Modulation Formats: Overcoming CD at 40 Gbit/s

One of the primary disadvantages with increasing speed to 40 Gbit/s is that this transmission rate has a very low CD tolerance, which can lead to non-linear effects (e.g., four-wave mixing, self-phase and cross-phase modulation, etc.). Non-linear effects occur when the network exhibits both low CD values and high peak power.

As mentioned in our previous article, non-return-to-zero (NRZ) modulation is not well-suited for 40 Gbit/s due to its inherent 6 dB penalty; i.e., the pulse is four times shorter, carries four times less power, and tolerates 16 times less CD. One way to deal with the 6 dB penalty would be to use return-to-zero (RZ) modulation. The low duty cycle allows for a slightly higher CD tolerance, thus enabling the effective length of a “0” bit value to be longer:

However, a low duty cycle also decreases average power, which affects the optical signal-to-noise ratio (OSNR), on top of the 6 dB loss mentioned above. And, increasing the peak power is not an option, as it would amplify the risk of non-linear effects.

With the standard RZ scheme, there is always that same dilemma. A low duty cycle improves CD tolerance, but generates a bad OSNR, whereas a high duty cycle simply does not solve the CD problem. Conclusion: the standard RZ scheme is not the answer.

Alternative Solutions

The key is to find a scheme that offers both a high duty cycle and CD resistance. One solution is to change the phase of the light content (i.e., applying positive or negative voltage changes the phase response from 0 to π). For example, the second graph below shows a phase change in each bit.

would result in this:

Although there is light in the “0” time slot, it is not of the proper phase content, so it is discarded and the “0” remains intact. This allows the duty cycle to increase to 66%, instead of the 33% that is set by the traditional RZ method, which helps overcome some of the power budget issues. Changing the phase of the light content can be achieved in several ways by using various modulator formats such as carrier-suppressed (CS-RZ), duo-binary or differential phase-shift keying (DPSK).

Carrier-Suppressed (CS-RZ)

CS-RZ is a modulation format in which each time slot has a phase change, as shown in this sequence of “0-1-0-1-1-1”:

This phase change allows for better CD tolerance and a suitable duty cycle. However, the pulse in the spectral domain widens, which limits DWDM to 100 GHz spacing, as shown here between two 10 Gbit/s NRZ signals:

CS-RZ offers an easy and inexpensive solution, but due to the limitations in DWDM spacing, it is not suitable for high-density routes or long-haul transmissions.

Duo-Binary

Duo-binary modulation is a format in which the content of the phase, rather than constantly alternating, is dependent on the amount of “0s” that precede the “1” bit (e.g., “1” has a positive phase if the amount of “0s” preceding it is odd, and it has a negative phase if the amount of “0s” preceding it is even). Less switching back and forth means that the pulse is less distorted, as seen on this duo-binary 40 Gbit/s signal surrounded by two 10 Gbit/s NRZ signals:

Instead of being four times larger, the pulse is only approximately two-and-a-half times larger. This allows for a 50 GHz DWDM signal, while providing an increase in CD tolerance of two-and-a-half times.

The duo-binary scheme therefore offers a cost-effective solution that is somewhat more resistant to dispersion. Nonetheless, it remains limited to short-haul transmissions.

Differential Phase-Shift Keying (DPSK)

DPSK is the most promising modulation format for 40 Gbit/s transmission. In DPSK, the “1” is represented by a phase shift, but more importantly, the “0” has light as well (i.e., the laser is always ON—only the phase determines the value of the bit). Therefore, not modulating the laser greatly reduces chirp, significantly limiting CD problems. Since both “1” and “0” have light, the average power is 3 dB higher, which solves most of the power budget problems.

Here is an example with the same sequence as described earlier:

Although its implementation is complex and expensive, DPSK is the modulation format with the most overall potential due to its degree of sensitivity, its resistance to dispersion effects and its suitability for long-haul transmission,

In the next edition, we will examine agile compensation, pre-chirp and other ways to deal with CD.