Traffic demand continues to increase, but revenue doesn’t. That puts a squeeze on operators.
Demand means that operators must invest in new transport equipment and technology innovation. But this spending must be carefully planned so that the business is still profitable.
The use of higher channel speeds is one of the most common solutions to grow capacity. Transporting more data traffic per channel over the same network infrastructure leads to a lower cost per bit. But capacity growth and performance must go hand in hand to achieve the desired cost-efficiency. Let’s see why.
The capacity and reach trade-off
Coherent-based technology evolution is enabling a tremendous boost of transport capacities. It also offers freedom of choice among different modulation formats and baud rate options. Why is that important?
By choosing a higher order coherent modulation format (i.e., a higher number of bits per symbol) carriers can increase channel speed. At a given symbol or baud rate, the higher number of bits per symbol provides a higher transmission capacity (a higher number of bits per second). But it also decreases the channel’s reach. As the number of constellation points increases, they get closer. And the channel is more sensitive to noise and transmission impairments. This is usually not a problem on short routes. However, on long routes it may force the use of expensive regeneration. And that doesn’t lead to a lower cost per bit per kilometer.
Coherent-based technology evolution is enabling a tremendous boost of transport capacities.
The benefits of using high baud rates
Network operators can also increase channel data rates by choosing a higher baud rate. This means transmitting a higher number of symbols per second. Contrary to modulation order, increasing the baud rate doesn’t have a significant impact on the wavelength reach. But it increases the spectral width of the channel. That might be incompatible with the filters’ passbands. The good news is that operators built most modern long-haul networks using flexgrid ROADMs. With flexgrid technology operators have control over spectrum allocation. This enables the transport of channels with larger bandwidth requirements.
Optical terminals with ultra-high baud rates and flexgrid ROADMs are the most efficient solution to deploy high-capacity long-haul networks – for example, ADVA’s FSP 3000 TeraFlex™ CoreChannel™ has 140GBd sub-carrier technology. It enables the transport of 800Gbit/s over 1600km without regeneration. This means doubling the reach compared to 95Gbd-based solutions.
Variable baud rate and fractional QAM: a winning combination
What’s the best solution to grow capacity at minimum cost? At this point, you might answer that there’s no silver bullet. You would use high-order modulation at short distances and higher baud rates on long-haul routes. I believe that the use of high baud rates always provides the most cost-efficient solution, even in short-haul networks. By using high baud rates, the network capacity increases with a lower number of channels. That also means fewer optical terminals and interfaces. This reduces capital as well as operational expenses and leads to the lowest cost per bit.
Only bandwidth limitations or grid restrictions would make it necessary to use lower baud rates. If we go to lower baud rates, then we should use flexible coherent interfaces with multiple baud rate setting options in conjunction with fractional QAM technology. That combination offers the most cost-efficient solution. Fractional QAM provides the closest adaption to filters’ passbands and facilitates the use of higher baud rates. This gives network operators a way to boost capacity over legacy infrastructure – no upgrade required!
Flexibility is required
Economical bandwidth growth demands ultra-flexible and software-defined coherent optical terminals. And these terminals must support high baud rates and fractional QAM. With that set of features we can achieve the desired capacity growth at lowest cost and with maximum operational simplicity. Network operators can choose the optimum configuration for each channel path over any type of network infrastructure. That means they can address different distances and data rates – all while minimizing the variety of equipment and spares. That’s how we can enable profitable growth!