Abstract:
A system and method for expanding traffic channels in a wireless communications system, such as CDMA2000, includes a method for generating a forward link signal including encoding each of a first set of channels with a mutually corresponding channel code, such as a Walsh code, selected from a predetermined set of channel codes, and encoding each of a second set of channels with the same mutually corresponding channel codes. A scramble code is applied to the encoded second set of traffic channels, which is combined with the encoded first set of traffic channels to form an extended forward link signal. Each channel code corresponds to two forward link channels, which are selectively decoded using the scramble code.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to wireless communications systems and more particularly to systems and methods for expanding available communications channels in a wireless communications system. 
     BACKGROUND OF THE INVENTION 
     CDMA2000 (code division multiple access 2000) is a third-generation (3G) wireless communications standard that uses Code Division Multiple Access (CDMA) to facilitate wireless communications between CDMA base stations and mobile devices. Variants of CDMA2000 include CDMA2000 1x, which provides both voice and data services over a standard CDMA channel, and CDMA2000 Evolution Data-Optimized (1xEV-DO) which is optimized for High Data Rate (HDR) capability. Current versions of the CDMA2000 standards are published and approved by the Telecommunications Industry Association and the International Telecommunication Union. 
     In a CDMA2000 1x system, communications channels in a forward link signal transmitted by a base station are identified using Walsh codes. A voice user is assigned one of 64 unique Walsh codes, allowing the voice user to receive and decode data on a corresponding traffic channel. A pilot channel, a sync channel and a plurality of paging channels are also identified using Walsh codes, which reduces the number of Walsh codes available for voice communications. In a system implementing the CDMA2000 1x data mode, each user is similarly assigned a unique Walsh code for each assigned channel which may include a fundamental channel (FCH) or a dedicated control channel (DCCH) for data rates up to 9.6 kpbs. If the user&#39;s date rate is greater than 9.6 kbps, then the user is further assigned a second Walsh code associated with a supplemental channel. 
     As all IP mobile networks become more popular, the number of mobile users attempting wireless packet communications could exceed the system capacity which is limited by the 64 Walsh codes that are typically available. Further, during operation, packet data users assigned Walsh codes may be in a control/hold state during short idle periods between packet bursts. These Walsh codes are not available for use by other users during these periods of inactivity. 
     The 64 Walsh codes used in CDMA2000 systems are mutually orthogonal and were selected to avoid unwanted interference between traffic channel signals transmitted from the same base station. Thus, there are practical limitations associated with adding more Walsh codes. In view of the above, there is a need for an improved system and method for utilizing Walsh codes in wireless communications systems. 
     SUMMARY OF THE INVENTION 
     The present invention is a system and method for expanding traffic channels in a wireless communications system, such as CDMA2000. In one aspect of the present invention, a wireless communications system includes a base station and a subscriber unit. A method for generating a forward link signal includes encoding each of a first set of communications channels with a unique channel code, such as a Walsh code, selected from a predetermined set of codes, and encoding each of a second set of communications channels with the same set of channel codes. A scramble code is applied to the encoded second set of traffic channels, which is combined with the encoded first set of traffic channels to form an extended forward link signal. A pilot channel, sync channel and paging channel may also be coded into the first set of traffic channels. A pseudorandom number is applied to the extended forward link signal to identify the transmitting base station. 
     In another aspect of the present invention, a wireless base station includes means for encoding each of a first set of communications channels with a unique channel code selected from a predetermined set of channel codes, and means for encoding each of a second set of traffic channels with distinct ones of the unique channel codes. A means for applying a scramble code to the encoded second set of traffic channels and means for combining the encoded first set of traffic channels with the encoded and scrambled second set of traffic channels produce an extended forward link signal. 
     In a wireless communications system, a method for decoding a forward link signal includes receiving an identifier of a selected traffic channel, the identifier including a pseudorandom number associated with a transmitting base station, a channel code, such as a Walsh code, associated with a forward link channel in each of a first and second set of communications channels, and a scramble code if the selected traffic channel is in the second set of communications channels. A signal broadcast by the base station is received and decoded using the pseudorandom number. If the selected traffic channel is in the second set of communications channels, the signal is further decoded using the scramble code, which scrambles the first set of traffic channels as a result. The channel code is applied to isolate the selected traffic channel. 
     A more complete understanding of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description. Reference will be made to the appended sheets of drawings, which will first be described briefly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: 
         FIG. 1  is a wireless communications system in accordance with an embodiment of the present invention; 
         FIG. 2  illustrates an embodiment of an extended forward link structure; 
         FIG. 3  is a logical circuit diagram illustrating a base station in accordance with an embodiment of the present invention; 
         FIG. 4  is a flow diagram illustrating an embodiment of a channel selection logic; 
         FIG. 5  is a flow diagram illustrating an operation of a subscriber unit in accordance with an embodiment of the present invention; and 
         FIG. 6  is a logical circuit diagram illustrating a subscriber unit in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     An exemplary embodiment of the present invention will now be described with reference to  FIG. 1 . A wireless communications system  2  is shown as a cell-based communication system including at least one base transceiver system (BTS)  8  including an antenna  4  providing wireless communications services to a plurality of subscriber units  10   a - d , in a geographical coverage area  6 . In operation, each subscriber unit  10   a - d  positioned within the cell  6  communicates with the local antenna  4  by exchanging data packets according to a multiple-access wireless communication standard, such as code division multiple access 2000 (CDMA2000). The wireless communications system  2  facilitates at least one mode of communication such as interconnect, push-to-talk (PTT), email, short messaging service (SMS), multimedia messaging service (MMS) and packet data communications. Each subscriber unit  10   a - d  is a device adapted for communication with the antenna  4 , and may include mobile phones, pagers, personal digital assistants (PDAs), Personal Computer Memory Card International Association (PCMCIA) cards, portable computers and other wireless communications devices. 
     The BTS  8  is connected to a base station controller (BSC)  30 , which is interfaced with a mobile switching center (MSC)  32  and a packet data serving node  34 . The mobile switching center (MSC)  32  manages the wireless communications in the cell  6 , including call set-up, routing calls between the subscriber units  10   a - d  and routing calls between the subscriber units  10   a - d  and at least one communications network, such as a public switched telephone network  38  (PTSN) or a data network  40  such as the Internet. The PDSN  34  facilitates wireless data communications between the subscriber units  10   a - d  and the data network  40 . It will be appreciated by those skilled in the art that the wireless communications system  2  of the exemplary embodiment may include other system components, including a plurality of BTSs, BSCs and MSCs, and may operate using other protocols and modes of communication, consistent with the teachings of the present invention. 
     In the exemplary embodiment, the BTS  8  includes Channel Allocation Logic  12  for managing communications resources including allocating traffic channels to the subscriber units  10   a - d  that request communications resources from the BTS  8 . The Channel Allocation Logic  12  manages at least 128 communications channels including 64 unique Walsh Codes  14  and 64 extended Walsh Codes  16 . 
     Referring to  FIG. 2 , an embodiment of a forward link structure implementing extended Walsh Codes is illustrated. The forward link channel structure includes a plurality of channels  100  each coded with one of 64 Walsh Codes. The channels include a pilot channel  102 , a paging channel  104 , a sync channel  106  and sixty-one traffic channels  108 . The coded channels are combined on a forward link channel  120 . An additional sixty-four traffic channels  110  are each coded with one of the 64 Walsh Codes. The extended traffic channels  110  are combined on a forward link channel and coded with a scrambling code  122 , such as a predefined pseudorandom number. The 128 channels are then combined to form an extended forward link channel  124  which includes 125 traffic channels in the exemplary embodiment. 
     Referring to  FIG. 3 , an embodiment of circuitry  110  for encoding extended traffic channels is logically illustrated. A pilot channel  120  is encoded with a first Walsh code  124 , a sync channel  130  is coded with a second Walsh code  134 , and a paging channel  140  is coded with a third Walsh code  144 . A plurality of traffic channels N 1   150  are each coded by unique Walsh codes  154 . A second plurality of traffic channels N 2  are each coded by Walsh codes  164  which are subsequently coded with a scrambling code  166 . All of the codes are combined at  170  and subsequently coded by a pseudorandom number (PN) code  172  identifying the transmitted base station. A modulation signal  182  is applied before transmission through an antenna  180  for reception by a subscriber unit  182 . 
     Referring to  FIG. 4 , in one embodiment of the Channel Allocation Logic, the BTS tracks channel usage among neighboring base stations in step  150 . A base station receives a request for a traffic channel in step  152  and, in step  154 , determines whether the requesting subscriber unit is capable of decoding Extended Walsh codes and verifies the quality of service (QoS) characteristics of the subscriber. An available traffic channel is selected in step  156  based on the availability of a traffic channel on the transmitting base station, availability of the traffic channel on neighboring base stations, the ability of the subscriber unit to decode extended traffic channels, and the subscriber&#39;s QoS characteristics. In one embodiment, extended traffic channels are allocated to mobile subscribers having subscriber units compatible with the extended Walsh code implementation of the present embodiment. The extended traffic channels, which are coded with the scramble code, will appear as noise to traditional subscriber units allowing backward compatibility. In step  158 , the Walsh Code associated with the selected traffic channel is returned to the requesting subscriber unit, along with a scramble code if the selected traffic channel is an extended traffic channel. 
     Referring to  FIGS. 5 and 6 , an embodiment of subscriber unit logic and circuitry for receiving an extended forward link signal from the base station is illustrated. In step  200 , the subscriber unit receives the Walsh code, the base station&#39;s PN offset and, if used, the scramble code from the base station for use in acquiring the traffic channel assigned to the subscriber unit. In step  202 , the subscriber unit decodes the forward link message using the PN offset of the base station. For each of the traffic channels assigned to the subscriber unit, if the traffic channel has a scramble code (step  204 ) then the forward link is decoded using the received scramble code in step  206 . Next, in step  208 , the received forward link message is decoded using the received Walsh code for the assigned traffic channel. If a scramble code is used, the extended forward link signals will be unscrambled, and the original unscrambled forward link signals will be scrambled, allowing the extended traffic channel to be decoded using the Wash code. The data is then extracted in step  210 . 
     Referring to  FIG. 6 , a base station  240  transmits an extended forward link signal  242  which is received at an antenna  252  of the subscriber unit  250 . A PN selector  254  is set to the base station  240 &#39;s PN and applied to the incoming signal to isolate the signals transmitted by the base station  240 . A scramble code selector  260  is set to the scramble code received by the base station and applied to the signal to unscramble the received signal if an extended traffic channel is used. If no scramble code has been assigned, then the signal passes through without modification. The Walsh Code selector  270  is set to the received Walsh code for the assigned traffic channel and applied to the signal to isolate the traffic channel data  280 . It will be appreciated by those skilled in the art that the logical circuitry illustrated in  FIG. 6  is exemplary and that other subscriber unit configurations may be used consistent with the present embodiment. For example, in one embodiment, the subscriber unit  250  may include separate logic and circuitry for receiving and decoding the pilot channel, sync channel and paging channels. 
     Having thus described various embodiments of the present invention, it should be apparent to those skilled in the art that certain advantages of the within described system have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention.