# Planning of Optical Connections in 5G Packet-Optical xHaul Access Network
## 2 Related Works  
Optimization of C-RANs connected using optical fiber networks has been frequently addressed jointly with the problem of placement of BBU/DU/CU processing resources. In Reference [14; 15], the authors studied a BBU placement problem in a C-RAN connected using a WDM optical network. The optimization problem was formulated as an ILP problem with the objective to minimize the total network cost represented by either the number of active BBU sites or the number of fibers used to transport the traffic in the network. In the study, the underlying optical network was given, and a generic WDM system was considered, in which transmission distance of optical paths (lightpaths) was not constrained by a physical-layer transmission model. The authors of Reference [18] proposed an ILP formulation for the problem of dimensioning of BBU processing and optical transponder resources in a CPRI-based C-RAN connected using an active WDM optical network equipped with sliceable bandwidth-variable transponders. In that work, dedicated point-to-point connections were assumed, without traffic aggregation, and there were not any transmission reach-related constraints. The problem of designing dense WDM (DWDM) rings in metro and access segments of a survivable 5G transport network was studied in Reference [19]. The authors proposed different schemes for survivable 5G transport and made use of a heuristic approach for planning and dimensioning of ring-based fiber connections. In Reference [20], the BBU location problem with planning of survivable (i.e., primary and backup) lightpath connections in a 5G fronthaul network was addressed. In that work, a generic WDM optical network without any constraints on transmission distance was assumed. The authors of Reference [21] focused on ILP modeling of the problem of DU and CU placement with lightpath provisioning in a ring-based WDM metro/aggregation network. Similar as in other aforementioned works, a generic WDM system was assumed, and transmission reach was not modeled. A literature survey on resource allocation-related problems and solutions in centralized RANs can be found [22]. Different solutions have been considered for passive WDM networks. Coarse wavelength division multiplexing (CWDM) has been widely used in local and metropolitan networks. In CWDM, the channel spacing equals 20 nm, which enables the use of 18 channels in the wavelength range from 1271 nm to 1611 nm. Due to the wide range of dispersion, CWDM has limited use in systems with an extreme bit rate of 100 Gb/s per wavelength. Zero dispersion of the G.652D fiber [23] lies in the range of 1312 \(\pm\) 12 nm. For this reason, in the IEEE 802.3 cu standard [24], the wavelength range is limited to 4 channels: 1271, 1291, 1311, and 1331 nm. A two times denser spacing, i.e., 10 nm, was proposed in the MWDM system [13]--12 channels were located in the range from 1267.5 to 1374.5 nm. In addition, 12-channel LAN-WDM system (LWDM) with an 800 GHz inter-channel spacing is used, which, in the O transmission window, corresponds to a spacing of about 4.5 nm [13] and the center of channels passband equals, respectively, 1269.23, 1273.55, 1277.89, 1282.26, 1286.66, 1291.10, 1295.56, 1300.05, 1304.58, 1309.14, 1313.73, and 1318.35 nm. Recently, an 8-channel system with a 400 GHz channel spacing, i.e., about 2.25 nm in the O transmission window (called nWDM--narrow WDM) has been proposed [16], in which the center of channels passband equals, respectively, 1295.56, 1297.80, 1300.05, 1302.31, 1304.58, 1306.85, 1309.14, and 1311.43 nm. This system, under the trade name of flexiHaul xWave 400G, offers a 20 km transmission distance and a 17 dB power budget. Four-level amplitude modulation (PAM-4) is used, which enables two bits of information to be encoded in one code symbol. Symbols are transferred at 53.125 Gbaud. Forward error correction RS (544,514) is applied to allow error-free transmission for the input bit error rate less than \(2.4\times 10^{-4}\)[25].