MULTICARRIER CDMA SYSTEM

BACKGROUND

The present invention relates generally to mobile communication systems, and, more particularly to multicarrier communication systems.

Conventional CDMA communication systems transmit data to users on a single carrier. To meet the demand for wireless high-speed data services, multicarrier CDMA systems will be required. Multicarrier CDMA systems, as the name implies, use multiple carriers to transmit data between a network station and a mobile station. Multicarrier CDMA offers advantages over conventional single carrier CDMA systems such as higher data rates, reduced latency, and improved support for Quality of Service (QoS). Multicarrier CDMA systems will also provide higher spectral efficiency by exploiting frequency selective fading across multiple carriers and through adaptive load balancing.

Existing proposals for multicarrier CDMA systems assume that all carriers employed use the same air interface. However, there may be circumstances where a communication system employs different air interfaces for different carriers. Therefore, there is a need for a multicarrier CDMA system that can accommodate two or more different air interfaces.

SUMMARY OF THE INVENTION The present invention relates to a multicarrier communication system for transmitting data from a transmitting system to a receiving system over multiple carriers. An input data stream is divided into multiple substreams to be assigned to different carriers. An RLP context is established for each carrier to provide a reliable transport mechanism over each carrier. Each substream is transmitted within a corresponding RLP context over an assigned carrier. The RLP contexts may be negotiated and managed independent of the carriers. When a carrier is dropped, the corresponding RLP context may be persisted for a short period to ensure delivery of data. If any data packets are missing at the receiver, another carrier may be selected to send a NAK to the transmitting system. The missing packets may also be transmitted over the selected carrier. When the missing packets are received, the RLP context for the dropped carrier may be closed.

In some embodiments, different air interfaces may be employed on different carriers. For example, the communication system may comprise a mix of cdma2000 and EVDO carriers. Other access technologies, such as WIMAX, may also be supported.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates an exemplary multicarrier communication system. Fig. 2 illustrates an exemplary multiplexing circuit for a transmitting system in a multicarrier communication system.

Ffg\ ^ll # MlistrlateS 'ϋfeximplary de-multiplexing circuit for a receiving system in a multicarrier communication system.

Fig. 4 illustrates an exemplary protocol stack for a multicarrier communication system.

Fig. 5 illustrates the transmission of an octet stream in a multicarrier communication system.

Fig. 6 illustrates an exemplary RLP packet for a multicarrier communication system.

Fig. 7 illustrates the transmission of a packet stream in a multicarrier communication system.

Fig. 8 illustrates the transmission of a packet stream with fragmentation in a multicarrier communication system.

Figs. 9A & 9B illustrate retransmission of RLP packets in a multicarrier communication system.

Fig. 10 illustrates a multicarrier communication system with two or more different air interfaces. Fig. 11 illustrates a multicarrier communication system with a multilink PPP layer.

Fig. 12 illustrates a multicarrier communication system with IP fragmentation and multilink PPP layers.

DETAILED DESCRIPTION Referring now to the drawings, Figure 1 illustrates a multicarrier communication system 10 according to one embodiment of the invention. An information stream I is input to a transmission system 20 and transmitted over a communication channel 14 to a receiving system 40. A multiplexing circuit 22 in the transmission system 20 divides the information stream I into N substreams for transmission to the receiving system 40 over N carriers. The number of carriers may be determined based on criteria such as channel conditions, data transmission rates, quality of service, etc., and may be dynamically varied during a communication session. Each substream is assigned to a respective one of the N carriers, which may be referred to in the context of the present invention as the frequency assignment (FA) of the substream. There is a 1 -to-1 mapping of substreams to carriers. A separate RLP context is established for each substream/carrier, which are managed by one or more RLP controllers 24. Transmit circuits 26 encode, modulate, and transmit each substream over the assigned carrier.

At the receiving system 40, receive circuits 42 receive, demodulate, and decode each substream. One or more RLP controllers 44 manage the RLP context for each carrier and send a negative acknowledgment when RLP packets are missed. A de-multiplexing circuit 46 reassembles the original information stream I from the received substreams.

Figure 2 illustrates the multiplexing circuit 22 in more detail. The multiplexing circuit 22 includes a framing circuit 28 and a scheduler 30. Framing circuit 28 receives the input

intσrmaticw-streamrWhiOTmay comprise an octet stream or packet stream. The framing circuit 28 divides the information stream into frames and adds a header to each frame. The frames are referred to herein as multicarrier frames. The header of each multicarrier frame includes a sequence number that is used by the de-multiplexing circuit 46 to reassemble frames at the receiving system 40. As will be described in more detail below, the multicarrier frames output by the framing circuit 28 may be further segmented. The multicarrier frames, or frame segments, are passed to a scheduler 30.

The scheduler 30 assigns each multicarrier frame or frame segment to one of N carriers. The simplest form of scheduling would be to assign the multicarrier frames in round robin fashion to each of the N carriers. However, this approach would not make the most efficient use of resources. A better approach is to distribute the frames across the N carriers to balance the load based on data transmission rates, data throughput rates, latency, or other performance criterion. For example, the scheduler 30 could distribute the multicarrier frames across the N carriers proportionately based on the effective throughput rate on each carrier. Another approach would be to distribute the frames so as to maintain the packet latency roughly the same across all carriers. Those skilled in the art will appreciate that numerous other scheduling algorithms could be employed and that the present invention is not limited to a particular scheduling algorithm.

Figure 3 illustrates an exemplary de-multiplexing 46 at the receiving system 40. The de- multiplexing circuit 46 includes a reassembly circuit 48 for reassembling the information stream from the individual substreams. The reassembly circuit 48 removes the header from the multicarrier frames received over each substream and reassembles the original information stream from the received data.

Figure 4 illustrates an exemplary protocol stack for a multicarrier communication system. The protocol stack generally follows the OSI Reference Model. The protocol layers include the physical layer (layer 1), the link layer (layer 2), and the upper layers (layers 3 - 7). 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 despreading of received signals. An advantage of the present invention is that no changes in the physical layer are required because the division of the information stream occurs in higher layers of the protocol stack. The link layer includes the medium access control (MAC) layer, the Radio Link Protocol (RLP) layer, and the multicarrier framing (MCF) layer. The MAC layer provides multiplexing of logical data and signaling channels onto the physical channels and provides quality of service (QoS) management for each active service. The RLP layer, which may be incorporated into the MAC layer, implements a retransmission protocol such as an automatic repeat request (ARQ) protocol to provide reliable transport of user data over the radio link. The MCF layer (MCFL) is a new protocol layer that supports multicarrier transmission of data as

layers (layers 3- 7) provide services such as signaling services, voice services, and data services.

Figure 5 schematically illustrates an implementation of the MCFL for transmission of an octet stream. In this example, the information stream comprises an octet stream. The octet stream is divided into blocks and a header is added to each block to generate the multicarrier frames. The header comprises a start bit, a 14-bit sequence number, and a stop bit. The start and stop bits are used to delimit the multicarrier frames. The sequence number is used for resequencing the frames at the receiving system 40. A wrapping or circular sequence number space is used. Thus, when the last sequence number is used, numbering restarts at the beginning of the sequence number space. The multicarrier scheduler 30 distributes the multicarrier frames across the available carriers. In the example shown in Figure 5, two carriers or frequency assignments are shown. An RLP context is established for each carrier or frequency assignment. For each frequency assignment, the RLP controller 24 adds an RLP header to the multicarrier frame as shown in Figure 6. Thus, each RLP packet includes both an RLP header and multicarrier framing header. In some implementations, multiple RLP packets may be included in a single physical layer frame. In other implementations, a single RLP frame may be divided among multiple physical layer packets.

As is well known in the art, the RLP controller 24 implements a retransmission protocol. When the RLP packets are received out of sequence, the RLP controller 44 at the receiving system 40 sends a negative acknowledgment (NAK) to the RLP controller 24 at the transmission system 20. The NAK' ed RLP packet is then scheduled for retransmission. The retransmitted packet carries the same multicarrier framing header.

Figure 7 schematically illustrates an implementation of the MFC layer for transmission of a packet stream. In this example, an Internet protocol (IP) packet stream is being transmitted. Each IP packet is inserted into a multicarrier frame. Each multicarrier frame carries a single IP packet. As in the previous embodiments, the header of the multicarrier frame includes a multicarrier sequence number. The multicarrier scheduler 30 distributes the multicarrier frames 30 across the available carriers. In this example, two carriers or frequency assignments are shown. A separate RLP context is created for each carrier or frequency assignment. Figure 8 illustrates an implementation of the MFC layer in which multicarrier frames are fragmented. For delay/jitter-sensitive traffic, additional efficiency can be gained by fragmenting or segmenting a multicarrier frame and sending the frame segments over different carriers or frequency assignments. The fragmentation or segmentation of the multicarrier frames may be performed by the framing circuit 28, and the reassembly of the multicarrier frames may be performed by the reassembly circuit 48. When the multicarrier frames are fragmented, a fragmentation header is added to each frame segment to enable reassembly of the frames at the receiver.

..PfWcϋr'Ifiife'erfibefclim'ihte, the frequency assignments may be changed during a communication session. When a frequency assignment is dropped, there may still be missing packets within the corresponding RLP context. In this case, the RLP context is persisted or maintained after the carrier is dropped. The RLP controller 44 at the receiving system 40 may tunnel signaling (e.g., NAKs) to one of the remaining carriers. The RLP controller 24 at the transmitting system 20 may tunnel the retransmitted RLP packets to one of the remaining carriers. Thus, the receiver may send NAKs for packets transmitted over a dropped carrier to one of the remaining frequencies. Similarly, the retransmitted packets are tunneled to an existing carrier for transmission to the receiving system 40. Figures 9A and 9B illustrate exemplary operations when a carrier is dropped. Figures

9A and 9B illustrate transmission of data over two carriers: FAx and FAy. Multicarrier frames 1 , 2, 4, and 5 are transmitted over FAx. Multicarrier frames 3 and 6 are transmitted over FAy. Portions of frames 2, 3, and 6 are lost. After the transmission of frames 3 and 6, the carrier associated with FAy is dropped. Referring to Figure 9B, the receiving system 40 NAKs the missing portion of frame 2 in a conventional manner by sending the NAK over FAx. Because the carrier associated with FAy has been dropped, the NAK for frames 3 and 6 are tunneled to the transmission station over the carrier associated with FAx. Similarly, the retransmitted packets are tunneled to the carrier for FAx and transmitted to the receiving system 40. The RLP context for FAy can thus be maintained after the carrier for FAy is dropped until the missing packets are received at the receiving system 40, or until the receiving system times out and forwards the RLP block with missing packets.

Figure 10 illustrates an implementation in which the multicarrier framing level is configured to operate across multiple carriers with different air interfaces. In this example, carrier FAx supports communications with a mobile station according to TIA-2000 standard. The carrier for FAy supports communications with the mobile station according to TIA-856. Figures 11 and 12 illustrate implementations using the multilink point-to-point (PPP) protocol to divide an IP packet stream into multiple streams. Multilink PPP is based on RFC 1717 published by the Internet Engineering Task Force (IETF). This embodiment has the added complexity in maintaining a PPP link for each carrier, but avoids the additional overhead introduced by the multicarrier framing layer. In this embodiment, the PPP layer manages multiple PPP instances, each of which is associated with a particular carrier. In situations where IP packets are large, a fragmentation layer may be interposed between the IP layer and PPP layer to fragment the IP packets as shown in Figure 12.

The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.