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The traditional WDM-based optical network assigns spectrum resources to optical paths without considering the appropriate modulation technique, which leads to an inefficient utilization of the spectrum. However, the OFDM-based EON allocates optical paths with consideration of adaptive modulation and bit rate to further improve the spectrum efficiency. In the modulation-based spectrum allocation scheme [29,115], the necessary minimum spectral resource is adaptively allocated to an optical path. The adaptation considers the physical conditions while ensuring a constant data rate. The modulation-based spectrum allocation scheme improves the spectrum efficiency, as the allocated spectral bandwidth can be reduced for shorter paths by increasing the number of modulated bits per symbol.

In this direction, Jinno et al. [29] have presented a distance-adaptive spectrum allocation scheme that adopts a high-level modulation format for long distance paths, and a low-level modulation format to shorter paths. As the optical signal- to-noise ratio (OSNR) tolerance of 64-QAM is lower than that of QPSK, it suits shorter distance lightpaths as shown in Fig. 5.2.

Modulation level versus transmission distance

Figure 5.2: Modulation level versus transmission distance.

The modulation based spectrum allocation schemes can be classified into two categories, namely—(i) offline modulation based spectrum allocation, and (ii) online modulation based spectrum allocation, and are discussed below.

Offline Modulation Based Spectrum Allocation

Christodoulopoulos et al. [31] have presented an offline modulation based spectrum allocation scheme, where a mapping function is provided as input to the problem. In their scheme, each demand is mapped to a modulation level according to the requested data rate and the distance of the end-to-end path. Initially, they presented a path-based ILP formulation for their scheme, and then decomposed the problem into two sub-problems, namely (i) routing and modulation level (RML), and (ii) spectrum allocation. They solved the sub-problems sequentially using ILPs. Finally, a sequential algorithm was presented to serve connections one-by-one, and to solve the planning problem sequentially.

Online Modulation Based Spectrum Allocation

Most of the studies on online modulation based spectrum allocation [116-119] have introduced heuristic algorithms, which deal with randomly arriving connection requests. Initially, these algorithms compute a number of fixed-alternate paths for each source-destination pair, and arrange them in decreasing order of their end-to-end path length. In the second step, a spectrum allocation policy is used to allocate a lightpath to each connection request by considering alternate path routing and modulation.

Recent studies [32,116,117] on modulation based spectrum allocation claim that this type of spectrum allocation scheme increases the spectral utilization by approximately 9%-60% compared to fixed-modulation based spectrum allocation in the EON. Fixed-modulation based spectrum allocation schemes do not consider the most appropriate modulation technique for different connection requests according to their lightpath distance. Typically, they select, conservatively, one modulation technique for all connection requests regardless of their lightpath distance. As an example, a fixed-modulation based spectrum allocation scheme adopts the BPSK modulation format for all connection requests regardless of their lightpath distance. As a result, this type of spectrum allocation schemes does not utilize the spectrum efficiently. On the other hand, modulation-based spectrum allocation schemes determine the modulation technique that best suits each lightpath distance. As an example, a modulation based spectrum allocation scheme adopts BPSK for long distance lightpaths, and 16-QAM for shorter distance lightpaths. This minimizes the number of spectrum slots that must be assigned, which yields better utilization of spectrum resources compared to fixed- modulation based spectrum allocation schemes.

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