KR20140116189A - A mobile ad-hoc network with reduced guard-time - Google Patents

Abstract

The present invention relates to a method for reducing guard time in a mobile ad hoc networking (MANET) system comprising a hopping transmitter and a combination of a plurality of hopping narrowband independent receivers capable of receiving and processing the entire operating band allocated to the system at one time , A transceiver architecture provided at each node, including a channel frequency that is dynamically selected within a wide operating bandwidth, follows the method. Alternatively, such a transceiver architecture includes a non-hopping wideband receiver capable of receiving and processing the entire operating band allocated to the system at a time. Several channels arbitrarily spread over the frequency bands allocated to this system are simultaneously received while maintaining the architecture of the MANET system. The transmit hopping pattern is determined to use the lowest possible number of frequencies. For each transport node, a time slot for which the corresponding receiver in each of the remaining active nodes is not transmitting, and a frequency channel for which no other active node is transmitting is found. The transmission frequency is then determined and if the other node does not select a slot at the same time and the transceiver is not transmitting in that slot, the transmission is received and allows the relay nodes to transmit at the same time using different channels. If multiple narrowband independent receivers are used for reception, the guard time is allocated to the time slot on request. Otherwise, no guard time is assigned to the time slot.

Description

A MOBILE AD-HOC NETWORK WITH REDUCED GUARD-TIME < RTI ID = 0.0 >

The present invention relates to the field of mobile communication. More particularly, the present invention relates to a system and method for improving the spectral efficiency and scalability of a mobile ad-hoc network.

Mobile Ad-hoc NETworks (MANET) is a self-formed and self-healing wireless network that supports data and / or voice communication between mobile or fixed nodes without any physical infrastructure. Network.

MANET is a kind of "mesh network" with additional mobility capabilities. In a mesh network, each node in the network can act as an independent router regardless of whether it is connected to another network. It provides continuous connection and reconfiguration by bypassing a disconnected or blocked path by "hopping" from one node to another until it arrives at its destination. Figure 1 (prior art) shows an exemplary connection path between an "A" node and a "B" node in a typical mesh network.

The mesh network self-heals. This network can still operate even when one node is disconnected or the connection quality is low. MANET is a mesh network that can solve the problems introduced by the mobility of nodes. One disadvantage of the mesh network is that when a number of subscribers need to hop through the same node, a "bottleneck" occurs at the node and the data rate of the subscribers is significantly reduced.

Each mobile node manages a dynamic routing table that tracks the MANET topology. This routing table may be used for any routing protocol suitable for MANET, such as an Optimized Link State Routing Protocol (OLSR) or an Ad Hoc On Demand Distance Vector Routing Protocol (AODV: Ad) hoc On-Demand Distance Vector routing protocol - a routing protocol for a mobile ad hoc network). OLSR is a proactive link state (LS) algorithm that holds radio link state information, and AODV is a reactive distance vector (DV) algorithm that holds only distances to all other loads. Such a topology may be possible to include additional parameters such as link quality, physical location, and channel frequency.

MANET can be used in military environments, or in areas where existing infrastructure has collapsed or is insufficient (eg, disaster areas). Recently, there is an increasing need for a broadband MANET function to support more users (nodes) and more burdensome applications. However, lack of spectrum resources and current MANET algorithms have not utilized the spectrum efficiently enough to meet the increasing demand.

The MANET node is identified by the node ID and executes Distributed Medium Access Control (MAC) that allocates time resources to the nodes for each MANET channel. The system uses a limited number of radio channels, manages each channel individually, and splits the time line of each channel between MANET nodes. If this time line is divided into slots (as shown in FIG. 2), the time division between the MANET nodes can be divided into "TDMA ", i.e. time division multiple access (by splitting the signal into different time slots, A method of channel access to a shared media network that allows the user to share the information.

In a wireless MANET network, a sophisticated distributed algorithm needs to manage access to several radio channels, which is determined for each node when received and transmitted in any channel for each time period.

In a standard MANET setup (where the transceiver can receive only one channel at a time), after the channel frequency is set, the mesh network is only run between subscribers tuned to that channel, It has meaning only within, and can not see all the other channels. The channel frequency may be selected from a plurality of frequencies, but the frequency is independent of the networking operation after the frequency is selected. Thus, a MANET system with one set of channels is a collection of virtually unconnected parallel MANET systems, each of which operates only on its own channel, and has a data rate and reliability performance limited by the width of a single channel I have. This observation is also true when the time line is divided into time slots. During each time slot, within each channel, each MANET is managed, and the participants of the MANET are assigned to a distributed type (e.g., to determine which node to transmit when) to determine how to divide the channel between them Algorithm. Therefore, whenever a hopping receiver is used, the "guard time" (the maximum propagation delay that can occur in the system) is a safety margin before the receiver is allowed to hop to the next frequency Is used. However, this guard time adds an idle period to the time slot, which slows the TDMA cycle and degrades the system's time efficiency.

It is therefore an object of the present invention to improve the spectral efficiency and scalability of the MANET system while reducing the guard time allocated to the time slot.

It is another object of the present invention to provide an improved MANET system that can reduce latency (the time it takes for one data packet to travel over a network connection).

Other objects and advantages of the present invention will become apparent from the following description.

The present invention relates to a method for reducing guard time in a mobile ad hoc networking (MANET) system comprising a hopping transmitter and a combination of a plurality of hopping narrowband independent receivers capable of receiving and processing the entire operating band allocated to the system at one time , A channel frequency that is dynamically selected within a wide operating bandwidth, and a transceiver architecture provided to each node follows the method. Alternatively, such a transceiver architecture includes a non-hopping broadband receiver capable of receiving and processing the entire operating band allocated to the system at one time.

Several channels arbitrarily spread over the frequency bands allocated to the system are received simultaneously, while maintaining the architecture of the MANET system. The transmit hopping pattern is determined to use the lowest possible number of frequencies. For each transport node, a time slot in which the counterpart receiver in each remaining active node is not transmitting, and a frequency channel in which the other active node is not transmitting are found. Then, if the transmission frequency is determined and the other node does not select a slot at the same time and the transceiver is not transmitting in that slot, the transmission is received and allows relay nodes to transmit at the same time using different channels. Each time a plurality of narrowband independent receivers are used for reception, the guard time is allocated on a time slot on demand. Otherwise, no guard time is assigned to the time slot.

According to one embodiment, a broadband receiver receives several channels at the same time,

a) a global band preselector for selecting a frequency range;

b) a global low noise amplifier for amplifying the selected range;

c) a global gain control unit for controlling the global gain of the reference receiver;

d) an anti-aliasing filter for filtering the amplified signal;

e) at least one analog-to-digital converter (ADC) for sampling the received signal;

f) an ADC driver for controlling the operation of the ADC;

g) a control unit for controlling the gain of the global gain control unit; And

h) a digital signal processing unit for processing samples from the RF band at one time.

Nodes can be received and transmitted by full-duplex or half-duplex transceivers.

The protection time is:

a) sequentially indexing all RF channels according to the hopping pattern to align all RF channels;

b) causing the receiver to hop on the frequency of the even index and to maintain each frequency for a time period equal to the sum of the maximum propagation delay differences that can occur between one time slot and any two nodes; And

c) causing the other receiver to hop on the frequency of the odd index and maintain each frequency for a time period equal to the sum of the maximum propagation delay differences that can occur between one time slot and any two nodes, .

Optionally, the guard time is not assigned to the time slot.

If a conventional receiver with two or more independent simultaneous receive channels is used, the guard time allocated to the time slot is:

a) sequentially indexing all RF channels according to the hopping pattern to align all RF channels;

b) causing the first receiver to hop on the frequency of the even index and to remain on each frequency for a time period equal to the sum of the maximum propagation delay differences that can occur between one time slot and any two nodes; And

c) causing a second receiver staggered in time by one time slot relative to the first receiver to hop on the frequency of the odd index, and to allow one time slot and a maximum Is kept on the frequency for a time period equal to the sum of the propagation delay differences, respectively.

If the duration of the maximum possible propagation delay difference is longer than one time slot, the guard time may be omitted using two or more receivers.

If a single wideband receiver is used, the guard time may be omitted without channel indexing or receiver time slot synchronization.

The guard time may be reduced to have a maximum delay of less than one frequency hop or frequency hop period per time slot.

The present invention relates to a mobile ad hoc networking (MANET) system having a reduced protection time during reception, the system comprising:

a. A transceiver architecture provided at each node, the transceiver architecture comprising:

a. a plurality of hopping narrowband independent receivers capable of receiving and processing the entire operating band assigned to the system at a time and including a channel frequency dynamically selected within a wide operating bandwidth,

a.2) the transceiver architecture comprising a combination of non-hopping wideband receivers capable of receiving and processing the entire operating band assigned to the system at a time,

The MANET system comprises:

b. Simultaneously receiving at least several channels randomly spread over a frequency band allocated to the MANET system while maintaining the architecture of the MANET system;

c. Determine a transmission hopping pattern to use a minimum possible number of frequencies;

d. For each transport node, looking for a time slot not transmitted by the corresponding receiver at each remaining active node and a frequency channel not transmitted by another active node;

e. For each transport node, determining a transmission frequency; And

f. For each forwarding node, if the other node has not selected a slot at the same time, and if the transceiver is not transmitting in the slot, it receives the forwarding and allows relay nodes to concurrently transmit using different channels ;

g. Allocate a guard time to a time slot on demand, or otherwise not assign a guard time to the time slot so that a plurality of narrow band independent receivers can be used for reception at any time.

Figure 1 (prior art) shows a typical conventional mesh connection.
Figure 2 (prior art) shows a typical TDMA cycle.
Figure 3A is a block diagram of a reference receiver with two independent receive channels in accordance with one embodiment of the present invention.
Figure 3B shows an expanded version of the block diagram with four independent receive channels in accordance with one embodiment of the present invention.
3C shows a block diagram of a reference receiver according to one embodiment of the present invention.
4 is a block diagram of a broadband reference transmitter in accordance with one embodiment of the present invention.
FIG. 5 (prior art) shows a possible near-far node placement in a MANET system where the node is close to one node and farther from the other node.
6A (prior art) shows a special case of the MANET of FIG. 6, where the fixed guard time allocation consumes 50% of the cycle time.
Figure 6B illustrates a special case of the MANET of Figure 5 without guard time according to one embodiment of the present invention.
Figure 6C illustrates one embodiment of the present invention implemented using an improved MANET transceiver architecture.

The improved MANET system proposed by the present invention uses a wideband receiver with sophisticated signal processing to take advantage of the simultaneous reception of several channels arbitrarily spread over the wideband frequency range while remaining within the environment of the MANET architecture do.

The transceiver architecture of the improved MANET system includes an independent transmitter capable of operating on a predetermined fixed channel or hopping according to some hopping sequence and rate.

A spectrum efficient MANET with improved time efficiency and reduced latency uses simultaneous reception and includes a reference receiver and a reference transmitter.

The prior art reference receiver

For the purpose of explaining the basic implementation of the improved MANET algorithm, one of the two-transmitter reference configuration or the four-receiver reference configuration is selected as the reference receiver.

Figure 3A shows a block diagram of a prior art reference receiver with two independent receive channels. Solid lines indicate radio frequency (RF) paths and dotted lines indicate control paths. The transmitter architecture includes at least two independent receive paths, each receive path providing an individually programmed receive channel. The reference receiver 30 includes a global band pre-selector 31 for selecting a range, and a global low noise amplifier (LNA) 32 for amplification. The global range is divided into sub-bands by programmable pre-selector filters 33a and 33b. Mixers 34a and 34b with variable frequencies f ₁ and f ₂ (the two-channel version shown in FIG. 3A) and f ₁ , f ₂ , f ₃ and f ₄ (the four- Are generated by independent synthesizers 35a and 35b, each of which is programmed by the control unit 36. The local oscillator (LO) The LO signal, subband range, and gain control level are dynamically determined by the digital control unit 36, depending on the receiving method used and the frequency and intensity of the desired receiving channel. The output of the mixer is a common low intermediate frequency (IF) and is connected to a signal processing unit 37, which is typically of FPGA type, to perform signal sampling (to IF frequency) and simultaneous digital processing of all channels.

Multi-channel receivers with up to four channels are already commercially available. For example, Rockwell-Collins offers "FlexNet-Four" with up to four independently programmable receivers in the HF / VHF / UHF band (2 ÷ 2000 MHz) , And the IAI / ELTA (Elta) provides "ARC-840D" that comes in two-receiver and four-receiver configurations that can be programmed independently in the VHF / UHF band (30 ÷ 1220 MHz). Thus, it has been demonstrated that a receiver architecture with up to four independent receivers is feasible. More independent channels can be easily added by duplicating a parallel branch.

Figure 3B shows an expanded version of the block diagram with four independent receive channels (Figure 3A). Solid lines indicate radio frequency (RF) paths and dotted lines indicate control paths. Since the complexity of the hardware increases sharply, channels over 4 may not be realistic. A similar array may be known by one of ordinary skill in the art.

Improved MANET reference receiver

3C shows a block diagram of a reference receiver according to one embodiment of the present invention. The solid line represents the RF path, the dotted line represents the control path, and the wide arrow represents the data bus. For example, it is possible to construct a receiver capable of simultaneously receiving an entire band with sufficient dynamic range, using the 225-400 MHz UHF band, which is generally assigned to airborne communication. Techniques that enable such receivers include (1) analog-to-digital converters (ADCs) that perform direct signal sampling at radio frequency (RF) and practically limit the bandwidth and dynamic range of the system; , (2) a digital signal processor that is implemented using a field programmable gate array (FPGA) to process the entire RF band at one time and typically handle high bandwidth systems. Such a receiver architecture is currently feasible for various systems. Digital processing power is sufficient for ADC technology as well as almost all applications today, which is sufficient for most systems that require the highest possible dynamic range. An example of a modern ADC that can be used to implement a broadband reference radio receiver is National Semiconductors 'ADC12D1800', a dual channel ADC with a maximum sampling frequency of 1800 MHz on each channel. These devices are specifically aimed at broadband software defined radios. Thus, unlike prior art receivers that only receive a few channels at a time, the reference receiver processes the entire band, i.e. all channels simultaneously, at one time, by dividing the band into subbands and then reprogramming the frequencies of the channels.

Reference transmitter

4 shows a block diagram of a reference transmitter that can operate both in a wideband and frequency hopping mode according to one embodiment of the present invention. The solid line represents the RF path, the dotted line represents the control path, and the wide arrow represents the data bus. This transmitter architecture is most likely to be selected for broadband applications by those skilled in the art. Samples of the signal modulated right near the final frequency are mathematically generated in digital form by the signal processing unit and can be processed at a rate of up to 2.5 giga-samples per second, such as 'AD9739A' (Analog Devices) It is converted to analog form by high-speed DAC (digital-analog converter). Signal samples from the DAC are converted to analog form by a global band reconstruction filter and pre-amplified by a low-level RF amplifier. This signal is then fed to a power-control attenuator whose damping level is dynamically controlled by the control unit. The global band-pass filter at the attenuator output removes the far-out distortion product produced by the pre-amplifier. The filter output is supplied to the RF driver which amplifies it to a level sufficient to drive the final PA (power amplifier) at the maximum allowed transmission power. The output from the PA passes through a global harmonic filter that removes the PA component, which is a multiple of the transmit frequency, and then the signal reaches the transmit antenna.

Improving Spectrum Efficiency of MANET

The present invention proposes a new MANET implementation method that uses the spectrum more efficiently than the prior art and provides a higher data rate and shorter latency than the existing MANET implementation method. The improved MANET system proposed by the present invention simultaneously receives several channels (using prior art reference receivers) over the entire frequency range (when an enhanced MANET reference receiver is employed) or over a wideband frequency range in the environment of the MANET architecture The function of the reference receiver is utilized.

Practical feasibility

The improved MANET algorithm proposed by the present invention uses a half-duplex transceiver. Half-duplex transceivers are flexible and easy to implement since they only consider protecting the receiver from burning-out during transmission. In one possible implementation, this protection can be achieved easily and at low cost using a simple unit known as an "antenna switch " which can operate correctly regardless of operating frequency.

Use of improved MANET to increase time efficiency of conventional MANET

In conventional MANET systems, the relative physical distance between all two nodes is a result of operational requirements in the field. Typically, the distances are unknown a-priori, have random lengths, and can be very different from each other. For example, in the MANET system shown in FIG. 5, the D node is close to the B node, but farther from the A node. The time delay due to electromagnetic wave propagation from node A to node D is d.

If both B and A nodes are to transmit to the D node, the propagation time of the transmission from A to D will be longer than the propagation time of the transmission from B to D. According to the conventional TDMA scheme, if the transmission of A node arrives delayed because the D node listens for a fixed-length time slot to receive each node in the system, some or even all of the data packets Can be lost. This phenomenon occurs because the D node will switch to the frequency of the other node before all the data from the A node reaches the D node. Also, even in the frequency hopping mode where a node hopes over several frequencies during a single TDMA time slot, since the D node will hop to the new frequency before all transmissions from A on the current frequency hop arrive at the destination, Lt; / RTI > To overcome this problem, in the prior art frequency-hopping / TDMA scheme, at the end of every time slot (whether there is only one frequency hop in the time slot or more than one exists) By pre-allocating a "guard time" equal to the maximum propagation delay that can occur, both the short-delayed and long-delayed transmissions can arrive at the destination before hopping to the next frequency is allowed. This guard time adds an idle period to the time slot, slowing the TDMA cycle and making the system time efficiency worse.

The system according to an embodiment of the present invention allows to omit the guard time pre-allocated in the physical layer, and instead allocates the guard time according to the request. This embodiment may be implemented in any prior art receiver having two or more independent simultaneous receive channels as follows. If all the RF channels are ordered by indexing them sequentially according to the hopping pattern, the first receiver hopes on the frequency of the even index, and the maximum receiver propagation delay difference between one time slot and any two nodes And is maintained at each frequency for an equivalent period of time. The second receiver is exactly the same except that it hopes on the frequency of the odd index and is time differential in time relative to the first receiver by one time slot. With this arrangement, it can be easily understood from the example of FIG. 6B that both the near transmitter with short delay and the far transmitter with long delay are correctly received by one receiver.

However, it will be readily appreciated that if the duration of the maximum possible propagation delay difference is greater than the length of one time slot, more than two receivers will be needed to bypass the guard time using prior art receivers. In addition, even the basic two-receiver configuration adds considerable hardware and software complexity. The challenge is overcome by using an improved single MANET receiver architecture that bypasses guard time using a single receiver and a low complexity algorithm, using an appropriate method, as described below.

Figure 6C illustrates one embodiment of the present invention implemented using an enhanced MANET transceiver architecture. Here, since the receiver is always "watching" all the channels and the receiver will always be prepared regardless of the delay length, there is no limitation on the delay length, and unlike the conventional transceiver implementation method, There is no need for receiver time slot synchronization, substantially reducing both hardware and software complexity substantially. In addition, the receiver need not know the frequency hopping sequence of the transmitter.

Omission of fixed guard time significantly improves time usability. Figures 6a, 6b and 6c show the special case of the MANET of Figure 5 where there is only one frequency hop in each transmission and the maximum propagation length, d (corresponding to physical distance), is equal to the duration of a single time slot. In Figure 6a, the fixed guard time allocation consumes 50% of the cycle time. The time efficiency improvement due to the omission of the guard time in Figs. 6B and 6C is significant. The guard time may also be saved in other TDMA schemes with one or more frequency hopping per time slot, and / or with a maximum delay shorter than the frequency hop period.

Since the enhanced MANET operates in a half-duplex mode, all the receive paths are disconnected when switching from receive to transmit. Therefore, Forward Error Correction (FEC) is introduced to compensate for possible loss of the "tail" of the last received time slot before reaching the transmit state. However, since the enhanced MANET system generally remains in the receive mode for a number of time slots before switching to transmit mode, the FEC will be needed to correct only a low percentage of the total data blocks. Therefore, the FEC rate can be made high, and the net data bit rate is not noticeably reduced.

The examples and descriptions are provided for illustrative purposes only, and are not intended to limit the invention in any manner. It will be appreciated by those skilled in the art that the present invention may be carried out in a wide variety of ways from the foregoing to one or more techniques, all of which are not outside the scope of the present invention.

Claims (11)

CLAIMS 1. A method for reducing guard time in a mobile ad hoc networking (MANET) system during reception,
a) at each node, a hopping transmitter and:
a plurality of hopping narrowband independent receivers capable of receiving and processing the entire operating band assigned to the system at a time and including channel frequencies dynamically selected within a wide operating bandwidth,
a.2) providing a transceiver architecture including a combination of non-hopping wideband receivers capable of receiving and processing an entire operating band assigned to the system at one time;
b) simultaneously receiving at least several channels randomly spread over a frequency band allocated to the system while maintaining the architecture of the MANET system;
c) determining a transmission hopping pattern to use a minimum possible number of frequencies;
For each transport node,
d) finding a time slot for which the corresponding receiver in each of the remaining active nodes is not transmitting and a frequency channel for which the other active node does not transmit;
e) determining a transmission frequency; And
f) receiving the transmission, allowing relay nodes to transmit simultaneously using different channels if the other nodes have not selected the same time slot, and if the transceiver is not transmitting in the slot,
Characterized in that when a plurality of narrowband independent receivers are used for reception, they allocate a guard time to the time slot or otherwise do not allocate guard time in the time slot according to the request. . 2. The method of claim 1, wherein the broadband receiver simultaneously receives several channels,
a) a global band pre-selector for selecting a frequency range;
b) a global low noise amplifier for amplifying the selected range;
c) a global gain control unit for controlling the global gain of the reference receiver;
d) an anti-aliasing filter for filtering the amplified signal;
e) at least one analog-to-digital converter (ADC) for sampling the received signal;
f) an ADC driver for controlling operation of the ADC;
g) a control unit for controlling the gain of the global gain control unit; And
h) a digital signal processing unit for processing samples from the RF band at one time. 2. The method of claim 1, wherein the nodes receive and transmit using a full-duplex or half-duplex transceiver. The method of claim 1,
a) sequentially indexing RF channels according to a hopping pattern to align all RF channels;
b) cause the receiver to hop on the frequency of the even index and remain on the frequency for a time period equal to the sum of one time slot and the maximum propagation delay difference that can occur between any two nodes; And
c) causing the other receiver to hop on the frequency of the odd index and remain on each frequency for a time period equal to the sum of one time slot and the maximum propagation delay difference that can occur between any two nodes,
Wherein the at least one mobile MANET system is allocated upon request. 2. The method of claim 1, wherein no guard time is assigned to a time slot. 3. The method of claim 1, wherein when a conventional receiver with two or more independent simultaneous receive channels is used, the guard time allocated to the time slot is:
a) aligning all RF channels by sequentially indexing according to a hopping pattern;
b) causing the first receiver to hop on the frequency of the even index and to remain on the frequency for a time period equal to the sum of one time slot and a maximum propagation delay difference that may occur between any two nodes; And
c) causing a second receiver staggered in time by one time slot relative to the first receiver to hop on the frequency of the odd indexes, and one time slot, By maintaining them on the frequency for a time period equal to the sum of the maximum propagation delay differences,
Characterized in that the protection time of the mobile MANET system is reduced during reception. 2. The method of claim 1, wherein, if the duration of the maximum possible propagation delay difference is greater than the length of one time slot, the guard time is skipped by using two or more receivers. How to reduce protection time. 2. The method of claim 1, wherein when a single wideband receiver is used, the guard time is omitted without channel indexing or receiver time slot synchronization. 2. The method of claim 1, wherein the guard time is reduced to have more than one frequency hop per time slot. 2. The method of claim 1, wherein the guard time is reduced to have a maximum delay less than the frequency hopping period. 1. A mobile ad hoc networking (MANET) system having reduced protection time during reception,
a. A transceiver architecture provided at each node, the transceiver architecture comprising:
a plurality of hopping narrowband independent receivers capable of receiving and processing the entire operating band assigned to the system at a time and including channel frequencies dynamically selected within a wide operating bandwidth,
a.2) the transceiver architecture with a combination of non-hopping wideband receivers capable of receiving and processing the entire operating band assigned to the system at a time,
The MANET system comprises:
b. Simultaneously receiving at least several channels randomly spread over a frequency band allocated to the MANET system while maintaining the architecture of the MANET system;
c. Determine a transmission hopping pattern to use a minimum possible number of frequencies;
d. For each transport node, a time slot in which the corresponding receiver in each remaining active node is not transmitting and a frequency channel in which the other active node is not transmitting is looking for;
e. For each transport node, determining a transmission frequency; And
f. For each forwarding node, if the other node has not selected a slot at the same time, and if the transceiver is not in transit in the slot, then it is possible to receive the forwarding and at the same time relay nodes simultaneously using different channels Allow;
g. Wherein the guard time is allocated to the time slot on request or else the guard time is not allocated to the time slot so that a plurality of narrow band independent receivers can be used for reception at any time. Mobile MANET system with time.

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