AVIAT SPACE DIVERS ITY WHITE PAPER

SPACE DIVERSITY AND LOWERING TCO

INTRODUCTION AND PURPOSE Space diversity has been an important feature in long haul systems for many years to counteract the effects of multi-path . Space diversity enables very long links over highly reflective surfaces such as bodies of water for example, where a non-diversity link would not deliver the high availability required. However, most operators are striving to lower their microwave TCO, specifically site and tower related costs, so the requirement for space diversity links to have an second at each site is coming under increased scrutiny. This paper describes the traditional space diversity and techniques and examines several ‘new’ techniques to reduce site requirements and improve diversity link performance to decrease overall costs. This includes three-antenna diversity and transmit diversity (also known as ‘Advanced Space Diversity’).

WHAT IS SPACE DIVERSITY? On microwave paths operating in frequency bands below 13 GHz, propagation outages due to multipath (also referred to as selective) fading are usually of short duration and are typically caused by signal reflections that interfere with and degrade the main received signal. An outage of one hour per year due to multipath fading might represent 1,000 or more individual outages, each averaging 1 second or less (1 Errored Second, or Severely Errored Second), on a properly engineered path. On the other hand, propagation outages totaling an hour per hop due to rain attenuation for links above 13 GHz may consist of only four or five longer individual outages per year, averaging five to fifteen minutes each.

Wanted Signal Path

Unwanted

(Reflected) Signal Path

Reflective Surface

Figure 1. Non-Diversity Path

The negative effects of multi-path fading can be countered by using Space Diversity (SD), which employs two antennas separated (usually vertically) on the tower by a pre-determined distance.

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Even though the system may employ two radios at each end, only one radio transmits at any one time (typically the main antenna). For a 1+1 hot standby diversity configuration TR/TR), if the main Tx fails the system will switch to the standby on the diversity or lower antenna. Alternatively, you can have a single transmitter/dual receiver system (TRR) that supports diversity without Tx protection.

Main Wanted Signal Path

Diversity Wanted Signal Path Diversity Unwanted (Reflected) Signal Path (Reflected)Mai Signal Path n Unwanted

Reflective Surface

Figure 2. Space Diversity Path

During multipath fading event conditions, the combination of the direct (or wanted) and indirect (or unwanted) received signal at each antenna will result in a distorted receive spectrum due to a frequency selective notch. Besides this distortion, there is also the addition or subtraction of the amplitude of the signal, depending on the phase of the received reflected signal. Since this distortion is uncorrelated between the upper/main antenna, and the lower/diversity antenna, Space Diversity involves the selection of the best of the two received signals or a process of combining the signals from both antennas, to create a significantly improved, good quality signal. Space Diversity does not remove all multi-path effects but can increase the availability of an unreliable and poorly performing microwave link to in excess of five-nines uptime.

DIVERSITY SELECTION - SWITCHING OR COMBINING Diversity selection can be done in one of two methods, Baseband Switching (BBS) or Maximum Ratio Combining (MRC, also referred to as IF Combining). Baseband Switching involves demodulating both of the received signals, and then selecting between the two digital streams on an errorless, frame-by-frame basis, based on uncorrectable frame detection. MRC can be performed at the IF (Intermediate Frequency) level, before the demodulator, or it can be performed in baseband after the demodulator and before the equalizer. In both cases it can result in a theoretical improvement in system gain improvement of 3 dB when both signals are in perfect condition without impairment (see Figure 3 below).

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Received Signal, Main Antenna

Combined Signal, after MRC

Received Signal, Diversity Antenna

Figure 3. Space Diversity Maximum Ratio Combining

Aviat uses MRC in our modem technology and diversity protection today, and achieve equivalent system gain improvements of a more realistic 2 to 2.5 dB in critical real-life scenarios when diversity is actually needed. An important thing to remember about the 3dB improvement from using IF/Combining/MRC, is that it requires two perfect or undistorted signals. If either or both of the signals are impaired during frequency selective fading, then there will be little to no improvement. However, the additional 3dB will be helpful during flat fading, which generally occurs over longer periods and affect all frequencies more or less equally. It is up to the link designer whether to take this extra gain into account when planning diversity paths. In LOS path design the extra 3 dB are usually not considered in the link budget, but a “diversity improvement factor” is considered to improve the multipath availability. For NLOS design (eg: LTE, WiMAX) the 3 dB is considered in link budget for MIMO 2x2 paths.

ONE-WAY SPACE DIVERSITY USING THREE ANTENNAS The introduction of a second parabolic antenna on a microwave path can impose increased costs, since to support a second antenna towers may need to be strengthened, or even replaced altogether, along with the increased costs to purchase and install the second antenna (at each end of the link), and any potential tower leasing charges. It is possible to fully implement space diversity using only three antennas, but in this case the diversity and fade margin improvement is only available in one direction (from the single to two antenna side).

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Main Antenna, Rx Main Signal Path

Diversity Signal Path Diversity Antenna, Rx

Figure 4. 3-Antenna Space Diversity

Three antenna diversity systems have been popular in the past, but in these cases frequency diversity is used in the single to the two-antenna direction to achieve the diversity improvement required without the need for a second antenna at that site. However, this does require the use of a second frequency allocation, which may not be available, may not be allowed by the National Regulator, or may be prohibitively expensive.

TRANSMITTER DIVERSITY A further, but little used, technique is referred to as transmitter diversity. In this case, in Figure 4 above both in the direction of the single antenna transmit simultaneously the same signal. This is similar to frequency diversity, but both transmitters are using the same frequency, and transmitting the same signal over separate paths that converge at the single antenna in the far end. A similar technique was developed for NLOS connections between base stations and the remote subscriber, which works in the case of completely decorrelated NLOS signals. For an LOS path, the technique attempts to replicate the same gain by artificially decorrelating the strong correlated signal present in LOS paths. However, in the latter (LOS) case, both transmitted signals must be accurately synchronized and phase-aligned so that they theoretically result in a quadrupling (or a +6dB improvement) of the signal level at the single receiving antenna. While you can accurately phase align the signals at the transmitter end through signal processing (similar to MIMO), any small divergence of these signals caused by anomalies on each path will result in the signals interfering (instead of reinforcing) with each other at the receiving antenna. If the two signals are just one-quarter of a symbol out of alignment then the result will be the cancelling out of the gain improvement, but if the mis-alignment is one-half of a symbol then the two signals will cancel each other out completely and the entire link will be lost. Furthermore, depending on the transmitter diversity antenna separation, the interference between the two transmitters will also need to be cancelled, which is not a perfect process, particularly at high modulation levels.

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This phase mis-alignment cannot be controlled nor predicted, so it is Extremely difficult for a link planner to know the proportion of time when the link will have the additional +6dB gain, when there will be partial or no gain at all, or when the link may not even be available. For this reason, transmit diversity is not, and has not been employed in the microwave industry, with the exception of the form of frequency diversity.

CONCLUSIONS Space diversity continues to be a critical technique for long challenging microwave paths. In some cases, where an operator can tolerate reduced performance in one direction, or if hybrid diversity can be employed, then only three antennas can be used, reducing site related costs at one end of the link. Transmitter diversity can theoretically deliver a significant boost in system fade margin under perfectly ideal conditions (up to +6dB) but is not predictable and can be just as easily catastrophic to link performance and is thus not an accepted nor recommended approach.

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Data subject to change without notice. 5 AVIAT NETWORKS OCTOBER 2018