Patent Publication Number: US-2006013182-A1

Title: Selective multicarrier CDMA network

Description:
RELATED APPLICATIONS  
      This application claims priority to Provisional U.S. Patent Application 60/588,975 filed Jul. 19, 2004, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates generally to wireless communication networks, and more particularly, to a selective multicarrier CDMA network.  
      The cdma2000 radio channels are designed to permit multicarrier operation in the same spectrum as existing 2G networks based on the IS95 standard. The cdma2000 standard defines two spreading rates—spreading rate  1  (SR 1 ) and spreading rate  2  (SR 2 ). SR 1  implies a single radio channel system with a total bandwidth of 1.25 MHz and a chip rate of 1.228 Mcps. SR 2  denotes a multicarrier system with a bandwidth of 3.75 MHz. The forward link in multicarrier cdma2000 uses three separate direct-spread carriers, each spread at a chip rate of 1.288 Mcps. The reverse link of multicarrier cdma2000 combines three cdma2000 radio channels to create a single carrier with a bandwidth of 3.75 MHz.  
      The existing multicarrier framework for cdma2000 has a number of drawbacks. First, the cdma2000 standard makes no provision for varying the number of carriers during transmission. Once a communication link between a mobile station and BS is established, the number of carriers (either 1× or 3×) supporting the connection is fixed. Second, the cdma2000 standard requires contiguous radio channels for a 3× carrier for both forward and reverse links. Moreover, only 1× is supported for the forward packet data channel (F-PDCH) in the IS-2000 Rev C and D and IS-856 standards.  
     SUMMARY OF THE INVENTION  
      A multicarrier CDMA system is provided that allows the number of carriers supporting a connection to be dynamically varied during active packet data communications. A new multicarrier sublayer is added to the cdma2000 protocol stack to divide and recombine packet data streams. In one embodiment of the invention, the packet data stream from the RLP sublayer is divided into multiple streams depending on the number of carrier frequencies supporting forward and reverse link communications. Each substream is separately coded and modulated, and transmitted over a corresponding carrier frequency. The number of carrier frequencies supporting forward and reverse link communications can be different.  
      In one embodiment of the invention, one carrier frequency may be designated as an anchor frequency with the remaining carrier frequencies serving as supplemental frequencies. Circuit-swtiched voice and data are carried only on the anchor frequency. Packet-swtiched data may be carried on both the anchor frequency and supplemental frequencies. At any given time, there is only one anchor frequency, but the anchor frequency may be changed over time.  
      The present invention does not require any modifications in the physical layer of existing standards. Therefore, it is possible to make a multicarrier system by combining carrier frequencies supporting different air interfaces. In one embodiment of the invention, the channel structure of the anchor frequency conforms to the IS2000 standard, while the channel structure of the supplemental frequencies conforms to the IS856 standard. Alternatively, the supplemental frequencies may comprise a data only carrier with a newly-defined channel structure to transport data more efficiently. The present invention, however, is not limited to use of particular standards.  
      In one embodiment of the invention, the supplemental carrier frequencies may comprise data only carriers. All related control information is carried over the anchor frequency for all carriers. Network operators may define new channel structures for the data only carriers frequencies without the burden of defining physical channels to carry control information.  
      The ability to dynamically vary the number of carriers supporting a connection provides greater flexibility in resource allocation. Further, the present invention does not require the carriers supporting a connection to be contiguous. Backward compatibility with existing 2G and 3G systems is preserved while providing a migration path for new multicarrier CDMA technology. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates protocol layers for a multicarrier CDMA network according to one exemplary embodiment of the present invention.  
       FIG. 2  illustrates the carrier frequencies in a multicarrier CDMA network according to one exemplary embodiment of the present invention.  
       FIG. 3  illustrates the carrier frequencies in a multicarrier CDMA network according to another exemplary embodiment of the present invention.  
       FIG. 4  illustrates the carrier frequencies in a multicarrier CDMA network according to another exemplary embodiment of the present invention.  
       FIG. 5  illustrates a multicarrier CDMA network according to one embodiment of the present invention.  
       FIG. 6  illustrates a transmitter for a mobile station and/or base station according to one exemplary embodiment of the present invention.  
       FIG. 7  illustrates a receiver for a mobile station and/or base station according to one exemplary embodiment of the present invention.  
       FIG. 8  illustrates a multicarrier sublayer for a multicarrier CDMA network for dividing a RLP stream into multiple substreams according to one embodiment of the present invention.  
       FIG. 9  illustrates protocol layers for a multicarrier CDMA network with mixed air interfaces according to one exemplary embodiment of the present invention.  
       FIG. 10  illustrates protocol layers for a multicarrier CDMA network with mixed air interfaces according to another exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The physical layer channelization for cdma2000 networks is designed to allow backward compatibility with 2G systems and gradual multicarrier deployment in the same spectrum. A conventional cdma2000 radio channel is 1.25 MHz. For multicarrier CDMA, multiple cdma2000 radio channels are used to support a single connection. A “1×” system uses a single cdma2000 radio channel spread at a chip rate of 1.228 Mcps occupying 1.25 MHz of bandwidth. A “3×” system uses three cdma2000 radio channels occupying 3.75 MHz of bandwidth. The forward link in multicarrier cdma2000 uses three separate direct-spread carriers, each spread at a chip rate of 1.288 Mcps. The reverse link of multicarrier cdma2000 combines three cdma2000 radio channels to create a single carrier with a bandwidth of 3.75 MHz.  
      The existing multicarrier framework for cdma2000 has a number of drawbacks. First, the cdma2000 standard makes no provision for varying the number of carriers during transmission. Once a communication link between a mobile station and BS is established, the number of carriers (either 1× and 3×) supporting the connection is fixed. Second, the cdma2000 standard requires contiguous radio channels for a 3× carrier.  
      According to the present invention, a multicarrier CDMA system is provided that allows the number of carriers supporting a connection to be dynamically varied. The present invention may be implemented in stages. In a first stage, changes are made only in layer  3  to support dynamic allocation of carrier frequencies. In a second stage, modifications may be made to the MAC protocols and/or new control channels may be added to optimize behavior. For example, load-balancing functions may be added to the MAC layer to balance the load across multiple carriers. In the third stage, new physical channels may be defined, such as a data only carrier to make data transfer more efficient.  
       FIG. 1  illustrates exemplary modifications to the cdma2000 protocol according to one embodiment of the present invention. The layering of the protocol stack generally follows the Open Systems Interconnection (OSI) reference model. The protocol layers include the physical layer (Layer  1 ), the link layer (Link  2 ), and upper layers (Layers  3 - 7 ). The link layer includes Medium Access Control (MAC) and Link Access Control (LAC) sublayers. Except as noted below, the functional entities described conform to the cdma2000 standard. While the cdma2000 protocol stack is used herein to illustrate the present invention, those skilled in the art will appreciate that the present invention may also be used in networks based on other standards.  
      The upper layers provide three basic services—upper layer signaling  50 , data services  52 , and voice services  54 . Upper layer signaling  50 , which resides in the signaling layer (Layer  3 ), comprises base station and mobile station interoperability procedures. The upper layer signaling function  50  performs call processing to set up, maintain, and release connections. The upper layer signaling function  50  is also responsible for mobility management, user identification, and security. Layer  3  also includes messaging needed to support packet data services. Voice services  52  and data services  54  are higher layer services that, as the names imply, provide circuit-switched voice services and packet-switched data services, respectively.  
      The LAC sublayer provides services to the signaling layer (Layer  3 ). The LAC sublayer includes protocols for the transmission of signaling messages across the MAC sublayer. The LAC sublayer includes a number of LAC functions  56  which assure reliable delivery of signaling data units (SDUs) to Layer  3 , generate packet data units (PDUs) to transport SDUs, and perform segmentation and reassembly of LAC PDU, authentication, and address control. These functions are not material to the present invention and therefore are not described herein.  
      The MAC sublayer provides reliable transport of user data over the radio link. The MAC sub layer allows multiple data service state machines, one for each packet or circuit-switched data application used in an active session. The reliable transmission of user data is made possible by the radio link protocol (RLP) sublayer  60 , which implements a specialized form of selective repeat automatic repeat request (ARQ) protocol. The Signaling Radio Burst Protocol (SRBP)  58  provides best effort delivery of Layer  3  signaling information. The MAC sublayer further includes a multiplexing and QoS sublayer  62  that performs multiplexing of logical data and signaling channels onto physical channels and provides quality of service (QoS) management for each active service. The MAC sublayer also includes a packet data control function  64  that performs, among other things, rate control for forward and reverse packet data channels.  
      The physical layer defines the air interface and contains the communication channels by which a base station and mobile station communicate. The physical layer performs coding, modulation, and spreading of signals for transmission, and decoding, demodulation, and dispreading of received signals. The cdma2000 standard specifies both dedicated channels, e.g. fundamental channels and supplemental channels, for voice and low to moderate speed packet data, and shared packet data channels for high speed packet data communications. An advantage of the present invention is that no changes in the physical layer are required because the division of the RLP stream occurs in higher layers of the protocol stack.  
      As can be seen in  FIG. 1 , a new multiplexing sublayer, referred to herein as the multicarrier (MC) sublayer  70  is interposed between the RLP sublayer  60  and multiplexing and QoS sublayer  62 . Alternatively, the RLP sublayer  60  could be modified to incorporate the function of the MC sublayer  70 . The MC sublayer  70  divides a single RLP stream into multiple substreams for forward link traffic and reassembles multiple substreams into a single RLP stream for reverse link traffic. As will be described in greater detail below, each substream is transmitted over a different carrier. The MC sublayer  70  incorporates load balancing and flow control mechanisms to distribute RLP packets across multiple carriers as will be hereinafter described. The MAC functions below the MC sublayer  70 , including the multiplexing and QoS sublayer  62  and packet data control function  64 , are duplicated for each carrier frequency. The multiplexing and QoS sublayer  62  for each carrier frequency is independent of the other carrier frequencies so the information carried by the different carrier frequencies will typically be different. A separate physical layer is also defined for each carrier frequency. Each RLP substream is separately coded and modulated. The separate physical layers ensure backward compatibility with existing 2G systems.  
      The physical layer channel structure for different carrier frequencies may be the same or may be different. In one embodiment of the invention, one of the carrier frequencies is designated as an anchor frequency. The anchor frequency contains legacy circuit-switched channels to maintain backward compatibility with existing 2G networks. The anchor frequency may also include channels designed to carry packet-switched data. The remaining carrier frequencies, referred to herein as supplemental carrier frequencies, include channel structures that are designed to carry packet-switched data.  
       FIG. 2  illustrates an exemplary multicarrier system with three carrier frequencies, although those skilled in the art will appreciate that the multicarrier CDMA system according to the present invention may have more carrier frequencies. The anchor frequency includes channel structures conforming to existing cdma2000 standards. The anchor frequency includes a full array of circuit-switched and packet-switched channels as specified in Revisions C and D of the cdma2000 standard. The anchor frequency includes forward and reverse fundamental channels (F/R-FCH), supplemental channels (F/R-SCH), and common control channels to preserve backward compatibility with 2G networks. The anchor frequency further includes forward and reverse packet data channels (F/R-PDCH) and associated control channels to provide high speed packet data services as specified in Revisions C and D of the cdma2000 standard. The supplemental carrier frequencies include F/R-PDCHs and associated control channels conforming to the cdma2000 standard without any dedicated channels. Revisions C and D of the cdma2000 standard are generally known as 1×EV-DV. Therefore, the F/R-PDCHs and associated control channels as specified in Revisions C and D of the cdma2000 standard are referred to herein as DV channels.  
      Using the channel structures shown in  FIG. 2 , circuit-switched voice and data is carried only over the anchor frequency and the supplemental carrier frequencies are used only for packet-switched data traffic. The anchor frequency also carries packet-switched data traffic over DV channels. Those skilled in the art will appreciate, however, that each carrier frequency could include the full array of cdma2000 channels the same as the anchor frequency. When two or more carrier frequencies include circuit switched channels, the circuit switched channels on each separate carrier frequency should be independent. That is, there is no multiplexing of circuit switched channels across multiple carriers. This limitation is necessary to maintain backward compatibility with existing cdma2000 standards.  
      The number of possible carrier frequencies for forward and reverse links may be different. In general, more data is transmitted on the forward link than on the reverse link. Therefore, the pool of available carrier frequencies for the forward link may be larger than the pool of available carrier frequencies for the reverse link. The same carrier frequencies may be used for both forward and reverse links. However, some carrier frequencies may be used for only forward link or reverse link communications. The carrier frequencies for reverse link communications may be a subset of the carrier frequencies for forward link communications. The selective multicarrier transmission scheme of the present invention employs an independent RF chain for each carrier frequency for both transmit and receive paths.  
      The present invention makes it possible to use different air interfaces for different carrier frequencies as shown in  FIG. 3 . As shown in  FIG. 3 , the anchor frequency conforms to the cdma2000 standard with a full array of circuit-switched and packet-switched channels as specified in Revisions C and D of the cdma2000 standard to maintain backward compatibility. The supplemental frequencies include channel structures based on the IS856 standard, which is generally known as 1×EV-DO (1st Evolution Data Only). The IS856 standard specifies a high data rate (HDR) forward and reverse packet data channels for packet-data may be overlaid on an existing circuit-switched or packet-switched network. A 1×EV-DO network does not include support for legacy circuit-switched channels. The F/R-PDCHs and associated control channels as specified in the IS856 standard are referred to herein as DO channels.  
       FIG. 4  illustrates a multicarrier system that combines a 1×EV-DV carrier with one or more user data carriers. In this embodiment, the 1×EV-DV carrier serves as the anchor frequency and maintains a backward compatibility with existing 1× systems. Alternatively, the anchor frequency may include a channel structure as specified in 1×EV-DO. All of the control channels are located on the anchor frequency. The supplemental frequencies may use a newly-defined channel structure to efficiently carry user data without having the burden of defining physical channels to carry control signaling. The supplemental frequencies can be dynamically added/removed from a connection. The supplemental frequencies are designed to carry only user data with all the related control information being carried over newly defined control channels (UDC control channel) on the anchor frequency. The supplemental frequencies are used to augment the user data traffic carried over the anchor frequency.  
       FIG. 5  illustrates a wireless communication network  10  according to one embodiment of the present invention.  FIG. 5  illustrates a wireless communication network  10  configured according to the cdma2000 (IS2000) standards. Wireless communication network  10  comprises a packet-switched core network  20  and a radio access network (RAN)  30 . The core network  20  includes a Packet Data Serving Node (PDSN)  22  that connects to an external packet data network (PDN)  16 , such as the Internet, and supports PPP connections to and from the mobile station  12 . Core network  20  adds and removes IP streams to and from the RAN  30  and routes packets between the external packet data network  16  and the RAN  30 .  
      RAN  30  connects to the core network  20  and gives mobile stations  12  access to the core network  20 . RAN  30  includes a Packet Control Function (PCF)  32  and one or more Base Stations (BSs)  34 . The primary function of the PCF  32  is to establish, maintain, and terminate connections to the PDSN  22 . The BSs  34  provide the mobile stations  12  with radio access to the network  10 . The BSs  34  include a Base Station Controller (BSC)  36  and one or more Base Transceiver Stations (BTS)  38 . The BSC  36  is the control part of the base station  34 . The BSC  36  manages the radio resources within their respective coverage areas. BTSs  38  are the part of the base station  34  that includes the radio equipment for communicating over the air interface with mobile stations  12  and is normally associated with a cell site. While  FIG. 5  illustrates a separate BSC  36  for each BS  34 , those skilled in the art will appreciate that a single BSC  36  may comprise the control part of multiple base stations  34 . In other network architectures based on other standards, the network components comprising the base station  34  may be different but the overall functionality will be the same or similar. In one exemplary embodiment of the invention, the RLP and MC  60 ,  70  sublayers are implemented at the BSC  36 , while the multiplexing and QoS sublayer  62  and packet data control functions  64  are implemented at the BTS  38 .  
       FIG. 6  illustrates a transmitter  100  according to one embodiment of the present invention for transmitting signals from a first station to a second station. The transmitter  100  may be incorporated into a mobile station  12  for transmitting signals on an uplink channel to a base station  34 , or may be incorporated in a base station  34  for transmitting signals on a downlink channel to a mobile station  12 . Transmitter  100  comprises a control and interface circuit  102  that divides and reassembles RLP streams as previously described. Each substream is applied to a corresponding transmit chain  104 . Each transmit chain  104  includes an encoder  106 , spreader  108 , modulator  110 , and transmit antenna  112 . The encoder  106  encodes the substream using error detection and/or correction codes. The coded substream output from the encoder  106  is spread in spreader  108  using a spreading code assigned to the mobile station  12  to generate a wideband signal. The same spreading code may be used to spread the signal in each transmit chain  104 . Alternatively, different transmit chains  104  may use different spreading codes. After spreading, the modulator  110  modulates the wideband signal output by the spreader  108  onto an RF carrier for transmission to a remote station. Each transmit chain  104  is assigned a different carrier frequency. There is no need for the carrier frequencies to be contiguous, though some embodiments may benefit from having contiguous carrier frequencies.  
      The aggregate transmit data rate for the transmitter  100  is equal to the sum of the data rates for each transmit chain  104 . The data rate for each transmit chain  104  may be independently controlled. This is useful when one carrier frequency is congested. In such circumstances, the control and interface circuit  102  may increase the data rate on the least congested carrier frequencies and decrease the data rate on the most congested carrier frequencies.  
       FIG. 7  illustrates a receiver  200  according to one embodiment of the present invention for receiving signals transmitted by the transmitter  100  shown in  FIG. 6 . The receiver  200  comprises a plurality of receive antennas  202 , each of which is coupled to a receive chain  204 . Each receive chain  204  comprises a receiver front end  206 , despreader  208 , and decoder  210 . The front end  206  filters, amplifies, and downconverts the received signal to the baseband frequency. Despreader  208  despreads the received baseband signal and outputs received signal samples to the decoder  210 . Decoder  210  decodes the received signal samples to produce an output bitstream. In the absence of bit errors, the output bitstream from the decoder  210  will be the same as a substream input to a corresponding transmit chain  104  at the transmitter  100 . The output bitstreams from each receive chain  204  are recombined by the control interface circuits  212  at the receiving station to produce the original input bitstream.  
      In operation, the BSC  36  selects the number of carrier frequencies for a communication link between a mobile station  12  and a base station  34 . The selective multicarrier transmission scheme may be used on a forward link, a reverse link, or both. Different carrier frequencies may be used for the forward and reverse links. Further, the number of carriers assigned to forward and reverse links may be different. The BSC  36  may use upper layer (Layer  3 ) signaling to notify the mobile station  12  of the number and identities of the selected carrier frequencies. Examples of messages that can carry this information include the Universal Handoff Direction Message (UHDM), Service Connect message, and In-Traffic System Parameters Message (ITSPM).  
      Alternatively, the BSC  36  can send signaling messages to the mobile station  12  over the forward packet data control channel (F-PDCCH) and/or over the Reverse Grant Channel (R-GCH) for the forward and reverse links respectively. In one embodiment of the invention, a new frequency assignment message is used to indicate forward link frequency assignments for the F-PDCH. The frequency assignment message includes the assigned MAC_ID of the user and an 8-bit frequency assignment bitmap identifying the carrier frequencies to use for subsequent F-PDCH transmissions. An 8-bit bitmap allows up to 8 supplemental frequencies to be activated for forward link communications. If more frequencies need to be assigned, then Layer  3  signaling can be used.  
      On the reverse link, the grant channel can be used to indicate the available frequencies to use. The frequency assignment message for the reverse link includes the MAC_ID of the user and a frequency assignment bitmap indicating the assigned carrier frequencies. The frequency assignment bitmap may comprise 2 bits to indicated the following: 
          ‘00’: deactivate supplemental frequencies     ‘01’: activate Frequency Group 1 (deactivate Frequency Groups 2 &amp; 3)     ‘10’: activate Frequency Group 2 (deactivate Frequency Groups 1 &amp; 3)     ‘11’: activate Frequency Group 3 (deactivate Frequency Groups 1 &amp; 2) 
 
 A frequency group is a group of frequencies that are available for the reverse link. The frequency groups are not necessarily disjoint and may include one or more of the same carrier frequencies. The mobile station  12  is granted the frequencies to use, but is allowed to determine the carrier frequencies that it wants to use. The mobile station  12  indicates the carrier frequencies on the R-PDCCH. 
       

      To determine the number of carrier frequencies to assign to a connection between a mobile station  12  and a base station  34 , the BSC  36  can take into account factors such as the RLP queue level for each carrier frequency, QoS requirements, the average effective data transmission rate on each carrier frequency, a desired data transmission rate, and the loading on each carrier frequency. When the number of carriers assigned to a communication link (either forward or reverse link) is insufficient to support a desired data rate or when the queue level is high, the BSC  36  can add additional carrier frequencies to the communication link. Conversely, when the bandwidth of the assigned carrier frequencies is not being fully utilized or the queue level is low, the BSC  36  can drop a carrier frequency from a communication link. The BSC  36  could also substitute one carrier frequency with another while maintaining the number of carrier frequencies the same. The substitution of one carrier frequency for another may be useful when one carrier frequency is congested.  
      In one embodiment of the invention, the BSC  36  may add a carrier frequency to the forward link when the buffer level exceeds a predetermined high level for a predetermined period of time, or may drop a carrier frequency if the buffer levels drop below a predetermined low level for a predetermined period of time. For the reverse link, the mobile station can request that a new carrier frequency be added or dropped based on its buffer levels, or it can report its buffer levels to the BSC  36 . Alternatively, the BSC  36  may predict expected traffic levels on the reverse link based on the amount of traffic experienced. The buffer level values used for making add/drop decisions may comprise filtered buffered values based on some time constant and not instantaneous values.  
      Rate control for forward and reverse packet data channels is handled at the BTS  38  by the packet data control function  64 . As shown in  FIG. 1 , the packet data control function  64  is duplicated for each carrier frequency. The forward and reverse packet data channel on each carrier may be as specified in Revision D of the cdma2000 standard, commonly known as 1×EV-DV. In this case, each carrier frequency may include all the channels supporting forward and/or reverse packet data channels as specified by the cdma2000 standard. The data transmission rate for each carrier frequency is independently controlled.  
      For the forward F-PDCH, the Forward Packet Data Control Channel (F-PDCCH) indicates separately for each carrier frequency whether the mobile station  12  is scheduled to receive data. The mobile station  12  sends Channel Quality Indicator (CQI) reports to the base station  34  for each carrier frequency over the Reverse Channel Quality Indication Channel (R-CQICH). Based on the CQI reports from the mobile station  12 , the packet data control function  64  schedules the mobile station  12  and determines the data transmission rate. Data packets transmitted by the base station  34  to the mobile station  12  on each carrier frequency are acknowledged by the mobile station  12  on a Reverse Acknowledgement Channel (R-ACKCH).  
      For the R-PDCH, the number of frequencies used for the reverse link connection depends of the amount of data to be sent by the mobile station  12  and the congestion experienced by the base station  34 . The mobile station  12  reports the power headroom and buffer levels to the base station  34  the same as in a “1×” system and the base station  34  schedules the data transmission rate separately for each carrier frequency assigned to the reverse link connection. Rate scheduling is performed by the packet data control function  64  at the BTS  38 . The mobile station  12  reports power headroom separately for each carrier frequency, and sends a common buffer level report for all carrier frequencies. The buffer level may be reported only on the anchor frequency, or on all carrier frequencies. Rate assignments and rate grant decisions are made on a carrier frequency basis. Thus, the data transmission rate of the mobile station  12  may differ on each carrier frequency. Further, the mobile station  12  may be denied permission to transmit on one carrier frequency due to congestion at the base station  34  even though the carrier frequency has been assigned by the BSC  36 .  
      Load balancing is performed by the MC sublayer  70 , which as noted above, may be located at the BSC  36 .  FIG. 8  illustrates the MC sublayer  70  in more detail. The MC sublayer  70  includes load balancing function  72  to distribute RLP packets across multiple carrier frequencies. A separate transmit queue  74  is maintained for each carrier frequency supporting a connection. The buffer size for each carrier frequency is variable depending on the data rate that the mobile station  12  is achieving for each carrier frequency. A flow control function  76  monitors the average data rate on each carrier frequency and adjusts the buffer sizes for each carrier frequency accordingly. As the average data rate increases, the buffer size is increased and as the average data rate decreases, the buffer size decreases. The flow control function  76  indicates the buffer size for each carrier frequency to the load balancing function  72 , which apportions the incoming RLP packets across the carrier frequencies in a proportional round-robin fashion. That is, the load balancing function  72  apportions packets according to the average data rate on each carrier frequency in a sector. When there is a difference in the average data rate for each carrier frequency, the load balancing function  72  gives the better performing carrier frequencies proportionally more RLP packets. If the average data rate is equal, the load balancing function  72  allocates each carrier frequency an equal number of packets.  
      As the mobile station  12  moves out of the coverage area provided by one of its assigned carrier frequencies, the load balancing function  72  may reassign RLP packets queued for that carrier frequency to another carrier frequency. As the mobile station  12  moves out of the coverage area of a particular carrier frequency, the average data rate will tend towards zero thereby reducing the number of RLP packets that need to be reassigned. If the BS  34  can anticipate when the mobile station  12  is moving out of the coverage area for a particular carrier frequency, it can gradually reduce the window size for that carrier frequency to zero before the mobile station  12  completely loses coverage on that carrier frequency so that no RLP packets need to be reassigned.  
      The load balancing function  72  may also redistribute RLP packets if the packet latency for one of the carrier frequencies increases above some predetermined threshold. The flow control function  76  monitors the packet latency for each carrier frequency and can signal the load balancing function  72  if the packet latency for one carrier frequency exceeds the packet latency for the others by more than a predetermined amount. The load balancing function  72  can redistribute RLP packets from the lagging carrier frequency to other carrier frequencies.  
      The present invention is particularly useful for transmitting high speed or ultra high speed packet data between a mobile station  12  and a base station  34 . In general, the number of carrier frequencies allocated to a communication link (either forward link or reverse link) between a mobile station  12  and a base station  34  will depend on a desired aggregate data transmission rate. The desired aggregate data transmission rate may be based on factors such as the amount of data that needs to be transmitted, quality of service, and network congestion. Once a desired aggregate data transmission rate is determined, the BSC  36  selects the carrier frequencies for the communication link. The specific carrier frequencies may be selected to balance the load across the available carrier frequencies.  
       FIGS. 9 and 10  illustrate techniques that may be employed where a mixture of IS2000 and IS856 carriers are employed. Referring to  FIG. 9 , an RLP stream for a mobile station  12  is divided into multiple parallel substreams by the MC sublayer  70 .  FIG. 9  illustrates two substreams denoted substream  1  (SS 1 ) and substream  2  (SS 2 ). Substream  1  passes through an IS2000 protocol stack and is transmitted over an IS2000 carrier to the mobile station  12 . Similarly, substream  2  passes through an IS876 protocol stack and is transmitted over an IS856 carrier to the mobile station  12 . Transmissions received from the mobile station  12  pass upward through the protocol stacks to the RLP layer.  
      The partitioning of the RLP streams over the different carrier frequencies happens below the RLP sublayer  60 . The RLP sublayer  60  ensures that there is in-order delivery of packets to the upper layer independent of the multiplexing sublayer layer  62  or the physical channels. In this embodiment, the RLP sublayer  60  accounts for the different radio interfaces and ensures that air interface compatible packets are delivered to the different IS2000 and IS856 protocol stacks. Because existing implementations of the RLP sublayer  60  are air interface dependent, this approach requires modification of the existing RLP protocol to handle multiple air interfaces. According to the present invention, a uniform RLP protocol is provided to account for differences in the air interfaces.  
       FIG. 10  illustrates an alternate approach that allows existing RLP implementations for IS2000 and IS856 air interfaces to be used without changes. In the embodiment shown in  FIG. 10 , a new protocol layer, referred to herein as the air interface synchronization layer (ASL), is defined to ensure the ordered sequencing of bits delivered to different carrier frequencies. This approach simplifies standards development and reduces development costs but will add additional overhead to accommodate ASL headers.  
      In any case, those skilled in the art should appreciate that the present invention is not limited by the foregoing discussion, nor by the accompanying figures. Rather, the present invention is limited only by the following claims and their reasonable legal equivalents.