WO2010059816A1 - Techniques de gestion de brouillage et d'accès à un canal décentralisé dans des réseaux cellulaires soutenus par des points d'accès sans fil - Google Patents

Techniques de gestion de brouillage et d'accès à un canal décentralisé dans des réseaux cellulaires soutenus par des points d'accès sans fil Download PDF

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Publication number
WO2010059816A1
WO2010059816A1 PCT/US2009/065135 US2009065135W WO2010059816A1 WO 2010059816 A1 WO2010059816 A1 WO 2010059816A1 US 2009065135 W US2009065135 W US 2009065135W WO 2010059816 A1 WO2010059816 A1 WO 2010059816A1
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Prior art keywords
femtocell
base station
femtocell base
base stations
cellular
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PCT/US2009/065135
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English (en)
Inventor
Jeffrey G. Andrews
Vikram Chandrasekhar
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Board Of Regents, The University Of Texas System
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Publication of WO2010059816A1 publication Critical patent/WO2010059816A1/fr

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    • 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
    • H04W52/24—TPC being performed according to specific parameters using SIR [Signal to Interference Ratio] or other wireless path parameters
    • H04W52/243—TPC being performed according to specific parameters using SIR [Signal to Interference Ratio] or other wireless path parameters taking into account interferences
    • H04W52/244—Interferences in heterogeneous networks, e.g. among macro and femto or pico cells or other sector / system interference [OSI]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J11/00—Orthogonal multiplex systems, e.g. using WALSH codes
    • H04J11/0023—Interference mitigation or co-ordination
    • H04J11/005—Interference mitigation or co-ordination of intercell interference
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W16/00—Network planning, e.g. coverage or traffic planning tools; Network deployment, e.g. resource partitioning or cells structures
    • H04W16/02—Resource partitioning among network components, e.g. reuse partitioning
    • H04W16/10—Dynamic resource partitioning
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W36/00—Hand-off or reselection arrangements
    • H04W36/04—Reselecting a cell layer in multi-layered cells
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W72/00—Local resource management
    • H04W72/04—Wireless resource allocation
    • H04W72/044—Wireless resource allocation based on the type of the allocated resource
    • H04W72/0446—Resources in time domain, e.g. slots or frames
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W72/00—Local resource management
    • H04W72/04—Wireless resource allocation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W72/00—Local resource management
    • H04W72/04—Wireless resource allocation
    • H04W72/044—Wireless resource allocation based on the type of the allocated resource
    • H04W72/0473—Wireless resource allocation based on the type of the allocated resource the resource being transmission power
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W84/00—Network topologies
    • H04W84/02—Hierarchically pre-organised networks, e.g. paging networks, cellular networks, WLAN [Wireless Local Area Network] or WLL [Wireless Local Loop]
    • H04W84/04—Large scale networks; Deep hierarchical networks
    • H04W84/042—Public Land Mobile systems, e.g. cellular systems
    • H04W84/045—Public Land Mobile systems, e.g. cellular systems using private Base Stations, e.g. femto Base Stations, home Node B

Definitions

  • Embodiments of the present methods and system are generally directed toward cellular wireless networks. More specifically, embodiments of the invention are directed toward two-tier cellular networks consisting of a typical macrocell overlaid or underlaid with supplemental infrastructure that has a shorter range than the macrocell base station, and, in particular, toward reducing interference and efficiently accommodating multiple users in these two-tier cellular networks.
  • Wireless communication systems are constantly being pushed to accommodate the conflicting goals of higher data rates per user, and improved coverage area.
  • a primary way to meet both of these desirable goals is by deploying more infrastructures (connected into the wired backbone, and eventually the PSTN and/or Internet). This infrastructure is generally quite expensive, in particular the base stations that comprise most of the infrastructure in cellular networks.
  • Two-tier networks comprising a conventional cellular network overlaid with shorter range hotspots (e.g. femtocells, distributed antennas, or wired relays), offer an economically viable way to improve cellular system capacity.
  • Femtocells are recently attracting interest for increasing overall system capacity and coverage, particularly for subscribers who are at home or in another common location.
  • Femtocells are small virtual base stations that are usually deployed by the end user (perhaps with subsidy or logistical help from the service provider). This can result in a win-win: the subscriber gets high speed, reliable access at their most common locations (many subscribers currently complain about their service experience at home), and the service provider unloads considerable traffic off their expensive large-scale network. Because this results in two spatially overlaid networks (Base Stations being tier 1 and femtocells being tier 2), the composite network is often referred to as a "two-tier" network.
  • Femtocells also known as home base stations (BTSs) or access point base stations, can connect to a service provider's network via a broadband backhaul connection (such as digital subscriber lines (DSL), cable, or even a radio link).
  • BTSs home base stations
  • DSL digital subscriber lines
  • Femtocells can allow service providers to extend service coverage indoors, where access would otherwise be limited or unavailable.
  • Femtocells can incorporate the functionality of a typical base station while allowing for a simpler, self contained deployment.
  • interference between femtocells and macrocells in such networks can be a capacity-limiting factor if the femtocells and macrocells share the same spectrum.
  • the cross-tier interference between macrocells and femtocells can suffocate the capacity due to the near-far problem, so in practice hotspots would typically want to use a different frequency channel than the potentially nearby high-power macrocell users.
  • Centralized or coordinated frequency planning which is difficult and inefficient even in conventional cellular networks, is even more difficult in a two-tier network.
  • using expensive wireless spectrum to coordinate between the cellular network and hotspots may be self-defeating as it undermines the principle argument - low capital and operating expenditures - for deploying femtocells in the first place.
  • femtocells and other types of supplemental infrastructure are likely to be deployed either randomly by users of the cellular network, or on an as- needed basis by the service provider.
  • Allocating dedicated spectrum just for these devices and the mobile stations (MSs) interacting with them is highly undesirable since they may be sparse in many areas, rolled out slowly, and the demands on the available spectrum are intense, which is largely what motivates these hotspots in the first place. Therefore, methods and techniques that achieve frequency reuse between the two tiers are highly desirable.
  • Wireless operators are in the process of augmenting the macrocellular network with supplemental infrastructure such as microcells, distributed antennas and relays.
  • An alternative with lower upfront costs is to improve indoor coverage and capacity using the concept of end-consumer installed femtocells or home base stations.
  • a femtocell can serve as a low power, short range data access point that provides high quality in-building coverage to home users, while backhauling their traffic over the IP network.
  • the femtocell radio range can be much smaller than the macrocell radius. Users transmitting to femtocells can experience superior indoor signal reception and can lower their transmit power, thus prolonging battery life.
  • femtocells can provide higher spatial reuse and cause less cochannel interference (CCI) to other users.
  • CCI cochannel interference
  • femtocells can provide higher spatial reuse and cause less cochannel interference (CCI) to other users.
  • CCI cochannel interference
  • near-far effects arising from cross-tier interference can create problems due to conventional signal strength based power control and can be particularly severe in a "closed access" deployment, where a femtocell allows only licensed subscribers to communicate with it.
  • Embodiments of the present invention provide solutions to tackle cross-tier interference in such a network — with two sets of users with dramatically uneven channel powers in their BSs - including: (1) enabling femtocell users to decrease their target data rates or employ adaptive power strategies such as decreasing their target data rate in the presence of a nearby cellular user, (2) employing interference avoidance through "time-hopping” (or frequency hopping) for both macrocell and femtocell users in order to "avoid” transmitting in the same time (frequency) interval, (3) requiring femtocells to perform "open-access” through vertical handoffs by servicing outdoor cellular users with poor channel powers to their central macrocell, (4) eliminating cross-tier interference by assigning macrocell and femtocell users with orthogonal frequency resources using a spectrum partitioning strategy, (5) providing multiple transmit antennas at femtocells and utilizing the extra spatial degrees of freedom to eliminate interference to cellular users (through beamforming techniques) and tackling cross-tier interference from the macrocell, and/or
  • a system comprising one or more femtocell base stations deployed within a range of a cellular base station, wherein the cellular base station utilizes a frequency band and respective femtocell base stations utilize a frequency band that is substantially the same as as the cellular base station frequency band.
  • each femtocell base station is configured to employ one or more interference avoidance techniques such that coexistence between the cellular base station and the corresponding femtocell base station is enabled.
  • the interference avoidance techniques employed may include use of randomized time or frequency hopping when transmitting signals to and from one or more femtocell users, randomly selecting a predetermined number of available frequency subchannels for transmitting signals to and from the femtocell users, identifying one or more unutilized frequency subchannels for transmitting signals to and from the femtocell users, using two or more transmit and two or more receive antennas, nulling one or more transmissions in a direction of a nearby cellular base station user, handing off at least one user associated with the cellular base station to one of the femtocell base stations and vice versa, and/or reducing the transmission power of at least one of the femtocell base stations.
  • a method of operating one or more femtocell base stations in conjunction with a cellular base station may include: (1) establishing one or more femtocell abse stations within a range of a cellular base station, wherein the cellular base station utilizes a frequency band and wherein respective femtocell base stations utilize a frequency band that is substantially the same as the cellular base station frequency band; and (2) employing, by respective femtocell base stations, one or more interference avoidance techniques such that coexistence between the cellular base station and the corresponding femtocell base station is enabled.
  • the interfence avoidance techniques may include any one or more of the techniques mentioned above and described in more detail below.
  • Figure 1 illustrates an example of a cellular system with macrocells and femtocells
  • Figure 2 illustrates potential interference between macrocell and femtocell transmissions
  • Figure 3 illustrates an exemplary operating environment for performing the methods of embodiments of the present invention
  • Figure 4 illustrates a two-tier femtocell network with DS-CDMA transmissions
  • Figure 5 illustrates a table of system parameters
  • Figure 6 illustrates intra-tier and cross-tier co-channel interference (CCI) at each tier
  • Figure 7 illustrates a comparison of joint and independent hopping protocols at a femtocell base station with antenna sectoring
  • Figure 8 illustrates outage lower bounds for interior and corner femtocells
  • Figure 9 illustrates macrocell outage performance with femtocell exclusion
  • Figure 10 illustrates a macrocell operating contour (OC)
  • Figure 11 illustrates interior femtocell OC
  • Figure 12 illustrates network OCs for different macrocell-femtocell received power ratios and fixed hopping slots
  • Figure 13 illustrates network OCs with different hopping slots and corner femtocell reference
  • Figure 14 illustrates network OCs with different hopping slots and interior femtocell reference
  • Figure 15 illustrates network OCs with tier selection and femtocell exclusion and interior femtocell
  • Figure 16 illustrates spatially averaged macrocell subchannel throughput
  • Figure 17 illustrates theoretical and empirical throughput per femtocell PfTf
  • Figure 18 illustrates femtocell area spectral efficiencies
  • Figure 19 illustrates optimal spectrum allocation p for varying Quality of Service (QoS) parameter ⁇ ;
  • Figure 20 illustrates area spectral efficiencies in a two-tier network for varying QoS parameter ⁇
  • Figure 21 illustrates the average network- wide throughput p/2/(l-p) provided by femtocells in their allocated spectrum (1-p);
  • Figure 23 illustrates another table of system parameters
  • Figure 24 illustrates dead zones caused by cross-tier interference in a shared spectrum two-tier network
  • Figure 25 illustrates transmission scheme at femtocell
  • Figure 26 illustrates examples showing two cellular users experiencing equal path-losses to BQ and B ⁇ respectively;
  • Figure 27 illustrates two antennas per femtocell and one indoor user served per femtocell, cancelling cross-tier interference requires coordinated design of precoder F;
  • Figure 28 illustrates system parameters
  • Figure 29 illustrates outage probability at femtocell as a function of its distance from the central macrocell
  • Figure 30 illustrates threshold distance of cellular user at which QoS requirement
  • ⁇ 0.1 is violated as a function of the number of femtocells per cell site and the number of transmit antennas at the macrocell M.
  • embodiments of the present invention provide an improvement over the known prior art by providing, among other things, several techniques for reducing or managing the interference associated with two-tier cellular networks consisting of a central macrocell and one or more femtocells, or other similar supplemental infrastructure. Further, embodiments of the present invention are practically implementable in two-tier networks irrespective of whether they employ a "closed- access" scheme (i.e., wherein each femtocell provides service to a few licensed privileged in-home users), or an "open-access” scheme (i.e., wherein a cellular user experiencing a poor channel to its macrocell may perform vertical handoff and receive service from a nearby femtocell).
  • a "closed- access” scheme i.e., wherein each femtocell provides service to a few licensed privileged in-home users
  • an "open-access” scheme i.e., wherein a cellular user experiencing a poor channel to its macrocell may perform vertical handoff and receive service from
  • co-channel interference between the outdoor cellular macrocell transmissions and indoor femtocell transmissions may be reduced by employing directional antennas at each femtocell.
  • Directional femtocell antennas may reduce the geographic region within which a femtocell user "sees" cross-tier interference from cellular users.
  • a randomized time-hopped (TH) or frequency hopped (FH) transmission strategy in each tier can be used to reduce interference. Since TH uses randomized channel access, the probability that a cellular user and a femtocell user accessing a common frequency resource at the same time instant may be significantly reduced.
  • a TH strategy can be augmented with additional interference suppression in the form of code-division-multiple-access (CDMA) to further reduce co-channel interference.
  • CDMA code-division-multiple-access
  • a FH-CDMA strategy may divide a given region of spectrum into multiple subchannels. Randomized frequency sub channel access by cellular [resp. indoor femtocell] users may significantly reduce the probability of collision - and hence cross-tier interference caused by simultaneous cellular and femtocell transmissions - in the same subchannel.
  • spectrum allocation or partitioning can be employed.
  • Spectrum partitioning can provide an efficient method for allocating spectrum between tiers by taking into account the loading of users at each tier.
  • the methods and systems can dynamically load balance spectrum by varying allocated spectrum between the macrocell and femtocell networks.
  • Frequency- ALOHA distinguishable from the conventional ALOHA, which refers to transmissions conducted in the time domain
  • the yield in the optimal portion of spectrum which can be accessed by each femtocell in order to maximize the spatial reuse of spectrum among co-channel femtocells can be determined.
  • carrier sensed spectrum access can be employed to enable a femtocell to opportunistically detect and transmit over unutilized subchannels in order to avoid interfering with nearby femtocells.
  • MIMO Multiple-Input-Multiple-Output
  • using multiple antennas may enable a femtocell to (a) eliminate cross-tier interference to nearby cellular users in a closed-access system by nulling their beams in the direction of a nearby cellular user, (b) handoff nearby cellular users and boost their signal strength through array processing, (c) utilize the extra spatial degrees of freedom to eliminate interference to cellular users (through beamforming techniques) and tackle cross-tier interference from the macrocell and/or (d) use techniques for handoffs (in open access) and adaptive power control at femtocells (in closed access).
  • FIG. 1 is a simplifiled illustration of a two-tier cellular system according to one embodiment of the present invention.
  • a macrocell 201 is a cell in a cellular network that provides radio coverage served by a power cellular base station 201 (tower) to one or more wireless devices 210, 211 (e.g., cellular telephones, personal digital assistants (PDAs), laptops, etc.).
  • Each cell within the cellular network typically employs a standardized method for each distributed station to distinguish the signal emanating from its own transmitter from the signals received from other transmitters.
  • Two standards known to those of ordinary skill in the art, are frequency division multiple access (FDMA) and code division multiple access (CDMA).
  • FDMA frequency division multiple access
  • CDMA code division multiple access
  • FDMA uses a channel access method in multiple-access protocols as a channelization protocol. FDMA can give users an individual allocation of one or several frequency bands, allowing them to utilize the allocated radio spectrum without interfering with each other. CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) that can allow multiple users to be multiplexed over the same physical channel.
  • Macrocells generally provide the widest geographic area coverage. However, some locations at the boundaries of an individual macrocell's geographic area of a plurality of macrocells or within the boundaries (such as inside a home or business) can suffer from low signal strength. To improve coverage at these locations, femtocells 220, 230, 240 can be deployed.
  • Femtocells 220, 230, 240 can be small cellular base stations designed for use in residential or small business environments. Each femtocell may provide coverage to one or more wireless devices 221, 222, 231, 232, 241, 242. Femtocells can connect to a service provider's network 250 via a broadband backhaul connection 262, 263, 264 (such as DSL, cable or radio link). A femtocell can allow service providers to extend service coverage indoors, where access would otherwise be limited or unavailable. Femtocells can incorporate the functionality of a typical base station with a simpler, self contained deployment. However, because femtocells use the same transmission frequencies as macrocells, inteference can occur.
  • FIG. 2 is a simplified illustration of when interference can occur. If femtocell 240 and macrocell 201 are using the same frequencies, as a user 211 travels to the boundary of a macrocell 201 the signal 301 from macrocell 201 and the signal 241 from femtocell 240 can inteferre with each other, causing problems with communication with both macrocell 201 and femtocell 240. Similarly, while not specifically shown, interference may occur from macrocell 201 at femtocell 220, which is very close to central macrocell 201) or 230, affecting users 221, 222, 231 and 232.
  • each femtocell may be configured to manage/reduce this type of interference using one or more interfernce avoidance techniques.
  • each femtocell may comprise, among other things, a processor configured to perform the interference management techniques described herein and/or memory storing an application which, when executed, causes the processor to perform some combination of those techniques.
  • the femtocell base station may comprise a computer 101 like the one shown in FIG. 3. Referring to FIG. 3, one skilled in the art will appreciate that provided herein is a functional description, and that respective functions can be performed by software, hardware, or a combination of software and hardware associated with the femtocell base station.
  • a unit can be software, hardware, or a combination of software and hardware operably connected to a base station.
  • the units can comprise interference avoidance and decentralized channel access schemes software 106 as illustrated in FIG. 3 and described below.
  • FIG. 3 is a block diagram illustrating an exemplary operating environment for performing the disclosed method.
  • This exemplary operating environment is only an example of an operating environment and is not intended to suggest any limitation as to the scope of use or functionality of operating environment architecture. Neither should the operating environment be interpreted as having any dependency or requirement relating to any one or combination of components illustrated in the exemplary operating environment.
  • Embodiments of the present methods and systems can be operational with numerous other general purpose or special purpose computing system environments or configurations.
  • Examples of well known computing systems, environments, and/or configurations that can be suitable for use with the system and method comprise, but are not limited to, personal computers, server computers, laptop devices, and multiprocessor systems. Additional examples comprise set top boxes, programmable consumer electronics, network PCs, minicomputers, mainframe computers, distributed computing environments that comprise any of the above systems or devices, and the like.
  • the processing of the disclosed methods and systems can be performed by software components.
  • the disclosed system and methods can be described in the general context of computer-executable instructions, such as program modules, being executed by one or more computers or other devices.
  • program modules comprise computer code, routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types.
  • the disclosed methods can also be practiced in grid-based and distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network.
  • program modules can be located in both local and remote computer storage media including memory storage devices.
  • the components of the computer 101 can comprise, but are not limited to, one or more processors or processing units 103, a system memory 112, and a system bus 113 that couples various system components including the processor 103 to the system memory 112.
  • the processor 103 can contain non-identical multiple cores. Such a processor 103 is referred to as a heterogeneous multi-core processor. In the case of multiple processing units 103, the system can utilize parallel computing.
  • the system bus 113 represents one or more of several possible types of bus structures, including a memory bus or memory controller, a peripheral bus, an accelerated graphics port, and a processor or local bus using any of a variety of bus architectures.
  • bus architectures can comprise an Industry Standard Architecture (ISA) bus, a Micro Channel Architecture (MCA) bus, an Enhanced ISA (EISA) bus, a Video Electronics Standards Association (VESA) local bus, an Accelerated Graphics Port (AGP) bus, a Peripheral Component Interconnects (PCI) bus, a PCI-Express bus, a Personal Computer Memory Card Industry Association (PCMCIA) bus, Universal Serial Bus (USB) and the like.
  • ISA Industry Standard Architecture
  • MCA Micro Channel Architecture
  • EISA Enhanced ISA
  • VESA Video Electronics Standards Association
  • AGP Accelerated Graphics Port
  • PCI Peripheral Component Interconnects
  • PCMCIA Personal Computer Memory Card Industry Association
  • USB Universal Serial Bus
  • the bus 113, and all buses specified in this description can also be implemented over a wired or wireless network connection and each of the subsystems, including the processor 103, a mass storage device 104, an operating system 105, interference avoidance and decentralized channel access schemes software 106, interference avoidance and decentralized channel access schemes data 107, a network adapter 108, system memory 112, an Input/Output Interface 116, a display adapter 109, a display device 111, and a human machine interface 102, can be contained within one or more remote computing devices 114a,b,c at physically separate locations, connected through buses of this form, in effect implementing a fully distributed system.
  • the computer 101 typically comprises a variety of computer readable media.
  • Exemplary readable media can be any available media that is accessible by the computer 101 and comprises, for example and not meant to be limiting, both volatile and non- volatile media, removable and non-removable media.
  • the system memory 112 comprises computer readable media in the form of volatile memory, such as random access memory (RAM), and/or non-volatile memory, such as read only memory (ROM).
  • RAM random access memory
  • ROM read only memory
  • the system memory 112 typically contains data such as interference avoidance and decentralized channel access schemes data 107 and/or program modules such as operating system 105 and interference avoidance and decentralized channel access schemes software 106 that are immediately accessible to and/or are presently operated on by the processing unit 103.
  • the computer 101 can also comprise other removable/nonremovable, volatile/non-volatile computer storage media.
  • FIG. 3 illustrates a mass storage device 104 which can provide non- volatile storage of computer code, computer readable instructions, data structures, program modules, and other data for the computer 101.
  • a mass storage device 104 can be a hard disk, a removable magnetic disk, a removable optical disk, magnetic cassettes or other magnetic storage devices, flash memory cards, CD-ROM, digital versatile disks (DVD) or other optical storage, random access memories (RAM), read only memories (ROM), electrically erasable programmable read-only memory (EEPROM), and the like.
  • any number of program modules can be stored on the mass storage device 104, including by way of example, an operating system 105 and interference avoidance and decentralized channel access schemes software 106.
  • Each of the operating system 105 and interference avoidance and decentralized channel access schemes software 106 (or some combination thereof) can comprise elements of the programming.
  • Interference avoidance and decentralized channel access schemes data 107 can also be stored on the mass storage device 104.
  • Interference avoidance and decentralized channel access schemes data 107 can be stored in any of one or more databases known in the art. Examples of such databases comprise, DB2®, Microsoft® Access, Microsoft® SQL Server, Oracle®, mySQL, PostgreSQL, and the like. The databases can be centralized or distributed across multiple systems.
  • the user can enter commands and information into the computer 101 via an input device (not shown).
  • input devices comprise, but are not limited to, a keyboard, pointing device (e.g., a "mouse"), a microphone, a joystick, a scanner, tactile input devices such as gloves, and other body coverings, and the like
  • a human machine interface 102 that is coupled to the system bus 113, but can be connected by other interface and bus structures, such as a parallel port, game port, an IEEE 1394 Port (also known as a Firewire port), a serial port, or a universal serial bus (USB).
  • a display device 111 can also be connected to the system bus 113 via an interface, such as a display adapter 109. It is contemplated that the computer 101 can have more than one display adapter 109 and the computer 101 can have more than one display device 111.
  • a display device can be a monitor, an LCD (Liquid Crystal Display), or a projector.
  • other output peripheral devices can comprise components such as speakers (not shown) and a printer (not shown) which can be connected to the computer 101 via Input/Output Interface 116. Any step and/or result of the methods can be output in any form to an output device.
  • the computer 101 can operate in a networked environment using logical connections to one or more remote computing devices 114a, b, c.
  • a remote computing device can be a personal computer, portable computer, a server, a router, a network computer, a peer device or other common network node, and so on.
  • Logical connections between the computer 101 and a remote computing device 114a, b, c can be made via a local area network (LAN) and a general wide area network (WAN).
  • LAN local area network
  • WAN general wide area network
  • Such network connections can be through a network adapter 108.
  • a network adapter 108 can be implemented in both wired and wireless environments. Such networking environments are conventional and commonplace in offices, enterprise-wide computer networks, intranets, and the Internet 117.
  • Computer readable media can comprise “computer storage media” and “communications media.”
  • “Computer storage media” comprise volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules, or other data.
  • Exemplary computer storage media comprises, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by a computer.
  • the methods and systems can employ Artificial Intelligence techniques such as machine learning and iterative learning.
  • Artificial Intelligence techniques such as machine learning and iterative learning. Examples of such techniques include, but are not limited to, expert systems, case based reasoning, Bayesian networks, behavior based AI, neural networks, fuzzy systems, evolutionary computation (e.g. genetic algorithms), swarm intelligence (e.g. ant algorithms), and hybrid intelligent systems (e.g. Expert inference rules generated through a neural network or production rules from statistical learning).
  • Computer processors are often designed with multiple processor cores.
  • a plurality of computer programs may run on a computer and an operating system on the computer can be tasked with determining which program runs on which core, hi one embodiment of the methods and systems a mathematical system that analyzes programs to determine which core they should be assigned to.
  • the mathematical system referred to as a fuzzy system or fuzzy logic, can be utilized to determine a desired assignment of applications to cores.
  • a two-tier network with universal frequency reuse such as a two-tier network employing code division multiple access (CDMA) transmission (e.g., IS-95, CDMA2000, UMTS, etc.)
  • CDMA code division multiple access
  • directional antennas at the femtocells may be used.
  • directional antennas refer to antennas which radiate greater power in one or more directions allowing for increased performance on transmit and receive, and reduced interference from unwanted sources.
  • this embodiment of the invention utilizes directional antennas at the femtocell base station level. Additionally, by using directional antennas at the femtocell level, the cross-tier cochannel interference (CCI) caused by nearby macrocellular users which can lead to outage performance over the femtocell uplink can be reduced.
  • CCI cross-tier cochannel interference
  • Providing directional antennas may benefit indoor femtocell users by restricting the region within which they experience interference from co-channel outdoor cellular transmissions. In addition, there may be zero associated protocol overhead.
  • randomized time-hopping may be used.
  • the CDMA duration G T 0 where G can be the processing gain and T c can be the chip period, can be divided into N hop hopping slots, where each user can randomly select a hopping slot for transmission and remain silent during the remaining slots.
  • time hopping may enable users to avoid rather than suppress mutual interference in a decentralized manner. Since time hopping uses randomized channel access, the probability that a cellular user and a femtocell user will access the common frequency resource as the same time instant may be significantly reduced. According to one embodiment, time hopping may be augmented with additional interference suppression in the form of CDMA to further reduce co-channel interference.
  • frequency hopping may be used.
  • the femtocell may rapidly, and randomly, switch carriers among many frequency channels using a pseudorandom sequence known to both the transmitter and the receiver.
  • H ⁇ R 2 may be denoted as the interior of a reference hexagonal macrocell C of radius R c .
  • the tier 1 network can consist of low density macrocellular users that are communicating with the central base station in each cell site.
  • the macrocellular users are distributed on R 2 according to a homogeneous SPPP ⁇ c of intensity ⁇ c .
  • the overlaid tier 2 network containing the femtocell base stations forms a homogeneous SPPP ⁇ / with intensity ⁇ f (the system model allows a macrocellular user to be present inside a femtocell as the governing process ⁇ c is homogeneous).
  • Each femtocell hotspot includes a Poisson distributed population of actively transmitting users with mean Uf in a circular coverage area of radius Rf Rf » R c (a hard handoff may be assumed to allocate subscribed hotspot users to a femtocell, provided they fall within its radio range).
  • Rf Rf » R c a hard handoff may be assumed to allocate subscribed hotspot users to a femtocell, provided they fall within its radio range.
  • FIG. 5 shows an exemplary summary of system parameters and typical values for them, which are used later in numerical simulations.
  • users in each tier employ DS- CDMA with processing gain G.
  • Uplink power control adjusts for propagation losses and log-normal shadowing, which is standard in contemporary CDMA networks.
  • the macrocell and femtocell receive powers are denoted as Pf and P/ respectively. Any power control errors and short-term fading effects are ignored for analytical convenience. This assumption has been affirmed as reasonable, especially in a wideband system with significant frequency diversity and robust reception (through RAKE receiver, coding and interleaving).
  • each of duration T/Nh op - Macrocell users and femtocell users (active users within a femtocell may transmit in the same hopping slot) can independently choose to transmit over any one slot, and remain silent over the remaining N hop - 1 slots.
  • the resulting intra-tier and cross-tier interference can be "thinned" by a factor of N hop .
  • T ⁇ -CDMA Time ⁇ opping-CDMA
  • FIG. 6 which illustrates intra-tier and cross-tier CCI at each tier and in which the arrows denote the CCI arising from either tier 1 (i.e., the macrocell) or tier 2 (i.e., the femtocell), assume sectored antenna reception in both the macrocell and femtocell base station, with antenna alignment angle ⁇ and sector width equaling 2 ⁇ /N sec - While antenna sectoring is a common feature at the macrocell BS in practical cellular systems, embodiments of the present invention use sectored antennas at femtocell BS's as well.
  • Lemma 1 (Spatial thinning by interference avoidance): With TH-CDMA transmission over Nh op slots and antenna sectoring with N sec directed base station antennas in each tier, the interfering field at a given base station antenna sector can be
  • ⁇ c ⁇ c /(N hop ⁇ N sec ) and respectively.
  • the SPPP is a natural model arising from mobility of macrocellular users and placement of femtocell base station in densely populated areas, and has been confirmed in empirical studies and used by those of ordinary skill in the art. [0056] The following definitions may be useful.
  • Definition 1 Denote *** — as the region within ⁇ covered by a base station antenna sector corresponding to a macrocell base station or a femtocell base
  • Definition 2 Denote ⁇ c and ⁇ - as the heterogeneous SPPPs composed of active macrocell and femtocell interferers as seen at a base station antenna sector in each tier, whose intensities are given by ⁇ c and ⁇ f . Denote the equivalent mapped
  • Definition 3 Denote the restriction of ⁇ c and ⁇ f to ⁇ by the SPPPs ⁇ c and ⁇ f respectively.
  • the channel can be represented as a combination of path-loss and log-normal shadowing.
  • the path-loss exponents are denoted by a (outdoor transmission) and ⁇ (indoor femtocell transmission) while lognormal shadowing can be parameterized by its standard deviation ⁇ dB .
  • ) can be the attenuation function defined as
  • e * ⁇ e ** ⁇ » can be a unitless constant that depends on the wavelength of the RF carrier c/f c and outdoor reference distance do c .
  • K c and Kf can be empirically determined.
  • the interference in each tier (FIG. 6) can be grouped as: [0062] Macrocell interference at a macrocell base station (I c ,i n jc,ou t )'- Through power control, all macro-cell users within Fl ⁇ e can be received with constant power P r c , so the in-cell interference equals (N - 1) • P r c , where iV ⁇ Poisson(N c / Nh op ). As such, inferring the exact statistics of out-of-cell macrocellular interference I c _ out is analytically intractable. It can be assumed that I c out can be distributed according to a scaled Gaussian pdf. Defining ⁇ and ⁇ 2 to be the empirically determined parameters of the Gaussian, the
  • pdf of I c ,out can be given as where [0063] Femtocell interference at a macrocell base station (I C J-
  • femtocell F,- with Ui ⁇ Poisson(L ⁇ ) users can be located at random position Xi with respect to reference macrocell base station C.
  • Pf (j) Pf I g f (Y j ) .
  • the interference caused at C from user ; inside F t can be given as:
  • Neighboring femtocell interference at a femtocell base station (IfJ):
  • the interference caused at a base station antenna sector of femtocell F j from other femtocells F 1 , where * ⁇ j, can be a Poisson SNP given by 7 /V ⁇ F ⁇ ,e ⁇ f ⁇ / ⁇ ' X ⁇ ,where
  • Macrocell interference at a femtocell BS (If rC ):
  • outage probability at a femtocell base station F 7 -, located on the hexagonal axis can be analyzed by considering the effect of in-cell macrocellular CCI.
  • the interference /* c arising from users in II C can form a lower bound on the cumulative tier 1 CCI If iC and
  • LF S i,c> « ⁇ o i ⁇ * mt can be the LN shadowing term and ⁇ Xi ⁇ t
  • ⁇ Y ⁇ can represent the distances of macrocell user i to the macrocell base station and femtocell base station respectively. Observe that a corner femtocell can experience a significantly higher macrocell CCI relative to an interior femtocell, therefore the cdf F j (•) may not be a stationary distribution.
  • an uplink outage probability constraint can be formulated in each tier.
  • Nf and N c can be defined as the average number of femtocell base stations and macrocell users per cellsite respectively.
  • a user can experience outage if the received instantaneous Signal-to- Interference Ratio (SIR) over a transmission is below a threshold ⁇ .
  • SIR Signal-to- Interference Ratio
  • tier. ⁇ /V > c 1) can be defined as the probabilities that the despread narrowband SIR for a macrocell user [femtocell user] at the Tier 1 [Tier 2] base station antenna sector is below ⁇ .
  • VUNf , JV c ) Pr ( ⁇ /*W? ⁇ 7 1 ⁇ ,1 > 1
  • N f (N c ) SUp(JV/ : P ⁇ (JV/, N c ) ⁇ e ⁇
  • N c (N f ) sup ⁇ N c : PL(JV/. JV C ) ⁇ e ⁇ (4)
  • the OCs for the two-tier network can be obtained corresponding to those feasible
  • Theorem 1 For small femtocell sizes, the statistics of the cross-tier femtocell CCI / c /(and intra-tier femtocell CCI Iff) at a base station antenna sector can be given by a
  • increasing femtocell size can increase the outage probabilities arising from the femtocell CCI Iff and I c ,f in a two-tier network.
  • the cdfs i /'/ v n lc J y - ' can decrease as K/ increases, causing a higher outage probability per tier.
  • a femtocell user located on the edge of a femtocell can cause excessive CCI at a nearby femtocell base station.
  • R ⁇ effect of the CCI can appear as a power control penalty factor in J equation (5).
  • Tier 2 users within a femtocell can be assumed to jointly choose a hopping slot. Comparing this against an independent hopping protocol, where users within a femtocell are independently assigned a hopping slot. With independent hopping, the
  • intensity of ⁇ / can equal J ⁇ ec v J (note the difference of from ⁇ in Lemma 1) and the average number of interfering users in an actively transmitting
  • femtocell can equal l -e ⁇ U f /Nh ⁇ ⁇ .
  • an outage threshold r W U V ⁇ op7J (equation (3)) at a femtocell base station two observations are in order: G > 1
  • TH-CDMA transmission When ⁇ W , joint hopping can be preferable from an outage probability perspective. Intuitively, joint hopping reduces )yby a factor N hop , which can cause a quadratic decrease in Kf in equation (5); independent hopping can decrease the number of interfering users per active femtocell, causing a sub-
  • FIG. 7 illustrates this intuition, where the gap in outage performance can be dictated by the hotspot user density.
  • the solid lines 410 represent the joint hopping performance when all users within a femtocell share a common hopping slot.
  • the dotted lines 420 indicate the performance when every femtocell user is assigned an independent CDMA hopping slot. In heavily loaded femtocells (£//>> 1), a joint hopping scheme can be desirable. For lightly loaded
  • Theorems 1 and 2 can provide the tools to quantify the largest N/ that can be accommodated at a given N c subject to an outage constraint e.
  • the next step is to compute the outage probability at a femtocell as defined in equation (3). To do so, assume that the femtocell is located on the axis at a distance Ro from the macrocell center and the receive antenna at the femtocell base station is aligned at angle ⁇ with respect to the hexagonal axis (FIG. 6). The following theorem can be used to derive a lower bound on the statistics of the tier 1 CCI I/ iC at any femtocell located along the hexagonal axis. [0077] Theorem 3 (Lower bound on Macrocellular CCI): For any femtocell base station, antenna sector located at distance 0 ⁇ Ro ⁇ R C from the macrocell base station along the hexagonal axis:
  • the lower bounds on the femtocell outage probability can be derived analogously as stated in the following corollary.
  • FIG. 8 depicts the outage lower bounds to evaluate the impact of macrocellular CCI If x .
  • Tightness of lower bound In one embodiment, the tightness of equations (7) and (8) can show that the cross-tier CCI If x can primarily be impacted by the set of dominant macrocellular interferers.
  • One implication is that one can perform accurate outage analysis at a femtocell by considering only the nearest tier 1 users that individually cause outage.
  • the femtocell outage probability ⁇ out can be lower bounded as:
  • Theorem 4 can compute the largest N c which can ensure the SIR threshold ⁇ can be satisfied for a fraction (1 - e ) of the time. Furthermore, the lower
  • the reference macrocell BS has a femtocell exclusion region
  • RJ C cz H surrounding it.
  • This idea may be motivated by the need to silence neighboring femtocell transmissions which may be strong enough to individually cause outage at a macrocell base station; similar schemes have been proposed and adopted in the CSMA scheduler in the 802.11 standard.
  • the tier 2 femtocell network may then form a heterogeneous SPPP on H with the average number of femtocells in each cell-site equaling ⁇ f " U ' ⁇ 1 1 ⁇ f Ii.
  • the following theorem derives a lower bound on the ccdf of the cross-tier femtocell interference I c / considering the effect of a femtocell exclusion region.
  • Lemma 2 (Femtocell exclusion region): With a femtocell exclusion region of radius R ⁇ c around the reference macrocell base station, the ccdf of cross-tier femtocell CCI / c /is lower bounded as:
  • a femtocell exclusion region can increase the number of simultaneous active femtocell transmissions, while satisfying the macrocell outage constraint ⁇ o «* — e .
  • the network OCs considering the effects of a femtocell exclusion region and tier selection can be obtained by applying Lemmas 2 and 3 in Theorems 2 and 4 respectively. In doing so, we approximate Iff as a Poisson SNP whose cdf is described by equation (1).
  • the close agreement between the theoretical and empirical OC curves indicates the accuracy of the analysis. Observe that the outage constraints oppose one another: increasing P ⁇ r /P° r decreases the largest N/Sustainable for a given N c from the macrocell BS perspective.
  • the corner femtocell may achieve greater than 7x improvement in Nf relative to the split spectrum network.
  • a tier selection policy may succeed in curbing tier 1 CCI mainly for a large Nf which may be sustainable when N c is small (to satisfy ⁇ aut — e ). This may explain the dominant gains in Nf at a low-to-moderate N c .
  • a relevant question is to ask: "How does the system capacity with randomly placed users and hotspots compare against a two-tier network with a given configuration?" Results appear to show that there is a decline in the system capacity, because the configuration contains high levels of cross-tier CCI.
  • Embodiments of the present invention assume that femtocells (placed by end consumer) operate with minimal information exchange with the macrocell base station. Due to reasons of security and scalability-there may be hundreds of embedded femtocells within a densely populated macrocell- handing off unsubscribed users from macrocell to a femtocell hotspot may not be practical. Moreover, femtocell hotspots have a small radio range ( ⁇ 50 meters). This further makes an interference avoidance strategy desirable.
  • the foregoing has presented an uplink capacity analysis and interference avoidance strategy for a shared spectrum two-tier DS-CDMA network in accordance with one embodiment of the present invention.
  • the exact outage probability at a macrocell base station and tight lower bounds on the ccdf of the CCI at a femtocell were derived.
  • Interference avoidance through a TH-CDMA physical layer coupled with sectorized receive antennas is shown to consistently outperform a split spectrum two-tier network with omnidirectional femtocell antennas.
  • the network OCs show a 7x improvement in femtocell density.
  • Load balancing users in each tier may be achievable through an orderwise difference in receive powers in each tier. Additional interference avoidance using a femtocell exclusion region and a tier selection based femtocell handoff may offer conservative improvements in the OCs.
  • Interference avoidance strategies can make shared spectrum two-tier networks a viable proposition in practical wireless systems. Techniques for Reducing Interference in FDMA Cellular Network
  • FDMA frequency division multiple access
  • 3GPP LTE 3GPP LTE
  • 802.16 WiMAX 3GPP2 UMB
  • OFDMA orthogonal frequency division multiple access
  • one or more of the following techniques may be provided in order to reduce interference in FDMA two-tier networks: (1) a spectrum allocation scheme to partition spectrum between cellular and hotspot, or femtocell, base stations; (2) a decentralized spectrum access protocol by femtocells; and (3) a carrier sensing scheme for femtocells to detect idle, or unoccupied, subchannels and avoid creating mutual interference for neighboring femtocells.
  • a spectrum allocation scheme may be provided to partition the spectrum between cellular and femtocell base stations.
  • spectrum allocation can maximize the network-wide Area Spectral Efficiency (ASE) in a two-tier network, assuming each macrocell can transmit to a single user per frequency subchannel, while femtocells may access the spectrum using Frequency- ALOHA (F-ALOHA), a method to be discussed below.
  • ASE which can be measured in b/s/Hz/m 2 , can be defined as the network-wide spatially averaged throughput per frequency subchannel divided by the product of the subchannel bandwidth and the area over which the transmissions take place.
  • the allocation can be determined by qualifying the per-tier ASEs based on the propagation environment and neighboring cell interference.
  • the ASE of the macrocell can either be fixed (for a channel blind scheduler) or increasing (by opportunistic scheduling).
  • the ASEs can be derived based on the optimal spectrum access using F-ALOHA.
  • the optimal allocation can be determined as one that maximizes the weighted mean of the per-tier ASEs. The weights can be given by the fraction of spectrum accessed by each tier.
  • the expected per-tier throughput can be derived for the macrocell and femtocell users accounting for interference from neighboring cells.
  • the maximum ASE of the femtocell network can be shown to be unchanged with addition of hotspots beyond a threshold.
  • a high level of femtocell ASEs can be attained when each femtocell can access most of the available spectrum.
  • femtocells can use a decreasing fraction of the spectrum; e.g. with an average of 100 femtocells in each cell site, each femtocell may access 30% of the available spectrum.
  • the proposed spectrum allocation can allocate the desired level of spatial reuse in a two-tier network, subject to a network- wide QoS requirement, which can create the desirable expected throughput per-user. Differing QoS constraints can produce different spectrum allocations due to the competing spatial coverage scales in each tier. A QoS requiring equal per user throughputs in each tier can mean assigning greater than 90% of spectrum to the macrocell. Conversely, an even division of spectrum can occur when the QoS constraints favor femtocells to provide higher data rates.
  • Frequency- ALOHA a decentralized spectrum access protocol, referred to as Frequency- ALOHA, may be used to access the allocated spectrum.
  • Frequency-ALOHA there can be an optimal fraction of spectrum access for each femtocell in order to maximize the spatial reuse of spectrum, or in effect the net number of simultaneous transmissions per unit area.
  • the spatial reuse is readily expressible using the ASE in b/s/Hz/m 2 .
  • the spectrum may be allocated based on a carrier sensing scheme, wherein the femtocell may opportunistically detect and transmit over unutilized subchannels, in order to avoid colliding with neighboring femtocells.
  • the femtocell may detect unutilized subchannels by using energy detection in each sub band (or source block in commercial OFDMA standards such as 3GPP LTE).
  • the energy detection in a frequency subchannel may be conducted by measuring the total power (signal plus interference plus noise) in that subchannel; hypothesis testing can be performed to decide whether the measured power in the subchannel corresponds to an occupied subchannel or not.
  • the null hypothesis corresponds to an unoccupied subchannel which is therefore available for transmission by the femtocell.
  • the Significant hypothesis corresponds to an occupied subchannel (i.e. there is an ongoing transmission intended for either a cellular user or an indoor user in a neighboring femtocell in that subchannel).
  • the cellular system setup can consist of a hexagonal region H of radius R c with a central macrocell base station C providing coverage area which can be surrounded by two rings of interfering macrocells.
  • the macro-cellular network can be overlaid with femtocell hotspots of radius Rf which can be randomly distributed on R 2 according to a homogeneous SPPP ⁇ / with intensity >y.
  • Macrocell users can be assumed to be uniformly distributed inside each cell site. Femtocells can be assumed to provide "closed access" to licensed indoor users who fall within the radio range Rf of their respective home base stations.
  • U U c + NfUf denote the average number of users in each cell site. These U users can be distributed into U c uniformly distributed tier 1 mobile outdoor users and Uf users per femtocell hotspot.
  • the available spectrum can comprise F frequency subchannels each with bandwidth JFHz.
  • p F c /F as the fraction of spectrum assigned to the macrocell base station with the following assumptions:
  • Each femtocell schedules its users in a round-robin (RR) fashion.
  • the macrocell schedules its users according to either a channel blind RR or a channel aware proportional fair (PF) scheduler.
  • RR round-robin
  • PF proportional fair
  • each femtocell transmits over exactly k frequency subchannels among their allotted Ff subchannels, the net portion of accessed spectrum per femtocell equals p/1 - p) where @ f ⁇ " ' L If femtocells choose their frequency subchannels independently and with equal probability, F-ALOHA can effectively "thin" the mean number of interfering femtocells in each frequency subchannel.
  • the probability p of a femtocell selecting a given frequency subchannel for transmission can be given as:
  • the set of interfering femtocells per frequency subchannel can be a marked SPPP A/ with intensity ⁇ yk/Ff.
  • femtocells may transmit in a small region of spectrum and avoid causing mutual interference. This strategy can provide a higher spectral efficiency over each frequency subchannel, but can incur reduced spectrum utilization because femtocells do not transmit over the entire available spectrum.
  • the downlink channel between each base station and its users can be composed of a fixed distance dependent path loss, a slowly varying component modeled by lognormal shadowing and Rayleigh fast fading with unit average power.
  • thermal noise may be neglected at the receiver since cellular systems, by nature, are interference limited.
  • imperfect feedback and/or channel estimation may have a potentially big impact on system capacity, the following does not account for these effects for the sake of analytical tractability.
  • the expected throughput provided by each macrocell [resp. femtocell] can be obtained by multiplying the expected throughput in equation (14) by their respective spectrum allocation p [resp. p/l-p)].
  • T c (p,U c ) be the long term throughput (in b/s/Hz) in each subchannel provided by the macrocell.
  • Obtaining T c can require calculating the average rate per subchannel in equation (14) after spatially averaging the SIR over all locations, and accounting for the interference from two rings of transmitting macrocells.
  • each femtocell access a portion p/ of its allotted spectrum using F-ALOHA, servicing its users in a RR schedule.
  • F-ALOHA F-ALOHA
  • the expected femtocell throughput in each frequency subchannel which can be determined by the intensity / 0/?y of the marked SPPP ⁇ /.
  • the ASE of the macrocell (resp. femtocell) network can be given as:
  • the factor N j P/ can represent the mean number of transmitting femtocells in each subchannel.
  • the per-tier throughputs (in b/s) per subchannel can be calculated by multiplying the ASEs in equation (15) by W
  • the network-wide ASE can therefore be given as:
  • the problem can be to determine the optimal spectrum allocation p over all possible spectrum partitioning strategies ⁇ € [0, IJ between the macrocell and femtocells.
  • the spectrum allocation can maximize the network-wide ASE with a QoS constraint ⁇ on the minimum expected per-tier throughput/user, as shown below:
  • T CM ⁇ ) and Tf, u ⁇ ) are the expected throughputs for a macrocell and femtocell user respectively.
  • the objective function in equation (17) can be an affine function with respect to ⁇ .
  • the following proposition derives the optimizing p considering the maximum may be obtained at the extremal points of the constraint set: [00117] In an embodiment, if the expected macrocell throughput per subchannel is independent of the total spectrum allocated to the macrocell ⁇ , i.e.
  • equation (17) can be a convex combination of the macrocell and femtocell throughputs which can be maximized at the extreme points pe ⁇ 0, 1 ⁇ . Such a partitioning can be unfair since it can result in a greedy allocation of the entire spectrum to one tier.
  • equation (17) is one dimensional optimization problem that can be solved efficiently for a given ⁇ using a numerical search.
  • c ⁇ represents the outdoor path loss exponent and
  • Averaging equation (26) over a hexagonal cell region may be difficult.
  • the spatially averaged CDF of SIR c can be obtained approximately by
  • the region may be divided into M non-overlapping annuli.
  • a simplifying assumption is that all users inside an annulus experience identical shadowing statistics (i.e. identical Denot j ng me distance of the user from C 0 by
  • R, the following lemma derives the expected spatial throughput by averaging SIR c (i?) inside a circular annulus with inner radius R ⁇ and outer radius i? 2 .
  • Lemma 1 The spatially averaged SIR distribution inside a circular annulus with inner radius Ri and outer radius U 2 may be given as:
  • Lemma 1 provides a simple method for estimating the cell-averaged macrocell throughput per sub-channel.
  • Equation (33) is obtained by substituting (27) inside the conditional expectation in (32) and the corresponding probability that the user lies in annulus m, 1 ⁇ m ⁇ M. Combining equations (14) and (31), the average macrocell throughput T c in a given subchannel is expressed as
  • Tc Y 1 I - E R [Pr(r ; ⁇ Sm r (R) ⁇ T 1+1 )] + L ⁇ E R [P ⁇ S ⁇ R c (R) ⁇ T L )]
  • FIG. 16(a) plots T c (in b/s/Hz) with RR scheduling as a function of the outdoor path-loss exponent O 0 for the system parameters in FIG. 23.
  • the close agreement between theory and numerical simulations indicates that the theoretically obtained SIR distribution is an accurate approximation for practical throughput in a macrocellular environment.
  • a PF scheduler in contrast to a RR scheduler, can enable macrocell users to compete for resources based on their requested rates normalized by their average throughput. Consequently, the macrocell can select the user with the highest rate relative to their average rate.
  • R k ⁇ m, n ⁇ as the requested rate for user k, 1 ⁇ k ⁇ U C , located at position J 36 me windowed mean throughput obtained by user k over the F c frequency subchannels allocated for macrocell transmission.
  • the PF scheduler can select the "- whose current supportable rate may be high relative to their mean rate.
  • the scheduling policy per subchannel m with equal per- subchannel transmission powers (Assumption 4) can be described as:
  • k(m, n) arg max — ⁇ . (35) b l ⁇ k ⁇ U c R k ⁇ n ⁇
  • mobile user k calculates R k ⁇ m, n ⁇ using equations (12) and (21), respectively.
  • the windowed throughput per user prior to transmission interval (n + 1) can be updated according to the following rule,
  • the window size N can be a parameter that can be selected considering the delay tolerance for each user. Choosing a smaller N can enable a given user to be scheduled more often, whereas choosing larger N can relax the fairness constraint and can allow the scheduler to wait longer before scheduling a user.
  • the average throughput per frequency subchannel for a given set of user positions can be obtained from the sample average over a long duration and expressed as:
  • the spatial averaged subchannel macrocell throughput can be obtained by averaging equation (37) with
  • T c (p, U c ) can be analytically intractable.
  • numerical simulation can be used to empirically estimate Tc(p, U c ), which can be used to derive the bandwidth partitioning.
  • the interference experienced by a femtocell user can depend on the distances of these interfering base stations relative to the user and their respective channel gains.
  • the interfering femtocells can form a marked SPPP ⁇ f Q ⁇ / with intensity pj) ⁇ .
  • the cochannel interference /// experienced by a user 0 within femtocell F 0 can be given as:
  • I fj ⁇ A f ® ok ⁇ h ok f ⁇ x ok n , (39) keA f
  • the received SIR can be given as:
  • xok represents the locations of the interfering femtocells F k with respect to user 0.
  • ⁇ o is the e ff ec tive channel gains from the desired [resp. interfering base station].
  • the terms Of [resp. ⁇ j ⁇ represent the path-loss exponents resulting from interfering transmissions [resp. in-home transmissions) to the user of interest.
  • a simple model can be used to distinguish between the fixed losses arising from in-home and interfering transmissions. Specifically, home users can be insulated against interfering femtocell transmissions through double penetration losses arising from external wall partitions. Consequently, Af and Bf (in dB) are related as is the wall penetration loss.
  • Equation (40) then can be simplified to:
  • Lemma 2 can provide the relationship between the density fycy of interfering femtocells in ⁇ / and the distribution of the CCI at a femtocell.
  • the tail probability Prf ⁇ / > y) ⁇ 0 in equation (42) indicating that selecting fewer subchannels using F-ALOHA transmission can provide greater resilience against persistent collisions from nearby femtocells.
  • the distribution of the femtocell SIR in equation (40) can be obtained as:
  • Equation (45) and (46) follow by conditioning on ⁇ O, assuming independence of ⁇ o and ofe " ⁇ /, and applying equation (42). Although it may not be possible to obtain a closed form expression for the expectation in equation (47), the distribution SIR/ can be calculated numerically.
  • the mean subchannel throughput Tf can be calculated by combining equations (14) and (47):
  • the femtocell throughput falls from approximately 4.5 b/s/Hz in a HA environment to nearly 0.5 b/s/Hz in an LA scenario, including the sensitivity of femtocell throughput to propagation from nearby femtocells.
  • equation (50) can compute the F-ALOHA spectrum access p/ which can maximize the mean overall throughput (1 — p) p/T j (p ⁇ per femtocell.
  • the ASE in equation (50) only depends on the effective intensity )y ⁇ of interfering femtocells per subchannel.
  • the intensity of ⁇ / can be given as f"f and remains constant, implying that the optimal p/is a monotone decreasing function of )y Consequently, if p/ ⁇ 1 for a given ⁇ y, the maximum ASE per subchannel is fixed. This means that with increasing ⁇ y, the network-wide femtocell throughput equaling « ' " ⁇ ⁇ P ) s / grow linearly with
  • T ⁇ i and P ⁇ be the mean subchannel throughput and the optimal F-ALOHA access respectively.
  • T ⁇ i and P ⁇ be the mean subchannel throughput and the optimal F-ALOHA access respectively.
  • Tf 1 ⁇ and P ⁇ h the corresponding quantities.
  • the femtocell network can be defined as fully-utilized [resp. sub-utilized] if a marginal increment in the femtocell density reduces [resp. improves] the mean throughput per femtocell as given below:
  • Equation (53) reflects the competing effects of increasing allocation (1 — p) and decreasing F-ALOHA access p/ (or increasing ⁇ y) in determining the net femtocell throughput.
  • FIG. 19 shows the allocation using (19) with RR scheduling per-tier for varying QoS parameter ⁇ .
  • ⁇ 0.5
  • ⁇ 0.5
  • FIG. 20 plots the ASEs of the two-tier network using (16) for varying QoS parameter ⁇ .
  • ⁇ 0.01
  • Nf 50 femtocells.
  • the ASEs monotonically increase with Nf indicating increasing spatial reuse with addition of femtocells.
  • a PF scheduler provides nearly 2.3x [resp.
  • FIG. 21 plots the expected throughput per femtocell (1- p) PfTf as a function of Nf and ⁇ .
  • ⁇ 0.5
  • the throughputs monotonically increase with Nf indicating that increasing spectrum allocation (1 - p) counteracts the effects of decreasing p/7/, in effect, the femtocell network is sub-utilized.
  • ⁇ 0.01 in a LA environment however, the femtocell throughputs decrease with increasing Nf, indicating that the femtocell network is fully-utilized.
  • D c 0.1 Mbps
  • spatial reuse and spectrum requirements with the addition of femtocells may be markedly different depending on attenuation from neighboring femtocells. For example, in a LA [resp. HA] scenario, the spectrum requirement WF increases [resp. decreases] with increasing hotspot density indicating the femtocell network is fully- utilized [resp. sub-utilized] with the per-tier spectrum allocation in equation (19).
  • Embodiments of the present invention propose a decentralized spectrum allocation strategy as an alternative to centralized/coordinated frequency assignment in a two-tier network.
  • the proposed allocation may depend on the per-tier throughputs, the loading of users in each tier and the QoS requirements, accounting for co-channel interference and path-losses.
  • femtocells should, according to one embodiment, access a decreasing fraction of their allocated spectrum with increasing femtocell density, in order to maximize spatial reuse. Spatial reuse benefits derived from channel aware macrocell scheduling result in nearly 50% spectrum reduction for meeting target per-tier data rates.
  • MIMO Multiple-Input-Multiple-Output
  • MIMO provides extra degrees of freedom (spatial diversity) to combat cross-tier interference in a two-tier network with universal frequency reuse.
  • using multiple antennas may enable a femtocell to either (a) eliminate cross-tier interference to nearby cellular users in a closed-access system by nulling their beams in the direction of a nearby cellular user, or (b) handoff nearby cellular users and boost their signal strength through array processing.
  • another embodiment of the invention may enable a group of geographically close femtocells - to reduce communication latency - to combine their antenna resources for collaboratively canceling cross-tier interference to nearby cellular user(s). So, even if a single femtocell cannot cancel its interference because it does not have sufficient antennas (i.e., degrees of freedom), if there are too many cellular users in its vicinity, this technique may overcome this difficulty as the collaborating femtocells possess extra degrees of freedom to eliminate cross-tier interference. This technique may require the collaborating femtocells to exchange their respective channel information (CI) with a nearby cellular user over the backhaul network.
  • CI channel information
  • yet another embodiment of the invention may provide an adaptive power control technique (with closed-access) and a handoff policy to femtocells (with open access) in a two-tier MIMO cellular system with universal frequency reuse.
  • this technique may enable femtocells to determine their maximum transmit power in order to guarantee a certain minimum Quality-of-Service (QoS) to a nearby cellular user.
  • QoS Quality-of-Service
  • the transmit power may be derived as a function of the number of antennas at the macrocell/femtocells, fixed and variable components of indoor and outdoor path-losses which arise during terrestrial propagation of wireless signals, and fast fading channel scenarios (e.g. Rayleigh fading).
  • the same embodiment may be used to determine the locations of cellular users with respect to their central macrocell, wherein a nearby femtocell may provide the outdoor cellular user with better QoS (through a handoff of the cellular user to the nearby femtocell).
  • a nearby femtocell may provide the outdoor cellular user with better QoS (through a handoff of the cellular user to the nearby femtocell).
  • Yet another embodiment of the invention with MIMO may provide a policy to handoff an indoor user from its femtocell to the central macrocell as a function of the number of antennas at the macrocell/femtocells, fixed and variable components of indoor and outdoor path-losses which arise during terrestrial propagation of wireless signals, and fast fading channel scenarios (e.g. Rayleigh fading).
  • cross-tier interference may become a capacity-limiting factor.
  • a cellular user communicates with the central macrocell in the vicinity of an active femtocell may suffer unacceptable interference.
  • Embodiments of the present invention propose two prior approaches to tackle cross-tier interference in a shared spectrum two-tier network namely 1) interference avoidance and 2) interference aware power control schemes at each femtocell.
  • Employing multiple antennas at a femtocell base station (BS) can offer a third possibility namely to utilize the available spatial degrees of freedom at the femtocell to serve multiple indoor users and eliminate interference at outdoor cellular users.
  • multiple antennas at femtocells can be used to serve multiple indoor users-providing multiplexing gain, and eliminating cross-tier interference by nulling transmissions in the direction of a nearby cellular user. Consequently, an outdoor cellular user can obtain reliable reception even though it is in the vicinity of an active femtocell. Additionally, indoor users can continue to experience the superior reception which was a primary reason why they installed a femtocell in the first place.
  • a femtocell may have only a limited number of antennas, sacrificing these degrees of freedom for interference cancellation may result in a tradeoff due to smaller multiplexing gain.
  • the co-channel interference at a nearby cellular user may be severe enough to force femtocells to expend resources to tackle the interference issue.
  • the questions to be addressed are as follows: [00151] 1) Given a performance metric which is the weighted sum of the achievable rates provided by the macrocell and femtocell respectively, to how many cellular users should a femtocell cancel its interference? Alternatively, how many users should a femtocell serve in each signaling interval? How does this depend on the path losses between the femtocell and the macrocell to the cellular user?
  • Assumption 1 Perfect channel information is assumed at the femtocell regarding its channels to users in Sf as well as S c /.
  • the precoder F can be chosen to be the orthogonal projector In the general case, one chooses to equal the T f - S Clf right singular vectors in the full SVD
  • the received signal model for a femtocell user can be represented as:
  • ⁇ is the channel power from the femtocell to indoor users.
  • Assumption 2 can be justified if one assumes that users in S c ,f are situated in the vicinity of a femtocell (see FIG. 26 for example wherein D c represents the distance of cellular user to macrocell, and D f represents the distance of cellular user to femtocell), or alternatively assuming that these users are co-located from each BS's perspective.
  • the ergodic sum rate for the macrocell can be given as:
  • One objective is to determine the optimal number of femtocell users that should be scheduled in order to maximize the weighted ergodic sum rates provided to indoor users and cellular users in ScJ. Allocate weights ⁇ , 0 ⁇ ⁇ 1 and 1 - ⁇ to the femtocell and macrocell rates in equations (56) and (58) respectively. Then, maximizing the convex combination of the per-tier rates results in the Pareto-frontier of the system.
  • One question posed herein is: Given path-losses a and ⁇ and a weighting parameter ⁇ , how many users should a femtocell serve? According to embodiments of the present invention, the number of served indoor users may be given as the solution to the following optimization problem:
  • the quantity * is a function of the channel powers parameterized by ⁇ and ⁇ , respectively.
  • a femtocell decides a certain set of indoor users Sf and a set of cellular users S c ,f, in order to maximize the instantaneous sum rate.
  • the user selection may be conducted either with full CSIT at the femtocell, or assuming limited feedback from indoor users to the femtocell using Random Vector Quantization (RVQ) based code-books.
  • RVQ Random Vector Quantization
  • embodiments of the present invention propose that neighboring femtocells communicate (possibly with a centralized entity over the internet backhaul) and use distributed MIMO to form a "meta-femtocell" which can cancel interference to both cellular users simultaneously.
  • each femtocell can cancel interference to just 1 cellular user.
  • the precoder F By jointly designing the precoder F, resulting 4 antenna BTS can cancel interference to both cellular users simultaneously (see FIG. 27).
  • deploying more femtocells may strategically help an operator reduce interference caused by a rogue femtocell, by inducing that femtocell to coordinate with other femtocells in its immediate vicinity. Perfect synchronization is assumed here which, along with IP latency, may be one of the key issues facing a practical implementation. Outage Probability and Threshold Distances in Two-tier Networks with Spatial Diversity
  • cross-tier interference may become a capacity-limiting factor.
  • a cellular user communicating with the central macrocell in the vicinity of an active femtocell may suffer unacceptable interference.
  • FIG. 24 around the femtocell can result in poor outdoor cellular reception.
  • This embodiment of the present invention considers a scenario wherein the central macrocell and the femtocells share a common region of spectrum and transmit using multiple antennas to their respective users. Femtocells are assumed to be randomly located on the plane according to a Spatial Poisson Point Process (SPPP) ⁇ f with intensity
  • SPPP Spatial Poisson Point Process
  • Each BS transmits to exactly one user per slot-WLOG, the slot may refer to either a time or frequency or an orthogonal code resource-for transmission by beamforming (abbreviated as SUBF) in the direction of the vector channel to its user.
  • SUBF orthogonal code resource-for transmission by beamforming
  • the terrestrial propagation losses between each BS to surrounding users can be modeled using the IMT-2000 channel model.
  • Embodiments of the present invention consider the following scenarios:
  • Path-loss between a given femtocell to a neighboring femtocell can be obtained by adding an extra P dB term to model the double wall partition losses during indoor-to- indoor propagation.
  • the beamforming vector wo can be chosen to be unit norm and in the direction of the vector downlink channel h 0 whose entries are distributed as CN(O 5 I).
  • the received signal for a cellular user 0 at distance D to the central macrocell BQ can be written as:
  • s 0 refers to the data symbol intended for cellular user 0 with -"- 1 Ll 3 O l J L c .
  • * ' / refers to the data symbol for the scheduled user in femtocell
  • the Signal-to-Interference (SIR) ratio for the cellular user 0 can be given as:
  • a Quality-of-Service (QoS) requirement for user 0 stipulates that the target SIR is satisfied with a probability of at least - »• ⁇ .
  • QoS Quality-of-Service
  • a target SIR ⁇ , a QoS requirement *- and an average of Nf )y)C
  • femtocells per cell site, (72) can provide the threshold distance of a cellular user to its macrocell D max beyond which the cellular user suffers unacceptable cross-tier interference.
  • a reference femtocell F 0 at distance D from the central macrocell 2? 0 is considered.
  • the outage probability at FQ may be computed considering only the cross-tier interference from BQ.
  • the signal received at user 0 inside femtocell F 0 can be given as: where go [resp. f c ] are the vector downlink channels from femtocell F 0 [resp. macrocell
  • the outage probability P 0 UtOS D) P ⁇ (SIRF, CS IT- SU BT(£0 ⁇ ) can be given as: ⁇ ( ⁇ , D) F 1 + ⁇ ( ⁇ , D)
  • the above expression can provide the threshold distance of a femtocell with respect to the central macrocell ⁇ 0 within which cross-tier interference prevents in-home users from satisfying their QoS constraint ⁇ - .
  • embodiments of the present invention propose that all in-home users located at D ⁇ D m i n communicate with the central macrocell.
  • FIG. 28, 29 and 30 illustrate simulation results associated with the foregoing embodiments of the present invention.
  • FIG. 28 provides the system parameters used for the simulations;
  • FIG. 29 illustrates the outage probability at a femtocell as a function of its distance from the central macrocell;
  • embodiments of the present invention may be configured as a system, method or network entity. Accordingly, embodiments of the present invention may be comprised of various means including entirely of hardware, entirely of software, or any combination of software and hardware. Furthermore, embodiments of the present invention may take the form of a computer program product on a computer-readable storage medium having computer-readable program instructions (e.g., computer software) embodied in the storage medium. Any suitable computer-readable storage medium may be utilized including hard disks, CD-ROMs, optical storage devices, or magnetic storage devices.
  • These computer program instructions may also be stored in a computer- readable memory that can direct a computer or other programmable data processing apparatus (e.g., processor 103 of FIG. 3) to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including computer-readable instructions for implementing the function specified in the flowchart block or blocks.
  • the computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions that execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks.
  • blocks of the block diagrams and flowchart illustrations support combinations of means for performing the specified functions, combinations of steps for performing the specified functions and program instruction means for performing the specified functions. It will also be understood that each block of the block diagrams and flowchart illustrations, and combinations of blocks in the block diagrams and flowchart illustrations, can be implemented by special purpose hardware-based computer systems that perform the specified functions or steps, or combinations of special purpose hardware and computer instructions.

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Abstract

L'invention porte sur un système et un procédé dans lesquels une ou plusieurs stations de base femtocellulaires sont déployées à portée d'une station de base cellulaire et utilisent sensiblement la même bande de fréquence que la station de base cellulaire. Chaque station de base femtocellulaire peut être configurée pour employer une ou plusieurs techniques d'annulation de brouillage pour qu'une coexistence entre la station de base cellulaire et la station de base femtocellulaire correspondante soit possible. Les techniques d'annulation de brouillage employées peuvent comprendre l'utilisation d'un saut de temps ou de fréquence aléatoire ; la sélection aléatoire d'un nombre prédéterminé de sous-canaux de fréquence, ou l'identification d'un ou plusieurs sous-canaux de fréquence non utilisés, pour une transmission de signaux ; l'utilisation de deux antennes d'émission ou plus et de deux antennes de réception ou plus ; l'annulation d'une ou plusieurs émissions dans une direction d'un utilisateur de station de base cellulaire proche ; le transfert d'au moins un utilisateur cellulaire à l'une des stations de base femtocellulaires et vice versa ; et/ou la réduction de la puissance d'émission d'au moins une station de base femtocellulaire.
PCT/US2009/065135 2008-11-20 2009-11-19 Techniques de gestion de brouillage et d'accès à un canal décentralisé dans des réseaux cellulaires soutenus par des points d'accès sans fil WO2010059816A1 (fr)

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