Adaptive Coding and Modulation (ACM) in Microwave Communications

26 2018 January

Wireless data transmission is known to be susceptible to varying atmospheric conditions to a large extent. Such weather conditions as rain or snow can significantly reduce the propagation characteristics of the microwaves thus reducing received signal level (RSL) and quality. Another source of signal quality deterioration is the interference. One of the widely used techniques of increasing link reliability in degraded conditions is Adaptive Coding and Modulation (ACM). ACM technology allows operators to achieve high-capacity data transmission over microwave links and maintain the highest link spectral efficiency possible at any given time in any link condition (see Fig.1).

Fig. 1. Modulation adaptation in different weather conditions.

In combination with QoS (Quality of Service) prioritizing it is possible to define which services should be transmitted under any link condition (high-priority services such as voice data) and which services should be adapted whenever the link condition is degraded and the link payload is decreased (low- and mid-priority services such as Internet browsing).

Now let us take a look at the ACM performance concept which allows choosing the modulation state suitable for particular link condition. The choice is made based on signal quality indicator – Mean Square Error (MSE). This parameter in turn is dependent on many other factors like RSL, interference and current modulation state. For simplicity purposes, we will consider RSL as the main factor for ACM decision-making.

RSL drop results in MSE drop and vice versa. Thus decrease of RSL will automatically trigger ACM downshift when RSL value surpasses certain threshold.

There are two types of the threshold values used in ACM algorithm: modulation step-down threshold and modulation step-up threshold. Step-down threshold is considered in case of link performance degradation, when ACM will switch to the lower modulation decreasing the link bitrate. When RSL and MSE improve, step-up threshold will be used to switch to higher modulation.

The ACM switching rate is hitless, meaning that no data will be lost during the change of modulation.

Fig. 2. ACM modulation switching algorithm.

The ACM prevents link from losing the synchronization by switching radio to lower modulation before RSL reaches the sensitivity threshold (for 10-6 BER) of current modulation. As the signal improves, ACM ensures to keep some reserve above the sensitivity threshold while shifting the modulation up. Thus we obtain step-up/step-down reserves being the margins between radio sensitivity threshold and step-up/step-down thresholds accordingly.

This concept is presented graphically in Figure 2. X-axis represents RSL value. The modulation states are shown on the Y-axis. Following the ACM switching path it is possible to see that modulation down-shifts at the step-down threshold to lower state before sensitivity threshold is reached. Conversely, the modulation is switched up when RSL arrives at step-up threshold point. Gap between step-up and step-down thresholds is called ACM hysteresis. It prevents algorithm from arbitrary shifting the modulation up and down as RSL fluctuates.

The idea of the effective threshold (average value of step-up and step-down) is used in link budget calculation applications (such as Pathloss software) for the ACM calculations. The resulting availability of any particular modulation state in ACM is lower comparing to fixed modulation performance. However, this gives us a benefit of being secured from complete synchronization loss. Thus overall link availability improves.

As a result, different link planning recommendations should be applied to the ACM calculation results with the relaxed values of fade margin and annual availability. SAF Tehnika recommendations regarding the ACM path calculations are the following:

Mode

ACM

Stable*

Best effort

Fade Margin (dB)

14

10

Availability (%)

99.99

99.95

Downtime per year (hh:mm)

0:52

4:21

*default recommendation for ACM

ACM engine has become a must-have technique of improving the link performance for the majority of Internet service providers. A variable modulation is basically considered acceptable in IP networks compared to legacy PDH and SDH applications demanding fixed capacity. It allows to effectively adapting the radio performance to any weather conditions. This ensures minimizing of the link total outage time and keeping high-priority traffic up and running at expense of worsening the sensitivity of particular modulation.

The more thorough description of availability requirements and applying different SLA (Service Level Agreement) standards to the ACM calculations will be covered in the upcoming second part of this article.

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