**5. Ultra high performance FEC schemes for long-haul network**

For long-haul optical transport networks (OTN), because the performance loss mainly results from long distance transmission, the requirement on coding gain is very strict. This requirement even gets more and more strict when optical backbone networks enter 100Gbps era. Based on OIF whitepaper, the new FEC schemes applied in 100G long-haul systems should achieve waterfall performance at very low BER region. Meanwhile the other requirements for OTN FEC such as capable of achieving very high speed and having very low error floor still remain. Therefore, 100Gbps era puts more challenges on FEC schemes. In this section, some recent FEC schemes targeted to 100Gbps applications are introduced.

**Figure 13.** The hard-decision product BCH scheme.

## **5.1. Hard-decision BCH product codes**

One candidate for 100Gbps application is BCH-based binary product codes such as the one presented in [25]. The component BCH (992, 960) and (987, 956) codes are constructed carefully over *GF*(210) for hardware amenity, which have the 3-error correction capability, therefore its decoder design can be developed based on simple PGZ algorithm in [4]. The simulation results show the performance of this FEC scheme can be very close to the Shannon limit.

**Figure 14.** Decoding performance of product BCH codes based on maximum 7 iterations.

#### **5.2. Some other soft-decision based concatenated codes**

284 Optical Communication

are introduced.

correcting capability, while UHD decoder can be still effective. In short, the UHD design provides an efficient and attractive unified solution for multi-mode RS decoding in optical

applications that demands enhanced error correcting capability.

**5. Ultra high performance FEC schemes for long-haul network** 

For long-haul optical transport networks (OTN), because the performance loss mainly results from long distance transmission, the requirement on coding gain is very strict. This requirement even gets more and more strict when optical backbone networks enter 100Gbps era. Based on OIF whitepaper, the new FEC schemes applied in 100G long-haul systems should achieve waterfall performance at very low BER region. Meanwhile the other requirements for OTN FEC such as capable of achieving very high speed and having very low error floor still remain. Therefore, 100Gbps era puts more challenges on FEC schemes. In this section, some recent FEC schemes targeted to 100Gbps applications

One candidate for 100Gbps application is BCH-based binary product codes such as the one presented in [25]. The component BCH (992, 960) and (987, 956) codes are constructed carefully over *GF*(210) for hardware amenity, which have the 3-error correction capability,

**Figure 12.** The timing charts for RiBC architecture.

**Figure 13.** The hard-decision product BCH scheme.

**5.1. Hard-decision BCH product codes** 

The above binary product scheme is based on hard-decision. If soft information is available in the system, soft-decision decoding approach can work with the product codes to enhance the overall decoding performance. Fig. 15 illustrates a LDPC code concatenated with BCHbased product code for long-haul network systems in [26]. In this scheme, LDPC code is used as inner code and BCH-based product code is used as outer code. Some other softdecision based concatenated FEC scheme such as RS code concatenated with LDPC coding system in [27] can also provide significant coding gain for targeted ultra high-speed optical communication.

**Figure 15.** Product BCH-LDPC concatenated scheme in [26].

### **6. Conclusion**

With the evolution of optical network, the employed FEC scheme has been developed in several generations. The requirement on high data rata and large coding gain is always challenging the design for efficient FEC decoder. In this chapter, targeted to different types of optical transmission networks, ranging from local Ethernet to long-haul backbone system, different FEC solutions with efficient VLSI implementations are discussed. For shortdistance networks, two kinds of area-efficient high-speed RS decoders are analyzed for the scenario. For mediate distance networks, which require some tradeoff between decoding performance and hardware efficiency, the introduced RS burst-error decoder can be employed to meet such requirement. For long-haul systems, which have stringent requirement on decoding performance, some candidate FEC schemes targeted to the future 100Gbps era are discussed. In summary, these various FEC architectures and schemes are good candidates for their specific targeted optical transmission applications.
