In wireless communication systems, an antenna unit (RRH: Remote Radio Head) and a signal processing unit (BBU: Baseband Unit) of a wireless base station may be separate. The RRH and the BBU communicate via a mobile fronthaul.
FIG. 12 is a diagram showing an example of a configuration of a mobile fronthaul. In the mobile fronthaul, communication of a wavelength division multiplex (WDM) scheme is used. In the WDM scheme, a wavelength of light used in an uplink is different from a wavelength of light used in a downlink. The uplink is a link from a lower-order device that is a mobile wireless terminal or the like to a higher-order device that is a BBU or the like. The downlink is a link from the higher-order device to the lower-order device. By using the WDM scheme, the transmission system simultaneously transmits an uplink signal and a downlink signal via a single optical fiber (see Non-Patent Document 1).
The transmission system may relay a signal of a communication system based on a time division duplex (TDD) scheme according to a frequency division duplex (FDD) scheme. In the FDD scheme, different frequency bands are used for the uplink and the downlink. In the TDD scheme, the same frequency band is used for the uplink and the downlink. In the TDD scheme, the uplink signal and the downlink signal are switched on a time axis.
FIG. 13 is a diagram showing a configuration of a TDD frame of Long Term Evolution (LTE). In LTE, the TDD frame includes 10 TDD subframes. There are seven types of combination of allocation of a downlink subframe, an uplink subframe, and a special subframe with respect to the TDD subframes. In LTE, the time length of the TDD subframe is 1 millisecond. In the TDD scheme, allocation to each TDD subframe in the TDD frame is determined in accordance with uplink and downlink traffic. A ratio between uplink and downlink communication times in the TDD frame can be flexibly changed because allocation to the TDD subframe is determined in accordance with the traffic.
The special subframe (S) includes a downlink pilot time slot (DwPTS), a guard period (GP), and an uplink pilot time slot (UpPTS) (see Non-Patent Document 2). DwPTS is a time slot used for transmission of a downlink control signal. UpPTS is a time slot used for transmission of an uplink control signal. GP is a protection time for switching between an uplink signal and a downlink signal.
FIG. 14 is a diagram showing an example of bandwidth utilization of a wireless section and an optical section in a mobile fronthaul when a wireless base station of a TDD scheme is accommodated. In the wireless section, data signals are bidirectionally transmitted by performing switching between the transmission of the downlink signal and the transmission of the uplink signal on the time axis. Also, in the optical section, data signals are bidirectionally transmitted by alternately performing transmission of the uplink signal and transmission of the downlink signal on the time axis in accordance with the switching in the wireless section. Therefore, in an optical section adopting the WDM scheme, there may occur a period during which no signal is transmitted according to the TDD scheme (hereinafter referred to as a “TDD non-transmission period”) on the time axis. That is, the TDD non-transmission period is a signal transmission stop period of the wireless communication system. The TDD non-transmission period occurs in both the uplink and the downlink.
There is a transmission system configured to superimpose a signal of a mobile system and a signal of another communication system which is Fiber To The Home (FFTH) or the like during the TDD non-transmission period (see Non-Patent Document 3).
FIG. 15 is a diagram showing an example of a network system using a passive optical network (PON) in an optical section. An optical line terminal (OLT) (an optical subscriber line termination device) is an optical device configured to estimate the TDD non-transmission period on the basis of a signal transmitted from a wireless communication system via an optical fiber.
FIG. 16 is a block diagram showing an example of a configuration of the OLT. In an initial state, the OLT allocates a bandwidth to the uplink signal of the wireless communication system in a fixed manner. Hereinafter, allocating a bandwidth in a fixed manner is called “fixed bandwidth allocation (FBA).” In the initial state, the OLT stops the transmission of a signal of another communication system. A TDD frame information estimation unit estimates a configuration of a TDD subframe. That is, the TDD frame information estimation unit determines whether the TDD subframe is a TDD subframe allocated to an uplink or a downlink on the basis of an estimation result.
The TDD frame information estimation unit estimates the TDD non-transmission period on the basis of a result of estimating the configuration of the TDD subframe and a result of estimating a timing of the TDD subframe. In order to estimate the TDD non-transmission period, the TDD frame information estimation unit needs to acquire a signal of a certain amount of traffic or more which is required (hereinafter referred to as a “required amount of traffic”).
The traffic monitoring unit adjusts a length of time for which the traffic of the uplink signal of the wireless communication system is monitored so that the TDD frame information estimation unit can acquire a signal having an amount of traffic greater than or equal to the required amount of traffic. Also, the traffic monitoring unit may monitor the uplink signal or the downlink signal.
The TDD frame information estimation unit notifies a timing calculation unit of TDD frame pattern information. That is, the TDD frame information estimation unit transmits the result of estimating the configuration of the TDD subframe and information indicating the timing of the TDD subframe to the timing calculation unit. The timing calculation unit calculates a timing capable of being used by the wireless communication system for signal transmission (hereinafter referred to as a “usable timing”) on the basis of the TDD frame pattern information. The timing calculation unit notifies a timing indication unit of the usable timing.
The timing indication unit determines a timing to be used by the wireless communication system for signal transmission (hereinafter referred to as a “use timing”) on the basis of the usable timing. The timing indication unit notifies a downlink layer 2 switch (L2SW) of the use timing. The timing indication unit transmits the use timing to a bandwidth allocation method selection unit so as to switch a method in which the bandwidth allocation method selection unit allocates the bandwidth to the uplink signal.
In a state in which the TDD non-transmission period is estimated correctly (hereinafter referred to as a “normal state”), the bandwidth allocation method selection unit allocates a bandwidth to an uplink signal of the wireless communication system according to FBA during a period in which a signal is transmitted according to the TDD scheme on the time axis (hereinafter referred to as a “TDD transmission period”). In the normal state, the bandwidth allocation method selection unit allocates a bandwidth to a signal of another communication system according to FBA or dynamic bandwidth allocation (DBA) during the TDD non-transmission period. The bandwidth allocation method selection unit transmits information indicating a method of allocating a bandwidth (hereinafter referred to as “bandwidth allocation method information”) to the bandwidth allocation unit. On the basis of the bandwidth allocation method information, the bandwidth allocation unit allocates an uplink signal of the wireless communication system to the bandwidth.