WO2001073972A1 - Ordonnanceur de paquets et procede associe - Google Patents

Ordonnanceur de paquets et procede associe Download PDF

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Publication number
WO2001073972A1
WO2001073972A1 PCT/EP2001/003548 EP0103548W WO0173972A1 WO 2001073972 A1 WO2001073972 A1 WO 2001073972A1 EP 0103548 W EP0103548 W EP 0103548W WO 0173972 A1 WO0173972 A1 WO 0173972A1
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WO
WIPO (PCT)
Prior art keywords
power
cell
cells
max
packets
Prior art date
Application number
PCT/EP2001/003548
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English (en)
Inventor
Stephen Hill
Original Assignee
Motorola Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Motorola Inc. filed Critical Motorola Inc.
Priority to AU2001246517A priority Critical patent/AU2001246517A1/en
Publication of WO2001073972A1 publication Critical patent/WO2001073972A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W52/00—Power management, e.g. TPC [Transmission Power Control], power saving or power classes
    • H04W52/04—TPC
    • H04W52/18—TPC being performed according to specific parameters
    • H04W52/26—TPC being performed according to specific parameters using transmission rate or quality of service QoS [Quality of Service]
    • H04W52/265—TPC being performed according to specific parameters using transmission rate or quality of service QoS [Quality of Service] taking into account the quality of service QoS
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W52/00—Power management, e.g. TPC [Transmission Power Control], power saving or power classes
    • H04W52/04—TPC
    • H04W52/30—TPC using constraints in the total amount of available transmission power
    • H04W52/34—TPC management, i.e. sharing limited amount of power among users or channels or data types, e.g. cell loading

Definitions

  • This invention relates to a packet scheduler and method therefor, and in particular to a packet scheduler for a cellular communication system for mobile communication.
  • remote terminals In a cellular communication system remote terminals (typically mobile stations) communicate with a fixed base station. Communication from the mobile stations to the base station is known as uplink and communication from the base station to the mobile stations is known as downlink.
  • the total coverage area of the system is divided into a number of separate cells, each covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells.
  • the communication link will change from being between the mobile station and the base station of the first cell, to being between the mobile station and the base station of the second cell. This is known as a handover or handoff.
  • All base stations are interconnected by a fixed network.
  • This fixed network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network.
  • a call from a mobile station is routed through the fixed network to the destination specific for this call. If the call is between two mobile stations of the same communication system the call will be routed through the fixed network to the base station of the cell in which the other mobile station currently is. A connection is thus established between the two serving cells through the fixed network.
  • PSTN Public Switched Telephone Network
  • the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.
  • PSTN Public Switched Telephone Network
  • a cellular mobile communication system is allocated a frequency spectrum for the radio communication between the mobile stations and the base stations. This spectrum must be shared between all mobile stations simultaneously using the system.
  • CDMA Code Division Multiple Access
  • DS-CDMA Direct Sequence CDMA
  • the signals are multiplied by a high rate code whereby the signal is spread over a larger frequency spectrum.
  • a narrowband signal is thus spread and transmitted as a wideband signal.
  • the original narrowband signal is regenerated by multiplication of the received signal with the same code.
  • a signal spread by use of a different code will at the receiver not be de- spread but will remain a wide band signal.
  • the majority of interference caused by interfering signals received in the same frequency spectrum as the wanted signal can thus be removed by filtering.
  • CDMA communication systems can be found in ⁇ Spread Spectrum CDMA Systems for Wireless Communications' , Glisic & Vucetic, Artech house Publishers, 1997, ISBN 0- 89006-858-5. Examples of CDMA cellular communication systems are IS 95 standardised in North America and the Universal Mobile Telecommunication System (UMTS) currently under standardisation in Europe.
  • UMTS Universal Mobile Telecommunication System
  • schedulers have been optimised for different environments than CDMA systems.
  • scheduling algorithms used for GPRS are optimised for a Time Division Multiple Access (TDMA) system and therefore not optimal for CDMA systems where codes and power must be shared.
  • TDMA Time Division Multiple Access
  • This invention aims to provide a packet scheduler suitable for use in a CDMA system and which enables an efficient sharing of network resource.
  • the present invention comprises apparatus for scheduling queued data packets for transmission between a plurality of base stations, each serving a respective cell in a cellular communications network, and a plurality of mobile stations located therein, the apparatus comprising: means for assigning an associated power bin to each cell into which at least some of the queued data packets are to be placed, means for setting a maximum power level value for each power bin and for adjusting the maximum power level value for each power bin dependent on the quality of service experienced in the respective cell.
  • the present invention comprises a method for scheduling queued data packets for transmission between a plurality of base stations, each serving a respective cell in a cellular communications system, and a plurality of mobile stations located therein, the method including the steps of:
  • Fig.l is a schematic block diagram of a cellular communications system operating in accordance with the invention.
  • FIGs. 2, 3 & 4 are schematic diagrams illustrating the operation of a packet scheduler in accordance with the invention.
  • Fig. 5 is a flow chart illustrating operation of a specific embodiment of the invention.
  • Fig. 6 shows a table illustrating a 'bottom up' adaptive technique used to determine the maximum power budget that can be used for scheduling packets within a distributed uplink scheduler.
  • FIG 1 shows a schematic diagram of an embodiment of a CDMA communication system in accordance with an embodiment of the invention.
  • the communication system has a number of base stations 1, 2, 3 each covering a geographical area and thereby defining a cell.
  • a number of mobile stations 4, 5 are associated with the communication system and communicate to each other or to other systems via the base stations 1, 2, 3.
  • the base stations are connected to a common Radio Network Controller (RNC) 6.
  • RNC Radio Network Controller
  • the RNC further provides gateways to other communication systems such as the fixed public telephone system (not shown) and contains a scheduler 7.
  • Each of the mobile stations 4, 5 have independent communication needs and communicate by use of data packets.
  • the mobile stations 4, 5 may require different services and can for example be Internet browsers, telephones or data terminals.
  • Each remote terminal may also request different services at different times.
  • the resource requirement for each individual mobile station may vary significantly over time so that a mobile station may sometimes require no transmissions and at other times require long transmissions at high data rate.
  • the resource requirement for each mobile station from the communication network can thus vary significantly and in order to ensure that the available network capacity is used optimally an efficient scheduling of packets for the different mobile stations is required. This task is performed by the scheduler 7.
  • a frame based communication system such as UMTS the communication is divided into discrete time intervals or frames and the communication resource is allocated on a per frame basis.
  • packets to be transmitted are scheduled during one time frame and transmitted during a subsequent frame.
  • the total data throughput is affected by the interference at a mobile station' s receiver caused by transmissions from base stations other than its serving base station.
  • a successful sharing of the available resources of the system requires a knowledge of these interference levels so that the signal to noise ratio for each connection is appropriate for the desired quality of service (QoS) .
  • QoS quality of service
  • transmissions from non-serving base stations must therefore be taken into account and scheduling in all cells must be done simultaneously.
  • the scheduler' s task is to divide the available transmit power of the base station (BS) between data packets removed in sequence from the queue. For each transmission the signal to noise ratio (SNR) at the destination mobile station must equal that necessary to achieve the agreed QoS.
  • SNR signal to noise ratio
  • equation ( 1 ) To determine the transmit power to a mobile, equation ( 1 ) must be solved for ⁇ l , giving
  • the path loss to a mobile station, the loss of orthogonality factor and the thermal noise power can all be measured by known techniques and reported to the scheduler.
  • P c can be set by the network operator and so can a target
  • Intercell interference can be calculated from knowledge of transmit power of the other base stations in the network and from the path loss.
  • Equation (2) a value for fractional power of every mobile station operative in a cell can be found.
  • the total power transmitted (during a frame) from a base station will then be the sum of the individual ⁇ , values ( ⁇ , ) multiplied by
  • the number of packets that can be transmitted by a particular base station during a frame is similarly limited.
  • Fig. 2 shows a schematic representation of the constituent parts of the scheduler 7.
  • a single queue of packets 8 is allocated as necessary to one of three power bins 9, 10, 11 which relate respectively to base stations 1, 2 and 3.
  • Packets labelled 1 are destined for transmission by base station 1
  • packets labelled 2 are destined for base station 2
  • packets labelled 3 are destined for base station 3.
  • a maximum value for the power bins 9, 10, 11 is set at
  • a power bin represents the total available power at a base station, for either reception or transmission, which is to be shared by i users.
  • the height of a data packet shown in the power bins of Figs. 2, 3 and 4 represents the amount of power required by a user per frame/scheduling period.
  • Fig. 3 shows the situation part-way through a frame/scheduling period.
  • the scheduler has scheduled and calculated the values for ⁇ ( . for a number of packets destined for base stations 1, 2 3 and begun to fill up the power bins 9, 10, 11 accordingly.
  • the value for power transmitted by every base station is taken to be P 0 max.
  • Fig. 3 the power bin 9 for base station 1 is completely full with some packets in the queue 8 still remaining to be scheduled.
  • power bins 10 and 11 are not yet full, so more packets are required to be scheduled for base stations 2 and 3 during this frame.
  • base stations 2 and 3 have spare resource which could be utilised by the over-loaded base station 1.
  • base stations 2 and 3 would transmit at a lower power because there would not be sufficient packets for the cells to reach P 0 max (no packets queued for the cells would remain in the single queue.) Therefore, there resulted some unused capacity in the network.
  • the present embodiment can utilise the unused capacity and further accommodate hot spots as follows.
  • P c max for power bins 10 and 11 is lowered to a new value P 0 max (new) [chosen by one of several methods to be discussed below] .
  • the scheduler 7 then re-calculates the values for ⁇ , for the base station 1 based on values for intercell interference determined from the adjusted values P 0 max (new) .
  • the sum of the new ⁇ , values will now have a value of less than unity. This means that more packets (remaining in queue 8) can be scheduled to base station 1 into power bin 9) yet still keeping total transmit power within the P 0 max limit.
  • Fig. 4 The re-configuration of the power bins at the end of the re-calculation procedure is shown in Fig. 4.
  • base station 1 transmits all the queued packets for its cell at a total power of P c max.
  • Base stations 2 and 3 transit at a total power level of P 2 and P 3 both being less than P 0 max (new) .
  • all packets in queue 8 have been successfully scheduled during the one frame.
  • the embodiment permits the borrowing of downlink capacity from neighbouring cells.
  • One option for selecting which cells should have their maximum power reduced is to choose those cells which generate the greatest interference to mobile stations lying within the hot spot cell.
  • the scheduler can be configured to redistribute any unused capacity within the network based on a re-calculation of the ⁇ ,- values (and of ⁇ ,) in one or more neighbouring cells or all cells in the network.
  • a hot-spot is defined as a cell in which there is high priority traffic queued which cannot all be serviced if the network capacity is shared equally amongst all cells (P c max equal in all cells) .
  • Hot-spots have limited lifespans, otherwise the network operator would deploy greater infrastructure in these areas to handle the greater demand. Examples include cells around a football ground (such a cell becomes ⁇ hot' at half-time and the end of play as people phone home) ; and cells covering a stretch of motorway when there has been an accident and a queue of cars develops. It is of considerable benefit to an operator if hot-spots can be handled efficiently by careful radio resource management over a number of cells.
  • the scheduler may keep reducing P 0 max of the appropriate base station (s) and carry out the re-calculation of ⁇ , values procedure until enough space has been created in the over-subscribed power bin to accommodate the queued packets. It is preferable to put a lower limit on P 0 max (new) so that the capacity of the reduced power bins is not disadvantaged, i.e., there should be enough resource left available in the reduced bins so that their own packets which are already scheduled can be transmitted.
  • P 0 max new
  • P 0 max in cells for which the path loss from the base station to the mobile stations in the overloaded cell is smallest. This will require a smaller reduction than for cells which are 'further' (in the path loss sense) from the overloaded cell. In other words, it is best to borrow capacity from the neighbouring cells.
  • P 0 max in proportion to the remaining available transmit power (designated Pr in Fig. 3) in the cells which have the greatest spare capacity.
  • the buffered traffic in queue 8 for each cell may be determined by the scheduler 7 prior to filling the power bins. Using this information ' the scheduler 7 can more accurately determine which cells will have spare capacity and therefore should have their P 0 max re-defined.
  • the scheduler 7 forms an equation for the sum of ⁇ i (including an additional packet [s] ) in the overloaded cell, and solves this for a P 0 max (new) value of the single cell whose maximum power has been adjusted. If the scheduler were to adjust the P 0 max in a number of cells simultaneously the mathematics is more complex and an iterative approach as outlined in Figure 5 (see below) is more appropriate. Following the accommodation of the additional packet (s) the scheduler recalculates the ⁇ i ' values allocated in cells other than the over-loaded cell (since the intercell interference may have changed) .
  • ⁇ i : 1- margin.
  • Figure 5 shows a flow chart illustrating the adjustment of the maximum transmit power (P 0 max) in one or more cells to accommodate the allocation of an additional packet to a cell M A" which is a "hot spot” . All steps are performed in the scheduler 7.
  • step 12 P D max is reduced for one or more neighbouring base stations.
  • step 13 the scheduler 7 re-calculates ⁇ , including an additional data packet in cell A.
  • step 14 it tests the value of ⁇ ( for cell A to see if it lies within a pre-defined margin of value 1. If it does, then the scheduler 7 adds the additional data packet to the power bin of cell A (step 15) . (It may also go on to calculate ⁇ ; values in all other cells [step 6] ) .
  • step 16 it checks to see if further changes in P 0 max for neighbours is possible. If so, the process repeats. If not, the process ends.
  • gamma values of between zero and 1 can be used where gamma controls either the maximum reduction of the transmit power below Pmax (i.e., this is a hard limit), or aspects of a probability density function for the transmit power reduction, for example, the standard deviation (i.e., this is a soft limit) . In this configuration there is both an attempt to follow the strict priority order and also to achieve good utilisation of the network.
  • the scheduler freezes the value of P 0 max (in a power bin which is unfilled) to the level reached at some pre-determined point during the frame, and then considers re-allocation of the further queued packets to cells whose power bins are already full.
  • P 0 max new
  • the scheduler 7 recalculates ⁇ ; values in cells which have P 0 max (new) equal to P 0 max (i.e., full bins). The scheduler then continues scheduling and adds more packets where possible to the power bins not "frozen".
  • a power bin now represents received power at a base station, and this includes both useful power and intercell interference.
  • the total uplink power resource in a cell (P tot ai) is a constant based on the characteristics of the UMTS uplink.
  • the capacity of the uplink is limited by the need to meet the signal to noise ratio for every transmission (SNR) .
  • Pto ta i is set to such a level as to ensure a high sustainable throughput.
  • a maximum useable power for resource scheduling in any cell (P m ax) is set to an appropriate level based on the mean ratio of the received power at a base station from the mobile station it serves to the received power from all other mobile stations. (This ratio is set a approximately 1:0.6). This ensures that there is adequate headroom within any cell to allow for the neighbouring cells to be fully loaded and thus use their power resource fully.
  • the techniques as described above in the downlink case can be similarly applied in the uplink case in order to transfer power resource between cells whilst maintaining the Signal to Noise Ratio (SNR) and therefore the desired QoS.
  • SNR Signal to Noise Ratio
  • the principle employed here is to reduce the P a x in cells neighbouring a highly loaded cell. This means that while P tota i remains constant, the split between usable uplink power P max at the receiver of a highly loaded base station and intercell interference is altered by lowering the amount of allowed intercell interference. This reduces the interference expected in the highly loaded cell, allowing P max to be increased, and thus allowing additional packets to be scheduled.
  • an alternative resource allocation strategy uses a 'bottom up' adaptive approach, in which every cell decides its own P max based upon its quality of service (QoS) in the previous few frames.
  • QoS quality of service
  • the problem of uplink power allocation across cells can be seen as a problem of managing a scarce, common resource or good. This type of problem is tackled using an adaptive approach. Because the problem is iterated (i.e. the same agents must split the resource repeatedly) , adaptive strategies can lead to stable, efficient solutions.
  • the problem of determining P max is efficiently tackled by a genetic algorithm which in effect holds a series of decision rules for setting the P max of a cell. These rules are updated at regular intervals using a genetic algorithm.
  • the result is an adaptive ruleset capable of reacting to variations in load and traffic statistics.
  • an adaptive algorithm is used to determine the maximum power budget that can be used for scheduling packets within a distributed uplink scheduler.
  • a genetic algorithm is an optimisation technique particularly suited to rugged multipeaked search spaces (full details of a GA are well known - see Holland, "Adaptation in Natural and Artificial Systems", MIT Press, 1975) . It is a simplification and abstraction of the biological processes of sexual reproduction and mutation under a selective pressure. It is particularly applicable in a situation such as the one presented, where there is naturally a population of agents (in this case base stations, one per cell), and a well defined measure of agent utility (usually referred to as fitness) exits; in this case a measure of attained QoS within any cell. It uses an analogue of the Darwinian principles of selection plus sexual reproduction to maintain a population of possible solutions.
  • the genetic algorithm consists of a population of possible solutions, each encoded as a genotype.
  • the genotype usually consists of a fixed length string, each position on the string being drawn from a limited alphabet (usually binary) .
  • a problem specific decoding algorithm is used to translate the genotype into a phenotype (a solution) whose utility may then be evaluated.
  • the genotype is an encoding of a rule-set used to alter the usage of the uplink bandwidth within a given cell.
  • the coding that has been used to encode the rule-set is based on the recent history of QoS within a given cell over the past three frames. In each frame the QoS is assessed as adequate/inadequate. Interpreting adequate as 1, and inadequate as 0, the history of the QoS over the past few (e.g., 3) frames can be written as a 3 digit binary number, i.e. within the range [0,7]. This is used as a key to access a lookup table of alterations (e.g., by, say, 5%) to the power budget within a cell: (I)ncrease, (U)nchanged, or (D)ecrease. Hence the lookup table can be written as an 8-digit string, as demonstrated in the table of Fig. 6.
  • a static lookup table is not intelligent; however, by dynamically altering this lookup table to suit the prevailing circumstances, both individual cells and the entire network can be made adaptive.
  • a Genetic Algorithm is used for this purpose.
  • the GA acts to regularly (e.g., every, say, 100 frames) replace the weakest rulesets (the replacement of at most 5% of the population is recommended) .
  • the strength of the ruleset used by each Base Station is defined as the QoS achieved by that base-station since the GA was last run.
  • an update regularly occurs of the lookup tables of cells that are performing badly relative to the population of cells as a whole.
  • a new lookup table is formed by combining aspects of the lookup tables of other, better performing cells.
  • a background element of random variation is also introduced (e.g., swapping 1% of positions in the genotype) . This ensures that the system does not converge completely, and thus can respond better to a changing environment.
  • This type of coding has been shown to give impressive results in noisy environments. Not only can such a schema achieve high levels of co-operation between base stations with conflicting priorities, it has also been shown to be extremely stable to noise in the system, resulting from operating conditions such as external conditions (such as a rise in the background noise levels), or changes to the system (e.g. changes in the cell topography, or malfunction in a cell Base Station) .

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  • Engineering & Computer Science (AREA)
  • Quality & Reliability (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Mobile Radio Communication Systems (AREA)

Abstract

L'invention concerne un ordonnanceur (7) destiné à ordonnancer des paquets de données mis en attente de transmission entre une pluralité de compartiments d'énergie (9, 10, 11) associés à des stations de base respectives (1, 2, 3) dans un réseau de communication cellulaire, ledit ordonnanceur facilitant l'adaptation des niveaux d'énergie aux variations des conditions d'utilisation. Le fonctionnement dudit ordonnanceur peut s'adapter efficacement aux environnements bruyants par réglage de la valeur maximale de la puissance de transmission d'une station de base en fonction de la qualité nécessaire des services, et par application d'un algorithme génétique dépendant de la qualité des services de manière que les niveaux d'énergies maximum s'adaptent aux conditions d'utilisation.
PCT/EP2001/003548 2000-03-31 2001-03-28 Ordonnanceur de paquets et procede associe WO2001073972A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU2001246517A AU2001246517A1 (en) 2000-03-31 2001-03-28 A packet scheduler and method

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0007853.5 2000-03-31
GB0007853A GB2361147B (en) 2000-03-31 2000-03-31 A packet scheduler and method

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Publication Number Publication Date
WO2001073972A1 true WO2001073972A1 (fr) 2001-10-04

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GB (1) GB2361147B (fr)
WO (1) WO2001073972A1 (fr)

Cited By (4)

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WO2003094384A2 (fr) * 2002-04-30 2003-11-13 Qualcomm Incorporated Conception de repartition de boucle externe amelioree pour des systemes de communications a mecanisme de retroaction de la qualite de voies
GB2402580A (en) * 2003-06-06 2004-12-08 Motorola Inc Scheduling one or more data packets in a cdma communication system based on computed power resource availability
DE10300495B4 (de) * 2002-08-08 2006-06-29 Fg Microtec Gmbh Verfahren zur Steuerung des Datenflusses bei WAP-basierten Übertragungen
EP2288047A3 (fr) * 2001-11-13 2012-04-18 Qualcomm Incorporated Sélection de combinaison de format de transport pour mode compressé dans un système W-CDMA

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GB2381406A (en) * 2001-10-24 2003-04-30 Ipwireless Inc Packet data queuing and processing
GB2384138B (en) * 2002-01-10 2005-06-08 Roke Manor Research Intelligent scheduling for adaptive modulation
GB2390775B (en) * 2002-07-12 2005-07-20 Fujitsu Ltd Packet scheduling
GB2390779B (en) * 2002-07-12 2006-02-22 Fujitsu Ltd Packet scheduling
GB2392055A (en) * 2002-08-14 2004-02-18 Hutchison Whampoa Three G Ip Adjusting resource threshold (maximum available resource per terminal) according to cell load

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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2288047A3 (fr) * 2001-11-13 2012-04-18 Qualcomm Incorporated Sélection de combinaison de format de transport pour mode compressé dans un système W-CDMA
WO2003094384A2 (fr) * 2002-04-30 2003-11-13 Qualcomm Incorporated Conception de repartition de boucle externe amelioree pour des systemes de communications a mecanisme de retroaction de la qualite de voies
WO2003094384A3 (fr) * 2002-04-30 2004-12-23 Qualcomm Inc Conception de repartition de boucle externe amelioree pour des systemes de communications a mecanisme de retroaction de la qualite de voies
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DE10300495B4 (de) * 2002-08-08 2006-06-29 Fg Microtec Gmbh Verfahren zur Steuerung des Datenflusses bei WAP-basierten Übertragungen
GB2402580A (en) * 2003-06-06 2004-12-08 Motorola Inc Scheduling one or more data packets in a cdma communication system based on computed power resource availability
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GB2361147A (en) 2001-10-10
GB0007853D0 (en) 2000-05-17
AU2001246517A1 (en) 2001-10-08
GB2361147B (en) 2004-01-07

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