Nigeria-based CobraNet is a leading provider of broadband Internet services. Since 2003, the Company has provided the Nigerian Market with a highly reliable Internet service and plans further expansions in this region as well as into West Africa in the near future.

In order to continue its pattern of growth, CobraNet needed to improve the bandwidth efficiency of its satellite link between the Intelsat Teleport in Fuchsstadt, Germany, and its own teleport in Lagos, Nigeria. When dimensioning the initial satellite link, the average and extreme climatic conditions at the transmission sites and the acceptable probability of losing the signal due to rain fade had to be considered. The transmission power and the level of error correction overhead were selected accordingly in order that the signal-to noise-ratio remained above the minimum threshold to guarantee an error-free transmission at all times.




This means that most of the time, when the weather is clear or fair, the signal to noise-ratio is well above the minimum threshold. During this period, the additional margin corresponded to a significant portion of the available data throughput that was wasted with unnecessary error correction overhead. The satellite link uses 18 MHz of satellite capacity on the forward link (towards Nigeria) and 9 MHz on the return and was dimensioned to take rain, back-off, and interference into account.

In the case of CobraNet, due to rain fall which occurs only a few days each year, the service had to use the minimum threshold to guarantee constant, error-free transmissions. Instead of using the highest possible modulation scheme and the lowest possible level of error correction, CobraNet needed to dimension its link for heavy rainfall. A less efficient transmission method (8PSK) had to be used, limiting the throughput to 38 Mbit/sec in forward and 14 Mbit/sec in return. In some cases, very heavy rainfall would cause the link to degrade completely—unacceptable to CobraNet.

In general, link conditions are influenced by rain fade as well as other interference conditions. The latter includes ground noise due to extreme heat, low elevation angles, phase noise, C-band interference with WiMAX and microwave, adjacent satellite interference, pointing losses, satellite gain variations… all can cause important quality changes of the link. The challenge for CobraNet was to find a solution that increased the efficiency of the link and also offered a stable, flexible, and reliable solution.

The ACM Solution
Adaptive Coding and Modulation (ACM) is a technology incorporated into the DVB-S2 standard and provides significant increases in bandwidth efficiency for a number of satellite applications. For IP trunking and satellite backbone links, this represents a doubling of the data throughput, or possible savings of up to 50 percent of the satellite costs.

ACM allows modification of the modulation parameters of a satellite signal on the fly without interrupting the transmission and without losing data. When combined with a measurement of the instantaneous link conditions and a system that automatically adjusts the modulation parameters every few seconds.

ACM allows use of the highest possible modulation schemes and the lowest possible level of error correction at all times. In some instances, the amount of data transmitted in a given satellite segment can be doubled (on average), compared to a fixed modulation system. When the condition of the link gets worse due to rain fade or other interference conditions, the system automatically changes the parameters to avoid loss of signal reception. An ACM system, therefore, always maximizes the throughput of a satellite link, while guaranteeing 100 percent availability of the link reception. This is particularly useful for IP transmission, where higher throughput is always welcome, but where users never want to lose their connection.

The only downside of ACM was the complexity of its implementation: the technology requires special transmission and reception equipment, together with a control system that enables the communication between the downlink and the uplink and manages the variation in the traffic rate. To this date, there had been no commercial ACM system available for point-to-point and point-to-multipoint SCPC/MCPC systems.

For point-to-point applications, the ACM solution is particularly simple. FlexACM consists of a software controller option enabled on the IP modulator and a software client option to be enabled on the IP demodulator. For bidirectional links, a combined ACM controller and client is available on the Newtec modems. The ACM controller automatically communicates with the ACM client on the other end of the transmission chain. This communication can be in-band (if there is a return satellite channel), or via a terrestrial IP connection. If modems are installed at both ends of the satellite link, ACM can be implemented in both directions.

CobraNet upgraded its network from DVB-S to DVB-S2 modems and implemented the ACM solution of Newtec. Newtec’s technology was incorporated into both ends of the link introducing 16APSK and 32APSK capabilities for the modulators and demodulators coupled for the first time with Newtec’s Adaptive Coding and Modulation (ACM) FlexACM technology.

Following the upgrade, CobraNet tested the link for differences to the efficiency readings and found that after the implementation, the satellite link gained 70 percent in throughput for an average of 300 days a year. Plus, the link was able to achieve, in clear skies, a forwards efficiency of 65Mbps with 32APSK and a return efficiency of 32 Mbps with 32APSK. The satellite transmission is now much more robust, guaranteeing 100 percent service availability. The system is able to sustain very heavy rain fades by dynamically changing the modulation parameters and progressively adding more error correction when needed. During severe thunderstorms with rain fades as deep as 20dB, the modulation parameters are changed gradually to QPSK 2/3 and the data throughput drops momentarily to 20Mbits/sec in the forward, but the system does not loose synchronization. This is the only satellite system in the region to maintain its service in the presence of such heavy rain.

This configuration turned out to be particularly simple to operate — all functions are integrated in the modems that talk to each other and automatically configure themselves, negating the need for link budgets or transmission tests.

In other network topologies, such as in point-to-multipoint systems, ACM may also be used: the IP traffic is distributed to several receiving points on a single satellite carrier, and each receiving point provides feedback on the local reception condition. The ACM system at the uplink will automatically and dynamically select modulation parameters for each IP packet, depending on the destination of this packet.

If one of the receiving points is affected by rain fade, the traffic for that point will use more bandwidth, resulting in a reduction of the total available throughput on the forward. The system must then decide how the remaining capacity is distributed among the services and receiving points. This requires a dynamic shaping of the IP services and a QoS management in line with the bandwidth variations caused by the ACM behavior. For these reasons, the ACM controller, the IP encapsulation, and the traffic shaping function are best combined for optimum interworking.

The remote sites of the network are implemented with IP demodulators equipped with the ACM client option (one way system) or with the modems with a combined ACM controller and client options.

The secrets that make these ACM implementations particularly efficient are an accurate measurement of the link margin, which allows the ACM system to work optimally while guaranteeing the link availability. Second, when adding IP traffic enhancement and acceleration technology that takes the ACM behavior of the system into consideration, a full synchronization between the ACM part and the IP software part can be achieved. This ensures the optimum use of the available bandwidth and avoids any traffic congestion in the system.

As Nigeria’s leading Internet provider, it was important to continue to stay ahead of the competition, especially in terms of connection speed and technology. That is exactly what was achieved according to Karim Boulos, Chairman of CobraNet: “Newtec has given us the chance to experience the significant enhancements of the DVB-S2 standard providing major gains in performance and throughput and with the addition of Newtec’s adaptive coding and modulation technology, our customers can be sure that they will receive a stable, flexible and highly reliable connection during all weather conditions.”

About the author
Tom De Baere joined Newtec in 2007 and holds the position of Director of Marketing Communications. He is an engineer with more than 12 years of professional experience in telecommunications, specialized in converged networks.