Patent Publication Number: US-2012026974-A1

Title: Load balancing for an air interface protocol architecture with a plurality of heterogeneous physical layer modes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present U.S. Utility patent application claims priority pursuant to 35 U.S.C. §§120, 121, as a divisional, to U.S. Utility application Ser. No. 12/066,096, entitled “LOAD BALANCING FOR AN AIR INTERFACE PROTOCOL ARCHITECTURE WITH A PLURALITY OF HETEROGENEOUS PHYSICAL LAYER MODES,” (Attorney Docket No. 17936ROUS05N), filed Mar. 7, 2008, pending, which claims priority pursuant to 35 U.S.C. §371 to International Application No. PCT/CA2006/001480, entitled “LOAD BALANCING FOR AN AIR INTERFACE PROTOCOL ARCHITECTURE WITH A PLURALITY OF HETEROGENEOUS PHYSICAL LAYER MODES,” filed Sep. 8, 2006, which claims priority pursuant to 35 U.S.C. §119(e) to the following U.S. Provisional patent applications:
         1. U.S. Provisional Application No. 60/715,281, entitled “WIRELESS MULTI-MODE ACCESS SYSTEMS AND METHODS,” filed Sep. 8, 2005, expired;   2. U.S. Provisional Application No. 60/751,848, entitled “LOAD BALANCING FOR MULTI-MODE ACCESS SYSTEM FOR BEYOND 3G WIRELESS NETWORKS,” filed Dec. 20, 2005, expired; and   3. U.S. Provisional Application No. 60/804,343, entitled “WIRELESS MULTI-MODE ACCESS SYSTEMS AND METHODS,” filed Jun. 9, 2006, expired;
 
all of which are hereby incorporated herein by reference in their entirety and made part of the present U.S. Utility patent application for all purposes.
       

    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates generally to cellular wireless networks, and more particularly to the loading of multiple physical layer modes to service a cellular wireless network. 
     2. Related Art 
     Wireless networks are known. Cellular wireless networks support wireless communication services in many populated areas of the world. While wireless networks were initially constructed to service voice circuit-switched voice communications, they are now called upon to support packet-switched data communications as well. 
     The transmission of packetized data communications within a wireless network places different demands on networks than does the transmission of voice communications. Voice communications require a sustained bandwidth with minimum signal-to-noise ratio (SNR) and continuity requirements. Data communications, on the other hand, typically are latency tolerant but have higher total throughput requirements. Conventional circuit-switched wireless networks were designed to support the well-known voice communication requirements. Thus, wireless networks (as well as conventional circuit switched telephone networks) have been adapted to service data communications, with such adaptation providing mixed results. 
     As the variety of devices that support wireless communications both on a circuit-switched basis and on a packet-switched basis are increasing, as well as devices having multiple modes of operation to support voice and multimedia communications. With respect to the infrastructure to support these devices, the ability to service all manners of circuit and packet communications exist while sustaining the Quality of Service, and complying with the Service Level Agreements despite the load placed upon components of the network infrastructure. 
     Thus, there exists a need for traffic load balancing of multiple physical layer modes of an air interface protocol architecture to accommodate the varying single-mode and multi-mode mobile terminals that may be present within a wireless communications system. 
     SUMMARY OF THE INVENTION 
     To overcome these shortcomings, among others, by providing dynamic traffic load balancing with a multiple physical layer modes of an air interface protocol architecture residing in a base station and/or an access network that includes a base station and access gateway that include distributed and centralized components of the air interface protocol architecture. 
     According to an embodiment of the invention, the traffic load balancing supports various multiplexing scenarios for scheduling communication data of a time slot of multiple time slots for use in the multi-carrier wireless network. This operation includes, for each time slot of the multiple time slots for transmitting the data, to assign priorities to each mobile terminal coupled to the base station based upon a scheduling priority criteria. Each of the mobile terminals include at least one physical layer mode. The mobile terminals are sorted in order of their assigned priority for data communications. 
     When the physical layer mode multiplexing scenario is a Time Division Multiplexing (TDM) scenario, the physical layer mode is selected based upon the mobile terminal having the highest assigned priority for data communications and the physical layer mode that it supports. If the time slot can accommodate an additional mobile terminal, the next highest assigned priority mobile terminal, which supports the selected physical layer mode, is selected. This operation continues until the time slot cannot accommodate further mobile terminals, or no further mobile terminals are available for data communications. 
     When the physical layer mode multiplexing scenario is a Frequency Division Multiplexing (FDM) scenario, the physical layer mode is selected based upon the highest assigned priority mobile terminal for data communication and whether the highest assigned priority mobile terminal is a single physical-layer mode mobile terminal or a multi physical-layer mode mobile terminal. 
     When physical layer resources can accommodate an additional mobile terminal, a next highest assigned priority terminal is selected for data communication. The process continues until the physical layer resources cannot accommodate an additional mobile terminal or no additional mobile terminals remain. 
     With respect to the FDM scenario, when the highest assigned priority mobile terminal is a physical layer multi-mode terminal, and the multi-mode mobile terminal is not capable of data communications with each of the plurality of physical layer modes of interest, the selected physical layer mode is a least loaded physical layer mode. When the multi-mode terminal is capable of data communications with each of the plurality of physical layer modes of interest, the selected physical layer mode is the physical layer mode that occupies a least amount of spectrum resources. 
     In a further aspect, when the selected physical layer mode changes to a subsequent selected physical layer mode, the set of MAC states supported by the subsequent physical layer mode is determined. When a present MAC state of the mobile terminal is not supported by the new set of MAC states, the mobile terminal switches to a valid MAC state for the selected physical layer mode. 
     In yet a further aspect, an access network for a multi-carrier wireless communications network to simplify context transfer in a handoff of a mobile terminal. The access network includes an access gateway and a plurality of base stations anchored to the access gateway. The access gateway is coupled to a wireless core network infrastructure, where the access gateway includes a centralized layer 2/3 protocol structure. Each of the base stations provide a coverage area for data communications with the mobile terminal, wherein each base station includes a distributed layer 2/3 protocol structure. The access gateway stores a centralized layer 2/3 context of the mobile terminal. By storing a centralized layer 2/3 context of the mobile terminal, context transfers are not necessary for an intra-base station handoff between the base stations. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a system diagram illustrating a portion of a communications network constructed in accordance with an embodiment of the present invention; 
         FIG. 2  illustrates a protocol stack for a high-speed multi-carrier wireless network in accordance with an embodiment of the present invention; 
         FIG. 3  illustrates the spectrum allocations scenarios with respect to a multimode physical layer resource sharing according to the time domain, frequency domain, and CDMA code domain; 
         FIG. 4  is a block diagram illustrating a common layer 2/3 protocol structure anchoring a multi-mode physical layer with heterogeneous physical layer modes in accordance with an embodiment of the present invention; 
         FIGS. 5 and 6  are a logic diagram illustrating dynamic traffic load balancing with a plurality of heterogeneous physical layer modes of an air interface protocol architecture in accordance with an embodiment of the present invention; 
         FIG. 7  is an illustration of a MAC state diagram in accordance with an embodiment of the present invention; 
         FIG. 8  is a logic diagram illustrating MAC state transitions upon for physical layer mode transitions in accordance with an embodiment of the present invention; 
         FIG. 9  is a block diagram illustrating a base station constructed according to an embodiment of the invention; and 
         FIG. 10  is a block diagram illustrating an access gateway constructed according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a system diagram illustrating a portion of a communications network  10  constructed according to an embodiment of the present invention. The communications network  10  includes a wireless core network infrastructure  24 , access gateway  28  and access gateway  30 . The wireless core network infrastructure operates according to an operating standards specification that may have been modified according to the embodiment of the present invention (for example, High Speed Downlink Packet Access (HSPDA), 1xEV, et cetera). The wireless core network infrastructure  24  couples to a local packet data network  18  and also to the Public Switched Telephone Network (PSTN)  14 . The wireless core network infrastructure may be provided as a packet-switched infrastructure, which couples to the local packet data network via a gateway  27 , and couples to the PSTN  14  via an interworking function (IWF)  22 . 
     A conventional wireline terminal  12  couples to the PSTN  14 . A Voice-over-IP (VoIP) terminal  16  and a personal computer  20  couple to the local packet data network  18 . Mobile terminals  40 ,  41 ,  52 , and  54  wirelessly couple to the communications network  10  via wireless links with the base stations  32 ,  34 ,  36 , and  38 , which provide wireless coverage via a hierarchical cell layout  42 . As illustrated, mobile terminals may include a cellular telephone  52 , a data terminal  54  (for example, a personal digital assistant), a laptop computer  41 , and a desktop computer  40 . The wireless network, however, supports communications with other types of mobile stations as well. 
     Each of the base stations  32 ,  34 ,  36 , and  38  has associated coverage areas  62 ,  60 ,  58 , and  56 , respectively, that services a cell/set of sectors. The coverage by the base stations  32 ,  34 ,  36 , and  38  provides a hierarchical cell layout  42 , which includes a combined coverage of both macrocells, microcells, and picocells, such as the macrocell  44 , the microcell  48 , and picocell  50 . 
     An access network  66  to the hierarchical cell layout  42  includes the access gateway  28  and the access gateway  30 , and the base stations  32 ,  34 ,  36 , and  38 . Wireless communication between the mobile terminals and the access network  66  occur via a protocol stack that includes a common layer 2/3 protocol structure and a multi-mode physical layer. The multi-mode physical layer includes a plurality of heterogeneous physical layer modes to support a variety of physical layer configurations of the various mobile terminals. 
     The common layer 2/3 protocol functionality may be partitioned between an access gateway and the base stations, but may also reside wholly in a base station. In this regard, the base stations  32 ,  34 ,  36 , and  38  include a multi-mode physical layer and a “distributed” common layer 2/3 function. Each of the base stations may support multiple physical layer modes in the same coverage area  56 ,  59 ,  60 , and  62 . The access gateways  28  and  30  provide a “centralized” component of the layer 2/3 function, and also serve as a router within the access network  66  and provide interface to the wireless core network infrastructure  24  (as well as public IP networks). 
     The centralized component of the layer 2/3 increases the efficiency by simplifying or removing the transfer of contexts in handoff operations. That is, each of the base stations  32 ,  34 ,  36 , and  38  belong to different cell hierarchies, which may be anchored by the same access gateway or different access gateways. When handoffs occur between the base stations  32 ,  34 , and/or  36  that are anchored by the same access gateway  28 , the centralized layer 2/3 context of the mobile terminal is contained within the access gateway  28 , thus eliminating a need to carry out a context transfer with the handoff. The distributed layer 2/3 context of the mobile terminal contained within the previous serving base station (such as base station  32 ) may be transferred to the target base station (such as base station  60 ), through the access gateway  28  or through a base station-to-base station link. As a result of having a centralized and a distributed common layer 2/3 protocol structure, a loss of layer 2/3 context information during handoff can be avoided, thus avoiding unnecessary packet loss and latency. 
     When there are handoffs between base stations anchored by different access gateways  28  and  30 , the layer 2/3 context information relating to a mobile terminal may be transferred from the previous serving access gateway/base station to the target access gateway/base station through links between the devices. 
     The wireless links include both forward link components and reverse link components support wireless communications between the base stations and their serviced mobile terminals. These wireless links support both data communications and multimedia communications, such as VoIP. The teachings of the present invention may be applied equally to any type of packetized communication. 
     The common layer 2/3 protocol structure and the multi-mode physical layer provide Beyond 3G (B3G) services that support user terminals having a single physical layer mode (such as legacy user terminals) and those terminals having a multi physical layer mode terminal (such as recent terminals incorporating multimedia capabilities with increased data throughput requirements). In other words, the common layer 2/3 protocol stack may be provided by extending the existing 3G cellular protocol stacks to support backward compatibility for legacy user terminals. The protocol stack and common layer 2/3 protocol structure, and the multi-mode physical layer, are discussed in detail with reference to  FIGS. 2 through 10 . 
       FIG. 2  illustrates a protocol stack  102  for a high-speed multi-carrier wireless network. The protocol stack includes wireline upper level protocol structures  103 , a common layer 2/3 protocol structure  104 , and a multi-mode physical layer  116 . The common layer 2/3 protocol structure  104  includes a centralized layer 2/3 protocol structure  106 , which may reside in an access gateway, and a distributed layer 2/3 protocol structure  108  that may reside in each of the base stations anchored by the access gateway. The multi-mode physical layer  116  includes a plurality of heterogeneous physical layer modes  118 ,  120 ,  122 ,  124 ,  126 , and  128 . Illustrated are centralized layer 2/3  305 , distributed layer 2/3  307 , and physical layer 309. The centralized layer 2/3 protocol structure  106  is included in an access gateway (for example, a base station controller), and the distributed layer 2/3 protocol structure  108  in the base station. The radio transceivers of the base stations include implement the multi-mode physical layer  116 . 
     In the forward link, the physical layer in resource assignment to each mobile terminal is performed according to the radio channel condition experienced in the forward link of the mobile terminal, as well as service requirements which are defined by upper protocol layers, that is, the air-interface common layer 2/3 protocol structure  104 . The layer 2 and 3 protocols provide a common interface with the wireline upper layer protocol structures  103  such as Point-to-Point Protocol (PPP)  112 , Internet Protocol (IP) layer 301  110  and Transmission Control Protocol (TCP). The layer 2 and 3 protocols interface with the multi-mode physical layer  116  by selecting the appropriate physical layer resource in both frequency domain and time domain to meet the Quality of Service (QoS) required by upper layer applications as well as the subscriber profile for the mobile terminal. The invention is fully applicable to any multi-carrier protocol structure with a centralized layer 2/3 as described herein. 
     Consistent with the multi-carrier configuration of the communications network  10 , the multi-mode physical layer  116  includes one to N carriers, each of which can be configured differently in terms of modulation and coding schemes. Each of the carriers can also be configured differently in terms of the QoS (or the set of QoS) the carrier provides to the wireline upper level protocol structures  103 . The physical resource is divided into two domains: frequency domain in terms of carrier(s); and time domain in terms of time slots(s). 
     The next generation wireless systems, also referred to as Beyond Third Generation (B3G) wireless systems, include existing 2G/3G wireless access systems and emerging B3G wireless access systems, including, for example, fixed and mobile networks; pico-, micro- and macro-cellular networks; relay, mesh and ad hoc networks; broadcast/multicast and unicast networks. To support the wide variety of heterogeneous wireless access systems, the protocol stack  102  supports dynamic radio resources, and load and spectrum management across the various access systems via the multi-mode physical layer  116 , to maximize the spectrum efficiency while meeting the Quality of Service (QoS) requirements of the user&#39;s applications and the user&#39;s Service Level Agreement (SLA). The spectrum allocation scenarios are discussed in detail with reference to  FIG. 3 . 
       FIG. 3  illustrates the spectrum allocations scenarios with respect to a multimode physical layer  116  resource sharing according to the time domain, frequency domain, and CDMA code domain. With the given variety of existing 2G/3G wireless access systems and emerging B3G systems, the different physical layer modes  118  through  128  may overlap in frequency. 
     For data communications, spectrum sharing the various physical layer modes may be performed in Time Division Multiplex (TDM) fashion, or Code Division Multiplex (CDM) fashion. The TDM approach may be used when different physical layer modes overlap in spectrum, such as when the same spectrum is chosen for emerging B3G access systems so as to enable support for both existing 2G/3G mobile terminals and new advanced mobile terminals during the transitional period when the deployment migrates from one physical layer mode to another. Spectrum sharing may also occur to support different service types that require different physical layer modes (for example, broadcast services and unicast services that may have different requirements on physical layer performance (such as coverage, cyclic prefix length in OFDMA case)), or fixed services and mobile services that are optimized under the different physical layer configurations available from the multi-mode physical layer  116 . Accordingly, CDM may be applied to Code Division Multiple Access (CDMA) based physical layer modes overlapping in spectrum. Frequency Division Multiplexing (FDM) may be used when different physical layer modes are assigned to non-overlapping frequency spectrums. 
     Accordingly, the 1xEV-DV carrier  132  and the MC-DV carrier  134  are overlaid in CDM fashion on the same spectrum. The 1xEV-DV carrier  132  and OFDMA (Orthogonal Frequency Division Multiple Access) with different configurations are overlaid in FDM fashion. The OFDMA mode with different configurations (for example, OFDMA for macro-cellular fixed/nomadic carrier  138  and OFDMA for mobile macro-cell carrier  140 ) are also be overlaid in TDM fashion. 
     A common signaling channel (physical and logical) may be shared by all the physical layer modes overlaid in the TDM fashion to allow mobile terminals to determine the physical layer mode used in a particular time slot. 
       FIG. 4  is a block diagram illustrating a common layer 2/3 protocol structure  104  anchoring a multi-mode physical layer  116  to provide, among other functions, mobility management, radio resource management, load control and/or spectrum management across the heterogeneous physical layer modes  118  through  128 . 
     The control plane  180  of the common layer 2/3 protocol structure  104  includes a mobility management module  162 , a MAC state control module  164 , and a radio resource and load management module  166 . 
     The data plane  182  of the common layer 2/3 protocol structure  104  includes a layer 3 signaling module  168  and a layer 2 module  170 . The layer 2 module  170  includes a SARQ (Selective Automatic Repeat reQuest)  172 , an ARQ (Automatic Repeat reQuest)  174 , and a multiplex/demultiplex sublayer module  176  that also includes a HARQ (Hybrid ARQ) control module  178 . The data plane  182  provides dynamic multiplexing and de-multiplexing of layer 2 frames from one or more mobile terminals to/from physical layer frames. 
     The multiplex/demultiplex sublayer module  176  of the data plane  182  may provide dynamic mapping of layer 2/3 data and signaling to/from the resource pools of the various physical layer modes  118  through  128 . The physical layer modes  118  through  128  include a resource pool relating to the configuration of the physical layer mode  118 . Each physical layer mode  188  is defined based on several parameters, which includes: the quality of service supported, a list of manageable resources, a carrier identification, and an air interface configuration (such as CDMA, OFDMA, MC-CDMA, et cetera). 
     The quality of service supported may include, but is not restricted to, data rate (minimum, maximum, and/or mean), packet loss rate, and service type such as real-time or delay tolerant service. The manageable resources are selected from among time slots, spreading codes, power, modulation and coding set, etc. The air-interface configuration may comprise IS-95, CDMA2000 1xRTT, CMDA2000 1xEV-DO, the evolution of CDMA2000 air interfaces, or other configuration types. One or more physical layer modes may reside on the same carrier. On the other hand, a resource pool may consist of multiple carriers. 
     Referring to the control plan  180 , the mobility management module  162  provides handoff support while the radio resource and load management module  166  manages how the multi-mode physical layer resource pools are used and load control between the resource pools. The MAC (Medium Access Control) state control module  164  includes a MAC state machine of each mobile terminal and controls how the mobile terminal transitions from one MAC state to the other. 
     The mobility management module  162  supports inter-base station handoffs, intra-base station handoff between the physical layer modes  118  through  128 , and inter-access gateway handoffs (such as between access gateway  28  and  30 ). The handoff may be initiated by a mobile terminal  40 ,  41 ,  52 , and/or  54  or by the access network  66 , which includes the access gateways  28  and  30 , and base stations  32  through  38 . 
     With respect to intra-base station handoffs, the handoffs may occur with physical layer modes having different frequency spectrums, and with physical layer modes sharing a frequency spectrum. In an intra-base station handoff between physical layer modes occupying different frequency spectrums, a handoff mechanism may be used. In an intra-base station handoff between physical layer modes sharing a frequency spectrum in either CDM fashion or TDM fashion, an explicit handoff mechanism may not be required if the mobile terminal has the capability to dynamically switch between the physical layer modes on a frame-by-frame basis based on the common signaling information from the base station. If the mobile terminal cannot dynamically switch between different physical layer modes, an explicit handoff mechanism may be used to ‘handoff’ the mobile terminal from one physical layer mode to another. 
     With handoffs, the respective layer 2/3 contexts relating to a mobile terminal are also transferred. For intra-BS handoff between physical layer modes  118  through  128 , a layer 2/3 context transfer is not necessary when a centralized layer 2/3 protocol structure  106  resides in an access gateway. For inter-base station handoff, the context for the distributed layer 2/3 protocol structure  108  is transferred from the serving base station to the target base station. For inter-access gateway handoff, both the contexts for the distributed layer 2/3 protocol structure  108  and the centralized layer 2/3 protocol structure  106  are transferred from the serving base station/access gateway to the target base station/access gateway, accordingly. 
     The radio resource and load management module  166  performs resource allocations including: semi-static and/or dynamic resource allocation within a physical layer mode, semi-static and/or dynamic resource allocation across physical layer modes sharing the same spectrum, or semi-static and/or dynamic resource allocation and load balancing across physical layer modes belonging to different spectrum and different cell hierarchies. Under semi-static resource allocation, resources allocated to each physical layer mode changes slowly, on the order of hours based on large scale loading condition of the physical layer modes. Under dynamic resource allocation, resources allocated to each physical layer mode can change as frequent as a per-frame basis. For dynamic resource allocation, the use of a multi-mode mobile terminal is not required. 
     Under semi-static and/or dynamic resource allocation across physical layer modes sharing the same spectrum, the same spectrum may be shared by multiple physical layer modes in either CDM fashion or TDM fashion. The resource partition (that is, the code space partition for the CDM case, the time slot partition for the TDM case) between physical layer modes may be configured semi-statically based on the loading condition on each physical layer modes. The resource partition may also be dynamic on a frame-by-frame basis based on the real-time QoS requirements and buffer condition of the mobile terminal traffic in each of the physical layer modes  118  through  128 . The frame-by-frame multiplexing and demultiplexing of the mobile terminal traffic to different physical layer modes may be performed at the multiplex/demultiplex sublayer module  176  based on the scheduling policy defined by the radio resource management and load management module  166 . 
     Under semi-static and/or dynamic resource allocation and load balancing across the physical layer modes  118  through  128 , each of the physical layer modes may belong to different spectrum and different cell hierarchies. Accordingly, the radio resource and load management module  166  serves to maximize the overall spectral efficiency across the physical layer modes according to a scheduling priority criteria based upon the QoS requirements of the mobile terminal&#39;s data traffic and/or mobility condition (that is, whether the mobile terminal is a user is a fixed, nomadic or mobile). In addition to load balancing, the radio resource management function makes handoff decisions based on the mobile terminal&#39;s location and channel condition with respect to the hierarchical cell layout  42  of the different physical layer modes  118  through  128 . Load balancing is discussed in further detail with respect to  FIGS. 5 and 6 . 
     The MAC state control module  164  provides power saving capabilities with respect to each of the physical layer modes  118  through  128  and is also accessed during mobile terminal handoffs to from one physical layer mode to another so as to place the physical layer mode into a MAC appropriate state (that is, a state recognized by the characteristics of that physical layer mode. 
     For power saving purposes, the MAC state control module  164  provides different operational states, including an active state, a suspended state, a dormant state, and a power-saving state. Each of the mobile terminals coupled to the base station may be in different operational states, or accommodate some, but not all, of the states. For example, a 1xRTT or 1xEV-DV systems may operate on the active state, suspended state and dormant state. A 1xEV-DO system may operate on the idle state and the active state. As another example, an IEEE 802.16e system may operate in a suspended mode, dormant mode, and an active mode (such as in IEEE 802.16e systems). 
     Accordingly, a mobile terminal with multi-mode access may include an existing 3G 1xEV-DO physical layer mode, MC-DO (that is, Multi-Carrier DO, a short-term evolution of 1xEV-DO analogous to the MC-DV concept described in) physical layer mode, OFDMA overlay physical layer mode and OFDMA standalone physical layer mode. 
     On the forward link, the 1xEV-DO, MC-DO and OFDMA overlay modes may share the same frequency spectrum in TDM fashion. On the reverse link, the 1xEV-DO and MC-DO modes may share the same frequency spectrum in CDM fashion, while the OFDMA overlay mode may occupy a separate frequency spectrum to avoid any mutual interference between a CDMA waveform and an OFDMA waveform. Alternatively, the OFDMA overlay mode may share the same spectrum as a 1xEV-DO/MC-DO at the expense of degraded overall system performance due to mutual interference. The OFDMA standalone mode may occupy separate frequency spectrums on the forward link and the reverse link. These physical layer modes may share a common layer 2/3 protocol structure  104  based on the existing 1xEV-DO layer 2/3 protocol structure. 
     Continuing with the example, the overlay of 1xEV-DO, MC-DO and OFDMA overlay modes onto the same spectrum supports a mixed of legacy 1xEV-DO terminals, MC-DO terminals, and OFDMA terminals while the deployment migrates towards broadband and OFDMA for a high-speed data service offering. The TDM pilot channel and MAC channel in the 1xEV-DO system may be shared by all three of the physical layer modes as common pilot and signaling channels for system acquisition and to carry information regarding the physical layer mode in use for a particular time slot, as well as the mobile terminal(s) scheduled on a particular time slot. 
     The physical layer mode and the mobile terminal(s) scheduled on each time slot can be dynamically changed based using fast TDM scheduling across the three physical layer modes. The fast TDM scheduling at the multiplex/demultiplex sub-layer module  176  dynamically maps the mobile terminals&#39; data communications to different physical layer modes per scheduling priority criteria based on the QoS requirements of the data communications, the mobile terminals&#39; SLA, the mobile terminals&#39; channel conditions, and the physical layer modes supported by the user terminals. 
     As noted, a terminal may support one or multiple physical layer modes. To provide universal system access, a set of primary common channels (that is, paging, access and synchronization channels) are used by each of the physical layer modes  118  through  128  to allow mobile terminals supporting that support multiple physical layer modes to perform initial access to the system. 
     Each of the physical layer modes  118  through  128  designated as anchor carrier(s) within the spectrum may be defined to carry the primary common channels. In addition to the primary common channels used for initial system access, each physical layer mode may also use an additional set of supplemental common channels used by terminals that have already acquired and operate in the physical layer mode. The supplemental common channels may be used for MAC state transition from the a dormant MAC state or other intermediate states to the active MAC state. Unlike the primary common channels, the supplemental common channels may not reside in the anchor carrier. Dividing the common channels into primary set and a supplemental set allows dynamic resource provisioning of the supplemental set based on loading of each physical layer mode. The MAC state control module is discussed in further detail with respect to  FIGS. 7 and 8 . 
       FIGS. 5 and 6  are a logic diagram  200  illustrating dynamic traffic load balancing with a plurality of heterogeneous physical layer modes of an air interface protocol architecture. The traffic load balancing serves to dynamically assign TDM and FDM resources to the heterogeneous physical layer modes on a time-slot basis for multiplexed communication of data communications to the multi-mode physical layer  116 . In operation, load balancing and fast scheduling may be simultaneously performed across the heterogeneous physical layer modes, allowing per time-slot and per packet opportunistic scheduling and load balancing to maximize the system capacity while meeting scheduling priority criteria (such as QoS, SLA, et cetera). 
     For each time slot of the plurality of time slots, beginning at step  202 , the base station or device deploying the protocol stack  102 , assigns priorities to each of a plurality of mobile terminals based upon a scheduling priority criteria, where each of the plurality of mobile terminals include at least one physical layer mode. The criteria may be based on various elements, such as QoS, SLA, et cetera. 
     For example, when a TDM scenario is expected, such as when sharing the frequency spectrum between the physical layer modes, the scheduling priority criteria may include the instantaneous data rate supported by the mobile terminal at the time slot, the physical layer mode based on an instantaneous channel condition being experienced by the mobile terminal in the physical layer mode, the average data rate experienced by the mobile terminal up to the present time slot, the queuing delay experienced at the time slot (applicable to delay sensitive service), and the minimum guaranteed data rate for the mobile terminal based on the SLA. 
     The scheduler for the multiplex/demultiplex sublayer module  176  may compute the priority of all the mobile terminals&#39; data packets in a buffer of the layer 2 module  170  for the corresponding physical layer modes supported by the mobile terminals. The set of physical layer modes supported by a user during an active call session may be configured during call setup based on the user terminal capability and the users requested service types and mobility conditions. Also, purposes of the scheduling priority criteria, which may be established under a priority equation or algorithm, a proportional fairness scheduler or a fairness plus delay scheduler may be used to optimize the overall system capacity across the physical layer modes for each of the mobile terminals&#39; QoS and SLA requirements. 
     With the priorities assigned, the mobile terminals are sorted in order of the assigned priority for data communication at step  204 . When the physical layer mode multiplexing scenario is a Time Division Multiplexing (TDM) scenario at step  206 , then the base station selects the physical layer mode based upon the mobile terminal with the highest assigned priority for data communication and the physical layer mode that the highest assigned priority mobile terminal supports at step  208 . 
     At step  210 , when the time slot can accommodate an additional mobile terminal, at step  212  the base station selects a next highest assigned priority mobile terminal for data communication that supports the selected physical layer mode. The base station continues this process until the time slot can not accommodate additional mobile terminals or no additional mobile terminals remain that support the selected physical layer mode. 
     At step  206 , when the multiplexing scenario is not a TDM scenario, the multiplexing scenario is a Frequency Division Multiplexing (FDM) scenario. In this instance, different physical layer modes occupy different non-overlapping frequency bands. Load balancing between the physical layer modes can be performed either dynamically or semi-statically. 
     With respect to dynamic load balancing under the FDM scenario, each physical layer mode may be dynamically changed each for each data packet or frame. In this manner, wideband receivers capable of receiving the radio frequency signal from the entire spectrum across the physical layer modes of interest can be used by the base station to service compatible multi-mode mobile terminals. 
     The dynamic spectrum load balancing between different OFDMA modes may be used for fixed users, medium speed users and high speed users, and for unicast and multicast services. The spectrum or OFDM sub-carriers in this case may be dynamically assigned to different OFDMA modes from one time slot to another. To facilitate efficient multiplexing of fixed and mobile users, a fixed Fast Fourier Transform size may be used, thus the same basic sub-carrier spacing for all OFDMA physical layer modes may be used (for example, physical layer mode  122  and physical layer mode  124  of  FIG. 4 ). The basic sub-carrier spacing may be based on the requirement to support fixed users. For mobile users, null sub-carriers are introduced to increase the effective sub-carrier spacing to integer number of the basic sub-carrier spacing. Unicast and multicast services are TDM as well because unicast and multicast transmissions may require different cyclic prefix length, thus different OFDM symbol durations. The specific time slots used for multicast transmission may be broadcast through the common signal channels to all mobile terminals in the communications system  10 . 
     Referring to  FIG. 6 , the base station selects, at step  214 , the physical layer mode based upon the next highest assigned priority mobile terminal for data communication and whether the next highest assigned priority mobile terminal is a single physical-layer mode mobile terminal or a multi physical-layer mode mobile terminal. When at step  216 , the mobile terminal is a single mode terminal, the physical layer mode selected is the physical layer mode supported by the mobile terminal. When at step  216 , the mobile terminal is not a single mode terminal but a multi-mode terminal, a determination is made at step  218  as to whether the mobile terminal is able to communicate with the physical layer modes of interest. When not, the mobile station then selects the physical layer mode to be the least loaded physical layer mode at step  222 . 
     When the mobile terminal is able to communicate with the physical layer modes of interest at step  218 , then the base station selects the physical layer mode occupying the least amount of spectrum resources at step  224 . At step  226 , when the time slot can accommodate an additional mobile terminal, the base station at step  228  selects the next highest assigned priority terminal for data communication until the physical layer resources cannot accommodate an additional mobile terminal or no additional mobile terminals remain. 
     The method  200  returns to step  202  (see  FIG. 5 ) when either the time slot cannot accommodate an additional mobile terminal at step  226 , or the condition of no further physical layer resources are available or no additional mobile terminals remain in step  228 . 
     An advantage of dynamic FDM load-balancing with respect to dynamic TDM load balancing is that the resource allocation to each physical layer mode is more flexible because there is no restriction to schedule mobile terminals with the same physical layer mode within each time slot. Frequency selective scheduling of the physical layer modes can be allocated for a particular sub-band based upon physical layer mode compatibility. For the case of frequency non-selective scheduling, compatibility other than the bandwidth granularity of the frequency division multiplexing (for example, 1.25 MHz for the case of FDM between 1xEV-DO/MC-DO and OFDMA) need not be considered. 
       FIG. 7  is an illustration of a MAC state diagram  240 . The MAC states are used for purposes of power saving and improved resource utilization to support packet data services to mobile terminals. Because of the varying physical layer modes (such as those supporting nomadic services, fixed services, mobile services), different layer modes may have different MAC state requirements (such as those services requiring power saving states versus those that do not). The MAC state diagram includes an active state  242 , a dormant state  246 , and intermediate states that are the power-saving state  248  and the suspended state  244 . 
     The active state  242  is typically the state where the mobile terminal is readily transmitting and receiving traffic in the forward link and the reverse link, respectively, when instructed by a base station. The mobile terminal is synchronized to the base station in the physical layer in terms of time and frequency, and in layers 2 and 3 in terms of protocol states. The mobile terminal monitors the control signaling from the base station on each time slot. In addition, the mobile terminal and the base station maintain a good estimate of the forward link channel quality of the mobile terminal, while the mobile terminal maintains a good estimate of the required transmit power in the reverse link to achieve the target signal-to-noise ratio (SNR). 
     A mobile terminal in the active state  242  typically consumes the most power, radio resource and network resource. Accordingly, maintaining a mobile terminal in an active state  242  at all times is not efficient in terms power consumption, radio resource utilization and network resource utilization when supporting packet data applications because packet data traffic is “bursty” in nature. For a large amount of time, the base station may not have data to transmit to the mobile terminal nor the mobile terminal to the base station. As an example, a typical reading time of a web-page is 30 seconds. During this buffer-empty period, the mobile terminal can be put into a state where no transmission and reception activities are required. 
     The dormant state  246  is the state where the mobile terminal is typically unavailable for forward link data reception and reverse link data transmission. The base station has no knowledge of the forward link channel condition of the mobile terminal, and the mobile terminal does not maintain a good estimate of the required reverse link transmit power and frequency/time transmission offset (for the case of OFDMA systems). The layers 2 and 3 protocol states of the protocol stack  102  may not be retained or synchronized between base station and the mobile terminal. The mobile terminal receiver may turn on periodically (in order of hundreds of milliseconds or seconds) to monitor the paging information from the base station. For the mobile terminal to receive data on the forward link or transmit data on the reverse link, a base station initiated (that is, paging) or mobile terminal initiated call setup procedure typically needs to be performed to transition the mobile terminal from the dormant state  246  to the active state  242 . 
     The intermediate states, that is, the power-saving state  248  and suspended state  244  provide different levels of tradeoff between power/resource consumption and QoS. The power-saving state  248  (also known as a Control-Hold state or Sleep mode) can be viewed as a pseudo-active state. The mobile terminal and the base station may maintain the same physical layer, layer 2 and layer 3 states synchronization as in the active state  242 . The difference is that a sleep interval is typically introduced in the power-saving state to allow the mobile terminal to disable its transmitter and receiver during this interval. The sleep intervals may be interlaced with listening intervals, where during listening intervals the mobile terminal operates as in the active state  242 . Although the power-saving state  248  consumes less power and radio/network resource than the active state  242 , it still consumes a relatively large amount of power and radio/network resource compared to the dormant state  246 . An advantage of the power-saving state  248  is the faster transition to the active state  242  (for example, on the order of tens of milliseconds) when there is data to be exchanged between the base station and the mobile terminal. 
     The suspended state  244  is a state that is similar to the dormant state  246  but provides faster or expedited call setup into the active state  242  (with latency, for example, on the order of hundreds of milliseconds). The difference between the suspended state  244  and the dormant state  246  is that for the suspended state  244 , some semi-static layer 2 and layer 3 protocol context information is retained in the mobile terminal and the base station. Accordingly, during call setup, the corresponding signaling handshake between the mobile terminal and the base station to establish or fill in context information that may be omitted. The layer 2/3 protocol context information that may be retained in the suspended state  244  include security context, service flow context, and mobile terminal/base station capability and preferred protocol configurations. In addition, location updates may be performed by the mobile terminal in the suspended state  244  to allow the base station to track the locations of the mobile terminals with respect to a particular cell/sector or paging zone. 
     The possible state transitions between the various MAC states are shown by the ‘arrows’ in  FIG. 7 . In general, each MAC state may not be present and/or supported in a system. A base station can decide which MAC state to transition a mobile terminal to, based on such factors as buffer status, QoS requirements, SLA, radio resource management policy, et cetera. 
     The MAC state machine  240  operates such that, when a user switches from one physical layer mode to another either within the same or different cell hierarchies, the MAC states and the associated context information of the mobile terminal may be retained. In this way, packet loss and physical layer mode switching/handoff latency can be minimized. 
     The MAC state machine  240  includes superset of MAC states relevant to the heterogeneous physical layer modes that are present in the multi-mode physical layer  116 . For a separate example, physical layer mode  118  supports fixed services. In this regard, only the active state  242  may be applicable because power saving is not an issue for fixed services. The physical layer mode  120  supports mobility services. Accordingly, all four MAC states are applicable. The physical layer mode  122  supports nomadic services. Accordingly, only the active state  242  and the dormant state  246  may be applicable because power consumption is less of an issue for nomadic devices (such as a laptop computer  41  as compared to a handheld type device  54 ). When a mobile terminal operates in a selected physical layer mode, only a subset of the MAC states and corresponding state transition may be in operation. 
     Thus, different MAC state transition scenarios when a mobile terminal switches from one physical layer mode to another, depending on the MAC state of the mobile terminal in the previous physical layer mode and the MAC states supported by the new physical layer mode. 
       FIG. 8  is a logic diagram  260  illustrating MAC state transitions upon for physical layer mode transitions. At step  262 , the set of MAC states S are dormant, suspended, power-saving, and active, where S j  is a member of the set for j equals 0, 1, 2, and 3. When the mobile terminal transitions switches from one PL mode to another, the current MAC state at the previous physical layer mode may be retained if the new physical layer mode supports the same MAC state. Otherwise, the MS may transition to the next higher MAC state supported by the new PL mode and the allowable state transition flow. 
     While in the active state  242 , a mobile terminal may continuously monitor the signal strength of the surrounding base stations and their associated physical layer modes (from step  262 ). The physical layer mode transition at step  264  may be initiated by either the base station or the mobile terminal based on the change in signal strength, mobility condition, service needs or system loading. 
     When at step  264  there is not a physical layer mode transition, the base station determines whether a MAC state transition has occurred at step  266 . If not, then the base station returns to step  262 . Otherwise, at step  268 , the base station transitions the mobile terminal to a new MAC state S k , where k equals j and set j to k. 
     When at step  264  there is a physical layer mode transition, the base station and/or access gateway determines at step  270  the set of MAC states supported by the new physical layer mode. As an example, when the mobile terminal transitions to other available physical layer modes while in the same state (for example, active state  242 ), the mobile terminal may remain in that state unless there is insufficient resource in the target physical layer mode to accommodate the mobile terminal. When there are insufficient resources, the mobile terminal may be downgraded (when in an active state) into a lower MAC state supported by the target physical layer mode. 
     With the set of MAC states determined for the physical layer mode at step  270 , the base station determines whether the state of the mobile terminal, state j, is supported by the new physical layer device at step  272 . If so, then the state for the mobile terminal is set to the present state. Otherwise, at step  274 , the base station transitions the terminal to a valid MAC state, in that the state Sj is changed such that i is the minimum value greater than j and set j=i. The adjustment to a valid MAC state accounts for the situations where the physical layer mode of a mobile terminal is transitioned to a new physical layer mode with a smaller set of valid MAC states (that is, referring briefly back to the example of  FIG. 7 , where physical layer mode  120  has four valid MAC states, and the physical layer mode  122  has two valid MAC states). With a valid MAC state, the base station returns to state  262  for a subsequent change to the MAC state and/or the physical layer mode. 
     In operation, the MAC states provides seamless transitions from a first physical layer mode to another, while avoiding unnecessary call re-establishment overhead and latency that would otherwise be incurred when a mobile terminal moves between heterogeneous systems that lack a common layer 2/3 protocol structure  104 . 
       FIG. 9  is a block diagram illustrating a base station  300  constructed according to an embodiment of the invention. The base station  300  supports a plurality of heterogeneous physical layer modes (for example, IS-95A, IS-95B, IS-2000, GSM-EDGE and/or various 3G and 4G standards specifications that are compatible with the teachings herein). The base station  300  supports protocol layer operations such as those described with reference to  FIGS. 2 through 4 . 
     The base station  300  includes a processor  302 , dynamic RAM  306 , static RAM  308 , EPROM  310 , and at least one data storage device  312 , such as a hard drive, optical drive, tape drive, et cetera. These components (which may be contained on a peripheral processing card or module) intercouple via a local bus  336  and couple to a peripheral bus  338  (which may be a back plane) via an interface  316 . These peripheral cards couple to the peripheral bus  338 . These peripheral cards include a network infrastructure interface card  320 , which couples the base station  300  to an access gateway  28  and/or  30 . 
     Digital processing cards  322 ,  324 , and  326  couple to the radio frequency (RF) units  328 ,  330 , and  332 , respectively. Each of these digital processing cards  322 ,  324 , and  326  performs digital processing for a respective sector (for example, sector one, sector two, or sector three) serviced by the base station  300 . Thus, each of the digital processing cards  322 ,  324 , and  326  will perform some or all of the processing operations described with reference to  FIGS. 5 through 8 . The RF units  328 ,  330 , and  332  couple to antennas  340 ,  342 , and  344 , respectively, and support wireless communication between the base station  300  and the mobile terminals. The base station  300  may include other cards  318  as well. 
     As noted, the common layer 2/3 protocol functionality may be partitioned between the access gateway and the base stations, but may also reside wholly in a base station. In this regard, the base stations  300  includes a multi-mode physical layer via the RF units and digital processing cards  322 ,  324 , and  326 , and a “distributed” common layer 2/3 function, and in this manner supports multiple physical layer modes in the same coverage area  56 ,  59 ,  60 , and  62  (see  FIG. 1 ). The access gateways  28  and  30  may provide a “centralized” component of the layer 2/3 function, and also serve as a router within the access network  66  and provide interface to the wireless core network infrastructure  24  (as well as public IP networks). 
     Structures and operational instructions regarding the protocol stack  314  are stored in storage  312 . The protocol stack  314  is downloaded to the processor  302  and/or the DRAM  306  as the protocol stack  304  for execution by the processor  302  while the protocol stack is shown to reside within storage  312  contained in the base station  300 , the protocol stack may be loaded onto portable media such as magnetic media, optical media, or electronic media. Further, the protocol stack  314  structure and/or operational instructions may be electronically transmitted from one computer to another across a data communication path. 
     Upon execution of the operational instructions and structures regarding the protocol stack  304 , the base station  300  performs operations according to the methods and processes described herein with reference to  FIGS. 1 through 10 . The protocol stack  304  structure and/or operational instructions may be partially executed by the digital processing cards  322 ,  324 , and  326  and/or other components of the base station  300 . Further, the structure of the base station  300  illustrated is only one of may varied base station structures that could be operated according to the descriptions contained herein. 
       FIG. 10  is a block diagram illustrating an access gateway, which may also be referred to as a Base Station Controller, constructed according to an embodiment of the present invention. The structure and operation of an access gateway is generally understood. The access gateway  360  may service both circuit switched and packet switched operations, and in some cases is called upon to convert data between circuit switched and data switched formats, depending upon the types of equipment coupled to the access gateway  360 . The components illustrated in  FIG. 10 , their function, and the interconnectivity may vary without departing from the teachings of the present invention. 
     The access gateway includes a processor  362 , dynamic RAM  366 , static RAM  368 , EPROM  370  and at least one data storage device  372 , such as a hard drive, optical drive, tape drive, et cetera. These components intercouple via a local bus  380  and couple to a peripheral bus  378  via an interface  376 . Various peripheral cards couple to the peripheral bus  378 . These peripheral cards include a wireless core network interface card  382 , a base station manager (BSM) interface card  384 , at least one selector card  386 , a mobile station controller (MSC) interface card  388 , and a plurality of base station interface cards  390 ,  392 , and  394 . 
     The wireless core network interface card couples the access gateway  360  to a wireless core network infrastructure  24 . The base station manager interface card  384  couples the access gateway  360  to a Base Station Manager  396 . The selector card  386  and MSC interface card  388  couple the access gateway  360  to the MSC/HLR/VLR  398 . The base station interface cards  390 ,  392 , and  394  couple the access gateway  360  to the base stations  32 ,  34 , and  36 , respectively. 
     Structures and operational instructions regarding the protocol stack  374  are stored in storage  372 . The protocol stack  374  is downloaded to the processor  362  and/or the DRAM  366  as the protocol stack  364  for execution by the processor  362 . While the protocol stack is shown to reside within storage  372  contained in the access gateway  360 , the protocol stack may be loaded onto portable media such as magnetic media, optical media, or electronic media. Further, the protocol stack  274  structure and/or operational instructions may be electronically transmitted from one computer to another across a data communication path. 
     The protocol stack  364  may be implemented as a centralized layer 2/3 protocol structure  106  (see  FIG. 2 ) to complement the distributed layer 2/3 protocol structure  108  within each of the base stations  32 ,  34 , and  36 . In operation, the access gateway  360  stores a centralized layer 2/3 context of each mobile terminal coupled to the access network formed by the access gateway  360  and the base stations  32 ,  34 , and  36  to eliminate a need for context transfer for intra-base station handoffs between the base stations  32 ,  34 , and  36 . 
     The embodiments of the invention disclosed herein are susceptible to various modifications and alternative forms. Specific embodiments therefore have been shown by way of example in the drawings and detailed description. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the claims.