Abstract:
We present a novel method, Virtual Soft Hand-off for wireless IP-centric CDMA networks whose routers, hosts and mobile stations have small group multicast (SGM) capability. The underlying ideas of the virtual soft hand-off method are that it views soft hand-off support as setting up an explicit synchronous multicast session among a small group of participants with dynamic group membership, the packets transmitted over the multicast session are time stamped to help maintain synchronization and the receivers recover multiple the packet flows from the multiple received signals concurrently, and combine them to create a single packet flow at higher layers.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a non-provisional application of provisional application Serial No. 60/278,014 filed Mar. 22, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to the use of CDMA for the design of IP-Centric wireless networks. Specifically, our invention is a method and apparatus for providing a virtual soft-hand-off technique in an end-to-end Internet Protocol (IP) wireless communications system, which uses code division multiple access (CDMA) transmission technology at the physical layer.  
         BACKGROUND OF THE INVENTION  
         [0003]    Today, many different wireless systems exist, ranging from indoor wireless LANs (Local Area Networks) to outdoor cellular systems. Generally, the numerous wireless systems are not compatible with each other, making it difficult to roam from one system to another. Although there have been attempts to unify third-generation wireless systems, incompatible systems are expected to co-exist in the future. Furthermore, wireless LANs and cellular wireless systems are being developed independently, and such systems are also evolving independently. So far, no wireless technology has emerged as a common and long-term universal solution.  
           [0004]    IP (Internet Protocol), which is already a universal network-layer protocol for packet networks, is rapidly becoming a promising universal network-layer protocol for wireless systems. An IP terminal, with multiple radio interfaces, can roam between different wireless systems if they all support IP as a common network layer. Unlike today&#39;s wireless systems in which Radio Access Networks (RANs) are mostly proprietary, IP provides an open interface and promotes an open market. IP will also enable widely adopted and rapidly growing IP-based applications to run over wireless networks. Moreover, distributed, autonomous IP-based wireless base stations have the potential of making the wireless systems more robust, scalable, and cost effective. There is, as a result, a current trend towards the design of Internet Protocol (IP) centric wireless networks comprising a large number of cells each covered by an autonomous IP router base station.  
           [0005]    There are, however, many challenges to realizing distributed all-IP wireless networks. For the sake of specificity in discussing these challenges as well as pointing out problem areas, reference is made to FIG. 1. The depiction of network  100  in FIG. 1 illustrates an exemplary configuration of a network that uses IP-based wireless base stations (designated BSs). The coverage area of the wireless network is defined by a multiplicity of cells (e.g., cells  101 ,  102 ,  103 ). The geographical area covered by each wireless base station is referred to as a cell (e.g., BS  111  serves cell  101 , and so forth). When mobile station  104  moves from one cell (e.g., cell  101  originally) into the overlapping regions (e.g., overlap of cells  101  and  102 ) of the coverage areas of multiple base stations, base station  111  may perform a “hand-off” of mobile station  104  to base station  112 . Hand-off is a process whereby a mobile station communicating with one wireless base station is switched to another base station during a session. Overlap regions  117  and  118  are coverage areas where hand-off is effected. For example, as mobile station  104  moves into region  117  while roaming in cell  101 , the radio signal strength from BS  2  (depicted by reference numeral  115 ) may be greater than the radio signal strength ( 114 ) from BS  1 , so hand-off is warranted to maintain the quality of the established session.  
           [0006]    Among the key challenges in a distributed all-IP wireless network is how to support “soft hand-off”. As suggested above, hand-off is the process that allows a mobile station&#39;s session-in-progress to continue without interruption when a mobile station (MS) moves from one wireless cell to another. Soft hand-off is a form of hand-off whereby a mobile station can start communication with the target base stations without interrupting the communication with the serving base station. Thus, soft hand-off allows a MS to communicate with multiple base stations (BSs) simultaneously. In particular, soft hand-off has been shown to be an effective way for increasing the capacity, reliability, and coverage range of CDMA-based wireless networks. Soft hand-off also provides more time for carrying out the hand-off procedure.  
           [0007]    Soft hand-off in a CDMA-based wireless system is the focus of the subject matter of the present invention. In Code Division Multiple Access (CDMA) radio systems, a narrowband user message signal is multiplied by a very large bandwidth signal called the spreading signal. The spreading signal is a pseudo-noise code sequence that has a communication signal rate, which is an order of magnitude greater than the data rate of the user message signal. All users in a CDMA system may transmit simultaneously. Each user has its own pseudorandom code for coding its own message signal - each code is approximately orthogonal to all other codes. A receiver is assigned a code to detect a desired user message signal, and performs a time correlation operation to detect only the specific assigned code. All other codes appear as noise due to de-correlation. CDMA is effective in wireless systems because a receiver can be assigned a multiplicity of codes to detect message signals from a corresponding multiplicity of transmitters, thereby engendering the soft hand-off process.  
           [0008]    An IP router is an IP network device that runs IP layer routing protocol (e.g., OSPF and BGP) and forwards IP packets. The running of a routing protocol decides the “routing policy ”, and the forwarding of IP packets realizes the “routing mechanism”. IP packets arriving from the wireline IP network ( 121 ) at a given base station (e.g., BS  111  over wireline path  122  or BS  112  over path  123 ) can be routed by the routing mechanism of the base station to mobile station  104  (or other appropriate wireline devices that connect directly to the base station).  
           [0009]    The fact that mobile stations (MSs) in different cells belong to different IP subnets suggests that a CDMA MS has multiple IP addresses as well as changes at least one of these IP address every time it moves into a new cell. Apart from additional delay due to getting and changing of IP addresses, as well as possible modification of the end-to-end session between an MS and its corresponding host, having multiple IP addresses per MS makes soft hand-off more difficult to implement because an MS receives multiple IP packet streams from different BSs with different traffic loads. The contents of these IP packet streams coming to the MS from different BSs will not be synchronized because copies of the same packet on each of these streams either experience different delays at different BSs, or may be lost altogether before reaching the MS. Consequently, copies of the same packets from different base stations may not be correctly combined by the MS&#39;s radio system. As a matter of fact, in this case, it is also possible that the MS&#39;s radio system combines copies of different packets received from different BSs.  
           [0010]    From another viewpoint, in today&#39;s circuit-switched CDMA networks such as IS-95, a centralized Base Station Controller (BSC) is responsible for data distribution in the forward direction (from BS to MS). The BSC creates and distributes multiple streams of the same data over layer- 2  circuits to multiple BSs that in turn relay the data to the MS. The MS&#39;s radio system (typically working below the IP layer) collaborates with the BSs to synchronize the radio channel frames and combine the radio signals received from different BSs to generate a single final copy of received data. The BSC helps ensure data content synchronization by ensuring that the matching layer- 2  frames sent to different base stations contain copies of the same data. In the reverse direction (from MS to BS), the MS ensures that the matching layer- 2  frames sent to different BSs contain copies of the same data. The BSC then selects one of the frames received from different base stations as the final copy of the data.  
           [0011]    One problem already alluded to is loss of data content synchronization. With distributed BSs, centralized control entities, such as the BSC in circuit switched wireless networks, will no longer exist. Consequently, even though the CDMA radio system is capable of synchronizing the link and physical layer frames on the radio channel, it cannot, on its own, guarantee that the matching frames from different base stations will carry copies of the same data. For example, IP packets can be lost on their way to the MS, creating random gaps in the packet streams received by the MS from different BSs. Furthermore, copies of the same data may arrive at the MS at different times due to the random delays suffered by the packets. Random gaps and delays can lead to a loss of data content synchronization. Suppose that packet X is lost at BS  1  (due to, for example, buffer overflow) but is not lost at BS  2 . Then, another totally unrelated packet Y from BS  1  and packet X from BS  2  may arrive at the MS at the same time and the MS&#39;s radio system will not be able to tell that they are not copies of the same data and will hence erroneously combine X with Y.  
           [0012]    The art is devoid of a methodology and concomitant systems that effect soft hand-off in an all-IP wireless network that uses autonomous BSs in a configuration having the following characteristics that differentiate the configuration from existing wireless networks: (a) the BSs use IP protocols for both signaling and transport of user traffic. For example, they may route/forward IP packets based on information carried in the IP headers, perform IP-layer signaling, mobility management and Quality of Service (QoS) management functions; (b) the BSs function autonomously. There is no centralized signaling and control over the behaviors of the BSs; (c) the BSs are interconnected via an IP network, which could have arbitrary network topology such as bus, ring, star, tree, etc.; and (d) the cells (a cell is a geographical radio coverage area of a BS) can be arranged in any arbitrary configuration.  
           [0013]    In order to support soft hand-off and satisfy its stringent synchronization requirements, current CDMA RANs usually take the following measures. A CDMA RAN provides means of stringent synchronization among its elements. The BSC manages the transmission power of MSs and BSs to ensure low error rate as well as minimize the power consumption. Moreover, as the MS moves, the BSC interacts with it to select and maintain an “optimum” set of BSs with which the MS remains in contact. On the forward link the BSC receives packets destined for the MS segments and assembles them into radio frames, and replicates the radio frames and transmits copies to BSs that are currently in contact with the MS. On the reverse link, the BSC collects copies of the radio frames received from BSs that are currently in contact with the MS, selects one of them, and synthesizes IP packets for forwarding to the wireline backbone.  
           [0014]    Today&#39;s CDMA networks use BSCs as the focal anchoring points for frame distribution and selection as well as content synchronization during soft hand-off. However, IP-centric networks do not employ a BSC and as such there is no focal anchoring point for control of soft hand-offs. However, without a BSC process to control the transmissions from the Base Stations, the soft hand-off process in a IP-centric CDMA environment suffer from: (i) the lack of a means for efficient distribution of packet flows to multiple BSs, (ii) loss of content synchronization because the multiple packet flows destined for the MS experience different loss and delay across the network, (iii) inaccuracy of signal combination at the radio (i.e. physical) layer caused by the combination of radio signals at the physical layer of the MS resulting in erroneous synthesis of packets at higher layers because contents of their packet flows within received signals are not synchronized, and (iv) the need to have the MS select and maintain their active set of BSs.  
           [0015]    Thus there is a need in the art for a soft hand-off process to be used in IP centric CDMA networks that overcome the limitations of the prior art as set forth above.  
         SUMMARY OF THE INVENTION  
         [0016]    These shortcomings and other limitations and deficiencies are obviated in accordance with the present invention for a wireless IP-centric CDMA networks whose routers, hosts and mobile stations have: small group multicast (SGM) capability, the ability to time stamp all packets transmitted, and the ability to mix multiple flows at a higher layer than the physical layer in the protocol stack. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 illustrates the architecture of an IP centric CDMA wireless network.  
         [0018]    [0018]FIG. 2 illustrates the operation of soft hand-off in accordance with one embodiment of our invention.  
         [0019]    [0019]FIG. 3 illustrates a schematic diagram of a CDMA with a soft hand-off capability in accordance with our invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]    Our invention is a virtual soft hand-off method and transceiver that resolves aforementioned issues in IP-centric CDMA environments. The virtual soft hand-off is built upon two simple ideas and a basic requirement. Let us start with the ideas and then explain the requirement. First, each Mobile station receives and maintains a separate IP address for each cell (i.e. BS) with which it is active and the MS sets up an explicit synchronous multicast session among each of its addresses and the device (another MS or host) to which it would connect. Second, unlike current art soft hand-off schemes, it recovers multiple packet flows from the multiple received signals concurrently, and combines them at a higher layer in the protocol stack to create a single packet flow. The basic requirement for the realization of virtual soft hand-off is that each element in the network (e.g. routers, hosts, and MSs) must be small group multicasting (SGM) capable. SGM is a known process used for small group multicasting (e.g., teleconferencing) in an IP environment.  
         [0021]    To be SGM capable a host and/or MS keeps track of destinations that it wants to send packets to, and create packet headers that contain the list of destination addresses. We refer to these headers as SGM headers, hereafter. Each router must be able to parse the SGM header, partition destinations based on each destinations next hop, and forward an appropriate SGM packet to the next hops.  
         [0022]    As an MS moves within a wireless network it will move into new radio cells serviced by a BS for which it must establish contact. When an MS detects a new pilot signal from the new BS it determines whether the signal strength of the pilot signal is sufficient to sustain communication, then the MS itself signals this new BS to commence communication with it. This differs from prior art CDMA implementations because in the prior art the MS contacts its current BS and the BSC acts as the intermediary between the MS and the new BS. In our inventive network, a BS receives confirmation from the new BS/ERC directly, the MS updates its list of active BSs accordingly. As the MS commences communication with a new BS/ERC, it obtains a new IP address in the new subnet as well as keeps (or extends the lease of) its old IP address. In an IPv6 environment, an MS may use either Dynamic Host Configuration Protocol (DHCP) or a Forward Address Assignment scheme for obtaining new addresses. In our IPcentric CDMA environment joining a new cell is the same as joining a new subnet. Typically, the CDMA transceiver employs a RAKE receiver, where each of the fingers in the Rake receiver has an IP address as if it were a separate MS.  
         [0023]    A CDMA Rake receiver may also have a Session Initiation Protocol (SIP) Uniform Resource Locator (URL) as well. For instance, if the Rake receiver of an MS has 3 fingers, they may be referred to as Y1≡“user@host 1”, Y2≡“user@host2”, and Y3≡“user@host3”. This paradigm requires that a SIP user agent be able to support multiple URLs as well as IP addresses.  
         [0024]    Once a MS establishes its IP address and multicasting session with the select set of BSs, it periodically sends a “request message” to all BSs in its active list, and requests them to inform the MS about the strength of signals on its reverse links. The MS transmits its outbound packets to the BS whose reverse path signal strength is the strongest. Whenever an MS decides to remove a BS from its active list, it releases the corresponding IP address, if necessary.  
         [0025]    In our invention, the radio signals are not mixed at the physical layer (i.e., the radio signal mixer of a typical CDMA receiver is disabled). The multiple packet flows received by each IP address resident in the Mobile station are forwarded to the IP layer where they will be mixed (at IP++layer) to synthesize a single IP packet flow for higher layers. In order to effect this mixing capability, each MS and host in our IP-centric CDMA network, must time stamp each IP packet before forwarding it to the network to ensure strict synchronization at the receiving MSs. This time stamp serves as a sequence number within each flow (one may use RTP to provide this time-stamp for real-time applications or place the stamp in an extension header of IPv6), and may also be used for forward error correction to protect against errors on the wireless media. An end-to-end delay equalization method [see J. Escobar, C. Partirdge, and D. Deutsch. “Flow Synchronization Protocol”, IEEE/ACM Trans. On Networking, Vol.2 No. 4, Apr. 1994] is used for content synchronization.  
         [0026]    Each packet is forwarded to the higher layer mixer at time (Ts+T), where Ts is the time stamp within the packet, and T is an acceptable buffering delay within the MS to compensate for the different delays a mulitcasted packets may endure over the different paths may take. This mixer is at the IP++layer in the protocol stack. The mixer uses a simple majority rule to combine the content of packets with identical time stamps. To illustrate, suppose there are N soft hand-off legs and therefore N packets are forwarded to MS. The mixer in the MS reads each bit of these N packets concurrently. If the number of legs is odd (i.e. N is an odd number) then the value of each bit in a packet is determined by majority rule. As an example, if there are three legs of the transmission at a MS and the mixer was attempting to ascertain the value of the first bit in the packet. The mixer would like at the first bit in each packet and select as the value of the first bit, the value that appears at least two times. If the number of legs is even (i.e. N is an even number) then the mixer uses a weighted average based on the normalized signal strength of the received radio signal on each leg. Specifically, if B j (k), 1≦j≦M where M is the packet size, and 1≦k≦N, represents the value of the jth bit of a packet on k-th packet stream and if and P(k) represents the signal strength of the k th  leg, if X=Σ k P(k) B j (k)&gt;N/2 the jth bit is set to 1. Otherwise, the jth bit is set to 0. Under the third circumstance when N is an even number and X=N/2, the mixer sets the value of the bit to the one received on the channel with strongest pilot.  
         [0027]    The operation of our soft hand-off method is illustrated using the network depicted in FIG. 2. For the sake of discussion, suppose that an MS  201  and its corresponding host (CH)  202  are already engaged in an ongoing SIP session over links  205   a  and  205   b  in an established multicast session through BS/ERC-  204   a  and  204   b  respectfully where the members of the multicast session are the CH  202  and the MS IP address  206  and MS IP address  207 . When the MS enters the intersection of cells  203   b  and  203   c  and it decides to initiate communication with BS/ERC- 204   c , on the forward link (i.e., the link from the BSs to the MSs), virtual soft hand-off operates as follows:  
         [0028]    The MS  201  detects the pilot  205   c  of BS/ERC 204   c  assesses whether it is strong enough to establish communication. It then sends a request for communication to BS/ERC- 204   c . The BS/ERC- 204   c  accepts the request and responds with necessary radio parameters. The MS  201  requests for a new IP  208  address from DHCP process within the BS/ERC- 204   c  in subnet  3  (i.e., cell  3 ) as well as extends the lease of its current IP address  207  in subnet  2   203   b  (note that the subnets are synonymous the individual Pico-cells). The MS  201  sends a SIP REFER message with the new IP address of its link with the BS/ERC  202   c  in its Contact field to the CH user agent indicating that MS  201  wants CH  202  to invite this new IP address  208  to the session. In this scenario, the MS  201  IP address  207 , the CH  202 , and the MS IP address  208  on its link to BS/ERC  204   c , play the roles of the “Transferor”, the “Transferee”, and the “Transfer Target”, respectively. Note that this REFER request has no impact on the current ongoing session between the MS  201  and the CH  202  from IP address  206  and IP address  207 , and leave them intact. The CH invites IP address  208  to the session by sending a SIP INVITE message. After the receipt of  200  OK from MS&#39;s IP address  207 , the CH SIP  202  sends the ACK to IP address  208 , and IP address  208  becomes part of the multicast session. Next, the CH  202  SIP user agent send a NOTIFY message to the IP address  207  to inform it about the result of its REFER request. The IP address of  208  is added to the SGM destination list at the CH  202 . The multicast session is established and the data transfer continues on it. The CH  202  sends time-stamped packets with SGM headers to IP address  206 ,  207  and  208 . The MS  201  receives the multiple identical packets, and forwards the multiple packet flows to the IP layer so that it synthesizes a single packet flow for higher layer in accordance with our inventive process described above. When a BS/ERC is removed from the active list and replaced with a new one, the associated finger gets a new IP address in the new subnet. Then, MS  201  sends a REFER message to the CH 202  indicating that the CH should invite the finger at its new address to join the session. The CH should also update its list of SGM destinations accordingly.  
         [0029]    On the reverse link, the MS measures the signal strength of all BS/ERCs on the reverse link, and transmits packets to the one that has best signal quality. Of course, the source address in the outbound IP packets will be the same as that of the finger that transmits on the selected reverse link.  
         [0030]    [0030]FIG. 3 depicts a schematic diagram of a CDMA receiver in accordance with our invention. Specifically, the receiver  301  a means for receiving the radio signals at the physical layer  303 . Each finger ( 304   a, b , and  c ) can be considered a separate physical receiver. Each finger ( 304   a ,  304   b  and  304   c ) receives a signal from a different base station. The fingers forward their received signals to the a corresponding function at the IP layer ( 305   a ,  305   b , and  305   c ). Each corresponding IP layer for each finger sends its received packet flows to the mixer  306  which combines the received packet flows in accordance with the method described above into a single packet flow  307  for forwarding to the higher layers  308 .  
         [0031]    The above description has been presented only to illustrate and describe the invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. The applications described were chosen and described in order to best explain the principles of the invention and its practical application to enable others skilled in the art to best utilize the invention on various applications and with various modifications as are suited to the particular use contemplated.