JP4213466B2 - Multi-carrier communication with adaptive cluster configuration and switching - Google Patents

Multi-carrier communication with adaptive cluster configuration and switching Download PDF

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JP4213466B2
JP4213466B2 JP2002550747A JP2002550747A JP4213466B2 JP 4213466 B2 JP4213466 B2 JP 4213466B2 JP 2002550747 A JP2002550747 A JP 2002550747A JP 2002550747 A JP2002550747 A JP 2002550747A JP 4213466 B2 JP4213466 B2 JP 4213466B2
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cluster
subscriber
channel
clusters
subcarriers
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JP2004529524A (en
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ウェンツォン ツァン
ケミン リ
シアオドン リ
フイ リウ
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アダプティックス インコーポレイテッド
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    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W72/00Local resource management, e.g. wireless traffic scheduling or selection or allocation of wireless resources
    • H04W72/04Wireless resource allocation
    • H04W72/08Wireless resource allocation where an allocation plan is defined based on quality criteria
    • H04W72/085Wireless resource allocation where an allocation plan is defined based on quality criteria using measured or perceived quality
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00Arrangements for detecting or preventing errors in the information received
    • H04L1/0001Systems modifying transmission characteristics according to link quality, e.g. power backoff
    • H04L1/0009Systems modifying transmission characteristics according to link quality, e.g. power backoff by adapting the channel coding
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00Arrangements for detecting or preventing errors in the information received
    • H04L1/0001Systems modifying transmission characteristics according to link quality, e.g. power backoff
    • H04L1/0023Systems modifying transmission characteristics according to link quality, e.g. power backoff characterised by the signalling
    • H04L1/0025Transmission of mode-switching indication
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00Arrangements for detecting or preventing errors in the information received
    • H04L1/0001Systems modifying transmission characteristics according to link quality, e.g. power backoff
    • H04L1/0023Systems modifying transmission characteristics according to link quality, e.g. power backoff characterised by the signalling
    • H04L1/0026Transmission of channel quality indication
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00Arrangements affording multiple use of the transmission path
    • H04L5/02Channels characterised by the type of signal
    • H04L5/023Multiplexing of multicarrier modulation signals
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W72/00Local resource management, e.g. wireless traffic scheduling or selection or allocation of wireless resources
    • H04W72/04Wireless resource allocation
    • H04W72/0406Wireless resource allocation involving control information exchange between nodes
    • H04W72/0413Wireless resource allocation involving control information exchange between nodes in uplink direction of a wireless link, i.e. towards network

Description

This patent application is a continuation-in-part (CIP) of US patent application Ser. No. 09 / 738,086, filed Dec. 15, 2000, entitled “OFDMA with Adaptive Subcarrier-Cluster Configuration and Selective Loading”. It is.
The present invention relates to the field of wireless communications, and more specifically, the present invention relates to a multi-cell multiple subscriber radio system using orthogonal frequency division multiplexing (OFDM).

  Orthogonal frequency division multiplexing (OFDM) is an efficient modulation scheme for transmitting signals over frequency selective channels. In OFDM, a wide bandwidth is divided into a number of narrowband subcarriers, but the subcarriers are configured to be orthogonal to each other. Signals modulated on subcarriers are transmitted in parallel. More specifically, Cimini, Jr. "Analysis and Simulation of Digital Mobile Channels Using Orthogonal Frequency Division Multiplexing" IEEE Trans. Commun. COM-33, No. 7, July 1985, pages 665-75; “Beyond 3G: Broadband Wireless Data Access Based on OFDM and Dynamic Packet Allocation” by Chung and Sollenberger, IEEE Communications Magazine, Volume 38, No. 7, pp. 78-87, July 2000.

  One way to use OFDM to support multiple access for multiple subscribers is time division multiple access (TDMA), where each subscriber uses all the subcarriers in the assigned time slot. . Orthogonal frequency division multiple access (OFDMA) is another method of multiple access using the basic format of OFDM. In OFDMA, many subscribers use different subcarriers simultaneously in a manner similar to frequency division multiple access (FDMA). More specifically, Sari and Karam, “Orthogonal Frequency Division Multiple Access and its Application to CATV Networks”, European Transactions on Telecommunications, Vol. 9 (6), pages 507-516, November / December 1998, and See “Improved Performance of Random OFDMA Mobile Communication Systems” IEEE VTC '98, pages 2502-2506, by Nogueroles, Bossert, Donder, and Zyablov.

  Multipath causes frequency selective fading. The channel gain is different for each subcarrier. Furthermore, the channels are usually not correlated for different subscribers. A subcarrier with large fading for one subscriber may provide a high channel gain for another subscriber. Therefore, in the OFDMA system, it is advantageous if subcarriers are adaptively allocated to subscribers so that each subscriber can enjoy a high channel gain. More particularly, Wong et al., “Multiuser OFDM with Adaptive Subcarrier, Bit and Power Allocation” IEEE J. Select. Areas Commun. 17 (10), 1747-1758, October 1999.

  Within a cell, each subscriber can be adjusted to have a different subcarrier in OFDMA. Since the signals for each subscriber can be orthogonal to each other, there is little intra-cell interference. However, when an aggressive frequency reuse plan, for example, the same spectrum is used in many adjacent cells, the problem of inter-cell interference occurs. It is clear that inter-cell interference in an OFDMA system is also frequency selective, and it is useful to mitigate the effects of inter-cell interference by performing adaptive allocation of subcarriers.

  One approach to do subcarrier allocation for OFDMA is a joint optimization operation, which not only requires knowledge of the behavior and channels of all subscribers in all cells, but also allows current subscribers to network If you leave the network or a new subscriber joins the network, you will need to readjust the frequency each time. This is often impractical in actual wireless systems, mainly due to bandwidth costs for updating subscriber information and computational costs for integrated optimization.

  A method and apparatus for allocating subcarriers in an orthogonal frequency division multiple access (OFDMA) system is described. In some embodiments, the method comprises the steps of assigning at least one subcarrier diversity cluster to a first subscriber and assigning at least one coherence cluster to a second subscriber.

  The present invention will become more fully understood from the following detailed description of the various embodiments of the invention and the accompanying drawings, which are not intended to limit the invention to the specific embodiments, It is presented only to help explain and understand it.

A method and apparatus for allocating subcarriers in an orthogonal frequency division multiple access (OFDMA) system is described. In an embodiment, the method comprises the steps of assigning at least one subcarrier diversity cluster to a first subscriber and assigning at least one coherence cluster to a second subscriber.
The technique disclosed herein will be described using OFDMA (cluster) as an example. However, the technique is not limited to OFDMA based systems. The present technique can be generally applied to a multi-carrier system. For example, the carrier is an OFDMA cluster, a CDMA spreading code, an SDMA (space division multiple access) antenna beam, or the like. In some embodiments, subcarrier allocation is performed separately within each cell. Within each cell, the allocation to individual subscribers (for example, mobile phones, etc.) is also determined by taking into account all the subscribers in the cell for each allocation, and the integrated allocation for the subscribers in each cell. On the contrary, it is done progressively as new subscribers are added to the system.

  For the downlink channel, each subscriber first measures channel and interference information for all subcarriers and performs well (eg, signal-to-interference plus noise ratio (SINR)). A plurality of subcarriers are selected), and information on the candidate subcarriers is fed back to the base station. This feedback includes channel and interference information (eg, signal-to-interference-plus-noise-ratio information) for all subcarriers or only for some subcarriers. . When providing information about only some of the subcarriers, the subscribers should be able to use the subcarriers in order, starting with the subcarrier they want to use, because they usually have better performance or better than other subcarriers. Provide a list of

  Upon receiving this information from the subscriber, the base station may receive additional information available at the base station, such as traffic load information for each subcarrier, amount of traffic requests waiting at the base station for each frequency band, and excessive frequency bandwidth. Subcarriers are further selected from the candidates by using information such as whether the subscribers are not used for and / or how long the subscriber is waiting for information transmission. In some embodiments, subcarrier loading information of neighboring cells is also exchanged between base stations. The base station uses this information for subcarrier allocation to reduce inter-cell interference.

  In some embodiments, the selection of the encoding / modulation rate is performed as a result of the base station selecting an assigned channel based on feedback. Such a coding / modulation rate may be specified by the subscriber when identifying the subcarriers found to be preferred for use. For example, if the SINR is below a certain threshold (eg 12 dB), quadrature phase shift keying (QPSK) modulation is used, otherwise 16 quadrature amplitude modulation (QAM) is used. The base station then informs the subscriber of the subcarrier allocation and / or encoding / modulation rate used.

  In some embodiments, feedback information regarding downlink subcarrier allocation is transmitted to the base station through an uplink access channel, which is a short time per transmission time slot, eg, 400 within each 10 millisecond time slot. Get up in microseconds. In some embodiments, the access channel occupies the entire frequency bandwidth. The base station can collect the uplink SINR of each subcarrier directly from the access channel. The uplink subcarrier SINR and traffic load information are used for uplink subcarrier allocation.

  In either direction, the base station makes a final determination of subcarrier allocation for each subscriber.

  In the following description, selective subcarrier allocation is described, including channel and interference detection methods, information feedback methods from subscribers to the base station, and algorithms used by the base station for subcarrier selection. Is also disclosed.

  In the following description, numerous details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.

  Some portions of the detailed descriptions that follow are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. Where an algorithm is here, it is generally considered to be a consistent sequence of steps leading to the desired result. Steps are those requiring physical manipulation of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transmitted, combined, compared, and otherwise manipulated. Sometimes it has proven convenient to refer to these signals as bits, numbers, elements, symbols, characters, terms, numbers, etc. mainly because of their common use.

  However, it should be noted that the above and similar terms are associated with appropriate physical quantities and are merely convenient labels used for these quantities. Unless otherwise specified in the following description, throughout the description, explanations using terms such as `` process '' or `` compute '' or `` calculate '' or `` determine '' or `` display '' Manipulate data represented as physical (electronic) quantities in computer system registers and memory, and so on as physical quantities in computer system memory or registers or other such information storage, transmission or display devices It should be understood that it refers to the operation and processing of a computer system or similar electronic computing device that translates into other data expressed in

  The present invention also relates to an apparatus for performing this operation. The apparatus may be specially configured for the required purpose, or it may comprise a general purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such computer programs include, but are not limited to, all types of disks including floppy disks, optical disks, CD-ROMs, and magnetic optical disks, read only memory (ROM), random access memory (RAM), EPROM. , EEPROM, magnetic or optical card, all types of media suitable for storing electronic instructions, etc., each stored on a computer readable storage medium coupled to a computer system bus.

  The algorithms and displays presented here are not inherently related to any particular computer or other apparatus. Various general purpose systems may be used with programs according to the teachings described herein, and it will prove convenient to construct more specialized equipment to perform the necessary method steps. Yes. The required structure for a variety of the above systems will appear from the description below. In addition, the present invention is not described with reference to any particular programming language. It should be understood that various programming languages may be used to implement the teachings of the invention described herein.

  A machine-readable medium includes any mechanism for storing or transmitting information in a form readable by a machine (eg, a computer). For example, machine-readable media may include read only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, electrical, optical, acoustic or other forms of propagation Signals (for example, carrier waves, infrared signals, digital signals, etc.) are included.

Subcarrier Clustering The technique described here focuses on subcarrier allocation for data traffic channels. In cellular systems, there is usually another channel pre-assigned for control information exchange and other purposes. These channels often include downlink and uplink control channels, uplink access channels, and time / frequency synchronization channels.

  FIG. 1A shows a plurality of subcarriers such as subcarrier 101 and a cluster 102. A cluster, such as cluster 102, is defined as a logical unit possessing at least one physical subcarrier, as shown in FIG. 1A. A cluster can have contiguous or disjoint subcarriers. The mapping between a cluster and its subcarriers may be fixed or reconfigurable. In the latter case, the base station informs the subscriber when the cluster will be redefined. In some embodiments, the frequency spectrum includes 512 subcarriers, and each cluster includes 4 consecutive subcarriers, resulting in 128 clusters.

Exemplary Subcarrier / Cluster Assignment Procedure FIG. 1B is a flow diagram illustrating one embodiment of a process for assigning clusters to subscribers. The process is performed by processing logic comprised of hardware (eg, dedicated logic, circuitry, etc.), software (eg, running on a general purpose computer system or dedicated machine, etc.), or a combination of both.

  As shown in FIG. 1B, each base station periodically broadcasts a pilot OFDM symbol to each subscriber in that cell (or sector) (processing block 101). Pilot symbols, also called sounding sequences or signals, are known to both base stations and subscribers. In some embodiments, each pilot symbol covers the entire OFDM frequency bandwidth. The pilot symbol may be different for each cell (or sector). Pilot symbols are used for multiple purposes, such as time and frequency synchronization, channel estimation for cluster assignment and signal-to-interference / noise (SINR) ratio measurements.

  Each subscriber then continuously monitors the reception of pilot symbols and measures the SINR and / or other parameters of each cluster, including inter-cell interference and intra-cell traffic (processing block 102). Based on this information, each subscriber selects one or more clusters with relatively good performance (eg, high SINR and low traffic loading) and passes information about these candidate clusters to a predetermined uplink. Feedback is provided to the base station through the access channel (processing block 103). For example, an SINR value higher than 10 dB indicates good performance. Similarly, a cluster utilization factor of less than 50% represents good performance. Each subscriber selects a cluster that performs better than others. With this selection, each subscriber will select a cluster that may be desirable to use based on the measured parameters.

  In some embodiments, each subscriber measures the SINR of each subcarrier cluster and reports those SINR measurements to the base station through the access channel. The SINR value includes the average of the SINR values of each subcarrier in the cluster. Alternatively, the SINR value of the cluster may be the worst SINR among the SINR values of the subcarriers in the cluster. In yet another embodiment, a weighted average of SINR values of subcarriers within the cluster is used to generate SINR for the cluster. This is particularly useful in diversity clusters with different weightings applied to subcarriers.

  The feedback of information from each subscriber to the base station includes the SINR value for each cluster and also indicates the coding / modulation rate that the subscriber wishes to use. As long as the base station knows the order of the information in the feedback, no cluster index is needed to indicate which SINR value in the feedback corresponds to which cluster. In another embodiment, the information in the feedback is ordered according to which cluster has the best performance for the subscriber. In such a case, an index is necessary to indicate which cluster the attached SINR value corresponds to.

  Upon receiving feedback from the subscriber, the base station then selects one or more clusters for the subscriber from the candidates (processing block 104). The base station sends additional information available at the base station, eg traffic load information for each subcarrier, the amount of traffic requests waiting at the base station for each frequency band, whether the frequency band is overused, Use information such as how long the subscriber is waiting for. Subcarrier loading information of adjacent cells can also be exchanged between base stations. The base station uses this information for subcarrier allocation to reduce inter-cell interference.

  After cluster selection, the base station informs the subscriber about the cluster assignment through the downlink common control channel or through a dedicated downlink traffic channel if a connection to the subscriber has already been established (processing block). 105). In some embodiments, the base station also informs the subscriber about the appropriate modulation / coding rate.

  Once the basic communication link is established, each subscriber continues to send feedback to the base station using a dedicated traffic channel (eg, one or more predetermined uplink access channels). can do.

  In some embodiments, the base station assigns all clusters that the subscriber will use at one time. In another embodiment, the base station initially allocates multiple clusters, referred to herein as basic clusters, and sets up a data link between the base station and the subscriber. The base station then allocates more clusters, referred to herein as auxiliary clusters, to subscribers to increase communication bandwidth. High priority is given to basic cluster assignments, and low priority is given to auxiliary cluster assignments. For example, the base station first ensures the allocation of the basic cluster to the subscriber and then tries to satisfy further requests for auxiliary clusters from the subscriber. Alternatively, the base station may assign the auxiliary cluster to one or more subscribers before assigning the base cluster to other subscribers. For example, the base station may assign the base and auxiliary clusters to one subscriber before assigning any cluster to other subscribers. In some embodiments, the base station assigns a basic cluster to a new subscriber and then determines whether there are other subscribers requesting the cluster. If not, the base station assigns an auxiliary cluster to the new subscriber.

  In some cases, processing logic performs retraining by repeating the above process (processing block 106). Re-education is carried out periodically. This retraining compensates for subscriber movement and any interference changes. In some embodiments, each subscriber reports its updated cluster selection and associated SINR to the base station. The base station then reselects and notifies the subscriber of the new cluster assignment. Retraining can be initiated by the base station, in which case the base station requests a particular subscriber to report its updated cluster selection. Retraining can also be initiated from the subscriber side if channel degradation is observed.

Adaptive Modulation and Coding In certain embodiments, different modulation and coding rates are used to support reliable transmission on channels with different SINRs. Signal spreading with multiple subcarriers may be used to improve reliability with very low SINR.

An example of the encoding / modulation table is shown in Table 1 below.

  In the above example, 1/8 spreading indicates that one QPSK modulation symbol is repeated over 8 subcarriers. Iteration / spreading is further extended to the time domain. For example, one QPSK symbol can be repeated over four subcarriers of two OFDM codes, resulting in 1/8 spreading.

  The coding / modulation rate is adaptively changed according to the channel state observed on the receiver side after the initial cluster allocation and rate selection.

Pilot Symbol and SINR Measurement In one embodiment, each base station transmits pilot symbols simultaneously, and each pilot symbol occupies the entire OFDM frequency bandwidth as shown in FIGS. 2A-C. As shown in FIGS. 2A-C, it can be seen that pilot symbol 201 extends across the entire OFDM frequency band for cells A, B, and C, respectively. In one embodiment, each pilot symbol is approximately 152 microseconds in total with a guard time of 128 microseconds in length or duration. Each pilot period is followed by a predetermined number of data periods, followed by another set of pilot symbols. In one embodiment, there are four data periods used for data transmission after each pilot, each data period being 152 microseconds.

  The subscriber evaluates the SINR for each cluster from the pilot symbols. In some embodiments, the subscriber first evaluates the channel response, including amplitude and phase, without interference or noise. Once the channel is evaluated, the subscriber calculates interference / noise from the received signal.

  The evaluated SINR values are arranged in descending order, and a cluster having a large SINR value is selected. In some embodiments, the selected cluster has a SINR value that is greater than the minimum SINR that allows reliable (albeit slow) transmissions supported by the system. The number of clusters selected depends on the feedback bandwidth and the request transmission rate. In some embodiments, the subscriber always tries to send information about as many clusters as possible, from which the base station selects.

  The estimated SINR value is also used to select the appropriate encoding / modulation rate for each cluster as described above. Using a suitable SINR indexing scheme, the SINR index can also indicate the specific coding and modulation rate that the subscriber wishes to use. Note that even for the same subscriber, different clusters may have different modulation / coding rates.

  Pilot symbols have an additional purpose in determining inter-cell interference. Since the pilots of multiple cells are broadcast simultaneously, they interfere with each other (since they occupy the entire frequency band). This collision of pilot symbols can be used to determine the amount of interference as a worst case scenario. Thus, in one embodiment, the SINR estimation using this method is that the measured interference level is the worst case scenario assuming that all interferers are operating, Be cautious. Thus, the structure of the pilot symbols occupies the entire frequency band and causes collisions between different cells to be used to detect the worst-case SINR in a packet transmission system.

  During the data traffic period, the subscriber can again determine the level of interference. The data traffic period is used to evaluate intra-cell traffic as well as inter-cell interference level. Specifically, the pilot and the power difference during the traffic period are used to sense (in-cell) traffic loading and inter-cell interference to select the desired cluster.

  In some clusters, the interference level is relatively low because it is not used in an adjacent cell. For example, in cell A, there is almost no interference with respect to cluster A because it is not used in cell B (although it is used in cell C). Similarly, in cell A, cluster B suffers slight interference from cell B because it is not used in cell C but is used in cell B.

  The modulation / coding rate based on this evaluation is strong against frequent interference changes due to explosive packet transmissions. This is because the speed prediction is based on the worst case scenario where all interferers are transmitting.

  In some embodiments, the subscriber uses information available from both the pilot symbol period and the data traffic period to analyze the presence of both intra-cell traffic load and inter-cell interference. The purpose of the subscriber is to provide an indication to the base station about the cluster that the subscriber wants to use. Ideally, the subscriber selection results in a cluster with high channel gain, low interference from other cells, and high availability. The subscriber provides feedback information including the results and lists the desired clusters in order or in a manner not described here.

  FIG. 3 illustrates one embodiment of subscriber processing. Processing is performed by processing logic comprising hardware (eg, dedicated logic, circuitry, etc.), software (eg, executed on a general purpose computer system or a dedicated machine), or a combination of both.

  As shown in FIG. 3, channel / interference estimation processing block 301 performs channel and interference estimation within a pilot period in response to pilot symbols. The traffic / interference analysis processing block 302 performs traffic and interference analysis within the data period in response to the signal information and information from the channel / interference evaluation block 301.

  Cluster ordering and speed prediction processing block 303 is coupled to the output of channel / interference evaluation processing block 301 and traffic / interference analysis processing block 302 to perform cluster ordering and selection along with speed prediction.

  The output of cluster ordering processing block 303 is input to cluster request processing block 304, which requires clusters and modulation / coding rates. An indication of these selections is sent to the base station. In some embodiments, the SINR of each cluster is reported to the base station through the access channel. This information is used for cluster selection to avoid clusters falling into severe intra-cell traffic loading and / or experiencing severe interference from other cells. That is, a new subscriber is not assigned to use a cluster if severe intra-cell traffic loading already exists for a particular cluster. In addition, if the interference is intense and the SINR is low and only low-speed transmission is possible or reliable transmission is not possible at all, the cluster is not assigned.

Channel / interference assessment by processing block 301 is well known in the art by monitoring interference that occurs due to simultaneous broadcast of full bandwidth pilot symbols in multiple cells. The interference information is sent to processing block 302, which uses that information to solve the following equation:
H i S i + I i + n i = y i
Here, S i represents a signal of subcarrier (freq.band) i, I i is interference of subcarrier i, n i is noise associated with subcarrier i, and y i is subcarrier. This is an observation of i. For 512 subcarriers, i is in the range of 0 to 511. I i and n i are not separated and may be considered as one quantity. Interference / noise and channel gain Hi are not known. During the pilot period, the signal S i representing the pilot symbol and the observation y i are known, so that the channel gain H i for the case of no interference or noise can be determined. Once this is known, since H i , S i , and y i are all known, the equation can be applied to determine the interference / noise during the data period.

  Using the interference information from processing blocks 301 and 302, the subscriber selects the desired cluster. In one embodiment, using processing block 303, the subscriber orders the clusters and predicts the data rate that would be available using such clusters. Predicted data rate information can be obtained from a look-up table with pre-calculated data rate values. Such a lookup table stores each SINR and the desired transmission rate pair associated with it. Based on this information, the subscriber selects a desired cluster based on predetermined performance criteria. Using the cluster order list, the subscriber requests the desired cluster along with the coding and modulation rates known to the subscriber to achieve the desired data rate.

  FIG. 4 is an embodiment of an apparatus for selecting clusters based on power differences. This approach performs energy detection using information available during both the pilot symbol period and the data traffic period. The processing of FIG. 4 is implemented in hardware (eg, dedicated logic, circuitry, etc.), software (eg, executed on a general purpose computer system or a dedicated machine), or a combination of both.

  As shown in FIG. 4, the subscriber performs SINR evaluation processing block 401 for performing SINR evaluation for each cluster within the pilot period, and power calculation processing block 402 for performing power calculation for each cluster within the pilot period. , And a power calculation processing block 403 for performing power calculation for each cluster within the data period. The subtractor 404 subtracts the power calculation during the data period from processing block 403 from the power calculation during the pilot period from processing block 402. The output of the subtractor 404 is input to a power difference ordering (and group selection) processing block 405, which performs cluster ordering and selection based on the SINR and the power difference between the pilot and data periods. Do. Once a cluster is selected, the subscriber requests the selected cluster and encoding / modulation rate at processing block 406.

  More specifically, in some embodiments, the signal power of each cluster during the pilot period is compared to that during the traffic period based on:

Where P P is the power corresponding to each cluster measured during the pilot period, P 0 is the power measured during the traffic period, P S is the signal power, and P I is the interference power. Yes, P N is the noise power.
In some embodiments, the subscriber selects a cluster with a relatively large P P / (P P −P D ), if possible, (eg, greater than a threshold, such as 10 dB), and P P / ( Avoid clusters where P P -P D ) is relatively small (eg, less than a threshold such as 10 dB).
Alternatively, the difference may be based on the energy difference between samples observed during the pilot period and samples observed during the data traffic period for each subscriber in the cluster, such as:

In this way, the subscriber sums the differences for all subcarriers.

Depending on the actual embodiment, the subscriber uses the following measures to select a cluster: SINR and P P -P D combined function.
β = f (SINR, P P / (P P −P D )
Here, f is a function of two inputs. One example of f is a weighted average (eg, equal weight). Alternatively, the subscriber may select clusters based on SINR and use only the power difference P P -P D to determine clusters with similar SINR. The difference may be smaller than a threshold value (for example, 1 dB).

And reduce the dispersion in order to increase the accuracy, the measurement of SINR and P P -P D it may be averaged over time. Some embodiments use a moving average time window, which is long enough to average statistical anomalies and short enough to capture the time-varying characteristics of the channel and interference, for example 10 Milliseconds.

Feedback Format for Downlink Cluster Assignment In one embodiment, for the downlink, the feedback carries both the index of the selected cluster and its SINR. A typical format for arbitrary cluster feedback is shown in FIG. As shown in FIG. 5, the subscriber provides a cluster index (ID) to indicate the cluster and its associated SINR value. For example, in the feedback, the subscriber has the cluster ID 1 (501) and the SINR1 (502) that is the SINR of the cluster, the cluster ID2 (503) and the SINR2 (504) that is the SINR of the cluster, and the cluster ID3 (505) and SINR3 (506) that is the SINR of the cluster is provided. The cluster SINR can be made using the average of the subcarrier SINRs. Thus, a plurality of arbitrary clusters can be selected as candidates. As described above, the selected clusters can also be ordered in the feedback to indicate priority. In some embodiments, the subscriber creates a priority list of clusters and returns SINR information in descending order of priority.

  In general, displaying the index for the SINR level instead of the SINR itself is sufficient to indicate the appropriate encoding / modulation for the cluster. For example, a 3 bit field may be used for SINR indexing to indicate 8 different rates of adaptive encoding / modulation.

The representative base station base station assigns the desired cluster to the requesting subscriber. In some embodiments, the availability of clusters allocated to subscribers depends on the total traffic load on the cluster. Therefore, the base station selects a cluster that not only has a high SINR but also a low traffic load.

  FIG. 13 is a block diagram of an embodiment of a base station. As shown in FIG. 13, the cluster allocation and load scheduling controller 1301 (cluster allocator) is configured to specify the downlink / uplink SINR (for example, the SINR / rate index received from the OFDM receiver 1305) of the cluster designated for each subscriber. Signal 1313) and all necessary information including user data, ie queue fullness / traffic load (eg, via user data buffer information 1311 from multi-user data buffer 1302). Using this information, the control device 1301 determines cluster allocation and load scheduling for each user, and stores the determination information in a memory (not shown). Controller 1301 informs the subscriber about the decision through a control signal channel (eg, control signal / cluster assignment 1312 via OFDM receiver 1305).

  In some embodiments, the controller 1301 also knows the traffic load of the system and thus performs ingress control for user access. This is done by controlling user data buffer 1302 using entry control signal 1310.

  The user 1-N packet data is stored in the user data buffer 1302. For the downlink, under the control of the control device 1301, the multiplexer 1301 loads user data into the cluster data buffer (in cluster 1-M) waiting for transmission. For the uplink, the multiplexer 1303 transmits the data in the cluster buffer to the corresponding user buffer. Cluster buffer 1304 stores signals to be transmitted through (downlink) OFDM transceiver 1305 and signals received from transceiver 1305. In some embodiments, each user occupies multiple clusters, and each cluster is shared by multiple users (in the case of time division multiplexing).

In another embodiment of group-based cluster allocation , in the case of the downlink, the clusters are divided into groups. Each group may include a plurality of clusters. FIG. 6 shows a typical division example. As shown in FIG. 6, groups 1-4 are indicated by arrows pointing to the clusters that are placed in each group as a result of grouping. In some embodiments, the clusters in each group are far apart across the entire bandwidth. In some embodiments, the clusters within each group are further separated than the channel coherence bandwidth, i.e., the bandwidth within which the channel response is approximately the same. A typical value for coherent bandwidth is 100 kHz for many cellular systems. This will improve frequency diversity within each group and increase the probability that at least some of the clusters in the group can provide high SINR. Clusters are assigned to groups. The purpose of group-based cluster allocation is to reduce the data bits for cluster indexing, thereby relaxing the bandwidth requirements of the feedback channel (information) and control channel (information) for cluster allocation. Group-based cluster assignment is also used to reduce inter-cell interference.

  After receiving the pilot signal from the base station, the subscriber returns channel information for one or more cluster groups simultaneously or sequentially. In some embodiments, only information about some of the groups is returned to the base station. A number of criteria can be used for group selection and ordering based on channel information, inter-cell interference levels, and intra-cell traffic load at each cluster.

  In some embodiments, the subscriber first selects the group with the best overall performance and feeds back the SINR information for the cluster in that group. The subscriber orders the groups based on the number of clusters whose SINR is higher than a predefined threshold. By transmitting the SINRs of all the clusters in the group, it is only necessary to transmit the group indexes, not all the cluster indexes. As described above, the feedback of each group generally holds two types of information, that is, a group index and an SINR value of each cluster in the group. FIG. 7 shows an exemplary format for displaying group-based cluster assignment. As illustrated in FIG. 7, the SINR value of each cluster in the group follows ID1 that is the group ID. This greatly reduces the feedback overhead.

  Upon receiving feedback information from the subscriber, the base station cluster allocator selects multiple clusters, if available, from one or more groups and then assigns the clusters to the subscriber. This selection is performed by assignment in the medium access control unit of the base station.

  Further, in a multi-cell environment, groups may have different priorities associated with different cells. In some embodiments, the group selection by a subscriber is biased by the priority of the group, which means that some subscribers have a higher priority than other subscribers with respect to the right to use a group. It means having.

  In some embodiments, there is no fixed relationship between a subscriber and a cluster group, but in another embodiment there is such a fixed relationship. In embodiments having a fixed relationship between a subscriber and one or more cluster groups, the group index in the feedback information is omitted because this information is known by default to both the subscriber and the base station. can do.

  In another embodiment, the pilot signal transmitted from the base station to the subscriber also indicates the availability of each cluster, e.g., the pilot signal is assigned to which other clusters are already assigned to other subscribers. And which clusters are available for new assignments. For example, the base station transmits a pilot sequence 1111 1111 regarding the subcarriers of the cluster, indicating that the cluster is available, and transmits 1111-1-1-1-1 and the cluster is not available. Represents the effect. At the receiver, the subscriber first distinguishes the two sequences using signal processing methods well known in the art, eg, correlation methods, and then evaluates the channel and interference level.

  By combining the present invention with the channel characteristics obtained by a subscriber, the subscriber can prioritize groups and achieve both high SINR and good load balance.

  In some embodiments, the subscriber protects the feedback information by using an error correction code. In some embodiments, the SINR information in the feedback is first compressed using a source encoding technique, eg, differential encoding, and then encoded with a channel code.

  FIG. 8 illustrates one embodiment of a frequency reuse pattern for a typical cellular setup. Each cell has a hexagonal structure with six sectors using directional antennas at the base station. Between cells, the frequency reuse factor is 1. Within each cell, the frequency reuse factor is 2, in which case the sector uses two frequencies alternately. As shown in FIG. 8, each shaded sector uses half of the available OFDMA clusters, and each unshaded sector uses the other half of the cluster. Without loss of generality, clusters used in shaded sectors are referred to herein as odd clusters, and those used in unshaded sectors are referred to as even clusters herein.

  Consider downlink signal transmission by an omnidirectional antenna on the subscriber side. From FIG. 8, for the shaded sector downlink, cell A interferes with cell B, cell B interferes with cell C, and cell C interferes with cell A, ie A-> B-> C-> A. It is clear that For the shadowless sector, cell A interferes with cell C, cell C interferes with cell B, and cell B interferes with cell A, ie A-> C-> B-> A.

  Sector A1 receives interference from sector C1, but its transmission interferes with sector B. That is, the source of the interference and the victim it interferes with are not the same. This creates a stability problem for a distributed cluster-assignment system using interference avoidance, because if a frequency cluster is assigned to sector B1 instead of sector C1, that cluster will appear clean in AI. Assigned to A1. However, this allocation of cluster A1 causes an interference problem with the existing allocation of B1.

  In some embodiments, different cluster groups are assigned different priorities for use in different cells to alleviate the problem when traffic load is progressively added to the sector. This priority is assigned in an integrated manner to selectively assign clusters to avoid interference from their interference sources and to reduce, and possibly minimize, the possibility of causing interference problems for existing assignments of other cells. It is prescribed.

  Using the example described above, odd clusters (used for shaded sectors) are divided into three groups, groups 1, 2, and 3. Table 2 shows the priority order.

  Consider sector A1. Initially, the group 1 clusters are selectively assigned. If there are still more subscribers requesting a cluster, the cluster in group 2 is selectively assigned to the subscriber by the measured SINR (avoids clusters subject to strong interference from sector C1). The cluster newly assigned to sector A1 from group 2 is in a state where the load of sector B1 is very heavy, both the groups 3 and 1 are used up, and the cluster of group 2 is also used. Unless this is the case, it will not cause interference problems in sector B1. Table 3 shows cluster usage when less than 2/3 of all available clusters are used in sectors A1, B1, and C1.

  FIG. 4 shows the priority order of the non-shaded sector, but is different from the shaded sector because the interference relationship is reversed.

The intelligent switching some embodiments between coherence clusters and diversity clusters, two categories of clusters, i.e. the coherence clusters to hold a plurality of sub-carriers are close to each other, at least a portion far across the spectrum There are diversity clusters that have subcarriers that are spread apart. The proximity of the subcarriers within the coherence cluster is preferably within the channel coherence bandwidth, i.e., within the bandwidth where the channel response is approximately the same, specifically within most 100 kHz in many cellular systems. In contrast, the spread of subcarriers in the diversity cluster is desirably wider than the channel coherence bandwidth, which is typically within 100 kHz in many cellular systems. Thus, the general goal in such cases is to maximize diffusion.

  FIG. 9 shows an exemplary cluster format of coherence clusters and diversity clusters for cells A-C. As shown in FIG. 9, for cells A-C, frequency (subcarrier) labeling indicates whether the frequency is part of a coherence cluster or part of a diversity cluster. For example, the frequency labeled 1-8 is a diversity cluster and the cluster labeled 9-16 is a coherence cluster. For example, all frequencies labeled 1 in a cell are part of one diversity cluster and all frequencies labeled 2 in a cell are part of another diversity cluster, On the other hand, the frequency group labeled 9 is one coherence cluster, the frequency group labeled 10 is another coherence cluster, and so on. Diversity clusters can also be configured differently for different cells in order to reduce the effects of inter-cell interference through interference averaging.

  FIG. 9 shows an example cluster configuration of three adjacent cells. Interference from a specific cluster in one cell is distributed to many clusters in other cells, for example, interference from cluster 1 in cell A is distributed to clusters 1, 8, 7, 6 in cell B The This significantly reduces the interference power for any particular cluster in cell B. Similarly, interference for any particular cluster in one cell comes from many different clusters in other cells. A diversity cluster that performs channel coding across subcarriers provides interference diversity gain because not all clusters cause strong interference. Therefore, it is effective to assign diversity clusters to subscribers that are close to the cell boundary (eg, within the coherent bandwidth) and are susceptible to inter-cell interference.

  Since the subcarriers in the coherence cluster are contiguous or close to each other (eg, within the coherent bandwidth), they are often within the coherent bandwidth of channel fading. Therefore, the channel gain of the coherence cluster varies greatly, and its performance can be greatly improved by cluster selection. On the other hand, the average channel gain of the diversity cluster has a small variation due to the inherent frequency diversity among the plurality of subcarriers spread over the spectrum. Since channel coding is performed over the entire subcarriers in the cluster, the diversity cluster is strong against cluster selection errors (due to its diversification nature), but the gain from cluster selection is small. Channel coding over the entire subcarrier means that each codeword includes bits transmitted from multiple subcarriers, and more specifically, the difference bits (error vectors) between codewords. This means that it is distributed among a plurality of subcarriers.

  More frequency diversity can be obtained through subcarrier hopping over time, and subscribers occupy one set of subcarriers in one time slot and another different set of subcarriers in another time slot It will be. One coding unit (frame) has a plurality of such time slots, and transmitted bits are encoded over the entire frame.

  FIG. 10 shows a diversity cluster with subcarrier hopping. As shown in FIG. 10, each of cell A and cell B shown in the figure has four diversity clusters, and each subcarrier in each diversity cluster has the same label (1, 2, 3, or 4). is doing. Although four separate time slots are shown, the subcarriers for each diversity cluster change during each time slot. For example, in cell A, subcarrier 1 is part of diversity cluster 1 during time slot 1, part of diversity cluster 2 during time slot 2, and diversity cluster 3 during time slot 3. The time slot 4 is part of the diversity cluster 4. In this way, more interference diversity can be obtained through subcarrier hopping over time, and by using different hopping patterns for different cells, as shown in FIG. 10, more interference diversity can be obtained. realizable.

  The manner in which the subscriber changes the subcarrier (hopping sequence) may vary from cell to cell in order to achieve better interference averaging through coding.

  For stationary subscribers such as fixed wireless access, the channel rarely changes over time. Selective cluster assignment using coherence clusters achieves good performance. On the other hand, for mobile subscribers, channel time fluctuations (fluctuations due to channel changes over time) may be very large. A cluster with high gain at one time can fall into severe fading at another time. Therefore, the cluster allocation must be updated at a high speed, and the control overhead becomes enormous. In this case, if a diversity cluster is used, strength can be reinforced and overhead of frequent cluster reassignment can be reduced. In some embodiments, the cluster allocation is often the channel change rate measured in channel Doppler rate (Hz), i.e., how many cycles per second the channel is changed if the channel is completely different after one cycle. Is also performed at high speed. Note that selective cluster assignment can be performed for both coherence clusters and diversity clusters.

  In some embodiments, a channel / interference variation detector can be implemented in a subscriber or base station, or both, in the case of a mixed cell of mobile and stationary subscribers. Using the detection results, subscribers and base stations can intelligently identify diversity clusters for mobile or stationary subscribers on cell boundaries and coherence clusters for stationary subscribers close to the base station. select. The channel / interference variation detector measures channel (SINR) variation from time to time for each cluster. For example, in one embodiment, the channel / interference detector measures the power difference between the pilot symbols of each cluster and averages the difference over the moving window (eg, four time slots). A large difference indicates that the channel / interference changes frequently and the subcarrier assignment is unreliable. In such cases, a diversity cluster would be more desirable for the subscriber.

  FIG. 11 is a flow diagram of an embodiment of a process for making an intelligent selection between a diversity cluster and a coherence cluster based on subscriber movement. The process is performed by processing logic comprised of hardware (eg, circuitry, dedicated logic, etc.), software (eg, executed on a general purpose computer system or a dedicated machine), or a combination thereof.

  As shown in FIG. 11, the processing logic of the base station performs channel / interference variation detection (processing block 1101). Processing logic then tests whether the channel / interference variation detection results indicate that the user is moving or is in a stationary position near the edge of the cell (processing block 1102). If the user is not moving or not at a stationary position near the edge of the cell, the process moves to processing block 1103 where the base station logic selects a coherence cluster, otherwise the process Moving to block 1104, the base station processing logic selects a diversity cluster.

  In some embodiments, the base station detects the rate of change of the pilot signal, i.e., normalized channel variation, and determines that the rate of change is greater than a pre-set threshold so that the subscriber Determine whether you are moving or stationary. The normalized instantaneous difference between channels is expressed as:

Here, H i represents a channel, and i is an index representing an individual channel.

  The threshold varies from system to system. For example, if the rate of change is greater than 10% (although it can be any percentage (eg, 20%)), the base station concludes that the subscriber is moving. In some embodiments, if the invariant period in signal transmission is not greater than several times the round trip delay (eg, five times the round trip delay), the base station determines that the subscriber is moving. A diversity cluster is assigned, otherwise the base station assigns a coherence cluster.

  Selections can be updated and intelligently switched during retraining.

  The ratio / allocation of the number of coherence clusters and diversity clusters in a cell depends on the ratio of mobile subscribers to stationary subscribers. As the number of people changes as the system evolves, the allocation of coherence and diversity clusters is reconfigured to accommodate the needs of the new system. FIG. 12 shows a cluster classification reconfiguration that can support more mobile subscribers than FIG.

  It should be obvious to those skilled in the art after reading the above description that various changes and modifications can be made to the present invention. It should be understood that all embodiments are not intended to be limiting. Accordingly, the details of various embodiments are not intended to limit the scope of the claims that detail the features considered essential to the invention.

Subcarriers and clusters are shown. FIG. 4 is a flow diagram of an embodiment of a process for assigning subcarriers. The time and frequency grid of OFDM symbols, pilots and clusters is shown. The processing of the subscriber is shown. An example of FIG. 3 is shown. Fig. 4 illustrates an embodiment of a format for arbitrary cluster feedback. Fig. 4 illustrates an embodiment for dividing a cluster into groups. Fig. 4 illustrates an embodiment of a feedback format for group-based cluster assignment. Figure 2 illustrates frequency reuse and interference in a multi-cell multi-sector network. 2 shows separate cluster formats for coherence clusters and diversity clusters. Fig. 2 shows a diversity cluster with subcarrier hopping. Fig. 4 illustrates intelligent switching between diversity and coherence clusters due to subscriber movement. Fig. 4 illustrates an embodiment of cluster classification reconfiguration. 1 illustrates an embodiment of a base station.

Claims (10)

  1. In a method for use in allocating subcarriers in an OFDMA system,
    Assigning a diversity cluster of at least one subcarrier to a first subscriber;
    Allocating at least one coherence cluster to a second subscriber, wherein the at least one diversity cluster and the at least one coherence cluster are simultaneously used to communicate with the first and second subscribers, respectively. Said step in which
    A method comprising reconfiguring a cluster classification when the population of moving and stationary subscribers in a cell changes .
  2.   The method of claim 1, further comprising transmitting information using one diversity cluster while performing frequency hopping.
  3.   The method of claim 1, wherein using the one diversity cluster includes channel coding over all subcarriers of the one diversity cluster.
  4.   The method of claim 1, wherein the subcarriers of one coherence cluster are within the coherence bandwidth of the channel between the base station and the subscriber.
  5. In a method for use in allocating subcarriers within an OFDMA system,
    Determining whether the subscriber is moving or stationary;
    If it is determined that the subscriber is moving, assigning the subscriber a diversity cluster of at least one subcarrier;
    If the subscriber is determined to be stationary, the steps allocating the coherence cluster of at least one subcarrier to the subscriber, formed Ri from
    The step of determining whether a subscriber is moving or stationary includes measuring a power difference between pilot symbols for each cluster and averaging the difference over a window of time slots. A method characterized by comprising .
  6. The step of determining whether the subscriber is moving or stationary includes detecting the rate of change of the pilot signal, and if the rate of change is greater than a preset amount, the subscriber is moving 6. The method of claim 5 , further comprising the step of displaying:
  7. 6. The method of claim 5 , wherein the step of determining whether a subscriber is moving or stationary comprises the step of periodically measuring channel variations.
  8. 6. The method of claim 5 , wherein the time slot window comprises a time slot moving window.
  9. With subscribers,
    A base station that is communicably connected to the subscriber and comprises a subcarrier allocator and a fluctuation detector;
    The fluctuation detector is for detecting channel fluctuation, and the subcarrier allocator is a diversity cluster of one or more subcarriers based on a result of channel fluctuation detection by the fluctuation detector, or Assigning one or more sub-carrier coherence clusters to the subscriber;
    The variation detector measures the power difference between pilot symbols for each cluster and averages the difference over a window of time slots.
  10. The apparatus of claim 9 , wherein the time slot window comprises a time slot moving window.
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US09/837,701 US7146172B2 (en) 2000-12-15 2001-04-17 Multi-carrier communications with adaptive cluster configuration and switching
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