Optical OFDMA network with dynamic sub-carrier allocation

A system and method for dynamically allocating sub-carriers between the nodes of an optical OFDMA ring network or an OFDMA passive optical network. A carrier allocation system assigns sub-carriers according to a utility function based on real-time measurements of arrival data rates and queue length variance.

BACKGROUND

1. Technical Field

The present invention relates to optical orthogonal Frequency Division Multiple Access (OFDMA) networks and, more particularly, to a system and method for dynamically allocating sub-carriers between nodes.

2. Description of the Related Art

Metro core networks are frequently based on fiber optic rings, stemming from legacy Synchronous Optical Network equipment. These networks are often built on a Unidirectional Path Switched Ring structure, having two redundant optical channels that allow for extremely fast recovery in the case of disruption of service. A metro core network serves a relatively large area, with the rings often being hundreds of kilometers in circumference, and provides connection between the local access networks and the long-haul (or backbone) networks.

Prior art implementations of optical metro core networks have been built using time-based resource sharing, as seen in the use of network structures such as RPR, HORNET, and OBT. These resource-sharing schemes schedule transmission such that individual nodes transmit sequentially for a short period of time, using the full bandwidth of the fiber. However, this leads to inefficient use of the network's bandwidth, as it is not responsive to to individual nodes' Quality of Service (QoS) needs. For instance, if a particular node has little data in its queue, its time slot (and hence network bandwidth) will be underused.

There is a similar problem in the implementation of Passive Optical Networks (PONs) such as those used to provide access to homes and businesses. These networks use unpowered optical splitters to share a fiber optic link from a single Optical Line Terminal (located at the service provider) between a plurality of Optical Network Units (located at the end user). These systems typically use time-division to share the link between the users, which presents the same inefficiencies as when time-division is used in a metro core network.

It is therefore advantageous to implement a resource sharing scheme which allows all nodes to transmit simultaneously and which flexibly allocates bandwidth based on QoS needs. One implementation of a metro core network involves the use of an Orthogonal Frequency-Division Multiple Access (OFDMA) scheme. This technique uses a plurality of orthogonal (i.e., non-interfering) sub-carrier frequencies to serve a plurality of nodes. By splitting traffic between the sub-carriers, the bandwidth on the channel is increased without having to alter the infrastructure. In addition, different sub-carriers can be assigned to different nodes on the network, effectively splitting the available bandwidth and allowing all nodes to transmit simultaneously.

However, using a static allocation of sub-carriers leads to a problem similar to that presented in the time-division protocols. If a node is underusing its allocated sub-carriers, then that node's bandwidth is being wasted. In the wireless communications context, OFDMA has several proposed schemes for dynamically allocating sub-carriers between nodes in order to respond to QoS needs. However, these techniques are not effective in the optical domain due to its greater complexity, different fading channel, and low bandwidth flows.

SUMMARY

A system for dynamically allocating sub-carriers to optical transmitters in an optical OFDMA network. The system has a dynamic sub-carrier allocation and assignment module, which is configured to dynamically assign sub-carriers to a plurality of optical transmitters according to a utility function, and a control module, which is configured to communicate sub-carrier assignments to the plurality of optical transmitters. The allocation determinations are made based on real-time measurements of arrival data rates, queue length variance, and Signal to Noise Ratio.

One embodiment of the present principles is in an optical network in a ring topology which uses OFDMA to share bandwidth resources between a plurality of nodes, each node transmitting over an assigned set of sub-carrier frequencies.

Another embodiment is in a passive optical network which uses OFDMA to share bandwidth resources between a plurality of Optical Network Units, each unit transmitting over an assigned set of sub-carrier frequencies.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In order to address the difficulties inherent in dynamically allocating sub-carrier addresses in an optical Orthogonal Frequency Division Multiple Access (OFDMA) network, there exists a need for a sub-carrier allocation scheme that takes into account the particular QoS needs and physical properties of OFDMA communications in optical metro core networks and PONs.

Referring now in detail to the figures in which like numerals represent the same or similar elements and initially toFIG. 1, an exemplary optical Orthogonal Frequency Division Multiple Access (OFDMA) network in a ring topology according to the present principles is shown. The network comprises a plurality of nodes102-1through102-N, connected by fiber optic links104. Each node is connected to its respective network(s) through one or more network interfaces108and each has at least one queue110(shown only for node102-1for simplicity) in which the node stores received data until the data can be transmitted along the fiber. The fiber links104may comprise one or more separate fibers and are arranged in a ring topology, such that each node102receives the transmissions of every node102on the ring. In addition to the nodes102, there is a carrier allocation system106.

In an optical OFDMA network, each node102is assigned a set of carrier frequencies to use for transmission. These frequencies are selected to be orthogonal, such that the transmissions of the nodes102do not interfere with each other. The use of multiple carrier frequencies allows for greater flexibility than is possible in time-based allocation schemes. The carrier allocation system106assigns sub-carriers to the nodes based on a determination of the most efficient distribution of bandwidth. This determination is made according to a utility function which takes into account queue length, data arrival rate, and Signal to Noise Ratios (SNR) from each of the nodes. The carrier allocation system106may operate on a dedicated control carrier and may be a stand-alone device, as shown inFIG. 1, or it may be a component of one or more of the nodes102. Having a plurality of nodes which comprise a carrier allocation system leads to additional failure resistance at the price of higher node cost.

FIG. 2depicts an OFDMA Passive Optical Network (PON) according to the present principles. One Optical Line Terminal (OLT)202is connected to a larger network through its network interface. It is also connected via a single fiber203to a passive coupler204. The passive coupler204splits the signals from the OLT202, transmitted at a wavelength λ, and sends them along further fibers to a plurality of Optical Network Units (ONUs)206-1through206-N. The ONUs206may be homogeneous and operate according to one PON standard (e.g., Gigabit PON (GPON) or Ethernet PON (EPON)), or they may operate according to different PON standards, with ONUs using the same differing standard comprising different “slices” of the network. Furthermore, the ONUs206may be separated from the OLT202by many kilometers. Each ONU206transmits back along the same fibers203, through passive coupler204, to communicate with the OLT202. The ONUs represent end users, and each ONU has a queue210(shown only for ONU206-1for simplicity) in which it stores data until it can be transmitted to the OLT.

Conventionally, ONUs used a single frequency and transmitted to the OLT according to a time-based sharing of the fiber. While the OLT's transmissions were sent at a different wavelength from the ONUs', the ONUs all shared a single carrier.

According to the present principles, each of the ONUs206-1through206-N is assigned a set of sub-carriers by the carrier allocation system208. In the case ofFIG. 2, the carrier allocation system208is depicted as a stand-alone device which communicates with the OLT202, but it is also contemplated that the carrier allocation system208may be a component of the OLT.

FIG. 2shows that each ONU206-1through206-N transmits signals in its own respective set of carriers, λ1-λN. Along with the data usually sent by the OLT202is sent control information comprising carrier assignments. The carrier allocation system208periodically revises carrier assignments based on queue length, data arrival rate, and measured SNR. In this way, all of the ONUs206may transmit simultaneously, each with a bandwidth appropriate to its Quality of Service (QoS) requirements.

FIG. 3provides further detail on the Carrier Allocation System106. Each node102-1through102-N receives an input traffic flow302-1through302-N, which accumulates in each node's queue110-1through110-N, before being transmitted on optical OFDMA link308. Information on the length of each node's queue is collected by the dynamic sub-carrier allocation and assignment module304, which uses a utility function305to allocate sub-carriers to the nodes, and comprises a modulation selection module306to select the modulation scheme (such as, e.g., Quadrature Phase Shift Keying (QPSK) or 16-ary Quadrature Amplitude Modulation (16-QAM)) that is most appropriate for a given node based on a signal to noise threshold table. A control module307then communicates with each of the nodes to instruct them as to which sub-carriers and which modulation scheme to use in transmission. Each node then transmits over optical OFDMA link308according to its assigned sub-carriers and modulation scheme.

Embodiments described herein may be entirely hardware, entirely software or including both hardware and software elements. In a preferred embodiment, part of the present invention is implemented in software, which includes but is not limited to firmware, resident software, microcode, etc., and controls a network hardware.

Referring now toFIG. 4, a method for dynamically allocating carriers in optical OFDMA networks is shown. Sub-carrier allocation schemes proposed for wireless applications, such as the multi-user diversity method and the cross-layer method, are ill-suited to the optical domain, due to its high complexity, different fading channel, and low bandwidth flows.FIG. 4depicts a method according to the present principles for allocating carriers. The method is designed for use in an optical OFDMA network.

The method begins with measuring in real time the arrival data rate, the queue length variance, and the SNR for each of the nodes (or ONUs) at block402. If the queue length variance does not exceed a threshold at block404, the method goes back to measuring block402. If the queue length variance does exceed a threshold (i.e., if the queue lengths of the nodes are significantly unbalanced), the method begins to reassign sub-carriers at405. If there are unassigned sub-carriers (block406), the method describes using a utility function at block408. The utility function step chooses a node k which maximizes the utility function based on real-time measurements of queue length, arrival data rate, SNR, and the number of sub-carriers already assigned to each node. At block410a sub-carrier j is assigned to the node k, and at block412a modulation scheme is assigned to the sub-carrier j. The method then returns to block406. If there are no unassigned sub-carriers remaining, the method returns to the measuring block402.

The utility function used in block408is a part of this process. The basic idea of dynamic sub-carrier allocation in optical OFDMA systems is to maximize each sub-carrier's utility during each short time period (a scheduling interval, e.g., 100 ms) according to: 1) the measured real data arrival rate in each node; 2) an adaptive modulation scheme sensitive to SNR; and 3) queuing length (delay). The randomly arriving incoming packets in each node are buffered in a FIFO queue. The scheduling interval may be chosen by monitoring the queuing length variance across the nodes. This allows the tracking of rapid rate variance in traffic flows. Generally, the scheduling interval falls in the range of one millisecond to one second, depending on the traffic flow patterns.

Several parameters are defined as follows: M is number of optical OFDMA nodes; N is total number of sub-carriers in the optical ring; λi(t) the measured real arrival data rate for node i during the last scheduling intervalt. The serving rate is defined as

μi⁡(t)=∑j⁢(xi,j×di,j)
where xi,j=1 if the sub-carrier j is assigned to node i, otherwise xi,j=0; di,jis the corresponding data rate of each sub-carrier when using an adaptive modulation scheme based on the transmission quality (i.e., SNR). The buffer occupancy of node i is modeled as ebi(t)/Bi, wherebi(t) is the measured average queuing length during the last scheduling intervaltand Biis the node i buffer size. Let Δi(t) be the set of carriers assigned to node i in current scheduling cycle t. The utility function305can then be implemented as follows:

2. k=argMax(λi(t)/μi(t)×ebi(t)/Bi)// choose a node k that make sub-carrier j has maximal utility.

As noted above, after a sub-carrier has been assigned, an appropriate modulation scheme is chosen. Different modulation schemes have different levels of sensitivity to noise, where higher-bandwidth schemes use higher SNRs. This can be accomplished using a series of thresholds, stored in a table, whereby the highest threshold that the SNR exceeds dictates the modulation scheme to use.FIG. 5shows a qualitative graph which illustrates this. For very low SNRs, Binary Phase Shift Keying (BPSK) may be used, because it has a high noise tolerance. After a first SNR threshold, the SNR is high enough to allow the use of QPSK. After a second SNR threshold, 16-QAM is permitted. Higher thresholds permit higher-bandwidth modulations.

Having described preferred embodiments of a system and method for dynamically allocating sub-carriers between the nodes of an optical OFDMA network (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.