It is expected in the near future that an individual customer’s need of symmetric bandwidth of Bi-Directional (BiDi) signals is common place with communication systems of optical transport networks, access networks, wireless backhaul networks, and private transmission networks. Network operators have to meet the customer’s need whereas making every effort to save in CAPEX and OPEX. The single wavelength BiDi transmission technology offers a unique solution to meet these apparently conflicting goals at the same time, particularly in access networks such as FTTx and in wireless backhaul networks between a base station and antenna tower or a Remote Radio Head (RRH), compared with the two-wavelength BiDi transmission and the duplex transmission which are currently in use. This article presents pros and cons between competing technologies, operating principles of the single wavelength transmission technology and its applications, and Fiberstore’s BiDi transmission products.
A full duplex transmission technology uses a pair of fibers for a simultaneous communication in both directions. For example, in a P2P upstream signal from the subscriber to the CO. The optical transceivers at two ends of a transmission link can be identical if one wavelength is used for both directions. However, the CAPEX and OPEX are much higher due to the cost for two fibers and their installation compared with other BiDi technologies described below which use a single fiber. This technology can be used in the Wavelength Division Multiplexing (WDM) communication as well as in the P2P communication.
A two-wavelength BiDi transmission system uses one fiber, but two wavelengths for a simultaneous communication in both directions. These wavelengths are separated widely from each other. For example, in a P2P access network, the downstream signal from the CO to a subscriber is at 1550 nm and the upstream signal from a subscriber to the CO is at 1310 nm. The fact that a different signal wavelength must be used in each opposite direction of transmission imposes on the network operators two disadvantages.
A single wavelength BiDi transmission system, on the other hand, uses one fiber and one wavelength for a simultaneous communication in both directions. For example, in a P2P access network , the wavelength can be at 1550nm (or 1310 nm) for both downstream and upstream signals. This reduces CAPEX and OPEX for the network operators since they need to deploy only one kind of optical transceivers at 1550 nm (or 1310 nm). This also guarantees a foolproof installation of transceivers without any confusion since all the transceivers are identical and there is one fiber. In a WDM BiDi system, this is only a viable approach for providing each channel a fully bi-directionally dedicated (or symmetric) bandwidth. This technology may face between upstream and downstream signals a crosstalk and an interferometric beat noise, both coming from reflections at the interface between a transceiver and a channel link fiber with PC (or UPC) type connectors , which may impose a limit on the maximum allowable channel loss, or in other words, the maximum transmission distance. These reflections, however, can be mitigated by using APC type connectors.
Here is a table that summarizes pros and cons of various BiDi transmission technologies. The single wavelength BiDi clearly shows its own unique advantages over two other competing technologies, two-wavelength BiDi and Duplex.
|Single Wavelength BiDi||Two-Wavelength BiDi||Duplex|
|Transmission Distance Limited By||Return Loss||Allowable Channel Loss||Allowable Channel Loss|
|Allowable Channel Loss|
|P2P||Number of Fibers||1||1||2|
|Minimum Number of Transceiver Types||1||2||1|
|Foolproof Installation of Transceiver||Yes||No||No|
 If PC or UPC type connectors are used, the transmission distance may be limited by return loss. If APC type connectors are used, the transmission distance is limited mainly by allowable channel loss.
 There is always a chance that a wrong type of transceiver can be installed if other different type of transceivers is available.
 Each duplex transceiver has two optical receptacles, one for the Tx and the other for the Rx. There is always a chance that the Tx at the CO is connected to the fiber for the upstream signal for the subscriber.
 A TDM for one direction (e.g. upstream) is necessary.
 CAPEX and OPEX are high due to two pairs of optical MUX and DEMUX for a link.
The single wavelength BiDi transmission technology allows over a single fiber a simultaneous communication in both directions at the almost same wavelength. Here is a figure that shows a simple of such transmission system: a P2P optical communication system which is composed of an OutSide Plant (OSP) fiber link over a single fiber as a medium of transmission and identical transceivers at both ends of the fiber link. The signal wavelengths from two transceivers, downstream signal from Tx 1 and upstream signal from Tx 2, are very close to each other, which explains why this approach is named as “a single wavelength BiDi transmission”.
The single wavelength BiDi transmission technology finds its applications broadly in the optical transport networks, access network s such as FTTx networks, wireless backhaul networks, and private transmission networks even though the transmission distance may be limited since most deployed optical transmission networks are equipped with PC type connectors and might have finite reflections. However, it may be still very attractive for P2P and WDM transmission systems with the distance up to 20 km because of its unique advantages over other technologies. Furthermore, the transmission distance can be extended much longer up to 120 km once the reflection is minimized using APC connectors.
In the WDM BiDi transmission application, this single wavelength BiDi transmission is only a viable approach for providing each channel a fully bi-directionally dedicated (or symmetric) bandwidth. The two-wavelength BiDi transmission technology cannot allocate for each channel a fully dedicated bandwidth in both directions simultaneously since all the subscribers must share a common wavelength in one direction, e.g., 1310 nm in upstream with the TDM technology.
The single wavelength BiDi transmission technology is also well poised to support the wireless backhaul networks, such as links between a CO and a base station, a base station and a RRH connected through an optical WDM BiDi system shown in the figure below, a base station and many picocells along the streets in metropolitan areas, and a link between a base station and antennas on a tower.
The single wavelength bi-directional transmission can be very cost-effective for a P2P system with link length up to 20 km and for a WDM system with link length up to 120 km. The transmission distance can be extended much longer once the reflection is minimized using APC connectors. This technology will be only viable solution for WDM BiDi systems when each channel needs a fully bi-directionally dedicated bandwidth. Particularly, it is well poised for the wireless backhaul networks to meet the ever increasing demand of bandwidth and volume of traffic.