4m (or 3m) antenna is installed on a low tower or building structure at Alajero. Dual 2.4m antennas vertically spaced 3.5-4m are positioned on the 50m tower at Cemisore. The 3.54m antenna separation at Cemisore provides anti-correlated fading between diversity receivers in both directions from the specular seawater reflection 5 km from Cemisore. With 3.5-4m diversity antenna separations at Cemisore, the Fresnel zone clearances on the main and diversity paths in both directions at this specular reflection point are F41 and F38 respectively at k = 4/3rds. These even and odd Fresnel zone clearances ensure that seawater reflection fade activity between all receivers are non-correlative and thus are disregarded in the path’s performance computations. Both Cemisore 2.4m antennas are uptilted 1-2 dB to accommodate path’s ray arrival angle changes with k-factor variations, and also to provide additional discrimination to the specular reflection located 10 below the wanted signal at this site. The Alajero – Cemisore link’s large 11mrad inclination angle mitigates the incidence of ducting and other power fade activity.
Some Special Path Engineering Considerations for Overwater Links Harris Microwave Communications Division has deployed worldwide a large number of PDH and SDH digital microwave links, in all capacities from 4E1/4T1 to NxSTM-1/OC-3, on long overwater paths in geoclimatic areas far more difficult than in the Canary Islands. Each geoclimatic region and each path geometry imposes a unique set of conditions that must be exactly accommodated. Harris MCD has commissioned many digital microwave links approaching 150km in length configured to easily meet ITU-R and North American performance and availability objectives. Hybrid Diversity is often an integral part of this solution. Adequate path clearances are especially important on long paths in some difficult geoclimatic areas, such as overwater, to avoid any incidence of defocusing and obstruction power fading and entrapments in ground-based and elevated atmospheric boundary layer. Power fades lower fade margins that increases frantic refractive multipath fade activity and destroys link performance. Therefore, antenna sizes must be constrained and precise uptilts are necessary in these and other areas to minimize antenna decoupling power fades caused by ray arrival angle changes with k-factor variations. Further, adequate antenna discriminations to long-delayed (>10 nsec) multipath signals from specular ground and water reflections are necessary to maintain the >50 dB link dispersive fade margin required to not degrade the link’s error performance.
Vertical polarizations are recommended on high-clearance paths with a >0.20 grazing (reflection) Engineering angle at an exposed reflection point. V-pol lowers the amplitudeNetwork of the specularly reflected multipath signal to further reduce link outage.
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