JP7463651B2 - IFoF transmission system, base station device, signal processing device, and transmission method - Google Patents

Description

The present invention relates to technology applied to analog RoF (Radio over Fiber) systems that convert analog electrical signals into optical signals and transmit them using optical fibers.

Analog RoF systems have been known for some time. For example, Patent Document 1 discloses a "Mobile fronthaul configuration" consisting of cascaded IFoF lines. In this "Mobile fronthaul configuration," IF radio signals for multiple antenna sites are transmitted at the same wavelength through the same fiber in a certain fiber transmission section. In the technology described in Patent Document 1, one optical access fiber is used from the "Central office (CO)" to the relay station, and different optical fibers are used from the relay station to each antenna site. Here, from the CO to the relay station, IF signals for all antenna sites must be multiplexed on the frequency axis for large-volume transmission, and signal quality degradation is likely to occur. In particular, high-quality transmission is difficult in this section. In Patent Document 1, "Digital signal processing (DSP)" is used to separate and convert IF signals at each antenna site, improving the quality of transmission characteristics and simplifying the RF component configuration.

Non-Patent Document 1 also discloses "Time division multiplexing passive optical network (TDM-PON)". In this technology, a single optical access fiber is shared to accommodate multiple TDD wireless base stations. In this TDD wireless base station, data flows in only one direction, either uplink or downlink, which reduces the efficiency of bandwidth usage in optical transmission. For this reason, the TDD wireless base station is used as a primer system, and data from another second system is transmitted during non-communication times, thereby improving the efficiency of bandwidth usage in optical transmission.

JP 2019-212983 A

ECOC2016 998-1000

However, in "Intermediate frequency over fiber (IFoF) transmission systems" that use the "Time division duplex (TDD)" method to transmit large volumes of wireless signals from wireless base stations, there is an issue of significant degradation in the quality of IF-band wireless signals. In other words, with conventional technology, it is difficult to transmit high-level QAM signals, and it is also difficult to ensure a high SNR.

The present invention has been made in consideration of these circumstances, and aims to provide an IFoF transmission system, base station device, signal processing device, and transmission method that can effectively utilize the upstream transmission timing of a wireless base station in optical analog transmission.

(1) In order to achieve the above object, the present invention employs the following means. That is, the IFoF transmission system of the present invention is an IFoF (IF-over-Fiber) transmission system that transmits radio waves emitted from an antenna as IF (Intermediate Frequency) signals over optical fiber, and is characterized in that it comprises an accommodation station that assigns a downstream IF signal to be emitted from the antenna during a downstream transmission period of TDD (Time Division Duplex) during an upstream transmission period of TDD, and transmits the downstream IF signal over optical fiber, and an antenna site that buffers the downstream IF signal received via optical fiber and controls the transmission timing to emit the buffered downstream IF signal from an antenna, and that the downstream IF signal from the accommodation station to the antenna site is transmitted during the upstream transmission period and downstream transmission period of TDD.

This configuration makes it possible to effectively utilize the upstream transmission timing of wireless base stations in optical analog transmission. As a result, the number of IF channels for IFoF transmission can be reduced, enabling IF transmission at a high modulation degree (i.e., high SNR), and making it possible to suppress deterioration of wireless signal quality. Furthermore, if the IF channel arrangement for transmission at the uplink and downlink communication timing of a TDD base station is made the same, it is possible to reduce the bandwidth required for RF and optical components, and costs can also be reduced.

(2) The base station device of the present invention is a base station device applied to a host station of the IFoF transmission system described in (1) above, and is characterized in that it includes a waveform division unit that divides a downstream IF signal to be transmitted into a plurality of downstream IF signals so that the time required to transmit the downstream IF signal to be transmitted fits the upstream transmission period, and outputs the divided downstream IF signals by aligning the transmission timing of each of the divided downstream IF signals, and transmits each of the divided downstream IF signals during the upstream transmission period.

This configuration makes it possible to transmit downstream IF signals in accordance with the upstream transmission period. This makes it possible to effectively utilize the upstream transmission timing of wireless base stations in optical analog transmission.

(3) In addition, in the base station device of the present invention, the waveform division unit is characterized in that it determines the number of divisions of the downstream IF signal to be transmitted during the upstream transmission period according to the ratio between the upstream transmission period and the downstream transmission period.

This configuration makes it possible to transmit downstream IF signals in accordance with the upstream transmission period. This makes it possible to effectively utilize the upstream transmission timing of wireless base stations in optical analog transmission.

(4) The signal processing device of the present invention is a signal processing device applied to an antenna site of the IFoF transmission system described in (1) above, and is characterized in that it comprises a buffering unit that restores each downstream IF signal, which has been divided by the receiving station, has its transmission timing adjusted, and is received during an upstream transmission period, to the downstream IF signal before division, and buffers the restored downstream IF signal, and a timing controller that instructs the buffering unit to output the buffered downstream IF signal based on a control signal obtained from a base station device.

This configuration makes it possible to wirelessly transmit downstream IF signals at appropriate timing. This makes it possible to effectively utilize the upstream transmission timing of wireless base stations in optical analog transmission.

(5) The IFoF transmission system of the present invention is an IFoF (IF-over-Fiber) transmission system that transmits radio waves emitted from an antenna as IF (Intermediate Frequency) signals over optical fiber, and includes an accommodation station that assigns a downstream IF signal to be emitted from the antenna during a downstream transmission period of TDD (Time Division Duplex) during an upstream transmission period of TDD and transmits the downstream IF signal over optical fiber, and a relay station that buffers the downstream IF signal received via optical fiber and controls the transmission timing to output the buffered downstream IF signal to an antenna site, and is characterized in that the downstream IF signal from the accommodation station to the antenna site is transmitted during the upstream transmission period and downstream transmission period of TDD.

This configuration makes it possible to effectively utilize the upstream transmission timing of wireless base stations in optical analog transmission. As a result, the number of IF channels for IFoF transmission can be reduced, enabling IF transmission at a high modulation degree (i.e., high SNR), and making it possible to suppress deterioration of wireless signal quality. Furthermore, if the IF channel arrangement for transmission at the uplink and downlink communication timing of a TDD base station is made the same, it is possible to reduce the bandwidth required for RF and optical components, and costs can also be reduced.

(6) The transmission method of the present invention is a transmission method for an IFoF (IF-over Fiber) transmission system that transmits radio waves emitted from an antenna as IF (Intermediate Frequency) signals over optical fiber, and includes the steps of: in a accommodating station, allocating, to a TDD (Time Division Duplex) upstream transmission period, a downstream IF signal to be emitted from the antenna in a subsequent TDD downstream transmission period, and transmitting the downstream IF signal over optical fiber; and in an antenna site, buffering the downstream IF signal received over optical fiber, controlling the transmission timing, and emitting the buffered downstream IF signal from an antenna, characterized in that the downstream IF signal from the accommodating station to the antenna site is transmitted during the TDD upstream transmission period and downstream transmission period.

This configuration makes it possible to effectively utilize the upstream transmission timing of wireless base stations in optical analog transmission. As a result, the number of IF channels for IFoF transmission can be reduced, enabling IF transmission at a high modulation degree (i.e., high SNR), and making it possible to suppress deterioration of wireless signal quality. Furthermore, if the IF channel arrangement for transmission at the uplink and downlink communication timing of a TDD base station is made the same, it is possible to reduce the bandwidth required for RF and optical components, and costs can also be reduced.

According to the present invention, it is possible to effectively utilize the upstream transmission timing of a wireless base station in optical analog transmission. As a result, it is possible to reduce the number of IF channels for IFoF transmission, making it possible to perform IF transmission at a high modulation degree (i.e., a high SNR), and to suppress deterioration of wireless signal quality. In addition, if the IF channel arrangement for transmission at the uplink and downlink communication timing of a TDD base station is made the same, it is possible to reduce the bandwidth required for RF and optical components, and costs can also be reduced.

1 is a diagram showing a schematic configuration of an IFoF transmission system according to an embodiment of the present invention. FIG. 11 is a diagram showing a schematic configuration of another example of an IFoF transmission system according to the present embodiment. 1 is a diagram showing an IF signal transmitted through IFoF on a time axis and a frequency axis. 1 is a diagram showing an IF signal transmitted through IFoF on a time axis and a frequency axis. FIG. 2 is a diagram showing a schematic configuration of a TDD base station according to the present embodiment. FIG. 13 is a diagram showing an example of an output signal (1). FIG. 13 is a diagram showing an example of an output signal (1). FIG. 13 is a diagram showing an example of an output signal (2). FIG. 13 is a diagram showing an example of an output signal (2). 1 is a diagram showing a schematic configuration of Wave form div. 117. 1 is a diagram showing a schematic configuration of Wave form div. 117. 2 is a diagram showing a schematic configuration of a signal processing device 23 of an antenna site 20.

The inventors noticed that in an IFoF transmission system, downstream data communication via optical fiber is not performed during upstream data communication by a TDD base station device, and discovered that during this upstream transmission period, a downstream IF signal to be emitted at a later timing can be transmitted via IFoF, buffered at the antenna site, and output at the appropriate TDD downstream timing, leading to the present invention.

That is, the IFoF transmission system of the present invention is an IFoF (IF-over-Fiber) transmission system that transmits radio waves emitted from an antenna as IF (Intermediate Frequency) signals over optical fiber, and is characterized in that it comprises an accommodation station that assigns a downstream IF signal to be emitted from the antenna during a downstream transmission period of TDD (Time Division Duplex) during an upstream transmission period of TDD, and transmits the downstream IF signal over optical fiber, and an antenna site that buffers the downstream IF signal received via optical fiber and controls the transmission timing to emit the buffered downstream IF signal from an antenna, and that the downstream IF signal from the accommodation station to the antenna site is transmitted during the upstream transmission period and downstream transmission period of TDD.

As a result, the inventors have made it possible to effectively utilize the upstream transmission timing of wireless base stations in optical analog transmission. Below, an embodiment of the present invention will be specifically described with reference to the drawings.

Figure 1A is a diagram showing a schematic configuration of an IFoF transmission system according to this embodiment. The IFoF transmission system 1 according to this embodiment is configured by connecting an accommodation station 10 and an antenna site 20 by a wideband IFoF 30. During TDD upstream data transmission, a TDD base station 11 transmits a downstream IF signal that is to be emitted from an antenna at a later timing via fiber. This makes it possible to reduce the number and bandwidth of IF signals transmitted via downstream IFoF at a certain time, and enables IFoF transmission at a high modulation degree (high input power) for IF signals with a high degree of multi-value.

Conventionally, in a TDD wireless base station, downstream data communication is not performed during upstream data communication, but in the IFoF transmission system 1 according to this embodiment, downstream signals are buffered at the antenna site 20, and radio waves are emitted at the timing of TDD downstream data communication. In this embodiment, it is effective to transmit a small number of downstream IF signals with high multi-levels at a high modulation degree by IFoF during TDD upstream data communication.

During TDD uplink data communication, the TDD base station 11 generates a wireless IF signal to be emitted from the antenna at a later timing for downlink data communication, and transmits it to the driver circuit 13 and E/O device 15 as IFoF transmission devices. Because the time available for downlink data communication at the antenna site 20 is limited, it is necessary to shorten the transmission time at the actual downlink transmission timing by the time during which the downlink IF signal was generated during TDD uplink data communication and transmitted to the driver circuit 13 and E/O device 15 as IFoF transmission devices. When IFoF transmission is performed only at the timing of TDD uplink data communication, it is necessary to transmit the signal divided into multiple IF channels to make up for the shortened time. In addition to the downlink IF signal, a control signal for notifying the actual timing for antenna emission is also transmitted using a different IF band, etc.

After detecting the control signal, the antenna site 20 can issue a command to launch "after Δt has elapsed" or "at time t", or it can be triggered by a predetermined signal pattern, etc.

Note that if the signal processing device 23 described below has the function of reliably transmitting wirelessly at the timing of wireless downstream data communication, this control signal is not necessarily required. For example, one possible method is to monitor the presence or absence of an upstream signal, and use the absence of an upstream signal as a trigger to transmit a buffered signal. Another possible method is to use the reception of an IF signal that has been transmitted at the original downstream timing as a trigger.

In FIG. 1A, at antenna site 20, multiple downstream IF signals transmitted via IFoF are acquired by O/E device 21, and only the desired signal is filtered from the multiple downstream IF signals received in signal processing device 23, and frequency converted to IF or RF. In this signal processing device 23, the digitized radio signal is buffered, and the buffered signal is output at the timing of TDD downstream data communication, the signal is amplified by Amp 25, and is emitted as a radio signal by ANT 27.

The timing of transmission of the signal buffered in the signal processing device 23 may be determined based on the control signal transmitted from the upper network and the presence or absence of an upstream signal transmitted from the lower network. It may also be determined based on the time within the digital signal processing circuit, which is time-synchronized with the GPS signal.

Figure 1B is a diagram showing a schematic configuration of another example of the IFoF transmission system according to this embodiment. In Figure 1B, the relay station 101 is equipped with an O/E 21 and a signal processing device 23, and the relay station 101 and the antenna site 102 are connected by an RF cable. In this way, it is also possible to separate the antenna site 102 from the relay station 101. The other configurations are the same as those in Figure 1A. In addition, the present invention may adopt a configuration in which the relay station 101 and the antenna site 102 are connected by optical fiber. In this case, an "E/O" is installed on the antenna site 102 side of the signal processing device 23 of the relay station 101, and an "O/E" is installed on the opposing antenna site 102. Furthermore, as shown in Patent Document 1, it is also possible to configure the relay station 101 and the antenna site 102 to transmit with narrowband IFoF. In this case, the relay station 101 is provided with a function of extracting an IF signal for each antenna site 102 and down-converting the frequency. That is, narrowband IFoF transmission is performed to each antenna site 102, and each IF signal is extracted and converted to the desired frequency at the antenna site 102. The function of restoring the IF signal divided at the accommodation station 10 to its original state may be possessed by either the relay station 101 or the antenna site 102. Buffering to adjust the emission timing may also be performed by either the relay station 101 or the antenna site 102, or by both.

Next, the operation of the IFoF transmission system according to this embodiment will be described. In the TDD base station 11 on the accommodation station 10 side, intermediate frequency signals (IF signals) of multiple radio signals to be emitted from the antenna site 20 are generated. Normally, downstream data communication is not performed during upstream data communication in the TDD base station 11, but a downstream IF signal is generated even during upstream data communication, and IFoF transmission is started. On the antenna site 20 side, the IF signal transmitted at the timing of the upstream data communication of the TDD base station 11 is buffered by the signal processing device 23, and is output at the downstream emission timing of the TDD base station 11 based on the timing information.

Figure 2 shows the IF signals transmitted by IFoF on the time axis and frequency axis. Here, the ratio of the TDD upstream communication period to the downstream communication period is shown as 1:2. This reduces the number of signals (bandwidth) transmitted at a certain time, making it possible to transmit at a high SNR. In the case shown in Figure 2, the optical transmission band can also be reduced to 2/3. In addition, in Figure 2, the number of IF signals (number of channels) transmitted at the upstream/downstream timing is the same, but if it is desired to transmit only a specific IF signal with high quality, it may be configured to transmit only that channel at the upstream timing. In this case, N-1 channels are transmitted at the conventional downstream timing. Also, if the upstream/downstream communication periods are asymmetric (for example, 1:N), N channels are used for transmission at the TDD upstream transmission timing (1/N) using IF signals.

Figure 3 shows the IF signal transmitted by IFoF on the time axis and frequency axis. Here, the ratio of the TDD upstream communication period to the downstream communication period is shown to be 1:1. This reduces the number of signals (bandwidth) transmitted at a given time, enabling transmission at a high SNR. As shown in Figure 3, the optical transmission bandwidth is halved.

Figure 4 is a diagram showing the schematic configuration of a TDD base station according to this embodiment. Here, the ratio of the TDD uplink communication period to the downlink communication period is shown to be 1:2. Also, functional blocks with the same name have the same function, and redundant explanations will be omitted. In Figure 4, the signal "Data #1" input to the TDD base station 11 is serial-to-parallel converted by S/P 111, and is converted from a frequency domain signal to a time domain signal by inverse Fourier transform by IFFT 113. After cyclic prefix processing is performed by CP 115, the signal is divided into waveforms by Wave form div. 117. Here, "Data #1" is divided into "Data #1_1" and "Data #1_2".

Next, the pre-emphasis 119 performs processing to emphasize the high frequencies of the modulated signal, the up-converted signal is up-converted by the Up comb. 121, and the signal is multiplexed by the Mux 123 and 125. The pre-emphasis 119 can also perform processing to correct the frequency response associated with RF components, optical components, and optical fiber transmission, but this is not a required component of the present invention. For this reason, the pre-emphasis 119 can be omitted in the present invention. In this sense, the pre-emphasis 119 is shown by a dotted line in FIG. 4. The signal is switched by the SW 127, and digital-to-analog converted by the DAC 129 to obtain the output signal (1) shown in FIG. 5A or FIG. 5B. Furthermore, a control signal generated by the control signal generation 130 is added by the Mux 131 to obtain the output signal (2) shown in FIG. 5C or FIG. 5D. Data from 1 to N/3 is divided in this way and output as output signals (1) and (2) as shown in FIG. 5A or FIG. 5C. Data from N/3+1 to N is not divided and is processed as in the conventional manner and output as output signals (1) and (2) as shown in FIG. 5B or FIG. 5D. Note that, although an example in which the control signal is multiplexed and separated by an analog circuit is shown here, the present invention is not limited to this, and similar processing can also be performed by a digital circuit.

6A and 6B are diagrams showing the schematic configuration of the Wave form div. 117. Here, the waveform is cut out every T/2 and processed to match the latest timing. When the uplink and downlink are asymmetric, it is not necessary to divide it into a plurality of parts, but it is sufficient to simply add a delay of T seconds. The Wave form div. 117a according to the configuration 1 shown in FIG. 6A is equipped with a SW (switch) and a Delay (delay circuit), and the SW switches every T/2 seconds. When the waveform input at t= _t0 is switched to t= _t1 and delayed by T/2 seconds, the heads of the two divided waveforms coincide at t= _t2 . On the other hand, the Wave form div. according to the configuration 2 shown in FIG. 6B has a delay of T/2 seconds. 117b has two filters (window functions) and a delay (delay circuit) connected in parallel, and the upper filter (window function) outputs only the first T/2 seconds every T seconds, and the lower filter (window function) outputs only the next T/2 seconds every T seconds. When the waveform input at t= _t0 is input to each filter (window function), and a waveform is output from each at t= _t1 , and the previous waveform (upper side) is delayed by T/2 seconds, the beginnings of the two divided waveforms coincide at t= _t2 . By performing such processing, it becomes possible to transmit a downstream IF signal during the upstream transmission period.

Figure 7 is a diagram showing a schematic configuration of the signal processing device 23 of the antenna site 20. Here, the ratio of the TDD uplink communication period to the downlink communication period is shown to be 1:2. In addition, functional blocks with the same name have the same function, so duplicated explanations will be omitted. The input signal is separated by Demaux 231, and a control signal is detected by Control signal Detection 232. Next, analog-to-digital conversion is performed by ADC 233. Next, frequency conversion is performed by Freq. comv. 237 via filter 235, which extracts only the desired signal. For example, the signals #1_1 and #1_2 are originally one signal, so this is restored. When #1_2 is delayed by Delay 238 and multiplexed by Mux 239, the original signal is restored. This is buffered by Buffering 241, and is output from Buffering 241 at the appropriate timing under control of Timing Controller 240. The control from Timing Controller 240 can use a control signal from TDD base station 11, an uplink signal, a GPS signal, etc. Finally, a desired output signal is obtained by digital-to-analog conversion in DAC 243. By performing such processing, it becomes possible to receive a downlink IF signal during the uplink transmission period and transmit a radio signal at the appropriate timing. Note that, although an example in which a control signal is multiplexed and separated by an analog circuit is shown here, the present invention is not limited to this, and similar processing can also be performed by a digital circuit.

As described above, according to this embodiment, by effectively utilizing the uplink communication timing of the TDD radio base station, the number of IF channels for IFoF transmission can be reduced, IF transmission at a high modulation degree (high SNR) becomes possible, and degradation of wireless signal quality can be suppressed. Furthermore, if the IF channel arrangement for transmission at the uplink and downlink communication timing of the TDD base station is the same, the bandwidth required for RF and optical components can also be reduced, and cost reductions can also be expected.

1, 100 IFoF transmission system 10 accommodation station 11 TDD base station 13 driver circuit 15 E/O device 20, 102 antenna site 21 O/E device 23 signal processing device 25 Amp
27 ANT
30 Broadband IFoF
101 Relay station

Claims (6)

An IFoF (IF-over-Fiber) transmission system that transmits radio waves emitted from an antenna as IF (Intermediate Frequency) signals over optical fiber,
an accommodating station which allocates, to an upstream transmission period of TDD (Time Division Duplex), a downstream IF signal to be emitted from the antenna in a downstream transmission period of the subsequent TDD, and transmits the downstream IF signal through an optical fiber;
an antenna site that buffers the downstream IF signal received via an optical fiber and controls a transmission timing to emit the buffered downstream IF signal from an antenna;
An IFoF transmission system, characterized in that a downstream IF signal from said accommodation station to said antenna site is transmitted in said TDD upstream transmission period and said TDD downstream transmission period. A base station device applied to an accommodation station of the IFoF transmission system according to claim 1,
a waveform division unit that divides a downstream IF signal to be transmitted into a plurality of downstream IF signals so that a time required for transmitting the downstream IF signal to be transmitted fits into an upstream transmission period of the TDD , and outputs the divided downstream IF signals by aligning the transmission timings of the divided downstream IF signals;
A base station apparatus characterized in that each of the divided downstream IF signals is transmitted during an upstream transmission period of the TDD . 3. The base station apparatus according to claim 2, wherein the waveform division unit determines the number of divisions of the downstream IF signal to be transmitted in the upstream transmission period of the TDD in accordance with a ratio between the upstream transmission period of the TDD and the downstream transmission period of the TDD. A signal processing device applied to an antenna site of the IFoF transmission system according to claim 1,
a buffering unit that restores each downstream IF signal, which has been divided by the accommodating station, has its transmission timing adjusted, and is received during the upstream transmission period of the TDD , to a downstream IF signal before division, and buffers the restored downstream IF signal;
a timing controller that instructs the buffering unit to output a buffered downstream IF signal based on a control signal obtained from a base station device. An IFoF (IF-over-Fiber) transmission system that transmits radio waves emitted from an antenna as IF (Intermediate Frequency) signals over optical fiber,
an accommodating station which allocates, to an upstream transmission period of TDD (Time Division Duplex), a downstream IF signal to be emitted from the antenna in a downstream transmission period of the subsequent TDD, and transmits the downstream IF signal through an optical fiber;
a relay station that buffers the downstream IF signal received via an optical fiber and outputs the buffered downstream IF signal to an antenna site by controlling a transmission timing,
An IFoF transmission system, characterized in that a downstream IF signal from said accommodation station to said antenna site is transmitted in said TDD upstream transmission period and said TDD downstream transmission period. A transmission method for an IFoF (IF-over Fiber) transmission system that transmits radio waves emitted from an antenna as IF (Intermediate Frequency) signals over an optical fiber, comprising:
a step of allocating, in a accommodating station, to an upstream transmission period of TDD (Time Division Duplex), a downstream IF signal to be emitted from the antenna in a downstream transmission period of the subsequent TDD, and transmitting the downstream IF signal through an optical fiber;
At an antenna site, buffering the downstream IF signal received through an optical fiber, and controlling a transmission timing to emit the buffered downstream IF signal from an antenna;
A transmission method, characterized in that a downstream IF signal from said accommodation station to said antenna site is transmitted in said TDD upstream transmission period and said TDD downstream transmission period.

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